
==== Front
ACS Omega
ACS Omega
ao
acsodf
ACS Omega
2470-1343
American Chemical Society

10.1021/acsomega.4c03333
Mini-Review
Novel Strategy for Efficient Recovery of CuO-Based Multiple-Metals from Copper Smelter Dust toward CO2 Electrocatalytic Reduction
Li Ken †‡§
Li Qingzhu †‡§
https://orcid.org/0000-0001-5457-6732
Zhao Feiping †‡§
Chen Qin ∥
https://orcid.org/0000-0003-0190-1663
Fu Junwei ∥
Min Xiaobo †‡§
Li Yun †‡§
https://orcid.org/0000-0003-2992-3442
Xiang Kaisong †‡§
Wang Qingwei †‡§
Shi Meiqing †‡§
https://orcid.org/0000-0001-9812-2150
Yan Xu *†‡§
https://orcid.org/0000-0001-8641-9774
Chai Liyuan †‡§
† School of Metallurgy and Environment, Central South University, Changsha 410083, China
‡ State Key Laboratory of Advanced Metallurgy for Nonferrous Metals, Changsha 410083, China
§ Chinese National Engineering Research Center for Control & Treatment of Heavy Metal Pollution, Changsha 410083, China
∥ Hunan Joint International Research Center for Carbon Dioxide Resource Utilization, School of Physics, Central South University, Changsha 410083, China
* Email: yanxu1202@csu.edu.cn.
31 08 2024
17 09 2024
9 37 3831638326
17 04 2024
31 07 2024
28 07 2024
© 2024 The Authors. Published by American Chemical Society
2024
The Authors
https://creativecommons.org/licenses/by-nc-nd/4.0/ Permits non-commercial access and re-use, provided that author attribution and integrity are maintained; but does not permit creation of adaptations or other derivative works (https://creativecommons.org/licenses/by-nc-nd/4.0/).

Copper smelter dust, a typical hazardous waste that is abundant in valuable heavy metals, holds the potential to be regarded as a promising resource. This study introduces a new approach that integrates chlorination roasting and cascade condensation to efficiently recover heavy metals from copper smelter dust. The findings demonstrate the successful separation of heavy metals (Cu, Pb, and Zn) as chlorides at nearly 100% efficiency while also effectively converting trivalent arsenic (As(III)) into pentavalent arsenic (As(V)) and immobilizing it in the roasting residues, thereby reducing environmental risk. Through the utilization of thermogravimetric mass spectrum analysis and thermodynamic equilibrium calculations, the chlorination process for heavy metals was investigated, revealing both direct and indirect chlorination processes. Additionally, the study resulted in the development of a CuO-based multiple-metals electrocatalyst from the oxidized roasting-recovered heavy metal chlorides, exhibiting significantly enhanced catalytic activity and faradaic efficiency for the electroreduction of CO2 into CO and CH4 compared to pure CuO electrocatalyst under similar electrocatalytic conditions. Overall, this work presents a sustainable and scalable method and new insights for addressing environmental risks while repurposing copper smelter dust.

National Natural Science Foundation of China 10.13039/501100001809 52022111 Natural Science Foundation of Hunan Province 10.13039/501100004735 2021JC0001 National Natural Science Foundation of China 10.13039/501100001809 52121004 document-id-old-9ao4c03333
document-id-new-14ao4c03333
ccc-price
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pmcIntroduction

Over the past decades, the treatment of hazardous waste containing high levels of heavy metals has garnered significant attention.1−4 The existence of various heavy metals in hazardous waste also presents an opportunity for value utilization. Copper smelting dust (CSD), a major byproduct of copper smelting, always contains a high concentration of heavy metals and is regarded as an environmentally hazardous waste.5−7 Furthermore, it easily enters the human respiration system and causes disease because of its ultrafine size.8,9 Therefore, the massive accumulation of CSD not only causes some potential environmental pollution but also results in the enormous waste of various metal resources.10

At present, CSD is mostly recycled back into the smelting process, leading to issues such as the accumulation of toxic elements, e.g., arsenic (As), lead (Pb), and zinc (Zn), increased energy requirements, and a decrease in the quality of refined copper.11−14 Furthermore, researchers have explored various methods, such as oxidative roasting,15 pressure leaching,16 bioleaching11 and supergravity12 methods, to recover valuable metals from CSD. Arsenic can be volatilized in the form of As4O6, whose purity reached 97% under a weak oxidizing atmosphere when heating the tube furnace to 800–1200 °C.15 However, other heavy metals, such as copper, lead, and tin (Sn), did not volatile from CSD and remained in the slag, which caused environmental pollution and waste of metal resources. Copper, cadmium and zinc can be efficiently leached from CSD under the conditions of high pressure and a large solid–liquid ratio.16 However, a significant amount of acid waste liquid production resulted in an increase in the total treatment cost. Gao et al. studied the recovery of crown zinc and metallic copper from CSD by evaporation, condensation and supergravity separation.12 However, the residue contains fairly high levels of arsenic. In other words, these approaches face challenges, including high energy consumption, low efficiency, and poor selectivity. Hence, it is of significance to develop novel methods for managing these heavy metals in the CSD and converting them into valuable products.

Chlorination roasting has shown promise in removing heavy metals from various solid wastes, especially fly ash.17,18 This is because heavy metals readily combine with highly chemically active chlorine under certain conditions by adding chlorinating agents, generating heavy metal chlorides with high volatility and low melting points. As a result, these heavy metal chlorides volatilize from incineration residues during thermal treatment.19−21 Nowak22 mixes CaCl2 with waste incineration fly ash (MSWF) and roasts at 1000 °C, which can remove more than 90% of Cd 85% of Pb, 60% of Cu and 80% of Zn in the MSWF. Furthermore, HCl is also an effective chlorination agent to remove Cu, Pb, Zn and Cd in sewage sludge ash under high-temperature conditions.23 However, the heavy metal and sulfur contents in those fly ash are much lower than those in CSD, which will obviously affect the chlorination process of heavy metals in CSD.24,25 Moreover, the high-value utilization of various volatilized heavy metal chlorides has been overlooked.

Recently, waste from the metallurgical industry has been used to prepare electrocatalysts for CO2 conversion, especially residue from the copper metallurgical industry.26−28 This is because copper is a unique metal with a moderate binding energy of crucial reaction intermediates. An example of this is bimetallic copper-based electrocatalysts,29−32 which can generate both simple (i.e., CO and CH4) and complex (i.e., C2+ hydrocarbons and oxygenates) products through CO2 electroreduction.33,34 The synergistic effects of copper with other metals have been found to greatly enhance selectivity toward specific products such as CO, CH4, and C2H4.35−37 Yang et al.38 report that the preparation of bimetallic copper–lead (Cu9.0Pb1.0) from industrial metallurgical waste. It shows the maximum Faradaic efficiency toward CO, which was four times higher than that of pure Cu, under the same electrocatalytic. Huang et al.39 show that the addition of Ag to Cu in the form of segregated nanodomain can significantly enhance the Faradaic efficiency for C2H4 by 3.4-fold and in the partial current density for CO2 reduction by 2-fold compared with the pure Cu counterpart, which is attributed to the tandem catalysis and electronic effects. CSD, in addition to being rich in copper, also contains diverse heavy metals, which renders it a promising copper-based electrocatalyst.

In this work, we developed a new method to handle CSD, which involves chlorination roasting, cascade condensation, and oxidation roasting. This method allows for the recovery of copper, lead, and zinc from dust, while also solidifying the toxic elements of arsenic. Furthermore, the heavy metal chlorides collected through cascade condensation can be further transformed into CuO-based multiple-metals through oxidation roasting. It has been shown to be effective in CO2 electroreduction, with higher faradic efficiencies for CO and CH4 compared to analytical grade CuO. This proof-of-concept demonstration (Figure 1) offers a sustainable and scalable route for simultaneously addressing environmental risk, recycling heavy metals, and generating CH4 and CO through the recycling of CSD.

Figure 1 Schematic illustration of the conversion of CSD into CuO-based multiple metals for the electroreduction of CO2.

Experimental Section

Chloritizing Roasting and Cascade Condensation

The chlorination roasting and cascade condensation were conducted in a three-temperature zone tube furnace. A quartz tube (I.D. 30 mm) was divided into three constant temperature zones, and each zone was controlled by a thermocouple. The first constant temperature zone was employed for evaporation of heavy metals from copper smelter dust, and the last two were used as the condensation zones for condensing different heavy metal chlorides. Furthermore, nitrogen was used to prevent the oxidation of heavy metals and drive them from the evaporation zone to the condensation zone. Copper smelter dust (1.75 g) was mixed with solid chlorination agent on a special ratio (10–40%) based on the mole fraction of chlorine in different chlorination agents, and then the mixed powders were billed and dried in an oven at 110 °C for 20 min. The mixture was filled into an alumina boat, which was put in the first constant temperature zone of the tube furnace and then heated to a special temperature (600–1000 °C) at a rate of 10 K/min under nitrogen temperature. Meanwhile, the last two constant temperature zones were heated to 600 and 300 °C. After being kept at various constant temperatures for 10–60 min, the residues in the alumina boats were cooled to room temperature and analyzed by XRD and ICP to ascertain the variations in the mineral and chemical compositions of the residues. Then, the condensates from the last two constant temperature zones were collected to investigate the condensation behaviors of the heavy metal chlorides. The macrostructures, microstructures, and chemical compositions of the condensate were characterized by SEM–EDS, ICP, and XRD methods. In addition, the evaporation ratio of heavy metals from copper smelter dust was calculated via Equation 1:1

where ηi represents the removal efficiency of element i (%), Cio is the content of element i in the original copper smelter dust (mg/kg), Ci1 is the content of element i in the roasted slag (mg/kg), and m and m1 are the masses of copper smelter dust and roasted slag (g), respectively.

Oxidizing Roasting and Purification

The oxidation roasting experiment was conducted in a single temperature zone horizontal tube furnace, and a quartz tube (I.D. 28 mm) with 950 mm was placed in the tube furnace to carry the sample. The primary condensate and secondary condensate collected in the cascade condensation experiments were mixed and placed in an aluminum boat, which was placed in the constant temperature zone of the tube furnace and then heated to 600 °C for 5 h under an oxygen atmosphere. The temperature was raised to 800 °C and then roasted for 2 h under a nitrogen atmosphere. The residues (CuO-based multiple-metal electrocatalyst) in the alumina boats were cooled to room temperature and analyzed by TEM, XRD, ICP and XRF to investigate the microstructures, chemical state, and mineral and chemical compositions of the electrocatalyst.

Electrochemical Measurements

The CO2RR experiment was carried out in a typical three-electrode system in a sealed H-cell consisting of the electrode to be tested by an electrochemical workstation (AUT50783), platinum plate (2 cm × 2 cm), Ag/AgCl (saturated KCl solution filling) electrode used as working electrodes, a counter electrode, and reference electrode. The H-cell used the 117-Nafion membrane as a proton exchange membrane. Before the CO2RR electrochemical measurements, they were blowing CO2 (99.999%) into 0.5 M KHCO3 at a 20 sccm flow rate for 30 min to be saturated (pH 7.3). The electrode potential was converted to the potential of the reversible hydrogen electrode following the conversion relationship: ERHE = EAg/AgCl + 0.0592 × pH + 0.197 V. Each H-cell chamber was equipped with 60 mL of 0.5 M KHCO3. The stirring magnet rotates at a speed of 750 rpm in the cathode chamber during the CO2RR measurement and keeps CO2 (99.999%) flowing at 20 sccm. The exhaust trachea was connected to a gas chromatograph (GC, Shimadzu 2014C) to analyze the gas product quantitatively. The voltage window of the scanning linear vamp method measured the curve was −0.4 to – 1.4 V vs RHE with 0.01 V s–1 scan rate. The frequency range of electrochemical impedance spectroscopy was from 10–1 Hz to 105 Hz with an amplitude of 10 mV at −0.5 V vs Ag/AgCl. The electrochemically active surface area (ECSA) of the electrode was measured by the double-layer capacitor method, which was obtained by the CV method. The potential window of the CV curves was 0.66–0.78 V vs RHE for CuO-based multiple-metal and pure CuO with different scan rates from 10 to 120 mV s–1. The geometric area of the CuO-based multiple-metal electrodes exposed to electrolytes was controlled by 0.25 cm2.

Analysis and Calculations

All samples were ground and passed through a 100 -mesh sieve before testing to ensure sample homogeneity. To accurately determine the content of heavy metals in all dust samples, 200 mg of each dust sample was digested by a microwave digester (Milestone ETHOS UP, Italy) with the addition of 6 mL of HNO3 + 2 mL of HCl + 2 mL of HF. The digested solution was diluted after cooling to room temperature, and the concentration of heavy metals in the prepared solution was analyzed by inductively coupled plasma optical emission spectroscopy (ICP–OES, Agilent 5100, Agilent Technologies, USA). The crystalline phase composition of all samples was identified by an X-ray diffractometer (XRD, Bruker D8A-A25 X, Germany) using Cu (40 kV, 400 mA, λ = 1.54060 Å) as a radiation source, with steps of 0.02° and a scanning speed of 10° min–1 in the 2θ range from 10 to 80 degrees. Phase identification was recognized with MDI Jade 6.5 software. To understand the microstructures of the samples, field emission scanning electron microscopy (JSM-7900F, JEOL, Japan) and transmission electron microscopy (TEM, JEOL Jem-2100, Japan) equipped with an energy dispersive spectrum analyzer were used. TG-DTG-MS (STA8000-FTIR-GCMS-ATD; PerkinElmer; American) was utilized to analyze the thermal stability of the sample during chlorination process, and the volatilized gas products were analyzed by Gas chromatography–mass spectrometry (GCMS). The sample weight is set at 10 mg and a special alumina crucible is used to avoid corrosion. Furthermore, the atmosphere is set to helium, and the heating rate is set to 20 °C/min. X-ray photoelectron spectroscopy (XPS, K-Alpha, Thermo Scientific, USA) using an Al Kα X X-ray source (1486.6 eV) was utilized to analyze the chemical state of the surface of the pristine electrocatalysts. To compensate for the charging effects, all XPS spectra were calibrated with graphitic carbon as the reference at a binding energy (BE) of 284.8 eV. TGA-GCMS (TGA8000, PerkinElmer, USA) was used to analyze the mass change and the types of gases emitted during the chlorination roasting process. The fractionations of heavy metals in copper smelter dust were investigated through sequential extraction procedures. The Gibbs free energy values (ΔG) of possible thermodynamic reactions throughout the experiment were calculated using FactSage 8.1. Gas product emitted from the electrolytic cell enters the GC for analysis and sampling every 15 min. According to the peak area in the gas chromatography quantitative analysis of gas products. Faraday efficiency characterizes the selectivity of the reduction product, and the formula for calculating FE (%) for quantitative analysis of gases is as eq 2:2

where n is the number of transferred electrons of the target product and V is the volume of the target product of the 1 mL sample detected by GC, S is the gas flow rate, 20 mL min–1, P is pressure, F is the Faradaic constant, 96485 C mol–1, and R is the ideal gas constant, 8.314 Pa m3 mol–1 K–1, Itotal is the current recorded by the electrochemical workstation.

Results and Discussion

Characterization of CSD

The chemical composition of the CSD was determined by X-ray fluorescence spectrometry (XRF) and inductively coupled plasma optical emission spectroscopy ICP–OES (Figure S1). The results indicated that oxygen (33%) and sulfur (8%) were the main nonmetal constituents. This is because CSD is generated by the combustion of sulfur-rich copper ore under oxygen-enriched conditions. For metal constituents, the main elements include copper (18%), iron (14%), arsenic (10%), lead (4%), and zinc (3%). Their crystalline phases were mainly oxides (CuO and Fe3O4), spinel (ZnFe2O4), and sulfate (CuSO4 and PbSO4), as identified by X-ray diffraction (XRD) analysis (Figure 2(a)). Chemical step extraction analysis also confirmed that heavy metals exist in the CSD with various crystalline phases (Table S1).

Figure 2 (a) XRD results of the original CSD and chlorination roasting slag and (b) the volatilization rate of heavy metals in the chlorination roasting process.

Separation of Heavy Metals from CSD

The initial step in separating various metals in CSD is chlorination roasting, wherein selection of the chlorination agent is of utmost importance. The superiority of CaCl2 over other chlorides (MgCl2, NaCl, KCl, and FeCl3) was verified through chlorination experiments. CaCl2-assisted roasting not only promotes the release of copper, lead, and zinc from CSD but also effectively immobilizes the toxic element arsenic in the residue (Figure S2). Furthermore, CaCl2 is cheaper and has advantages in industrial applications. The optimal conditions for CaCl2-assisted roasting were determined as follows: roasting at 1000 °C for 1 h with a 40% CaCl2 addition (Figures S4–S6). In addition, the effects of oxygen concentrations on the volatilization of heavy metals during the chlorination roasting process was investigated. It shows that the volatilization rates of copper, zinc, and lead were close to 100% under different oxygen concentrations (Figure S7). However, the moderate oxygen addition (10–20%) can further inhibit the volatilization of arsenic and iron. Under the optimal conditions, the removal efficiencies of copper, lead, and zinc are close to 100%, whereas the removal efficiencies of arsenic and iron are below 5% as shown in Figure 2(b). Furthermore, the XRD results (Figure 2(b)) show that CaSO4 appears in the residue after CaCl2-assisted roasting. This could enhance the immobilization of arsenic, and as a stable phase, CaHAsO4(H2O) was also identified. More importantly, the phases containing copper, lead, and zinc all disappeared, and the iron component was transformed into Fe2O3, confirming the effective extraction of copper, lead, and zinc from the CSD.

To further separate the volatilized heavy metal chlorides, a three-stage constant temperature zone (CTZ) is constructed through a three-zone tube furnace, as shown in Figure 1 (1000 °C–600 °C–300 °C). In the 1000 °C CTZ, the heavy metals in the CSD are volatilized in the form of heavy metal chlorides, which are sequentially condensed in subsequent CTZs. Furthermore, a small amount of condensate was observed at the outlet of the quartz tube at room temperature. Digital photographs of condensates in different positions are shown in Figure 3(a). The XRD patterns of the three condensates (Figure 3(b)) show that the primary condensate (deposited in 600 °C CTZ) is mainly composed of CuCl and PbCl2, and the secondary condensate is principally CuCl (deposited in 300 °C CTZ). In addition, a small portion of ZnCl2 existed in the secondary condensate according to the SEM results (Figure S8). However, ZnCl2 is tend to convert into Zn(OH)2 and ZnO after absorbing water, and the concentration of a single substance is lower than the detection limits of XRD (5%). Therefore, no Zn-related products were found in the XRD results (Figure 3(b)). In the room temperature zone, BiOCl is seen as the only condensate. The results of ICP-OES and XRF of CSD both indicated that about 1.8% Cr existed in copper smelter dust, which evaporates in the form of BiOCl during chlorination roasting and is deposited in the room temperature zone due to its low-melting point. ICP–OES was utilized to analyze the heavy metal concentrations of the primary and secondary condensates (Table S2). The ratio of copper/diameter in the primary condensate is 5:1, and the content of other heavy metals is less than 1%. Meanwhile, the content of lead in the secondary condensate is less than 0.01%, and the ratio of copper to zinc in the secondary condensate is 18:1. This result indicates that the zinc and lead in the CSD can be separated into ZnCl2 and PbCl2 in the different CTZs, which is due to the difference in vapor pressure40,41 (Figure 3(c)). The saturated vapor pressure of ZnCl2 is much higher than that of PbCl2 and CuCl. Therefore, PbCl2 and CuCl were deposited preferentially at 600 °C CTZ. However, the concentration of CuCl is much higher than that of PbCl2 and ZnCl2, resulting in the existence of CuCl in the 300 °C CTZ.

Figure 3 (a) Digital photographs and (b) XRD patterns of different condensates and saturated vapor pressures of CuCl, PbCl2, and ZnCl2 at different temperatures (c).

Mechanism of Chlorination Volatilization of Heavy Metals

The identification of intermediate products in the chlorination process is the shortcoming of current studies, and many studies speculate on the intermediate products based on the experience. In our study, the TG-DTG-Ms was utilized to directly measure and determine the form of the intermediate products. In addition, this technology can also monitor the release amount and temperature of intermediate products, which is beneficial for further analysis of the overall chlorination process. TG-DTG curves are provided in Figure 4(a), and the gas evolution resulting from the roasting of samples (CSD + 40% CaCl2) is presented in Figure 4(b). Two downward peaks at 99 and 162 °C were observed in the DTG curve, which were likely to be associated with the release of the adsorbed water of CaCl2, and the corresponding mass loss was 10%. The peak of H2O release was also clearly observed in Figure 4(b). Furthermore, a significant weight loss was found in the TG curve from 438 to 1000 °C, associated with the continuous release of HCl. The downward peaks at 577 and 610 °C were mainly attributed to the vaporization of various heavy metal chlorides generated from the reaction between CSD and CaCl2 (HCl). The mass change rate of the sample reached its maximum at 576.8 °C, as indicated by the DTG curve, suggesting that the volatilization rate of heavy metal chlorides is the highest at this temperature. In addition, when the temperature reached 688 °C, SO2 also started to be released, possibly as the reaction product.

Figure 4 (a) TGA curve and derivative thermogravimetric curve and (b) the release of gas in the chlorination roasting process and (c, d) the As XPS spectrum of the original copper smelter dust and roasting slag.

According to the results of TG-DTG-MS, it can be speculated that there are two main mechanisms for the chlorination reaction. One is the direct reaction between CaCl2 and heavy metal sulfate to generate heavy metal chlorides (such as reactions 3–5), which should be the main process of heavy metal chlorination due to the large proportion of sulfates in the CSD.3

4

5

Another chlorination pathway is that CaCl2 decomposes to release HCl, which can convert heavy metal oxides into heavy metal chlorides (such as reactions 6 and 9):6

7

8

9

Among these heavy metal oxides, CuO plays a significant role as an oxygen agent in an argon atmosphere. It can react with HCl to release O2 to promote the chlorination transformation of heavy metal sulfides (such as reactions 10 and 12). In addition, a part of oxygen can be used for the oxidation of magnetite.10

11

12

Furthermore, Cu(II) oxidized As(III) to produce As(V) at high temperature (∼600 °C),42 which can be explained using redox reaction 13, and As(V) could be captured by calcium to form stable CaHAsO4(H2O).13

The XPS spectra of As 3d obtained before and after chlorination confirmed this conclusion. As shown in Figure 4 (c), the As 3d spectra are divided into As 3d and As 3d3/2 doublet peaks located at 43–44 and 44.4–45.8 eV, which were assigned to As(III) and As(V).43 Based on the XPS results, As(III) accounted for approximately 74.2% [MAs(III) (%) = SAs(V)/(SAs(III) + SAs(V)), where S is the peak area and M is the percentage] of all the arsenic in the CSD. After chlorination roasting, the proportion of As(III) decreased to 15.3%, while that of As(V) increased to 84.7%, as presented in Figure 4(d). The main iron phase of the CSD are Fe3O4 and Fe2O3, which are difficult to chlorinate when some other components that are more likely to combine with chlorine are presented in the CSD.44 However, Fe3O4 is easily oxidized to Fe2O3 at high temperature as shown in reaction 14. In addition, CaO produced by the decomposition of CaCl2 can easily generate CaSO4 in the presence of O2 and SO2 as presented in reaction 15.14

15

The standard Gibbs free energy changes of these reactions at different temperatures were calculated through FactSage 8.1, indicating that all reactions can proceed spontaneously at high temperature (ΔG < 0) (Figure S6). The mechanism of the CaCl2 additive promoting the chlorination volatilization of heavy metals and the solidification of arsenic is shown in Figure 5.

Figure 5 Mechanism of the CaCl2 additive promoting the chlorination volatilization of heavy metals and the solidification of arsenic.

Catalyst Synthesis and Characterization

Simple oxidation roasting was conducted to separate lead and zinc from condensates, and a CuO-based multiple-metal electrocatalyst was synthesized without any additives. The morphologies of the as-prepared CuO-based multiple-metal electrocatalyst were examined using transmission electron microscopy (TEM). The micron-sized particles in the electrocatalyst are mostly irregular polygons with a relatively compact texture (Figure S9), which is considered copper oxide according to the results of energy dispersive spectrometry (EDS). In contrast, the TEM-EDS mappings (Figure 6(a)) show the homogeneous distribution of Pb, Fe and Cr in the copper oxide nanoparticles, suggesting that these heavy metals do not form individual nanoparticles. Many studies have proven that Cr is commonly found in copper ores. As copper smelting proceeds, a part of Cr existing in the copper ore will volatilize and eventually appear in the CSD. The results of ICP-OES and XRF of CSD indicated that about 0.1% Cr existed in CSD, which evaporates during chlorination roasting and is deposited in the condensate. Then, it is homogeneous distribution in the copper the copper oxide nanoparticles during oxidative roasting as shown in Figure 6(a). The ICP-OES was utilized to analyze the element distribution of the CuO-based multiple-metals. The average values of Cu, Cr, Pb and Fe are 75.31%, 2.03%, 1.15%, and 0.12%, respectively. It is worth mentioning that all tests were performed on three catalysts produced under the same experimental conditions. In addition, the XRD result of the CuO-based multiple-metallic electrocatalyst (Figure 6(b)) exhibits three characteristic diffraction peaks located at 35.6, 38.7 and 48.7, which are assigned to the (−111), (111) and (−202) crystal facets of CuO (JCPDS 48–1548), respectively. To gain insight into the chemical valence states of the surface of the CuO-based multiple-metal electrocatalyst, X-ray photoelectron spectroscopy (XPS) measurements were performed. As shown in Figure 6(c), the peak of the Cu 2p3/2 spectrum is located at 933.5 eV with a typical shock peak of Cu2+, which is consistent with the XRD results. It can be judged that Cu exists in the form of CuO on the surface of the materials, and there is no metallic copper or Cu+.30 The Pb 4f spectra are deconvoluted into peaks at 138 and 138.9 eV, corresponding to PbO and PbCl2, respectively (Figure S10). PbO is transformed from incompletely volatilized PbCl2.38 In addition, a negligibly small peak of metallic Pb (136.8 eV) is observed in the XPS measurements. The Fe 2p spectrum (Figure S11) is fitted with two symmetric peaks at approximately 709.9 eV (attributed to Fe2+) and 713.4 eV (attributed to Fe3+),45 which may be caused by incomplete oxidation. The chemical state of Cr cannot be accurately determined by the XPS results due to the existence of the Cu Augur peak (568.1 eV). Therefore, it is speculated that the existence of multiple heavy metals in nanoparticles will significantly affect the efficiency and selectivity of the CO2 electroreduction.

Figure 6 (a) TEM-EDS images, (b) XRD pattern, and (c) Cu 2P XPS spectrum of CuO-based multiple-metal electrocatalyst.

Surface Reaction Kinetics Studies

The electrochemical properties of CuO-based multiple-metals and analytical grade CuO electrocatalysts were investigated and compared to study the activity of the catalyst surface. The linear scanning voltammetry (LSV) results are shown in Figure 7(a, b). This demonstrates that CuO-based multiple-metal materials exhibit a higher current density (60 mA cm–2 at −1.6 V vs RHE) than analytical grade CuO, suggesting higher CO2 reduction reaction kinetics in CuO-based multiple-metals. In addition, the electrochemical surface areas (ECSAs) of the two catalysts were measured via cyclic voltammetry (CV). The ECSA of CuO-based multiple-metallics is slightly higher than that of analytical grade CuO, indicating that CuO-based multiple-metallics can provide more active sites, as shown in Figure 7(c, d). Furthermore, the XRD results, particle size distributions, and surface morphologies of analytical grade CuO were similar to those of CuO-based multiple-metals (Figure S12), indicating that the heavy metal composition may play an important role in the surface activity of CuO-based multiple-metals. Electrochemical impedance spectroscopy (EIS) studies were conducted to study the transfer of the two catalysts. The CuO-based multiple-metal material exhibits a smaller arc than that of analytical grade CuO. This suggests that the electron transfer in CuO-based multiple-metal materials is smoother and the resistance is smaller (Figure S13).

Figure 7 (a, b) LSV curves and (c, d) ECSA of CuO-based multiple-metals and analytical grade CuO.

Electrochemical CO2 Reduction Performance

The eCO2R performance of the as-prepared CuO-based multiple-metal electrocatalyst at different applied voltages was investigated using a three electrode H-type cell containing a CO2-saturated 0.1 M KHCO3 aqueous electrolyte solution, while analytical grade CuO was employed for comparison. More experimental details are demonstrated in the Supporting Information. The CuO-based multiple-metal material shows higher CO and CH4 faradaic efficiencies (FEs) than analytical grade CuO in the given potential window (Figure 8(a, b)). The maximum FE values of CO and CH4 are 12.2% and 19.1% at −0.6 V vs RHE, respectively, which are approximately four and eight times higher than that of analytical grade CuO at this applied potential. However, the HER is greatly enhanced as the applied voltage increases, leading to a decrease in the FE values of CO and CH4. Combined with the results of TEM and XPS, the enrichment of CO* intermediates can be attributed to the existence of lead species in CuO-based multiple-metals electrocatalysts,38 and the introduction of chromium sites can intensify the hydrogenation of CO*, which enhances the pathway for methane production.31 Therefore, it exhibits higher CH4 selectivity than analytical grade CuO. The presence of iron leads to the enhancement of HER at a high potential.46 Hence, the FEs of the other products are greatly reduced.

Figure 8 (a, b) FEs of the main products (H2, C2H4, and CO and CH4) for (a) CuO-based multiple-metals and (b) analytical grade CuO in different voltages.

Conclusions

In this study, we demonstrate a novel approach for cascade separation of heavy metals in CSD and high-value utilization of their products. Nearly all copper, lead, and zinc can be separated from CSD, and toxic trivalent arsenic is oxidized to pentavalent arsenic and immobilized in roasting residues, reducing the potential environmental risk. Furthermore, the chlorination processes of different heavy metals were analyzed. This indicates that there are two mechanisms of direct and indirect chlorination in the chlorination process. To improve the value of recycled products, they are converted into CuO-based multiple-metal products through simple oxidative roasting. It exhibits high catalytic activity and high faradaic efficiency to achieve the electroreduction of CO2 into CO and CH4. In conclusion, this proof-of-concept demonstration offers a sustainable and facile route toward simultaneous environmental risk elimination, efficient recovery of heavy metals, and the generation of CO and CH4.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.4c03333.Element composition of copper smelter dust; screening of different chlorination agents; influence of different experimental conditions on the volatilization of heavy metals; calculation of Gipps free energy of reaction; analysis of different condensates; characterization of CuO-based multiple-metallic (PDF)

Supplementary Material

ao4c03333_si_001.pdf

The authors declare no competing financial interest.

Acknowledgments

This work was supported by the Innovative Research Groups of the National Natural Science Foundation of China (No. 52121004), the National Natural Science Foundation of China (No.52022111), and the Major Program Natural Science Foundation of Hunan Province of China (No. 2021JC0001).
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