土壤中鐵錳結核對鉻的氧化還原反應之影響

Abstract

先前研究中已發現具鐵錳結核的土壤具有將Cr(III)氧化為Cr(VI)之能力，本研究欲探討於田間容水量下，鐵錳結核之含量與粒徑大小對其氧化Cr(III)能力之影響。並利用土壤之易還原性錳含量預估鐵錳結核之氧化能力，並與純的二氧化錳進行比較；另外亦探討鐵錳結核對有機資材復育Cr(VI)污染土壤之影響，觀察鐵錳結核是否具將Cr(III)再氧化之能力。研究鐵錳結核含量對Cr(III)氧化能力之影響的實驗可發現隨鐵錳結核含量增加，總Cr(VI)含量明顯增加，產生Cr(VI)之速率亦較快，但與純二氧化錳的反應能力相較，土壤中鐵錳結核僅產生純二氧化錳可氧化產生量約30 % 的Cr(VI)，表示土壤中鐵錳結核之氧化能力較純二氧化錳差，且易還原性錳量無法有效代表土壤中鐵錳結核之氧化能力。隨時間增加，鉻被鐵鋁氧化物吸附，其有效性Cr(VI)含量僅佔總Cr(VI)產生量之15 %。研究鐵錳結核粒徑對Cr(III)氧化能力之影響的實驗中，發現鐵錳結核粒徑較小者 (0.2 mm) 相較之下容易與Cr(III)反應產生Cr(VI)，推測是因鐵錳結核粒徑不同，使其具有不同之比表面積，較小的顆粒具有較大的比表面積，而可使Cr(III)與鐵錳結核均勻反應。但鐵錳結核粒徑較大之土壤 (4 mm) 在反應過程中最高仍可將500 mg-Cr(III) kg-1氧化產生27.6 % 之Cr(VI)。本研究亦觀察鐵錳結核對添加3 % 有機質 (大豆粕) 還原Cr(VI)之影響，反應第20天，高鐵錳結核含量之土壤比低鐵錳結核含量之土壤多測得8 % Cr(VI)，推測是因鐵錳結核將經有機質還原Cr(VI)產生之Cr(III)再氧化，而使該土壤可測得之Cr(VI)含量較中、低鐵錳結核含量土壤高。經觀察得知土壤原有有機質在不同鐵錳結核含量之土壤中還原Cr(VI)之能力不顯著，推測是因即使土壤中之Cr(VI)被還原，所產生之Cr(III)也可能被鐵錳結核氧化。高鐵錳結核含量之土壤中低有機質添加量 (1、2 %) 還原Cr(VI)的實驗中可發現添加少量有機質對Cr(VI)之還原作用並不顯著，推測是因鐵錳結核對Cr(III)之氧化作用所致。

The previous study had found that the soil with high content of Fe-Mn nodules can oxidize Cr(III) into Cr(VI). The effects of content and size of Fe-Mn nodules on its ability of oxidizing Cr(III) were investigated in this research. Besides, we predicted the oxidizing ability of Fe-Mn nodules based on its content of easily reducible manganese (Mnr), and compared with that of pure manganese dioxide. Furthermore, the effects of Fe-Mn nodules on Cr(VI) reduction by organic amendment added to Cr(VI)-contaminated soil was also studied to observe whether Fe-Mn nodules could re-oxidize Cr(III) or not. The results showed the total Cr(VI) content increased with the Fe-Mn nodules content during the oxidation of 500 mg-Cr(III) kg-1 by different contents of Fe-Mn nodules. Compared with pure MnO2, Fe-Mn nodules could only produce 30 % Cr(VI) of pure MnO2 could. And we also knew that Mnr could not evaluate the oxidizing ability of Fe-Mn nodules. Cr would be adsorbed by soil particles and the available Cr(VI) was only 15 % of total Cr(VI). The results also showed the total Cr(VI) content increased with the Fe-Mn nodules content during the oxidation of 500 mg-Cr(III) kg-1 by different sizes of Fe-Mn nodules, the soil with smaller Fe-Mn nodules (0.2 mm) could produce more Cr(VI) than the larger one (4 mm), because of the well mixed soil and Cr(III) solution and the larger specific surface area. The larger one could still produce 27.6 % Cr(VI) of oxidizing 500 mg-Cr(III) kg-1 during the reaction. To study the effects of Fe-Mn nodules on the reduction of Cr(VI) by organic matter, we added the soybean meal, which has confirmed its high ability to reduce Cr(VI), into the studied soils to raise 3 % organic matter and to reduce Cr(VI)-contaminated soil which has high content of Fe-Mn nodules. We found the total Cr(VI) was decreased with time and higher Cr(VI) was detected in the soil with higher content of Fe-Mn nodules. It was probably because of the re-oxidation of Cr(III) from Cr(VI) reduced by organic matter by Fe-Mn nodules. We also observed that the soil organic matter has low ability to reduce Cr(VI)-contaminated soil which has high content of Fe-Mn nodules. If we used less amount of organic matter, we could not reduce Cr(VI) obviously.