Simultaneous Cu(II)-EDTA decomplexation and Cu(II) recovery using integrated contact-electro-catalysis and capacitive deionization from electroplating wastewater

The complex of heavy metals and organic acids leads to high difficulty in heavy metals separation by traditional technologies. Meanwhile, alkaline precipitation commonly used in industry causes the great consumption of resources and extra pollution. Herein, the effective decomplexation of Cu(Ⅱ)-EDTA and synchronous recycling of Cu2+ were realized by contact-electro-catalysis (CEC) coupled with capacitive deionization (CDI) innovatively. In particular, fluorinated ethylene propylene (FEP) as dielectric powders could generate reactive oxygen species under ultrasonic stimulation, realizing continuous deaminization and decarboxylation of Cu(Ⅱ)-EDTA and accelerating the totally breakage of Cu-O and Cu-N bonds. Additionally, the degradation pathway and intermediates evolution of Cu(Ⅱ)-EDTA were investigated using various characterization methods. It was confirmed that decarboxylation predominantly governed the degradation process of Cu(Ⅱ)-EDTA in CEC. During the course of treatment, the degradation ratio of Cu(Ⅱ)-EDTA reached 86.4 % within 150 min. Impressively, this strategy had satisfactory applicability to other metal combinations and excellent cycle stability. Subsequently, the released Cu ions were captured by CuSe cathode electrode through CDI. This research elucidated the degradation mechanism of persistent organic pollutant during CEC, and provided a novel approach for efficiently treating industrial wastewater containing metal complexes and advancing the exploitation and utilization of new technologies for metal recovery.

Ball-milled zero-valent iron with formic acid for effectively removing Cu(II)-EDTA accomplished by EDTA ligands oxidative degradation and Cu(II) removal

Heavy metal complexes in industrial wastewater are challenging to be removed by conventional methods arising from their stable chelating structure. In this study, zero-valent iron (ZVI) was ball-milled with tiny formic acid (FA), and the as-prepared sample (FA-ZVIbm) was attempted to eliminate a model heavy metal complex of Cu(II)-ethylenediaminetetraacetic acid (Cu(II)-EDTA). The addition of FA to ball-milling could dramatically enhance the performance of ball-milled ZVI (ZVIbm) towards Cu(II)-EDTA removal and increase the removal rate constant by 80 times. This conspicuous improvement of Cu(II)-EDTA elimination was attributed to the ferrous formate (Fe(HCOO)2) shell formed on the surface of FA-ZVIbm. Results revealed that the Fe(HCOO)2 shell facilitated the activation of O2 to reactive oxygen species (ROS) and the leaching of Fe3+. Cu(II)-EDTA was decomplexed through both oxidative destruction and Fe3+ replacement, and the released Cu2+ was reduced by FA-ZVIbm and immobilized synchronously. Meanwhile, the ligands underwent oxidative degradation by ROS, thus avoiding the re-chelation ecological risk. Impressively, FA-ZVIbm could achieve cyclic treatment of actual copper complex wastewater and possessed promising advantage in treatment cost. This study would offer a promising approach for eliminating Cu(II)-EDTA through EDTA ligands degradation and synchronous Cu(II) removal, moreover to shed light on the decomplexation mechanism.

Self-catalytic decomplexation of Cu-TEPA and simultaneous recovery of Cu by an electrochemical ozone production system using heterojunction Ni-Sb-SnO2 anode

Heavy metal complexes from the industrial wastewater induce risks for the humans and ecosystems, yet are valuable metal resources. For energy saving and emission reduction goals, the simultaneous decomplexation and recovery of metal resources is the ideal disposal of wastewater with heavy metal complexes. Herein, a self-catalytic decomplexation scheme is developed via an electrochemical ozone production (EOP) system to achieve efficient decomplexation and Cu recovery. The EOP system could achieve 94.36% decomplexation of Cu-TEPA, which is a typical complex in catalyst industrial wastewater, and 86.52% recovery of Cu within 60 min at a current density of 10 mA/cm2. The O3 and •OH generated at the anode would first attack Cu-TEPA to produce Cu-organic nitrogen intermediates, which further catalyze O3 to generate •OH, thus self-enhancing the decomposition process in the EOP system. The released Cu2+ was gradually reduced to Cu+ and finally deposited as Cu2O and Cu to the stainless steel cathode. The technological feasibility was confirmed with other Cu-complexes such as Cu-EDTA and Cu-citrate, and the actual Cu-TEPA-containing industrial wastewater. The results provide new insights regarding the application of EOP in the simultaneous treatment of heavy metal complex wastewater and resource recovery.

Decomposition of metal-organic complexes and metal recovery in wastewater: A systematic review and meta-synthesis

Metals are rarely found as free ions in natural and anthropogenic environments, but they are often associated with organic matter and minerals. Under the context of circular economy, metals should be recycled, yet they are difficult to extract for their complex forms in real situations. Based on the protocols of review methodology and the analysis of VOS viewer, there are few reviews on the properties of metal-organic complexes, decomplexation methods, the effect of coexisting ions, the pH influence, and metal recovery methods for the increasingly complicated metal-organic complexes wastewater. Conventional treatment methods such as flocculation, adsorption, biological degradation, and ion exchange fail to decompose metal-organic complexes completely without causing secondary pollution in wastewater. To enhance comprehension of the behavior and morphology exhibited by metal-organic complexes within aqueous solutions, we presented the molecular structure and properties of metal-organic complexes, the decomplexation mechanisms that encompassed both radical and non-radical oxidizing species, including hydroxyl radical (OH), sulfate radical (SO˙4-), superoxide radical (O˙2-), hydrogen peroxide (H2O2), ozone (O3), and singlet oxygen (1O2). More importantly, we reviewed novel aspects that have not been covered by previous reviews considering the impact of operational parameters and coexisting ions. Finally, the potential avenues and challenges were proposed for future research.

Synchronous decomplexation and mineralization of copper complexes by activating peroxymonosulfate with magnetic bimetallic biochar derived from municipal sludge

Efficient removal of copper complexes is a challenging issue due to their robust stability and solubility. In this study, CoFe2O4-Co0 loaded sludge-derived biochar (MSBC), a magnetic heterogeneous catalyst, was prepared to activate peroxymonosulfate (PMS) for the decomplexation and mineralization of some typical copper complexes (including Cu(Ⅱ)-EDTA, Cu(Ⅱ)-NTA, Cu(Ⅱ)-citrate, and Cu(Ⅱ)-tartrate). The results showed that abundant cobalt ferrite and cobalt nanoparticles were decorated in the plate-like carbonaceous matrix, making it a higher degree of graphitization, better conductivity and more excellent catalytic activity than the raw biochar. Cu(Ⅱ)-EDTA was chosen as the representative copper complex. Under the optimum condition, the decomplexation and mineralization efficiency of Cu(Ⅱ)-EDTA in MSBC/PMS system were 98% and 68% within 20 min, respectively. The mechanistic investigation confirmed that the activation of PMS by MSBC followed both a radical pathway contributed by SO4•- and •OH and a nonradical pathway contributed by 1O2. In addition, the electron transfer pathway between Cu(Ⅱ)-EDTA and PMS facilitated the decomplexation of Cu(Ⅱ)-EDTA. Jointly, CO, Co0, and the redox cycles of Co(Ⅲ)/Co(Ⅱ) and Fe (Ⅲ)/Fe (Ⅱ) were found to play a critical role in the decomplexation process. Overall, the MSBC/PMS system provides a new strategy for efficient decomplexation and mineralization of copper complexes.

Decreasing dissolved oxygen enhances in situ curtailment of intermediate Cr(VI) during photo-oxidative decomplexation of Cr(III)-EDTA

Cr(III)-organic complexes are stably presented in tanning, electroplating, and other industrial wastewaters, and their safe and efficient removal remains a current challenge. Available oxidation processes can remove Cr(III) complexes but readily result in highly toxic Cr(VI) accumulation. Herein, negligible Cr(VI) accumulation was achieved during photo-oxidation of Cr(III) complexes using a simple strategy of decreasing dissolved oxygen (DO). At the DO concentration of 5.0 mg·L^-1 or less, the in-process formation of intermediate Cr(VI) was totally abated by in situ formed reductive species, and total Cr was reduced from 9.0-11.0 mg·L^-1 to below 1.0 mg·L^-1. A complete curtailment of Cr(VI) was observed after 30-60 min at pH 6.0-9.0. Increasing Cr(III)-EDTA concentration and decreasing pH value facilitated the in situ reduction of intermediate Cr(VI). Based on the identification of intermediates and additional Cr(II) and quenching experiments, the possible key species involved in intermediate Cr(VI) reduction were the photogenerated Cr(II) and some C-centered radicals from Cr(III)-EDTA decomplexation, and the possible mechanisms of Cr(III)-EDTA decomplexation and intermediate Cr(VI) reduction were thus proposed. The process also showed efficient treatment on other Cr(III) complexes (citrate, oxalate, and tartrate) and realistic Cr(III) complexed wastewater. This study would provide an insignificant Cr(VI)-accumulated alternative for efficient and safe removal of Cr(III) complexes from contaminated water.

Enhanced decomplexation of Cu-EDTA and simultaneous removal of Cu(II) by electron beam irradiation accompanied with autocatalytic fenton-like reaction: Synergistic performance and mechanism

Widely existing heavy metal complexes with high stability and poor biodegradability are intractable to be eliminated by conventional methods. In this study, electron beam (EB) irradiation characterized by rapidly producing strong oxidizing radicals was employed to effectively decompose Cu-ethylenediaminetetraacetic acid (Cu-EDTA) with almost complete elimination at 5 kGy. In terms of heavy metal removal, EB irradiation at relatively low doses was insufficient to remove copper ions, which was only 17.2% under 15 kGy. However, with the extra addition of 8 mM H₂O₂, such an irradiation dose could result in 99.0% copper ions removal. Mechanism analysis indicated that EB irradiation combined with spontaneously induced Fenton-like reactions were responsible for its excellent performance. The prime function of EB irradiation was to destroy the structure of Cu-EDTA with in-situ produced ·OH, and the subsequent released Cu-based intermediates could activate H₂O₂ to initiate autocatalytic chain reactions, correspondingly accelerating the degradation of complexes and the liberation of metal ions. Highly oxidative ·OH and O₂·^- were demonstrated as main active species acted on different positions of Cu-EDTA to realize gradual decarboxylation, synchronously generating low molecular weight compounds. XRD and XPS analysis showed that the released copper ions were mainly precipitated in the form of CuO, Cu(OH)₂ and Cu₂(OH)₂CO₃. In general, EB/H₂O₂ was an adoptable strategy for the disposal of such refractory heavy metal complexes.

Comparative study of PMS oxidation with Fenton oxidation as an advanced oxidation process for Co-EDTA decomplexation

In nuclear industry, Co-EDTA complex is generated due to the decontamination activities of nuclear power plants (NPPs). This complex is extremely refractory to the convention methods and can escalate the mobility of Co radionuclide in the environment. Due to its hazardous impact on human and environment, the effective treatments of Co-EDTA complexes are highly recommended. In this study, for the first time, we applied both hydroxyl (OH) and sulfate radical (SO₄^-) based advanced oxidation processes (AOPs) namely Fenton and peroxymonosulfate (PMS) reactions for the Co-EDTA decomplexation. Both reactions exhibited higher Co-EDTA decomplexation at pH = 3, however, the PMS based reaction was found to be superior, which showed highest decomplexation efficiency (without pH adjustment) over Fenton reaction (pH = 1-13). Moreover, PMS based system was found to be more suitable than Fenton reaction, because PMS showed best Co-EDTA decomplexation efficiency without any additional catalyst dosages at the shorter reaction time. XRD data confirmed the presence of both CoO and Co(OH)₂ in the precipitates after treatment. The electron spin resonance spectroscopy (ESR) analysis identified OH and SO₄^- in Fenton and PMS system, respectively. From this study, we believe that PMS based reaction is a superior alternative of Fenton reaction for the Co-EDTA decomplexation.

Complexation behaviour and removal of organic-Cr(III) complexes from the environment: A review

Chromium (Cr) is mainly found in the form of organic-Cr(III) complexes in the natural environment and industrial waste. The widespread existence of composite contaminants composed of organic matter (OM) and Cr pose a serious ecological threat, and its potential interaction and removal need to be further summarised. Organic ligands, such as carbohydrates, nitrogen compounds, phenolic compounds, humus substances (HS), and low molecular weight organic acids (LMWOAs), play an important role in governing the speciation, mobility, and absorption and desorption of Cr in the environment. Moreover, growing evidence indicates that oxygen-containing functional groups (e.g., carboxyl, hydroxyl, and phosphate) are closely related to the complexation of Cr(III). Advanced oxidation processes (AOPs) are efficient and widely applicable technologies. However, the re-complexation of oxidation intermediates with Cr(III) and the formation and accumulation of much more toxic Cr(VI) species hinder the possible utilisation of AOPs. In this paper, the sources and harmful effects of organic-Cr(III) complexes are reported in detail. The complexation behaviour and structure of the organic-Cr(III) complexes are also described. Subsequently, the application of AOPs in the decomplexation and degradation of organic-Cr(III) complexes is summarised. This review can be helpful for developing technologies that are more efficient for organic-Cr(III) complex removal and establishing the scientific background for reducing Cr discharge Cr into the environment.

Minimizing toxic chlorinated byproducts during electrochemical oxidation of Ni-EDTA: Importance of active chlorine-triggered Fe(II) transition to Fe(IV)

The formation of chlorinated byproducts represents a significant threat to the quality of the effluent treated using electrochemical advanced oxidation processes (EAOPs), thus spurring investigation into alleviating their production. This study presents a new strategy to minimize the release of chlorinated intermediates during the electrochemical oxidation of Ni-EDTA by establishing a dual mixed metal oxide (MMO)/Fe anode system. The results indicate that the dual-anode system achieved a substantially higher rate (0.141 min^-1) of Ni-EDTA destruction and accordingly allowed a more pronounced removal of aqueous Ni (from 39.85 to 0.63 mg L^-1) after alkaline precipitation, compared with its single MMO anode (0.017 min^-1 of Ni-EDTA removal, with 14.38 mg L^-1 Ni remaining) and single Fe anode (insignificant Ni-EDTA removal, with 38.37 mg L^-1 Ni remaining) counterparts. Compared to reactive chlorine species (RCS) produced from the single MMO anode system, Fe(IV) was in situ generated from the dual-anode system and was predominantly responsible for the attenuation of chlorinated byproducts and thus the decrease in the acute toxicity of the treated solution (evaluated using luminescent bacteria). The Fe(IV)-dominated dual-anode system also exhibited superior performance in removing multiple pollutants (including organic ligands, Ni, and phosphite) in the real electroless plating effluent. The findings suggest that the strategy for Fe(II) transition to Fe(IV) by active chlorine paves a new avenue for yielding less chlorinated products with lower toxicity when EAOPs are used to treat chloride-containing organic wastewater.

Electro-peroxone enables efficient Cr removal and recovery from Cr(III) complexes and inhibits intermediate Cr(VI) generation in wastewater: Performance and mechanism

Available oxidation processes for removing Cr(III) complexes from water/wastewater usually encounter the formation of highly toxic Cr(VI) and the generation of Cr enriched waste sludge, posing challenges on the subsequent disposal. Herein, we achieve efficient removal of Cr(III)-organic complexes and simultaneous recovery of Cr from wastewater with enhanced curtailment of intermediate Cr(VI), by using an electrochemically driven peroxone (i.e., electro-peroxone) process with activated carbon fiber (ACF) electrodes. For Cr(III)-EDTA, electro-peroxone could remove ∼90% total Cr from 11.50 mg/L to 1.20 mg/L and ∼80% total organic carbon, with a strong curtailment of Cr(VI) to less than 0.2 mg/L. Additionally, the process could obtain a complete recovery of the removable Cr, of which 78.3% are enriched at ACF cathode as amorphous Cr(OH)₃ deposits and the remaining 21.7% are adsorbed at the anode, thus avoiding the generation of Cr laden sludge. Mechanism studies show the electro-generated H₂O₂ reacts with O₃ to generate abundant HO· for decomplexation, which sequentially oxidizes Cr(III) to Cr(VI), and degrades the released EDTA via stepwise decarboxylated process, as confirmed by HPLC analysis. Multiple pathways including electro-reduction, H₂O₂ reduction and electro-adsorption synergistically curtail and immobilize the formed intermediate Cr(VI). ACF characterizations and continuous 5-cycle experiments substantiate the excellent reusability of the ACF electrodes. Moreover, this process exhibits satisfactory effectiveness to Cr(III) complexed with other ligands (e.g., citrate and oxalate), and complexed Cr(III) in the real electroplating wastewater. We believe this study would provide an efficient and eco-friendly alternative for Cr(III) complexes removal from wastewater.

Decomplexation of Cu(II)-EDTA by synergistic activation of persulfate with alkali and CuO: Kinetics and activation mechanism

Heavy metals usually coexist with a variety of chelating agents to form heavy metal complexes in industrial wastewater. The decomplexation of heavy metal complexes is the crucial step before the removal of heavy metals via alkaline precipitation process. An efficient synergistic activation of persulfate (PS) with alkali and CuO was used for the simultaneous decomplexation of Cu-ethylenediamine tetraacetic acid (Cu(II)-EDTA) (3.14 mM) and the Cu(II) precipitation. The experimental results demonstrated that nearly complete removal of Cu(II) could be achieved by synergistic activation of PS with alkali and CuO at pH 11 after 2 h of decomplexation reaction. However, sole PS could not effectively decomplex Cu(II)-EDTA (13.5%), while the alkaline activation of PS could accomplish 57.0% removal of Cu(II). Radical scavenger tests indicated that reactive oxygen species (ROS) including SO₄^•-, •OH and O₂^•- were responsible for the decomplexation of Cu(II)-EDTA in the synergistic activation of PS with alkali and CuO. As a heterogeneous activator, CuO possessed excellent reusability and long-lasting catalytic activity and the rate constant value (k) of Cu(II) removal showed an increase (from 0.0326 min^-1 in the first cycle to 0.0491 min^-1 in the 24^th cycle) with 24 cycles experiments. Furthermore, the biotoxicity evaluation of treated solution revealed that the biotoxicity of Cu(II)-EDTA contaminated wastewater could be effectively mitigated by the synergistic activation of PS with alkali and CuO because of the efficient precipitation of Cu(II) and oxidative degradation of EDTA organic ligands, which was favorable for the subsequent biochemical treatment.

Decomplexation of heterogeneous catalytic ozonation assisted with heavy metal chelation for advanced treatment of coordination complexes of Ni

Following the conventional physicochemical treatment of electroless nickel (Ni) plating wastewater (ENPW) in electroplating wastewater treatment plants, highly stable and recalcitrant coordination complexes of Ni (CCN) still remain. This results in various technical problems, leading to the treatment difficulty, poor wastewater biochemistry, and failure to meet effluent standards. Therefore, an efficient decomplexation system involving heterogeneous catalytic ozonation assisted with heavy metal chelation (O₃/SAO3II-MDCR) was proposed in this study for the advanced treatment of CCN. The catalyst SAO3II was characterized by various methods, which revealed the mechanism of catalytic ozonation. Hydroxyl radicals (OH) and other reactive oxygen species (ROS) groups were detected, proving that catalytic ozonation was a complicated reaction process and also a foundation process of the entire system. These ROS are vital for decomplexation via heterogeneous catalytic ozonation of the system. During the catalytic decomplexation process via ozonation, CCN first underwent gradual decomposition from a highly stable macromolecular state to a volatile micromolecular state (or even completely mineralized state). Then Ni was chelated to form an insoluble and stable chelate via competitive coordination. The optimum conditions for the O₃/SAO3II-MDCR system were determined by single factor static experiments. After treatment with the O₃/SAO3II-MDCR system, the effluent concentration of total Ni was found to be <0.1 mg L^-1, exhibiting a removal rate of up to 95.6% and achieving effective removal of total Ni from ENPW and stably meeting the discharge standard. O₃/SAO3II-MDCR system can easily and hopefully be extended to practical engineering applications.

Removal of lead complexes by ferrous phosphate and iron phosphate: Unexpected favorable role of ferrous ions

The high chemical stability of lead complexes in solution precludes most traditional removal methods. Achieving the efficient, cost-effective, and environmentally friendly removal of metal complexes from wastewater is a challenge. In this study, ferrous phosphate and iron phosphate were used to treat wastewater containing EDTA-Pb, and the differences in their removal processes were compared. Both materials enabled efficient removal of the EDTA-Pb complex from 50 mg Pb/L to <1 mg Pb/L, and the leaching of Fe was <50 mg/L. More attractively, the maximum adsorption capacity of ferrous phosphate significantly increased from 80.44 mg Pb/g to 436.68 mg Pb/g as the reaction environment changed from aerobic to anoxic. The concentration of Pb was reduced to the sub-ppm level by ferrous phosphate even when the initial concentration of EDTA-Pb was 300 mg/L. In-depth exploration of the removal mechanism of EDTA-Pb demonstrated that the synergistic effect of Fe²⁺ and Fe³⁺ contributed to the high removal efficiency of EDTA-Pb by ferrous phosphate. Moreover, ferrous phosphate was minimally affected by salinity and organics, but the iron phosphate performance was significantly suppressed. The potential application of ferrous phosphate was further explored by processing explosive wastewater containing lead complexes. The results showed that the residual Pb content was 0.94 mg/L (lower than the discharge standard of China) and the removal performance of iron phosphate was suppressed. The results demonstrate that ferrous phosphate is a promising material for the decontamination of EDTA-Pb-contaminated water.

Decomplexation of Cu(II)-natural organic matter complex by non-thermal plasma oxidation: Process and mechanisms

Heavy metals and natural organic matters (NOM) form very stable heavy metal-NOM complexes in aqueous, facilitating the migration of heavy metals and enhancing their potential risks. In this study, non-thermal plasma oxidation was attempted to destroy the heavy metal-NOM complexes, with Cu-humate (Cu-HA) as a model. The decomplexation efficiency reached 86.1 % within 50 min of plasma oxidation at 16 kV. The generated reactive species by the non-thermal plasma, including O₂^-, ¹O₂, OH, attacked the carboxyl and hydroxyl functional groups of HA, leading to cleavage of the Cu-O bonds, decomplexation of Cu-HA, and release of free Cu(II). Meanwhile, a variety of small molecular intermediates, including phenols, benzoic acids, esters, amines, ketones, acetic acid, formic acid, and oxalic acid, were generated due to attack by the oxidative species on the aromatic moiety and double bonds in Cu-HA. As a consequence of decomplexation, the residual toxicity of Cu-HA to Scenedesmus obliquus was distinctly reduced. This study provides a potential technique to decomplex heavy metal-NOM complexes, and reduces their toxicity to typical Scenedesmus obliquus.

Sludge reduction and cost saving in removal of Cu(II)-EDTA from electroplating wastewater by introducing a low dose of acetylacetone into the Fe(III)/UV/NaOH process

Cu(II)-EDTA is highly stable in a wide pH range (3.0∼12.0) and hard to be removed by the conventional precipitation method. Fe(III) displacement/UV photolysis/alkaline precipitation [Fe(III)/UV/NaOH] has been proposed as a promising method for the removal of Cu(II)-EDTA. Nevertheless, a high dose of Fe(III) is needed in this combined process, resulting in the production of a large amount of hazardous sludge. The photochemistry of Fe(III) is known to be ligand-dependent. Fe(III)-oxalate complexes are strongly photoactive. However, the addition of oxalic acid to the Fe(III)/UV/NaOH process was of little help. Acetylacetone (AA) is a good chelating ligand for many metals and has been proved as an efficient photo-activator. By introducing a low dose of AA ([AA]/[Cu] = 1.5) into the Fe(III)/UV/NaOH process, the Fe(III) dosage ([Fe]/[Cu]) was reduced from 10.4 to 3.2. As a result, the chemical cost was reduced from 13.9 to 7.6 kW h/m³. Meanwhile, the energy cost in the UV photolysis was reduced from 1066.5 to 752.4 kW h/m³. Most importantly, the sludge yields were reduced from 8.3 to 2.7 kg/m³ in a simulated wastewater and from 101.8 to 30.8 kg/m³ in a real electroplating wastewater. Such a sludge reduction is of great significance in mitigating the load of landfill.

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.

Decomposition of complexed Pb(II) and subsequent adsorption of Pb(II) with yolk-shell Fe3O4@ hydrous zirconium oxide sphere

Purification of water containing heavy metals that are complexed by organic chelating agents remains a challenging task. In this study, a yolk-shell Fe₃O₄@hydrous zirconium oxide (Zr(OH)_x) sphere sphere (YHZOs) nanomaterial was evaluated for its ability to remove ethylene diamine tetraacetic acid complexed Pb²⁺ (Pb-EDTA) from aqueous solution. Specifically, it is hypothesized that upon addition of H₂O₂, the Fe₃O₄ core of YHZOs served as a Fenton-type catalyst that results in oxidation of the Pb-complexed EDTA, and the Zr(OH)_x shell acted as an adsorbent, removing the released Pb²⁺ from solution. From an aqueous solution containing 0.1 mM Pb-EDTA at pH 5, 0.5 g/L YHZOs, and 20 mM H₂O_2, TOC reduction and Pb removal were determined to be 65.3% and 89.8%, respectively. HPLC-MS, IC and continuous flow analyzer results identified major intermediates of EDTA decay to be ethylenediaminetriacetate, (ED3A), ethylenediamine-N,N'-diacetate (ED2A), nitrilotriacetate (NTA), iminodiacetate (IDA), ethylenediamine (EDA), acetic acid, formic acid, oxalic acid, ammonia, and nitrate, with the first 5 species having some affinity to remain complexed to Pb²⁺. The adsorption of Pb²⁺ onto the Zr(OH)_x shells was confirmed by scanning transmission electron microscopy (STEM) with mapping and X-ray photoelectron spectra (XPS). Moreover, the Pb²⁺-adsorbed YHZOs could be easily recovered due to their magnetic properties, with the Pb²⁺ rinsed from them at low pH. Indeed, reused for five cycles showed only minor capacity loss. These findings suggest that the removal of chelated Pb²⁺ from water, and presumably other heavy metals, by yolk-shell Fe₃O₄@Zr(OH)_x may prove to be a useful technology for some contaminated waters.

Decomplexation removal of Ni(II)-citrate complexes through heterogeneous Fenton-like process using novel CuO-CeO2-CoOx composite nanocatalyst

A novel CuO-CeO₂-CoO_x nanocatalyst was prepared for heterogeneous Fenton-like reaction to break the Ni²⁺-chelate into free Ni²⁺ in electroless nickel plating wastewater, followed by separation of Ni²⁺ in an insoluble form. The composite nanocatalysts prepared by co-precipitation method were characterized by XRD, TEM, and XPS, et al. Its catalytic activity as Fenton-like reagent was evaluated by the removal efficiency of Ni(II)-citrate after decomplexation and postprecipitation treatment. Subsequently, the effects of operating parameters on the decomplexation efficiency of the nanocatalysts were investigated including calcination temperature of catalysts, H₂O₂ concentration, catalyst dosage, initial pH and reaction temperature. Under optimized condition, the Ni(II)-citrate complexes (C₀ = 1.0 mM) achieved the complete decomplexation (>99.9%) within 60-min reaction using 0.3 g/L of CuO-CeO₂-CoO_x calcined at 450 °C and 75 mM of H₂O₂ at pH 3.0 under 50 °C of reaction. Then, Ni²⁺ after decomplexation could be completely removed by the subsequent precipitation at pH 11.0. In addition, the life test of CuO-CeO₂-CoO_x catalyst indicated that, after recycled 10 times, its activity for decomplexation of Ni(II)-citrate decreased no more than 8%. As a result, this new heterogeneous Fenton-like process is promising for decomplexation and purification of electroless nickel plating wastewater as a sludge reduction technology.

Decomplexation efficiency and mechanism of Cu(II)-EDTA by H2O2 coupled internal micro-electrolysis process

Internal micro-electrolysis (IE) coupled with Fenton oxidation (IEF) was a very effective technology for copper (Cu)-ethylenediaminetetraacetic acid (EDTA) wastewater treatment. However, the mechanisms of Cu²⁺ removal and EDTA degradation were scarce and lack persuasion in the IEF process. In this paper, the decomplexation and removal efficiency of Cu-EDTA and the corresponding mechanisms during the IEF process were investigated by batch test. An empirical equation and the oxidation reduction potential (ORP) index were proposed to flexibly control IE and the Fenton process, respectively. The results showed that Cu²⁺, total organic carbon (TOC), and EDTA removal efficiencies were 99.6, 80.3, and 83.4%, respectively, under the proper operation conditions of iron dosage of 30 g/L, Fe/C of 3/1, initial pH of 3.0, Fe²⁺/H₂O₂ molar ratio of 1/4, and reaction time of 20 min, respectively for IE and the Fenton process. The contributions of IE and Fenton to Cu²⁺ removal were 91.2 and 8.4%, respectively, and those to TOC and EDTA removal were 23.3, 25.1, and 57, 58.3%, respectively. It was found that Fe²⁺-based replacement-precipitation and hydroxyl radical (•OH) were the most important effects during the IEF process. •OH played an important role in the degradation of EDTA, whose yield and productive rate were 3.13 mg/L and 0.157 mg/(L min^-1), respectively. Based on the intermediates detected by GC-MS, including acetic acid, propionic acid, pentanoic acid, amino acetic acid, 3-(diethylamino)-1,2-propanediol, and nitrilotriacetic acid (NTA), a possible degradation pathway of Cu-EDTA in the IEF process was proposed. Graphical abstract The mechanism diagram of IEF process.

Simultaneous decomplexation in blended Cu(II)/Ni(II)-EDTA systems by electro-Fenton process using iron sacrificing electrodes

This research explored the application of electro-Fenton (E-Fenton) technique for the simultaneous decomplexation in blended Cu(II)/Ni(II)-EDTA systems by using iron sacrificing electrodes. Standard discharge (0.3 mg L^-1 for Cu and 0.1 mg L^-1 for Ni in China) could be achieved after 30 min reaction under the optimum conditions (i.e. initial solution pH of 2.0, H₂O₂ dosage of 6 mL L^-1 h^-1, current density of 20 mA/cm², inter-electrode distance of 2 cm, and sulfate electrolyte concentration of 2000 mg L^-1). The distinct differences in apparent kinetic rate constants (k_app) and intermediate removal efficiencies corresponding to mere and blended systems indicated the mutual promotion effect toward the decomplexation between Cu(II) and Ni(II). Massive accumulation of Fe(Ⅲ) favored the further removal of Cu(II) and Ni(II) by metal ion substitution. Species distribution results demonstrated that the decomplexation of metal-EDTA in E-Fenton process was mainly contributed to the combination of various reactions, including Fenton reaction together with the anodic oxidation, electro-coagulation (E-coagulation) and electrodeposition. Unlike hypophosphite and citrate, the presence of chlorine ion displayed favorable effects on the removal efficiencies of Cu(II) and Ni(II) at low dosage, but facilitated the ammonia nitrogen (NH₄⁺-N) removal only at high dosage.

Efficient removal of Cr(III)-organic complexes from water using UV/Fe(III) system: Negligible Cr(VI) accumulation and mechanism

Most available processes are incapable of removing Cr(III)-organic complexes from water due to their high solubility, extremely slow decomplexation rate, and possible formation of more toxic Cr(VI) during oxidation. Herein, we proposed a new combined process, i.e., UV/Fe(III) followed by alkaline precipitation (namely UV/Fe(III)+OH), to achieve highly efficient and environmentally benign removal of Cr(III)-organic complexes from water. The combined process could remove Cr(III)-citrate from 10.4 mg Cr/L to 0.36 mg Cr/L and ∼60% total organic carbon as well. More attractively, negligible Cr(VI) (<0.06 mg/L) was formed during the process. In the viewpoint of mechanism, the added Fe(III) generates ·OH radicals to transform Cr(III) into Cr(VI) and simultaneously released the citrate ligand to form Fe(III)-citrate simultaneously. Then, the photolysis of Fe(III)-citrate under UV irradiation involved the citrate degradation and the production of massive Fe(II) species, which in turn transformed the formed Cr(VI) back to Cr(III). The free metal ions, including Cr(III), Fe(II) and Fe(III) were removed by the subsequent alkaline precipitation. Also, the combined process is applicable to other Cr(III) complexes with EDTA, tartrate, oxalate, acetate. The applicability of the combined process was further demonstrated by treating two real tanning effluents, resulting in the residual Cr(III) below 1.5 mg/L (the discharge standard of China) and negligible formation of Cr(VI) (<0.004 mg/L) as well. In general, the combined process has a great potential for efficient removal of Cr(III) complexes from contaminated waters.

Sequestration of chelated copper by structural Fe(II): Reductive decomplexation and transformation of Cu(II)-EDTA

Chelated coppers, such as Cu(II)-EDTA, are characteristically refractory and difficult to break down because of their high stability and solubility. Cu(II)-EDTA sequestration by structural Fe(II) (Fe(II)) was investigated intensively in this study. Up to 101.21mgCu(II)/gFe(II) was obtained by Fe(II) in chelated copper sequestration under near neutral pH condition (pH 7.70). The mechanism of Cu(II)-EDTA sequestration by Fe(II) was concluded as follows: 3Cu(II)-EDTA+7Fe(II)+9H2O → Cu(0)↓+ Cu2O↓(the major product)+2Fe2O3·H2O↓+3Fe(II)-EDTA +14H(+) Novel results strongly indicate that Cu(II) reductive transformation induced by surface Fe(II) was mainly responsible for chelated copper sequestration. Cu(0) generation was initially facilitated, and subsequent reduction of Cu(II) into Cu(I) was closely combined with the gradual increase of ORP (Oxidation-Reduction Potential). Cu-containing products were inherently stable, but Cu2O would be reoxidized to Cu(II) with extra-aeration, resulting in the release of copper, which was beneficial to Cu reclamation. Concentration diminution of Cu(II)-EDTA within the electric double layer and competitive adsorption were responsible for the negative effects of Ca(2+), Mg(2+). By generating vivianite, PO4(3-) was found to decrease surface Fe(II) content. This study is among the first ones to identify the indispensible role of reductive decomplexation in chelated copper sequestration. Given the high feasibility and reactivity, Fe(II) may provide a potential alternative in chelated metals pollution controlling.

Decomplexation and subsequent reductive removal of EDTA-chelated Cu II by zero-valent iron coupled with a weak magnetic field: Performances and mechanisms

The feasibility of EDTA-chelated Cu(II) (Cu(II)-EDTA) removal by zero-valent iron (Fe(0)) in the presence of a weak magnetic field (WMF) and the involved mechanisms were systematically investigated. Fe(0) combined with WMF (Fe(0)/WMF) was very effective for removing Cu(II)-EDTA at pH 4.0-6.0 with the rate constants ranging from 0.1190 min(-1) to 0.0704 min(-1). Little passivation of Fe(0) was observed during Cu(II)-EDTA removal by Fe(0)/WMF in 8 consecutive runs when 10.0 mg L(-1) Cu(II)-EDTA was dosed before the initiation of each run. The evidences presented in this study verified that Cu(II)-EDTA was removed by decomplexation followed by reduction/adsorption. In brief, Fe(II) released from Fe(0) corrosion was rapidly oxidized by oxygen to Fe(III) to chelate with EDTA and release free Cu(II), and the detached Cu(II) ions were subsequently reduced/removed by Fe(0)/Fe(II) and co-precipitated by the generated iron (hydr)-oxides. To advance the application of Fe(0)/WMF technology in real practice, a magnetic propeller agitator was designed to offer WMF inside the reactor, which could greatly improve Cu(II)-EDTA removal by Fe(0) and be easily amplified.