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Photocatalytic degradation of pharmaceutical wastes by alginate supported TiO2 nanoparticles in packed bed photo reactor (PBPR)
Ecotoxicology and Environmental Safety ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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Photocatalytic degradation of pharmaceutical wastes by alginate supported TiO2 nanoparticles in packed bed photo reactor (PBPR) Santanu Sarkar a, Sudip Chakraborty b, Chiranjib Bhattacharjee a,n a
Department of Chemical Engineering, Jadavpur University, Kolkata 700032, India Department of Informatics, Modeling, Electronics and Systems Engineering (D.I.M.E.S.), University of Calabria, Via-P. Bucci, Cubo 42a, 87036 Rende (CS), Italy b
art ic l e i nf o
a b s t r a c t
Article history: Received 29 November 2014 Received in revised form 19 February 2015 Accepted 24 February 2015
In recent years deposal of pharmaceutical wastes has become a major problem globally. Therefore, it is necessary to removes pharmaceutical waste from the municipal as well as industrial effluents before its discharge. The convectional wastewater and biological treatments are generally failed to separate different drugs from wastewater streams. Thus, heterogeneous photocatalysis process becomes lucrative method for reduction of detrimental effects of pharmaceutical compounds. The main disadvantage of the process is the reuse or recycle of photocatalysis is a tedious job. In this work, the degradation of aqueous solution of chlorhexidine digluconate (CHD), an antibiotic drug, by heterogeneous photocatalysis was study using supported TiO2 nanoparticle. The major concern of this study is to bring down the limitations of suspension mode heterogeneous photocatalysis by implementation of immobilized TiO2 with help of calcium alginate beads. The alginate supported catalyst beads was characterized by scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM/EDAX) as well as the characteristic crystalline forms of TiO2 nanoparticle was confirmed by XRD. The degradation efficiency of TiO2 impregnated alginate beads (TIAB) was compared with the performance of free TiO2 suspension. Although, the degradation efficiency was reduced considerably using TIAB but the recycle and reuse of catalyst was increased quite appreciably. The kinetic parameters related to this work have also been measure. Moreover, to study the susceptibility of the present system photocatalysis of other three drugs ibuprofen (IBP), atenolol (ATL) and carbamazepine (CBZ) has been carried out using immobilized TiO2. The continuous mode operation in PBPR has ensured the applicability of alginate beads along with TiO2 in wastewater treatment. The variation of residence time has significant impact on the performance of PBPR. & 2015 Published by Elsevier Inc.
Keywords: Photocatalysis Alginate beads Pharmaceutical compounds Packed bed photo reactor
1. Introduction Wastewaters from pharmaceutical industries and from households contain lots of drugs especially antibiotics are vigorously entering to water environment. Therefore, threats due to pharmaceutical drugs to the aquatic life as well as all living element become major concern of research work (Carballa et al., 2004; Hirsch et al., 1998; Ternes, 1998; Kidd et al., 2007; Lange et al., 2001; Oaks et al., 2004). For remediation of a disease pharmaceutical drugs are frequently used and thereafter without alternation major part of those medicines enters to the environment through municipal sewage system. Moreover, sometime medicines are thrown into environment directly and pharmaceutical wastes n
Corresponding author. E-mail addresses: [email protected], [email protected] (C. Bhattacharjee).
from the respective industries directly disposed off to the water bodies (Hirsch et al., 1998; Ternes, 1998; Kidd et al., 2007; HallingSørensen et al., 1998). As a result a gradual growth pharmaceutical component is observed in water environment. However, it is impossible to separate pharmaceutical compounds i.e. antibiotics, hormones, steroids, etc. through wastewater treatment and cannot be degraded by means of biological treatment (Daughton and Ternes, 1999; Zwiener and Frimmel, 2000). Several groups of researcher have adopted photocatalysis in presence of nanoparticle, one of the main categories of advanced oxidation process (AOP) to eliminate detrimental effects of pharmaceutical compounds (Klavarioti et al., 2009). Heterogeneous photocatalysis in presence of nanoparticle mainly TiO2 has become a promising pathway for separation of several micropollutants form wastewater streams (Devipriya and Yesodharan, 2005; Woo et al., 2009; Zayani et al., 2009; Hsu et al., 2008; Calza et al., 2006; Sakkas et al., 2007; Zhang et al., 2010; An et al., 2011; Sarkar et al., 2014a,b). Small size nanoparticle provides
http://dx.doi.org/10.1016/j.ecoenv.2015.02.035 0147-6513/& 2015 Published by Elsevier Inc.
Please cite this article as: Sarkar, S., et al., Photocatalytic degradation of pharmaceutical wastes by alginate supported TiO2 nanoparticles in packed bed photo reactor (PBPR). Ecotoxicol. Environ. Saf. (2015), http://dx.doi.org/10.1016/j.ecoenv.2015.02.035i
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higher surface to volume ratio thus it offers better surface reaction. Titanium oxide nanoparticle is used frequently due to its strong oxidizing power, higher photo stability and non-toxic nature (Gupta and Tripathi, 2011). Many researchers have highlighted many advantageous sides of photocatalysis in presence of TiO2 nanoparticle although the scale up of heterogeneous photocatalysis process is the main challenging task. The recovery and reuse of TiO2 nanoparticle at the end of photocatalysis process is the main obligation of photocatalysis process and eventually it is very tedious job in practice. The problem related to the larger scale implementation of heterogeneous photocatalysis is cumbersome separation and recycling of photo catalyst during the wastewater treatment. This problem can be obliterated by implementation of immobilization of TiO2 on solid support. The immobilization is very simple and easy to execute for immobilization of cells and enzyme (Santos et al., 2008). However, mentioned method by Santos et al. (2008) can be adopted to entrap TiO2 nanoparticle. Moreover, it is necessary to develop a cost effective and environment friendly entrapment method so that it can be easily adopted by research fraternity. Harikumar et al. (2011) described that calcium alginate was nontoxic, biodegradable, non-immunogenic, water insoluble and thermally irreversible type of polymer matrix which could provide better support for immobilization of nanoparticle. Therefore, calcium alginate beads can be used for environment friendly immobilization of TiO2 nanoparticle and those photo active beads should be used in wastewater treatment in large scale. The main goal of this current research work is to develop an immobilized photocatalytic system to remove pharmaceutical components from the wastewater. The current research group has already established that photocatalysis in presence of TiO2 nanoparticle is very much effective in removal of anti-biotic like chlorhexidine digluconate (CHD) (Das et al., 2014, Sarkar et al., 2014a,b). Although, the last successful attempt of photocatalytic degradation of CHD has been done using suspension of photo catalyst (Das et al., 2014, Sarkar et al., 2014a,b) but it prevails some limitations which have been described earlier. As per literature survey, this research work has made the first attempt to implement immobilized TiO2 using alginate in the field of pharmaceutical wastewater treatment and at the same time elimination of disadvantages of suspension mode has been tried so far. Furthermore, a new concept of packed bed photo reactor (PBPR) has been introduced here in which nanoparticle impregnated alginate beads have been used as packing material. Introduction of PBPR has been made to eliminate the major limitation of batch mode during treatment of large volume of wastewater as PBPR has been operated in continuous mode. Moreover, to ascertain the applicability of immobilized system as well as PBPR, the present research work has tried to carry out photocatalytic degradation of different drugs in continuous mode. Other three pharmaceutical drugs i.e. ibuprofen (IBP), atenolol (ATL) and carbamazepine (CBZ) which have also some other detrimental effects on environment (Hapeshi et al., 2010, Georgaki et al., 2014), were also treated with immobilized TiO2 system to check the viability of present treatment method using PBPR.
(C15H12N2O) were purchased from Sigma-Aldrich to prepare the simulated solutions for experimental purpose. All experiments were carried out with ultrapure water from Ariums Pro VF (Sartorius Stedim Biotech) of 18.2 MΩ cm resistivity. All other chemicals i.e. sodium alginate, calcium chloride (di-hydrate) were purchased from Sigma Aldrich Chemical Co., USA. 2.2. Immobilization of TiO2 in calcium alginate beads The titanium dioxide impregnated beads were prepared by entrapping TiO2 nanoparticles in the calcium alginate beads. About 100 mL of casting solution was prepared by mixing 4.0 g (4%) of sodium alginate powder and 1.0–4.0 g of TiO2 nanoparticle in ultrapure water and stirring until a homogenous solution was achieved. First, TiO2 nanoparticle was added to 100 ml water and stirred for 30 min to form homogeneous suspension and then sodium alginate was introduced in that solution. The mixture (100 mL) was injected drop wise into 400 mL CaCl2 solution (0.5 M) using a syringe (10 mL) with a needle (0.8 mm in diameter, 38 mm in length) to form TiO2 impregnated alginate beads (TIAB). The TIAB were cured in the CaCl2 solution for overnight at room temperature and then rinsed with ultrapure water for several times. Prepared TIAB stored in ultrapure water and kept in 4° C for future use. Blank beads were also prepared by above mentioned method but without TiO2 addition. 2.3. Adsorption and photocatalysis The photocatalytic degradation using suspension mode of TiO2 was carried out under artificial UV source in a quartz reactor in the batch mode. The experimental details have been already described by Das et al. (2014) and Sarkar et al. (2014a) in suspension mode. The batch study using TIAB was carried out maintaining the favorable conditions similar to the batch study in suspension mode. The pH and the temperature were maintained at 10.5 and 30° C and substrate to catalyst ratio (S/C) was fixed at 2.5. The irradiation time for both systems maintained for 1 h. With certain interval aliquot solution was piped out from the reaction broth to measure the concentration of antibiotic, CHD. For the adsorption process, same type of experimental study was carried out as mentioned above but without UV irradiation. To increase the efficacy of TIAB and to eliminate the limitations of batch mode operation, the photocatalysis has been carried out in continuous mode using Packed Bed Photo Reactor (PBPR). The present mode of operation has been illustrated schematically in Fig. 1. The residence time (τ) and all other physical parameters were selected according to the best removal condition achieved in batch mode. To maintain constant temperature during the photocatalysis reaction the reactor is jacketed for the circulation of coolant and to protect UV
2. Experimental 2.1. Chemical and reagents Titanium dioxide photocatalyst nanopowder AeroxidesP25 (mixture of rutile and anatase, 718467) of particle size 21 nm with surface area (BET) 35–65 m2 g 1 from Sigma-Aldrich, were used as photo catalysis. Chlorhexidine digluconate solution (20% w/v), Ibuprofen (CH3H18O2, purity Z 99%), atenolol and carbamazepine
Fig. 1. Schematic representation of PBPR.
Please cite this article as: Sarkar, S., et al., Photocatalytic degradation of pharmaceutical wastes by alginate supported TiO2 nanoparticles in packed bed photo reactor (PBPR). Ecotoxicol. Environ. Saf. (2015), http://dx.doi.org/10.1016/j.ecoenv.2015.02.035i
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light, it is covered with quartz glass so that the wavelength of UV irradiation is unaltered. An artificial UVA tube having 365 nm wavelengths has been used as a source of UV illumination and the intensity of UV are fixed for PBPR. The void fraction of PBPR is 0.37 under fully loaded with TIAB and according to that the flow rate was kept constant using peristaltic pump to ascertain a fixed residence time. The residence time was varied with change of pharmaceutical drugs and the residence time was a variable parameter during photocatalysis of individual drug. The experimental study was continued till the outlet concentration of pharmaceutical drug became constant. To study the reusability of TIAB multiple experimental runs were carried out using same beads. In case of other three drugs, the photocatalytic degradation was carried in continuous mode only. The variable process parameters of the best fit were taken from the earlier batch studies, carried out by other research groups (Hapeshi et al., 2010; Georgaki et al., 2014) using TiO2 suspension. 2.4. Analytical methods 2.4.1. Characterization of immobilized photo catalyst The physical appearance of TIAB was white color spherical particles with average diameter of 3.33 mm. The surface morphologies of the entrapped catalysts inside alginate matrix were evaluated by using scanning electron microscopy (SEM) in combination with energy dispersive X-ray analysis (EDAX). The X-ray powder diffraction (XRD) pattern for the TIAB was recorded using Shimadzu XRD-6000 diffractometer using Cu Kα radiation (l ¼1.54 Å) operating at 40 kV, 20 mA and scanning rate of 2° min 1. The BET specific surface areas of the TIAB was determined by the N2 adsorption–desorption method. 2.4.2. Measurement of degradation The concentration of chlorhexidine digluconate concentration
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in the reaction mixtures was determined using spectrophotometer at 275 nm. The details of identification have been elaborately described in the recent publication by the same research group (Das et al., 2014; Sarkar et al., 2014a). To validate the concentration measurements and for the identification of product pattern, RPHPLC system (Cyber Lab, Millbury, USA) with Zorbax SB Phenyl column (4.8 250 mm2, 5 mm, Agilent, USA) was used. The details of HPLC analysis and the chromatogram from HPLC were described earlier (Das et al., 2014) and chromatogram ensured the generation of by-product. To characterize the degraded by-products from CHD, the mass spectra analysis was done in Quadrapole-TOF Micromass Spectrometer (Waters Co., USA). The details of chromatogram and mass spectroscopy had been indicated in the experimental research work by the present research group (Das et al., 2014). The residue of other three drugs and identification & measurement of degraded by-products from ATL, IBP and CBZ were carried out according to the principles that described by Hapeshi et al. (2010) and Georgaki et al. (2014) respectively. In all three cases HPLC (Perkin Elmer, Series 200) was used with essential C18 column for IBP and ATL and Hypersil BDS C8 column for CBZ (250 mm 4 mm 5 μm). The removal percentage of pharmaceutical components has been calculated during the whole study using Eq. (1).
⎛ Ct ⎞ % Removal of pharmaceutical component = ⎜1 − ⎟ × 100% C ⎝ 0⎠
(1)
where, Ct is the concentration of any drug at any time t and C0 is corresponding value at t¼ 0.
Fig. 2. (a) Photograph of TIAB and (b) and (C) SEM images at different magnifications, (d) EDXA spectra of a single TIAB congaing 4% alginate beads and 2% TiO2 nanoparticles by weight.
Please cite this article as: Sarkar, S., et al., Photocatalytic degradation of pharmaceutical wastes by alginate supported TiO2 nanoparticles in packed bed photo reactor (PBPR). Ecotoxicol. Environ. Saf. (2015), http://dx.doi.org/10.1016/j.ecoenv.2015.02.035i
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3. Results and discussions 3.1. Characterization of TIAB The formation of nano-TiO2 impregnated bead (Fig. 2a) was confirmed by SEM with EDXA and XRD patterns. TiO2 nanoparticles were entrapped in a polymeric matrix to study its activity towards the photo catalytic degradation of antibiotic drugs. Calcium alginate was used as the solid support in this research topic. Calcium alginate beads in diluted acidic and alkaline solutions were mechanically stable. SEM analysis was carried out to confirm the presence of nanoparticles, and its distribution pattern in polymeric matrix. A representative SEM image (Fig. 2b and c) shows that most of the particles are well distributed. The quantitative compositional analysis of the TiO2 nanoparticle entrapped beads was carried out using EDXA spectroscopy measurements. The spectra confirm the presence of TiO2 in the structure. The spectra were recorded from a single bead which was produced from casting solution containing 4% alginate and 2% TiO2 nanoparticles by weight. From the measurements, it was ensured that the each bead consisted of an average 6.21% Ti, 18.14% O2, 15.43% Ca, 9.83% Na and 47.28% Cl2 and the EDXA spectrum has been shown in Fig. 2(d). EDXA analysis showed minimum level of impurities were present inside the alginate beads. The characterization by XRD evidenced the amorphous nature of the TIAB. Therefore, it was difficult to identify the crystalline phases of TiO2. Under reduce scanning velocity two phases of TiO2, anatase and rutile phases were observed. The BET surface area of the same TIAB was measured to be 21.43 m2 g 1. 3.2. Effect of alginate to catalyst (A/C) ratio The main process parameter of the current system is alginate to catalyst ratio as all other parameters are kept same with the parametric values from the earlier study (Das et al., 2014). It refers that at S/C ratio (substrate to catalyst ratio) 2.5, pH of 10.5, temperature of 30° C and UV intensity of 80 μW/cm2 the removal percentage of CHD reached its steady state value after 1 h. To study the effect of alginate to catalyst (A/C) ratio, it was varied in four different ways from ratio 0.5 to 4.0 and the effect has been clearly indicated in Fig. 3 though so many trial experiments were conducted with the variation of A/C ratio. Fig. 3 reveals that with decrease of A/C ratio adsorption efficiency of the system decreases for the current system. With increase of A/C ratio concentration of nanoparticle inside alginate matrix was decreased, therefore more
Fig. 3. Effects of A/C ratio on both adsorptions an photocatalysis process and the interdependency between adsorption and photocatalysis.
porous vacant sites were available on outer surface of beads to adsorb the substrate molecules. Thus, higher adsorption was observed for lower catalyst concentration. TiO2 nanoparticles were entrapped inside porous matrix of beads therefore; during photocatalysis a different observation was identified. Both at lower and higher value of A/C ratio photocatalytic activity were reduced. Lower value of A/C ratio refers higher catalyst concentration and higher catalyst concentration causes problem for UV penetration (Das et al., 2014; Sarkar et al., 2014a). As a result at lower value of A/C ratio photocatalytic degradation of CHD was reduced. But at higher A/C ratio, available active site on alginate beads for photocatalytic reaction was not sufficient. Moreover, at similar situation TiO2 distribution on surface of the alginate beads was inadequate which referred poor amount of active sites was available for photocatalysis. Larger value of A/C refers higher concentration of alginate present in TIAB and as a result the opacity of the alginate beads increases which causes lower UV penetration through alginate surface to reach the active site of the TiO2 nanoparticle. Therefore, at that particular ratio it was impossible to degrade CHD completely by photocatalysis. On the contrary the residual amount of CHD after adsorption process could be degraded completely by photocatalysis using alginate beads with A/C ratio of 2. However, for this type of reaction A/C ratio played the major role for percentage removal of pharmaceutical component and it should be optimized to achieve better removal of the pollutants from the system. Minute observation on Fig. 3 could reveal that at lower value of A/C ratio during the photocatalysis same percentage of CHD was removed. This was due to at lower value of such ratio the distributions of TiO2 on the upper surface of TIAB was similar and at same time the difficulty of UV penetration through alginate beads reduced to some extent. From the above discussion, it may be concluded that A/C ratio should be maintained in such way that both the stability and the catalytic activity of TIBA can reach their optimum value. Then only TIBA can be reused several times without losing catalytic activity in photocatalytic wastewater treatment. 3.3. Adsorption and photocatalysis of CHD Simultaneous adsorption and photocatalysis process was carried out at 30° C and pH of 10.5 with S/C of 2.5 in batch mode. Those process parameters have been chosen from the previous study (Das et al., 2014) carried out using TiO2 suspension to evaluate the comparison between suspension and immobilized mode of operations. Moreover, at that parametric condition maximum removal of CHD was possible. After 1 h adsorption process, photocatalysis was carried out for 1 h to find out the removal efficiency of photocatalysis alone. From Fig. 3 it has been observed that under same operating condition and maintaining A/C ratio at 2, the maximum number of CHD molecules have been adsorbed on the surface of the TIAB and after that left amount of CHD has been degraded with help of photocatalysis reaction. This observation can be justified with help of porous structure of TIAB. As it has already been mentioned that TIAB offered higher BET surface area therefore CHD was adsorbed inside the porous matrix of alginate beads. As a consequence free molecule of CHD was not available for photocatalytic degradation. The main advantage of this process with the help of TIAB almost all molecules of the pharmaceutical component (CHD) can be removed within 2 h. Though, in current process adsorption played the major role to remove pharmaceutical waste from the system. Therefore, to understand efficacy of TIAB, the fresh pharmaceutical component was continually fed to the system for photocatalytic degradation. In later part of the present article it would be understandable that the extent of adsorption on photocatalytic degradation using TIAB.
Please cite this article as: Sarkar, S., et al., Photocatalytic degradation of pharmaceutical wastes by alginate supported TiO2 nanoparticles in packed bed photo reactor (PBPR). Ecotoxicol. Environ. Saf. (2015), http://dx.doi.org/10.1016/j.ecoenv.2015.02.035i
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3.4. Adsorption and reaction kinetics In the present study two types of kinetics are involved, one is adsorption kinetics and other is reaction kinetics during photocatalysis. Both are necessary to understand the direction and extent of the removal process. First adsorption of target molecule takes place on TIAB surface and then photocatalytic degradation of that molecule occurs. Here, CHD was adsorbed on TIAB surface and simultaneously it was disappeared from the system due photocatalytic reaction. The phenomenon of adsorption of a substrate on any adsorbent can be explained through isotherms. For the liquid–solid adsorption the amount of adsorbate absorbs on the solid adsorbent surface is the function of concentration of adsorbate at constant temperature. In the present study TIAB is absorbent and CHD is absorbate. Though, different types of isotherm have already been developed; among those Langmuir isotherms is quite popular to describe the adsorption phenomenon on the solid surface. Moreover, this was developed considering that adsorbate was inert to active surface without no phase change and single molecule could occupy one active site thus mono layer formation took place. In this study, the experimental adsorption data has been well fitted with Langmuir isotherm than all other isotherms, which can be expressed mathematically using Eq. (2) (Stumn and Morgan, 1996) and in the isotherm study, initial CHD concentration was the variable parameter, whereas the others were kept constant.
C0 C0 1 = + qequ qmaxKCHDG qmax
(2)
where, qequ is the amount of CHD adsorbed at equilibrium per gram of TIAB, C0 initial equilibrium concentration of CHD (main driving force), qmax is the maximum amount of CHD adsorbed at equilibrium per gram of TIAB; KCHD adsorption rate constant (g 1). The fitted curve of C0 vs C0/qequ has linearity with the correlation coefficient of 0.897 that has been shown in Fig. 4(a). The value qmax and KCHD have been obtained as 0.286 and 0.035 g 1 respectively from the same plot. The heterogeneous photocatalysis reactions generally follow pseudo first order reaction kinetics (Sarkar et al., 2014b). A good agreement between theoretical pseudo first order kinetic model and experimental observation has been observed and that can be expressed using Eq. (3). ‵ −rphoto =
dC = kCt dt
(3)
‵ where, rphoto and k is pseudo fist order reaction rate and rate constant respectively. The above equation can be written after integrating with boundary condition
⎛ Ct ⎞ − ln ⎜ ⎟ = kt ⎝ C0 ⎠
(4)
Initial concentration of pharmaceutical waste (CHD) is represented as C0. The rate constant values can be calculated from the slope of the plot of ln(Ct/C0) vs time From Fig. 4(b) the value of rate constant (k) was calculated as 0.0555 min 1. 3.5. Comparison between suspension and entrapment mode The present research group already observed in suspension mode maximum 30% CHD was removed due to adsorption and up to 70% removal was possible during photocatalytic degradation (Das et al., 2014) at earlier specified condition. In that case photocatalysis was predominant for removal of CHD. Although at same parametric condition, in the present study, the adsorption process on TIAB played the major contribution (79%) for removal of CHD
Fig. 4. Measurement of (b) photocatalysis of CHD.
kinetics
parameters
for
(a)
adsorption
and
from the system. Moreover, with combination of both adsorption and photocatalysis could remove 99% of present antibiotic form the reaction mixture. Therefore, in batch study adsorption played the important role for the removal of pharmaceutical component, CHD from the simulated solution. 3.6. Photocatalytic degradation using PBPR In PBPR, TIAB was used as packing material. In the center of the packed column UV source was there. The sufficient time was provided for photocatalytic degradation. The operating parameters were chosen from the batch studies which were performed by several researchers (Das et al., 2014; Hapeshi et al., 2010; Georgaki et al., 2014). During all experimental observation A/C ratio of 2 was maintained. The photocatalytic degradation of several pharmaceutical drugs has been carried out using PBPR and experimental observation has been represented clearly in the current context. 3.6.1. Degradation of CHD in PBPR In the earlier batch study in suspension mode the present research group (Das et al., 2014) has revealed that maximum percentage of CHD can be removed at S/C ratio of 2.5, pH 10.5 and ambient temperature (30 °C). In the present case same parametric condition has been maintained during the photocatalytic degradation in PBPR and outlet concentration of CHD has been measured to calculate its removal percentage with time which has been plotted in Fig. 5(a). At the initial stage almost 99% removal has been observed for 60 min residence time but as the time
Please cite this article as: Sarkar, S., et al., Photocatalytic degradation of pharmaceutical wastes by alginate supported TiO2 nanoparticles in packed bed photo reactor (PBPR). Ecotoxicol. Environ. Saf. (2015), http://dx.doi.org/10.1016/j.ecoenv.2015.02.035i
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Fig. 5. Percentage removal of different pharmaceutical drugs using TIAB in PBPR at specified conditions.
progress the removal percentage has decreased gradually. In the connection with that it should be noted, the treated simulated waste water first came out from PBPR after 60 min from the start of removal process. This observation may be explained with the help of earlier batch study. According to Fig. 3 initially maximum removals of CHD was possible with help of surface adsorption on TIAB but as the time progress active sites present in porous matrix of alginate beads were getting occupied by CHD molecules. Therefore, effective adsorption was reduced after some time and at that time only photocatalysis became the predominant factor to remove pharmaceutical components. Moreover, the substrate molecules were also available to take part in photocatalysis. It has also been observed from Fig. 5(a) that removal percentage of CHD reached to its steady state value after some time. Though, in continuous mode only 55% steady state removal of CHD was possible using TIAB in PBPR whereas nearly 70% removal was achieved (Das et al., 2014) using TiO2 suspension in batch mode. This contradictory behavior was observed as in suspension mode all TiO2 nanoparticles were freely available for photocatalysis but in case of alginate beads catalyst particles were in entrapped condition. Thus, all nanoparticles could not take part in such reaction. Moreover, lower UV penetration through the alginate surface was one of the major drawbacks of the system and beads were placed around the UV source in multi-layered condition, which also caused lower light penetration through TIABs which were far away from UV light. 3.6.2. Degradation of other drugs To establish the applicability of PBPR for photocatalytic degradation of pharmaceutical wastes, three other pharmaceutical
components namely ATL, IBP and CBZ were degraded using TIAB in continuous mode. The removal percentage of those three components has been shown in Fig. 5(b)–(d). For all the cases temperature, pH and S/C ratio was maintained at 25 °C, 7 and 0.04 respectively. The parametric conditions for ATL was taken as described by Hapeshi et al. (2010) and for other two, it was opted from the research work Georgaki et al. (2014). The most interesting think is that Hapeshi et al. (2010) and Georgaki et al. (2014) was experimented with lower value of substrate concentration such that it could replicate the environmental pollution load due to those drugs. It has been observed from their studies that at mentioned condition ATL, IBP and CBZ can be removed up to 85%, 99% and 99% after 60 min, 20 min and 40 min respectively in suspension mode of operation. For all cases initial concentration was kept constant at 10 mg/L. In the current study, the continuous mode of operation has been preformed considering the residence time is equal to the time after which the maximum removal percentage was obtained by Hapeshi et al. (2010) and Georgaki et al. (2014) in batch mode. In the present system the steady state percentage of removal was achieved 58%, 85% and 80% for ATL, IBP and CBZ when residence times were 60 min, 20 min and 40 min respectively and those values were much lesser than suspension mode. The similar observation was observed in case of CHD. Therefore, similar type of explanation prevails for the removal of ATL, IBP and CBZ using TIAB. From the above discussion it is clear that TIAB as well as PBPR has failed to achieve maximum percentage of removal of pharmaceutical components from the simulated solution. Though, one thing is confirmed that an appreciable amount of drugs can be removed by such type of continuous mode of photocatalysis
Please cite this article as: Sarkar, S., et al., Photocatalytic degradation of pharmaceutical wastes by alginate supported TiO2 nanoparticles in packed bed photo reactor (PBPR). Ecotoxicol. Environ. Saf. (2015), http://dx.doi.org/10.1016/j.ecoenv.2015.02.035i
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degradation process for pharmaceutical wastewater treatment. The obtained results showed that the efficiency decreases from 99% to 85%. This is likely due to the fouling of porous surface TIAB as well as the catalyst surface and loss of catalyst due to repeated acid wash.
4. Conclusions
Fig. 6. The recycling efficiency of TIAB in continuous mode during CHD removal.
process and therefore, large volume of wastewater can be treated by the present system. Moreover, TIBA also ensures the reduction of loss of valuable photocatalyst and its contamination. Accordingly, the reusability of nanoparticle in immobilized condition has been studied here. 3.6.3. Effect of residence time in PBPR Residence time (τ) is the time period that spends by a molecule in a system. Higher residence time refers longer staying and vice versa. To understand the effect of τ on the performance of PBPR, τ has been decreased from the value of residence time in batch mode operation (Das et al., 2014, Hapeshi et al., 2010; Georgaki et al., 2014) for each drug i.e. CHD, ATL, IBP and CBZ. The τ has been varied in three different ways for each antibiotic and results have been plotted in Fig. 5. It has been observed that at a specified condition with decrease of τ the removal percentage of any drug has decreased very rapidly. This was happened during removal of any pharmaceutical components because lower value of τ provided short time period of adsorption and photocatalysis as well. A sufficient amount time is required to come in contact between catalyst and target molecule. Thus, in case PBPR higher value of residence time is favorable though it is not possible to increase that value infinitely. Moreover, higher loading concentration of pharmaceutical components in waste effluents leads to increase residence time because at that particular condition more time is required for adsorption and photocatalytic degradation. At larger value of τ, the inlet and outlet flow rate of reactor becomes lower hence the volume of wastewater which can be handled by PBPR per unit time also reduces at a particular condition. The liquid chandelling capacity by a PBPR can be kept constant at higher residence time; the volume of the reactor should be increased accordingly which is not feasible in practice. Therefore, the residence time should be maintained in such a way that the removal percentage of pharmaceutical component can reach to desired value and at the same time the PBPR can handle large volume of pharmaceutical wastewater. 3.7. Recycling of photocatalyst TIAB can be recycled effectively which makes the process cost effective. To study the reusability, the catalyst was recycled five times in continuous mode of operation using PBPR during degradation of CHD and degradation efficiency was recorded which had been plotted in Fig. 6. At optimized conditions for photocatalysis the degradation of CHD were determined, the catalyst was recovered by giving mild acid water wash and again used in
The above study is very much important considering the aspects of recycling and reuse of TiO2 nanoparticles in continuous mode operation. After suspension mode study, A/C ratio is the most important parameter which is needed to be optimized for entrapment mode photocatalysis. TIAB can efficiently remove the pharmaceutical components from the reaction mixture though the reduction of antibiotic activity is less significant. The uses of alginate beads as immobilization of photo catalysts show some difficulty, mainly lower degradation efficiency when compared with suspension mode photocatalysis. But with the help of PBPR appreciable percentage of pharmaceutical drugs can be removed and large amount of wastewater can be treated using such system. The residence time has significant effect on the performance of PBPR and at higher value of it produces better efficiency of PBPR. Finally, TIAB can be reused without losing its effectiveness for at least five cycles in PBPR which is main advantage to make the process more cost effective over the suspension mode of photocatalysis. Some necessary modifications should be incorporated to enhance the photocatalytic performance of alginate beads. Then only this methodology can be viable for large scale pharmaceutical wastewater treatment.
Acknowledgement The work reported in this article is part of an Indo-Bulgarian project (vide sanction letter no. INT/BULGARIP-09/2012), funded by Department of Science & Technology (Government of India). Accordingly, the contributions of DST (India) are gratefully acknowledged.
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Ecotoxicology and Environmental Safety journal homepage: www.elsevier.com/locate/ecoenv
Photocatalytic degradation of pharmaceutical wastes by alginate supported TiO2 nanoparticles in packed bed photo reactor (PBPR) Santanu Sarkar a, Sudip Chakraborty b, Chiranjib Bhattacharjee a,n a
Department of Chemical Engineering, Jadavpur University, Kolkata 700032, India Department of Informatics, Modeling, Electronics and Systems Engineering (D.I.M.E.S.), University of Calabria, Via-P. Bucci, Cubo 42a, 87036 Rende (CS), Italy b
art ic l e i nf o
a b s t r a c t
Article history: Received 29 November 2014 Received in revised form 19 February 2015 Accepted 24 February 2015
In recent years deposal of pharmaceutical wastes has become a major problem globally. Therefore, it is necessary to removes pharmaceutical waste from the municipal as well as industrial effluents before its discharge. The convectional wastewater and biological treatments are generally failed to separate different drugs from wastewater streams. Thus, heterogeneous photocatalysis process becomes lucrative method for reduction of detrimental effects of pharmaceutical compounds. The main disadvantage of the process is the reuse or recycle of photocatalysis is a tedious job. In this work, the degradation of aqueous solution of chlorhexidine digluconate (CHD), an antibiotic drug, by heterogeneous photocatalysis was study using supported TiO2 nanoparticle. The major concern of this study is to bring down the limitations of suspension mode heterogeneous photocatalysis by implementation of immobilized TiO2 with help of calcium alginate beads. The alginate supported catalyst beads was characterized by scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM/EDAX) as well as the characteristic crystalline forms of TiO2 nanoparticle was confirmed by XRD. The degradation efficiency of TiO2 impregnated alginate beads (TIAB) was compared with the performance of free TiO2 suspension. Although, the degradation efficiency was reduced considerably using TIAB but the recycle and reuse of catalyst was increased quite appreciably. The kinetic parameters related to this work have also been measure. Moreover, to study the susceptibility of the present system photocatalysis of other three drugs ibuprofen (IBP), atenolol (ATL) and carbamazepine (CBZ) has been carried out using immobilized TiO2. The continuous mode operation in PBPR has ensured the applicability of alginate beads along with TiO2 in wastewater treatment. The variation of residence time has significant impact on the performance of PBPR. & 2015 Published by Elsevier Inc.
Keywords: Photocatalysis Alginate beads Pharmaceutical compounds Packed bed photo reactor
1. Introduction Wastewaters from pharmaceutical industries and from households contain lots of drugs especially antibiotics are vigorously entering to water environment. Therefore, threats due to pharmaceutical drugs to the aquatic life as well as all living element become major concern of research work (Carballa et al., 2004; Hirsch et al., 1998; Ternes, 1998; Kidd et al., 2007; Lange et al., 2001; Oaks et al., 2004). For remediation of a disease pharmaceutical drugs are frequently used and thereafter without alternation major part of those medicines enters to the environment through municipal sewage system. Moreover, sometime medicines are thrown into environment directly and pharmaceutical wastes n
Corresponding author. E-mail addresses: [email protected], [email protected] (C. Bhattacharjee).
from the respective industries directly disposed off to the water bodies (Hirsch et al., 1998; Ternes, 1998; Kidd et al., 2007; HallingSørensen et al., 1998). As a result a gradual growth pharmaceutical component is observed in water environment. However, it is impossible to separate pharmaceutical compounds i.e. antibiotics, hormones, steroids, etc. through wastewater treatment and cannot be degraded by means of biological treatment (Daughton and Ternes, 1999; Zwiener and Frimmel, 2000). Several groups of researcher have adopted photocatalysis in presence of nanoparticle, one of the main categories of advanced oxidation process (AOP) to eliminate detrimental effects of pharmaceutical compounds (Klavarioti et al., 2009). Heterogeneous photocatalysis in presence of nanoparticle mainly TiO2 has become a promising pathway for separation of several micropollutants form wastewater streams (Devipriya and Yesodharan, 2005; Woo et al., 2009; Zayani et al., 2009; Hsu et al., 2008; Calza et al., 2006; Sakkas et al., 2007; Zhang et al., 2010; An et al., 2011; Sarkar et al., 2014a,b). Small size nanoparticle provides
http://dx.doi.org/10.1016/j.ecoenv.2015.02.035 0147-6513/& 2015 Published by Elsevier Inc.
Please cite this article as: Sarkar, S., et al., Photocatalytic degradation of pharmaceutical wastes by alginate supported TiO2 nanoparticles in packed bed photo reactor (PBPR). Ecotoxicol. Environ. Saf. (2015), http://dx.doi.org/10.1016/j.ecoenv.2015.02.035i
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higher surface to volume ratio thus it offers better surface reaction. Titanium oxide nanoparticle is used frequently due to its strong oxidizing power, higher photo stability and non-toxic nature (Gupta and Tripathi, 2011). Many researchers have highlighted many advantageous sides of photocatalysis in presence of TiO2 nanoparticle although the scale up of heterogeneous photocatalysis process is the main challenging task. The recovery and reuse of TiO2 nanoparticle at the end of photocatalysis process is the main obligation of photocatalysis process and eventually it is very tedious job in practice. The problem related to the larger scale implementation of heterogeneous photocatalysis is cumbersome separation and recycling of photo catalyst during the wastewater treatment. This problem can be obliterated by implementation of immobilization of TiO2 on solid support. The immobilization is very simple and easy to execute for immobilization of cells and enzyme (Santos et al., 2008). However, mentioned method by Santos et al. (2008) can be adopted to entrap TiO2 nanoparticle. Moreover, it is necessary to develop a cost effective and environment friendly entrapment method so that it can be easily adopted by research fraternity. Harikumar et al. (2011) described that calcium alginate was nontoxic, biodegradable, non-immunogenic, water insoluble and thermally irreversible type of polymer matrix which could provide better support for immobilization of nanoparticle. Therefore, calcium alginate beads can be used for environment friendly immobilization of TiO2 nanoparticle and those photo active beads should be used in wastewater treatment in large scale. The main goal of this current research work is to develop an immobilized photocatalytic system to remove pharmaceutical components from the wastewater. The current research group has already established that photocatalysis in presence of TiO2 nanoparticle is very much effective in removal of anti-biotic like chlorhexidine digluconate (CHD) (Das et al., 2014, Sarkar et al., 2014a,b). Although, the last successful attempt of photocatalytic degradation of CHD has been done using suspension of photo catalyst (Das et al., 2014, Sarkar et al., 2014a,b) but it prevails some limitations which have been described earlier. As per literature survey, this research work has made the first attempt to implement immobilized TiO2 using alginate in the field of pharmaceutical wastewater treatment and at the same time elimination of disadvantages of suspension mode has been tried so far. Furthermore, a new concept of packed bed photo reactor (PBPR) has been introduced here in which nanoparticle impregnated alginate beads have been used as packing material. Introduction of PBPR has been made to eliminate the major limitation of batch mode during treatment of large volume of wastewater as PBPR has been operated in continuous mode. Moreover, to ascertain the applicability of immobilized system as well as PBPR, the present research work has tried to carry out photocatalytic degradation of different drugs in continuous mode. Other three pharmaceutical drugs i.e. ibuprofen (IBP), atenolol (ATL) and carbamazepine (CBZ) which have also some other detrimental effects on environment (Hapeshi et al., 2010, Georgaki et al., 2014), were also treated with immobilized TiO2 system to check the viability of present treatment method using PBPR.
(C15H12N2O) were purchased from Sigma-Aldrich to prepare the simulated solutions for experimental purpose. All experiments were carried out with ultrapure water from Ariums Pro VF (Sartorius Stedim Biotech) of 18.2 MΩ cm resistivity. All other chemicals i.e. sodium alginate, calcium chloride (di-hydrate) were purchased from Sigma Aldrich Chemical Co., USA. 2.2. Immobilization of TiO2 in calcium alginate beads The titanium dioxide impregnated beads were prepared by entrapping TiO2 nanoparticles in the calcium alginate beads. About 100 mL of casting solution was prepared by mixing 4.0 g (4%) of sodium alginate powder and 1.0–4.0 g of TiO2 nanoparticle in ultrapure water and stirring until a homogenous solution was achieved. First, TiO2 nanoparticle was added to 100 ml water and stirred for 30 min to form homogeneous suspension and then sodium alginate was introduced in that solution. The mixture (100 mL) was injected drop wise into 400 mL CaCl2 solution (0.5 M) using a syringe (10 mL) with a needle (0.8 mm in diameter, 38 mm in length) to form TiO2 impregnated alginate beads (TIAB). The TIAB were cured in the CaCl2 solution for overnight at room temperature and then rinsed with ultrapure water for several times. Prepared TIAB stored in ultrapure water and kept in 4° C for future use. Blank beads were also prepared by above mentioned method but without TiO2 addition. 2.3. Adsorption and photocatalysis The photocatalytic degradation using suspension mode of TiO2 was carried out under artificial UV source in a quartz reactor in the batch mode. The experimental details have been already described by Das et al. (2014) and Sarkar et al. (2014a) in suspension mode. The batch study using TIAB was carried out maintaining the favorable conditions similar to the batch study in suspension mode. The pH and the temperature were maintained at 10.5 and 30° C and substrate to catalyst ratio (S/C) was fixed at 2.5. The irradiation time for both systems maintained for 1 h. With certain interval aliquot solution was piped out from the reaction broth to measure the concentration of antibiotic, CHD. For the adsorption process, same type of experimental study was carried out as mentioned above but without UV irradiation. To increase the efficacy of TIAB and to eliminate the limitations of batch mode operation, the photocatalysis has been carried out in continuous mode using Packed Bed Photo Reactor (PBPR). The present mode of operation has been illustrated schematically in Fig. 1. The residence time (τ) and all other physical parameters were selected according to the best removal condition achieved in batch mode. To maintain constant temperature during the photocatalysis reaction the reactor is jacketed for the circulation of coolant and to protect UV
2. Experimental 2.1. Chemical and reagents Titanium dioxide photocatalyst nanopowder AeroxidesP25 (mixture of rutile and anatase, 718467) of particle size 21 nm with surface area (BET) 35–65 m2 g 1 from Sigma-Aldrich, were used as photo catalysis. Chlorhexidine digluconate solution (20% w/v), Ibuprofen (CH3H18O2, purity Z 99%), atenolol and carbamazepine
Fig. 1. Schematic representation of PBPR.
Please cite this article as: Sarkar, S., et al., Photocatalytic degradation of pharmaceutical wastes by alginate supported TiO2 nanoparticles in packed bed photo reactor (PBPR). Ecotoxicol. Environ. Saf. (2015), http://dx.doi.org/10.1016/j.ecoenv.2015.02.035i
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light, it is covered with quartz glass so that the wavelength of UV irradiation is unaltered. An artificial UVA tube having 365 nm wavelengths has been used as a source of UV illumination and the intensity of UV are fixed for PBPR. The void fraction of PBPR is 0.37 under fully loaded with TIAB and according to that the flow rate was kept constant using peristaltic pump to ascertain a fixed residence time. The residence time was varied with change of pharmaceutical drugs and the residence time was a variable parameter during photocatalysis of individual drug. The experimental study was continued till the outlet concentration of pharmaceutical drug became constant. To study the reusability of TIAB multiple experimental runs were carried out using same beads. In case of other three drugs, the photocatalytic degradation was carried in continuous mode only. The variable process parameters of the best fit were taken from the earlier batch studies, carried out by other research groups (Hapeshi et al., 2010; Georgaki et al., 2014) using TiO2 suspension. 2.4. Analytical methods 2.4.1. Characterization of immobilized photo catalyst The physical appearance of TIAB was white color spherical particles with average diameter of 3.33 mm. The surface morphologies of the entrapped catalysts inside alginate matrix were evaluated by using scanning electron microscopy (SEM) in combination with energy dispersive X-ray analysis (EDAX). The X-ray powder diffraction (XRD) pattern for the TIAB was recorded using Shimadzu XRD-6000 diffractometer using Cu Kα radiation (l ¼1.54 Å) operating at 40 kV, 20 mA and scanning rate of 2° min 1. The BET specific surface areas of the TIAB was determined by the N2 adsorption–desorption method. 2.4.2. Measurement of degradation The concentration of chlorhexidine digluconate concentration
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in the reaction mixtures was determined using spectrophotometer at 275 nm. The details of identification have been elaborately described in the recent publication by the same research group (Das et al., 2014; Sarkar et al., 2014a). To validate the concentration measurements and for the identification of product pattern, RPHPLC system (Cyber Lab, Millbury, USA) with Zorbax SB Phenyl column (4.8 250 mm2, 5 mm, Agilent, USA) was used. The details of HPLC analysis and the chromatogram from HPLC were described earlier (Das et al., 2014) and chromatogram ensured the generation of by-product. To characterize the degraded by-products from CHD, the mass spectra analysis was done in Quadrapole-TOF Micromass Spectrometer (Waters Co., USA). The details of chromatogram and mass spectroscopy had been indicated in the experimental research work by the present research group (Das et al., 2014). The residue of other three drugs and identification & measurement of degraded by-products from ATL, IBP and CBZ were carried out according to the principles that described by Hapeshi et al. (2010) and Georgaki et al. (2014) respectively. In all three cases HPLC (Perkin Elmer, Series 200) was used with essential C18 column for IBP and ATL and Hypersil BDS C8 column for CBZ (250 mm 4 mm 5 μm). The removal percentage of pharmaceutical components has been calculated during the whole study using Eq. (1).
⎛ Ct ⎞ % Removal of pharmaceutical component = ⎜1 − ⎟ × 100% C ⎝ 0⎠
(1)
where, Ct is the concentration of any drug at any time t and C0 is corresponding value at t¼ 0.
Fig. 2. (a) Photograph of TIAB and (b) and (C) SEM images at different magnifications, (d) EDXA spectra of a single TIAB congaing 4% alginate beads and 2% TiO2 nanoparticles by weight.
Please cite this article as: Sarkar, S., et al., Photocatalytic degradation of pharmaceutical wastes by alginate supported TiO2 nanoparticles in packed bed photo reactor (PBPR). Ecotoxicol. Environ. Saf. (2015), http://dx.doi.org/10.1016/j.ecoenv.2015.02.035i
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3. Results and discussions 3.1. Characterization of TIAB The formation of nano-TiO2 impregnated bead (Fig. 2a) was confirmed by SEM with EDXA and XRD patterns. TiO2 nanoparticles were entrapped in a polymeric matrix to study its activity towards the photo catalytic degradation of antibiotic drugs. Calcium alginate was used as the solid support in this research topic. Calcium alginate beads in diluted acidic and alkaline solutions were mechanically stable. SEM analysis was carried out to confirm the presence of nanoparticles, and its distribution pattern in polymeric matrix. A representative SEM image (Fig. 2b and c) shows that most of the particles are well distributed. The quantitative compositional analysis of the TiO2 nanoparticle entrapped beads was carried out using EDXA spectroscopy measurements. The spectra confirm the presence of TiO2 in the structure. The spectra were recorded from a single bead which was produced from casting solution containing 4% alginate and 2% TiO2 nanoparticles by weight. From the measurements, it was ensured that the each bead consisted of an average 6.21% Ti, 18.14% O2, 15.43% Ca, 9.83% Na and 47.28% Cl2 and the EDXA spectrum has been shown in Fig. 2(d). EDXA analysis showed minimum level of impurities were present inside the alginate beads. The characterization by XRD evidenced the amorphous nature of the TIAB. Therefore, it was difficult to identify the crystalline phases of TiO2. Under reduce scanning velocity two phases of TiO2, anatase and rutile phases were observed. The BET surface area of the same TIAB was measured to be 21.43 m2 g 1. 3.2. Effect of alginate to catalyst (A/C) ratio The main process parameter of the current system is alginate to catalyst ratio as all other parameters are kept same with the parametric values from the earlier study (Das et al., 2014). It refers that at S/C ratio (substrate to catalyst ratio) 2.5, pH of 10.5, temperature of 30° C and UV intensity of 80 μW/cm2 the removal percentage of CHD reached its steady state value after 1 h. To study the effect of alginate to catalyst (A/C) ratio, it was varied in four different ways from ratio 0.5 to 4.0 and the effect has been clearly indicated in Fig. 3 though so many trial experiments were conducted with the variation of A/C ratio. Fig. 3 reveals that with decrease of A/C ratio adsorption efficiency of the system decreases for the current system. With increase of A/C ratio concentration of nanoparticle inside alginate matrix was decreased, therefore more
Fig. 3. Effects of A/C ratio on both adsorptions an photocatalysis process and the interdependency between adsorption and photocatalysis.
porous vacant sites were available on outer surface of beads to adsorb the substrate molecules. Thus, higher adsorption was observed for lower catalyst concentration. TiO2 nanoparticles were entrapped inside porous matrix of beads therefore; during photocatalysis a different observation was identified. Both at lower and higher value of A/C ratio photocatalytic activity were reduced. Lower value of A/C ratio refers higher catalyst concentration and higher catalyst concentration causes problem for UV penetration (Das et al., 2014; Sarkar et al., 2014a). As a result at lower value of A/C ratio photocatalytic degradation of CHD was reduced. But at higher A/C ratio, available active site on alginate beads for photocatalytic reaction was not sufficient. Moreover, at similar situation TiO2 distribution on surface of the alginate beads was inadequate which referred poor amount of active sites was available for photocatalysis. Larger value of A/C refers higher concentration of alginate present in TIAB and as a result the opacity of the alginate beads increases which causes lower UV penetration through alginate surface to reach the active site of the TiO2 nanoparticle. Therefore, at that particular ratio it was impossible to degrade CHD completely by photocatalysis. On the contrary the residual amount of CHD after adsorption process could be degraded completely by photocatalysis using alginate beads with A/C ratio of 2. However, for this type of reaction A/C ratio played the major role for percentage removal of pharmaceutical component and it should be optimized to achieve better removal of the pollutants from the system. Minute observation on Fig. 3 could reveal that at lower value of A/C ratio during the photocatalysis same percentage of CHD was removed. This was due to at lower value of such ratio the distributions of TiO2 on the upper surface of TIAB was similar and at same time the difficulty of UV penetration through alginate beads reduced to some extent. From the above discussion, it may be concluded that A/C ratio should be maintained in such way that both the stability and the catalytic activity of TIBA can reach their optimum value. Then only TIBA can be reused several times without losing catalytic activity in photocatalytic wastewater treatment. 3.3. Adsorption and photocatalysis of CHD Simultaneous adsorption and photocatalysis process was carried out at 30° C and pH of 10.5 with S/C of 2.5 in batch mode. Those process parameters have been chosen from the previous study (Das et al., 2014) carried out using TiO2 suspension to evaluate the comparison between suspension and immobilized mode of operations. Moreover, at that parametric condition maximum removal of CHD was possible. After 1 h adsorption process, photocatalysis was carried out for 1 h to find out the removal efficiency of photocatalysis alone. From Fig. 3 it has been observed that under same operating condition and maintaining A/C ratio at 2, the maximum number of CHD molecules have been adsorbed on the surface of the TIAB and after that left amount of CHD has been degraded with help of photocatalysis reaction. This observation can be justified with help of porous structure of TIAB. As it has already been mentioned that TIAB offered higher BET surface area therefore CHD was adsorbed inside the porous matrix of alginate beads. As a consequence free molecule of CHD was not available for photocatalytic degradation. The main advantage of this process with the help of TIAB almost all molecules of the pharmaceutical component (CHD) can be removed within 2 h. Though, in current process adsorption played the major role to remove pharmaceutical waste from the system. Therefore, to understand efficacy of TIAB, the fresh pharmaceutical component was continually fed to the system for photocatalytic degradation. In later part of the present article it would be understandable that the extent of adsorption on photocatalytic degradation using TIAB.
Please cite this article as: Sarkar, S., et al., Photocatalytic degradation of pharmaceutical wastes by alginate supported TiO2 nanoparticles in packed bed photo reactor (PBPR). Ecotoxicol. Environ. Saf. (2015), http://dx.doi.org/10.1016/j.ecoenv.2015.02.035i
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3.4. Adsorption and reaction kinetics In the present study two types of kinetics are involved, one is adsorption kinetics and other is reaction kinetics during photocatalysis. Both are necessary to understand the direction and extent of the removal process. First adsorption of target molecule takes place on TIAB surface and then photocatalytic degradation of that molecule occurs. Here, CHD was adsorbed on TIAB surface and simultaneously it was disappeared from the system due photocatalytic reaction. The phenomenon of adsorption of a substrate on any adsorbent can be explained through isotherms. For the liquid–solid adsorption the amount of adsorbate absorbs on the solid adsorbent surface is the function of concentration of adsorbate at constant temperature. In the present study TIAB is absorbent and CHD is absorbate. Though, different types of isotherm have already been developed; among those Langmuir isotherms is quite popular to describe the adsorption phenomenon on the solid surface. Moreover, this was developed considering that adsorbate was inert to active surface without no phase change and single molecule could occupy one active site thus mono layer formation took place. In this study, the experimental adsorption data has been well fitted with Langmuir isotherm than all other isotherms, which can be expressed mathematically using Eq. (2) (Stumn and Morgan, 1996) and in the isotherm study, initial CHD concentration was the variable parameter, whereas the others were kept constant.
C0 C0 1 = + qequ qmaxKCHDG qmax
(2)
where, qequ is the amount of CHD adsorbed at equilibrium per gram of TIAB, C0 initial equilibrium concentration of CHD (main driving force), qmax is the maximum amount of CHD adsorbed at equilibrium per gram of TIAB; KCHD adsorption rate constant (g 1). The fitted curve of C0 vs C0/qequ has linearity with the correlation coefficient of 0.897 that has been shown in Fig. 4(a). The value qmax and KCHD have been obtained as 0.286 and 0.035 g 1 respectively from the same plot. The heterogeneous photocatalysis reactions generally follow pseudo first order reaction kinetics (Sarkar et al., 2014b). A good agreement between theoretical pseudo first order kinetic model and experimental observation has been observed and that can be expressed using Eq. (3). ‵ −rphoto =
dC = kCt dt
(3)
‵ where, rphoto and k is pseudo fist order reaction rate and rate constant respectively. The above equation can be written after integrating with boundary condition
⎛ Ct ⎞ − ln ⎜ ⎟ = kt ⎝ C0 ⎠
(4)
Initial concentration of pharmaceutical waste (CHD) is represented as C0. The rate constant values can be calculated from the slope of the plot of ln(Ct/C0) vs time From Fig. 4(b) the value of rate constant (k) was calculated as 0.0555 min 1. 3.5. Comparison between suspension and entrapment mode The present research group already observed in suspension mode maximum 30% CHD was removed due to adsorption and up to 70% removal was possible during photocatalytic degradation (Das et al., 2014) at earlier specified condition. In that case photocatalysis was predominant for removal of CHD. Although at same parametric condition, in the present study, the adsorption process on TIAB played the major contribution (79%) for removal of CHD
Fig. 4. Measurement of (b) photocatalysis of CHD.
kinetics
parameters
for
(a)
adsorption
and
from the system. Moreover, with combination of both adsorption and photocatalysis could remove 99% of present antibiotic form the reaction mixture. Therefore, in batch study adsorption played the important role for the removal of pharmaceutical component, CHD from the simulated solution. 3.6. Photocatalytic degradation using PBPR In PBPR, TIAB was used as packing material. In the center of the packed column UV source was there. The sufficient time was provided for photocatalytic degradation. The operating parameters were chosen from the batch studies which were performed by several researchers (Das et al., 2014; Hapeshi et al., 2010; Georgaki et al., 2014). During all experimental observation A/C ratio of 2 was maintained. The photocatalytic degradation of several pharmaceutical drugs has been carried out using PBPR and experimental observation has been represented clearly in the current context. 3.6.1. Degradation of CHD in PBPR In the earlier batch study in suspension mode the present research group (Das et al., 2014) has revealed that maximum percentage of CHD can be removed at S/C ratio of 2.5, pH 10.5 and ambient temperature (30 °C). In the present case same parametric condition has been maintained during the photocatalytic degradation in PBPR and outlet concentration of CHD has been measured to calculate its removal percentage with time which has been plotted in Fig. 5(a). At the initial stage almost 99% removal has been observed for 60 min residence time but as the time
Please cite this article as: Sarkar, S., et al., Photocatalytic degradation of pharmaceutical wastes by alginate supported TiO2 nanoparticles in packed bed photo reactor (PBPR). Ecotoxicol. Environ. Saf. (2015), http://dx.doi.org/10.1016/j.ecoenv.2015.02.035i
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Fig. 5. Percentage removal of different pharmaceutical drugs using TIAB in PBPR at specified conditions.
progress the removal percentage has decreased gradually. In the connection with that it should be noted, the treated simulated waste water first came out from PBPR after 60 min from the start of removal process. This observation may be explained with the help of earlier batch study. According to Fig. 3 initially maximum removals of CHD was possible with help of surface adsorption on TIAB but as the time progress active sites present in porous matrix of alginate beads were getting occupied by CHD molecules. Therefore, effective adsorption was reduced after some time and at that time only photocatalysis became the predominant factor to remove pharmaceutical components. Moreover, the substrate molecules were also available to take part in photocatalysis. It has also been observed from Fig. 5(a) that removal percentage of CHD reached to its steady state value after some time. Though, in continuous mode only 55% steady state removal of CHD was possible using TIAB in PBPR whereas nearly 70% removal was achieved (Das et al., 2014) using TiO2 suspension in batch mode. This contradictory behavior was observed as in suspension mode all TiO2 nanoparticles were freely available for photocatalysis but in case of alginate beads catalyst particles were in entrapped condition. Thus, all nanoparticles could not take part in such reaction. Moreover, lower UV penetration through the alginate surface was one of the major drawbacks of the system and beads were placed around the UV source in multi-layered condition, which also caused lower light penetration through TIABs which were far away from UV light. 3.6.2. Degradation of other drugs To establish the applicability of PBPR for photocatalytic degradation of pharmaceutical wastes, three other pharmaceutical
components namely ATL, IBP and CBZ were degraded using TIAB in continuous mode. The removal percentage of those three components has been shown in Fig. 5(b)–(d). For all the cases temperature, pH and S/C ratio was maintained at 25 °C, 7 and 0.04 respectively. The parametric conditions for ATL was taken as described by Hapeshi et al. (2010) and for other two, it was opted from the research work Georgaki et al. (2014). The most interesting think is that Hapeshi et al. (2010) and Georgaki et al. (2014) was experimented with lower value of substrate concentration such that it could replicate the environmental pollution load due to those drugs. It has been observed from their studies that at mentioned condition ATL, IBP and CBZ can be removed up to 85%, 99% and 99% after 60 min, 20 min and 40 min respectively in suspension mode of operation. For all cases initial concentration was kept constant at 10 mg/L. In the current study, the continuous mode of operation has been preformed considering the residence time is equal to the time after which the maximum removal percentage was obtained by Hapeshi et al. (2010) and Georgaki et al. (2014) in batch mode. In the present system the steady state percentage of removal was achieved 58%, 85% and 80% for ATL, IBP and CBZ when residence times were 60 min, 20 min and 40 min respectively and those values were much lesser than suspension mode. The similar observation was observed in case of CHD. Therefore, similar type of explanation prevails for the removal of ATL, IBP and CBZ using TIAB. From the above discussion it is clear that TIAB as well as PBPR has failed to achieve maximum percentage of removal of pharmaceutical components from the simulated solution. Though, one thing is confirmed that an appreciable amount of drugs can be removed by such type of continuous mode of photocatalysis
Please cite this article as: Sarkar, S., et al., Photocatalytic degradation of pharmaceutical wastes by alginate supported TiO2 nanoparticles in packed bed photo reactor (PBPR). Ecotoxicol. Environ. Saf. (2015), http://dx.doi.org/10.1016/j.ecoenv.2015.02.035i
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degradation process for pharmaceutical wastewater treatment. The obtained results showed that the efficiency decreases from 99% to 85%. This is likely due to the fouling of porous surface TIAB as well as the catalyst surface and loss of catalyst due to repeated acid wash.
4. Conclusions
Fig. 6. The recycling efficiency of TIAB in continuous mode during CHD removal.
process and therefore, large volume of wastewater can be treated by the present system. Moreover, TIBA also ensures the reduction of loss of valuable photocatalyst and its contamination. Accordingly, the reusability of nanoparticle in immobilized condition has been studied here. 3.6.3. Effect of residence time in PBPR Residence time (τ) is the time period that spends by a molecule in a system. Higher residence time refers longer staying and vice versa. To understand the effect of τ on the performance of PBPR, τ has been decreased from the value of residence time in batch mode operation (Das et al., 2014, Hapeshi et al., 2010; Georgaki et al., 2014) for each drug i.e. CHD, ATL, IBP and CBZ. The τ has been varied in three different ways for each antibiotic and results have been plotted in Fig. 5. It has been observed that at a specified condition with decrease of τ the removal percentage of any drug has decreased very rapidly. This was happened during removal of any pharmaceutical components because lower value of τ provided short time period of adsorption and photocatalysis as well. A sufficient amount time is required to come in contact between catalyst and target molecule. Thus, in case PBPR higher value of residence time is favorable though it is not possible to increase that value infinitely. Moreover, higher loading concentration of pharmaceutical components in waste effluents leads to increase residence time because at that particular condition more time is required for adsorption and photocatalytic degradation. At larger value of τ, the inlet and outlet flow rate of reactor becomes lower hence the volume of wastewater which can be handled by PBPR per unit time also reduces at a particular condition. The liquid chandelling capacity by a PBPR can be kept constant at higher residence time; the volume of the reactor should be increased accordingly which is not feasible in practice. Therefore, the residence time should be maintained in such a way that the removal percentage of pharmaceutical component can reach to desired value and at the same time the PBPR can handle large volume of pharmaceutical wastewater. 3.7. Recycling of photocatalyst TIAB can be recycled effectively which makes the process cost effective. To study the reusability, the catalyst was recycled five times in continuous mode of operation using PBPR during degradation of CHD and degradation efficiency was recorded which had been plotted in Fig. 6. At optimized conditions for photocatalysis the degradation of CHD were determined, the catalyst was recovered by giving mild acid water wash and again used in
The above study is very much important considering the aspects of recycling and reuse of TiO2 nanoparticles in continuous mode operation. After suspension mode study, A/C ratio is the most important parameter which is needed to be optimized for entrapment mode photocatalysis. TIAB can efficiently remove the pharmaceutical components from the reaction mixture though the reduction of antibiotic activity is less significant. The uses of alginate beads as immobilization of photo catalysts show some difficulty, mainly lower degradation efficiency when compared with suspension mode photocatalysis. But with the help of PBPR appreciable percentage of pharmaceutical drugs can be removed and large amount of wastewater can be treated using such system. The residence time has significant effect on the performance of PBPR and at higher value of it produces better efficiency of PBPR. Finally, TIAB can be reused without losing its effectiveness for at least five cycles in PBPR which is main advantage to make the process more cost effective over the suspension mode of photocatalysis. Some necessary modifications should be incorporated to enhance the photocatalytic performance of alginate beads. Then only this methodology can be viable for large scale pharmaceutical wastewater treatment.
Acknowledgement The work reported in this article is part of an Indo-Bulgarian project (vide sanction letter no. INT/BULGARIP-09/2012), funded by Department of Science & Technology (Government of India). Accordingly, the contributions of DST (India) are gratefully acknowledged.
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