Arsenic Cycling during Chlorine Oxidation of Pyrite from the St. Peter Sandstone Aquifer, Eastern Wisconsin

High arsenic (As) concentrations (>100 ppb) have been measured in wells completed in the Ordovician St. Peter sandstone aquifer of eastern Wisconsin. The primary source of As is pyrite occurring within the aquifer. There is concern that periodic disinfection of wells by chlorination may facilitate As release to groundwater by increasing the rate of sulfide mineral oxidation. This study focuses on abiotic processes that can mobilize As from pyrite during controlled exposure to chlorinated solutions similar to those occurring in wells during disinfection.

A series of parallel experiments were completed on thin sections and crushed material made from sample collected at the Leonard's Michael quarry, located in Winnebago County, Wisconsin. Samples were exposed to solutions of 60 mg/L free chlorine, 1200 mg/L free chlorine, and nanopure water (control) at pH 7.0 and pH 8.5 for 24 hours. Thin sections were analyzed before and after chlorine exposure to measure changes in the pyrite surfaces. Solution samples were collected at designated time steps during the 24 hour exposure of crushed material to chlorine solutions and analyzed for dissolved (0.22 micron) As, iron, and sulfate.

Pyrite oxidation was shown to dramatically increase with increasing chlorine concentrations as shown by measurements of released sulfate. Electron microprobe (EPMA) maps revealed that increasing chlorine concentrations also increased iron-oxyhydroxide formation. Complete oxidation of pyrite cements and replacement by iron-oxyhydroxides occurred during exposure to the high dose chlorine solution at pH 7.0. Arsenic release to solution remained low due to iron-oxyhydroxide nucleation, which adsorbs dissolved As. Acid digestion of the oxyhydroxides formed during batch reactor experiments revealed higher As concentrations than those measured in the initial pyrite host. Although these secondary minerals provided a sink for As under these strongly oxidizing conditions, they could potentially become a source for As under reducing conditions.