RATES AND PRODUCTS OF ARSENOJAROSITE DISSOLUTION

Jarosite (KFe₃(SO₄)₂(OH)₆) is an environmentally important mineral because it contributes to the natural attenuation of arsenic in some acid mine/rock drainage (AMD/ARD) environments by incorporating arsenate into its crystal structure. However, when solution conditions are altered beyond the stability region for jarosite (e.g., by heavy rain events or remediation) the mineral will dissolve and potentially release the incorporated arsenic back into the environment. With as many as 500,000 inactive or abandoned mine sites in the United States, it is important to understand the complex geochemical reactions that may enhance or reduce the effects of AMD/ARD.

For this study, arsenojarosite and potassium jarosite were synthesized in the laboratory with varying amounts of arsenic. X-ray diffraction was used to verify the formation of jarosite, while EMPA and ICP were used to measure the amount of arsenic incorporated into the jarosite. Batch experiments were then undertaken to measure potassium release rates, aqueous arsenic concentrations, and to identify the nanoscale reaction products in ultra-pure water. Potassium release rates (log k, mol m^-2 s^-1 ) for jarosite containing 0%, 0.5%, and 1.8 wt% arsenic were -8.22 (+/- 0.32), -9.02 (+/- 0.14), and -8.78 (+/- 0.06), respectively. Aqueous arsenic concentrations fluctuate between a few ppb in the two arsenic jarosite experiments and show no trend over time. Using transmission electron microscopy, the nanoscale reaction products have been identified to be iron (oxyhydr)oxides, predominately maghemite, hematite, goethite, and ferrihydrite.

Arsenic incorporation into jarosite potentially alters the rate of potassium release. However, the difference between the potassium jarosite and arsenojarosite release rates may also be due to the synthesis rinsing techniques for arsenic containing jarosite. Fluctuations in arsenic concentrations are likely due to sorption/desorption of arsenic onto the newly formed iron oxides. These results show that the dissolution of arsenojarosite can have a significant environmental impact on AMD/ARD environments through the release of arsenic sorbed on the surfaces of nanoparticulate iron oxides.