不同因子影響具鐵錳結核之土壤對 Cr(III) 的氧化反應

Abstract

土壤中存在之 Cr(VI) 對植物的毒性較強，在土壤中移動性較大，而 Cr(III) 的毒性較弱，在土壤中移動性較小，但是當土壤存在高量錳氧化物和高濃度 Cr(III) 時，可能會促使土壤中 Cr(III) 氧化成毒性更高的 Cr(VI)，故本篇目的是探討自然存在鐵錳結核含量高之土壤是否具有將 Cr(III) 氧化成毒性更高的 Cr(VI) 能力，及不同土壤、pH 值和錳量是否對氧化 Cr(III) 的能力產生不同影響，與在田間容水量孵育狀態下，探討高錳含量土壤氧化 Cr(III) 所產生 Cr(VI) 的量。試驗土壤為鐵錳結核含量高之土壤，竹圍系土壤 A、竹圍系土壤 B 與後湖系土壤，土壤中總錳量 (王水消化法) 分別為 9.8、1.0 和 0.3 g kg-1，首先發現在 1 mM Cr(III) 溶液，土水比為 1：75 的反應下，竹圍系土壤 A 和竹圍系土壤 B 經過 64 小時後，可分別將 2.65和1.86％ 添加的 Cr(III) 氧化成 Cr(VI)，而後湖系土壤則測不到 Cr(VI)，其原因為竹圍系土壤 A 的錳量 > 竹圍系土壤 B > 後湖系土壤，所以導致此研究結果。其次在pH值為 3-5 的動力反應中，反應約在 64 小時後達到平衡，另外，在反應 192 小時後，竹圍系土壤 A 可將所添加 Cr(III) 的 2.72-3.23% 轉變為 Cr(VI)，但是在不同 pH 值下所產生的總 Cr(VI) 量差異並不大，而且所產生的 Cr(VI) 大多數被吸附在土壤固相。在實驗中也觀察到可溶性錳 (MnL) 會隨氧化 Cr(III) 反應時間的增加而增加，顯示反應中所產生的 Cr(VI) 是由於土壤錳氧化物還原所造成的。另外以添加不同量竹圍系土壤模擬不同錳量對氧化 Cr(III) 之影響，當添加相同量 Cr(III) 時，添加愈多錳量土壤會產生愈多的 Cr(VI)，然而產生的 Cr(VI) 與土壤所含總錳莫耳數比為 0.015 到 0.010 之間，與理論上 1 mole MnO2 可以產生 0.67 mole Cr(VI) 比較，表示土壤中所含的錳只有一部份錳可將 Cr(III) 氧化，所以導致比 MnO2 氧化 Cr(III) 產生還要少的 Cr(VI)。且本研究也在竹圍系土壤 B 中添加 0、250、500和1000 mg Cr(III) kg-1，並維持在田間容水量孵育下，測定土壤中產生有效性 Cr(VI) 的量，竹圍系土壤 B 在 24 小時後即可用銅飽和 Dowex M4195 和 0.01 M KH2PO4 抽出 18.5-23.8 mg kg-1 的有效性 Cr(VI)，經過1個月後總共可產生 24.8-29.7 mg kg-1 的有效性 Cr(VI)，隨著 Cr(III) 添加量的改變，產生的 Cr(VI) 差異不顯著，所以土壤中可氧化 Cr(III) 的錳含量多寡為氧化 Cr(III) 成 Cr(VI) 之主要限制因子。因此，當鐵錳結核含量高之土壤一旦遭受到 Cr(III) 汙染時，有可能會產生危害性更大的 Cr(VI)。

Chromium was used in metal plating, wood preservation, and leather tanning, and had been commonly found in contaminated sites. Chromium exists in two oxidation states, Cr(III) and Cr(VI). Chromium(VI) is more hazardous and mobile than Cr(III) in soils. The Mn oxide minerals had been proven to be able to oxidize Cr(III) to Cr(VI). The study investigated the Cr(III) oxidation by natural soils which had high amounts of Fe-Mn nodules under various conditions. Three soils, Chuwei A and B, and Houhu, were used. The results showed that Chuwei A and B soils could oxidize 2.65% and 1.86% of added Cr(III) to Cr(VI) at the soil/water ratio of 1/75, however, no detectable Cr(VI) was found in Houhu soil. The results were mainly due to that the order of total soil Mn contents was Chuwei A > Chuwei B > Houhu. In addition, the amounts of Cr(VI) produced by Chuwei A soil was not affected by pH in the pH range of 3-5. Moreover, most of the produced Cr(VI) were adsorbed by soil solids. We also found that dissolved Mn (MnL) increased with time during the oxidation of Cr(III) by Chuwei A soil, suggesting that the Cr(VI) production is resulted from the reduction of soil Mn oxides. As we increased the amounts of soil added into Cr(III) solutions, the amounts of Cr(VI) production increased. It was due to the increase of quantities of Mn oxides, thus increasing the amounts of Cr(VI) production. The mole ratio of Cr(VI) produced/soil Mn of the Chuwei A soil was 0.015-0.010, which was much smaller than theoretical value of mole ratio of 0.67 of Cr(VI) produced/soil Mn for Cr(III) to be oxidized to Cr(VI) by pure MnO2. The results of Cr(III) oxidation indicated that only a small portion of soil Mn was able to oxidize Cr(III). In order to mimic field conditions, the soil available Cr(VI) produced from the Cr(III) oxidation by the Cr(III)-spiked Chuwei B soils (0, 250, 500, and 1000 mg Cr(III) kg-1) incubated at field capacity as a function of time was extractred by KH2PO4 or DOWEX M4195 resins. The results showed that the amounts of soil available Cr(VI) increased rapidly in one day and reached the level of 24.8-29.7 mg kg-1 after 30 days no matter what the amounts of Cr(III) were spiked, suggesting that the main factor controlling the extent of Cr(VI) production is the contents of soil reducible-Mn for oxidizing Cr(III). Therefore, the Cr(VI) hazard could occur if soils containing high amounts of reducible-Mn were contaminated by Cr(III).