The Effect Of Iron-bearing Minerals Phase Transformation On Antimony(Ⅴ) Behavior In Aqueous Media

Antimony（Sb） is a heavy metal with growing environmental concern because of severe toxicity and potential carcinogenicity.The wide applications in various products（e.g.,flame retardants,alloys,semiconductors,plastics）and uncontrolled release into the aqueous media,which have brought hazard to the ecological environment and human being health.Iron oxyhydroxides are the most important and reactive components in aqueous media,influencing the geochemical cycle and mobility of Sb and other metals.It is significant to clarify the speciation of Sb in the iron minerals transformation process.In this study,the behavior of Sb（Ⅴ）during Fe（II）-induced ferrihydrite transformation,including interaction of Sb（Ⅴ）and secondary minerals and the corresponding mechanism were investigated.The main conclusions are as follows:（1）The distributions of Sb in typical rivers were analyzed.The results show that the high concentration of Sb located to an abandoned mining area,reaching 1.66 mg·L^-1,which only 7.44μg·L^-1 in a forest area,owing to luxuriant plants.The Sb content in minerals around the rivers was also analyzed,and strong positive relationship between iron oxyhydroxides and Sb（Ⅴ）was observed.The removal efficiency of typical iron oxyhydroxides towards Sb（Ⅴ）under different dosages and temperatures was investigated.The removal efficiency of Sb（Ⅴ）as follow:pyrite>siderite>goethite>hematite,implying the important effect of iron-bearing mineral species on the Sb（Ⅴ）behaviors.（2）The speciation of Sb（Ⅴ） during the phase transformation of ferrihydrite was investigated.First,as the Fe（II）inducement,metastable ferrihydrite was easily transformed into more thermodynamically stable minerals,such as goethite,hematite,and magnetite.And different transformation products were obtained under different p H conditions.Moreover,the binding mechanisms of Sb（Ⅴ）on iron-bearing minerals in different transformation pathway were clarified.At p H 4～8,electrostatic attraction was the dominant contribution for the binding of Sb（Ⅴ）on ferrihydrite and the transformation products.At p H 10～11,the electrostatic attraction decreased and turned to electrostatic repulsion,which diminished the binding ability of Sb（Ⅴ）.Furthermore,a series of characterizations were used to investigate the speciation of Sb（Ⅴ）binding with the transformation products.The results show that the binding mainly included surface precipitation,surface complexation,and structural incorporation.Sb（Ⅴ）existed partly on the surface of goethite,hematite and magnetite formed as Fe Sb O₄.Part of Sb（Ⅴ）combined with transformed goethite,hematite,and magnetite through inner-sphere bidentate binuclear complex,and the remaining Sb（Ⅴ）entered the lattice structure.（3）The effect of phase transformation of iron-bearing minerals on Sb（Ⅴ）behavior in real aquatic environment was investigated.The result shows that the phase transformation greatly decreased the concentration of Sb（Ⅴ）with removal efficiency of more than 98%,indicating its dominant effect.The effect of humic acid on phase transformation of iron-bearing minerals to influence the behavior of Sb（Ⅴ）was investigated.The results proved that the surface of iron-bearing minerals was coated by humic acid at p H 4～7,which inhibited the iron-bearing phase transformation,but enhanced the coordination contribution of Sb（Ⅴ）by numbers of active functional groups（o-phenolic hydroxyl,o-carboxyl,etc.）on the surface of humic acid.Furthermore,the surface electronegativity of iron-bearing minerals was enhanced by dissociation of humic acid,inhibiting the binding of Sb（Ⅴ） at pH 8～10.