以生物溶出法處理受重金屬污染底泥之研究

Abstract

底泥為水環境中重要之介質，長久已來底泥皆承受著廢、污水之排放，以及廢棄物之棄置、大氣沉降與農業區逕流之排入，使底泥成為廢、污水、空氣及廢棄物中污染物之最終儲存槽。另外，底泥中污染物可能因河水性質改變而再釋放進入河水中，即使濃度不高，經由生物食物鏈濃縮之作用，亦會對生態系統造成危害。目前國內對於受污染河川之整治極為重視，在河川整治過程中將會面臨大量且含高濃度毒性污染物底泥之問題，而此等受污染底泥必需經過處理後才能進行最終處置，以免造成二次污染。在底泥污染物中，由於重金屬具有累積性、毒害性及穩定性，因此重金屬生物處理技術之建立則為本文主要目的。 受重金污染底泥以生物溶出法處理時，發現硫氧化菌於生長代謝過程中，利用其氧化能力與產酸之能力能有效地將底泥中之重金屬移除。當底泥之固體物含量愈高時，其具有較高之緩衝能力而使底泥之 pH 值下降速率愈慢，硫氧化菌具有較高之產酸能力。若不考慮反應時間，當底泥固體物含量為 1-10%，底泥 pH 值下降最低時，鉛、鎳與鉻之最大溶出效率將會受到底泥固體物含量之影響，反之，銅、鋅與錳則無此現象。若考慮反應時間時，當底泥固體物含量越大時，其重金屬溶出速率越小。同時可利用一階反應式模擬底泥固體物含量影響重金屬溶出效率之情形。 生物溶出法中，常利用粉末狀元素硫作為生物生長所需之基質，當所添加元素硫基質之量改變時，硫氧化菌之氧化產酸能力與底泥中重金屬去除效率將受到影響。當元素硫含量愈高時，硫氧化菌具有較高產酸能力而使底泥之 pH 值下降速率愈快，此因反應槽中元素硫含有較大之面積可供硫氧化菌吸附，進而將元素硫氧化。但元素硫含量高於 0.5% (w/v) 時，pH 值下降速率變慢，顯示硫氧化菌產酸能力受到抑制。底泥中重金屬溶出速率隨元素硫含量增加而增快，若元素硫含量高於 0.5% (w/v) 時，因硫氧化菌活性受到抑制，其重金屬溶出速率變慢。由研究結果得知，以生物溶出法移除受污染底泥中重金屬之最佳硫基質含量為 0.5% (w/v)。 一般於生物溶出法處理完成後，部份粉末狀元素硫未被生物氧化，殘留於底泥中無法順利回收，此將造成底泥於最終處置之過程中產生再酸化問題。利用圓球狀硫與片狀硫取代粉末狀硫進行底泥重金屬之生物溶出時，硫氧化菌吸附至不同型態之硫顆粒上皆符合 Langmiur 等溫吸附模式。利用所得的飽和吸附量 (XAM) 可推得三種硫型態的比表面積，此比表面積在本研究中為影響生物溶出效率之最主要的因素。圓球狀硫與片狀硫的比表面積雖然較小，但並不影響硫氧化菌對硫的氧化代謝，三種不同型態硫基質的最終 pH值、硫酸根濃度及重金屬溶出效率並沒有顯著的差異。初次生物溶出後所回收的硫片與硫顆粒再進行溶出實驗時，發現其結果比原先基質的溶出速率加快許多，且以第二次回收後的片狀硫，其反應速率與粉末狀硫相當，同時片狀硫的回收率高達 70%，可有效地降低處理後底泥中硫的殘留量，減少底泥再酸化的現象。 生物溶出法中造成重金屬溶出之最主要原因為生物氧化及酸化作用。而底泥 pH 值之變化則為生物溶出法進行過程中最直接且較易觀察到之指標，因此量測系統中之 pH 值將可監測生物溶出法進行之過程。本文中所建立之修正型飽和曲線法將可正確地推估生物溶出法過程中 pH 值變化情形。另外，因為生物產酸速率為生物溶出法中重金屬溶出之限制步驟，所以本文中亦發展與 pH 值相關之重金屬溶出效率模式。因此利用此 pH 值預測模式與重金屬溶出效率模式之結合，將可不用進行重金屬分析實驗，而能較容易地預測生物溶出法中重金屬之溶出效率。

The management of contaminated sediments in the aquatic environment is one of the most important environmental issues. In future, the remediation of a contaminated river will be faced with two typical problems-- increasing volumes of dredged materials and high concentrations of toxic substances. The objective of this dissertation is to develop a technique for treatment of the large quantity of metal contaminated sediments in the remediation of contaminated rivers. A bacterial mediated leaching process with a mixed culture of two sulfur-oxidizing bacteria for removal of heavy metals form contaminated sediments was established in this dissertation. The effects of operational parameters on solubilization of metals from sediments were assessed. It was found that continuous growth of two species of thiobacilli resulted in sediment acidification and metal solubilization. Because of higher buffer capacity of sediment, the rate of decline in pH decreased with increasing solids concentration of sediment. The rates of sulfate production of bacteria increased as sediment solids concentrations increased. In the heavy metals of concern, the maximum leaching efficiencies of Pb, Ni and Cr were apparently influenced by sediment solids concentrations. The metal solubilization from sediments appeared to follow a first order reaction related with the sediment solids concentration. In the bioleaching process, elemental sulfur is usually used as the substrate for bacterial growth. Adsorption of bacteria to sulfur particles is the first step for oxidaion of sulfur. The more the concentrations of elemental sulfur, the faster the rates of acid production and metal solubilization. But sulfur concentrations in excess of 5% (w/v) were found to be inhibitory to bacterial activity and metal solubilization in the bioleaching process. The optimum concentration of sulfur fed in the bioleaching process is recognized to be 5% (w/v). A first-order reaction related to sulfur concentration is also used to describe the metal solubilization in this bioleaching process when the sulfur concentration are below 5% (w/v). To prevent the reacidification of treated sediments and to recover the remaining sulfur, the sulfur particles in the form of pastilles and pellets are used as the energy source for thiobacilli in the bioleaching process for replacing sulfur powder. The maximum adsorption obtained from the Langmuir isotherm capacity is applied to calculate the specific surface area of sulfur particle. These results of specific surface areas are significant to interpret the sediment acidification and metal solubilization. The pH reduction and metal solubilization are significantly enhanced while reusing of the recovered sulfur particles. It is very possible to reuse the recovered sulfur particles in the bioleaching, sulfur pastilles especially. Finally, a modified logistic model was successfully developed to estimate the variations of pH in the bioleaching process. Besides, the solubilization of heavy metals from sediments is highly pH-dependent and a non-linear efficiency equation of metal solubilization related to pH value in the bioleaching process was established. Therefore, a more simple and faster method of measuring pH is able to simulating the metal solubilization in the bioleaching process.