| Summary: | 碩士 === 國立臺灣大學 === 農業化學研究所 === 105 === Serpentine soils are characterized with high concentration of geogenic Cr, Ni, and Co. Although these metals are mainly bound in the mineral frameworks, they could be released into groundwater or absorbed by plant through weathering and oxidation-reduction cycles and further pose potential risk to the environment and human health, in particular paddy soil. The objectives of this study were to monitor the solubility of Cr, Ni and Co under a continuous range of pre-defined redox conditions as well as how soil biogeochemical factors regulate the dynamics of these metals, and to provide critical information on potential risk of metals released from serpentine soil. Two serpentine soils (0-20 cm in depth) were collected from eastern Taiwan and they were Ta and Wm soils. An automated biogeochemical microcosm (MC) system was used to simulate flooding condition in the soil, which equipped with an automatic-valve gas regulation system control of EH by adding N2 to lower EH or O2 to increase EH. Each MC was filled with 300 g soil mixed with 60 g straw powder and ultrapure water in 1:8 ratio. EH was set from 200 mV to -200 mV, then returned to 250 mV. The slurry samples were centrifuged and the supernatants were filtered. Metals, dissolved organic carbon (DOC), specific UV absorbance (SUVA254 nm), and anions were determined in the supernatant.
The experimental results indicated that the temporal course of EH and pH in the MCs revealed converse trends in both soils due to the consumption of H+ accompany with the Fe-(hydr)oxides reductive dissolution. DOC increased along with straw breakdown and further complexed with Fe, Mn, Cr, Ni, and Co, and thus the solubile metals became higher. The SUVA254 nm values indicated the clear aromaticity of DOC but showed different components of DOC in two studied soils. The decline in SUVA254 nm with time was resulted from the increase of organic acid generated by the growth of microbial population. Anion cycles seemed to be related to OM degradation, microbial mineralization, and immobilization. Cr existed as Cr2O3(s) in the tested soils and thus the concentration of soluble Cr was very low depending on redox change and sorption ability. Soluble Co increased with Ni particularly in the Ta soil with higher CEC, indicating that Co and Ni were both controlled by redox process because of their similar ionic diameters and the same adsorptive sites on soil colloid surfaces. However, Co might be precipitated as Co(OH)2 when pH > 8.5 in the Wm soil. The soluble Cr, Ni, and Co trends with time were different in the two soils, because of the soil characteristics and mineral composition identified by the factor analysis. The solubility of Cr was very low and increase with DOC as well as Ni and Co, and thus we should pay attention to the water quality when DOC concentration becomes high in the study area.
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