Theoretical Study of Formation of Secondary Arsenic Minerals: Scorodite and Pharmacosiderite

This study models the formation of two important secondary arsenic minerals, scorodite and pharmacosiderite, from Fe(III) and As(V) ions using density functional theory, exploring small representative molecular clusters of the respective minerals. The formation enthalpies and free energies of multinuclear clusters representing the minerals are calculated in gas phase and in water using implicit solvent-based continuum model. The results indicate that the formation of small clusters of scorodite mineral from hydrated Fe(III) and hydrated As(V) ions is spontaneous and the formation enthalpies are negative when equimolar Fe(III) and As(V) ions are present. Formation of multinuclear closed ringlike clusters of Fe(III) and As(III) bridged by oxygen atoms were also studied, which are observed in extended mineral structure of scorodite. We have further studied the binding of arsenate to dimeric Fe(III) oxyhydroxide, also termed as arsenical ferrihydrite, and predicted the formation enthalpies for smallest unit of pharmacosiderite mineral from dimeric Fe(III) oxyhydroxide. The spontaneous formation of the unique Fe-4(OH)(4)O-12 core of pharmacosiderite mineral is expected only when the formation of larger Fe(III) oxyhydroxides are arrested by successive binding with arsenate ions present in the medium.