Controls On Antimony And Arsenic Speciation Via Sorption And Redox Chemistry At The Clay Mineral - Water Interface In Natural And Laboratory Settings

Abstract

Adsorption and redox transformations of contaminants in soil and aqueous environments are often controlled by the available mineral substrates. Aluminosilicates and aluminum oxides are ubiquitous and can influence the speciation and, therefore, the transport and bio-availability of toxic elements such as arsenic (As) and antimony (Sb). It is important to understand the partitioning and redox reactions promoted by these substrates in order to understand and model the transport of As and Sb in soils and surface waters. This study provides a detailed investigation of the sorption and redox behavior of As and Sb in clay-rich natural and laboratory systems. Since Fe 3+ is often found substituting for Al3+ in clay mineral structures, we also investigated the role of structural Fe in redox transformations of As and Sb adsorbed at the clay surface in controlled laboratory experiments. In a natural system affected by the release of spent geothermal fluids (Mutnovsky geothermal fields, Kamchatka, Russia), As concentrations are elevated above background levels in the Falshivaia River water and sediments (< 65 microm size fraction). Arsenic from the geothermal source fluids is in the reduced As3+ form, and is oxidized to As5+ after mixing with river water. Both As3+ and As5+ are found in aqueous and adsorbed forms. Analysis of the extended x-ray absorption fine structure (EXAFS) spectra shows that sediment-phase arsenic is associated with both Al- and Fe-rich phases with a bi-dentate corner-sharing local geometry. A series of laboratory experiments were performed in order to investigate Sb adsorption by Al-rich mineral substrates at a macroscopic and molecular level. The EXAFS analysis of the experimental samples concluded that both Sb3+ and Sb5+ form inner-sphere sorption complexes on the surfaces of hydrous aluminum oxide (HAO), and the clay minerals kaolinite (KGa-1b) and nontronite (NAu-1). Primarily, bi-dentate corner-sharing with a minor amount of mono-dentate corner-sharing complexes were formed. The oxidation state of the clay structural Fe affects the adsorption capacity of nontronite; if the clay is partially reduced, the Sb5+ uptake is increased significantly. The long term dynamics of the As aqueous speciation in clay suspensions where reduced arsenic (As3+) was added initially is complex. A fast disappearance of As3+ due to oxidation to As 5+ was followed by a slow increase of aqueous As3+. This behavior is explained by two simultaneous reactions: fast oxidation of As3+ by structural Fe3+ (anaerobic) or Fe 3+ and dissolved O2 (aerobic) and the slow reduction of As5+ by dissolved Fe2+. The ability of the structural Fe in nontronite clay to promote oxidation of As3+/Sb 3+ was greatly affected by its oxidation state: if all structural Fe was in an oxidized Fe3+ form, no oxidation was observed; however, when ~ 20 % of structural Fe was reduced to Fe2+, the clay promoted the most extensive oxidation under both aerobic and anaerobic conditions. The structural Fe2+ is not able to reduce As 5+/Sb5+, but reduction was seen when aqueous Fe 2+ was present in the systems. These research findings indicate that As and Sb can be effectively immobilized by Al-rich phases, while the substrate nature and oxidation state of structural Fe, along with the presence of dissolved Fe2+, can greatly affect the fate and transport of As and Sb. The increase in Sb5+ uptake in response to reducing structural Fe, possible increase or decrease in uptake of As due to As5+ reduction by aqueous Fe 2+, or oxidation of As3+ by clay structural Fe 3+, is likely to take place in a natural clay-rich soil or aquifer environment in moderate to slightly reducing conditions.