Arsenic release from marine sedimentary rock after excavation from urbanized coastal areas: Oxidation of framboidal pyrite and subsequent natural suppression of arsenic release

Abstract

Massive quantities of marine sedimentary rock are excavated from urban coastal areas for making underground spaces available for infrastructure projects globally. The excavated rock often contains low levels of framboidal pyrite, which can release arsenic (As) to levels that exceed environmental standards. In the present study, changes in As release and its phases were investigated during oxidation of framboidal pyrite in marine sedimentary rock following exposure to the atmosphere. Batch leaching tests showed that As release increases with atmospheric exposures over 14 days, and then decreases. In unexposed rock, 79% of As released was As(III), but the proportion of As(V) increased with atmospheric exposure times. The ratio of readily soluble As fractions (fraction 1 + 2) following sequential extraction also increased during the first 14 days of atmospheric exposures, and then decreased. In comparisons of As phases before and after leaching tests of atmosphere-exposed rocks, ratios of readily soluble As fractions decreased over 90-day atmospheric exposures. Amounts of amorphous iron also increased with the duration of atmospheric exposures. However, X-ray diffraction and scanning electron microscope analyses showed that framboidal pyrite morphology was maintained after 90 days of atmospheric exposure. Herein, we identify factors that control As release and its phases from marine sedimentary rock after excavation. The results will inform future studies that will indicate when and how to evaluate the risks of As release from marine sedimentary rocks containing framboidal pyrite.