1. Introduction
Volatile organic compounds (VOCs) constitute a group of organic chemicals having low Tb (generally less than 250℃) and have been concerned widely for adverse impacts on environment and human health (McDonald et al., 2018; Sheu et al., 2020). Due to rapid urbanization and industrialization, there has been a rapid increase of VOC emission within the last century (He et al., 2019). Globally, VOC emissions contribute significantly to environmental pollutions and climate changes, generating photochemical smog and destroying stratospheric ozone. Exposure to VOCs can also cause severe health consequences (Almomani et al., 2021; Bravo et al., 2017; Hein et al., 2018; Matějová et al., 2013). Short-term exposure to VOCs can cause headaches, dizziness, fatigue, nausea, and respiratory irritation, while long-term exposure can lead to damages to the kidney, liver, and central nervous system (Rumchev, 2004; Wu et al., 2007; Yang et al., 2020). VOCs, especially aromatic compounds, are often strongly recalcitrant to biodegradation in environment. A series of restrict legislations and standards have been formulated for VOC emission control worldwide; however, VOCs generated in manufacturing, agriculture and transportation industries still account for a large portion of gaseous pollutants (Schiavon et al., 2017). To date, a variety of technologies have been proposed for VOC treatment which can be categorized into two groups: VOC recovery (sorption, condensation, and membrane separation) and VOC degradation (incineration, catalytic oxidation, biodegradation, photooxidation, and non-thermal plasma oxidation) (He et al., 2019). Among these technologies, biodegradation is particularly appealing due to its environmentally-benign nature and high energy efficiency over other physical or chemical technologies (Estrada et al., 2015).
Previous research had demonstrated the feasibility of using conventional bioreactors that typically involved microorganisms hosted in liquid media for VOC degradation, in form of biofilters, biotrickling reactors, and bioscrubbers (Mudliar et al., 2010; Delhoménie & Heitz, 2005; Detchanamurthy & Gostomski, 2012). However, most VOCs are hydrophobic with very limited solubility and suffer significant mass transfer resistance in aqueous media (Cheng et al., 2016; Khan et al., 2018). Accordingly, various bioreactor designs have been suggested and examined to intensify VOC biodegradation by promoting substrate and biomass interactions. That included the use of organic solvents to form two-phase partitioning bioreactors. To our opinion, these organic phases (including silicone oil, hexadecane, and polymeric compounds) can help with the removal of VOC from gas phase, but do not necessary improve VOC solubility in aqueous media hosting the biomass, as they usually offer only limited partitioning coefficients between the two phases (Hernández et al., 2012; Muñoz et al., 2012; Muñoz et al., 2007). Another common approach is the use of membrane bioreactors in which porous membranes serve as contacting surfaces interfacing gas, liquid media and biomass for improved VOC solvation and degradation kinetics. However, the clogging and the high cost of the membrane supports generally limited large-scale applications of membrane bioreactors (Lebrero et al., 2013; Reij et al., 1998). An alternative strategy is to pretreat the VOCs and break them down to more soluble chemicals that are more vulnerable to biodegradation. Pretreatment technologies such as UV photooxidation and non-thermal plasma treatment have successfully been coupled with conventional bioreactors (Almomani et al., 2021; Schiavon et al., 2015). Such pretreatment processes may suffer from formation of toxic byproducts, high cost and complications in scaleup operations.
As far as mass transfer resistance concerned, VOC biodegradation may implicate gas phase diffusion, bulk liquid phase transfer, and cell membrane adsorption and diffusion. The liquid phase mass transfer resistance is often regarded the primary limiting factor (Mudliar et al., 2010; Delhoménie & Heitz, 2005; Detchanamurthy & Gostomski, 2012; Cheng et al., 2016). Therefore, we assume here that elimination of the bulk liquid phase would substantially improve biodegradation efficiency. The aim of this work is therefore to examine the feasibility of growing microbial biofilms on solid supports without any bulk liquid phase for VOC biodegradation. P. putida F1 and toluene are selected as the model bacterium and VOC, respectively. Factors regulating the growth of biofilm on the solid support and VOC degradation efficiency are examined. The long-term operational stability of the supported biofilm is also demonstrated in a tubular packed bed reactor.