Enhanced remediation of PAHs-contaminated site soil by bioaugmentation with graphene oxide immobilized bacterial pellets

Highlights

• Graphene oxide-immobilized bacterial pellets (GOBP) was explored.

• GOBP exhibited greatly improved mechanical and structural properties.

• GOBP significantly promoted the removal of PAHs in site soil.

• GOBP greatly increased the abundance of embedded degrading bacterial in soil.

• GOBP obviously enhanced the enrichment of potential indigenous degrading bacteria.

Abstract

Bioaugmentation is considered as a promising technology for cleanup of polycyclic aromatic hydrocarbons (PAHs) from contaminated site soil, however, available high-efficiency microbial agents remain very limited. Herein, we explored graphene oxide (GO)-immobilized bacterial pellets (JGOLB) by embedding high-efficiency degrading bacteria Paracoccus aminovorans HPD-2 in alginate-GO-Luria-Bertani medium (LB) composites. Microcosm culture experiments were performed with contaminated site soil to assess the effect of JGOLB on the removal of PAHs. The results showed that JGOLB exhibited greatly improved mechanical strength, larger specific surface area and more enriched mesopores, compared with traditional immobilized bacterial pellets. They significantly increased the removal rate of PAHs by 18.51% compared with traditional bacterial pellets, reaching the removal rate at 62.86% over 35 days of incubation. Moreover, the increase mainly focused on high-molecular-weight PAHs. JGOLB not only greatly increased the abundance of embedded degrading bacteria in soil, but also significantly enhanced the enrichment of potential indigenous degrading bacteria (Pseudarthrobacter and Arthrobacter), the functional genes involved in PAHs degradation and a number of ATP transport genes in the soil. Overall, such nanocomposite bacterial pellets provide a novel microbial immobilization option for remediating organic pollutants in harsh soil environment.

Introduction

Soil pollution caused by polycyclic aromatic hydrocarbons (PAHs) has become one of the main environmental problems faced currently by most countries in the world. Especially in China, the point exceeded rate of PAHs in soil has reach up to 1.4%. Therefore, it is quite urgent to develop high-efficiency and sustainable remediation strategies for cleanup of these pollutants in soil environment. Bioremediation has gradually attracted increasing attention because of many advantages, such as economic, green, environmentally friendly and so on, particularly microbial bioaugmentation technology (Liao et al., 2019, Sun et al., 2012). However, artificially added degrading bacteria are often difficult to resist high-concentration pollution stress, complex environmental conditions (e.g., extreme pH and high salt) and competition from indigenous microbial communities, resulting in a rapid decrease in abundance of added degrading bacteria and low degradation efficiency of PAHs (Stefaniuk et al., 2018, Suszek-Lopatka et al., 2019, Zeng et al., 2021).

Immobilization technology is one of important ways to solve this problem by providing more comfortable shelter for degrading bacteria. A large number of immobilized materials, including biochar (Bandara et al., 2019, Deng et al., 2021), polyvinyl alcohol (Chen et al., 2021), sodium alginate (Ravikumar et al., 2016), organoclay (Kim et al., 2012) and chitosan/alginate (Dou et al., 2021), etc., have been explored for the improvement in removal efficiency of organic pollutants in complex environment. Compared with other materials, sodium alginate, with many advantages including high biocompatibility and low toxicity, has been considered as an excellent microbial immobilization carrier for growth and proliferation of degrading bacteria in water treatment or wastewater field (Dai et al., 2020, Nie et al., 2015; Park et al., 2021). However, sodium alginate is still comparatively vulnerable to ruptures and maintain limited protective effect in more complex soil environment. In addition, relatively small adsorption ability of sodium alginate for pollutants is one of crucial factors influencing removal efficiency of pollutants by embedded degrading bacteria (Dai et al., 2020, Ouyang et al., 2021). Although some high hardness materials (e.g., Fe and polyvinyl alcohol) have been investigated to enhance mechanical strength of sodium alginate (Chen et al., 2021, Funada et al., 2018, Shanker et al., 2017), effective materials for promoting removal of PAHs in soil environment remain still relatively limited. Therefore, developing novel immobilized bacterial agents with higher mechanical strength and stronger adsorption performance are urgently needed in bioremediation of contaminated soil, especially highly-contaminated site soil.

In recent years, nanomaterials have displayed great potential to remove or transform recalcitrant pollutants in complex environmental media due to their unique features of large specific surface area, high reactivity, selectivity and versatility, which is attracting an increasing research and industrial attentions (Fernando et al., 2020, Ge et al., 2018, Queiroz et al., 2021). Graphene oxide (GO), as a typical carbon nanomaterial, harbors an excellent adsorption ability for PAHs, especially high-molecular-weight PAHs, due to the strong interaction with the carboxyl groups attaching to the edges of GO (Wang et al., 2014, Zhang et al., 2013). Besides, the addition of GO has been observed to augment the mechanical strength and chemical stability of alginate hydrogels (Choe et al., 2019, Valentin et al., 2019). Alginate-GO composite-based thin films and hydrogel beads have been explored in many fields (Zhu et al., 2017), such as water purification (Yang et al., 2018). Moreover, our previous study demonstrated that GO was highly biocompatible for resistant bacteria and able to promote the growth and biofilm formation of Paracoccus aminovorans HPD-2, a high-efficiency PAH-degrading bacterium (Mao et al., 2020). Therefore, we hypothesized that GO-immobilized bacterial agents, prepared by embedding PAH-degrading bacteria in alginate-GO nanocomposites, could better protect degrading bacteria from stress of complex soil environment and enhance the degradation of high-concentration PAHs in site soil.

The objectives of this study were to (1) develop novel high-efficiency nano-immobilized bacterial pellets for bioaugmentation in PAH-contaminated site soil; (2) decipher the underlying mechanisms driving the promotion effect of the nano-immobilized bacterial pellets. To address these objectives, nano-immobilized bacterial pellets were constructed by embedding a high-efficiency PAH-degrading bacterium in alginate-GO nanocomposites. A series of physical, microscopic and spectroscopic technologies were applied to characterize the bacterial pellets. Microcosm culture experiments were performed with a high-concentration-PAH- contaminated site soil spiked with different types of bacterial pellets. High-throughput sequencing and real-time PCR were used to investigate the changes in bacterial community structure and abundance of functional genes.