Immobilization of nickel and other metals in contaminated sediments by hydroxyapatite addition

Batch experiments were conducted to evaluate the ability of hydroxyapatite (HA) to reduce the solubility of metals, including the primary contaminants of concern, Ni and U, from contaminated sediments located on the Departmen t of Energy's Savannah River Site, near Aiken, SC. Hydroxyapatite was added to the sediments at application rates of 0, 5, 15.8, and 50 g kg(-1). Afte r equilibrating in either 0.02 M KCl or 0.01 M CaCl2, the samples were cent rifuged and the supernatants filtered prior to metal, dissolved organic C, and PO4 analyses. The treated soils were then air-dried and changes in soli d-phase metal distribution were evaluated using sequential extractions and electron-based microanalysis techniques. Hydroxyapatite was effective at re ducing the solubility of U and, to a lesser degree, Ni. Hydroxyapatite was also effective in reducing the solubility of Al, Ba, Cd, Co, Mn, and ph. Se quential extractions indicate that HA transfers such metals from more chemi cally labile forms, such as the water-soluble and exchangeable fractions, b y altering solid-phase speciation in favor of secondary phosphate precipita tes. Hydroxyapatite effectiveness was somewhat reduced in the presence of s oluble organics that likely increased contaminant metal solubility through complexation. Arsenic and Cr solubility increased with HA addition, suggest ing that the increase in pH and competition from PO4 reduced sorption of ox yanion contaminants. Energy dispersive x-ray (EDXA) analysis conducted in t he transmission electron microscope (TEM) confirmed that HA amendment seque sters U, Ni, Pb, and possibly other contaminant metals in association with secondary Al-phosphates.