ARSENIC CHEMISTRY IN MARINE ENVIRONMENTS - A COMPARISON BETWEEN THEORETICAL AND FIELD OBSERVATIONS

The objective of this paper is to compare the results of thermodynamic calculations with the field data. Thermodynamic diagrams have been developed to depict the solubility relationships of arsenate and arsenite minerals, the effect of redox on As transformations, and As speciation in the marine sediments and seawater. Fe3(AsO4)2 was the most stable solid phase in oxic and suboxic marine systems whereas realgar (AsS) was more stable under anoxic conditions with pe+pH < 4. Arsenate species were in abundance in oxic seawaters whereas arsenite species were more prevalent in anoxic seawaters. The chemical forms HAsO4(2-) and HASO2(0) were the most important arsenate and arsenite species in seawater, respectively. The results of the thermodynamic As speciation calculations and the redox transformations model of As were in agreement with the literature. The activity of HAsO4(2-) was calculated from the published data, which showed that formation and/or dissolution of Fe3(AsO4)2 could control concentrations of dissolved As in suboxic and oxic marine systems. A model which combines adsorption and formation of Fe3(AsO4)2 is proposed. The proposed model supports both the published literature and the thermodynamic calculations.