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TECHNICAL REPORTS
Heavy Metals in the Environment

In Situ Treatment of Metals in Contaminated Soils with Phytate

Advanced Analytical Center for Environmental Sciences, Savannah River Ecology Laboratory, The University of Georgia, Drawer E, Aiken, SC 29802

* Corresponding author (seaman{at}srel.edu)

Received for publication January 23, 2002. Batch experiments were conducted to evaluate the ability of various forms of phytate, the hexaphosphoric form of myo-inositol (IP6), to immobilize U, Ni, and other inorganic contaminants in soils and sediments. A Ca–phytate precipitate (Ca_n–IP6), dodeca sodium–phytate (Na₁₂–IP6), and hydroxyapatite (HA) were added to contaminated soil at rates of 0, 10, 25, and 50 g kg^-1 and equilibrated in 0.001 M CaCl₂. The samples were then centrifuged, the solution pH was measured, and the supernatants were filtered prior to analysis for dissolved organic carbon (DOC), U, Ni, P, and other inorganic contaminants, such as As, Cr, Se, and Pb. The residual sediments were air-dried prior to characterization by analytical electron microscopy and extraction with the Toxicity Characteristic Leaching Procedure (TCLP). The solubility of several metals (e.g., U, Pb, Cu) increased with increasing Na₁₂–IP6 when compared with the nonamended control. In some cases immobilization was observed at the lowest Na₁₂–IP6 application rate (10 g kg^-1) with an increase in solubility observed at the higher rates, demonstrating the importance of metal to ligand ratio. In contrast, Ca_n–IP6 and HA decreased the solubility of U, Ni, Al, Pb, Ba, Co, Mn, and Zn. For example, soluble U decreased from 2242 to 76 µg kg^-1 and Ni from 58 to 9.6 mg kg^-1 with the Ca_n–IP6 addition, similar to the results observed for HA. Arsenic and Se solubility increased for HA and both forms of IP6, but to a much greater degree for Na₁₂–IP6, suggesting that the increase in pH observed for HA and Na₁₂–IP6, combined with added competition from PO₄ and IP6 for sorption sites, resulted in the release of sorbed oxyanion contaminants. The analytical electron microscopy results indicated that metals such as U and Ni were closely associated with secondary Al-rich precipitates in the HA-treated soils, rather than unreacted HA. The analytical electron microscopy results were less definitive for the Ca_n–IP6-treated soil, although the residual P-containing material was enriched in Al, with lesser amounts of U and Ni.

Abbreviations: Ca_n–IP6, calcium–phytate precipitate • Ca₁–IP6, reagent-grade calcium–phytate (CaC₆H₁₆O₂₄P₆·nH₂O, n = approximately 3.1) • DOC, dissolved organic carbon • EDXA, energy dispersive X-ray analysis • HA, hydroxyapatite • IP6, phytate • Na₁₂–IP6, dodeca sodium–phytate • TCLP, Toxicity Characteristic Leaching Procedure

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