含銅重金屬污泥螯合萃取及資源化研究

Abstract

含重金屬廢水污泥之具有產生源業別多、分布廣、種類複雜且數量龐大等特性，其TCLP測試往往無法符合有害事業廢棄物認定標準的規定，因此，如何將有害重金屬污泥資源化及回收有價重金屬，並作為環境融合之綠色資材，已是刻不容緩之研究課題。為解決國內長久存在之重金屬污泥污染的問題，本研究利用螯合劑（生物易分解、生物不易分解兩類）萃取印刷電路板及電鍍污泥中之重金屬，並探討化學置換反應對生物不易分解螯合劑回收再利用之可行性；同時評估添加螯合劑處理後有害重金屬污泥之資源化研究。研究內容包括：(1)螯合萃取污泥中有害重金屬；(2)有害重金屬回收及螯合劑之再利用；(3)添加螯合劑處理後有害重金屬污泥資源化研究；(4)螯合後重金屬污泥之綠色資材的環境溶出效應。 研究顯示，A、 C及D污泥偏鹼性，B污泥為中性偏酸性。各廠污泥中均含有極高之Cu（約7.2-28.2%），其次為Ni與Zn。各廠污泥中Cu的溶出遠高於有害事業廢棄物的認定標準，故需妥善處理，以避免造成環境之危害與衝擊。A與C污泥中重金屬結合型態多存在於鐵錳氧化態與有機態，具有較強的鍵結而不易被萃出，即具有較高之生物不可利用性；B污泥中重金屬結合型態多存在於可交換態與碳酸鹽態，最易溶出或萃出，即具有較高之生物可利用性。 在螯合萃取方面，以0.1 M EDTA、DTPA或EDDS即可有效萃取B與C污泥中之重金屬，對A污泥則需提高EDTA、DTPA或EDDS之濃度至0.25 M才能達到較佳之萃取率；這與MINEQL+之模擬結果相似；且不論螯合劑的種類，萃取效率隨著液固比的增加而上升。A污泥連續萃取實驗，以使用EDTA連續萃取效果較佳；B與C污泥，則不論連續萃取的組合，均可獲得不錯的效果。0.1 M EDTA、DTPA或EDDS萃取後，污泥中重金屬結合型態均偏向生物不可利用，即萃取後會趨於穩定，這與萃取後污泥之TCLP試驗結果相符。比較EDTA、DTPA或EDDS對A、B與C污泥中Cu之萃取效率，發現EDDS對Cu的萃取效率與EDTA或DTPA相當且大於NTA，因此，EDDS的生物易分解性將使螯合萃取技術更具環境競爭力。 在化學置換反應方面，Fe置換螯合後溶液之銅隨Fe:Cu莫爾比增加而上升，但逐漸趨於平緩；較佳pH為3、Fe:Cu莫爾比為6，此時對Cu的置換率可達80%左右。在Fe置換螯合銅的過程中，亦有少量的Zn及Ni沈澱，這是因為Fe置換螯合銅會產生共沉澱效應，使得Zn及Ni亦有部分的沉澱。另外，Fe的濃度在Fe:Cu莫爾比大於4時，即趨近穩定狀態。從固體物之XRD及SEM分析可知，置換後固體物為Cu及Fe之結晶。 在螯合劑再利用方面，置換後懸浮液中螯合劑濃度隨Fe用量增加而上升，且置換後懸浮液之螯合劑濃度均無法達到原始的濃度。Fe置換螯合銅後之DTPA懸浮液可再利用於萃取A、B或C污泥中之重金屬，而Fe置換螯合銅後之EDTA懸浮液則僅可再利用於萃取B污泥中之重金屬，這是B污泥中重金屬結合型態多為易萃出之可交換態與碳酸鹽態所致。另外，有經Fe沈澱處理之萃取溶液其萃取效率明顯高於未經Fe沈澱處理之萃取溶液，這是Fe會與Cu競爭自由的EDTA或DTPA所致。而回收的萃取溶液經三次的再萃取並不會降低其萃取效果，因此，有助於螯合萃取劑的再生利用。 硫沈澱對螯合後萃取溶液中之Cu、Zn與Ni均有良好的沈澱效果，與A、B或C污泥或螯合劑的種類無關，這是因為硫離子跟重金屬間具有較高錯和能力（高pK值）。而硫沈澱後之懸浮溶液則可再利用於萃取新鮮之重金屬污泥，但在三次重複利用後，以EDTA懸浮液萃取效率降低較多，對A污泥之萃取率會降低36~54%、B污泥則為6~10%、C污泥則為16~24%。 螯合後重金屬污泥資源化研究方面，混合40%之螯合後重金屬污泥與60%之石材污泥，在燒結時間15 min、溫度1150oC時，可資源化成密度0.74 g/cm3、抗壓強度4.41 MPa之輕質骨材。序列萃取分析重金屬污泥為基礎的輕質骨材則發現，污泥中重金屬多鍵結在鐵錳氧化態、有機態與矽酸鹽態；隨燒結溫度愈高，燒結輕質骨材之溶出濃度愈低，在燒結溫度達1150oC時，序列萃取之溶出總量即可符合TCLP之標準。

Sludge containing heavy metals is a widespread and complicated headache for many related industries. The TCLP leaching concentration of sludge is higher than the standards for defining hazardous waste. Thus, the resource recovery of heavy metal from sludge is an emergent environmental issue. In this study, we evaluate the performances of novel copper removal processes for printed circuit board and electroplating wastewater sludge applying chelant extraction (Biodegradable chelate and Persistent chelate) and powdered iron cementation, followed with the reuse of chelating agents to chelate supplementary fresh copper-containing sludge. The contents of this study are: (1) to extract the heavy metals from sludge; (2) to recover the heavy metal and to recycle the chelating solution; (3) to recycle the hazardous heavy metal sludge that is pretreated by chelating extraction; (4) to evaluate the leaching behavior of heavy metals from green materials that is produced from the heavy metal sludge after chelating extraction. The results of this study indicated that sludges A, C, and D were slightly alkaline, but sludge B was very slightly acidic. The study showed that the sludges contained copper of high total concentrations (about 7.2-28.2 wt. %), with small total concentrations of nickel and zinc. The leaching concentrations of copper in all sludges were extremely high, especially in sludge B. Based on this data, the recovery of copper from sludges appears to be of practical, as well as environmental, value. The results of sequential extraction indicated that heavy metals in sludge A and C existed as the forms of Fe/Mn-oxide bound and organically bound mostly, but the forms of exchangeable bound and carbonate bound mostly for sludge B. Thus, the metal mobility and potential bioavailability was lower for sludge A and C, but contrary to sludge B. For the extraction experiments, the results indicated that the best extraction efficiency of heavy metals was 0.25 M EDTA or DTPA or EDDS for sludge A, 0.1 M chelating agents for sludge B and sludge C. The experimental results were similar to the simulated results using MINEQL+. The extraction efficiency of heavy metals increased when the ratio of liquid to solid increased, irrespective of the kind of chelating agent. The successive extraction using EDTA would achieve the better extraction efficiency for sludge A. The distribution of the metal fractions in the sludge would become stable after chelating extraction. For Cu, the order of extraction efficiency was EDDS ≥ EDTA ≥ DTPA > NTA. The easily biodegradable chelating agent EDDS has been proposed as a safe and environmentally benign replacement for EDTA in sludge extraction. Results of the cementation experiments showed that precipitation efficiencies of Cu of were higher than 80% when the Fe:Cu molar ratio was as high as 6:1 at pH 3 for each sludge sample. The deposit of zinc and nickel results from the coprecipitation on copper precipitated by cementation processes. The XRD analysis results of recovered copper from the chelated cupric wastewater indicated that copper deposits on the iron surface almost entirely in the form of the copper molecules. The more powdered iron used, the higher the recovered efficiency of EDTA and DTPA. The efficiency of re-extraction using reused EDTA reached the original level of chelating extraction only for sludge B. However, the copper extraction efficiency for each sludge is quite approximate when using DTPA recovered at various iron concentrations. This is because the leachability of sludge B was superior to that of sludge A or C. The removal of Cu, Zn, and Ni from clelated wastewater by sulfide precipitation was well, irrespective of the kind of chelating agent or sludge. This may be related to the fact that CuS has a higher pKa value than CuEDTA or CuDTPA. The supernatant could be recovered and reused again as chelants for sludge extracting solutions. However, the extraction efficiency of the supernatant after being recycled over three cycles was lower than that of fresh chelating agents. Reduction ratios of copper from supplementary sludge using the extract from the metal-sulfide precipitation were 36-54% for sludge A, 6-10% for sludge B, and 16-24% for sludge C in comparison with previous extraction. The heavy metal sludge after chelating extraction and mining residues were evenly mixed at a weight ratio of 40% : 60% into raw aggregate pellets of 3-5 mm diameter. The lowest density of 0.74 g/cm3 and low compressive strength of 4.41 MPa could be obtained at sintering temperature of 1150°C for 15 min. The concentrations of heavy metals leached tend to decrease with increasing sintering temperature. Results obtained by sequential extraction show that concentrations of Cd, Cr, Cu, and Pb in LWA sintered at 1150°C for 15 min dropped significantly to the regulatory threshold.