Formation and occurrence of metal-arsenate precipitates in the environment
The reactivity of metals and arsenic (As) was investigated to determine how differently behaving contaminants react in co-contaminated environments and in model systems. In order to minimize the bioavailability of several co-occurring contaminants, their interactions must be well understood. Micro-focused synchrotron x-ray fluorescence mapping and x-ray absorption spectroscopy (μSXRF, μXAS, respectively) were used to investigate a copper chromated arsenate contaminated soil. Micro-SXRF mapping showed As to be well correlated to the co-contaminants Cu, Cr, and Zn (>80%). Principal component, target transformation, and linear combination fit analyses of μXAS spectra showed a continuum of partially mineralized species, precipitates and co-adsorbates of arsenate (As(V)) with Zn, Cu, and/or Cr sorbed on Al and Fe-oxides.
Co-sorbing As(V) and Zn solutions in goethite suspensions were investigated to determine solid phase partitioning and kinetic factors. A 100 ppm goethite suspension was titrated with As(V) and Zn stock solution at pH 4 and 7 and showed increased uptake of As(V) and Zn from solution at pH 7. EXAFS spectroscopy determined that the increased uptake at pH 7 occurred due to the formation of an adamite-like surface precipitate, while at pH 4, As(V) and Zn only formed co-adsorbed surface complexes. Precipitates did not form below site-saturation at pH 7 suggesting that the number of surface sites may influence precipitation reactions.
Subsequently, As(V) and Zn were reacted at pH 7 in either 10, 100, or 1000 ppm goethite suspensions and their sorption was monitored as a function time for six months. Arsenic and Zn K-edge EXAFS data suggested that a continuum of surface processes occurred (adsorption, precipitation, and structural refinement) leading to the formation of a koettigite-like and an adamite-like surface precipitate in 10 and 100 ppm goethite suspensions, respectively, after 6 months. We ascribed the precipitation reactions to a catalytic effect from goethite, which provided a good steric match for the surface-precipitates. In 1000 ppm goethite suspensions, co-adsorbed surface complexes dominated, because adsorption lowered As(V) and Zn concentrations in solution such that a precipitation reaction could not occur. Co-precipitated contaminants are less bio-available and hence more favorable for remediation and disposal purposes.
0768: Environmental science