Environ. Sci. Technol. 2000, 34, 4614-4619
In Situ Stabilization of Soil Lead Using Phosphorus and Manganese Oxide GANGA M. HETTIARACHCHI, GARY M. PIERZYNSKI,* AND MICHEL D. RANSOM Department of Agronomy, Throckmorton Plant Sciences Center, Kansas State University, Manhattan, Kansas 66506-5501
In addition to the formation of insoluble lead (Pb) compounds, adsorption is another potentially important process controlling the bioavailability of Pb in soils. Less attention has been given to manganese (Mn) oxides, even though they are known to adsorb Pb more strongly than any other metal (hydr)oxides. This study was conducted to evaluate the effects of P and Mn oxide on bioavailable Pb in five metal-contaminated soils or mine spoils from Kansas and Missouri. Cryptomelane was used as the representative Mn oxide. Nine treatments were used: zero P, 5000 mg of P as triple superphosphate (TSP) or phosphate rock (PR), 2500 and 5000 mg of Mn oxide/kg, and combinations of Mn oxide and P as TSP or PR. Changes in bioavailable Pb over time were measured using a modified physiologically based extraction procedure (PBET), and mineralogical changes were observed using X-ray diffraction. The addition of P or cryptomelane reduced bioavailable Pb in all five materials. The addition of P and cryptomelane together was more effective in reducing bioavailable Pb than the addition of either amendment alone in all materials. Reductions in bioavailable Pb in stomach phase extractions upon addition of P or P and cryptomelane ranged from 15 to 41% and 23 to 67%, respectively, compared to the unamended control. Similarly, a modified toxicity characteristic leaching procedure (TCLP) indicated less soluble Pb in materials receiving P and cryptomelane compared to the control, P- or cryptomelane-treated samples. X-ray diffractometry analysis supported the PBET and TCLP results, indicating that more “pyromorphite-like minerals” formed in the presence of both P and Mn oxide compared to the control. This new improved technique to remediate Pb-contaminated soil and mine wastes has advantages over standard methods.
Introduction Lead (Pb) is toxic to humans, especially young children, and to animals. Lead-contaminated soil is a primary source of Pb exposure to young children. Excavation is the currently accepted technique to clean up Pb-contaminated soils in residential areas. This procedure is costly and disruptive. In addition, soil excavation requires a source of clean soil and a repository for the contaminated soil. In situ remediation techniques overcome many of these disadvantages (1, 2). * Corresponding author phone: (785)532-7209; fax: (785)532-6094; e-mail:
[email protected]. 4614
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 21, 2000
The goal of soil amendment as an in situ remediation technology is to produce the maximum reduction in soil Pb bioavailability. Lead phosphates, especially pyromorphites, are the most insoluble forms of Pb in soils under a wide range of environmental conditions (3, 4). Experimental evidence supports the hypothesis that Pb phosphates can form rapidly when adequate Pb and phosphate are present in aqueous systems (2, 5, 6). In vivo (animal feeding) studies have indicated that Pb availability in mammalian gastrointestinal systems is dependent on the form and relative dissolution rates of Pb solids (7, 8). Moreover, the formation of Pb phosphates in soils contaminated with both Pb and P may be responsible for immobilizing Pb, thereby reducing the bioavailability of Pb (7). Therefore, P amendment has been suggested as a cost-effective remediation option for Pbcontaminated soils in residential areas (9). An in vitro bioaccessibility test (also known as the physiologically based extraction test-PBET) that mimics the human digestive system has been developed (8). The PBET has been validated for use in determining soil Pb bioavailability with animal feeding studies done with weanling rats (r 2 ) 0.93 for the stomach phase, (8)) and young swine (r 2 ) 0.85 for the stomach phase (10)). Formation of pyromorphites upon additions of apatite or soluble P has been observed in Pb contaminated soils (11, 12). Hettiarachchi et al. (12) reported significant reductions in in vitro Pb bioavailability measured by PBET when various amounts and sources of P were added to Pb-contaminated soils. They concluded that P amendment to Pb-contaminated soils might not provide sufficient reductions in risk, and soil amendments in addition to, or instead of, P also warrant investigation. Adsorption of Pb is another potentially important process for reducing Pb bioavailability. Because these adsorption processes can be essentially irreversible, the presence of a strong adsorbent in the soil also could provide reductions in Pb bioavailability. Specific adsorption of Pb to Mn(IV) (hydr)oxide is known to be greater than that to any other metal (hydr)oxide. McKenzie (13) demonstrated that Pb adsorbs more or less irreversibly to Mn (hydr)oxides over iron oxides by a factor of 40, suggesting that Mn oxides can be used as a strong adsorbent or scavenger for Pb. In addition to strong oxide-metal complexes, the formation of ternary complexes (oxide surface-metal-ligand) is also possible and can involve both inorganic and organic ligands. Enhanced adsorption of Zn2+, Cu2+, Cd2+, and Pb2+ by Fe and Al oxides and soils in the presence of phosphate or sulfate provide evidence for the possibility of forming ternary complexes (14-17). The metal/ligand ratio seems to be the key factor in determining whether metal adsorption at oxide surfaces is enhanced or inhibited by the presence of ligands (15). Weesner and Bleam (17) found similarities between pyromorphite and Pb adsorbed to phosphatetreated geothite using X-ray absorption spectroscopy. They concluded that phosphate adsorbed on geothite acts as a reactant to form Pb phosphates on the geothite surfaces. Similarly, Mn oxides also can act as retaining matrices for both Pb and P; therefore, their presence also may have synergistic effects on the formation of insoluble Pb phosphate minerals. The objective of the current study was to evaluate the effects of P and Mn oxides on soil Pb bioavailability. Mineralogical changes over time in soils with in situ treatments were also observed with X-ray diffraction. 10.1021/es001228p CCC: $19.00
2000 American Chemical Society Published on Web 09/29/2000
TABLE 1. Selected Chemical Properties of the e2-mm Particle Size Fraction of Soil Materials mg/kg soil material
CdTa
PbT
ZnT
bioavailable Pb,b %
TCR AR Joplin Chat Dearing
48.0 31.2 25.7 101 189
3291 1521 2832 1179 12414
8649 6338 4463 28616 42592
36.6 29.5 22.6 24.9 18.3
a Total metals by 4 M HNO digestion (18). b Measured using PBET 3 procedure. Percentages were calculated using soluble Pb in the stomach phase divided by total soil Pb in the
