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TECHNICAL REPORTS

Heavy Metals in the Environment

Phosphate Treatment of Firing Range Soils: Lead Fixation or Phosphorus Release?

W.M. Keck Geoenvironmental Lab., Stevens Inst. of Technology, Hoboken, NJ 07030, USA

^* Corresponding author (xxu{at}stevens.edu).

Received for publication March 27, 2007. Phosphate treatment of lead (Pb)-contaminated soils relies on the premise that Pb converts to the thermodynamically stable, insoluble mineral class of pyromorphites. Recent research showed that treatment performance is kinetically controlled and strongly dependent on soil pH; this study employed an acidic phosphate (P) form, monobasic calcium phosphate (MCP), to investigate treatment performance of Pb occurring in an alkaline-buffered and an acidic firing range soil. The results of leaching, X-ray powder diffraction (XRPD), and modeling analyses showed that P and Pb dissolution in the alkaline soil and transformation reactions were kinetically controlled, so that: (i) TCLP (toxicity characteristic leaching procedure) and SPLP (synthetic precipitation leaching procedure) results were poor to marginal even at high MCP dosages; (ii) brushite (Ca(HPO₄)·2H₂O) and cerussite (PbCO₃) persisted in XRPD patterns; and, (iii) geochemical modeling failed to predict leaching and phase assemblages. In the acidic soil, Pb-P reactions promoted further soil acidification, improved TCLP performance, and generated better agreement with the equilibrium-based model; however, SPLP and modeling results showed that Pb concentrations could not be reduced below 15 µg/L mainly due to the low soil pH. The marginal or inadequate Pb immobilization was observed in both soils despite the elevated MCP dosages, which were well in excess of the pyromorphite stoichiometric ratio (P/Pb = 0.6). Additionally, P leaching concentrations and rates were extremely high (>300 mg/L), under both SPLP and deionized (DI) water extraction conditions, and as predicted by thermodynamic equilibrium. The performance and sustainability of phosphate-based treatment therefore seem questionable.

Abbreviations: MCP, monobasic calcium phosphate • TCLP, toxicity characteristic leaching procedure • SPLP, synthetic precipitation leaching procedure • DIW, deionized water • PATF, Picatinny Arsenal Technology Facility • FDR26, Fort Dix Range 26 • HP, hydroxypyromorphite • CP, chloropyromorphite