Chromium Speciation Using Sequential Extraction Procedure and Synchrotron Radiation Technique in Contaminated IrrigationChannel Sediments and Agricultural Soils

摘要

Recently, the relationship between the heavy metals pollution of the irrigation soil and the sediment from the irrigation channel has drawn great attention from the scientific communities. Traditionally, heavy metal pollution are assessed by total extraction. Since the toxicity of a metal depends more on its chemical forms than on its total amount, determination of metal speciation is essential to evaluate metal toxicity in the environment. It has been speculated that irrigation water is the major source of metal contamination on agricultural soils. However, no information is available on the transformations of heavy metals, variations in metal speciation, and transport mechanisms between the sediment of the irrigation channel and the soil of the farm land. This research was intended to assess the speciation of chromium in soils and sediments of contamination sites. Two experiments, the sequential extraction procedure and synchrotron radiation, were conducted to assess the source of contamination. Irrigation channel sediments and agricultural soils were collected from sites in Central Taiwan in polyethylene bags. The samples were immediately air dried and stored in the laboratory before further study. Large debris such as stones and plant fragments were removed from the dried samples by screening using 2mm sieve. The metal extraction procedure was modified from that proposed by Tessier. The sequential extraction produced five fractions: exchangeable, carbonate, Fe–Mn oxides, organic material, and silicate residues. Results of sequential extraction indicated that chromium speciation mostly occurred in the residual fraction, about 70% in the sediment whereas 2045% in the Fe–Mn oxide fraction for the corresponding soil. Results of EXAFS spectra analysis indicated that the chromium in the sample of soil profile (015cm and 1530cm) were the same species, and the chromium in other samples were the same species.