环境水质学国家重点实验室知识库

       高分子量聚丙烯酰胺（分子量在107 Da以上）具有增加溶液粘度并抵抗高的机械力的特性，因此被广泛应用于海上和陆地油田强化采油。大量使用的聚丙烯酰胺在采油后会随着采出水回到海上平台或地面上，如果直接排放会带来潜在的环境风险。短期的好氧和厌氧评价实验表明聚丙烯酰胺很难生物处理，然而废水中聚丙烯酰胺的长期生物降解性还不清楚。聚丙烯酰胺可能的生物降解途径主要包括酰胺基水解和碳链断裂，以往的研究表明酰胺基可以水解产生氨氮用于微生物生长，但由于缺少合适的表征107 Da以上分子量PAM的方法，有关碳链能否发生断裂还缺乏相关证据。另一方面，油田废水达标外排也需要快速有效去除废水中的聚丙烯酰胺。因此，研究废水中高分子量聚丙烯酰胺的长期生物可降解性以及快速去除技术具有重要的意义。
      本研究首先利用两个不同运行温度下好氧悬浮污泥反应器和厌氧升流式污泥床反应器（运行接近500 d）研究了重均分子量分别为2. 17 ×10 7 和 1.4 0 ×10 6 Da 的 聚丙烯酰胺的生物可降解性，并利用场流光散射等多种方法综合分析了废水中高分子聚丙烯酰胺的碳链断裂和酰胺基水解过程；其次，研究了利用气液界面放电等离子体快速去除废水中聚丙烯酰胺的效果、条件和去除机理。本研究取得的主要成果如下：
      （1）高分子量聚丙烯酰胺（2.17×107 Da）在添加淀粉作为外加碳源条件下可以进行好氧和厌氧生物降解，并且发生主链碳碳单键的断裂：
        建立并运行了两个好氧悬浮污泥反应器（25和40℃）和两个厌氧升流式污泥床反应器（35和55℃），利用场流光散射方法分析聚丙烯酰胺的分子量变化，发现淀粉作为外加碳源和水力停留时间为2 d的条件下好氧和厌氧生物处理都能够将聚丙烯酰胺分子量从2.17×107 Da 降低到3.76-5.87×106 Da，同时粘度也从进水中的2.02 cp降低到了1.45–1.60 cp，表明碳链发生了断裂。红外光谱分析也进一步确认聚丙烯酰胺的酰胺基水解和碳碳单键断裂。批量实验结果发现在25-55℃条件下，温度对聚丙烯酰胺分子量变化影响很小，因此推测碳链断裂主要是生物作用。
        （2）聚丙烯酰胺可以作为唯一碳源 进行 厌氧 处理 （运行温度 55 ℃）℃），而好氧系统不能利用聚丙烯酰 胺作为唯一碳源 ：
        去除淀粉之后，利用聚丙烯酰胺作为唯一碳源的好氧生物处理系统崩溃，但是厌氧生物处理（运行温度 55 却可以利用聚丙烯酰胺作为唯一碳源运行。延长水力停留时间和串联生物膜反应器可以进一步提高聚丙烯酰胺的生物处理效率，延长水力停留时间从2到8 d出水分子质量从3.35×106 Da降低到1.60×106 Da。
（3）小分子量聚丙烯酰胺（1.40×106 Da）生物处理酰胺基更容易发生水解：
研究发现小分子量聚丙烯酰胺（1.40×106 Da）经过厌氧生物处理后分子量降低了一个数量级（从1.40×106降低到6.46-9.31×105 Da），和大分子量聚丙烯酰胺（2.17×107 Da）结果相似。基于酰胺基上化学反应检测的聚丙烯酰胺浓度测定结果表明，小分子量聚丙烯酰胺最大去除率（52.6%）高于大分子量聚丙烯酰胺（17%），表明小分子量聚丙烯酰胺酰胺基更容易水解。处理两种分子量聚丙烯酰胺的反应器中微生物群落结构相似，但是处理小分子量聚丙烯酰胺反应器中聚丙烯酰胺降解相关功能微生物丰度增加。
        （4）气液界面放电等离子体可以快速有效地去除废水中的聚丙烯酰胺：
        在放电电压为25 kV条件下，等离子体能够在 20 min 内把水溶液中聚丙烯
酰胺（ 100 mg/L pH=7 ）快速去除 总有机碳去除遵循零级动力学模型在240 min内从49.57降低到1.23 mg/L。在废水中存在其他有机物（总有机碳为152.2 mg/L）的条件下，等离子体技术仍然能够有效的去除废水中的聚丙烯酰胺（100 mg/L），等离子体处理后废水的可生化性得到提高。羟基自由基在等离子体处理聚丙烯酰胺过程中发挥着关键作用，聚丙烯酰胺降解过程中伴随着解聚、氨化和矿化作用同时进行。
        本研究为油田废水及相关水环境中聚丙烯酰胺危险评价和环境管理提供了数据和科学基础。

            High molecular weight (more than 10 7 Da ) polyacrylamide (PAM) could increase the viscosity of solution and tolerate high mechanical forces therefore it was widely used in the onshore and offshore oil field s to enhance oil recovery . The produced water with a large amount of PAM was brought to the o ffshore platforms or ground after oil recovery . It will pose potentially dangerous if the produced water was discharged into the environment directly. The PAM was recalcitran t accrording to the short term evaluation standard for aerobic and anaerobic biodegradabilit y , however, the biodegradability of PAM in the wastewater in the long term biological treatment was uncertainy.
          The potential biodegradation pathway of PAM mainly comprised the amino hydrolysis and the cleavage of main carbon chain. Previous study had demonstrated that the amino could be hydrolyzed to generate the ammonia for microbial growth.Becau se few method s could effectively analyze the PAM with the molecular weight more than 10 7 Da, there is a lack of solid evidence about the cleavage of carbon chain.On the other hand, there is necessary to rapidly and effectively remove the PAM from the oilfield wastewater to meet the discharge requirement. Thus, it was significant to investigate the biodegradability of high molecular weight PAM during the long term treatment and how to rapidly remove the PAM from wastewater.
        Firstly the biodegradation of PAM with the molecular weight of 2. 17 ×10 7 and 1.4 0 ×10 6 Da was investigated in the two aerobic suspended sludge and two u pflow a naerobic s ludge b lanket reactors operated at different temperature (over 500 days).Multiple methods , such as the flow field fractionation coupled with the multi angles
laser light scatting etc, were used to analyze the amino hydrolysis and carbon chain cleavage of high molecular weight PAM in wastewater. S econdly, the performance conditions and removal mechanisms was analyzed during the removal of PAM by a plasma operated in the gas liquid interface apparatus The main conclusions in this study
as follows:
         (1) High molecular weight PAM 2.17×107 Da) was biodegraded in the aerobic and anaerobic condition with the addition of starch as the carbon sources, and the main carbon chain was fragment ed.

        Two aerobic suspend sludge (25 and 40 °C ) and two upflow anaerobic sludge blanket (35 and 55 °C reactors were installed and operated. Flow field fractionation coupled with multi angles laser light scattering was used to analyze the molecular weight change of PAM. The results showed that b oth anaerobic and aerobic biological treatment reduced the molecular weight of PAM from 2. 17 ×10 7 to 3.76-5.87×106 Da u nder a hydraulic residence time of 2 d and the addition of starch as the carbon sources. T he viscosity was also declined from 2.02 cp in the influent to 1.45 1.60 cp ,which indicated the cleavage of main carbon chain. T he amino hydrolysis and the cleavage of main carbon chain of PAM were further supported by the FTIR analysis.It was found that the temperature had little effect on the molecular weight change of PAM over the range 25 55 °C by batch experiment Therefore it was speculated that the cleavage of the main carbon chain was attributed to biological processes .
        (2)The PAM could be used as the sole carbon sources for anaerobic treatment, while it could not be used for aerobic treatment.
        The aerobic suspended sludge reactor failed to operate used the PAM as the sole carbon sources after removal of starch , while the anaerobic reactors could use the PAM as the sole carbon sources. E xtending the hydraulic residence time and the combination of biofilm reactors could further improve the treatment efficiency of PAM, and the molecular weight of PAM decreased from 3.35×106 to 1.60×106 Da after extending the hydraulic residence time from 2 to 8 d
        (3) The hydrolysis of amino became easier during the biological treatment of the low molecular weight PAM ( 1.40×106 Da).
        The molecular weight of low molecular weight PAM (1.40×106 Da) decreased with a level (from 1.40×106 to 6.46-9.31×105 Da), which was similar with the high molecular weight PAM 2.17×107 Da). It was found that the maximum PAM concentration removal of low molecular weight PAM 52 .6 %%) was higher than the high molecular weight PAM 17 .0 %%), indicating that the amino hydrolysis of low molecular weight PAM became easier. T he microbial community in the reactors treating with the two different molecular weights PAM were similar , however, the abundance of PAM degrading bacteria increased in the reactors with the low molecular weight PAM.
        (4)The plasma operated in the gas liquid interface could r apid and effective removal of PAM from wastewater
The plasma could rapidly remove PAM 100 mg/L pH 7 ) from the aqueous solution within 20 min under the discharge voltage of 25 kV, and the total organic carbon was removed from 49.57 to 1.2 3 mg/L within 240 min following zero order reaction kinetics. Plasma c ould also effectively remove the PAM from the wastewater even in the presence of other organic pollutions ( background TOC as high as 152.2 mg/L ), and the biodegradability of PAM improved after plasma treatment. The ·OH radicals play a key role in t he removal of PAM by plasma, and d epolymerization, ammonification and mineralization of PAM proceeded over the whole duration of PAM degradation

        This research provide s the data and scientific basis for the risk assessment and environmental management of PAM in the oilfield wastewater and relevant aquatic environment