Metal solubility in phosphogypsum-amended sediment under controlled pH andredox conditions

The storage of phosphogypsum (PG, CaSO4 xH(2)O) has occupied vast areas of land that remain abandoned and subject to leaching and erosion by wind and water. The major constraints for pc use in the environment are the presence of several heavy metals and its radioactivity. Mississippi River alluvial sediment amended with PG (1%) was equilibrated under controlled redox (-250 , -100, 0, 150, 300, and 500 mV) and pH (5.0 and 7.0) conditions to evaluat e the feasibility of using PG to reduce aqueous concentrations of potential ly toxic heavy metals and as a nutrients source. Phosphogypsum is an effect ive nutrient source since it increased water soluble Ca, Mg, K, and P conce ntrations (essential plant nutrients). At the same time, PG significantly r educed soluble Al at pH 7.0 (normal pH value of anaerobic wetland sediments ), thereby reducing Al toxicity to plants growing in the sediment. Phosphog ypsum addition resulted in a large increase in sulfide levels in reduced se diment suspensions. As a result, at low redox potential values, the solubil ity of spiked heavy metals (Cd and Ni at rates of 1000 mg kg(-1)) and natur al trace elements was substantially reduced by precipitation as insoluble s ulfides. Low sediment pH (pH 5.0) resulted in the highest release of all th e studied metals [Ca, Mg, P, K, Pe, Mn, Cd, and Ni] into the sediment solut ion, likely due to H+, Fe2+, Mn2+, and Al3+ displacing these metals from th e exchange complex. This study, therefore, indicates that PG application to Louisiana Mississippi River alluvial sediments is useful for alleviating A l toxicity (neutral pH), increasing nutrient availability under watterlogge d conditions, and reducing aqueous concentrations of toxic heavy metals to trace levels under anoxic conditions by precipitating these toxic elements as insoluble sulfides. If PG or PG-products are placed in neutral to alkali ne sediments/soils and/or reducing environments, metals release at toxic le vels should be of little concern to the wetland environment.