Impacts of Phosphate Amendments on Lead Biogeochemistry at a Contaminated Site

Abstract

Soil amendments can be used to cost-effectively reduce the bioavailability and mobility of toxic metals in contaminated soils. In this study a field demonstration was conducted at a Pb-contaminated site to evaluate the effectiveness of P-induced Pb immobilization. Phosphate was applied at a 4.0 molar ratio of P to Pb with three treatments: T1, 100% of P from H3PO4; T2, 50% P from H3PO4 + 50% P from Ca(H2PO4)2; and T3, 50% P from H3PO4 + 5% phosphate rock. Phosphate amendments effectively transformed soil Pb from the nonresidual (sum of exchangeable, carbonate, Fe/Mn, and organic) to the residual fraction, with residual Pb increase by 19-48% for T1, 22-50% for T2, and 11-55% for T3, respectively. Lead immobilization was attributed to the P-induced formation of chloropyromorphite [Pb10(PO4)6Cl2], which was identified in the surface soil, subsurface soil, and plant rhizosphere soil. Occurrence of chloropyromorphite was evident 220 days after P addition for T1 and T2 treatments and 330 days for T3. Visual MINTEQ model and activity-ratio diagram indicated that lead phosphate minerals controlled Pb2+ activities in the P-treated soils. Phosphate treatments significantly reduced Pb translocation from the roots to the shoots in the St. Augustine grass (Stenotaphrum secundatum), possibly via the formation of chloropyromorphite on the cell walls of roots. This field observation suggested that P amendments are efficient in reducing Pb mobility via in situ formation of insoluble chloropyromorphite minerals at a field setting. Lead immobilization shows a long-term stability. A mixture of H3PO4 and phosphate rock yields the best overall results for in situ Pb immobilization, with less soil pH change and less P leaching. Application of combined H3PO4 with phosphate rock may provide an effective alternative to the current phosphate remediation technologies for contaminated soils.

Immobilization of lead in shooting range soils by means of cement, quicklime, and phosphate amendments

“…For example, upon phosphate treatment from 5% to 10%, cerussite was reduced from 1.7% to 0.1%, whereas PbHPO 4 was increased from 0.2% to 3.4% (see Table 2). Note, many of previous studies have shown in situ or ex situ formation of most stable pyromorphite-like minerals (Pb 5 (PO 4 ) 3 X, X=Cl, OH, F) in the contaminated soils upon phosphate addition (Laperche et al 1997, Zhang & Ryan 1999, Cao et al 2002, Scheckel & Ryan 2004. However, pyromorphite-like minerals were not identified in the present study.…”

“…Chemical-based S/S approach is based on the formation of thermodynamically stable and insoluble precipitate end-products with the contaminants. More effective chemical additives include phosphates and Fe-Mn oxides (Ma et al 1995, Hettiarachchi et al 2000, Basta et al 2001, Seaman et al 2001, Cao et al 2002. Phosphate has been proven to be effective in immobilizing Pb from labile soil Pb forms via formation of lead phosphate compounds such as pyromorphite-like minerals (Pb 5 (PO 4 ) 3 X, X=F, Cl, OH) by scavenging Pb from soluble Pb compounds such as cerussite (PbCO 3 ) and litharge (PbO) (Zhang & Ryan 1999, Cao et al 2002, Scheckel & Ryan 2004.…”

“…More effective chemical additives include phosphates and Fe-Mn oxides (Ma et al 1995, Hettiarachchi et al 2000, Basta et al 2001, Seaman et al 2001, Cao et al 2002. Phosphate has been proven to be effective in immobilizing Pb from labile soil Pb forms via formation of lead phosphate compounds such as pyromorphite-like minerals (Pb 5 (PO 4 ) 3 X, X=F, Cl, OH) by scavenging Pb from soluble Pb compounds such as cerussite (PbCO 3 ) and litharge (PbO) (Zhang & Ryan 1999, Cao et al 2002, Scheckel & Ryan 2004. Field application of this stabilization method was demonstrated at an abandoned battery recycling site by Cao et al (2002) who found that phosphate rock significantly reduced leachability and phytoavailability of Pb due to formation of hydroxypyromorphite (Pb 5 (PO 4 ) 3 OH).…”

Effect of phosphate on the adsorption of Cu and Cd on natural hematite

“…Phosphate in soil may affect the chemical reactions of metals on mineral surfaces. Recent studies (Traina and Laperche, 1999;Cao et al, 2002) indicated that phosphate minerals, such as apatite, could sequester heavy metals, metalloids and radionuclide through adsorption and/or the formation of secondary PO 3À 4 precipitates, which remained stable under a wide range of geochemical conditions. Therefore, it is necessary to examine the influence of phosphate on the reactions of heavy metals on the surface of soil minerals.…”

Mechanisms of lead, copper, and zinc retention by phosphate rock

“…Phosphate minerals, i.e. phosphate rock (PR) and hydroxyapatite, have been shown to effectively immobilize Pb from various contam inated waters and soils (Cao et al, 2002;Ma et al, 1995;Ryan et al, 2001). Takeuchi and Arai (1990) suggested that Pb immobilization resulted from the formation of a more stable lead phosphate, such as Ca (10-x) Pb x (PO 4 ) 6 (OH) 2 via an ion exchange mechanism where aqueous Pb replaces Ca on the hydroxyapatite lattice.…”