mineralsArticleDetoxification of Arsenic-Containing Copper Smelting Dust byElectrochemical Advanced Oxidation TechnologyMeng Li 1,2,3, Junfan Yuan 1, Bingbing Liu 1 , Hao Du 2, David Dreisinger 3, Yijun Cao 1,4,* and Guihong Han 1,*



Citation: Li, M.; Yuan, J.; Liu, B.; Du,H.; Dreisinger, D.; Cao, Y.; Han, G.Detoxification of Arsenic-ContainingCopper Smelting Dust byElectrochemical Advanced OxidationTechnology. Minerals 2021, 11, 1311.https://doi.org/10.3390/min11121311Academic Editors: Shuai Wang,Xingjie Wang, Jia Yang andCarlito TabelinReceived: 15 October 2021Accepted: 22 November 2021Published: 24 November 2021Publisher’s Note: MDPI stays neutralwith regard to jurisdictional claims inpublished maps and institutional affil-iations.Copyright: © 2021 by the authors.Licensee MDPI, Basel, Switzerland.This article is an open access articledistributed under the terms andconditions of the Creative CommonsAttribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).1 School of Chemical Engineering, Zhengzhou University, Zhengzhou 450001, China; li.meng@zzu.edu (M.L.);moguizihuan@163.com (J.Y.); liubingbing@zzu.edu.cn (B.L.)2 National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Key Laboratory ofGreen Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences,Beijing 100190, China; hdu@ipe.ac.cn3 Department of Materials Engineering, The University of British Columbia, Vancouver, BC V6T1Z4, Canada;david.dreisinger@ubc.ca4 Henan Province Industrial Technology Research Institute of Resources and Materials, Zhengzhou University,Zhengzhou 450001, China* Correspondence: yijuncao@126.com (Y.C.); hanguihong@zzu.edu.cn (G.H.)Abstract: A large amount of arsenic-containing solid waste is produced in the metallurgical processof heavy nonferrous metals (copper, lead, and zinc). The landfill disposal of these arsenic-containingsolid waste will cause serious environmental problems and endanger people’s health. An electro-chemical advanced oxidation experiment was carried out with the cathode modified by addingcarbon black and polytetrafluoroethylene (PTFE) emulsion. The removal rate of arsenic using ad-vanced electrochemical oxidation with the modified cathode in 75 g/L NaOH at 25 ◦C for 90 minreached 98.4%, which was significantly higher than 80.69% of the alkaline leaching arsenic removalprocess. The use of electrochemical advanced oxidation technology can efficiently deal with theproblem of arsenic-containing toxic solid waste, considered as a cleaner and efficient method.Keywords: arsenic; copper smelting dust; electrochemical advanced oxidation technology; iron-freeFenton-like reaction1. IntroductionThe output of nonferrous metals is increasing by the year, and most of the heavynonferrous metal concentrates contain arsenic. The arsenic in these heavy nonferrous metalminerals is mainly combined with copper, lead, zinc, and sulfur [1,2]. In the nonferrousmetal metallurgical process, arsenic usually enters smoke, smelting residues, and acid inthe form of waste slag, wastewater, and exhaust gas, respectively [3]. Compared withelemental arsenic, arsenic compounds are more toxic. Arsenic can enter the human bodythrough air, water, and other ways, causing great harm to the human skin, digestive, andrespiratory system [4]. Long-term exposure to arsenic substances can cause skin cancer,lung cancer, bladder cancer, and kidney cancer [5]. Tumor tissue analyses confirmed thatit is one of the human carcinogens. Arsenic-containing copper smelting dust (ARCD) isa solid hazardous waste formed through processes such as copper smelting, converting,and anode slime smelting [6]. Arsenic mainly forms compounds with copper and sulfurin copper sulfide concentrates. During the copper smelting process, part of the arsenicwill enter the copper smelting dust. If copper smelting dust is recycled directly to coppersmelting without arsenic removal, arsenic will accumulate, reducing the main metallurgyproduction efficiency and affecting the entire smelting process [7]. Further, if the ARCD isnot appropriately treated, it will have a more significant impact on the environment andhuman health.The main methods for removing arsenic from arsenic-containing solid waste includealkaline leaching, acid leaching, and roasting. The alkaline leaching process mainly con-verts arsenic from solid waste residue to the liquid phase by alkali to realize the separationMinerals 2021, 11, 1311. https://doi.org/10.3390/min11121311 https://www.mdpi.com/journal/mineralsMinerals 2021, 11, 1311 2 of 14of arsenic from other metals [8–10]. The acid leaching process usually uses sulfuric acidand hydrochloric acid to leach the arsenic-containing material and then separates thearsenic from the arsenic-containing material through solid–liquid separation [11]. Theroasting method mainly includes oxidation roasting, reduction roasting, and solidificationcalcination. The roasting arsenic removal process is used mainly to volatilize the arsenicin the form of arsenic trioxide at high temperatures to separate it from other valuablemetals [12,13]. The As(III) in the solid waste is converted to As(V) into the liquid phaseand then removed by precipitation, adsorption, or other methods [14–17]. Most of thearsenic in ARCD exists as As(III). Converting As(III) to As(V) can reduce its toxicity. Thestandard methods for oxidizing As(III) to As(V) include H2O2 oxidation, O3 oxidation,and others [18–21]. However, the oxidation of these oxidants may lead to the formation oftoxic by-products, and the cost is relatively high. The electrochemical advanced oxidationtechnology is characterized by the generation of H2O2, hydroxyl radicals, active chlorinespecies with strong oxidizing ability, etc. [22]. The use of electrochemical advanced oxida-tion technology can be even more efficient and convenient for the oxidation of As(III) toAs(V) [23–25].The electrochemical advanced oxidation method mainly generates H2O2 in situ;namely, when the oxygen passes into the cathode, the H2O2 is in-situ generated throughthe two-electron oxygen reduction reaction (ORR) [26]. The H2O2 generated in situ at thecathode can reduce the transportation and storage costs of H2O2 during the reaction pro-cess, and the oxidation efficiency may increase [26]. Advanced electrochemical oxidationhas the advantages of high efficiency, green environmental protection, and controllableprocess of oxidation method. This approach uses different electrode materials to generateoxidants. Compared with oxidation methods such as ozone oxidation, ultraviolet oxidation,and microwave oxidation catalysis, the electrochemical advanced oxidation method hasthe advantages of rapid reaction and simple operation and can generate active oxygengroups in situ for material oxidation. At present, the most widely used cathode materialsare carbonaceous due to their high stability, corrosion resistance, and high conductivity.Carbonaceous materials mainly use graphite, carbon nanotubes, and carbon felt (CF).The carbon felt electrode has a porous structure conducive to the transmission of oxygen,causing higher in situ generation of H2O2 [27,28].The modification of carbon felt can increase the electrochemical activity of the elec-trode and promote the generation of H2O2 by introducing active oxygen-containing groupson the surface. Treated with KOH firstly, then calcined at a high temperature, the oxygen-containing functional groups, the specific surface area, and the microporous structure ofthe carbon felt will be increased [29]. The surface of carbon felt is doped with nonmetallicelements, and the electrochemical performance of the electrode can be enhanced by dopingwith nonmetallic elements such as N, P, and S. After doping these nonmetal elements,the number of H2O2 produced can be increased, the secondary pollution in the electro-chemical reaction can be reduced, and the pH range of electrochemical experiments canbe expanded [30,31]. Metal materials have a strong electrical conductivity and catalyticactivity, so the electrical conductivity and electrochemical performance of the electrodeare enhanced by introducing metal ions. By introducing Fe, Cu, and other elements, theelectrode gains higher electrochemical performance [32].The carbon felt is modified by carbon black and PTFE [2]. Under the action of carbonblack, the conductivity of the carbon felt electrode, the active sites of the reaction, and theelectrochemical performance of the carbon felt cathode would be improved. The carbon feltelectrode was characterized and analyzed using a scanning electron microscope, nitrogenadsorption–desorption test, and water contact angle. The modified carbon felt electrodewas used to remove arsenic from the ARCD by electrochemical advanced oxidation method.Minerals 2021, 11, 1311 3 of 142. Experimental Procedures2.1. MaterialsThe ARCD collected from Western Mining Group Co., Ltd. (Xining, China) was driedat 60 ◦C for 24 h, then it was crushed, ground, and sieved. The particle size of −74~+48 µmwas selected as the experimental material. All chemicals used in this experiment were ofanalytical grade. The experimental water had high purity. The experimental carbon felt ofInner Mongolia Wanxing Carbon Co., Ltd., the carbon black of the US CABOT Cabot BlackVXC-72, and the PTFE of Daikin Fluorochemicals (China) Co., Ltd. (Changshu, China)were used in the experiment.2.2. MethodsThe carbon felt was ultrasonically cleaned in a mixed solution of acetone, ethanol, andultrapure water for 30 min and then dried in a vacuum drying oven at 60 ◦C for 24 h. Thecarbon felt sample was labeled as the original carbon felt. A certain proportion of PTFEemulsion (60%) and carbon black was mixed with 30 mL of ultrapure water and 1 mL ofn-butanol and then ultrasonically mixed for 10 min. The carbon felt was immersed in themixture and ultrasonicated for 30 min. The ultrasonic carbon felt was dried in a vacuumfor 24 h. The dried carbon felt was calcined at 360 ◦C for 1 h. The calcined carbon felt waslabeled as modified carbon felt.The experiment was carried out in a 250 mL undivided three-electrode cell at 25 ◦C,and the temperature was controlled by an electrochemical workstation (PARSTAT 4000A).The original carbon felt and the modified carbon felt were used as the working electrode,the platinum sheet was used as the counter electrode, and the mercury-oxide mercuryelectrode was used as the reference electrode. The electrolyte was the NaOH solution witha concentration of 75 g/L. Before electrochemical experiments, oxygen was pumped intothe solution for 30 min to saturate the oxygen in the electrolyte. The ARCD was addedto the electrolyte at a concentration of 500 mg/L. The arsenic removal experiment wasconducted at the condition of original carbon felt and modified carbon felt.The arsenic in the leaching solution was removed by calcium salt, and Ca(OH)2 wasadded to an exact amount of 100 mL arsenic leaching solution. The mixed solution wasplaced in a constant temperature water bath at 65 ◦C for 2 h with the rotation speed of300 r/min and then filtered by a water circulating vacuum pump. The filtrate was added0.075 mol/L aluminum chloride as a flocculant in a constant temperature water bath forfurther arsenic removal [33,34]. The filtrate was stirred at a speed of 120 r/min for 5 minand then stirred at a speed of 80 r/min for 15 min. After the mixture was complete, thefiltering operation was performed. Finally, the obtained final filtrate can be recycled for theleaching process after adjusting the concentration, and the final residue can be recycled forthe copper smelting process.2.3. Characterization and AnalysisThe phases of ARCD were determined by X-ray diffraction (XRD, PANalytical Empyrean,Almelo, The Netherlands), and the chemical composition was determined by inductivelycoupled optical emission spectrometry (ICP-OES, PerkinElmer Optima 7300 V, Richmond,CA, USA). The carbon felt and modified carbon felt were characterized by a scanningelectron microscope (SEM, FEI Quanta 250, Hillsboro, OH, USA) equipped with energydispersive X-ray spectrometry (EDS, EDAX Genesis, Richmond, CA, USA). Further, itsmicroscopic morphology, the specific surface area, and pore size distribution were mea-sured by the BET with N2 adsorption and desorption analyzer. The hydrophilicity andhydrophobicity of the electrode surface were measured by a contact angle meter (Theta,Biolin, Espoo, Finland), and the pH value of the electrolyte was obtained using a pH meter.The concentration of H2O2 generated in the electrolyte was measured with a double-beam UV-visible spectrophotometer using a potassium titanium oxalate developer. Electronspin resonance spectroscopy (ESR, Bruker EMX12, Karlsruhe, Germany) was used toMinerals 2021, 11, 1311 4 of 14measure the ·OH content in the electrolyte, and DMPO was added as a quencher for OHfor comparison experiments.The leaching efficiencies of the target metal were obtained as follows:η =CAs(solution)·VCAs(solid)·M× 100% (1)where CAs(solution) represents the As content in the solution, V represents the electrolytevolume, CAs(solid) corresponds to the arsenic content in the ARCD, and M is the content ofthe ARCD in the electrolyte.3. Results and Discussion3.1. Characterization of the Spent Catalyst of the ARCDThe ARCD contains a variety of metal elements. As shown in Table 1, the contentof As2O3 reaches 8.78%, and the content of CuO, ZnO, PbO, and Bi2O3 is 15.31%, 1.95%,17.7%, and 6.62%, respectively. The XRD shows that the main phases in the smelting dustare As2O5, As2O3, and CuAsS. The XPS analysis shows that the primary forms of arsenicin the material are As(III) and As(V). The contents of As(V) and As(III) in the ARCD are59% and 41%, respectively. As shown in Figure 1d, the copper in the ARCD mainly existsin Cu(I) and Cu(II). The ARCD was analyzed by SEM-EDS, which shows that As coexistedwith Cu, Pb, and Zn.Table 1. The main compositions of the ARCD (* Data from XRF and other data from ICP, wt.%).Element As2O3 CuO ZnO PbO Bi2O3 Fe2O3 * SO3 OthersContent 8.78 15.31 1.95 17.7 6.62 1.63 20.73 27.38Minerals 2021, 11, x FOR PEER REVIEW 4 of 15   used to measure the ·OH content in the electrolyte, and DMPO was added as a quencher for OH for comparison experiments. The leaching efficiencies of the target metal were obtained as follows: s(solution)s( )100%AA solidC VC M  (1)where CAs (solution) represents the As content in the solution, V represents the electrolyte volume, CAs (Solid) corresponds to the arsenic content in the ARCD, and M is the content of the ARCD in the electrolyte. 3. Results nd Discussion 3.1. Characterization of the Spent Catalyst of the ARCD The ARCD contains a variety of metal elements. As shown in Table 1, the content of As2O3 reaches 8.78%, and the content of CuO, ZnO, PbO, and Bi2O3 is 15.31%, 1.95%, 17.7%, and 6.62%, respectively. The XRD shows that the main phases in the smelting dust are As2O5, As2O3, and CuAsS. The XPS analysis shows that the primary forms of arsenic in the material are As(Ⅲ) and As(Ⅴ). The contents of As(Ⅴ) and As(Ⅲ) in the ARCD are 59% and 41%, respectively. As shown in Figure 1d, the copper in the ARCD mainly exists in Cu(Ⅰ) and Cu(Ⅱ). The ARCD was analyzed by SEM-EDS, which shows that As coexisted with Cu, Pb, and Zn. Table 1. The main compositions of the ARCD (* Data from XRF and other data from ICP, wt.%). Element As2O3 CuO ZnO PbO Bi2O3 Fe2O3 * SO3 others Content 8.78 15.31 1.95 17.7 6.62 1.63 20.73 27.38  Figure 1. Cont.Minerals 2021, 11, 1311 5 of 14Minerals 2021, 11, x FOR PEER REVIEW 5 of 15    Figure 1. (a) XRD pattern of ARCD, (b) XPS spectra of ARCD, full spectrum, (c) As 3d, (d) Cu 2p3/2, (e) SEM-EDS of ARCD. 3.2. Alkaline Leaching Mechanisms The arsenic in the ARCD was converted to arsenate by the NaOH leaching process and entered the liquid phase. The leaching process was mainly conducted according to the following (2)–(4) reactions. The thermodynamic analysis of the reaction shows that the reaction can be carried out at 10–100 °C. As can be seen from Figure 2, reactions (2)–(4) can be carried out under the experimental conditions. As2O3 + 2NaOH = 2NaAsO2 + H2O (2)As2O3 + 6NaOH + O2 = 2Na3AsO4 + 3H2O (3)As2O5 + 6NaOH = 2Na3AsO4 + 3H2O (4) Figure 2. The plots of ΔrG°~T for the Equations (2)–(4) (Drawn by HSC 5.0 chemistry software, Outokumpu Research, Finland). The Eh-pH diagram was used to analyze the thermodynamics of the elements in the leaching process of ARCD. Figure 3 shows the Eh—pH diagrams of As, Pb, Zn, and Cu, respectively. Arsenic exists in the form of As(s), HAsO2(aq), H2AsO3−, HasO42−, and AsO43− under alkaline conditions. At higher pH and higher potential, H2AsO3− oxidized to AsO43− which shows that the As in the ARCD will enter the solution. Pb exists in the form of Pb2+, Figure 1. (a) XRD pattern of ARCD, (b) XPS spectra of ARCD, full spectrum, (c) As 3d, (d) Cu 2p3/2,(e) SEM-EDS of ARCD.3.2. Alkaline Leaching MechanismsThe arsenic in the ARCD was converted to arsenate by the NaOH leaching processand entered the liquid phase. The leaching process was mainly conducted according tothe following (2)–(4) reactions. The thermodynamic analysis of the reaction shows that thereaction can be carried out at 10–100 ◦C. As can be seen from Figure 2, reactions (2)–(4) canbe carried out under the experimental conditions.As2O3 + 2NaOH = 2NaAsO2 + H2O (2)As2O3 + 6NaOH + O2 = 2Na3AsO4 + 3H2O (3)As2O5 + 6NaOH = 2Na3AsO4 + 3H2O (4)Minerals 2021, 1, x FOR PEER REVIEW  of 14    Figure 1. (a) XRD pattern of ARCD, (b) XPS spectra of ARCD, full spectrum, (c) As 3d, (d) Cu 2p3/2, (e) SEM-EDS of ARCD. 3.2. Alkaline Leaching Mechanisms The arsenic in the ARCD was converted to arsenate by the NaOH leaching process and entered the liquid phase. The leaching process was mainly conducted according to the following (2)–(4) reactions. The thermodynamic analysis of the reaction shows that the reaction can be carried out at 10–100 °C. As can be seen from Figure 2, reactions (2)–(4) can be carried out under the experimental conditions. As2O3 + 2NaOH = 2NaAsO2 + H2O (2)As2O3 + 6NaOH + O2 = 2Na3AsO4 + 3H2O (3)As2O5 + 6NaOH = 2Na3AsO4 + 3H2O (4) Figure 2. The plots of ΔrG°~T for the Equations (2)–(4) (Drawn by HSC 5.0 chemistry software, Outokumpu Research, Finland). The Eh-pH diagram was used to analyze the thermodynamics of the elements in the leaching process of ARCD. Figure 3 shows the Eh—pH diagrams of As, Pb, Zn, and Cu, respectively. Arsenic exists in the form of As(s), HAsO2(aq), H2AsO3−, HasO42−, and AsO43− under alkaline conditions. At higher pH and higher potential, H2AsO3− oxidized to AsO43− which shows that the As in the ARCD will enter the solution. Pb exists in the form of Pb2+, Figure 2. The plots of ∆rG◦~T for the Equations (2)–(4) (Drawn by HSC 5.0 chemistry software,Outokum u Research, Finland).The Eh-pH diagram was used to analyze the thermodynamics of the elements in theleaching proces of ARCD. Figure 3 shows the Eh—pH diagrams of As, Pb, Zn, and Cu,respectively. i the form of As(s), HAsO2(aq), H2AsO3−, HasO42−, andAsO43− under alkaline conditions. At higher pH and higher potential, H2AsO3− oxidizedto AsO43− hich shows tha the As in the ARCD will en er the solution. Pb exists in theMinerals 2021, 11, 1311 6 of 14form of Pb2+, PbOH+, HPbO2−, and PbO(s) under alkaline conditions. Under higher pHconditions, Pb mainly exists in the form of HPbO2−, which means Pb can dissolve in thealkaline solution. With the change of pH, Zn varies in the form of Zn(OH)2, HZnO2−, andZnO22−, and Zn could dissolve in the solution when the alkaline strength increases.Minerals 2021, 11, x FOR PEER REVIEW 6 of 14   PbOH+, HPbO2−, and PbO(s) under alkaline conditions. Under higher pH conditions, Pb mainly exists in the form of HPbO2−, which means Pb can dissolve in the alkalin  solu-tion. With the change of pH, Zn varies in the form of Zn(OH)2, HZnO2−, a d ZnO22−, and Zn could dissolve in the solution when the alkali e strength increases.  Figure 3. (a) Eh—pH diagrams of As, (b) Pb, (c) Zn, and (d) Cu, respectively. 3.3. The Alkaline Leaching Process of the ARCD The alkaline leaching process was optimized to increase the leaching rate of arsenic. The leaching rate of arsenic was optimized via varying the temperature, NaOH concen-tration, and time to finally obtain the optimum conditions, as can be seen from Figure 4; NaOH of 75 g/L, the temperature of 70 °C, and leaching time of 90 min. The arsenic re-moval rate from ARCD of 94% was obtained. After alkaline leaching, the leaching residue was analyzed by XRD and XPS. Figure 5a,b shows that as the NaOH concentration increases, the peak intensity of arsenic de-creases until it finally disappears. At the same time, Cu(OH)2 phase was found in the slag, which indicates that Cu(OH)2 is formed after NaOH leaching and remains in the slag. The XPS shows that the arsenic in the ARCD exists in the form of trivalent and pentavalent. Figure 5d indicates that, after alkaline leaching, arsenic exists in the trivalent form in the leaching residue. This means that this part of trivalent arsenic needs to be removed by the oxidation leaching process. Figure 3. (a) Eh—pH diagrams of b, (c) Zn, and (d) Cu, respectively.3.3. The Alkaline Leaching Pr f the ARCDThe alkaline leaching process as optimized to increase the leaching rate of arsenic.The leaching rate of arsenic was optimized via varying the temperature, NaOH concen-tration, and time to finally obtain the optimum conditions, as can be seen from Figure 4;NaOH of 75 g/L, the temperature of 70 ◦C, and leaching time of 90 min. The arsenicremoval rate from ARCD of 94% was obtained.After alkaline leaching, the leaching residue was analyzed by XRD and XPS. Figure 5a,bshows that as the NaOH concentration increases, the peak intensity of arsenic decreasesuntil it finally disapp ars. At the same ime, Cu(OH)2 phase was found in the slag, whichindicates that Cu(OH)2 is formed after NaOH leaching and remains in the lag. TheXPS shows that the arsenic in the ARCD exists in the form of trivalent and pentavalent.Figure 5d indicates that, after alkaline leaching, arsenic exists in the trivalent form in theleaching residue. This means that this part of trivalent arsenic needs to be removed by theoxidation leaching process.Minerals 2021, 11, 1311 7 of 14Minerals 2021, 11, x FOR PEER REVIEW 7 of 15    Figure 4. (a) Effect of NaOH concentration on the leaching efficiencies (70 °C, 10:1 mL/g and 90 min), (b) experimental reaction duration (70 °C, 10:1 mL/g and 75 g/L) and (c) reaction temperature (75 g/L, 10:1 mL/g and 90 min). After alkaline leaching, the leaching residue was analyzed by XRD and XPS. Figure 5a,b shows that as the NaOH concentration increases, the peak intensity of arsenic de-creases until it finally disappears. At the same time, Cu(OH)2 phase was found in the slag, which indicates that Cu(OH)2 is formed after NaOH leaching and remains in the slag. The XPS shows that the arsenic in the ARCD exists in the form of trivalent and pentavalent. Figure 5d indicates that, after alkaline leaching, arsenic exists in the trivalent form in the leaching residue. This means that this part of trivalent arsenic needs to be removed by the oxidation leaching process. Figure 4. (a) Effect of NaOH concentration on the leaching efficiencies (70 ◦C, 10:1 mL/g and 90 min),(b) experimental reaction duration (70 ◦C, 10:1 mL/g and 75 g/L) and (c) reaction temperature(75 g/L, 10:1 mL/g and 90 min).Minerals 2021, 11, x FOR PEER REVIEW  7  of  14    Figure 4. (a) Effect of NaOH concentration on the leaching efficiencies  (70 °C, 10:1 mL/g and 90 min),  (b) experimental reaction duration (70 °C, 10:1 mL/g and 75 g/L) and (c) r action temperature (75 g/L, 10:1 mL/g and 90 min).  Figure 5. (a,b) XRD pattern, (c) XPS spectra, full spectrum, (d) As 3d of the alkaline leaching residue.    Figure 5. (a,b) XRD pattern, (c) XPS spectra, full spectrum, (d) As 3d of the alkaline leaching residue.3.4. Electrochemical Advanced Oxidation Treatment of ARCD3.4.1. Characteristics of the Cf and Modified Carbon FeltThe morphologies of carbon felt and modified carbon felt were obtained by SEManalysis. Figure 6a,b shows that the fibers on the surface of the original carbon felt (RCF)Minerals 2021, 11, 1311 8 of 14are smooth, and the surface of the pretreated carbon felt has no adhesion and impurities.As shown in Figure 6c,d, there are many particles on the surface of the carbon felt modifiedby carbon black and PTFE. The interconnection of these particles dramatically changes thesurface structure of the carbon felt electrode, increasing the surface area of the electrodeand the gas—liquid contact interface of the carbon felt electrode [2]. The modified carbonfelt may enhance the generation of H2O2 when used as a cathode. As shown in Figure 6c,d,after modification, the contact angle increased from 109.64◦ to 122.33◦, and the cathodeoverflow reduced [2].Figure 6g,h shows the N2 adsorption/desorption isotherm and pore size distributionof the original carbon felt and the modified carbon felt, respectively. Compared withthe original carbon felt, the modified carbon felt shows a larger specific surface area.The specific surface areas of the modified carbon felt CF—1:1, CF—1:3, and CF—1:5 are2.3978 m2/g, 3.7009 m2/g, and 6.9583 m2/g, respectively. As shown in Figure 6g,h, the totalpore volume is 0.015595 cm3/g, 0.066591 cm3/g, and 0.135 cm3/g, respectively. Accordingto the pore size distribution, the modified carbon felt has a smaller pore size, and thenanopore structure on the electrode surface is increased.3.4.2. Performance of the Modified CF ElectrodeThe cyclic voltammetry (CV) curves of the original carbon felt and modified carbonfelt is shown in Figure 7. After the CV test, it can be seen that an oxygen reduction peakappears at −0.5 V, indicating an oxygen reduction reaction in Figure 7a. Compared withthe original carbon felt, the modified carbon felt has a greater peak current density. Withthe increase of the scanning speed, the oxidation peak moves in the direction of positivevoltage, and the reduction peak moves in the direction of negative voltage (Figure 7b).The oxygen reduction peak current of the modified carbon felt electrode was significantlyimproved, meaning the improved electrochemical performance.Minerals 2021, 11, x FOR PEER REVIEW  9  of  15     Figure 6. Morphological changes of modified CF are as follows: (a) low magnification and (b) high magnification SEM images of RCF, (c) low‐power and (d) high—power SEM images of modified CF, (e) Contact angle of the original CF electrode, (f) Contact angle of the modified CF electrode, (g) pore size distribution, and (h) N2 adsorption/desorption isotherms. 3.4.2. Performance of the Modified CF Electrode The cyclic voltammetry (CV) curves of the original carbon felt and modified carbon felt is shown in Figure 7. After the CV test, it can be seen that an oxygen reduction peak Figure 6. Cont.Minerals 2021, 11, 1311 9 of 14Minerals 2021, 11, x FOR PEER REVIEW  9  of  15     Figure 6. Morphological changes of modified CF are as follows: (a) low magnification and (b) high magnification SEM images of RCF, (c) low‐power and (d) high—power SEM images of modified CF, (e) Contact angle of the original CF electrode, (f) Contact angle of the modified CF electrode, (g) pore size distribution, and (h) N2 adsorption/desorption isotherms. 3.4.2. Performance of the Modified CF Electrode The cyclic voltammetry (CV) curves of the original carbon felt and modified carbon felt is shown in Figure 7. After the CV test, it can be seen that an oxygen reduction peak Figure 6. orphological changes of odified CF are as follows: (a) low agnification and (b) highagnification SEM images of RCF, (c) low-power and ( ) hig —power SEM images of modified CF,(e) Contact angle of the original CF electrode, (f) Contact angle of the modified CF electrode, (g) poresize distribution, and (h) N2 adsorption/desorpti n is therms.Minerals 2021, 11, x FOR PEER REVIEW 9 of 14    Figure 6. Morphological changes of modified CF are as follows: (a) low magnification and (b) high magnification SEM images of RCF, (c) low-power and (d) high—power SEM images of modified CF, (e) Contact angle of the original CF electrode, (f) Contact angle of the modified CF electrode, (g) pore size distribution, and (h) N2 adsorption/desorption isotherms. 3.4.2. Performance of the Modified CF Electrode The cyclic voltammetry (CV) curves of the original carbon felt and modified carbon felt is shown in Figure 7. After the CV test, it can be seen that an oxygen reduction peak appears at −0.5 V, indicating an oxygen reduction reaction in Figure 7a. Compared with the original carbon felt, the modifie  carbon felt has a greater peak current density. Wit  the i crease of the scanning speed, the oxidation peak moves in the direction of positive voltage, a d the reductio  peak moves in the direction of negative voltage (Figure 7b). The oxygen reduction peak curre t of th  modified ca bon felt electrode was signifi-cantly improved, meaning the improved electrochemical performance.  Figure 7. (a) CV diagram of RCF and modified CF, (b) changes of CF—1:5 electrode with scavenging speed electrode CV. 3.4.3. Influencing Factors of Hydrogen Peroxide Generation The carbon felt cathode was modified by adjusting CB:PTFE ratio. The influence of the electrodes on the generation of H2O2 was carried out through different modified Fi re 7. (a) diagra of F and odifie , ( ) c a es f 1:5 electr e it sca e i s ee electr e .3.4.3. Influencing Factors of Hydrogen Peroxide GenerationThe carbon felt cathode was modified by adjusting CB:PTFE ratio. The influenceof the electrodes on the generation of H2O2 was carried out through different odifiedelectrodes. As the addition amount continued to increase, the oxygen reduction currentand the current density both gradually increased, resulting in the increase in generatedH2O2 content. The content of H2O2 generated at CB:PTFE of 1:5 reached 148.048 mg/L. Asshown in Figure 8a, the amount of H2O2 produced by CF—1:5 is almost 2.6 times that ofunmodified carbon felt.Minerals 2021, 11, 1311 10 of 14Minerals 2021, 11, x FOR PEER REVIEW 10 of 14   electrodes. As the addition amount continued to increase, the oxygen reduction current and the current density both gradually increased, resulting in the increase in generated H2O2 content. The content of H2O2 generated at CB:PTFE of 1:5 reached 148.048 mg/L. As shown in Figure 8a, the amount of H2O2 produced by CF—1:5 is almost 2.6 times that of unmodified carbon felt. The increasing trend of H2O2 production was carried out by adjusting the oxygen flow rate, and the results were given in Figure 8b. As the oxygen flow rate increases from 0.2 L/min to 0.6 L/min, the generated H2O2 also increases, and 0.4 L/min was selected as the optimized oxygen flow rate.  Figure 8. (a) Content of H2O2 in situ generated by different electrodes, (b) the change of H2O2 gen-erated by CF—1:5 with different oxygen flow rate, (c) the change of arsenic removal rate from the ACRD by different electrodes, (d) and the difference between alkaline leaching and electrochemical advanced oxidation leaching. 3.4.4. Performance of the Modified Electrode in Detoxification of ARCD A two—electrode system is adopted in the detoxification experiments; a carbon felt electrode, a modified electrode (working electrode), and a graphite rod electrode (coun-ter electrode). The electrochemical oxidation experiment was carried out at the voltage of 1 V. The experimental conditions were as follows: 150 mL of 75 g/L NaOH, solid to liquid ratio of 500 mg/L, experiment temperature of 25 °C, experiment time of 90 min, and the oxygen flow rate of 0.4 L/min. After 90 min of the electrolysis experiment, the removal rate of arsenic reached 98.04%. By comparing pretreated carbon felt and modi-fied carbon felt, the removal rate of arsenic increased from 84.5% to 98.04% at CF of 1:5 (shown in Figure 8c). 3.5. Electrochemical Oxidation Leaching Mechanisms The ESR tests were applied in the electrochemical oxidation leaching process to de-tect the reactive oxygen species, and DMPO was used as a trapping reagent. As shown in Figure 9, when 0.02 mol/L DMPO was added to the electrolyte, significant active ·OH signals were detected under the condition of the oxygen flow rate of 0.4 L/min. Due to the Figure 8. ( ) t t f 2O2 in situ generated by diff rent electrodes, (b) the change of H2O2generated by CF—1:5 with different oxygen flow rate, (c) the change of arsenic removal rate from theACRD by different electrodes, (d) and the difference between alkaline leaching and electrochemicaladvanced oxidation leaching.The increasing trend of H2O2 production was carried out by adjusting the oxygenflow rate, and the result were given in Figure 8b. As the oxygen flow rate increases from0.2 L/min to 0.6 L/min, the generated H2O2 also incre ses, and 0.4 L/min was selected asthe optimized oxygen flow rate.3.4.4. Performance of the Modified Electrode in Detoxification of ARCDA two—electr de system is adopted in the detoxification experiments; a carbon feltel ctrode, m dified electrode (working electrode), a d a graphite rod electrode (counterelectrode). The electrochemical oxidation experiment was carried out at the voltage of 1 V.The experimental conditions were as follows: 150 mL of 75 g/L NaOH, solid to liquid ratioof 500 mg/L, experiment temperature of 25 ◦C, experiment time of 90 min, and the oxygenflow rate f 0.4 L/min. After 90 min of the electrolysis experiment, the removal rate ofarsenic reached 98.04%. By comparing pretreated carbon felt and modified carbon felt, theremoval rate of arsenic increased from 84.5% to 98.04% at CF of 1:5 (shown in Figure 8c).3.5. Electrochemical Oxidation Leaching MechanismsThe ESR tests were applied in the electrochemical oxidation leaching process to detectthe reactive oxygen species, and DMPO was used as a trapping reagent. As shown inFigure 9, when 0.02 mol/L DMPO was added to the electrolyte, significant active ·OHsignals were detected under the condition of the oxygen flow rate of 0.4 L/min. Due to thepresence of OH, the leaching rate of arsenic can be further improved with respect to alkalineleaching. Furthermore, the arsenic leaching process can be considered as an advancedoxidation process (AOPs). When oxygen is not supplied, the generation of ·OH cannot bedetected, further showing that only the transition metals can catalyze the conversion ofHO2− into ·OH.Minerals 2021, 11, 1311 11 of 14Minerals 2021, 11, x FOR PEER REVIEW 11 of 14   presence of OH, the leaching rate of arsenic can be further improved with respect to al-kaline leaching. Furthermore, the arsenic leaching process can be considered as an ad-vanced oxidation process (AOPs). When oxygen is not supplied, the generation of ·OH cannot be detected, further showing that only the transition metals can catalyze the conversion of HO2- into ·OH. Figure 10 shows the schematic diagram of the experimental setup and arsenic oxi-dation by reactive oxygen species. A large amount of HO2− with oxidizing properties was in situ generated in the alkaline solution system through the oxidation reaction (5) at the cathode [35–37]. In addition, Cu(Ⅰ) was oxidized by HO2− to Cu(Ⅱ), triggering the pro-duction of ·OH, followed by Cu(Ⅱ) conversion into copper hydroxide by OH− into the final residue, also detected by XRD (Figure 5), and no Cu(Ⅰ) was detected in the final residue. Therefore, the electrochemical AOPs can be considered an iron-free Fenton-like reaction and can be described by reaction (6) [38,39]. The OH species have very high ac-tivity and can oxidize As(Ⅲ) through reaction (7). The arsenic in the ACRD is oxidized into the liquid phase, and the removal rate of arsenic is further improved compared with alkaline leaching; - - -2 2 2O +H O+2e HO +OH→  (5)+ 2+2Cu +HO Cu OH− → + ⋅  (6)3- 3-3 4 2AsO + OH AsO +H O⋅ →  (7) Figure 9. ESR tests of ·OH during the electrochemical oxidation leaching process. 3460 3480 3500 3520 3540 3560Intensity (CPS)Magnetic field (mT) 0 O2 Flow rate 0.4 L/min O2 Flow rateFigure 9. ESR tests of ·OH during the electrochemical oxidation leaching process.Figure 10 shows the schematic diagram of the experimental setup and arsenic oxi-dation by reactive oxygen species. A large amount of HO2− with oxidizing propertieswas in situ generated in the alkaline solution system through the oxidation reaction (5)at the cathode [35–37]. In addition, Cu(I) was oxidized by HO2− to Cu(II), triggering theproduction of ·OH, followed by Cu(II) conversion into copper hydroxide by OH− intothe final residue, also detected by XRD (Figure 5), and no Cu(I) was detected in the finalresidue. Therefore, the electrochemical AOPs can be considered an iron-free Fenton-likereaction and can be described by reaction (6) [38,39]. The OH species have very highactivity and can oxidize As(III) through reaction (7). The arsenic in the ACRD is oxidizedinto the liquid phase, and the removal rate of arsenic is further improved compared withalkaline leaching;O2+H2O + 2e− → HO2−+OH− (5)Cu++HO2− → Cu2+ + ·OH (6)AsO33− + ·OH→ AsO43−+H2O (7)Minerals 2021, 11, x FOR PEER REVIEW 12 of 14    Figure 10. (a) Schematic diagram of the experimental setup and (b) Schematic diagram of the oxi-dation process of low arsenic by reactive oxygen species. 3.6. The Difference between Alkaline Leaching Process and Electrochemical Advanced Oxidation Leaching Process The purpose of alkaline leaching of arsenic-containing waste is to transfer arsenic from solid waste to the leachate. However, the alkaline leaching process has disad-vantages such as a large amount of waste liquid, easy generation of H2S gas, and low arsenic leaching efficiency. The oxidation treatment of the waste residue containing low valent arsenic is beneficial to transforming the arsenic from a stable form to an unstable form (e.g., a water-soluble state, an exchange state, and a carbonate bond state), thereby increasing the leaching rate of arsenic. The electrochemical advanced oxidation method uses reactive oxygen groups with strong oxidation properties generated in situ to oxidize low arsenic in the ARCD to the leaching solution, which improves the removal rate of arsenic. The alkaline leaching process has the problem of incomplete removal of arsenic. The use of an electrochemical advanced oxidation process can increase the removal rate of arsenic without secondary pollution. The alkaline leaching process cannot oxidize the As(Ⅲ). The advanced oxidation technology is used to oxidize the low valence arsenic in the solid waste and transfer it into the leaching solution, increasing the arsenic removal rate. As shown in Figure 8d, the removal rate of arsenic is only 80.69%, but it reaches 98.04% through advanced electrochemical oxidation. 4. Conclusions The use of an electrochemical advanced oxidation leaching process can effectively remove arsenic from arsenic-containing solid waste. By adding carbon black and PTFE to improve the electrochemical performance of the carbon felt, the specific surface area of the carbon felt, the electrochemical performance, and the content of in suit generated H2O2 are all increased. The arsenic in the ARCD can be transferred to the solution by the electrochemical AOPs; therefore, the high-efficiency detoxification of the ARCD is real-ized. Under the condition of the voltage of 1.0 V, the electrolysis duration of 90 min, the oxygen flow rate of 0.4 L/min, and the CF of 1:5, the removal rate of arsenic reaches 98.4%. In addition, the electrochemical oxidation leaching method avoids secondary pollution and can be considered a clean and efficient arsenic removal method. Author Contributions: Data curation, B.L.; writing—original draft preparation, M.L.; writ-ing—review and editing, J.Y.; supervision, D.D. and H.D.; project administration, Y.C.; funding acquisition, G.H. All authors have read and agreed to the published version of the manuscript. Funding: This work was funded by [the National Natural Science Foundation of China] (Grant No. 52004252), [the National Key Research and Development Program of China] (Grant No. 2018YFC1901601), [the Postdoctoral Research Grant in Henan Province] (Grant No. 201902016), [Henan Province Key R&D and Promotion Special (Scientific and Technical) Project] (Grant Figure 10. (a) Schematic diagram of the experimental setup and (b) Schematic diagram of theoxidation process of low arsenic by reactive oxygen species.Minerals 2021, 11, 1311 12 of 143.6. The Difference between Alkaline Leaching Process and Electrochemical Advanced OxidationLeaching ProcessThe purpose of alkaline leaching of arsenic-containing waste is to transfer arsenic fromsolid waste to the leachate. However, the alkaline leaching process has disadvantages suchas a large amount of waste liquid, easy generation of H2S gas, and low arsenic leachingefficiency. The oxidation treatment of the waste residue containing low valent arsenicis beneficial to transforming the arsenic from a stable form to an unstable form (e.g., awater-soluble state, an exchange state, and a carbonate bond state), thereby increasing theleaching rate of arsenic. The electrochemical advanced oxidation method uses reactiveoxygen groups with strong oxidation properties generated in situ to oxidize low arsenicin the ARCD to the leaching solution, which improves the removal rate of arsenic. Thealkaline leaching process has the problem of incomplete removal of arsenic. The use ofan electrochemical advanced oxidation process can increase the removal rate of arsenicwithout secondary pollution. The alkaline leaching process cannot oxidize the As(III). Theadvanced oxidation technology is used to oxidize the low valence arsenic in the solid wasteand transfer it into the leaching solution, increasing the arsenic removal rate. As shownin Figure 8d, the removal rate of arsenic is only 80.69%, but it reaches 98.04% throughadvanced electrochemical oxidation.4. ConclusionsThe use of an electrochemical advanced oxidation leaching process can effectivelyremove arsenic from arsenic-containing solid waste. By adding carbon black and PTFEto improve the electrochemical performance of the carbon felt, the specific surface areaof the carbon felt, the electrochemical performance, and the content of in suit generatedH2O2 are all increased. The arsenic in the ARCD can be transferred to the solution by theelectrochemical AOPs; therefore, the high-efficiency detoxification of the ARCD is realized.Under the condition of the voltage of 1.0 V, the electrolysis duration of 90 min, the oxygenflow rate of 0.4 L/min, and the CF of 1:5, the removal rate of arsenic reaches 98.4%. Inaddition, the electrochemical oxidation leaching method avoids secondary pollution andcan be considered a clean and efficient arsenic removal method.Author Contributions: Data curation, B.L.; writing—original draft preparation, M.L.; writing—review and editing, J.Y.; supervision, D.D. and H.D.; project administration, Y.C.; funding acquisition,G.H. All authors have read and agreed to the published version of the manuscript.Funding: This work was funded by [the National Natural Science Foundation of China] (GrantNo. 52004252), [the National Key Research and Development Program of China] (Grant No.2018YFC1901601), [the Postdoctoral Research Grant in Henan Province] (Grant No. 201902016),[Henan Province Key R&D and Promotion Special (Scientific and Technical) Project] (Grant No.212102310600) and [the Key Research Project of Henan Province Colleges and Universities] (GrantNo. 20A440011).Institutional Review Board Statement: Not applicable.Informed Consent Statement: Not applicable.Data Availability Statement: Not applicable.Acknowledgments: This work was financially supported by the National Natural Science Foun-dation of China (Grant No. 52004252), the National Key Research and Development Program ofChina (Grant No. 2018YFC1901601), the Postdoctoral Research Grant in Henan Province (Grant No.201902016), Henan Province Key R&D and Promotion Special (Scientific and Technical) Project (GrantNo. 212102310600) and the Key Research Project of Henan Province Colleges and Universities (GrantNo. 20A440011).Conflicts of Interest: The authors declare no conflict of interest.Minerals 2021, 11, 1311 13 of 14References1. 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