Effect of surfactant type and concentration on the gas-liquid mass transfer in biotrickling filters used for air pollution control

Journal of Environmental Management 367 (2024) 121968 Available online 27 July 2024 0301-4797/© 2024 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/bync-nd/4.0/). Research article Effect of surfactant type and concentration on the gas-liquid mass transfer in biotrickling filters used for air pollution control Paula Alejandra Lamprea-Pineda a , b , Francisco Javier Carmona b , c , Kristof Demeestere a , Jose Joaquin Gonz´ alez-Cort´ es a , d , Herman Van Langenhove a , Christophe Walgraeve a , ** , Raquel Lebrero b , * a Research Group EnVOC, Department of Green Chemistry and Technology, Faculty of Bioscience Engineering, Ghent University, Coupure Links 653, 9000, Ghent, Belgium b Institute of Sustainable Processes (ISP), University of Valladolid, C/Dr. Margelina, s/n, 47011, Valladolid, Spain c Surfaces and Porous Materials (SMAP), Associated Research Unit to CSIC, University of Valladolid, Paseo Bel´ en 7, E-47011, Valladolid, Spain d Department of Chemical Engineering and Food Technology, Vine and Agri-Food Research Institute (IVAGRO), University of C´ adiz, Pol. Río San Pedro s/n, Puerto Real, 11510, C´ adiz, Spain ARTICLE INFO Handling editor: Raf Dewil Keywords: Biotrickling filtration Critical micelle concentration Gas treatment Hydrophobic VOCs Oxygen transfer rate ABSTRACT Volatile organic compounds (VOCs) emitted into the atmosphere negatively affect the environment and human health. Biotrickling filtration, an effective technology for treating VOC-laden waste gases, faces challenges in removing hydrophobic VOCs due to their low water solubility and therefore limited bioavailability to microorganisms. Consequently, the addition of (bio)surfactants has proven to be a promising strategy to enhance the removal of hydrophobic VOCs in biotrickling filters (BTFs). Yet, up to now, no single study has ever performed a mass transfer characterization of a BTF under (bio)surfactants addition. In this study, the effect of (bio)surfactant addition on the gas-liquid mass transfer characteristics of two BTFs was measured by using oxygen (O 2 ) as a model gas. Through an empirical correlation, the mass transfer coefficients (k L a) of two hydrophobic VOCs, toluene and hexane, which are of industrial and environmental significance, were estimated. One BTF was filled with expanded perlite, while the other with a mixture of compost and wood chips (C +WC). Both BTFs were operated under different liquid velocities (U L : 0.95 and 1.53 m h −1 ). Saponin, a biological surfactant, and Tween 80, a synthetic surfactant, were added to the recirculating liquid at different critical micelle concentrations (CMCs: 0–3 CMC). The higher interfacial and surface area of the perlite BTF compared to the C +WC BTF led to higher k L aO 2 values regardless of the operational condition: 308 ±18–612 ±19 h −1 versus 42 ±4–177 ±24 h −1 , respectively. Saponin addition at 0.5 and 1 CMC had positive effects on the perlite BTF, with k L aO 2 values two times higher compared to those at 0 CMC. Tween 80 exhibited a neutral or slightly positive effect on the mass transfer of both BTFs under all conditions. Overall, the CMC, along with the physical characteristics of the packing materials and the operational conditions evaluated explained the results obtained. This study provides fundamental data essential to improve the performance and design of BTFs for hydrophobic VOCs abatement. 1. Introduction The emission of volatile organic compounds (VOCs) into the atmosphere represents an environmental and human health threat. Biofiltration is recognized as a cost-effective and sustainable biological treatment technique for the removal of VOCs from waste gas streams (Rybarczyk, 2022). Biotrickling filters (BTFs) are of particular interest because of the advantages they offer, including low operating costs, excellent operational control and robustness (Estrada et al., 2014). BTFs are fixed-bed bioreactors where active microorganisms grow as a biofilm on a packing material and degrade the VOCs present in the waste stream. The pollutants serve as a source of carbon and energy to the microorganisms and, under aerobic conditions, they are typically converted into biomass, carbon dioxide (CO 2 ) and water (H 2 O). An * Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (C. Walgraeve), [email protected] (R. Lebrero). Contents lists available at ScienceDirect Journal of Environmental Management journal homepage: www.elsevier.com/locate/jenvman https://doi.org/10.1016/j.jenvman.2024.121968 Received 4 April 2024; Received in revised form 18 July 2024; Accepted 18 July 2024 Journal of Environmental Management 367 (2024) 121968 2 aqueous solution containing the essential nutrients for the microorganisms is trickled over the bed and continuously recirculated at a specific velocity (Rybarczyk, 2022). The removal of VOCs in BTFs involves a combination of various physical-chemical and biological phenomena, such as absorption, adsorption, diffusion and biodegradation. Initially, the VOCs present in the gas phase and passing through the bed must be transferred to the aqueous and biofilm phases. Subsequently, biodegradation occurs (Lamprea Pineda et al., 2021). Therefore, the removal of VOCs in BTFs is governed by (i) the mass transfer rate of VOCs from the gas phase to the biofilm, and (ii) the biodegradation reaction. In this sense, depending mainly on the type of VOC and operational conditions, the process can be mass-transfer or kinetically limited (Ferdowsi et al., 2016;Sarkar et al., 2022). Hydrophobic VOCs play an important role in the production of a wide range of products and are released into the atmosphere by multiple industries. They are often used as solvents and raw materials or as intermediates in the pharmaceutical, textile, printing and coating, petrochemical, and cosmetics and personal care industries, among others (Wang et al., 2013). Moreover, hydrophobic VOCs such as toluene and xylene have the potential to cause neurological harm. In contrast, others such as benzene, ethylbenzene and trichloroethylene are known to be toxic and carcinogenic (David and Niculescu, 2021). The low aqueous solubility of hydrophobic VOCs is translated into limited bioavailability to the microorganism (i.e., mass transfer limitation) and, therefore, poor removal performances in BTFs. In the last decades, different strategies to enhance the removal of hydrophobic VOCs in BTFs have been evaluated (Cheng et al., 2016;Rybarczyk, 2022). Among them, the addition of surfactants has shown promising results in terms of removal performance by increasing the removal efficiencies (REs) and elimination capacities (ECs) of hydrophobic pollutants (Lamprea Pineda et al., 2021). Surfactants are amphiphilic compounds (i.e., with both a hydrophobic -tail- and hydrophilic -head- moiety). They reduce surface and interfacial tensions between two immiscible phases (e.g., liquid-gas, liquid-liquid and/or solid-liquid) and significantly increase the solubility of a compound in a medium when added at concentrations beyond their critical micelle concentration (CMC). The CMC corresponds to the concentration at which micelles are formed, and is characteristic for each type of surfactant. Micelles are aggregates of surfactant monomers with their hydrophobic heads arranged on the outer periphery and the hydrophilic tails positioned in the center (Singh et al., 2007). As an example, the addition of Tween 80 (100 mg L −1 ; CMC: 15.7 mg L −1 ), a synthetic surfactant, to a BTF filled with polyurethane foam (PUF) and treating a gaseous stream of m-xylene (inlet load (IL): 216 g m −3 h −1 at an empty bed residence time (EBRT) of 20 s) increased the m-xylene RE and EC by 20% and 24%, respectively (Wang et al., 2018). The authors concluded that Tween 80 could enhance the bioavailability of m-xylene and therefore improve its degradation efficiency. The biofiltration studies carried out so far to evaluate the effect of surfactants addition have mainly focused on the efficacy of the process, and a comprehensive compilation of these studies can be found in Lamprea Pineda et al. (2021). In such studies, complementary measurements of Henry’s law constants (or gas-liquid partitioning coefficients) (Dewidar and Sorial, 2021;Wu et al., 2022) and/or surface tension (He et al., 2020;Yang et al., 2022) are usually conducted, although typically under ideal conditions far from real biofiltration experiments, i.e., using mostly water instead of a mineral medium and not considering the presence of packing material. However, despite this strategy aiming to alleviate the mass transfer limitation of hydrophobic VOCs, to date, no study has conducted a mass transfer assessment of a BTF under surfactant addition. To the best of our knowledge, mass transfer studies involving the addition of surfactants have only been conducted in bubble columns (Belo et al., 2011;Jia et al., 2015;Mcclure et al., 2015;Nekoeian et al., 2019;Painmanakul et al., 2005;Sardeing et al., 2006), stirred tanks (Alves et al., 2004;Garcia-Ochoa and Gomez, 2005;Hebrard et al., 2009) and airlift reactors (Moraveji et al., 2012a, 2012b). All of them, except for Garcia-Ochoa and Gomez (2005), employed synthetic surfactants. Moreover, in each case, the evaluation focused on the effect of surfactants addition on the mass transfer of either CO 2 or oxygen (O 2 ) as model compounds, with the former presenting safety concerns due to the use of caustic water (Estrada et al., 2014). Therefore, enhancing the performance and design of BTFs for hydrophobic VOCs abatement requires a thorough understanding of the degree to which the VOC mass transfer rate improves with the addition of both synthetic and biological surfactants. The latter are particularly important due to their recognized biodegradability, environmental friendliness, and lower toxicity in comparison to their synthetic counterparts (Mulligan et al., 2001;Souza et al., 2014;Tu et al., 2015). In this study, we evaluated the effect of surfactant addition by using O 2 as a model gas for which the volumetric mass transfer coefficient (k L a) was experimentally measured in two non-inoculated BTFs, i.e., one filled with expanded perlite and one with a mixture of compost and wood chips (C +WC), both operated under different trickling liquid velocities (U L ). To this aim, the oxygen transfer rate (OTR) method, based on the oxidation of sulfite in a liquid medium, was implemented. This method is considered as a reliable and rapid technique for the determination of k L a coefficients (Mu˜ noz et al., 2018). Two surfactants were chosen, i.e., a synthetic (Tween 80) and a biological (saponin) one. They are both commercially available and have already been evaluated in biofiltration studies, but only in terms of performances, biofilm characteristics, and microbial growth and identification ((Lamprea-Pineda et al., 2024;Qian et al., 2018;Tu et al., 2015;Wang et al., 2018)). Moreover, the k L a values of two common hydrophobic VOCs (i. e., toluene and hexane) –widely investigated under surfactants’addition and of industrial and environmental significance–were estimated from the experimental values obtained for O 2 . 2. Materials and methods 2.1. Biotrickling filters (BTFs) set-up The mass transfer characterization was performed in two identical lab-scale BTFs (Fig. 1). Each BTF consisted of a cylindrical PVC column with a working packed bed volume of 2 L (12 cm of internal diameter and 18 cm of height). One BTF was packed with a mixture of compost (5% v/v) (Compo Sana, COMPO IBERIA S.L., Spain) and wood chips Fig. 1. Biotrickling filter (BTF) set-up. C G stands for gas concentration, while C L for liquid concentration. P.A. Lamprea-Pineda et al. Journal of Environmental Management 367 (2024) 121968 3 (95% v/v) (pine bark; granulometry: 15–20 mm) (GEOLIA S.L., Madrid, Spain) (C +WC), while the other BTF was packed with expanded perlite (granulometry: 2–6 mm) (MASS´ O Garden, Barcelona, Spain). An air stream supplied by an air compressor (ABAC B2500-50 2, Robassomero, Italy) flowed from bottom to top at a constant flow rate of 0.8 L min −1 , corresponding to an EBRT of 153 s and a gas velocity (U G ) of 4.24 m h −1 . A 1 L liquid solution containing sodium sulfite 0.05 M (Na 2 SO 3 assay = 98–100%; Sigma-Aldrich, Madrid, Spain) was continuously agitated at 600 rpm in an external 1.5 L mixing tank and recirculated through the BTFs at a U L of 0.95 or 1.53 m h −1 . A long EBRT and low U L s were selected to mimic the desired conditions for the removal of hydrophobic VOCs. These compounds require extended contact times for efficient removal, while low water percolation is desired to reduce gas-liquid mass transfer limitation (Estrada et al., 2014;Rybarczyk, 2022). To guarantee a uniform distribution of the liquid solution over the packed bed, a PVC cone nozzle (S.S. CO. B ¼HHSJ-PVC6007; Spaying Systems Co., Madrid, Spain) was installed in the upper part of the BTF. Two sampling ports allowed the collection of liquid samples at the bottom of the BTF and in the mixing tank. Tween 80 (assay: not reported; Sigma-Aldrich, Madrid, Spain) and saponin (Saponin Quillaja sp. - Sapogenin content: 20–35%; Sigma-Aldrich, Madrid, Spain) were added to the mixing tank at different CMCs (Table S1) and tested at each U L . Each test was performed in triplicate (Table 1) and the experimental set-up was kept at a constant temperature of 25 ◦C. The packing materials were characterized in terms of bulk density ( ρ b ), moisture content (Mc), water holding capacity (WHC) and interparticle porosity (Φ) according to Lamprea Pineda et al. (2023) and TMECC (2002) (Table 2). 2.2. Surface tension measurements To evaluate the effect of surfactant addition at different CMCs on the surface tension of the liquid solutions employed in this study (i.e., 0.05 M Na 2 SO 3 and 0.05 M Na 2 SO 3 with saponin or Tween 80), surface tension measurements were carried out by the pendant drop method using a FTA200 instrument (First Ten Ångstroms Inc., Portsmouth, VA, USA). Additionally, to get some insights into the interaction of the packing materials with the (bio)surfactants, measurements of the remaining liquid (with and without (bio)surfactant) after contact with the packing material were conducted. To this aim, samples of C +WC and expanded perlite were mixed in 150 mL plastic containers with liquid solutions containing either 0.05 M Na 2 SO 3 or 0.05 M Na 2 SO 3 with 0.5–3 CMC saponin or Tween 80. The packing material-liquid volume ratio (2:1) used was the same as in the BTFs. The solution was mixed at 180 rpm for 2 min and settled for 1 h. Afterward, a sample of the supernatant was taken and measured in the FTA200. In both cases, liquid densities were experimentally measured at room temperature by weighing a precise volume of the solution to later correct the surface tension measurements. The surface tension of each sample was measured five times, and reported values represent their arithmetic average. 2.3. Determination of the volumetric mass transfer coefficients The OTR method was used to determine the k L aO 2 . The OTR method is based on monitoring the oxidation of sulfite (SO 3 2− ) in the mixing tank according to Mu˜ noz et al. (2018). At the beginning of each experiment, new packing material was placed in the BTF. A nitrogen (N 2 ) stream (1 L min −1 for ~10 min) was used to eliminate the dissolved O 2 present in the mixing tank that contained Na 2 SO 3 with or without a (bio)surfactant (see Table 1). Afterward, 5 mL of a catalyst solution containing 0.05 M CoSO 4 ⋅7H 2 O (assay ≥99%; Sigma-Aldrich, Madrid, Spain) was injected to support the instantaneous oxidation of SO 3 2− according to Eq. (1). SO2− 3+1 2O2→ Co2+SO2− 4(1) The injection of the catalyst solution was considered as time zero. Simultaneously, an aqueous sample of 3 mL was taken from the bulk phase of the mixing tank, and the recirculation of the liquid solution through the BTF was started. Subsequently, 3 mL samples were taken at the bottom of the BTF every 2–3 min (see Fig. 1), and the kinetics of O 2 absorption were recorded until complete SO 3 2− depletion. The SO 3 2− concentration in each sample was determined by iodometric backtitration according to Zhao et al. (1999). Briefly, each sample was placed in a 50 mL Erlenmeyer containing 1 mL of an iodine solution (i.e., a titrant solution containing: 0.08 M KIO 3 (assay: 99.5%; Sigma-Aldrich, Madrid, Spain), 0.06 M KI (assay: 99%; Sigma-Aldrich, Madrid, Spain) and 0.3 M H 2 SO 4 (assay: 96%; Sigma-Aldrich, Madrid, Spain)). The sample was titrated with 0.03 M sodium thiosulfate (Na 2 S 2 O 3 ⋅5H 2 O) (assay ≥99.5%; Sigma-Aldrich, Madrid, Spain) using a solution of 1% on a mass basis of potato starch (assay: not reported; Panreac, Barcelona, Spain) as indicator. Each sample was immediately titrated after sampling to minimize contact with ambient air. Moreover, a sample of distilled water was processed as a blank control every day. The absorbed O 2 (mg L −1 ) was estimated from Eq. (1) and Eq. (2) considering the sample volume. By plotting the mg O 2 L −1 versus time, the OTR (mg O 2 L −1 min −1 ) was calculated from the slope of 4–8 data points. SO2− 3+I2+H2O→2I−+SO2− 4+2I−+2H+(2) Next, the k L aO 2 was determined using Eq. (3): kLaO2=OTR CO2,G HO2 −CO2,L (3) Table 1 Set of experiments carried out in this study. CMC stands for critical micelle concentration, U L for liquid velocity and U G for gas velocity. Surfactant Surfactant concentration (CMC) a U L (m h −1 )U G (m h −1 ) Tween 80 0 0.95 4.24 1 3 Saponin 0 0.5 1 3 Tween 80 0 1.53 1 3 Saponin 0 1 3 a The CMC value in pure water and at room temperature (25 ◦C) is used in this manuscript and corresponds to 15.7 mg L −1 for Tween 80 and 1000 mg L −1 for saponin (see Table S1). Table 2 Packing material (PM) characteristics. The table shows average values ±standard deviations (SD) for measurements carried out in triplicate. Property Expanded perlite Mixture C + WC Bulk density - ρ b (g mL −1 ) 0.107 ±0.004 0.32 ±0.01 Moisture content - Mc (%) 0.63 ±0.13 5.7 ±1.0 Water holding capacity - WHC (g H 2 O g PM −1 ) a 3.7 ±0.4 0.496 ±0.002 Interparticle porosity - Φ(%) b 61 ±1 58 ±2 Specific surface area (m b g −1 ) 1.2–6.5 c Unknown a Expressed per unit of packing material at ambient conditions. b Measured for each packing material “as received”i.e., without drying or water saturation. c Range of values reported by Bruneel et al. (2020),Celik et al. (2013) and Hern´ andez-Mel´ endez et al. (2008) for expanded perlite. P.A. Lamprea-Pineda et al. Journal of Environmental Management 367 (2024) 121968 4 where HO2stands for the dimensionless Henry’s law constant of O 2 at 25 ◦C (i.e., 32.08 (Rettlch et al., 1981)), CO2,Gfor the O 2 concentration in the exhaust gas (i.e., O 2 concentration in the air stream), and CO2,Lfor the liquid O 2 concentration in the mixing tank (i.e., 0 due to N 2 flushing at the beginning). Finally, the k L a of two model hydrophobic VOCs (i.e., toluene and hexane) was estimated from the k L a of O 2 (i.e., reference compound, ref) according to Estrada et al. (2014) and Eq. (4). kLax kLaref =(1/Vm,x)0.4 (1/Vm,ref)0.4(4) where V m,x and V m,ref stand for the molecular volume at the boiling point of the model hydrophobic VOC and the reference compound, respectively. In this study, V m values of 25.6, 118.2 and 140.6 mL mol −1 were used for O 2 , toluene and hexane, respectively (Mackay et al., 2006a, 2006b;Welty et al., 2008). All experiments (OTR determinations) were carried out in triplicate. 3. Results and discussion 3.1. Surface tension measurements The addition of saponin and Tween 80 induced a decrease in the surface tension of the aqueous solutions compared to the surface tension of pure water, i.e., 0 CMC (72.5 ±0.2 mN m −1 ) (Fig. 2). A sharp decrease was observed when saponin was added, with an almost constant surface tension at surfactant concentrations above 1 CMC. The highest saponin concentration tested (3 CMC) resulted in a surface tension of 44.6 ±0.1 mN m −1 , more than 1.6 times lower than that of pure water. On the contrary, the addition of Tween 80 resulted in only a slight decrease in surface tension when compared to pure water, with no significant differences between 2 (60.2 ±0.8 mN m −1 ) and 3 CMC (58.1 ±0.9 mN m −1 ). The presence of 0.05 M Na 2 SO 3 did not influence the surface tension of pure water and showed no significant differences for all Tween 80 concentrations tested. In the aqueous solutions containing saponin, a slight difference was observed upon the addition of 0.05 M Na 2 SO 3 , but only at 0.5 and 1 CMC. Previous publications have already reported a notable decrease in the surface tension of aqueous solutions with the addition of (bio)surfactants, as well as its subsequent stabilization regardless of further (bio) surfactant increments. For example, Mitra and Dungan (1997) observed a surface tension decrease from ~50 up to ~36 mN m −1 when Quillaja saponin was added at a concentration of 0.5 g L −1 , while higher biosurfactant concentrations (e.g., up to 10 g L −1 ) did not lead to any further decrease in surface tension. Similar findings have been achieved by Patist et al. (2000) for Tween 20, 22, 40, 60 and 80; Triton X-100; Brij 35, 58 and 78; pentaethyleneglycol mono n-dodecyl ether, and octaethyleneglycol mono n-dodecyl ether. The CMC, which is a characteristic value for each surfactant, is in fact represented by the break in the plot of the measured physical property (surface tension in this case) against surfactant concentration. However, depending on the measuring device (e.g., surface tention versus dye solubilization) and surfactant type (e.g., (bio)surfactant source and impurities present), a clear break is not always observed and some authors prefer to report a range of CMC instead (Garidel et al., 2021;Mitra and Dungan, 1997;Qazi et al., 2020). The sharp decreases in surface tention observed for both Tween 80 and saponon in Fig. 2 indicate that the CMC value of each (bio)surfactant is within the order of magnitude of the values reported in the literature and used in this study (see Table S1). However, the purpose of these measurements was not to identify the CMC of both (bio)surfactants but merely to corroborate the decrease in surface tension when (bio) surfactants are added and, more importantly, to quantify the effect of the addition of Na 2 SO 3 in the surface tension of the aqueous solutions. The latter is of particular interest because salts are known to modify significantly the CMC of (bio)surfactants. Yet, this depends on the (bio)surfactant (i.e., the hydrophobic and hydrophilic groups), the salt and its concentration (Miyagishi et al., 2001). For instance, Qazi et al. (2020) have experimentally demonstrated that the addition of NaCl up to 6.5 M to aqueous solutions of Tween 80 does not affect the CMC. In contrast, Mitra and Dungan (1997) observed that the addition of 0.1–1 M NaCl notably decreases the saponin CMC (up to 8.5 times lower), although this decrease appeared less pronounced for NaCl concentrations below 0.2 M (1.5 times lower). The interaction between the packing material (perlite and C +WC) with the aqueous solutions containing Na 2 SO 3 and saponin or Tween 80 altered the surface tension of the aqueous solutions (Fig. 3). At 0 CMC, the surface tension recorded for the perlite supernatant was the same as that of pure water (Fig. 3 A), while for the C +WC supernatant, a slight decrease (8.4% lower) was observed. Such a decrease has been reported before for soil water, due to the presence of dissolved organic matter (Tschapek et al., 1978). At saponin and Tween 80 concentrations of 0.5, 1 and 3 CMC, both the perlite and C +WC supernatant showed slightly higher surface tensions compared to the aqueous solutions without perlite (4–20% higher) and C +WC (max. 13% higher), respectively (Fig. 2). This observation suggests a potential interaction between the surfactant and the packing material, such as adsorption. Fig. 2. Surface tension as a function of surfactant concentration (critical micelle concentration (CMC) and mg L −1 ) for saponin (A) and Tween 80 (B) in pure water (dots) and in water with 0.05 M sodium sulfite (Na 2 SO 3 ) (triangles). The error bars represent the standard deviation (n =5) of the measured surface tension (see Section 2.2). Note: the dotted lines do not represent continuous values of surface tension for the #CMC but merely connect the measured points to guide the eye. P.A. Lamprea-Pineda et al. Journal of Environmental Management 367 (2024) 121968 5 Fig. 3. Surface tension as a function of surfactant concentration (critical micelle concentration (CMC) and mg L −1 ) of the supernatant of expanded perlite (A) and the mixture of compost and wood chips (C +WC) (B) with saponin and 0.05 M sodium sulfite (Na 2 SO 3 ) (green dots), Tween 80 and 0.05 M Na 2 SO 3 (orange triangles), and pure water and 0.05 M Na 2 SO 3 (purple squares). The error bars represent the standard deviation (n =5) of the measured surface tension (see Section 2.2). Note: the dotted lines do not represent continuous values of surface tension for the #CMC but merely connect the measured points to guide the eye. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) Fig. 4. Measured volumetric mass transfer coefficients of O 2 (k L aO 2 ) in the BTF filled with expanded perlite (A, C) and with a mixture of compost and wood chips (C +WC) (B, D) and as a function of saponin (red dots) and Tween 80 (blue triangles) concentrations. Measurements were carried out at two different liquid velocities (U L ) and in triplicate. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) P.A. Lamprea-Pineda et al. Journal of Environmental Management 367 (2024) 121968 6 3.2. Mass transfer characterization 3.2.1. Oxygen mass transfer coefficients At 0 CMC and both liquid velocities, the k L aO 2 values for the perlite BTF were 2 times higher than those obtained for the C +WC BTF (Fig. 4 and Table S2). A k L aO 2 increase of 1.2 times was observed in both BTFs when the U L increased from 0.95 m h −1 (Perlite: 308 ±18 h −1 ; C +WC: 158 ±5 h −1 ) to 1.53 m h −1 (Perlite: 361 ±39 h −1 ; C +WC: 177 ±24 h −1 ). The lower k L aO 2 values observed in the C +WC BTF are attributed to the lower interfacial and surface area of this packing material compared to the expanded perlite. Additionally, as highlighted by Kim and Deshusses (2008), the heterogeneous nature of the C +WC bed can be translated into air channeling, leading to a subsequent decrease in the surface area for mass transfer due to compost relocation in the bed. On the other hand, the increase in k L aO 2 obtained with increasing U L is consistent with the results observed in other studies (Estrada et al., 2014;Kim and Deshusses, 2008;Moreno-Casas et al., 2020). An increase in the liquid trickling velocity over the bed results in higher O 2 concentration differences between the two phases, and therefore in a higher concentration gradient for mass transfer. Moreover, higher turbulence at higher liquid velocities increases the contact between both phases (Kim and Deshusses, 2008;Moreno-Casas et al., 2020). The k L aO 2 values here obtained at 0 CMC are higher than those reported in the literature for BTFs filled with PUF and operated under similar conditions. For example, Moreno-Casas et al. (2020) experimentally measured a k L aO 2 of 113 ±3 h −1 for a PUF BTF at an U L of 2 m h −1 and an EBRT of 240 s. The highest recorded k L aO 2 value of 381 ±30 h −1 was achieved at 17 m h −1 and 17 s, comparable to that of the perlite BTF in our study. Estrada et al. (2014) reported values of ~140 h −1 and 30 h −1 in a PUF BTF operated at 120 s and 5 m h −1 , and at 240 s and 0.6 m h −1 , respectively, with k L aO 2 values of up to ~300 h −1 when operating at 12 s and 5 m h −1 . In addition, Kim and Deshusses (2008) reported k L aO 2 values as high as ~275 h −1 for a BTF filled with porous ceramic beads and operated at an EBRT of 1.8 s and a U L of 10 m h −1 . This indicates a high mass transfer potential for our BTFs configurations and packing materials, particularly for the perlite BTF. According to the literature (see Table 2), perlite exhibits a specific surface area that can be between 50 and 3000 times higher than those reported for common packing materials used in BTFs such as PUF, pall rings and porous ceramic beads (Kim and Deshusses, 2008). The addition of (bio)surfactants to the mixing tank solution showed different k L aO 2 trends depending on the BTF packing material and the (bio)surfactant (Fig. 4 and Table S2). In the perlite BTF, adding saponin up to 1 CMC improved the k L aO 2 by a factor of 2.0 (612 ±19 h −1 ) and 1.6 (579 ±36 h −1 ) compared to 0 CMC at 0.95 and 1.53 m h −1 , respectively. On the contrary, a further increase in saponin concentration up to 3 CMC led to a substantial decrease in the k L aO 2 value (394 ± 23 h −1 at U L : 0.95 m h −1 and 346 ±34 h −1 at U L : 1.53 m h −1 ) compared to 1 CMC. In the case of Tween 80 and at both U L s, the k L aO 2 increased by only a factor of ~1.2 from 0 to 1 CMC, and no significant differences were observed between 1 and 3 CMC. The C +WC BTF, on the other hand, exhibited a decrease in k L aO 2 with increasing saponin concentrations, reaching values as low as 67 ±13 h −1 and 42 ±4 h −1 at 3 CMC and 0.95 and 1.53 h −1 , respectively, i.e., 2.4 and 4.2 times lower than without surfactant addition. The addition of Tween 80 at each CMC showed a very limited effect at both U L s. By plotting the k L aO 2 as a function of the U L (Fig. 5), for the perlite BTF, it is possible to observe a slight, although not significant, decrease in k L aO 2 at 1.53 m h −1 when saponin was added at both 1 and 3 CMC, compared to 0.95 m h −1 (Fig. 5 A). The opposite trend (increasing k L aO 2 with increasing U L ) was observed for Tween 80 (Fig. 5 C), with significant differences only at 3 CMC (382 ±34 h −1 at 0.95 m h −1 versus 441 Fig. 5. Measured volumetric mass transfer coefficients of O 2 (k L aO 2 ) in the BTF filled with expanded perlite (A, C) and with the mixture of compost and wood chips (C +WC) (B, D) and as a function of the liquid velocity (U L ) when saponin and Tween 80 are added at 0 (red dots), 1 (blue triangles) and 3 (green squares) critical micelle concentrations (CMC). Measurements were carried out in triplicate. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) P.A. Lamprea-Pineda et al. Journal of Environmental Management 367 (2024) 121968 7 ±19 h −1 at 1.53 m h −1 ). The increase in U L in the C +WC BTF resulted in slight decreases in the k L aO 2 when either saponin or Tween 80 were added, although these changes were not statistically significant (Fig. 5B–D). Overall, the results demonstrate that, under the conditions here evaluated (i.e., U L : 0.95 and 1.53 m h −1 EBRT: 153 s and U G : 4.24 m h −1 ), there is an optimal saponin concentration (1 CMC in this study) that boosted the mass transfer in the perlite BTF. On the contrary, negligible and even negative effects are obtained at higher CMCs and/or other packing materials. For example, in the C +WC BTF, a decrease in mass transfer is observed after the addition of saponin at 0.5–3 CMC. In the case of Tween 80 and at both 1 and 3 CMC, only a slight enhancement was observed in the perlite BTF compared to the C +WC BTF where the effect was almost negligible. The dissimilarity between both BTFs might be explained by the differences in their packing materials and the amount of (bio)surfactants dosed. At the highest U L tested, both the perlite and the C +WC BTFs showed foaming when saponin was added, either at the top of the bed (perlite BTF) or within the packed bed (C +WC BTF). This was associated with the higher compaction of the perlite bed compared to the C + WC bed, and the non-uniform distribution of wood chips, leading to significant void spaces in the C +WC BTF that resulted in preferential flow and channeling. Besides this, in terms of mass, higher amounts of saponin (~64 times higher) were added compared to Tween 80 (see Table S1), which likely contributed to the foaming observed at 3 CMC in the perlite BTF and at 1 and 3 CMC in the C +WC BTF, at both U L s, and compared to 0–1 and 0 CMC, respectively. This could indeed explain the decrease in the k L aO 2 value in both BTFs at the corresponding CMCs. Based on this rationale, it is evident that a higher U L would result in increased foaming and thus lower k L aO 2 values, as illustrated in Fig. 5. 3.2.2. Estimation of mass transfer coefficients for different gas pollutants The k L a estimation of the two model hydrophobic VOCs, i.e., toluene and hexane, using the experimental k L aO 2 values shows that toluene mass transfer is slightly higher (1.1 times), although not significantly different, than that of hexane (see Fig. S1). For toluene, the estimated k L a values range between 86 ±3 (C +WC BTF, U L : of 0.95) and 196 ± 21 h −1 (perlite BTF, U L of 153 m h −1 ) at 0 CMC, and an EBRT of 153 s. These are in the same order of magnitude than those reported in literature for toluene in PUF BTFs ranging between 35 and 113 h −1 (EBRTs: 50-7 s and U L : 0.6 m h −1 ) (Lebrero et al., 2012) and 12-258 h −1 (EBRTs: 100-6 s and U L : 2 m h −1 ) (Dorado et al., 2009). On the contrary, the estimated hexane k L a values correspond to 80 ±3 (C +WC BTF, U L : 0.95) and 183 ±20 h −1 (perlite BTF, U L : 153 m h −1 ) at 0 CMC, and an EBRT of 153 s, which are considerably higher than those reported in the literature by for example Hern´ andez et al. (2011) in a PUF BTF (from 25 to 60 h −1 at EBRTs between 40 and 120 s and a U L of 2 m h −1 ). As this estimation (Eq. (4)) is based on the molecular volume and not the hydrophobicity of the compounds of interest, O 2 shows a higher mass transfer than toluene (i.e., almost two times higher), even though its dimensionless Henry’s law constant is more than 100 times higher than that of toluene (i.e., 0.27 at 25 ◦C (Howard and Meylan, 1997). This approach represents a simplified method for estimating the mass transfer of model VOCs relative to O 2 and attention should be paid to more complex systems where external factors could influence the expected values. 4. Conclusions and recommendations The results of the evaluation of (bio)surfactants addition showed a high mass transfer potential for both BTFs, i.e., expanded perlite and a mixture of C +WC, compared to other commonly used packing materials such as PUF. Different trends were observed depending on the packing material, the type of (bio)surfactant and its concentration. Overall, the perlite BTF exhibited higher k L aO 2 values (308-612 h −1 ) than the C +WC BTF (42-177 h −1 ) regardless of the operating condition. In this BTF, an increase in the U L from 0.95 to 1.53 m h −1 resulted in an enhanced mass transfer in case of no (bio)surfactant addition. The addition of saponin at 0.5 and 1 CMC had clearly positive effects on the perlite BTF, with the k L aO 2 increasing up to 2 times compared to 0 CMC. However, saponin addition at 3 CMC resulted in foaming in the perlite BTF and at any CMC in the C +WC BTF, which was then translated into a decreased mass transfer. An increase in foaming was observed with increasing U L . In contrast, the addition of Tween 80 at both U L s had a positive effect in the perlite BTF (e.g., 1.3 times higher k L aO 2 at 1 CMC and 0.95 m h −1 compared to 0 CMC) and a neutral effect in the C +WC BTF. Although saponin decreased the surface tension of aqueous solutions to a greater extent, the CMC of each (bio)surfactant (Tween 80: 15.7 mg L −1 versus saponin: 1000 mg L −1 ), together with the physical characteristics of the packing materials (e.g., higher surface area in the perlite BTF and non-uniform void spaces in the C +WC BTF) seemed to play an important role in the mass transfer experiments. The results here obtained are of great importance to (i) improve the understanding of surfactants addition in BTFs as a strategy to enhance the removal of hydrophobic VOCs, and (ii) facilitate the selection of packing materials for future studies. Additionally, this fundamental data holds significant value for future modeling tools aiming for the design and optimization of bioreactors for air pollution control. Future studies should evaluate the effect of the presence of microorganisms in the trickling solution. For instance, nutrients added to the medium and substances excreted by the microorganisms affect the physical-chemical properties of the liquid solution, and could therefore have an impact on the mass transfer of pollutants. In this study, relatively high EBRTs and low U L s were selected to represent operational conditions that favor the removal of hydrophobic VOCs. Future studies could expand these conditions and evaluate the effect of both parameters on their mass transfer. In particular, lower EBRTs are important to reduce the footprint of future installations. CRediT authorship contribution statement Paula Alejandra Lamprea-Pineda: Writing –review &editing, Writing –original draft, Visualization, Methodology, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization. Francisco Javier Carmona: Writing –review &editing, Validation, Methodology. Kristof Demeestere: Writing –review &editing, Supervision, Methodology. Jose Joaquin Gonz´ alez-Cort´ es: Writing –review &editing, Supervision, Methodology. Herman Van Langenhove: Writing –review &editing, Supervision, Methodology. Christophe Walgraeve: Writing –review &editing, Supervision, Resources, Project administration, Methodology, Funding acquisition, Conceptualization. Raquel Lebrero: Writing –review &editing, Supervision, Resources, Project administration, Methodology, Formal analysis, Conceptualization. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Data availability Data will be made available on request. Acknowledgments This work was financed by the Fonds Wetenschappelijk Onderzoek (FWO) through a long stay abroad grant (V410823N) awarded to Paula Alejandra Lamprea-Pineda, the ReCOVery project (TED 2021-129340B- I00, Spanish Ministry of Science and Innovation) and a special research P.A. Lamprea-Pineda et al. Journal of Environmental Management 367 (2024) 121968 8 grant (BOFSTG2019005701) awarded to Christophe Walgraeve. Jos´ e Joaquin Gonzalez-Cortes thanks the Spanish Ministry of Universities for his postdoctoral position at the University of C´ adiz through the Next- GenerationEU program, as a Margarita Salas fellowship (ref. 2021–067/ PN/MS-RECUAL/CD). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jenvman.2024.121968. References Alves, S.S., Maia, C.I., Vasconcelos, J.M.T., 2004. Gas-liquid mass transfer coefficient in stirred tanks interpreted through bubble contamination kinetics. Chem. Eng. Process: Process Intensif. 43, 823–830. https://doi.org/10.1016/S0255-2701(03) 00100-4. Belo, I., García-Abuín, A., G´ omez-Díaz, D., Navaza, J.M., Vidal-Tato, sabel I., 2011. Effect of tween 80 on bubble size and mass transfer in a bubble contactor. Chem. Eng. Technol. 34, 1790–1796. https://doi.org/10.1002/ceat.201100140. Bruneel, J., Huepe Follert, J.L., Laforce, B., Vincze, L., Van Langenhove, H., Walgraeve, C., 2020. Dynamic performance of a fungal biofilter packed with perlite for the abatement of hexane polluted gas streams using SIFT-MS and packing characterization with advanced X-ray spectroscopy. Chemosphere 253, 126684. https://doi.org/10.1016/j.chemosphere.2020.126684. Celik, A.G., Kilic, A.M., Cakal, G.O., 2013. Expanded perlite aggregate characterization for use as a lightweight construction raw material. Physicochem. Probl. Miner. Process. 49, 689–700. https://doi.org/10.5277/ppmp130227. Cheng, Y., He, H., Yang, C., Zeng, G., Li, X., Chen, H., Yu, G., 2016. Challenges and solutions for biofiltration of hydrophobic volatile organic compounds. Biotechnol. Adv. 34, 1091–1102. https://doi.org/10.1016/j.biotechadv.2016.06.007. David, E., Niculescu, V.C., 2021. Volatile organic compounds (VOCs) as environmental pollutants: occurrence and mitigation using nanomaterials. Int. J. Environ. Res. Publ. Health 18, 1–15. https://doi.org/10.3390/ijerph182413147. Dewidar, A.A., Sorial, G.A., 2021. Effect of surfactin on removal of semi-volatile organic compound: emphasis on enhanced biofiltration performance. Environ. Res. 193, 110532 https://doi.org/10.1016/j.envres.2020.110532. Dorado, A.D., Rodríguez, G., Ribera, G., Bonsfills, A., Gabriel, D., Lafuente, J., Gamisans, X., 2009. Evaluation of mass transfer coefficients in biotrickling filters: experimental determination and comparison to correlations. Chem. Eng. Technol. 32, 1941–1950. https://doi.org/10.1002/ceat.200900275. Estrada, J.M., Dudek, A., Mu˜ noz, R., Quijano, G., 2014. Fundamental study on gas-liquid mass transfer in a biotrickling filter packed with polyurethane foam. J. Chem. Technol. Biotechnol. 89, 1419–1424. https://doi.org/10.1002/jctb.4226. Ferdowsi, M., Avalos Ramirez, A., Jones, J.P., Heitz, M., 2016. Elimination of mass transfer and kinetic limited organic pollutants in biofilters: a review. Int. Biodeterior. Biodegrad. 119, 336–348. https://doi.org/10.1016/j.ibiod.2016.10.015. Garcia-Ochoa, F., Gomez, E., 2005. Prediction of gas-liquid mass transfer coefficient in sparged stirred tank bioreactors. Biotechnol. Bioeng. 92, 761–772. https://doi.org/ 10.1002/bit.20638. Garidel, P., Blech, M., Buske, J., Blume, A., 2021. Surface tension and self-association properties of aqueous polysorbate 20 HP and 80 HP Solutions: insights into protein stabilisation mechanisms. J Pharm Innov 16, 726–734. https://doi.org/10.1007/ s12247-020-09488-4. He, S., Ni, Y., Lu, L., Chai, Q., Yu, T., Shen, Z., Yang, C., 2020. Simultaneous degradation of n-hexane and production of biosurfactants by Pseudomonas sp. strain NEE2 isolated from oil-contaminated soils. Chemosphere 242, 125237. https://doi.org/ 10.1016/j.chemosphere.2019.125237. Hebrard, G., Zeng, J., Loubiere, K., 2009. Effect of surfactants on liquid side mass transfer coefficients: a new insight. Chem. Eng. J. 148, 132–138. https://doi.org/10.1016/j. cej.2008.08.027. Hern´ andez, M., Quijano, G., Mu˜ noz, R., Bordel, S., 2011. Modeling of VOC mass transfer in two-liquid phase stirred tank, biotrickling filter and airlift reactors. Chem. Eng. J. 172, 961–969. https://doi.org/10.1016/j.cej.2011.07.008. Hern´ andez-Mel´ endez, O., B´ arzana, E., Arriaga, S., Hern´ andez-Luna, M., Revah, S., 2008. Fungal removal of gaseous hexane in biofilters packed with poly(ethylene carbonate) pine sawdust or peat composites. Biotechnol. Bioeng. 100, 864–871. https://doi.org/ 10.1002/bit.21825. Howard, P.H., Meylan, W.M., 1997. Handbook of Physical Properties of Organic Chemicals. CRC Press, Inc, Boca Raton, The United States of America. Jia, X., Hu, W., Yuan, X., Yu, K., 2015. Effect of surfactant type on interfacial area and liquid mass transfer for CO2 absorption in a bubble column. Chin. J. Chem. Eng. 23, 476–481. https://doi.org/10.1016/j.cjche.2014.11.027. Kim, S., Deshusses, M.A., 2008. Determination of mass transfer coefficients for packing materials used in biofilters and biotrickling filters for air pollution control. 1. Experimental results. Chem. Eng. Sci. 63, 841–855. https://doi.org/10.1016/j. ces.2007.10.011. Lamprea Pineda, P.A., Demeestere, K., Toledo, M., Boon, N., Van Langenhove, H., Walgraeve, C., 2023. Long-term biofiltration of gaseous N,N-dimethylformamide: operational performance and microbial diversity analysis at different conditions. J. Hazard Mater. 447, 130767 https://doi.org/10.1016/j.jhazmat.2023.130767. Lamprea Pineda, P.A., Demeestere, K., Toledo, M., Van Langenhove, H., Walgraeve, C., 2021. Enhanced removal of hydrophobic volatile organic compounds in biofilters and biotrickling filters: a review on the use of surfactants and the addition of hydrophilic compounds. Chemosphere 279, 130757. https://doi.org/10.1016/j. chemosphere.2021.130757. Lamprea-Pineda, P.A., Demeestere, K., Gonz´ alez-Cort´ es, J.J., Boon, N., Devlieghere, F., Van Langenhove, H., Walgraeve, C., 2024. Addition of (bio)surfactants in the biofiltration of hydrophobic volatile organic compounds in air. J. Environ. Manag. 353, 120132 https://doi.org/10.1016/j.jenvman.2024.120132. Lebrero, R., Estrada, J.M., Mu˜ noz, R., Quijano, G., 2012. Toluene mass transfer characterization in a biotrickling filter. Biochem. Eng. J. 60, 44–49. https://doi.org/ 10.1016/j.bej.2011.09.017. Mackay, D., Shiu, W.Y., Ma, K.-C., Lee, S.C., 2006a. Mononuclear aromatic hydrocarbons, in: handbook of physical-chemical properties and environmental fate for organic chemicals. Volume III Oxygen Containing Compounds. Taylor &Francis Group, Boca Rat´ on, The United States of America, pp. 406–616. Mackay, D., Shiu, W.-Y., Ma, K.-C., Lee, S.C., 2006b. Aliphatic and cyclic hydrocarbons. In: Handbook of Physical-Chemical Properties and Environmental Fate for Organic Chemicals. Volume III Oxygen Containing Compounds. Taylor &Francis Group, Boca Rat´ on, The United States of America, pp. 211–269. https://doi.org/10.1201/ 9781420044393. Mcclure, D.D., Lee, A.C., Kavanagh, J.M., Fletcher, D.F., Barton, G.W., 2015. Impact of surfactant addition on oxygen mass transfer in a bubble column. Chem. Eng. Technol. 38, 44–52. https://doi.org/10.1002/ceat.201400403. Mitra, S., Dungan, S.R., 1997. Micellar properties of quillaja saponin. 1. Effects of temperature, salt, and pH on solution properties. J. Agric. Food Chem. 45, 1587–1595. https://doi.org/10.1021/jf960349z. Miyagishi, S., Okada, K., Asakawa, T., 2001. Salt effect on critical micelle concentrations of nonionic surfactants, N-acyl-N-methylglucamides (MEGA-n). J. Colloid Interface Sci. 238, 91–95. https://doi.org/10.1006/jcis.2001.7503. Moraveji, M.K., Mohsenzadeh, E., Fakhari, M.E., Davarnejad, R., 2012a. Effects of surface active agents on hydrodynamics and mass transfer characteristics in a splitcylinder airlift bioreactor with packed bed. Chem. Eng. Res. Des. 90, 899–905. https://doi.org/10.1016/j.cherd.2011.10.008. Moraveji, M.K., Pasand, M.M., Davarnejad, R., Chisti, Y., 2012b. Effects of surfactants on hydrodynamics and mass transfer in a split-cylinder airlift reactor. Can. J. Chem. Eng. 90, 93–99. https://doi.org/10.1002/cjce.20516. Moreno-Casas, P.A., Scott, F., Delpiano, J., Abell, J.A., Caicedo, F., Mu˜ noz, R., Vergara- Fern´ andez, A., 2020. Mechanistic description of convective gas−liquid mass transfer in biotrickling filters using CFD modeling. Environ. Sci. Technol. 54, 419–426. https://doi.org/10.1021/acs.est.9b02662. Mulligan, C.N., Yong, R.N., Gibbs, B.F., 2001. Surfactant-enhanced remediation of contaminated soil: a review. Eng. Geol. 60, 371–380. https://doi.org/10.1016/ S0013-7952(00)00117-4. Mu˜ noz, R., Soto, C., Zu˜ niga, C., Revah, S., 2018. A systematic comparison of two empirical gas-liquid mass transfer determination methodologies to characterize methane biodegradation in stirred tank bioreactors. J. Environ. Manag. 217, 247–252. https://doi.org/10.1016/j.jenvman.2018.03.097. Nekoeian, S., Aghajani, M., Alavi, S.M., Sotoudeh, F., 2019. Effect of surfactants on mass transfer coefficients in bubble column contactors: an interpretative critical review study. Rev. Chem. Eng. https://doi.org/10.1515/revce-2018-0089. Painmanakul, P., Loubi` ere, K., H´ ebrard, G., Mietton-Peuchot, M., Roustan, M., 2005. Effect of surfactants on liquid-side mass transfer coefficients. Chem. Eng. Sci. 60, 6480–6491. https://doi.org/10.1016/j.ces.2005.04.053. Patist, A., Bhagwat, S.S., Penfield, K.W., Aikens, P., Shah, D.O., 2000. On the measurement of critical micelle concentrations of pure and technical-grade nonionic surfactants. J. Surfactants Deterg. 3, 53–58. https://doi.org/10.1007/s11743-000- 0113-4. Qazi, M.J., Schlegel, S.J., Backus, E.H.G., Bonn, M., Bonn, D., Shahidzadeh, N., 2020. Dynamic surface tension of surfactants in the presence of high salt concentrations. Langmuir 36, 7956–7964. https://doi.org/10.1021/acs.langmuir.0c01211. Qian, H., Cheng, Y., Yang, C., Wu, S., Zeng, G., Xi, J., 2018. Performance and biofilm characteristics of biotrickling filters for ethylbenzene removal in the presence of saponins. Environ. Sci. Pollut. Control Ser. 25, 30021–30030. https://doi.org/ 10.1007/s11356-017-0776-6. Rettlch, T.R., Handa, Y.P., Battino, R., Wilhelm, E., 1981. Solubility of gases in liquids. 13. High-precision determination of Henry’s constants for methane and ethane in liquid water at 275 to 328 K. J. Phys. Chem. 85, 3230–3237. Rybarczyk, P., 2022. Removal of volatile organic compounds (VOCs) from air: focus on biotrickling filtration and process modeling. Processes 10, 1–39. https://doi.org/ 10.3390/pr10122531. Sardeing, R., Painmanakul, P., H´ ebrard, G., 2006. Effect of surfactants on liquid-side mass transfer coefficients in gas-liquid systems: a first step to modeling. Chem. Eng. Sci. 61, 6249–6260. https://doi.org/10.1016/j.ces.2006.05.051. Sarkar, P., Tiwari, H., Garkoti, P., Neogi, S., Biswas, J.K., Dey, A., 2022. Biofiltration as a green technology for abatement of volatile organic compounds (VOCs): a synoptic review. In: Shah, M., Rodriguez-Couto, S., Biswas, J. (Eds.), An Innovative Role of Biofiltration in Wastewater Treatment Plants (WWTPs). Elsevier Inc., pp. 477–496. https://doi.org/10.1016/b978-0-12-823946-9.00019-x Singh, A., Van Hamme, J.D., Ward, O.P., 2007. Surfactants in microbiology and biotechnology: Part 2. Application aspects. Biotechnol. Adv. 25, 99–121. https://doi. org/10.1016/j.biotechadv.2006.10.004. Souza, E.C., Vessoni-Penna, T.C., De Souza Oliveira, R.P., 2014. Biosurfactant-enhanced hydrocarbon bioremediation: an overview. Int. Biodeterior. Biodegrad. 89, 88–94. https://doi.org/10.1016/j.ibiod.2014.01.007. P.A. Lamprea-Pineda et al. Journal of Environmental Management 367 (2024) 121968 9 (TMECC) Test Methods for the Examination of Composting and Compost, 2002. The US Composting Council Research and Education Foundation &the US Department of Agriculture. Tschapek, M., Scoppa, C.0, Wasowski, C., 1978. The surface tension of soil water. J. Soil Sci. 29, 17–19. Tu, Y., Yang, C., Cheng, Y., Zeng, G., Lu, L., Wang, L., 2015. Effect of saponins on nhexane removal in biotrickling filters. Bioresour. Technol. 175, 231–238. https:// doi.org/10.1016/j.biortech.2014.10.039. Wang, H.L., Nie, L., Li, J., Wang, Y.F., Wang, G., Wang, J.H., Hao, Z.P., 2013. Characterization and assessment of volatile organic compounds (VOCs) emissions from typical industries. Chin. Sci. Bull. 58, 724–730. https://doi.org/10.1007/ s11434-012-5345-2. Wang, L., Xu, R., Yang, B., Wei, S., Yin, N., Cao, C., 2018. Nonionic surfactant enhanced biodegradation of m-xylene by mixed bacteria and its application in biotrickling filter. J. Air Waste Manage. Assoc. 68, 1065–1076. https://doi.org/10.1080/ 10962247.2018.1466741. Welty, J.R., Wicks, C.E., Wilson, R.E., Rorrer, G.L., 2008. Fundamentals of mass transfer. In: Fundamentals of Momentum, Heat, and Mass Transfer. John Wiley &Sons, Inc., pp. 398–431 Wu, X., Lin, Y., Wang, Y., Wu, S., Li, X., Yang, C., 2022. Enhanced removal of hydrophobic short-chain n-alkanes from gas streams in biotrickling filters in presence of surfactant. Environ. Sci. Technol. 56, 10349–10360. https://doi.org/ 10.1021/acs.est.2c02022. Yang, B., Wang, J., Wu, M., Shang, Q., Zhang, H., 2022. Effect of rhamnolipids on the biodegradation of m-dichlorobenzene in biotrickling filters: performance and mechanism. J. Environ. Manag. 320, 115951 https://doi.org/10.1016/j. jenvman.2022.115951. Zhao, S., Kuttuva, S.G., Ju, L.-K., Zhao, Su, Kuttuva, Srikumar G., Ju, Lu-Kwang, 1999. Oxygen transfer characteristics of multiple-phase dispersions simulating water-in-oil xanthan fermentations. Bioprocess Eng. 20, 313–323. P.A. Lamprea-Pineda et al.