施用稻殼生物炭於污染土壤中探討氧化還原作用對土壤重金屬溶解度的影響

Abstract

汙染土壤中的潛在有毒元素(Potentially toxic elements, PTEs)，例如鉻、鎳、銅和鋅等重金屬，可能會透過淋洗或氧化還原作用從土壤固相中釋出，進一步地被植物吸收或滲入至地下水體中，影響到作物安全性以及人體健康，尤其是被PTEs汙染的水稻田土壤。生物炭可用來改善PTEs汙染土壤，但氧化還原循環對PTEs釋出的知識仍不充足，因此，本研究藉由模擬浸水土壤的氧化還原變化，探討施用稻殼生物炭時，生物地球化學因子對鉻、鎳、銅、鋅溶解度之影響。研究目的為: (1)量化預設氧化還原條件下，生物炭的添加對供試土壤鉻、鎳、銅、鋅溶解度之影響，(2)了解5%生物炭的添加及氧化還原電位(Eh)和其他因子，包括pH值、可溶性有機碳(Dissolved organic carbon, DOC)、專一性紫外光吸收度(Specific ultraviolet absorbance, SUVA254 nm)、Fe、Mn、Cl-、PO43-和SO42-等如何對動態氧化還原條件下重金屬從土壤固相釋出產生影響。供試汙染土壤採自彰化縣和美系20公分表土，實驗分為供試土壤處理組(S)和試驗土壤-稻殼生物炭處理組(S+BC)，以土水比1:8在微系統反應槽(microcosm)進行孵育實驗，生物炭處理組(S+BC)額外添加5%的稻殼生物炭，並控制氧化還原電位從250 mV還原至-200 mV，再氧化至250 mV，供試土壤處理組(S)和生物炭處理組(S+BC)孵育時間分別為35天及48天。在預設氧化還原電位平衡24小時候，收集不同氧化還原電位之土壤溶液樣品，經離心後，在通氮操作檯中進行過濾，並測定濾液中PTEs、可溶性有機碳(DOC)、陰離子濃度及專一性紫外光吸收度(SUVA 254 nm)。研究結果顯示，在兩種處理土壤中，土壤pH值和氧化還原電位兩者呈負相關，推測主要受到錳(氫)氧化物釋出及微生物活動有關，DOC和PTEs間存在顯著正相關性，表示金屬可能會因為DOC的溶解釋出而增加其溶解度。陰離子濃度大多隨反應時間的增加而增加，且陰離子可能會和金屬離子反應生成可溶性錯合物，間接增加金屬溶解度。本研究結果為系統性改變Eh，會促進兩種處理土壤中溶解態Cr、Ni、Cu、Zn、Fe及陰離子的釋放，添加5%生物炭後，液相重金屬的溶解度大幅增加，主要是因為可溶性有機碳會和金屬形成可溶的錯合物，因此，在供試土壤中施用5%稻殼生物炭可能會釋出較多的重金屬。

Soil contamination with potentially toxic elements (PTEs) in Taiwan is still a noteworthy environmental issue, such as chromium, nickel, copper, and zinc. These PTEs may be absorbed by plants or infiltrated into groundwater through weathering, leaching or redox conditions, and affecting the safety of food crops, microbial community abundance and human health risks, in particular PTEs contaminated paddy soil. Biochar (BC) as a carbon-rich material, is a cost-effective soil amendment that has been recommended for remediation of PTEs contaminated soils. However, the efficiency of BC to immobilize PTEs in contaminated paddy soils under dynamic redox conditions has not been clearly up to date. Thus, in this study, we have (i) quantified the impact of pre-defined redox conditions on the release dynamic of dissolved Cr, Ni, Cu, and Zn in the studied soil as affected by biochar addition, (ii) determined the impact of biochar addition, redox potential (Eh), pH, dissolved organic carbon (DOC), metals, anions and specific UV absorbance (SUVA254nm) on dynamics of heavy metals. In this study, the soil was collected from the alluvial plain of Changhua county in western Taiwan, and the soil was analyzed for basic soil characteristics and total metal content corresponding to the sequential extraction. An automated biogeochemical microcosm system was added with 300 g soil mixed with 30 g straw powder and ultrapure water in 1:8 ratio. The range of the pre-defined redox conditions was 250 mV to - 200 mV, and oxidized to 250 mV. The total incubation period was approximately 35 to 48 days. The pre-set EH windows were achieved at least 24 h before sampling and automatically maintained with the flushing of N2 and O2. Incubated soil samples were collected at different oxidation-reduction potentials, after centrifugation, filtering in a nitrogen glove box, and measuring heavy metal, dissolve organic carbon, anion concentration and specific ultraviolet absorbance (SUVA254 nm) in the filtrate. According to the experimental results, the range of the pre-defined redox conditions was +250 mV to - 200 mV, and oxidized to +250 mV. The temporal course of Eh and pH in the MCs revealed converse relationship in S and S+BC was detected, which was presumed to the reduction and dissolution of Mn hydroxides and microbial activity. The concentration of DOC in S and S+BC under reducing conditions increased might be due to release of the bounded organic matter onto the reductively dissolved-Fe/Mn and the release of DOC from soluble organic metabolites produced by reducing bacteria under reductive conditions. Moreover, the concentration of DOC has consistent trends with metals. The increase in SUVA254nm with time related to the community of microbial population. Anions had a significant positive relationship with DOC. Biochars have the potential to immobilize heavy metals in contaminated soils; however, in this study, the solubility of metals in S+BC were much higher than in S. It might be due to DOC, more DOC could provide more sites for metals to complex. When Eh declined, DOC released by reduction and dissolution; thus, the solubility of metals increased. To sum up, adding 5% rice husk biochar might increase the concentration of PTEs.