利用逐級萃取程序分析關渡平原土壤砷之型態分佈

Abstract

關渡平原的水田土壤因早期（約20-250年前）引用含砷溫泉水灌溉而導致土壤受到砷污染，超過土壤砷含量管制標準（60 mg kg-1）的農地面積達128公頃。本研究以逐級萃取程序（SEP）分析水平表土與垂直剖面土壤中重金屬型態分佈之變化情形，藉由代表不同化學性質的萃取條件，模擬土體中重金屬型態分佈之吸附過程，依序為非特定吸附型態、特定吸附型態、無定形及弱結晶鐵鋁氧化物型態、強結晶鐵鋁氧化物型態與殘餘型態。結果顯示表層土壤在不同型態下的砷濃度分佈，係以無定形及弱結晶鐵、鋁氧化物型態與殘餘型態所佔比例較高。關渡平原水田表土砷的非特定吸附型態及特定吸附型態所佔比例較低（5%），濃度平均值分別為0.19 mg kg-1（0.03-0.41 mg kg-1）與4.9 mg kg-1（0.6-10 mg kg-1）。深層土壤（60-120公分）非特定吸附型態及特定吸附型態的砷濃度值高於相對應淺層土壤（0-30公分）。關渡平原長期受到水田耕作影響，造成淺層土壤的砷濃度含量，受到作物吸收、根圈吸附及淋洗作用而有降低趨勢。逐級萃取程序有助於理解受污染農地及土壤中重金屬的型態分佈、移動狀況及釋出機制，以評估土壤污染程度與影響。

About 128 hectares of paddy soil was heavily contaminated at Guandu and exceeded the national standard of 60 mg As kg-1, due to irrigated with arsenic-rich hot spring water in the past 20-250 years. The purpose of this study is to analyze the form and the concentrations of arsenic and other heavy metals, including lead, silicon, iron, manganese and aluminum, of the horizontal and vertical contaminated paddy soil samples by using the sequential extraction procedure(SEP). The SEP could obtain five chemical fractionations: the non-specifically sorbed, specifically-sorbed, amorphous and poorly-crystalline hydrous oxides of Fe and Al, well-crystallized hydrous oxides of Fe and Al, and residual phases. These results showed that the amorphous and poorly-crystalline hydrous oxides of Fe and Al and residual phases dominated the arsenic fractionations of paddy soils. Only 5% non-specifically sorbed and specifically-sorbed were in the top-soils. The mean arsenic concentrations in the non-specifically sorbed phase and the specifically sorbed phase were 0.19 mg kg-1 (0.03-0.41 mg kg-1) and 4.9 mg kg-1 (0.6-10 mg kg-1). Notably, the arsenic value in the deep soils (60-120 cm) was higher than that in the top-soils (0-30 cm). Top-soil arsenic concentration decreases due to long-term of paddy cultivation, such as the crop absorption, adsorption of rhizosphere and leaching. Arsenic the demonstration of this case study, performing a SEP can target all potential primary chemical forms of arsenic in the soil solid phase, provide useful information to explain the agricultural land and soil heavy metal contamination, and then propose countermeasures.