Effects Of Organic Matter On The Behavior Of As During Microbially Mediated Iron Redox Processes

Arsenic（As）is a widespread metalloid pollutant in the environment.Due to the abundant surface active sites of iron oxides in soil,As can be immobilized by means of adsorption or coprecipitation.However,the migration and transformation behaviors of As is affected by dissimilatory iron reduction.Under anoxic condition,dissimilatory iron reducing bacteria causes the reduction and dissolution of iron mineral,leading to the release of As.After converting to oxic conditions,ferrous iron（Fe（II））can be oxidized followed by the production of reactive oxygen species（ROS）,and the secondary minerals can immobilize the dissolved As.Organic matter（OM）significantly affects the microbially mediated coupling transformation of Fe and As.However,the mechanism of these processes needs further investigation.Therefore,in this study,the synthetic ferrihydrite was used as the model mineral to study the effect of OM on the fixation of arsenic through adsorption and coprecipitation,to clarify the effect of OM on the production of Fe（II）and the formation path of hydroxyl radical（·OH）during microbial iron reduction-oxidation processes,and to reveal the mechanism of arsenic migration and transformation in different OM-mineral-arsenic systems during redox processes.In order to clarify the influence of OM on As（V）immobilization by iron mineral,the ternary adsorption and coprecipitation systems of OM-mineral-arsenic were established to explore the efficiency of As（V）fixation by different combination methods,as well as the influence of humic acid（HA）and fulvic acid（FA）on As（V）adsorption by ferrihydrite.The results showed that the pseudo second-order kinetics fitted better for the As adsorption process by ferrihydrite,which was mainly controlled by chemisorption,and the intra-partical diffusion process was not the only rate-limiting step.The Langmuir isotherms model had a better fitting effect on the adsorption of As（V）with different initial concentrations by ferrihydrite,indicating the adsorption process was closer to monolayer adsorption.Additionally,the hydroxyl and carboxyl groups of OM competed with As（V）for adsorption sites on the surface of ferriydrite.Due to the larger molecular weight of HA than that of FA,HA blocked a large number of adsorption sites,resulting in a more obvious competitive adsorption between HA and As（V）.As-O-Fe bond was formed through coordination and ligand exchange between As（V）and H-O-Fe bond on ferrihydrite surface.Moreover,the removal rate of As（V）in the coprecipitation process was much higher than that in the adsorption process,but the coprecipitated minerals were easy to transform into hematite with higher crystallinity,leading to the slow release of As（V）.In the coprecipitation process,carboxyl group of OM exhibited a good affinity with minerals,inhibited the aggregation of minerals,and thereby enhanced the As（V）immobilization.In order to reveal the influence of different OM components on microbially mediated iron reduction and·OH production mechanism,Shewanella oneidensis MR-1 was used as dissimilatory iron reducing strain,and microbe-mineral-OM ternary systems were established with three different OM components,humic acid,fulvic acid and humin（HM）,extracted from black soil and peat soil.The results showed that OM as electron shuttles could promote the microbially mediated iron reduction process under anoxic conditions,depending on the electron transfer ability of OM components.Under oxic conditions,HA had a stronger inhibitory effect on·OH production due to its higher electron donating capacity.Furthermore,by adding ROS quencher,it was found that the mechanisms of·OH production in different OM-containing systems were different.The production of·OH in FA-containing system was mainly through one electron transfer process,while one-and two-electron transfer processes were present in HA-and HM-containing system.In addition,one electron transfer process dominated·OH production during the oxidation of microbially reduced OM.Moreover,the complexation between OM and Fe（II）inhibited the Fe（II）-catalyzed transformation of secondary minerals to more stable forms.Based on the above results,the OM-mineral-arsenic complexes was prepared with different OM components（HA and FA respectively）by adsorption and coprecipitation,and co-cultured with Shewanella oneidensis MR-1 to explore the effects of microbially mediated iron reduction and ROS production on the migration and transformation of As in different OM-containing systems.The results showed that HA promoted the reduction of As（V）and inhibited the reduction of Fe（III）in adsorption and coprecipitation systems.Under oxic conditions,Fe（II）and As（III）were oxidized,and the secondary minerals generated could immobilize As（V）,reducing the content of acid extracted As（V）.By adding ROS quencher to the system,it was found that·OH,superoxide radical(O₂^·-)and hydrogen peroxide（H₂O₂）played an important role in the transformation of arsenic.Among them,·OH was the main contributor to the oxidation of As（III）due to the higher reaction rate of·OH with As（III）than that of O₂^·-,while H₂O₂mainly generated·OH through Fenton-like reaction with Fe（II）,which indirectly oxidized As（III）.In conclusion,this dissertation studied the effects of OM on As（V）immobilization by iron mineral by means of adsorption and coprecipitation,clarified the mechanisms of microbially mediated iron reduction and ROS production in different OM-containing systems,and revealed the effects of microbial iron reduction and ROS production on the migration and transformation of arsenic under redox conditions.The results will be helpful for the in-depth understanding of the biogeochemical behavior of As and provide theoretical basis for the prevention and control of As pollution in environment.