評估淨水污泥及硫酸亞鐵對降低污染土壤中砷的有效性之效果

Abstract

砷是自然界中分佈極廣且具有毒性的類金屬元素，在環境中除了天然礦物外，過去人類因農業、工業活動使用含砷化合物使得環境中砷濃度提高，藉由風化作用，砷可能會從礦物中釋放至地下水和土壤中。砷酸鹽會與鐵鋁氫氧化物所富含的－OH官能基發生配位交換，且不論As(III)和As(V)皆可與針鐵礦(FeOOH)以強鍵結的雙核雙橋方式形成錯合物，降低土壤中砷的有效性。 由於鐵鋁氫氧化物是影響土壤中砷的有效性之重要因素，因此本試驗利用含有高量鐵鋁氫氧化物的淨水污泥作為固定污染土壤中砷的資材，期能降低土壤中砷的有效性。另一方面，探討硫酸亞鐵處理後對砷污染土壤的砷固定效果，以此改善並提高淨水污泥固定砷污染土壤中砷的能力。在添加淨水污泥 (5-20% (w/w) ) 或硫酸亞鐵處理後，再以去離子水、氧化鐵濾紙及磷酸二氫鈉等三種抽出方法，評估其對砷在人工添加砷孵育的將軍系土壤 (480 mg As kg-1) 及天然含砷關渡平原土壤 (520 mg As kg-1) 中之有效性影響。 結果顯示，淨水污泥對砷有很大的吸附容量，且對土壤中砷的固定效果並不會隨乾濕交替的孵育次數而影響。試驗中砷污染土壤經淨水污泥處理後，不論是以去離子水、氧化鐵濾紙和磷酸二氫鈉抽出之可抽出砷皆顯著隨淨水污泥量的增加而下降，表示淨水污泥的添加可有效降低砷污染土壤中砷的有效性。另外，以硫酸亞鐵處理後，將軍系砷污染土壤中抽出之砷對去離子水抽出砷量下降達87%，其餘抽出方法僅下降22-30%，顯示硫酸亞鐵處理可有效降低污染土壤中可溶性砷，並將砷固定於土壤中。而關渡平原砷污染土壤添加硫酸亞鐵處理之對砷的固定效果不明顯，所以進一步探討混合添加淨水污泥與硫酸亞鐵的結果發現，對添加10% (w/w) 淨水污泥的關渡平原砷污染土壤而言，再加入硫酸亞鐵後，對土壤以三種抽出法之砷抽出量均較單一使用硫酸亞鐵或單一添加10% (w/w) 淨水污泥的抽出量低。另一方面，混合添加淨水污泥與硫酸亞鐵，對將軍系砷污染土壤之效果，與添加20% (w/w) 淨水污泥之結果相仿。 因此，利用淨水污泥處理之砷污染土壤，均可有效固定土壤中的砷，且配合硫酸亞鐵的使用更可提高砷的固定效果。

Arsenic is one of the most hazardous metalloid, ubiquitously present in the environment. Because amorphous ferric and aluminum hydroxides can affect mobility and availability of arsenic, both As(III) and As(V) could be held on amorphous ferric and aluminum hydroxides strongly. In the study, a waste by-product of the drinking water treatment process, namely drinking water treatment residuals (WTRs) were evaluated for their ability on decreasing As availability with or without ferrous sulfate (FeSO4). After adding WTRs (5-20% (w/w) ) or ferrous sulfate to As(V)-spiked Chengchung soil (Cf soil) and Guandu naturally As-contained soil (Gd soil), three extraction methods including deionized water extraction, iron oxide-impregnated filter paper and NaH2PO4 extraction were used to evaluate the effect of WTRs on arsenic mobility and availability. WTRs have very huge adsorbing capacity to As, and the ability of decreasing As availability is almost the same during longer incubation times. And also, in the two As-contaminated soils after adding WTRs, no matter which extraction method was used, the extractable arsenic decreased with increasing WTRs contents. WTRs not only showed greatest affinity for As, but also reduced As availability of As-contaminated soils effectively by adding them into soils. In addition, the results of FeSO4 treatment to As(V)-spiked Cf soil show that extractable arsenic by deionized water extraction method is decreased about 87%, and that by other extraction methods only decreased 22-30%. Furthermore, extraction rate of Cf soil mixed with 10% (w/w) WTRs and FeSO4 is similar to Cf soil mixed with 20% (w/w) WTRs. It indicated that FeSO4 could effectively fix As into immobile forms and lower soluble As content in soil. But the effect of FeSO4 on naturally As-contained Gd soil is not distinct. In spite of the extraction rate of FeSO4 treatment was not obviously different from the control Gd soil, in the results of mixing 10% (w/w) WTRs with FeSO4 into Gd soil, all the extractable arsenic of three extraction methods are lower than only use FeSO4 or 10% (w/w) WTRs. The result of this study showed that adding WTRs into As-contaminated soils could immobilize arsenic and decrease its availability, and additionally mixing FeSO4 and WTRs into As-contaminated soils could enhance the effect.