Stabilization Of Lead,Cadmium,and Arsenic In Soil By Slow-release Iron Phosphate And Its Mechanisms Of Action

The pollution caused by the combined presence of lead（Pb）,cadmium（Cd）,and arsenic（As）in Chinese soil is severe.This problem is particularly prominent in areas such as metal mines,smelting areas,and metal processing plants,thereby becoming a major source of risk in watershed regions.Mainstream stabilization technologies for soil remediation of the complex pollution of lead,cadmium,and arsenic face difficulties achieving simultaneous stabilization of the three contaminants,due to significant differences in the chemical behavior between cationic lead,cadmium and anionic arsenic.Thus,exhibits poor long-term stability both domestically and internationally.To address this issue,a new strategy called"slow-release iron-phosphate synergistic stabilization of soil lead,cadmium,and arsenic"has been proposed in this paper.The strategy is based on the development of various slow-release materials,starting from the perspective of forming stable phases for lead,cadmium,and arsenic.The release behavior of phosphate,Fe²⁺,and Fe³⁺in slow-release materials was investigated,and the stabilizing effect of slow-release ferrous on soil lead,cadmium,and arsenic was clarified.This study explores the stabilizing effect and formation of stable phases of lead,cadmium,and arsenic under the synergistic action of slow-release trivalent iron/ferrous iron and phosphate.Furthermore,this study elucidates the phase transition of amorphous iron minerals（ferrihydrite）in soil under the action of slow-release iron and discusses in-depth the mechanism of the fixation of lead,cadmium,and arsenic during the transformation of iron minerals.This strategy provides a promising approach to simultaneously stabilize soil lead,cadmium,and arsenic pollution and achieve long-term stability,which is crucial for environmental protection and human health.The main innovative research results are as follows:（1）The slow-release ferrous effect of ball-milled zero-valent iron B-Fe⁰ and its stabilization effect on lead,cadmium,and arsenic in soil were determined.B-Fe⁰ was found to release ferrous ions sustainably in water,with a higher concentration than that of the original Fe⁰,reaching 4.34mg/L within 60 minutes.The addition of B-Fe⁰ resulted in a significant improvement in the stabilization efficiency of lead,cadmium,and arsenic in the soil,with water-extractable contaminants being stabilized up to 99%,and Na HCO₃-extractable arsenic and DTPA-extractable lead and cadmium being stabilized up to 72.52%,43.63%,and 34.71%,respectively after 56days.Additionally,the proportion of residual lead,cadmium,and arsenic increased from 5.58%,25.69%,and 58.01%to 28.94%,32.01%,and83.5%,respectively,indicating their transformation into more stable forms.Moreover,the study observed a continuous increase in the concentration of ferrous ions in the soil,reaching 938.22 mg/L within 56 days,after the addition of B-Fe⁰.The increase in ferrous ions led to an increase in the content of oxidized iron in the soil,promoting the transformation of iron oxides and facilitating the stabilization of soil contaminants.Further analysis using ferrihydrite,an important component of soil,as a simulation object confirmed that the production of ferrous ions resulted from the electron transfer between zero-valent iron and the surface Fe（III）of ferrihydrite.The released ferrous ions then promoted the transformation of ferrihydrite minerals into stable oxidized iron,（such as magnetite or lepidocrocite）.The contaminants,namely arsenic,cadmium,and lead,were then stabilized in the crystal structure of secondary minerals during the transformation process（2）The synergistic effect of slow-release trivalent iron and phosphate has resulted in the formation of stable phases of lead,cadmium,and arsenic in soil.Under the action of citric acid dissolution,CA-Fe HP-5 sustained released phosphate and trivalent iron,achieved concentrations of 9.98mg/L and 27.74 mg/L,respectively,within 60 minutes.The maximum stabilization efficiency of water-extractable lead,cadmium,and arsenic in soil by CA-Fe HP-5 were 80%,55%,and 99%,respectively.The stabilization efficiency of Na HCO₃-extractable As and DTPA-extractable Cd and Pb by CA-Fe HP-5 were 44.62%,40.83%,and 48.13%,respectively.The results of the BCR stepwise extraction indicated that lead,cadmium,and arsenic were transformed into more stable forms,and the residual forms increased by 12.11%,14.53%,and 36.42%after 28 days of reaction.Analysis of the chemical behavior and phase transformation of lead,cadmium,and arsenic during the reaction showed that the released phosphate competed with soil-adsorbed arsenic,transforming it into a dissolved state and reacting with the released trivalent iron to form iron-arsenic compounds.At the same time,the released phosphate reacted with cadmium and lead to form cadmium/lead-phosphate.（3）Based on the effect of slow-release iron in catalyzing the crystal phase transition of iron oxide to fix lead,cadmium,and arsenic and understanding the mechanism of slow-release phosphate to promote the formation of stable phases of these elements,researchers have explored the development of materials that can release iron and phosphate to simultaneously stabilize these pollutants in soil.In this study,under the action of citric acid dissolution,CA-Fe⁰-HAP-5 was found to sustainably release ferrous ions and phosphate,with concentrations reaching 26.01mg/L and 1.72 mg/L,respectively,within 60 minutes.The material’s effectiveness in stabilizing water-extractable lead,cadmium,and arsenic in soil was observed to reach 99%within 7 days,while its effectiveness in stabilizing Na HCO₃-extractable As,DTPA-extractable Cd and Pb was found to reach 92.61%,57.84%,and 62.81%,respectively.The chemical speciation analysis shows that lead,cadmium,and arsenic in soil were transformed into more stable forms over time,with residual forms increasing from 58.01%,25.69%,and 5.58%to 93.82%,47.86%,and48.54%,respectively,after 56 days.Furthermore,using ferrihydrite as a representative soil component,the mechanism of slow-release ferrous ions and phosphate was elucidated,and it was found that slow-release phosphate and ferrous ions have a synergistic effect on stabilizing lead,cadmium,and arsenic,as they react with these pollutants to form stable iron-arsenic compounds and cadmium/lead-phosphate.The slow-release phosphate also transforms adsorbed arsenic into dissolved arsenic,which then reacts with slow-release ferrous ions to form more stable forms.During the ferrous ion-catalyzed ferrihydrite phase transformation process,it was discovered that lead,cadmium,and arsenic were structurally bound to iron oxides.（4）An in-depth analysis of the phase transformation mechanism induced by slow-release ferrous ion on ferrihydrite and its interaction with lead,cadmium,and arsenic was explored.Slow-release ferrous ion undergoes interfacial electron transfer on the surface of ferrihydrite,forming unstable Fe（III）as the nucleation precursor of new-formed ferrihydrite,while promoting the continuous accumulation of adsorbed Fe（II）on the mineral surface,raising the system p H and lowering the redox potential and catalyzing the further transformation of ferrihydrite into a more stable phase.Under anaerobic conditions,the main minerals in the system are magnetite and lepidocrocite,while under aerobic conditions,the main minerals are goethite and magnetite.The chelation of citric acid with Fe²⁺inhibits the interfacial electron transfer between slow-release ferrous ion and the mineral,resulting in lepidocrocite phase as the final product.The phosphate ion reduces the electron transfer potential of ferrihydrite,leading to a decrease in the release of Fe²⁺,and a reduction in the extent of mineral phase transformation.Density functional theory calculations show that the energy gaps(E_gap)of ferrihydrite after adsorption of As,Cd,and Pb are 3.49 e V,3.45 e V,and 2.04 e V,respectively,all of which are higher than the E_gap of pure ferrihydrite（1.57 e V）,indicating an increase in the stability and a decrease in the reactivity of ferrihydrite.Cyclic voltammetry experiments show that the adsorption of lead,cadmium,and arsenic results in a decrease in the reduction potential of ferrihydrite:Fh（-0.537 V）>Pb/Fh（-0.545 V）>As/Fh（-0.562 V）>Cd/Fh（-0.566 V）,causing a decrease in the electron donating capacity of ferrihydrite and a reduction in the amount of Fe²⁺produced,thus inhibiting the slow-release ferrous ion-induced phase transformation of ferrihydrite.During the phase transformation of ferrihydrite,the proportion of stable lead,cadmium,and arsenic increases,and the structured bonding of lead,cadmium,and arsenic is fixed in the newly formed iron minerals,with Pb coordinating with O and Fe,Cd coordinating with O,and As coordinating with Fe-O-As.73 Figures,9 Tables,258 References...