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Environmental Science and PollutionResearch ISSN 0944-1344 Environ Sci Pollut ResDOI 10.1007/s11356-014-3411-9

UV-induced photocatalytic degradationof aqueous acetaminophen: the role ofadsorption and reaction kinetics

Shaik Basha, David Keane, KieranNolan, Michael Oelgemöller, JennyLawler, John M. Tobin & AnneMorrissey

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RESEARCH ARTICLE

UV-induced photocatalytic degradation of aqueousacetaminophen: the role of adsorption and reaction kinetics

Shaik Basha & David Keane & Kieran Nolan & Michael Oelgemöller &

Jenny Lawler & John M. Tobin & Anne Morrissey

Received: 6 February 2014 /Accepted: 4 August 2014# Springer-Verlag Berlin Heidelberg 2014

Abstract Nanostructured titania supported on activated car-bon (AC), termed as integrated photocatalytic adsorbents(IPCAs), were prepared by ultrasonication and investigatedfor the photocatalytic degradation of acetaminophen (AMP), acommon analgesic and antipyretic drug. The IPCAs showedhigh affinity towards AMP (in dark adsorption studies), withthe amount adsorbed proportional to the TiO2 content; thehighest adsorption was at 10 wt% TiO2. Equilibrium isothermstudies showed that the adsorption followed the Langmuirmodel, indicating the dependence of the reaction on an initialadsorption step, with maximum adsorption capacity of28.4 mg/g for 10 % TiO2 IPCA. The effects of initial pH,catalyst amount and initial AMP concentration on the photo-catalytic degradation rates were studied. Generally, the AMPphotodegradation activity of the IPCAs was better than that ofbare TiO2. Kinetic studies on the photocatalytic degradation ofAMP under UV suggest that the degradation followedLangmuir–Hinshelwood (L–H) kinetics, with an adsorption

rate constant (K) that was considerably higher than the pho-tocatalytic rate constant (kr), indicating that the photocatalysisof AMP is the rate-determining step during the adsorption/photocatalysis process.

Keywords Adsorption . Photocatalysis . Kinetics .

Acetaminophen . Integrated photocatalytic adsorbent

Introduction

The presence of active pharmaceutical ingredients (APIs) insurface water is an emerging environmental problem through-out the world, due to their continuous input and persistence inthe aquatic ecosystem (Santos et al. 2010). The productionvolume of APIs has been estimated as several thousands oftons per year (Federsel 2013), which may enter the aquaticenvironment after ingestion and subsequent excretion, eitherwithout modification or in a partially metabolised form(Halling-Sørensen et al. 1998; Stackelberg et al. 2007).Several studies have shown that some pharmaceutical com-pounds are neither eliminated during wastewater treatmentnor biodegraded in the environment (Daughton and Ternes1999, Deegan et al. 2011). Many studies have reported a largevariety of pharmaceutical compounds at concentrations rangingfrom nanogram per liter to milligram per liter in sewage treat-ment plant effluents, natural waters and even in drinking water(Ternes 1998; Heberer et al. 2002; Nikolaou et al. 2007; Laceyet al. 2012). Whilst some of the reported levels are much lowerthan those applied for therapeutic use, the potential humanhealth effects associated with chronic exposure to trace levelsof these compounds are still unknown (Kummerer 2008).

Acetaminophen (AMP), also known as paracetamol (N-acetyl-4-aminophenol) (Fig. 1), is a common analgesic andantipyretic drug that is used for the relief of fever, headaches,and other minor aches and pains. It is one of the most widely

Responsible editor: Philippe Garrigues

A. Morrissey (*)Business School, Dublin City University, Dublin 9, Irelande-mail: anne.morrissey@dcu.ie

S. BashaDiscipline of Marine Biotechnology and Ecology, CSIR-Central Saltand Marine Chemicals Research Institute,Bhavagnar 364002, Gujarat, India

D. Keane : J. Lawler : J. M. TobinSchool of Biotechnology, Dublin City University, Dublin 9, Ireland

K. NolanSchool of Chemical Sciences, Dublin City University, Dublin 9,Ireland

M. OelgemöllerCollege of Science, Technology and Engineering, James CookUniversity, Townsville, Queensland 4811, Australia

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used over-the-counter medications, and AMP-containing an-algesics are one of the most prescribed medications in Irelandwith 0.88 million prescriptions during 2003 (Usher et al.2005). Most (85–95 %) of AMP taken is excreted from thebody during therapeutic use (Forrest et al. 1982; Usher et al.2005). AMP has been detected in the effluent of conventionalsecondary WWTPs ranging from several hundred nanogramsper liter up to 65 μg L−1, despite exposure to a variety ofprocesses including photolysis, hydrolysis, biodegradationand sorption (Kolpin et al. 2002; Roberts and Thomas 2006;Yamamoto et al. 2009; Chen et al. 2012; Salgado et al. 2012).

Water treatment plants usually use granular activated car-bon to ‘soak up’ API molecules, such as acetaminophen. Thecarbon is then regenerated or disposed of by burning off thepollutants. However, this can result in a wide range of volatile,semi-volatile and non-volatile organic products, which cancontribute to anthropogenic secondary organic aerosol(SOA) formation, so the problem changes from one of waterpollution to one of atmospheric pollution. Moreover, SOASare also formed by the degradation of anthropogenic organicchemicals (Flores et al. 2014). On the other hand, heteroge-neous photocatalysis has also received a great deal of attentionas an advanced oxidation process (AOP) for the treatment ofthese pharmaceutical compounds including AMP (Yang et al.2008; Klavarioti et al. 2009; Liu et al. 2010; Peuravuori 2012).It relies on the generation of highly reactive •OH radicals asthe main oxidative species for the potential destruction andconversion of organics into harmless substances (Chiron et al.2000; Klavarioti et al. 2009). Hydroxyl radicals also play avital role in the sunlight-induced photolysis of organic con-taminants, including pharmaceuticals (Jasper and Sedlak2013; Zeng and Arnold 2013). Nano-sized TiO2 is one ofthe most widely used semiconductors for photocatalytic deg-radation, due to its availability, low cost, high chemical sta-bility, and low toxicity (Hoffmann et al. 1995; Herrmann1999; Liu et al. 2006; Fujishima et al. 2008). However,nanoparticles tend to aggregate in suspension, leading to arapid loss in active sites and photocatalytic efficiency. Inaddition, subsequent separation and recovery of the nanopar-ticles poses a key obstacle to large-scale implementation ofthis technology for water treatment. Therefore, many alterna-tives have been proposed using a variety of support materialsand different immobilization methods to prepare supportedTiO2 for the effective degradation of organic pollutants

(Shimizu et al. 2002; Vohra and Tanaka 2003; Chen et al.2006). Activated carbons (ACs) are an attractive supportoption due to their high surface area, suitable pore structure,high adsorption capability and low cost (Matos et al. 2001; Liet al. 2005; Wang et al. 2009). The physical properties of theAC support strongly affect the dynamics of photo-inducedcharges and adsorption (Cunningham et al. 1994).

In this work, granular activated carbonwas coated with TiO2

powder by ultrasonication to synthesize TiO2/AC compositephotocatalysts (integrated photocatalytic adsorbents (IPCAs)).These IPCAs have been used successfully for degradation ofvarious pharmaceutical compounds (Basha et al. 2010, 2011;Keane et al. 2011). The use of the IPCAs for removal ofacetaminophen from an aqueous system was investigated, andboth the adsorption characteristics and photocatalytic activity ofvarious IPCAs are reported. The kinetic analysis and modellingof AMP removal via adsorption and photodegradation, as wellas the influence of pH and photocatalyst concentration on AMPdegradation are discussed.

Experimental

The photocatalyst, TiO2 (AEROXIDE® P25, anatase/rutile (8:2)mixture with an average particle size of 21 nm) manufactured byEvonik Industries, and activated carbon, Aquasorb 2000manufactured by Jacobi Carbons, were used in this study.Acetaminophen was purchased from Sigma-Aldrich Inc.,Ireland and its structure is shown in Fig. 1. HPLC-grade meth-anol and water were purchased from Fisher Scientific Ltd.,Dublin, Ireland. Amber silanised HPLC vials and 90 mm diam-eter glass fibre filter paper (FB59077 equivalent toWhatmanNo.3) were purchased from Fisher Scientific, Ireland whilst 0.22 μmnylon syringe filterswere purchased fromPhenomenex Inc., UK.Pall nylon filters (0.2 μm pore size and 47 mm diameter) werepurchased from Sigma-Aldrich. A Bransonic® ultrasonic cleaner(5510 E-Mt) was used for mobile phase degassing and IPCApreparation.

Preparation of IPCAs

A low-tempera ture impregnat ion method us ingultrasonication was developed for applying TiO2 to the ACs(Ao et al. 2008). Aquasorb 2000 (6 g) and a mass of P25between 0.06 and 1.5 g was added to a 200-mL solution ofdeionised water and sonicated for 1 h in an ultrasonic bath.The IPCAs are denoted by their TiO2 loading ranging from 1to 25 wt% TiO2 to AC. Following overnight drying at 110 °Cthe resulting IPCAs were washed with deionised water toremove any excess P25. Finally, the photocatalysts were againdried at 110 °C and stored in sealed glass vials before use.

HO NH

OCH3

Fig 1 Acetaminophen (AMP): Molecular formula: C8H9NO2, molecularweight: 151.17, density: 1.263 g/cm3

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Apparatus and experimental procedures

Adsorption/photodegradation experiments used a borosilicateglass photochemical reactor with a cut-off of about 285 nmmanufactured by Ace Glass composed of a model 7841-06reactor vessel with a 1-L capacity and a model 7857 immersionwell, 290 mm in length with water cooling. To determine theeffects of adsorption and photocatalysis, adsorption experimentsin complete darkness were performed before photocatalysis. Allof the adsorption/photocatalysis studies utilized 1.5 g/L of IPCAin concentrations of AMP varying from 10 to 115 mg/L. WhilstAMP levels in treated effluents are typically in the range ofnanogram per liter to microgram per liter, in some instances,many industries discharge without proper treatment giving rise toconcentrations in the milligram per liter range. In the design ofany wastewater treatment system, the system is generally con-sidered for treating high concentration effluents. As a result, highconcentrations of AMP as a worst case scenario were used fortesting the IPCAs in this work. This indicates that IPCAs have amuch greater potential for treating low concentration effluents.The IPCA and AMP solution in the reactor was mixed using amagnetic stirrer. Aliquots were taken in duplicate atpredetermined times and syringe filtered with 0.22 μm nylonfilters and stored for analysis. The adsorption temperature andcontact time were 20 °C and 3 h, respectively. The pH wasadjusted using HCl and NaOH in the presence of 1 mM potas-sium phosphate buffer.

After dark adsorption for a period of 3 h, photodegradationof AMP on various IPCAs was conducted under UV illumi-nation. A 125 W medium pressure mercury lamp (TQ 150Heraeus Noblelight), with a light intensity in the range of4.86–4.88 mW/cm2, inserted in the centre of the reactor wasused as the light source. The change of the AMP concentrationduring UV irradiation was measured by withdrawing 3 mLsamples of the solution from the reactor at appropriate inter-vals. These samples were syringe filtered and analysed asdescribed in following section. Direct photolysis studies wereundertaken using UV irradiation without any catalyst to de-termine the baseline AMP photodegradation rate.

All experiments were conducted in replicate (at a minimumn=2). The average values are used in the study for clarity(without error bars being included in the figures), as thereproducibility of repeated runs was found to be within anacceptable limit (±5 %). Several specimens of IPCAs witheach loading of TiO2 were prepared and tested for consistency.

Analysis

The concentration of AMP was determined by a HPLC sys-tem consisting of an Agilent 1100 (Agilent Technologies, PaloAlto, Ca, USA) equipped with a UV–VIS detector. A 150×4.6 mm, 3.5 μm particle SunFire pentaflourophenyl propylreverse phase column was used for separation of the analytes.

The wavelength of the detector was set at 242 nm. Mobilephase consisted of 30 % methanol to to 70 % water, wasfiltered through nylon filters and degassed by ultrasonicationfor 30 min. The eluent flow rate was 1.0 mL min−1 withinjection volume 50 μL. The quantitative determination ofAMP was performed by using an external standard and thecalculations were based on the average peak areas of thestandard. The data was processed using Agilent ChemStation software B.02.01SR1.

Infrared spectra of the IPCAwere obtained using a Fourier-transform infrared spectrometer (FT-IR GX 2000, Perkin-Elmer). For the FT-IR study, 30 mg of finely ground IPCAwas pelleted with 300 mg of KBr (Sigma) in order to preparetranslucent sample disks. The FT-IR spectra were recordedwith 10 scans at a resolution of 4 cm−1. Analysis was per-formed on a freshly prepared 10 % TiO2 IPCA, a 10 % TiO2

IPCA that was used only to adsorb acetaminophen and a 10%TiO2 IPCA used for photodegradation.

The adsorption/photodegradation capacity of the IPCA, qe(mg/g) was calculated from the difference in AMP concentra-tion in the aqueous phase before and after adsorption/photodegradation, as per Eq. (1):

qe ¼V Co−Ceð Þ

Wð1Þ

where V is the volume of AMP solution (L), Co and Ce arethe initial and equilibrium concentration of AMP in solution(mg/L), respectively, and W is the mass of IPCA (g).

The removal efficiency of 10 % TiO2 IPCA for AMP wascalculated from Eq. (2):

Removal %ð Þ ¼ 100 Co−C fð ÞCo

ð2Þ

where Cf is the final concentration of AMP in solution (mg/L). The final concentration after adsorption is the initial con-centration for the calculation of photodegradation efficiency.

All of the model parameters were evaluated by non-linearregression using DATAFIT® software (Oakdale Engineering,USA). The optimization procedure requires an error functionto be defined in order to be able to evaluate the fitness of theequation to the experimental data. Apart from the regressioncoefficient (R2), the residual or sum of square error (SSE) andthe standard error (SE) of the estimate were also used to gaugethe goodness-of-fit. SSE can be defined as:

SSE ¼Xi¼1

m

Qi−qið Þ2 ð3Þ

SE can be defined as:

SE ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1

m−p

Xi¼1

m

Qi−qið Þ2s

ð4Þ

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where qi is the observation from the batch experiment i, Qi

is the estimate from the isotherm for corresponding qi,m is thenumber of observations in the experimental isotherm, and p isnumber of parameters in the regression model. Minimisationof SE and SSE values indicate better curve fitting. In thepresent study, the correlation coefficient, R2, SE, SSE, andpredicted qe/qm values (wherever applicable) were used todetermine the best fit model.

Results and discussion

Role of adsorption and photocatalytic degradation

To understand and compare adsorption mechanisms onto theIPCAs, adsorbate/adsorbent equilibrium properties were eval-uated in dark conditions. AMP adsorption studies were per-formed at 20 °C on various IPCAs over 3 h under stirredconditions for initial concentrations of AMP ranging from 10to 115 mg/L. The experimental adsorption equilibrium datafor the various IPCAs showed that the 10 % TiO2 IPCA hadthe highest capacity to adsorb AMP (Fig. 2). The capacity ofAMP adsorption on the IPCAs followed the order: 10 % TiO2

IPCA>5 % TiO2 IPCA>1 % TiO2 IPCA>25 % TiO2 IPCA.With the TiO2 content in the IPCA increasing from 1 to 10 %,the adsorption capacity increased from 10.8 to 19.9 mg/g.Further increase in TiO2 content resulted in lower adsorptioncapacity due to greater aggregation of the TiO2 particles on thesurface of the AC (Wang et al. 2008).

Experimental data for the AMP adsorption equilibrium onthe IPCAs were interpreted using the adsorption isotherm

models of Freundlich, Langmuir and Prausnitz-Radke, repre-sented by the Eqs. (5) to (7) (Ocampo-Pereza et al. 2011). TheLangmuir isotherm considers sorption as a chemical phenom-enon and restricted to a monolayer whereas the Freundlichisotherm reasonably applied in the low to intermediate con-centration ranges. At low concentrations, the Prausnitz-Radkeisotherm reduces to a linear isotherm and at high concentra-tions, it becomes the Freundlich isotherm.

qe ¼ K FC1ne ð5Þ

qe ¼qmKLCe

1þ KLCeand RL ¼ 1

1þ KLC0ð6Þ

qe ¼aRrRCe

1þ rRCeð Þα ð7Þ

where qe (mg/g) and Ce (mg/L) are the amount of AMPadsorbed per unit mass of IPCA, and the unadsorbed AMPconcentration in solution at equilibrium, respectively. KF

(L/mg) and n (dimensionless) signify the adsorption capacityand adsorption intensity for the Freundlich model. The max-imum amount of AMP adsorbed per adsorbent unit mass is qm(mg/g), KL is the Langmuir constant related to the affinity ofthe binding sites (L/mg) and RL is separation factor for theLangmuir model. The Prausnitz-Radke isotherm constants, aR(mg/g) and rR (L/mg) represent the maximum adsorptioncapacity and affinity, whilst α is the model exponent. TheFreundlich isotherm is empirical in nature, but was laterinterpreted as adsorption to heterogeneous surface or surfacessupporting sites of varied affinities, and has been used widelyto fit experimental data of liquid phase sorption, whereas theLangmuir isotherm model is an analytical equation basicallydeveloped for gas-phase adsorption on homogeneous surfacesand predicts a single maximum binding capacity (Fritz andSchluender 1974; Aksu and Kutsal 1991; Khan et al. 1997). Avalue of n in the range of 1–10 in the Freundlich modelindicates favourable adsorption, whilst KL in the Langmuirmodel is a coefficient attributed to the affinity between thesorbent and sorbate (Vadivelan and Kumar 2005).

The values of the model constants of all three isothermmodels along with the corresponding regression coefficient(R2), SE and SSE values for all IPCA-AMP systems arepresented in Table 1. Statistically, the Langmuir model wasthe best fit to the experimental data, with higher R2 and lowerSE and SSE values than the Freundlich and Prausnitz-Radkemodels. Moreover, the predicted adsorption capacities (32.3 to52.5 mg/g) of IPCAs by the Prausnitz-Radke model werehigher than both the experimental (10.8 to 19.9 mg/g) andthe Langmuir model (17.1 to 28.4 mg/g). The Freundlichmodel predicts that the AMP concentration on the sorbentwill increase as long as there is an increase in the AMP

0

5

10

15

20

25

0 20 40 60 80 100

qe (m

g/g

)

Ce (mg/L)

Experimental-1%TiO2

IPCA

Experimental-5%TiO2

IPCA

Experimental-10%TiO2

IPCA

Experimental-25%TiO2 IPCA

Langmuir model

Fig. 2 A comparison of the fitting of the experimental data with theLangmuir model adsorption isotherm for IPCAs. (open diamond) Exper-imental-1 % TiO2 IPCA, (filled upright triangle) Experimental-5 % TiO2

IPCA, (open square) Experimental-10 % TiO2 IPCA (plus sign) Exper-imental-25 % TiO2 IPCA, (solid line) Langmuir model

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concentration in the liquid phase. However, the experimentaldata indicate that an isotherm plateau is reached at a limitingvalue of the solid-phase concentration. This plateau is notpredicted by the Freundlich equation (Allen et al. 1988). Then values were in the range of 1.19 to 1.86, implying that thedistribution of TiO2 on the surface of activated carbon (AC) iseven (Xue et al. 2011). In addition, the separation factor (RL)values of the Langmuir model indicate that AMP adsorptiononto IPCAwas favourable (Table 1). Figure 2 shows that thetheoretical plots of the Langmuir isotherm compare well withexperimental data.

The solute uptake rate (governing the contact time of theadsorption reaction) is one of the important kinetic character-istics that define the efficiency of adsorption. Pseudo-first-order (Lagergren and Svenska 1898) and pseudo-second-order (Ho and McKay 1998) equations were fitted to theexperimental data and thus elucidate the adsorption kineticprocess.

The pseudo-first-order model is expressed as:

qt ¼ qe 1−e−k1t� � ð8Þ

where qt and qe are the amount adsorbed at time t and atequilibrium (mg/g), and k1 is the pseudo-first-order rate con-stant for the adsorption process (min−1).

The pseudo-second-order model can be represented in thefollowing form:

qt ¼k2q2e t

1þ k2qeth ¼ k2q

2e

ð9Þ

where k2 and h are the pseudo-second-order rate constant(g/mg/min) and the initial adsorption rate (mg/g/min),respectively.

The fitted parameters to the kinetic models are listed inTable 2. The regression coefficients (R2) for the pseudo-first-order kinetic model were relatively low (0.722–0.792) andalso both SE and SSE values were >1 for adsorption of AMPon IPCAs. In comparison, the R2 values of pseudo-second-order model were significantly higher, ranging from 0.985 to0.994. Correspondingly, SE and SSE values were lower andranged from 0.252 to 0.521 and 0.158 to 0.712, respectively.Furthermore, the agreement between the experimental (10.8 to19.9 mg g−1) and predicted equilibrium sorption capacities(14.1 to 22.3 mg/g) confirm a better fit to the pseudo-second-order model (Fig. 3). These results support the model assump-tion that the adsorption is mainly monolayer adsorption(Kumar et al. 2005). Generally, the rate constant k2 increasedwith increase in TiO2 content up to 10 % (Table 2). The rateconstant and the initial adsorption rate, h, as well as theequilibrium adsorption level, qe, were higher for the 10 %IPCA than for any of the other IPCAs. This may be attributedto the increased adsorption active sites on the surface withincreasing TiO2 content (Basha et al. 2011). It can be con-cluded from the kinetics study that the TiO2 content on thesurface of the IPCA significantly affects the adsorption kinet-ics of AMP on IPCAs.

Table 1 Isotherm model parameters for AMP adsorption on variousIPCAs

Models % (weight) of TiO2 in IPCA

1 5 10 25

Freundlich

KF (L/g) 1.665 1.576 1.599 2.031

n 1.861 1.316 1.197 1.672

R2 0.873 0.848 0.925 0.855

SE 1.637 1.941 1.293 1.859

SSE 3.226 4.085 2.574 3.885

Langmuir

qm (mg/g) 18.238 24.344 28.429 17.134

KL (L/mg) 0.031 0.042 0.044 0.014

RL 0.448 0.385 0.376 0.615

R2 0.982 0.986 0.991 0.980

SE 0.124 0.115 0.102 0.132

SSE 0.074 0.062 0.017 0.091

Prausnitz-Radke

aR (mg/g) 46.175 49.304 52.547 32.348

rR (L/mg) 0.046 0.061 0.069 0.037

αR 0.764 0.782 0.812 0.767

R2 0.790 0.781 0.794 0.789

SE 4.288 5.095 4.059 4.868

SSE 18.019 23.012 16.004 21.096

Table 2 Kinetic parameters for AMP adsorption onto various IPCAs

Models 1 % IPCA 5 % IPCA 10 % IPCA 25 % IPCA

Kinetic

Pseudo-first-order

k1 (min−1) 0. 151 0.242 0.374 0.117

qe (mg/g) 36.74 39.41 46.29 33.47

R2 0.792 0.781 0.722 0.730

SE 2.119 2.722 3.412 3.027

SSE 5.210 6.257 10.118 9.457

Pseudo-second-order

qe (mg/g) 16.34 19.86 22.31 14.12

k2 (g/mg/min) 0.056 0.077 0.082 0.042

h (mg/g/min) 14.952 30.370 40.814 8.374

R2 0.994 0.990 0.992 0.985

SE 0.252 0.341 0.311 0.521

SSE 0.158 0.229 0.203 0.712

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Photocatalytic degradation of AMP

The effect of irradiation with UVon the photodegradation rate ofAMP in the presence of suspended TiO2 and 10%TiO2 IPCA isshown in Fig. 4. Dark conditions (no UV) with 10 % IPCA afterequilibrium adsorption was included as a control, showing aminor decrease in AMP concentration. Irradiation in the absenceof IPCA/TiO2 (photolysis) showed insignificant conversion ofAMP. The UV/free-TiO2 system degraded 52 % of the initialAMP during the first 180 min of irradiation with no subsequent

additional AMP degradation. The photodegradation rate of AMPon 10% TiO2 IPCAwas higher than TiO2 alone, with no plateauindicated over the 240min. In addition, the performance of 10%TiO2 is better than TiO2 on its own and also uses lessphotocatalyst with only 10 wt% TiO2 being used in the prepara-tion of 10 % TiO2 IPCAs. The main advantage of IPCAs overTiO2 is that the IPCA can be easily separated from the solutionafter irradiation and recycled whilst the separation and recyclingof free TiO2 is an extremely difficult task. Whilst it has beenshown that the particle size of the photocatalyst in composites ofthis nature has an impact (Zhang et al. (2013)), the TiO2 used inthis study is at the size that is readily available commercially.Since the composites prepared in this study showed increasedphotoactivity in comparison with the free TiO2, no further parti-cle sizes were investigated.

The results of the photocatalytic degradation of AMP overthe range of IPCAs are shown in Fig. 5. The degradation ratewas highest with the 10 % TiO2 IPCA system and decreased inthe order: 10 % TiO2 IPCA>5% TiO2 IPCA 1% TiO2 IPCA>25 % TiO2 IPCA. This sequence is identical to the orderexhibited with respect to adsorption capacity (Table 1), and isstrongly related to the TiO2 content of IPCAs. There was littledifference in the photodegradation rate between 1 and 25 %TiO2 loadings, however, a significant variation between 1 and10 % and 10 and 25 % TiO2 loadings was observed. Above acertain level of TiO2, excess TiO2 particles hinder the UV lightpenetration (Lu et al. 1999). As the increase in thephotodegradation rate between 5 and 10 % TiO2 loadings wasmarginal, further TiO2 loadings between 11 to 24 % were notstudied. In both adsorption and photodegradation, optimal per-formance was achieved at a TiO2 loading of 10 %. The higherefficiency of the photocatalyst can be attributed to the greateradsorption of AMP on the photocatalyst. The adsorption ca-pacity of the AC enhances the chance of •OH radicals attacking

Fig. 3 Comparison of the pseudo-second-order kinetic model for AMPadsorption onto IPCAs. (open diamond) Experimental-1 % TiO2 IPCA,(filled upright triangle) Experimental-5 % TiO2 IPCA, (open square)Experimental-10 % TiO2 IPCA (plus sign) Experimental-25 % TiO2

IPCA, (solid line) Pseudo-second order model

Fig. 4 Photocatalytic degradation of AMP. (filled square) Photolysis,(filled upright triangle) Adsorption in dark with 10 % TiO2 IPCA, (opendiamond) Photocatalysis with TiO2, (ex symbol) Photocatalysis with 10%TiO2 IPCA

Fig. 5 Photocatalytic activity of IPCAs (filled square) 1 % TiO2 IPCA,(filled upright triangle) 5 % TiO2 IPCA, (open diamond) 10 % TiO2

IPCA, (ex symbol) 25 % TiO2 IPCA

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the adsorbed AMP, resulting in faster degradation. Also, thedelocalization capacity of the AC framework can efficientlyseparate the electrons and holes produced during photoexcita-tion of TiO2, thus enhancing the photocatalytic efficiency(Cojocaru et al. 2009).

Generally, the greater the amount of TiO2 present, the higherthe photocatalytic reaction rate due to increased generation ofholes and hydroxyl radicals. Whilst the size of the TiO2 parti-cles and therefore the surface available for absorption is impor-tant in photodegradation studies, this is not as relevant in thecurrent study because the TiO2 was immobilised on activatedcarbon for preparing the IPCAs, which were then employed foradsorption/photodegradation of AMP. Initially, the AMP wasadsorbed and then the adsorbed AMP was photodegraded byhydroxyl radicals generated through TiO2 photocatalysis. Thisillustrates the unique features of the IPCAs, which have bothadsorption and photodegradation capabilities with the TiO2

playing a crucial role in both processes.In addition, more TiO2may also induce greater aggregation

of the TiO2 particles at the surface of the AC, leading to areduction in reaction rates as seen here at 25 % TiO2 loading(Zhu et al. 2000). Furthermore, the high degradation rate of10 % TiO2 IPCA can be attributed to the synergistic effects ofadsorptive properties of the AC and photocatalytic activity ofthe TiO2 in the composite. A concentration effect for thechemical reactions is seen due to the surface adsorption ofAMP on the IPCA substrate, whilst degradation intermediateson the site of the TiO2 enhance the photocatalytic degradativeactivity (Xue et al. 2011).

Kinetics of AMP degradation

In general, the photocatalytic kinetics follows the Langmuir–Hinshelwood (L–H)model in which the rate of a unimolecularsurface reaction, r, is proportional to the surface coverage, θ:

r ¼ −dC

dt¼ krθ ¼ krKC0

1þ KC0ð10Þ

where kr is the reaction rate constant, θ is the fraction of thesurface covered by the reactant, K is the adsorption coefficientof the reactant and C0 is the initial concentration of thereactant. Equation (10) is only applicable when both thereactant and solvent are adsorbed on the surface withoutcompeting for the same active sites.

Integration of Eq. (10) yields Eq. (11):

lnC0

C

� �þ K C0−Cð Þ ¼ krKt ð11Þ

Owing to the complex mechanisms of the reactions involved,it is difficult to describe entire photocatalytic degradation rate

using a single model. Thus, kinetic modelling of a photocatalyticprocess is usually evaluated by the half time of the reaction(Guettai and Amar 2005). This can be obtained from the slopeand the initial concentration in an experiment in which thevariation of the AMP concentration is measured as a functionof time. Half-life, t1/2, describes the time taken for the concentra-tion of a reactant to fall to half of its initial value. It is observed tobe a function of initial AMP concentration (C0) as per Eq. (12).

t1=2 ¼ C0

2krþ ln2

krKð12Þ

Figure 6 shows the variation of AMP concentration with10 % TiO2 IPCA as a function of reaction time for fourdifferent initial concentrations. The half time increased withincreasing initial concentration of AMP and there was a linearrelationship between t1/2 and C0. Non-linear regression wasused to estimate the parameters involved in the Langmuir–Hinshelwood (L–H) kinetic expressions (Table 3). High re-gression coefficients (R2=0.984–0.998) and low SE (0.164–0.288) and SSE (0.629–0.922) values for all IPCAs and TiO2

were obtained, indicating that the reaction kinetics of AMPdegradation obeyed the L–H model.

The dependence of the rate constant kr and adsorption equi-librium constant K on the TiO2 content of IPCAs is shown inFig. 7. The rate constant showed an initial increase and then adecrease with increasing content of TiO2 (with a maximum at aTiO2 content of 10 %), whilst the inverse relationship wasobserved for the adsorption equilibrium constant. The degrada-tion rate would be expected to depend on the TiO2 content, withkr being lower for a smaller TiO2 content (Kim et al. 2008).However, the adsorption strength of the substrate is an importantfactor affecting the photoactivity of a catalyst. The IPCA con-taining 1 % TiO2 content had a high adsorption equilibriumconstant, but exhibited lower degradation rates than the 10 %

90

100

110

120

130

0 20 40 60 80 100 120

t 1/2

C0 (mg/L)

Fig. 6 The Langmuir–Hinshelwood plot for AMP photodegradationwith 10 % TiO2 IPCA

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TiO2 IPCA, presumably due to low TiO2 content and possiblyretardation of diffusion of the adsorbed AMP by high adsorptionstrength (Bhattacharyya et al. 2004; Li et al. 2006).

Effect of initial pH

The solution pH is an important operating parameter that canaffect photocatalytic reactions. The influence of pH onphotodegradation of AMP was evaluated using the 10 %TiO2 IPCA. Experiments were conducted at a pH range from3 to 11 with an initial AMP concentration of 25 mg/L (Fig. 8).Photodegradation efficiency increased from 55 % at pH 3 to80 % at pH 9, decreasing to 46 % at pH 11. It has beenreported that the elimination of AMP from wastewater anddrinking water samples by suspended TiO2-photocatalysis ishighly efficient at pH 9.0 (Yang et al. 2008; Zhang et al. 2008).In general, the TiO2 in an IPCA presents an amphotericcharacter, and either a positive or a negative charge can be

developed on its surface (Bayarri et al. 2005). AMP (pKa=9.38) is undissociated and present in a neutral form at asolution pH lower than pKa (Brunner et al. 1998; Yang et al.2008). So, when the reaction takes place in an acidic solution(pH 2 or 4), multilayers of AMP molecules are formed on thesurface of TiO2 particles due to the electrostatic attractionbetween the positively charged TiO2 particles and AMP mol-ecule (Velegraki and Mantzavinos 2008). This multilayer actsas a blocking mechanism, which prevents further interactionsbetween catalytic active centres and AMP molecules.Moreover, in acidic solution, the redox potentials are greaterthan 2.8 and 2.0 V according to Eqs. 13 and 14 respectively(Tang and Huang 1995):

hþ VBð Þ þ H2O→•OHþ Hþ ð13Þ

hþ VBð Þ þ OH−→•OHad ð14Þ

Table 3 Kinetic parameters andEEO for photodegradation ofAMP on various IPCAs

Model TiO2 content in IPCAs (%) 100 % TiO2

1 5 10 25

Langmuir–Hinshelwood

kr (mg/L/min) 0.838 1.241 1.511 0.936 0.742

K (L/mg) 0.0079 0.0059 0.0050 0.0071 0.0079

kapp (=kr×K) 0.0066 0.0073 0.0076 0.0066 0.0059

R2 0.991 0.998 0.989 0.984 0.990

SE 0.196 0.164 0.212 0.288 0.204

SSE 0.802 0.629 0.825 0.922 0.726

EEM (kWh/kg/order) 121.04 107.21 96.70 116.82 135.24

Fig. 7 Rate constant (kr) of AMP degradation reaction and the adsorptionequilibrium constant (K) as a function of the TiO2 content in IPCAcatalyst (filled diamond) kr (open upright triangle) K

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 100 200 300

C/C

0

Irradiation time (min)

pH 3

pH 5.9

pH 9

pH 11

Fig. 8 Effect of pH on photocatalytic degradation of AMP by 10% TiO2

IPCA, initial AMP concentration: 25 mg/L

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Hence, the formation of hydroxyl radicals will be thermo-dynamically unfavourable and thus suppressed. As a result, theformation of hydroxyl radicals increases with an increase of pHfrom 3 to 9, leading to higher removal efficiency forphotodegradation of AMP. Further increase in pH to 11 resultsin the formation of superoxide radical anions (O2

•−) throughoxygen reduction, which suppresses the generation of H2O2

and •OH. This phenomenon causes a reduction in the degrada-tion of AMP. Furthermore, at pH 11 the IPCA surface becomesmore negative, and the hydroxyl group in AMP is changed intoa phenoxide ion. Thus, the higher repulsion between the nega-tive surface and AMP contributes to the lower degradation rate.

Influence of the photocatalyst concentration

To optimize AMP degradation using 10 % TiO2 IPCA, theeffect of increasing the photocatalyst concentration from 0.75to 2.50 g/L on the photocatalytic degradation rate of AMPwasinvestigated (Fig. 9). The AMP photodegradation increasedwith increasing photocatalyst concentration up to a concen-tration of 2.0 g/L and then decreased. The increase in thephotodegradation rate from 0.75 to 2.0 g/L catalyst concen-tration is attributable to increased •OH production. As theconcentration of the IPCA is increased, the number of photonsabsorbed and the number of AMP molecules adsorbed areincreased with respect to an increase in the number of catalystmolecules. The density of the molecules in the area of illumi-nation also increases and thus the photodegradation rate isenhanced. Increasing IPCA concentration to 2.5 g/L resultedin a decrease in degradation rate. This can be attributed to theAMP molecules available not being sufficient for adsorptionby the increased number of catalyst molecules. The additionalcatalyst is not involved in the photocatalytic activity and the

rate does not increase with an increase in the amount ofcatalyst beyond a certain limit. The aggregation of the catalystmolecules at high concentrations must also be considered,which causes a decrease in the number of active surface sites(Goncalves et al. 1999; Wong and Chu 2003; Gogate andPandit 2004). Therefore, an optimum concentration of 1.5 g/L was considered in the present study.

Catalyst reuse

In order to test the feasibility of cyclic use of the 10 % TiO2

IPCA, four cycles of photocatalytic degradation of AMP wereperformed (Fig. 10). Each experiment was carried out underidentical conditions (1 L AMP solution with an initial con-centration of 74.5 mg/L, 1.5 g photocatalyst, and 240 minirradiation time). After each experiment, the solution residuefrom the photocatalytic degradation was filtered, washed andthe solid was dried. The dried catalyst samples were usedagain for AMP degradation. The percentage AMP removalremains at approximately 69.5 % after the four cycles, with adecrease in IPCA activity of not greater than 4 %. The ob-served loss of photocatalyst efficiency can be due to thepresence of reaction intermediates adsorbed onto the activesites of the catalyst. Concurrently, mechanical stress on thecatalyst particles can occur due to stirred reactor conditions.Both factors account for a reduction of the number of availablephotoactive sites and therefore a decrease in thephotodegradation capability of the IPCA.

Electrical energy for photodegradation

The cost involved in the treatment of wastewater is one of thekey aspects that must be considered in the evaluation of a newtechnology. Since photocatalysis is an energy-intensive process,electricity can represent a major part of the operating costs. The

0

0.2

0.4

0.6

0.8

1

0 50 100 150 200 250

C/C

0

Irradiation time (min)

Fig. 9 Effect of photocatalyst concentration on degradation of AMP by10 % TiO2 IPCA, initial AMP concentration: 25 mg/L (filled diamond)0.75 g/L (open square) 1.5 g/L (filled upright triangle) 2.0 g/L (exsymbol) 2.5 g/L

0

20

40

60

80

100

30 60 120 180 240

Ph

oto

deg

rad

ati

on

of

AM

P (

%)

Irradiation time (min)

Cycle1

Cycle2

Cycle3

Cycle4

Fig. 10 Removal of AMP by 10 % TiO2 IPCA in multiple cycles

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International Union of Pure and Applied Chemistry (IUPAC)has proposed two figures-of-merit for advanced oxidation pro-cesses (AOPs) on the use of electricity (Daneshvar et al. 2005).In the case of high-pollutant concentrations, the appropriatefigure-of-merit is the electrical energy per order (EEM), definedas the number of kilowatt hours of electrical energy required toreduce the concentration of a pollutant by 1 order of magnitudein a unit mass of contaminant (Bolton et al. 2001).

The EEM (kWh/kg/order) may be calculated from:

EEO ¼ P � t � 1; 000

V �M � C0−C fð Þ ð15Þ

where P is the input power (kW) to the photocatalysissystem, t is the irradiation time (min), V is the volume of water(L) in the reactor,M is the molar mass of pollutant (g/mol) andC0 andCf are the initial and final pollutant concentrations (mg/L), respectively.

The EEM values for degradation of AMP by various IPCAsand TiO2 are listed in Table 3. They range from 96.70 kW/kg/order for 10 % TiO2 IPCA to 135.24 kW/kg/order for freesuspended TiO2. The 10 % TiO2 IPCA exhibited the highestelectrical efficiency as expected. There is potential scope forimprovement of the photoreactor system to further reduce theEEO.

Fourier-transform infrared spectra

FT-IR analysis was employed to elucidate the process ofadsorption and photodegradation of AMP on IPCA. The FT-IR spectra of the 10%TiO2 IPCA after the adsorption of AMPin the dark and after photodegradation along with the spectrumof virgin 10 % TiO2 IPCA are presented in Fig. 11. After AMP

adsorption, new absorption bands between 1,450–1,850 cm−1

appeared in the spectra, which may be assigned to the variousC=O (carbonyl) and C=C (aromatic) stretching, as well as N-H(amide) bending vibrations, respectively (Terzyk 2001)(Fig. 11b). In general, the acetaminophen molecule consistsof phenol (ArylOH), amide (CONH) and arene functionalgroups (Danten et al. 2006). These functionalities exhibitstrong absorptions in the region 3,438–3,600 cm−1 as well as1,400–1,680 cm−1 and 950–1,300 cm−1, respectively (Burginaet al. 2004; Danten et al. 2006). After irradiation of the AMPloaded 10 % TiO2 IPCA, peaks in this range disappeared(Fig. 11c) and the spectrum resembles that of virgin material,which indirectly confirms the adsorption/photodegradationmechanism of the IPCA action.

Conclusion

Adsorption experiments showed that the IPCAs had a highadsorption capacity for AMP with the amount adsorbed pro-portional to the TiO2 loading with a maximum at 10wt% TiO2

loading. The adsorption isotherms followed the Langmuirmodel whilst the kinetics were best described by a pseudo-second-order model. The effects of photocatalyst dosage,initial solution pH and irradiation time on the degradation ofAMP were studied. The photocatalytic degradation kineticswas found to follow the Langmuir–Hinshelwood model andthe values of the rate constants, KAds and kL–H, were depen-dent on the TiO2 loadings. The 10 % TiO2 IPCA exhibited thehighest rate constant, kL–H of 1.511 mg/L/min, a low adsorp-tion constant, Kads of 0.005 L/mg and achieved an overallAMP removal of 72 % within 240 min. Nitro and carbonylfunctional groups present in the 10 % TiO2 IPCA responsiblefor the photodegradation-adsorption effect were confirmed byFT-IR analysis. The reusability of the 10 % TiO2 IPCA wasonly slightly decreased (4 %) after four photodegradationcycles, and it exhibited the lowest electrical energy consump-tion per order of magnitude for photocatalytic degradation ofAMP. The 10 % TiO2 IPCA is a very promising photocatalystfor the degradation of acetaminophen due to its high perfor-mance and potential for recycling. The results clearly showthat these bifunctional materials could potentially be used inthe decontamination of organic pollutants from water/wastewater and highlights the practical application in environ-mental pollution management.

Acknowledgments SB wishes to gratefully acknowledge financialsupport from the Irish Research Council for Science, Engineering andTechnology (IRCSET) in the form of IRCSET EMPOWER postdoctoralfellowship and the Central Salt and Marine Chemicals Research Institute(CSMCRI) for grant of study/earned leave. DK gratefully acknowledgesthe financial support from the STRIVE program from the EnvironmentalProtection Agency (EPA) Ireland and technical assistance from ENVAIreland Ltd and National Chemical Company of Ireland.

Fig. 11 FT-IR spectra (percentage transmission versus scanning wave-length) of (a) virgin 10 % TiO2 IPCA, (b) 10 % TiO2 IPCA after AMPadsorption and (c) 10 % TiO2 IPCA after photodegradation of AMP

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