Arsenic uptake by gypsum and calcite: Modelling and probing by neutron and X-ray scattering

Abstract

Uptaking of contaminants by solid phases is relevant to many issues in environmental science as this process can remove them from solutions and retard their transport into the hydrosphere. Here we report on two structural studies performed on As-doped gypsum (CaSO 4 2H 2 O) and calcite (CaCO 3 ), using neutron (D20-ILL) and X-ray (ID11-ESRF) diffraction data and EXAFS (BM8-ESRF). The aim of this study is to determine whether As gets into the bulk of gypsum and calcite structures or is simply adsorbed on the surface. Different mechanisms of substitution are used as hypotheses. The combined Rietveld analysis of neutron and X-ray diffraction data shows an expansion of the unit cell volume proportional to the As concentration within the samples. DFT-based simulations confirm the increase of the unit cell volume proportional to the amount of carbonate or sulphate groups substituted. Interpolation of the experimental Rietveld data allows us to distinguish As substituted within the structure from that adsorbed on the surface of both minerals. Results obtained by EXAFS analysis from calcite samples show good agreement with the hypothesis of replacement of As into the C crystallographic site. r

Geochemical modelling of arsenic and selenium leaching in alkaline water treatment sludge from the production of non-ferrous metals

“…Much progress has been made in the study of solid solution formation and adsorption of oxyanions to common minerals of alkaline matrices. It is, for example, well known that arsenate and selenate can be incorporated in ettringite (Ca 6 [Al(OH) 6 ] 2 (SO 4 ) 3 ·26H 2 O) [12,13], gypsum (CaSO 4 ·2H 2 O) [14,15]. Moreover, arsenite and selenate can be taken up in the structure of calcite (CaCO 3 ) [14,[16][17][18], whereas selenite interacts with calcite and ettringite through surface exchange reactions [12,19,20].…”

Exploration of As(III)/As(V) Uptake from Aqueous Solution by Synthesized Calcium Sulfate Whisker

“…Roman-Ross and co-workers [31,42] have recently studied the precipitation of gypsum by mixing CaCl 2 and Na 2 SO 4 aqueous solutions in the presence of dissolved Na 2 HAsO 4 , and detected the increase of the unit cell volume with the amount of sulfate groups substituted. The incorporation of arsenic into the gypsum structure has been confirmed by DFTbased simulations.…”

“…Recently, calcic minerals such as gypsum and calcite have attracted the interest due to their universal and easy availability. Arsenic present in groundwater may be trapped by calcite [30][31][32][33] or gypsum [31,34,35], either through adsorption or co-precipitation. The interaction leads to the formation of a variety of Ca-arsenate compounds and many of them with unknown X-ray diffraction (XRD) patterns and compositions [36].…”

Arsenic in Natural Environments

“…Calcite is the dominant mineral in limestones, whereas dolomite is prevalent in dolostones. Although As(III) may partially substitute for carbon in the carbonate of calcite (Di Benedetto et al, 2006;Fernández-Martínez et al, 2006), most limestones unaltered by hydrothermal or other fluids contain very little arsenic. Pure dolomite, like quartz, is also ineffective in sorbing arsenic when compared with iron (oxy)(hydr)oxides (Thornburg and Sahai, 2004).…”

“…Elemental sulfur (S 0 ) also occurs in some subsurface salt deposits and may result from the microbial reduction of anhydrite (Blatt, Middleton and Murray, 1980), 564. Although As(V) may partially substitute for sulfate in gypsum and anhydrite (Fernández-Martínez et al, 2006), most salt deposits contain 10 mg kg −1 or less of arsenic (Table 3.23).…”