Treatment Of Etching Terminal Wastewater With Simultaneous Mineralization Of Recalcitrant Organics And Sequential Recovery Of Heavy Metals Using Photo-assisted Bioelectrochemical Systems

Etching Terminal Wastewater（ETW）is one typical effluent of electroplating industrial wastewaters treated by flocculation,ion exchange and other processes,and still contains a certain amount of heavy metals such as Cu（II）,Ni（II）and Zn（II）,as well as a variety of recalcitrant organics.While this ETW was normally discharged to municipal wastewater treatment plants for further dilution and treatment,it is urgently significant great environmental,ecological and social significance to find effective approaches for simultaneous mineralization of these recalcitrant organics with recovery and separation of these heavy metals in ETW due to the increasingly strict standards for this effluent discharge.Combined with the latest research progress in photoassisted bioelectrochemical systems（BES）,this study successfully achieved the simultaneous recovery and separation of heavy metals with mineralization of recalcitrant organics in ETW using either sequentical photo-assisted BESs or self-driven photo-assisted BESs,reaching the effluent meeting China wastewater discharge standard（GB 39731-2020）.Main results included the following:（1）Under the optimal condition（HRT 6 h+no N₂sparging）,the dominant recovery of copper（85.8±2.3%）in one microbial fuel cell(MFC_Cu),nickel（71.6±0.2%）in one microbial electrolysis cell(MEC_Ni),and zinc（67.7±1.3%）in the other MEC(MEC_Zn)of the sequential photo-assisted BESs were achieved under the optimal conditions of an HRT of 6 h with no N₂sparging.Accordingly,efficient mineralization of recalcitrant organics(MFC_Cu:34.1±0.8%,MEC_Ni:22.8±1.3%,MEC_Zn:13.7±2.7%)was achieved with the effluent meeting China wastewater discharge standard（GB 39731-2020）.Light irradiation and circuital current played synergistic roles in recalcitrant organics mineralization and heavy metals recovery,1.9–4.2 folds（organics mineralization）and 1.2–1.4 times（heavy metals recovery）of sole circuital current,and 3.5–7.1 times（organics mineralization）and 35.8–112.8 folds（heavy metals recovery）of light irradiation only.Oxidative active species(^˙OH,O₂^˙-,holes and H₂O₂)variably contributed to recalcitrant organics mineralization whereas heavy metals recovery was dominantly ascribed to circuital current.SEM-EDS confirmed the different speices of heavy metals with various ratios on each cathodes of MFC_Cu,MEC_Niand MEC_Znwhereas XRD demonstrated the crystal forms of different heavy metals on the corresponding cathodes.（2）A self-driven photo-assisted BES composed of two MFCs in serial connection driving one MEC unit(MFC_Cu-MFC_Cu-MEC_Ni+Zn)was purposely scheduled to achieve dominant recovery of Cu in each MFC units(MFC_Cu)with the residual Ni and Zn in the MEC unit(MEC_Ni+Zn).Under the optimal HRT of 6 h withouout anaerobic sparging,Cu（II）was dominantly removed in MFC_Cu（69.1±7.0%）with a separation factor of 7.67±0.31,whereas Ni（II）（64.9±2.0%）and Zn（II）（75.7±7.9%）were mainly removed in MEC_Ni+Znwith separation factors of 1.61±0.05（Ni）and 3.44±0.27（Zn）.In parallel,mineralization of recalcitrant organics was achieved(MFC_Cu:38.2±1.0%,MEC_Ni+Zn:29.9±0.4%).The total removals of heavy metals and mineralization of recalcitrant organics in the effluent of ETW were 98.6±0.6%（Cu（II））,100±0.1%（Ni（II））,86.0±7.4%（Zn（II））,and 67.9±0.3%（recalcitrant organics）,with the effluent meeting China wastewater discharge standard（GB39731-2020）.Light illumination improved recalcitrant organics mineralization and heavy metal recovery,shown as 4.1 fold（recalcitrant organics）and 1.2-3.3 times（heavy metals）as those in the controls.XPS confirmed the dominant species of Cu（0）（73%）and Cu（II）（27%）on the cathodes of the MFC_Cuwhereas the cathodic surfaces of the MEC_Ni+Znwere mainly covered by Ni（0）（69%）and Ni（II）（31%）,compared to the complete Zn（OH）₂precipitants（100%）at the bottom of the MEC_Ni+Zn.From an energy point of view,this self-driven BES produced 0.030 k Wh/m³ETW,compared to the 0.022 k Wh/m³in the controls in the absence of light.Overall,these studies provided sustainable and energy-saving approaches for simultaneous efficient mineralization of recalcitrant organics and complete recovery of value-added heavy metals of Cu（II）,Ni（II）and Zn（II）from the ETW.