以氧化鐵濾紙抽出法評估五價砷污染土壤中砷的植物有效性及其毒性

Abstract

砷在土壤中之植物有效性及其毒性受土壤性質所影響，因此如何測定污染土壤中砷之植物有效性，是評估危害之首要步驟。土壤中之砷一般多以無機 As(V) 陰離子型態存在且其行為模式與磷相似。已有前人研究建立氧化鐵濾紙抽出法用以測定土壤有效性磷含量，因此本研究評估以此方法測定土壤中植物有效性砷之可行性。 首先進行不同時間吸附As(V)溶液之試驗，結果發現氧化鐵濾紙吸附量隨時間增加而增加，於48小時後吸附量已無顯著變化，表示吸附已達平衡，因此以48小時為氧化鐵濾紙抽出法所需之吸附時間，且以0.2 M H2SO4 脫附後，評估其脫附回收率皆達96 % 以上。其次在不同濃度 As(V) 溶液（pH值3-7）吸附試驗中，發現pH 值愈低氧化鐵濾紙所吸附之砷量愈多，此係因鐵氧化物 ZPC 約為7，且不同 pH 時砷物種不同（H3AsO4，pKa1＝2.2，pKa2＝7.0）所致。由結果發現，在 pH 值 3-7 的範圍內氧化鐵濾紙吸附容量為 6.2 - 6.4 μmol As，因此在供試土壤 pH 值範圍內一條氧化鐵濾紙（2 cm*10 cm）即可將砷含量最高（480 mg As/kg）之土壤之有效性砷完全吸附。因此訂定氧化鐵濾紙抽出法所需時間為 48 小時，且以一條濾紙即足夠吸附供試土壤中的砷。當其他陰離子（Cl- 、SO42-）存在時，氧化鐵濾紙之吸附量並無下降之趨勢，且在一般土壤中磷酸鹽含量對於氧化鐵濾紙吸附砷不造成影響。 在土壤試驗，本研究選用三種代表性台灣土壤添加Na2AsO4 並經三次乾溼交替，使土壤中含有0, 30, 60, 120, 240 及480 mg As(V) kg-1 soil，及關渡平原三種不同砷含量之土壤為供試土壤，進行氧化鐵濾紙抽出法抽出砷之測試，並以小麥幼苗毒性試驗法，進行砷植物有效性及毒性之測試。同一添加濃度下氧化鐵濾紙抽出量為將軍系＞太康系＞平鎮系，而關渡平原土壤總量雖高抽出量卻較低。植物幼苗毒性試驗結果發現小麥株高與氧化鐵濾紙抽出法測得之土壤中砷有效性含量有顯著負相關（r ＝ 0.8026**），且抽出後之土壤溶液 pH 值與原土壤 pH 值間無顯著變化。其它抽出法（去離子水、NaH2PO4）與小麥株高之相關性則不如氧化鐵濾紙，且於抽出過程會改變土壤溶液之pH值，影響原有土壤中砷的植物有效性。因此，以氧化鐵濾紙抽出法所測得之土壤中砷含量，可作為As(Ⅴ) 污染土壤中植物可利用性砷之參考指標，以評估土壤有效性砷對於植物之毒害。

The availability and phytotoxicity of arsenic in contaminated soil is dependent a on a soil properties. To assess the phytoavailability of arsenic in soils is essential for risk assessment. The major arsenic form in soils was inorganic arsenate and its behavior in soils is similar to that of phosphate. The iron oxide-impregnated filter paper extraction method was used to estimate the amounts of soil available phosphorus. The objective of this study is to assess the feasibility of using the iron oxide-impregnated filter paper for determining the availability and phytotoxicity of As(Ⅴ) in soils. In the adsorption experiment of As(Ⅴ) by the filter paper, one strip of filter paper (2 cm × 10 cm) was used to adsorb As(Ⅴ) at various times and the results showed that the amounts of As(Ⅴ) adsorbed increased as the increase of reaction time. After 48 hours, the amounts of adsorbed As(Ⅴ) reached a constant value obviously. Therefore, we set 48 hours as the reaction time to extract soil As(Ⅴ) by iron oxide-impregnated filter papers. The amounts of As(Ⅴ) adsorbed as a function of pH was also tested and the results showed that the lower pH, the more As(Ⅴ) adsorbed by the filter paper. It may due to the presence of different species of As(Ⅴ) at various pH and the zero point of charge of iron oxides. When the pH is between 3 and 7, one filter paper could adsorb at least 6.2-6.4 μmol As. Thus, the capacity of one filter paper is enough for extracting soil available As from one gram of test soils. In addition, the amounts of adsorbed As(Ⅴ) were not affected by the presence of other anions, such as chloride, sulfate and phosphate in soil solutions. In the soil experiments, three representative soils of Taiwan, Pingchung (Pc), Taikang (Tk) and Chingchung (Cf) were treated with Na2AsO4 solutions to reach the level of 0, 30, 60, 120, 240 and 480 mg As kg-1 soil respectively. These soils then underwent three wetting-drying cycles at room temperature. Three Guandu soils that had high concentrations of arsenic were also used in this study. The As(Ⅴ) spiked soils and Guandu soils were determined for available As(Ⅴ) by using the filter paper extraction method. Amount the tested soils at the same level of As(Ⅴ) addition, the amounts of As in soils extracted by filter paper were in the order of Cf-soil > Tk-soil > Pc-soil. Although the Guandu soil contained higher concentration of As than other As(Ⅴ) spiked soils, the filter paper extractable As were lower. In the phytotoxicity test, the amounts of soil extractable As by the iron oxide-impregnated filter paper had significantly negative correlation with plant height of wheat seedlings (r = 0.8026**). When compared with extraction methods (H2O and NaH2PO4), and correlation between filter paper extractable As and plant height of wheat seedlings was the highest among the tested extraction methods. Therefore, the iron oxide-impregnated filter paper extraction method could be a used for the assessment of availability and phytotoxicity As in soils.