Metal Solubility in Phosphogypsum‐Amended Sediment under Controlled pH and Redox Conditions

Abstract The storage of phosphogypsum (PG, CaSO 4 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 PG 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 evaluate the feasibility of using PG to reduce aqueous concentrations of potentially toxic heavy metals and as a nutrients source. Phosphogypsum is an effective nutrient source since it increased water soluble Ca, Mg, K, and P concentrations (essential plant nutrients). At the same time, PG significantly reduced soluble Al at pH 7.0 (normal pH value of anaerobic wetland sediments), thereby reducing Al toxicity to plants growing in the sediment. Phosphogypsum addition resulted in a large increase in sulfide levels in reduced sediment suspensions. As a result, at low redox potential values, the solubility of spiked heavy metals (Cd and Ni at rates of 1000 mg kg −1 ) and natural trace elements was substantially reduced by precipitation as insoluble sulfides. Low sediment pH (pH 5.0) resulted in the highest release of all the studied metals [Ca, Mg, P, K, Fe, Mn, Cd, and Ni] into the sediment solution, likely due to H + , Fe 2+ , Mn 2+ , and Al 3+ displacing these metals from the exchange complex. This study, therefore, indicates that PG application to Louisiana Mississippi River alluvial sediments is useful for alleviating Al toxicity (neutral pH), increasing nutrient availability under watterlogged 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 alkaline sediments/soils and/or reducing environments, metals release at toxic levels should be of little concern to the wetland environment.