Environ. Sci. Technol. 1997, 31, 2745-2753
Effect of Apatite Amendments on Plant Uptake of Lead from Contaminated Soil VALE ´ RIE LAPERCHE,* TERRY J. LOGAN, PRANITHA GADDAM, AND SAMUEL J. TRAINA School of Natural Resources, Ohio State University, 2021 Coffey Road, Columbus, Ohio 43210
Phosphate compounds of Pb [e.g., pyromorphite Pb5(PO4)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, J.; Caporn, S. Appl. Geochem. 1996, 11, 335) and as apatite (Laperche et al. Environ. Sci. Technol. 1996, 30, 3321), to Pb-contaminated soils. In the present study, the effect of apatite amendments on the bioavailability of Pb in contaminated soil and the stability of pyromorphite were examined. A Pb-contaminated soil was treated with natural and synthetic apatites, and the bioavailability of Pb was determined in plant uptake studies with sudax (Sorghum bicolor L. Moench). The Pb content in shoot tissue decreased as the quantity of added apatite increased. However, Pb and P contents in the plant roots increased as the quantity of added apatite increased. In the absence of apatite amendments, Pb content in the shoot was 170 mg of Pb/kg dry weight; apatite decreased the shoot Pb concentration to 3 mg/kg. XRD and SEM analysis indicated that apatite reacted with Pb in the contaminated soil to form pyromorphite, in situ. However, accumulation of Pb in the roots and formation of pyromorphite on root surfaces was also noted. This study indicates that apatite amendments to contaminated soils can lower the bioavailability and increase the geochemical stability of soil Pb.
Introduction Contamination by trace elements such as lead (Pb) represents one of the most pressing and potentially costly threats to water and soil resources as well as a serious threat to human health. Previous studies by Ma and co-workers (3-6) and by Laperche et al. (2) have demonstrated that apatite amendments can convert dissolved, solid, and soil Pb species to pyromorphite. The rationale behind this research was to develop a process for in situ stabilization of Pb in contaminated soils and sediments. Formation of pyromorphite results in equilibrium aqueous Pb concentrations approximately equal to EPA drinking water limits (15 µg L-1); however, information on the bioavailability of lead phosphate is lacking. The bioavailability of Pb in contaminated soils has been shown to vary with the mineralogical form of Pb (7), and in vivo and in vitro assays have indicated that the mammalian gastrointestinal availability of Pb is controlled by the form * Corresponding author fax: 614-292-7432; e-mail: laperche.
[email protected].
S0013-936X(96)01011-5 CCC: $14.00
1997 American Chemical Society
and relative dissolution rates of Pb solids (8). Dissolution is necessary for Pb to pass from the soil phase into the blood stream. Similarly, dissolution of Pb solids should control the phytoavailability of Pb in contaminated soils. Decreases in the relative solubility of Pb solids, as can occur during the conversion of cerussite (PbCO3) to pyromorphite (2), should cause reduced availability of Pb to plants. The dissolution of pyromorphite can be expressed as
Pb5(PO4)3(X)(s) + 3H+(aq) w 5Pb2+(aq) + 3HPO42+(aq) + X-(aq) (1) This reaction will be driven to the right by decreases in solution pH and by removal of Pb or P from solution. The latter condition may be of particular importance in P-deficient, Pb-contaminated soils where pyromorphite formed from apatite amendments could represent the dominant P forms. The present study investigated the use of apatite minerals to induce in situ formation of stable lead phosphates in contaminated soil, and determined the impact of apatites on Pb uptake by plants. Additionally we examined the phytoavailability of Pb as hydroxypyromorphite (HP) in the presence and absence of natural and synthetic apatites. Finally, electron microscopy and X-ray diffraction (XRD) were used to examine lead phosphate particles present on the roots of plants grown in contaminated soil that had been previously amended with synthetic and natural apatites.
Experimental Section Materials. The soil used for this study was from a residential area in Oakland, CA. This soil was contaminated by paint spills, and the total contaminated area encompassed approximately 50% of a typical residential lot. The soil contained 37 026 mg of Pb kg-1, along with high levels of Zn, Cr, Cu, and Cd. This soil was collected from between 0 and 10 cm depth in the profile. The soil was air-dried and sieved to an effective diameter of e2 mm (see ref 2 for further description of this soil). This soil was chosen for its high Pb content to facilitate the identification of the Pb forms and to increase the potential to detect the formation of pyromorphite in a short time period (3-4 months, duration of the bioassays). The minerals used in this study are all members of the apatite group (9) with the following general formula A5(XO4)3(F,Cl,OH); A ) Ba, Ca, Ce, Na, Pb, and Sr; X ) As5+, P5+, Si4+, and V5+; (CO3) may partially replace (PO4). Some of these minerals have the specific names hydroxylapatite [Ca5(PO4)3OH; HA], fluorapatite [Ca5(PO4)3F; FAP], chlorapatite [Ca5(PO4)3Cl; ClAP], and pyromorphite [Pb5(PO4)3Cl]. There are no recognized names for the minerals with the formulas Pb5(PO4)3F and Pb5(PO4)3OH (9). To facilitate the understanding of this study, we used the term apatite for the calcium phosphates [Ca5(PO4)3(OH,F,Cl)] and we extended the term pyromorphite to include all the lead phosphates [Pb5(PO4)3(OH,F,Cl]. We used the names hydroxypyromorphite, fluoropyromorphite, and chloropyromorphite for the formulas Pb5(PO4)3OH (HP), Pb5(PO4)3F (FP), and Pb5(PO4)3Cl (ClP), respectively. The minerals used as phosphate sources were a synthetic HA (particle diameter
