ARSENIC UPTAKE MECHANISM IN AN ARSENIC-BEARING SMECTITE FROM THE GEOTHERMAL ENVIRONMENT

An arsenic-bearing Mg-rich trioctahedral smectite was found in the geothermal environment. . Sequential extractions in the natural samples suggest that arsenic is stably contained within the smectite. Adsorption experiments show that aqueous arsenic (both AsV and AsIII) is not taken up by a similar crystalline Mg-rich trioctahedral smectite. In order to utilize this mineral for remediation of arsenic-contaminated environments, it is necessary to understand the mechanism how aqueous arsenic is taken up by the smectite.

There are two mechanisms explored in this study to explain the occurrence of the documented arsenic-bearing smectite. First, precursory phases could be involved in the initial uptake of arsenic and stabilized during ageing of the smectite. Secondly, aqueous arsenic could form complexes with dissolved major components (e.g. silica) and later incorporated during smectite precipitation. A poorly crystalline Mg-rich clay-like mineral phase was prepared by mixing different proportions of Mg and Si solutions. The resulting solid was used as an analogue to a poorly crystalline precursory phase in the first mechanism. In the second mechanism, aqueous arsenic initially would have to form complexes with Mg, Al or Si prior to smectite precipitation. An additional constraint would be the order of mixing and ratios of these three major chemical components of smectites (i.e. Al, Si and Mg). This would also give an insight to the timing of aqueous arsenic uptake during the precipitation of smectite.

Results show that both mechanisms can uptake a considerable amount of aqueous arsenic. XRD analyses of the solid samples from the second mechanism show that arsenic uptake inhibits smectite formation resulting to generally amorphous solids. This is in contrast with the first mechanism since the crystallinity of the precursory phase did not degrade with increased uptake of aqueous arsenic. The formation of an arsenic-bearing smectite is suggested to be mediated by a poorly crystalline precursory phase.