Phosphogypsum chemistry under highly anoxic conditions.

Phosphogypsum (PG), primary byproduct from phosphoric acid production, is accumulated in large stockpiles and occupies vast areas of land. Contaminants emanating from PG stacks can impact the environment including waterbodies. The major constraint for PG use in the environment is the presence of metals in high concentrations. Reduction of sulfate found in PG and significance of sulfide production in reducing aqueous concentrations of toxic metals were studied. Mississippi River alluvial sediment amended with PG was equilibrated under controlled redox (-250 mV) and pH (5.5, 6.5, and 7.5) conditions. Phosphogypsum addition resulted in a large increase in sulfide levels in sediment suspensions. As a result, the solubility of spiked heavy metals (Cd and Cr, 100 and 1000 mg kg(-1)) and natural trace elements (As, Ba, and Cd) was significantly reduced by precipitation as insoluble sulfides. Sediment pH also influenced sulfate reduction and sulfide formation in both PG-amended and control sediment. Low sediment pH (5.5) resulted in the highest release of all studied metals and sulfate into sediment solution. This study indicates that if PG or PG-products are placed in neutral to alkaline sediments/soils and/or reducing environments, metals released at toxic levels should be of little concern to the wetland environment.