Reducing Lead Bioavalability In Soil By Adding Apatite II
Alkandary, Dhary Saad
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This study was an examination of phosphate-induced immobilization of lead (Pb) in soils through addition of apatite II (fish bone material). The sources of Pb minerals in the soils examined in this in situ remediation study included anglesite (PbSO4), litharge (PbO), and cerrusite (PbCO3). Phosphate-induced Pb immobilization occurs when Pb ions are exchanged with calcium ions to form insoluble lead-phosphate (pyromorphite (Pb5(PO4)3Cl). Commercially available A- horizon soil was spiked with ten different Pb paints that contained different forms of Pb (e.g. PbO, PbCO3, PbSO4) and then sieved through a <250 micron mesh. Two and three replicates were prepared for each Pb paint sample type for apatite and no apatite additions, respectively. A series of large syringe tubes were filled separately with each of the spiked test soils to create a series of soil columns. In all, a set to sixty seven soil columns amended nominally with 5% w/w of apatite, and 2% Pb paint were tested. The change in potential bioavailability of lead in each soil column was evaluated after each column was eluted with Milli-Q water for one year. Soil samples were taken from each column at times zero, six, and twelve months. An in vitro bioaccessibility assessment of each soil column sample was accomplished using a simulated gastric fluid (SGF) digestion to determine the amount of Pb that could potentially be released into solution in the acid conditions in the human stomach. The assay involved using a SGF solution of 100 mL of Milli-Q water, 3 grams of glycine, and hydrochloric acid (HCl) in sufficient quantity to adjust the pH to a value 2.5. To this solution was added 0.5 gram or 1 gram of soil collected at each of the selected time points. Each SGF and soil suspension was tumbled for one hour at 37 °C in a modified Toxicity Characteristic Leaching Procedure (TCLP) system. A Scanning Electron Microscopy (SEM) spectrometer with a Back-Scattered Electron (BSE) detector and an Energy Dispersive X-ray Spectrometer (EDS) was used to obtain photomicrographs of soil samples and to determine the chemical nature of constituent Pb-bearing particles at time zero and at twelve months. A hand-held X-ray florescence (XRF) detector was used to determine the total Pb concentrations in soil samples at time zero. The study found that there was a general reduction in the amount of bioaccessible Pb in the soils at twelve months compared to time zero. This was supported by the identification of Pb-phosphate particle phases in the soils samples at twelve months. The effectiveness of using apatite II to remediate Pb-paint contaminated soil, through the immobilization of Pb through the formation of the sparingly soluble Pb-phosphate mineral pyromorphite (Pb5(PO4)3Cl) appears to be justified.