蛇紋岩土壤中氧化還原循環對重金屬溶解度的影響

Abstract

蛇紋岩含有高濃度的鉻、鎳和鈷，可能透過風化或氧化還原作用釋放到土壤中，進而被植物吸收或滲入地下水中，造成環境與人體健康的風險，尤其頻繁浸水的蛇紋岩水田土壤。本研究藉由模擬浸水土壤的氧化還原變化，以了解生物地球化學因子對鉻、鎳及鈷溶解度之影響，並提供蛇紋岩土壤的風險評估參考。供試土壤採自臺東縣池上鄉銅安山 (Ta) 與花蓮縣富里鄉無毛山 (Wm)，進行基本土壤特性與金屬全量分析。微系統裝置反應槽中加入風乾土與去離子水，土水比1:8，並添加20%水稻稈作為微生物碳源，另以氮氣與氧氣控制EH從200 mV還原至-200 mV，再氧化至250 mV，採集不同EH下之泥漿樣品，經離心與過濾後，測定濾液中重金屬、可溶性有機碳 (DOC)、陰離子濃度，以及專一性紫外光吸光度 (SUVA254 nm)。結果顯示，兩種土壤pH均隨EH降低而提高，主要受到鐵(氫)氧化物溶解與生成以及微生物活動影響。DOC濃度在氧化還原過程中增加，來自鐵(氫)氧化物溶解釋出、有機質降解與微生物代謝產物，由SUVA254 nm值可看出微生物代謝產物占DOC的比例隨時間增加。鉻主要以Cr2O3(s)沉澱，溶解度極低，而鎳與鈷多以二價陽離子存在，受氧化還原作用影響較明顯，尤其CEC較大的Ta土壤，且當pH大於8.5時，可能發生Co(OH)2沉澱。總體而言，蛇紋岩土壤中可溶性鉻濃度很低，但鉻、鎳及鈷均可能透過與DOC錯合而提高溶解度與穩定性，故蛇紋岩土壤地區水體中若含有高濃度DOC，可能會增加這些重金屬的環境風險問題。

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.