| Summary: | 博士 === 國立中興大學 === 土壤環境科學系所 === 103 === Soil quality deteriorated by heavy metals has become a serious environmental problem in Taiwan. Previous studies had proven that the application of metal-contaminated irrigation water during the growing season was one of the major sources of heavy metals on the farmlands. Soil solids, such as those derived from sediments in the irrigation ditch, may be the major carriers of the heavy metals, and thus, it had been observed that the distributions of heavy metals decreased with an increase in the distance away from the entrance of irrigation water on the farmlands. Once the heavy metals were deposited in the soils, the particle sizes of the carrier and the size distributions of soil particles would affect greatly the behaviors of heavy metals, including both sorption and desorption characteristics. This study investigated mainly the influence of particle sizes on the sorption and desorption of heavy metals on soils. The particle sizes were separated into 3 fractionations, i.e.,clay, silt, and sand, using ultrasonic or conventional chemical treatment. The sorption and desorption of heavy metals on the particles with various sizes was then examined to evaluated the behavior of heavy metals in soils as influenced by the particle sizes. The results showed that the kinetic sorption of heavy metals was very fast and was independent to the particle sizes. The sorption reaction could reach an apparent equilibrium within 30 min reaction. Two-step adsorption isotherm was observed in the study. The first step was attributed to the rapid adsorption of heavy metals, and the second step may be related to the precipitation of heavy metals on the particle surfaces. X-ray absorption near edge structure (XANES) spectroscopy was used to classify Cr species based on its molecular form. These results complement the conventional Cr fractionation derived from the Tessier extraction method. The linear combination fitting (LCF) for the Cr-XANES spectra indicated that the Cr species in the soils could generally be described as Cr(III) sorbed on ferrihydrite (Cr-FH), Cr(III) complexed with humic acid (Cr-HA), and precipitated Cr. While the sum of the adsorbed Cr(III) and Cr(III) coprecipitated with Fe showed a nearly 1:1 relationship with reducible Cr, the sum of Cr precipitates and organic Cr also followed the same trend with oxidizable Cr. This result indicated that the reducible reagent in the Tessier method only extracts the Cr that is adsorbed and coprecipitated with Fe and not the pure Cr(III) precipitates on surfaces of Fe minerals. In addition, the oxidizable reagents were capable of dissolving the Cr precipitates that were associated with the organic matter and Fe minerals.
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