Research articleAnoxic denitrification of BTEX: Biodegradation kinetics and pollutant interactions
Graphical abstract
Introduction
Benzene, toluene, ethylbenzene and xylene (BTEX) are volatile organic compounds (VOCs) commonly used in petrochemical industry and in many industrial sectors as reagents for the synthesis of multiple C-based products (e.g. plastics, synthetic fibers, pesticides) (El-Naas et al., 2014; Mazzeo et al., 2010). The extensive use of BTEX is often criticized due to their hazardous effects on the environment and to human health (Morlett-Chávez et al., 2010): they have genotoxic properties, human exposure to BTEX-laden emissions can cause central nervous system depression and negative effects on the respiratory system (Rahul and Balomajumder, 2013), and benzene has been classified within Group A-known human carcinogens of medium carcinogenic hazard by US EPA (United States Environmental Protection Agency).
Due to the detrimental effects of BTEX on both the environment and human health, there is an urgent need to develop cost-efficient treatment technologies able to minimize or abate the emissions of these gas pollutants (El-Naas et al., 2014). In this sense, biological techniques are preferred over traditional physical–chemical processes such as absorption, adsorption or combustion, due to their lower operating costs and their environmentally-friendly nature (Estrada et al., 2011; Trigueros et al., 2010).
Aerobic BTEX biodegradation has been successfully achieved in different bioreactor configurations such as biofilters (Leili et al., 2017; Singh et al., 2010), biotrickling filters (Chen et al., 2010) or airlifts (Lebrero et al., 2016). However, most VOC emissions from the petrochemical industry are characterized by their O2-free nature, BTEX accounting for up to 59% (w/w) of gasoline pollutants and representing ≈80% of the total VOC emissions in petrochemical plants (Barona et al., 2007; El-Naas et al., 2014). The absence of oxygen in these off-gas streams limits the implementation of conventional biotechnologies as end-of-pipe abatement methods (Muñoz et al., 2013). This requires the engineering of alternative biotechnologies able to use an electron acceptor other than O2 during pollutant oxidation. In this sense, anoxic BTEX mineralization via NO3−/NO2− denitrification represents a promising platform for the removal of these pollutants from O2-deprived emissions. However, the number of studies devoted to anoxic BTEX removal is still scarce and little is known about the kinetics underlying BTEX biodegradation, which is central for an optimum bioreactor design (Annesini et al., 2014). Moreover, whereas most previous studies have focused on the aerobic biodegradation of individual BTEX compounds (either benzene, toluene or xylene alone), and their metabolic pathways for aerobic degradation have been fully elucidated (El-Naas et al., 2014; Oh et al., 1994), substrate interactions such as competition, inhibition and co-metabolism during BTEX mixture treatment can modify the biodegradation kinetics, thus resulting in the enhancement or inhibition of BTEX biodegradation (Deeb and Alvarez-Cohen, 1999; Littlejohns and Daugulis, 2008).
Culture enrichment aiming at isolating and identifying specific degrading species is usually performed before kinetic assays. However, the experimental conditions selected for the enrichment process commonly differ from those used in real treatment systems, thus the results obtained in the kinetic assays are not an accurate representation of the treatment process (Dou et al., 2008; Jiang et al., 2015). Therefore, it is important to set similar conditions for both acclimation and kinetic tests in terms of pollutant concentration in the headspace, temperature and source of inoculum.
This work aimed at evaluating the biodegradation of BTEX by acclimated microbial communities under anoxic conditions when BTEX were present as single substrates and as dual/quaternary mixtures. Moreover, the biodegradation kinetic parameters of single, dual and quaternary BTEX mixtures are estimated for the first time using two mathematical models: Monod model and a Modified Gompertz model. In addition, the influence of the type of BTEX compound or BTEX mixture on the structure of the microbial community was also assessed.
Section snippets
Inoculum
Activated sludge from the anoxic denitrification tank of the urban wastewater treatment plant (WWTP) of Valladolid (Spain) was used as inoculum. One sample of the fresh sludge was stored at 253 K for microbial characterization.
Chemicals and mineral salt medium
All chemicals for mineral salt medium (MSM) preparation were purchased from PANREAC (Barcelona, Spain) with at least 99% purity. Benzene, toluene, ethylbenzene and o-xylene (99.0% purity) were obtained from Sigma–Aldrich (Madrid, Spain). The MSM was composed of (kg m−3):
Kinetics of individual BTEX biodegradation
After the 5th pollutant amendment acclimation period (lasting 29, 27, 76 and 115 d for toluene, ethyl-benzene, benzene and xylene, respectively), kinetic assays were performed obtaining characteristic substrate degradation profiles for individual substrates (Fig. 2). E and T were rapidly and completely metabolized when present as single carbon and energy sources, their degradation being characterized by μmax values of 0.473 h−1 and 0.318 h−1, respectively, and elapsed biodegradation periods (t∞
Conclusions
This study evaluated for the first time the biodegradation kinetics and the interactions during the anoxic biodegradation of BTEX present as single compounds or in dual or quaternary mixtures, by a previously acclimated bacterial consortium. A complete set of kinetic parameters and their confidence intervals, calculated with the Monod model and a modified Gomperzt model, are provided. Results showed that both T and E are readily biodegradable by the bacterial consortium under anoxic conditions,
Acknowledgments
This research was supported by the Spanish Ministry of Economy and Competitiveness and the European Union through the FEDER Funding Program (RED NOVEDAR project, CTM2015-70442-R) and the Regional Government of Castilla y León (UIC71).
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