| 摘要: | 隨著工業發展、人為活動增加，許多重金屬物種及有害物質被排放至土壤環境中，構成人體及生態健康的威脅，而其中土壤重金屬污染是現今備受關注的議題之一，因為當污染物達一定濃度時可直接毒害栽種其上的植物及生長其中的微生物外，污染土壤也可能成為固定污染來源，使得重金屬從土壤基質逕流及滲出，導致表面及地下水環中重金屬含量升高，進而對安全供水造成很大的風險。然而，重金屬不同於有機污染物，因無法被降解，故其化學特性明顯地限制許多整治技術的應用。根據美國環保署所公布的資料，固化/穩定化工法在整治工業廢棄物和污染土壤已被證明為最佳可利用的技術之一。本研究利用三種生物性聚合物(包含海藻酸鈉、黃原膠、瓜爾豆膠)調查和評估其施用在土壤中穩定鉛、鉻、鎘的能力，主要是因生物性聚合物對於重金屬有高度親和力，可作為屏障以防止有害廢棄物在環境中的遷移，且已被認定為是可再生及具成本效益之材料。研究首先測試利用硼砂及鹼土金屬離子(如鈣和鋇離子)作為交聯劑，將生物性聚合物形成相互穿透交聯網絡(IPNs)後，其抵抗生物降解的程度是否得以提升，以作為後續可否長久存在於環境的適用性；交聯後的產物以重量溶脹比作為IPNs 交聯程度之指標，生物降解程度以所生成的CO2 定量。實驗結果顯示交聯反應後的生物性聚合物具有高交聯密度及低含水率，暗示其具有複雜結構的存在，可減弱其生物降解性。此外，初步的土壤試驗結果顯示，藉由硼砂交聯的瓜爾豆膠當處理含鉛污染之土壤，可將未經處理的土壤中鉛溶出濃度從13.47 mg/L 降低至2.67 mg/L，代表81%的鉛被成功固定；另外，混合黃原膠及瓜爾豆膠並利用硼砂進行交聯可讓土壤中鎘溶出濃度由原本6.05 mg/L 降低至0.63 mg/L，穩固鎘之能力達85%。這些結果表明當此技術應用於人工合成鉛、鉻、鎘的污染土壤時，交聯生物性聚合物在現地系統中可產生交聯網絡以牢籠方式困住重金屬，並具有抗生物降解特性，可看出此方法在穩定土壤重金屬的技術上提供另一個具前景且可行的整治工法。;Many species of heavy metals released from anthropogenic activities are highly mobile in the environment, thereby posing a potential threat to human health and wildlife reproduction. Soil contamination with heavy metals is of particular concern, as the pollutants may reach concentrations that are directly toxic to plants/crops and microorganisms. Moreover, contaminated soil may act as a constant pollution source where runoff and leachate from soil matrix may lead to elevated levels of heavy metals in surface and groundwater environments, causing a great risk for safe water supplies. Therefore, a considerable amount of work has been carried out to address the remedies that may efficiently control or cleanup the contaminated sites. However, unlike organic pollutants, metals cannot be degraded and thus their chemical characteristics significantly limit the application of many remediation techniques. Nonetheless, according to the U.S. EPA, solidification/stabilization is one of the best demonstrated available technologies to treat certain industrial wastes and contaminated soil. In this study, stabilization of lead, chromium, and cadmium in soil with three types of biopolymers, including alginate, xanthan and guar gum, was demonstrated and evaluated. The reason for biopolymers being selected was because they have high affinity for binding heavy metals and have been considered as renewable and cost-effective materials. These materials can also potentially be used as barriers to prevent the migration of hazardous waste, which requires slow or non-biodegradability in order to permanently immobilize the pollutants in soil. Indeed, using borax and alkaline earth metal ions such as calcium and barium ions as crosslinking agents, the biodegradability of crosslinked biopolymers used in this study was significantly decreased presumably due to the formation of interpenetrating polymer networks (IPNs), which are able to make the crosslinked products persistent in the environment. The results of weight swelling ratio, an indication for the extent of crosslinking of the IPNs, also supported the presence of complex polymer structures in terms of the high crosslinking density and the low water content. When this crosslinked biopolymer technology was applied to synthetic polluted soil of lead, chromium and cadmium to create an in situ system that acted as cages for trapping these metals in soil, remarkable results were obtained: under the optimum conditions, up to 81% of lead was successfully immobilized in soil using guar crosslinked by borax, and 85% of cadmium was stabilized with xanthan and guar crosslinked by borax. These results show that the soil treatment with crosslinked biopolymer networks to stabilize lead, chromium and cadmium was effective. The unique property of the networks to be resistant to biodegradation may add another value to this technology, providing a promising means to permanently stabilize heavy metals in soil. |
