Mechanisms Of Heavy Metals Adsorption On Typical Iron And Manganese Oxides Under Electrochemical Control And Its Application In Contaminated Soils Remediation

The quality and safety of the soil and surface/ground water environment is essential for maintaining the rapid development of agriculture,society and the economy and protecting human health.With the rapid development of urbanization and industrialization,heavy metal pollution caused by industrial activities is also worsening.Heavy metals in environment have strong mobility,high toxicity and are nondegradable,which makes them easily absorbed and enriched by crops in production activities.Therefore,heavy metals seriously affect the yield and quality of crops and accumulate in the human body via the food chain to endanger human health.Recently,a variety of effective methods have been developed to remove heavy metals.However,these existing methods require the addition of environmentally unfriendly chemicals and excessive pretreatment steps.Hence,it is very necessary to design an efficient means for removal of heavy metals for the sustainable development of the environment.The adsorption technology based on environmental materials such as iron and manganese oxides has attracted wide attentions.Their heavy metal adsorption performance is closely related to their chemical composition and surface structure.Electrochemical methods can drive the oxidation-reduction reaction of transition metal oxides.This process may be used to generate high-performance adsorbents or regulate the chemical components and micromorphology of adsorbents,so as to improve heavy metals removal.This dissertation explores the mechanism of enhancing the adsorption capacity of heavy metals in the formation and transformation process of iron and manganese oxides under electrochemical regulation from the two gradations of principle and application.The mechanisms of electrochemical redox reaction were studied to improve the adsorption capacity of heavy metals on iron and manganese oxides,the iron and manganese resources in the natural environment were made use of to generate iron and manganese oxides for heavy metals adsorption,and the electrochemical adsorption technologies were expanded to the remediation of polluted soil combination with soil leaching.The specific research contents and results are as follows:1.The adsorption of heavy metal cation(Cu²⁺)on typical manganese oxides（birnessite）was enhanced by multi-cycle redox reaction under electrochemical control,and the effects of solution p H and potential window were further investigated.The results showed that the electrosorption capacity for Cu²⁺reached as high as 372.3?mg?g^-1 by electrochemical redox,which was remarkably larger than the adsorption isotherm capacity(44.3?mg?g^-1).In addition,birnessite electrode could be reused for many times after electrochemical activation.In the process of electrosorption,the amount of copper electrodeposited on the counter electrode accounted for less than 3.2%of the total removal capacity.The enhancement of Cu²⁺adsorption capacity could be attributed to the changes in the chemical composition and the dissolution-recrystallization processes of birnessite during the electrochemical redox reactions.The electrosorption capacity increased with increasing p H from 3.0 to 5.0 and potential window width.This study revealed the key role of reduction reaction of manganese oxide to heavy metal cations adsorption.2.The adsorption of heavy metal anion（Cr（Ⅵ））on typical iron oxides（magnetite）was improved by electrochemical reduction reaction,and the influence of p H,potential,and coexisting anions including NO₃^-,Cl^-,and SO₄^2-on the adsorption capacity was also investigated.The results showed that the highest Cr（Ⅵ）reduction percentage reached93.7%with a total Cr removal capacity of 514.7?mg?g^-1 at optimized p H 2 and-0.2?V（vs.SCE）in supporting electrolyte of KNO₃.Cr（Ⅵ）was reduced to Cr（III）on the surface of magnetite due to the direct electrochemical reduction at low potentials and reduction by Fe_aq²⁺electrochemically generated from magnetite.The Cr（III）was subsequently removed and easily separated due to the formation of Cr（OH）₃ precipitate on magnetite surface when KNO₃ and KCl were used as supporting electrolyte;however,when K₂SO₄was used instead,Cr（OH）₃ precipitate was not observed.The decrease in p H（≤2.0）and electrical potential ranging from-0.6–0.2 V（vs.SCE）was found to facilitate the reduction and removal of Cr（Ⅵ）.This study clarified the mechanisms for improving heavy metal anions adsorption on iron oxides under electrochemical reduction.3.The mixed anionic and cationic heavy metals in soil washing effluent extracted using organic acid were electrochemically removed by inert electrodes,the mechanisms for the electrochemical removal of various complexed heavy metals were studied,and the effect of applied current density and the regeneration performance of electrodes were further investigated.Soil extraction by 200 mmol kg^-1 of citrate yielded leachate solution with241,107,29,and 15 mg L^-1 of As,Zn,Pb,and Cd,respectively,and then the soluble heavy metals were electrochemically removed using the commercial Ti/mixed metal oxides（MMO）anode and graphite felt cathode(current density of 26 m A cm^-2).The local alkaline environment provided by electro mediated water reduction caused the precipitation of Zn（II）and Pb（II）at cathode surface,the hydrolytic precipitation of Fe（III）washed out from Fe-riched soil led the coprecipitation of As on cathode,and the low potential of cathode aroused the cathodic electrodeposition of Cd（II）.These combined cathodic reaction processes decreased the toxic heavy metal concentrations by over99.4%,and the anode show less degradation effect to citrate.The electrode materials exhibited high stability during the multiple rounds of reuse.Increasing current density accelerated heavy metals removal,but also aggravated the oxidative degradation of citrate.This work opens up the possibility to develop efficient and sustainable heavy metals removal method from soil leachates.4.A remediation method composed of soil washing using low-molecular-weight organic acids and electrochemical adsorption was used to remove Cu（II）and Zn（II）from polluted soils,the electrochemical adsorption mechanism as well as the influence of p H,organic acid type and voltage were investigated,and the soil remediation effect was further evaluated by the cultivation of rape.After extraction by citrate at initial p H 8.3 and electrochemical adsorption at 0.9 V for 7 d,the concentrations of total and bioavailable Cu（II）in soils decreased from 1090 mg kg^-1 and 281 mg kg^-1 to 391 mg kg^-1 and 52 mg kg^-1,and those of Zn（II）decreased from 262 mg kg^-1 and 39 mg kg^-1 to 208 mg kg^-1 and30 mg kg^-1,respectively.Cu（II）and Zn（II）ions were mainly electrochemically adsorbed on the carbon cathode and anode,respectively,resulting in decreases of their concentrations to below 1 mg L^-1 in the leachate.The presence of organic acids improved the remediation performance in the order of citrate>oxalate>acetate.The decrease in the initial p H of citrate solution enhanced the removal rate of Zn（II）,while seemed to have no effect on that of Cu（II）.The removal capacity for heavy metals decreased with decreasing cell voltage from 0.9 to 0.3 V.In the rape cultivation experiment,the Cu（II）and Zn（II）contents in shoot and root were decreased by more than 50%,validating the soil remediation effect.This study explored the coupling mechanisms of organic acid soil leaching and electrochemical adsorption in the remediation of heavy metal cation contaminated soil and the effects of various operating conditions.5.Organic acid（citrate）washing and electrochemical removal（manganese oxide cathode）were combined to remediate highly As-contaminated soils,and the effect of voltage was investigated as well.Citrate could extract the As bound to iron and aluminum oxides and enhance As mobility by indirectly reducing As（V）to As（III）in the soils.During the electrochemical removal of As,the rhodochrosite produced from the reduction of birnessite at the cathode,the birnessite generated from the re-oxidation of released Mn（II）and the ferrihydrite formed from the hydrolysis of Fe（III）at the anode together contributed to the adsorption and fixation of As in the leachate.After three successive rounds of combined remediation by citrate(0.1 mol L^-1)washing and electrochemical removal with birnessite electrode at 1.5 V,the As was totally removed in the leachate and the content of As bound to iron and aluminum（hydr）oxides was reduced by 84.2%in soils.Correspondingly,the contents of total and bioavailable As in the soil decreased from1980 and 242 to 563 and 86 mg kg^-1,respectively.The As removal efficiency from the leachate and soil increased with increasing voltage from 0.5 to 1.5 V.This study clarified the mechanism for heavy metal anions adsorption on the electrogenerated iron and manganese oxides in the process of heavy metal anion-contaminated soil remediation by combined soil leaching and electrochemical removal technology.