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聚丙烯酰胺(PAM)是一种由丙烯酰胺和其他共聚物组成的高分子质量聚合物,按照其所带电荷属性的不同可以分为阳离子PAM、阴离子PAM和非离子PAM。其中阴离子型PAM在油田中应用最为广泛,通过在注入水中加入PAM可以提高溶液的黏度从而强化油田的采收率,有研究[1]表明通过添加PAM可以提高大约25%的采收率。油田所用的阴离子型PAM分子质量一般在107 Da上,PAM在采油之后会随采出水来到地面上,因此,研究PAM的生物可降解性可以为PAM排放到环境中可能带来的环境影响提供数据基础。
PAM生物处理包括2个部分:碳链断裂和酰胺基的水解。以往的研究[2-3]发现,PAM的酰胺基可以被微生物水解释放出氮,进而被产甲烷菌和硫酸盐还原菌作为氮源用于生长。芽孢杆菌属、不动杆菌属和恶臭假单胞杆菌属等利用PAM作为唯一碳源的好氧功能菌已经被分离出来[4-6],同时也分离出在厌氧的双酶梭状芽孢杆菌[7]。通过混合多种纯培养的微生物联合降解PAM,有发现微生物之间的协同效应可以提高PAM的生物处理效果,这表明PAM有望作为唯一碳源被厌氧或好氧反应器里复合微生物用于生长[8-9]。SONG等[10]通过添加共存碳源淀粉促进PAM生物处理,结果表明好氧和厌氧反应器长期运行中PAM发生降解,并且生物处理后分子质量降低碳链发生断裂,然而生物处理能否利用PAM作为唯一碳源还不清楚,需要进一步的研究[11]。
本研究选择2.17×107 Da阴离子型高分子质量PAM作为目标污染物,在以PAM为唯一碳源和共存碳源淀粉为对照的实验条件下,研究了2个方面内容:1)好氧悬浮污泥反应器和厌氧升流式污泥反应器长期运行过程中PAM的生物降解效果; 2)延长水力停留时间(HRT)和串联生物膜反应器对生物处理PAM性能的影响。由于目前的排阻色谱柱只能用来分析分子质量107 Da以下聚合物,而对分子质量过高的聚合物则会因为剪切、吸附和不规则的孔洞分布等原因导致分析结果产生偏差[12]。流场流分离技术利用2个半渗透膜组成上下两壁面形成细小的毛细管流道(宽高比 > 100),这有效避免了排阻色谱柱中凝胶孔洞可能带来的偏差,因此,本研究选择非对称流场流耦合激光光散射技术分析PAM的分子质量[13]。本研究综合利用流场流分离耦合多维角度激光光散射、浊度法和黏度法表征PAM作为唯一碳源的生物处理效果。采出水中PAM外排到环境中可能带来的环境影响的研究可为相关环境管理决策部门提供参考。
聚丙烯酰胺作为唯一碳源的好氧和厌氧生物降解
Aerobic and anaerobic biodegradation of polyacrylamide as the sole carbon source
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摘要: 油田为提高原油采收率而采用聚合物驱油作业,产生的采出水中残留着阴离子型高分子质量聚丙烯酰胺(PAM)。废水中PAM和淀粉共存时PAM可发生碳链断裂和生物降解,然而以PAM作为唯一碳源的生物降解性还不清楚。利用好氧悬浮污泥和厌氧升流式反应器,分别处理PAM为唯一碳源的模拟废水(水力停留时间(HRT)为2 d,PAM浓度为200 mg·L-1),结果表明,好氧反应器出水的PAM浓度和黏度均没有降低,同时运行84 d后污泥流失,造成系统崩溃。而厌氧反应器出水PAM浓度和黏度分别降为169.81 mg·L-1和1.50 mPa·s,流场流分离耦合多维角度激光光散射分析发现PAM的分子质量从2.17×107 Da降低到3.35×106 Da,表明厌氧条件下可以利用PAM作为唯一碳源进行生物降解,并发生碳链断裂。延长HRT从2~8 d可以提高利用PAM作为唯一碳源的厌氧处理效果,出水分子质量进一步降低到1.60×106 Da,同时黏度也从1.50 mPa·s降低到1.21 mPa·s。串联生物膜反应器也可以提高利用PAM作为唯一碳源的厌氧生物处理效果,在HRT为4 d条件下PAM的分子质量和黏度降低到1.87×106 Da和1.26 mPa·s。Abstract: Polymer flooding was used to enhance the oil recovery in the oilfield, and the partial hydrolyzed anionic high molecular weight polyacrylamide (PAM) would remained in the produced wastewater. It was found the cleavage of the main carbon backbone and biodegradability of PAM will occur under aerobic and anaerobic conditions using the PAM and coexisting starch as the carbon sources, while the biodegradability of PAM is uncertainty when it was used as the sole carbon source. Aerobic suspended sludge reactor (ASSR) and upflow anaerobic sludge blanket (UASB) were used to treat the synthetic wastewater containing PAM as the sole carbon source (hydraulic residence time (HRT) 2 d, PAM concentration 200 mg·L-1), respectively. The results indicated that PAM concentration and viscosity of ASSR effluent did not decrease, after 84 d running, the biomass was washed out and ASSR system collapsed. However, PAM concentration and the viscosity of UASB effluent decreased to 169.81 mg·L-1 and 1.50 mPa·s, respectively. Through the analysis of the flow field fractionation coupled with multi-angles laser lights method, the molecular weight of PAM decreased from 2.17×107 Da to 3.35×106 Da by. This indicated that the anaerobic biodegradation of PAM could occur when it was used as the sole carbon source, as well as the disruption of its main carbon backbone. Extending the HRT from 2 d to 8 d could improve the anaerobic biodegradation of PAM as the sole carbon source, and PAM molecular weight in UASB effluent further declined to 1.60×106 Da. At the same time, the viscosity of UASB effluent decreased from 1.50 mPa·s to 1.21 mPa·s. The tanks-in-series biofilm reactors could also improve the anaerobic biodegradation of PAM as the sole carbon source, and PAM molecular weight and the viscosity of reactor effluent were 1.87×106 Da and 1.26 mPa·s at the HRT of 4 d, respectively.
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表 1 延长HRT对生物处理PAM黏度和分子质量的影响
Table 1. Effect of extension the HRT on the changes of viscosity and molecular weight during the microbial treatment
水样 分子质量/(106 Da) 黏度/(mPa·s) HRT=2 d HRT=4 d HRT=8 d HRT=16 d HRT =2 d HRT=4 d HRT=8 d HRT=16 d 厌氧出水(进水PAM) 3.35±0.56 2.65±0.18 1.60±0.10 1.62±0.26 1.50±0.13 1.40±0.09 1.21±0.05 1.28±0.06 厌氧出水(进水淀粉+PAM) 3.96±0.95 2.27±0.49 1.29±0.42 1.49±0.18 1.55±0.14 1.35±0.10 1.19±0.07 1.23±0.05 生物膜1出水 2.74±0.77 1.87±0.77 — — 1.47±0.11 1.29±0.068 — — 生物膜2出水 3.23±0.63 1.96±0.88 — — 1.43±0.09 1.30±0.11 — — 表 2 处理时间(55 ℃)对分子质量和黏度的影响
Table 2. Effect of treatment time (55 ℃) on molecular weight and viscosity
处理时间/d 分子质量/(107 Da) 黏度/(mPa·s) 0 2.17±0.03 2.02±0.02 2 2.16±0.03 1.99±0.03 4 2.14±0.04 1.94±0.05 8 2.10±0.06 1.88±0.08 16 2.01±0.09 1.80±0.14 -
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