{'fraction_non_alphanumeric': 0.042453306410903585, 'fraction_numerical': 0.02700656234225139, 'mean_word_length': 4.472651933701657, 'pattern_counts': {'":': 0, '<': 3, '<?xml version=': 0, '>': 0, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 22, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
Metal mobility and toxicity of reclaimed copper smelting fly ash and smelting slag

Copper is a nonferrous metal closely connected to humans. Approximately 40% of copper is produced by reclaimed copper smelting (RCS). Reclaimed copper smelting fly ash and smelting slag are generated during the RCS process, posing a serious threat to the ecosystem and environment as they contain many heavy metals, such as Cu and Zn. In this study, the metal mobility and toxicity of RCS fly ash and smelting slag were analyzed using standard leaching toxicity procedures, sequential extraction procedures, and bioavailability tests. The results showed that the main phases of RCS fly ash were Cu2(OH)3Cl, FeCl2·2H2O, CuS2, C, CuO, Cu, Ca2SiO4, ZnClO42, Zn(OH)2·0.5H2O, and KFeCl3, and those for smelting slag were SiO2, CaCO3, SiS2, CaAl2Si2O8·4H2O, Cu4O3, CuO, ZnO, NiSO4·6H2O, AlPO4, and Na3Mn(PO4)(CO)3. These two slags contain high contents of Cu, Zn and Fe and trace amounts of heavy metals, such as Ba, Be, Cd, Cr, Ni, As, Pb, Au, Se and Sb. RCS fly ash is classified as hazardous waste in both China and the USA as the toxic leaching concentrations of Pb and Cd exceed the thresholds of 5 and 1 mg L−1. Cu and Zn contained in these two slags can easily be released into the environment, although the residual fraction of Cu and Zn was found to be higher than 65%. Additionally, RCS fly ash and smelting slag also show significant biohazardous potential as the EDTA- and DTPA-extractable Zn, Cu and Se of these two residues are considerably high. The results described above could provide reclaimed copper smelting companies and governments with a better understanding of the risk of RCS fly ash and smelting slag, urging them to stop the slag from harming ecosystems and humans.


Introduction
Copper is a nonferrous metal strongly related to humans. It is widely used in the electrical, light, machinery manufacturing, construction, and national defense industries, as well as other elds. 1 In 2019, the output of global copper mines was 20.69 million tons, and the output of rened copper production was 23.72 million tons. China is the world's largest rened copper producer, with an annual rened copper output of 9.784 million tons in 2019, accounting for 41.24% of the world's total. China is also the largest consumer of rened copper, accounting for approximately 48% of the world's total. 2 Copper has an excellent recycling performance, and it is an important raw material for copper rening to directly utilize copper waste and indirect smelting of waste copper. 3 Low energy consumption, material consumption, and waste discharge per unit of rened copper are produced from waste copper regeneration. They are also benecial to energy conservation, emission reduction, and resource recycling compared with copper smelting. 4 In 2019, the proportion of reclaimed copper production was very high according to data from the International Copper Research Organization (ICSG), among which the United States accounted for approximately 60%, Japan accounted for approximately 45%, and Germany accounted for approximately 80%, while China accounted for only 33.73%.
The most economical utilization method is determined by the composition of raw materials and different copper grades. 5 The smelting process mainly includes one-stage and two-stage smelting. 6 The one-stage smelting method is aimed at highgrade waste copper and miscellaneous copper. If the copper content is higher than 90%, it can be rened directly into anode copper through a re rening furnace. The two-stage smelting process focuses on high-grade scrap copper and miscellaneous copper. If the copper content is lower than 90%, the smelting furnace rst uses a melting furnace to remove impurities; if, aer processing, the material has a copper content higher than 80%, a two-stage smelting method is adopted for direct input into the rening furnace for smelting anode copper. A three-stage smelting process is used if the copper content is lower than 80%.
Purple miscellaneous scrap copper is the primary reclaimed copper, including tin-plated steel waste, tin-bronze, brass, and various alloys. 7 The copper content is higher than 95%, and the specic smelting process is shown in Fig. S1. † As presented in Fig. S1, † the ue gas produced in the melting furnace is mainly removed by the cloth bag dedusting process. The reclaimed copper smelting (RCS) y ash and smelting slag contains many heavy metal ions because the purple miscellaneous scrap copper contains copper, lead, zinc, tin, protective iron, oxygen and other impurities. If RCS y ash and smelting slag are piled up, they may contaminate the soil and water, effecting the growth of plants, and even endangering human health. The area of cultivated land polluted by heavy metals such as arsenic, cadmium, lead, and chromium accounts for about one h of the total area of cultivated land in China. 8 As metal in farmland soil is the most important risk factor for carcinogenesis, with contribution rates of 99% in children and 98% in adults. 9 Plants around mining areas are also severely contaminated by Sb, As, Cd and Pb. The edible parts of vegetables can take up 2.05 mg kg À1 of Sb, which has high risks for human ingestion, moreover, the metals of As and Pb in vegetables signicantly exceed the standard guidelines. 10 Direct discharge of RCS y ash and smelting slag causes severe environmental pollution.
Researchers have performed a signicant amount of research into the treatment and resource utilization of RCS y ash and smelting slag to make it harmless. 4,11,12 This research mainly includes water leaching, 13 acid leaching, 14-17 oxidation leaching, 18,19 bioleaching, 20,21 pressure oxidative leaching, 14 chloride leaching, 22 alkali leaching, 1 controlling potential leaching, 23 the leaching-extraction method and the leachingblast furnace smelting method. 24 The copper and zinc in RCS y ash and smelting slag can be leached using water or dilute sulfuric acid, and the leaching solution and leach residue can be treated using other methods. [25][26][27] However, limited information was found regarding the metal mobility and toxicity of RCS y ash and smelting slag.
In this study, typical RCS y ash and smelting slag were collected and their metal levels, phases and morphologies were analyzed using X-ray diffractometry (XRD), X-ray uorescence spectroscopy (XRF) and inductively coupled plasma optical emission spectroscopy (ICPOES). The metal mobility was examined using bioavailability tests and sequential extraction procedures; the leaching toxicity was classied according to the Identication Standards for Hazardous Waste-Leaching Toxicity (GB5085.3-2007) and the Toxicity Characteristic Leaching Procedure (TCLP). We expect that the results of this study will provide valuable information for managing and reusing RCS y ash and smelting slag, in addition to reducing environmental risks.

Preparation of the materials
Reclaimed copper smelting y ash and smelting slag were collected from an RCS factory in Sichuan province in China. RCS y ash and smelting slag were dried at 80 C for 24 h to achieve a constant weight. The carbon content of the RCS y ash and smelting slag were 30.27% and 2.67%, respectively. The density of the RCS y ash and smelting slag were 0.5769 and 1.0649 g cm À3 , respectively. In this study, all the chemical reagents (HNO 3 , HCI, HF, H 2 SO 4 , CH 3 COOH, NaOH, NH 4 2 HCl, H 2 O 2 , and EDTA-Na 2 ) were purchased from Mianyang Xinjie Trading Co. Ltd, China (Analytical grade) and applied without further purication. Deionized water was provided using the Water Purication System (Advantage A10, Millipore, Burlington, MA, USA), and all experiments were carried out at room temperature.

Chemical composition
The chemical component of RCS y ash and smelting slag were analyzed using XRF (Axios, PANalytical, Holland). In addition, these residues were digested and analyzed using ICPOES (OPTIMA 8000, PerkinElmer, U.S.) to quantify the concentrations of Al, Ba, Be, Cr, Cd, Cu, Fe, Ni, Pb, Sb, Sn, Tl, Zn, and Au. The chemical phase of the RCS y ash and smelting slag were examined using XRD (D/MAX2500, Rigaku, Almelo, Holland).

Hazardous waste classication
The heavy metal leaching toxicities of RCS y ash and smelting slag were examined using the Identication Standards for Hazardous Waste-Leaching toxicity (GB5085.3-2007, Ministry of Ecology and Environment of the People's Republic of China, 2007, Chinese standards) and the TCLP (US EPA 1992; American standards). [28][29][30] The solid waste classications of the RCS y ash and smelting slag were examined using HJ/T 299-2007 (Solid waste-extraction for leaching toxicity, sulfuric and nitric acid method) and the TCLP (Method 1311; 40 CFR §261.24). Based on HJ/T 299-2007, only Al, As, Ba, Be, Cd, Cu, Ni, Pb, Sb, In, Se, Zn and Au were analyzed, while for TCLP, seven metals, Al, As, Ba, Be, Cd, Cr, Cu, Fe, Ni, Pb, Sb, In, Se and Zn were examined.
For HJ/T 299-2007, 10.00 g of RCS y ash and 10.00 g smelting slag were placed into extraction bottles, respectively. 100 mL of sulfuric and nitric acid solution (the mass ratio of the concentrated sulfuric acids and nitric acids was 2 : 1) was used as the extraction liquid, and the pH of the extraction liquid was 3.20 AE 0.05. The bottles were xed on a rotary extractor (extraction time, 18 AE 2 h; speed, 32 AE 2 rpm), and then the concentrations of heavy metals in the extraction liquid were analyzed using ICPOES aer ltration through a 0.45 mm microporous membrane.
For the TCLP, 5 g RCS y ash and 5 g smelting slag were placed into extraction bottles, respectively. Acetic acid was used as the extraction liquid, and the pH of the extraction liquid was 2.88 AE 0.05, when the solid-liquid ratio was 20 : 1 (kg L À1 ). The bottles were xed on a rotary extractor (extraction time, 18 AE 2 h; speed, 32 AE 2 rpm), and then the concentrations of heavy metals in extraction liquid were analyzed using ICPOES aer ltration through a 0.45 mm microporous membrane.

Metal chemical speciation and bioavailability
The chemical speciation of Al, Ba, Be, Cr, Cd, Cu, Fe, Ni, Pb, Sb, Sn, Tl and Zn in RCS y ash and smelting slag were performed according to the three steps outlined in the Community Bureau of Reference (BCR) protocol of the European Standards Measurements and Testing Program, respectively. 31 The details are shown in Table A in the ESI. † Aer each step, the suspensions were ltered using a 0.45 mm microporous membrane and tested using ICPOES. Metal bioavailability tests on RCS y ash and smelting slag were tested using the Wear and Evans method, as well as the method proposed by Lindsay and Norvell. For the Wear and Evans method, 2.0 g of RCS y ash and smelting slag were placed in a 50 mL centrifuge tube and mixed with 20 mL 0.05 mol L À1 EDTA at pH 7.0, respectively, as well as shaking the suspension in an oscillating device for 1 h. For the Lindsay and Norvell method, 10 g of the RCS y ash and smelting slag were mixed in a 50 mL centrifuge tube and mixed with 20 mL 0.005 mol L À1 DTPA and 0.01 mol L À1 CaCl 2 at pH 7.3, respectively, 32,33 as well as shaking the suspension in a oscillating device for 2 h. Aer extraction, the suspensions were centrifuged at 3000 rpm for 20 min, and the suspensions were then ltered using a 0.45 mm microporous membrane and tested using ICPOES. All the tests were repeated three times, and the mean value of the three experiments was obtained without error bars in this study.

Chemical composition
The XRF chemical composition analysis of the RCS y ash and smelting slag are shown in Table 1 As shown in Table 2, both RCS y ash and smelting slag contain valuable metallic resources. For example, the concentrations of Cu, Zn, Fe and Al in RCS y ash and smelting slag were 172.24, 124.54, 21.66, and 6.00 mg g À1 and 11.47, 13.70, 19.73, and 17.20 mg g À1 , respectively. The levels of Cu and Zn in RCS y ash were found to be much higher than their corresponding ores, 34,35 and can be used as resources for resource recovery. In addition, smelting slag can also be reused as a raw material for construction materials as it contains minerals of Si, Al, Fe and Ca. RCS y ash and smelting slag are classied as hazardous waste by China owing to the high concentrations of copper and zinc in both residues. Other heavy metals such as Ba, Be, Cd, Cr, Ni, Pb, and Sb are also observed in Table 2. Therefore, when recycling valuable metals from RCS y ash and smelting slag, the effect of heavy metals should also be considered. The results described above show that the elemental composition of RCS y ash and smelting slag are different.
The XRD phase analysis of RCS y ash and smelting slag are shown in Fig. 1. As shown in Fig. 1, RCS y ash and smelting slag have completely different phases. In Fig. 1a, the main  The microstructure and compositional distribution of the RCS y ash and smelting slag were further investigated using SEM and energy dispersive X-ray analysis (EDX) mapping. In Fig. 1c, the SEM images of RCS y ash showed spherical particles of uneven sizes piled up together. EDX analysis showed that the surface of the spherical particles mainly contains C, Ca, O, Zn, Fe, Cu, S, Si and Cl. The EDX elemental maps reveals that the compositional distributions of the ve elements (Cu, O, Zn, S, Ca and Al) in RCS y ash are uniform, suggesting a mixed mineral structure. In Fig. 1c, SEM images of the smelting slag showed a lumpy structure of unequal size with rounded grooves of unequal size on the surface. EDX analysis showed that the surface of the spherical particles mainly contain Si, O, Ca, Zn, Fe, Cd, Cu, S and Cl. The EDX elemental maps further reveal that the compositional distributions of the three elements (Cu, O and Ca) in the smelting slag are uniform, suggesting a mixed mineral structure.

Hazardous waste assessment
Leachates from RCS y ash and smelting slag according to HJ/T 299-2007 are shown in Table 3. As shown in Table 3, the concentrations of Cd, Pb and Zn in RCS y ash were 12.13, 6.46 and 1373.0 mg L À1 , respectively. All concentrations exceeded the threshold values of 1.0, 5.0 and 100 mg L À1 for Cd, Pb and Zn, respectively. The most signicant leaching toxicity was Zn which was orders of magnitude higher than Pb and Cd, as well as Al, Be, In and Se, which were not detected in RCS y ash. The concentrations of metal ions in smelting slag were lower than the threshold. The results showed that both RCS y ash types are classied as hazardous waste because of the Cd, Zn and Pb contamination. Furthermore, the result also suggests that the smelting industry and government should focus on hazardous elements other than Zn when recycling RCS y ash. Table 4 presents leachates from RCS y ash and smelting slag according to the TCLP. As shown in Table 4, RCS y ash is identied as hazardous waste owing to the excessive levels of Cd and Pb. The TCLP leaching concentrations of Ba, Cd, Cu, Pb and Zn for RCS y ash are 1.6, 13.35, 554.6, 211.35 and 3644.5 mg L À1 , respectively, and the concentrations of metal ions in smelting slag were lower than the threshold. Cd, Zn, Pb and Cu showed the most signicant leaching toxicity, and Al, As, Ba, Be, Cr, Fe, Ni, Sb and Se were not detected in RCS y ash. Tables 3 and 4 demonstrate that the concentrations of Cd, Pb, Cu and Zn by HJ/T 299-2007 in RCS y ash were lower than those obtained using the TCLP. The results conrmed that TCLP is much stricter than HJ/T 299-2007, suggesting that China should consider stricter regulation protocols for identifying hazardous waste when considering the environment and human health.  Table B in the ESI † provides detailed data. As shown in Fig. 2a, the residual fraction of Sn and Fe in RCS y ash was almost 100%, while the fractions of Ba, Pb, Cu and Be were 90.97%, 75.51%, 71.78% and 56.36%, respectively. 38 The contents of Ba, Cd, Au, Se, Ni, Sb and Be were less than 1&, as displayed in Table 2. Therefore, the risks associated with these seven metals in RCS y ash are insignicant. The contents of Cu and Zn are higher than 10% in Table 2. Cu poses signicant environmental and human health risks, although its residual fraction was 71.78%. The copper concentration in RCS y ash is relatively high, and its exchangeable and soluble fractions are approximately 8.10%. Zn poses a signicant risk as the sum of exchangeable and soluble fractions and reducible and oxidizable fractions surpasses 67.4%. Therefore, the risks associated with Cu and Zn metals in RCS y ash are signicant. Pb poses the highest risk as the sum of its exchangeable and soluble fractions and reducible and oxidizable fractions is approaching 24.5%. Both levels are consistent with the TCLP results. Data from Tables 3 and 4 and Fig. 1a and 2b indicate that Cu, Pb, Cr and Zn are also the primary concerns when dealing with RCS y ash.

Chemical speciation analysis
For smelting slag (Fig. 2b), most metals are more stable than in RCS y ash as their residual fractions are higher, with the exception of Fe and Pb. The contents of Cu and Zn are higher than 1% in Table 2. The residual fraction of Zn in RCS y ash was only 32.60% but this increased signicantly to 78.63% in smelting slag. Metal mobility increases in the following sequence: exchangeable and soluble < reducible < oxidizable < residual, 39,40 indicating that the mobility of Zn in RCS y ash is much higher than that in smelting slag. Conversely, the residual   Table 2. Therefore, the hazards associated with these 11 metals in RCS smelting slag are insignicant.

Bioavailability assessment
The method of EDTA-and/or DTPA-extractions was applied to examine the metal bioavailability of RCS y ash and smelting slag. 41 As shown in Table 5 (Table 5). The bio-hazards of Cd and Se also cause concern because EDTA and DTPA extractable concentrations of RCS y ash and smelting slag were the third highest values obtained. Meanwhile, the levels of EDTA and DTPA extractable Ba, Be, As, Cr, Ni, Sb and Au in RCS y ash and smelting slag were also low; for example, DTPA extractable As from RCS y ash was only 8 mg kg À1 . The above described results suggest that Cu and Zn in RCS y ash and smelting slag pose signicant bio-hazardous risks and directly impact the vegetation that surrounds the stacks.

Conclusions
Reclaimed copper smelting y ash and smelting slag are generated during the reclaimed copper smelting process and contain a large amount of valuable metals such as Cu and Zn, which have a certain recovery value. In this study, the metal mobility and toxicity of RCS y ash and smelting slag were analyzed. The results showed that RCS y ash and smelting slag contain high contents of Cu, Zn and Fe and trace amounts of heavy metals, such as Ba, Be, Cd, Cr, Ni, As, Pb, Au, Se and Sb. RCS y ash is classied as hazardous waste in both China and the USA as the toxic leaching concentrations of Pb and Cd exceed the thresholds of 5 and 1 mg L À1 . Cu and Zn contained in these two slags can easily be released into the environment. Additionally, RCS y ash and smelting slag also show signicant biohazardous potential as the EDTA-and DTPA-extractable Zn, Cu and Se of these two residues are considerably high. The above described results not only provide a certain reference basis for the eld of RCS y ash and smelting slag, but will also enable recycled copper smelting enterprises and government departments to better understand RCS y ash and smelting slag and the associated levels of risk. Recycling copper smelting enterprises should consider the leaching toxicity and bioavailability when stacking RCS y ash and smelting slag.

Conflicts of interest
There are no conicts to declare.