Arsenic trapping by iron oxyhydroxides and carbonates at hydrothermal spring outlets

Active precipitation of Fe oxyhydroxides and carbonates is associated with the oxygenation and CO2 degassing of hydrothermal spring waters in the Cézallier area of the French Massif Central. In order to study the trapping of trace elements, including As, by these mineral precipitates, the authors carried out batch and dynamic laboratory and field experiments, spiking the Cézallier natural waters with selected trace elements. The system was then modelled using the Pimage code, taking into account aqueous speciation, dissolution/precipitation and gaseous exchange, as well as sorption reactions. In addition to thermodynamic equilibrium reactions, kinetic aspects were introduced into the gaseous exchange and dissolution/precipitation reactions. The experiments show that As is trapped during the precipitation of Fe oxyhydroxides. During the laboratory experiments, As release after initial trapping is associated with a pH increase of the solution and is interpreted as desorption from the Fe oxyhydroxide surfaces. The results of this study suggest that calcite plays only a minor role in As trapping. The model provides a good reproduction of the major chemical trends observed experimentally.