評估添加各種有機資材降低六價鉻污染土壤中有效性六價鉻之效果

Abstract

鉻在土壤中以三價陽離子或六價陰離子的型態存在，而六價鉻在土壤中的毒性、移動性及溶解度都遠高於三價鉻。前人研究發現土壤中的有機質能將六價鉻還原成三價鉻，進而降低六價鉻在土壤中的毒性，然而添加不同有機資材於土壤中時，會因其本身之理化性質不同，使其分解速率不同進而影響六價鉻之還原效果。因此本研究之目的為探討添加不同有機資材處理 (大豆粕、牛糞堆肥、蔗渣堆肥及米糠) 對土壤中六價鉻之有效性之影響。 本試驗選定三種台灣代表性土系 (平鎮系(Pc)、將軍系（Cf）、太康系（Tk）) 作為供試土壤。將三種供試土壤分別添加K2Cr2O7溶液使其含有 0 及 500 mg Cr(VI) kg-1，加水至飽和狀態，進行三次乾濕交替。之後分別添加蔗渣堆肥 (SCDC)、牛糞堆肥(CC)、大豆粕 (SBM)及米糠 (RB)等四種有機資材於三種供試土壤中，使每公斤供試土壤增加 0、1 % 及 2 % 的有機質，於 25 ℃，田間容水量下孵育 40 天。各處理分別於孵育之第 0、4、8、12、20、30、40 天取出部份土樣，經風乾研磨過80 mesh的篩網之後，利用DOWEX M4195選擇性離子交換樹脂抽出土壤中有效性之六價鉻。另外取出孵育第 21 天之土壤，經風乾研磨過230 mesh的篩網後，進行X光吸收近邊緣結構光譜(XANES)測定。 由 DOWEX M4195 選擇性離子交換樹脂抽出結果發現，平鎮系土壤在不添加有機資材孵育下即能使土壤中有效性六價鉻降低至 10 mg kg-1。而在太康系與將軍系土壤中添加四種有機資材處理之後，土壤中之有效性六價鉻皆明顯下降，其中以添加大豆粕與米糠的效果最為快速。添加六價鉻之太康系與將軍系的土壤經三次乾濕交替後樹脂抽出有效性六價鉻分別為 149.8 及 160.7 mg kg-1，在添加 1 % 的大豆粕與米糠孵育 8 天後，能使有效性六價鉻降低至10 mg kg-1以下，而添加 2 % 的大豆粕與米糠孵育4天後即能達相同之效果。將添加不同有機資材孵育不同天數後之土壤樹脂可抽出六價鉻含量利用一級動力方程式進行套配，求得太康系土壤中六價鉻之還原速率係數分別為蔗渣堆肥 0.052 day-1、牛糞堆肥 0.18 day-1、米糠0.47 day-1及大豆粕 0.5 day-1，在將軍系土壤中六價鉻之還原速率係數分別為蔗渣堆肥 0.09 day-1、牛糞堆肥 0.174 day-1、米糠 0.44 day-1 及大豆粕 0.28 day-1。由有機資材之基本性質可知有機資材在土壤中之分解難易程度，大豆粕與米糠都是屬於較易分解之有機資材，而在土壤中大豆粕又比米糠更容易分解，但是我們從試驗結果中發現添加米糠孵育後六價鉻之還原速率大於大豆粕，可能的原因是米糠 C/N 比高於大豆粕且米糠中之有機碳比大豆粕多，因此能提供更多電子使六價鉻還原更快速。 XANES 測定結果我們發現，三種添加六價鉻但未添加有機資材土壤之譜圖於 5992-5994 eV 的位置皆有明顯之 pre-edge，表示在土壤中有六價鉻存在。而添加有機資材孵育後，pre-edge 有下降的趨勢，表示有機資材能將土壤中之六價鉻還原成三價鉻。 利用有機資材添加還原污染土壤中之六價鉻是常被使用的一種整治方法。綜合本研究結果，建議在有機資材的挑選上，首先要考慮能在土壤中快速分解之資材，並且也要有適當之 C/N 比，如此才能更快速的還原土壤中之有效性六價鉻。

Chromium is present in soils in Cr(III) cation or Cr(VI) oxyanion forms. Cr(VI) is more toxic and mobile than Cr(III). Adding organic materials into soil can reduce Cr(VI) to Cr(III) which in turn reduces the toxicity of Cr(VI). The reduction effectiveness of Cr(VI) is influenced by the rate of decomposition of added organic materials. The characteristics of organic materials will vary in how they affect their rate of decomposition. The purpose of this study is to evaluate the effectiveness of various organic materials on decreasing Cr(VI) availability of Cr(VI)-contaminated soils. Three representative soils of Taiwan [Pinchung (Pc), Chingchung (Cf), and Taikang (Tk)] were used in this study. These soils were added with K2Cr2O7 solution to reach the level of 0 and 500 mg Cr(VI) kg-1 respectively and gone through 3 drying and wetting cycles to prepare Cr(VI)-spiked soil. Organic matter was added into Cr(VI)-spiked soils with 0, 1 % and 2 % by dry weight in the form of sugarcane dregs compost (SCDC), cattle-dung compost, soybean meal (SBM) and rice bran (RB). The organic materials amended soils were then incubated in field capacity at 20 ℃. Soils were taken out after 0, 4, 8, 12, 20, 30 and 40 day of incubation. For extracting soil available Cr(VI) the DOWEX M4195 selective ion exchange resin was used. The degree of soil Cr(VI) reduction to Cr(III) after incubation 21 days was identified by X-ray adsorption near edge structure spectroscopy (XANES). Results from this study indicate that only 10 mg Cr(VI) kg-1 were extracted in Pc Cr(VI)-spiked soil (500 mg Cr(VI) kg-1 ) without adding organic materials after 40 days of incubation.Most of the Cr(VI) was reduced by the natyral organic matter of Pc soils.. The addition of organic materials into Cf and Tk soils significantly decreased the amount of extractable Cr(VI). Among the four organic materials, SBM and RB were the most effective ones in reducing Cr(VI). Less than 10 mg Cr(VI) kg-1 were extracted from Cr(VI)-spiked soil amended with 1 % organic matter in the form of SBM and RB after 8 days of incubation and with 2 % organic matter in the form of SBM and RB after 4 days of incubation. The relationship between the soil extractable Cr(VI) and the incubation time was described by the first-order kinetic equation, and the rate coefficients of Cr(VI) for adding SCDC, CC, RB and SBM were 0.052 day-1, 0.18 day-1, 0.47 day-1 and 0.5 day-1 in TK soil, and 0.09day-1, 0.174 day-1, 0.44 day-1 and0.28 day-1 in Cf soil respectively. The different rate coefficients are due to the different rates of decomposition of the organic materials in soils. Although both the SBM and RB were easily decomposed in soil, the higher C/N ratio of the RB resulted in the higher effectiveness of Cr(VI) reduction than SBM. Comparing the XANES spectra of Cr(VI)-spiked soils with and without organic material amendments for Cf and Tk-soils, it is evident that the intensity of the Cr(VI) peak was significantly lower with organic materials amendments. The XANES results suggested that the reduction of Cr(VI) to Cr(III) was the dominant mechanism for the decrease of soil available Cr(VI) when organic materials were added. The above results suggest that when adding organic materials into Cr(VI)- contaminated soil to reduce Cr(VI), the materials that can be decomposed easily in soil must be chosen. Other than the rate of the decomposition of organic materials, the C/N ratio of organic material is also an important factor that must be taken into consideration.