Effects Of Phosphate On The Formation Of Secondary Iron Minerals And Cadmium Transformation Induced By Dissimilatory Fe(?) Reduction

Heavy metals in soil have the characteristics of persistant accumlation,high toxicity,difficult migration and so on.The morphological transformation is an important process to influence the toxicity and migration of heavy metals.Simultaneously,it could be harmful to human health that heavy metals transfer into crops,water and atmosphere.Under anaerobic conditions,the abundant iron and microbial Fe(?)dissimilatory reduction in soil provide favorable conditions for the formation of secondary iron minerals with different crystalline forms in the natural environment.The dissimilatory reduction of ferrihydrite is influenced by various environmental factors(inorganic oxygenated salt ions,organic matter,microbial species and quantity,etc.)and ferrihydrite transformed into secondary iron minerals with different microscopic properties and crystal structure,such as magnetite,green rust or vivianite.These secondary iron minerals have significant differences in their morphological characteristics(different surface areas,surface reactivity,etc.)and have different effects on the morphological transformation of heavy metal cadmium.For example,the secondary mineralization process of ferrihydrite is promoted and ferrihydrite could be tranformed into vivianite or green rust and siderite in the presence of phosphate.The microcrystalline structure of secondary iron minerals can absorb large amount of Cd(?)or ion cluster,or coprecipitate with Cd(?)or ion clusters in solution,thus changing the morphology of cadmium.While ferrihydrite could be transformed to conductive magnetite or goethite and hematite without phosphate and part of Cd(?)was adsorbed by secondary iron minerals,so the available Cd(?)concentration in the system decreased.In this paper,Geobacter soli GSS01 as the driving force of ferrihydrite reduction,isolated and identified in our laboratory.Secondary biomineralization of ferrihydrite was coupled with cadmium speciation transformation to study the effect of the crystallinity transformation of ferrihydrite on the morphological transformation of cadmium.The results showed that the difference of iron reducing rates and the mineralization products led to different changes of cadmium morphology.In addition,cadmium-contaminated paddy soil in Hunan Zhuzhou as the inoculum,we further investigated the response of microbial community structure to the transformation of cadmium mophorlogy induced by secondary bio transformation of ferrihydrite and the microbial community structure.The main conclusions are as follows:(1)The ferrihydrite mineralization process with or without phosphates and the effect of secondary minerals on the transformation of cadmium by GSS01 were investigated.The results of this study indicated that phosphate affects ferrihydrite mineralization and the secondary minerals have an obvious impact on transformation of cadmium.Ferrihydrite was transformed into vivanite under phosphate amendment by dissimilatory reduction bacteria GSS01,in which most of soluble cadmium was absorbed onto the surface of vivianite:the soluble phase of cadmium decreased and exchangeable phase,iron oxide surface-bound phase and amorphous iron oxide-bound phase cadmium increased.In the absence of phosphate,the dissimilatory Fe(?)reduction resulted in the formation of goethite.The goethite produced under Cd stress has no obvious effect on the morphological transformation of cadmium.(2)We constructed the anaerobic coexistence system of'paddy soil inoculum-ferrihydrite-cadmium",to study the impact of secondary mineralization of ferrihyrite impact on mophorlogical transformation of cadmium with phosphate and the response of microbial community structure to the secondary mineralization and mophorlogical transformation of cadmium.The results showed that phosphate accelerates the reduction of amorphous ferrihydrite and the main secondary product was siderite,in which the morphology of cadmium was transformed into stable forms.While in the absence of phosphate treatment,ferrihydrite was transformed into magnetite and hematite,and the concentration of available cadmium in the experimental system is significantly reduced by magnetite produced under Cd stress.As the reaction proceeds,Cd stress causes the decrease in the abundance of typical dissimilatory iron reducing bacteria Geobacter in the system;while in the presence of phosphates,Geothrix and Desulforispora replace part of Geobacter and become the dominant driving force for dissimilatory Fe(III)reduction.