Environ Sci Pollut Res DOI 10.1007/s11356-014-3411-9

RESEARCH ARTICLE

UV-induced photocatalytic degradation of aqueous acetaminophen: 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 carbon (AC), termed as integrated photocatalytic adsorbents (IPCAs), were prepared by ultrasonication and investigated for the photocatalytic degradation of acetaminophen (AMP), a common analgesic and antipyretic drug. The IPCAs showed high affinity towards AMP (in dark adsorption studies), with the amount adsorbed proportional to the TiO2 content; the highest adsorption was at 10 wt% TiO2. Equilibrium isotherm studies showed that the adsorption followed the Langmuir model, indicating the dependence of the reaction on an initial adsorption step, with maximum adsorption capacity of 28.4 mg/g for 10 % TiO2 IPCA. The effects of initial pH, catalyst amount and initial AMP concentration on the photocatalytic degradation rates were studied. Generally, the AMP photodegradation activity of the IPCAs was better than that of bare TiO2. Kinetic studies on the photocatalytic degradation of AMP under UV suggest that the degradation followed Langmuir–Hinshelwood (L–H) kinetics, with an adsorption Responsible editor: Philippe Garrigues A. Morrissey (*) Business School, Dublin City University, Dublin 9, Ireland e-mail: [email protected] S. Basha Discipline of Marine Biotechnology and Ecology, CSIR-Central Salt and Marine Chemicals Research Institute, Bhavagnar 364002, Gujarat, India D. Keane : J. Lawler : J. M. Tobin School of Biotechnology, Dublin City University, Dublin 9, Ireland K. Nolan School of Chemical Sciences, Dublin City University, Dublin 9, Ireland M. Oelgemöller College of Science, Technology and Engineering, James Cook University, Townsville, Queensland 4811, Australia

rate constant (K) that was considerably higher than the photocatalytic rate constant (kr), indicating that the photocatalysis of 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) in surface water is an emerging environmental problem throughout the world, due to their continuous input and persistence in the aquatic ecosystem (Santos et al. 2010). The production volume of APIs has been estimated as several thousands of tons per year (Federsel 2013), which may enter the aquatic environment after ingestion and subsequent excretion, either without modification or in a partially metabolised form (Halling-Sørensen et al. 1998; Stackelberg et al. 2007). Several studies have shown that some pharmaceutical compounds are neither eliminated during wastewater treatment nor biodegraded in the environment (Daughton and Ternes 1999, Deegan et al. 2011). Many studies have reported a large variety of pharmaceutical compounds at concentrations ranging from nanogram per liter to milligram per liter in sewage treatment plant effluents, natural waters and even in drinking water (Ternes 1998; Heberer et al. 2002; Nikolaou et al. 2007; Lacey et al. 2012). Whilst some of the reported levels are much lower than those applied for therapeutic use, the potential human health effects associated with chronic exposure to trace levels of these compounds are still unknown (Kummerer 2008). Acetaminophen (AMP), also known as paracetamol (Nacetyl-4-aminophenol) (Fig. 1), is a common analgesic and antipyretic drug that is used for the relief of fever, headaches, and other minor aches and pains. It is one of the most widely

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O CH3 HO

NH

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

used over-the-counter medications, and AMP-containing analgesics are one of the most prescribed medications in Ireland with 0.88 million prescriptions during 2003 (Usher et al. 2005). Most (85–95 %) of AMP taken is excreted from the body during therapeutic use (Forrest et al. 1982; Usher et al. 2005). AMP has been detected in the effluent of conventional secondary WWTPs ranging from several hundred nanograms per liter up to 65 μg L−1, despite exposure to a variety of processes including photolysis, hydrolysis, biodegradation and 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 carbon to ‘soak up’ API molecules, such as acetaminophen. The carbon is then regenerated or disposed of by burning off the pollutants. However, this can result in a wide range of volatile, semi-volatile and non-volatile organic products, which can contribute to anthropogenic secondary organic aerosol (SOA) formation, so the problem changes from one of water pollution to one of atmospheric pollution. Moreover, SOAS are also formed by the degradation of anthropogenic organic chemicals (Flores et al. 2014). On the other hand, heterogeneous photocatalysis has also received a great deal of attention as an advanced oxidation process (AOP) for the treatment of these 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 as the main oxidative species for the potential destruction and conversion of organics into harmless substances (Chiron et al. 2000; Klavarioti et al. 2009). Hydroxyl radicals also play a vital role in the sunlight-induced photolysis of organic contaminants, including pharmaceuticals (Jasper and Sedlak 2013; Zeng and Arnold 2013). Nano-sized TiO2 is one of the most widely used semiconductors for photocatalytic degradation, due to its availability, low cost, high chemical stability, and low toxicity (Hoffmann et al. 1995; Herrmann 1999; Liu et al. 2006; Fujishima et al. 2008). However, nanoparticles tend to aggregate in suspension, leading to a rapid loss in active sites and photocatalytic efficiency. In addition, subsequent separation and recovery of the nanoparticles poses a key obstacle to large-scale implementation of this technology for water treatment. Therefore, many alternatives have been proposed using a variety of support materials and different immobilization methods to prepare supported TiO2 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 support option due to their high surface area, suitable pore structure, high adsorption capability and low cost (Matos et al. 2001; Li et al. 2005; Wang et al. 2009). The physical properties of the AC support strongly affect the dynamics of photo-induced charges and adsorption (Cunningham et al. 1994). In this work, granular activated carbon was coated with TiO2 powder by ultrasonication to synthesize TiO2/AC composite photocatalysts (integrated photocatalytic adsorbents (IPCAs)). These IPCAs have been used successfully for degradation of various pharmaceutical compounds (Basha et al. 2010, 2011; Keane et al. 2011). The use of the IPCAs for removal of acetaminophen from an aqueous system was investigated, and both the adsorption characteristics and photocatalytic activity of various IPCAs are reported. The kinetic analysis and modelling of AMP removal via adsorption and photodegradation, as well as the influence of pH and photocatalyst concentration on AMP degradation are discussed.

Experimental The photocatalyst, TiO2 (AEROXIDE® P25, anatase/rutile (8:2) mixture with an average particle size of 21 nm) manufactured by Evonik Industries, and activated carbon, Aquasorb 2000 manufactured 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 methanol and water were purchased from Fisher Scientific Ltd., Dublin, Ireland. Amber silanised HPLC vials and 90 mm diameter glass fibre filter paper (FB59077 equivalent to Whatman No. 3) were purchased from Fisher Scientific, Ireland whilst 0.22 μm nylon syringe filters were purchased from Phenomenex Inc., UK. Pall nylon filters (0.2 μm pore size and 47 mm diameter) were purchased from Sigma-Aldrich. A Bransonic® ultrasonic cleaner (5510 E-Mt) was used for mobile phase degassing and IPCA preparation.

Preparation of IPCAs A low-temperature impregnation method using ultrasonication was developed for applying TiO2 to the ACs (Ao et al. 2008). Aquasorb 2000 (6 g) and a mass of P25 between 0.06 and 1.5 g was added to a 200-mL solution of deionised water and sonicated for 1 h in an ultrasonic bath. The IPCAs are denoted by their TiO2 loading ranging from 1 to 25 wt% TiO2 to AC. Following overnight drying at 110 °C the resulting IPCAs were washed with deionised water to remove any excess P25. Finally, the photocatalysts were again dried at 110 °C and stored in sealed glass vials before use.

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Apparatus and experimental procedures Adsorption/photodegradation experiments used a borosilicate glass photochemical reactor with a cut-off of about 285 nm manufactured by Ace Glass composed of a model 7841-06 reactor vessel with a 1-L capacity and a model 7857 immersion well, 290 mm in length with water cooling. To determine the effects of adsorption and photocatalysis, adsorption experiments in complete darkness were performed before photocatalysis. All of the adsorption/photocatalysis studies utilized 1.5 g/L of IPCA in concentrations of AMP varying from 10 to 115 mg/L. Whilst AMP levels in treated effluents are typically in the range of nanogram per liter to microgram per liter, in some instances, many industries discharge without proper treatment giving rise to concentrations in the milligram per liter range. In the design of any wastewater treatment system, the system is generally considered for treating high concentration effluents. As a result, high concentrations of AMP as a worst case scenario were used for testing the IPCAs in this work. This indicates that IPCAs have a much greater potential for treating low concentration effluents. The IPCA and AMP solution in the reactor was mixed using a magnetic stirrer. Aliquots were taken in duplicate at predetermined times and syringe filtered with 0.22 μm nylon filters and stored for analysis. The adsorption temperature and contact time were 20 °C and 3 h, respectively. The pH was adjusted using HCl and NaOH in the presence of 1 mM potassium phosphate buffer. After dark adsorption for a period of 3 h, photodegradation of AMP on various IPCAs was conducted under UV illumination. A 125 W medium pressure mercury lamp (TQ 150 Heraeus Noblelight), with a light intensity in the range of 4.86–4.88 mW/cm2, inserted in the centre of the reactor was used as the light source. The change of the AMP concentration during UV irradiation was measured by withdrawing 3 mL samples of the solution from the reactor at appropriate intervals. These samples were syringe filtered and analysed as described in following section. Direct photolysis studies were undertaken using UV irradiation without any catalyst to determine the baseline AMP photodegradation rate. All experiments were conducted in replicate (at a minimum n=2). The average values are used in the study for clarity (without error bars being included in the figures), as the reproducibility of repeated runs was found to be within an acceptable limit (±5 %). Several specimens of IPCAs with each loading of TiO2 were prepared and tested for consistency. Analysis The concentration of AMP was determined by a HPLC system consisting of an Agilent 1100 (Agilent Technologies, Palo Alto, Ca, USA) equipped with a UV–VIS detector. A 150× 4.6 mm, 3.5 μm particle SunFire pentaflourophenyl propyl reverse phase column was used for separation of the analytes.

The wavelength of the detector was set at 242 nm. Mobile phase consisted of 30 % methanol to to 70 % water, was filtered through nylon filters and degassed by ultrasonication for 30 min. The eluent flow rate was 1.0 mL min−1 with injection volume 50 μL. The quantitative determination of AMP was performed by using an external standard and the calculations were based on the average peak areas of the standard. The data was processed using Agilent Chem Station software B.02.01SR1. Infrared spectra of the IPCA were obtained using a Fouriertransform infrared spectrometer (FT-IR GX 2000, PerkinElmer). For the FT-IR study, 30 mg of finely ground IPCA was pelleted with 300 mg of KBr (Sigma) in order to prepare translucent sample disks. The FT-IR spectra were recorded with 10 scans at a resolution of 4 cm−1. Analysis was performed 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 concentration in the aqueous phase before and after adsorption/ photodegradation, as per Eq. (1): V ðC o −C e Þ qe ¼ ð1Þ W where V is the volume of AMP solution (L), Co and Ce are the 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 was calculated from Eq. (2): Removalð%Þ ¼

100ðC o −C f Þ Co

ð2Þ

where Cf is the final concentration of AMP in solution (mg/ L). The final concentration after adsorption is the initial concentration for the calculation of photodegradation efficiency. All of the model parameters were evaluated by non-linear regression using DATAFIT® software (Oakdale Engineering, USA). The optimization procedure requires an error function to be defined in order to be able to evaluate the fitness of the equation to the experimental data. Apart from the regression coefficient (R2), the residual or sum of square error (SSE) and the standard error (SE) of the estimate were also used to gauge the goodness-of-fit. SSE can be defined as: SSE ¼

m X

ðQi −qi Þ2

ð3Þ

i¼1

SE can be defined as: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi m 1 X ðQi −qi Þ2 SE ¼ m−p i¼1

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 the number of observations in the experimental isotherm, and p is number of parameters in the regression model. Minimisation of SE and SSE values indicate better curve fitting. In the present study, the correlation coefficient, R2, SE, SSE, and predicted qe/qm values (wherever applicable) were used to determine the best fit model.

models of Freundlich, Langmuir and Prausnitz-Radke, represented by the Eqs. (5) to (7) (Ocampo-Pereza et al. 2011). The Langmuir isotherm considers sorption as a chemical phenomenon and restricted to a monolayer whereas the Freundlich isotherm reasonably applied in the low to intermediate concentration ranges. At low concentrations, the Prausnitz-Radke isotherm reduces to a linear isotherm and at high concentrations, it becomes the Freundlich isotherm. 1

qe ¼ K F C en

ð5Þ

Results and discussion Role of adsorption and photocatalytic degradation To understand and compare adsorption mechanisms onto the IPCAs, adsorbate/adsorbent equilibrium properties were evaluated in dark conditions. AMP adsorption studies were performed at 20 °C on various IPCAs over 3 h under stirred conditions for initial concentrations of AMP ranging from 10 to 115 mg/L. The experimental adsorption equilibrium data for the various IPCAs showed that the 10 % TiO2 IPCA had the highest capacity to adsorb AMP (Fig. 2). The capacity of AMP 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 adsorption capacity due to greater aggregation of the TiO2 particles on the surface of the AC (Wang et al. 2008). Experimental data for the AMP adsorption equilibrium on the IPCAs were interpreted using the adsorption isotherm 25

20

qe (mg/g)

15

10

Experimental-1%TiO2 IPCA Experimental-5%TiO2 IPCA Experimental-10%TiO2 IPCA Experimental-25%TiO2 IPCA Langmuir model

5

0 0

20

40

60

80

100

Ce (mg/L)

Fig. 2 A comparison of the fitting of the experimental data with the Langmuir model adsorption isotherm for 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) Langmuir model

qe ¼

qm K L C e 1 and RL ¼ 1 þ K LC0 1 þ K LCe

ð6Þ

qe ¼

aR r R C e ð1 þ r R C e Þα

ð7Þ

where qe (mg/g) and Ce (mg/L) are the amount of AMP adsorbed per unit mass of IPCA, and the unadsorbed AMP concentration in solution at equilibrium, respectively. KF (L/mg) and n (dimensionless) signify the adsorption capacity and adsorption intensity for the Freundlich model. The maximum amount of AMP adsorbed per adsorbent unit mass is qm (mg/g), KL is the Langmuir constant related to the affinity of the binding sites (L/mg) and RL is separation factor for the Langmuir model. The Prausnitz-Radke isotherm constants, aR (mg/g) and rR (L/mg) represent the maximum adsorption capacity and affinity, whilst α is the model exponent. The Freundlich isotherm is empirical in nature, but was later interpreted as adsorption to heterogeneous surface or surfaces supporting sites of varied affinities, and has been used widely to fit experimental data of liquid phase sorption, whereas the Langmuir isotherm model is an analytical equation basically developed for gas-phase adsorption on homogeneous surfaces and predicts a single maximum binding capacity (Fritz and Schluender 1974; Aksu and Kutsal 1991; Khan et al. 1997). A value of n in the range of 1–10 in the Freundlich model indicates favourable adsorption, whilst KL in the Langmuir model is a coefficient attributed to the affinity between the sorbent and sorbate (Vadivelan and Kumar 2005). The values of the model constants of all three isotherm models along with the corresponding regression coefficient (R2), SE and SSE values for all IPCA-AMP systems are presented in Table 1. Statistically, the Langmuir model was the best fit to the experimental data, with higher R2 and lower SE and SSE values than the Freundlich and Prausnitz-Radke models. Moreover, the predicted adsorption capacities (32.3 to 52.5 mg/g) of IPCAs by the Prausnitz-Radke model were higher than both the experimental (10.8 to 19.9 mg/g) and the Langmuir model (17.1 to 28.4 mg/g). The Freundlich model predicts that the AMP concentration on the sorbent will increase as long as there is an increase in the AMP

Environ Sci Pollut Res Table 1 Isotherm model parameters for AMP adsorption on various IPCAs

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

% (weight) of TiO2 in IPCA

Models

k 2 q2e t 1 þ k 2 qe t h ¼ k 2 q2e

qt ¼

1

5

10

25

1.665 1.861 0.873 1.637 3.226

1.576 1.316 0.848 1.941 4.085

1.599 1.197 0.925 1.293 2.574

2.031 1.672 0.855 1.859 3.885

18.238 0.031 0.448 0.982 0.124 0.074

24.344 0.042 0.385 0.986 0.115 0.062

28.429 0.044 0.376 0.991 0.102 0.017

17.134 0.014 0.615 0.980 0.132 0.091

46.175 0.046 0.764 0.790 4.288

49.304 0.061 0.782 0.781 5.095

52.547 0.069 0.812 0.794 4.059

32.348 0.037 0.767 0.789 4.868

18.019

23.012

16.004

21.096

Freundlich KF (L/g) n R2 SE SSE Langmuir qm (mg/g) KL (L/mg) RL R2 SE SSE Prausnitz-Radke aR (mg/g) rR (L/mg) αR R2 SE SSE

concentration in the liquid phase. However, the experimental data indicate that an isotherm plateau is reached at a limiting value of the solid-phase concentration. This plateau is not predicted by the Freundlich equation (Allen et al. 1988). The n values were in the range of 1.19 to 1.86, implying that the distribution of TiO2 on the surface of activated carbon (AC) is even (Xue et al. 2011). In addition, the separation factor (RL) values of the Langmuir model indicate that AMP adsorption onto IPCA was favourable (Table 1). Figure 2 shows that the theoretical plots of the Langmuir isotherm compare well with experimental data. The solute uptake rate (governing the contact time of the adsorption reaction) is one of the important kinetic characteristics that define the efficiency of adsorption. Pseudo-firstorder (Lagergren and Svenska 1898) and pseudo-secondorder (Ho and McKay 1998) equations were fitted to the experimental data and thus elucidate the adsorption kinetic process. The pseudo-first-order model is expressed as: 

qt ¼ qe 1−e−k 1

 t

ð8Þ

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

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 in Table 2. The regression coefficients (R2) for the pseudo-firstorder kinetic model were relatively low (0.722–0.792) and also both SE and SSE values were >1 for adsorption of AMP on IPCAs. In comparison, the R2 values of pseudo-secondorder model were significantly higher, ranging from 0.985 to 0.994. Correspondingly, SE and SSE values were lower and ranged from 0.252 to 0.521 and 0.158 to 0.712, respectively. Furthermore, the agreement between the experimental (10.8 to 19.9 mg g−1) and predicted equilibrium sorption capacities (14.1 to 22.3 mg/g) confirm a better fit to the pseudo-secondorder model (Fig. 3). These results support the model assumption that the adsorption is mainly monolayer adsorption (Kumar et al. 2005). Generally, the rate constant k2 increased with increase in TiO2 content up to 10 % (Table 2). The rate constant and the initial adsorption rate, h, as well as the equilibrium adsorption level, qe, were higher for the 10 % IPCA than for any of the other IPCAs. This may be attributed to the increased adsorption active sites on the surface with increasing TiO2 content (Basha et al. 2011). It can be concluded from the kinetics study that the TiO2 content on the surface of the IPCA significantly affects the adsorption kinetics of AMP on IPCAs. Table 2 Kinetic parameters for AMP adsorption onto various IPCAs Models

1 % IPCA

Kinetic Pseudo-first-order k1 (min−1) 0. 151 qe (mg/g) 36.74 R2 0.792 SE 2.119 SSE 5.210 Pseudo-second-order qe (mg/g) 16.34 k2 (g/mg/min) 0.056 h (mg/g/min) 14.952 R2 0.994 SE 0.252 SSE 0.158

5 % IPCA

10 % IPCA

25 % IPCA

0.242 39.41 0.781 2.722 6.257

0.374 46.29 0.722 3.412 10.118

0.117 33.47 0.730 3.027 9.457

19.86 0.077 30.370 0.990 0.341 0.229

22.31 0.082 40.814 0.992 0.311 0.203

14.12 0.042 8.374 0.985 0.521 0.712

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Fig. 3 Comparison of the pseudo-second-order kinetic model for AMP adsorption 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

Photocatalytic degradation of AMP The effect of irradiation with UVon the photodegradation rate of AMP in the presence of suspended TiO2 and 10 % TiO2 IPCA is shown in Fig. 4. Dark conditions (no UV) with 10 % IPCA after equilibrium adsorption was included as a control, showing a minor decrease in AMP concentration. Irradiation in the absence of IPCA/TiO2 (photolysis) showed insignificant conversion of AMP. The UV/free-TiO2 system degraded 52 % of the initial AMP during the first 180 min of irradiation with no subsequent

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

additional AMP degradation. The photodegradation rate of AMP on 10 % TiO2 IPCA was higher than TiO2 alone, with no plateau indicated over the 240 min. In addition, the performance of 10 % TiO2 is better than TiO2 on its own and also uses less photocatalyst with only 10 wt% TiO2 being used in the preparation of 10 % TiO2 IPCAs. The main advantage of IPCAs over TiO2 is that the IPCA can be easily separated from the solution after irradiation and recycled whilst the separation and recycling of free TiO2 is an extremely difficult task. Whilst it has been shown that the particle size of the photocatalyst in composites of this nature has an impact (Zhang et al. (2013)), the TiO2 used in this study is at the size that is readily available commercially. Since the composites prepared in this study showed increased photoactivity in comparison with the free TiO2, no further particle sizes were investigated. The results of the photocatalytic degradation of AMP over the range of IPCAs are shown in Fig. 5. The degradation rate was highest with the 10 % TiO2 IPCA system and decreased in the order: 10 % TiO2 IPCA>5 % TiO2 IPCA 1 % TiO2 IPCA> 25 % TiO2 IPCA. This sequence is identical to the order exhibited with respect to adsorption capacity (Table 1), and is strongly related to the TiO2 content of IPCAs. There was little difference in the photodegradation rate between 1 and 25 % TiO2 loadings, however, a significant variation between 1 and 10 % and 10 and 25 % TiO2 loadings was observed. Above a certain level of TiO2, excess TiO2 particles hinder the UV light penetration (Lu et al. 1999). As the increase in the photodegradation rate between 5 and 10 % TiO2 loadings was marginal, further TiO2 loadings between 11 to 24 % were not studied. In both adsorption and photodegradation, optimal performance was achieved at a TiO2 loading of 10 %. The higher efficiency of the photocatalyst can be attributed to the greater adsorption of AMP on the photocatalyst. The adsorption capacity of the AC enhances the chance of •OH radicals attacking

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, the delocalization capacity of the AC framework can efficiently separate the electrons and holes produced during photoexcitation of TiO2, thus enhancing the photocatalytic efficiency (Cojocaru et al. 2009). Generally, the greater the amount of TiO2 present, the higher the photocatalytic reaction rate due to increased generation of holes and hydroxyl radicals. Whilst the size of the TiO2 particles and therefore the surface available for absorption is important in photodegradation studies, this is not as relevant in the current study because the TiO2 was immobilised on activated carbon for preparing the IPCAs, which were then employed for adsorption/photodegradation of AMP. Initially, the AMP was adsorbed and then the adsorbed AMP was photodegraded by hydroxyl radicals generated through TiO2 photocatalysis. This illustrates the unique features of the IPCAs, which have both adsorption and photodegradation capabilities with the TiO2 playing a crucial role in both processes. In addition, more TiO2 may also induce greater aggregation of the TiO2 particles at the surface of the AC, leading to a reduction in reaction rates as seen here at 25 % TiO2 loading (Zhu et al. 2000). Furthermore, the high degradation rate of 10 % TiO2 IPCA can be attributed to the synergistic effects of adsorptive properties of the AC and photocatalytic activity of the TiO2 in the composite. A concentration effect for the chemical reactions is seen due to the surface adsorption of AMP on the IPCA substrate, whilst degradation intermediates on the site of the TiO2 enhance the photocatalytic degradative activity (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 unimolecular surface reaction, r, is proportional to the surface coverage, θ: dC k r KC 0 ¼ krθ ¼ r¼− dt 1 þ KC 0

using a single model. Thus, kinetic modelling of a photocatalytic process is usually evaluated by the half time of the reaction (Guettai and Amar 2005). This can be obtained from the slope and the initial concentration in an experiment in which the variation of the AMP concentration is measured as a function of time. Half-life, t1/2, describes the time taken for the concentration of a reactant to fall to half of its initial value. It is observed to be a function of initial AMP concentration (C0) as per Eq. (12). t 1=2 ¼

C0 ln2 þ 2k r k r K

ð12Þ

Figure 6 shows the variation of AMP concentration with 10 % TiO2 IPCA as a function of reaction time for four different initial concentrations. The half time increased with increasing initial concentration of AMP and there was a linear relationship between t1/2 and C0. Non-linear regression was used to estimate the parameters involved in the Langmuir– Hinshelwood (L–H) kinetic expressions (Table 3). High regression 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 AMP degradation obeyed the L–H model. The dependence of the rate constant kr and adsorption equilibrium constant K on the TiO2 content of IPCAs is shown in Fig. 7. The rate constant showed an initial increase and then a decrease with increasing content of TiO2 (with a maximum at a TiO2 content of 10 %), whilst the inverse relationship was observed for the adsorption equilibrium constant. The degradation rate would be expected to depend on the TiO2 content, with kr being lower for a smaller TiO2 content (Kim et al. 2008). However, the adsorption strength of the substrate is an important factor affecting the photoactivity of a catalyst. The IPCA containing 1 % TiO2 content had a high adsorption equilibrium constant, but exhibited lower degradation rates than the 10 % 130

ð10Þ

where kr is the reaction rate constant, θ is the fraction of the surface covered by the reactant, K is the adsorption coefficient of the reactant and C0 is the initial concentration of the reactant. Equation (10) is only applicable when both the reactant and solvent are adsorbed on the surface without competing for the same active sites. Integration of Eq. (10) yields Eq. (11):   C0 ln ð11Þ þ K ðC 0 −C Þ ¼ k r Kt C

t1/2

120

110

100

90 0

20

40

60

80

100

120

C0 (mg/L)

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

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

Environ Sci Pollut Res Table 3 Kinetic parameters and EEO for photodegradation of AMP on various IPCAs

Model

TiO2 content in IPCAs (%)

100 % TiO2

1

5

10

25

0.838 0.0079 0.0066 0.991 0.196 0.802 121.04

1.241 0.0059 0.0073 0.998 0.164 0.629 107.21

1.511 0.0050 0.0076 0.989 0.212 0.825 96.70

0.936 0.0071 0.0066 0.984 0.288 0.922 116.82

Langmuir–Hinshelwood kr (mg/L/min) K (L/mg) kapp (=kr ×K) R2 SE SSE EEM (kWh/kg/order)

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

Effect of initial pH The solution pH is an important operating parameter that can affect photocatalytic reactions. The influence of pH on photodegradation of AMP was evaluated using the 10 % TiO2 IPCA. Experiments were conducted at a pH range from 3 to 11 with an initial AMP concentration of 25 mg/L (Fig. 8). Photodegradation efficiency increased from 55 % at pH 3 to 80 % at pH 9, decreasing to 46 % at pH 11. It has been reported that the elimination of AMP from wastewater and drinking water samples by suspended TiO2-photocatalysis is highly efficient at pH 9.0 (Yang et al. 2008; Zhang et al. 2008). In general, the TiO2 in an IPCA presents an amphoteric character, and either a positive or a negative charge can be

0.742 0.0079 0.0059 0.990 0.204 0.726 135.24

developed on its surface (Bayarri et al. 2005). AMP (pKa = 9.38) is undissociated and present in a neutral form at a solution 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 the surface of TiO2 particles due to the electrostatic attraction between the positively charged TiO2 particles and AMP molecule (Velegraki and Mantzavinos 2008). This multilayer acts as a blocking mechanism, which prevents further interactions between catalytic active centres and AMP molecules. Moreover, in acidic solution, the redox potentials are greater than 2.8 and 2.0 V according to Eqs. 13 and 14 respectively (Tang and Huang 1995): hþ ðVBÞ þ H2 O→• OH þ Hþ

ð13Þ

hþ ðVBÞ þ OH− →• OHad

ð14Þ

1 0.9 0.8 0.7

C/C0

0.6 0.5 0.4

pH 3

0.3

pH 5.9

0.2

pH 9

0.1

pH 11

0 0

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

100

200

300

Irradiation time (min)

Fig. 8 Effect of pH on photocatalytic degradation of AMP by 10 % TiO2 IPCA, initial AMP concentration: 25 mg/L

Environ Sci Pollut Res

Hence, the formation of hydroxyl radicals will be thermodynamically unfavourable and thus suppressed. As a result, the formation of hydroxyl radicals increases with an increase of pH from 3 to 9, leading to higher removal efficiency for photodegradation of AMP. Further increase in pH to 11 results in the formation of superoxide radical anions (O2•−) through oxygen reduction, which suppresses the generation of H2O2 and •OH. This phenomenon causes a reduction in the degradation of AMP. Furthermore, at pH 11 the IPCA surface becomes more negative, and the hydroxyl group in AMP is changed into a phenoxide ion. Thus, the higher repulsion between the negative surface and AMP contributes to the lower degradation rate. Influence of the photocatalyst concentration To optimize AMP degradation using 10 % TiO2 IPCA, the effect of increasing the photocatalyst concentration from 0.75 to 2.50 g/L on the photocatalytic degradation rate of AMP was investigated (Fig. 9). The AMP photodegradation increased with increasing photocatalyst concentration up to a concentration of 2.0 g/L and then decreased. The increase in the photodegradation rate from 0.75 to 2.0 g/L catalyst concentration is attributable to increased •OH production. As the concentration of the IPCA is increased, the number of photons absorbed and the number of AMP molecules adsorbed are increased with respect to an increase in the number of catalyst molecules. The density of the molecules in the area of illumination also increases and thus the photodegradation rate is enhanced. Increasing IPCA concentration to 2.5 g/L resulted in a decrease in degradation rate. This can be attributed to the AMP molecules available not being sufficient for adsorption by the increased number of catalyst molecules. The additional catalyst is not involved in the photocatalytic activity and the 1

rate does not increase with an increase in the amount of catalyst beyond a certain limit. The aggregation of the catalyst molecules 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 and Pandit 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 were performed (Fig. 10). Each experiment was carried out under identical conditions (1 L AMP solution with an initial concentration of 74.5 mg/L, 1.5 g photocatalyst, and 240 min irradiation time). After each experiment, the solution residue from the photocatalytic degradation was filtered, washed and the solid was dried. The dried catalyst samples were used again for AMP degradation. The percentage AMP removal remains at approximately 69.5 % after the four cycles, with a decrease in IPCA activity of not greater than 4 %. The observed loss of photocatalyst efficiency can be due to the presence of reaction intermediates adsorbed onto the active sites of the catalyst. Concurrently, mechanical stress on the catalyst particles can occur due to stirred reactor conditions. Both factors account for a reduction of the number of available photoactive sites and therefore a decrease in the photodegradation capability of the IPCA. Electrical energy for photodegradation The cost involved in the treatment of wastewater is one of the key aspects that must be considered in the evaluation of a new technology. Since photocatalysis is an energy-intensive process, electricity can represent a major part of the operating costs. The 100

0.8

Photodegradation of AMP ( %)

Cycle1

C/C0

0.6

0.4

0.2

0 0

50

100

150

200

80

Cycle2 Cycle3 Cycle4

60

40

20

250

Irradiation time (min)

Fig. 9 Effect of photocatalyst concentration on degradation of AMP by 10 % 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 (ex symbol) 2.5 g/L

0 30

60

120

180

240

Irradiation time (min)

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 processes (AOPs) on the use of electricity (Daneshvar et al. 2005). In the case of high-pollutant concentrations, the appropriate figure-of-merit is the electrical energy per order (EEM), defined as the number of kilowatt hours of electrical energy required to reduce the concentration of a pollutant by 1 order of magnitude in a unit mass of contaminant (Bolton et al. 2001). The EEM (kWh/kg/order) may be calculated from: E EO ¼

P  t  1; 000 V  M  ðC 0 −C f Þ

ð15Þ

where P is the input power (kW) to the photocatalysis system, 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) and C0 and Cf are the initial and final pollutant concentrations (mg/ L), respectively. The EEM values for degradation of AMP by various IPCAs and 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 free suspended TiO2. The 10 % TiO2 IPCA exhibited the highest electrical efficiency as expected. There is potential scope for improvement of the photoreactor system to further reduce the EEO. Fourier-transform infrared spectra FT-IR analysis was employed to elucidate the process of adsorption and photodegradation of AMP on IPCA. The FTIR spectra of the 10 % TiO2 IPCA after the adsorption of AMP in the dark and after photodegradation along with the spectrum of virgin 10 % TiO2 IPCA are presented in Fig. 11. After AMP

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

adsorption, new absorption bands between 1,450–1,850 cm−1 appeared in the spectra, which may be assigned to the various C=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 consists of phenol (ArylOH), amide (CONH) and arene functional groups (Danten et al. 2006). These functionalities exhibit strong absorptions in the region 3,438–3,600 cm−1 as well as 1,400–1,680 cm−1 and 950–1,300 cm−1, respectively (Burgina et al. 2004; Danten et al. 2006). After irradiation of the AMP loaded 10 % TiO2 IPCA, peaks in this range disappeared (Fig. 11c) and the spectrum resembles that of virgin material, which indirectly confirms the adsorption/photodegradation mechanism of the IPCA action.

Conclusion Adsorption experiments showed that the IPCAs had a high adsorption capacity for AMP with the amount adsorbed proportional to the TiO2 loading with a maximum at 10 wt% TiO2 loading. The adsorption isotherms followed the Langmuir model whilst the kinetics were best described by a pseudosecond-order model. The effects of photocatalyst dosage, initial solution pH and irradiation time on the degradation of AMP were studied. The photocatalytic degradation kinetics was found to follow the Langmuir–Hinshelwood model and the values of the rate constants, KAds and kL–H, were dependent on the TiO2 loadings. The 10 % TiO2 IPCA exhibited the highest rate constant, kL–H of 1.511 mg/L/min, a low adsorption constant, Kads of 0.005 L/mg and achieved an overall AMP removal of 72 % within 240 min. Nitro and carbonyl functional groups present in the 10 % TiO2 IPCA responsible for the photodegradation-adsorption effect were confirmed by FT-IR analysis. The reusability of the 10 % TiO2 IPCA was only slightly decreased (4 %) after four photodegradation cycles, and it exhibited the lowest electrical energy consumption per order of magnitude for photocatalytic degradation of AMP. The 10 % TiO2 IPCA is a very promising photocatalyst for the degradation of acetaminophen due to its high performance and potential for recycling. The results clearly show that these bifunctional materials could potentially be used in the decontamination of organic pollutants from water/ wastewater and highlights the practical application in environmental pollution management. Acknowledgments SB wishes to gratefully acknowledge financial support from the Irish Research Council for Science, Engineering and Technology (IRCSET) in the form of IRCSET EMPOWER postdoctoral fellowship and the Central Salt and Marine Chemicals Research Institute (CSMCRI) for grant of study/earned leave. DK gratefully acknowledges the financial support from the STRIVE program from the Environmental Protection Agency (EPA) Ireland and technical assistance from ENVA Ireland Ltd and National Chemical Company of Ireland.

Environ Sci Pollut Res

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