Geochemistry and mineralogy of arsenic in (natural) anaerobic groundwaters

摘要

Here new data from field bioremediation experiments and geochemical modeling are reported to illustrate the principal geochemical behavior of As in anaerobic groundwaters. In the field bioremediation experiments, groundwater in Holocene alluvial aquifers in Bangladesh was amended with labile water-soluble organic C (molasses) and MgSO4 to Stimulate metabolism of indigenous SO4-reducing bacteria (SRB). In the USA, the groundwater was contaminated by Zn, Cd and SO4, and contained <10 mu g/L As under oxidized conditions, and a mixture of sucrose and methanol were injected to stimulate SRB metabolism. In Bangladesh, groundwater was under moderately reducing conditions and contained similar to 10 mg/L Fe and similar to 100 mu g/L As. In the USA experiment, groundwater rapidly became anaerobic, and dissolved Fe and As increased dramatically (As > 1000 mu g/L) under geochemical conditions consistent with bacteria] Fe-reducing conditions. With time, groundwater became more reducing and biogenic SO4 reduction began, and Cd and Zn were virtually completely removed due to precipitation of sphalerite (ZnS) and other metal sulfide mineral(s). Following precipitation of chalcophile elements Zn and Cd, the concentrations of Fe and As both began to decrease in groundwater, presumably due to formation of As-bearing FeS/FeS2. By the end of the six-month experiment, dissolved As had returned to below background levels. In the initial Bangladesh experiment, As decreased to virtually zero once biogenic SO4 reduction commenced but increased to pre-experiment level once SO4 reduction ended. In the ongoing experiment, both SO4 and Fe(II) were amended to groundwater to evaluate if FeS/FeS2 formation causes longer-lived As removal. Because As-bearing pyrite is the common product of SRB metabolism in Holocene alluvial aquifers in both the USA and Southeast Asia, it was endeavored to derive thermodynamic data for arsenian pyrite to better predict geochemical processes in naturally reducing groundwaters. Including the new data for arsenian pyrite into Geochemist's Workbench, its stability field completely dominates in reducing Eh-pH space and "displaces" other As-sulfides (orpiment, realgar) that have been implied to be important in previous modeling exercises and reported in rare field conditions. In summary, when anaerobic bacterial metabolism is optimized by providing both electron donors and acceptors, As is mobile under Fe-reducing conditions, immobile under SO4-reducing conditions, and arsenian pyrite is the likely stable mineral phase formed under SO4-reducing conditions, instead of pure As-S phases such as realgar or orpiment. (C) 2008 Elsevier Ltd. All rights reserved.