Arsenic uptake by gypsum and calcite: Modeling and probing by neutron and x-ray scattering

TL;DR

This study investigates arsenic incorporation in gypsum and calcite using neutron and x-ray diffraction, along with EXAFS, to determine if arsenic is structurally integrated or surface-adsorbed, revealing volume expansion linked to arsenic content.

Contribution

It combines experimental diffraction data with DFT simulations to distinguish arsenic substitution within mineral structures from surface adsorption.

Findings

Unit cell volume increases with arsenic concentration.

DFT confirms substitution correlates with volume expansion.

Experimental data distinguishes bulk incorporation from surface adsorption.

Abstract

Here we report on two structural studies performed on As-doped gypsum (CaSO4 2H2O) and calcite (CaCO3), 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.