Engineering multiscale polypyrrole/carbon nanotubes interface to boost electron utilization in a bioelectrochemical system coupled with chemical absorption for NO removal

Highlights

• An interface was engineered by microscale polypyrrole and nanoscale carbon nanotubes.

• Synergistic effect existed between the multiscale interface and microbial nanowires.

• Faraday efficiency along Fe(III)EDTA reduction increased 40.7%–302.6% by PPy/CNTs.

• Functional microorganisms were also enriched in the CABER system for NO removal.

Abstract

The chemical absorption-bioelectrochemical reduction (CABER) integrated system provides an alternative of good potential for NO removal. The efficient utilization of cathode electrons directly determines the system performance and operating cost. Herein, we synthesize a polypyrrole/carbon nanotubes (PPy/CNTs) composite to engineer a micro-and nanoscale interface with low resistance and high biocompatibility between the cathode and biofilms in the CABER system. The resulting PPy/CNTs biocathodes exhibit 36.4% increase in biomass density, 40.7%–302.6% increase in Faraday efficiency along Fe(III)EDTA reduction, and 204% increase in Fe(II)EDTA-NO reduction rate. The enrichment of functional microorganisms is validated to be a key strengthening factor, as the proportion of which increased from 57.9% to 84.6%. Moreover, for efficient electron transfer and utilization, a low-resistance electron transfer route, “electrode substrate → PPy (→ CNTs) → microbial cells → Fe(III)EDTA or Fe(II)EDTA-NO”, is realized in the multiscale conductive networks constructed of PPy/CNTs composite and microbial nanowires.

Introduction

The reduction of NOx emission is crucial for the control of combined air pollution caused by ozone and PM_2.5 (Wang et al., 2016; Zhang et al., 2016). As a good alternative of physical or chemical denitrification technologies, biological process has the advantages of low cost, good safety, and high flexibility (Niu and Leung, 2010). However, as a kind of hydrophobic pollutant, NOx (95% of which was NO in flue gas) became a challenge for effective biological control due to the mass transfer limitation. To enhance the absorption of NO from gas phase to liquid phase, ferrous ethylenediaminetetraacetate (Fe(II)EDTA) was always utilized, and the integration of chemical absorption and biological reduction has been developed recently (Li et al., 2006; Lovley, 2008). There were two basic processes in the chemical absorption-biological reduction (CABR) system, namely absorption of NO and O₂ and bioreduction of Fe(II)EDTA-NO and Fe(III)EDTA. It has been confirmed that the rate-limiting step in the CABR system was bioreduction, which meant the enhancement of biological reduction was key for further development of CABR system (Zhang et al., 2008).

Bioelectrochemical systems (BESs) could enhance the bioreactions in microbes immobilized on electrode due to the fast electron transfer between microbial cell and electrode (Xia et al., 2013). Microbial electrolytic cell (MEC) was a typical BESs configuration, in which an additional potential on electrode with the same polarity was applied (Borole et al., 2011). Taking advantages of MEC, the chemical absorption-bioelectrochemical reduction (CABER) integrated system has been developed (Kadier et al., 2014; Xia et al., 2014). The bioreduction of Fe(II)EDTA-NO and Fe(III)EDTA in biocathode could be further strengthened because of the reaction driving force provided by additional potential. It has been proved that, microbes accepted electrons from cathode through physical contact, and utilized them for Fe(III)EDTA reduction (Mi et al., 2009). Besides, the produced Fe(II)EDTA was the main electron donor for Fe(II)EDTA-NO reduction (Zhao et al., 2018). Hence, the improvement of electron transfer rate in the interface between microbial cell and cathode was an effective approach for bioelectrochemical reduction enhancement in CABER integrated system.

Although redox intermediates could be used to transfer electrons (van der Maas et al., 2003; von Canstein et al., 2008; Yong et al., 2011), mediated electron transfer was susceptible to the solution environment (Bond et al., 2012; Chen et al., 2021), which made it less stable than direct electron transfer (DET) via physical contact. The c-type cytochromes and bacterial nanowires were the main extracellular conductive structures for DET (Creasey et al., 2018; Lu et al., 2022). Therefore, increasing the density of the conductive structure between microbial cell and cathode to a certain extent could effectively enhance the electron transfer rate. Microscale conductive polymers have certain advantages in conductivity and stability (Apetrei et al., 2018), which thereby meet the requirement for electron transfer efficiency improvement in bioelectrodes. For instance, a polypyrrole (PPy) modified stainless steel contributed to a 29 times higher current density in a microbial fuel cell (MFC) (Pu et al., 2018). Moreover, the utilization of conductive polymers also has the potential to increase the biomass attached on the electrode (Lai et al., 2011; Zhao et al., 2013; Wang et al., 2019). Except for the great conductivity and chemical stability, carbon nanotubes (CNTs) could promote electron transfer on the microbial cell surface directly due to their size effect (Wu et al., 2017). Recently, CNTs are widely used in biosensors (Ding et al., 2016; Hassan and Wollenberger, 2016), but rarely applied in bioelectrode modification for pollution control. Consequently, microscale conductive polymer and nanoscale CNTs have the synergistic effect on electron transfer enhancement in bioelectrode. As an efficient cathode catalyst in fuel cells, the PPy/CNTs composite was generally utilized for chemical reaction promotion in chemical fuel cells or electron transfer enhancement in bioanodes in microbial fuel cells (Ghasemi et al., 2016). The research on pollutant removal in BESs constructed with PPy/CNTs composite biocathode is still relatively shallow now, and its application in biocathode for biological NO reduction has not been reported yet. The complicated interaction between the composite and microbial cell during pollutant degradation remains to be studied.

In this work, we aimed to engineer a micro-and nanoscale PPy/CNTs interface in biocathodes to boost the bioreduction rate and electron utilization in the CABER system. The improvements of biofilm formation and denitrification performance in the CABER system were validated. The reduction rate and columbic efficiency along Fe(III)EDTA and Fe(II)EDTA-NO were investigated individually to explore the strengthening mechanism. Meanwhile, the electron transfer pathway was proposed to give an in-depth insight into the bioelectrochemical processes. Overall, it is hoped that this work could provide a new strategy for bioelectrode design in the CABER integrated system for NO removal and provide foundational information for its practical application.