Geochemical Modeling of Arsenic Speciation, Sorption, and Precipitation Under Changing Redox Conditions

Geochemical modeling techniques were used to examine the principal geochemical behavior of arsenic in aerobic and anaerobic groundwaters. Thermodynamic data for thioarsenite species, amorphous As and Fe sulfide phases, and solid solution of arsenian pyrite (FeS1.99As0.01 – FeS1.90As0.10) were compiled into a revised Geochemist's Workbench database Thermo08-As. This new thermodynamic database is more realistic in characterizing and predicting the arsenic behavior in changing redox conditions. We modeled: 1) the adsorption and desorption of As onto the surface of hydrous ferric oxides (HFO's) in stream beds under aerobic conditions; 2) reductive dissolution of HFO in anaerobic conditions; and 3) precipitation of As under sulfate-reducing conditions. The modeling results indicate that reductive dissolution of HFO, rather than desorption, is the main trigger leading to the release of As under near-neutral pH conditions. Dissolved arsenic may be removed by co-precipitation or precipitation with iron or arsenic sulfides under reducing conditions. However, the formation of soluble thioarsenite species at high H2S/Fe ratios would enhance As mobility. Moreover, As concentrations would remain high in Fe-free solutions when the precipitation of arsenic sulfide solids such as orpiment (As2S3) or realgar (AsS) is kinetically prohibited or when their amorphous precursors are formed. Under Fe-rich geochemical conditions, the stability field of arsenian pyrite solid solution completely dominates in reducing Eh-pH space and “displaces” other As-sulfides (orpiment, realgar) that have been implied to be important in previous modeling and field studies. In summary, our field data and geochemical modeling results clearly indicate that As is mobile under Fe-reducing conditions, immobile under aerobic and sulfate-reducing conditions. Iron oxyhydroxides and arsenian pyrite are the likely stable mineral phase that serve as major sinks for arsenic under aerobic and sulfate reducing conditions, respectively.