Photo-regenerable multi-walled carbon nanotube membranes for the removal of pharmaceutical micropollutants from water

Environmental ScienceProcesses & Impacts

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aBioEnergy & Environmental Laboratory (BE

Technology and Environmental Research (iW

Technology, PO Box 54224, Abu Dhabi, UA

+971 2 810 9901; Tel: +971 2 810 9114bMaterials Science and Engineering Prog

Technology, PO Box 54224, Abu Dhabi, UAE

† Electronic supplementary informa10.1039/c3em00150d

Cite this: Environ. Sci.: ProcessesImpacts, 2013, 15, 1582

Received 20th March 2013Accepted 5th June 2013

DOI: 10.1039/c3em00150d

rsc.li/process-impacts

1582 | Environ. Sci.: Processes Impacts

Photo-regenerable multi-walled carbon nanotubemembranes for the removal of pharmaceuticalmicropollutants from water†

Qammer Zaib,a Bilal Mansoorb and Farrukh Ahmad*a

Pharmaceutical micropollutants fall in the category of “emerging contaminants” in water because of their

prevalence and persistence in the aqueous environment, and because of a poor understanding of their

low-dose exposure effects on human and animal populations. In this study, photo-regenerable

multiwalled carbon nanotube membranes with variable water permeabilities were produced by

embedding hierarchical TiO2 structures (having porous, spherical morphology) onto a pre-deposited bed

of multi-walled carbon nanotubes (MWNTs) using a modified sol–gel technique. These MWNT–TiO2

composites and their constituent materials were characterized by analytical electron microscopy, surface

charge measurement, thermogravimetric analysis, and hydrophobicity determination. The adsorption

removal potential of MWNT–TiO2 membranes was demonstrated for three representative

pharmaceuticals: acetaminophen, carbamazepine and ibuprofen. The peak initial removal percentages

of the pharmaceuticals by the MWNT–TiO2 membranes were 80%, 45%, and 24% for carbamazepine,

ibuprofen, and acetaminophen, respectively. The ability of the membranes to be regenerated, once they

were saturated with the pharmaceutical compounds, was verified by repeating the adsorption removal

experiment on the same membranes after exposure to UV light at 254 nm. Peak removal efficiencies

after regeneration were 55%, 32%, and 19% for carbamazepine, ibuprofen, and acetaminophen,

respectively, indicating some loss in sorptive capacity upon regeneration. Furthermore, the effect of pH

on adsorption of ibuprofen, the pharmaceutical that attained the highest mass loading on the sorbent

at equilibrium saturation, was studied and its mechanism of adsorption was proposed at pH below pKa.

Environmental impact

Pharmaceutical micropollutants, having various ecotoxicological effects, persist in the aqueous environment because conventional wastewater treatment plantsinadequately remove them. In this work, we offer a proof-of-concept sustainable solution for pharmaceutical micropollutant removal from water. The solutionentails a photo-regenerable multi-walled carbon nanotube–titanium dioxide (MWNT–TiO2) composite membrane that is water permeable and can sorb selectivepharmaceuticals. Once saturated, the membrane can then be regenerated photocatalytically. Prior to this work, electrically regenerable MWNTmembranes havebeen applied to biological disinfection of water, and non-water-permeable TiO2 coatings stabilized with carbon nanotubes (CNT) have been applied for pho-tocatalytic oxidation of phenol. However, to the best of our knowledge, photo-regenerable, water-permeable membranes of carbon nanotubes have neither beendeveloped nor applied to remove pharmaceuticals from water.

Introduction

Pharmaceuticals have been classied as emerging contami-nants because of their frequent occurrence in surface water,groundwater, seawater, and treated sewage effluent.1–6 The

EL), Institute Center for Water Advanced

ATER), Masdar Institute of Science and

E. E-mail: [email protected]; Fax:

ram, Masdar Institute of Science and

tion (ESI) available. See DOI:

, 2013, 15, 1582–1589

production of oral and intravenous pharmaceutical compoundsfor use in veterinary and human medicines is increasing glob-ally due to ageing populations and enhancement in the qualityof life.5,6 Pharmaceuticals usually enter the environment eitherthrough excretion from humans and animals, or through directdisposal of unused or expired medicines in wastewater.3,5

In wastewater treatment plants, many pharmaceuticals arenot effectively mineralized to innocuous products. Instead, theycan become partially metabolized or biotransformed beforepassing through wastewater treatment plants.7 Generally,wastewaters contain 10�3 to 10�6 mg L�1 of pharmaceuticalsdepending upon physical and chemical characteristics of theparticular pharmaceutical as well as the wastewater treatment

This journal is ª The Royal Society of Chemistry 2013

http://dx.doi.org/10.1039/c3em00150d

http://pubs.rsc.org/en/journals/journal/EM

http://pubs.rsc.org/en/journals/journal/EM?issueid=EM015008

Paper Environmental Science: Processes & Impacts

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technologies employed.8,9 Their occurrence in water andwastewater is of concern owing to their ability to adversely affectgrowth, reproduction, and behavior of non-target organismsespecially aquatic ones.10

The inability of conventional wastewater treatment plants toremove these compounds and their biotransformed daughterproducts has attracted the attention of researchers around theworld in the past two decades.2,3,11 Since then, a number oftreatments have been proposed to address this problem,including biodegradation, volatilization, hydrolysis, chemicaloxidation, and adsorption.12,13 Among these unit processes,adsorption is of special importance owing to its ability toremove pharmaceutical metabolites, which form during thewastewater treatment, and which might actually be moreharmful than their parent compounds.14,15

The discovery of carbon nanotubes (CNTs)16 paved a way totheir applications in energy storage devices, composite materials,sensors, hydrogen storage media, probes, semiconductors, andwater treatment.17,18 These applications are a result of CNTsexceptional optical, mechanical, electrical, and thermal proper-ties.17 In water treatment, CNTs have been successfully employedto remove heavy metals,19 metalloids,20 polycyclic aromatichydrocarbons,21 endocrine disrupting compounds,22,23 andpharmaceuticals,24 chiey through adsorption.18 CNTs can beapplied for adsorption of target compounds either (i) by sus-pending freely in solution as demonstrated in the case of dis-solved aqueous contamination,23 or (ii) by modifying them intogas permeable at sheets or membranes.25 Prior to this work,electrically regenerable MWNT membranes have been applied tobiological disinfection of water,26 and non-water-permeable TiO2

coatings stabilized with CNTs have been applied for the photo-lytic decay of phenol.27 However, to the best of our knowledge,photocatalytically regenerable water-permeable membranes ofcarbon nanotubes have not been applied to remove pharma-ceuticals from water.

In this work, membranes of various thicknesses wereprepared from multi-walled carbon nanotubes (MWNTs) fol-lowed by embedding with in-house pretreated titanium dioxide(TiO2) spheres. Thorough characterization of these membraneswas carried out by a number of techniques, including analyticalscanning electron microscopy (SEM), electron dispersive X-rayspectroscopy (EDS), zeta potential measurements, thermogra-vimetric analysis (TGA), and hydrophobicity estimation. Then,the efficacy of these membranes for removing common phar-maceuticals from water was evaluated aer measuring theirpermeate ux under constant hydraulic head conditions. Later,the potential for these membranes to be regenerated photo-catalytically was studied. Finally, the effect of pH on removal ofibuprofen was explored and the adsorption mechanism of thatparticular pharmaceutical on MWNTs was proposed.

Materials and methodsMaterials

MWNTs were purchased from Cheaptubes Inc. (Brattleboro, VT,USA). The MWNTs were >99% pure, with an outer diameterrange of 13–18 nm and lengths of up to 30 mm according to the


manufacturer. Sodium hydroxide (98.8%) was provided by PochBasic (Gliwice, Poland). Reagent-grade sodium dodecyl sulfate(SDS, $99%) and sodium chloride (cell culture tested) wereobtained from Sigma Aldrich (St. Louis, MO, USA). Vacuumltration was carried out using Millipore (Millipore Corp.,Billerica, MA, USA) mixed cellulose acetate lter papers with apore size of 220 nm. Fresh deionized water with an averageresistivity of 18.2 MU cm was used throughout the course ofexperimentation. In-house pretreated TiO2 spheres wereproduced by autoclaving anatase acquired from Sigma Aldrich(St. Louis, MO, USA) using the procedure reported in the liter-ature28 with modications. Pharmaceuticals were also obtainedfrom Sigma Aldrich (St. Louis, MO, USA). Acetaminophenquality met US Pharmacopeial Convention (USP) testing speci-cations by having at least 98% purity; ibuprofen was phar-maceutical secondary standard which was traceable to USP(1335508), PhEur (I0020000), and BP (539); and carbamazepinewas $98% pure powder.

Preparation of multi-walled carbon nanotube-titaniumdioxide (MWNT–TiO2) membranes

Each MWNT–TiO2 membrane was prepared in two steps. Step(1) MWNT membrane was prepared by following the previouslydeveloped protocol29 with some modications. 1 mg mL�1

MWNTs were dissolved in 1% SDS solution with the help ofultrasonication.30 The MWNT solution was then lteredthrough 220 nm pore size cellulose acetate lter paper with theaid of vacuum ltration. This resulted in the deposition of athin layer on the surface of the lter paper. Step (2) embeddingof TiO2 spheres was performed by ltration of aqueous disper-sion of in-house pretreated TiO2 spheres through the previouslyprepared MWNT membrane. The TiO2 retained on the surfaceof the MWNT membrane resulting in a MWNT–TiO2

membrane. The resulting membrane was washed with acopious amount of deionized water to remove the surfactantand other soluble impurities until the conductivity of theeffluent water passing through the membrane decreasedfrom >400 mS cm�1 to <1 mS cm�1. The relationship betweenpermeate ux and thickness of MWNTs–TiO2 membranes ispresented in Fig. 1. A number of membranes having varyingthicknesses were prepared by increasing the mass of MWNTsand TiO2, while keeping their mass ratio near unity (see Fig. S1in the ESI†). The membrane thickness was observed to beinversely related to the permeate ux. Membranes with MWNTand TiO2 masses of 10 mg each and having an aqueouspermeate ux of 25 L h�1 m�2 were selected for further study.

Characterization of MWNT–TiO2 membranes

Analytical electron microscopy. The structures of MWNTs,in-house pretreated TiO2 spheres, andMWNT–TiO2membraneswere examined by scanning electron microscopy. An FEI,Quanta FEG 250 SEM, operating at �5 to 30 keV was employedfor this purpose. The microscopy samples were coated with a�50 nm thick gold and palladium layer using a GATAN Model682 Precision Etching Coating System (PECS). The elementalmicroanalysis of specied regions on the samples was

Environ. Sci.: Processes Impacts, 2013, 15, 1582–1589 | 1583


Fig. 1 Permeate fluxes throughMWNTs–TiO2 membranes for water were inverselyproportional to their thicknesses. The plot shows the flux of deionized water throughMWNT–TiO2 membranes of various thicknesses under the effect of gravity. Thepressure was kept constant at 3.43 N cm�2 by controlling the water head.

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performed using an EDAX Energy Dispersive Spectroscopy(EDS) system and TEAM soware.

Zeta potential measurements. The charge on MWNTs wasmeasured using a ZetaPALS analyzer (Brookhaven, NY, USA).The Smoluchowski equation was used to calculate zeta poten-tials from electrophoretic mobilities.23,30 Electrophoreticmobilities of MWNTs were measured by varying conductivity ofbackground solution from deionized water to 1, 10, 50, and100 mM sodium chloride solution.

TGA. Thermogravimetric analysis of MWNTs, in-house pre-treated TiO2 spheres, and MWNT–TiO2 membranes was carriedout using a Perkin-Elmer Thermogravimetric Analyzer (Wal-tham, MA, USA) using nitrogen as a carrier gas. The tempera-ture was gradually increased from 30 �C to 800 �C usingapproximately 10 mg of sample. The difference in weight overthe temperature gradient provided the information about thesample.

Hydrophobicity measurements. The hydrophobicity ofMWNTs and MWNT–TiO2 membranes was determined bycontact angle measurement using KRUSS apparatus (Hamburg,Germany). Initially, a micro-syringe was used to manuallygenerate deionized water drops on the membrane surface. Thecontact angle was calculated from a static image of the droplet

Table 1 Physical properties of compounds34

Compound CAS Formulae Structure35Mol. W(g mol

Acetaminophen 103-90-2 C8H9NO2 151.17

Ibuprofen 15687-27-1 C13H18O2 206.29

Carbamazepine 298-46-4 C15H12N2O 236.28

1584 | Environ. Sci.: Processes Impacts, 2013, 15, 1582–1589

taken immediately aer contact with the surface using animage-processing program DSA1. The soware estimated thecontact angle by circle tting the drop using the sessile dropmethod.

Adsorption experiments

Selection of pharmaceuticals and adsorption experiments.The pharmaceuticals, acetaminophen, ibuprofen, and carba-mazepine, were selected on the basis of their high frequency ofoccurrence in water systems2,15,31–33 and their physicochemicalproperties (Table 1). A broad range of parameters such as logKow (0.46 to 3.97) and water solubilities (17.7 to 14000 mg L�1)were covered by using these compounds as presented inTable 1.

The pharmaceuticals, from solutions made in deionizedwater, were allowed to adsorb onto the MWNT membrane(control) and MWNT–TiO2 membranes under gravity ow whilemaintaining a constant head. The ux of the pharmaceuticalsolution remained approximately constant over the course ofthe experiments by maintaining a constant hydraulic head.Effluent fractions were collected at regular intervals to deter-mine the mass of pharmaceuticals removed with respect totime. Characteristic absorption wavelengths were used todevelop calibration curves for pharmaceutical concentrations inwater (Table 1). Themass concentration of pharmaceuticals wasdetermined using a UV-Vis spectrophotometer, Model Evolu-tion 300 manufactured by Thermo Fisher Scientic (Madison,WI, USA).

Photo-regeneration of the membrane. The MWNT–TiO2

membranes were photo-regenerated (cleaned) using UV-Vis254 nm light. The UV light source was 6� 8Watt – 312 nm tube.The membrane was dipped in 50 mL deionized water in analuminum dish (10.5 cm diameter). The average UV exposuretime was 2 h.

Effect of pH on adsorption of ibuprofen on the MWNTmembrane. The effect of pH on adsorption of ibuprofen wasstudied by employing aqueous ibuprofen solutions with threedifferent pH values. One solution was below (pH 4.0) and twowere above (pH 7.0 and pH 11.0) the pKa of ibuprofen (pKa ¼4.9). The conductivity was kept constant at 500 mS cm�1 andadjusted with monovalent salt (NaCl). The pH was adjustedusing HCl and NaOH in the presence of 1 mM potassiumphosphate buffer.

t�1) log Kow

Water solubilitymg L�1 (25 �C)

Absorptionwavelength (nm) pKa

0.46 1.40 � 104 244 9.5

3.97 21 222 4.9

2.45 17.7 284 13.9 (ref. 36)




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Results and discussionCharacterization of MWNTs, TiO2, and MWNT–TiO2

membrane

Electron microscopy, zeta potential technique, and TGA wereapplied to characterize MWNTs, in-house pretreated TiO2

spheres, and MWNT–TiO2 membrane. The microstructure andmorphology of the three materials were analyzed using an SEM.The electron micrograph in Fig. 2a shows the overall structureof a loosely packed dense mesh or a layer of MWNTs that acttogether as a water-permeable membrane. The higher resolu-tion image in Fig. 2b shows that nanotubes stack into a non-uniform, random arrangement. Each square centimeter of the

Fig. 2 SEM micrographs of (a) MWNT membrane, (b) high magnification imageof theMWNTmembrane, (c) TiO2 anatase before treatment, (d) TiO2 spheres aftertreatment, (e) TiO2 spheres distributed in the MWNT membrane (f) MWNTsentangle TiO2 spheres (g) EDS of the MWNT–TiO2 membrane confirming thepresence of carbon, titanium, and oxygen.


membrane contains a large number of MWNTs in which theinterstices generated between the randomly ordered MWNTsmay allow water and other small molecules to pass through.

The SEM image in Fig. 2d shows the in-house pretreatedTiO2 hierarchical structures obtained from the randomly sizedas-received TiO2 anatase particles (Fig. 2c) (see Fig. S2 in theESI†). These samples were porous and exhibit a sphericalmorphology (diameter approximately 1–3 microns) with super-imposed nano-rods and nano-ribbons. The size of nano-rodsand ribbons was between 100 and 200 nm. The in-house pre-treated TiO2 structures consisting of nano-rods and nano-ribbons will be referred to as “TiO2 spheres” throughout theremainder of the text. The morphology of the TiO2 spheres,however, could not be maintained during their pressure assis-ted deposition on the MWNT layer. Fig. 2e and f indicate thedisintegration of these TiO2 spheres into relatively smallfractions.

The porous morphology and the ultra-thin, superimposedstructures (nano-rods and nano-ribbons) signicantly increasedthe overall specic surface area of the TiO2 spheres. This mighthave positively impacted the photocatalytic character of theTiO2. Fig. 2e presents the successful embedding of functional-ized TiO2 spheres on the surface of the MWNT membrane.

The TiO2 spheres appear to be uniformly distributed and insome cases they cluster together to form a network. The higherresolution image of the selected area in Fig. 2f shows nanotubesentangling the TiO2 spheres and effectively binding the twotogether. This arrangement is expected to offer minimumresistance to the uid ow. It can be observed that theembedded TiO2 did not maintain the exactly same upper mostsurface morphology as the treated TiO2 presented in Fig. 2d.This might be due to the loss of the needle like upper mostsurfaces of these TiO2 spheres when deposition was attemptedon the MWNT layer through vacuum ltration. The MWNTmembranes embedded with TiO2 spheres were selected forfurther investigation. The EDS analysis was performed on theMWNT–TiO2 membrane to look into the elemental compositionof the membrane. Fig. 2g, the EDS elemental analysis result,indicated the presence of carbon, titanium, and oxygen, therebyconrming these elemental components of the MWNT–TiO2

membrane.Zeta potential measurements revealed that MWNTs were

negatively charged (Fig. 3a). This negative charge on MWNTsdecreased with an increase in concentration of sodium chloridein the background medium (i.e., water). The negative surfacecharge of MWNTs decreased from �40.6 � 1.5 mV to �22 � 3.9upon increasing the concentration of sodium chloride from�0 to 100 mM. One possible explanation for the presence ofnegative charge on MWNTs is the existence of defects on themwhich are prone to oxidation and/or attachment of othernegative functionalities such as carboxyl and hydroxylgroups.37,38 As the concentration of background salt increases,these negative charges neutralize. This results in a decrease innegative zeta potential of MWNTs with increasing backgroundsalt concentration. The values of zeta potential observed in thisstudy are comparable to earlier observed electrophoreticmobilities of similar materials.38



Fig. 3 (a) Zeta potential ofMWNTs decreasedwith the increase of background saltconcentration. The MWNT concentration was �1 mg L�1. NaCl was added imme-diately before measuring the electrophoretic motilities of MWNTs in the sample. (b)TGA of MWNTs (A) and TiO2 spheres (,) was obtained using nitrogen as a carriergas. The analysis indicates least amount of impurities in bothMWNTs and TiO2. Thisconfirms manufacturer's claim of >99% pure MWNTs and high yield of in-housepretreated TiO2 nanowires. The sample mass was >8 mg and the temperature wasgradually increased from 30 �C to 800 �C with step increments of 10 �C.


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The TGA analysis was carried out on MWNTs and TiO2 in anitrogen atmosphere in order to monitor their weight loss andthermal stability (Fig. 3b). In MWNTs, a 10% weight loss wasobserved between 53 and 100 �C. This can be attributed tomoisture, volatiles, and other impurities that have a low ashpoint.39 Insignicant transitions from 100–600 �C indicate thethermal stability of MWNTs up to 600 �C. Aer 600 �C, a steepinclination of the weight loss curve indicates rapid degradationof MWNTs.39 Our equipment was limited to a temperature of800 �C; therefore, complete gasication of MWNTs could not beevidenced. For TiO2, 22% weight loss was observed for the rst100 �C rise in temperature. Again, this can be due to the pres-ence of moisture and other volatile impurities.40,41 This highermoisture content in TiO2 spheres indicates the hydrophilicity ofthe material which is also evident from the increased hydro-philicity of the MWNT–TiO2 membrane. Aer 100 �C, TiO2

appeared to be stable to thermal degradation. Hence, TGAanalysis yielded the purity of MWNTs and TiO2 to be 90% and


78%, respectively, while accounting for all impurities includingmoisture and other volatiles.

Fig. S3 (in ESI†) presents the surface hydrophilicity ofMWNTs-only and MWNT–TiO2 membranes. Contact anglemeasurements were applied to measure the extent of hydro-philicity.42 The contact angles of both MWNT (Fig. S3a†) andMWNT–TiO2 (Fig. S3b†) membranes were found to be less than90� showing that the membranes are hydrophilic. However, theMWNT–TiO2 membrane appeared to be more hydrophilicbecause the water droplet attened quicker on it than on theMWNTs-only membrane surface.

Removal of pharmaceuticals from water by the MWNT andMWNT–TiO2 membranes

The ability of the MWNT–TiO2 membrane to remove pharma-ceuticals from water was investigated. Experiments were alsoconducted with the MWNTs-only membrane (i.e., without anyembedded TiO2 spheres) to compare pharmaceutical removalefficiencies and to establish controls. Fig. 4 shows the removalof pharmaceuticals from water by the MWNT and MWNT–TiO2

membranes. The inuent pharmaceutical concentration (10 mgL�1) and solution ux (40 L m�2 h�1) were kept constant, andeffluent fractions were analyzed to assess the performance ofmembranes and their saturation over time.

The pharmaceutical removal efficiencies of MWNT–TiO2

membranes were consistently higher than those of MWNTs-only membranes. These can be explained by simultaneousadsorption and degradation of pharmaceuticals in the presenceof TiO2.40,43,44 Fig. 4 shows that acetaminophen exhibits the leastaffinity towards MWNTs when compared to the other twocompounds. The initial sorptive removal for acetaminophenwas �10% when 10 mg L�1 acetaminophen solution wasallowed to ow through the MWNT membrane (Fig. 4a).However, in the case of the MWNT–TiO2 membrane, thisremoval increased to�24% (2.4 mg L�1 effluent from 10mg L�1

inuent solution). Later, this removal gradually decreasedto <1% when 20 mL of acetaminophen solution was passedthrough. The MWNT–TiO2 membrane was regenerated by UV at254 nm as described earlier (Materials and Methods) andexamined for its effectiveness in removing the same (10 mg L�1)acetaminophen solution. Aer regeneration, the initial removalefficiency of the MWNT–TiO2 membrane decreased to �18%(compared to 24% in the 1st run) removal of acetaminophen. Itshould be noted that, even aer regeneration, the adsorptionremoval of the MWNT–TiO2 membrane was eighty percenthigher than in the case of the MWNTs-only membrane whichsupports the hypothesis of continued adsorption and photolysisof acetaminophen on the MWNT–TiO2 membrane.

The MWNTs-only and MWNT–TiO2 membranes were testedfor a more hydrophobic pharmaceutical, ibuprofen, as pre-sented in Fig. 4b. The initial sorptive removal efficiency andsaturation time of both (MWNTs-only and MWNT–TiO2)membranes for ibuprofen were higher than that of acetamino-phen. A maximum of 30% ibuprofen was observed to beremoved initially by the MWNT membrane for the rst 5 mL of10 mg L�1 ibuprofen solution. This removal efficiency gradually



Fig. 4 Removal of (a) acetaminophen, (b) ibuprofen, and (c) carbamazepinefrom water by the MWNTs-only membrane (>), MWNT–TiO2 membrane (D) andMWNT–TiO2 membrane after photo-regeneration (B) under same conditions.The influent pharmaceutical's concentration was 10 mg L�1. Adsorption wasperformed at room temperature and concentrations of influents and effluentswere determined using UV-Vis. The removal of pharmaceuticals decreased withthe saturation of membranes. However, the membranes were successfullyregenerated by exposure to UV.

Fig. 5 The mass loadings of pharmaceuticals on (i) MWNT membranes, (ii)MWNT–TiO2 membranes during the 1st run, and (iii) MWNT–TiO2 membranesafter photo-regeneration. The pharmaceuticals mass removal was calculatedwhen the membranes fully saturated with pharmaceuticals and their pharma-ceutical removal efficiency reached zero. The influent concentration of acet-aminophen, ibuprofen, and carbamazepine was 10 mg L�1 each in deionizedwater (18 mS cm�1).


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decreased to one-third (10%) upon the passage of an additional10 mL of the same solution. Finally, the adsorption removaldecreased to �5% aer the continuous ow of (a total) 40 mLibuprofen solution, indicating a slow attainment of sorbentsaturation. In the case of the MWNT–TiO2 membrane, initially�45% (4.5 mL) ibuprofen was removed. Aer 20 mL of thesolution had passed through the membrane, the removal ofibuprofen was still 37%, signicantly higher than in the case ofthe MWNTs-only membrane. The adsorption removal efficiencyof the MWNT–TiO2 membrane decreased to 6% aer passing100 mL ibuprofen (10 mg L�1) solution. Aer regeneration, theMWNT–TiO2 membrane was effective in removing only 32% ofinuent ibuprofen, initially. The slow sorbent saturationtrend of ibuprofen removal was similar in the MWNT andMWNT–TiO2 (before and aer regeneration) membranes.


Carbamazepine exhibited high affinity towards both MWNTs-only and MWNT–TiO2 membranes (Fig. 4c). Carbamazepineremoval by MWNT–TiO2 started from 80% of the inuentconcentration (8 mg L�1 was removed from 10 mg L�1 solution)which was approximately double that of the MWNTs-onlymembrane (39% removal). Aer 20 mL, the MWNT–TiO2

membrane was still able to remove �45% of the carbamazepinefrom the solution compared to only 5% removal by the MWNTs-only membrane.

However, in the case of carbamazepine, the membranessaturated earlier when compared to ibuprofen. It was alsoobserved that the removal efficiency of the membranedecreased to <5% aer passing 50 mL of carbamazepine solu-tion from the two membranes. Whereas, at the same time stage(aer 50 mL solution passed), �20% of the ibuprofen was stillbeing removed from the solution by the MWNT–TiO2

membrane. The regenerated MWNT–TiO2 membrane initiallyremoved 53% of carbamazepine from solution which was still13% higher than the removal by the non-photoregenerableMWNTs-only membrane. Fig. S4† presents the removal ofpharmaceuticals from water (C/C0) over time (see ESI†).

The membranes exhibited varied trends towards removal ofthe different pharmaceuticals tested. Therefore, the totalmasses of acetaminophen, ibuprofen, and carbamazepine,removed by MWNT membranes, MWNT–TiO2 membranesduring the 1st run, and MWNT–TiO2 aer photo-regenerationwere calculated to better understand this phenomenon. Fig. 5shows mass loading of membranes upon complete saturationwith pharmaceuticals. The results exhibit a superiority of theMWNT–TiO2 membrane to the MWNTs-only membrane for theremoval capacity of selected pharmaceuticals. For each gram ofMWNTs, a total of 2.9, 6.7, and 4.8 mg of acetaminophen,ibuprofen, and carbamazepine were removed by MWNTmembranes, respectively, with the order correlating with the logKow values of the pharmaceuticals tested. This removalincreased to 4.13, 22.1, and 17.2 mg in the case of the MWNT–TiO2 membrane for the rst run and ultimately decreased to




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0.78, 12.4, and 8.4 mg for the MWNT–TiO2 membrane aerphoto-regeneration. This removal, whichmay not necessarily bedue solely to sorption, is comparable to reported data of acet-aminophen,31 ibuprofen,33 and carbamazepine32 adsorption bycarbonaceous materials. However, these researchers chieyperformed experiments using activated carbon suspensionswhich were hard to recover from the system and almostimpossible to regenerate for further use. In contrast, theMWNTs-only and MWNT–TiO2 membranes, developed andapplied in this work, could be easily recovered and regeneratedfrom the water system. This higher adsorption removal ofibuprofen on MWNTs-only and MWNT–TiO2 membranes pavedthe way to further investigate the effect of pH on its adsorption.In addition, the difference in removal between the rst-timesorption (or the 1st run) and the post-regeneration sorption forall three sorbates warrants further study, specically to opti-mize the efficacy of the photo-regeneration parameters such asregeneration wavelength and exposure time. Quantum yields ofanatase TiO2 from irradiation with long-wave UV have beenreported to be higher than those obtained at 254 nm.45

Effects of pH on adsorption of ibuprofen on MWNTs

The effect of pH on the adsorption of ibuprofen by MWNTmembranes was investigated at pH 4, 7 and 11. The pKa ofibuprofen is reported to be 4.9.33,46 As shown in Fig. 6a, pHplayed a signicant role in the adsorption of ibuprofen on themembranes. The surface charge density of carbon nanotubeswas negative as shown in Fig. 3a and reported elsewhere.47

Therefore, their isoelectric point should exist below pH 7. In

Fig. 6 (a) Adsorption of ibuprofen on the MWNT membrane at pH 4 (A), pH 7(,), and pH 11 (:). pKa of ibuprofen is 4.9. The influent concentration ofibuprofen was 8.46 mg L�1, NaCl conductivity was 500 mS cm�1, and pH wasadjusted using HCl and NaOH with 1 mM phosphate buffer. (b) Mechanism ofadsorption of ibuprofen on MWNTs. Deprotonated ibuprofen molecule wasprotonated at pH 4 below pKa (4.9) of ibuprofen. Electrostatic interactionsbetween positively charged ibuprofen and negatively charged MWNTs wereresponsible for attachment.


fact, it is reported in the literature to be even below pH 4.0.46,48

Therefore, the surface charge of the MWNT membrane wasessentially negative throughout the course of this work i.e., frompH 4 to 11. The decrease in pH of solution led to the decrease insurface charge density of carbon nanotubes through the tran-sition from –COO� groups to –COOH groups23,46 located atbroken edges and sidewall defects of MWNTs. In contrast, theibuprofen molecule was more protonated at pH below 4.9 andexisted as a carboxylate ion at pH 7 and 11.

Based on the observations listed above, it can be proposedthat at pH 7 and pH 11 the surface of MWNTs and ibuprofenmolecule were negatively charged and underwent electrostaticrepulsion which resulted in minimal adsorption of ibuprofenon the MWNT membranes. Upon decreasing the pH ofibuprofen solution below its pKa (i.e. at pH 4), a signicantquantity of ibuprofen adsorbed on the MWNT membrane(Fig. 6a). The MWNTs-only membrane stabilized aer thepassage of 40 mL ibuprofen solution and continuously adsor-bed over 25% of inuent ibuprofen (8.64 mg L�1) from (at least)another 25 mL of ibuprofen solution. This adsorption removalat pH 4 was signicantly higher than adsorption removal at pH7 and pH 11 (Fig. 6a) and at the pH (5.5) of deionized water(Fig. 4b) under ambient conditions. A preliminary adsorptionmechanism of ibuprofen on MWNTs at pH 4 is proposed inFig. 6b. It can be inferred from the reported literature46,47 andFig. 3a that the MWNTs were negatively charged at pH 4,therefore, only the protonation of ibuprofen was responsible forelectrostatic attraction between positively charged ibuprofenand negatively charged MWNTs. Ultimately, the negativelycharged MWNTs adhered to protonated ibuprofen at pH 4which was below pKa (4.9) of ibuprofen, resulting in increasedadsorption of ibuprofen on the MWNT membrane at pH 4compared to pH 7 and pH 11.

Conclusions

Following are the main conclusions of this study:(1) Hierarchical TiO2 structures with porous, spherical

morphology and large specic surface area were prepared fromcommercially available material and then embedded onto alayer of MWNTs to produce water-permeable, photo-regener-able membranes. (2) The MWNT–TiO2 membranes and theirconstituent materials were characterized by analytical SEM,surface charge measurement, TGA, and hydrophobicity deter-mination. Furthermore, the membranes were analyzed forwater permeability before using them for contaminants removalfrom water. (3) The removal potential of MWNT–TiO2

membranes was successfully demonstrated for three represen-tative pharmaceuticals: acetaminophen, carbamazepine andibuprofen. In addition, the ability of the membranes to re-generate was veried by repeating the adsorption removalprocedures aer exposure to UV light. The initial order ofremoval of pharmaceuticals from water was carbamazepine >ibuprofen > acetaminophen even though ibuprofen carries thehighest log Kow (Fig. 4). However, the order of the total mass ofpharmaceuticals removed from water by the membranes wasibuprofen > carbamazepine > acetaminophen (Fig. 5). (4) The




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effect of pH on the adsorption of ibuprofen was studied and thecorrelation between pKa of ibuprofen and its dependence onsolution chemistry was veried. (5) A mechanism for adsorptionof ibuprofen on MWNTs was proposed. (6) Further studies areneeded to fully establish the benets offered by photo-regen-erable MWNT–TiO2 membranes for removal of pharmaceuticalcontaminants from water.

Acknowledgements

The authors are thankful to the Masdar Institute of Science andTechnology for funding this research. We gratefully acknowl-edge valuable feedback from Dr Philip M. Gschwend of theMassachusetts Institute of Technology during the course of thisstudy.

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Photo-regenerable multi-walled carbon nanotube membranes for the removal of pharmaceutical micropollutants from water