Arsenic in natural carbonates: The role of the biogeochemical conditions in its solid speciation

Abstract

The interaction of geogenic arsenic (As) with carbonate minerals depends on the biological and geochemical conditions of the environment. This study sheds light on how different environmental conditions influence the carbonate mineral formation and the interaction with As. Three different samples were analyzed: carbonates associated with thermal springs, oncolites and a soil carbonate sample. Synchrotron-based XAS was used to evaluate the As solid speciation while micro-focused XPS analysis was performed to determine the surface speciation of Fe and S. Chemical and mineralogical characterization were performed using conventional techniques. The results revealed that the calcium carbonates minerals are a consequence of the environmental chemical composition (Sr2+, Mg2+ and arsenate), although physical factors (i.e., temperature) and the influence of microorganisms could not be ruled out. These conditions, identified in thermal springs samples and oncolites, determine the precipitation of aragonite, calcite and Sr/Mg-rich calcite and the inclusion of As(V)/(III) within crystal lattice. As(III) could be a product of the arsenate reduction by microbial activities. Conversely, in the soil carbonate sample, As(V) is adsorbed onto calcite. In addition, the As(1-) species was determined in one sample from Laguna Negra; this species likely corresponds to an As-bearing sulfide compound (i.e., arsenian pyrite).