Extraction of Chromium, Copper, and Arsenic from CCA Treated Wood by Using Chelating Agents

鉻化砷酸銅（Chromated copper arsenate, CCA）是一種被廣泛使用的水溶性木材防腐藥劑，由鉻、砷及銅三種金屬氧化物混合而成。CCA防腐材廢棄物在焚燒或掩埋之後，容易釋放重金屬，因此CCA防腐材廢棄物若能先移除一部分重金屬，將能有效降低對環境的污染。CCA防腐材之處理有許多方法，其中水溶液萃取是最有效且被廣泛使用的方法之一。本研究使用CCA-C型防腐劑（含有氧化鉻47.5%，氧化銅18.5%，五氧化二砷34.0%）經滿胞法處理之南方松(Pinus taeda)，螯合劑使用具生物可分解性之螯合劑：EDTA(Ethylenediaminetetracetic acid)， EDDS（S, S-Ethylenediaminedisuccinic acid）及NTA（Nitrilotriacetic acid）等三種。萃取條件包括：溶液酸鹼值、濃度、萃取溫度與萃取時間。三種金屬均在酸性條件下（pH 2.0-4.0）的萃取效果最好，因此推論此三種螯合劑較適合於酸性條件下反應。螯合劑濃度結果顯示在濃度範圍為0.01至0.1 M時，0.1M可以達到最好的萃取效果。溫度之結果可發現在75oC時即可達到十分良好之重金屬移除效率。萃取時間可看出各金屬之萃取速度並不一致，鉻和砷為萃取速率較慢之重金屬，萃取時間對其結果影響較大；銅的萃取速率較快，5~6小時左右就可以移除93%。綜合以上最適條件，最好之移除效率分別為鉻53%-66%，銅84%-93%，砷55%-62%。固液比對EDDS-CCA化學藥劑比之指數函數結果可看出EDDS對於銅之螯合能力最佳，鉻次之，砷最弱;且固液比為10時萃取效果較好。序列萃取結果顯示未以EDDS萃取之CCA防腐材，85.6%之鉻存在於有機相分離部及殘餘相分離部；銅有49%存在於可交換分離部及殘餘相分離部。有機相分離部及殘餘相分離部則含有80.2%的砷。本研究顯示以生物可分解性較強之螯合劑EDDS與NTA，可有效去除CCA防腐材中之重金屬，且效果比一般經常使用之生物可分解性螯合劑EDTA更佳，除了可降低CCA防腐材廢棄物對於環境之污染，也能使此技術應用層面更廣。Chromated copper arsenate (CCA) is the most widely used wood preservative. It is formulated with copper, chromium, and arsenic oxides. Better disposal methods of CCA treated wood waste would be needed to alleviate secondary pollution induced by waste land-filling or incineration. There are many methods to remediate CCA treated wood waste. Aqueous extraction is the most one. In this study, the materials are southern pine (Pinus taeda) that is pressure-treated with CCA type C preservative (hexavalent chromium as CrO3, 47.5%; copper as CuO, 18.5%; arsenic as As2O5, 34.0%). Chelating agents are biodegradable chelating agents: ethylenediaminetetraacetic acid (EDTA), nitrilotriacetic acid (NTA) and [S, S]-Ethylenediaminedisuccinic acid (EDDS). This study investigates impact of various parameters including pH, chelating concentration, temperature, and chelating duration on chromium, copper, and arsenic extraction efficiencies. Results indicates that lower pH (pH 2.0-4.0) are more effective than higher pH (pH>6) for extraction of chromium, copper, and arsenic. By changing solution concentration from 0.01 to 0.1M, the optimal extraction efficiencies for three metals were obtained at 0.1 M. Optimal extraction temperature for CCA metals was 75oC. The results of duration of extraction presented the chelating rates of metals. There is great extraction efficiencies of copper between 5 and 6 hours extraction. Under these best conditions, the highest percentage removals of chromium, copper and arsenic are 53%-66%, 84%-93%, 55%-62%, respectively. In the results of EDDS-CCA stoichiometric ratios, EDDS concentrations above unit stoichiometric requirement are required to remove CCA metals at optimal. And the extraction efficiencies are illustrated when solution to wood ratio is 10. The results of sequential extractions demonstrated that chromium was mainly existed in the organic and residual fractions (85.6%); copper is mainly existed in exchangeable and residual fractions (49%); arsenic is mainly existed in organic and residual fractions (80.2%). This study indicates the extraction efficiencies for chromium, copper, and arsenic by EDTA, EDDS and NTA are satisfactory. Besides, EDDS and NTA are more biodegradable, and more effect in CCA treated wood remediation. In the near future, EDDS and NTA could be applied broadly to remove heavy metals from wastes