三價鉻在水田土壤中之有效性與其對水稻之毒害評估

Abstract

鉻在土壤中以三價陽離子或是六價陰離子的型態存在，雖然三價鉻的毒性、有效性和移動性都比六價鉻低，然而在土壤中之有效性三價鉻濃度高時是否會造成作物的危害也值得關切。另外，國內皮革業廢棄物資源化所製成之蒸製皮革粉也含有高含量之三價鉻，曾被用來配製有機質肥料，若大量而長期施用由皮革粉配製而成之有機質肥料至土壤中是否會造成三價鉻的累積而造成作物毒害也值得探討。本研究主要以鉗合性樹脂 (Chelex100) 抽出法評估添加三價鉻和皮革粉至台灣代表性水田土壤中Cr(Ⅲ) 之有效性，並種植水稻幼苗，進行供試土壤中樹脂可抽出性Cr(Ⅲ) 含量與水稻幼苗生長之相關性評估。本研究選用平鎮系、將軍系及太康系三種代表性土壤的表土，作為供試土壤，添加CrCl3至土壤中，製備成0、175、250、500、1000、2000 Cr(Ⅲ) mg kg-1soil六種處理之Cr(Ⅲ) 添加土壤。結果顯示平鎮系土壤、將軍系土壤及太康系土壤之Cr(Ⅲ)添加量分別大於175、500及1000 mg kg-1soil時，水稻幼苗生長有毒害癥狀。在三個供試土壤造成水稻生長毒害不同之原因，推測是由於土壤pH：平鎮系 < 將軍系 < 太康系，Cr(Ⅲ) 之植物有效性為平鎮系>將軍系>太康系所造成。在添加皮革粉之試驗，三種供試土壤分別添加1 %、5 %、10 %、20 % 之皮革粉，相當於製備成含有89.8 ~ 1796.2 Cr(Ⅲ) mg kg-1soil的土壤。平鎮系土壤添加大於1 % 之皮革粉，就產生毒害，將軍系和太康系皆在大於5 % 之皮革粉產生毒害。推測由於酸性土壤中皮革粉中的Cr(Ⅲ) 較容易釋出，因此較易造成作物毒害。結果也顯示供試土壤中樹脂可抽出性Cr(Ⅲ) 含量與水稻幼苗生長呈現顯著負相關，表示利用土壤中鉗合性樹脂可抽出Cr(Ⅲ) 的含量，能作為植物有效性Cr(Ⅲ) 含量的指標。由土壤樹脂可抽出之Cr(Ⅲ) 含量與水稻株高之劑量-反應關係曲線，推估降低水稻生長20 % 之土壤樹脂可抽出之Cr(Ⅲ)含量 (Effective concentration, EC20)，添加三價鉻處理之EC20為2.75 mg kg-1；添加皮革粉處理之EC20為2.67 mg kg-1。綜合以上結果顯示土壤中Cr(Ⅲ)對於作物生長可能造成危害，其對環境或植物之威脅還是不可忽視。施用皮革粉造成高量Cr(Ⅲ) 之釋出亦會使作物產生毒害，尤其在酸性土壤，皮革粉的施用對作物危害之風險也不容忽視。

Chromium is present in soils in Cr(Ⅲ) cation or Cr(Ⅵ) oxyanion forms. Although the toxicity, availability, and mobility of Cr(Ⅲ) is smaller than those of Cr(Ⅲ), the phytotoxicity of Cr(Ⅲ) resulting from high amounts of available Cr(Ⅵ) in soils is needed to be concerned. The leather meal containing large amounts of Cr(Ⅲ), produced from residues of tanning industry, was used for making organic fertilizers. There is concern that large amounts of Cr(Ⅲ) accumulated in soils due to long term’s application of large amounts of leather meal may cause toxic effect on crop growth. Therefore, in this study, a chelating ion exchange resin (Chelex 100) extraction method was used to evaluate the availability of Cr(Ⅲ) in representative paddy soils of Taiwan spiked with Cr(Ⅲ) or added with leather meal. The relationship between the soil resin-extractable Cr(Ⅲ) and the growth of rice seedlings grown on the tested Cr(Ⅲ)-spiked or leather-meal-added soils was investigated. Three soils, Pinchen (Pc), Chengchung (Cf), and Taikang (Tk), were used and treated with five levels of Cr(Ⅲ), i.e. 0, 175, 250, 500, 1000, and 2000 Cr(Ⅲ) mg kg-1soil in the form of CrCl3. The results showed that the growth of rice seedlings was inhibited when the amounts of Cr(Ⅲ) spiked were larger than 175 mg kg-1soil for Pc soil, 500 mg kg-1soil for Cf soil, and 1000 mg kg-1soil for Tk soil respectively. The results was due to the availability of Cr(Ⅲ) in there tested soils is in the order of Pc > Cf > Tk, resulting from the order of soil pH Pc < Cf < Tk. In the experiments of soils added with various amounts of leather meal (0, 1, 5, 10, and 20 %), the results showed that the growth of rice seedlings was inhibited when the amounts of leather meal added were larger than 1 % for Pc soil, and 5 % for Cf and Tk soils. The results was also due to that the availability of Cr(Ⅲ) released from leather-meal-added acidic Pc soils is larger than that of the other two soils. In addition, the results showed that there was a sigmoidal relationship between soil resin-extractable Cr(Ⅲ) and the plant height of rice seedlings and the obtained effective concentrations of resin extractable Cr(Ⅲ) resulting in 20% growth inhibition (EC20) were 2.75 and 2.67 mg kg-1 respectively for Cr(Ⅲ)-spiked and leather-meal-added soils. The above results suggest that the hazardous effect of Cr(Ⅲ) on crop growth and the risk caused by large amounts of Cr(Ⅲ) released from leather-meal-added soils, especially in acid soils, can not be ignored.