以化學試劑及模擬垃圾掩埋場滲出水抽出評估玄武岩及蛇紋岩土壤鉻及鎳之溶出性

Abstract

澎湖群島之玄武岩土壤及台灣本島東部之蛇紋岩土壤，是分別由基性火成岩及超基性變質岩所風化而成，此類土壤具有一共同特性，即為土壤中鉻(chromium, Cr)及鎳(nickel, Ni)全量較台灣一般土壤背景值高，甚至有些蛇紋岩土壤Cr、Ni含量超過台灣土壤汙染管制標準。而在一實際發生於澎湖之掩埋場案件中，因疑似掩埋場滲出水外漏使玄武岩土壤中Ni溶出後造成鄰近之地下水監測井中測得地下水Ni含量超過第一類地下水汙染管制標準。因此本研究目的以化學試劑及模擬掩埋場滲出水來評估此兩種土壤Cr、Ni之溶出性。在本研究中所使用之玄武岩及蛇紋岩土壤Cr之化學試劑之抽出量均低於總量1%，而Ni則均低於3%，Ni可抽出量高於Cr之原因在於Ni存在土壤中之無定型(氫)氧化物鍵結型態所佔比例較Cr高，但對照台灣過去使用之0.1 N HCl抽出等級區分表，試驗土壤中的Cr、Ni只屬於背景值或低於背景值，因此這類土壤仍可作為種植作物之用途。但是，在模擬酸化期掩埋場滲出水溶出試驗結果中，有Ni溶出之現象，此結果亦與澎湖實際發生之案例相符，且若存在EDTA，會使Cr、Ni釋出量增加，而黃酸亦有類似現象，但對Ni溶出的量並不如EDTA高，不過也可由此推測在自然環境中，黃酸是有可能在高濃度醋酸造成Ni溶出後進而導致地下水之污染。因此若是往後在此類基性或超基性土壤建蓋垃圾掩埋場，對於不透水層的設置及垃圾滲出水的收集與處理應需要更謹慎的執行方式。

The basaltic soils in Peng Hu archipelago and serpentine soils in east of Taiwan are derived from mafic igneous rock and ultramafic metamorphic rock respectively. The contents of Cr and Ni in these soils are much higher than the background contents of soils in Taiwan, even in some serpentine soils, contents of Cr and Ni exceed the Taiwan soil contaminant control standards. In addition, a case of high Ni concentration found in the groundwater in Peng Hu is suspected due to the release of Ni from basaltic soils by the landfill leachates. Therefore, the objective of study is to assess the release of Cr and Ni in basaltic and serpentine soils by extraction with chemical reagents and simulated landfill leachates. The amounts of Cr and Ni extracted from basaltic and serpentine soils in this study by 0.1 M HCl, 0.01 M EDTA and 0.005 M DTPA are all less than 1% and 3% of the total contents respectively. That relatively large amounts of Ni compared to Cr were extracted is due to that higher percentage of amorphous oxide bounded Ni than Cr in the tested soils. The amounts of Cr and Ni extracted by 0.1 M HCl in the tested soils are all lower than background levels of Taiwan soils, suggesting that the availability of Cr and Ni in these soils is low. However, the results of significant amounts of Ni released into solutions by simulated landfill leachates were found. The results might support the case found in Peng Hu that Ni was released from basaltic soils by landfill leachates and thus increasing Ni concentration in the groundwater. The release of Cr and Ni from these soils was enhanced by EDTA or fulvic acid. The extent of release enhancement by fulvic acid was lower than by EDTA. The results suggest that fulvic acid in the landfill leachates may enhance the release of Ni from basaltic and serpentine soils. Therefore, in order to avoid groundwater contamination by Ni, it is essential to set up an impermeable layer and collection and treatment of leachates if the landfill site is build on mafic or ultramafic soils.