Effect Of Al Substitution In Ferrihydrite On The Arsenic/iron Migration And Transformation Mediated By Reducing Bacteria

Arsenic is a highly toxic heavy metal element.Arsenic pollution in groundwater has threatened the health of millions of people around the world.The enrichment of arsenic by iron oxides/hydroxides is one of the most important geochemical processes controlling its transformation,migration,and bioavailability in groundwater environments.As(V)/Fe(III)microbial reduction,as one of the main factors affecting the distribution of As between solid and liquid phases in groundwater systems,has gradually become a hot topic and research focus in arsenic contamination of groundwater.Under natural conditions,aluminum commonly exists in various iron minerals in the form of isomorphic substitution due to its ionic radius being similar to that of Fe.The combination of Al and ferrihydrite often occurs in alternating anoxic-oxic environments.Considering that the isomorphic substitution of Al in ferrihydrite will affect its physical/chemical properties and the dissolution rate of biotic/abiotic reduction,understanding the effect of isomorphic substitution of Al on the migration and transformation of arsenic/iron mediated by reducing bacteria is of great significance.In this study,ferrihydrite with different Al substitution levels(Al Fh)were used as the experimental materials to investigate the effect of reducing bacteria(Shewanella oneidensis MR-1 and Citrobacter sp.JH012-1)on the biomigration and transformation of arsenic/iron in As-containing Al Fh or sediments with exogenous Al Fh.After clarifying the influence mechanism of the isomorphic substitution Al on the bio-reduction of ferrihydrite,the process of biologically arsenate and iron reduction,release and transformation with different amounts of Al substitution,different arsenic binding forms and different arsenic binding amounts in ferrihydrite were explored by microbial anaerobic batch experimental systematically.On this basis,the effects of exogenous Al Fh addition on arsenic biomigration and fixation in arsenic-containing sediments were discussed.Our results will contribute to a more precise description of the release,transformation,and refixation of arsenic caused by As/Fe reducing bacteria in arsenic-contaminated groundwater.The research has made the following advances:1.The mechanism of aluminum isomorphous substitution in bioreduction process of ferrihydrite was clarified.The bioreduction of Al-substituted ferrihydrite(Al Fh)by Shewanella oneidensis MR-1(MR-1)and Citrobacter sp.JH012-1(JH012-1)was studied.The effect of aluminum substitution in Fe(III)reduction and mineral transformat ion was quantified by calculating the efficiency of Fe(II)production.The results showed that the substitution of Al inhibited the bio-reduction of ferrihydrite.This was mainly due to the unequal dissolution of iron and aluminum in Al Fh during the bioreduction process,which resulted in a surface cover of aluminum oxides and further hinder the contact between bacteria and unexposed Fe(Ⅲ).Characterization of secondary minerals showed that under the incubation with JH012-1,the phase of ferrihydrite did not changes,while under the incubation with MR-1,ferrihydrite transfomed to akaganeite and magnetite and the substitution of Al changed the secondary mineral type from magnetite to akaganeite.2.The adsorption of arsenic in Al Fh and the migration and transformation of adsorbed arsenic under bacterial influence were investigated.It was found that with the increase of Al content,the amount of As(V)adsorbed on the surface of ferrihydrite was also promoted and the As/(Al+Fe)mol% increased from 3.44% to 3.69%.The adsorption kinetics and adsorption edge experiments of As(V)in Al Fh showed that the presence of Al can enhance the adsorption of As(V)by increasing the protonation degree of ferrihydrite.When incubated with reducing bacteria,the Fe(Ⅲ)reduction in Al Fh containing adsorbed arsenic decreases with the increase of Al substitution.In addition,MR-1 induced the transition of ferrihydrite to akaganeite and parasymplesite,these secondary minerals favor the refixation of arsenic released into solution.For strain JH012-1,the substitution of Al promoted the migration of arsenic,which was mainly due to the weak affinity of As(Ⅲ)towards Al containing ferrihydrite.3.Experimental studies were conducted on the biological migration and transforma-tion of arsenic in As-Fe-Al ternary complexes.Elemental analysis found that there is a competitive relationship between As and Al in the complex mineral materials.By controlling the content of As and Al in composite minerals,the reduction and release of arsenic and iron from As-Fe-Al with the participation of reducing bacteria were analyzed.The results showed that the precipitation of As(V)could weaken the inhibitory effect of Al substitution on bacterial iron reduction through reducing the content of Al in ferrihydrite or changing the surface charge of the ferrihydrite.In addition,the precipitated As and Al in As-Fe-Al ternary complexes jointly regulated the types and morphology of biogenic secondary minerals.For the MR-1 reduction experiment,the content of As in As-Fe-Al complex changed the type of secondary mineral and the rule of As(V)release.The increase of Al content result in the morphology of the secondary mineral parasymplesite transform from rod to flaky.For the JH012-1 reduction experiment,only when As:(Fe+Al)mol%=0.1,a weak characteristic peak of parasymplesite could be observed in the Al Fh-0 experimental group,and the substitution aluminum promoted the release of arsenic.4.The influence of exogenous Al Fh addition on the biotransferability and stability of arsenic in sediment was examined.The biomigration and transformation of arsenic/iron in sediment C2-11 were explored,and it was found that the participation of reducing bacteria could lead to a drastic reduction of Fe(Ⅲ),in which the release of As(V)was simultaneous with the production of soultion Fe(II).When exogenous Al Fh was introduced,bacteria mediated As release was significantly inhibited,while the increase of Al content in ferrihydrite promoted the release of As(Ⅲ).Arsenic extraction experiments of sediment showed that exogenous Al Fh could promote the fixation of arsenic and enhance the stability of arsenic in sediments after bioreduction.