Biodegradation of hydrolyzed polyacrylamide by a Bacillus megaterium strain SZK-5: Functional enzymes and antioxidant defense mechanism
Graphical abstract
Introduction
Petroleum is chemical raw material and fossil fuel with great economic value. In order to keep pace with the rapidly growing demand for energy, various enhanced oil recovery (EOR) technologies have been studied and performed over the past few decades. One of the most vital techniques for EOR is polymer flooding, which is categorized as a third recovery method. Hydrolyzed polyacrylamide (HPAM) is one of the most widely used polymers in oil fields because of its high water solubility, high temperature resistance, salt resistance, easy synthesis and low cost (Yang et al., 2010; Liu et al., 2017). In general, HPAM is recognized as nontoxic, however under certain chemical or physical conditions, it may be decomposed and produce acrylamide (AM), which exhibits toxicity to the nervous system and poses a great threat to human and animal health (Rayburn and Friedman, 2010; Huang et al., 2018). AM and its main metabolite, glycidamide (which has more carcinogenic and genotoxic properties), are able to pass through the placenta in humans and can be transferred into breast milk (Mojska et al., 2012; Li et al., 2017). AM has been classified as a Group 2A carcinogen by the International Agency for Research on Cancer and the European Food Safety Authority has announced that developmental toxicity has been identified as a probable critical endpoint for AM toxicity (Effects, 2011; Chain, 2015). As such, HPAM-containing wastewater has attracted growing concern in terms of its serious threats to humans and ecological environments. Hence, it is urgent and significant to conduct studies on finding an efficient technology to treat HPAM-containing wastewater and minimize the risk of HPAM contamination in the environment.
Various methods have been studied to treat HPAM-containing wastewater, such as physical disposal (Silva et al., 2000), chemical oxidation (Liu et al., 2009), biodegradation and the combination of these methods (Song et al., 2018; Zhang et al., 2019). Among these methods, bioremediation or biodegradation has always been a concern of researchers owing to its eco-friendly and cost-effective advantages (Sen et al., 2014; Ren et al., 2018b, 2018c; Banerjee et al., 2019). Bao et al. (2010) isolated two HPAM-degrading bacterial strains from the produced water of polymer flooding, which could breakdown the carbon backbone and the amide group. Sang et al. (2015) inoculated HPAM-degrading bacteria strains into the activated sludge of the bioreactor. The results suggested that the inoculated bacteria strains could adapt to the environment and became the dominant bacteria in the bioreactor. Song et al. (2017) revealed that both anaerobic and aerobic biodegradation were effective in the hydrolysis of HPAM. Song et al. (2018) explored the feasibility of using the combined expanded granular sludge bed reactor-aerobic biofilm reactor biosystem to treat HPAM-containing wastewater. The experimental results showed that the HPAM removal rate was improved by the cooperation of anaerobic microorganisms and aerobic microorganisms. Also, some researchers have studied the combination of biodegradation and other methods to treat HPAM-containing wastewater. For example, Zhang et al. (2019) reported that Fenton pretreatment improved the biodegradability of HPAM and HPAM was successfully removed in subsequent biological treatment. However, in the face of multiple pollutants and impurities in oilfield produced water, most studies only focus on the biodegradation of HPAM, ignoring other disadvantageous conditions, such as the presence of crude oil (Wen et al., 2010; Sang et al., 2015; Song et al., 2017). In oilfield produced water, HPAM enhances the emulsifying ability of crude oil, resulting in smaller oil droplets in the produced water, which makes the wastewater more stable and reduces the separation effect of oil and water. As a result, the concentration of crude oil in oilfield produced water is much higher than that in ordinary oily wastewater. Hence, the effects of crude oil on HPAM biodegradation are essential to be taken into account.
The objectives of this study are (Ⅰ) to isolate and identify HPAM-degrading bacteria from soil contaminated by oilfield produced water, (Ⅱ) to determine the factors that have significant effects on HPAM biodegradation, (Ⅲ) to investigate particular enzymes and cell fractions of the strain SZK-5 with respect to their abilities to degrade HPAM and (Ⅳ) to study the effects of crude oil and HPAM on the strain SZK-5.
Section snippets
Samples and reagents
The soil sample used in this study was isolated from soil contaminated by oilfield produced water. The wastewater was discharged from Gu Liu joint station (Dongying, China), the concentration of crude oil in which ranged from 0.1% to 3% (w/v). The maximum concentration of HPAM in the wastewater was 500 mg L−1.
The HPAM sample was obtained from Chang'an Polymer Co. Ltd. (Dongying, China). The relative molecular weight of HPAM was about 2.2 × 107 and it was 10% hydrolyzed. The crude oil sample was
Isolation of HPAM-degrading strains
Isolation of HPAM-degrading strains from the soil sample was carried out over a 5-week incubation period by gradually increasing HPAM concentrations from 100 mg L−1 to 500 mg L−1 at 100 mg L−1 intervals. Six bacterial strains that could rely on just HPAM for carbon and energy were obtained. These HPAM-degrading strains exhibited different biodegradation capacities to HPAM, and the strain SZK-5 performed best. Therefore, it was chosen for further study.
Identification of the strain SZK-5
Colonies of the strain SZK-5 which grew at
Conclusions
Biodegradation of HPAM by B. megaterium was investigated for the first time in this study. The strain SZK-5 could rely on just HPAM as for carbon and energy and exhibited exceptional adaptability to the crude oil-containing environment. The enzymes inhibition experiments suggested that CYP was involved in the cleavage and biotransformation of the carbon chain of HPAM molecule and urease was significant to the nitrogen removal. Changes of MDA content and the activities of SOD and CAT under the
Acknowledgements
This study was financially supported by the Key Research and Development Program of Shandong Province (public welfare special project) (2017GSF217012), Major Projects of the National High Technology Research and Development Program 863 (2013AA064401) and the National Natural Science Foundation of China (51174181).
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This is MCTL Contribution No. 202.