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Ind. Eng. Chem. Res. 2010, 49, 11302–11309
Studies on the Adsorption and Kinetics of Photodegradation of Pharmaceutical Compound, Indomethacin Using Novel Photocatalytic Adsorbents (IPCAs) Shaik Basha,†,⊥ David Keane,† Anne Morrissey,‡ Kieran Nolan,§ Michael Oelgemöller,| and John Tobin*,† School of Biotechnology, Dublin City UniVersity, Dublin 9, Ireland, Oscail, Dublin City UniVersity, Dublin 9, Ireland, School of Chemical Sciences, Dublin City UniVersity, Dublin 9, Ireland, and School of Pharmacy and Molecular Sciences, James Cook UniVersity, TownsVille, Queensland 4811, Australia
Integrated photocatalytic adsorbents (IPCAs) based on TiO2-activated carbon synthesized by an ultrasonic impregnation technique have been used for the photodegradation of indomethacin (IND) in aqueous solutions. The IPCAs in dark adsorption studies had high affinity toward IND with the amount adsorbed proportional to the TiO2 loading. The adsorption capacity increased from 0.597 to 0.657 mmol/g with increase in TiO2 content from 0.5 to 10% in IPCAs. Three adsorption models, Langmuir, Freundlich and Sips, were used to describe the adsorption isotherms while the adsorption kinetic data were fitted to pseudofirst order and pseudosecond order models. The adsorption isotherm study showed that the adsorption followed both Sips and Langmuir models with high regression coefficients (R2) and low standard error (SE) and sum of residual square error (SSE) values. The adsorption kinetic data are well represented by pseudosecond order model. The kinetics of photocatalytic degradation under UV were found to follow a Langmuir-Hinshelwood model for the various IPCAs. The adsorption rate constant (Kads) was considerably higher than the photocatalytic rate constant (kL-H), suggesting that the photocatalysis of IND is the rate-determining step during the adsorption/ photocatalysis process. The proportion of TiO2 played a significant role upon the photoefficiency of the IPCAs. The photocatalytic efficiency of the 10% TiO2 IPCA remained greater than 70% after five cycles of use. 1. Introduction Pharmaceutical compounds in surface waters are an emerging environmental concern due to their biological activity and consequently provide a new challenge to drinking water and wastewater treatment systems.1 Most of the pharmaceuticals administered to patients are not entirely metabolized in the human body, and unmetabolized amounts are excreted to water effluents to be treated at wastewater treatment plants. The persistence of their residues in surface waters is of great concern, in particular because of their potential impact on ecosystem and human health.2-4 Due to low removal levels in conventional wastewater treatment plants, there is a need to develop technologies that promote an easier degradation of these pollutants. Among different treatments, “advanced oxidation processes” (AOPs) heterogeneous photocatalysis has become very popular and demonstrated high effectiveness in the degradation of pharmaceuticals in water and wastewater.5-8 Within a wide research program focused on the removal of pharmaceuticals from waters by heterogeneous photocatalysis, a nonsteroid anti-inflammatory drug, indomethacin (IND) was selected as a model compound for the study. IND is used for the relief of mild to moderately severe pain accompanied by inflammation, rheumatoid arthritis, osteoarthritis, and ankylosing spondylitis. In the United States, IND is included in a list of the top 200 prescription drugs for 2009.9 In sewage and surface * Corresponding Author, Phone: +353-1-700-5408, Fax: +353-1700-5412, E-mail:
[email protected]. † School of Biotechnology, Dublin City University. ‡ Oscail, Dublin City University. § School of Chemical Sciences, Dublin City University. | School of Pharmacy and Molecular Sciences, James Cook University. ⊥ Permanent address: Discipline of Marine Biotechnology and Ecology, Central Salt and Marine Chemicals Research Institute, Council of Scientific and Industrial Research, Bhavnagar 364002, Gujarat, India.
water samples collected in the UK and Ireland IND has been detected in the range of 5-792 ng/L.10,11 Titanium dioxide is the most widely used photocatalyst due to its large photocatalytic activity, high stability, nonenvironmental impact and low cost, particularly when sunlight is used as the source of irradiation.12,13 However, the shortcomings of conventional powder catalysts lie in the low efficiency in making use of light irradiation, and the difficulty in separation after photocatalysis.14 These disadvantages of TiO2 result in a low efficiency of the photocatalytic activity in practical applications. To achieve rapid and efficient decomposition of organic pollutants and easy manipulation in a total photocatalytic process, it may be effective to load photocatalysts onto suitably fine adsorbents to concentrate the pollutants around the photocatalysts. Various support materials have been used to prepare supported TiO2 for the degradation of organic pollutants.14-17 Of these, porous activated carbons (ACs) show considerable potential due to their high porosity and adsorption capability.18 A synergistic effect has been reported in using powdered TiO2 and AC in the photocatalytic degradation phenol, 4-chlorophenol and 2,4-dichlorophenoxyacetic acid.19-23 Cordero et al.24 studied the associative effects between TiO2 and activated carbon during 4-chlorophenol (4CP) photodegradation employing different types of AC; they found that an increase of electronic density in carbon support clearly introduces an enhancement in TiO2 photoactivity for 4CP photodegradation. In addition, the property of AC support is important since it strongly affects the dynamics of photoinduced charges and the adsorption. In light of this, the aim of this work is to assess the adsorption and photocatalytic activity of various IPCAs for the removal of indomethacin from aqueous solution. The IPCAs were synthesized by impregnating TiO2 particles on AC by ultrasonication. The kinetics of IND removal via adsorption and photodegradation were monitored and modeled. In addition, the reuse capability of IPCAs was investigated.
10.1021/ie101304a 2010 American Chemical Society Published on Web 10/19/2010
Ind. Eng. Chem. Res., Vol. 49, No. 22, 2010 Table 1. Characteristics of Photocatalyst TiO2 Used specifications structure components average aggregate particle diameter primary crystal size (µm) mean pore diameter (nm) band gap apparent density (kg/m3) surface area (m2/g)
Degussa P25 TiO2 photocatalyst nonporous 65% anatase, 25% rutile, 0.2% SiO2, 0.3% Al2O3, 0.3% HCl, 0.01% Fe2O3 nonporous 3.0 6.9 3.03 (from 500 to 300 nm) with UV-vis 130 42.32 ( 0.18
Table 2. Characteristics of Activated Carbon (AC) Used specifications
Aquasorb 2000
type ash content (%) moisture content (%) bulk density (kg/m3) surface area (m2/g) nominal size mean pore diameter (A°) micropore volume (cm3/g) mean diameter (µm) iodine number (mg/g min)
coal based 13 max. 5 max. 290-390 1050 80% min finer than 75 µm 30.61 0.34 19.71 1000
2. Experimental Section 2.1. Materials. The photocatalyst, TiO2 (AEROXIDE P25) manufactured by Evonik Industries, donated by the National Chemical Company of Ireland, was used in this study. Activated carbon, Aquasorb 2000 manufactured by Jacobi Carbons was donated by ENVA Water Treatment, Cork, Ireland. The characteristics of TiO2 and AC used in this study are given in Tables 1 and 2. Indomethacin (C19H16ClNO4) was purchased from Sigma-Aldrich Inc., Ireland and its structure is shown in Figure 1. HPLC grade acetonitrile and water were purchased from Fisher Scientific Ltd., Dublin, Ireland. Orthophosphoric acid (>98%) was purchased from Aldrich-Chemie GmbH, Germany. Amber silanized HPLC vials and 90 mm diameter glass fiber filter paper (FB59077 equivalent to Whatman No. 3) were purchased from Fisher Scientific, Ireland while 0.22 µm nylon syringe filters were purchased from Phenomenex Inc., United Kingdom. Pall nylon filters (0.2 µm pore size 47 mm diameter) were purchased from Sigma Aldrich. A Bransonic ultrasonic cleaner (5510 E-Mt) was used for mobile phase degassing and IPCA preparation. 2.1. Preparation of IPCAs. A low temperature impregnation method using ultrasonication was developed for applying TiO2 to the ACs.25,26 Six grams of Aquasorb 2000 and a mass of P25 between 0.03 and 0.6 g was added to a 200 mL solution of deionized water and sonicated for 1 h in an ultrasonic bath. The IPCAs are denoted by their TiO2 loading ranging from 0.5 to 50 wt % TiO2 to AC. Following overnight drying at 110 °C the resulting IPCAs were washed with deionized water to remove any excess P25. Finally, the photocatalysts were again dried at 110 °C and stored in sealed glass vials before use.
Figure 1. Structure of indomethacin (IND).
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2.2. Apparatus and Experimental Procedures. Adsorption/ photodegradation experiments used a borosilicate glass photochemical reactor 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 diameter with water cooling. To determine the effects of adsorption and photocatalysis, adsorption experiments in complete darkness were performed before photocatalysis. All of the adsorption studies utilized 1.2 g/L of IPCA in concentrations of IND varying from 0.1 to 1.5 mmol/L. The IPCA and IND 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 2 h, respectively. After dark adsorption, photodegradation of IND on various IPCAs was conducted under UV illumination. A 125 W medium pressure mercury lamp (TQ 150 Heraeus Noblelight) inserted in the center of the reactor was used as the light source. The change of the IND 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 analyzed as described in section 2.3. Direct photolysis studies were undertaken using UV irradiation without any catalyst to determine the baseline IND photodegradation rate. 2.3. Analysis. The concentration of IND was measured using a HPLC system consisting of an Agilent 1100 (Agilent Technologies, Palo Alto, Ca, USA) low-pressure gradient pump 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 analyte. Mobile phase consisted of 63% acetonitrile to 37% water with 0.2% orthophosphoric acid. This solution was filtered by nylon filters and degassed by ultrasonication for 30 min. The eluant flow rate was 0.8 mL/ min, injection volume 50 µL and stop time was 9.0 min. The wavelength of the detector was set at 265 nm. The data were processed by Agilent Chem Station software B.02.01SR1. The adsorption/photodegradation capacity of IPCA, qe (mmol/ g) was calculated from the difference in IND concentration in the aqueous phase before and after adsorption/photodegradation, as per eq 1: qe )
V(Ci - Ce) W
(1)
where, V is the volume of IND solution (L), Ci and Ce are the initial and equilibrium concentration of IND in solution (mmol/ L), respectively, and W is the mass of IPCA (g). The removal efficiency of 10% TiO2 IPCA for IND was calculated from the eq 2: removal (%)
100(Ci - Cf) Ci
(2)
where Cf is the final concentration of IND in solution. The final concentration after adsorption is the initial concentration for the calculation of photodegradation efficiency. All of the model parameters were evaluated by nonlinear regression using the 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.27 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 follows:
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Ind. Eng. Chem. Res., Vol. 49, No. 22, 2010 m
SSE )
∑ (Q
i
- qi)2
(3)
i)1
SE can be defined as follows: SE )
1 m-p
The experimental data here were fitted to the above models as described below. The Langmuir isotherm is valid for monolayer adsorption onto a surface with a finite number of identical sites and it is given by eq 5:
m
∑ (Q
i
- qi)2
(4)
qe )
i)1
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. The smaller SE and SSE values indicate the better curve fitting. In the present study, the correlation coefficient, R2, SE, SSE, and predicted qe/qm (wherever applicable) values were used to determine the best fit model. 3. Results and Discussion 3.1. Adsorption Isotherm Studies. The adsorption of IND on the surface of IPCA is an important parameter in determining photocatalytic degradation rate. As the lifetime of the hydroxyl radicals formed during the photodegradation process is very short, they can only react at or very near where they were formed.28 In the same way, in the interface of IPCA-IND, excited electrons from theπbonds of IND can be promoted to the conduction band of the photocatalyst under UV irradiation. In summary, increasing the number of active sites leads to increased adsorption of IND and, in consequence, increases the photodegradation rate. Therefore, adsorption tests in the dark were performed to evaluate the equilibrium constants of the adsorption of the IND on the IPCA surface. The adsorption isotherms of IND to various IPCAs, shown in Figure 2, are of type L-shape according to the classification of Giles et al.29 The L-shape of the isotherm indicates that there is no strong competition between the solvent and the adsorbate to occupy the adsorbent surface sites. Many models have been proposed to describe experimental data of adsorption isotherms. Most of the published literature employs two or three isotherm models, mainly Freundlich, Langmuir, and/or Sips. Simplicity and easy interpretability are some of the important reasons for extensive use of these models.
Figure 2. Dark adsorption of IND, followed by visible-light photocatalysis kinetic curves for various IPCAs (() 0.5% TiO2 IPCA, (0) 1.0% TiO2 IPCA, (∆) 2.5% TiO2 IPCA, (×) 5.0% TiO2 IPCA, (*) 7.5% TiO2 IPCA, (]) 10% TiO2 IPCA, (+) 25% TiO2 IPCA, and (-) 50% TiO2 IPCA.
qmKLCe 1 + KLCe
(5)
where, qm (mg/g) is the amount of adsorption corresponding to complete monolayer coverage and KL (L/mg) is the Langmuir constant. The Freundlich isotherm is expressed as follows: qe ) KFC1/n e
(6)
where, the mechanism and the rate of adsorption are functions of the constants 1/n and KF (L/g). For a good adsorbent, 0.2 < 1/n < 0.8, and a smaller value of 1/n indicates better adsorption and formation of rather strong bond between the adsorbate and adsorbent. The Sips isotherm30 has the following form: qe )
KSCγe 1 + asCγe
(7)
where, KS is the Sips isotherm constant (L/g), as the Sips model constant (L/mg) and γ is the model exponent. At low sorbate concentrations, the Sips equation reduces to the Freundlich isotherm. At high sorbate concentrations, the Sips model predicts monolayer sorption capacity, similar to the Langmuir isotherm. The isotherm constants including statistical parameters of studied models for various IPCAs are presented in Table 3. The Langmuir model served to estimate the maximum uptake values where they were not reached experimentally and it contains the two important parameters of the adsorption system (qm and KL).31 The qm is the maximum uptake upon complete saturation of the sorbent and KL is a coefficient related to the affinity between the sorbent and sorbate. The experimental data fit the Langmuir isotherm extremely well with high values of the R2 (>0.9792) and low SSE and SE values for all IPCAs (Table 3). The adsorption performances of the various IPCAs were compared by their respective qm values calculated from fitting the Langmuir isotherm to the experimental data. On the basis of the qm values, given in Table 3, the adsorption performance of 10% TiO2 IPCA was higher than other IPCAs. Both qm and KL values increase with increase in TiO2 content in IPCAs until 10% and decreased with increase TiO2 content from 25 to 50%. The Freundlich isotherm is originally empirical in nature, but was later interpreted as adsorption to heterogeneous surfaces or surfaces supporting sites of varied affinities and has been used widely to fit experimental data.32,33 The present study results indicate that the Freundlich model does not fit the experimental data well, however, the value of n falling in the range of 1-10 indicates that IND was adsorbed favorably by all IPCAs (Table 3). It is not a suitable model for describing these adsorption processes, as R2 values are generally lower (R2 < 0.8395) than both Langmuir and Sips models with very high SE and SSE values. At low sorbate concentrations, the Sips isotherm effectively reduces to the Freundlich isotherm.30 The Sips isotherm constants for sorption of IND onto various IPCAs are given in Table 3 and Figure 3 shows the theoretical plots of this isotherm
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Table 3. Adsorption Isotherm Parameters for Sorption of IND on Various IPCAs TiO2 content in IPCAs (%) isotherm models
0.5
1.0
2.5
0.7997 0.2245 0.9260 0.9793 0.0690 0.0286
0.8183 0.2591 0.9093 0.9850 0.0428 0.0108
0.8548 0.3697 0.8745 0.9822 0.0619 0.0229
30.680 2.2580 0.8108 32.483 7386.3
30.183 2.2002 0.8434 27.075 5131.4
37.723 2.4358 0.8275 29.288 6004.8
0.6155 0.0424 0.5616 0.9798 0.0688 0.0879
0.6715 0.0486 0.6336 0.9871 0.0457 0.0606
0.7099 0.0438 0.7313 0.9901 0.0382 0.0154
5.0
7.5
10
25
50
0.8981 0.4822 0.7614 0.9901 0.0175 0.0032
0.9275 0.7931 0.7219 0.9953 0.0135 0.0013
0.8270 0.4425 0.8245 0.9792 0.0690 0.0286
0.8408 0.4772 0.7915 0.9850 0.0424 0.0108
32.764 2.4167 0.7937 51.892 18850
39.931 2.4074 0.7478 54.642 20900
27.186 1.6336 0.8395 43.606 13310
25.48 1.7313 0.8388 45.869 14728
0.7820 0.0581 0.8403 0.9921 0.0320 0.0173
0.8178 0.0625 0.8611 0.9910 0.0256 0.0075
0.6240 0.0470 0.7842 0.9879 0.0694 0.0290
0.5980 0.0440 0.7778 0.9840 0.0619 0.0792
Langmuir qm (mmol/g) KL (L/mmol) RL R2 SE SSE
0.8782 0.3824 0.8262 0.9898 0.0234 0.0134 Freundlich
KF (L/g) n R2 SE SSE
35.344 2.3055 0.7446 55.015 21186 Sips
KS (L/g) aS (L/mmol) γ R2 SE SSE
0.7423 0.0463 0.7749 0.9904 0.0353 0.0258
compared with experimental data for 10% TiO2 IPCA. The results indicate that the Sips isotherm model is marginally better for describing the adsorption equilibrium for all IPCAs although the Langmuir model also fitted the experimental data well. The Sips regression coefficients were in the range of 0.9798-0.9910, while the SE and SSE values varied between 0.0256-0.0688 and 0.0075-0.0879, respectively, for all the IPCAs. 3.2. Adsorption Kinetic Studies. The transient behavior of the adsorption kinetics was analyzed using pseudo first-order and pseudosecond order models.34 Pseudofirst order rate equation is expressed as follows:35 dqt ) kads,1(qe - qt) dt
(8)
where qt (mmol/g) and qe (mmol/g) is the sorption capacity at time t and equilibrium, respectively, and kads,1 (min-1) is pseudofirst order rate constant. After integration and applying
Figure 3. Adsorption isotherm of IND by 10% TiO2 IPCA without UV light (∆) Experimental 10% TiO2 IPCA, (s) Sips model 10% TiO2 IPCA.
boundary conditions t ) 0 and qt ) 0 to t ) t and qt ) qe at equilibrium, the above equation becomes, log(qe - qt) ) log qe -
kads,1t 2.303
(9)
The pseudosecond order model35 is given by the following: dq ) kads,2(qe - qt)2 dt
(10)
where kads,2 is pseudosecond order rate constant. After integration and applying the same boundary conditions t ) 0 and qt )0 to t ) t and qt ) qe at equilibrium, eq 10 becomes, t 1 t ) + 2 qt q kads,2(qe) e
(11)
Adsorption studies with regular sampling of the liquid phase were carried out in absence of light in order to evaluate the kinetic constants. The rate constants evaluated together with statistical parameters are given in Table 4. The pseudofirst order model assumes the rate of occupation of sorption sites to be proportional to the number of unoccupied sites. Pseudofirst order plots for selected IPCAs are shown in Figure 4. The adsorption data are well represented by the pseudofirst order model for only the first 60 min approximately and thereafter deviate from the linear relationship predicted by eq 8. This deviation from pseudofirst order behavior to the slower adsorption which occurs in the IPCA pores preceding the very fast initial uptake of IND on to the surface of the IPCA.36 Ho and McKay35 reported similar observation as the sorption data were represented well by the pseudofirst order model only for the rapid initial phase that occurs for a contact time of 0-30 min for basic dyes onto peat particles. The kinetic data were further analyzed using a pseudosecond order model and Figure 5 shows the pseudosecond order plot (eq 10) for IND adsorption. For all the IPCAs and for the entire
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Table 4. Adsorption Kinetic Parameters for Sorption of IND on Various IPCAs TiO2 content in IPCAs (%) kinetic models experimental capacity (mmol/g) qe (mmol/g) kads,1 (min-1) R2 SE SSE qe (mmol/g) kads,2 (g/mmol/min) h (mmol/g/min) R2 SE SSE
0.5
1.0
0.1537
0.2334
2.5
5.0
7.5
0.1977
0.2788
0.1965
0.2062
0.2094 0.0852 0.9927 14.97 1121.4
0.1914 0.0873 0.9961 10.75 578.02
0.1952 0.0863 0.8942 70.15 24611
0.2057 0.0877 0.9146 60.54 18327
0.2819 0.5528 0.0439 0.9849 0.1779 0.0023
0.2865 0.6953 0.0571 0.9672 0.2710 0.0035
0.2410 0.1624 0.0094 0.9684 0.2625 0.0045
0.2482 0.1734 0.0107 0.9774 0.2247 0.0029
0.1990 0.0918 0.9839 24.91 3102.5
0.1691 0.4325 0.0124 0.9918 0.1284 0.0017
0.2528 0.3804 0.0243 0.9818 0.2108 0.0027
Pseudosecond order 0.2108 0.2137 0.4533 0.3668 0.0201 0.0168 0.9930 0.9787 0.1128 0.1896 0.0009 0.0024
the rate contant, kads,2 increased with increase in TiO2 content up to 10% in IPCAs (Table 4). Both the rate contant, kads,2 and the initial adsorption rate, h () kads,2qe2), as well as the equilibrium adsorption level, qe, were higher for the 10% TiO2 IPCA than for any of the other IPCAs. This would suggest an activated adsorption between IND and functional groups on IPCA surface involving valence forces through sharing or exchange of electrons between IPCA and IND.37 3.3. Photocatalysis Kinetic Studies. The kinetics of IND photodegradation on various IPCAs (after attaining adsorption equilibrium) are illustrated in the right-hand side panel of Figure 2. As expected, the photocatalytic capability of IPCAs increased with increase in TiO2 content. However a maximum was attained at 10% TiO2 loading and photodegradation rates decreased with further increases in TiO2 content in IPCAs beyond 10%. The photocatalytic capacities of various IPCAs (calculated using eq 1) with an initial concentration of 0.25 mmol after 4 h UV illumination were of the following order: 10% TiO2 IPCA (2.28 mmol/g) > 7.5% TiO2 IPCA (0.971 mmol/g) > 5.0% TiO2 IPCA (0.950 mmol/g) > 2.5% TiO2 IPCA (0.943 mmol/g) > 25% TiO2 IPCA (0.913 mmol/g) > 1.0% TiO2 IPCA (0.909 mmol/g) > 50% TiO2 IPCA (0.892 mmol/g) > 100% TiO2 (0.847 mmol/g) > 0.5% TiO2 IPCA (0.813 mmol/g). This sequence is identical with the order exhibited with respect to adsorption capacity (Table 3), which is strongly related to the TiO2 content of IPCAs. In both cases optimal performance was achieved at a TiO2 loading (10%) which is less than the maximum investigated. Generally, the greater the amount of TiO2 present the higher the photocatalytic reaction rate due to increased generation of holes and hydroxyl radicals. However, 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 and 50% TiO2 loadings.38 For quantitative evaluation, the first-order, second-order and Langmuir-Hinshelwood kinetic equations were fitted to the experimental data.39,40 These models are given by the following equations: dC ) kphoto,1C dt
(12)
dC ) kphoto,2C2 dt
(13)
KadkL-HC dC ) dt 1 + KadC
(14)
r)r)r)Figure 5. Pseudosecond-order model for adsorption of IND on various IPCAs (() 0.5% TiO2 IPCA, (0) 1.0% TiO2 IPCA, (∆) 2.5% TiO2 IPCA, (×) 5.0% TiO2 IPCA, (*) 7.5% TiO2 IPCA, (]) 10% TiO2 IPCA, (+) 25% TiO2 IPCA, and (-) 50% TiO2 IPCA.
50
0.2726
0.1457 0.0838 0.9743 47.22 11149
Figure 4. Pseudofirst-order model for adsorption of IND on various IPCAs (() 0.5% TiO2 IPCA, (0) 1.0% TiO2 IPCA, (∆) 2.5% TiO2 IPCA, (×) 5.0% TiO2 IPCA, (*) 7.5% TiO2 IPCA, (]) 10% TiO2 IPCA, (+) 25% TiO2 IPCA, and (-) 50% TiO2 IPCA.
25
0.1971
Pseudofirst order 0.1731 0.1753 0.0796 0.0762 0.9843 0.9767 29.49 35.55 4349.4 6321.3
sorption period, correlation coefficient (R2) values were found to be higher, and ranged from 0.9684 to 0.9930, while SE and SSE values were low and ranged from 0.1128 to 0.2625 and 0.0009 to 0.0045, respectively. The high R2 and low SE and SSE values as well as the good agreement between the experimental and predicted equilibrium sorption capacities confirm good fit to the pseudosecond order model. Generally,
10
where r is the rate of IND concentration (min/mg/L), C the concentration at any time (mg/L), kphoto,1 the first-order rate
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Table 5. Photocatalysis Kinetics Parameters of IND on Various IPCAs TiO2 content in IPCAs (%) kinetic models
0.5
1.0
2.5
5.0
7.5
10
25
50
0.0946 0.8083 1.3109 3.8174
0.1059 0.8193 1.3014 3.8674
0.0592 0.7721 1.4723 4.4156
0.0232 0.7952 1.5126 4.9542
0.0912 0.9093 0.8027 2.9158
0.1524 0.9172 0.7948 2.8986
0.0743 0.8544 1.0122 3.5792
0.0421 0.8881 1.1129 3.5124
0.0514 2.2715 0.9577 0.1958 0.6478
0.0846 1.1843 0.9643 0.2246 0.6149
0.0315 2.5562 0.9451 0.3245 1.0765
0.0375 3.1425 0.9342 0.3025 0.9829
first-order kphoto,1 R2 SE SSE
0.0766 0.7642 1.4146 4.2471
0.0791 0.7953 1.5546 6.4471
0.0812 0.7014 1.3125 3.8927
kphoto,2 R2 SE SSE
0.0542 0.8913 0.9215 3.3894
0.0631 0.8945 0.8125 2.9471
0.0782 0.9011 0.8122 2.9012
kL-H (mmol/L/min) KAds (L/mmol) R2 SE SSE
0.0312 6.8297 0.9011 0.4426 1.1125
0.0345 4.5314 0.9052 0.2719 0.9184
0.0372 3.1412 0.9313 0.2097 0.7411
0.0854 0.8022 1.3124 3.8194 second-order 0.0817 0.9073 0.8891 2.7598
Langmuir-Hinshelwood
constant (min-1), kphoto,2 the second-order rate constant (h-1), Kad and kL-H are the limiting rate constants of reaction at maximum coverage under the given experimental conditions and the equilibrium constant for adsorption of IND onto various IPCAs. Integrating eqs 12-14 with respect to the limits CdCe at time t ) 0 and CdC at time t, the nonlinearized forms of the first-order, second-order and the Langmuir-Hinshelwood kinetic expression are obtained as follows: C ) Ceexp-kphoto,1t C)
Ce kphoto,2Cet + 1
C ) Ceexp-Kad(kL-Ht+(Ce-C))
0.0412 2.8567 0.9423 0.3146 1.0146
content and possibly retardation of diffusion of the adsorbed IND by high adsorption strength.41,42 Above 10% TiO2 the rate constants decrease with increasing TiO2 content, which may be due to decreasing amount of adsorbed IND.21,42
(15) (16) (17)
where Ce is the concentration of solution at equilibrium without UV light (mg/L), Kads is equivalent to KL of the Langmuir model, and is the apparent kinetic constant. Nonlinear regression was used to estimate the parameters involved in the first order, second-order and Langmuir-Hinshelwood (L-H) kinetic expressions and the results are presented in Table 5. Clearly, the first-order kinetics do not provide good fit to the experimental data for all IPCAs as regression coefficients were low and ranged from 0.7642 to 0.8183. Moreover, the SE and SSE values were greater than 1 confirming the unsatisfactory fit of experimental data to the first-order model. While the second-order kinetic expression exhibited improved fit, the again improved R2 and low SE and SSE values for L-H model for all IPCAs show that it best represent the kinetics of IND degradation by IPCAs. The predicted L-H kinetics along with experimental data for 10% TiO2 IPCA are plotted in Figure 6. The dependence of the two L-H parameters, the rate constant kL-H and adsorption equilibrium constant Kads, on the TiO2 content of IPCAs is shown in Figure 7. While the rate constant first increases with increasing content of TiO2 (to a maximum at a TiO2 content of 10%), but then decreases, the adsorption equilibrium constant decreases with increasing TiO2 content. Clearly, degradation rate would be expected to depend on the TiO2 content with kL-H being the lower with the smaller TiO2 content.40 But adsorption strength of substrates is an important factor affecting photoactivity of catalysts. IPCA containing 0.5% TiO2 content had high adsorption equilibrium constant but exhibited lower degradation rates presumably due to low TiO2
Figure 6. Photocatalytic degradation of IND on 10% TiO2 IPCA (∆) Experimental 10% TiO2 IPCA, and (-) L-H model 10% TiO2 IPCA.
Figure 7. Rate constant (kL-H) of IND degradation reaction and the adsorption equilibrium constant (Kads) as a function of the TiO2 content in IPCA catalyst (() kL-H (2) Kads.
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POWER postdoctoral fellowship and the Central Salt and Marine Chemicals Research Institute (CSMCRI) for grant of study/earned leave. D.K. acknowledges the financial support from the STRIVE program from the Environmental Protection Agency (EPA) Ireland and technical assistance from ENVA environmental. Supporting Information Available: Figures related to experimental and predicted Sips isotherm as well as L-H model for various IPCAs. This information is available free of charge via the Internet at http://pubs.acs.org. Literature Cited
Figure 8. Removal of IND by 10% TiO2 IPCA in multiple cycles.
3.4. Reuse of IPCA. In order to evaluate the viability of the IPCAs for long-term use recycling of the 10% TiO2 IPCA for IND degradation over multiple cycles was investigated. After each cycle the catalyst was recovered and the percentage weight loss was found to be negligible (less than 1.5%) in each case. The same IPCA weight to IND solution volume ratios were maintained in each subsequent cycle. The results are shown in Figure 8. In the first cycle, 73.3% IND was removed by photodegradation. As increasing recycling times, the efficiency decreased slightly. The catalysis efficiency of 10% TiO2 IPCA was still higher than 70.5% after completion of five cycles while adsorption efficincy decreased from 26.2 to 24.3%. The reduced catalysis efficiency may be ascribed to the minute loss of the photocatalyst during the experiments. It was also observed that the TiO2 on activated carbon did not release or dissolve into solution. If the titania released or dissolved into solution, then in the reuse of the IPCA the photocatalytic activity would decreased sharply because activated carbon has no photocatalytic activity. 4. Conclusions The removal of IND by adsorption and photocatalysis on various integrated photocatalytic adsorbents was investigated in the present work. The adsorption capacity of the IPCAs increased with increasing TiO2 loadings up to 10 wt %. The adsorption isotherms were fitted to the Langmuir, Freundlich, and Sips models. Adsorption kinetic studies suggested that the kinetic data follow a pseudosecond order model. The kinetics of the IND degradation by IPCAs fit well to the L-H kinetic model. The values of the adsorption equilibrium constants, Kads, and the rate constants, kL-H, are dependent on the amount of TiO2 in IPCAs. The 10% TiO2 IPCA exhibited the highest rate constant, kL-H of 0.0846 mg/L/min, and low adsorption constant, Kads of 1.1843 L/mg. The enhanced photocatalytic activity of 10% TiO2 IPCA is likely due to the increased adsorption and better separation. These novel IPCAs demonstrate potential for a variety of photocatalysis applications because of the combined process advantages of a high porosity activated carbon with highly photocatalytically active TiO2. Acknowledgment S.B. wishes to gratefully acknowledge financial support from the Irish Research Council for Science, Engineering and Technology (IRCSET) in the form of IRCSET EM-
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ReceiVed for reView June 17, 2010 ReVised manuscript receiVed September 1, 2010 Accepted September 13, 2010 IE101304A
