| The interaction zone between surface water and groundwater is a dynamic redox fluctuation zone,in which the hydrologic and biogeochemical processes change dynamically that that may affect arsenic migration and transformation.The geochemical cycling behavior of arsenic in sedimentary environments can be interacted with the geochemical cycling of iron through processes such as adsorption-desorption,coprecipitation,and redox transformation.The sulfates(or sulfides)carried by surface water and groundwater exchange,together with the microbial reduction of sulfates in the interactive zone can lead to the enrichment of inorganic sulfur,which can change the properties of sedimentary iron minerals and ultimately change the migration behavior and fate of arsenic.The existing researches mainly focus on the spatiotemporal variation characteristics of arsenic species and content in the water system,and the mechanism of arsenic migration and transformation in the iron-sulfur-arsenic subterranean system is still unclear.However,there have been many mechanism studies on the single system of ironarsenic,iron-sulfur and sulfur-arsenic systems,but relatively few studies on the coupling dynamic process of the complex system of iron-sulfur – arsenic.Iron,sulfur,and arsenic are all redox sensitive elements,the investigation of the dynamic changes of this ironsulfur-arsenic system under redox-variable environments is critical for understanding arsenic transformation and transport in surface water and groundwater interaction zones.This study focused on the mechanisms of iron mineral transformation dynamics in coupling arsenic transformation and transport in the iron-sulfur-arsenic system.The study is mainly carried out with batch and column apparatus.A variety of macroscopic and microscopic characterizations(such as XRD,FTIR,BET,XPS,SEM,HIM,Cs-STEM,etc.)were used to provide insights into the dynamic changes of the iron-sulfur-arsenic system and their effects on arsenic migration and transformation.The specific research contents and relevant conclusions are as follows.(1)The coupled transformation of As-containing ferrihydrite and the kinetic arsenic release/re-adsorption in the presence of reducing S(-Ⅱ)were studied.It was found that the reduction of ferrihydrite by S(-II)was faster than the reduction of As(V)by S(-II)so that the reduction of ferrihydrite by S(-II)had the kinetic advantage over the reduction of As(V)in the iron-sulfur-arsenic anaerobic reaction system with p H value of 7.The result was the production of Fe(II),which was in reaction with residual ferrihydrite and S(-II)to form Fe S and other secondary Fe minerals.The As(V)was,however,not reduced as revealed by no observation of As(III).The newly formed Fe S and other secondary minerals covered on the surface of residual As-containing ferrihydrite,protecting it from further reduction.During the reaction,less than 10% of the adsorbed arsenic was quickly desorbed and released to the solution,and then gradually re-adsorbed by the secondary minerals.The rate and degree of arsenic re-adsorption depended on the dynamics of secondary mineral formations and their affinity to arsenic.(2)The morphology and properties of the iron minerals generated under different iron-sulfur-arsenic reaction sequences were analyzed,and the complexation forms of iron and arsenic and the adsorption amount of arsenic in each system was compared.In the system in which ferrihydrite reacted first with S(-II)and then with As(i.e.,pre-sulfidation system),lepidocrocite and goethite were found to be the crystalline secondary minerals.In the system in which ferrihydrite was first adsorbed with arsenic,and then reacted with S(-II)(i.e.,pre-adsorption system),no crystalline minerals were generated.When the molar ratio of S/Fe is greater than 0.5,a large number of amorphous Fe S and a small amount of magnetite were generated in both the pre-adsorption system and the presulfidation system,and arsenic could also form complexes with iron in the form of coprecipitation.Sulfidation of ferrihydrite greatly reduced its surface activity and the affinity to arsenic.However,weak sulfidation of As-containing ferrihydrite can provide new adsorption sites for arsenic,enhancing arsenic sorption.(3)The influence of S(-II)on mixed arsenic-containing mineral and the kinetic behavior of arsenic was investigated under the condition of doping non-sensitive redox mineral(alumina).It was found that the mechanism of S(-II)on arsenic-containing alumina was completely different from that of arsenic-containing ferrihydrite.In the anaerobic reaction system of Al-S-As with p H of 7,75.6% of As(V)on the surface of alumina was reduced to As(III)by S(-II)and gradually desorbed out,while arsenic desorbed out was no longer adsorbed.In a reduction system with a mixture of ferrihydrite and alumina,the concentration of sulfide and Fe/Al ratio all affected the release of arsenic.The mixing of alumina makes it harder for the ferrihydrite in the system to be converted into other minerals of better crystalline form.Compared with pure iron oxides,the structure of aluminum-iron co-precipitation minerals embedded with aluminum ions is more stable,and the doping of aluminum ions can reduce its reduction degree by S(-Ⅱ).Under the dynamic change of redox condition,the mixture of non-sensitive minerals can enhance the stability of the reaction system,making arsenic less likely to be released from the solid phase and reducing the risk of arsenic contamination in the water environment.(4)The influence of dynamic cycling changes of redox conditions on the migration and transformation of arsenic-bearing minerals and the migration characteristics of arsenic-bearing minerals in saturated porous media were studied.The results indicated that multiple cyclic redox condition changes caused frequent transformation of iron minerals,which decreased the capacity of the minerals to adsorb As.The reduction of As(Ⅴ)to As(Ⅲ)in the cyclic system also decrease As affinity,enhancing As migration.In saturated porous media,S(-Ⅱ)can reduced As-containing ferrihydrite locally and reduced Fe(II),together with S(-II),migrated downstream.In the migration,part of the Fe(Ⅱ)reacted with the S(-Ⅱ)to form Fe S,which mainly attached to the surface of the residual ferrihydrite.Another part of the Fe(Ⅱ)were attached to the residual ferrihydrite surfaces,promoting the formation of lepidocrocite and goethite.The released arsenic during the reductive dissolution of ferrihydrite also migrated with water flow.When oxygen-containing solution injected into the column,the oxygen oxidized Fe(Ⅱ)minerals in the column to form Fe(Ⅲ)minerals,enhancing arsenic adsorption and form ferric arsenate,which keeps the arsenic in the solid phase and makes it difficult to migrate.Overall,this study investigated the dynamic coupling of mineral and arsenic transformation under the changing redox conditions,revealed different behaviors of arsenic in redox sensitive/non-sensitive mineral systems in the presence of S(-Ⅱ),and found different transformation and migration behaviors of arsenic under cyclic changes of redox conditions.The results enhanced our understanding of As-Fe-S geochemical systems in the environment,and have implication to the As fate and transport in groundwater-surface water interaction zones,and to the prevention and mitigation of arsenic pollution in groundwater. |
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