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

Abstract

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(HPO(4)).2H(2)O) and cerussite (PbCO(3)) 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 microg/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.

Lead contamination of an agricultural soil in the vicinity of a shooting range

“…Moreover, the higher TOC content at the control site could have, to some extent, influenced the pH value as well (which alters mobility of metals). Due to the fact that oxidized Pb originating from Pb shot is present mainly in the form of (hydro)cerussite or Pb oxide in soils (Lin et al 1995;Cao et al 2003b;Rooney et al 2007;Dermatas et al 2008), these are unstable under reducing acidic conditions of the hydroxylamine hydrochloric extraction (fraction B). For this reason, Pb at the contaminated site was mainly associated with the reducible soil fraction.…”

Phosphorus: Chemism and Interactions

“…The amount of Pb in the applied phosphate ranged between 10 and 20 mg kg −1 (Stevanovic et al, 2009). It is known that lead-containing compounds are weakly soluble (Davies, 1995), but application of phosphates affects their solubility, resulting in their greater immobilization (McGowen et al, 2001;Dermatas et al, 2008). Hettiarachchi & Pierzynski (1996) had more success with the solubility equilibrium approach.…”

Environmental Long Term Impact on a Romanian Military Testing Range

“…A number of studies [4][5][6] have evaluated the environmental impact of contaminants from the firing of bombs, rockets, projectiles, and pyrotechnics, on the quality of soil, ground water and surface water. Depending on the depth of the ground water, climate, soil parameters, or the proximity of surface water on the range, contaminants can reach ground or surface waters.…”