- 學位論文
以選擇性離子交換樹脂埋入法測定水田土壤中六價鉻的有效性
Determination of available chromium(VI) in paddy soils by using selective ion exchange resin embedded in soils
摘要
重金屬鉻在土壤中多以三價陽離子或是六價陰離子的形態存在，而六價鉻之毒性、有效性和移動性都比三價鉻高，所以評估土壤中六價鉻的有效性為目前值得重視的問題之一。重金屬鉻會因土壤氧化還原電位之變化而改變其三價或六價之型態，所以在水田土壤中必須要有一不改變土壤原有性質而可測定水田土壤中有效性六價鉻之方法，才可以較準確地評估六價鉻對水稻生長及環境之危害性。本研究方法主要是利用選擇性離子交換樹脂 (DOWEX M4195) ，評估以離子交換樹脂埋入法測定水田土壤中有效性六價鉻之可行性，且與水稻幼苗生長之生物試驗做相關分析，以建立利用離子交換樹脂埋入法來現址 (in situ) 測定水田土壤有效性六價鉻含量之方法，評估鉻污染土壤對水稻毒害之程度。 本試驗土壤樣品選用pH值及質地等土壤性質差異較大且具代表性之三種土系，分別為平鎮系土壤 (Pc) 、將軍系土壤 (Cf) 、太康系土壤 (Tk) 。將三種供試土壤分別添加不同量之K2Cr2O7溶液經乾溼交替三次後，製備成含有0、75、175、200、250及300 mg Cr(Ⅵ)/kg soil之鉻污染土壤。將裝有銅飽和樹脂 (DOWEX M4195) 之樹脂袋埋入已製備好之水飽和土壤中，分別於1、2、3及4天後取出樹脂，振盪後測定樹脂抽出土壤中有效性六價鉻之含量，以確定抽出水田土壤中有效性六價鉻所需平衡之時間。由結果顯示，隨埋入時間 (一至四天) 的增加，樹脂上六價鉻的累積量亦隨之增加，且在經過兩天後逐漸趨於一定值；而在添加等量六價鉻之三種土壤中，以太康系土壤所測得之樹脂可抽出六價鉻含量最多，將軍系土壤次之，平鎮系土壤則是幾乎抽不到六價鉻含量。另外，將製備好之鉻污染土壤先行浸水不同時間 (七天、十五天及三十天) 後，再將樹脂埋入兩天後取出，測定樹脂可抽出土壤有效性六價鉻之含量，以探討水田土壤浸水狀況對於土壤中有效性六價鉻含量之影響。結果，樹脂上所抽到之六價鉻含量隨著土壤先行浸水時間 (七至三十天) 的增加而逐漸減少。 在評估樹脂抽出土壤中六價鉻含量對水稻幼苗毒害程度之相關性方面，本試驗先將三種製備之鉻污染土壤水飽和後靜置孵育七天。之後，於人工氣候室中種植水稻幼苗，種植期間 (三十天) 並做生育調查。生育調查的結果發現六價鉻會對水稻幼苗造成毒害；而在同一種鉻污染土壤中，隨著添加六價鉻濃度增加，抑制水稻幼苗生長之情形愈嚴重；且在添加等量六價鉻之處理下，太康系土壤水稻幼苗毒害最嚴重，將軍系土壤次之，平鎮系土壤之水稻幼苗則幾乎沒受到六價鉻之毒害，由此顯示水稻幼苗毒害程度與樹脂埋入法測得之六價鉻含量呈正相關。 綜合以上之結果顯示，離子交換樹脂埋入法可適用於各種性質不同之鉻污染土壤，具有測定水田土壤有效性六價鉻含量之可行性，並可評估污染土壤中鉻對水稻毒害之程度。
並列摘要
Chromium in soils is presented in the form of Cr(VI) oxyanions or Cr(III) cation. The toxicity, availability and mobility of Cr(VI) is higher than those of Cr(III), thus it is worth to evaluate the Cr(VI) availability of soils. Since soil redox potential can influence the distribution of Cr between Cr(VI) and Cr(III) forms, an in situ method that will not affected by the change of soil properties due to soil preparation needs to be developed for determining Cr(VI) availability in paddy fields. In this study, a selective ion exchange resin (DOWEX M4195) was used to test its feasibility for determining Cr(VI) availability by embedding the resin in paddy soils. The amounts of soil Cr(VI) extracted by the resin were correlated with growth and Cr uptake of rice seedlings grown on the studied Cr(VI)-added soils. Three Taiwan representative agriculture soils [Pingchung (Pc)、Chringchung (Cf) and Taikang (Tk)] were used in this study. Three studied soils were added with K2CrO7 solutions to reach the level of 0、75、175、200、250、300 mg Cr/kg soil respectively. One gram of Cu-saturated selective ion exchange resin was horizontally embedded in saturated soil pastes and recovered after various time ( 1、2、3、4 days) intervals, and the amounts of Cr(VI) adsorbed on the ion exchange resin were measured in order to determine the time needed for equilibrium. The results showed that the amounts of Cr(VI) accumulated on the ion exchange resin increased as the length of embedded time increased, and reached a nearly constant value after 2 days. Among the three studied soils at the same level of Cr(VI) addition, the amounts of Cr(VI) in soils extracted by selective ion exchange resin method are in the order of Tk-soil＞Cf-soil＞Pc-soil. In addition, the studied soils were pre-flooded for 7、15、30 days respectively before the ion exchange resin was embedded in saturated soils and recovered after 2 days. The amounts of Cr(VI) adsorbed on the ion exchange resin were measured to determine the contents of soil available Cr(VI) are affected by pre-flooding time in paddy soils. The results showed that the amounts of soil Cr(VI) extracted by the ion exchange resin decreased as the pre-flooding time (7 to 30 days) increased. The amounts of soil Cr(VI) extracted by the resin were correlated with growth and Cr uptake of rice seedlings grown on the studied Cr(VI)-added soils. Three studied soils were pre-flooded for 7 days and used to grow rice seedlings in the phytotron. The growth of rice seedlings were investigated. The results showed that the growth of rice seedlings were found to be inhibited by the added Cr(VI). The extent of inhibition increased with the amounts of Cr(VI) added to soils. Among the three studied soils, the growth of rice seedlings in the Tk-soil and Cf-soil were much inhibited by Cr than that in the Pc-soil. The amounts of soil extractable Cr(VI) by ion exchange resin had significantly positive correlation with the toxic effect of rice seedlings are affected by Cr. The above results indicated that the ion exchange resin embedded method is suitable for determining Cr(VI) availability of paddy soils across a wide range of properties. In addition, the method may be feasible for in situ determining the Cr(VI) availability of paddy soils.