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A standard P adsorption procedure was proposed and the ability of four laboratories to produce consistent results over a wide range of soils was determined. For this procedure, 0.5 to 1.0 g of soil were shaken in 0.01 mot L CaCI at a soil/solution ratio of 1:25 in containers allowing a 50% head space for 24 h at 24 to 26°C on an end-over-end shaker. Initial dissolved inorganic P concentrations of 0 to 323 smol P L (as KHPO4 or NaH1`04) were used and microbial activity inhibited by 20 g L chloroform. Excellent agreement between the four laboratories was obtained for the amount of P adsorbed by the 12 soils studied, with a mean coefficient of variation (CV) over all P levels and soils of 0.91 01o. The laboratories also exhibited a high degree of replication of individual treatments with no laboratory showing a strong consistent bias across all soils and P levels in terms of P adsorption. Langmuir, Freundlich, and Tempkin adsorption models were highly correlated with the adsorption data. Respective mean correlations for the 12 soils were 0.98, 0.97, and 0.95. The proposed method, therefore, has the potential to produce consistent results that can be used to predict partitioning of dissolved inorganic P between solid and solution phases in the environment.

Phosphate compounds of Pb [e.g., pyromorphite Pb 5 (PO 4 ) 3 -(X) where X ) OH, F, or Cl] are comparatively insoluble, and inducing their formation in contaminated soils may be a means of reducing the bioavailability and chemical lability of Pb in soil. Previous research has documented the formation of pyromorphite subsequent to the addition of phosphates, as soluble phosphate (Cotter-Howells,

Previous studies have shown that the interactions of apatite with dissolved Pb are caused by the dissolution of apatite grains concomitant with the precipitation of lead orthophosphates (pyromorphites). The present study extends this work by examining the interactions of selected Pb minerals and a Pbcontaminated soil with apatite. Specimen-grade PbO and PbCO 3 were reacted separately with hydroxylapatite (HA) in controlled pH reactors. Hydroxypyromorphite (HP) formed at the expense of HA, PbO, and PbCO 3 after a reaction period of 2 days, causing significant decreases in aqueous Pb concentrations. The extent of reaction was pH dependent, with more HP formation at pH 5 than at pH 6 or pH 7. Equilibrium modeling with MINEQL + indicated the stoichiometric conversion of the native Pb solids to HP at all pH values examined in laboratory experiments. In companion experiments, particle size and density separation techniques were used to obtain Pb-enriched fractions from a contaminated soil. These were identified as PbO and PbCO 3 with X-ray diffraction (XRD) and scanning electron microscopy (SEM). The Pb-enriched fractions were reacted with HA, and the formation of HP (at the expense of "native" Pb solids) was observed by XRD and SEM. Clearly, apatite amendments to Pb-contaminated soil materials can induce the formation of pyromorphites.