Removal, recycle and reutilization of multiple heavy metal ions from electroplating wastewater using super-stable mineralizer Ca-based layered double hydroxides
Graphical abstract
Introduction
Nowadays, serious pollution of wastewater with heavy metal ions (such as Cu2+, Zn2+, Co2+, Ni2+, AsO2- etc.) is caused by discharge from traditional industries like mining refining, electroplating, chip manufacturing, etc., stimulating great attention from scientific researchers and industry (Guo et al., 2016, Kaur et al., 2022, Kirichenko et al., 2020, Li et al., 2022, Li et al., 2014). These undisposed heavy metals cannot be self-degraded by nature and would be enriched in the ecosystem and harmful to humans and the environment (Fu and Wang, 2011, Kong et al., 2021). Furthermore, considering the high cost of the heavy metal resource, such discharge treatment also results in a waste of metal resources (Yu et al., 2021). In China, the electroplating industry annually discharges 400 million tons of electroplating wastewater, the discharge of electroplating wastewater has caused serious environmental pollution and resource waste (Gong et al., 2021). The soil polluted by electroplating wastewater can transfer heavy metals from crops to human beings, causing great harm to human life and health. It is imperative to develop effective and economical technology to eliminate and recycle toxic heavy metals from wastewater to establish a green and sustainable society.
Recently, several methods have been extensively employed in treating heavy metal ions-contained wastewater, including chemical precipitation (Kurniawan et al., 2006), electrochemical redox (Wei et al., 2018, Wu et al., 2019), bioremediation (Leong and Chang, 2020, Yang et al., 2020), membrane filtration technology (Abdullah et al., 2016, Bruno et al., 2021, Vilela et al., 2016), photocatalysis (Yu et al., 2020), and ion exchange (Dong et al., 2021). Among these methods, chemical precipitation requires the addition of excessive hydroxide or sulfide, which results in a high pH value of the effluent or H2S pollution (Fu and Wang, 2011); Other methods such as electrochemical redox are constrained by energy consumption and efficiency. Adsorption has been seen as a more economical option than the aforementioned methods. Adsorption materials such as carbon materials (Chen et al., 2009, Vuković et al., 2010), clay materials (Liu et al., 2021), hydrogel (Godiya et al., 2019), modified metal oxide (Lingamdinne et al., 2017), metal–organic frameworks (Bui et al., 2020, Sun et al., 2018, Xu et al., 2020, Zhang et al., 2021), have been widely studied recently. However, their affinity and adsorption capacity for Cd is still insufficient, which results in high capital cost. It remains challenging for these materials to quickly decrease the concentration of heavy metal ions to the WHO drinking water standard (like 50 ppb) levels in a complex environment. Thus, it is significantly urgent to develop a new type of materials to satisfy the actual requirements that can adapt to complex environments and recycling.
Layered double hydroxides (LDHs) are a type of 2D layered clay, which have garnered significant interest as adsorbents or mineralizers due to their distinctive features. The formula of layered double hydroxides (LDHs) is [M2+1-xM3+x(OH)2]x+·(An−)x/n·mH2O, in which M2+ and M3+ are bivalent and trivalent metal cations and An− is the anion that compensates for the positive charge. LDHs have been extensively used in the removal of heavy metal ions and anion molecules (like Cu2+, Cr2O72− etc.) (Celik et al., 2021; Changjuan et al., 2023; Gore et al., 2016, Lai et al., 2022, Rojas, 2016, Wang et al., 2022, Wang and O’Hare, 2012, Yang et al., 2022). Mineralization of heavy metal ions using LDHs is more effective and stable due to the existing covalent bonds, van der Waals forces, and electrostatic interactions in the LDHs structure than other clay materials, providing a pathway for the efficient super stable mineralization removal of heavy metal ions. Recently, the Cd2+ ions can be immobilized by forming a super-stable mineralization structure (SSMS) CdAl-LDH.(Kong et al., 2021) The average content of Cd in wheat grain is below the national standard with consecutive 5-years for one-time remediation, showing robust long-term stabilization ability as reported by Kong using CaAl-LDH as a mineralizer. Our group recently reported that Ni2+ could be mineralized using CaFe-LDH with a capacity of 321 mg/g.(Kong et al., 2021; Chi et al., 2022) However, the effect of trivalent metal components in LDH was barely investigated. There is still a lot of room for optimization of the current materials, like enhancing the removal ability of heavy metals and application in complex mixed/actual environments (like in the solution with thousands ppm content of various heavy metals), and study on the recovery and reuse of the mineralization products (Zhao et al., 2020).
Herein, CaAl/Fe-based LDHs were synthesized by a simple one-step co-precipitation approach (Scheme 1). CaAl-LDH provided a remarkable removal capacity over individual or mixed Cu2+, Ni2+, Zn2+, and Co2+ ions within a short time (5 min) of >400 mg/g. Additionally, it can be treated to the ppb level, far lower than the WHO drinking water standard for the actual electroplating wastewater, with >9000 ppm of Cu2+ and >7500 ppm of Ni2+. X-ray fine structure (XAFS) and density functional theory (DFT) findings further examined the mineralization mechanism, and CaAl-LDH mineralized Cu2+ into the layer of LDH and converted it into CuAl-LDH using isomorphous substitution. Similar findings can also be found in Zn2+, Ni2+, and Co2+ ions. CaAl-LDH also showed its potential in the remediation of heavy metal-contaminated soil. The mineralization product from Ni2+ indicated enhanced photocatalytic CO2 reduction activities, and the mineralization product from Zn2+ showed good potential in the removal of arsenite ions. From a resource recovery point of view, Cu foil can be electro-reduced from the Cu2+ mineralization product. This research suggests a strategy for the efficient removal of heavy metal ions and the optimal use of available resources.
Section snippets
Synthesis of LDHs nanosheets
A 40.0 mL solution composed of 13 mmol CaCl2 and 6.5 mmol FeCl3·6H2O was added drop by drop to a 10.0 mL solution. Simultaneously, NaOH solution (1 mol/L) was added dropwise to maintain the system at a pH value of 12.3 under magnetic stirring at room temperature. The precipitations, denoted as CaAl-LDH, were collected by washing, centrifugation, and drying. The CaFeAl-LDH and CaFe-LDH were prepared by the same method except for the type of metal salts, and products were denoted as CaFeAl-LDH
Characterization of Ca-based LDHs
To address the practical issue, eco-friendly bivalent Ca and trivalent Al and Fe salts were chosen as raw materials for the coprecipitation-based industrial scale-up synthesis of LDHs. Thus, three kinds of Ca-based LDHs (CaAl-LDH, CaFeAl-LDH, and CaFe-LDH) were selected as the heavy metal ions treatment mineralizers. Fig. 1a showed the X-ray diffraction (XRD) patterns of CaAl-LDH, CaFeAl-LDH, and CaFe-LDH, proving the successful synthesis of Ca-based LDH nanosheets with obvious (00 l) peaks.
Conclusion
Heavy metal ions in electroplating wastewater and soil were treated using Ca-based LDHs with different Al/Fe ratios. CaAl-LDH showed the best performance, removing over 400 mg/g of Cu2+, Zn2+, Ni2+, and Co2+, which can also reduce real heavy metal ions in electroplating effluent to ppb levels. The experimental and DFT calculation results showed that CaAl-LDH mineralized Cu2+ into the main layer of LDH and isomorphous transformed into CuAl-LDH. The treatment of Zn2+, Ni2+, and Co2+ ions also
CRediT authorship contribution statement
Wenbo Xiong: Formal analysis, Investigation, Writing – original draft. Mufei Yang: Formal analysis, Writing – original draft, Investigation. Jikang Wang: Data curation, Software. Huijuan Wang: Investigation. Pu Zhao: Investigation, Resources. Zixian Li: Investigation, Software. Bin Liu: Investigation, Resources. Xianggui Kong: Resources. Haohong Duan: Project administration, Resources, Writing – review & editing. Yufei Zhao: Conceptualization, Formal analysis, Investigation, Supervision,
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This research was supported by the National Natural Science Foundation of China (22278030, 22090032, 22090030, 22288102, 22242019), the Fundamental Research Funds for the Central Universities (buctrc202119, 2312018RC07), Major Program of Qingyuan Innovation Laboratory (Grant No. 001220005), and State Key Laboratory of Fluorine & Nitrogen Chemicals, Xi’an Modern Chemistry Research Institute. Post-graduate Science Popularization Capability Promotion Program for 2022 sponsored by China Association
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These authors contributed equally.