Acid treatment and chemical extracting agents can be used to remove heavy metals from wastewater and sludge: **Highlights:** * The concentration of heavy metals in excess sludge from municipal wastewater treatment plants has restricted its conversion to soil fertilizer, so it is necessary to remove the heavy metals from sludge. In this article, **the effects of heavy metal removal by some mineral acids such as HCl, H2SO4, H3PO4, HNO3 were analysed**, when treating excess sludge from a second sediment tank in municipal wastewater treatment plants. To improve the treatment, a combination of H2O2 and acid was also investigated. Aided by H2O2, the removal by the acid was enhanced. The suitable dosage of H2O2 was specially related to the solid content of sludge, pH and the acid kind.[ \[Art. #ARTNUM\]](#article-96099-2081073710) * For chemical extraction, because of the adverse impacts that can result from the use of inorganic acids and complexing agents, interest can be directed more toward **utilizing organic acids as extracting agents** because of their biodegradability and capability to remove metals at mildly acidic condition, hence requiring less acid. [\[Art. #ARTNUM\]](#article-96099-2140127378) * Demetalization of Pb, Ni, Zn, Cu and Cr heavy metal ions from sea sand and real samples of sewage sludge by subcritical water and supercritical carbon dioxide was investigated. **The best extraction efficiencies were obtained using acetyl acetonate as a chelating agent in both subcritical water** and supercritical CO2 extractions for real and artificial samples. The highest recoveries obtained from real samples for Cr, Cu, Ni, Pb and Zn were 77.25, 95.1, 84.82, 94.92 and 98.39 %, respectively, using the chelating agent in the subcritical water extraction.[\[Paper\]](https://www.researchgate.net/publication/261859713_Subcritical_and_supercritical_fluid_extraction_of_heavy_metals_from_sand_and_sewage_sludge)

Supercritical fluid extraction is the process of separating one component from another using supercritical fluids as the extracting solvent. Extraction is usually from a solid matrix, but can also be from liquids.[\[Wiki\]](https://en.wikipedia.org/wiki/Supercritical_fluid_extraction) **Highlights:** * SFE also gave higher Cr removal efficiency than bioleaching.  The SFE process also gave good results for Cu, Mn and Zn removal.  the SFE method, on the other hand, is limited by the complexity of the process and the cost of ligands suitable for effective metal extraction. [\[Art. #ARTNUM\]](#article-96103-2140127378) * Demetalization of Pb, Ni, Zn, Cu and Cr heavy metal ions from sea sand and real samples of sewage sludge by subcritical water and **supercritical carbon dioxide** was investigated. Experimental parameters such as temperature, pressure, extraction time in the static and dynamic mode and sampling were optimized in order to determine the suitable conditions for high removal of metals. The best extraction efficiencies were obtained using acetyl acetonate as a chelating agent in both subcritical water and supercritical CO2 extractions for real and artificial samples. Samples collected from extraction system using both subcritical water and supercritical carbon dioxide were analyzed by inductively coupled plasma mass spectrometry (ICP-MS). The highest recoveries obtained from real samples for Cr, Cu, Ni, Pb and Zn were 77.25, 95.1, 84.82, 94.92 and 98.39 %, respectively, using the chelating agent in the subcritical water extraction.[\[Paper\]](https://www.researchgate.net/publication/261859713_Subcritical_and_supercritical_fluid_extraction_of_heavy_metals_from_sand_and_sewage_sludge)

Precipitation using a precipitation agent (usually in the form of hydroxide/ lime) is a common separation technique, which can both remove heavy metals and phosphorus compounds. Other (inorganic) chemical precipitants have also been used, such as alum (for phosphorus removal) and ferric salts (FeCl3). **Highlights:** * Chemical precipitation is the most widely used for heavy metal removal from inorganic effluent. Adjustment of pH to the basic conditions (pH 9–11) is the major parameter that significantly improves heavy metal removal by chemical precipitation. **Lime and limestone are the most commonly employed precipitant agents due to their availability and low-cost in most countries.** [\[Paper\]](https://www.sciencedirect.com/science/article/pii/S1878535210001334#:\~:text=2.3.&text=The%20conventional%20processes%20for%20removing,metal%20removal%20from%20inorganic%20effluent.) * Presently, phosphorus is traditionally removed from wastewater using chemical precipitation or biological methods, ending up in unusable products or excess sludge. **Chemical (nonmetallic) precipitation, under controlled condition, could constitute a sustainable approach for achieving this goal, with the formation of struvite (magnesium ammonium phosphate hexahydrate) or other calcium phosphate compounds**, which have great potential of being used as fertilizers. In this study, experimental and pilot-test conditions for recovery of P-based fertilizers from aerobically treated sludge are analyzed, estimating at the same time the economic and energy consequences of several process approaches on the treatment facility operation. [\[Art. #ARTNUM\]](#article-96072-2920892624) * The reduction of sulfate ions from the industrial wastewater using advanced **calcium-aluminum precipitation** method (ACAPM) has been evaluated. The ACAPM involved the precipitation of sulfate ions as minerals like gypsum and ettringite via calcium/ aluminum compounds. Experimental results indicate that the ACAPM was effectively reduced sulfate with removal efficiency exceed 98%. It also effectively removed heavy metals including: Cr, Ni, Cd, Pb, Fe, Mn & Zn, with removal efficiencies up to 99%. Results showed that the ACAPM has remarkable sulfate ions removal efficiency with high concentrations.[ \[Art. #ARTNUM\]](#article-96072-2989754776) * Phosphorus removal is an important issue in the field of water and wastewater treatment. In this study, a starch-based flocculant (St-CTA), as an assisting agent, was fed after the application of traditional inorganic precipitant of **ferric chloride (FeCl3) to remove inorganic and organic phosphorus** from their respective simulated turbid wastewaters.[ \[Art. #ARTNUM\]](#article-96072-2991298135) * Wastewater samples from battery, paint and textile industries were treated with different doses of locally **available alum, aluminum sulphate and ferric chloride** in order to determine and compare their effectiveness in removing heavy metal contents from the wastewaters.[ \[Art. #ARTNUM\]](#article-96072-2008496022)

Sulfide precipitation uses a sulfide precipitant, such as Na2S, NaHS and H2S to precipitate heavy metals, which generally results in a more complete removal than using OH. **Highlights:** * **Sulfide as a product of dissimilatory sulphate reduction may remove heavy metals from wastewaters via the precipitation of metal sulphide and the use of CWs in treating heavy metal containing waters (e.g. acid mine drainage) is increasing.** [ \[Art. #ARTNUM\]](#article-96097-2052401730) * Sulfide precipitation of heavy metal containing wastewaters results in low effluent concentrations. However, sulphide precipitation is not widely applied in practice because the dosing of sulphide cannot adequately be controlled. **A new process was developed where the combination of a sulphide-selective electrode (pS-electrode) and pH electrode controls the sulphide addition. The metals were removed to levels <0.05 mg.l^-1^ at pH 6.0 by sulphide precipitation while maintaining a total sulphide concentration <0.02 mg.l^-1^.** [\[Paper\]](https://iwaponline.com/wst/article/47/10/9/10266/Innovative-developments-in-the-selective-removal)

Ferrite (Fe~3~O~4~), a magnetic iron oxide containing Fe^2+^ and Fe^3+^ in the structure, has spinel structure and can be synthesized through the reaction depicted by Eq: 3Fe^2+^ + 6OH^−^ + 1/2O~2~ → Fe~3~O~4~ + 3H~2~O When heavy metal ions coexist with Fe^2+^, they can be incorporated into the structure through co-precipitation. The principle of FP to treat heavy metals is presented in Eq.:  *x*M^2+^ + (3 − *x*)Fe^2+^+ 6OH^−^ + 1/2O~2~ → M\*~x~\*Fe~(3−\*x\*)~O~4~ + 3H~2~O **Highlights:** * Complete removal of heavy metal from complex heavy-metal wastewater (CHMW) requires advanced technology. **This study investigated the feasibility of a multi-staged ferrite process (MSFP) for treating CHMW, containing Cd, Cu, Pb, Cr, Zn, Ag, Hg, Ni, Sn and Mn.** After the three-staged procedures, all heavy metals in supernatant and sludge could fulfill the contamination levels regulated by law. [ \[Art. #ARTNUM\]](#article-96100-2048778130) * **The ferrite process (FP), which is another effective approach of treating metal ion-containing wastewater, can crystallize metal ions into ferrites; the sludge easily precipitates, is stable and can be recycled.** [\[Paper\]](https://link.springer.com/article/10.1007/s10661-008-0266-x)

In water treatment, coagulation flocculation involves the addition of compounds that promote the clumping of fines into larger floc so that they can be more easily separated from the water. Coagulation is a chemical process that involves neutralization of charge whereas flocculation is a physical process and does not involve neutralization of charge.[\[Wiki\]](https://en.wikipedia.org/wiki/Coagulation_(water_treatment)) **Highlights:** * Coagulation–flocculation process is regarded as one of the most important and widely used treatment processes of industrial wastewaters due to its simplicity and effectiveness. This paper provides a critical review on recent studies of coagulation–flocculation treatment processes of various industrial wastewaters. The limitations and challenges for the coagulation–flocculation process such as the toxicity and health hazard posed by inorganic coagulants, production of large amount of toxic sludge, ineffectiveness in removing heavy metals and emerging contaminants, increase in effluent color, inefficient pollutant removal using natural coagulants, and complexity of scaling up procedure are presented. In addition, an overview on the influence of process parameters on treatment efficiency is included in this revie...[ \[Art. #ARTNUM\]](#article-96092-2333262005) * Hematite wastewater containing dissolved heavy metals is a potential threat to eco-environment and human health. Flocculation as an efficient and cost effective water treatment technology should be applied in its treatment. A sulfonated chitosan-based flocculant (CS-g-P(AM-AMPS)) was prepared via graft copolymerization. **Dissolved heavy metals in the wastewater were removed by the chelation of sulfonate group in CS-g-P(AM-AMPS), the adsorption of other functional groups and the co-settlement with flocs.** [ \[Art. #ARTNUM\]](#article-96092-2979282107) * In a previous work, a novel effluent treatment process was developed and applied to remove heavy metal ions from dilute aqueous solutions using polymer–surfactant aggregates (PSAs) (Shen et al., 2015). A subsequent pH adjustment method for the polymer–surfactant aggregate process has been developed to recover the heavy metal ions from the flocculated PSAs into a concentrated solution and to regenerate the PSAs for recycle. The PSA is a colloidal structure that is formed by micelle-like aggregates associating with the oppositely charged polymer chains. The PSA can then bind with heavy metal ions and precipitate out of the solution. In the work presented, the flocs are firstly acidified using a small volume of 0.05 M H 2 SO 4 . The bound metal ion are leached out in 15 min, and pass through a coarse filter to report as a metallic ions solution 20–50 times more concentrated than the original effluent. After the acid leaching, the flocs can be completely dissolved in a small volume of 0.1 M NaOH. This basic polymer and surfactant solution is then reused in the next treatment cycle; meanwhile, the pH of the solution is neutralised by adding H 2 SO 4 . The results show that the removal efficiency of 11.2 ppm Cd(II) solution remains above 97% after 5 cycles without the need for make-up of the removal agent. The recovery efficiencies of Cd(II), Zn(II), and Cr(III) remain above 91%, 86% and 73%, respectively, after 6 cycles. In addition, a MATLAB simulation shows that the accumulations of Na 2 SO 4 are stabilised at 280 ppm after the third cycle. Thus, the pH adjustment method is able to recover and concentrate the metal ions from the flocculated PSAs, and also to regenerate the removal agent such that it can be recycled with little deterioration of removal ability. [\[Art. #ARTNUM\]](#article-96092-2214003663)

Biosurfactants are amphiphilic molecules which showed application in the food, medical, and cosmetics industries and in bioremediation. **Highlights:** * **The MSI 54 lipopeptide biosurfactant was an anionic molecule which showed high affinity toward cationic heavy metals including Pb, Hg, Mn, and Cd.** When MSI 54 lipopeptide biosurfactant was added to heavy metals, this resulted in a white co-precipitate of the metal–biosurfactant complex. The heavy metal remediation efficacy of the biosurfactant at a 2.0 × critical micelle concentration (CMC) showed removal of 75.5% Hg, 97.73% Pb, 89.5% Mn, and 99.93% Cd, respectively, in 1,000 ppm of the respective metal solution. The surface treatment of farm fresh cabbage, carrot, and lettuce with 2.0 × CMC of the lipopeptide showed effective removal of the surface heavy metal contaminants. [\[Paper\]](https://www.frontiersin.org/articles/10.3389/fmicb.2020.00222/full) * The purpose of this research is to study effectiveness of biosurfactant with concentration=CMC for the removal of heavy metals, lead, zinc and copper in batch washing test under four different biosurfactant production by microbial origin; Pseudomonas putida T1(8), Bacillus subtilis 3K, Acinetobacter sp, and Actinobacillus sp was grown on mineral salt medium that had been already added with 2% concentration of molasses that it is a low cost application. The samples were kept in a shaker 120 rpm at room temperature for 3 days. Biosurfactant has proven its ability as a washing agent in heavy metals removal from sediments, but more research is needed to optimize the process of removal heavy metals.[ \[Art. #ARTNUM\]](#article-96096-2321006402) * Bio-surfactants are commonly used in the treatment of wastewaters due to their favorable characteristics such as biodegradability and low toxicity. In this work, the bio-surfactants are extracted from low cost substrates like Ziziphus spina-christi (ZSC) plant leaves to reduce the cost of production. Thereafter, **the bio-surfactant extracts were used for the preparation of colloidal gas aphrons (CGAs) to remove heavy metal ions (Cu 2+ , Pb 2+ )** from aqueous solutions. CGA bubbles were generated using the bio-surfactant extracts and employed in single stage and multistage cascaded flotation units to remove Cu 2+ and Pb 2+ from aqueous solutions. In the single stage flotation, the removal of Pb 2+ and Cu 2+ from the single and equimolar mixed solution were low. However, in the multistage cascade flotation, the removal efficiency of Pb 2+ and Cu 2+ increased significantly to 90.7% and 88.2%, respectively. Interestingly, the collected foamate from the flotation columns was regenerated using calcium alginate adsorbent and reused further to prepare the CGAs. Bio-surfactant solution obtained from the leaves of ZSC proved to be useful in the preparation of CGAs and the treatment of heavy metal containing wastewater. [\[Art. #ARTNUM\]](#article-96096-2913233077)

Ion exchange usually describes a processes of purification of aqueous solutions using solid polymeric ion exchange resin. More precisely, the term encompasses a large variety of processes where ions are exchanged between two electrolytes.[\[Wiki\]](https://en.wikipedia.org/wiki/Ion_exchange) **Highlights:** * This study aims at using ion-exchange resin to remove heavy metals such as copper, zinc, cadmium, and chromium from sludge generated by a PCB manufacturing plant. The ion-exchange resin plays an important role in the sludge extraction or metal recovery. [\[Art. #ARTNUM\]](#article-96073-2155454150) * The conventional techniques used for the removal of heavy metals are chemical precipitation, chemical oxidation, coagulation, evaporation, ion exchange, membrane separation, reverse osmosis, electrolytic and adsorption. However, **composite ion exchangers have proven to be versatile and efficient for removing heavy metals from contaminated water.** [\[Art. #ARTNUM\]](#article-96073-2911818693) * An innovative phosphorus removal filter made of silica granules was designed and evaluated for advanced wastewater treatment. The silica granules from natural mineral stones were formed into a porous body by sintering with addition of glass powder. The phosphorus was removed by the ionic exchange mechanism between OH − in Si(OH) 4 and PO 4 3− in wastewater. The silica filter was very effective in removing phosphorus in wastewater by ion exchange. We found the possibility of a phosphorus removal filter with no sludge production characteristics of flocculation treatment techniques, and it seems to be quite meaningful as a new wastewater treatment. [\[Art. #ARTNUM\]](#article-96073-1970619429) * Ion exchange is another method used successfully in the industry for the removal of heavy metals from effluent. An ion exchanger is a solid capable of exchanging either cations or anions from the surrounding materials. Commonly used matrices for ion exchange are synthetic organic ion exchange resins. The disadvantage of this method is that it cannot handle concentrated metal solution as the matrix gets easily fouled by organics and other solids in the wastewater. Moreover ion exchange is nonselective and is highly sensitive to the pH of the solution.[\[Paper\]](https://www.sciencedirect.com/science/article/pii/S1878535210001334#:\~:text=2.3.&text=The%20conventional%20processes%20for%20removing,metal%20removal%20from%20inorganic%20effluent.)

Ion flotation is a separation technology for recovering and removing metal ions from dilute aqueous solutions. In this process, an ionic collector is utilized to transport non-surface active colligend ions of the opposite charge from a bulk solution to the solution–vapor interface.[\[Paper\]](https://www.sciencedirect.com/science/article/pii/S0301751603001133#:\~:text=Ion%20flotation%20is%20a%20separation,to%20the%20solution%E2%80%93vapor%20interface.) **Highlights:** * Ion flotation is a promising process for the separation of toxic metals from industrial wastewater. It is a separation and elimination process for the treatment of surface-inactive ions from the aqueous media by the addition of a surfactant which has an opposite charge to the removal metal ion. When the surfactants are added into the industrial effluent, the metal ion is transferring into the hydrophobic phase. These metal ions in the hydrophobic phase react with air bubbles to form flocks and elimination of flocks takes place respectively. Ion flotation technique was employed for the remediation of toxic heavy metal ions from an aqueous solution which has little concentration. [\[Paper\]](https://www.researchgate.net/publication/331952883_Remediation_of_Nickel_ion_from_wastewater_by_applying_various_techniques_a_review) * The objective of the present work is to extend the application of adsorbing colloid flotation techniques to remove mixtures of metal ions. The systems studied are: 1) Co(II) and Cr(VI); 2) Co(II), Ni(II), and Cr(VI); 3) Cr(VI), Cu(II), and Zn(II); 4) Cr(VI), Cu(II), Zn(II), and Ni(II); 5) Cd(II), Pd(II), and Cu(II). Ferric hydroxide and aluminum hydroxide were used as the coprecipitant, and sodium lauryl sulfate was used as the collector and frother. The ionic strength of the solution was adjusted with NaNO3 or Na2SO4. It was found that all the heavy metals can be removed effectively by a single step foam flotation treatment. [\[Art. #ARTNUM\]](#article-96231-2028743979)

Advanced oxidation processes (AOPs), in a broad sense, are a set of chemical treatment procedures designed to remove organic (and sometimes inorganic) materials in water and wastewater by oxidation through reactions with hydroxyl radicals (·OH). In real-world applications of wastewater treatment, however, this term usually refers more specifically to a subset of such chemical processes that employ ozone (O3), hydrogen peroxide (H2O2) and/or UV light.[\[Wiki\]](https://en.wikipedia.org/wiki/Advanced_oxidation_process) To remove various toxic and hazardous chemicals especially endocrine-disrupting chemicals from wastewater, chemical oxidation techniques are preferred, and it is a promising technology for the treatment of wastewaters containing pharmaceuticals products. Organic compounds that are oxidized by oxidation of readily degradable species such as alcohols and carboxylic acids are the main components of this process. Ozone, hydrogen peroxide, and Fenton’s reagent are commonly used as chemical oxidation reagent.[\[Paper\]](https://www.intechopen.com/online-first/heavy-metal-removal-techniques-using-response-surface-methodology-water-wastewater-treatment) * When Ca(ClO)₂ and CaO₂ were applied, an increase of the supernatant conductivity was achieved. COD removal ≥71% and turbidity elimination in the range of 85–100% were achieved by using oxidation processes. CaO₂ was very effective to remove total phosphorus (81–96%). The increase of the oxidant concentration in H2O2 and Ca(ClO)₂ oxidation processes led to a decrease in the removal of total phenols and bicarbonates species. Optical density of the microorganism cultures was efficiently eliminated (up to 100%) by oxidation processes. [\[Art. #ARTNUM\]](#article-96289-2974412099) * Therefore, the aim of this study is to remove heavy lead metal from aqueous solution by Fenton method. This study is an experimental study. The samples are synthesized in a collection of laboratories at the Faculty of Health, and the pH parameters, contact time and lead elimination rate and the optimal Fenton ratio are investigated. Results are analyzed using SPSS software and Charts are plotted by excel. The results showed that the best pH for removal of lead 5 and its best concentration is 30 mg/ L, and the optimal Fenton ratio is 1 to 5, as well as the best contact time for removal of lead from aqueous media for 10 minutes. The present study showed that the pH factor, contact time, Fenton concentration and lead in all four were effective in removing lead from aquatic solution.[\[Paper\]](http://www.journalcsij.com/index.php/CSIJ/article/view/30111) * Phosphoric acid of 8% concentration with hydrogen peroxide showed good removal rates of heavy metals comparable to those by 1 N hydrochloric acid. Copper is easily removed from the sludge in the presence of hydrogen peroxide. From the results of the investigation of the solubilities of several copper compounds into phosphoric acid with or without hydrogen peroxide, the copper in the sludge does not exist as copper sulfides, but it seems to exist as copper thiolates containing Cu(I) in biopolymers. In this process, the phosphoric acid used for the removal of heavy metals is almost entirely recovered and is reused. Sewage sludge with low content of heavy metals can be recycled as a useful resource.[\[Paper\]](https://www.researchgate.net/publication/231292183_Principle_and_Process_of_Heavy_Metal_Removal_from_Sewage_Sludge)

Activated carbon, also called activated charcoal, is a form of carbon processed to have small, low-volume pores that increase the surface area available for adsorption or chemical reactions.[\[Wiki\]](https://en.wikipedia.org/wiki/Activated_carbon#:\~:text=Activated%20carbon%2C%20also%20called%20activated,is%20sometimes%20substituted%20with%20active.) Activated carbons have been used in many adsorption processes, among which nutrient removal and heavy metal removal. **Highlights:** * The effect of temperature and contact time on the removal of these heavy metals using the activated carbon produced was investigated. The activated carbon showed a significant ability in removing heavy metals; Cadmium, Copper, Nickel, and Lead from the wastewater. [\[Paper\]](https://link.springer.com/article/10.1007/s13201-016-0460-x#:\~:text=The%20effect%20of%20temperature%20and,and%20Lead%20from%20the%20wastewater.) * Batch experiments were conducted to test the ability of activated carbon for the removal of lead, cadmium, nickel, chromium and zinc from water. Nickel showed the highest removal percentages by activated carbon at all concentrations and the removal percentages decreased as the concentration of heavy metal increased. [\[Paper\]](https://www.sciencedirect.com/science/article/pii/S1876610214007504) * Nutrients (P, N) in stormwater runoff are a major cause of eutrophication and algal blooms. **A promising solution to this problem is to amend the rain garden growing medium (RGGM) with sewage sludge-based activated carbon (SBAC).** To optimize the SBAC production process, different metals, pyrolysis conditions (temperature, heating time, carrier gas), and post-treatments were explored. When pyrolyzed at 400 °C for two hours, Zn-activated SBAC removed up to 41% of PO4-P (initial concentration of 1 mg/L) and 72% of NO3-N (initial concentration of 2 mg/L), at a dose of 1 g sorbent/L of nutrient-spiked distilled water. When the same dosage was applied to stormwater leachate made from RGGM and spiked with nutrients, the removal efficiencies were reduced to 20% for PO4-P and 38% for NO3-N. [\[Paper\]](https://www.sciencedirect.com/science/article/pii/S0301479718308521)

Ashes (such as fly ash or biomass ash), are a common byproduct of thermochemical processes. **Highlights:** * Fly ash had been widely used in the process of sewage treatment due to its wide source,high porosity structure,large surface area and so on. It has been cleared about the application value of fly ash in removing heavy metals,which lay the foundation for the comprehensive investigation of fly ash on heavy metals treatment in the future.[ \[Art. #ARTNUM\]](#article-96094-2376033866) * **Adsorption of Chromium and lead ions onto sewage sludge ash from aqueous solutions** was studied to enable comparison with alternative commonly available absorbents. Highest metal uptake of 45 and 62 mg/g were observed for Cr and Pb, respectively. Pb showed higher affinity and adsorption rate compared to Cr under all the experimental conditions. Kinetic studies revealed that Pb and Cr uptake was up with 90% or more of the adsorption occurring at 60 min of contact time. [\[Paper\]](https://papers.ssrn.com/sol3/papers.cfm?abstract_id=3164094) * This work studied the effectiveness of fly ash in removing phosphate from aqueous solution and its related removal mechanism. **The adsorption and precipitation of phosphate by fly ash** were investigated separately in order to evaluate their role in the removal of phosphate. Results showed that the removal of phosphate by fly ash was rapid. The removal percentage of phosphate in the first 5min reached 68-96% of the maximum removal of phosphate by fly ash. The removal processes of phosphate by fly ash included a fast and large removal representing precipitation, then a slower and longer removal due to adsorption. [\[Paper\]](https://pubmed.ncbi.nlm.nih.gov/18434007/#:\~:text=Therefore%2C%20the%20removal%20of%20phosphate,wastewater%20treatment%20and%20pollution%20control.)

Clays and different types of minerals have been used as sorbent and ion exchangers. Other types of mineral compounds are zeolites, brick materials, and concrete-like materials. **Highlights:** * **Vermiculite is a kind of clay mineral which has big specific surface area and strong cation exchange capacity and can effectively remove heavy metal ions from wastewater.** It was pointed out that, it should deepen the study of the vermiculite adsorption mechanism and the disposal technology after the adsorption, and promote the application of vermiculite in actual projects. [\[Art. #ARTNUM\]](#article-96084-2391872728) * The highest adsorption capacity (q = 4.92 mg/g) of ammonium ions with the maximum removal efficiency (52.3%) was obtained for bentonite, with a 0–0.05 mm particle size. After pretreatment with a 1 mol/L NaCl solution, maximum efficiency increments were observed (55.7%). **Based on these results it was concluded that natural sorbents may be effectively applied in wastewater treatment.** [\[Art. #ARTNUM\]](#article-96084-2979571471) * In this study, **zeolite adsorption** and anaerobic digestion (AD) were integrated to improve the methane production and energy recovery of PHWW from Chlorella 1067. A statistical design for maximum toxicants removal by zeolite was applied before AD process. Zeolite could mitigate the inhibition associated to compounds such as ammonia, N-heterocyclic compounds, etc. in PHWW and thereby shortening the lag phase and increasing methane production by 32–117% compared with that without zeolite adsorption. Zeolite adsorption also increased energy recovery efficiency (up to 70.5%) for this integrated system. Integration of HTL and AD brought higher energetic return from feedstock via oil and biomethane production, which may offer insight into industrial application of microalgae biomass in the circular economy. In addition, carbon and nitrogen flow for the integrated process was determined. [\[Paper\]](https://www.sciencedirect.com/science/article/pii/S0048969718336192?via%3Dihub) * In this paper, a batch experiment was conducted to evaluate the water quality obtained from **using pervious concrete (PERVC) technology to treat acid mine drainage (AMD).** [\[Art. #ARTNUM\]](#article-96084-2982363937)

**Hydrotalcite** is a Layered Double Hydroxide whose name is derived from its resemblance with talc and its high water content. Layered double hydroxides (LDH) comprise an unusual class of layered materials with positively charged hydroxide layers and charge balancing, mobile anions located in the interlayer region.[\[Source\]](https://www.kisuma.com/materials/hydrotalcite#:\~:text=Hydrotalcite%20is%20a%20Layered%20Double,located%20in%20the%20interlayer%20region.) **Highlights:** * Hydrotalcite materials are generally utilized for anionic pollutants due to its interlayered anion exchange ability. Their potentiality for cationic contaminants is rarely explored. In this study, **disulfide (S2-) intercalated LDH material demonstrated capability to remove both heavy metal cations and oxyanions simultaneously from water.** The S2- intercalation of LDH significantly improved its adsorption capability towards both heavy metal cations (Co2+ and Ni2+) and oxyanion (CrO4 2-). This work provided a highly practical adsorption technology based on the S2- modification hydrotalcite material for the purification of heavy metal ions contaminated wastewater.[ \[Art. #ARTNUM\]](#article-96083-2971605206) * Hydrotalcites are minerals, sometimes found in stomach antacids, which are able to absorb a variety of contaminants including arsenic, cadmium, and iron. We found that hydrotalcites could be formed by adjusting the concentrations of common wastewater contaminants, aluminium and magnesium, to an ideal ratio, and increasing the pH.[\[Virtual curtain\]](https://www.csiro.au/en/Research/MRF/Areas/Community-and-environment/Minesite-environmental-management/Virtual-curtain) * A ZnO-ZnAl hydrotalcite (ZZA) was prepared by a new centrifugal method \[...\]. The adsorption by ZZA of phosphate in a simulated wastewater was studied in detail. [\[Paper\]](https://www.tandfonline.com/doi/abs/10.1080/03067319.2019.1622696?journalCode=geac20) * Hydrotalcites (HT*x*) of different Mg/Al ratio (*x* was Mg/Al ratio; 2, 3, 4) were synthesized by co-precipitation method. Calcined hydrotalcites (CHT*x*) at 500 °C have been shown to recover their original layered structure by **adsorbing nitrate or nitrite from aqueous solution.** [\[Paper\]](https://www.sciencedirect.com/science/article/abs/pii/S1385894712005529)

Zerovalent iron (ZVI) is the most commonly used zerovalent metal (ZVM) for environmental remediation. ZVI is typically applied as a reductant and is capable of transforming (degrading) or sequestering a variety of contaminants found in groundwater and soil.[\[Source\]](https://www.enviro.wiki/index.php?title=Zerovalent_Iron_(ZVI)_(Chemical_Reduction_-_ISCR)#:\~:text=From%20Enviro%20Wiki,found%20in%20groundwater%20and%20soil.) **Highlights:** * Reducing the concentration of heavy metals including lead (Pb) and nickel (Ni) in organic contaminants such as municipal wastes and sewage sludge is of health and environmental importance. **Nanoscale zero-valent iron (NZVI) particles can effectively remove heavy metals from contaminated aqueous and solid media.** [\[Art. #ARTNUM\]](#article-96751-2744894627) * **The hybrid zero-valent-iron (hZVI) process is a novel chemical treatment process that has shown promise for removing heavy metals and nutrients from industrial wastewaters.** In this study, a pilot-scale demonstration was conducted to continuously treat 3.8–7.6 L/min (1–2 gpm) of the flue-gas-desulfurization (FGD) wastewater at a coal-fired power plant for 5 months. \[...\] the process simultaneously removed a broad spectrum of heavy metals such as As(III), As(V), Cr(VI), Cd(II), Pb(II) and Cu(II) from mg/L to near or sub-ppb (μg/L) level after a single-stage treatment. The process consumed about 0.3 kg ZVI per 1 m 3 FGD wastewater treated at a cost of about US$0.6/m 3 . Solid waste production and energy consumption were reasonably low. The successful pilot study demonstrated that the hZVI technology can be a low-cost, high-performance treatment platform for solving some of the toughest heavy metal water problems.[ \[Art. #ARTNUM\]](#article-96751-2094605413) * The present study investigated the reactivity and ability of permeable reactive barriers \[zero-valent iron (ZVI)-barrier plus biobarrier) to remove various contaminants (Cd, As, Zn, Cu, Mn, Cr, NO3 −, NH4 +, and CODcr) from synthetic leachate. Two different reactive materials were used in this study, namely ZVI and autoclaved lightweight concrete (ALC).[ \[Art. #ARTNUM\]](#article-96751-1990602760)

Nanomaterials are chemical substances or materials that are manufactured and used at a very small scale. Nanomaterials are developed to exhibit novel characteristics compared to the same material without nanoscale features, such as increased strength, chemical reactivity or conductivity. [\[Source\]](http://www.safenano.org/knowledgebase/resources/faqs/what-is-a-nanomaterial/#:\~:text=Nanomaterials%20are%20chemical%20substances%20or,strength%2C%20chemical%20reactivity%20or%20conductivity.) Commonly researched nanomaterials in waste remediation are nanoscale zero valent iron, carbon nanotubes, nanoparticles etc. **Highlights:** * In this work, novel nanomaterials, including **carbon-based nanomaterials, zero-valent metal, metal-oxide based nanomaterials, and nanocomposites, and their applications for the removal of heavy metal ions from wastewater were systematically reviewed.** Their efficiency, limitations, and advantages were compared and discussed. Furthermore, the promising perspective of nanomaterials in environmental applications was also discussed and potential directions for future work were suggested.[ \[Art. #ARTNUM\]](#article-96119-2921291027) * **The treatment columns for phosphorus sorption consisted of two 2.54 cm diameter PVC columns containing porous ceramic media nano coated with iron formed into a monolith**, manufactured by MetaMateria Technologies, LLC, Columbus, Ohio. The denitrification columns were 5.1 cm diameter PVC buckets containing loosely packed porous ceramic media (manufactured from MetaMateria) crushed to have a diameter of roughly 1 cm. Results showed that the phosphorus media effectively removed phosphorus from simulated tile drain water from 0.12 mg/L-P to undetectable levels. Initially, denitrification did not occur and was not encouraged by the additional of a microbial inoculum. However, once ethanol was added to increase the carbon to nitrogen ratio, treatment was very effective. The nitrate was reduced from 83 mg/L-N to 0.6 mg/L-N. [\[Art. #ARTNUM\]](#article-96119-2755455545) * This work highlights up-to-date methods for the removal of heavy metals from water using the technique of adsorption. After a comprehensive review of the range of materials used as adsorbents to remove heavy metals, the paper focuses on one particular technique, involving **carbon nanotubes (CNTs)**. This study shows that using deep eutectic solvents (DESs) as a functionalization agent to replace ionic liquids (ILs) has a number of advantages, including lower costs, easier availability of materials and environmentally friendliness. In sum, it suggests that the most efficient adsorption process is to use functionalized CNTs with DESs as an adsorbent. The critical analysis and conclusions emerging from this study should be of benefit to engineers, environmental scientists and chemists interested in the use of carbon nanotubes for use in environmental remediation. [\[Art. #ARTNUM\]](#article-96119-2945202622)

Sludge particles, such as (dry) activated sludge and alum sludge can be used to remediate waste streams from heavy metals and some minerals.. **Highlights:** * A wide range of batch experiments were carried out for estimation of the key process parameters in competitive biosorption of Pb 2+ , Hg 2+ , Cr 3+ and As 5+ from simulated of wastewater onto **dry activated sludge** in batch adsorber. [ \[Art. #ARTNUM\]](#article-96098-2008237009) * This study attempts to determine effectiveness of **dried alum sludge as media to remove phosphorus from different kind of water samples.** Three types of wastewater were selected; synthetic phosphate solution, wastewater and river water. The continuous flow test approach with constant hydraulic loading is used in this study. The results indicate that dried alum sludge from water treatment plant has great potential as to remove the phosphate from synthetic water, river water and wastewater. [\[Art. #ARTNUM\]](#article-96098-1822564579)

Biopolymers are industrially attractive because they are, capable of lowering transition metal ion concentrations to sub-part per billion concentrations, widely available, and environmentally safe. Another attractive feature of biopolymers is that they possess a number of different functional groups, such as hydroxyls and amines, which increase the efficiency of metal ion uptake and the maximum chemical loading possibility. New polysaccharide-based-materials were described as modified biopolymer adsorbents (derived from chitin, chitosan, and starch) for the removal of heavy metals from the wastewater.[\[Paper\]](https://www.sciencedirect.com/science/article/pii/S1878535210001334#s0030) **Highlights:** * Many biopolymer materials which are capable of adsorbing have been developed as modified materials and applied for the removal of heavy metals from metal-contaminated wastewater, especially modified natural rubber, modified Lyocell fiber, and chitosan-based adsorbents. Moreover, chitosan derivatives such as chitosan-containing nitrogen, phosphorus, sulfur, and ethers have been studied as adsorbents as well. [\[Paper\]](https://link.springer.com/article/10.1007%2Fs00253-016-7646-x) * Keratin protein fiber (AKF TM ) is a renewable source of biosorbent that can be used for filtration of heavy metal and nanosize contaminants. The major composition of keratin fiber is the structured protein micofibril, consisting of numerous nanosized pores. The combination of nanostructure and metal binding capacity protein sites make it an ideal material for removal of heavy metals from solutions.[ \[Art. #ARTNUM\]](#article-96323-2036889827) * In this study, the preparation, characterization and heavy metal removal ability of chitosan–ethyl acrylate (CEA) biopolymer were investigated. Ethyl acrylate was successfully grafted to chitosan, which was characterized by Fourier transform infrared spectroscopy (FTIR) and nuclear magnetic resonance (NMR). Three heavy metal cations (Pb 2+ , Cd 2+ and Zn 2+ ) were removed from the simulated wastewater in batch mode, and the effect of initial metal ions concentration, adsorbent dosage, solution temperature, contact time and initial pH on the adsorption efficiency was illustrated. The maximum Pb 2+ , Cd 2+ and Zn 2+ removal efficiency of 92%, 86% and 98% were achieved, respectively.[ \[Art. #ARTNUM\]](#article-96323-2015768761) * Heavy metal pollution has raised a lot of concerns because of its bioaccumulation and non-degradability. A variety of methods have been applied in removal of heavy metals. This paper focused on the method of biosorption using biopolymer-based adsorbents in removing heavy metals from waster water. **Cellulose and chitin/chitosan are abundant in supply and contain reactive functional groups.** Cellulose-based adsorbents prepared from direct chemical modification or graft copolymerization are proved to show chelating ability to bind heavy metals. Chitin-and chitosan-based adsorbents have improved mechanical strength and better resistance to chemical conditions after physical or chemical modification. The regeneration and reusability of biopolymer-based adsorbents were also discussed in this paper.[ \[Art. #ARTNUM\]](#article-96323-2012650961)

Hydrogels, which are crosslinked hydrophilic polymers, are capable of expanding their volumes due to their high swelling in water. Accordingly they are widely used in the purification of wastewater. Various hydrogels were synthesized and their adsorption behavior for heavy metals was investigated. The removal is basically governed by the water diffusion into the hydrogel, carrying the heavy metals inside especially in the absence of strongly binding sites.[\[Paper\]](https://www.sciencedirect.com/science/article/pii/S1878535210001334#s0030) **Highlights:** * heavy metal ions are removed effectively by forming hydrogen and chelating bonds with excess hydroxyl groups in the hydrogel.[ \[Art. #ARTNUM\]](#article-96696-3005157882) * In this study, a polyampholyte hydrogel was well designed and prepared via a simple radical polymerization procedure. Due to the remarkable mechanical strength, the three-dimensional polyampholyte hydrogel could be fast separated, easily regenerated and highly reused. The sorption capacities were as high as 216.1 mg/g for Pb(II) and 153.8 mg/g for Cd(II) owing to the existence of the large number of active groups. The adsorption could be conducted in a wide pH range of 3–6 and the equilibrium fast reached in 30 min due to its excellent water penetration for highly accessible to metal ions. The fixed-bed column sorption results indicated that the polyampholyte hydrogel was particularly effective in removing Pb(II) and Cd(II) from actual industrial effluent to meet the regulatory requirements.[ \[Art. #ARTNUM\]](#article-96696-2183368156)

Biochar is charcoal used as a soil amendment for both carbon sequestration and soil health benefits. Biochar is a stable solid, rich in carbon, and can endure in soil for thousands of years. Like most charcoal, biochar is made from biomass via pyrolysis. [\[Wiki\]](https://en.wikipedia.org/wiki/Biochar) **Highlights:** * In this study, a redox precipitation method was used to load manganese dioxide (MnO2) nanoparticles on biochar (BC) (BC@MnO2) pyrolyzed from the invasive water hyacinth, and the adsorption of Cd(II),Cu(II), Zn(II), and Pb(II) was investigated. \[...\] This study could lay an essential foundation to develop a win-win strategy for heavy metal ions removal from wastewater using biochar derived from water hyacinth. [\[Art. #ARTNUM\]](#article-96085-3015508606) * The physicochemical properties and adsorption capacities of yak manure biochar (SP350-YMB) and modified yak manure biochar by H2O2 (AC-YMB) were investigated. Compared to SP350-YMB, the adsorption capacities of heavy metal ions (Pb2+, Cu2+, Cd2+ and Zn2+) were enhanced by AC-YMB in single-metal and multi-metal solutions. H2O2 modification reduced ash content and increased carboxyl content resulting in the major mechanism of heavy metal sorption shifting from precipitation with carbonate/phosphate to complexation with carboxyl. **Results from this study indicated that H2O2 modified biochar could act as effective surface sorbent to remove heavy metals, but its ability to remove multi-metal ions needs to be carefully evaluated on an individual basis.** [\[Paper\]](https://www.sciencedirect.com/science/article/pii/S0048969718305278)

It is reported that there are many agricultural and biological wastes materials can be used as adsorbents for the removal of heavy metals from wastewater. **New resources such as rice husk, eggshell, Moringa pods, bamboo leaf powder, cashew nut shells, palm oil fruit shells, and barley straw show great capacity of heavy metal removal.** **Highlights:** * And there are many materials which were studied including the mechanism, kinetic, and efficiency. The kinetic test of **Moringa pods** demonstrates that biosorption equilibrium reached within 30 min for nickel and 40 min for copper and chromium, respectively. **Bamboo leaf powder** modified by using anionic surfactant SDS and non-ionic surfactant Triton X-100 achieved the maximum adsorption of mercury around 30 mg/g. Rice husk can be fruitfully used for the removal of heavy metals with a concentration range of 20–60 mg/L. The removal of Pb and Cu can reach 100 % at pH 10 by **palm fruit shells**, while the maximum adsorption of Pb by **okra waste** was optimized at pH 5. Based on Langmuir isotherm, the maximum adsorption of Cu, Cd, Zn, and Ni was 406.6, 436.7, 455.7, and 456.3 mg/g by using **cashew nut shells**.[\[Paper\]](https://link.springer.com/article/10.1007%2Fs00253-016-7646-x) * Therefore, this study investigated whether **inexpensive humic substances (HS) from sewage sludge compost could effectively remove copper (Cu) and cadmium (Cd) from highly contaminated sandy clay loam (S1) and clay (S2).** [\[Art. #ARTNUM\]](#article-96093-281661659) * **Many types of lignocellulosic biomass show effective binding of toxic heavy metals from industrial and environmental effluents.** Biosorption is an emerging option for conventional methods to remove heavy metals, some of them with even better efficiencies compared to conventional methods. Raw material for biosorption is typically low-cost and easily available, including agricultural waste or forest residues such as sawdust, bark, or needles. **This review concentrates on the accumulation of heavy metals by lignocellulosic biosorbents.** Thus far, biosorption has not been economically feasible on a large scale and needs further development for profitability. Industrial-scale wood-based biosorbent applications are especially still lacking. Moreover, due to legislative demands, there is an increasing need for accurate and reliable analytical methods for metal analysis of environmental and industrial effluents. In the future, biosorption processes are likely to become common, and the requirement for environmental monitoring will increase due to ever restricting regulations. [\[Art. #ARTNUM\]](#article-96093-2920469717)

Biosorption is sorption using (usually inactive) biomass. Numerous studies have been carried out with bacterial, fungal and algal biomass as biosorbents. Here the biomass does not actively take up the pollutants (as in bioleaching). **Highlights:** * Biosorption using algae biomass can be an alternative method to eliminate heavy metals. The objective of the project is to investigate the capability of Marine Algae (MA) and Freshwater Algae (FA) biomass in adsorbing heavy metals of Cu, Pb, Zn and Cd from water medium using synthetic water and industrial water. The result showed that FA and MA had a higher capability in adsorbing a total metal of about 40 ppm level from an industrial water, or 4 times than synthetic water concentration, at the same adsorbent dosage of 50 mg. In con-clusion, the presence of various functional groups, hydroxyl, carboxylic and amine groups, in all MA and FA samples had enabled the algae biomass to adsorb heavy metals of Cu, Pb, Cd and Zn from synthetic and industrial water. Due to their bio-sorptive properties and fast adsorption capability, algae could be a potential method for cleaning up surface water or post-treatment of wastewater and minimise the cost of eutrophication.[ \[Art. #ARTNUM\]](#article-96076-2540464837) * Biosorption method by microalgae is one of the newest methods which have the properties such as cost, selective adsorption, high processing rate and no sludge production. In this process in order to clean up wastewater by adsorption, the two algae Chlorella and Spirulina are optimal. In comparing these two algaes, Chlorella algaehas better ability to attract as well as more balanced with the absorption effect. [\[Art. #ARTNUM\]](#article-96076-2268465717) * A variety of biomaterials are known to bind these pollutants, including bacteria, fungi, algae, and industrial and agricultural wastes. In this review, the biosorption abilities of bacterial and fungal biomass towards metal ions are emphasized. [\[Art. #ARTNUM\]](#article-96076-2185808385)

Electrochemical techniques, such as electrolysis use electrical currents to facilitate chemical reactions that can lead to separation. **Highlights:** * Electrochemical techniques have been extensively applied to treat heavy metal pollution in our daily life. Traditional electrochemistry is always driven by continuous DC (CDC), but the energy harvested in environment from water using nanogenerator is always a pulsed DC (PDC). Here, we systematically investigated the electrochemical performance of hexavalent chromium (Cr(VI)) removal from wasted as powered by pulsed output of triboelectric nanogenerator (TENG) using the energy harvested from environment. **By optimizing the frequency and on-off ratio of PDC, the removal efficiency of Cr(VI) can be maximally enhanced by 53.5% compared to that driven by a CDC under equal amount of electric charges, which is further confirmed by electrochemical experiments driven by layered TENG.** \[...\] This study demonstrate the feasibility and effectiveness of self-powered electrochemical for cleaning environment pollution using the harvested energy.[ \[Art. #ARTNUM\]](#article-96116-2963263812) * Recently electrochemical process for removing phosphorous (P) from effluents has received more attention to control water resource pollution. The energy demand was 7.69 KWh per equivalent of removed P and 0.45 kWh/m3. The data showed that the proposed method is economical and certainly more efficient in comparison with conventional P removal methods. Therefore, the electrochemical process alone by steel electrodes may be a capable process for P removal from filtered activated sludge effluent. [\[Art. #ARTNUM\]](#article-96116-3015328989) * Electrolytic recovery or electro-winning is one of the many technologies used to remove metals from process water streams. This process uses electricity to pass a current through an aqueous metal-bearing solution containing a cathode plate and an insoluble anode. Positively charged metallic ions cling to the negatively charged cathodes leaving behind a metal deposit that is strippable and recoverable. A noticeable disadvantage was that corrosion could become a significant limiting factor, where electrodes would frequently have to be replaced[\[Paper\]](https://www.sciencedirect.com/science/article/pii/S1878535210001334#:\~:text=2.3.&text=The%20conventional%20processes%20for%20removing,metal%20removal%20from%20inorganic%20effluent.)

In electrokinetic methods, compounds are separated based on their charge. Electrophoresis and electroosmosis are two examples of electrokinetic methods, and its is also the pinciple behind electrodialysis (see [\[Electrodialysis\]](#technology-96117)) **Highlights:** * **In this study, an ex situ method for removing heavy metals from sludge using electroosmosis** was designed, and experiments were conducted to study the effects of electroosmosis voltages (30 V, 40 V and 50 V), citric acid concentrations (0.03 mol/kg, 0.06 mol/kg, 0.09 mol/kg and 0.12 mol/kg) and the power supply (continuous and interrupted) on the removal rate of heavy metals (Cu, Cr, Cd, Zn and Pb). **The study found that the combination of the citric acid pretreatment and electroosmosis can effectively improve the removal rate of heavy metals.** [\[Art. #ARTNUM\]](#article-96089-3011564947) * Sewage sludge ash is rich in phosphorus, but the direct use as fertilizer is limited because of inorganic contaminants such as heavy metals and strong bonding of phosphorous in the ash. **Electrodialysis (ED) can be used to recover phosphorus and simultaneously remove heavy metals.** Simplifying this experimental setup by removing the anion exchange membrane brings the anode in direct contact with the stirred ash suspension. Through this adjustment, half-reactions at the anode contribute to the acidity of the stirred suspension resulting in increased dissolution of both phosphorus and heavy metals (Cd, Cu, Cr, Pb, Zn, Ni) and better separation of most heavy metals from the stirred ash suspension. [\[Art. #ARTNUM\]](#article-96089-1980378680) * The electrophoretic deposition (EPD) is found to be one of the potential techniques that can be used to minimize these issues.EPD is the process of colloidal particles and regularly carried out in two electrode cells that dispersed in suspension medium with the existence of electric field. To obtain the optimum condition of EPD; the pH, current, particle size of heavy metals, type of electrode, and voltage are determined key factors. [\[Art. #ARTNUM\]](#article-96089-2969982782) * Dielectrophoresis (DEP) was combined with adsorption (ADS) to simultaneously and effectively remove Cd2+ and Pb2+ species from aqueous solution. To implement the process, bentonite particles of submicro-meter size were used to first adsorb the heavy metal ions. These particles were subsequently trapped and removed by DEP. The high removal efficiency, 97.3% and 99.9% for Cd2+ and Pb2+, respectively, were achieved when the ions are coexisting in the system. The microstructure of bentonite particles before and after ADS/DEP was examined by scanning electron microscopy. Our results suggest that the dielectrophoresis-assisted adsorption method has a high capability to remove the heavy metals from wastewater. [\[Art. #ARTNUM\]](#article-96089-2908932138)

Electrocoagulation (EC), is a technique used for wastewater treatment, wash water treatment, industrial processed water, and medical treatment. Electrocoagulation has become a rapidly growing area of wastewater treatment due to its ability to remove contaminants that are generally more difficult to remove by filtration or chemical treatment systems, such as emulsified oil, total petroleum hydrocarbons, refractory organics, suspended solids, and heavy metals.[\[Wiki\]](https://en.wikipedia.org/wiki/Electrocoagulation) **Highlights:** * Heavy metals have frequently been detected in metal plating wastewater. **In this study, electrocoagulation method using iron (Fe) and aluminum (Al) electrodes was applied to simultaneously remove four heavy metals (Cu, Ni, Zn, Cr) in artificial metal plating wastewater.** In electrocoagulation experiments using Fe electrodes, the mass of sludge formed was 0.68–2.50 kg/m3 and the amount of energy consumed was 0.37–2.78 kW h/m3, respectively, during the treatment of artificial metal plating wastewater containing four heavy metals in the absence of cyanide, which increased to 3.64–4.74 kg/m3 and 4.80–5.04 kW h/m3 for artificial wastewater in the presence of cyanide. [\[Art. #ARTNUM\]](#article-96077-2995529317) * Inspired by the wastewater treatment in such industries as those of textiles, food, and petrochemistry, in the present work, electrocoagulation (EC) is applied for the first time to explore its feasibility in the treatment of wastewater with an initial COD of 424.29 mg/L from a Pb/Zn sulfide mineral flotation plant and its effect on water reuse. Typical parameters, such as anode materials, current density, initial pH, and additives, were characterized to evaluate the performance of the EC method. This study confirmed that EC is a promising method for the treatment and reuse of high-COD-containing wastewater in the mining industry, and it possesses great potential for wide industrial applications.[\[Paper\]](https://www.mdpi.com/2073-4441/12/2/595) * Electrocoagulation is an electrochemical technique with many applications. This process has recently attracted attention as a potential technique for treating industrial wastewater due to its versatility and environmental compatibility. This process has been applied for the treatment of many kinds of wastewater such as landfill leachate, restaurant, carwash, slaughterhouse, textile, laundry, tannery, petroleum refinery wastewater and for removal of bacteria, arsenic, fluoride, pesticides and heavy metals from aqueous environments. The objective of the present manuscript is to review the potential of electrocoagulation process for the treatment of domestic, industrial and agricultural effluents, especially removal of heavy metals from aqueous environments.[\[Paper\]](https://link.springer.com/article/10.1186/s40201-015-0233-8)

***Electroflotation*** is a process of floating of pollutants to water surface by tiny bubbles of hydrogen and oxygen gases generated from water electrolysis. **Highlights:** * Electro-flotation is a removal process which produced tiny bubbles of hydrogen and oxygen gases emerged from water electrolysis to float the contaminants at the surface of effluent. The electro-flotation process can be used with the combination of aluminium electrocoagulation. This process was employed for the remediation of Ni(II) from the polymetallic solution. The removal efficiency was obtained 99% at the pH of 8.0. [\[Paper\]](https://www.researchgate.net/publication/331952883_Remediation_of_Nickel_ion_from_wastewater_by_applying_various_techniques_a_review) * Flotation is a known gravity separation process originated from mineral processing; concerning the used bubble generation method, this could be typically by dispersed-air flotation, dissolved-air flotation or electroflotation. The later technique constitutes the scope of the present review article, commenting recent selected papers published in the literature, and focusing on applications such as waters, heavy metals, biological wastes, etc. Examples given (from our lab) are, among others, minerals like magnesite and pyrite, and pollutant metal ions, as Cd(II) and Cr(VI). The process advantages were discussed in-depth, such as the electric field gradient and the fine gas bubbles; and also, certain design aspects (as kinetics, hydrodynamics, electrolytic cell). Electroflotation was shown to contribute well for the recycling of by-products and waste materials, including water.[\[Paper\]](https://www.sciencedirect.com/science/article/abs/pii/S0167732216301660)

Electro-deionization is the combination of ion exchange and electrodialysis. It is often used as a polishing step. **Highlights:** * Dermentzis et al. employed an electro-deionization method for the removal of nickel from electroplating rinse water. The removal efficiency was obtained 100% at pH 4.0 by using platinum graphite powder platinum as an anode and platinum platinized titanium platinum as a cathode. The initial concentration of nickel was 0.1 mg/L at the flow rate of 2.02 x 10-4 and the current density was 30 A/m2. * In this work, the treatability of nickel containing wastewater by membrane-free electrodeionization (MFEDI) was experimentally investigated. The study concentrated on the effect of different resin bed configurations in MFEDI system in term of nickel removal, precipitation generation and water recovery, as well as energy consumption. The experiment results show that excellent purification and effective regeneration were achieved by MFEDI with a layered mixed bed. It was observed that, in the purification stage, with an inlet conductivity of 42.5 μS/cm and a nickel concentration of 10.0 mg/L, the effluent conductivity was 0.3–1.0 μS/cm and the nickel concentration of the effluent was below the detection limit. [\[Paper\]](https://www.sciencedirect.com/science/article/abs/pii/S1385894715009043)

bioelectrochemical systems (BES) have emerged as a potential technological platform for recovery of metal ions from metallurgical waste (end-of-life products), process streams and wastewaters. In bioelectrochemical systems, microbial oxidation of organic substrate at the anode is coupled to abiotic or biotic reduction of metal ions at the cathode.[\[Paper\]](https://link.springer.com/chapter/10.1007/978-3-319-58622-9_6) **Highlights:** * MFC technology arises as an attractive alternative for the treatment of high strength animal wastewater, such as pig slurries, to potentially improve energetic valorisation of organic wastes, **concomitantly to carbon and nitrogen content reduction or recovery**.[ \[Art. #ARTNUM\]](#article-96158-2202793691) * the bioelectrochemical approach, which is based on the integration of microbiology with electrochemistry, is **emerging as a potential technology for the efficient removal or recovery of various metals. It offers a flexible platform based on the oxidation and reduction reactions of different heavy metals at the electrodes.** This chapter covers different aspects associated with the use of microbial (bio)electrochemical systems for the removal and recovery of various metals.[\[Paper\]](https://www.sciencedirect.com/science/article/pii/B9780128174937000035)

Bioleaching is the extraction of metals from their ores through the use of living organisms. [\[Wiki\]](https://en.wikipedia.org/wiki/Bioleaching) It has also been developed for sludge treatment. **Highlights:** * Three kinds of mixed culture of acidophilic bacteria were enriched and prepared from natural acid mine drainage and used in bioleaching experiments.The heavy metals (Cu,Zn and Cd) removal f rom sewage sludge and the improvement of sewage sludge dewaterability in bioleaching processes were investigated.The results showed that all the three prepared mixed culture of acidophilic bacteria could efficiently remove heavy metals from sewage sludge(P0.01).After 12 days of bioleaching,82.0% of copper and 82.9% of Cd could be removed by the mixed culture of acidophilic bacteria enriched by modified Starkey medium,and 87.5% of zinc could be leached out when inoculating the mixed culture of acidophilic bacteria enriched by 9K medium. [\[Art. #ARTNUM\]](#article-96079-2359122416) * The bioleaching process, although it seems to give a higher yield of metal extraction with lower chemical cost than chemical extraction, may be limited by the inability of the system to cope with the natural environmental conditions, requires strict monitoring of aeration rate and temperature and has applicability to only low sludge solids concentration. [\[Art. #ARTNUM\]](#article-96079-2140127378) * The purpose of this work was to study the sulfur concentration on bioleaching of heavy metals from pig manure employing indigenous sulfur-oxidizing bacteria. Also, the variations in physicochemical properties of pig manure before and after bioleaching were investigated. The results showed that sulfur concentration significantly affected the rate of acidification, sulfate production and metal solubilization during pig manure bioleaching process. After 12 days of bioleaching, 93%–97% of Zn, 96%–98% of Mn and 48%–94% of Cu were leached out from pig manure, respectively. [\[Paper\]](https://www.sciencedirect.com/science/article/pii/S2405844017339725#:\~:text=In%20the%20bioleaching%20process%2C%20the,efficiency%20during%20the%20bioleaching%20process.)

These examples combine bioleaching with different technologies: advanced oxidation, filtration, electro-techniques. **Highlights:** * Feasibility of **bioleaching combining with Fenton-like reaction** to remove heavy metals from sewage sludge was investigated. After 5-day bioleaching, the sludge pH decreased from 6.95 to 2.50, which satisfied the acidic conditions for Fenton-like reaction. Meanwhile, more than 50% of sludge-borne heavy metals were dissolved except for Pb. The bioleached sludge was further oxidized with Fenton-like reaction, with an optimal H2O2 dosage of 5 g/L, the Cu, Zn, Pb and Cd removal reached up to 75.3%, 72.6%, 34.5% and 65.4%, respectively, and the residual content of heavy metals in treated sludge meets the requirement of Disposal of Sludge from Municipal Wastewater Treatment Plant – Control Standards for Agricultural Use (CJ/T 309-2009) of China for A grade sludge. Bioleaching combined with Fenton-like reaction was the most effective method for heavy metal removal, compared with 15-day bioleaching and inorganic acid leaching with 10% H2SO4, 10% HCl and 10% HNO3. [\[Art. #ARTNUM\]](#article-96090-1979915077) * The combination of **bioleaching and electrokinetic** remediation has been proved to be an effective method to remove heavy metals from municipal sewage sludge. The results showed that using electrokinetic remediation for six days after bioleaching for four days, the contents of Cu and Zn in sewage sludge decreased from 296.4 mg kg − 1 and 3756.2 mg kg − 1 to 63.4 mg kg − 1 and 33.3 mg kg − 1 , respectively, which could meet the Chinese standard for land application of the heavy metals in sewage sludge. [\[Art. #ARTNUM\]](#article-96090-2070246701) * There is needed for treatment to remove chloride from wastewater before discharged to river or water bodies. Furthermore, chloride is also a very corrosive agent, and elevated levels pose a threat to infrastructure, such as road beds, bridges, and industrial pipes. The effect caused by these hazardous pollutants and growing concerns to environmental issues led to remove chloride concentration from rare earth wastewater by using bioremediation hybrid with electrocoagulation system. The application of yeast in the wastewater treatment has potential in the treatment and reuse of wastes containing solids and high concentrations of salt, fat and antibiotics. The information obtained from this study is useful for scale up purpose in the rare earth industry that choose bioremediation hybrid with electrocoagulation system method to remove chloride concentration from rare earth wastewater. [\[Art. #ARTNUM\]](#article-96090-2963928864) * Trickling filters (TFs) constitute a cost-effective post-treatment option, assuring low sludge production, low operational costs and maintenance simplicity compared to other post-treatment technologies (e.g. activated sludge). This paper reviews the experience of the last 20 years of research, design and operation of UASB/TF systems. Three main topics are addressed: i) the development of trickling filters for UASB reactor effluent treatment, building on first experiences with TFs preceded by primary settlers; ii) the design criteria, performance and empirical models for predicting the efficiency of TFs post-UASB reactors; and iii) the future challenges associated with elimination of secondary settlers and nitrogen removal in sponge-bed trickling filters (SBTFs). [\[Art. #ARTNUM\]](#article-96090-2892119691) * A hybrid process (**coupling biological and chemical processes**) has been explored in laboratory pilot-scale experiments for heavy metals (cadmium \[Cd\], copper \[Cu\], chromium \[Cr\], and zinc \[Zn\]) removal from three types of sludge (primary sludge, secondary activated sludge, and a mixture of primary and secondary sludge). The hybrid process consisted of producing a concentrate ferric ion solution followed by chemical treatment of sludges. Ferric iron solution was produced biologically via oxidation of ferrous iron by A. ferrooxidans in a continuous-flow stirred tank (5.2 L) reactor (CSTR). Wastewater sludge filtrate (WSF) containing nutrients (phosphorus and nitrogen) has been used as culture media to support the growth and activity of indigenous iron-oxidizing bacteria. [\[Art. #ARTNUM\]](#article-96090-2412106185)

Vermiremediation is an earthworm-based bioremediation technology. **Highlights:** * This work presents the results of removing heavy metals from paper mill wastewater (PMS) sludge spiked with cow dung (CD) employing Eisenia fetida . A total of seven set-ups were prepared: CD (100 percent), PMS: CD (1:3), PMS:CD (1:2), PMS:CD (1:1), PMS (100 percent), PMS:CD (3:1) and PMS:CD (2:1) and changes in chemical parameters were observed for 60 days. Vermistabilization caused the significant decrease in the level of Cd (32–37 percent), Cr (47.3–80.9 percent), Cu (68.8–88.4 percent), and Pb (95.3–97.5 percent) and substantial increase in EC, total-N, available P and K at the end. At the end, the tissues of inoculated worms showed the high load (mg kg −1 , dry biomass) of Pb (8.81–9.69), Cd (2.31–2.71), Cr (20.7–35.9) and Cu (9.94–11.6), respectively which indicated bioaccumulation of metals by worms. The PMS:CD (2:1 and/or 3:1) appeared to be suitable waste mixture in terms of high metal removal and earthworm growth rates. Bioaccumulation, as quantified using BCF, was in the order: Cd>Cr>Pb>Cu. Results suggested vermiremediation as appropriate technology for bioremediation of heavy metals from PMS.[ \[Art. #ARTNUM\]](#article-96095-2117836668) * Efficacy of vermi-transformation for metal partitioning and transformation from **Upflow Anaerobic Sludge Blanket (UASB)** and Activated Sludge (AS) was investigated. Sludge samples were mixed with cow dung (CD) in two combinations (1:1 (UASB/AS:CD)) & (2:1(UASB/AS: CD)). Fractionation study revealed that Zn, Cd & Pb were associated with reducible fractions, and Cr, Cu with oxidizable fractions. Higher removal efficiency for 1:1 (UASB/AS: CD) combination over 2:1 (UASB/AS: CD) implies the non-significant contribution of cow dung during the metal stabilization process. **After vermi-remediation, maximum metal removal was achieved at 1:1 ratio than 2:1 in AS. In UASB, 1:1 ratio worked better for Cr, Zn & Cd, whereas for Cu & Zn 2:1 ratio resulted in efficient removal.** [ \[Art. #ARTNUM\]](#article-96095-2911869462)

These approaches use a certain kind of bioreactor for separation. **Highlights:** * We proposed a method for sewage water purification of nitrogen and phosphorus compounds on a **disk bioreactor** of full displacement. The developed method of sewage water purification on a bioreactor of full displacement is very promising and we can use it for purification of sewage water, which contains a large amount of organic substances. **The efficiency of purification of mineral nitrogen reaches 98.9 %, phosphates up to 40‒50 %. The total nitrogen content decreases by 4‒6 times and the total phosphorus content ‒ by 2‒2.5 times.** We can use the scheme of purification on troughs and a bioreactor of full displacement for sewage water of dairy industry, livestock farms, communal services, and surface runoff. [ \[Art. #ARTNUM\]](#article-96136-2899297251) * Compost leachate forms during the composting process of organic material. It is rich in oxidizable organics, ammonia and metals, which pose a risk to the environment if released without proper treatment. An innovative method based on the m**embrane bioreactor (MBR)** technology was developed to treat compost leachate over 39 days. A decrease of more than 99% was achieved for a COD of 116 g/L in the initial leachate. **Ammonia was decreased from 2720 mg/L to 0.046 mg/L**, while the nitrate concentration in the effluent rose to 710 mg/L. The bacteria in the MBR system adjusted to the presence of the leachate, and increased 4 orders of magnitude. **Heavy metals were removed by at least 82.7% except copper.** These successful results demonstrated the membrane bioreactor technology is feasible, efficient method for the treatment of compost leachate.[ \[Art. #ARTNUM\]](#article-96136-2040836780)

Ultrafiltration (UF) is a pressure-driven barrier to suspended solids, bacteria, viruses, endotoxins and other pathogens to produce water with very high purity and low silt density. Ultrafiltration (UF) is a variety of membrane filtration in which hydrostatic pressure forces a liquid against a semi permeable membrane.[\[Source\]](https://crystalquest.com/pages/what-is-ultrafiltration#:\~:text=Ultrafiltration%20(UF)%20is%20a%20pressure,against%20a%20semi%20permeable%20membrane.) **Highlights:** * Micellar-enhanced ultrafiltration(MEUF) is a powerful treatment process developed recently to remove heavy metals from wastewater,which combines ultrafiltration with surfactant technology. Competitive adsorption of Cd2+,Zn2+,Pb2+ to SDS micelle micellar-enhanced ultrafiltration(MEUF) was studied for single solute and mixed solute containing cadmium ions,zinc ions and plumbum ions with sodium dodecyl sulfate(SDS) as surfactant. Results show that with the wastewater containing a variety of bivalence heavy metal ions, competitive adsorption to SDS micelle exists among various ions, and the sequence of competitive adsorption is Pb2+Zn2+Cd2+. [\[Art. #ARTNUM\]](#article-96074-2353167906) * Based on this concept, we developed an integrated process taking advantage of the **strong adsorption abilities of in-situ Al or Fe hydrolytic flocs and excellent separation properties of ultrafiltration (UF) membranes.** **By controlling the aeration rate, injection frequency and the solution pH, membrane fouling was alleviated, especially under weakly acidic conditions. Additionally, owing to the higher rejection efficiency of the UF membrane, the effluent quality was improved, including the iron concentration, turbidity, and chromaticity.** This innovative separation method shows promising potential for application in removing heavy metals in water treatment.[ \[Art. #ARTNUM\]](#article-96074-2611225203) * In this work, a novel positively charged tight ultrafiltration (PCTUF) membrane was developed to remove heavy metal cations (Mn 2+ , Co 2+ , Ni 2+ , Zn 2+ and Cd 2+ ) from contaminated waters via electrostatic repulsion mechanism. The results revealed that the prepared PCTUF membrane with its high perm-selectivity performance provides a worthy reference for highly efficient removal of heavy metal cations. [\[Art. #ARTNUM\]](#article-96074-2922823182)

Nanofiltration is a membrane filtration-based method that uses nanometer sized through-pores that pass through the membrane. Nanofiltration membranes have pore sizes from 1-10 nanometers, smaller than that used in microfiltration and ultrafiltration, but just larger than that in reverse osmosis.[\[Wiki\]](https://en.wikipedia.org/wiki/Nanofiltration#:\~:text=Nanofiltration%20is%20a%20membrane%20filtration,than%20that%20in%20reverse%20osmosis.) **Highlights:** * One must separate phosphorus from the heavy metals in order to produce fertilizers of high quality. Among various available methods, nanofiltration (NF) has been demonstrated to be a feasible and promising option when the sewage sludge undergoes acidic dissolution and the operating pH is around 2. Because the performance of commercially available thin film composite (TFC) NF membranes reported thus far has great room for improvement, the development of highly permeable positively charged NF membranes is recommended. To this aim, a NF membrane that is desirable for phosphorus recovery was fabricated via interfacial polymerization of polyethylenimine (PEI) and trimesoyl chloride (TMC) on a porous poly(ether sulfone) (PES) membrane substrate.[ \[Art. #ARTNUM\]](#article-96121-2510602979) * This is a report of the first attempt to develop a composite NF membrane comprising a molecularly designed pentablock copolymer selective layer for the removal of heavy metal ions. The resultant NF membrane has a mean effective pore diameter of 0.50 nm, a molecular weight cutoff of 255 Da, and a reasonably high pure water permeability (A) of 2.4 LMH/bar. The newly developed NF membrane can effectively remove heavy metal cations such as Pb2+, Cd2+, Zn2+, and Ni2+ with a rejection of >98.0%.[ \[Art. #ARTNUM\]](#article-96121-2314982393) * In order to effectively separate chloride (such as NaCl) and sulfate (such as Na 2 SO 4 ) for high saline wastewater recycling, one commercial NF membrane (named Desal-DL) was employed in the permeation of the single and binary salt solutions of NaCl and Na 2 SO 4 with a lab-scale cross-flow batch module, where the salt concentration ranged from 4 to 96 g L −1 and the operating pressures varied from 0.6 to 2.4 MPa as well as the temperature was kept under room temperature. The experimental results showed that the Desal-DL NF membrane had a low rejection to NaCl and a high rejection to Na 2 SO 4 for single salt solutions. [\[Art. #ARTNUM\]](#article-96121-2324824251) * For the first time, thin film nanocomposite (TFN) nanofiltration membranes incorporated with graphene oxide (GO) were synthesized and used to separate phosphorus from water source of different properties. [\[Art. #ARTNUM\]](#article-96121-2767186783)

Formally, reverse osmosis is the process of forcing a solvent from a region of high solute concentration through a semipermeable membrane to a region of low-solute concentration by applying a pressure in excess of the osmotic pressure.[\[Wiki\]](https://en.wikipedia.org/wiki/Reverse_osmosis#:\~:text=Formally%2C%20reverse%20osmosis%20is%20the,excess%20of%20the%20osmotic%20pressure.) **Highlights:** * In this study, low-pressure reverse osmosis was analyzed to remove heavy metal ions (nickel and copper) from highly diluted feed flows. Given the ability of these ions to form complexes with EDTA, the effectiveness of a preliminary stage of complexation was also evaluated. The experimental system consisted of a reactor with commercial flat membrane of polyamide and cross flow. Studies employing solutions of single ions and also mixture of ions in different concentration ranges reached removals of 99%. [ \[Art. #ARTNUM\]](#article-96290-2311502662) * The results showed that high removal efficiency of the heavy metals could be achieved by RO process (98% and 99% for copper and cadmium, respectively). [\[Paper\]](https://www.sciencedirect.com/science/article/pii/S1878535210001334#s0055) * R.O is a membrane filtration process where water can easily pass through a membrane however cationic compounds are retained. The pore size of the R.O membrane is 0.1nm. R.O can be used for all solutions which have less quantity of metal dissolved in the millimolar or micromolar range. [\[Paper\]](https://www.researchgate.net/publication/331952883_Remediation_of_Nickel_ion_from_wastewater_by_applying_various_techniques_a_review)

Electrodialysis is used to transport salt ions from one solution through ion-exchange membranes to another solution under the influence of an applied electric potential difference. This is done in a configuration called an electrodialysis cell. [\[Wiki\]](https://en.wikipedia.org/wiki/Electrodialysis) **Highlights:** * The raffinate generated during copper ore hydrometallurgical processing is difficult to be treated because it is strongly acidic, and has high concentrations of heavy metals (Iron, Zinc, Copper etc). **In this study, a bipolar membrane electrodialysis (BMED) system was studied for treatment of this challenging stream because in which salts can be converted into their corresponding acids and bases, which enables resource recovery in raffinate.** The target was to reuse the raffinate as a leaching influent, to achieve zero discharge of wastewater. It was found that 85.9% of SO42- in the raffinate could be recovered by the formation of H2SO4. The removal rates of heavy metals were 99.3% (iron), 99.1% (zinc), 99.0% (copper), 84.9% (nickel), 70.6% (chromium), 95.8% (cadmium), and 94.8% (arsenic). The heavy metal cations were mainly removed in the heavy metals chamber (HMC) and anions were mainly removed in the acid compartment. It was concluded that the raffinate can be used as a leaching solution after treatment and the studied BMED process is an effective technique for the treatment of raffinate.[ \[Art. #ARTNUM\]](#article-96117-2977176722) * Electro-dialysis is separation method where ions are shifted over ion exchange resin with the help of electricity between two electrodes. Tzanetakis et al. investigate the removal of Cobalt and Nickel from their sulphate solution by using two cation exchange and an anion exchange membrane, a stainless steel cathode and platinum oxide based coated titanium cobalt, a suitable complexing agent is EDTA. [\[Paper\]](https://www.researchgate.net/publication/331952883_Remediation_of_Nickel_ion_from_wastewater_by_applying_various_techniques_a_review)

New concepts of separation based on electrochemical interactions were realized due to the successful developments of the electrically conductive membrane (called electromembrane). This electromembrane differs from the conventional ion exchange membrane due to the nature of the separation mechanism. The separation in an electromembrane is based on the electrical potential and/or magnetic field “barrier” generated by passing current through the membrane, while the separation in a conventional ion-exchange membrane relies on the relative ionic affinity of the compounds being separated. It is this characteristic which provides a unique opportunity for this new class of membrane.[\[Paper\]](https://www.sciencedirect.com/science/article/abs/pii/S0376738800808844) **Highlights:** * The goal of this dissertation is to demonstrate fouling prevention and foulant removal on carbon nanotube-modified (CNT) electrically conductive UF, NF and RO membranes. First, mineral scaling was prevented and removed by applying an external anodic electrical potential to an electrically conducting CNT – polyamide RO membrane. The results demonstrate that CaCO3 scaling was efficiently removed by the intermittent application of 2.5V and CaSO4 scaling can be prevented by the continuous application of 1.5V with the membrane as the anode. Second, biofouling and organic fouling were prevented while treating anaerobic sequencing batch reactor effluent by using electrically conducting UF and NF membranes. The modified membrane can be used to degrade organic compounds and perform in situ oxidative cleaning of the fouled membrane without any additional oxidizing chemical reagents[ \[Art. #ARTNUM\]](#article-96155-2898647488) * In the present study, the electromembrane extraction (EME) procedure was used for extraction of heavy metal cations from aqueous samples. The obtained results showed that more than 71.2% of the free ions were extracted from primeval aqueous samples. Moreover, the results indicated that this technique can be used in large-scale extraction of heavy metal cations from aqueous solutions.[\[Paper\]](https://pubs.rsc.org/en/content/articlelanding/2015/ay/c5ay00243e#!divAbstract)

A membrane adsorbent is made by connecting functional groups to the surface and pore wall of polymer membranes; the target pollutants are selectively adsorbed to the functional group. The membrane adsorbent effectively combines the filtration performance of the membrane.[\[Paper\]](https://link.springer.com/article/10.1007/s13201-018-0661-6#:\~:text=A%20membrane%20adsorbent%20is%20made,filtration%20performance%20of%20the%20membrane.) **Highlights:** * Application of polymeric membranes for the adsorption of hazardous pollutants may lead to the development of next-generation reusable and portable water purification appliances. **Membranes for membrane adsorption (MA) have the dual function of membrane filtration and adsorption to be very effective to remove trace amounts of pollutants such as cationic heavy metals, anionic phosphates and nitrates.**  [\[Paper\]](https://link.springer.com/article/10.1007/s13201-018-0661-6) * High performance nanofiltration (NF) membranes for heavy metal removal have been molecularly designed by adsorption of chelating polymers containing negatively charged functional groups such as poly (acrylic acid-co-maleic acid) (PAM), poly (acrylic acid) (PAA) and poly (dimethylamine-co-epichlorohydrin-co-ethylenediamine) (PDMED) on the positively charged polyethyleneimine (PEI) cross-linked P84 hollow fiber substrates. **Not only do these chelating polymers change the membrane surface charge and pore size, but also provide an extra mean to remove heavy metal ions through adsorption in addition to traditional steric effect and Donnan exclusion.** The adsorbed membranes have comparable water permeability and superior rejections to heavy metals, for instance, Pb(NO 3 ) 2 , CuSO 4 , NiCl 2 , CdCl 2 , ZnCl 2 , Na 2 Cr 2 O 7 and Na 2 HAsO 4 , with rejections higher than 98%. The membranes also display excellent rejections to mixed ions with rejections more than 99%. The newly developed membranes show reasonably stability during 60-h tests as well as multiple washes. [\[Art. #ARTNUM\]](#article-96282-2066359298)

Biomembranes are membranes from biological sources, such as eggshell membranes, nanocellulose (bacterial) etc.  **Highlights:** * **Eggshell membrane (ESM) was selected and modified to adsorb heavy metals. Adsorption of metal ions on the modified ESM (MESM) might be attributed to electrostatic interaction, ion exchange and coordination effect with chelating ligands containing N and S on the surface of the MESM.** [ \[Art. #ARTNUM\]](#article-96087-2890836740) * Here, we demonstrate a **novel membrane composed of polydopamine (PDA) particles and bacterial nanocellulose (BNC), which can efficiently remove a variety of metal ions and organic dyes from contaminated water.** The biocompatible and biodegradable PDA/BNC membrane is synthesized by in situ incorporation of PDA particles into BNC matrix during its bacteria-mediated growth. We show that the PDA/BNC membrane can effectively remove heavy metal ions such as lead and cadmium, and organic dyes as surrogate markers of organic pollutants such as rhodamine 6G (R6G), methylene blue (MB), and methyl orange (MO). The removal efficiencies of contaminants were tested separately or simultaneously via simple filtration at pH values ranging from 4 to 7. [\[Art. #ARTNUM\]](#article-96087-2910415304)

Photocatalysis is a type of catalysis that results in the modification of the rate of a photoreaction - a chemical reaction that involves the absorption of light by one or more reacting species - by adding substances (catalysts) that participate in the chemical reaction without being consumed.[\[Source\]](https://www.nature.com/subjects/photocatalysis#:\~:text=Photocatalysis%20is%20a%20type%20of,chemical%20reaction%20without%20being%20consumed.) **Highlights:** * With the help of semiconductor, photocatalytic process has caused considerable attention in removing heavy metal in wastewater for its high efficiency and rapid destruction. As generally observed, titanium dioxide can obtain the best photocatalytic performances. Upon of illumination of semiconductor-electrolyte, electron-hole pairs are formed in the conduction and the valence band of the semiconductor, respectively. Migrating to the surface of titanium dioxide, these charge carriers have redox potential. The hydroxylions (OH-) likely leads to the formation of hydroxyl radicals which has strong oxidizing property. And the traps for electrons are adsorbed oxygen species at the same time.[\[Art. #ARTNUM\]](#article-96288-2421754045) * Based on biosorption and photodegradation coupling technology, a novel adsorbent, which not only adsorbs the heavy metal ions but also degrades organic compound, was prepared by immobilization of nano-TiO2 on chitosan matrix. [\[Art. #ARTNUM\]](#article-96288-2147241663)

A hydrocyclone is a device to classify, separate or sort particles in a liquid suspension based on the ratio of their centripetal force to fluid resistance. This ratio is high for dense and coarse particles, and low for light and fine particles.[\[Wiki\]](https://en.wikipedia.org/wiki/Hydrocyclone) **Highlights:** * Heavy metals usually aggregate as fine particles in dredging slurry and need to be removed or reduced before recycling of the slurry. However, traditional hydrocyclone method often leads to low separation efficiency because of the difficulties in the complete separation of fine particles. To address this problem**, this paper proposes a new type of parabolic hydrocyclone for the removal of heavy metals from dredging slurry.** Therefore, after the parabolic hydrocyclone separation process, the underflow products contain less heavy metals and become both non-hazardous and recyclable. [\[Paper\]](https://www.revistascca.unam.mx/rica/index.php/rica/article/viewFile/RICA.2019.35.esp01.09/46853)

The basis of the Eutectic Freeze Crystallization (EFC) technology is the existence of the eutectic point. The eutectic point is a characteristic point in the phase diagram of a salt-water mixture. At the eutectic point an equilibrium exists between ice, salt and a solution with a specific concentration. This specific concentration is called the eutectic concentration and the temperature at which this equilibrium is found is the eutectic temperature.[\[Source\]](https://www.coolseparations.nl/efc-technology-how-it-works/#:\~:text=The%20basis%20of%20the%20Eutectic,solution%20with%20a%20specific%20concentration.) **Highlights:** * This paper focuses on the application of EFC for the purification of a typical brine containing high levels of sodium, chlorine, sulphate and ammonia that cannot be achieved with other separation techniques. The presence of ammonia prevents the application of membrane technology to treat the brine, leaving only cooling or evaporation as other possible options. Evaporation produces a mixed salt that requires further treatment. Modelling tools were applied to describe the phase behaviour of the complex saline systems under different process conditions and were experimentally validated. The results showed that Eutectic Freeze Crystallization could be used to selectively recover the sodium as a sodium sulphate salt. The simulation tools were especially useful in the design and optimisation of the process.[\[Paper\]](https://www.sciencedirect.com/science/article/abs/pii/S0263876210000377)