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Research ArticleReceived: 15 July 2009 Revised: 14 September 2009 Accepted: 14 September 2009 Published online in Wiley Interscience: 12 January 2010

(www.interscience.wiley.com) DOI 10.1002/jctb.2331

Silicone oil: An effective absorbentfor the removal of hydrophobic volatile organiccompoundsGuillaume Darracq,a,b∗ Annabelle Couvert,a,b Catherine Couriol,a,b

Abdeltif Amrane,a,b Diane Thomas,c Eric Dumont,d Yves Andresd andPierre Le Cloireca,b

Abstract

BACKGROUND: Hydrophobic volatile organic compounds (VOCs), such as toluene, dimethyl sulfide (DMS) and dimethyl disulfide(DMDS), are poorly soluble in water and classical air treatment processes like chemical scrubbers are not efficient. An alternativetechnique involving an absorption step in an organic solvent followed by a biodegradation phase was proposed. The solventmust fulfil several characteristics, which are key factors of process efficiency, and a previous study allowed polydimethylsiloxane(or PDMS, i.e. silicone oil) to be selected for this purpose. The aim of this paper was to determine some of its characteristicslike absorption capacity and velocity performances (Henry’s constant, diffusivity and mass transfer coefficient), and to verifyits non-biodegradability.

RESULTS: For the three targeted VOCs, Henry’s constants in silicone oil were very low compared to those in water, and solubilitywas infinite. Diffusivity values were found to be in the range 10−10 to 10−11 m2 s−1 and mass transfer coefficients did not showsignificant differences between the values in pure water and pure silicone oil, in the range 1.0 × 10−3 to 4.0 × 10−3 s−1 for allthe VOCs considered. Silicone oil was also found to be non-biodegradable, since its biological oxygen demand (BOD5) valuewas zero.

CONCLUSION: Absorption performances of silicone oil towards toluene, DMS and DMDS were determined and showed that thissolvent could be used during the first step of the process. Moreover, its low biodegradability and its absence of toxicity justifyits use as an absorbent phase for the integrated process being considered.c© 2010 Society of Chemical Industry

Keywords: absorption; silicone oil (PDMS); hydrophobic VOC; mass transfer coefficient; diffusivities; biodegradation

INTRODUCTIONSince several world conferences such as New York (1997) or Kyoto(1998), which highlighted the influence of anthropogenic VOCemissions on the environment and the climate imbalance, airtreatment has become an important research topic. Hydrophiliccompounds can usually be removed by chemical scrubbing,biological treatment (biofilter) or thermal oxidation. But theseprocesses are not really efficient for hydrophobic compoundslike toluene, dimethyl sulfide (DMS) and dimethyl disulfide(DMDS), which have low olfactory thresholds, 8.2 mg m−3, 1.5and 0.1 µg m−3, respectively, according to Hartikainen et al.,1 andcan be toxic for humans at low levels.1

An alternative process (Fig. 1), which consisted in coupling anabsorption step involving an organic solvent2 and a biodegra-dation step has been developed. The aim was to achieve theconsumption of the VOCs and the regeneration of the absorbentphase. In 1991, Bruce and Daugulis set up a method to choose thebest organic phase for implementation in a multiphase bioreactor.3

The solvent must fulfil several characteristics: high absorption ca-pacity and velocity performances (Henry’s constant, diffusivityand mass transfer coefficient), low emulsion-forming tendency,

low solubility in water, good chemical and thermal stabilities,non-biodegradability, biocompatibility and, of course, cheapness.Silicone oil (polydimethylsiloxane or PDMS) was used by severalauthors4,5 as a pollutant reservoir and seemed to be suitable.Hayachi et al.6 and Matsumoto et al.7 showed that the partitioningcoefficient octanol/water (log Kow) allowed to check the toxicity of asolvent towards microorganisms. Since log Kow of silicone oil is 4.25and silicone oil is not soluble in water, this solvent could be consid-

∗ Correspondence to: Guillaume Darracq, Ecole Nationale Superieure de Chimiede Rennes, CNRS, UMR 6226, Avenue du General Leclerc, CS 50837, 35708Rennes Cedex 7, France. E-mail: guillaume.darracq@ensc-rennes.fr

a Ecole Nationale Superieure de Chimie de Rennes, CNRS, UMR 6226, Avenue duGeneral Leclerc, CS 50837, 35708 Rennes Cedex 7, France

b Universite europeenne de Bretagne, 35000 RENNES, France

c Faculte Polytechnique de Mons, 56, Rue de l’Epargne, B-7000 Mons, Belgium

d UMR CNRS 6144 GEPEA, Ecole des Mines de Nantes, La Chantrerie, 4 rue AlfredKastler, B.P. 20722, 44307 Nantes Cedex 3, France

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Organic phase recyclingGas outlet

Organic phase inlet

Gas-liquidcontactor

Bioreactor

Settler

Organic phase

Aqueous phase +Activated sludgeOrganic phase

outlet + VOCs

Gas inlet

Air flow

Emulsionwater/solvent

Figure 1. Hybrid absorption–biodegradation process with regeneration of the organic phase.

ered as biocompatible. The aim of this work was to measure severalparameters such as dimensionless Henry’s constants (H), diffusivi-ties (DL) and global mass transfer coefficients (KLa) for each VOC inorder to characterise the absorption performances of silicone oil.

MATERIALS AND METHODSHenry’s constant and global mass transfer coefficientIn 2003, Roustan described the physical absorption between agaseous compound and a liquid phase8 (Equation 1). Mass transferis governed by the driving force (i.e. the difference between thepollutant concentrations in both phases).

dN = KLadV(CEL − CL) (1)

where dN (in mol s−1) is the transferred amount; dV (in m3) is thevolume element; KL (in m s−1) is the global liquid mass transfercoefficient; a (in m2 m−3) is the volumetric interfacial area; CL is thepollutant liquid concentration; and CE

L (in mol m−3) is the pollutantliquid concentration at the equilibrium with the gas phase givenby Equation 2:

CEL = p

H′′ = CGRT

H′′ (2)

where p (in Pa) is the VOC partial pressure; R is the gas constant(8.314 Pa.m3.mol−1.K−1); T is temperature (K); and H′′ Henry’sconstant (Pa.m3.mol−1).

Dimensionless Henry’s constants, H, were obtained using a staticmethod. A known quantity of solvent (or water) was introducedinto a specific flask (vial), whose exact volume was measured.After closing it and making it gastight, a known quantity of VOCswas added through the septum. The vial was shaken for 3 days at25 ◦C by using a swivel support. Once the equilibrium was reached(checking of the stability of the gaseous VOC concentration), theVOC concentration in the gas phase (CG) was determined bygas chromatography (flame ionisation detector type focus). Theconcentration in the liquid phase (CE

L ) was then deduced from themass balance and the dimensionless constant, H, was calculatedby using Equation 2.

The gas chromatography methods used in this work are shownin Table 1.

Table 1. Analytical conditions implemented to determine the VOCconcentration in the gas phase

Pollutant Oven temperature, Toven Carrier gas

Toluene 100 ◦C (1.5 min)20 ◦C min−1 → 180 ◦C(0 min)

N23.7 mL min−1

Dimethyl disulfide 100 ◦C (1.4 min)50 ◦C min−1 → 200 ◦C(3 min)

N23.3 mL min−1

Dimethyl sulfide 50 ◦C (1.2 min)50 ◦C min−1 → 200 ◦C(3 min)

N23.3 mL min−1

For all samples, the following equipment and conditions were used:Thermo Focus GC with an RTx-1.15 m ×0.32 mm column and FID. Theinjector temperature (Tinj) was 150 ◦C and the detector temperature(Tdet) was 250 ◦C.

For measurements of KLa, the absorption of toluene, DMS andDMDS in water and in silicone oil was independently studied inlaboratory-scale reactors at a constant gas flow rate (1 m3 h−1).A dynamic method9 was used to determine the volumetricmass transfer coefficients of pollutants during absorption. In thismethod, a known air volume (VG =215 L) loaded with the pollutantwas continuously flowed via a circulating loop through a watervolume (VL = 1 L) at 25 ◦C. The operation was batchwise withrespect to the liquid system and the decrease in time course of thepollutant concentration (toluene, DMS or DMDS) in the gas phasewas monitored.

DiffusivityA specific mathematical model was developed to simulate the VOCabsorption into viscous solvents in a vertical wetted-wall columnwith a co-current gas–liquid downflow. Considering a plane flow,the liquid stream was considered as laminar and exempt of ripplesfor Reynolds numbers lower than 4, requiring a low liquid flowrate if the viscosity was not so high.10

In these hydrodynamic conditions, the liquid velocity profilewas parabolic, with a well-known estimation of the liquid film

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thickness. The model involved a general equation describing thesteady-state diffusion of the VOC from the interface into thediffusion falling film, with adequate boundary conditions, anda classical mass balance between the top of the column and agiven level, used to compute the average VOC concentration inthe liquid film along the column height. More details, includingequations and assumptions, concerning the model can be foundin a previous paper.11

The simulation of the absorber required the knowledge of:

• The gas-phase mass-transfer characteristics in the column.For sake of precision, this coefficient was not estimatedfrom literature correlations12 but experimentally measuredby absorption of n-pentylbenzene; a classical correction of thevalue taking into account the ratio of the corresponding gasphase diffusivities can be applied. The diffusivities of the VOCin the gaseous phase were estimated thanks to the correlationof Fuller et al.13

• The physicochemical properties of the silicone oil. The density wasexperimentally determined using a densimeter; the viscosity(mPa s) was experimentally measured using a falling sphereviscometer in the range 15–30 ◦C:

ηSO = 7005.3

T− 19.4 (3)

• The Henry’s constants of the absorbed VOCs.Starting from the top, the program finally provided the VOCoutlet gas concentration. Computation of DL was made byminimizing for each absorption test run the deviation betweenexperimental and calculated VOC outlet gas concentrations.Absorption experiments were achieved in an experimentalset-up including:

• A scrubber. The VOC scrubber was a small glass wetted-wallcolumn of inside diameter 0.02 m with an effective heightof 0.66 m, characterised by a co-current contacting modebetween liquid and gas streams entering at the top and leavingat the bottom of the column. An external jacket flowed by athermostated fluid allowed to keep constant the temperaturein the absorber.

• A gas supply. A precise rate of VOC was injected by a syringedispenser in the air stream, supplied by a compressor, whoseflow rate was measured and controlled by a gas mass-flowcontroller. Valves allowed to direct the mixture toward theby-pass; this system was useful for the determination of theinitial VOC concentration in the inlet gas or the wetted-wallcolumn during absorption test runs. The exit gas was ventedout through a laboratory fume hood.

• A liquid supply. The liquid flow rate was fed by using a gearpump and regulated by a mass-flow controller. The liquidphase, namely the silicone oil, was pumped from a container,previously ran through heat exchange coils immersed in thethermostatic bath and flowed at the inner side of the column.

• A gas sampling part. The gas analysis, upstream and down-stream of the wetted-wall absorption column, was performedby a flame ionisation detector (FID). A certified standard(nitrogen–propane gas mixture) was used for the calibrationof the FID.

Inlet and outlet liquid and gas temperatures were registered bytemperature sensors.

The system was allowed to reach a steady-state indicated by nosignificant changes of average temperatures for liquid and gasphases, gas flow rate G, liquid flow rate L and VOC inlet and outlet

gas concentrations, cG,in and cG,out, respectively. Experimentalinvestigations were conducted for typical operating conditions:L = 0.4 kg h−1, G = 1 N m3 h−1, temperature = 25 ◦C and cG,in =650–1500 ppm. Data collected were required for interpretationand estimation of liquid diffusivities of VOC into viscous solvents.More details on the experimental apparatus and procedure can befound in Bourgois et al.11

BiodegradabilityBiodegradability was deduced from measurements of thebiological oxygen demand (BOD5), carried out in Oxitop IS6 (WTW,Ales, France). Activated sludge from a wastewater treatment plantwas used to inoculate samples, which consisted in a solution of80 mg.L−1 of silicone oil in water, and the control solution. Theinitial bacterial concentration was 0.5 g L−1.

The following mineral basis was used for all experiments (g L−1):MgSO4· 7H2O, 22.5; CaCl2, 27.5; FeCl3, 0.15; NH4Cl, 2.0; Na2HPO4,6.80; KH2PO4, 2.80.

The BOD5 value was initially estimated based on the chemicaloxygen demand (COD) value experimentally measured by meansof a Test Nanocolor CSB 160 (Macherey–Nagel, Duren, Germany)or calculated, BOD5 = COD/1.46. The range of expected BOD5

measurements was then deduced and hence led to the volumes ofsample, of activated sludge solution and of nitrification inhibitor(10 mg L−l solution of N-allylthiourea) which have to be added tothe shaker flask of the Oxitop apparatus.

A similar protocol was applied for the control flask except thatit was replaced by a solution of easily biodegradable compounds,namely glutamic acid (130 mg L−1) and glucose (130 mg L−1). Be-fore use, KOH was added to achieve neutral pH (7.0±0.2). A similarprotocol was also considered for the blank solution, for which thesample was replaced by water to have a negligible BOD5 value.

RESULTS AND DISCUSSIONHenry’s constantsHenry’s constants in silicone oil (H′′

SO) and in water (H′′w) are

reported in Table 2. The three selected VOCs were preferentiallyabsorbed in silicone oil. Moreover, since each VOC has infinitesolubility in silicone oil, its absorption should be improved usingsilicone oil in a gas–liquid contactor. Henry’s constants werealso measured in a silicone oil/water emulsion to check for theabsence of VOC stripping in a two-phase partitioning bioreactor.The experimental value was then compared to the theoreticalvalue calculated by using Equation 4:

1

H′′mixture

= 1

H′′SO

x + 1

H′′w

(1 − x) (4)

where H′′SO is Henry’s constant of VOC in pure silicone oil; H′′

w isHenry’s constant of VOC in pure water; and x is the volumetricfraction of silicone oil in emulsion.

Results are reported in Fig. 2 for toluene. Since the relativeerror was below 15%, Henry’s constant for toluene in an oil/wateremulsion could be estimated by Equation 4.

In the case of toluene, the H′′SO value appeared very low if

compared to the H′′w value; hence the term 1

/H′′

w can be neglectedtowards 1

/H′′

SO and Equation 4 led to:

H′′mixture

∼= H′′SO

x. (5)

According to Equation 5, H′′mixture increases when the volumetric

fraction of silicone oil in emulsion decreases. This equation allows

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Table 2. Henry’s constants of the selected VOCs in silicone oil and inwater

Henry’s constant (Pa m3 mol−1)

Pollutant in silicone oil, H′′SO in water, H

′′w

Toluene 1.6 609∗

Dimethyl disulfide 2.3 119

Dimethyl sulfide 41.6 124

∗ Taken from Staudinger and Roberts.14

Figure 2. Henry’s constants of toluene in a silicone oil/water emulsion.Comparison between experimental and calculated values. (�) H′′ deter-mined by experiment; (�) H′′ calculated value.

Figure 3. Henry’s constants of dimethyl disulfide in a silicone oil/wateremulsion. Comparison between experimental and calculated values. (�)H′′ determined by experiment; (�) H′′ calculated value.

an easy estimation of the minimal volumetric fraction necessaryfor an efficient transfer. Results for DMDS, reported in Fig. 3, leadto the same conclusions.

For DMS, experimental results (Fig. 4) differed from thosecalculated by means of the theoretical Equation 4. A differentbehaviour for DMS if compared to the other VOCs was previouslyshown by Vuong et al.,15 who have calculated Henry’s constants forDMS in other organics solvents. However, preferential absorption

Figure 4. Henry’s constants of dimethyl sulfide in a silicone oil/wateremulsion. Comparison between experimental and calculated values. (�)H′′ determined by experiment; (�) H′′ calculated value.

Table 3. Global mass transfer coefficients of VOCs in silicone oil andin water

Pollutant KLa in pure water (s−1) KLa in pure silicone oil (s−1)

Toluene 1.4 × 10−3 1.6 × 10−3

Dimethyl sulfide 4.0 × 10−3 4.0 × 10−3

Dimethyl disulfide 1.8 × 10−3 1.6 × 10−3

of DMS in silicone oil, rather than in water, similarly to toluene andDMDS, was demonstrated.

Global mass transfer coefficientThe results summarised in Table 3 show that there was nosignificant difference between the values of KLa in pure waterand pure silicone oil. However, owing to the higher affinity ofpollutants for silicone oil than for water, higher KLa values wereexpected for silicone oil than for water. The dynamic viscosity ofthe silicone oil used, which was five times higher than that of water,could account for this unexpected result. Indeed, KLa values inbioreactors have generally to be correlated with the combinationof stirrer speed (N), superficial gas velocity (V) and viscosity of theliquid (η) according to the equation

KLa = CVα

(P

VL

)β

ηδ (6)

where C, α, β and δ are empirical constants depending onboth hydrodynamic conditions and geometrical parameters ofthe vessel and stirrer used, and (P

/VL) is the power input per liquid

volume.Since all the operating conditions (V , P, VL) were maintained

constant during the experiments, it should be assumed that thebenefit in using silicone oil to absorb pollutants more rapidly wascancelled by its dynamic viscosity. Nevertheless, the use of siliconeoil enabling the absorbtion of large amounts of pollutant shouldbe beneficial to increase the mass transfer rate.

Modelling of VOC absorption into silicone oil for the estima-tion of the liquid diffusivitiesThe liquid diffusivities resulting from these absorption testsachieved in the wetted wall column using the experimental

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Table 4. Liquid diffusivities of VOCs in silicone oil

SampleCG,in

(g N m−3)CG,out

(g N m−3) DL (25 ◦C) (m2 s−1)

Toluene 1.630 1.286 5.65 × 10−11

Dimethyl sulfide 2.615 2.486 Not given because of veryhigh H′′

Dimethyl disulfide 3.277 3.026 8.72 × 10−12

Table 5. COD and BOD5 for silicone oil in water (approx. 80 mg L−1)

Sample

Control flask (withgutamic acidand glucose)

Siliconeoil 1

Siliconeoil 2

COD (mg O2 L−1) – 116 117

BOD5 (mg O2 L−1) withendogenous breath

205 51 54

BOD5 (mg O2 L−1) withoutendogenous breath

121 0 0

Endogenous breath (mg O2 L−1) 84

procedure described above were in the range of 10−11 to10−10 m2 s−1. Absorption tests results and diffusivities values aregiven in Table 4.

A direct comparison with other studies was not possible, sinceno details of a similar VOC–silicone oil system was found inthe available literature. The comparison of these values withthose calculated with semi-empirical formulae (Wilke and Chang,Scheibel, Fedors)8 confirmed that they cannot be used to predictthe VOC diffusivity in viscous solvents.

BiodegradabilityThe results given in Table 5 are the average of two experimentswith two control flasks (glutamic acid and glucose) or twoexperiments with four samples (silicone oil in water). After 5 daysof culture, a ratio of 4 was found for the BOD5 values found in thecontrol flask on those found in presence of silicone oil (samples).However, the increase of the biological oxygen demand in theflasks containing silicone oil resulted from endogenous respirationby bacteria as shown in Table 5. At this concentration, siliconeoil was therefore non-biodegradable and non-toxic towardsmicroorganisms.

Moreover, during preliminary batch cultures, some analyseson the aqueous and gas phases were carried out by gaschromatography and UPLC/MS/MS. No silicone oil biodegradationby-products (dimethylsilandiol, trimethylsilandiol or silanol)16

were recorded in the aqueous phase, showing that silicone oilwas not assimilated by microorganisms.

CONCLUSIONSilicone oil seems to fulfil most of the required characteristics andwill subsequently be tested for hydrophobic VOC removal in anintegrated process coupling absorption and biodegradation.

For the three VOCs, the Henry’s constants in pure water weresignificantly higher than in pure silicone oil, leading to ratios of

400, 53 and 3 for toluene, DMDS and DMS, respectively. Siliconeoil appeared to be a good absorbent since it efficiently solubilisesDMDS and toluene. The results obtained for DMS were a little lessinteresting, most likely due to its high volatility and then to lossesduring experiments. This could explain the non-significant resultsrecorded for various emulsion ratios, while the Henry’s constant inpure silicone oil was high.

The results obtained for diffusivities in silicone oil showed thatDL values can be determined for two VOCs and were in the rangeof 5.65 × 10−10 and 8.72 × 10−11 m2 s−1 for toluene and DMDS,respectively. For DMS, the liquid diffusivity in silicone oil could notbe determined, owing to the high value of Henry’s constant.

Results for the global mass transfer coefficient did not show asignificant gap between the absorption in water and in siliconeoil. The rough estimate for the three selected VOCs was between1.6 × 10−3 and 4.0 × 10−3 s−1.

Finally, no biodegradability of silicone oil and the lack of toxicityjustify its use as an absorbent phase for the integrated processconsidered.

REFERENCES1 Hartikainen T, Martikainen PJ, Olkkonen M and Ruuskanen J, Peat

biofilters in long-term experiment for removing odorous sulphurcompounds. Water Air Soil Pollut 133:335–348 (2002).

2 Daugulis AJ and Boudreau NJ, Removal and destruction of high con-centrations of gaseous toluene in a two-phase partitioning bio-reactor by Alcaligenes xylosoxidans. Biotechnol Lett 25:1421–1424(2003).

3 Bruce LJ and Daugulis AJ, Solvent selection strategies for extractivebiocatalysis. Biotechnol Prog 7:116–124 (1991).

4 Ascon-Cabrera MA and Lebeault JM, Cell hydrophobicity influencingthe activity/stability of xenobiotic-degradind microorganisms in acontinuous biphasic aqueous–organic system. J Ferment Bioeng80:270–275 (1995).

5 Bouchez M, Blanchet D and Vandecasteele JP, Substrate availabiltiy inphenanthrene biodegradation: transfer mechanism and influenceon metabolism. App Microb Biotechnol 43:952–960 (1995).

6 Hayachi S, Kobayashi T and Honda H, Simple and rapid cell growthassay using tetrazolium violet coloring method for screening oforganic solvent tolerant bacteria. J Biosci Bioeng 96:360–363 (2003).

7 Matsumoto M, Mochiduki K and Kondo K, Toxicity of ionic liquids andorganic solvents to lactic acid-producing bacteria. J Biosci Bioeng98:344–347 (2004).

8 Roustan M, Lavoisier, Transfert gaz-liquide dans des procedes detraitement des eaux et des effluents gazeux, TEC&DOC, Paris (2003).

9 Dumont E, Andres Y and Le Cloirec P, Mass transfer coefficients ofstyrene and oxygen into silicone oil emulsions in a bubble reactor.Chem Eng Sci 61:5612–5619 (2006).

10 Bird RB, Stewart WE and Lightfoot EN, Transport Phenomena, 7thedition. John Wiley & Sons, New York (1966).

11 Bourgois D, Vanderschuren J and Thomas D, Determination of liquiddiffusivities of VOC (paraffins and aromatic hydrocarbons) inphthalates. Chem Eng Proc 47:1357–1364 (2008).

12 Crause JC and Nieuwoudt I, Mass transfer in a short wetted-wallcolumn. 1. Pure components. Ind Eng Chem Res 38:4928–4932(1999).

13 Perry RH and Green D, Perry’s Chemical Engineers’ Handbook, 7thedition. McGraw-Hill, New York (1997).

14 Staudinger J and Roberts PV, A critical review of Henry’s law constantsfor environmental applications. Crit Rev Environ Sci Technol26:205–297 (1996).

15 Vuong MD, Couvert A, Couriol C, Amrane A, Le Cloirec P and Renner C,Determination of the Henry’s constant and the mass transfer rateof VOCs in solvents. Chem Eng J 150:426–430 (2009).

16 Xu S, Lehmann R, Miller J and Chandra G, Degradation of poly-dimethylsiloxanes (silicones) as influenced by clay minerals. EnvironSci Technol 32:1199–1206 (1998).

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