Migration and transformation of Cu(II)-EDTA during electrodialysis accompanied by an electrochemical process with different compartment designs

Recovery of heavy metal complexes from wastewaters: A critical review of mechanisms and technologies

Heavy metal complexes (HMCs) pose significant ecological challenges while also holding attractive economic value. This review comprehensively summarizes advanced techniques for recovering HMCs from wastewater, including reductive recovery, oxidative decomplexation-recovery, and non-redox separation. Physical and chemical separation approaches utilize specific properties of metal complexes for efficient segregation. Specifically, we explore oxidative decomposition techniques, emphasizing the underlying mechanisms and practical application for selective and non-selective decomplexation techniques. The crucial role of cathodic potential on the efficiency and selectivity of electrochemical reduction processes is also examined. In addition, a comprehensive cost assessment, including energy consumption, associated with these recovering processes is investigated, and opinions on the inadequacy of current studies are provided. Overall, this review uniquely integrates findings on selective physical separation, oxidation, and reduction processes as well as the cost assessments for these techniques, providing a novel and comprehensive perspective on heavy metal recovery. It aims to bridge existing gaps in literature and advance the development of effective recovery methodologies for HMCs.

Progress and challenges in the use of electrochemical oxidation and reduction processes for heavy metals removal and recovery from wastewaters

Heavy metals-laden industrial wastewater represents both a threat to ecosystems and human health and, in some instances, a potential source of valuable metals however the presence of organic ligands that bind the metals in heavy metal complexes (HMCs) renders metal removal (and, where appropriate, recovery) difficult. Electrochemical-based oxidation and reduction processes represent a potentially promising means of degrading the organic ligands and reducing their ability to retain the metals in solution. In this state-of-the-art review, we provide a comprehensive overview of the current status on use of electrochemical redox technologies for organic ligand degradation and subsequent heavy metal removal and recovery from industrial wastewaters. The principles and degradation mechanism of common organic ligands by various types of electrochemical redox technologies are discussed in this review and consideration given to recent progress in electrode materials synthesis, cell architecture, and operation of electrochemical redox systems. Furthermore, we highlight the current challenges in application of electrochemical redox technologies for treatment of HMC-containing wastewaters including (i) limited understanding of the chemical composition of industrial wastewaters, (ii) constrained mass transfer process affecting the direct/indirect electron transfer, (iii) absence of approaches to convert recovered metal into high-value-added products, and (iv) restricted semi-or full-industrial-scale application of these technologies. Potential strategies for improvement are accordingly provided to guide efforts in addressing these challenges in future research.

Effective oxidation decomplexation of Cu-EDTA and Cu2+ electrodeposition from PCB manufacturing wastewater by persulfate-based electrochemical oxidation: Performance and mechanisms

Complex wastewater matrices such as printed circuit board (PCB) manufacturing wastewater present a major environmental concern. In this work, simultaneous decomplexation of metal complex Cu-EDTA and reduction/electrodeposition of Cu2+ was conducted in a persulfate-based electrochemical oxidation system. Oxidizing/reductive species were simultaneously produced in this system, which realized 99.8% of Cu-EDTA decomplexation, 94.5% of Cu2+ reduction/electrodeposition under the conditions of original solution pH = 3.2, electrode distance = 3 cm, [Na2S2O8]0 = 5 mM, current density = 12 mA/cm2, and reaction time = 180 min. The total treatment cost is as low as 0.80 USD/mol Cu-EDTA. Effective mineralization (74.1% total organic carbon removal) of the solution was obtained after 3 h of treatment. •OH and SO4•- drove the Cu-EDTA decomplexation, destroying the chelating sites and finally it was effectively mineralized to CO2, H2O and Cu2+. The mechanisms of copper electrodeposition on the stainless steel cathode and persulfate activation by the BDD anode were proposed based on the electrochemical measurements. The electrodes exhibited excellent reusability and low metal (total iron and Ni2+) leaching during 20 cycles of application. This study provide an effective and sustainable method for the application of the electro-persulfate process in treating complex wastewater matrices.

Electrochemical treatment of wastewaters containing metal-organic complexes: A one-step approach for efficient metal complex decomposition and selective metal recovery

Metal-organic complexes, especially those of ethylenediaminetetraacetic acid (EDTA) with metals such as copper (Cu) and nickel (Ni) (denoted here as Cu-EDTA and Ni-EDTA), are common contaminants in wastewaters from chemical and plating industries. In this study, a multi-electrode (ME) system using a two-chamber reactor and two pairs of electrodes is proposed for simultaneous electrochemical oxidation of a wastewater containing both Cu-EDTA and Ni-EDTA complexes as well as separation and selective recovery of Cu and Ni onto two different cathodes via electrodeposition. Our results demonstrate that the ME system successfully achieved 90% EDTA removal, 99% solid Cu recovery at the Cu recovery cathode and 56% Ni recovery (33.3% on the Ni recovery cathode and 22.6% in the solution) after a four-hour operation. The system further achieved 85.5% Ni recovery after consecutive five cycles of operation for 20 h. While Cu removal was mainly driven by the direct reduction of EDTA-complexed Cu(II) at the cathode, oxidation of EDTA within the Ni-EDTA complex at the anode was a prerequisite for Ni removal. The oxidation of metal-bound EDTA and free EDTA was driven by •OH and direct electron transfer on the PbO2 anode surface and graphite anode, respectively. We further show that ME system performs well for all pH conditions, treatment of real wastewaters as well as wastewaters containing other metals ions (Cr and Zn) along with Cu/Ni. The separation efficiency of Cu and Ni is dependent on applied electrode potential as well as nature and concentration of binding ligand present with comparatively lower separation efficiency achieved in the presence of weaker binding capacity and/or at lower ligand concentration and lower applied electrode potential. As such, some optimization of electrode potential is required depending on the nature/concentration of ligands in the wastewaters. Overall, this study provides new insights into the design and operation of EAOP technology for effective organic abatement and metal recovery from wastewaters containing mixtures of various metal-organic complexes.

Electrochemical Removal of Metal-Organic Complexes in Metal Plating Wastewater: A Comparative Study of Cu-EDTA and Ni-EDTA Removal Mechanisms

Cu and Ni complexes with ethylenediaminetetraacetic acid (Cu/Ni-EDTA), which are commonly present in metal plating industry wastewaters, pose a serious threat to both the environment and human health due to their high toxicity and low biodegradability. In this study, the treatment of solutions containing either or both Cu-EDTA and Ni-EDTA using an electrochemical process is investigated under both oxidizing and reducing electrolysis conditions. Our results indicate that Cu-EDTA is decomplexed as a result of the cathodic reduction of Cu(II) with subsequent electrodeposition of Cu(0) at the cathode when the cathode potential is more negative than the reduction potential of Cu-EDTA to Cu(0). In contrast, the very negative reduction potential of Ni-EDTA to Ni(0) renders the direct reduction of EDTA-complexed Ni(II) at the cathode unimportant. The removal of Ni during the electrolysis process mainly occurs via anodic oxidation of EDTA in Ni-EDTA, with the resulting formation of low-molecular-weight organic acids and the release of Ni2+, which is subsequently deposited as Ni0 on the cathode. A kinetic model incorporating the key reactions occurring in the electrolysis process has been developed, which satisfactorily describes EDTA, Cu, Ni, and TOC removal. Overall, this study improves our understanding of the mechanism of removal of heavy metals from solution during the electrochemical advanced oxidation of metal plating wastewaters.

Cadmium removal potential of hyperaccumulator Solanum nigrum L. under two planting modes in three years continuous phytoremediation

Solanum nigrum L. is a Cd hyperaccumulator, but the potential for continuous remediation, or different planting methods have not been fully characterized. The potential for continuous phytoremediation of Cd-contaminated farmland soil (2.08 mg kg^-1 Cd) by 2 planting methods (flowering harvest twice a year and maturity harvest once a year) was studied in a 3-year pot experiment. The total Cd accumulation (ug plant^-1) of the 3-year flowering stage treatments was 26.3% higher than that of the maturity stage treatments, which was mainly due to that flowering harvest twice a year caused 65.5% increase of shoot biomass. Similarly, the Cd decreased concentration in soil and Cd removal rate in the flowering stage treatments were 29.2% and 27.9% higher than that in the maturity stage treatments, respectively. After 3 years of phytoremediation, the extractable Cd concentration in soil was reduced by 36.4% in the flowering stage treatments and by 27.6% in the maturity stage treatments, which also led to the same decreasing trend of Cd accumulation of S. nigrum. In conclusion, the study results have demonstrated that the planting mode of two harvests a year at the flowering stage seems to be a viable option to apply for continuous phytoremediation of Cd-contaminated farmland soil.

Removal behavior and mechanisms of cadmium and lead by coupled ethylenediaminetetraacetic acid washing and electrochemical reduction: influence of current conditions

Ethylenediaminetetraacetic acid (EDTA) washing has been used extensively to remediate heavy metal-contaminated soils. Electrochemical reduction treatment of spent washing solution is an effective method of EDTA regeneration. However, at present, these two technologies are usually regarded as two independent treatment processes. This research raised a new heavy metal-contaminated soil treatment strategy-a combination technique of coupled EDTA washing and electrochemical reduction. We speculated that the combination of EDTA washing and electroreduction treatment could improve the efficiency of Cd and Pb removal from contaminated soil. In this study, the removal performance and mechanisms of Cd and Pb under different current conditions were investigated based on a coupling of EDTA washing and electrochemical reduction. The combination technique can increase Cd and Pb removal efficiencies by 13.37-15.24% and 14.91-27.05%, respectively, compared with EDTA washing alone. Sequential extraction analysis showed that the reducible fraction improved metal removal efficiency. The percentage of metal removed increased with an increased current value and EDTA concentration. In addition, pulse current mode removed more Cd and Pb than continuous current, although the difference was not significant (p > 0.05). However, pulse current could effectively eliminate the cathodic hydrogen evolution reaction, resulting in a further heavy metal deposition at the cathode. The combination technique exhibited enhanced removal efficiency due to EDTA regeneration in the suspension and the cathodic reduction reaction. The most cost-effective treatment in 48 h was a pulse current mode of 32 min on/16 min off-32 mA-EDTA-10 mM, where 47.56% of Cd and 77.00% of Pb were removed from the soil with an electric energy consumption of 8.24 Wh.

Electrochemical Ni-EDTA degradation and Ni removal from electroless plating wastewaters using an innovative Ni-doped PbO2 anode: Optimization and mechanism

In this work, a novel Ni-doped PbO₂ anode (Ni-PbO₂) was prepared via a co-electrodeposition method and used to remove Ni-ethylenediaminetetraacetic acid (Ni-EDTA) from solutions typical of electroless nickel plating wastewater. Compared with a pure PbO₂ electrode, Ni doping increased the oxygen evolution potential as well as the reactive surface area and reactive site concentration and reduced the electron transfer resistance thereby resulting in superior Ni-EDTA degradation performance. The 1% Ni-doped PbO₂ electrode exhibited the best electrochemical oxidation activity with a Ni-EDTA removal efficiency of 96.5 ± 1.2%, a Ni removal efficiency of 52.1 ± 1.4% and an energy consumption of 2.6 kWh m^-3. Further investigations revealed that 1% Ni doping enhanced both direct oxidation and hydroxyl radical mediated oxidation processes involved in Ni-EDTA degradation. A mechanism for Ni-EDTA degradation is proposed based on the identified products. The free nickel ion concentration initially increased as a result of the degradation of Ni-EDTA complexes and subsequently decreased as a consequence of nickel electrodeposition on the cathode surface. Further characterization of the cathode deposits by X-ray diffraction and X-ray photoelectron spectra indicated that the deposition products were a mixture of Ni⁰, NiO and Ni(OH)₂ with elemental Ni accounting for roughly 80% of the deposited nickel. Results of this study pave the way for the application of anodic oxidation processes for efficient degradation of Ni-containing complexes and recovery of Ni from nickel-containing wastewaters.

Two-compartment membrane electrochemical remediation of heavy metals from an aged electroplating-contaminated soil: A comparative study of anodic and cathodic processes

In this study, two-compartment membrane electrochemical remediation (MER) based on the anode process and the cathode process strategies were compared for treating a multi metal -contaminated soil. Remediation effect, as well as energy consumption and risk evaluation of the two strategies under different current density conditions of electroplating-contaminated soil suspension were performed, the following conclusions were drawn. MERs based on both the anode and cathode processes exhibited a synergetic effect because the DC electric field and extractants dissolved more metals from the soil phase into the liquid phase of the suspension compared to a usual soil washing treatment. The maximum Cr, Cu, and Ni removal efficiencies of MERs based on the anode process were 79.5%, 86.2%, and 85.0%, respectively, compared to 27.5%, 72.5%, and 65.9% based on the cathode process. Risk assessment results showed lower soil environmental risk after MER based on the cathode process than after MER based on the anode process. In this study, MER based on the cathode process as an evolving soil remediation strategy was found to present high simultaneous remediation ability for soil heavy metals and leaching materials, showing its advantages of environmental friendliness and economic effectiveness.

The treatment of electroplating wastewater using an integrated approach of interior microelectrolysis and Fenton combined with recycle ferrite

Heavy metal ions in chelated forms have aroused great concerns because of their high solubility, poor biodegradation and extreme stability. In this research, an efficient strategy, interior microelectrolysis-Fenton-recycle ferrite (IM-Fenton-RF), was developed to treat simulated electroplating wastewater containing chelated copper at room temperature. The decomplexation of chelated copper was carried out by both interior microelectrolysis and Fenton reactions. IM process can not only partly degrade the complexes of chelated copper via the microelectrolysis reaction but also it produces Fe²⁺ ions for the Fenton reaction. After decomplexation, the IM-Fenton effluent directly flowed into the RF reactor for copper ions removal. Under optimum reaction conditions (reflux ratio = 0.37, Fe²⁺ concentration = 9.20 g/L at pH 10.18), 99.9% copper was removed by the IM-Fenton-RF system. The produced IM-Fenton-RF sludge is based on ferrite precipitate and has several advantages over metal hydroxides sludge. Ferrite sludge is stable owing to the stability of ferrite's crystal structure, while the toxicity characteristic leaching procedure (TCLP) test meets relevant standards. The sedimentation rate and volume of ferrite sludge were 3.86 times faster and 11.0 times lower than those of metal hydroxides sludge. Furthermore, the yielding sludge of ferrite can be recovered and utilized for the synthesis of Fe-C metallic species, the main compound of IM packing for interior microelectrolysis reaction. All these results show that a combination of IM-Fenton and RF is an effective approach to treat wastewater containing chelated copper, showing great potential for industrial applications.

Chelating surfactant N-lauroyl ethylenediamine triacetate enhanced electrokinetic remediation of copper and decabromodiphenyl ether co-contaminated low permeability soil: Applicability analysis

In this study, chelating surfactant N-lauroyl ethylenediamine triacetate (N-LED3A) was used as strengthening agent for electrokinetic (EK) remediation of copper (Cu) and decabromodiphenyl ether (BDE209) co-contaminated low permeability soil. The results indicated that negligible amount of N-LED3A would be adsorbed on the experimental soil. The synchronous elution efficiencies (SEEs) of Cu and BDE209 had reached 65.4% and 49.9%, respectively, when the concentration of N-LED3A was 4000 mg/L, and they kept almost unchanged as the concentration of N-LED3A further increased. Meanwhile, the optimal SEEs were obtained at the pH condition within 6-8. The removal efficiencies of Cu (55.3%-65.8%) and BDE209 (31.4%-46.4%) would be increased with the applied voltage gradient and concentration of N-LED3A. In addition, BDE209 and Cu contaminants were also detected in the catholyte and anolyte, respectively, and their concentrations still showed an uptrend by the end of the experiments. While in the control experiments, the removal efficiency of Cu was in the range of 18.2%-23.6%, and almost no BDE209 was migrated out. The electric current would be increased with N-LED3A concentration increased, further resulting in the enhancement of cumulative electro-osmotic flow (EOF). However, the increment of EOF was limited after an 8-day treatment due to the declined capacity of the soil water supply, and the removal efficiency of BDE209 did not change proportionally to the cumulative EOF as a consequence. The accumulated (21 days) energy consumption under the optimal operation conditions (voltage gradient 1 V/cm, N-LED3A 1 g/L) was 377.28 KWh/m³. Efficiently synchronous removal of BDE209 and Cu could be achieved by the N-LED3A enhanced EK technique, exhibiting a promising application potential in the organic pollutant and heavy metal co-contaminated soil remediation.

Adsorption of lead and antimony in the presence and absence of EDTA by a new vermiculite product with potential recyclability

A new two-step modification method has been proposed where 1.8% HCl and 3.1% HNO₃ were applied to modify the interlayer of vermiculite (VMT). This product was given 90 °C of heat in 30% H₂SO₄ solution that was used for Pb (II) and Sb (III) adsorption. The EDTA presence on the individual adsorption was assessed. X-ray diffraction revealed that the VMT inter-stratified reflection through acid intercalation within the interlayer decreased the parallel gaps between the atoms, witnessing on the outer-sphere adsorption. The driving force was found electrostatic, which fits well with pseudo-second-order kinetics and Langmuir isotherm. The Pb (II) and Sb (III) uptake followed descending order adsorption with increasing concentration of chelating EDTA. Three consecutive desorption cycles revealed that the prepared adsorbent was suitable that may be regarded as a good candidate for complex wastewaters.

Hazardous wastes treatment technologies

A review of the literature published in 2019 on topics related to hazardous waste management in water, soils, sediments, and air. The review covered treatment technologies applying physical, chemical, and biological principles for the remediation of contaminated water, soils, sediments, and air. PRACTICAL POINTS: This report provides a review of technologies for the management of waters, wastewaters, air, sediments, and soils contaminated by various hazardous chemicals including inorganic (e.g., oxyanions, salts, and heavy metals), organic (e.g., halogenated, pharmaceuticals and personal care products, pesticides, and persistent organic chemicals) in three scientific areas of physical, chemical, and biological methods. Physical methods for the management of hazardous wastes including general adsorption, sand filtration, coagulation/flocculation, electrodialysis, electrokinetics, electro-sorption ( capacitive deionization, CDI), membrane (RO, NF, MF), photocatalysis, photoelectrochemical oxidation, sonochemical, non-thermal plasma, supercritical fluid, electrochemical oxidation, and electrochemical reduction processes were reviewed. Chemical methods including ozone-based, hydrogen peroxide-based, potassium permanganate processes, and Fenton and Fenton-like process were reviewed. Biological methods such as aerobic, anoxic, anaerobic, bioreactors, constructed wetlands, soil bioremediation and biofilter processes for the management of hazardous wastes, in mode of consortium and pure culture were reviewed. Case histories were reviewed in four areas including contaminated sediments, contaminated soils, mixed industrial solid wastes and radioactive wastes.

Electrodialysis Applications in Wastewater Treatment for Environmental Protection and Resources Recovery: A Systematic Review on Progress and Perspectives

This paper presents a comprehensive review of studies on electrodialysis (ED) applications in wastewater treatment, outlining the current status and the future prospect. ED is a membrane process of separation under the action of an electric field, where ions are selectively transported across ion-exchange membranes. ED of both conventional or unconventional fashion has been tested to treat several waste or spent aqueous solutions, including effluents from various industrial processes, municipal wastewater or salt water treatment plants, and animal farms. Properties such as selectivity, high separation efficiency, and chemical-free treatment make ED methods adequate for desalination and other treatments with significant environmental benefits. ED technologies can be used in operations of concentration, dilution, desalination, regeneration, and valorisation to reclaim wastewater and recover water and/or other products, e.g., heavy metal ions, salts, acids/bases, nutrients, and organics, or electrical energy. Intense research activity has been directed towards developing enhanced or novel systems, showing that zero or minimal liquid discharge approaches can be techno-economically affordable and competitive. Despite few real plants having been installed, recent developments are opening new routes for the large-scale use of ED techniques in a plethora of treatment processes for wastewater.

Low-Fe(III) driven UV/Air process for enhanced recovery of heavy metals from EDTA complexed system

The efficient recovery of heavy metals from complexed wastewater is an essential but challenging task. In this study, a novel low-Fe(III) driven UV/Air process (LFUA) was developed to break the strong complexation between ethylenediamine tetracetic acid (EDTA) and heavy metal ions (HMIs) and enable the enhanced recovery of HMIs via chelating resin adsorption (CRA). The inside mechanism of the LFUA process includes: 1) displacement of HMIs from HMI-EDTA complexes by Fe(III); 2) direct photolysis of Fe(III)-EDTA through a ligand-to-metal charge transition reaction (LMCT) and indirect photolysis of EDTA by HO₂·/O₂·^-. The iron dosage was orders of magnitude lower than that previously reported, due to the Fe(II)/Fe(III) redox cycle in the LFUA process. Fe(II) formed during the LMCT reaction of Fe(III)-EDTA was oxidized back to Fe(III) by O₂ and HO₂·, and the reformed Fe(III) was then recombined with EDTA to sustains the LMCT reaction. EDTA was completely removed in 20 min at a molar ratio of Fe(III)/EDTA = 0.05. In addition, following the LFUA process, the adsorption amounts of various HMIs onto D463 resin were at least two orders of magnitude higher than those reported using the direct adsorption process. Employing the integrated technique of LFUA + CRA enabled the efficient removal of up to 64.5 mg/L of Cu(II) from inlet wastewater, and residual Cu(II) was below 0.5 mg/L. The results of desorption experiments showed that over 90% of Cu(II) was recovered, and the desorption solution had a Cu concentration of 2.1 g/L and purity of 99%. Furthermore, the economic and practical feasibility of using the combined process of LFUA + CRA was analyzed to substantiate that the technique is highly efficient and clean (produces no harmful sludge). Therefore, it is an appropriate and practical process in removing HMIs-EDTA complexes and recovering HMIs from wastewater.

EDTA-enhanced electrokinetic remediation of aged electroplating contaminated soil assisted by combining dual cation-exchange membranes and circulation methods

This paper introduces a novel method for ethylenediaminetetraacetic acid (EDTA)-enhanced electrokinetic (EK) remediation by combining dual cation-exchange membranes and circulation methods for an aged electroplating soil contaminated by chrome (Cr), copper (Cu), and nickel (Ni). Three laboratory-scale EK experiments were carried out, including T1, the traditional EK process; T2, the traditional EDTA-enhanced EK process; and T3, the assisted EDTA-enhanced EK process. The results obtained show that removal of Cu and Ni in T3 was 3-10 times higher than after T1 and T2. However, the removal of Cr (total) was small in all experiments because of the high content of Cr(III). T3 eliminated the metal accumulation problem that existed for T1 and T2. Simultaneously, the highly acidified area (pH < 4) was reduced from 80% in T1 and T2 to only 20% in T3. The results obtained in T3 indicate that the chelating effect of EDTA has a greater ability to dissolve oxidizable Cu and Ni in the soil than the acidification effect. Toxicity evaluation confirmed that the soil treated by T3 presented a lower effect on a luminescent bacterium (Photobacterium phosphoreum T3) because soil pH tended to be more neutral after this treatment. This research provides a novel method for removing heavy metals from soil in a more environmentally friendly way and clarifies the cause of the existing problems of low removal efficiency and high accumulation in the traditional EK process.

Migration and decomplexation of metal-chelate complexes causing metal accumulation phenomenon after chelate-enhanced electrokinetic remediation

This study investigates the migration and decomplexation effects of metal-ethylenediaminetetraacetic acid (EDTA) complexes during an electrokinetic (EK) remediation process and the resulting metal accumulation phenomena. Six EK tests with control of the electrolyte pH and using ion-exchange membranes were performed to treat Pb-EDTA and Cd-EDTA co-contaminated red soil. The obtained results showed that a portion of free metal cations could be decomplexed from the metal-EDTA complexes due to the low pH and electrochemical degradation at the anode. These cations went back into the soil by electromigration and accumulated in separate locations according to their hydrolysis ability and the distribution of soil pH in different sections. Totals of 61% Cd and 83% Pb were removed from the soil after a 7-day treatment under the condition of controlling the electrolyte pH at 10. The removal efficiencies of metals under the anion-exchange membrane-assisted treatment were higher than those of the cation-exchange membrane-assisted treatment. Based on the mechanisms of metal accumulation phenomena, the migration of decomplexed free metal cations back to the soil is limited by using an anion-exchange membrane or pre-precipitation with alkaline conditions was confirmed to effectively reduce the effect of metal accumulation.