Potential adverse effects of applying phosphate amendments to immobilize soil contaminants

Seven-day batch equilibrium experiments were conducted to measure the efficacy of four phosphate amendments (trisodium trimetaphosphate TP3], dodecasodium phytate Na-IP6], precipitated calcium phytate Ca-IP6], and hydroxyapatite HA]) for immobilizing Ni and U in organic-rich sediment. Using the eight-step modified Miller's sequential extraction procedure and the USEPA's Toxicity Characteristic Leaching Procedure, the effect of these amendments on the distribution of Ni and U was assessed. Relative to unamended controls, equilibrium aqueous-phase U concentrations were lower following HA and Ca-IP6 additions but higher following TP3 and Na-IP6 amendments, whereas aqueous Ni concentrations were not decreased only in the Na-IP6 amended treatment relative to the control. The poor rates of contaminant immobilization following TP3 and Na-IP6 amendments correlate with the dispersion of organic matter and organo-mineral colloids, which probably contain sorbed U and Ni. While all amendments shifted the U distribution toward more recalcitrant soil fractions, Ni was redistributed to more labile soil fractions. This study cautions that the addition of orthophosphates and organophosphates as contaminant immobilizing amendments may in fact have adverse effects, especially in high-organic soils. Particular attention is warranted at sites with mixed contaminants with varying geochemical behaviors.