Research progress on the oxalic acid-regulated zero-valent iron for pollutant removal in soil and water column
Zero-valent iron(Fe0),which adsorbs and reduces typical heavy metals and organic pollutants,has emerged as a promising and environmentally friendly reductant for soil and water pollution remediation.However,Fe0 tends to form surface oxides and corrosion products that reduce its electron transfer rate and overall efficiency,thereby degrading its performance.Accordingly,researchers have modulated Fe0 with oxalic acid(OA)to improve its surface properties and enhance its pollutant-removal capabilities.This review systematically summarizes the research progress on OA-modulated Fe0 in soil and water pollution remediation.We begin by discussing the advantages of OA modulation,highlighting its ability to improve the electron transfer rate of Fe0 by altering the surface oxide layer on Fe0 and promoting the redox cycle of ferrous ions.Through these processes,OA modulation enhances the removal of heavy metals,organic pollutants,and inorganic anions.We discuss the removal of Cr(Ⅵ)as a case study of heavy-metal removal.OA effectively promotes the reduction of Cr(Ⅵ)to Cr(Ⅲ)by acting as an electron donor,forming stable complexes with Fe(Ⅱ).OA also facilitates the removal of Cr(Ⅲ)-EDTA complexes by generating singlet oxygen(1O2)and promoting Cr(OH)3 precipitation.We then explore the application of OA-modulated Fe0 in the removal of other heavy metals such as As(Ⅲ),Cd,Pb,and Zn,demonstrating its potential for eliminating a wide range of heavy-metal contaminants.In the realm of organic pollutants,OA-modulated Fe0 can potentially remove chlorinated hydrocarbons such as trichloroethylene(TCE),perchloroethylene(PCE),and organic dyes.OA effectively eliminates the passivating Fe(OH)3 layer on Fe0,facilitating direct electron transfer and enhancing the adsorption and reduction of chlorinated hydrocarbons.In addition,OA promotes the generation of hydroxyl radicals in advanced oxidation processes such as the Fenton reaction and peroxymonosulfate(PMS)activation,enabling the efficient degradation of organic dyes.OA-modulated Fe0 also promises to remove inorganic anions such as nitrate and bromate.OA donates electrons during nitrate and bromate reduction,promoting the formation of reactive intermediates and enhancing the overall removal efficiency.We conclude by discussing the key factors influencing the performance of OA-modulated Fe0,namely,the OA concentration,pH,reaction time,and the presence of other chemical species.As guides for future research,we propose optimizing OA-modulated Fe0 materials,investigating the removal of complex pollutants,and coupling OA-modulated Fe0 with other remediation technologies such as bioremediation and electrochemical remediation.Overall,this review demonstrates the considerable potential of OA-modulated Fe0 in soil and water pollution remediation.By improving the surface properties and electron transfer capacity of Fe0,OA offers a promising and sustainable solution to a wide range of environmental contaminants.OA-modulated Fe0 offers several advantages over traditional Fe0:improved stability,reduced Fe0 leaching,and enhanced removal efficiency.As OA is a naturally occurring organic acid,OA modulation is also more environmentally friendly than other modification methods such as metal doping or the immobilization of Fe0 on carriers.Future research should focus on optimizing the preparation of OA-modulated Fe0 materials,investigating the removal of complex pollutants,and exploring the coupling of OA-modulated Fe0 with other remediation technologies such as bioremediation and electrochemical remediation.Investigating the long-term stability of OA-modulated Fe0 under different environmental conditions is essential for ensuring the effectiveness and sustainability of this technology in real-world applications.
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