Treatment, reuse, leaching characteristics and genotoxicity evaluation of electroplating sludge

Catalytic reduction of nitrogen monoxide using iron-nickel oxygen carriers derived from electroplating sludge: Novel method for the collaborative emission decrease of polluting gases

The valorization of electroplating sludge (ES) for high added value presents greater economic and environmental benefits than conventional treatment methods such as thermal processing, solidification, and landfill. Inspired by the mechanism of chemical looping combustion (CLC), this study developed a novel cost-effective method for denitrification by preparing FeNi-OCs from ES to achieve the synergistic reduction of CO and NO emissions. The phase structure, micromorphology, and valence state changes of the FeNi-OC catalyst during the CO-catalyzed reduction of NO and the pathway for catalytic denitrification using FeNi-OCs were analyzed. Results showed that CO could reduce FeNi-OCs to FeNi, and the reduced FeNi was subsequently oxidized back to FeNi-OCs by NO, a process analogous to CLC. During experiments, the simultaneous consumption of CO and NO gases was observed at 350 °C. This phenomenon was highly pronounced at 600 °C, where the CO and NO concentrations decreased from initial values of 8550 and 470 ppm, respectively, to 6719 and 0 ppm, respectively, with conversion rates of 21.41 % and 100 %, respectively. Hence, synergistic emission reduction was achieved. Further experiments also indicated that the addition of 1.5 % ES during iron ore sintering could substantially reduce the CO and NO concentrations in the sintering flue gas from 1268.32 and 244.81 ppm, respectively, to 974.51 and 161.11 ppm, respectively.

A novel method for effective solidifying chromium and preparing crude stainless steel from multi-metallic electroplating sludge

Electroplating sludge (ES) is a globally prevalent hazardous waste that primarily contains Cr, Cu, Ni, and Fe. However, the residual Cr phases within the slag potentially poses an environmental risk in current vitrification. A novel method for effective recovering and solidifying Cr in ES is proposed in this work. ES was desulfurized and subsequently co-treated with ferrosilicon (Fe-Si) and spent carbon anode (SCA) for enhancing the recovery of Cr, Cu, Ni, and Fe to prepare crude stainless steel. Under optimal conditions, the recovery ratios of Cr, Cu, Ni, and Fe reached 96.96%, 99.45%, 99.92%, and 99.20%, respectively, signifying improvements of 21.4%, 0.2%, 1.5%, and 2.8%, respectively, compared with existing research. Meanwhile, the fluoride in SCA yielded CaF2, further progressing to the Si-Ca-F-Na-Al-O phase, with a solidification ratio of 97.87%. The Cr leaching content of the residual Cr-Cu-S phase in the slag remained below 5 mg/L across a pH range of 2-4, demonstrating enhanced stability compared to prior alloy, oxide, and chemically dissolved phases. An innovative approach for solidify Cr by forming matte holds implications for the treatment of Cr-containing solid wastes such as chromium slag, tannery sludge and stainless steel slag.

Study on dechlorination salt characteristics of pickling sludge by a water-washing process

Steel hydrochloric acid pickling sludge (SHPS), containing the heavy metals Fe, Zn, and Ni and a high chloride salt content, is considered a hazardous solid waste. With the gradual reduction of high-grade metal mineral resources such as Fe, Zn and Ni, it is particularly urgent to recycle valuable metals such as Fe, Zn and Ni in solid waste SHPS in order to realize the resource utilization of SHPS and reduce the environmental harm caused by SHPS. In addition, SHPS usually contains different amounts of alkali chloride, which will have a serious adverse impact on the subsequent extraction and smelting process of Fe, Zn and other metals. Therefore, the removal of chloride plays an important role in the resource utilization of valuable metals in SHPS. Thus, in this study, the effects of water washing dechlorination process parameters such as liquid-solid (L/S) ratio, SHPS particle size, washing time and washing frequency on the chloride removal rate were investigated. The best experimental parameters of SHPS washing were obtained. At the same time, the microscopic morphology and crystal phase composition of SHPS before and after washing were explored. The results showed that the optimized conditions were as follows: room temperature, a L/S ratio of 3 : 1, an SHPS particle size of 100 mesh, and 10 min of water washing, repeated two or three times; under these conditions, the removal rate of Cl, Na, Ca, K, Mg, and S reached 96.64-99.68%, 97.38-99.89%, 36.40-60.37%, 49.11-54.82%, 39.18-40.22%, and 36.98-42.13% respectively. The contents of Cl, K, and Na in filter residue (FR) meets the requirements in GB/T 36144-2018 and GB/T 32545-2016. Conversely, the contents of Fe, Zn, Mn and Ni in the FR are enriched, which is more conducive to the subsequent resource utilization of SHPS. The scanning electron microscope (SEM) image shows the particle size of the FR particles is reduced after washing. The X-ray diffractometer (XRD) results proved that the chlorine salt content in the FR after washing was significantly reduced, the diffraction peaks of Al2O3 appeared in the FR, and the diffraction peak intensity of CaCO3, Fe2O3 and SiO2 increased.

Acid-leaching mechanism of electroplating sludge: based on a comprehensive analysis of heavy-metal occurrence and the dynamic evolution of coexisting mineral phases

Electroplating sludge is a typical heavy metal-containing hazardous waste with tens of millions of tons produced annually in China. Acid leaching is the most common method to extract valuable heavy metals for resource recycling and environmental protection. However, the coexisting elements, which are released from electroplating sludge to the leaching solution, will hinder the recycling of valuable heavy metals. In this work, dynamic acid-leaching experiments, X-ray diffraction analysis, and simulation calculations were conducted. It was found that coexisting elements (mainly Ca, Fe, and Al) account for a large proportion, and calcium salts as coexisting mineral phase (especially CaCO3) are ubiquitous in electroplating sludge. Moreover, the evolution of coexisting mineral phase plays an essential role in the acid-leaching process: (1) the dissolution of CaCO3 contributed a strong acid-neutralization capability and released Ca2+; (2) H2SO4 is the optimal extracting reagent, since it triggered the transformation of calcium salts to CaSO4·2H2O, reducing the Ca2+ concentration; (3) the coexisting elements Fe and Al would form ferrous and aluminum salt minerals with the acid-leaching process, which reduces the leaching of low-value elements. This work provides a new perspective on the acid-leaching mechanism of electroplating sludge, where the evolution of the mineral phase effect the release of valuable heavy metals and coexisting elements. This work also provides as comprehensive information as possible on electroplating sludge and inspires the improvement of the acid-leaching method.

Characterization of ionic liquids removing heavy metals from electroplating sludge: Influencing factors, optimisation strategies and reaction mechanisms

The disposal of electroplating sludge (ES) is a common concern of researchers. Currently, it is difficult to achieve effective fixation of heavy metals (HMs) using traditional ES treatment. As green and effective HMs removal agents, ionic liquids can be used for the disposal of ES. In this study, 1-butyl-3-methyl-imidazole hydrogen sulphate ([Bmim]HSO₄) and 1-propyl sulphonic acid-3-methyl imidazole hydrogen sulphate ([PrSO₃Hmim]HSO₄) were used as washing solvents for the removal of Cr, Ni, and Cu from ES. In reaction with increased agent concentration, solid-liquid ratio, and duration, the amount of HMs eliminated from ES rises, whereas opposite patterns were shown in response to rising pH. The quadratic orthogonal regression optimisation analysis also revealed that the ideal washing specifications for [Bmim]HSO₄ were 60 g L^-1, 1:40, and 60 min, respectively, for agent concentration, solid-liquid ratio, and washing time, while those for [PrSO₃Hmim]HSO₄ were 60 g L^-1, 1:35, and 60 min, respectively. Under the optimal experimental conditions, the Cr, Ni, and Cu removal efficiencies for [Bmim]HSO₄ were 84.3, 78.6, and 89.7%, respectively, and those values for [PrSO₃Hmim]HSO₄ were 99.8, 90.1, and 91.3%, respectively. This was mainly attributed to that ionic liquids enhance metal desorption through acid solubilisation, chelation, and electrostatic attraction. Overall, ionic liquids are reliable washing reagents for ES contaminated by HMs.

Preparation of copper-based catalysts from electroplating sludge by ultrasound treatment and their antibiotic degradation performance

The recovery of heavy metals from electroplating sludge is important for alleviating heavy metal pollution and recycling metal resources. However, the selective recovery of metal resources is limited by the complexity of electroplating sludge. Herein, CuFe bimetallic Fenton-like catalysts were successfully prepared from electroplating sludge by a facile room-temperature ultrasonic-assisted co-precipitation method. The prepared CuFe-S mainly consisted of nanorods with diameters of 20-30 nm and lengths of 100-200 nm and a small number of irregular particles. Subsequently, we performed tetracycline (TC) degradation experiments, and the results showed that the product CuFe-S had very good performance over a wide pH range (2-11). At an initial pH = 2, CuFe-S could degrade 91.9% of 50 mg L^-1 TC aqueous solution within 30 min, which is better than that of a single metal catalyst. Free radical scavenging experiments and electron paramagnetic resonance (EPR) tests revealed that ·OH was the main active species for the degradation of TC by CuFe-S. In conclusion, a CuFe bimetallic Fenton-like catalyst was developed for the catalytic degradation of antibiotics, which provides a novel technical route for the resource utilization of electroplating sludge and shows an important practical application prospect.

Designing Magnesium Phosphate Cement for Stabilization/Solidification of Zn-Rich Electroplating Sludge

Electroplating sludge is a hazardous waste due to its high potential to leach toxic elements into the natural environment. To alleviate this issue, we tailored magnesium phosphate cement (MPC) as a low-carbon material for stabilization/solidification (S/S) of Zn-rich electroplating sludge. The interaction between MPC and ZnO was investigated to clarify the precipitate chemistry, microstructure transition, and chemical environment of Zn species in the MPC-treated Zn sludge system. Comprehensive characterization (by X-ray diffraction (XRD), ³¹P nuclear magnetic resonance (NMR), and extended X-ray absorption fine structure spectroscopy (EXAFS)) and thermodynamic modeling results revealed that the incorporated ZnO preferentially reacted with phosphate to form Zn₃(PO₄)₂·2H₂O/Zn₃(PO₄)₂·4H₂O, changing the orthophosphate environment in the MPC system. Stronger chemical bonding between Zn and phosphate in comparison to the bonding between Mg and phosphate also resulted in the formation of amorphous Zn₃(PO₄)₂·2H₂O/Zn₃(PO₄)₂·4H₂O. Zn₃(PO₄)₂·4H₂O precipitate appears to predominate at high {K⁺}{H⁺}{HPO₄^2-} values, and the formation of Zn₃(PO₄)₂·2H₂O/Zn₃(PO₄)₂·4H₂O competed for the Mg sites in the MPC system, leading to the inhibition of formation of Mg-phosphate precipitates. Overall, this work uncovers the precipitate chemistry and microstructure transition of Zn species in the MPC system, providing new insights into the sustainable S/S of Zn-contaminated wastes by adopting MPC.

Recent Progress in Sludge Co-Pyrolysis Technology

With the development of society and industry, the treatment and disposal of sludge have become a challenge for environmental protection. Co-pyrolysis is considered a sustainable technology to optimize the pyrolysis process and improve the quality and performance of pyrolysis products. Researchers have investigated the sludge co-pyrolysis process of sludge with other wastes, such as biomass, coal, and domestic waste, in laboratories. Co-pyrolysis technology has reduced pyrolysis energy consumption and improved the range and quality of pyrolysis product applications. In this paper, the various types of sludge and the factors influencing co-pyrolysis technology have been classified and summarized. Simultaneously, some reported studies have been conducted to investigate the co-pyrolysis characteristics of sludge with other wastes, such as biomass, coal, and domestic waste. In addition, the research on and development of sludge co-pyrolysis are expected to provide theoretical support for the development of sludge co-pyrolysis technology. However, the technological maturity of sludge pyrolysis and co-pyrolysis is far and needs further study to achieve industrial applications.

Reduction-Magnetic Separation of Pickling Sludge by Biomass Pyrolysis Reducing Gas

The neutralization process of carbon steel pickling wastewater produces a large amount of steel hydrochloric acid pickling sludge (SHPS), and improper treatment of this sludge poses a serious threat to the environment. Considering that SHPS contains a large amount of iron oxide and given the huge demand for iron concentrate in China's ironmaking industry, refining iron oxide in SHPS into iron concentrate will have great environmental and economic benefits. This paper proposes a new method that uses biomass (corncob) to replace conventional coal-based reductants for the recovery of iron components in SHPS to simultaneously utilize two kinds of solid waste resources. Factors that affect the iron recovery rate and iron grade of SHPS, such as the reaction temperature, corncob dosage, residence time, and magnetic field strength, were studied using a fixed bed and a magnetic separator. These studies were combined with thermodynamic analysis, thermogravimetric analysis, X-ray diffraction, inductively coupled plasma-mass spectrometry, gas chromatography, etc. The results showed that when the reaction temperature was 680 °C, the corncob dosage was 5%, the residence time was 20 min, and the magnetic field strength was 200 mT, the recovery rate of iron reached 91.83%, and the iron grade of the recovered products was 67.72%, meeting the level I requirements in GB/T 32545-2016. Based on this result, a process involving SHPS reduction roasting with corncob pyrolysis reducing gas-magnetic separation was established to recover iron from SHPS. This process not only effectively utilizes the iron oxide in SHPS by converting it into iron concentrate powder for the ironmaking industry but also proves that the pyrolysis gas of corncob has good reduction ability.

Composition and Leaching Toxicity of Hydrochloric Acid Pickling Sludge Generated from the Hot-Dip Galvanized Steel Industry

Steel hydrochloric acid pickling sludge (SHPS), containing the heavy metals Fe, Zn, and Ni and a high chloride salt content, is considered a type of hazardous solid waste because of its potential harm to human health and the environment. In addition, the SHPS yield is large, but the main treatment currently used is only safe for landfills. Although studying the composition and leaching toxicity of SHPS is of great importance, only a small amount of related literature is available. This paper can help compensate for this deficiency. SHPS is analyzed from the aspects of its formation mechanism, pH, moisture content, elemental concentration, phase composition, microstructure, and leaching toxicity. The results show that its pH ranges from 2.25 to 11.11, and the moisture content ranges from 45.47% to 83.34%. Additionally, the concentration of Fe is the highest, with values from 29.80% to 50.65%, while other alkali metal elements, namely, Ca, K, and Na, have values of 0.36% to 23.07%, 0.02% to 19.82%, and 0.38% to 3.31%, respectively. Heavy metal elements, namely, Zn, Ni, Mn, Cr, and Pb, have values of 0.02% to 14.88%, 0.001% to 0.05%, 0.03% to 0.38%, 0.01% to 0.09%, and 0.02% to 0.19%, respectively. Anions, namely, SO₄ ^2-, Cl^-, F^-, and NO₃ ^-, have contents of 0.09% to 0.34%, 0.54% to 5.73%, 0.001% to 0.04%, and 0.01% to 0.15%, respectively. X-ray diffraction (XRD) analysis shows that Fe and Zn are mainly present in oxides, Ca is present as CaO and CaCO₃, and chlorine is present in NaCl. Moreover, scanning electron microscopy (SEM) analysis shows that the microscopic structure consists mainly of bright and fluffy irregular spheres; stripes; flakes; and dark, very small irregular particles. The leaching toxicity test based on HJ/T 299-2007 (China) was performed, where SHPS samples were treated with a mixed solution of sulfuric acid, nitric acid, and pure water (pH = 3.20 ± 0.05) at a liquid-to-solid ratio of 10:1 for a period of 18 h. The leachate was filtered and analyzed for Cr, Ni, Mn, Zn, etc. The leaching results indicate that Zn and Ni are the main elements that cause SHPS to be hazardous to the environment. These research results can provide a reference for later researchers studying the effective treatment of SHPS, such as more effective treatments for reducing toxicity and resource utilization.

Long-term and high-bioavailable potentially toxic elements (PTEs) strongly influence the microbiota in electroplating sites

Multiple potentially toxic elements (PTEs) wastes are produced in the process of electroplating, which pollute the surrounding soils. However, the priority pollutants and critical risk factors in electroplating sites are still unclear. Hence, a typical demolished electroplating site (operation for 31 years) in the Yangtze River Delta was investigated. Results showed that the soil was severely polluted by Cr(VI) (1711.3 mg kg^-1), Ni (6754.0 mg kg^-1) and Pb (2784.4 mg kg^-1). The spatial distribution of soil PTEs performed by ArcGIS illustrated that the soil pollution varied with plating workshops. Hard Cr electroplating workshops (HCE), decorative Cr electroplating workshops (DCE) and sludge storage station (SS) were the hot spots in the site. Besides, the toxicity characteristic leaching procedure (TCLP) - extractable Cr and Ni contents in different workshops were significantly related (P < 0.05) to their bioavailable fractions (exchangeable fraction (F1) + bound to carbonate fraction (F2)), which pose potential risk to humans. Although the soil total Pb concentration was high, its mobility was very low (<0.007%). Moreover, the soil microbial community dynamics under the stress of long term and high contents of PTEs were further revealed. The soil microbiota was significantly disturbed by long term and high concentration of PTEs. A bit of bacteria (Caulobacter) and fungi (Cladosporium and Monocillium) showed tolerance potential to multiple metals. Furthermore, the canonical correspondence analysis (CCA) showed that the bioavailable fractions (F1 + F2) of Cr and Ni were the most critical environmental variables affecting microbiota. Therefore, remediation strategies are required urgently to reduce the bioavailability of soil Cr and Ni. The results of this study provide an overview of the pollution distribution and microbial dynamics of a typical plating site, laying a foundation for ecological remediation of electroplating sites in Yangtze River Delta of China.

Recycling electroplating sludge as a monolithic catalyst for effective catalytic purification of volatile organic compounds

Electroplating sludge had a high content of heavy metals and usually lacked high-value-added utilization. In this work, Cu-containing sludge was used to synthesize a spinel catalyst, which was applied in catalytic oxidization of toluene. As a result, the sludge-derived spinel removed 50% of toluene (1000 ppm, 9600 h^-1) at 280 °C. In comparison, a reagent-synthesized spinel with a similar component removed 50% of pollutant at 294 °C. The sludge-derived spinel also showed a stable performance for over 50 h at 370 °C. Even when the initial concentration was increased to 5000 ppm, or the gas hourly space velocity was increased to 40,000 h^-1, the temperature for 50% removal was only increased to 303 °C. According to characterizations, surface oxygens of the sludge-derived spinel were more active than those in the reagent-synthesized one. Besides, the former had more active surface oxygens (207.9 μmol/g) than the latter (183.1 μmol/g). Furthermore, the sludge-derived spinel was coated on a monolithic honeycomb, which were also effective in catalytic oxidization of toluene. The main results of this work were in favor of high-value-added utilization of hazardous solid waste and promoting its real industry application.