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Rapid Removal of Organics and Oil Spills from Waters Using Silicone Rubber“Sponges”Insun Park a; Kirill Efimenko a; Johan Sjöblom b; Jan Genzer ab

a Department of Chemical and Biomolecular Engineering, North Carolina State University, Raleigh, NorthCarolina, USA b Ugelstad Laboratory, Institute of Chemical Engineering, Norwegian University of Science &Technology, Trondheim, Norway

Online Publication Date: 01 March 2009

To cite this Article Park, Insun, Efimenko, Kirill, Sjöblom, Johan and Genzer, Jan(2009)'Rapid Removal of Organics and Oil Spills fromWaters Using Silicone Rubber “Sponges”',Journal of Dispersion Science and Technology,30:3,318 — 327

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Rapid Removal of Organics and Oil Spills from WatersUsing Silicone Rubber ‘‘Sponges’’

Insun Park,1 Kirill Efimenko,1 Johan Sjoblom,2 and Jan Genzer1,2

1Department of Chemical and Biomolecular Engineering, North Carolina State University, Raleigh,North Carolina, USA2Ugelstad Laboratory, Institute of Chemical Engineering, Norwegian University ofScience & Technology, Trondheim, Norway

We have developed a simple, robust, and efficient technology utilizing cheap and recoverablematerials based on commercially available silicone elastomer networks for removing organic sol-vents and crude oil from waters. Hydrophobic and oleophilic properties of silicone elastomersendow poly(dimethyl siloxane) (PDMS) with the capacity to absorb a large variety of organics,including benzene (B), toluene (T), ethylbenzene (E), and xylene (X), commonly referred to asBTEX, and also crude oils, while at the same time enabling the organic ‘‘sponges’’ to float onwaters, which facilitates straightforward handling. We developed a method for generating PDMSparticles with variable sizes (ranging from hundreds nanometers to few millimeters) by drop-wisedepositing siloxane/cross-linker mixtures into warm water, a process which leads to the cross-linking of the PDMS components. We have tested the capability of the PDMS particles toremove toluene and benzene from water. We also performed similar experiments by utilizingPDMS sheets. In both instances we observed a rapid sorption of the organic phase into PDMS;the amount of absorbed organic solvent depended on the concentration in water and the total mass(volume) of PDMS and did not depend on the geometry of the PDMS ‘‘sponge.’’ In addition, wehave examined the uptake of toluene and benzene from toluene/benzene (T/B) mixtures dissolvedin water. Our results indicate that the amount of benzene absorbed from the T/B mixtures intoPDMS increases relative to the uptake from pure benzene/water solutions. This behavior is asso-ciated with toluene acting as a ‘‘surfactant’’ that effectively replaces the more unfavorablePDMS/B contacts with less costly T/B ones. Finally, a simple experiment demonstrates qualita-tively that PDMS is also capable of removing crude oils from oil-contaminated waters.

Keywords Water purification, removal of organic from water, BTEX, PDMS, sylgard-184

INTRODUCTION

Oil constitutes one of the major sources of contamina-tion of waters. For instance, in 2000, produced watertotaled about 150 million m3 and approximately 3,500metric tons of oil (not including spills) was unintentionallydischarged to the sea.[1] Materials that are utilized in recov-ery and containment of oil spills typically involve a varietyof booms, barriers, and skimmers, as well as sorbent mate-rials. Being oleophilic, these materials readily absorb oiland can be, after application, disposed off. In addition to

crude oil, the removal of small organic molecules fromwater, such as organic solvents that accompany oil anddissolve in water is very important. Oil contains relativelylarge quantities of semi-soluble organic hydrocarbons,including: benzene, toluene, ethylbenzene, and xylene(commonly referred to as BTEX), polycyclic aromatichydrocarbons, naphtalenes, phenantrenes, and diben-zothiophenes, organic acids, phenol and alkylatedphenols.[2] Especially volatile organic compounds, such asBTEX, belong among chief environmental contaminantson account of their toxicity and widespread occurrence.The U.S. Environmental Protection Agency has includedBTEX compounds on the list of National PrimaryDrinking Water Standards and set a maximum contami-nant level (MCL) of 5 mg=L for benzene and 0.7–10 mg=Lfor the other BTEX compounds.[3]

Over the past few years, substantial effort has beendevoted to decrease the pollution of waters byBTEX.[4–8] Activated carbon adsorption systems havebeen identified as being among of the most effective

Received 19 November 2007; accepted 24 November 2007.The work at NCSU was supported in part by the grand from

the Korean research foundation. J. S. gratefully acknowledges theTOP WATER Program financed by industry and NorwegianResearch Council.

Address correspondence to Jan Genzer, Department ofChemical and Biomolecular Engineering, North Carolina StateUniversity, Raleigh, North Carolina 27695-7905, USA. E-mail:[email protected]

Journal of Dispersion Science and Technology, 30:318–327, 2009

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ISSN: 0193-2691 print=1532-2351 online

DOI: 10.1080/01932690802540384

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technologies for the removal of volatile organic compoundscompared with impractical traditional methods such asflocculation, sedimentation, and filtration.[9–12] It is,however, recognized that activated carbon suffers froma number of drawbacks, including, slow adsorptionkinetics, poor selectivity, the need for expensive contain-ment systems, and limited working capacity. Anotherproblem is the lack of design flexibility when usinggranulated carbons. To address some of these disadvan-tages, phenolic-based activated carbon fibers have beendeveloped, which demonstrated improved contact effi-ciency with the media, leading to greater rates of adsorp-tion, much higher surface areas, and potential for greatlysimplified in-situ regeneration through electrical resis-tance heating. However, higher cost and not very durablehandling limit wider application of these structures. Inspite of some success in developing methods for effectiveremoval of BTEX from waters, it is this vital to keepdeveloping alternative new methodologies for removingBTEX. In this article, we offer one such method, whichis based on utilizing silicone elastomers networks as‘‘sponges’’ for organic impurities and oils.

Our goal is to develop a simple, robust, and efficienttechnology utilizing low cost and recoverable materialsfor removing organic impurities including crude oil spills.In order to accomplish this goal, we identified cross-linkedpoly(dimethyl siloxane) (PDMS) as adsorbent material(stationary phase). PDMS satisfies our prerequisites: it iscommercially available, cheap, and it possesses a high affi-nity toward organic solvents (such as those present inBTEX) and oils. In addition, PDMS offers the potentialpossibility of recovering the organic solvents after absorp-tion thus facilitating reusing the stationary phase. Eventhough the strong affinity between BTEX and PDMS hasbeen well known,[13–15] the use of PDMS as alternative

adsorbent material to remove BTEX and oils from waterdirectly was not utilized widely. In our experiments we con-centrated on benzene and toluene, which possess the high-est solubility and abundance in water among all BTEXcomponents (cf. Table 1). While most experimental set-ups utilized to date have concentrated on investigatingthe removal of organics from waters by means of flowvia a column packed with the absorbing stationary phase,we have pursued an alternative route involving absorptionfrom standing waters. We appreciate that this method hassome limitations (further noted below); our goal was todevelop understanding of uptake of organics in situationswhere one does not have the capability of pumpingcontaminated waters through a column.

The amount of absorbed organic solvent in the PDMSwas determined from comparing the composition of thewater solution of the organic solvents both before andafter absorption using UV=Vis spectroscopy. In additionto determining the total absorbed amount of toluene andbenzene from water (equilibrium values) and calculatingthe corresponding partition coefficients, we attemptedto get a glimpse at the kinetic of absorption of these sol-vents in PDMS. The effects of the size and the shape ofPDMS phase as well as their total amount on absorptionwere rigorously studied. We studied competitive absorp-tion from benzene=toluene mixtures as a function of thesolute concentration in waters and observed that the pre-sence of toluene increases the amount of benzene absor-bed by the PDMS phase. We explain this observation byinvolving the solubility parameters of PDMS and bothorganic solvents and show that toluene acts as a‘‘compatibilizer’’ between PDMS and benzene. Finally, wehave performed a quick visual experiment demonstratingthe PDMS particles are capable of removing oil spillsfrom standing waters.

TABLE 1Selected characteristics of BTEX components

CompoundWater solubilitya

(mg=L)db

(cal=cm3)1=2Tb

c

(�C)p�d

(kPa)re

(mJ=m2)

Benzene 1740–1860 9.2 80.1 15.9 22.9Toluene 500–627 8.9 110.6 4.9 28.4Ethylbenzene 131–208 8.8 136.2 1.7 29.2o-xylene 167–196 8.8 144.4 1.2 30.1m-xylene 167–196 8.8 139.1 1.5 28.9p-xylene 167–196 8.8 138.4 1.5

aRefs.[18,19]

bRef.[20]

cRef.[21]

dVapor pressure at 30�C, calculated from Antoine equation using coefficients given in ref.[22]

ehttp://www.surface-tension.de

RAPID REMOVAL OF ORGANICS AND OIL SPILLS 319


EXPERIMENTAL

Toluene, benzene, and tetrahydrofuran (THF) werepurchased from Fisher. The PDMS network was preparedfrom commercially available kit (Sylgard-184) accordingto the procedure specified by the manufacturer (DowCorning). Specifically, the components A and B from thekit were mixed in specified properties, vigorously and airbubbles introduced during mixing were removed byvacuum suction. The PDMS particles were fabricated bythe procedure illustrated in Figure 1. Specifically, thePDMS mixture was diluted with THF (30%, w=v) anddeposited drop-wise using a syringe pump into stirred dis-tilled water containing surfactant, polyethylene glycol sor-bitan monolaurate. By heating the emulsion in water, thePDMS network was formed by cross-linking the PDMSchains. By adjusting the concentration of THF, the viscos-ity of the PDMS solution, and thus the size of the particles,can be adjusted; additional control can be achieved byvarying the stirring speed and the injection rate of thePDMS=THF mixtures. The cross-linked PDMS networkparticles were filtered and dried in vacuum at room tem-perature for 1 day. In addition, PDMS sheets were fabri-cated by pouring pre-mixed PDMS solutions into plasticPetri dishes and letting them cure at ambient temperaturefor 1 day. The thickness of the PDMS sheets was �1mm.

The adsorption experiments were performed by addingthe PDMS particles or PDMS sheets into the aqueoussolution of toluene or benzene or mixtures containing bothcomponents of varying concentration of the organic phase.The volume of the aqueous phase was held constant

(10 mL). The solution was stirred vigorously and theabsorbed amount of the organic phase was determinedby measuring the UV=Vis spectra of the aqueous phasebefore and after absorption. In the case of particles, thesolution was filtered with Millipore, PVDF Durapore fil-ters (pore size 0.22 mm) to remove the particles before theUV=Vis spectroscopy measurement.

RESULTS AND DISCUSSION

We employed UV=Vis spectroscopy to measure theconcentration of the organic phase in waters. By perform-ing the UV=Vis experiments both before and after theabsorption of the organics into the PDMS we were ableto determine the amount of the absorbed organic phasein PDMS, assuming that no evaporation of the organicphase took place during the course of the experiment. Wewill return to the issue of solvent evaporation later in thepaper. The presence of benzene ring in toluene and benzeneprovides a convenient natural tag for the UV=Vis experi-ments. The intensity of the resultant peaks in the UV=Visspectra (occurring at wavelengths between approximately200 and 300 nm) corresponds to the concentration of thecompound in the medium. In order to use UV=Vis to deter-mine quantitatively the amount of the organic phase inwater, we generated calibration curves; these involved mea-suring the UV=Vis absorbance for solutions of benzene andtoluene in water having different concentrations of theorganic phase. The main parts of Figures 2 and 3 depictthe UV=Vis spectra of aqueous toluene and benzenesolutions having known concentrations. In the insets to

FIG. 1. Schematic illustrating the formation of silicone elastomer (SE) particles. A surfactant (polyethylene glycol sorbitan mono-laurate) is added

into bath containing deionized water at room temperature and the mixture is agitated vigorously (a). A mixture of SE, cross-linker, and a catalyst is

deposited drop-wise into the vessel (b). The bath is heated to about 70�C and the SE breaks into small droplets that are stabilized with the surfactant.

Heat promotes the cross-linking reaction and stabilization of SE particles thus formed (c). After formation, the particles are dried and stored. SE

particles do not coagulate.

320 I. PARK ET AL.


Figures 2 and 3 we plot the calibration curves, which wereobtained by multiplying the individual UV=Vis spectra asto match the one measured for solutions having a concen-tration of 100 ppm of the solutes in water. The calibrationplots show that the scaling factor depends linearly on con-centration of the organic phase in water (up to at least 40mL of toluene in 100 mL H2O and 150 mL of benzene in100 mL H2O), as predicted by the Beer’s law.

Because of their relatively low boiling point and low sur-face tension (cf. Table 1), the organic solvent may evapo-rate from the aqueous solution with time thus changingthe original composition of the solution. We measuredthe UV=Vis spectra of toluene and benzene solutions hav-ing initial concentration of 20 mL=10 mL H2O as a functionof time. Our results (cf. Figure 4) indicate that while theconcentration of benzene in the solution remains constantfor at least 3 days, the concentration of toluene in waterdecreases rapidly during the first 24 hours and then satu-rates. At first, this result seems to be somehow counterin-tuitive, given the fact that the surface tension and normalboiling point of benzene are lower than that of toluene(cf. Table 1). The observed behavior can be explained byconsidering that the solubility of benzene in water is about3 times that of toluene, hence, benzene will not evaporateso rapidly from the mixture as toluene although it has morethan three times higher vapor pressure at 30�C (cf. Table1). Importantly, because the evaporation of toluene duringthe first 24 hours is smaller than 10%, we neglect it in outfuture experiments and assume that the concentration ofthe original solution remains the same. Because there isno more evaporation of toluene after 2 days tells that thereis not any significant leakage from the vial and the systemreaches to the saturated state of the organic phase.

In order to assess how fast the adsorption reaches toequilibrium state, we performed a simple kinetic test. Asshown by the data in Figure 5, the concentration of toluenedecreases very rapidly with increasing time and reaches anequilibrium state within the first 2 hours. The conclusion isindependent of the shape of the stationary phase; both par-ticles and sheets absorb the organic phase rather quickly.The concentration of the solute in water does not change

FIG. 4. UV=Vis spectra monitoring the concentration of benzene and

toluene in water as a function of time.FIG. 2. UV=Vis curves for toluene in water as a function of toluene

concentration. The inset shows a calibration curve obtained by scaling

the UV=Vis curves with respect to the solution having a concentration

of 100 ppm of toluene (obtained by dividing the corresponding concentra-

tion by that at 100 ppm).

FIG. 3. UV=Vis curves for benzene in water as a function of benzene

concentration. the inset shows a calibration curve obtained by scaling the

UV=Vis curves with respect to the solution having a concentration of

100 ppm of benzene (obtained by dividing the corresponding concentra-

tion by that at 100 ppm).



even after 3 days, indicating that there is no additionalabsorption of toluene into or desorption from PDMS.Because of this rapid uptake of toluene, our additionalexperiments were performed by exposing the PDMS phaseto organic solutions for 24 hours. We assume that this timeperiod was sufficiently long in order for the system to reachan equilibrium.

The ability of PDMS to absorb organic solvents wastested for both particles and sheets and in toluene andbenzene solutions of varying initial concentrations. InFigure 6, we plot the concentration of the solute remainingin water solutions after exposing the liquid to a givenamount of PDMS phase for 24 hours. The trends revealedby the data indicate that there is a rapid absorption of theorganic phase into PDMS upon adding a minute amountof PDMS into the solution. Further increase in the amountof PDMS further reduces the concentration of the organicsolvent in waters but the decrease at large PDMS loading isnot as rapid as that at small amounts of the ‘‘sponge’’phase. In order to compare the uptake by PDMS particlesand sheets and to compare the absorption trends collectedfrom the solutions of all initial concentrations, in Figure 7we plot the normalized solute concentrations on a 0–100%scale and plot them against the volume of the PDMSphase. The trends in the data in Figure 7 reveal that fora given solvent, the data collapse on a unique master curve,thus removing the effect of the initial concentration. Thisobservation is important as it reveals that what controlsthe update of the organic phase into PDMS is not the sur-face area of the absorbing ‘‘sponge’’ but rather the totalvolume, which determines the overall ability to removethe organic solvents from waters. The data in Figure 8 pro-vide further evidence to the latter observation. Here we

plot the toluene concentration in the solute from solutionson approximately the same initial concentration of toluenein water as a function of the amount of PDMS, whichcomes in the form of particles of two different sizes (smallerparticle nominal size�100 mm, larger particle nominalsize�3 mm, as determined from optical microscopyimages) and sheets. As anticipated from the trends pre-sented by the data in Figure 7, all data in Figure 8 essen-tially collapse onto a single curve. While the observationthat the amount of organic solvent absorbed depends onthe volume (or mass) of the PDMS phase rather than itssurface area is important, there may be reasons why oneshape would be preferred over the other one in variousapplications. For instance, PDMS in the form of particleswould be the preferred shape as a stationary phase forseparation in columns. One can take additional benefit

FIG. 6. Concentration of solute, (a) benzene, and (b) and (c) toluene,

for different initial concentrations of the solute in water as a function of

the amount of the stationary phase (PDMS), which comes in the form

of (a) and (b) particles and (c) sheets. The absorption took place for

24 hours. The symbols denote various initial concentration of the organic

phase in water.

FIG. 5. Concentration of solute in water as a function of the exposure

time for toluene solutions in the presence of 100 mg of PDMS particles

and sheets.

322 I. PARK ET AL.


from the fact that such particles would have a relativelylarge surface area (compared to sheets having the samevolume of PDMS), which can be utilized in situationswhere chemical grafts on top of the PDMS stationaryphases are present in large quantities. In this context onecan think of generating complex hybrid materials com-prising PDMS particles grafted with chemically tailoredpolymer chains. The PDMS phase will serve as a ‘‘sponge’’for collecting organics and the polymer brush can be usedto collect other impurities from waters, such as heavymetals, etc. Compared to particles, sheets have much smal-ler surface area; hence, the density of possible grafts on thesurface (relative to the volume of the PDMS phase) is muchsmaller. However, sheets have the benefit of easy handlingand can used in producing absorbing coatings, etc.

A convenient way to determine the capability of extract-ing a solute from one phase into another is based onevaluating the so-called partition or distribution coefficient

for component j (Kj), which represents the ratio of concen-trations of solute j in the two phases of a mixture of twoimmiscible substances at equilibrium:

Kj ¼½cj�PDMS

½cj�Aq

¼ ½mj�PDMS

½mj�Aq

VAq

VPDMS½1�

In Equation (1) [cj]PDMS and [mj]PDMS denote the concen-tration and mass of j in the PDMS phase, respectively,and [cj]Aq and [mj]Aq represent the concentration and massof j in the aqueous solution of j, respectively. VAq andVPDMS are the volumes of the aqueous and PDMS phasesafter absorption, respectively. Note that VPDMS representsthe volume of both the ‘‘dry’’ PDMS and the volume of theabsorbed organic phase. To this end, Kj is a measure of dif-ferential solubility of the solute j dispersed between the twophases. In this work we evaluated the partition coefficientof toluene (KT) and benzene (KB) between the respectiveorganic solution and the PDMS phase from the experimen-tally measured concentrations of the respective organicphase in water and in PDMS. [cj]Aq was determined directlyfrom the mass of j remaining in the solution (measured byUV=Vis spectroscopy) and the solution volume. [cj]PDMS

was calculated from the mass of j absorbed by PDMSand the volume of PDMS. [mj]PDMS was estimated by sub-tracting the remaining mass of j in the solution afterabsorption from that before the absorption; VPDMS wascalculated as a sum of the volume of pure PDMS, deter-mined from the known PDMS loading the density of

FIG. 7. Concentration of (a) benzene (B) and (b) toluene (T) remain-

ing in the aqueous phase normalized by the initial solute concentration

(marked in the legends) after exposing the solution to PDMS particles

(solid symbols) and PDMS sheets (open symbols) as a function of the

volume of the PDMS stationary phase. The absorption took place for

24 hours. The symbols denote various initial concentration of the organic

phase in water.

FIG. 8. Concentration of toluene in water as a function of the expo-

sure time of the solution to PDMS particles of various PDMS mass. The

data illustrate that the amount of absorbed toluene does not depend on

geometry of the PDMS phase but rather its amount. The absorption took

place for 24 hours.



PDMS (0.98 g=cm3),[16] and the volume of the absorbedorganic phase j, calculated from the mass of the absorbedsolution and the respective density. Note that the volumecontribution to the overall PDMS volume of the soaked‘‘sponge’’ was very small (1–5%). In our calculations wehave neglected any evaporation of toluene from the aqu-eous phase. In Figure 9, we plot the logarithm of K as afunction of the volume of the PDMS phase (i.e., PDMSplus absorbed solute) for benzene and toluene extractedby PDMS particles (Figures 9a and 9b, respectively) andtoluene extracted by PDMS sheets (Figure 9c). The datashow that KB<KT, as expected, because of the 1) highersolubility of benzene in water (relative to toluene) and 2)the higher solubility of toluene in PDMS (relative to ben-zene). Because the solubility parameter of PDMS, dPDMS,is 7.5 cal1=2=cm3=2, toluene will be more soluble in PDMSthan benzene because dT¼ 8.9 cal1=2=cm3=2 and dB¼9.2 cal1=2=cm3=2 (the solubility parameter of toluene is

closer to that of PDMS relative to the solubility parameterof benzene). There is a slight but non-negligible decrease inKj associated with extracting the organic phase by PDMSparticles. The decrease is more pronounced at higher initialconcentrations of the organic solute in water. The KT

values obtained by extracting toluene from water usingPDMS particles are the same as those evaluated from thePDMS particle extraction experiment. However, unlikeparticles, the extraction of toluene with PDMS sheetsprovides Kj values that do not vary with the volume ofthe PDMS phase. We thus tentative attribute the trendsin the PDMS particle data to sample handling. Specifically,we speculate that during the filtering process some of theorganic solute may have leaked back into the water solu-tion due to the applied pressure thus lowering the apparentKj value. If this were the case, one may envision using theprocedure of ‘‘mechanical extraction’’ of the solute torecover the organic and PDMS phases. More work is cur-rently underway to quantify this issue. Overall, KB and KT

obtained from our experiments are in a good agreementwith previously reported values.[17]

Until now we have considered the absorption of indivi-dual components of BTEX. We have established thattoluene uptake into PDMS is higher than that of benzene,because of 1) lower solubility of toluene in water and 2)higher solubility of toluene in PDMS. In addition toexploring the uptake of individual organic componentsinto PDMS, we also performed a model study evaluatingthe absorption of BTEX. Considering that benzene andtoluene constitute the major components of BTEX westudied the competitive absorption of benzene=toluenemixtures from waters with mixtures having a ratio ofbenzene to toluene around 30%. As previously, we usedUV=Vis spectroscopy to measure the amount of theorganic phase before and after absorption. We benefitedfrom the fact that benzene and toluene UV=Vis spectrahave different shapes, a feature that enabled us to detecteach component in the overall mixture spectra. InFigure 10a, we plot the UV=Vis spectra of toluene (T)(2.5 mL in 10 mL of water, black line), benzene (B)(7.5 mL in 10 mL of water, red line), the B=T mixture(7.5 mL of B and 2.5 mL of T in 10 mL of water, green line).In the same plot we also show the sum of the B and T spec-tra (blue line). As apparent, the sum spectrum matches wellwith the experimentally measured spectrum (green line).Next, we added 200 mg of PDMS particles into the pureB and T solutions and the B=T mixture, stirred the solu-tions in closed vial for 24 hrs, filtered the PDMS phaseand measured the UV=Vis spectra of the remaining solu-tions. In Figure 10b we plot the UV=Vis spectra of all threesolutions (B, T, and B=T mixture) using the same line col-ors as in Figure 10a. As shown by the data in Figure 10b,the remaining concentration of solutes decreased becauseof the absorption of B and T into PDMS.

FIG. 9. Logarithm of the partition coefficient for (a) benzene (B) and

(b,c) toluene (T) absorbing from aqueous solutions into (a,b) PDMS

particles (solid symbols) and (c) PDMS sheets (open symbols) as a func-

tion of the volume of the PDMS stationary phase for various initial con-

centration of the solute in the aqueous solution. The absorption took place

for 24 hours. the dotted lines denote the average values of log(KB) and

log(KT).

324 I. PARK ET AL.


In order to quantify the uptake of each component, weevaluated the ‘‘expected’’ ratio of the partition coefficientsof toluene and benzene, [KT=KB]:

KT

KB

� �¼ ½mT�PDMS

½mB�PDMS

½mB�Aq

½mT�Aq

½2�

Equation (2) was obtained by ratioing the expressions forKT and KB given by Equation (1). Because the value ofKT and KB were evaluated in our single component experi-ments and are given in Figure 9, we can compare [KT=KB]to KT=KB. If [KT=KB]<KT=KB more benzene getsabsorbed into PDMS from toluene=benzene mixtures rela-tive to what would be expected based on the uptake of indi-vidual components. The results from our experiments aresummarized in Table 2 and Figure 11. Note that in ouranalysis we have again neglected any evaporation oftoluene during the course of the experiment.

In Figure 11, we plot the total mass of the organic phasedissolved in water (a) and the amount of the organicsabsorbed into PDMS (b) as a function of the ratio of themass of toluene to that of benzene in the aqueous mixturebefore absorption, [mT]inp=[mB]inp. PDMS stationaryphases either in their particle (P) or sheet (S) form weretested; the amount of the PDMS ranged from 50 to300 mg and is given for each data set in the legend to Figure11. The weight fraction of toluene in the toluene=benzenemixture ranged from�0.3 to�0.4, corresponding to anincrease of [mT]inp=[mB]inp from� 0.437 to� 0.665, respec-tively. As apparent form the data in parts (a) and (b) inFigure 11, the amount of organic phase in water decreasedafter PDMS was added and exposed to the mixture for24 hours. The data in Figure 11b also demonstrate thatincreasing the amount of PDMS improved the removal

FIG. 10. UV=Vis curves (a) before and (b) after adding 200 mg of

PDMS particles into a solution comprising 2.5mL toluene (black line),

7.5mL benzene (red line) and a mixture of toluene and benzene (25=75

composition) (green line) after 24 hours. The blue line represents a curve

generated by weigh-averaging the toluene and benzene curves as described

in the text.

TABLE 2Results from competitive absorption of toluene (T)=benzene (B) mixtures into PDMS

Input Output

SystemaTin

(mL)Bin

(mL)mPDMS

(mg)TAq

(mL)BAq

(mL)TPDMS

(mL)BPDMS

(mL) [KT=KBb] [KT=KB]c

P 5.4 8 50 1.0 3.2 4.4 4.7 3.09 2.93P 3.1 7 100 0.6 2.3 2.5 2.5 3.09 2.04P 2.4 5.2 100 1.0 3.6 1.4 1.6 3.09 3.15S 3.7 6.1 100 1.2 3.5 2.5 2.6 2.95 2.80S 2.4 5.2 100 0.9 2.9 1.5 2.3 2.95 2.11P 5.4 8 200 0.5 2.2 4.9 5.8 3.09 3.71P 2.4 5.2 200 0.7 2.6 1.7 2.6 3.09 2.43P 2.4 5.2 300 0.5 2.0 1.9 3.2 3.09 2.38

aS: sheet, P: particle.bKT¼ ([mT]PDMS=[mT]Aq)=(VAq=VPDMS); KB¼ ([mB]PDMS=[mB]Aq)=(VAq=VPDMS).c[KT=KB]¼ ([mT]PDMS=[mB]PDMS)([mB]Aq=[mT]Aq).



of the organics from water as the uptake of the solventsinto PDMS increased with increasing the amount ofPDMS.

In order to compare the relative uptakes of toluene andbenzene into PDMS from toluene=benzene mixtures, inFigure 11c we plot the values [KT=KB] as a function of[mT]inp=[mB]inp and compare them to the ‘‘expected’’KT=KB ratios (solid and dotted lines) evaluated using thepartition coefficients of pure components listed in Figure9. While there is a scatter in the data, the trends observedare clear, namely, [KT=KB] is much smaller than KT=KB

for small values [mT]inp=[mB]inp and increases with increas-ing the fraction of toluene in the toluene=benzene mixture.This trend thus reveals that the amount of benzene that isbeing absorbed into the PDMS phase is higher relative tothat expected based on pure benzene uptake experiments.

This tendency gets stronger when the amount of toluenein the toluene=benzene mixture decreases. The explanationfor this behavior can again be based on the basis of solubi-lity parameters of the individual components (cf. Table 1).Recall that the solubility parameter of toluene is betweenthat of PDMS and benzene. Hence, when both tolueneand benzene are present in water and absorbed intoPDMS, toluene acts as a ‘‘surfactant’’ for PDMS=benzeneand facilitates a higher uptake of benzene into PDMSrelative to situation where only benzene is being absorbedinto the PDMS phase. This observation has important con-sequence on recovering both organic solvents from water.While benzene is present in waters more abundantly thantoluene, its lower solubility in PDMS does not allow a largeamount of benzene to be absorbed into PDMS. However,when toluene is present concurrently in the mixture, itcan assist in increasing the uptake of benzene into PDMSby acting as an efficient surfactant by replacing more unfa-vorable PDMS=benzene interactions with energetically lesscostly toluene=benzene ones. A comment needs to be madewith regard to our assumption that toluene evaporation,though observed to take place during the first 24 hours,can be neglected. If correction were made that wouldaccount for some toluene evaporation, it would likelydecrease the [KT=KB] even more because of lower [mT]PDMS

and higher [mT]Aq, cf. Equation (2). While we do not antici-pate that the actual amount of toluene in PDMS will belower (recall that we detected a very fact uptake of tolu-ene into PDMS in our kinetic tests), the value of [mT]PDMS

we report is calculated from the initial amount of toluenecharged into the solution and that remaining after theabsorption, [mT]Aq; the latter value being lower thanthe one not affected by evaporation. Having stated this,the observations and results presented in Figure 11 thusrepresent the upper bound of processes that actually takesplace; that is, in reality more benzene is likely absorbed inPDMS from T=B mixtures than reported by the datapresented here.

As a final step in our demonstration of the effectivenessof PDMS in recovering oleophilic phase from water, we per-formed visual test aiming at establishing the capability ofPDMS particles to remove spills of crude oil from water.The procedure is shown in Figure 12. After mixing a fewdrops of crude oil into water a small number of PDMSparticles (diameters ranging from a few micrometers toabout 1 mm). We observed an uptake of oil nearly imme-diately into the particles. Upon mixing the particles startedto agglomerate and could easily be removed from the con-taminated water. At that point a second batch of PDMSparticles was added to the suspension, which improved thecleaning even further. While we have made no attempts toquantify the results, even the qualitative observations madeclearly demonstrate the capabilities of PDMS stationaryphase to remove crude oils from contaminated water.

FIG. 11. (a) Total weight of toluene (T) and benzene (B) in T=B

mixtures in water before absorption into PDMS as a function of the ratio

of the weight of toluene and benzene in the initial mixture. (b) Total

weight of T and B absorbed into PDMS (type and amount given in the

legend) as a function of the ratio of the weight of toluene and benzene

in the initial mixture. (c) The ratio of partition coefficients of T and B

([KT=KB]) calculated using equation (2) as a function of the ratio of the

weight of toluene and benzene in the initial mixture. the dashed and dotted

lines denote the KT=KB values for the particle and sheet absorption due to

individual components; these were calculated using the KT and KB values

presented in Figure 9.

326 I. PARK ET AL.


CONCLUSIONS

To summarize, we have developed simple, robust, andefficient technology utilizing low cost, recoverable materi-als based on silicone elastomer networks for removingdissolved organic solvents and oil spills from waters. ThePDMS phase had either a shape of particles with variablesizes (ranging from hundreds nanometers to a fewmillimeters) or sheets. We observed a rapid sorption oforganic solutes into the PDMS networks; the rate andamount of organic solvent uptake did not depend on theshape of the PDMS ‘‘sponge.’’ The partition coefficientsobtained for both the toluene with PDMS particles andsheets are similar and the value for benzene is lower thanthat for toluene, which means relatively higher affinity oftoluene toward PDMS than benzene. Experiments invol-ving co-absorption of toluene=benzene mixtures revealedthat the amount of benzene absorbed into PDMS increasesrelatively to the case of pure benzene uptake into PDMS.This behavior results from toluene acting as a ‘‘surfactant’’capable of replacing PDMS=benzene contacts withenergetically more favorable toluene=benzene ones. Finally,a simple visual experiment demonstrated qualitativelythat PDMS is also capable of removing crude oils fromoil-contaminated waters.

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FIG. 12. Removal of crude oil spills from water. A drop of crude oil was deposited on water (a) and mixed thoroughly (b,c). A small number of

PDMS particles (diameters ranging from a few micrometers to about 1 mm) was deposited in the oil=water mixture; almost immediately we observed an

uptake of oil into the particles (d). Mixing the suspension led to enhanced oil uptake into the particles (e). A second batch of pure PDMS particles was

added to the suspension, which further improved the cleaning (f,g). The oil-containing particles formed film-like aggregates floating on top of water,

these could be readily picked up mechanically (h).



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