Coupled photocatalytic-bacterial degradation of pyrene: Removal enhancement and bacterial community responses

https://doi.org/10.1016/j.envres.2020.109135Get rights and content

Highlights

  • The combination of photocatalysis and biodegradation promoted PYR removal.

  • Dominant bacteria altered along with some functional bacteria quickly acclimatizing.

  • UV light-induced combined system altered bacteria community more than visible light.

  • Functional features adjusted to shield bacterial community and remove PYR.

Abstract

Polycyclic aromatic hydrocarbons (PAHs) are a class of pollutants that ubiquitously present in environment and hard to be degraded by microorganisms. Herein, we reported a novel photocatalytic-bacterial coupled removal system to treat PAH-polluted water. Using pyrene as the model pollutant, we demonstrated that the removal percentage of different groups was in order: 63.89% ± 1.03% (Vis-Biological) > 61.27% ± 1.08% (UV-Biological) > 59.58% ± 1.15% (UV) > 57.41% ± 1.13% (Vis) > 6.65% ± 0.72% (Biological) > 1.70% ± 0.34% (Control), showing the coupled system significantly improved the removal percentage of pyrene. Additionally, we observed that the coupled system driven by visible light showed higher removal percentage than UV light, exhibiting a good potential for future application. Sequencing analysis of 16S rRNA genes showed that alpha diversity (richness, evenness and diversity) got promoted and data of the relative abundance showed that Pseudomonadaceae was substituted as the dominant bacteria for Planococcaceae, with some other functional bacteria quickly acclimatizing in the bacterial community. Difference analysis indicated that over half of top fifteen genera were generally different significantly (p < 0.001) among two different samples, and UV light altered structure and composition of bacterial community more than visible light. Functional features’ change suggested that the bacterial community not only protected itself but also participated in degrading pyrene. Overall, our study offered a new method for PAH degradation and contributed to further understanding of coupled catalytic-bacterial degradation processes.

Introduction

As one group of the persistent organic pollutants, polycyclic aromatic hydrocarbons (PAHs) can be generated from both natural processes and anthropogenic activities such as coal combustion and vehicle emissions (Souza et al., 2018). PAHs have been listed as priority pollutants by the United States Environmental Protection Agency (USEPA) (Nam et al., 2008) because of their highly carcinogenic, teratogenic and mutagenic properties, which threaten ecological environment, organisms and mankind health (Jang et al., 2013; Polachova et al., 2019; Yu et al., 2013). A series of technologies have been proposed and developed to remove PAHs, which include adsorption (Zhu et al., 2014), photocatalytic degradation (Reddy et al., 2015; Zhou et al., 2019), bacterial degradation (Hedbavna et al., 2016) and hybrid removal processes (Song et al., 2017; Zhao et al., 2018).

Advanced-oxidation Processes (AOPs) have been extensively studied for degrading various organic pollutants effectively in wastewater such as dyes and pesticides (Ribeiro et al., 2019; Salmeron et al., 2019; Sharma et al., 2018). Among different AOPs, photocatalysis has emerged as one of the most reliable techniques for degrading organic pollutants (Sood et al., 2015). Zhang et al. (2008) reported that PAHs can be effectively degraded under ultraviolet (UV) light and TiO2 catalysis. However, in order to utilize solar light as the light source to drive photocatalytic processes, it is desirable to have photocatalysts that can be driven by visible light, which accounts for more than 95% solar irradiation (Zhang et al., 2006). In view of this, new emerging photocatalytic materials are prepared and applied to remove PAHs under visible light (Fu et al., 2018; Yang et al., 2018). However, a noticeable disadvantage of photocatalytic process is that toxic end-products may be generated in the degradation process (Solís et al., 2016).

Biological degradation of pollutants has been considered as a sustainable and environmentally benign approach to removal pollutants from different media due primarily to its low cost and high efficiency (Boruszko, 2017; Sriprapat and Thiravetyan, 2016). At present, the bioremediation of PAHs by applying microorganisms has been shown to be successful in the lab and also in the natural environment by employing various in-situ and ex-situ treatments (Larsen et al., 2009; Sharma et al., 2016). A salient feature of the biological degradation is that the produced end products are generally less-toxic than the initial pollutants, which has the net effect of reducing the environmental risks associated with the pollution (Baoune et al., 2019; Das and Kumar, 2016). Nevertheless, biological degradation processes also have their own shortcomings (Gaur et al., 2018). On the one hand, bacterial consortium that has high degradation capabilities need to selected and enriched (Bacosa and Inoue, 2015). On the other hand, the bacterial degradation process typically requires longer time to degrade PAHs, even for the enriched cultures (Azubuike et al., 2016; Bacosa and Inoue, 2015).

The two above-mentioned methods, photocatalysis and microbial degradation processes, each has their own merits and disadvantages in terms of degrading organic pollutants. Thus, it is expected to develop hybrid systems involving biological and photocatalytic processes to complement each other's advantages. Recently, approach of intimately coupled photocatalysis and biodegradation (ICPB) has been developed to research biodegradation and mineralization of a series of pollutants including tetracycline and phenol through employing both UV and visible light (Li et al., 2011; Zhou et al., 2015; Xiong et al., 2017). However, to the best of our knowledge, relatively little attention has been paid in coupling the AOPs with biological degradation processes for PAHs degradation. In this study, we investigated the removal of pyrene, which was used as a model pollutant of PAHs, through coupled photocatalytic-bacterial degradation process. A coupled system consisted of enriched PAH-degrading bacterial consortium and photocatalyst (Cu/N-codoped TiO2) was used to remove pyrene. In order to further shed light on the pyrene degradation process in the coupled system, we have also studied the pyrene removal in a series of control groups. In addition, the effect of light source on degradation process was also investigated. The dynamic change of microbial community in different systems were also revealed to gain further insight into our systems.

Section snippets

Material coating and biofilm cultivation

Cu/N-codoped TiO2 was used as photocatalyst. It was prepared and characterized according to our previous study (Zhao et al., 2019). The method of PAH-degrading bacteria isolation was described previously (Tao et al., 2007). More details and information were given in supplementary materials. The nanoparticle-coating procedure was as follows: 0.1 g of the dried Cu/N-codoped TiO2 powder were dispersed into the conical flask containing 20 mL pure ethanol. Then 32 pieces of polytetrafluoroethylene

Comparation of pyrene and TOC removal percentage in different groups

We plotted the removal percentage of pyrene in the four groups (C, B, M and MB) under different light sources and made a comparation among them (Fig. 1A). Generally, the removal percentage in different groups was in order of MB > M > B > C. Compared with group C, the removal percentage in group B increased by about 4.95% (p < 0.0005). Photocatalytic material driven by UV or visible light presented excellent effect on removing pyrene (group M) than microbes only (group B) (p < 0.0005). The

Outstanding of the coupled system on removing pyrene

The greater removal percentage of pyrene appearing in the coupled system verified the combination of photocatalytic and microbial degradation on persistent organic pollutants successfully. Meanwhile, we observed that pyrene removal was mainly contributed by photocatalysis process when comparing with biodegradation. Excellent photocatalytic performance was associated with the free radicals and holes (e.g. radical dotO2, radical dotOH and h+) created in photocatalytic reaction (Cai et al., 2019). Although many

Conclusions

Applying Cu/N-codoped TiO2 and PAH-degrading bacterial consortium to assemble coupled photocatalysis and biodegradation system prompted the removal and mineralization of pyrene under distinct light sources. It is worth noting that the coupled system irradiated by visible light presented enormous potential on removing pyrene, which has important practical application value. High-throughput sequencing proved that bacterial community diversity collected from samples used to remove pyrene was

Funding

This work was supported by the National Natural Science Foundation of China (Grants No. 51879080, 51509129, 41371307, 41907108), Natural Science Foundation of Jiangsu Province, China (BK20171435), National Key Research and Development Program of China (No. 2018YFC0407906), a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and the Top-notch Academic Programs Project (TAPP) of Jiangsu Higher Education Institutions.

Author contribution

ZQ, WJ and ZZ conceived and designed the experiments; ZQ, LX and JY performed the experiments and analyzed the data; All authors contributed to the manuscript development and revisions.

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