PVA固定化硫酸還原菌體顆粒處理含銅廢水之研究

Abstract

近數十年來，生物沉澱 (bio-precipitation) 技術利用硫酸還原菌 (sulfate reducing bacteria, SRB) 處理含高硫酸鹽及高重金屬濃度的酸礦廢水，為一具相當發展潛力的生物處理技術。但在實際應用上仍有多方面需改進之處，如降低添加碳源成本、增加SRB活性或減少對SRB毒性及抑制作用以增加重金屬處理效率等。 本研究以聚乙烯醇 (polyvinyl alcohol, PVA) 包覆馴養SRB製成PVA固定化菌體顆粒，探討在含不同銅離子濃度及微生物含量 (PVA包覆微生物量) 的條件下，對SRB生長抑制的影響，及探討生物沉澱處理含銅廢水的效率，並藉由中央合成設計法 (central composite design, CCD) 規劃實驗組數及實驗條件進行實驗。PVA包覆微生物量以測定PVA固定化菌體顆粒之蛋白質含量為參數，實驗規劃SRB含量介於19-235 mg VSS/L，即PVA之蛋白質添加量介於0.16-1.94 mg，設計銅離子濃度範圍介於10-100 mg/L。以批次實驗比較PVA固定化菌體顆粒及PVA空白顆粒之差異，於固定時間分析各試驗組之pH、ORP、硫酸鹽、硫離子及銅離子濃度之變化。以反應曲面法 (response surface method, RSM) 分析找出最適的評估指標及最佳的操作條件。 實驗結果顯示所有PVA固定化菌體顆粒試驗組在第168小時，硫酸鹽還原率在99% 以上，而銅離子濃度則小於1 mg/L，與PVA空白顆粒之對照組實驗結果有明顯差異。以PVA固定化菌體顆粒進行生物沉澱實驗，其去除重金屬機制包括物理化學沉澱 (含吸附作用) 及生物沉澱兩大部分。結果顯示銅離子之物理化學沉澱 (含吸附作用) 量介於17.0-64.6%，而生物沉澱量介於35.4-83%，由此可知重金屬銅離子去除途徑除了生物沉澱之外還包括物化作用的影響。針對試驗組之硫質量平衡計算得知，其回收率介於59.4-79.9%，推測可能原因有 (1) 硫離子與培養基內鐵離子形成硫化鐵沉澱，(2) 形成硫化氫揮發及 (3) 在採樣時硫離子暴露在空氣而造成損失有關。 由CCD實驗結果經ANOVA分析結果顯示R4評估指標為顯著。而RSM分析結果顯示其反應曲面呈收斂之趨勢，有最佳值存在，即當微生物濃度136 mg VSS/L (相當蛋白質添加量1.12 mg)，銅離子濃度57.9 mg/L，且反應溫度30±2oC時，有最高硫酸鹽還原之反應速率常數0.0423 h-1。經由廻歸係數分析得知硫酸鹽還原之反應速率常數配適二階反應模型，其R2值為85.1%。

Heavy metals are present in wastewaters released from battery, paint and chemical manufacturing industries. Also, considerable portion of heavy metal containing wastewater is discharged from mining industry i.e. acid mine drainage. Heavy metal disposal problems require urgent solution to avoid serious environmental contamination. The activity of sulfate reducing bacteria (SRB) offers interesting potentials for metal removal and recovery. In the present work, utilization of PVA as a gel matrix for immobilization of SRB was investigated. This study was also focused on the optimization of bio-precipitation process using a statistical method to provide information concerning the effect of amount of SRB immobilized on PVA and copper concentration in biotic experiments. A central composite (CCD) was used to develop a model for the responses. Response surface methodology (RSM) was used to optimize the amount of SRB immobilized (19-235 mg VSS/L), and copper concentrations (10-100 mg/L). The objective of this research consists of two parts. In the first part, the amount of SRB immobilized on PVA and the copper concentration are considered study their effect on SRB during bioprecipitation experiments as designed by CCD. In the second part, the four responses such as biological copper removal (R1), chemical precipitation and adsorption (R2), specific sulfate reduction (R3) and reaction rate constant (R4) are considered to optimize the amount of SRB immobilized on PVA and copper concentration using RSM. The bioprecipitation experimental results showed that the sulfate removal was more than 99% and residual copper concentration was less than 1 mg/L in a11 experimental runs within 7 days. The results demonstrated that the copper removal was by bioprecipitation and adsorption on PVA beads. The blank PVA adsorption experiments indicated that the blank PVA and SRB immobilized PVA have copper adsorption capacities of 0.355 and 0.363 mg-Cu2+/g PVA beads, respectively. According to the results of RSM, R1 and R2 responses were not significant but R3 and R4 responses were significant. Result of R4 showed better convergence in the contour plot. The optimal quantity of SRB immobilized on PVA and copper concentration were observed these as 136 mg VSS/L (equivalent to 1.12 mg protein) and 57.9 mg/L, respectively. At there conditions the highest sulfate reaction rate constant (k) as per first-order kinetic was observed as 0.0423 h-1 in 30±2oC.