An Electrochemical Strategy for Simultaneous Heavy Metal Complexes Wastewater Treatment and Resource Recovery

Abstract

Heavy metals chelated with coexisting organic ligands in wastewater impose severe risks to public health and the ambient ecosystem but are also valuable metal resources. For sustainable development goals, the treatment of heavy metal complexes wastewater requires simultaneous metal–organic bond destruction and metal resource recovery. In this study, we demonstrated that a neutral pH electro-Fenton (EF) system, which was composed of an iron anode, carbon cloth cathode, and sodium tetrapolyphosphate electrolyte (Na6TPP), could induce a successive single-electron activation pathway of molecular oxygen due to the formation of Fe­(II)-TPP complexes. The boosted •OH generation in the Na6TPP-EF process could decomplex 99.9% of copper ethylene diamine tetraacetate within 8 h; meanwhile, the released Cu ions were in situ deposited on the carbon cloth cathode in the form of Cu nanoparticles with a high energy efficiency of 2.45 g kWh–1. Impressively, the recovered Cu nanoparticles were of purity over 95.0%. More importantly, this neutral EF strategy could realize the simultaneous removal of Cu, Ni, and Cr complexes from real electroplating effluents. This study provides a promising neutral EF system for simultaneous heavy metal complexes wastewater treatment and resource recovery and sheds light on the importance of molecular oxygen activation in the field of pollutant control.

Targeted Decomplexation of Metal Complexes for Efficient Metal Recovery by Ozone/Percarbonate

“…We investigated the effects of typical influence factors (such as solution pH and coexisting substances) on the Cu recovery performance in the O 3 /SPC process. As shown in Figure a, the O 3 /SPC process exhibited excellent Cu recovery efficiency (95.7–100%) from the Cu­(II)–EDTA complex under broad initial pH range conditions (2.0–12.0), which was better than the previously mentioned processes, such as the electro-Fenton process, discharge plasma oxidation, and UV/persulfate. ,, The phenomenon was ascribed to the fact that SPC could maintain the alkalinity of the reaction solution regardless of the initial pH value (Figure S21). We further investigated the recovery performance of Cu in the O 3 /SPC process by preintroducing SPC before adjusting the solution pH (7.0–11.0) (Figure S22).…”

“…Thus, electron paramagnetic resonance (EPR) trapping measurement with the assistance of 5,5-dimethyl- 1 -pyrroline N-oxide (DMPO) or 2,2,6,6-tetramethylpiperidine (TEMP) as the spin-trapping agent was utilized to detect the generated reactive species in the O 3 /SPC process. As shown in Figure c, DMPO–CO 3 •– ( A N = 15.8 G, A H = 19.10 G), DMPO– • OH ( A N = A H = 14.86 G), DMPO–O 2 •– ( A N = 13.85 G, A H = 10.10 G), and TEMP– 1 O 2 ( A N = 17.32 G) spin adducts were observed in the O 3 /SPC process (). ,, Subsequently, the quenching experiments using different scavengers were conducted to verify the contributions of these active species in the O 3 /SPC process. As shown in Figure S7a, the Cu recovery efficiency was significantly decreased to 60.5% and 21.3% with the addition of tert -butyl alcohol (TBA) and N , N -dimethylaniline (DMA), respectively .…”

“…In pathway I of decarboxylation, the cleavage of N–CH 2 bond in the [−N–(CH 2 –COOH)] group occurs with the removal of acetic acid, resulting in the formation of Cu­(II)–ethylenediaminetriacetic acid (ED3A). Cu­(II)–ethylenediamine- N , N ′-diacetic acid (ED2A) and Cu­(II)–ethylenediamineacetic acid (EDMA) can be generated through the continuous decarboxylation process. − In pathway II of deamination, the CH 2 –N bond in the [CH 2 –N–(CH 2 –COOH) 2 ] group of Cu­(II)–EDTA is cleaved with the removal of iminodiacetic acid (IMDA) and then rapidly oxidized to generate Cu­(II)–nitrilotriacetate acid (NTA). − Pathways I and II have been previously reported in the decomplexation of Cu­(II)–EDTA. In pathway III of formic acid removal, the C–C bond is cleaved in the [C–COOH] group with formic acid broken off.…”

“…A large amount of metal wastewater containing ethylenediaminetetraacetic acid-chelated copper [Cu­(II)–EDTA] was produced from metal smelting, electrolysis, electroplating, and chemical cleaning industries. − However, traditional chemical precipitation and adsorption processes cannot eliminate Cu­(II)–EDTA . Fenton oxidation process reveals its capability in the abatement of Cu­(II)–EDTA but faces the problem caused by the secondary solid waste (i.e., iron sludge containing copper ions), thus impeding copper resource for reuse. , For sustainable industrial development, it is essential to develop feasible treatment processes to remove the Cu–organic complex and recover Cu.…”

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

“…At lower applied cell voltages (and cathodic and anodic potentials), the removal of Cu via both anodic oxidation of EDTA and cathodic reduction of EDTA-complexed Cu­(II) will be limited. However, a combination of the electrochemical process with other AOPs such as photocatalytic oxidation, persulfate, and/or the electron-Fenton process ,, may facilitate EDTA degradation and concomitant Cu removal. In contrast to the removal of Cu, the reduction of EDTA-complexed Ni­(II) to Ni(0) on the cathode surface is minimal without EDTA degradation due to the high reduction potential of EDTA-complexed Ni­(II) and low Ni 2+ concentration present in equilibrium EDTA-complexed Ni­(II).…”