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.