Mass Transfer Of Volatile Organic Compounds Research Articles

Green synthesis of biomass-derived porous carbon with hierarchical pores and enhanced surface area for superior VOCs adsorption

The release of volatile organic compounds (VOCs) during the production, storage, and transportation of petroleum and its derivatives can be hazardous to human health and lead to secondary pollution. Addressing this challenge, this study introduces a sustainable and low-energy approach for synthesizing high-efficiency porous carbon materials, leveraging the abundant and renewable watermelon rinds as a carbon source. The carbonization and activation processes were optimized to minimize energy consumption while maximizing adsorptive performance. In this research, watermelon rinds were used as a source of carbon production, and melamine phosphate (FR-MP) was employed as a modifying agent. The FR-MP modified porous biomass-derived activated carbon (PWAC) was created by subjecting it to physical milling, carbonization, and activation. This approach not only utilized a waste biomass effectively but also reduced the overall carbon footprint of the production process. An examination of the effects of FR-MP dosage on the pore structure, surface chemical properties, and VOCs adsorption abilities of PWAC revealed that a higher FR-MP dosage resulted in a wider range of pore sizes within PWAC. The incorporation of FR-MP intensified the development of internal porosity and pore expansion in PWAC. Specifically, the optimized PWAC-0.3 exhibited a significant increase in specific surface area (3149 m²/g) and pore volume (1.76 cm³/g), enhancing adsorption capacities for acetone and n-hexane up to 1046 mg/g and 760 mg/g, respectively. Furthermore, the PWAC demonstrated exceptional recyclability, maintaining 97% of its original adsorption capacity after five cycles, underscoring its sustainable and economic viability. The highly advanced hierarchical ultra-meso-macroporous structure of PWAC significantly reduced the diffusion resistance of VOCs, guaranteeing swift mass transfer and desorption of VOCs. Consequently, the developed PWAC represents a novel, cost-effective, and environmentally-friendly approach for VOCs adsorption, aligning with the principles of green chemistry and low-energy consumption. This research not only provides a promising solution for VOCs mitigation but also highlights the potential of waste biomass in producing high-performance carbon materials.

Analytical model for assessing mass transfer of VOC in liquid desiccant dehumidifier: A broad-range response to Henry's law constant

Removing indoor volatile organic compounds (VOCs) is crucial for human health. Liquid dehumidification is considered as a promising way for VOC removal. The methods for predicting its performance are limited to numerical simulations or experiments, which are time-consuming, expensive, effort-intensive, and unintuitive. Although analytical method is advantageous for resolving these problems, existing analytical models focus primarily on single-VOC benzene, neglecting the effect of Henry's law constant (HLC) and leading to inaccurate predictions for other VOCs. In this study, considering the effect of HLC, a new analytical model with a broad-range response to HLC is proposed. Moreover, a correction method is proposed to enhance the application range and precision of analytical model. Thus, the definition domain of the proposed model is segmented into various zones, considering accuracy and key parameters. The validity of the proposed model is demonstrated under two conditions: HLCs are on the orders of 10–4 and 10–5. Results show that the proposed analytical solutions show good agreement with published data with a deviation within ±20%. Thus, the proposed model exhibits a broad-range response to HLC, resolving the limitation of existing analytical solution. This study provides an analytical method for assessing VOC removal using liquid dehumidifier.

Diffusion of volatile organics and water in the epicuticular waxes of petunia petal epidermal cells.

Plant cuticles are a mixture of crystalline and amorphous waxes that restrict the exchange of molecules between the plant and the atmosphere. The multicomponent nature of cuticular waxes complicates the study of the relationship between the physical and transport properties. Here, a model cuticle based on the epicuticular waxes of Petunia hybrida flower petals was formulated to test the effect of wax composition on diffusion of water and volatile organic compounds (VOCs). The model cuticle was composed of an n-tetracosane (C24 H50 ), 1-docosanol (C22 H45 OH), and 3-methylbutyl dodecanoate (C17 H34 O2 ), reflecting the relative chain length, functional groups, molecular arrangements, and crystallinity of the natural waxes. Molecular dynamics simulations were performed to obtain diffusion coefficients for compounds moving through waxes of varying composition. Simulated VOC diffusivities of the model system were found to highly correlate with in vitro measurements in isolated petunia cuticles. VOC diffusivity increased up to 30-fold in completely amorphous waxes, indicating a significant effect of crystallinity on cuticular permeability. The crystallinity of the waxes was highly dependent on the elongation of the lattice length and decrease in gap width between crystalline unit cells. Diffusion of water and higher molecular weight VOCs were significantly affected by alterations in crystalline spacing and lengths, whereas the low molecular weight VOCs were less affected. Comparison of measured diffusion coefficients from atomistic simulations and emissions from petunia flowers indicates that the role of the plant cuticle in the VOC emission network is attributed to the differential control on mass transfer of individual VOCs by controlling the composition, amount, and dynamics of scent emission.

Optimization of acrylic-styrene latex-based biofilms as a platform for biological indoor air treatment

Biotechnologies have emerged as a promising solution for indoor air purification with the potential to overcome the inherent limitations of indoor air treatment. These limitations include the low concentrations and variability of pollutants and mass-transfer problems caused by pollutant hydrophobicity. A new latex-based biocoating was herein optimized for the abatement of the volatile organic compounds (VOCs) toluene, trichloroethylene, n-hexane, and α-pinene using acclimated activated sludge dominated by members of the phylum Patescibacteria. The influence of the water content, the presence of water absorbing compounds, the latex pretreatment, the biomass concentration, and the pollutant load was tested on VOC removal efficiency (RE) by varying the formulation of the mixtures. Overall, hexane and trichloroethylene removal was low (<30%), while high REs (>90%) were consistently recorded for toluene and pinene. The assays demonstrated the benefits of operating at high water content in the biocoating, either by including mineral medium or water absorbing compounds in the latex-biomass mixtures. The performance of the latex-based biocoating was likely limited by VOC mass-transfer rather than by biomass concentration in the biocoating. The latex-based biocoating supported a superior toluene and pinene removal than biomass in suspension when VOC loading rate was increased by a factor of 4.

Characterizing the partitioning behavior of formaldehyde, benzene and toluene on indoor fabrics: Effects of temperature and humidity

Fabrics are widely distributed in residential buildings. Due to their highly porous structures and large specific surface areas, they have strong adsorption properties for volatile organic compounds (VOCs). The secondary source effect that is induced by their desorption can aggravate indoor air pollution and prolong the pollution period. The partition coefficient, which is a characteristic parameter of VOC mass transfer, is sensitive to variations in environmental parameters. However, due to the inherent differences between fabrics and other indoor porous building materials, the relevant research conclusions on the VOC mass transfer parameters of building materials cannot be applied. In addition, the effects of temperature and humidity on the partitioning behavior of VOCs on fabrics have rarely been quantitatively analyzed. Based on an analysis of the porous structure and corresponding mass transfer process of fabrics, a novel prediction model of the fabric partition coefficient under the coupling effect of temperature and humidity is proposed. Three types of indoor typical fabrics and primary water-soluble VOC (formaldehyde) and water-insoluble VOC (benzene, toluene) are examined experimentally via hygroscopicity tests and environmental chamber tests. The experimental results demonstrate the reliability of the proposed model for a variety of conditions.

Cuticle thickness affects dynamics of volatile emission from petunia flowers.

The plant cuticle is the final barrier for volatile organic compounds (VOCs) to cross for release to the atmosphere, yet its role in the emission process is poorly understood. Here, using a combination of reverse-genetic and chemical approaches, we demonstrate that the cuticle imposes substantial resistance to VOC mass transfer, acting as a sink/concentrator for VOCs and hence protecting cells from the potentially toxic internal accumulation of these hydrophobic compounds. Reduction in cuticle thickness has differential effects on individual VOCs depending on their volatility, and leads to their internal cellular redistribution, a shift in mass transfer resistance sources and altered VOC synthesis. These results reveal that the cuticle is not simply a passive diffusion barrier for VOCs to cross, but plays the aforementioned complex roles in the emission process as an integral member of the overall VOC network.

Behaviour of individual VOCs in indoor environments: How ventilation affects emission from materials

Indoor air quality is affected by both emissions of Volatile Organic Compounds (VOC) from materials and ventilation. The purpose of this paper is to provide a multi-scale analysis of the impact of ventilation on VOC emissions to highlight the individual behaviours of VOCs emitted from a wood particleboard. Emissions were studied in an experimental chamber by (i) assessing the effect of ventilation on emission rates and (ii) determining intrinsic parameters (Ki, C0,i, Di) describing the VOC mass transfer from the material to air. The overall assessment of the effect of ventilation indicated that the air change rate could significantly affect the behaviour of individual compounds. Typically, the formaldehyde emission rate increased from 214.6 to 274.2 μg.m−2.h−1 when air change rate varies from 2.5 to 5.5 h−1, whereas the air speed had no influence on emission rates for any VOC monitored. These results agree with the key emission parameters (partitioning and diffusion coefficients) which were higher for formaldehyde than those for other compounds. VOC diffusion related to VOC mass transfer from a material's surface to the surrounding air was the limiting step in VOC emission for the solid material studied, and should therefore be considered when developing ventilation strategies.

Influence of groundwater table fluctuation on the non-equilibrium transport of volatile organic contaminants in the vadose zone

The migration of volatile organic compounds (VOCs) from groundwater is an environmental concern because of the risk of vapor intrusion. In coastal aquifers or areas under the influence of systematic pumping, groundwater table fluctuations may be significant, which may affect VOC flux. This study developed a one-dimensional numerical model to simulate VOC mass transfer from groundwater to the vadose zone to the atmosphere, which was validated with data gained from a previously performed soil tank experiment. The numerical model revealed that groundwater table fluctuations in a simulated sandy aquifer cause VOC flux levels to increase by more than one order of magnitude. Furthermore, groundwater table fluctuations of larger amplitude are associated with larger VOC flux levels. Contaminants associated with larger Henry’s law constants and smaller water-solid distribution coefficients are influenced to a greater extent by groundwater table fluctuations. In a coarse sand aquifer, the non-equilibrium model demonstrated that the VOC flux level is lower (18%) than that calculated if equilibrium is assumed, showing the equilibrium assumption to be somewhat conservative. Groundwater table fluctuations induce significant increases in VOC flux from soils of coarse or medium sand, while flux levels are not significantly affected in silt or clay soils. Rainfall only has a temporal effect on VOC flux during groundwater table fluctuations. The numerical model in this paper could help improve estimates of human health risk by VI in areas with groundwater table fluctuation. Further non-equilibrium studies are needed to estimate mass transfer coefficients among the three phases for different contaminants under different conditions.

KLa Determination Using the Effectiveness-NTU Method: Application to Countercurrent Absorbers in Operation Using Viscous Solvents for VOCs Mass Transfer

In this study, the Effectiveness-NTU method, which is usually applied to heat exchanger design, was adapted to gas–liquid countercurrent absorbers to determine the overall mass transfer coefficient, KLa, of the apparatus in operation. It was demonstrated that the ε-NTU method could be used to determine the KLa using the Henry coefficient of the solute to be transferred (HVOC), the gas flow-rate (QG), the liquid flow-rate (QL), the scrubber volume (V), and the effectiveness of the absorber (ε). These measures are calculated from the gaseous concentrations of the solute measured at the absorber inlet (CGin) and outlet (CGout), respectively. The ε-NTU method was validated from literature dedicated to the absorption of volatile organic compounds (VOCs) by heavy solvents. Therefore, this method could be a simple, robust, and reliable tool for the KLa determination of gas–liquid contactors in operation, despite the type of liquid used, i.e., water or viscous solvents.

Effect of Postharvest Dehydration on Content of Volatile Organic Compounds in the Epicarp of Cesanese Grape Berry

Postharvest dehydration is a technique to help increase polyphenol concentration and to modify volatile organic compounds (VOCs) in grape berry. Cesanese winegrape was dehydrated off the vine at 10°C, 45% relative humidity, and an air flow of 1.5 m/sec. Sampling was performed every week for 6 weeks, until it reached 37% mass loss. Proanthocyanidins in seeds increased progressively on both fresh and dry weight basis during dehydration until 30% mass loss. Total polyphenols extracted from the epicarp measured on a fresh weight basis increased during dehydration. A similar pattern was shown by anthocyanins. For VOCs, a high prevalence of benzenoids followed by norisoprenoids, terpenols, and C₆ compounds was observed. Benzenoids were more localized in the mesocarp (7352 μg/kg DW) than in the epicarp (3312 μg/kg DW) as were norisoprenoids. In contrast, as expected, terpenols were more concentrated in the epicarp (1787 μg/kg DW) than in the mesocarp (1370 μg/kg DW). During dehydration, a significant decrease was observed in these compounds in the mesocarp and a parallel increase in the epicarp, especially concerning benzyl alcohol and 2-phenylethanol among the benzenoids and vomifoliol and 3-OH-β-damascone among the norisoprenoids. Diendiol1, geraniol, and <i>trans</i>-8-OH-linalool in the mesocarp decreased progressively during dehydration, while the same compounds increased in the epicarp. C₆ increased significantly in the epicarp. In addition to epicarp biosynthesis of VOCs during postharvest dehydration, a mass transfer of VOCs from the mesocarp to the epicarp might also occur.

Abatement of odorant compounds in one- and two-phase biotrickling filters under steady and transient conditions

The removal of hydrophobic volatile organic compounds (VOCs) still remains the main restriction in the biological treatment of odorous emissions due to mass transfer limitations. The addition of a non-aqueous phase to conventional biotrickling filters (BTF) may overcome this limitation by enhancing VOCs transport from the gas to the microorganisms. This study compared the long-term and transient performance of a one- (1P) and two-liquid phase (2P; with silicone oil as non-aqueous phase) BTFs for the removal of four VOCs (butanone, toluene, alpha-pinene, and hexane) at empty bed residence times (EBRT) ranging from 47 to 6 s. Removal efficiencies (RE) >96 % were obtained for butanone, toluene, and alpha-pinene in both bioreactors regardless of the EBRT, while higher hexane REs were recorded in the 2P-BTF (81-92 %) compared to the 1P-BTF (60-97 %). The two-phase system always showed a more consistent performance, being able to better withstand step VOC concentration increases and starvation periods, although it was more affected by liquid recycling shutdowns due to a reduced VOC mass transfer. The analysis of the microbial communities showed a high biodiversity and richness despite the low C source spectrum and high community evenness and richness. In this context, the presence of silicone oil mediated the development of a highly different phylogenetic composition of the communities.

Intensification of volatile organic compounds mass transfer in a compact scrubber using the O 3/H 2O 2 advanced oxidation process: Kinetic study and hydroxyl radical tracking

This study assesses the potential of ozonation and advanced oxidation process O 3/H 2O 2 to enhance the dimethyldisulfide (DMDS) mass transfer in a compact chemical scrubber developed for air treatment applications. Theoretical calculations, through Hatta number and enhancement factor evaluations for two parallel irreversible reactions, were compared to experimental data and enabled the description of the mass transfer mechanisms. These calculations required the determination of the kinetic constant of the DMDS oxidation by molecular ozone ( k DMDS / O 3 ) and the measurement of the hydroxyl radical concentration within the scrubber. The competitive kinetic method using the 1,2-dihydroxybenzene (resorcinol) enabled to determine a value of the kinetic constant k DMDS / O 3 of 1.1 × 10 6 M −1 s −1 at 293 K. Then, experiments using para-chlorobenzoic acid in solution allowed measuring the average hydroxyl concentration in the scrubber between the inlet and the outlet depending on the chemical conditions (pH and inlet O 3 and H 2O 2 concentrations). High hydroxyl radical concentrations (10 −8 M) and ratio of the HO°-to-O 3 exposure ( R ct ≈ 10 −4) were put in evidence.

Kinetics of toluene and sulfur compounds removal by means of an integrated process involving the coupling of absorption and biodegradation

AbstractBACKGROUND: Scrubbing using an organic solution instead of an aqueous solution could be a useful way to improve the removal of hydrophobic compounds. Absorption of toluene, dimethyldisulfide (DMDS) and dimethylsulfide (DMS) in an organic solution (di‐2‐ethylhexyladipate—DEHA), followed by biodegradation by activated sludge was considered, with particular attention to kinetic aspects. DEHA was selected for its relevance in terms of absorption capacity and absorption velocity of the selected volatile organic compounds (VOCs). After the biodegradation step and owing to its cost, recycling of the VOC‐free solvent should be considered.RESULTS: Enhancement of VOC mass transfer from the organic to the aqueous phase due to bacterial activity was highlighted and the main driving force was found to be biosurfactant production rather than biodegradation reaction. However, the mass transfer rate between the two phases was shown to be lower than VOC biodegradation rate; hence, significant biodegradation of DMDS and toluene was recorded in a few days during batch experiments, 0.10 and 0.09 mmol respectively. Toluene showed higher biodegradation rates (about 0.05 and 0.10 mg h−1 for DMDS and toluene), leading to higher growth rates. Contrarily, owing to its high volatility, important DMS losses were observed.CONCLUSION: The relevance of the proposed integrated process was shown for hydrophobic VOC removal, at least for toluene and DMDS. Unfortunately, the absorbent phase was also degraded, proved by detection of by‐products during analyses of the aqueous phase headspace. The comparison of DEHA with other solvents or solid polymers available for multiphase bioreactor applications may be a reliable option to continue this work. Copyright © 2010 Society of Chemical Industry

Modeling breakthrough curves of volatile organic compounds on activated carbon fibers

Granular activated carbon (GAC) and more recently activated carbon fibers (ACF) are used for the treatment of volatile organic compounds (VOC) in industrial processes. The purpose of this study was to investigate the adsorption kinetics of ACF to eliminate VOC from polluted air. This approach is carried out by modeling experimental breakthrough curves with two kinds of models: an equilibrium model and a mass transfer model based on a linear driving force (LDF) in conjunction with the Langmuir equilibrium model. The results show the influence of the intraparticle diffusion on the adsorption kinetics of ACF, in spite of their small fiber diameter. Moreover, external diffusion kinetics is fast because of the influence of the large external surface area of ACF on the VOC mass transfer.

Performance of a Membrane Diffuser Bioreactor for the Removal of Gaseous Toluene

Hollow fiber membrane bioreactors have emerged as a promising technology for use in the treatment of volatile organic compounds (VOCs), due to their high surface area for VOC mass transfer and significant biodegradation capacity. In this study, a bioreactor operated in a diffuser mode using a submerged, hollow fiber membrane module (referred to as membrane diffuser bioreactor, MDBR) was employed in order to investigate its performance for the treatment of gaseous toluene. MDBR experiments were performed at different inlet toluene concentrations of 54, 160, and 750 ppmv, respectively. Overall, toluene removal efficiencies were maintained in the range of 60–70% at each operating condition. The biomass density in the liquid phase was changed according to the inlet loading rate, but the toluene concentrations in the liquid phase were always extremely low, indicating that the overall reaction was controlled by the mass transfer rate of toluene, rather than by the biodegradation rate. Additionally, elimination ...

Numerical simulation for volatile organic compound removal in rotating drum biofilter

Rotating drum biofilters (RDBs) could effectively remove volatile organic compounds (VOCs) from waste gas streams. A mathematical model was developed on the basis of mass transport and mass balance equations in an RDB, the two-film theory, and the Monod kinetics. This model took account of mass transfer and biodegradation of VOC in the gas-water-biofilm three-phase system in the biofilter, and could simulate variations of VOC removal efficiency with a changing specific surface area and porosity of the media due to the increasing of biofilm thickness in the biofilter. Toluene was used as a model VOC. This model was further simplified by introducing a coefficient of the gas velocity and neglecting the water phase due to the complexity of operating conditions. The equations for the biofilm phase, gas phase, and biofilm accumulation in this model were solved using colloction method, analytic method, and the Runge-Kutta method separately. A computer program was written down as MATLAB to solve this model. Results of numerical solutions showed that toluene removal efficiency in the RDB increased and reached the maximum values of 97% on day 4 after the startup, and then decreased the remained at 90% after 5 more days of operation. Toluene concentration was high at the outermost layer where more than 70% toluene was removed, and was low at the innermost layer where less than 10% toluene was removed. The dynamic removal efficiencies from this model correlated reasonably well with experimental results for toluene removal in a multi-layered RDB.

Absorption of a mixture of volatile organic compounds (VOCs) in aqueous solutions of soluble cutting oil

A semi-industrial bioscrubber was developed to treat a complex mixture of VOCs: oxygenated, aromatic and chlorinated compounds. In order to optimize the VOCs mass transfer, an original washing agent made up of water and cutting oil was tested, and the impact of this washing agent on bioscrubbing performances was investigated. The results obtained with a laboratory unit show that the addition of oil strongly increases the quantity of transferred aromatics. For these compounds, the apparent mass transfer coefficient k L a is lower than with water alone. In term of bioscrubbing performances, comparison of the results obtained with the water–oil mixture and water alone showed that the removal efficiency for aromatics is enhanced: from 12% to 36% (applied load of 852 g VOCs m −3 h −1); the elimination of chlorinated compounds is slightly improved. The addition of oil does not seem to lead to any dysfunction of the microbial communities that metabolize the transferred compounds.

Determining the equilibrium partitioning coefficients of volatile organic compounds at an air–water interface

The single equilibration technique (SET) was adopted to determine the partitioning coefficients ( p c) at an air–water interface for volatile organic compounds (VOCs), including ethanol, iso-propanol (IPA), iso-butanol (IBA), methyl ethyl ketone (MEK) and toluene, all extensively used in industrial processes. Standard SET procedures were established. The liquid concentrations ( C L) of tested VOCs ranged from 10 to 125 mg l −1 for alcohols and MEK, and from 0.5 to 20 mg l −1 for toluene. The temperatures ( T w) of aqueous VOC solutions were maintained at 27, 32, 38 and 42 °C to determine the gaseous concentrations at equilibrium ( C g ∗ ) and p c of VOCs, using the formula p c =(C g ∗/C L ) . Results reveal that the p c values of all tested components increase slowly with T w given a constant C L, and that the p c of alcohols and MEK fall as C L increases at a constant T w. In contrast, the p c of toluene is not significantly impacted by a variation in C L at a constant T w. However, the effect of C L concentration has seldom been discussed. The heats of liquid and gaseous phase transfer (Δ H tr) of VOC, and the highly linear regression (with squared correlation coefficients, R 2, from 0.901 to 0.999) between lnC g ∗ and T w −1 are also evaluated. The experimental results and the VOC mass transfer characteristics are helpful for evaluating the emission of VOC from the water surface of wastewater treatment facilities.

Pervaporation-biological oxidation hybrid process for removal of volatile organic compounds from wastewaters

A new pervaporation-biological oxidation hybrid process for the treatment of wastewaters containing volatile organic compounds (VOCs) has been investigated. The process combines pervaporation using a sweep gas with absorption and biological degradation of the permeate VOCs. A model system with monochlorobenzene (MCB) as the VOC and Pseudomonas JS150 as the degrading microorganism was used for the study. Relatively high temperatures for the pervaporation operation were used, allowing the use of lower membrane area and lower sweep gas flowrate. The resulting higher concentration of VOC in the sweep gas and the difference in temperatures between pervaporation unit and bioreactor, were expected to improve the mass transfer of VOC from the gas into the biomedium, leading to a better VOC removal from the gas stream. The performance of this system, working at a constant gas flowrate and a bioreactor temperature of 30°C, was studied for step increases in pervaporation temperature. Raising the temperature was shown to increase the mass transfer flux in the membrane module. Bioreactor removal efficiencies close to 100% were obtained throughout, and an elimination capacity of 84 g (MCB) m −3 h −1 was attained. The system was then run with a lower bioreactor temperature (15°C), in order to accurately quantify changes in removal efficiency with varying pervaporation temperature. In this case, a constant VOC load was fed to the bioreactor, with reductions in gas flowrates concomitant with increasing pervaporation temperatures. It was shown that using lower gas flowrates with higher VOC concentrations, at higher temperatures, resulted in significantly improved removal efficiencies. Concentrations of 17 g m −3 of MCB 1 Throughout the text, gas concentrations and gas flowrates are at standard temperature and pressure, STP (temperature=0°C, pressure=1atm). 1 were treated in the bioreactor working at 15°C, with removals of 95%.

Air sparging effectiveness: laboratory characterization of air-channel mass transfer zone for VOC volatilization

Air sparging in conjunction with soil vapor extraction is one of many technologies currently being applied for the remediation of groundwater contaminated with volatile organic compounds (VOCs). Mass transfer at the air–water interface during air sparging is affected by various soil and VOC properties. In this study with a single air-channel apparatus, mass transfer of VOCs was shown to occur within a thin layer of saturated porous media next to the air channel. In this zone, the VOCs were found to rapidly deplete during air sparging resulting in a steep concentration gradient while the VOC concentration outside the zone remained fairly constant. The sizes of the mass transfer zone were found to range from 17 to 41 mm or 70 d 50 and 215 d 50 ( d 50=mean particle size) for low organic carbon content media (<0.01% OC). The size of the mass transfer zone was found to be proportional to the square root of the aqueous diffusivity of the VOC, and was affected by the mean particle size, and the uniformity coefficient. Effects of the volatility of the VOCs as represented by the Henry’s law constants and the airflow rates on the mass transfer zone were found to be negligible but VOC mass transfer from air–water interface to bulk air phase seems to play a role. A general correlation for predicting the size of the mass transfer zone was developed. The model was developed using data from nine different VOCs and verified by two other VOCs. The existence of the mass transfer zone provides an explanation for the tailing effect of the air phase concentration under prolonged air sparging and the rebound in the VOC air phase concentration after the sparging system is turned off.