施用奈米零價鐵與石灰及堆肥於鎘污染土壤對水稻吸收鎘之影響

Abstract

水田土壤鎘污染問題普遍存在於許多亞洲國家，相較於其他微量重金屬，鎘較容易被吸收而累積於水稻植體中，進而危及稻米的食用安全。在眾多的土壤污染整治技術中，現地化學固定法為一相對成本較低且較易操作的方法；奈米零價鐵具有粒徑小與大比表面積之特性，在地下水污染整治中已被廣泛應用，但其在土壤中對於重金屬固定能力之相關研究卻較少。因此，本研究目的為探討施用奈米零價鐵於鎘污染土壤中對於水稻吸收鎘之影響，並與其他常被使用的土壤改良劑（堆肥或石灰）比較施用效果，以評估此改良劑的整治效益。盆栽試驗中使用三種鎘濃度（mg/kg）分別為2、10與20之土壤，並分別進行以下幾種改良劑添加處理：（1）對照組，（2）堆肥，施用量為40 ton/ha，（3）石灰，以提高土壤pH值至6.8，（4）0.1% (w/w)之奈米零價鐵粉與（5）0.2% (w/w)之奈米零價鐵粉。研究結果顯示，不同改良劑處理在正常鎘濃度土壤中對於水稻穀粒產量皆無顯著影響；鎘污染土壤中對照組水稻生長情形不受土壤高濃度鎘影響，然而，施用奈米零價鐵卻會使水稻生長不佳、分蘖數下降，稻藁與穀粒生質量亦隨之下降，水稻穀粒產量減產比例達15-75%，推測可能為奈米零價鐵影響土壤有效性養分或是本身毒害所致。石灰處理在所有土壤鎘濃度之下皆能降低水稻稻藁中鎘濃度，堆肥與奈米零價鐵則無顯著降低之影響。於本試驗條件下，由於盆栽長期淹水，因此即使土壤鎘濃度高達10 m/kg，所有處理糙米鎘濃度皆仍未超過衛生署所規範之食米中鎘限量標準（0.4 mg/kg）；高濃度鎘污染土壤中，僅石灰的施用可顯著降低糙米鎘濃度至低於限量標準，堆肥與奈米零價鐵處理糙米鎘濃度則無顯著降低且高於限量標準。綜合以上結果，在鎘污染土壤中添加奈米零價鐵會造成水稻生長不佳，且可能由於濃縮效應使得植體中鎘濃度相對較高，因此整體而言，相較於 石灰的施用，奈米零價鐵的整治效果並效果不顯著。

Cadmium（Cd）contamination of paddy soils has been reported for several Asian countries. Compared to other trace metals, Cd is rather mobile in soils. Consequently, Cd is more readily taken up by rice plants, which it can be translocated into the brown rice, and might cause serious human health problems. In-situ chemical stabilization is one of the most efficient and cost-effective remediation techniques for heavy metal contamination sites. Nano zero-valent iron（NZVI）is characterized by their small particle, large specific surface and high reducing power, and it can effectively degraded the organic pollutants or stabilized inorganic toxic heavy metals. Therefore, NZVI has been widely used as one of the remediation techniques for groundwater pollution sites. However, few studies have been evaluated as the stabilization of Cd-contaminated paddy soils by using NZVI. The objectives of this study are (1) to examine the effects of NZVI application on the Cd uptake by rice grown in Cd-contaminated soil and (2) to compare the remediation effectiveness with other soil amendments (by applying lime or compost). The pot experiments were conducted with 3 levels of soil Cd concentrations (2, 10 and 20 mg/kg, sampled from the Cd-contaminated sites) and five soil amendment treatments, including (1) control, (2) by applying lime materials to control the soil pH to 6.8, (3) by applying livestock manure compost at the rate of 40 ton/ha, (4) by applying 0.1% of NVZI and (5) by applying 0.2% of NVZI. The study results showed that the high concentration of soil Cd didn’t cause toxicity of rice productivity. However, the application of NZVI had severe effects on the growth of rice plants and decreased the tiller numbers of rice and also the yields of straw and grains by 15-75%. The possible reasons were proposed as the bioavailability of soil nutrition was affected by applying NZVI or NVZI was toxic to rice production. In all 3 levels of soil Cd concentration, only applying lime materials treatment can reduce the Cd concentration of rice straws. The compost and NZVI treatments can’t significantly decrease the Cd concentration of rice straws. In this study, even the soil Cd concentration is high as 10 mg/kg, the Cd concentration of brown rice was lower than the regulation of brown rice in Taiwan (0.4 mg/kg) announced by Department of Health and Welfare of Taiwan, due to the longer period of soil flooding than farmer conventional treatment. When soil Cd concentration was high as 20 mg/kg, the Cd concentration of brown rice might exceed 0.4 mg/kg and might have high human risk. The effect of soil amendment application was similar to the results of rice straw, only the application of lime treatment can significantly decrease the Cd concentration of brown rice. The Cd concentration of brown rice in compost and NZVI treatments were all higher than the regulation of brown rice. In conclusions, the application of NZVI had toxic effects on the growth of rice plants and it might produce the Cd uptake in rice plant. Therefore, the remediation by NVZI was not effective compared with by applying lime materials.