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The Use of Phosphorus and Other Soil Amendments for in Situ Stabilization of Soil Lead 1G.M

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The Use of Phosphorus and Other Soil Amendments for in Situ Stabilization of Soil Lead 1G.M THE USE OF PHOSPHORUS AND OTHER SOIL AMENDMENTS FOR IN SITU STABILIZATION OF SOIL LEAD 1G.M. Hettiarachchi and 2G.M. Pierzynski Dept. of Agronomy, Kansas State University, Manhattan, KS 66506, 1Phone: (785)532-7281; Fax:(785)-532-6094; 2Phone:(785)532-7209; Fax:(785 )532-6094. ABSTRACT This study was conducted to evaluate the effects of P, Mn oxide, and time on bioavailable Pb in five metal-contaminated soils or mine spoils in the absence or the presence of plants, and to evaluate the effect of those treatments on phytoavailability of Pb, Cd, and Zn. The addition of P or Mn oxide reduced bioavailable Pb compared to control, as measured by a modified physiologically based extraction procedure (PBET). The maximum reduction in bioavailable Pb was always observed with the addition of P and manganese oxides together. X-ray diffractometry analyses support the PBET results, indicating that more “pyromorphite-like minerals” may have formed in the presence of both P and Mn oxides. Synergistic effects from the presence of Mn oxide with P in reducing Pb bioavailability were still evident after extensive cropping of the soils. Several mechanisms, alone or in combination, may have been responsible for this synergistic effect. The concentrations of Pb, Cd, and Zn in sudax and Swiss chard plant tissues were reduced in the presence of soluble P, and soluble P or insoluble P with Mn oxides. This new improved in situ technique to remediate Pb-contaminated soil and mine wastes has advantages over standard methods. Key words: in situ stabilization, bioavailable PB, soil remediation INTRODUCTION addition to the formation of insoluble Pb com- Lead-contaminated soil is a primary pounds as a means of reducing Pb source of Pb exposure to young children. bioavailability, adsorption of Pb represents Remediation procedures used today for Pb- another potentially important process for contaminated soils are costly, disruptive, and not reducing Pb bioavailability. McKenzie (1980) sustainable. In situ remediation techniques demonstrated that Pb adsorbs more or less overcome many of these disadvantages. It has irreversibly to manganese (IV) (hydr)oxides been suggested that the formation of Pb phos- over iron oxides by a factor of 40, suggesting phates in soils contaminated with both Pb and P that manganese oxides can be used as a strong is responsible for immobilizing Pb, thereby adsorbent or scavenger for Pb. Two incubation reducing the bioavailability of Pb (Ruby et al., studies were conducted to evaluate the effects 1994). Lead phosphates, and in particular of (I) P source, level of P, and time and (II) P pyromorphites, are some of the most stable and/or Mn oxide on bioavailable Pb in five forms of Pb in soils under a wide range of metal-contaminated soils or mine spoils. A environmental conditions (Nriagu, 1973). greenhouse experiment was conducted to Experimental evidence supports the hypothesis evaluate the effect of a presence of plants on that lead phosphates can form rapidly in the bioavailable Pb in in situ-treated (P or/and Mn presence of adequate lead and phosphate in oxide), Pb-contaminated soils, and to evaluate aqueous systems (Ma et al., 1993; Zhang and the effect of those treatments on Ryan, 1999) and in Pb-contaminated soils phytoavailability of Pb, Cd, and Zn. (Laperche et al., 1996; Zhang et al., 1998). In Proceedings of the 2000 Conference on Hazardous Waste Research 125 MATERIALS AND METHODS bioavailable Pb by a modified in vitro Five contaminated soils/mine waste bioaccessibility test (physiologically based materials (TCR, AR, Joplin, Dearing, and extraction test- PBET) (Ruby et al., 1996). The Galena) were collected from the tri-state mining concentration of Pb in PBET extracts was area. Total metal concentrations ranged from analyzed using ICP-AES. X-ray diffraction 1200 to 9100 mg Pb/kg, 30 to 190 mg Cd/kg, data was collected for the <10 :m size fraction and 4500 to 42600 mg Zn/kg. Selected separated by using an ATM sonic sifter. physical and chemical properties of the materials Incubation Study II are given in Table 1. Nine treatments were used as follows: zero Incubation Study I P (control); 5000 mg P/kg as triple super Seven treatments were used as follows: phosphate (TSP) or phosphate rock (PR); zero P (control); 2500 mg P/kg as triple super- 2500 mg of MnO2/kg (X); 5000 mg of MnO2/ phosphate (TSP2500); phosphate rock kg (2X); PR (PR5000); 2500 mg of MnO2 + (PR2500); phosphoric acid (PA2500) or 5000 mg of P as TSP (TSP+X) or PR preacidification to pH 5.0 with acetic acid (PR+X); and 5000 mg of MnO2 + 5000 mg of followed by TSP (acetic); and 5000 mg of TSP P as TSP (TSP+2X) or PR (PR+2X). Prede- (TSP5000). Predetermined amounts of CaO termined amounts of CaO were added for all were added for all samples, except for the samples, except for the control and PR, 24 hrs control and PR, 24 hrs after P treatment to after P treatment to increase the soil pH. Tripli- increase the soil pH to 7.0 to 7.5. Triplicate cate samples were incubated for three different samples were incubated for five different sampling times (4, 12, and 24 weeks) at 20% sampling times (3, 28, 84, 252, and 365 days) gravimetric moisture content and 25oC. Air- at 20% gravimetric moisture content and 25oC. dried samples were analyzed for soil pH and Air-dried samples were analyzed for soil pH, bioavailable Pb. X-ray diffraction data was plant available P (Bray-1 extractable P), and collected as described before. Table 1. Selected chemical and physical properties of soil materials (#2 mm fraction) prior to treatment applications. Sdoil Material Stan Syil CCla CPE THotal pnOrganic Carbo %gcgmol/k mgg/k g/k T8CR 324023188. 909 71. 26. A4R 404611158. 805 76. 26. J6oplin 503410308. 775 65. 36. C8hat 761623. 9229 78. 5. D6earing 7024405. 6306 61. 37. 126 Proceedings of the 2000 Conference on Hazardous Waste Research Greenhouse Study materials except chat (data not shown). The Sudax [Sorghum vulgare (L.) Moench] addition of TSP2500 reduced soil pH to and Swiss chard variety Fordhook giant [Beta 5.7"0.2 in all materials, whereas PR had no vulgaris (L.) Koch] were used. Eight treat- effect on soil pH even at the highest level. ments in triplicate were evaluated as follows: no Calcium oxide additions increased soil pH to P (control); 2500 mg P/kg soil as TSP fertilizer near 7.0 in most samples. The samples that (TSP2500); 5000 mg P/kg soil as TSP received TSP5000 had the lowest pH of all (TSP5000) or PR (PR5000); 5000 mg of materials tested, indicating that the CaO added MnO2 /kg soil (2X); 2500 mg P as triple was not sufficient to neutralize the acidity superphosphate fertilizer plus 5000 mg of created by the dissolution of TSP. MnO2/kg of soil (TSP2500+2X); 5000 mg P as Plant-available P in soils can be esti- TSP plus 5000 mg of MnO2/kg of soil mated by one of several methods depending on (TSP5000+2X); and 5000 mg P as PR plus soil characteristics (Kuo, 1996). Bray-1 5000 mg of MnO2/kg of soil (PR5000+2X). extractable P is generally used for acidic to After the fourth cutting of sudax, soil samples near-neutral soils. Soils treated with TSP were collected, air dried, and analyzed for (TSP2500, TSP5000, and acetic) and PA2500 bioavailable Pb. Plant tissue Pb, Cd, Zn, and P had levels of Bray-1 P well above those re- concentrations were also measured. quired for normal plant growth (Table 2). RESULTS AND DISCUSSION However, PR2500 and PR5000 did not change Bray 1-P levels significantly, indicating the Incubation Study I relative insolubility of PR compared to the other The acetic, PA2500, and TSP5000 P sources. Although excessive levels of plant- treatments reduced soil pH to 5.2"0.3 in all Table 2. Bray-1 extractable P in soil materials at two sampling times. Soil Material TRCR AnJtopli Cgha Dearin Treatment 3dd 3d65 3d3d65 3d3d65 3d3d65 3d365 ----------------------------------------mg/kg---------------------------------------- C†ontrol 1e24 d 1e23 1c32 1e36 1e09 1d03 1e4 1e7 4e12 TbSP2500 1c705 1c728 1b796 1c336 1c759 1b565 1c479 1b092 8b62 685 TaSP5000 2a864 2a876 3a061 2a140 2a841 2a527 2a737 2a181 1a488 1133 accetic 1d427 1d407 1b148 1d191 1d507 1c419 7d32 8c59 7c46 561 PdR2500 1e46 1e42 1c58 1e60 1e32 1d21 3e2 4e8 1e1 15 PdR5000 1e59 1e62 1c76 2e03 1e62 1d55 4e4 7e9 2e0 22 PbA2500 1b855 1b846 2a239 1b988 1b935 1b809 1b375 1d316 4d87 437 † Means with the same letter within a column are not significantly different at P<0.05. Proceedings of the 2000 Conference on Hazardous Waste Research 127 Figure 1. Bioavailable Pb by PBET for the Dearing material. Bioavailable Pb is expressed as a percentage of bioavailable Pb in the control sample. Means with the same letter within a phase are not significantly different at P<0.05. Figure 2. Bioavailable Pb by PBET for the Dearing material. Bioavailable Pb is expressed as a percentage of bioavailable Pb in the control sample. Means with the same letter within a phase are not significantly different at P<0.05. available P are not harmful to plants, they have averaged over time. Without exception, all soil been correlated with higher P losses in runoff. amendments reduced bioavailable Pb in both Thus, PR may present less of an environmental the stomach and intestinal phases, compared to risk from enhanced eutrophication compared to the control (only Dearing and Joplin data are the other P sources. shown; Figures 1 and 2). In general, the reproducibility of For the Dearing material, the PA2500 bioavailable Pb values was higher in the treatment produced the greatest reduction in stomach phase than in the intestinal phase.
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