Technological trends in heavy metals removal from industrial wastewater: A review

Rapid industrialization, with economic prosperity set as the prime goal, has always created some secondary intolerable problems such as heavy metal contamination, wastewater that need remediation. Industrial wastewater is the major contributors to contamination of aquatic and terrestrial ecosystems with toxic heavy metals like arsenic, copper, chromium, cadmium, nickel, zinc, lead, and mercury whose hazardous bio-accumulative nature in biotic systems is attributed to their high solubility in the aquatic environments. There has, therefore, always been a need for the removal and/or recovery of these toxic, non-biodegradable, and persistent heavy metals from the industrial wastewater. For several decades, extensive investigations have been performed for easy, efficient, and economic removal of heavy metals with a varying degree of success. Chemical precipitation, adsorption, ion floatation, ion-exchange, coagulation/flocculation and electrochemical methods have been the most readily available conventional methods for the removal of these heavy metals. These methods however have posed some serious shortcomings such as high sludge production needing further treatment, low removal efficiency and high energy requirements. In the present years, newer more efficient, more economic and innovative technologies are being investigated. Recently photocatalysis, electrodialysis, hydrogels, membrane separation technique and introducing newer adsorbents have been developed for better adsorption. Hence in this paper, we have reviewed efforts and technological advances achieved so far in the pursuit of more efficient removal and recovery of heavy metals from industrial wastewaters and have evaluated their efficiency dependence on various parameters such as pH, temperature & initial dosing.

Journal of Environmental Chemical Engineering xxx (xxxx) xxx Contents lists available at ScienceDirect Journal of Environmental Chemical Engineering journal homepage: www.elsevier.com/locate/jece Technological trends in heavy metals removal from industrial wastewater: A review Rakesh Shrestha a, 1, Sagar Ban a, 1, Sijan Devkota a, 1, Sudip Sharma a, 1, Rajendra Joshi a, Arjun Prasad Tiwari b, *, Hak Yong Kim c, *, Mahesh Kumar Joshi a, d, ** a Department of Chemical Science and Engineering, School of Engineering, Kathmandu University, Dhulikhel, Kavre, Nepal Carbon Nano Convergence Technology Center for Next Generation Engineers (CNN), Jeonbuk National University, Republic of Korea Department of Nano Convergence Engineering, Jeonbuk National University, Jeonju 561-756, Republic of Korea d Department of Chemistry, Tri-Chandra Multiple Campus, Tribhuvan University, Kathmandu, Nepal b c A R T I C L E I N F O A B S T R A C T Editor: Teik Thye Lim Rapid industrialization, with economic prosperity set as the prime goal, has always created some secondary intolerable problems such as heavy metal contamination, wastewater that need remediation. Industrial wastewater is the major contributors to contamination of aquatic and terrestrial ecosystems with toxic heavy metals like arsenic, copper, chromium, cadmium, nickel, zinc, lead, and mercury whose hazardous bio-accumulative nature in biotic systems is attributed to their high solubility in the aquatic environments. There has, therefore, always been a need for the removal and/or recovery of these toxic, non-biodegradable, and persistent heavy metals from the industrial wastewater. For several decades, extensive investigations have been performed for easy, efficient, and economic removal of heavy metals with a varying degree of success. Chemical precipitation, adsorption, ion floatation, ion-exchange, coagulation/flocculation and electrochemical methods have been the most readily available conventional methods for the removal of these heavy metals. These methods however have posed some serious shortcomings such as high sludge production needing further treatment, low removal efficiency and high energy requirements. In the present years, newer more efficient, more economic and innovative technologies are being investigated. Recently photocatalysis, electrodialysis, hydrogels, membrane separation technique and introducing newer adsorbents have been developed for better adsorption. Hence in this paper, we have reviewed efforts and technological advances achieved so far in the pursuit of more efficient removal and recovery of heavy metals from industrial wastewaters and have evaluated their efficiency dependence on various parameters such as pH, temperature & initial dosing. Keywords: Heavy metals Wastewater treatment Chemical precipitation Electrodialysis Adsorption Photocatalysis Membrane separation 1. Introduction Chemical industries with their rapid growth have increasingly created nuisances for living beings and the environment. One of these nuisances is the generation of large volumes of wastewater contaminated with heavy metals whose high solubility in the aquatic environment poses serious threats to all living organisms. Several industries have contributed to this problem, with the most prevalent contaminants being Cadmium(Cd), Zinc(Zn), Lead(Pb), Chromium(Cr), Nickel(Ni), Copper(Cu), Vanadium(V), Platinum(Pt), Silver(Ag), Tin(Sn) and Titanium(Ti) [1–3]. These heavy metals, which are generally categorized with a density exceeding 5 g/cm3, are well-known to be toxic and can act as carcinogenic agents and are usually generated by electroplating, electrolytic depositing, conversion-coating, anodizing-cleaning, milling, and etching industries [4]. Other industries contributing notable amounts of Sn, Pb, Ni, As include printed circuit boards (PCBs) manufacturing industries and wood processing industries. Similarly, petroleum refinery generates Ni, V and Cr containing effluents from conversion catalysts, also films with high concentrations of Ag and ferrocyanide are produced from photographic operations [5]. The most common sources of heavy metals have been listed in Table 1. World health organization (WHO) has set guidelines for the * Corresponding authors. ** Correspondence to: Department of Chemical Science and Engineering, School of Engineering, Kathmandu University, PO BOX 6250, Dhulikhel, Kavre, Nepal. E-mail addresses: tiwariarjuna@gmail.com (A.P. Tiwari), dragon4875@gmail.com (H.Y. Kim), joshimj2003@yahoo.com (M.K. Joshi). 1 These authors contributed equally. https://doi.org/10.1016/j.jece.2021.105688 Received 7 January 2021; Received in revised form 8 May 2021; Accepted 12 May 2021 Available online 17 May 2021 2213-3437/© 2021 Elsevier Ltd. All rights reserved. Please cite this article as: Rakesh Shrestha, Journal of Environmental Chemical Engineering, https://doi.org/10.1016/j.jece.2021.105688 R. Shrestha et al. Journal of Environmental Chemical Engineering xxx (xxxx) xxx maximum permissible limits on these toxic heavy metals and their consequences to human health when those limits are surpassed in drinking water and industrial waste water, as listed in Table 2 [10,11]. Contamination of agricultural soils and crops by heavy metals produced from industries and municipal waste has been considered serious environmental problem due to their non-biodegradable nature and long biological half-life. Consumption of heavy metal content vegetables can seriously deplete some essential nutrients in body that are further responsible of decreasing immunological defenses, intrauterine growth retardation, impaired psychological faculties, disabilities related to malnutrition, and high prevalence of upper gastrointestinal cancer rates [12,13]. Many reports showed plants uptake preferentially the divalent, trivalent, or multivalent metal present in nutrient solution in the ionic or chelating form [14,15]. When human injested the plant source containing the heavy metals like Cd, it affects the adsorption of more essential metals such as Fe and Zn in gastrointestinal tract. In such a case, Fe content can be diminished while increasing Cd content, a toxic metal due to the common Fe transporter [16]. This results in the intoxication of the heavy metal and showed side effects associated with it finally. The level of toxicity of the selected metals for humans follows the order Zn<Fe<Cu<Mn< Ni<Cr <Pb< <Cd<Hg [10,17]. To achieve heavy metal concentrations in wastewater well below the limits set, it has to be subjected to a treatment process before discharging them into the environment or reusing them in the industry. Besides affecting humans and animals, heavy metals concentration should not exceed a permissible limit in plants as well and when exceeded, there can be serious consequences as seed germination & lipid content get declined by Cd, photosynthesis gets inhibited by Cu and Hg, chlorophyll production and plant growth get hindered by Pb [18]. Therefore, it is extremely necessary to treat these toxic metals contaminated wastewater before their discharge to the environment. Over several decades, heavy metals have been removed from these industrial effluents by conventional treatment methods such as chemical precipitation [21,22], ion flotation [23,24], ion exchange [25,26], coagulation/flocculation [27,28], adsorption [29], and electrochemical removal [30]. These low-cost advantage-oriented methods, however, have significant disadvantages and inadequacies, some of them being incomplete removal of heavy metals, high-energy requirements and toxic sludge production [31,32]. Several studies have been made on the development of a cheaper and more efficient technology. Substantial achievements have already been made on different types of membrane separation processes such as ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO) [33,34]. Newer adsorbents have been introduced for better adsorption. In the other hand, innovative technologies such as photocatalysis and electrodialysis techniques have been proven efficient and promising to not only organic waste management but also removal of heavy metal [35,36]. Fig. 1 provides a summary of different technologies that have been developed for heavy metals removal under adsorption, membrane separation, electrochemical, photocatalytic and gravity settling processes. In this review, we have reviewed some of these better changes that have been made on technologies over recent years along with their advantages and disadvantages on efficient removal of heavy metals from industrial wastewater. Before that, we have discussed briefly the techniques used conventionally. 2. Conventional technologies for heavy metals removal from industrial wastewater The physical and chemical conventional techniques that have been employed for decent amounts of removal of heavy metals include chemical precipitation, adsorption, ion floatation, ion-exchange, coagulation/flocculation and electrochemical processes. 2.1. Chemical precipitation Chemical precipitation is used to remove the ionic constituents in wastewater by the addition of precipitating agents resulting in a chemical reaction that converts the soluble compound into an insoluble form. It is always followed by some other separation techniques including coagulation or filtration to remove the precipitates. Most metals are precipitated as hydroxides, however other methods such as sulfide and carbonate precipitation are also in common use [37]. The mechanism of precipitation can be generalized as. M2+ + 2(OH)- = M(OH)2↓ Where, M2+, OH-, and M(OH)2 are the metal ions, the precipitating agent and finally the insoluble metal hydroxide formed, respectively. AbiD et al. compared the effectiveness of using magnesia (MgO) over Table 1 Some of the most common sources of heavy metals. Industries Metals Paper mills Organic chemistry Allies, Chlorine Fertilizers Petroleum refinery Steel works Aircraft Glass/Cement Textile mills Tanning Power plants Pharmaceutical Engineering Fine chemicals Dyes Pesticides Fungicides Metal smelters Welding Electroplating Nuclear fission Mining Ferromanganese production Batteries Brass manufacture Al As Cd Cr Cu Hg Pb Ni Zn ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ Co Fe Mn References ✓ [6] ✓ ✓ ✓ ✓ ✓ ✓ [7] [8] ✓ ✓ ✓ [9] ✓ ✓ ✓ ✓ ✓ ✓ 2 ✓ R. Shrestha et al. Journal of Environmental Chemical Engineering xxx (xxxx) xxx hydroxide as a precipitating agent and the final precipitate was absorbed by the biofilm absorbent [40]. They have studied the effect of pH, reaction time, adsorbent dosage and the initial concentration of metal ions on the removal efficiency and concluded that the removal efficiency was optimum at pH 5. The removal rate increased with an increase in the amount of adsorbent. From the above-depicted studies, it can be suggested that while the chemical precipitating technique is among the least expensive technology requiring simple and easily operable equipment, it is not quite effective for treating wastewater having high acid content besides also producing a large amount of toxic sludge that needs treatment with chemical stabilization followed by proper disposal. Some of the metal salts are not water-soluble, in this case, correct anion should be added to get precipitated. This can lead to the production of a high water content sludge and disposal process became expensive. On other hand, precipitation with lime and bisulphide lacks specificity. This process can be ineffective in the removal of metal ions of low concentration. Table 2 Permissible limits on toxic heavy metals and their toxic effects on human health [10,17]. Contaminants Recommeneded safe limits by WHO in drinking water Recommeneded safe limits by WHO in waste water Toxic effects Copper < 2 mg/L 1 mg/L Zinc < 3 mg/L 2–5 mg/L Manganese < 0.12 mg/L [11, 17] < 0.2 mg/L Arsenic < 0.01 mg/L n/a Gastrointestinal effects, carriers of the gene for Wilson disease, metabolic disorder of copper homeostasis Skin irritations, anemia, nausea and vomiting Neurological effects following inhalation exposure, psychological symptoms (irritability, emotional lability). Chronic arsenicism including dermal lesions such as hyperand hypopigmentation, peripheral neuropathy, skin cancer, bladder and lung cancers and peripheral vascular disease Carcinogenic by the inhalation route, kidney problem due to accumulation in the kidneys Chromium(VI) is carcinogenic to humans, lung cancer Accumulates in the skeleton and creates bone problems, adverse effects on central and peripheral nervous systems Long-term exposure is manifested in nails, hair and liver, effects on synthesis of a liver protein Oral ingestion creates problems in gastrointestine; kidney damage, increase the incidence of some benign tumors. Irritability, nausea, vomiting, difficulty sleeping Cadmium 0.003–0.005 mg/ L 0.003 mg/L Chromium < 0.05 mg/L 0.05 mg/L Lead < 0.01 mg/L 0.01 mg/L [20] Selenium < 0.01 mg/L n/a Mercury < 0.006 mg/L 0.05 mg/L [20] Nickel 0.02–0.07 mg/L 0.02 mg/L 2.2. Adsorption Adsorption is a surface phenomenon and is defined as an attachment of a particular compound at the surface of a solid object by physical forces or by chemical bonds [41]. Compound (pollutant) that attaches to the solid surface is called an adsorbate while the solid surface is known as an adsorbent. There are mainly three sequential steps involved in heavy metals adsorption: the transport of heavy metals from the bulk solution to the absorbent surface followed by adsorption on particle surface, and finally the transport within the adsorbent particle. Fig. 2 illustrates different types of adsorption. Factors affecting the adsorption are temperature, nature of the adsorbate and adsorbent, presence of other pollutants, and atmospheric & experimental conditions (pH, the concentration of pollutants, and contact time and particle size of the adsorbent). In addition, the presence of suspended particles, oils and greases reduces the efficiency of the process and, therefore, pre-filtration may sometimes be required [42]. Graphene, activated carbon, carbon nanotubes, rice husk, surfactant modified waste, modified sugarcane bagasse, modified wheat bran, modified coconut waste, modified orange peel waste, modified saw dust, modified eggshell, mesoporous silica, chitosan, zeolite, red mud, coffee residue, powdered olive stones, apple pomace, magnetic adsorbents, alumina, clay, fungal biomass, yeast, algal biomass and microbial (bacteria) are among the most studied adsorbents [43–46]. Since the introduction of adsorption in the 1940s, activated carbon has been the important and prime choice for the treatment and recycling of community and industrial wastewater to potable quality water because of its good adsorption capacity due to small particle size, active free valences and high surface area. However, large-scale utilization of activated carbon for water treatment is not feasible because of its high cost of production and regeneration [47,48]. Kadirvelu et al. demonstrated pH dependence of adsorption with maximum removal of 73% for Cu(II), 100% for Hg(II), Pb(II) and Cd(II) and 92% for Ni(II) by coir pith carbon at pH of 5.0 for Cu(II), 4.0 for Pb(II), 3.5 for Ni(II) & Hg(II), and 4.0 for Cd(II) [49]. The maximum adsorption was detected at a pH range of 4.0–5.0. Santhy et al. found that at pH value below 3, the adsorption of metal ions was very low and was effective only when the pH value was above 6 [50]. In the batch mode operation, the efficiency of adsorption of metal ions (Cu(II), Cd(II), and Zn(II)) was studied on porous carbon, increased with an increase in contact time, pH, carbon dose, and decrease in initial metal–ion concentrations. The presence of common anions such as chloride and sulfate did not affect the removal of metal ions up to 3000 mg/L. Nickel showed the highest removal of 90% by activated carbon at every concentrations but the removal percentage decreased with the increase in the concentrations of the heavy metal [51]. Table 3 summarizes some of the adsorbents, their specific heavy metal removal efficiencies and further weighing of merits with demerits. lime for the removal of Fe(III), Cr(III), Cu(II), Pb(II), Ni(II) & Cd(II) and reported that the removal efficiency was more than 97% at an optimum MgO dose [38]. With the use of MgO as a precipitating agent, the sludge was grainy, dense and easily settleable & dewatered, while lime usage resulted in a low settling rate and dewatering difficulty. Meunier et al. studied the electrocoagulation and chemical precipitation for the removal of heavy metals from acidic soil saline leachate and found that the chemical precipitation using Ca(OH)2 was effective in reducing Cr, Cu, Ni and Zn but not Cd and Pb [39]. While both the techniques needed to be operated at higher pH values; electrocoagulation was found to be effective at pH between 7 and 8. The electrochemical treatment is suitable for the removal of Pb and can be applicable of producing an effluent having a pH value close to the neutral. Wu et al. used calcium 3 R. Shrestha et al. Journal of Environmental Chemical Engineering xxx (xxxx) xxx Fig. 1. Commonly used technologies used for removal of heavy metals from industrial wastewater so far. kinetics. Recently, low-level removal of arsenic, mercury, lead, chromium and cadmium ions from aqueous solution have been achieved using cysteine reacted with octanoyl, decanoyl and dodecanoyl chloride [72]. The highest removal efficiency of 99.9% by re-crystallized octanoyl-cysteine (octanoyl-cys) surfactant for mercury ion using pure nitrogen gas and the removal of other ions was also around 99.1–99.7% efficient using either pure nitrogen or air. Twice recrystallized S-octanoyl-cys surfactant showed high efficiency in removing low levels of arsenic ions from aqueous solutions. Hoseinian et al. also investigated removal of Ni(II) and Zn(II) ions from wastewater using Ethyl-hexadecyl-dimethyl-ammonium bromide (EHDABR) and sodium dodecyl sulfate (SDS) as collector and Dowfroth250 and methyl isobutyl carbonyl (MIBC) as frothers [73]. In their study, the optimal conditions for removal of Ni(II) and Zn(II) at an initial concentration of 10 ppm were’ pH 3, SDS = 300 ppm Downforth 250 = 90 ppm and airflow rate of 108 mL/h. They reported an optimal recovery of Zn(II) and Ni(II) to be 92% and 88%, respectively further adding the optimal recovery of Zn(II) being achieved with a shorter period than Ni(II). The advantages include its simplicity, flexibility, low energy consumption, small volume sludge production, and acting selectively in industrial applications. However, metal ions are difficult to be separated from the collectors due to they remain still in the form of complexion. This limits large-scale application for the remediation of environmental pollution [74]. Fig. 2. Schematic diagram showing the different types of adsorption. 2.3. Ion flotation Ion flotation is one of the most accepted techniques in metal removal from industrial wastewater. Various lab scale and industrial scale removal of metals from wastewater have been reported with various efficiencies. Removal of achieved 90% Fe(III) and Mn(II) in flotation column has been achieved with biodegradable surfactant as collector at a lowest airflow rate of 1098 mL/min after 15 min of operation [69]. The synthetic surfactant like sodium oleate as a collector also showed a similar collection of heavy metals. The use of bio-surfactant is a more promising alternative to a synthetic surfactant with dissolved air flotation system compared to column flotation [69]. Bodagh et al. studied the potentiality of removal of cadmium from aqueous solution by foam flotation with rhamnolipid bio-surfactant [70]. In their study, cadmium separation from zinc and copper was investigated with different operating conditions like rhamnolipid and cadmium concentration, solution pH, aeration rate, frother type and concentration. Superior selectivity coefficient of cadmium over copper and zinc was found to be 57%, 36% and 48% respectively following first-order kinetics with rate of 0.0071 min-1 with maximum separation of cadmium from zinc and copper contaminated solution. Using sodium dodecyl sulfate (SDS) and hexadecyl trimethyl ammonium bromide (HTAB) as anionic and cationic surfactant resulted removal of 98% and 76% of copper at optimal operation condition, respectively [71]. The presence of NaCl in the solution reduced recoveries while addition of 1% ethanol increased removal efficiencies with removal kinetics following the first order 2.4. Ion exchange Another classical method for heavy metal removal from industrial effluent with a solid capable of exchanging cations/anions is known ion exchanger. Over the last two decades, performance investigations of several ion exchange resins have been done. Most advantages of using this method are that it can remove the parts per billion (PPB) levels while managing the relatively large volume. The other important advantage is that this technique can be effective to clean up either cations or anions. Rengaraj et al. studied the performance of cation exchange resins IRN77 and SKN1 in removing Cr from synthetic coolant water with study of the influence of parameters such as initial resin dosage, agitation time and pH, and reported that the resins were able to 4 R. Shrestha et al. Journal of Environmental Chemical Engineering xxx (xxxx) xxx Table 3 Adsorbents with their efficiencies compared alongside their shortcomings. Industry type Metals Efficiencies Merits Demerits References Waste sludge from industries Cd(II) 37.8 mg/g No recovery [52] Melon peel Cd 81.97 mg/g Metal-laden adsorbent immobilization in cement for ultimate disposal. Effective use of agricultural waste [53] Pyrophillite waste Silty clay Cd, Ni Cd(II), Ni(II) Biochars Cr(III), Cr (VI), As(V) Cr(VI) Cd = 75%, Pb = 98% Cd(II) = 5.48 mg/g Ni(II) = 3.60 mg/g Cr(III) = 95%, Cr(VI) = 89%, As(V) = 53% 22.96 mg/g Higher mixing rates needs higher energy Low Cd removal rate n/a Lower removal rates [56] Absorbent was derived from the green source Suitable substitute for commercial activated carbon Performance evaluation in short time High limiting capacity for adsorption [57] Effective environment friendly adsorbent Lacking of mechanism of adsorbtion Cr(VI) to Cr(III) slower reduction Difficult to harvest specific fungi Metal based-derivative to remove metal ions [59] Needs high temperature to prepare char Low scale production of adsorbent Low scale production of adsorbent Not suitable for highly concentrated samples Nature of adsorption not spontaneous n/a [63] Orthophosphoric acid-treated Green coconut shell Coconut coir and bituminious coal based activated carbon Cr(VI) Low cost adsorbent Metal ions removal by SC were spontaneous, and feasible Biochars could retain metal ions compared to soil Pea waste Cr For coconut coir = 27.8 mg/g For bituminious coal = 38.5 mg/g 3.56 mg/g Sulfur reducing bacteria and zerovalent iron Ectomycorrhizal fungi Cr(VI) Complete removal Complete removal for low Cr concentration Cr(VI) 99% Chromium doped nickel nano metal oxide Cd(II), Pb (II), Cu(II) Surface modified biochar Cr(VI) Cd(II) = 98%, Pb(II) = 99.5%, Cu(II) = 97.5% 45,366 µg/g pH reduced by Hydrogen ion present in the fungi OH- formation on Cr doped NiO surface for aqueous solutes Fe2O3 nanoparticles Cr(VI) Fe2O3/A1SBA-15 Fe3O4 nanoparticles Cr(VI) As(V) As(V) Cr(VI) = 85%, As(V) = 94% 99.2% Tea waste As(V) 72% Treated magnetite waste As > 90% Eco-friendly and economic adsorbent Cost-effective adsorbent with high removal efficiency Superior adsorbent for the industrial effluent Utilization of waste from the Aluminum production process Low-cost adsorbent over a wide concentrated range Cost-effective and potential candidate to captor other metals remove up to 98% of chromium from an aqueous solution of 100 mg/L with the influence of parameters being optimal pH of 3.5, decrease in equilibrium concentration with increasing resin doses and finally SKN1 resin requiring less agitation time than IRN77 [75]. Another study demonstrated nearly 99.9% Cd, Cu and Hg removal technique from waste sludge using commercially available magnetic ion exchange resin having a capacity of 4.5 meq/g and particle size of 100–300 µm [76]. Cavaco et al. evaluated the temperature dependence of a chelating [54] [55] [58] [60] [61] [62] [64] [65] [66] [67] [68] exchange resin Diaion CR11 and Amberlite IRC86 having capacity of 1.21 and 2.77 meq Na+/g resin, respectively on removing Cr (III) from industrial effluents [77]. Three basic anion exchange resins D301, D314 and D354 have been used to investigate the Chromium (VI) adsorption capacities in an aqueous solution and found their dependence on varying experimental conditions [78]. The result indicated more than 99.4% removal of Cr6+ ions in 1–5 pH range. At room temperature, D301 and D314 resins showed higher adsorption capacities than D314 but when Table 4 Ion exchange resins with their efficiencies compared with their shortcomings. Resin type Metals Efficiency Merits Demerits References Amorphous hydrous Manganese oxide (HMO) B. longum 46, L. fermentum ME3 and B. lactis Bb12 Golden Apple Snail (GAS) Cd(II), Ni (II), Zn(II) Increase in the amount of Ca(II) shows higher removal efficiency HMO can be regenerated by HCL, advantageous than other polymeric exchangers High pressure drop in column operation [82] Cd(II), Pb (II) Cd(II) = 54.7 mg/g and Pb (II) = 175.7 mg/g of dry biomass 81.301 mg/g Fast removal and metabolism-independent surface process Needs further water purification [83] Economic and effective biomaterial [84] Nitrated Styrofoam and sulfonated Styrofoam Cd(II), Pb (II), Hg(II Utilization of waste styrofoam leading to decrease in environmental pollution, potential use in the floatation system due to its low density Ferrous chloride Cr(VI) For Nitrated Styrofoam, Cd = 600 µmol/g, Pb = 1100 µmol/g Hg = 2450 µmol/g For sulfonated Styrofoam Cd = 500 µmol/g, Pb = 1000 µmol/g Hg = 2100 µmol/g > 77% Recovery of Harmful invasive gastropod, yet to be studied No Cr removal was measured in the bicarbonate field brine [85] Cd(II) Adsorption to the precipitated solids 5 [52] R. Shrestha et al. Journal of Environmental Chemical Engineering xxx (xxxx) xxx the temperature was 60 ◦ C. Alyüz et al. [79] reported more than 98% of Ni2+ and Zn+ ions removal from an aqueous solution using Dowex HCR S/S cation exchange resin. The agitation time for reaching equilibrium was 90 min for nickel and 120 min for zinc. The operation was 99% efficient when the initial resin dosage was 0.2–0.3 g/100 mL for both zinc and nickel. The effects of pH on the removal of Cd2+ by ion-exchange resin Amberjet 1200H from a metal solution revealed that metal removal was optimum within the pH range 4–7 [80]. Table 4 further lists some of the ion exchange resins with their specific heavy metal removal efficiencies and their merits and demerits. The drawback of this method is that it cannot be effective in the case of using concentrated metal solution due to exchange matrix gets easily fouled by organic and other solids in the waste [81]. 2.6. Electrochemical process Electrochemical treatment of wastewater deals with an electron transfer reaction that may be either electroreduction or electrooxidation [6]. Electrolytic metal recovery has been investigated for a long time now. In this method as illustrated in Fig. 4, a direct current is passed through an aqueous solution containing metal ions between the cathode plates and insoluble anodes. The positively charged metal ions adhere to the negatively charged cathode [87]. It can be summarized basically as a cathodic deposition given by the equation: Mn+ (aq) + neￚ = M (s) In the past, the electrochemical reactors for metal recovery, electrocoagulation, electrofloation and electrooxidation were popular [88]. The removal efficiencies of the metals have been reported to be influenced by the type of cell and the electrode used. The hexavalent chromium can be reduced to trivalent chromium and can be electrochemically precipitated in an alkaline medium [89]. The complete reduction after 5 h at pH of 1 and with 0.25 A current was achieved and after 8 h of electrolysis at pH 5.5, 98.6% removal of chromium can be achieved. Cheballah et al. studied the effect of pH, current density, number, and the types of electrodes used for the reduction of hexavalent to trivalent chromium by bipolar electrocoagulation, and found that 100% Cr(VI) was reduced to Cr(III) at pH 3, current density of 200 A/m2 and conductivity of 2.6 mS/cm [90]. The energy requirement is much lower with the use of iron electrode than with Al electrode [90]. Khattab et al. found that the removal efficiencies were directly proportional to the current density [91]. Maarof et al. concluded that electrosorption technology could be an emerging method that offers a cost-effective solution for wastewater treatment [92]. The 3D electrodes could also be brought into practice compared to 2D electrodes. Overall, electrochemical processes provide following advantages: (1) metal selectivity, (2) no additional requirements of chemicals, (3) rapid and well-controlled operation with high removal efficiency and (4) lower level of produced sludge. While limitation comprises: (1) pH-sensitive process, (2) need for replacement of sacrificial electrode and (3) requirement for high-cost electrodes and its subsequent electrical energy requirement make this technology of relatively higher cost [93]. 2.5. Coagulation and flocculation Coagulation and flocculation are essential treatment processes of wastewater disposal and subsequently achieving drinking water. Coagulation is obtained when a chemical reaction occurs after the use of a coagulant or a chemical in wastewater. In an aqueous solution, colloidal materials join together to form flocs or small aggregates. Suspended materials like metals are attracted to these small aggregates or flocs. Slow mixing of water could help form small flocs which could increase in size such that they settle down inside the solution. This process is known as flocculation. Both of these phenomena are illustrated in Fig. 3. Mostly organic contaminants are removed with these procedures. Hossain et al. investigated the performance of ferrous sulfate heptahydrate FeSO4⋅7H2O removing biochemical oxygen demand (BOD), chemical oxygen demand (COD) and total suspended solids (TSS) from palm oil mill effluent of waste from Titanium Oxide industry [86]. Freundlich isotherm model described the coagulation adsorption mechanism showing that coagulation adsorption occurred in a multilayer formation with non-uniform distribution of adsorbed particles. FeSO4⋅7H2O waste could remove about 70% COD, over 80% BOD, and over 85% TSS in a single-stage coagulation treatment. Although having the advantage of being a simple and non-metal selective heavy metal separation process, the method produces huge amount of sludge and further creates a separation problem by transferring the toxic compounds to solid phase. This method is suitable for the effective removal of dissolved metals, dyes and suspended solids. 3. Modern technologies for heavy metals removal from industrial wastewater Several inadequacies are attributed to each of the conventional methods such as chemical precipitation and coagulation/flocculation while being among the simplest and least expensive technology requiring easily operable equipment, are not quite effective for treating wastewater with high acid content. Besides conventional techniques produce large amount of toxic sludge (in solid-phase for coagulation/ flocculation) that needs treatment with chemical stabilization followed by proper disposal. Although previously studied adsorbents have shown good retention and almost complete removal of heavy metals, they have Fig. 3. Schematic diagram showing the coagulation & flocculation methods for heavy metal removal. Fig. 4. Electrolytic method for heavy metals recovery. 6 R. Shrestha et al. Journal of Environmental Chemical Engineering xxx (xxxx) xxx however also shown higher mixing rates needing higher energy, lower removal rates, high limiting capacity and slower reduction. IonExchange, despite being economic and effective, has several disadvantages such as the ion exchange media being fouled by the metals and also with the oil, grease, clay, organic materials and microbes [94]. This fouling should be removed with proper cleaning program that increases the maintenance cost. The recovery of metals can be difficult after the cleaning. The efficiency of this treatment is also reduced by the presence of free acids. There is high dependence of Ion floatation technology on pH and temperature. With an increase in ionic strength, a decrease in the removal efficiency has been observed [76–78,95]. The surfactant concentration is also affected by the temperature which affects the removal efficiency. Electrochemical methods with metal selectivity, rapid and well-controlled operation and high removal efficiency need replacement of high-cost sacrificial electrode. Hence, to fulfill the need for better technology to overcome these inadequacies several new methods, adsorbents and technologies have been developed which are described briefly. high performance and lower cost. Graphene [96,97], activated carbon [98], carbon nanotubes [98,99], and Zeolite [100] have been the most common and commercially studied adsorbents. Modification on shape, size, physical and chemical nature of these adsorbents have influenced the removal efficiency. Some of these newly developed adsorbents with their removal efficiency for the most prevalent heavy metals have been listed in Table 5. 3.2. Hydrogels A hydrogel is a three-dimensional (3D) network of hydrophilic polymers that maintaining the structure by the physical and chemical linking between the individual chain. The important property of the hydrogel is that it can swell in water and hold a large amount of water while maintaining the structure. For a material to be considered a hydrogel, at least 10% of total weight (or volume) must constitute water. In most of the cases, hydrogel can return to its initial state as soon as the stimulus is removed [160]. pH-sensitive hydrogels, temperature-sensitive hydrogels, electro-sensitive hydrogels, light sensitive hydrogels are some common types of hydrogels. Along with heavy metals removal, hydrogels have been used for drug delivery, as scaffolding in tissue engineering, in contact-lenses, pH sensors, biosensors, biomedical engineering [161]. 3.1. Nano-adsorbents Large numbers of studies have been focussed to develop high efficient nano-adsorbents to remove heavy metals from wastewater with Table 5 List of adsorbents and their removal parameters for each heavy metal. Adsorbents Graphene Activated carbon Carbon nanotubes Rice husk Zeolite Heavy metals Adsorption capacity, mg/g Conc., mg/L Optimum pH/ Temp, K Efficiency (%) Contact time, min. References Pb(II) Cd(II) Cr Cu++ As+++ Co(II) Ni(II) Hg++ Zn++ 256 136.98 92.65 207.27 42.75 21.28 66.01 280.8 208.33 3.225–0.01 3.225–0.01 52 50 5 2.0–25 10–100 100 10 5/313 6/313 5.5 5/298–313 5/298–313 5.5/298 4–9/318 5/298 7/293 99 98.46 92.65 86.77 80.39 93.8 78.31 98 100 120 120 12 300 900 30 120 120 1440 [101,102] [101,103,104] [105,106] [104,106,107] [104,107] [104,108] [104,109] [110,111] [112,113] Pb(II) Cd(II) Cr Cu++ As(V) Co(II) Ni(II) Hg++ Zn++ Mn++ Fe++ 134.22 38.03 21.57 38 14 22.57 67.56 154 31.11 51.23 58.76 40 200 50–100 37 40 100 20 40 200 20 20 6.5/298 6.8/298 5/293 5/300 6.5/303 6.0/303 5.8/293 5/ 4.5/298 5.8/293 5.8/293 97.95 95 98.2 97 99 90.3 94 100 95 88 90 50 120 60 75 150 100 120 40 120 120 120 [114,115] [115,116] [117–119] [115,120,121] [122] [121,123] [121,124] [125] [115] [124] [124,126] Pb(II) Cd(II) Cr Cu++ Co(II) Ni(II) Hg++ 102.04 14.09 264.5 8.84 69.63 47.86 35.89 540 26.5 100 0.01 100–1200 10–80 0.55 5/298 6/298 2/298 4/298 7/298 7/NA 6/298 96.03 97.25 95 96 58 93.4 26.5 80 600 240 30 NA 900 600 [127,128] [129] [130,131] [132,133] [134] [135,136] [129,137] Pb(II) Cd(II) Cr Fe++ Cu++ Ni(II) 4.0 125.94 63.69 0.22 0.10 0.029 20 20–400 50–300 11.78–0.088 5.43–0.099 1.74–0.053 11/298 6.5/298 2/NA 2–10/298–300 2–10/298–300 2–10/298–300 99.6 99.1 71 99.253 98.177 96.954 40 120 120 120–150 120–150 120–150 [138–140] [141,142] [143] [140,144] [144,145] [144,146] Pb(II) Cd(II) Cr Cu++ Co(II) Ni(II) Hg++ Zn++ 16.81 47 3.27 2.1 5.5–16.8 24.65 16.354 28.6 1–200 2500 2.89 100–400 50–100 20 13.20–575 20 5/293 4/298 3.5/300 5/NA 3–4/298 7.5/298 5–6/NA 7.5/298 92 97.5 96 99 66.10 99 90 99 25 30 240 60 60 60 420 30 [147,148] [149,150] [151] [152–154] [155,156] [155,157] [158,159] [154,155] 7 R. Shrestha et al. Journal of Environmental Chemical Engineering xxx (xxxx) xxx Cross-linked acrylamide (AM) and 2-acrylamido-2-methylpropanesulfonic acid (AMPS) homopolymers and copolymers have been prepared by free radical solution polymerization and studied to separate Cd (II), Cu(II) and Fe(III) from aqueous solutions [162]. The pH of the solution affects significantly on the capability of hydrogel to adsorb metal ions. Increasing pH also increased the adsorption capacity of AM/ AMPS hydrogels. The affinity order of metal ions adsorbed by hydrogel is Cd (II) > Cu(II) > Fe(III), which is suggested to be mainly dependent on polarity, electronic configuration, ionic radius, etc. and most importantly on nature of interaction with the functional groups of hydrogel [163,164]. The binding capacities of all metal ions decreased under competitive conditions; 730–800 mg/g for Cd(II), 650–720 mg/g for Cu (II), and 610–700 mg/g for Fe(III). The binding capacities for other metal ions Zn(II), Mn(II), Co(II), Pb(II), and Na(I), are 580–620, 610–680, 500–581, 300–350, and 200–280 mg/g, respectively. Desorption of the absorbed metal ions from saturated AM/AMPS hydrogels can be achieved by decreasing the pH of the medium. The recoveries of Cd(II), Cu(II) and Fe(III) using AM (50 mol%)/ AMPS (50 mol%) cross linked with 10% of N, N’ -methylenebisacrylamide (MBA) were reported to be 98.5%, 93% and 94%, respectively [162]. Self assembled TEMPO-oxidized cellulose nanofibers (TOCN)/ cationic guar gum (CGG) hydrogels was developed to remedy wastewater containing oil, heavy metal ions or organic dyes [165]. The monolithic TOCN/CGG hydrogel could also effectively remove copper ions (Cu II) and dyes (i.e. thioflavin T and methyl orange), based on an adsorption mechanism. CMC/PAM composite hydrogel has been found efficient for the removal of Cu(II), Pb(II) and Cd(II) ions from the wastewater [166]. The adsorption followed the Langmuir model and exhibited pseudo-second-order kinetics. Further, in situ reductions of adsorbed Cu(II) ions lead to a Cu NPs-loaded composite hydrogel, serving as a heterogeneous catalyst for the reduction of 4-NP. The cost-effectiveness, excellent adsorption and catalytic properties proved the hydrogel to be a promising material for the cascaded treatment/reuse of the heavy metal ions [166]. Super adsorbents with ultrahigh adsorption capabilities are highly beneficial for effective wastewater remediation. A super-adsorbent hydrogel spheres (SAHSs) are based on self-sacrificing micro-reactors obtained from the interior cross-linking polymerization of the hydrogel network and subsequent self-disintegration within only 150 s [167]. The as-prepared poly(2-acrylamido-2-methyl-propanesulfonic acid-co-acrylic acid) hydrogel spheres exhibited excellent adsorption capabilities towards organic dyes (4625 ± 231 mg g−1 for methylene blue) and heavy metallic ions (4312 ± 185 mg g−1 for Pb(II)). In addition, the hydrogel also showed good recyclability, selective adsorption and smart separation–regeneration. The chitosan/gelatin hydrogel particles prepared by inverse emulsion from the aqueous solutions of chitosan, gelatin, and glutaraldehyde showed a maximum removal efficiency of 98% for Hg (II) ions in a solution [168]. The results were affected by the composition of hydrogels rather than the pore size or degree of swelling. The removal efficiencies for Pb (II), Cd (II), Hg (II) and Cr (III) ions reached about 73−94% in a multiple metal ion solution. The result indicated that the CG hydrogel could be used to remove co-existing heavy metal ions from wastewater, providing a versatile method to remove multiple metal ions from natural or industrial wastes [168]. Cr, Cu, Cd, etc. [171]. Debnath et al. produced and tested a synthetic oxide as hydrated and agglomerated nanocrystallite (11–13 nm) titanium oxide for Ni(II) adsorption from aqueous solution [172]. Film diffusion seemed to control the reaction rate following a pseudo-first-order reaction. Maghemite (γ-Fe2O3) nanoparticles were investigated for selective removal of toxic heavy metals such as Cr(VI), Cu(II) and Ni(II) from electroplating wastewater [173]. Electrostatic attraction and ion exchange caused adsorption of Cr and Cu whereas Ni was adsorbed due to electrostatic attraction of maghemite. Wang et al. [174] separated lead and chromium from wastewater using magnetite nanoparticles (Fe3O4 NPs). Fe3O4 able to adsorb 90% Pb ion within 2 min from the solution containing 10 ppm lead (Pb) ion. 3.4. Membrane separation Membrane separation process is a technique in which feed water is forced through a semipermeable membrane at high pressure to separate specific materials from the solution. According to the pore size, this process can be categorized as microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO). Schematic representation of membrane separation process is given in Fig. 5. The molecules or ions are passed through the membrane by diffusion and the rate of diffusion depends upon the pressure, temperature, permeability of membrane and the concentration of molecules or ions present in the solution. Membrane separation process mainly be governed by three basic principles, namely adsorption, sieving and electrostatic phenomenon. Adsorption mechanism is based on the hydrophobic interactions of the membrane and the solute. The separation of materials through the membrane depends on membrane pore size and solute size. Based on these principles, various membrane processes with different separation mechanisms have been developed [175]. The uses of polymeric membranes are increasing due to their attractive properties such as flexibility, high mechanical integrity, and easy fabrication. Water-soluble polymers are commonly involved in polymer enhanced ultrafiltration processes (PEUF), and allowed rapid sorption of metallic cations in water. The interactions between metallic cations and functional acidic groups depend on pH of the solution. The sorption capacities decrease in the following order: SO3H > P (O)(OH)2 > COOH, whereas selectivity increases in the opposite way: COOH > P (O)(OH)2 > SO3H [176]. The cost-effective analysis and selectivity of materials are the most important parameters that need to be analyzed for use of these materials. The other efficient solution for the treatment of wastewater is the use of super adsorbent polymers. Recently, graft copolymers of polyacrylonitrile (PAN) onto Arabic gum (AG) are prepared in aqueous solution using (KMnO4/HNO3) as a redox initiator [177]. Chemical modification of the graft copolymer has been carried out by reaction with hydrazine hydrochloride and subsequent hydrolysis in the basic medium [178]. This polymer could be used to remove Pb2+, Cu2+ and Cd2+ from the wastewater. The maximum super adsorbent capacities were found to be 1017, 413 and 396 mg/g for Pb2+, Cu2+ and Cd2+ respectively. Further, treatment with 0.2 M HNO3 resulted in the 3.3. Multifunctional nanomaterials One of the most rapidly growing sectors in wastewater treatment technologies is the use of multifunctional nanomaterials that can solve many problems more efficiently. Magnetic nanoparticles can be easily and more efficiently recovered after adsorption compared to other typical adsorbents [169,170]. Zinc Oxide (ZnO), Titanium Dioxide (TiO2), Cerium Oxide (CeO2) are major metal oxides used as nano adsorbents since they have high surface area, and their affinity can be improved by using numerous functionalized groups. Zeolites are also used as an ion-exchange medium for heavy metals removal like Zn, Ni, Fig. 5.. Membrane separation process for heavy metal removal. 8 R. Shrestha et al. Journal of Environmental Chemical Engineering xxx (xxxx) xxx recovery of 96%, 99% and 99% for Pb2+, Cu2+ and Cd2+ respectively [178]. A large variety of materials have been observed recently for fabricating the membranes and the adsorbents to find the best removal efficiency. The filtration membranes and adsorbents containing aromatic conjugated polymers (ACPs) such as polyaniline and polypyrrole exhibited superior metal removal efficiency as they were found to possess high surface areas and charge and site density within their structures to attract and attach metal ions. The uses of the polymer membranes on the removal of metal ions from aqueous solution mostly dependents on the porosity of the membranes [179]. The performance of PVP modified membranes in removal of copper and iron ions from liquid phase revealed that the membranes with the lowest content of PVP were found to be most effective while the membranes with the highest content of PVP showed the lowest resistance [179]. The nanoparticles incorporated nanofibrous membranes allowed high adsorption and high filtration capacity due to nanofiber’s large surface area and porosity. The adsorption isotherm and kinetics used in membrane adsorption for the characterization of parameters are similar to that used in the adsorption techniques. Wang et al. showed polysulfonebenzylthiourea hollow fiber membrane could be operated at a high feed flow rate and large-scale removal of Cd(II) and Zn(II) were realized [180]. Novel chelating membrane, the PVA/poly(ethyleneimine) (PEI) has been prepared and studied for the removal of heavy metal ions from aqueous solutions. The maximum adsorption capacities of the membrane are 0.729 mmolg-1 for Pb(II), 0.692 mmolg-1 for Cu (II) and 0.525 mmolg-1 for Cd(II). Gao et al. examined the adsorption property of grafted polymer of polysulfone and sodium p-styrenesulfonate membrane for three heavy metal ions, Pb(II), Zn(II), and Hg(II) ions [182]. Results showed that adsorption capacities of all metal ions are dependent on the pH values. The adsorption capacity first increases and then decreases with the increase of pH, and there is a maximum at pH 5 for Pb2+ and at pH 5.5 for Zn2+ while adsorption is maximum at pH 2 for Hg2+ (Fig. 6). Removal of heavy metals using Polymer Inclusion Membranes (PIMs) has gained significant interest. The selectivity and the permeability of the PIMs can be enhanced by choosing the right components in PIMs which are base polymer, extractant and plasticizer [184]. PVC and CTA are the popular base polymers, and Aliquat 336 & D2EHPA are the popular extractant. The dual system of PIM could successfully transport two metal ions into their stripping compartment by leaving the third metal ion in the feed phase [184]. Nanofiltration (NF) is an advanced membrane separation technique that has the size of the pores smaller than microfiltration and ultrafiltration but larger than reverse osmosis. This method is effective for the removal of higher metal concentrations up to 12,000 ppm. NF membranes are capable of treating wastewater containing more than one heavy metal with a removal efficiency of more than 80%. The membrane filtration with the combination of floatation showed a high rejection rate and it can be used for any type of water for the treatment of inorganic effluent in the water with a metal concentration of more than 10,000 ppm [185]. Wei et al. prepared the NF hollow fiber membranes for the treatment of wastewater under different operating conditions. At 0.4 MPa and pH 2.31, the membrane showed good stability and the removal rate for Cr, Cu and Ni were 95.76%, 95.33% and 94.99% respectively [186]. Basaran et al. studied the ability of two commercially available membranes for the removal of Ni(II) and Cr(VI) from metal plating wastewater in the aviation industry [187]. The effects of the operating pressure (10–30 bar) and the feed pH (3.5–10) on the performance of the membranes revealed that the Ni(II) rejection increased more than the Cr(VI) rejection, as the pH value changed from 3.5 to 10. The membrane rejection values for both metals were over 95% using a cross-flow filtration system. The polyethersulfone (PES) nanofiltration membrane with 0.5 wt% MMGO nanoparticles had the highest copper removal of about 92% [188]. Chung et al. developed a novel dual-layer polybenzimidazole/polyethersulfone NF hollow fiber membrane to remove the heavy metal ions [189]. The simultaneous co-extrusion of PBI and PES polymers not only kept the high rejection property of the PBI material but also significantly reduced the transport resistance across the membrane as well as the material cost for the membrane manufacture. As-developed membrane showed superior rejection efficiency to metal ions. For example, the membrane had a rejection efficiency of 95% and 93% for Cd(II) and Pb (II), respectively at pH 12 While the rejection to Cr2O27 can reach 98%. Otero et al. made a thin film composite (TFC) on a porous polysulfonate substrate for the removal of chromium in an industrial pilot plant [190]. For a feed concentration from 1 to 120 ppm, the result achieved was above 95% for chromium. Composite NF membrane comprising a molecularly designed pentablock copolymer selective layer is a useful platform for the development of new NF for the removal of heavy metal ions [191]. The membrane showed an effective removal efficiency greater than 98% for Pb(II), Cd(II), Zn(II) and Cu(II). The high membrane rejection of these cations is ascribed to the small mean effective pore diameter of the composite membrane (0.5 nm), which is significantly smaller than the hydrated diameters of these cations. Liu et al. prepared a positively charged nanofiltration membrane by phase inversion method for the removal of heavy metals from pollutant water. The obtained membrane fabricated via ion complexation induced phase inversion process had high water permeability and possessed high Fig. 6.. Adsorption isotherms of Pb2+ on PSF-g-PSSS membrane at different pH values (A) and effect of pH value on adsorption capacities of three metal ions on PSFg-PSSS membrane Temperature: 25 ◦ C (B) [183]. 9 Journal of Environmental Chemical Engineering xxx (xxxx) xxx R. Shrestha et al. rejection towards divalent cations such as Mg2+, Ca2+ and Pb2+ that can be used for the removal of heavy metals from wastewater. At the optimal condition, the pure water reflux from the membrane was 24.3 Lm-2h-1 with MgCl2 rejection of 92.2% [192]. The building block of tunable quaternary ammonium groups was used to prepare quaternized polyelectrolyte complex membranes (QPECMs) for the nanofiltration process via surface coating and glutaraldehyde crosslinking method [193]. As prepared membrane had efficient and anti-scaling properties with a low decline ratio of 11.5% and a high recovery ratio of 96.5%. The design strategy of this membrane presents facile adaptability to other polyelectrolyte complex membranes for specific water treatments and environmental remediation. A positively charged nanofiltration membrane had been developed using 2-chloro-1-methyliodopyridine as an active agent onto the membrane surface via covalent bonding with surface carboxylic groups [194]. The prepared membrane had a highly selective separation for divalent and monovalent salts and also exhibited high removal efficiency for toxic metal ions and dyes. PES/B-Cur membranes were fabricated by incorporating boehmite nanoparticles functionalized with curcumin (B-Cur) into PES membrane via phase inversion method [195]. The PES/B-Cur exhibited higher pure water reflux of 120–140 kg/m2h than PES membrane due to an increase in pore size, and porosity. Surface functionalized ZnO nanoparticles were incorporated to prepare polyether-imide-based nanofiltration membranes using phase inversion method [196]. The prepared PEI/PZ membranes showed superior separation performance and anti-fouling properties compared with neat PEI and PEI/ZnO membranes. Zhu et al. prepared thin-film Table 6 Summary of removal of heavy metals from wastewater by membrane technologies. Membranes Metals Sources Removal efficiencies References Spiral bound RO membrane Ni(II), Pb(II), Cu(II) Industrial wastewater [198] Functional polymers Metallic cations Wastewater Super adsorbent polymer Pb(II), Cu(II), Cd(II) Industrial wastewater Polyethersulfone membrane with pore generating agent (PVP) Polysolfonebenzylthiourea reactive hollow fiber ultrafiltration membrane PVA/poly(ethyleneimine) (PEI) membrane Cu, Fe Cd(II), Zn(II) Aqueous solutions Aqueous solutions Ni(II) = 98.5% Pb(II) = 97.5% Cu(II) = 96% Sorption capacities: SO3H > P (O)(OH)2 > COOH Pb(II) = 1017 mg/g Cu(II) = 413 mg/g Cd(II) = 396 mg/g High removal efficiency High rejection rate Hg(II), Pb(II), Cd (II), Cu(II) Aqueous solutions Grafted polymer of polysulfone and sodium p-styrenesulfonate membrane Hollow fiber membrane with polyamide film on the inner side Pb(II), Zn(II), Hg(II) Aqueous solutions Cr, Cu, Ni Electroplating wastewater DL and DK membrane Cu, Zn, Cd Industrial waste water from Tunisian wiring industry NF90 and NF270 membranes Ni(II), Cr(VI) Aviation industry metal plating wastewater Magnetic grapheme based composite embedded polyethersulfone (PES) polymer Dual layer nanofiltration hollow fiber membrane Cu Aqueous solutions from Cu (NO3)2 Model wastewater Aromatic polyamide NF membrane Cr(VI) Pentablock copolymer Pb(II), Cd(II), Zn(II), Cu(II) Divalent cations Pb(II) = 0.729 mmolg-1 Cd(II) = 0.525 mmolg-1 Cu(II) = 0.692 mmolg-1 95% [199] [178] [179] [180] [182] Cr = 95.76% Cu = 95.33% Ni = 94.99% For DL membrane; Cu = > 84% Zn = < 93% Cd = 69–86% For DK membrane; Cu = 94–97% Zn = 80–92% Cd = 81–85% For NF90 membrane;Ni(II) = 99.2% Cr(VI) = 96.5% For NF270 membrane;Ni(II) = 98.7% Cr(VI) = 95.7% 92% [200] Mg(II) = 98% Cd(II) = 95% Pb(II) = 93% > 95% [189] > 98% [203] Pollutant water MgCl2 rejection of 92.2% [192] Metal cations Cu, Ni, Cr Wastewater Aqueous solution containing metal ions and dyes [193] [204] PES/B-Cur membranes Fe2+ Cu2+, Pb2+, Mn2+, Ni3+, Zn2+ Aqueous solution containing metal ions Polyether-imide based nanofiltration membranes Pb2+, Cu2+ Aqueous solution containing metal ions Rejection ratio of 96.5% Cu = 96% Ni = 95.8% Cr = 98% Fe2+ = 99.88% Cu2+ = 98.72% Pb2+ = 99.61% Mn2+ = 99.31% Ni3+ = 99.11% Zn2+ = 99.51% Pb(NO3)2 = 59% Cu(NO3)2 = 67% Positively charged nanofiltration membrane prepare from the mixture of polyacids and polybases by phase inversion method Quaternized polyelectrolyte complex membranes Nanofiltration membrane Mg(II), Cd(II), Pb(II) 10 Samples of varying concentration were prepared Aqueous solutions [201] [202] [188] [190] [195] [196] R. Shrestha et al. Journal of Environmental Chemical Engineering xxx (xxxx) xxx composite (TFC) hollow fiber nanofiltration (NF) membranes by grafting poly (amidoamine) dendrimer (PAMAM) on the interfacially polymerized layer of polyethersulfone (PES) membranes [197]. The membrane showed a rejection efficiency of over 99% for Pb(II), Cu(II), Zn(II), Ni(II) and Cd(II). On changing the pH of a solution, the rejection efficiency for As(V) reached about 97%. The water permeability gets increased with the presence of –NH2 groups on PAMAM that makes the surface of the TFC membrane more positively charged in the acidic medium (Tables 6 and 7). that can be applied for the removal of aqueous ions through electrically driven ion-exchange membranes [216]. The method further has the advantage of producing effluent water that can be reused besides removing even the lower concentrations of metal ions from the effluents. A laboratory-scale ED system is used in the treatment of synthetic effluent based on industrial nickel electroplating process for concentrating and extracting Nickel (Ni) and its salts and also can be used for the removal of organic compounds from the additives [217]. Simultaneous recovery of heavy metals and phosphorus has been achieved using 3-compartmental electrodialysis method at low pH with the electrodes separated by ion-exchange membranes from the sample [218]. The heavy metals extraction pattern, at 3.7 pH, followed Zn(85%) > Ni (56%) > Cd(31%) > Cu(22%) > Cr(6%) > Pb(1%) in the order of most to least released or extracted. Due to rise in pH caused by the half-reactions at cathode, followed by the precipitation, 2-compartmental ED cell setup was less effective in extraction [218]. Semerci et al. conducted a two-part experiment with the first part containing bioleaching process for solubilization of phosphorus at different conditions and the second part of electrodialysis separating heavy metal ions from phosphorus ions. The pH of the Sewage Sludge Ash (SSA) was 11.6 and conductivity of 2.04 mS cm-1 and electrodialysis experiments with copper electrodes coated with gold resulted in 24.6% phosphorus passage to the anode chamber and showed Zn, Cu and Ni separation efficiency of 64.2%, 100% and 68.6% respectively [219]. Kirkelund et al. subjected leachable heavy metals and salts which pose most problems in Municipal Solid Waste Incineration (MSWI) to electrodialytic remediation and reported a reduction in Cd, Cu, Pb and Zn leaching with a pH below 8 [220]. Santos et al. used ED process as a sequential treatment for removal of Cr(VI) from an effluent that was already subjected to an anaerobic biological treatment in a hybrid anaerobic bioreactor and was able to effectively remove more than 99% of Cr(VI) in 75 min operation time [221]. They studied the influence of electrical current as an adjustable operational factor on the performance of ED systems and the quality of treated wastewater. A bipolar membrane electrodialysis (BMED) system has been developed for the treatment of raffinate generated during copper ore hydrometallurgical processing for their reuse as leaching influent and reported removal rates of heavy metals as 99.3% (iron), 99.1% (zinc), 99.0% (copper), 84.9% (nickel), 70.6% (chromium), 95.8% (cadmium), and 94.8% (arsenic) during an optimal 40 h operating time. The study 3.5. Biosorption In recent years, biosorption is considered to be an efficient and ecofriendly alternative technology for the removal of heavy metals from wastewater effluent generated from different industries. Biosorption is a physiochemical process that involves binding of metal ions to the surface of a biosorbent. The potential metal sorbents could be algae, fungi, bacteria, yeasts, agricultural and industrial wastes and biopolymers [205,206]. Biosorption is considered to be a significant technology not just only for the removal of even lower concentrations of heavy metals but also their recovery. Sarrafzadeh et al. [46] developed a tool to measure the efficiency of nutrient removal, carbon captor and metabolite generation using the different ratios of Chlorella vulgaris and nitrifier-enriched-activated-sludge (NAS). The influent was based on the municipal wastewater and heavy metals adsorption were not considered. Kanamarlapudi et al. have inferred simple operation, no additional nutrients requirement, lower amount of sludge generation, lower operational cost, higher efficiency and regeneration of biosorbents as prime advantages of this process [207]. The percentage of biosorption (RE) also termed as biosorption efficiency for the metal can be evaluated as RE = Ci − Ce × 100% Ci where, Ci is the initial concentration of metal ions in the solution (mg/L) and Ce is the equilibrium concentration of metal ions in the solution (mg/L). 3.6. Electrodialysis Electrodialysis (ED) is among the most economic recent technology Table 7 Some of the biosorbents with their performance parameters and inadequacies. Psidium guvajava L. leaf powder Rice bran and pine sawdust Bellamya bengalensis (snail made up of CaCO3) Chlorella vulgaris and Scenedesmus acutus Metals Efficiencies Cd(II) 93.2% Cd(II), Pb (II) For rice bran, Cd(II) = 98.25%, Pb(II) = 99.25%For pine sawdust, Cd(II) = 90.21%, Pb (II) = 95.25% 30.33 mg/g Cd Cr For C. vulgaris 88.2% and for S. acutus 87.1% Lactobacillus acidophilus Dicerocaryum eriocarpum plant modified with sodium and potassium chlorides As(III) Zn(II), Cd (II), Ni(II), Cr(III), Fe (II) Klebsiella sp. 3S1 Floating (Azolla filiculoides) and submerged (Hydrilla vertcillata) macrophytes PEI-modified sericin beads Zn(II) Cu(II), Cr (VI), As(III), Pb(II) Cr(Vi) 60% Zn(II) = 99.94% Ni(II) = 99.87% Fe(II) = 95.54% Cr(III) = 96.21% Cd(II) = 93.7% 48.4 mg/g Pb(II)>Cu(II)>As(III)> Cr(VI) Merits Demerits References [208] Effective biosorbent for the electroplating industry and also potential removal for Zn, Ca and Mg Comparatively effective for Pb [209] Efficient, low cost, environment friendly, sustainable adsorbent Mostly influenced by pH than biomass dose and time Reduction in efficiency can be seen due to the interaction between functional groups of the microalgae and other compounds present in water that are competitor of chromium [210] Highly efficient in the removal of metal ions than unmodified mucilage Potential use of aquatic macrophytes for biosorption capability 11 Pb(II) and Cu(II) strongly inhibited Cu(II) removal and Cr(VI) inhibits Pb(II) removal Reduction of Cr(Vi) to Cr(III) [211] [212] [213] [214] [215] [206] R. Shrestha et al. Journal of Environmental Chemical Engineering xxx (xxxx) xxx degradation and best removal efficiency at pH 6 for both Ni2+ and Fe3+ metal ions. Recently, clay-TiO2 composite has been developed from Degussa P25 (80% anatase and 20% rutile) and clay, for advanced treatment of wastewaters by single-step combination of photocatalysis and adsorption [233]. The adsorption capacities and photocatalytic properties of as-synthesized composites on pollutant matrix containing Methylene Blue (MB) as dye and cadmium ion (Cd2+) as a heavy metal resulted that the composites could be efficiently used for simultaneous removal of MB and Cd2+ more simply and cost-effectively for industrial wastewater treatment processes [233]. TiO2/C3N4 heterojunctions on carbon-fiber cloth are developed by in situ growth of TiO2 particles via dip-coating/hydrothermal method on carbon-fiber (CF) cloth substrate followed by incorporation of C3N4 nanosheets on their surface by thermal polymerization. The prepared CF/TiO2/C3N4 cloth exhibited improved photocurrent and photocatalytic activity for various organic pollutants degradation and on reducing Cr(VI) heavy metal ions [234]. Multifunctional silver nanoparticles incorporated chitosan thin films has been studied photodegradation, heavy metals removal and its antibacterial activity [235]. The photodegradation rate of organic pollutants under sunlight irradiation with and without oxygen was found to be increased with raising catalyst concentrations using incorporated polymer thin films. As-synthesized composites also showed antimicrobial activity against gram-negative (Escherichia coli) and gram-positive (G. bacillus). CuCo2S4 modified Z-scheme MoSe2/BiVO4 hybrid composites have been synthesized via a facile hydrothermal method for the removal of heavy metals efficiently under visible light [236]. Incorporation of CuCo2S4 inhibited the electrons/holes pair recombination during photocatalysis. The highest photocatalytic removal was attained at pH of 9, 0.5 mg catalyst dosage and with 210 min of irradiation time for almost all metals. They attributed the excellent removal efficiency of photocatalyst to the wide adsorption of visible light region, high surface area and suitable content of CuCo2S4 [236]. Carbon nitride (CN) nanorods are synthesized by solvothermal route followed by growing Ag/AgCl nanoparticles on CN surface by a facile coprecipitation method [237]. The Ag/AgCl-decorated CN heterojunctions with diverse molar ratios (0.3,0.5,0.7) of Ag/CN precursor exhibited wide adsorption from UV to visible-light region. With 120 min of visible-light illumination, Ag/AgCl-CN nanorods degraded 98.5% rhodamine B (RhB), 75.4% tetracycline (TC) and adsorbed 39.5% Cr(VI) ions better than Ag/AgCl, CN and Ag-CN [237]. Wang et al. synthesized mesoporous zeolite Beta (mBeta) via one-step hydrothermal crystalline process investigating the removal efficiency of Cd2+ and Zn2+ from the wastewater. The mBeta adsorbed heavy metal ions and sulfurizing to obtain metal sulfide loaded mBeta [238]. In the coexisting solution of 100 ppm Cd2+ and 100 ppm Zn2+, the adsorption capacities reached 30 and 16 mg/g respectively, and the hydrogen evolution rate of 5.52 mmol g-1 was achieved on the sample CdS/ZnS-mBeta in 4 h under visible light irradiation [238]. An “adsorbent-to-photocatalyst” strategy has been proposed by Chen et al. connecting heavy metals removal from wastewater to photocatalytic reduction of CO2. In this strategy, heavy metals were collected by adsorbents are converted to photocatalysts without any secondary treatment [239]. They prepared calcium silicate hydrate (CSH) nanosheets by rate-controlled precipitation at ambient temperature and studied the Cu2+, Zn2+, Ni2+ and Pb2+ ions. Additionally, they have evaluated the photocatalytic activity of CO2 reduction of CSH-M that were recycled and reported that CSH-Pb was inert to CO2 reduction and very less gas generated for CSH-Cu and CSH-Zn whereas CSH-Ni showed superior photocatalytic activity with CO2 adsorbed being 31.2 cm3 g-1 than 29.1 cm3 g-1 and 28.5 cm3 g-1, for CSH-Zn and CSH-Cu, respectively [239]. Fenton process is another recommended advanced oxidation method that involves the oxidation of organic substrates by iron and hydrogen peroxide. The significance of this effective process is it produces non-toxic compounds, can be used in various scales, reduces the amount of sludge, increases the biodegradability of biological sludge, goal-focused on the removal of heavy metals and also H2SO4 [222]. 3.7. Photocatalysis One of the most innovative solutions for rapid and efficient destruction of environmental pollutants is with photocatalysis, a method in which a semiconductor-electrolyte interface is illuminated with light energy that exceeds the semiconductor bandgap resulting in a formation of electron-hole (e-/h+) pairs in both conduction and valence band of the semiconductors [223–227]. These migrating charge carriers can reduce or oxidize species with suitable redox potential in solution. TiO2, ZnO, CeO2, SnO2, CdS, ZnS are some of the commonly studied semiconductors. The photocatalysis mechanism over semiconductor particles follows a generation of electron-hole pairs with a subsequent need of a trap for these generated electron-hole pairs to avoid their recombination and the process is schematically represented in Fig. 7. The Hydroxyl ions (OH-) act as a trap for holes which leads to strong oxidizing capable hydroxyl radical’s formation while the adsorbed oxygen species act as a trap for electrons leading to the formation of unstable and reactive superoxides which may finally evolve through several ways [228]. + TiO2 + hv = TiO2 + e− CB + hVB + ⋅ TiO2(substrate) − OH− s + h = TiO2(substrate) − OHads −(ads) O2(ads) +e− = O2 Solar irradiated photocatalytic reductions of Cr(VI), Ni(II), Zn(II) and Cu(II) at different TiO2 concentrations [229]. Minimal reduction of Cr(VI) was reported in the absence of citric acid but the acidic pH favored rapid and higher reduction of Cr(VI). However, Ni(II), Zn(II) and Cu(II) showed their reduction rates independent of citric acid concentrations supporting thermodynamic feasibility and water acted as an equally efficient hole scavenger for these metals. All of Cr(VI) was reduced at a pH level of 2 in a 1.5 h minimum period and the period increased on increasing pH values. Adsorption and removal of Ni(II), Zn (II) and Cu(II) however were significant only in the alkaline pH range with maximum Ni(II) removal of 60.8% at 7 pH and Zn(II) removal of 70.6% at 8 pH and Cu(II) removal of 31%. On varying TiO2 concentrations for Cr(VI) reduction at its optimum removal pH of 2, all of the Cr (VI) was reduced with effect being increased reduction time with decreasing TiO2 concentrations [229]. Wahaab et al. compared three commercial TiO2 specimens namely TiO2 Degussa P25 (80% anatase, 20% rutile), TiO2 (100% anatase) and TiO2 (100% rutile) suspended in a constant amount of 0.25 g/l of cyanide solution with a medium pressure Hg lamp as UV-radiation source [230]. The results indicated TiO2 Degussa P25 gave the best photocatalytic activity in the removal of cyanides with 90% removal at 60 min. The removal was further enhanced by the addition of hydrogen peroxide along with a decrease in the complete removal time [230]. In another study Rahimi et al. showed the maximum removal of 99.8% and 99.2% for Cd2+ and Pb2+ respectively, when the TiO2 dosage was 0.9 g/l and at a pH of 11 [231]. Zhao et al. synthesized nanoparticles assembled SnO2 nanosheets and studied the organic dyes degradation ability and heavy metals such as Fe3+ and Ni2+ removal ability [232]. The SnO2 nanosheets showed an excellent catalytic property for organic dye Fig. 7. Photocatalytic method for heavy metal removal. 12 Journal of Environmental Chemical Engineering xxx (xxxx) xxx R. Shrestha et al. and can lead to a decrease in volatile solids. Moreover uses of this process synergistically with other remediation processes such as photocatalysis can enhance the adsorption for the heavy metal ions and degradation of the organic pollutants [240]. researchers. Due to the excessive pressure drop, the nanoparticles cannot be used in a packed column. A technique should be developed to incorporate nanoparticles in a nanofibrous membrane and also for the regeneration of heavy metals and these nanoparticles. Different techniques are being employed for the better and efficient performance of hydrogels for intensive use in industrial wastewater treatment. The shift in the preparation techniques from pure synthetic chemical polymers to bio-based and ultra-high adsorption polymers have seen significant improvement and utilization. Smart hydrogels have the capacity of being regenerated to their initial state as soon as the external stimulus is removed. Hydrogels were previously prepared from (AM) and 2-acrylamido-2-methylpropanesulfonic acid (AMPS) homopolymers and co-polymers by free radical solution polymerization using N, N’-methylenebisacrylamide (MBP) as the cross-linker and many other similar synthetic chemicals. Nowadays, hydrogels are being prepared by inverse emulsion from the aqueous solutions of chitosan, gelatin, and glutaraldehyde, making them more environmentally friendly. A super-adsorbent hydrogel sphere (SAHSs) based on selfsacrificing micro-reactors, which achieve the interior cross-linking polymerization of the hydrogel network and subsequent selfdisintegration within only 150 s was developed for ultra-high adsorption capabilities. A new bio-based bilateral hydrogel containing carboxymethyl cellulose (CMC) and polyacrylamide (PAM) can be considered for wastewater remediation in the future. Recent researches have been focused to develop cost-effective strategies. However, the overall cost can be varied upon the technologies based on the sources of wastewater and scale of remediation. Hence, it is a difficult task to compare the specific cost of remediation for different techniques. This suggests the future works should be considered the cost of operation while remediation that would make more useful for realworld applications. 4. Advantages/disadvantages of modern technologies In recent years, physiochemical treatments offer various advantages such as low cost, ease of operation, economic feasibility and flexibility in modification of a chemical plant wherever necessary. These benefits exceed the number of drawbacks such as high operational cost, high sludge production, high energy consumption and also the metal selectivity encountered in the conventional methods. However, if the cost for the disposal of these toxic sludge is reduced by any means, the physiochemical treatment becomes the most suitable treatment method for the wastewater with inorganic effluents. Studies on biological methods have also sought the interest of many researchers and companies. Various biological methods such as biosorption and activated sludge process have been used at present. Simple operation, low amount of sludge production, high metal removal efficiencies and regeneration of the biosorbents are the advantages of using biological methods. However, the advantages are followed by some drawbacks such as high pH and temperature dependence, high energy requirement and maintenance requirement. The technology targets to have high metal removal efficiency as well as the recovery of metals. The most promising and innovative technologies to treat the effluents at present have been the photocatalysis and electrodialysis. In photocatalysis, the photons serve as an electron rays from organic substrate to the metal ions whereas in electrodialysis, the ions are transported through a semi-permeable membrane under the influence of an electric potential. Table 8 summarizes the main advantages and disadvantages of the recent technologies for the removal of heavy metals from industrial effluents presented in this study. 6. Conclusions 5. Future perspective of the technologies Over the past few decades, with more stringent environmental regulations, there has been a need for an improved quality of treated industrial effluent. Technologies like adsorption, chemical precipitation, ion-flotation, ion-exchange, electrodialysis, membrane filtration, biosorption, coagulation/flocculation and photocatalysis have been used to treat the effluent containing heavy metals. Ion-exchange is recognized to be the most frequently used technique owing to its higher efficiency at even higher metal concentrations. Meanwhile, adsorption by low-cost adsorbents and biosorption is identified to be efficient and costeffective even applicable for lower concentrations of heavy metals. Furthermore, few technologies are also emerging as promising techniques for industrial heavy metals containing effluent treatment. Several polymeric materials and variety of nanomaterials have been developed to enhance the removal efficiency of the heavy metals. Factors including pH, temperature and pressure affect the cost and the removal efficiencies of the membrane technology. The parameters should be adjusted to their appropriate values so the cost could be lowered and the efficiency could be enhanced. Different heavy metal ions can be removed from wastewater by the use of polymer-based adsorbents. The selectivity and the cost analysis can be compared for different functional polymers for their role in the removal of heavy metals. The use of nanofiltration has grabbed the attention of many Table 8 Evaluation of recently developed technologies for heavy metals removal. Heavy metals removal technologies Advantages Disadvantages References Adsorption with recent adsorbents Hydrogels Low cost, high metal binding capacities, wide pH range, easy operability, flexibility and design simplicity Easy operability, low cost, more effective, biodegradable, reusable and recyclable Suitable for sulfate salts and hardness ions such as Cu(II) and Cd(II), lowpressure requirement Suitable for removal of organic and inorganic wastes, small space requirement, high separation selectivity, low pressure Requirement of adsorbent regeneration, low selectivity, excess waste production Highly dependent on pH, temperature, Metal conc. and nature of the material used Costly, prone to membrane fouling [41,241] High operational cost, high energy consumption due to membrane fouling, high concentration of sludge production Highly dependent on pH, temperature, type of bioadsorbent used, reactive site and agitation speed [186,188, 192,222] Membrane fouling and high energy consumption, formation of large particles, high sludge production Significant amount of O2 requirement, long duration time, limited application [222,246] Nanoparticles and nanotechnology Membrane separation Biosorption Electrodialysis Photocatalysis Simple operation, no additional nutrients requirement, low quantity sludge, low operational cost, high efficiency, bio-sorbent regeneration and low COD High separation selectivity, rapid process and effectiveness for certain metal ions, economic No sludge production, effective at lab scale, simultaneous removal of metals as well as organic pollutants, less harmful by-product 13 [166,167, 242] [243–245] [41,206,207] [247–249] R. Shrestha et al. Journal of Environmental Chemical Engineering xxx (xxxx) xxx Membrane filtration is widely studied and is recognized to have a high efficiency but there is a cost factor affecting its wider use. Photocatalysis has recently emerged as a promising innovative technique for clean and efficient treatment. Biosorption has also been given high importance keeping the environment-friendly nature of the biosorbents in mind. Although many techniques have been developed and used, most of the techniques depend on the critical factors like pH, initial concentration of heavy metals in the wastewater, temperature of operation and others. Environmental impact, overall treatment performance, compared to other technologies, as well as economic parameters such as capital investment and operational cost should be considered before its application. This review serves as a resource for further studies on the treatment of industrial wastewater for heavy metals removal. [18] J.L. 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