J Palau

A A Assadi

J M Penya-Roja

A Bouzaza

D Wolbert

Vicente Martínez-Soria

Isovaleraldehyde degradation using UV photocatalytic and dielectric barrier discharge reactors, and their combinations

The abatement of isovaleraldehyde present in air was carried out in UV photocatalytic and dielectric barrier discharge reactors (and their combinations) for concentrations up to 150 mg C m -3 and air flow rates ranging from 4 to 10 m 3 h -1 . A kinetic study was performed following a Langmuir-Hinshelwood model. Photocatalytic treatment of an isovaleraldehyde and isovaleric acid mixture showed a clear inhibition of isovaleric acid abatement in presence of isovaleraldehyde. Dielectric barrier discharge treatment of isovaleraldehyde showed an increase of removal efficiency with applied voltage and a decrease with air flow rate and inlet concentration. Moreover, introduction of a catalyst into the dielectric barrier discharge reactor did not produce a significant effect (UV light off). However, a combination of both techniques significantly increased isovaleraldehyde removal efficiency, indicating the treatment technique sequence had an effect on system performance. In conclusion, combined treatment showed promising results for the removal of VOCs, providing a synergic improvement in the removal of isovaleraldehyde.

Introduction

The harmful effects of pollution on the environment and health has led in recent decades and in many countries, to environmental policies aimed at the reduction of pollution (including air pollution) [1,[START_REF]Clean Air US Code, Environmental Protection Agency[END_REF]. Among the main air pollutants are volatile organic compounds (VOCs) released into the atmosphere due to various human activities, among which industrial activity accounts for a high percentage of the total emissions. Long periods of exposure to certain VOCs are linked to the development of cancer, eye irritation and breathing difficulties after short periods of contact.

VOCs are active in many atmospheric reactions contributing to the generation of tropospheric ozone and resulting in environmental problems [START_REF]ESIG[END_REF]4]. It is therefore necessary to reduce emissions, or if reduction is not possible, treatment with economically and environmentally-sustainable technologies. For this purpose, photocatalytic and dielectric barrier discharge (DBD) treatments are innovative techniques that have advantages (such as a reduced environmental impact) over conventional techniques, however more studies are necessary to develop/implement their industrial applications to make them viable alternatives to existing treatment technologies [START_REF] Parmar | Emerging Control Technologies for Volatile Organic Compounds[END_REF][6][START_REF] Lee | Abatement of Gas-phase p-Xylene via Dielectric Barrier Discharge[END_REF].

Heterogeneous photocatalysis is promoted by direct/indirect absorption of a photon with energy equal to or greater than the band gap of the semiconductor (typically TiO 2 ), generating a hole (h + ) -electron (e -) pair, which in turn produce species such as superoxide and hydroxyl radicals (OH˙) that can oxidize the VOCs to non-toxic end-products (primarily CO 2 and water) [START_REF] Chang | Photocatalytic oxidation of gaseous DMF using thin film TiO2 photocatalyst[END_REF][START_REF] Vincent | Abatement of volatile organic compounds using an annular photocatalytic reactor: Study of gaseous acetone[END_REF][10].

DBD uses an electrical discharge to treat gas present between electrodes separated by a dielectric material, which can ionize the gas, modifying the components. This generates an electrical breakdown in which the new species create an electronic cascade generating a large VOCs are active in many atmospheric reactions contributing to the generation of VOCs are active in many atmospheric reactions contributing to the generation of tropospheric ozone and resulting in environmental problems [START_REF]ESIG[END_REF]4]. It is therefore necessary to tropospheric ozone and resulting in environmental problems [START_REF]ESIG[END_REF]4]. It is therefore necessary to ssions, or if reduction is not possible, treatment with economically and ssions, or if reduction is not possible, treatment with economically and sustainable technologies. For this purpose, photocatalytic and dielectric sustainable technologies. For this purpose, photocatalytic and dielectric barrier discharge (DBD) treatments are innovative techniques that have advantages (such as a barrier discharge (DBD) treatments are innovative techniques that have advantages (such as a reduced environmental impact) over conventional techniques, however more studies are reduced environmental impact) over conventional techniques, however more studies are necessary to develop/implement their industrial applications to make them viable alternatives necessary to develop/implement their industrial applications to make them viable alternatives to existing treatment technologies [5 to existing treatment technologies [START_REF] Parmar | Emerging Control Technologies for Volatile Organic Compounds[END_REF][6][START_REF] Lee | Abatement of Gas-phase p-Xylene via Dielectric Barrier Discharge[END_REF].

Heterogeneous photocatalysis is pro Heterogeneous photocatalysis is pro with energy equal to or greater than the band gap of the semiconductor (typically TiO with energy equal to or greater than the band gap of the semiconductor (typically TiO generating a hole (h generating a hole (h + + generating a hole (h generating a hole (h generating a hole (h ) ) and hydroxyl radicals (OH and hydroxyl radicals (OH CO 2 and water) [8 and water) [8 compounds. This approach has gained increasing attention in recent years, and several researchers have studied this technology for use in the removal of nitrogen oxides, sulphur oxides and VOCs [11][12][13][START_REF] Park | NOx removal from the flue gas of oil-fired boiler using a multistage plasma-catalyst hybrid system[END_REF].

Although several studies have combined non-thermal plasma with heterogeneous catalysis in the abatement of VOCs and nitrogen oxides, the combination of dielectric barrier discharge with photocatalysis using external ultraviolet (UV) radiation in the removal of VOCs has not been extensively explored [START_REF] Vandenbroucke | Non-thermal plasmas for non-catalytic and catalytic VOC abatement[END_REF]. Increased removal efficiency for various VOCs [START_REF] Guaitella | C2H2 oxidation by plasma/TiO2 combination: Influence of the porosity, and photocatalytic mechanisms under plasma exposure[END_REF][START_REF] Maciuca | Synergetic by coupling with plasma for VOCs concentration removal from air[END_REF][18] has been described using this specific combination, however additional efforts are needed to explore the synergistic reactions, as the effects of plasma-triggered photocatalysis are not well understood.

Isovaleraldehyde and isovaleric acid (acid form) are used as reagents in the production of pesticides (e.g. pinacolone) and a wide range of pharmaceutical products, such as butizide [START_REF] Kohlpaintner | Ullmann's Encyclopedia of Industrial Chemistry[END_REF]. Although isovaleraldehyde is not very toxic, it can cause health and environmental problems. This compound produces oral, dermal and respiratory irritation after exposure as well as discomfort related to the strong odour, even at very low concentrations [START_REF]OECD screening information databases: 3-methyl butanal[END_REF][START_REF] Ottengraf | Exhaust gas purification[END_REF].

Despite these issues, studies on the removal of isovaleraldehyde from emissions are limited [START_REF] Maciuca | Synergetic by coupling with plasma for VOCs concentration removal from air[END_REF].

In a previous work [22], operational parameters such as geometry, UV radiation, humidity, and air gap were explored in the photocatalytic degradation of isovaleraldehyde, and the optimal operational conditions were used in this present work. A part of the present work adds to previous photocatalytic studies by evaluating reaction kinetics in addition to the inhibitory effects of isovaleric acid in the mixture. The main objective of this paper was the degradation of isovaleraldehyde using photocatalysis and dielectric barrier discharge (and their combinations) to determine the potential synergy between systems, the effects on the reactor performance as well as the sequence for applying the techniques.

A C C E P T E D M

A N U S C R I P T escribed using this specific combination, however additional efforts escribed using this specific combination, however additional efforts are needed to explore the synergistic reactions, as the effects of plasma are needed to explore the synergistic reactions, as the effects of plasma Isovaleraldehyde and isovaleric acid (acid form) are used as r Isovaleraldehyde and isovaleric acid (acid form) are used as reagents in the production eagents in the production of pesticides (e.g. pinacolone) and a wide range of pharmaceutical products, such as butizide of pesticides (e.g. pinacolone) and a wide range of pharmaceutical products, such as butizide [START_REF] Kohlpaintner | Ullmann's Encyclopedia of Industrial Chemistry[END_REF]. Although isovaleraldehyde is not very toxic, it can cause health and environmental [START_REF] Kohlpaintner | Ullmann's Encyclopedia of Industrial Chemistry[END_REF]. Although isovaleraldehyde is not very toxic, it can cause health and environmental problems. This compound produces oral, dermal a problems. This compound produces oral, dermal and respiratory irritation after exposure as nd respiratory irritation after exposure as problems. This compound produces oral, dermal a problems. This compound produces oral, dermal a problems. This compound produces oral, dermal a well as discomfort related to the strong odour, even at very low concentrations [START_REF]OECD screening information databases: 3-methyl butanal[END_REF][START_REF] Ottengraf | Exhaust gas purification[END_REF]. well as discomfort related to the strong odour, even at very low concentrations [START_REF]OECD screening information databases: 3-methyl butanal[END_REF][START_REF] Ottengraf | Exhaust gas purification[END_REF].

Despite these issues, studies on the removal of isovaleraldehyde from emissions are limited Despite these issues, studies on the removal of isovaleraldehyde from emissions are limited In a previous work [22] In a previous work [22] humidity, and air gap were explored in the photocatalytic degradation of isovaleraldehyde, humidity, and air gap were explored in the photocatalytic degradation of isovaleraldehyde, and the optimal operational conditions were used in this present work. A part of the present and the optimal operational conditions were used in this present work. A part of the present work adds to pr work adds to pr inhibitory effects of isovaleric acid in the mixture. The main objective of this paper was the inhibitory effects of isovaleric acid in the mixture. The main objective of this paper was the

Material and methods

Photoreactor unit

The reactor (with a planar geometry, 1000 x 135 x 135 mm) consisted of a rectangular polymethyl metracrylate (PMMA) chamber containing two parallel PMMA sheets (4 mm thick) where the distance between sheets could be adjusted to modify the space/air gap (GAP, 30 and 40 mm). In the centre of the reactor, four UV-A lamps (PL-S 9W/10/4P, spectrum centred at 365 nm, 0.012 m bulb diameter, 0.135 m bulb length, from Phillips, Netherlands) were equidistantly distributed to ensure a good distribution of radiation. The light intensity (20 W m -2 for 4 lamps with 40 mm of GAP) was measured using a UV Radiometer (VLX-3W equipped with a sensor CX 365, ALYS Technologies, Switzerland). The catalyst was placed in commercial fiberglass tissue (Ahlostrom, Finland) supporting 13 g m -2 TiO 2 at its surface [23] with a total length of 0.8 m, which was located on the internal face of the two PMMA sheets, yielding a photocatalytic surface of 0.19 m 2 [22,24].

Dielectric barrier discharge reactor

The DBD reactor employed the same external frame as the photocatalytic unit, containing two sheets of glass installed parallel to each other which supported the electrodes and dielectric barrier. As in the photocatalytic reactor, the distance between sheets was adjustable. Non-thermal plasma was obtained by applying a sinusoidal high voltage between 0-60 kV (peak to peak) with a frequency of 50 Hz to the electrodes and a 700 pf capacitator was connected in series with the reactor to determine the charge transferred to the system (Manley method). The voltage applied and the high voltage capacity were measured by highvoltage probes and recorded with a digital oscilloscope (Wave surfer 24Xs, 200 Mhz, LeCroy, EEUU) [24]. were equidistantly distributed to ensure a good distribution of radiation. The light intensity were equidistantly distributed to ensure a good distribution of radiation. The light intensity lamps with 40 mm of GAP) was measured using a UV Radiometer (VLX lamps with 40 mm of GAP) was measured using a UV Radiometer (VLX 3W equipped with a sensor CX 365, ALYS Technologies, Switzerland). The catalyst was 3W equipped with a sensor CX 365, ALYS Technologies, Switzerland). The catalyst was placed in commercial fiberglass tissue (Ahlostrom, Finland) supporting 13 g m placed in commercial fiberglass tissue (Ahlostrom, Finland) supporting 13 g m ith a total length of 0.8 m, which was located on the internal face of the two ith a total length of 0.8 m, which was located on the internal face of the two PMMA sheets, yielding a photocatalytic surface of 0.19 m PMMA sheets, yielding a photocatalytic surface of 0.19 m

Dielectric barrier discharge reactor Dielectric barrier discharge reactor

The DBD reactor employed the same external frame as the photocataly The DBD reactor employed the same external frame as the photocataly containing two sheets of glass installed parallel to each other which supported the electrodes containing two sheets of glass installed parallel to each other which supported the electrodes and dielectric barrier. As in the photocatalytic reactor, the distance between sheets was and dielectric barrier. As in the photocatalytic reactor, the distance between sheets was adjustable. Non adjustable. Non-thermal plasma was obtained by applying a si thermal plasma was obtained by applying a si 60 kV (peak to peak) with a frequency of 50 Hz to the electrodes and a 700 pf capacitator 60 kV (peak to peak) with a frequency of 50 Hz to the electrodes and a 700 pf capacitator was connected in series with the reactor to determine the charge transferred to the system was connected in series with the reactor to determine the charge transferred to the system 2.

Integrated system

The integrated reactor contained stacked elements for both technologies, using the dielectric barrier discharge reactor as the primary structure with the ultraviolet lamps installed in the centre of the reactor. The fiberglass/catalyst was placed on the inner face of the two glass sheets, between the glass sheet and the inner electrode.

Experimental set-up

Figure 1 shows a diagram of the system used. The air flowrate (air relative humidity of 5%) was regulated by a mass flow controller (EL-FLOW F-201AV, Hi-Tec Bronkhorst, Netherlands) reaching a maximum working value of 10 m 3 h -1 . The humidity of the air stream was regulated to avoid the influence of this parameter on the results (40% ± 15%). This was achieved by passing a fraction of the air stream through a countercurrent humidifier column.

The volatile organic compound was injected into the inlet air through a syringe pump with manual refill and a volume of 5 mL.

Before every experiment, the power was turned off (UV and/or electric discharge), and once inlet and outlet concentration of VOCs were the same (adsorption equilibrium state), the reactor power was turned on. Output samples were collected at 30 to 60 minute intervals until a steady state was achieved. At the conclusion of the experiment, the reactor was cleaned by a flow of clean air for at least one hour.

VOC concentration and air flow rate were 10-150 mg C m -3 and from 4-10 m 3 h -1 , respectively.

A C C E P T E D M

A N U S C R I P T

Figure 1 shows a diagram of the system used. The air flowrate (air relative humidity Figure 1 shows a diagram of the system used. The air flowrate (air relative humidity -201AV, Hi 201AV, Hi h h -1 . T . The humidity of the air stream he humidity of the air stream was regulated to avoid the influence of this parameter on the results (40% ± 15%). This was was regulated to avoid the influence of this parameter on the results (40% ± 15%). This was achieved by passing a fraction of the air stream through a countercurrent humidifier column. achieved by passing a fraction of the air stream through a countercurrent humidifier column.

The volatile organic compound was injecte

The volatile organic compound was injected into the inlet air through a syringe pump with d into the inlet air through a syringe pump with manual refill and a volume of 5 mL. manual refill and a volume of 5 mL.

Before every experiment, the power was turned off (UV and/or electric discharge), Before every experiment, the power was turned off (UV and/or electric discharge), and once inlet and outlet concentration of VOCs were the same (adsorption equilibrium and once inlet and outlet concentration of VOCs were the same (adsorption equilibrium ate), the reactor power was turned on. Output samples were collected at 30 to 60 minute ate), the reactor power was turned on. Output samples were collected at 30 to 60 minute intervals until a steady state was achieved. At the conclusion of the experiment, the reactor intervals until a steady state was achieved. At the conclusion of the experiment, the reactor was cleaned by a flow of clean air for at least one hour. was cleaned by a flow of clean air for at least one hour.

VOC concentratio VOC concentratio respectively. respectively.

The operational conditions/parameters used for the experimental series as well as the evaluation of photoreactor performance were:

• Inlet (C inlet ) and outlet (C outlet ) concentration of pollutant, as milligrams of carbon per cubic meter (mg C m -3 )

• Effective volume of reactor (V; m 3 ) and air flowrate (Q; m 3 h -1 )

• Removal efficiency (RE; %) = 100 (1-C outlet /C inlet ) • Inlet load (IL; mg C m -3 s -1 ) = C inlet Q/V • Elimination capacity (EC; mg C m -3 s -1 ) = (C inlet -C outlet ) Q/V 2.

Analytical methods

Isovaleraldehyde and isovaleric acid concentrations were determined by a gas chromatograph (GC Thermo Focus, USA) equipped with a flame ionization detector and a

Chrompact FFAP-CD capillary column (25 m of length), with nitrogen gas as the carrier.

Samples were collected with a 250 µL syringe.

Although intermediate organic compounds such as acetone or acetic acid [22] could be expected as by-products in these processes, they were not found above the detection limit of the analytical method (>0.2 mg C m -3 ), at least at the tested operational conditions. A simple pathway for the VOC removal is proposed as follow: Samples were collected with a 250 µL syringe. Samples were collected with a 250 µL syringe.

VOC à (intermediates à) CO 2 + H 2 O
Although intermediate organic compounds such as acetone or acetic acid [22] could Although intermediate organic compounds such as acetone or acetic acid [22] could be expected as by be expected as by--products in these processes, they were not f products in these processes, they were not f of the analytical method (>0.2 mg C m of the analytical method (>0.2 mg C m simple pathway for the VOC removal is proposed as follow: simple pathway for the VOC removal is proposed as follow:

The effects of inlet concentration and flow rate on the performance of the planar photoreactor were tested, and the removal efficiency (RE) of the isovaleraldehyde is shown (Figure 2) for different air flow rates and inlet concentrations. RE decreased when inlet concentration increased, and could be explained by taking into account the limited number of active sites on the catalyst surface available for adsorption and reaction with VOCs [25][START_REF] Bouzaza | Photocatalytic degradation of some VOCs in the gas phase using an annular flow reactor -determination of the contribution of mass transfer A C C[END_REF][START_REF] Kim | Kinetic study for photocatalytic degradation of volatile organic compounds in air using thin film of a TiO2 photocatalyst[END_REF].

Data indicated the higher air flow fed to the reactor, the lower the RE [START_REF] Jeong | Photodegradation of gaseous volatile organic compounds (VOCs) using TiO2 photoirradiated by an ozone producing UV lamp: decomposition characteristics, identification of by-products and water soluble organic intermediates[END_REF], which could be attributed to decreased contact time between the compound and active centres-hydroxyl radicals. The difference was greater at low concentrations and reduced at high concentrations, and could be explained by the ratio between the limited number of active centres and the compound molecules to be degraded decreasing with the inlet concentration of pollutant. At high inlet concentrations, the reaction rate, which determined the number of molecules degraded in a given time, became constant and independent of the inlet concentration (as can be observed and discussed in the next section).

Photocatalytic degradation of individual compounds: effect of inlet load and kinetics

Figure 3 shows the elimination capacity (EC) versus the inlet load (IL) for isovaleraldehyde and isovaleric acid. No significant differences in behaviour were observed at different air gaps and air flow rates (Fig 3a ) for isovaleraldehyde degradation, which was similar to results previously described for aromatic and oxygenated compounds [START_REF] Palau | UV photocatalytic oxidation of paint solvent compounds in air using an annular TiO 2supported reactor[END_REF]. For both compounds, a nearly complete elimination (RE > 90%) was observed for low IL values, while at high inlet loads, the EC remained relatively constant at a maximum EC of 12 mg C m -3 s -1 for isovaleraldehyde and 10-14 mg C m -3 s -1 for isovaleric acid. These values were comparable to those obtained previously in the removal of toluene and xylene [START_REF] Palau | UV photocatalytic oxidation of paint solvent compounds in air using an annular TiO 2supported reactor[END_REF].

A C C E P T E D M A N U S C R I P T
oncentrations and reduced at high concentrations, oncentrations and reduced at high concentrations, and could be explained by the ratio between the limited number of active centres and the and could be explained by the ratio between the limited number of active centres and the compound molecules to be degraded decreasing with the inlet concentration of pollutant. At compound molecules to be degraded decreasing with the inlet concentration of pollutant. At the reaction rate, which determined the number of molecules the reaction rate, which determined the number of molecules degraded in a given time, became constant and independent of the inlet concentration (as can degraded in a given time, became constant and independent of the inlet concentration (as can degradation of individual compounds: effect of inlet load and kinetics degradation of individual compounds: effect of inlet load and kinetics

Figure 3 shows the elimination capacity (EC) versus the inlet load (IL) for Figure 3 shows the elimination capacity (EC) versus the inlet load (IL) for isovaleraldehyde and isovaleric acid. No significant differences in behaviour were observed isovaleraldehyde and isovaleric acid. No significant differences in behaviour were observed at different ai at different air gaps and air flow rates (Fig 3a ) for isovaleraldehyde degradation, which was r gaps and air flow rates (Fig 3a ) for isovaleraldehyde degradation, which was at different ai at different ai at different ai similar to results previously described for aromatic and oxygenated compounds [START_REF] Palau | UV photocatalytic oxidation of paint solvent compounds in air using an annular TiO 2supported reactor[END_REF]. For similar to results previously described for aromatic and oxygenated compounds [START_REF] Palau | UV photocatalytic oxidation of paint solvent compounds in air using an annular TiO 2supported reactor[END_REF]. For both compounds, a nearly complete elimination (RE > 90%) was observed for low IL values, both compounds, a nearly complete elimination (RE > 90%) was observed for low IL values, while at high inlet loads, the EC remained relatively constant at a maximum EC of 12 mg C while at high inlet loads, the EC remained relatively constant at a maximum EC of 12 mg C

The photocatalytic process can be modelled by Langmuir-Hinshelwood (LH) kinetics.

Assuming the negligible mass transfer effects and considering a plug flow pattern in the reactor, the mass balance through the photocatalytic reactor can be described by the following equation [30]:

r d dC = ζ ( 1
)
where C is the VOC concentration (mg C m -3 ), r is the rate of disappearance for the component (mg C m -3 s -1 ) and ζ the retention time in the system(s).

Considering the variations in oxygen concentration and water vapour in air as negligible while assuming the reaction was the limiting step and the by-products had little or no action on the active adsorption centres, the LH model can be described by the following rate equation [31][START_REF] Demeestere | Heterogeneous photocatalysis as an advanced oxidation process for the abatement of chlorinated, monocyclic aromatic and A C C E P T E D M A N U S C R I P T photoirradiated by an ozone producing UV lamp: photoirradiated by an ozone producing UV lamp: products and water soluble organic products water soluble aldón[END_REF][START_REF] Wang | Photocatalytic degradation of gaseous benzene in air streams by using an optical fiber photoreactor[END_REF][START_REF] Belver | Palladium enhanced resistance to deactivation of titanium dioxide during the photocatalytic oxidation of toluene vapors[END_REF]:

C K k 1 C K k k r LH LH + = = θ ( 2
)
where k is the apparent kinetic constant (mg C m -3 s -1 ), θ is the fraction of active sites being covered by adsorbed reactant and K LH is the adsorption constant (m 3 mg C -1 ). Combining both equations yields:

ζ k dC C K 1 1 LH - =       + (3)
By integrating and rearranging this equation, it follows that:

LH outlet inlet LH outlet inlet outlet inlet K C C K k C C ) C / C ( ln - - = - ζ (4) A C C E P T E D M A N U S C R I P T
), r is the rate of disappearance for the ), r is the rate of disappearance for the the retention time in the system(s). the retention time in the system(s).

Considering the variations in oxygen concentration and water vapour in air as Considering the variations in oxygen concentration and water vapour in air as assuming the reaction was the limiting step and the by assuming the reaction was the limiting step and the by no action on the active adsorption centres, the LH model can be described by the fo no action on the active adsorption centres, the LH model can be described by the fo

where k is the apparent kinetic constant (mg C m where k is the apparent kinetic constant (mg C m covered by adsorbed reactant and K covered by adsorbed reactant and K both equations yields: both equations yields: 1.

A C C E P T E D M A N U S C R I P T dC dC C C K K 1 1 LH           +
As can be seen in Table 1, no significant differences were observed for the kinetic constant for isovaleraldehyde although this rate constant (k) should be slightly higher for the lower air gap, since this parameter should influence the light irradiance [22]. The adsorption Langmuir constant (K LH ) refers to the adsorption/desorption equilibrium and it is supposed to be independent of the light intensity during photocatalytic reaction. Nevertheless, it has been reported that there are some influence of the photon flow on both kinetic and the apparent adsorption constant [22]. Explanations for this behaviour were related with the hypothesis that irradiation influences adsorption equilibrium and therefore K LH values. In any case, the expected variation of the LH model constants due to the variation of irradiation is lower than the experimental error of the test of our study (Table 1).

Photocatalytic degradation of the mixture

For these experiments, a mixture of isovaleraldehyde and isovaleric acid (2:1 weight) was added to air, simulating industrial emissions. In Table 2, the individual REs of each compound in the mixture and the overall RE are shown. At the lowest inlet concentrations, the efficacy of both components was similar, but for the highest inlet concentrations (lowest global removal efficiency), the behaviour changed dramatically. As inlet concentration increased, the degradation of isovaleric acid rapidly decreased, and no degradation was observed at inlet concentrations higher than 70 mg C m -3 . At the highest concentrations, isovaleraldehyde was the only compound degraded and was responsible for the overall RE in [22]. The adsorption lower air gap, since this parameter should influence the light irradiance [22]. The adsorption ) refers to the adsorption/desorption equilibrium and it is supposed to ) refers to the adsorption/desorption equilibrium and it is supposed to intensity during photocatalytic reaction. Nevertheless, it has been intensity during photocatalytic reaction. Nevertheless, it has been reported that there are some influence of the photon flow on both kinetic and the apparent reported that there are some influence of the photon flow on both kinetic and the apparent adsorption constant [22]. Explanations for this behaviour were related with the hypothesis adsorption constant [22]. Explanations for this behaviour were related with the hypothesis irradiation influences adsorption equilibrium and therefore K irradiation influences adsorption equilibrium and therefore K expected variation of the LH model constants due to the variation of irradiation is lower than expected variation of the LH model constants due to the variation of irradiation is lower than the experimental error of the test of our study (Table 1). the experimental error of the test of our study (Table 1). atalytic degradation of the mixture atalytic degradation of the mixture For these experiments, a mixture of isovaleraldehyde and isovaleric acid (2:1 weight) For these experiments, a mixture of isovaleraldehyde and isovaleric acid (2:1 weight) was added to air, simulating industrial emissions. In Table 2, the individual REs of each was added to air, simulating industrial emissions. In Table 2, the individual REs of each compound in the mixture and the overall RE a compound in the mixture and the overall RE a the efficacy of both components was similar, but for the highest inlet concentrations (lowest the efficacy of both components was similar, but for the highest inlet concentrations (lowest global removal efficiency), the behaviour changed dramatically. As inlet concentration global removal efficiency), the behaviour changed dramatically. As inlet concentration the mixture. These observations may be explained by competition for active centres between these compounds.

Figure 5 shows the variations in EC versus inlet load for isovaleraldehyde and isovaleric acid, individually and as part of a mixture. Results seen for isovaleraldehyde were similar alone or as part of a mixture, and only at the highest concentrations, was the mixture EC slightly lower (compared to the sole compound), suggesting a slight inhibitory effect resulting from competition between both compounds for active sites on the catalyst. In case of isovaleric acid, the behaviour was very different for the individual compound compared to the mixture. Figure 5 shows the EC of isovaleric acid in the mixture decreased drastically with the inlet load, suggesting the compound might be strongly inhibited by the presence of the isovaleraldehyde, which agrees with the adsorption constant values for both compounds.

The K LH values for isovaleraldehyde were higher than those of isovaleric acid, and it could be assumed that most of the catalyst active centres were preferentially occupied by isovaleraldehyde, hindering (to some extent) the adsorption of the isovaleric acid and its degradation. Similar competitive adsorption phenomena are described in the literature [START_REF] Palau | UV photocatalytic oxidation of paint solvent compounds in air using an annular TiO 2supported reactor[END_REF]35].

Dielectric barrier discharge reactor

Effects of inlet concentration and air gap

Isovaleraldehyde removal efficiency for the DBD reactor at different operational conditions can be seen in Figure 6. As was expected, RE decreased with concentration and increased with applied voltage [START_REF] Lee | Abatement of Gas-phase p-Xylene via Dielectric Barrier Discharge[END_REF], which could be explained by a ratio reduction between the number of radicals and the energy electrons available per unit of VOC to be degraded under constant conditions [START_REF] Ma | H2S and NH3 Removal by Silent Discharge Plasma and Ozone Combo-System[END_REF]. Moreover, the increased removal efficiency observed when the applied voltage was increased, resulted from an increased number of radicals and electrons

A C C E P T E D M

A N U S C R I P T of isovaleric acid, the behaviour was very different for the individual compound compared to of isovaleric acid, the behaviour was very different for the individual compound compared to the mixture. Figure 5 shows the EC of isovaleric acid in the mixture decreased drastically the mixture. Figure 5 shows the EC of isovaleric acid in the mixture decreased drastically d might be strongly inhibited by the presence of d might be strongly inhibited by the presence of the isovaleraldehyde, which agrees with the adsorption constant values for both compounds. the isovaleraldehyde, which agrees with the adsorption constant values for both compounds. values for isovaleraldehyde were higher than those of isovaleric acid, and it could be values for isovaleraldehyde were higher than those of isovaleric acid, and it could be the catalyst active centres were preferentially occupied by the catalyst active centres were preferentially occupied by isovaleraldehyde, hindering (to some extent) the adsorption of the isovaleric acid and its isovaleraldehyde, hindering (to some extent) the adsorption of the isovaleric acid and its competitive adsorption competitive adsorption phenomena phenomena ectric barrier discharge reactor ectric barrier discharge reactor

Effects of inlet concentration and air gap Effects of inlet concentration and air gap

Isovaleraldehyde removal efficiency for the DBD reactor at different operational Isovaleraldehyde removal efficiency for the DBD reactor at different operational conditions can be seen in Figure 6. As was expected, RE decreased with concentration and conditions can be seen in Figure 6. As was expected, RE decreased with concentration and inc increased with applied voltage [START_REF] Lee | Abatement of Gas-phase p-Xylene via Dielectric Barrier Discharge[END_REF], which could be explained by a ratio reduction between the reased with applied voltage [START_REF] Lee | Abatement of Gas-phase p-Xylene via Dielectric Barrier Discharge[END_REF], which could be explained by a ratio reduction between the available in the system [11,[START_REF] Liu | Methane Conversion to Higher Hydrocarbons in the Presence of Carbon Dioxide Using Dielectric-Barrier Discharge Plasmas[END_REF].

The effect of distance between the sheets (GAP) that support the electrodes can be seen in Figure 6, where the results of experiments performed at 17 kV and at similar inlet concentration (31 and 35 mg C m -3 ) can be compared. In this case, to reach the same residence time of 1.3 s, the GAP was reduced from 40 to 30 mm and the flow rate was changed from 10 to 8 m 3 h -1 . A reduction in distance between electrode sheets resulted in an increased RE (17 to 29%), suggesting better ionization of the air due to increased contact between air and plasma. This observation was supported by the work of Cal and Schluep [38], that studied two different gap distances (3 and 5 mm) between electrodes and observed that the reactor with the 5 mm gap spacing required about three times the amount of power compared to the reactor with the 3 mm gap to achieve a benzene RE of 95%.

Effect of the presence of TiO 2 catalyst in the DBD reactor

Figure 7 shows the variations in RE with applied voltage for the DBD reactor combined with catalyst sheets in situ. Comparing the results between the two configurations, introduction of the catalyst did not appear to exert a positive effect on the RE of isovaleraldehyde, also seen for similar catalysts elsewhere [START_REF] Huang | Contribution of UV light to the decomposition of toluene in dielectric barrier discharge plasma/photocatalysis system[END_REF]. These results suggest the ultraviolet light produced by the ion plasma was not enough to appreciably activate the TiO 2 particles and create sufficient paired hole-electrons which could contribute (significantly) to the oxidation of VOCs [START_REF] Vandenbroucke | Non-thermal plasmas for non-catalytic and catalytic VOC abatement[END_REF]. Moreover, the slightly lower RE values for DBD combined with that the reactor with the 5 mm gap spacing required about three times the amount of power that the reactor with the 5 mm gap spacing required about three times the amount of power compared to the reactor with the 3 mm gap to achieve a benzene RE of 95%. compared to the reactor with the 3 mm gap to achieve a benzene RE of 95%.

ect of the presence of TiO ect of the presence of TiO 2 2 catalyst in the DBD reactor catalyst in the DBD reactor

Figure 7 shows the variations in RE with applied voltage for the DBD reactor Figure 7 shows the variations in RE with applied voltage for the DBD reactor combined with catalyst sheets combined with catalyst sheets introduction of the catalyst d introduction of the catalyst d isovaleraldehyde, also seen for similar catalysts elsewhere [START_REF] Huang | Contribution of UV light to the decomposition of toluene in dielectric barrier discharge plasma/photocatalysis system[END_REF]. These results suggest the isovaleraldehyde, also seen for similar catalysts elsewhere [START_REF] Huang | Contribution of UV light to the decomposition of toluene in dielectric barrier discharge plasma/photocatalysis system[END_REF]. These results suggest the ultraviolet light produced by the ion plasma was not enough to appreciably activate the TiO ultraviolet light produced by the ion plasma was not enough to appreciably activate the TiO particles and cre particles and cre indicating higher removal efficiencies for DBD combined with catalyst, which could be due to modification of the catalysts by the incorporation of metals (such as aluminium or silver), resulting in significant changes in catalytic behaviour [START_REF] Jarrige | Plasma-enhanced catalysis of propane and isopropyl alcohol at ambient temperature on a MnO2-based catalyst[END_REF][41][42].

Performance of the combined system

The performance of the combined system and its comparison with DBD and photocatalytic reactors is shown in Figure 8. These experiments were carried out at an inlet concentration of isovaleraldehyde around 50 mg C m -3 , an applied voltage of 12, 17 and 20 kV, flow rates of 4, 6 and 10 m 3 h -1 and an electrode gap of 30 mm (the 0 kV voltage corresponded to the operation of the photocatalytic reactor alone). As expected, RE increased for both systems with applied voltage, and was attributed to an increase in the total number of free radicals and high-energy electrons. Additionally, RE decreased with flow rate due to decreased contact time with ionized molecules, free radicals, high-energy electrons and the catalyst. Furthermore, the existence of a threshold voltage (~10 kV) was seen, where below this value, electric breakdown did not occur and the pollutant was not degraded by DBD. The RE of the combined system was considerably higher than the REs obtained for individual reactors (photocatalyst and DBD) (Figure 8).

The values of the elimination capacities obtained for combined and individual reactors are summarized in Table 3. As can be seen, the EC of the combined system was considerably higher (>10-20%) than the sum obtained for individual reactors and independent of the applied voltage (12 or 17 kV). This indicated that coupling of the photocatalytic and DBD reactors significantly improved elimination and, consequently, the energy efficiency. and an electrode gap of 30 mm (the 0 kV voltage and an electrode gap of 30 mm (the 0 kV voltage rresponded to the operation of the photocatalytic reactor alone). As expected, RE increased rresponded to the operation of the photocatalytic reactor alone). As expected, RE increased for both systems with applied voltage, and was attributed to an increase in the total number of for both systems with applied voltage, and was attributed to an increase in the total number of energy electrons. Additionally, RE decreased energy electrons. Additionally, RE decreased decreased contact time with ionized molecules, free radicals, high decreased contact time with ionized molecules, free radicals, high catalyst. Furthermore, the existence of a threshold voltage (~10 kV) was seen, where below catalyst. Furthermore, the existence of a threshold voltage (~10 kV) was seen, where below this value, electric breakdown did not occur and th this value, electric breakdown did not occur and th RE of the combined system was considerably higher than the REs obtained for individual RE of the combined system was considerably higher than the REs obtained for individual reactors (photocatalyst and DBD) (Figure 8). reactors (photocatalyst and DBD) (Figure 8).

The values of the elimination capacities obtained for combined and individual reactors

The values of the elimination capacities obtained for combined and individual reactors are summarized in Table 3. As can be seen, the EC of the combined system was considerably are summarized in Table 3. As can be seen, the EC of the combined system was considerably higher (>10 higher (>10 applied voltage (12 or 17 kV). This indicated that coupling of the photocatalytic and DBD applied voltage (12 or 17 kV). This indicated that coupling of the photocatalytic and DBD Studies using a combination of these technologies have been performed [START_REF] Guaitella | C2H2 oxidation by plasma/TiO2 combination: Influence of the porosity, and photocatalytic mechanisms under plasma exposure[END_REF]43,[START_REF] Thevenet | Comparison of the Plasma-Photocatalyst Synergy at Low and Atmospheric Pressure[END_REF],

and the results obtained by coupling these techniques show an improvement in elimination and a significant decrease in the production of by-products. The explanations of this synergistic effect have been attributed to diverse causes. It has been linked to intermediates produced by DBD [START_REF] Thevenet | Comparison of the Plasma-Photocatalyst Synergy at Low and Atmospheric Pressure[END_REF] which could be more easily degraded by photocatalysis than the original compounds, increasing the rate of degradation for the overall system. This hypothesis was confirmed by analysis of the compounds adsorbed on the hole-electron pairs. In another study [START_REF] Vandenbroucke | Non-thermal plasmas for non-catalytic and catalytic VOC abatement[END_REF], the improvement in the RE was linked to improvements in VOC degradation by the photocatalytic phase, where the free radicals, oxygen atoms and ozone molecules produced by DBD interact with catalyst and the pollutants [START_REF] Guaitella | C2H2 oxidation by plasma/TiO2 combination: Influence of the porosity, and photocatalytic mechanisms under plasma exposure[END_REF].

Effect of the treatment sequence

In order evaluate the effects of treatment sequence on RE, experiments were designed where the reactor was internally divided into halves. DBD was implemented in one half while the other half was operated as a combined DBD and photocatalytic reactor. Initially, experiments were performed using DBD treatment first, then combined (DBD / Photo + DBD) and the sequence reversed (Photo + DBD / DBD). Analyses of each half-reactor were carried out. Results were also compared to the full reactor operating with both technologies.

Figure 9 shows the RE values obtained for the three combinations. As expected, when the operation was performed with both technologies in the complete reactor, the performance was higher than when half the reactor operated with DBD. This result was consistent with the positive combination of these technologies (described above). When both sequential configurations were compared, for any voltage applied, higher removal efficiencies were study [START_REF] Vandenbroucke | Non-thermal plasmas for non-catalytic and catalytic VOC abatement[END_REF], the improvement in the RE was linked to improvements in VOC degradation by study [START_REF] Vandenbroucke | Non-thermal plasmas for non-catalytic and catalytic VOC abatement[END_REF], improvement in the RE was linked to improvements in VOC degradation by the photocatalytic phase, where the free radicals, oxygen atoms and ozone molecules the phase, where the free radicals, oxygen atoms and ozone molecules

In order evaluate the effects of treatment sequence on RE, experiments were designed In order evaluate effects of treatment sequence on RE, experiments were designed where the reactor was internally divided into halves. DBD was implemented in one half while where the reactor was internally divided into halves. DBD was implemented in one half while operated as a combined DBD and photocatalytic reactor. Initially, operated as a combined DBD and photocatalytic reactor. Initially, experiments were performed using DBD treatment first, then combined (DBD / Photo + experiments were performed using DBD treatment first, then combined (DBD / Photo + DBD) and the sequence reversed (Photo + DBD / DBD). Analyses of each half DBD) and the sequence reversed (Photo + DBD / DBD). Analyses of each half were also compared to the full reactor operating with both technologies. were compared to the full reactor operating with both technologies.

Figure 9 shows the RE values obtained for the three combinations. As expected, when Figure 9 shows the RE values obtained for the three combinations. As expected, when the operation was performed with both technologies in the complete reactor, the performance the was with both technologies in the complete reactor, the performance obtained when DBD treatment was placed in the first half, which could indicate treatment with this technique promoted/increased the rate of photocatalytic degradation in the second part. The results obtained from the first sequential reactor design (DBD alone and combined as DBD/Photo + DBD, second column of Figure 9) showed that degradation of the compound in the first part of the system contributed near the half of the global removal efficiency, and as voltage increased, the contribution percentage decreased (the combined portion increased the contribution of the applied voltage). The behaviour of the second sequential reactor (Photo + DBD / DBD, first column in Figure 9) was quite different, where the removal contribution from the first part of the reactor (combined) was notably higher than the second portion (DBD alone), except at the lowest voltage applied.

These results could be explained by assuming pretreatment with DBD could enhance photocatalytic activity. Several reasons have proposed the literature for this enhancement [41]. By combining both treatments, modifications in electron energy, the type of discharge and effects on catalytic properties occur. The presence of the catalyst in the plasma discharge could increase the production of active species, in some cases accelerating the formation of superoxide (O 2 -), reducing the recombination process and increasing total catalytic activity. Furthermore, the plasma could generate intermediate species with longer life-times which interacted with the surface of the catalyst (ozone, atomic oxygen and other reactive species) and increased the catalytic activity of the reactor. Moreover, the discharge could affect catalyst properties, improving the dispersion of the active catalytic components by modifying the stability and catalytic activity of the material. It is important to note the adsorption process for organic compounds was positively influenced by the presence of plasma discharge, and improved the affinity of the catalyst for the organic compounds.

Furthermore, reactions could occur through alternative reduction and oxidation at the surface of the catalyst, and a positive effect for heat activation by the dielectric barrier discharge. (Photo + DBD / DBD, first column in Figure 9) was quite different, where the removal (Photo + DBD / DBD, first column in Figure 9) was quite different, where the removal actor (combined) was notably higher than the second actor (combined) was notably higher than the second These results could be explained by assuming pretreatment with DBD could enhance These could be explained by assuming pretreatment with DBD could enhance photocatalytic activity. Several reasons have been proposed in photocatalytic activity. reasons have been proposed in enhancement [41]. By combining both treatments, modifications in electron energy, the type enhancement [41]. By combining both treatments, modifications in electron energy, the type of discharge and effects on catalytic properties occur. The presence of the catalyst in the of and effects on catalytic properties occur. The presence of the catalyst in the plasma discharge could increase the production of plasma discharge could the production of the formation of superoxide (O the formation of superoxide 2 --

), reducing the recombination process and increasing total ), reducing the recombination process and increasing total catalytic activity. Furthermore, the plasma could generate intermediate species with longer catalytic activity. Furthermore, the plasma could generate intermediate species with longer times which interacted w times which interacted with the surface of the catalyst (ozone, atomic oxygen and other reactive species) and increased the catalytic activity of the reactor. Moreover, the discharge reactive species) and increased catalytic activity of the reactor. Moreover, the discharge could affect catalyst properties, improving the dispersion of the active catalytic components could affect catalyst properties, improving the dispersion of the active catalytic components by modifying the stability and catalytic activity of the material. It is important to note the modifying the stability and catalytic activity of the material. It is important to note the adsorption process for organic compounds was positively influenced by the presence of adsorption process for organic compounds was positively influenced by the presence of

Conclusions

The application of photocatalytic treatment to degrade VOCs resulted in a maximum elimination capacity of around 12 and 14 mg C m -3 for isovaleraldehyde and isovaleric acid, respectively. The elimination capacity of isovaleric acid was negatively affected in presence of isovaleraldehyde, indicating inhibitory and competition effects between both compounds.

The dielectric barrier discharge treatment showed an increased removal efficiency when the applied voltage was increased, promoting the generation of high energy electrons and free radicals capable of degrading volatile organic compounds. As expected, an increase in air flow rate and inlet concentration of the VOC resulted in decreased removal efficiency At tested operational conditions, the coupled system combining dielectric barrier discharge and UV photocatalysis showed a significant improvement in VOC removal efficiency. This suggested the dielectric barrier discharge process promoted degradation of intermediates which facilitated the catalytic reaction, and the energy involved in the coupled system promoted the adsorption of organic compounds at catalytic active sites. The sequence of VOC dielectric barrier discharge and photocatalysis coupled with dielectric barrier discharge, was important to removal efficiency. The use of dielectric barrier discharge in the first half and in the second half of a coupled reactor system showed higher removal efficiency due to the generation of smaller compounds and more active molecules that could interact in the second half, activating the catalyst sites.

  number of reactive species (mainly O 2 -, OH˙ and O 3 ) which interact and degrade organic A

  nm, 0.012 m bulb diameter, 0.135 m bulb length, from Phillips, Netherlands) centred at 365 nm, 0.012 m bulb diameter, 0.135 m bulb length, from Phillips, Netherlands)

3. Results and discussions 3 . 1

 31 Photocatalytic reactor 3.1.1 Effects of inlet concentration and flow rate on VOC degradation A acid concentrations were determined by a gas Isovaleraldehyde and isovaleric acid concentrations were determined by a gas chromatograph (GC Thermo Focus, USA) equipped with a flame ionization detector and a chromatograph (GC Thermo Focus, USA) equipped with a flame ionization detector and a llary column (25 m of length), with nitrogen gas as the carrier. llary column (25 m of length), with nitrogen gas as the carrier.

  By integrating and rearranging this equation, it follows that: By integrating and rearranging this equation, it follows that: Plotting ln (C inlet /C outlet ) / (C inlet -C outlet ) versus ζ/(C inlet -C outlet ) should be linear if the LH model described the process, where the slope of the regression line represented the product of constants kK LH and the y-intercept -K LH . The correlation between experimental results for the two compounds according to the LH model are shown in Figure 4, and constant values and correlations obtained for the LH model are shown in Table

  I P T lower air gap, since this parameter should influence the light irradiance

  catalyst at low voltage (<20 kV) could be attributed to an extra-resistance due to the introduction of the TiO 2 sheets. In contrast, other researchers report different results, 1.3 s, the GAP was reduced from 40 to 30 mm and the flow rate was residence time of 1.3 s, the GAP was reduced from 40 to 30 mm and the flow rate was . A reduction in distance between electrode sheets resulted in an . A reduction in distance between electrode sheets resulted in an increased RE (17 to 29%), suggesting better ionization of the air due to increased contact increased RE (17 to 29%), suggesting better ionization of the air due to increased contact between air and plasma. This observation was supported by the work of Cal and Schluep between air and plasma. This observation was supported by the work of Cal and Schluep studied two different gap distances (3 and 5 mm) between electrodes and observed studied two different gap distances (3 and 5 mm) between electrodes and observed

  voltage of 12, 17 and 20 , an applied voltage of 12, 17 and 20

  titanium dioxide during the photocatalytic oxidation of toluene vapors, to of dioxide during the photocatalytic oxidation of toluene vapors,

Figure 1 .

 1 Figure 1. Schematics and layout of the integrated system: photocatalytic and DBD reactor.

Figure 2 .

 2 Figure 2. Isovaleraldehyde conversion versus inlet concentration at different air flow rates

Figure 3 .

 3 Figure 3. Elimination capacity versus inlet load for isovaleraldehyde (a) and isovaleric acid

Figure 4 .

 4 Figure 4. Regression of experimental results with the kinetic LH model.

Figure 5 .

 5 Figure 5. Comparison of elimination capacities for isovaleraldehyde (a) and isovaleric acid (b), individually and as a mixture. Inlet concentration of 34-140 mg C m -3 and a 4 m 3 h -1 air

Figure 6 .

 6 Figure 6. Removal efficiency of the dielectric barrier discharge reactor at different inlet

Figure 7 .

 7 Figure 7. Influence of catalyst on RE for various applied voltages. Inlet concentration of isovaleraldehyde = 35 mg C m -3 , air flow rate = 10 m 3 h -1 and GAP = 40 mm.

Figure 8 .Figure 5 .

 85 Figure 8. RE for dielectric barrier discharge and its combination with photocatalysis for different applied voltages and flow rates. Inlet concentration = 50 mg C m -3

Figure 7 .Figure 9 .

 79 Figure 7. Influence of catalyst on RE for vario Figure 7. Influence of on RE for vario

Table 2 .Table 3 .

 23 Global and individual removal efficiencies (RE%) in the photocatalytic reactor at Table 2. Global and individual removal efficiencies (RE%) in the photocatalytic reactor at different VOC inlet concentrations. Air flow rate = 4 m different VOC inlet concentrations. Air flow rate = EC (mg C m -3 s -1 ) values for isovaleraldehyde removal by DBD, photocatalysis and a combination of both. Inlet concentration = 40 mg C m -3 and GAP = 40.

Fig. 1 A

 1 Fig. 1 A C C E P T E D M A N U S C R I P T

  Fig. 2

Fig. 3 A

 3 Fig. 3 A C C E P T E D M A N U S C R I P T

Fig. 4 A

 4 Fig. 4 A C C E P T E D M A N U S C R I P T

Fig. 5

 5 Fig. 5

Fig. 6

 6 Fig. 6

Fig. 7

 7 Fig. 7

  Fig. 8

Fig. 9 A

 9 Fig. 9 A C C E P T E D M A N U S C R I P T

Table 1 .

 1 Values and estimated errors for LH model constants (obtained from regression of the experimental results, see Figure4).

	VOC	kK LH (s -1 )	K LH (m 3 mg C -1 ) k (mg C m -3 s -1 ) r 2
	Isovaleraldehyde (40 mm) 0.374±0.024 0.037±0.006	10.1±2.3	0.93
	Isovaleraldehyde (30 mm) 0.604±0.083 0.065±0.016	9.3±3.6	0.89
	Isovaleric acid (30 mm)	0.310±0.017 0.022±0.002	14.1±2.1	0.98

Table 2 .

 2 Global and individual removal efficiencies (RE%) in the photocatalytic reactor at different VOC inlet concentrations. Air flow rate = 4 m 3 h -1 and GAP = 30 mm.

	Inlet concentration	Global RE	Isovaleraldehyde RE	Isovaleric acid RE
	(mg C m -3 )	(%)	(%)	(%)
	34.6	34.6	36.5	32.9
	68.1	23.3	33.9	12.5
	104.6	10.7	21.4	0.0
	140.6	6.8	13.5	0.0

Acknowledgements

Assadi, Bouzaza and Wolbert gratefully acknowledge the financial support provided by the French National Research Agency (ANR) for this work. Palau, Penya-roja and Martinez-Soria acknowledge financial support from the Ministerio de Educación (Spain, Acknowledgements Acknowledgements Project CTM-2010-15031/TECNO) and Generalitat Valenciana (Spain, Project PROMETEO/2013/053).

A C C E P T E D M

A N U S C R I P T when the applied voltage was increased, promoting the generation of high energy electrons when the applied voltage was increased, promoting the generation of high energy electrons able of degrading volatile organic compounds. As expected, an increase able of degrading volatile organic compounds. As expected, an increase in air flow rate and inlet concentration of the VOC resulted in decreased removal efficiency in air flow rate and inlet concentration of the VOC resulted in decreased removal efficiency At tested operational conditions, the coupled system combining dielectric barrier At tested operational conditions, the coupled system combining dielectric barrier and UV photocatalysis showed a significant improvement in VOC removal and UV showed significant improvement in VOC removal efficiency. This suggested the dielectric barrier discharge process promoted degradation of efficiency. This suggested the dielectric barrier discharge process promoted degradation of intermediates which facilitated the catalytic reaction, and the energy involved in the couple intermediates which facilitated the catalytic reaction, and the energy involved in the couple system promoted the adsorption of organic compounds at catalytic active sites. The sequence system promoted the adsorption of organic compounds at catalytic active sites. The sequence of VOC treatments, dielectric barrier discharge and photocatalysis coupled with dielectric of VOC treatments, dielectric barrier discharge and photocatalysis coupled with dielectric barrier discharge, was important to removal efficiency. The use of diel barrier discharge, was important to removal efficiency. The use of diel in the first half and in the second half of a coupled reactor system showed higher removal in the half and in the second half of a coupled reactor system showed higher removal efficiency due to the generation of smaller compounds and more active molecules that could efficiency due to the generation of smaller compounds and more active molecules that could interact in the second half, activating the catal interact in the second half, activating the catal


Erwan Godefroy
email: erwan.godefroy@supelec.freric.totel@supelec.fr

Eric Totel

Michel Hurfin
email: michel.hurfin@inria.fr

Frédéric Majorczyk
email: frederic.majorczyk@intradef.gouv.fr

Automatic Generation of Correlation Rules to Detect Complex Attack Scenarios

Keywords:

In large distributed information systems, alert correlation systems are necessary to handle the huge amount of elementary security alerts and to identify complex multistep attacks within the flow of low level events and alerts. In this paper, we show that, once a human expert has provided an action tree derived from an attack tree, a fully automated transformation process can generate exhaustive correlation rules that would be tedious and error prone to enumerate by hand. The transformation relies on a detailed description of various aspects of the real execution environment (topology of the system, deployed services, etc.). Consequently, the generated correlation rules are tightly linked to the characteristics of the monitored information system. The proposed transformation process has been implemented in a prototype that generates correlation rules expressed in an attack description language.

I. INTRODUCTION

An alert correlation system aims at exploiting the known relationships between some elements that appear in the flow of low level notifications to generate high semantic meta-alerts. The main goal is to reduce the number of alerts returned to the security administrator and to allow a higher level analysis of the situation. However, producing correlation rules is a highly difficult operation, as it requires both the knowledge of an attacker, and the knowledge of the functionalities of all IDSes involved in the detection process. This paper focuses on the transformation process that allows to translate the description of a complex attack scenario into correlation rules. Obviously, the first phase (i.e., the identification of the elementary actions that compose the attack scenario) requires the knowledge of a human expert. In this work, we assume that this specification relies on attack trees [START_REF] Schneier | Attack trees: Modeling security threats[END_REF]. These structures are extended to identify the elementary actions that compose a multi-step attack. This hierarchical formalism allows to describe the logical relations between elementary actions and to indicate some temporal constraints between them.

From this extended attack tree, we define a transformation process for the creation of correlation rules. The sequential transformations take progressively into account various information about the execution context. Our correlation rules generator needs two kinds of information that can be provided by two kinds of independent experts. Some experts provide information about attack scenarios while the others provide information on the execution environment. Thus the areas of expertise that should be required for each of them is smaller. The initial representation of an attack has to be as compact and as general as possible while the resulting set of correlation rules has to be specific to a concrete environment. The goal is to ensure that this analysis remains valid even if the environment evolves from time to time. The expert has only to describe the main steps and characteristics of a complex attack without identifying neither the possible targeted nodes, nor the possible observations that may lead to detect that a given step of this attack has occurred.

Section II presents an overview of our approach for automatically generating correlation rules. Section III illustrates our starting point with an example. Section IV defines the notion of action tree and Section V describes the steps required to transform an action tree into a correlation tree. Section VI reviews the related works and Section VII concludes the paper.

II. FROM ATTACK TREES TO CORRELATION RULES

The generation process is divided into five steps (See Figure 1). Among the five steps, four can be entirely automated by using data stored in a knowledge base. Only the first step requires the involvement of a human expert during the transformation process. An attack tree (provided by a human expert) is the starting point of our transformation process. This tree includes the operators defined in [START_REF] Çamtepe | Modeling and detection of complex attacks[END_REF] (OR, AND, SAND). The attack scenario can be completely generic (in that case, the attack tree describes the steps of a known attack) or specific to the information system to be monitored.

The first step consists in transforming the attack tree into an action tree. An attack tree cannot be used directly as input to an automated transformation process because its description is too informal and because it refers only to goals and sub goals of the attack. The rewriting operated by the expert addresses these two problems. Broadly, the built action tree is quite similar to what has been proposed in [START_REF] Çamtepe | Modeling and detection of complex attacks[END_REF]: an action is associated with each leaf node. Obviously, any implicit information must be explicit in the action tree. An action description language (detailed later in Section IV) is a cornerstone in the proposed transformation process. The purpose of the next steps is to adapt the action tree to the specificities of the monitored information system by using information from the knowledge base. The second step aims at identifying the actors of the different actions (i.e., the hosts that can be the source or the target of an action). For each action of the action tree and for each unspecified host, the cartography (installed softwares and services) provided by the knowledge base is used to find the possible hosts. For example, if the action is a basic attack's step, only the hosts where a vulnerable process is running are considered. The third step aims at identifying observers that have the opportunity to observe the specified actions. Again the discovery process relies on the knowledge base which contains information about the different sensors, NIDS and HIDS. More details are provided in the Subsection V-B. The fourth step consists in transforming the action tree with observers into a correlation tree that describes the observable events that can be used to detect the whole attack scenario. IDSes and sensors will respectively raise alerts or send messages to the correlation system. This step is fully described in the Subsection V-C. During the fifth step of the process, the correlation rules are generated so that they can be included in the available correlation system. This step involves mostly syntax translations.

The generation process explained above relies on a knowledge base describing the monitored system and the deployment of the sensors and IDSes in this system. This description takes into account the topology of the network (the sub-networks where the nodes are placed, their addresses), the cartography (which services, processes and softwares are running on each node) and the different sensors monitoring the system. Some ontologies-based models ([4], [START_REF] Granadillo | An ontology-based model for siem environments[END_REF]) have been proposed as a knowledge base and can fulfill partially these requirements. We choose to use a knowledge base that extends M4D4 ( [START_REF] Morin | M4d4: a logical framework to support alert correlation in intrusion detection[END_REF]) because most of the objects and relationships we have to model are already taken into account. This framework is built upon a Prolog engine, which enables the definition of facts and rules in the same language.

III. INTRODUCING A PRACTICAL EXAMPLE

A. System architecture In order to illustrate our approach, we propose to apply each step in the context of an example. The reference system is shown on Figure 2a. For the sake of simplicity, our example is limited to the sub-part surrounded with the dotted rectangle (with the minimal number of machines required to run the scenario). The gateway firewall g1 makes the public DMZ (identified by dmz in Figure 2a) accessible from the outside but does not authorize any connection from the outside world or from the DMZ to the internal subnetwork. However, the internal network and the DMZ can access the internet. The DMZ contains two hosts s1 and s2 that run different services. Both hosts run web servers that are accessible from the internet. They also act as file servers by hosting FTP services. These services are only accessible from the internal network. The internal network is composed of two clients c1 and c2 and a file server s3 which is only accessible from the internal network. The role of s3 consists in storing confidential files.

We can define the topology and cartography of this system with facts defined in an extension of M4D4. In our implementation, such information is represented in the form of Prolog facts and can be accessed through queries. For example, the IP address of the node s1 is defined with node_address(s1, ipv4(10,0,0,1)).

B. Attack Tree Example

We suppose that an external attacker wants to steal some valuable information stored in the internal server s3. One possible scenario could consist in the following steps: as the attacker can only access the subnetwork dmz, its attack strategy consists in compromising one of the DMZ server in order to place a trojan horse then one user in the internal network will access this malicious file, and thus will compromise his machine in the internal network. The attack tree describing one possible multi-step attack is illustrated in Figure 2b. In order to illustrate our approach, we will focus on a small part of this attack tree, which consists in a possible attack to upload the initial trojan to the public DMZ server. This attack consists in a sequence of two actions: a code injection that is followed to a connection to the attacker machine. This small attack is described by the attack tree on Figure 2c.

IV. ACTION TREE

This section focuses on how an action tree can be derived from an attack tree by describing (1) each elementary action and (2) how these actions can be linked together.

A. Action Description Language

The attacker actions are specified using an action description language that describes the action and its actors (source and target). Each action is defined by its name. The possible names are identified in the following paragraph. An action can require to define the source and target of the action. This information constitutes the attributes of the action and are defined in the following.

Action Naming Convention: Elementary actions of an attack tree can be split in two categories: they can be attacks, which means that they match a known attack class, or they can be normal actions which are considered malicious only within the context of the scenario as a whole. As a consequence, we choose an appropriate naming convention for each of these two categories.

As actions that refer to attacks should refer to known categories of attacks, we need an attack taxonomy in order to name attacks. Among the taxonomies which have been proposed, CAPEC1 (Common Attack Pattern Enumeration and Classification) seems to be a suitable choice. Indeed, this taxonomy takes into account about 400 attack patterns organized in about 70 different categories. This taxonomy is built from 15 main attack mechanisms (from Data Leakage Attacks to Supply chain Attacks). Each of these mechanisms is then subdivided into other attack mechanisms until it reaches elementary attack patterns that compose the leaves of the CAPEC tree. During the definition of the knowledge base, this organization allows to express general mechanisms of attacks which may encompass several more detailed attack patterns. During the specification of an action, the chosen name of an attack action has to correspond to the name of the common ancestor attack mechanism that includes potentially all the attack patterns the attacker can perform. This enables us to specify an action with a variable level of precision. However, choosing a too general attack mechanism may lead to false positives, whereas choosing a too specific attack pattern may lead to false negatives. In our example, the first leaf is an attack action labelled Injection attack. Injection is one of the root attack mechanism provided by CAPEC. It is sub-classed in 17 different attack categories (from remote code inclusion to SQL injection). We can decide to stop here and choose injection for the name of the action. But if the context gives more restrictions about the possible attacks, we can for example choose the injection sub-class command injection and consequently reject all other types of injections.

Actions which are not attacks by themselves are supposed to be normal system actions. Common system actions are referenced in the CEE2 (Common Event Expression) taxonomy. CEE goal is to standardize the representation of logs. Consequently, it provides system primitives including a list of elementary actions which can be performed in a system. Contrary to CAPEC, the action taxonomy is reduced to a simple list of terms. In our example, the second leaf describes an outgoing connection, which can be described by the action initiate.

Attributes: The description of an action should allow to decide if the action is observable on the system, and how this observation can be performed. In order to answer these questions, it is necessary to know at different observation levels (network, system, application) if the action is visible.

In practice, to answer this question, it is necessary to know if an attack is remote (and involves packets on the (a) Action tree Figure 3. Action description language network), or if it is local (and is locally observable). To provide this information, we decided to define two attributes: source and target. The source and the target have locations and IP addresses on the network. If these attributes are equal, the attack is local to a node. Moreover, an action has always an impact on an entity on the system: this entity can be a file, a service listening on a port, or a process. A file is completely determined by its file name. A process has a name, and runs with the rights of a user (that is defined by his name and his user id). This set of attributes is sufficient to know what level of the system is targeted by the attack. It is important to note that each attribute of the action description has a type. If one of these attributes is represented by a variable, this variable inherits from the type of the attributes it represents.

B. Variable or constant attributes

The attack scenario consists of several actions with different kinds of attributes: some of them are constant and loosely coupled with the system characteristics and are known before the attack occurs. Some others are in the defender zone and are strongly coupled with the system topology and configuration. Finally, some attributes are under the control of the attacker, which means that their values cannot be determined before an attack occurs and can change for each attack instance. These several kinds of attributes conduct to the definition of the nature of each attribute: it is either constant or variable and variable attributes can be divided in two categories: static and dynamic.

Constant Attributes: An attribute is constant when its value is constant for all instances of the described attack (e.g., the network in which the web servers are located). This constant has the type of the attribute and can be used to define a value which is known to be an attack invariant. In the example Figure 3a, the target.location of the left action is always the dmz, as it is the only part of the network that is reachable by the external attacker.

Variable Attributes: An attribute must be represented by a variable when its value can vary depending on the instance of the attack. We can then distinguish two categories of variables: the variable whose possible values can be enumerated knowing the system configuration (topology, cartography, etc.), and the variables that take values completely independently of the system. The first category of variables is called static variable, and the second category is called dynamic variable. The category of each variable is defined in the declaration part of the variables.

In the example Figure 3a, the target.address of the action is one of the two servers in the DMZ. As a consequence, the target.address attribute is a variable whose values is in the set {10.0.0.1, 10.0.0.2}. This set can be completely enumerated using the knowledge of our network. This attribute is thus a static variable. On the contrary, the value of the source.address attribute of the left action cannot be guessed uniquely by relying on the knowledge of the system topology. This attribute will take a value only during the execution of the attack: it will be the IP address of the attacker machine. This attribute is thus a dynamic variable.

Relations between actions: A variable that has been declared in the variable declaration part can be used in several actions if necessary, as long as it is used for attributes of the same type. In the example Figure 3a, the variable Att (whose value is the IP address of the attacker machine) describes both the source.address of the left action and the target.address of the right action. This is a way to define that the value of the attribute source.address of the left action is equal to the attribute target.address of the right action.

V. AUTOMATIC GENERATION OF CORRELATION TREE A. Taking into account the target system

The purpose of this second step is to translate the generic action tree defined in IV-A into an action tree specific to the targeted system.

Action tree transformations: During this step all attributes parameters with static variables will be identified thanks to suitable requests to the knowledge base. These requests rely on a connection between the action attributes and the predicates that describe the different actors. This instantiation is subdivided in two operations. The first operation produces the list of all the possible trees given the information available in the knowledge base and the second operation merges these trees into a single one.

The first operation produces a list of trees whose action parameters previously defined with a static variable will be instantiated. Each time a static variable can receive more than one value, a new tree is created. As a consequence, each output tree reflects one possible specific attack tree in our current system. Then, given that all these trees refer to the same attack scenario, we consider that only one attack tree should include them. Therefore, a new common OR root node is added to link all these trees.

Illustration: Given the action tree of the previous section, the system will request the address, service port and process related to a running service for all nodes located in the DMZ. Moreover, when the two actions are remote actions, the filtering rules of the gateway will be taken into account. Indeed, the gateway allows external nodes to only access ports 80 and 8080 respectively of servers s1 and s2. Then, the services (and the related softwares and users information) associated to each of these ports for each server will be identified. Given the cartography of our example, two servers (s1 and s2) will be identified as potential actors. Consequently, the static variables will take these specific values, resulting in two trees (one dedicated for each server) that will be merged as shown in Figure 4.

B. Who can observe the actions?

This step focuses on selecting suitable observers for each actor. An observer is any device able to provide information about system or network events. They can be a simple sensor or a sophisticated IDS. During this step, a new parameter will be added to each action. This parameter will associate each action to its set of potential observers. A potential observer of a specific action is an observer for which the topological visibility and the functional visibility are compatible with the action specification. The topological visibility of an observer refers to the set of actors it can monitor and its functional visibility refers to the type of action it can detect based on its configuration. These definitions extend those provided in [START_REF] Morin | M4d4: a logical framework to support alert correlation in intrusion detection[END_REF].

The topological visibility of an observer can be a network or a node. Consequently, an action involving only one node can be potentially only observed by a HIDS or a local sensor, while actions involving two different nodes can be observed by local and remote sensors. The selection of all observers providing a suitable topological visibility is achieved in two stages: first, all suitable host-based sensors are selected.

Then, the suitable network sensors are selected based on the possible routes between the source and target of the action.

Then, we suppose that an observer can detect specific actions. The functional visibility is hence related to the name action field describing the type of action. The functional visibility is modeled by a double association. Each sensor is linked to a detection configuration and each detection configuration is linked to the type of attack or actions it can detect. Hence, the selection of observers given their functional visibility can be split in two cases. When the action refers to an attack class, all sensors able to detect one of the sub-classes or parent-classes of this action in the CAPEC taxonomy are selected. Indeed, a sensor can produce an alert referring to an attack which is a subclass of the specified action or it can produce a more generic alert. When the action is a common event (whose name is issued from CEE), all sensors whose configuration can observe the action are selected. These selections are illustrated in Figure 5.

C. From action tree to correlation tree

The goal of this section is to explain how the structure defined in Figure 5 can be transformed into a correlation tree, i.e., a structure describing only observables (called messages in our context) that can be used to detect an attack against the system. Three mechanisms are required to describe this step: how we describe messages produced by sensors, how an action is transformed into messages for a given sensor and how messages are managed when several sensors monitor the same action.

Message model: In our model, each message is composed of two parts: the message format and a set of fields. The format identifies the real format in which the message will be generated at run-time. The set of fields expresses the information that the message actually contains.

Converting actions to messages: This step takes into account the subset of attributes that messages generated by a given sensor can really include. As an example, a network sensor can generate messages containing IP source address, IP destination address, source and destination port values, but these messages will probably not include process or user related information. In addition, a given sensor can potentially produce different messages for a given action. For example, an IDS can provide different signatures for detecting sub-classes of a specific attack class. In this case, the possible messages are joined by an OR operator, meaning that we assume the action occurs if at least one of the messages is raised. When several sensors can detect the same action, we consider the possible messages for each of them. Then, they are joined together with an OR operator. As a consequence, the action will be successful if at least one sensor detects it.

Illustration: Figure 6 shows the correlation tree resulting from the identification of the sensor generated messages. The tree is non-symmetrical because the servers s1 and s2 are monitored by a different set of sensors. In our case, two Snort signatures match the injection action and have respectively the signature id 22063 and 22064. In the explicit alert correlation field, works focus mainly on the efficiency of detection algorithms and less in the configuration of these correlation systems with attack scenario. However some techniques we used in our approach to generate correlation rules are similar to those of systems built for different purpose.

The works [START_REF] Xu | Alert correlation through triggering events and common resources[END_REF] and [START_REF] Noel | Correlating intrusion events and building attack scenarios through attack graph distances[END_REF] use a model of the supervised system (including network services, host reachabilities and known vulnerabilities). However, their purpose is not to generate rules but to check the accuracy of received alerts at run time. They also propose different solutions to model alerts. [START_REF] Xu | Alert correlation through triggering events and common resources[END_REF] defines alert type but without relying on an existing taxonomy. The approach [START_REF] Noel | Correlating intrusion events and building attack scenarios through attack graph distances[END_REF] proposes a way to map alert generated by Snort with their defined attack scenario. It consists in a manual mapping between Snort signatures and Nessus vulnerability identifiers. Some works on correlation use an attack graph as input attack scenario ([3], [START_REF] Roschke | A new alert correlation algorithm based on attack graph[END_REF]). These graphs are automatically generated but take only known vulnerabilities referenced in the system into account. In general, method relying on attack graph can only take into account attack paths exploiting known and automatically identified vulnerabilities, whereas our approach allows a more global description without needing to know each vulnerability on each host. From our point of view, our approach is complementary, as attack tree could be built from information extracted from attack graphs.

VII. CONCLUSION AND FUTURE WORK

In this paper, we introduce a method for generating correlation rules from an attack scenario and a knowledge base. In order to reach this goal, we have defined an action description language to specify each elementary action and we have built an extension of the knowledge base M4D4. The benefits of this approach are twofold: first, it decouples the required expert knowledges for creating correlation rules in three low coupled domains (attack scenario specification, system cartography and sensor modeling). Then the evolutions of the system are more easily taken into account. Indeed, suitable changes involving cartography or sensors have to be made in the knowledge base to reflect the system evolutions. Then, these changes will be taken into account during a re-generation of the correlation rules.

Figure 1 .

 1 Figure 1. Transformation process of attack an tree to correlation rules: steps and used structures

  Attack scenario to steal confidential information (c) Subset of a global attack

Figure 2 .

 2 Figure 2. Topology and attack trees

Figure 4 .

 4 Figure 4. Actors identification

Figure 5 .

 5 Figure 5. Action tree enriched with observers

Figure 6 .

 6 Figure 6. Correlation tree

http://capec.mitre.org/

http://cee.mitre.org

This work was partially funded by the European project Panoptesec (FP7-GA 610416).