In this work, a new biotechnology based on latex biofilms was developed and tested for VOC abatement in the context of indoor air. Four VOCs – hexane, trichloroethylene, toluene and pinene – of different solubilities were selected as model pollutants. A mixed bacteria culture enriched from activated sludge was used as inoculum for the experiments. The removal efficiency (RE) of the pollutants was evaluated for different biofilm mixtures, which involved variations in the water content, the presence of water retainers, the latex pre-treatment, and the biomass concentration.
Additionally, the influence of the pollutant load was tested. Overall, toluene and pinene REs were high (<90%), while hexane and trichloroethylene did not achieve satisfactory REs (<30%). A high-water content in the latex-biofilm mixture was proven to increase the abatement, especially when provided as nutrient solution.
Javier González-Martín a, b, Raquel Lebrero a, b, Raúl Muñoz a, b*.
a Institute of Sustainable Processes, University of Valladolid, Dr. Mergelina s/n., Valladolid 47011, Spain
b Department of Chemical Engineering and Environmental Technology, University of Valladolid, Dr. Mergelina s/n., Valladolid 47011, Spain
*firstname.lastname@example.org. Phone number: +34 983186424
Competing interests: The author has declared that no competing interests exist.
Academic editor: Carlos N. Díaz
Content quality: This paper has been peer-reviewed by at least two reviewers. See scientific committee here.
Citation: Javier González-Martín, Raquel Lebrero, Raúl Muñoz, 2021, Latex-based biofilms for indoor air purification, 9th IWA Odour& VOC/Air Emission Conference, Bilbao, Spain, www.olores.org.
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Keyword: indoor air quality; biofiltration; biocoating; VOC; pollutant.
In this work, a new biotechnology based on latex biofilms was developed and tested for VOC abatement in the context of indoor air. Four VOCs – hexane, trichloroethylene, toluene and pinene – of different solubilities were selected as model pollutants. A mixed bacteria culture enriched from activated sludge was used as inoculum for the experiments. The removal efficiency (RE) of the pollutants was evaluated for different biofilm mixtures, which involved variations in the water content, the presence of water retainers, the latex pre-treatment, and the biomass concentration. Additionally, the influence of the pollutant load was tested. Overall, toluene and pinene REs were high (<90%), while hexane and trichloroethylene did not achieve satisfactory REs (<30%). A high-water content in the latex-biofilm mixture was proven to increase the abatement, especially when provided as nutrient solution. A 4- fold biomass content did not substantially increase RE, which suggested that the degradation of the pollutants was limited by a poor mass-transfer of the pollutants to the microorganisms. The latex-based biofilm showed improved robustness against desiccation when compared to regular biofilm at increasing pollutant loading rates.
Poor indoor air quality has become a priority concern in the last decades. Worldwide, people spend as much as 80% indoors, where the levels of pollution are usually higher than outdoors (González-Martín et al., 2021). Poor indoor air quality can cause mild to severe health problems, leading to an estimate of 4.3 million premature deaths every year (WHO, 2014). New energy-efficiency policies could worsen this problem, as it entails more sealed buildings and then an increase in the levels of indoor pollutants (European Parliament, 2010). In this context, purification methods are needed to decrease the levels of indoor pollution and raise the indoor air quality. Traditionally, physical-chemical technologies have been used for this purpose. However, the great number of pollutants in indoor air and the low and variable concentrations of them in indoor air decrease the suitability of these technologies for indoor air treatment (Guieysse et al., 2008). On the other hand, biotechnologies have emerged as a promising solution able to cope with this problematic (Luengas et al., 2015; González-Martín et al., 2021). Some problems are also associated with biotechnologies, including the limited mass-transfer of some hydrophobic pollutants and the undesired release of microorganisms to indoor air (Kraakman et al., 2011; Cheng et al., 2016). A possible solution could be embedding microorganisms as biocoatings, a configuration in which the microorganisms that carry out the degradation of pollutants are trapped and isolated in a polymer matrix. The microorganisms, in a non-growing state, would have direct contact with the gaseous pollutants, then increasing the mass-transfer even for hydrophobic compounds. Additionally, the risk of microorganism release would be avoided (Cortez et al., 2017; Estrada and Quijano, 2020). A few studies have been developed following this approach. In this context, a photosynthetic bacteria latex-based biofilm was developed for H2 production, where the biofilm reached remarkable hydrogen production even after dry or frozen storage (Gosse et al., 2012; Piskorska et al., 2013). Similarly. Estrada investigated a Pseudomonas putida F1 latex-based biofilm for toluene removal. The removal efficiency was 10-fold higher than traditional agarose biofilms (Estrada et al., 2015). However, these studies are restricted to very low scale and only include one pollutant, which are far from real indoor air conditions. In this study, a novel latex-based biocoating was developed and studied for the continuous treatment of four different model VOCs (hexane, trichloroethylene, toluene and pinene) representing different groups of pollutants that could be found in indoor air. The influence of different operational parameters and biofilm formulations on VOC removal was investigated.
2. Materials and methods
2.1. Chemical products
The model indoor air pollutants n-hexane (CAS-110-54-3), α-pinene (CAS-80-56- 8), toluene (CAS-108-88-3) and trichloroethylene (TCE) (CAS-79-01-6) were mixed at 30%, 25%, 30% and 15%, respectively. The mineral salt medium (MSM) contained 0.7 g L-1 KH2PO4, 0.92 g L-1 K2HPO4∙3H2O, 3.0 g L-1 KNO3, 0.2 g L-1 NaCl, 0.35 g L-1 MgSO4∙7H2O, 0.026 g L-1 CaCl2∙2H2O and 2 mL L-1 of a micronutrient solution (Muñoz et al., 2008). MSM salts, n-hexane, TCE and toluene were purchased from Panreac® (Barcelona, Spain). α-pinene was supplied by Sigma-Aldrich (Madrid, Spain). PRIMAL™ SF-208 ER (alkylphenol ethoxylates and biocide free, acrylic-styrene copolymer; solids content 48.05%; pH 8.0-9.5. Dow Chemical, Germany) was kindly supplied by Brenntag Química (Barcelona, Spain) and it was used as latex for biocoatings. Hydrogel beads (acrylate-acrylamide copolymer) were purchased from Concorde Ibérica S.L. (Madrid, Spain).
Activated sludge from Valladolid wastewater treatment plant was acclimated to the model pollutants in a 3 L stirred tank bioreactor for 8 months. For each experiment, a volume of the culture, containing 0.5 of biomass, was centrifuged (10000 rpm,10 min, 4 ºC). The biomass pellet was then washed with fresh MSM, centrifuged again, and supplied to the corresponding chambers as described below.
2.3. Experimental set-up
Four PVC flat chambers (70×10×2 cm3; 1.4L) were used as bioreactors. The chambers were filled with polyurethane foam (PUF) (density 0.01 g mL–1; specific PUF surface area 1000 m2 m–3; porosity 96%; water retention capacity 0.12 Lwater LPUF–1), which was supplied by Filtren TM 25280, Recticel Ibérica S.L., Spain. Humidified ambient air was used throughout the experiments. A syringe pump (Chemyx Fusion 100, USA) and a 100 µL liquid syringe (SGE 100R-GT-LC, Australia) were used for liquid VOC injection at 1 µL h-1. The polluted air stream was set at 50 mL min-1 (EBRT 28 minutes) resulting in gas concentrations of 9.6±2.4, 13.7±3.0, 14.7±3.5 and 21.3±3.9 mg m-3 for n-hexane, TCE, toluene and α-pinene. Samples of inlet and outlet gas streams were taken in 250 mL glass bulbs (Sigma-Aldrich, Madrid, Spain) by solid- phase microextraction (SPME) and analysed using GC-FID. Experiments were carried out at ambient temperature.
2.4. Experimental runs
The influence of several parameters on VOC removal was investigated throughout five experimental runs: the presence of latex in the biocoating in run 1, the presence of hydrogel (water absorbing material) in the biocoating in run 2, the water content of the latex-biomass mixture in run 3; the latex pre-treatment and MSM content in mixture in run 4, and the biomass concentration and VOC loading rate in run 5. The composition of each mixture is summarized in table 1.
Table 1. Composition of the mixtures used in the experimental runs.
2.5. Analytical procedures
Samples were taken by 10 min exposure to 85 µm CAR/PDMS SPME fibres (Supelco, Bellefonte, USA), and further analysed in a GC-FID (Varian 3900) equipped with an Agilent HP-5MSI capillary column (30m×0.25mm×0.25µm). Injector temperature was set at 150ºC. Oven temperature was initially set at 40ºC for 1.5 min, increased at 10ºC min-1 to 50ºC and held for 1 min, and finally increased at 40ºC min-1 to 250ºC and held for 1 min. Detector temperature was set at 200ºC. N2 was used as carrier gas (2.5 mL min-1), and as make-up gas (25 mL min-1). Hydrogen and air flowrates were set at 30 and 300 mL min-1. New fibres were conditioned at 300ºC for an hour before calibration. Calibration curves were prepared by external standard analysis of the model pollutants following the method above described. Biomass concentration was quantified according to Standard Method 2540 D (American Public Health Association et al., 2017). %RE values were calculated with inlet and outlet concentrations of a pollutant at a given time. Average and standard deviation of RE were calculated when a steady state of removal was reached.
3. Results and discussion
The REs obtained in the experimental runs are represented in table 2. These RE values correspond with the steady state removals observed during the experimental runs. The conclusions extracted come from the combined results of the five experimental runs, as some experimental runs were performed changing more than one parameter in the chambers.
Table 2: %REs obtained in the experimental runs for hexane, TCE, toluene and pinene.
*%REs decreased to 18.2±8.1 and 18.6±4.6 for toluene and pinene after inlet load rate increase at hour 200.
Absorption in water phase or in latex was considered not significant when a steady removal efficiency was reached under long term operation (i.e. abiotic studies: experimental runs 2 and 5 of chamber 3). Unexpected results were obtained in experimental run 1, chamber 3. The high REs in this abiotic study could be attributed to contamination of the experiment. Variability of REs between replicated experiments are attributed to fluctuations in the experimental conditions. TCE was the most recalcitrant compound (%REs <23%). The Henry law coefficient of TCE, which estimates the solubility of a compound in water, is very similar to that of pinene, a compound which experienced very high REs (HTCE = 9.7×10-4 mol m-3 Pa-1; HPinene = 2.1∙10-4 mol m-3 Pa-1) (Sander, 2015). This limited TCE removal was possibly due to physiological limitations of the microbial community. Indeed, scarce information is available for aerobic TCE degradation, only co-metabolism with other compounds has been reported (Dolinová et al., 2017; Gafni et al., 2020). Hexane REs were generally low (<30%) during the experimental runs. These low values could be explained by the low Henry coefficient of hexane (H = 6.0×10-6 mol m-3 Pa-1) (Sander, 2015), which reduced its bioavailability. Only two high REs were obtained for hexane: 98.4% in chamber 1, exp. run 2; and 64.4% in chamber 1, exp. run 3. These higher hexane REs could be attributed to a partial drying of the biofilm surface, which could have led to a direct uptake of hexane from gas phase even though high moisture content is essential to biofilm viability. Unexpectedly, the presence of latex in the mixtures did not increase hexane removals, possibly due to the high water content of the latex-biomass mixtures. Toluene and pinene consistently experienced high REs, usually higher than 80%. This could be explained by a combination of the moderate hydrophobicity of the pollutants (H = 1.5×10-4 and 2.1×10-4 mol m-3 Pa-1, respectively) (Sander, 2015) and the presence of the appropriate microbial community. Indeed, Rhizobiales, Xanthomonadales, Sphingomonadales and Burkholderiales have been previously reported as hydrocarbon degraders (Sun et al., 2013; McGenity, 2019; Morya et al., 2020), demonstrating that the structure of these pollutants does not involve a particular complexity for microbial attack.
The key influence of water content in the biofilm was demonstrated in several experimental runs. Overall, the mixtures that included the biomass in suspension supported much higher REs (56.4 to 94.4% for toluene; 93.6 to 99.7% for pinene) compared to the mixtures that included the biomass as a wet pellet (18.2 to 35.7% for toluene; 18.6 to 33.8% for pinene). For instance, the REs of toluene and pinene increased with increasing water contents of the mixtures in experimental run 4. Chamber 1, inoculated with a wet pellet of biomass, only supported REs of 21.7% and 21.2%, while chamber 3, inoculated with a mixture of 20 mL of biomass suspension and 60 mL of latex, supported REs of 56.4% and 94.4%. Likewise, chamber 4, inoculated with a mixture of 40 mL of biomass suspension and 60 mL of latex, supported REs of 72.9% and 96.1%. This effect is also observed in experimental run 3, when toluene and pinene REs increased when water content was increased (from chamber 4 with BM pellet and pre-dried latex to chamber 3 with a mixture of 40 mL of biomass suspension and 60 mL of latex). These results were in agreement with Estrada et al. (2015), who reported toluene elimination capacities by using latex-based biofilms slightly lower than those of suspended cultures of microorganisms with the same conditions. In experimental run 2, the benefit of hydrogel beads as moisture retainer was demonstrated. Pre-dried latex did not increase the REs. On the contrary, latex treated with activated carbon improved the REs of toluene and pinene, which could be explained by the removal of trace levels of biocide compounds from the latex or by the improvement of the bioavailability of the pollutants due to adsorption on residual activated carbon particles. An increased robustness was observed for the latex-based biofilms in experimental run 5. REs over 90% for toluene and pinene were achieved in chamber 1 (40 mL of biomass suspension) before the increase in inlet load rate. This increase mediated a decrease in REs to average values of 18.2% for toluene and 18.6% for pinene due to desiccation of the biofilm. Chambers 2 and 4, both containing latex-based biofilms, maintained REs over 86% for toluene and 93.6% for pinene during the entire experimental run. Two biomass concentrations in the biofilms were compared in experimental run 5 (0.5 g in chamber 2; 2.0 g in chamber 4). Overall, the performance of chamber 4 was slightly better, achieving higher REs for all the pollutants. This 4-fold increase in biomass concentration did not involve a considerable increase in REs, which suggested that the degradation of the pollutants was limited by mass-transfer.
Hexane and TCE experience low REs in latex-based biofilms, either due to mass- transfer limitations from the gas phase to the cells and the lack of metabolic pathways for its biodegradation, respectively. High REs were achieved for toluene and pinene when the mixtures contained high water and nutrient content, demonstrating the key role of moisture in the biofilm. Activated carbon pre-treated latex showed enhanced degradation of toluene and pinene. However, pre-dried latex negatively affected microbial activity. The concentration of the biomass poorly influenced the performance of the latex-based biofilm, suggesting that the degradation process is mainly controlled by mass-transfer limitations. Latex-based biofilms showed an improved robustness against dryness over a conventional biofilm, which could be an opportunity for higher air treatment capacity. This is the first study assessing the simultaneous treatment of several VOCs at low concentrations using a mixed microbial culture.
This work was supported by Ente Regional de la Energía de Castilla y León (Castilla y León, Spain).
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