- Email: [email protected]
Adsorption and photocatalytic removal of Ibuprofen by activated carbon impregnated with TiO2 by UV–Vis monitoring
Chemosphere 217 (2019) 724e731
Contents lists available at ScienceDirect
Chemosphere journal homepage: www.elsevier.com/locate/chemosphere
Adsorption and photocatalytic removal of Ibuprofen by activated carbon impregnated with TiO2 by UVeVis monitoring Ying Gu a, *, Jan Yperman b, Robert Carleer b, Jan D'Haen c, d, Jens Maggen b, Sara Vanderheyden b, Kenny Vanreppelen b, Roberto Machado Garcia e a
Zhejiang Ocean University, Zhoushan 316022, China Research Group of Applied and Analytical Chemistry, Hasselt University, Diepenbeek, Belgium Institute for Materials Research (IMO), Hasselt University, Diepenbeek, Belgium d IMOMEC, IMEC vzw, Diepenbeek, Belgium e Chemistry Department, Faculty of Natural and Exact Sciences, Universidad de Oriente, Cuba b c
h i g h l i g h t s Three kinds of activated carbon impregnated with TiO2 (ACT) were prepared. Removal of Ibuprofen includes effects of adsorption and photodegradation by ACT. ACT composites effectively improve removal of Ibuprofen under UV. Kinetic tests of Ibuprofen removal were studied onto different composite catalysts.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 July 2018 Received in revised form 6 November 2018 Accepted 10 November 2018 Available online 13 November 2018
The removal of Ibuprofen was investigated by activated carbon impregnated with TiO2. Emphasis was given on the effect of different parameters, such as composite type, initial Ibuprofen concentration (5 e25 mg/L), temperature (22e28 C) and pH (acidic and alkaline solution). The experiment was carried out in a self-made tubular flow reactor, with one 15 W monochromatic UV lamp (254 nm). The composite AC90T10 gives the highest removal degree of 92% of Ibuprofen solution under UV light within 4 h, due to synergy of adsorption and photodegradation. It was found that weight ratio of composite/Ibuprofen has limited effect on the removal degree within the concentration range (5e25 mg/L), but reaction time under UV light (4 h) and pH (acidic solution) are very important. The kinetic experimental data obtained at pH 4.3 at 25 C on different composites were fitted to pseudo-first, pseudo-second and Elovich models, obtaining a high accuracy based on R2 values. From the results, composites of granular activated carbon and TiO2 can enhance removal of Ibuprofen effectively, making recycle process much easier and less costly, which can be a promising method in future water treatment. © 2018 Elsevier Ltd. All rights reserved.
Handling Editor: Klaus Kümmerer Keywords: Activated carbon TiO2 Ibuprofen Photodegradation Adsorption
1. Introduction Sea water gets increasingly polluted by the widespread discharge of wastewater from industries, by chemical products and by extensive medical and cosmetic material consumption of human beings. These pollutants include pharmaceuticals, pesticides, organic dyes, phenols, etc. Even though the concentration of each pollutant is low the damage towards the ecosystem is very high due
* Corresponding author. Zhejiang ocean university, Zhoushan 316022, China. E-mail address: [email protected] (Y. Gu). https://doi.org/10.1016/j.chemosphere.2018.11.068 0045-6535/© 2018 Elsevier Ltd. All rights reserved.
to their toxicity. It is indeed a threat for drinking water supplies, as many pollutants might have serious adverse impact on human beings (Stackelberg et al., 2004; Jones et al., 2005). Therefore, combined advanced oxidation technologies for the removal of pollutants from wastewater are widely studied (Challis et al., 2014; Tijani et al., 2014). Ibuprofen is one of the pharmaceuticals which has been extensively used. Ibuprofen is slightly soluble in water (0.021 mg/ ml at 20 C), but soluble in most organic solvents. It is a nonsteroidal anti-inflammatory drug, mostly used for treating pain, fever, and inflammation, including painful menstrual periods, migraines, and rheumatoid arthritis. As other pharmaceuticals,
Y. Gu et al. / Chemosphere 217 (2019) 724e731
Ibuprofen can leach into ground water and soil through direct and indirect use in daily life. Since conventional wastewater treatment plants commonly do not include pharmaceutical degradation, removal of pharmaceuticals is important (Hernando et al., 2006). Photodegradation is an effective method for pharmaceutical removal (Boreen et al., 2003; Fatta-Kassinos et al., 2011; Fu et al., 2015). Recent years photochemical degradation using nanosized TiO2 as photo-catalyst has been studied by many researchers ndez-Arriaga et al., 2008; Madhavan et al., 2010; Wang et al., (Me 2012; Georgaki et al., 2014). The efficiency of this approach is very good for two reasons: 1) nanosized TiO2 has a high photocatalytic activity and 2) it can be made under ambient conditions. The degradation rate is related to several parameters, such as the loading of composite, the irradiation power, the initial concentration, the pH of solution, and the surface area of composite. Activated carbon (AC) is also effective to adsorb Ibuprofen, due to its physical and chemical adsorption ability, which has also been studied (Mestre et al., 2007, 2009; Snyder et al., 2007; Dubey et al., 2010; Baccar et al., 2012; Behera et al., 2012; Guedidi et al., 2013). The advantages of AC as adsorbent is due to its large surface area, developed pore structures and surface functional groups (Li et al., 2002). The adsorption ability matches to different amounts of AC, temperature, concentration of the pollutant and contact time. However, hybrid materials of AC and TiO2 for Ibuprofen degradation have to our knowledge not been studied yet. Using composites of AC and TiO2 for other species like benzalkonium chloride degradation, it was concluded that with proper weight ratio of AC and TiO2 a more effective degradation was realized than pure AC or pure TiO2(Suchithra et al., 2015). Within the field of AC applications, mostly batch experiments are carried out. Stirring or shaking methods were used to increase the interaction and improve the contact between composite and solution. However, this approach is not ideal since too fast stirring might deteriorate the composites in batch reactor. Flow reactor is a promising technology emerging in a variety of research areas (Schuster and Wipf, 2014; Junkers and Hoogenboom, 2016). In flow chemistry, reaction conditions have been improved significantly compared to batch experiments (Conradi and Junkers, 2013; Schuster and Wipf, 2014; Kermagoret et al., 2015; Junkers, 2016, 2017; Junkers and Hoogenboom, 2016; Junkers and Wenn, 2016; Wenn and Junkers, 2016): a) it takes much shorter time to reach acceptable yields, b) reaction yield is greatly improved, and c) it is possible to adjust reactor volume and flow rate in flow setup, in order to get high yields, which is difficult to achieve in batch system. However, Ibuprofen degradation directly by UV irradiation and by TiO2 impregnated AC under UV irradiation using flow reactor has not been applied yet. In this study, photocatalytic degradation of Ibuprofen by AC impregnated with TiO2 (ACxTy) in a UV-flow reactor was investigated. A self-made tubular flow UV-reactor was used. Ibuprofen concentration was monitored by UVeVIS spectrometry, SEM-EDX was used for morphology characterization and elemental analysis. 2. Experimental 2.1. Materials 2-(4-isobutylphenyl)propanoic acid (98%), also named as Ibuprofen (IBP) was purchased in Ark Pharm, USA. Titanium isopropoxide, 98þ% (ACROS Organics, Belgium), glacial acetic acid (MERCK Germany) and ethanol 96% (VWR international, Belgium) were of analytical grade and used as received. Other chemical reagents used were: sulfuric acid, 95e97% (MERCK Germany), ascorbic acid, 99% (ACROS Organics, Belgium), ammonium molybdate (MERCK Germany) and potassium antimony
725
tartrate (MERCK Germany). Commercial AC Filtrasorb 400, a granular AC for the removal of dissolved organic compounds from water and wastewater, was used in this study. It is made from bituminous coal and available from Calgon Carbon Corporation. All chemical reagents were of analytical grade and Milli-Q water (18.2 MU cm1 conductivity) was used in all working solutions. 2.2. Synthesis of ACxTy composites In this study, sol-gel method was used to prepare ACxTy. 17 g Ti(O-i-Pr)4 was dissolved in 28 ml glacial acetic acid and was magnetically stirred for 30 min. Then, 287 ml ethanol was added drop wise to this solution under continuous stirring for another 30 min until the clear homogeneous sol started to show slight turbid nature. At this stage, a certain amount of AC (Filtrasorb 400) was added in accordance of the required ACxTy composite under vigorous stirring for 1 h. Afterwards, 45 ml Milli-Q water was added to this solution and stirring was continued for 5 h for complete hydrolysis. The final solution was evaporated at 70 C to form gradually a precursor gel. In the end this gel was calcined at 500 C with a heating rate of 4 C/min for 2 h in nitrogen atmosphere with a flow rate of 70 ml/min, to obtain porous gray powders (ACxTy). Three composite samples were prepared with a weight percentage of 90%, 50% and 10% AC respectively. For ACxTy, where x and y (¼100-x) stands for the weight percentage of AC and TiO2. The three types of prepared composites are labelled as AC90T10, AC50T50, and AC10T90. Pure AC (Filtrasorb 400) was used as reference and designated as AC100. 2.3. Adsorption and photo-catalysis experiments Every experiment was carried out in a self-made tubular flow UV-reactor, with one 15 W monochromatic UV lamp (254 nm), shown in Fig. 1. An amount of 1.6 g/L composite was dispersed in a quartz tube (id:4 mm, od:6 mm, length:5 cm) which was placed onto the UV-lamp, in order to have the shortest distance between composite and UV light. 20 ml of Ibuprofen solution within a concentration range of 5e25 mg/L was kept in a double walled beaker, and the reaction temperature was strictly controlled by a thermostatic water bath. The Ibuprofen solution was pumped at a 16.37 ml/ min flow rate through a looping circuit (an UV-transparent PFA (perfluoroalkoxy) tubing connected to the quartz tube), getting optimal contact with the composite in the quartz tube under UV light. Small samples of around 3 ml were taken after defined time periods for the determination of Ibuprofen concentration by UVeVIS spectrometry. Total analysis time takes only 10e20 s, including sampling, measurement, and recycling the sample to the looping solution. It was supposed that the effect of sampling was a negligible factor in the entire adsorption process. Before and after this procedure, the measuring quartz cell used in UVeVIS spectrometer was washed 3 times by HCl solution (3.6e3.7%), 3 times by de-ionized water, 3 times by Milli-Q water, and dried by pressured air, to prevent memory effects between analyses, same as the sampling tube. Each experiment was done at least two times. The repeated tests provide reliable measurements and error control. The tubular flow UV-reactor has several advantages. Firstly, it can maintain the shortest distance between composite and UV light, reaction yield has been improved. Secondly, reaction temperature can be precisely controlled by a water bath as temperature is key effect for adsorption and fast flow rate. Thirdly, reactor volume and flow rate can be adjusted in flow reactor set-up, according to the purpose of study. It can be noticed that results for pure TiO2 are missing in our
726
Y. Gu et al. / Chemosphere 217 (2019) 724e731
Fig. 1. Flow UV-reactor.
experimental set-up. Comparison between different composites is very common in batch experiments, since TiO2 was left in solution during stirring and is easy to maintain. In this flow reactor, it is very difficult to keep all TiO2 particles in the quartz tube located directly on the lamp. Pure TiO2 contains very fine particles. Quartz wool which was used to block ACxTy does not prevent TiO2 to pass this barrier. Once some TiO2 particles escape from the quartz tube flow reactor on the lamp, the effect of UV irradiation for the photocatalysis process was partly lost. Filter paper was tried instead of quartz wool, however the problem still remains. TiO2 can also pass the filter paper, additional an extra resistance on the flow rate of looping was observed, both having a significant effect on the photodegradation process of Ibuprofen in the flow reactor. 2.4. Analysis Ibuprofen removal was quantified by UVeVIS spectrometry, using a quartz cell of 1.0 cm path length and using the absorbance at 225 nm. Five standard concentrations ranging from 5, 10, 15, 20 and 25 mg/L were measured under the same experimental conditions. Ibuprofen concentration can be reliable measured as low as 5 mg/L. A calibration curve with a correlation coefficient of 0.99942, clearly shows the effectiveness of this method in quantifying Ibuprofen within a concentration range of 5e25 mg/L. LC/MS, GC/MS and HPLC-PDA were carried out to detect possible by-products, however, no reliable results were obtained because of too low concentration. A possible explanation is the combined effect by ACxTy adsorption and photodegradation of ACxTy combined with UVeVis, which leads to a much smaller amount of degradation products. Additionally those degradation products might also partly be adsorbed by ACxTy. Scanning Electron Microscopy (SEM) was used to analyse the morphology of the surface of different composites. Energydispersive X-ray spectroscopy (EDX) was used in combination with SEM to provide selected area elemental analysis. For better explanation of Ibuprofen removal, SBET and porous structure of ACxTy evaluation were considered. However, TiO2 plays an important role in removal degree with ACxTy. When the surface of AC was covered by TiO2, blocking the road for adsorption
by AC. Even if a porous structure is present, the effect is quite limited. Therefore it is believed that SBET data and porous structure have no direct relationship with Ibuprofen removal degree in the experiment with ACxTy. The experimental data also proved that. TGA was used to check the synthesis of ACxTy composites using the ash residues as a measure for the TiO2 content. Samples were heated up to 900 C in an air atmosphere (20 C/min). The removal degree was calculated as:
Removal degreeð%Þ ¼
C0 Ci 100% C0
where C0 (mg/L) is initial Ibuprofen concentration, Ci (mg/L) is the Ibuprofen concentration after i min. B-spline curve fitting was used in figures with experimental data, if there was no specific explanation.
3. Results and discussion 3.1. Characterization of composites The weight % of TiO2 based on the synthesis input parameters and the TGA results for the three prepared ACxTy materials were studied, detailed data can be found in Supplementary Information. TGA results are in line with the amounts used in the synthesis based on solution concentrations confirming that synthesis procedure works well. SEM and EDX were used to evaluate the synthesis of composites: the morphology of the surface and the elemental composition of selected area respectively. From Fig. 2, it can be seen that for ACxTy, TiO2 is dispersed on the surface of the AC. The white area is assigned to Ti, while the grey and black areas refer to carbon. In the case of AC10T90 the SEM images demonstrate that a significant TiO2 content at the surface is observed. From the elemental analysis of AC50T50 and AC90T10, it is also demonstrated that AC90T10 has an enhanced white area at the surface compared to AC50T50. Based on the TGA results, this is apparently not reflected in the mixture ratio. The reason for this must be found in the fact that TiO2 has not been homogeneous distributed on the surface for each particle of activated carbon
Y. Gu et al. / Chemosphere 217 (2019) 724e731
727
Fig. 2. Morphology of the surface of different composites (SEM).
which is confirmed by less magnified SEM images in contrast with SEM images with larger magnifications. EDX data can be found in Supplementary Information. For AC100, mainly carbon is measured with very limited amounts of Al, Si, O, Ca, and Fe. For the composite AC90T10, the EDX data reveal that in the white part still peaks of carbon are detected. Similarly, in the black area, peaks of Ti could also be seen, reflecting that Ti and carbon cannot be simply differentiated by visible color in SEM images. The mixing of Ti and C resulted in a rather irregular covering, some area were heavily covered with Ti, while others contained only a minor amount of Ti. For the other two composites AC50T50 and AC10T90, EDX results are also very similar. 3.2. Effect of AC and composites used in the adsorption experiments Three composites and one pure AC have been used: AC10T90, AC50T50, AC90T10 and AC100 respectively. The quartz tube placed on the UV lamp was filled with 1.6 g/L AC or composite. 20 ml Ibuprofen solution (25 mg/L) was looping through the reactor for 4 h at 25 C under UV irradiation at 254 nm. As can be seen in Fig. 3, the removal degree of Ibuprofen is depending on the conditions and type of adsorbents (AC and composite). Blank UV curve shows decomposition of Ibuprofen without using AC or composite under UV irradiation. Apparently, UV light has a direct strong effect on Ibuprofen degradation, especially within the first half hour, the removal degree increases very sharply, even faster than any other combination. The photolysis of ibuprofen under UVC irradiation was already discussed in previous papers (Castell et al., 2010; Eskandarian et al., 2016). Another reason for this can be found in the used set-up of the quartz tube, irradiating the solution directly by UV without any
Fig. 3. The effect of different composites on removal degree of Ibuprofen (1.6 g/L composite/AC, 25 mg/L Ibuprofen initial solution, pH ¼ 4.3, 25 C). Removal degree values are the average values of duplicates.
“obstruction” of AC or composites in the reaction tube. However, the removal degree of Ibuprofen flattens faster compared to the curves with AC or composite. Almost no change in the last hour can be noticed, quite different compared to experiments with AC or composites. Working without UV and pure AC100 (AC100 dark) a worse removal degree and removal amount of Ibuprofen are found compared to the blank UV experiment within the first two hours,
728
Y. Gu et al. / Chemosphere 217 (2019) 724e731
due to the photolytic mechanism. By using pure AC100 under UV irradiation (AC100 UV) the same tendency can be noticed within the first two hours. However, after two hours, the positive effect of UV becomes visible, higher removal degree (Fig. 3) and removal amounts of Ibuprofen compared to the blank UV experiment (62%) are observed: 82% versus 71% after 4 h. This can be explained by a combined adsorption and self-decomposition process, while for AC100 dark only an adsorption on the AC takes place, additionally leading to a faster decrease in the number of unoccupied adsorption sites. AC50T50 and AC90T10 have very good and comparable removal degrees and removal amounts of Ibuprofen, both reaching quite high values of 90% and 92% respectively after 4 h. A saturation point of adsorption and decomposition is almost reached. Currently, decomposition time and efficiency of UV irradiation cannot be correctly estimated, according to Fig. 3, it seems ibuprofen removal degree does not reach 100%. Nevertheless, the synergetic effect of adsorption and photocatalytic degradation by TiO2 works very well for both composites. In contrast, the removal degree and removal amount by AC10T90 are the lowest. The following reason can be formulated and is supported by SEM images: the surface of AC10T90 is largely covered with TiO2, which decreases the effective adsorption surface of the AC. In addition, AC10T90 has a relatively larger average particle size due to the extra TiO2 layer on the AC. For the same weight of composite addition, AC10T90 has fewer amounts of particles for each experiment, which might cause lower removal degree of Ibuprofen. When AC100 (with and without UV light) is compared with AC50T50 and AC90T10 no flattening of the curve is noticed nearby the 4 h point. A possible reason could be that the oxygen groups from TiO2, being present at the entrance and on the walls of the pores of the AC, decreased the total specific surface area and micropore volume. Additionally, there is an indication that these two composites show a higher removal tendency at the beginning of the Ibuprofen removal experiment, which decreases their activity as more adsorption sites are occupied. Perhaps also explaining why a 100% removal percentage cannot be reached. AC10T90 behaves clearly different. It was found in performing batch experiments that granular TiO2 has an effective ability for degradation of organic species due to its larger surface area. However, setting up an industrial application in using this very fine TiO2 powder will render high filtration costs. Hybridization of granular AC and TiO2 can make recycle process much easier and less costly. At the same time a good removal yield is the result of a synergetic effect of both adsorption and photodegradation. 3.3. Effect of different initial concentrations of Ibuprofen Fig. 4 shows the effect of different initial concentrations on removal degree and residual amount of Ibuprofen. Three initial concentrations were chosen: 5 mg/L, 15 mg/L and 25 mg/L respectively. Reaction temperature was strictly controlled at 25 C, 1.6 g/L AC90T10 was used as reference and as the only composite, pH of the Ibuprofen solution is 4.3. As can be seen from Figs. 4 and 5 mg/L Ibuprofen solution reaches the highest removal degree within the first 2 h, while 25 mg/L Ibuprofen solution has the lowest indicating that the highest weight ratio of composite/Ibuprofen ruled this effect. However, after 2 h, the removal degree of 5 mg/L Ibuprofen solution starts to decrease slightly and reached a maximum value of 96% after 4 h. This cannot be noticed for the 15 mg/L and 25 mg/L Ibuprofen concentrations, where a further increase of % removal can be observed. For the 15 mg/L solution a maximum of 95% removal degree is reached similar for the 5 mg/L solution, while for the 25 mg/L solution “only” 92% is reached after 4 h. So within this
Fig. 4. The effect of different initial concentrations on removal degree of Ibuprofen (1.6 g/L composite AC90T10, pH ¼ 4.3, 25 C). Removal degree values are the average values of duplicates.
Fig. 5. The effect of different reaction temperatures on removal degree of Ibuprofen (1.6 g/L AC90T10, 25 mg/L Ibuprofen initial solution, pH ¼ 4.3). Removal degree values are the average values of duplicates.
concentration range, the weight ratio composite/Ibuprofen has rather a limited effect on the kinetics of adsorption and degradation at the initial state and on the maximum amount of Ibuprofen removed.
3.4. Effect of temperature From Fig. 5, the effect of three different temperatures on the removal degree and amount of Ibuprofen is shown. Although a quite small temperature range from 22 C to 28 C is studied, clearly some minor differences in removal mechanism can be noticed. After 4 h, surprisingly, almost the same removal degree is obtained. According to the theory of AC adsorption, one should expect that the qm value increases with increasing temperature. However, in this concept of flow reactor under UV irradiation, the experimental data do not follow this rule, because the removal is a result of a combination of adsorption and photodegradation. At the moment, it is not clear why at the initial state such a different behavior is
Y. Gu et al. / Chemosphere 217 (2019) 724e731
noticed with increasing temperature. Additional and specific experiments need to be developed and performed to unravel the reason behind. 3.5. Effect of different pH The pH of reaction solution has a significant effect on the adsorption process of Ibuprofen, because the surface charge of adsorbent (AC90T10) has been changed. The pH of original Ibuprofen solution of 25 mg/L without any adjustment is 4.3. The pKa of Ibuprofen will determine which molecular species will be dominant present at a given pH. Both experiments were run with 25 mg/L Ibuprofen solution with 1.6 g/L AC90T10 at 25 C, with UV light of 254 nm. From Fig. 6, the acidic environment shows better removal performance of Ibuprofen, the maximum removal is reached more quickly than that in the more basic solution. However under basic conditions, the removal degree is still increasing even after 4 h. Similar to the temperature experiment, the combination of adsorption and photodegradation must be the reason for this time effect. In acidic solution, composites surfaces were charged positively, while composites and Ibuprofen are negatively charged in basic solutions. The initial better performance in acidic solution can be explained by electrostatic interaction involving electrostatic repulsion between the adsorptive anion and the negative charged AC surface with increasing pH (Mestre et al., 2007). As it was suggested by Dubey et al., rather a physisorption than a chemisorption process might be involved, enforcing the combined removal mechanism (Dubey et al., 2010). 3.6. Adsorption and photo-catalytic kinetics of ACxTy The rate of adsorption and photodegradation of ACxTy in Ibuprofen solutions are also evaluated. During the past 30 years numerous sorption systems have been studied. The adsorption and photocatalytic kinetic data of ACxTy were fitted by three different models (Weber and Morris, 1963; Chien and Clayton, 1980; Ho and McKay, 1999; Worch, 2012; Guedidi et al., 2013; Kaur et al., 2015), the pseudo-first-order model, the pseudo-second-order model, and the Elovich model respectively.
Fig. 6. The effect of different pH on removal degree of Ibuprofen (1.6 g/L AC90T10, 25 mg/L Ibuprofen initial solution, 25 C). Removal degree values are the average values of duplicates.
729
The pseudo-first-order equation is given by:
Qt ¼ Qe Qe ek1 t
(1)
where Qe (mg g1)is the capacity at equilibrium, Qt (mg g1) is the amount of Ibuprofen decreased at time t (h), k1 (h1) is pseudofirst-order model rate constant. The pseudo-second-order equation is given by:
Qt ¼
Q 2e k2 t 1 þ Qe k 2 t
(2)
where k2 (g mg1 h1) is pseudo-second-order model rate constant. The Elovich equation is given by:
Qt ¼
1
b
1 lnða bÞ þ lnðt t0 Þ
b
(3)
where a (mg g1 h1) is the initial rate, b (g mg1) is a constant related to external surface area coverage and activation energy of chemisorption. Data were fitted by the above models and results were listed in Table 1. Except for AC100 in dark, all other experiments were carried out in a combined system of adsorption and photodegradation. So the actual constant will include two parts: adsorption and photodegradation. It is currently not possible to determine the rate of adsorption and the rate of photodegradation separately. AC90T10 gives the highest removal degree of 92% of Ibuprofen solution under 254 nm UV light within 4 h. Except AC90T10, fitted profiles of adsorption and photocatalytic kinetics of other composites by three models, were shown in Supplementary Information. From R2 values, it can be seen that the kinetic data are best fitted by a pseudo-second order model, however the other two models (first and Elovich) fits also quite well with the experimental data based on the R2 values. Having a closer look to Fig. 7, for AC90T10 the first order model describes better with the found experimental trend. For the other composites, the second order model describes slightly better for AC100 dark, AC100 UV and AC50T50. However, it's difficult to explain the outcome of AC10T90 since it did not reach an equilibrium within the performed experimental conditions. The rate constants for first and second order model in Table 1 are both in agreement with Fig. 3: AC50T50 > AC90T10 > AC100 dark > AC100 UV [ AC10T90. Based on Fig. 3 the maximum amount of Ibuprofen is probably the lowest for AC100 dark comparing to AC100 UV as it tends to go faster to a saturation situation. Additionally, the AC100 UV curve crossed the removal tendency of AC100 dark. AC100 UV tends to a removal degree of the best performing ones (AC50T50 and AC90T10), which seem to reach their maximum value much faster. However the highest maximum Ibuprofen uptake is found for the slowest removal composite material AC10T90, although it is not that obvious from Fig. 3. But one can observe that no clear flattening in the curve can be noticed and that the photodegradation mechanism is less efficient and slower compared to the combined mechanism of adsorption and photodegradation for the other composites. On long term base this seems in favor of AC10T90 as higher amounts of TiO2 are present. For Elovich models, the initial rates of a for different composites are also in the same order when comparing the rate constants for first and second order model, and the data in Fig. 3. As initial rate a increases, the combined reaction of adsorption and photodegradation is faster. The other constant b is related to external surface area coverage and activation energy of chemisorption. As in Supplementary Information, b for AC10T90 is only 0.085, very low comparing to the other adsorbents having values in
730
Y. Gu et al. / Chemosphere 217 (2019) 724e731
Table 1 The pseudo-first-order, pseudo-second-order and Elovich model by non-linear regression analysis and experimental data.
AC100 UV AC90T10 UV AC50T50 UV AC10T90 UV AC100 dark
Experimental
Pseudo-first-order
Pseudo-second-order
Elovich equation
Qe (mg g1)
Qe (mg g1)
Qe (mg g1)
a (mg g1 h1/2)
12.862 14.375 14.085 8.618 11.198
12.982 14.223 13.458 15.46 10.910
17.041 16.676 15.446 26.89 13.832
14.496 56.148 78.091 3.293 17.957
Fig. 8. Plot of intraparticle diffusion model at pH ¼ 4.3, 25 mg/L Ibuprofen solution with 1.6 g/L AC90T10. Fig. 7. Fitted profiles of Ibuprofen adsorption and photocatalytic kinetics of AC90T10 by pseudo-first-order, pseudo-second-order, and Elovich models (1.6 g/L AC90T10, 25 mg/L Ibuprofen initial solution, pH ¼ 4.3, 25 C).
the range of 0.2e0.3 g/mg. It also confirms that in 4 h reaction, 90% Ti coverage is too high for adsorption and degradation, long term experiments are recommended for this composite. 3.7. Adsorption and photo-catalytic mechanism of ACxTy The Weber-Morris intraparticle diffusion model was tested to investigate the mechanism. The intraparticle diffusion model rather refers to possible different mechanistic steps within the adsorption and photodegradation process. Weber-Morris intraparticle diffusion model is given by:
qt ¼ kp i t 0:5 þ Ci
(4)
where kp i (mg g1 h1/2) is the rate parameter of stage i, which is the slope of the straight line of qt versus t 0:5 , also named as intraparticle diffusion rate constant. C i (mg g1) represents the thickness of the boundary layer at the intercept of stage i. As can be seen from Fig. 8, there are three stages in the intraparticle diffusion plots. For pH ¼ 4.3, in the first stage the line looks like going through the origin, a small negative value was found because it's experimental data loaded with an error. In the first stage, there is an instantaneous adsorption and photodegradation. The second stage is less sharp and gradual adsorption and degradation take place with rate limiting intraparticle diffusion. The last stage is mild, and can be considered as the final equilibrium stage. As residual concentration of Ibuprofen is lower and lower, the intraparticle diffusion also occurs slower. From Fig. 8, it was confirmed that initially the intraparticle diffusion rate was quite
fast, but slowed down with contact time. The constant became larger as time increased, confirming that the boundary layer effect was more significant. The linear lines of second and third stage did not pass through the origin point, which might be caused by difference in mass transfer rate. Possible reason is that there are other rate limiting mechanisms in this process. For solution of pH ¼ 8.6 it was also tested, the intraparticle diffusion model was not in good agreement with the experimental kinetics, a linear relation was only found in third stage, no overall good fitting could be found.
4. Conclusions Different mixture ratios of AC and titanium dioxide were prepared. Adsorption and photo-catalytic experiments with different initial concentrations of Ibuprofen, at different temperatures, and at different pH were researched. Additionally morphological profiles were used to understand the mechanism of Ibuprofen removal from wastewater using these composites. The following conclusions can be drawn: (1) Due to synergy of adsorption and photodegradation, it is found that the composite AC90T10 gives the highest removal degree of 92% of Ibuprofen after UV light irradiation at 254 nm within 4 h. (2) Weight ratio of composite/Ibuprofen has limited effect on the removal degree within the concentration range of 5 mg/L and 25 mg/L. (3) Reaction time under UV light is very important for Ibuprofen removal. (4) The sorption capacity of AC90T10 for Ibuprofen removal is higher in acidic solution than in alkaline conditions.
Y. Gu et al. / Chemosphere 217 (2019) 724e731
(5) The pseudo-second order model gives the best fit for the kinetic data of AC90T10, however only minor differences are found with the other kinetic models. The combined adsorption and photodegradation effect could be responsible for that. Acknowledgements This work was supported by Hasselt University, Belgium(R8109), and Zhejiang Ocean University, China (21045012615). The Chinese author is grateful to Jenny Put for sample analysis, and Yvo Feytong for assistance during experiments. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.chemosphere.2018.11.068. References , M., Bouzid, J., Feki, M., Bla nquez, P., 2012. Removal of pharmaBaccar, R., Sarra ceutical compounds by activated carbon prepared from agricultural by-product. Chem. Eng. J. 211, 310e317. Behera, S.K., Oh, S.Y., Park, H.S., 2012. Sorptive removal of ibuprofen from water using selected soil minerals and activated carbon. Int. J. Environ. Sci. Technol. 9, 85e94. Boreen, A.L., Arnold, W.A., Mcneill, K., 2003. Photodegradation of pharmaceuticals in the aquatic environment: a review. Aquat. Sci. 65, 320e341. Castell, J.V., Gomez, M.J., Miranda, M.A., Morera, I.M., 2010. Photolytic degradation of ibuprofen. Toxicity of the isolated photoproducts on fibroblasts and erythrocytes. Photochem. Photobiol. 46, 991e996. Challis, J.K., Hanson, M.L., Friesen, K.J., Wong, C.S., 2014. A critical assessment of the photodegradation of pharmaceuticals in aquatic environments: defining our current understanding and identifying knowledge gaps. Environ. Sci. Process. Impacts 16, 672e696. Chien, S.H., Clayton, W.R., 1980. Application of Elovich equation to the kinetics of phosphate release and sorption in soils. Soil Sci. Soc. Am. J. 44, 265e268. Conradi, M., Junkers, T., 2013. Efficient [2þ2] photocycloadditions under equimolar conditions by employing a continuous UV-flow reactor. J. Photochem. Photobiol. Chem. 259, 41e46. €, M., Gopal, K., 2010. Artemisia vulgaris-derived Dubey, S.P., Dwivedi, A.D., Sillanp€ aa mesoporous honeycomb-shaped activated carbon for ibuprofen adsorption. Chem. Eng. J. 165, 537e544. Eskandarian, M.R., Choi, H., Fazli, M., Rasoulifard, M.H., 2016. Effect of UV-LED wavelengths on direct photolytic and TiO 2 photocatalytic degradation of emerging contaminants in water. Chem. Eng. J. 300, 414e422. Fatta-Kassinos, D., Vasquez, M.I., Kümmerer, K., 2011. Transformation products of pharmaceuticals in surface waters and wastewater formed during photolysis and advanced oxidation processes e degradation, elucidation of byproducts and assessment of their biological potency. Chemosphere 85, 693e709. Fu, H.L., Yao, K., Wen, Y.L., Guo, G.L., Ping, C., Hao, P.H., Kang, Y.P., 2015. Photodegradation of ibuprofen under UVeVis irradiation: mechanism and toxicity of photolysis products. Bull. Environ. Contam. Toxicol. 94, 479e483. Georgaki, I., Vasilaki, E., Katsarakis, N., 2014. A study on the degradation of carbamazepine and ibuprofen by TiO2&ZnO photocatalysis upon UV/Visible-Light irradiation. Am. J. Anal. Chem. 518e534. v e ^que, J.-M., Soneda, Y., Bellakhal, N., Duclaux, L., 2013. Guedidi, H., Reinert, L., Le
731
The effects of the surface oxidation of activated carbon, the solution pH and the temperature on adsorption of ibuprofen. Carbon 54, 432e443. ndez-Alba, A.R., Barcelo , D., 2006. Environmental Hernando, M.D., Mezcua, M., Ferna risk assessment of pharmaceutical residues in wastewater effluents, surface waters and sediments. Talanta 69, 334. Ho, Y.S., McKay, G., 1999. Pseudo-second order model for sorption processes. Process Biochem. 34, 451e465. Weber Jr., J., Morris, J.C., 1963. Kinetics of Adsorption on Carbon from Solution. Jones, O.A., Lester, J.N., Voulvoulis, N., 2005. Pharmaceuticals: a threat to drinking water? Trends Biotechnol. 23, 163e167. Junkers, T., 2016. Photo-induced Copper-mediated Acrylate Polymerization in Continuous-flow Reactors. Junkers, T., 2017. Precision polymer design in microstructured flow reactors: improved control and first upscale at once. Macromol. Chem. Phys. 218, 1600421en/a. Junkers, T., Hoogenboom, R., 2016. Advanced polymer flow synthesis. Eur. Polym. J. 80, 175e176. Junkers, T., Wenn, B., 2016. Continuous photoflow synthesis of precision polymers. React. Chem. Eng. 1, 60e64. Kaur, S., Rani, S., Mahajan, R.K., Asif, M., Gupta, V.K., 2015. Synthesis and adsorption properties of mesoporous material for the removal of dye safranin: kinetics, equilibrium, and thermodynamics. J. Ind. Eng. Chem. 22, 19e27. Kermagoret, A., Wenn, B., Debuigne, A., Jerome, C., Junkers, T., Detrembleur, C., 2015. Improved photo-induced cobalt-mediated radical polymerization in continuous flow photoreactors. Polym. Chem. 6, 3847e3857. Li, L., Quinlivan, P.A., Knappe, D.R.U., 2002. Effects of activated carbon surface chemistry and pore structure on the adsorption of organic contaminants from aqueous solution. Carbon 40, 2085e2100. Madhavan, J., Grieser, F., Ashokkumar, M., 2010. Combined advanced oxidation processes for the synergistic degradation of ibuprofen in aqueous environments. J. Hazard Mater. 178, 202e208. ndez-Arriaga, F., Esplugas, S., Gime nez, J., 2008. Photocatalytic degradation of Me non-steroidal anti-inflammatory drugs with TiO2 and simulated solar irradiation. Water Res. 42, 585e594. Mestre, A.S., Pires, J., Nogueira, J.M.F., Carvalho, A.P., 2007. Activated carbons for the adsorption of ibuprofen. Carbon 45, 1979e1988. Mestre, A.S., Pires, J., Nogueira, J.M.F., Parra, J.B., Carvalho, A.P., Ania, C.O., 2009. Waste-derived activated carbons for removal of ibuprofen from solution: role of surface chemistry and pore structure. Bioresour. Technol. 100, 1720e1726. Schuster, E.M., Wipf, P., 2014. Photochemical flow reactions. Isr. J. Chem. 54, 361e370. Snyder, S.A., Adham, S., Redding, A.M., Cannon, F.S., DeCarolis, J., Oppenheimer, J., Wert, E.C., Yoon, Y., 2007. Role of membranes and activated carbon in the removal of endocrine disruptors and pharmaceuticals. Desalination 202, 156e181. Stackelberg, P.E., Furlong, E.T., Meyer, M.T., Zaugg, S.D., Henderson, A.K., Reissman, D.B., 2004. Persistence of pharmaceutical compounds and other organic wastewater contaminants in a conventional drinking-water-treatment plant. Sci. Total Environ. 329, 99e113. Suchithra, P.S., Carleer, R., Ananthakumar, S., Yperman, J., 2015. A hybridization approach to efficient TiO 2 photodegradation of aqueous benzalkonium chloride. J. Hazard Mater. 293, 122e130. Tijani, J.O., Fatoba, O.O., Madzivire, G., Petrik, L.F., 2014. A review of combined advanced oxidation technologies for the removal of organic pollutants from water. Water Air Soil Pollut. 225, 1e30. Wang, X., Tang, Y., Chen, Z., Lim, T.-T., 2012. Highly stable heterostructured Ag-AgBr/ TiO2 composite: a bifunctional visible-light active photocatalyst for destruction of ibuprofen and bacteria. J. Mater. Chem. 22, 23149e23158. Wenn, B., Junkers, T., 2016. Continuous microflow PhotoRAFT polymerization. Macromolecules 49, 6888e6895. Worch, E., 2012. Adsorption Technology in Water Treatment, first ed. De Gruyter, Dresden.
Contents lists available at ScienceDirect
Chemosphere journal homepage: www.elsevier.com/locate/chemosphere
Adsorption and photocatalytic removal of Ibuprofen by activated carbon impregnated with TiO2 by UVeVis monitoring Ying Gu a, *, Jan Yperman b, Robert Carleer b, Jan D'Haen c, d, Jens Maggen b, Sara Vanderheyden b, Kenny Vanreppelen b, Roberto Machado Garcia e a
Zhejiang Ocean University, Zhoushan 316022, China Research Group of Applied and Analytical Chemistry, Hasselt University, Diepenbeek, Belgium Institute for Materials Research (IMO), Hasselt University, Diepenbeek, Belgium d IMOMEC, IMEC vzw, Diepenbeek, Belgium e Chemistry Department, Faculty of Natural and Exact Sciences, Universidad de Oriente, Cuba b c
h i g h l i g h t s Three kinds of activated carbon impregnated with TiO2 (ACT) were prepared. Removal of Ibuprofen includes effects of adsorption and photodegradation by ACT. ACT composites effectively improve removal of Ibuprofen under UV. Kinetic tests of Ibuprofen removal were studied onto different composite catalysts.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 July 2018 Received in revised form 6 November 2018 Accepted 10 November 2018 Available online 13 November 2018
The removal of Ibuprofen was investigated by activated carbon impregnated with TiO2. Emphasis was given on the effect of different parameters, such as composite type, initial Ibuprofen concentration (5 e25 mg/L), temperature (22e28 C) and pH (acidic and alkaline solution). The experiment was carried out in a self-made tubular flow reactor, with one 15 W monochromatic UV lamp (254 nm). The composite AC90T10 gives the highest removal degree of 92% of Ibuprofen solution under UV light within 4 h, due to synergy of adsorption and photodegradation. It was found that weight ratio of composite/Ibuprofen has limited effect on the removal degree within the concentration range (5e25 mg/L), but reaction time under UV light (4 h) and pH (acidic solution) are very important. The kinetic experimental data obtained at pH 4.3 at 25 C on different composites were fitted to pseudo-first, pseudo-second and Elovich models, obtaining a high accuracy based on R2 values. From the results, composites of granular activated carbon and TiO2 can enhance removal of Ibuprofen effectively, making recycle process much easier and less costly, which can be a promising method in future water treatment. © 2018 Elsevier Ltd. All rights reserved.
Handling Editor: Klaus Kümmerer Keywords: Activated carbon TiO2 Ibuprofen Photodegradation Adsorption
1. Introduction Sea water gets increasingly polluted by the widespread discharge of wastewater from industries, by chemical products and by extensive medical and cosmetic material consumption of human beings. These pollutants include pharmaceuticals, pesticides, organic dyes, phenols, etc. Even though the concentration of each pollutant is low the damage towards the ecosystem is very high due
* Corresponding author. Zhejiang ocean university, Zhoushan 316022, China. E-mail address: [email protected] (Y. Gu). https://doi.org/10.1016/j.chemosphere.2018.11.068 0045-6535/© 2018 Elsevier Ltd. All rights reserved.
to their toxicity. It is indeed a threat for drinking water supplies, as many pollutants might have serious adverse impact on human beings (Stackelberg et al., 2004; Jones et al., 2005). Therefore, combined advanced oxidation technologies for the removal of pollutants from wastewater are widely studied (Challis et al., 2014; Tijani et al., 2014). Ibuprofen is one of the pharmaceuticals which has been extensively used. Ibuprofen is slightly soluble in water (0.021 mg/ ml at 20 C), but soluble in most organic solvents. It is a nonsteroidal anti-inflammatory drug, mostly used for treating pain, fever, and inflammation, including painful menstrual periods, migraines, and rheumatoid arthritis. As other pharmaceuticals,
Y. Gu et al. / Chemosphere 217 (2019) 724e731
Ibuprofen can leach into ground water and soil through direct and indirect use in daily life. Since conventional wastewater treatment plants commonly do not include pharmaceutical degradation, removal of pharmaceuticals is important (Hernando et al., 2006). Photodegradation is an effective method for pharmaceutical removal (Boreen et al., 2003; Fatta-Kassinos et al., 2011; Fu et al., 2015). Recent years photochemical degradation using nanosized TiO2 as photo-catalyst has been studied by many researchers ndez-Arriaga et al., 2008; Madhavan et al., 2010; Wang et al., (Me 2012; Georgaki et al., 2014). The efficiency of this approach is very good for two reasons: 1) nanosized TiO2 has a high photocatalytic activity and 2) it can be made under ambient conditions. The degradation rate is related to several parameters, such as the loading of composite, the irradiation power, the initial concentration, the pH of solution, and the surface area of composite. Activated carbon (AC) is also effective to adsorb Ibuprofen, due to its physical and chemical adsorption ability, which has also been studied (Mestre et al., 2007, 2009; Snyder et al., 2007; Dubey et al., 2010; Baccar et al., 2012; Behera et al., 2012; Guedidi et al., 2013). The advantages of AC as adsorbent is due to its large surface area, developed pore structures and surface functional groups (Li et al., 2002). The adsorption ability matches to different amounts of AC, temperature, concentration of the pollutant and contact time. However, hybrid materials of AC and TiO2 for Ibuprofen degradation have to our knowledge not been studied yet. Using composites of AC and TiO2 for other species like benzalkonium chloride degradation, it was concluded that with proper weight ratio of AC and TiO2 a more effective degradation was realized than pure AC or pure TiO2(Suchithra et al., 2015). Within the field of AC applications, mostly batch experiments are carried out. Stirring or shaking methods were used to increase the interaction and improve the contact between composite and solution. However, this approach is not ideal since too fast stirring might deteriorate the composites in batch reactor. Flow reactor is a promising technology emerging in a variety of research areas (Schuster and Wipf, 2014; Junkers and Hoogenboom, 2016). In flow chemistry, reaction conditions have been improved significantly compared to batch experiments (Conradi and Junkers, 2013; Schuster and Wipf, 2014; Kermagoret et al., 2015; Junkers, 2016, 2017; Junkers and Hoogenboom, 2016; Junkers and Wenn, 2016; Wenn and Junkers, 2016): a) it takes much shorter time to reach acceptable yields, b) reaction yield is greatly improved, and c) it is possible to adjust reactor volume and flow rate in flow setup, in order to get high yields, which is difficult to achieve in batch system. However, Ibuprofen degradation directly by UV irradiation and by TiO2 impregnated AC under UV irradiation using flow reactor has not been applied yet. In this study, photocatalytic degradation of Ibuprofen by AC impregnated with TiO2 (ACxTy) in a UV-flow reactor was investigated. A self-made tubular flow UV-reactor was used. Ibuprofen concentration was monitored by UVeVIS spectrometry, SEM-EDX was used for morphology characterization and elemental analysis. 2. Experimental 2.1. Materials 2-(4-isobutylphenyl)propanoic acid (98%), also named as Ibuprofen (IBP) was purchased in Ark Pharm, USA. Titanium isopropoxide, 98þ% (ACROS Organics, Belgium), glacial acetic acid (MERCK Germany) and ethanol 96% (VWR international, Belgium) were of analytical grade and used as received. Other chemical reagents used were: sulfuric acid, 95e97% (MERCK Germany), ascorbic acid, 99% (ACROS Organics, Belgium), ammonium molybdate (MERCK Germany) and potassium antimony
725
tartrate (MERCK Germany). Commercial AC Filtrasorb 400, a granular AC for the removal of dissolved organic compounds from water and wastewater, was used in this study. It is made from bituminous coal and available from Calgon Carbon Corporation. All chemical reagents were of analytical grade and Milli-Q water (18.2 MU cm1 conductivity) was used in all working solutions. 2.2. Synthesis of ACxTy composites In this study, sol-gel method was used to prepare ACxTy. 17 g Ti(O-i-Pr)4 was dissolved in 28 ml glacial acetic acid and was magnetically stirred for 30 min. Then, 287 ml ethanol was added drop wise to this solution under continuous stirring for another 30 min until the clear homogeneous sol started to show slight turbid nature. At this stage, a certain amount of AC (Filtrasorb 400) was added in accordance of the required ACxTy composite under vigorous stirring for 1 h. Afterwards, 45 ml Milli-Q water was added to this solution and stirring was continued for 5 h for complete hydrolysis. The final solution was evaporated at 70 C to form gradually a precursor gel. In the end this gel was calcined at 500 C with a heating rate of 4 C/min for 2 h in nitrogen atmosphere with a flow rate of 70 ml/min, to obtain porous gray powders (ACxTy). Three composite samples were prepared with a weight percentage of 90%, 50% and 10% AC respectively. For ACxTy, where x and y (¼100-x) stands for the weight percentage of AC and TiO2. The three types of prepared composites are labelled as AC90T10, AC50T50, and AC10T90. Pure AC (Filtrasorb 400) was used as reference and designated as AC100. 2.3. Adsorption and photo-catalysis experiments Every experiment was carried out in a self-made tubular flow UV-reactor, with one 15 W monochromatic UV lamp (254 nm), shown in Fig. 1. An amount of 1.6 g/L composite was dispersed in a quartz tube (id:4 mm, od:6 mm, length:5 cm) which was placed onto the UV-lamp, in order to have the shortest distance between composite and UV light. 20 ml of Ibuprofen solution within a concentration range of 5e25 mg/L was kept in a double walled beaker, and the reaction temperature was strictly controlled by a thermostatic water bath. The Ibuprofen solution was pumped at a 16.37 ml/ min flow rate through a looping circuit (an UV-transparent PFA (perfluoroalkoxy) tubing connected to the quartz tube), getting optimal contact with the composite in the quartz tube under UV light. Small samples of around 3 ml were taken after defined time periods for the determination of Ibuprofen concentration by UVeVIS spectrometry. Total analysis time takes only 10e20 s, including sampling, measurement, and recycling the sample to the looping solution. It was supposed that the effect of sampling was a negligible factor in the entire adsorption process. Before and after this procedure, the measuring quartz cell used in UVeVIS spectrometer was washed 3 times by HCl solution (3.6e3.7%), 3 times by de-ionized water, 3 times by Milli-Q water, and dried by pressured air, to prevent memory effects between analyses, same as the sampling tube. Each experiment was done at least two times. The repeated tests provide reliable measurements and error control. The tubular flow UV-reactor has several advantages. Firstly, it can maintain the shortest distance between composite and UV light, reaction yield has been improved. Secondly, reaction temperature can be precisely controlled by a water bath as temperature is key effect for adsorption and fast flow rate. Thirdly, reactor volume and flow rate can be adjusted in flow reactor set-up, according to the purpose of study. It can be noticed that results for pure TiO2 are missing in our
726
Y. Gu et al. / Chemosphere 217 (2019) 724e731
Fig. 1. Flow UV-reactor.
experimental set-up. Comparison between different composites is very common in batch experiments, since TiO2 was left in solution during stirring and is easy to maintain. In this flow reactor, it is very difficult to keep all TiO2 particles in the quartz tube located directly on the lamp. Pure TiO2 contains very fine particles. Quartz wool which was used to block ACxTy does not prevent TiO2 to pass this barrier. Once some TiO2 particles escape from the quartz tube flow reactor on the lamp, the effect of UV irradiation for the photocatalysis process was partly lost. Filter paper was tried instead of quartz wool, however the problem still remains. TiO2 can also pass the filter paper, additional an extra resistance on the flow rate of looping was observed, both having a significant effect on the photodegradation process of Ibuprofen in the flow reactor. 2.4. Analysis Ibuprofen removal was quantified by UVeVIS spectrometry, using a quartz cell of 1.0 cm path length and using the absorbance at 225 nm. Five standard concentrations ranging from 5, 10, 15, 20 and 25 mg/L were measured under the same experimental conditions. Ibuprofen concentration can be reliable measured as low as 5 mg/L. A calibration curve with a correlation coefficient of 0.99942, clearly shows the effectiveness of this method in quantifying Ibuprofen within a concentration range of 5e25 mg/L. LC/MS, GC/MS and HPLC-PDA were carried out to detect possible by-products, however, no reliable results were obtained because of too low concentration. A possible explanation is the combined effect by ACxTy adsorption and photodegradation of ACxTy combined with UVeVis, which leads to a much smaller amount of degradation products. Additionally those degradation products might also partly be adsorbed by ACxTy. Scanning Electron Microscopy (SEM) was used to analyse the morphology of the surface of different composites. Energydispersive X-ray spectroscopy (EDX) was used in combination with SEM to provide selected area elemental analysis. For better explanation of Ibuprofen removal, SBET and porous structure of ACxTy evaluation were considered. However, TiO2 plays an important role in removal degree with ACxTy. When the surface of AC was covered by TiO2, blocking the road for adsorption
by AC. Even if a porous structure is present, the effect is quite limited. Therefore it is believed that SBET data and porous structure have no direct relationship with Ibuprofen removal degree in the experiment with ACxTy. The experimental data also proved that. TGA was used to check the synthesis of ACxTy composites using the ash residues as a measure for the TiO2 content. Samples were heated up to 900 C in an air atmosphere (20 C/min). The removal degree was calculated as:
Removal degreeð%Þ ¼
C0 Ci 100% C0
where C0 (mg/L) is initial Ibuprofen concentration, Ci (mg/L) is the Ibuprofen concentration after i min. B-spline curve fitting was used in figures with experimental data, if there was no specific explanation.
3. Results and discussion 3.1. Characterization of composites The weight % of TiO2 based on the synthesis input parameters and the TGA results for the three prepared ACxTy materials were studied, detailed data can be found in Supplementary Information. TGA results are in line with the amounts used in the synthesis based on solution concentrations confirming that synthesis procedure works well. SEM and EDX were used to evaluate the synthesis of composites: the morphology of the surface and the elemental composition of selected area respectively. From Fig. 2, it can be seen that for ACxTy, TiO2 is dispersed on the surface of the AC. The white area is assigned to Ti, while the grey and black areas refer to carbon. In the case of AC10T90 the SEM images demonstrate that a significant TiO2 content at the surface is observed. From the elemental analysis of AC50T50 and AC90T10, it is also demonstrated that AC90T10 has an enhanced white area at the surface compared to AC50T50. Based on the TGA results, this is apparently not reflected in the mixture ratio. The reason for this must be found in the fact that TiO2 has not been homogeneous distributed on the surface for each particle of activated carbon
Y. Gu et al. / Chemosphere 217 (2019) 724e731
727
Fig. 2. Morphology of the surface of different composites (SEM).
which is confirmed by less magnified SEM images in contrast with SEM images with larger magnifications. EDX data can be found in Supplementary Information. For AC100, mainly carbon is measured with very limited amounts of Al, Si, O, Ca, and Fe. For the composite AC90T10, the EDX data reveal that in the white part still peaks of carbon are detected. Similarly, in the black area, peaks of Ti could also be seen, reflecting that Ti and carbon cannot be simply differentiated by visible color in SEM images. The mixing of Ti and C resulted in a rather irregular covering, some area were heavily covered with Ti, while others contained only a minor amount of Ti. For the other two composites AC50T50 and AC10T90, EDX results are also very similar. 3.2. Effect of AC and composites used in the adsorption experiments Three composites and one pure AC have been used: AC10T90, AC50T50, AC90T10 and AC100 respectively. The quartz tube placed on the UV lamp was filled with 1.6 g/L AC or composite. 20 ml Ibuprofen solution (25 mg/L) was looping through the reactor for 4 h at 25 C under UV irradiation at 254 nm. As can be seen in Fig. 3, the removal degree of Ibuprofen is depending on the conditions and type of adsorbents (AC and composite). Blank UV curve shows decomposition of Ibuprofen without using AC or composite under UV irradiation. Apparently, UV light has a direct strong effect on Ibuprofen degradation, especially within the first half hour, the removal degree increases very sharply, even faster than any other combination. The photolysis of ibuprofen under UVC irradiation was already discussed in previous papers (Castell et al., 2010; Eskandarian et al., 2016). Another reason for this can be found in the used set-up of the quartz tube, irradiating the solution directly by UV without any
Fig. 3. The effect of different composites on removal degree of Ibuprofen (1.6 g/L composite/AC, 25 mg/L Ibuprofen initial solution, pH ¼ 4.3, 25 C). Removal degree values are the average values of duplicates.
“obstruction” of AC or composites in the reaction tube. However, the removal degree of Ibuprofen flattens faster compared to the curves with AC or composite. Almost no change in the last hour can be noticed, quite different compared to experiments with AC or composites. Working without UV and pure AC100 (AC100 dark) a worse removal degree and removal amount of Ibuprofen are found compared to the blank UV experiment within the first two hours,
728
Y. Gu et al. / Chemosphere 217 (2019) 724e731
due to the photolytic mechanism. By using pure AC100 under UV irradiation (AC100 UV) the same tendency can be noticed within the first two hours. However, after two hours, the positive effect of UV becomes visible, higher removal degree (Fig. 3) and removal amounts of Ibuprofen compared to the blank UV experiment (62%) are observed: 82% versus 71% after 4 h. This can be explained by a combined adsorption and self-decomposition process, while for AC100 dark only an adsorption on the AC takes place, additionally leading to a faster decrease in the number of unoccupied adsorption sites. AC50T50 and AC90T10 have very good and comparable removal degrees and removal amounts of Ibuprofen, both reaching quite high values of 90% and 92% respectively after 4 h. A saturation point of adsorption and decomposition is almost reached. Currently, decomposition time and efficiency of UV irradiation cannot be correctly estimated, according to Fig. 3, it seems ibuprofen removal degree does not reach 100%. Nevertheless, the synergetic effect of adsorption and photocatalytic degradation by TiO2 works very well for both composites. In contrast, the removal degree and removal amount by AC10T90 are the lowest. The following reason can be formulated and is supported by SEM images: the surface of AC10T90 is largely covered with TiO2, which decreases the effective adsorption surface of the AC. In addition, AC10T90 has a relatively larger average particle size due to the extra TiO2 layer on the AC. For the same weight of composite addition, AC10T90 has fewer amounts of particles for each experiment, which might cause lower removal degree of Ibuprofen. When AC100 (with and without UV light) is compared with AC50T50 and AC90T10 no flattening of the curve is noticed nearby the 4 h point. A possible reason could be that the oxygen groups from TiO2, being present at the entrance and on the walls of the pores of the AC, decreased the total specific surface area and micropore volume. Additionally, there is an indication that these two composites show a higher removal tendency at the beginning of the Ibuprofen removal experiment, which decreases their activity as more adsorption sites are occupied. Perhaps also explaining why a 100% removal percentage cannot be reached. AC10T90 behaves clearly different. It was found in performing batch experiments that granular TiO2 has an effective ability for degradation of organic species due to its larger surface area. However, setting up an industrial application in using this very fine TiO2 powder will render high filtration costs. Hybridization of granular AC and TiO2 can make recycle process much easier and less costly. At the same time a good removal yield is the result of a synergetic effect of both adsorption and photodegradation. 3.3. Effect of different initial concentrations of Ibuprofen Fig. 4 shows the effect of different initial concentrations on removal degree and residual amount of Ibuprofen. Three initial concentrations were chosen: 5 mg/L, 15 mg/L and 25 mg/L respectively. Reaction temperature was strictly controlled at 25 C, 1.6 g/L AC90T10 was used as reference and as the only composite, pH of the Ibuprofen solution is 4.3. As can be seen from Figs. 4 and 5 mg/L Ibuprofen solution reaches the highest removal degree within the first 2 h, while 25 mg/L Ibuprofen solution has the lowest indicating that the highest weight ratio of composite/Ibuprofen ruled this effect. However, after 2 h, the removal degree of 5 mg/L Ibuprofen solution starts to decrease slightly and reached a maximum value of 96% after 4 h. This cannot be noticed for the 15 mg/L and 25 mg/L Ibuprofen concentrations, where a further increase of % removal can be observed. For the 15 mg/L solution a maximum of 95% removal degree is reached similar for the 5 mg/L solution, while for the 25 mg/L solution “only” 92% is reached after 4 h. So within this
Fig. 4. The effect of different initial concentrations on removal degree of Ibuprofen (1.6 g/L composite AC90T10, pH ¼ 4.3, 25 C). Removal degree values are the average values of duplicates.
Fig. 5. The effect of different reaction temperatures on removal degree of Ibuprofen (1.6 g/L AC90T10, 25 mg/L Ibuprofen initial solution, pH ¼ 4.3). Removal degree values are the average values of duplicates.
concentration range, the weight ratio composite/Ibuprofen has rather a limited effect on the kinetics of adsorption and degradation at the initial state and on the maximum amount of Ibuprofen removed.
3.4. Effect of temperature From Fig. 5, the effect of three different temperatures on the removal degree and amount of Ibuprofen is shown. Although a quite small temperature range from 22 C to 28 C is studied, clearly some minor differences in removal mechanism can be noticed. After 4 h, surprisingly, almost the same removal degree is obtained. According to the theory of AC adsorption, one should expect that the qm value increases with increasing temperature. However, in this concept of flow reactor under UV irradiation, the experimental data do not follow this rule, because the removal is a result of a combination of adsorption and photodegradation. At the moment, it is not clear why at the initial state such a different behavior is
Y. Gu et al. / Chemosphere 217 (2019) 724e731
noticed with increasing temperature. Additional and specific experiments need to be developed and performed to unravel the reason behind. 3.5. Effect of different pH The pH of reaction solution has a significant effect on the adsorption process of Ibuprofen, because the surface charge of adsorbent (AC90T10) has been changed. The pH of original Ibuprofen solution of 25 mg/L without any adjustment is 4.3. The pKa of Ibuprofen will determine which molecular species will be dominant present at a given pH. Both experiments were run with 25 mg/L Ibuprofen solution with 1.6 g/L AC90T10 at 25 C, with UV light of 254 nm. From Fig. 6, the acidic environment shows better removal performance of Ibuprofen, the maximum removal is reached more quickly than that in the more basic solution. However under basic conditions, the removal degree is still increasing even after 4 h. Similar to the temperature experiment, the combination of adsorption and photodegradation must be the reason for this time effect. In acidic solution, composites surfaces were charged positively, while composites and Ibuprofen are negatively charged in basic solutions. The initial better performance in acidic solution can be explained by electrostatic interaction involving electrostatic repulsion between the adsorptive anion and the negative charged AC surface with increasing pH (Mestre et al., 2007). As it was suggested by Dubey et al., rather a physisorption than a chemisorption process might be involved, enforcing the combined removal mechanism (Dubey et al., 2010). 3.6. Adsorption and photo-catalytic kinetics of ACxTy The rate of adsorption and photodegradation of ACxTy in Ibuprofen solutions are also evaluated. During the past 30 years numerous sorption systems have been studied. The adsorption and photocatalytic kinetic data of ACxTy were fitted by three different models (Weber and Morris, 1963; Chien and Clayton, 1980; Ho and McKay, 1999; Worch, 2012; Guedidi et al., 2013; Kaur et al., 2015), the pseudo-first-order model, the pseudo-second-order model, and the Elovich model respectively.
Fig. 6. The effect of different pH on removal degree of Ibuprofen (1.6 g/L AC90T10, 25 mg/L Ibuprofen initial solution, 25 C). Removal degree values are the average values of duplicates.
729
The pseudo-first-order equation is given by:
Qt ¼ Qe Qe ek1 t
(1)
where Qe (mg g1)is the capacity at equilibrium, Qt (mg g1) is the amount of Ibuprofen decreased at time t (h), k1 (h1) is pseudofirst-order model rate constant. The pseudo-second-order equation is given by:
Qt ¼
Q 2e k2 t 1 þ Qe k 2 t
(2)
where k2 (g mg1 h1) is pseudo-second-order model rate constant. The Elovich equation is given by:
Qt ¼
1
b
1 lnða bÞ þ lnðt t0 Þ
b
(3)
where a (mg g1 h1) is the initial rate, b (g mg1) is a constant related to external surface area coverage and activation energy of chemisorption. Data were fitted by the above models and results were listed in Table 1. Except for AC100 in dark, all other experiments were carried out in a combined system of adsorption and photodegradation. So the actual constant will include two parts: adsorption and photodegradation. It is currently not possible to determine the rate of adsorption and the rate of photodegradation separately. AC90T10 gives the highest removal degree of 92% of Ibuprofen solution under 254 nm UV light within 4 h. Except AC90T10, fitted profiles of adsorption and photocatalytic kinetics of other composites by three models, were shown in Supplementary Information. From R2 values, it can be seen that the kinetic data are best fitted by a pseudo-second order model, however the other two models (first and Elovich) fits also quite well with the experimental data based on the R2 values. Having a closer look to Fig. 7, for AC90T10 the first order model describes better with the found experimental trend. For the other composites, the second order model describes slightly better for AC100 dark, AC100 UV and AC50T50. However, it's difficult to explain the outcome of AC10T90 since it did not reach an equilibrium within the performed experimental conditions. The rate constants for first and second order model in Table 1 are both in agreement with Fig. 3: AC50T50 > AC90T10 > AC100 dark > AC100 UV [ AC10T90. Based on Fig. 3 the maximum amount of Ibuprofen is probably the lowest for AC100 dark comparing to AC100 UV as it tends to go faster to a saturation situation. Additionally, the AC100 UV curve crossed the removal tendency of AC100 dark. AC100 UV tends to a removal degree of the best performing ones (AC50T50 and AC90T10), which seem to reach their maximum value much faster. However the highest maximum Ibuprofen uptake is found for the slowest removal composite material AC10T90, although it is not that obvious from Fig. 3. But one can observe that no clear flattening in the curve can be noticed and that the photodegradation mechanism is less efficient and slower compared to the combined mechanism of adsorption and photodegradation for the other composites. On long term base this seems in favor of AC10T90 as higher amounts of TiO2 are present. For Elovich models, the initial rates of a for different composites are also in the same order when comparing the rate constants for first and second order model, and the data in Fig. 3. As initial rate a increases, the combined reaction of adsorption and photodegradation is faster. The other constant b is related to external surface area coverage and activation energy of chemisorption. As in Supplementary Information, b for AC10T90 is only 0.085, very low comparing to the other adsorbents having values in
730
Y. Gu et al. / Chemosphere 217 (2019) 724e731
Table 1 The pseudo-first-order, pseudo-second-order and Elovich model by non-linear regression analysis and experimental data.
AC100 UV AC90T10 UV AC50T50 UV AC10T90 UV AC100 dark
Experimental
Pseudo-first-order
Pseudo-second-order
Elovich equation
Qe (mg g1)
Qe (mg g1)
Qe (mg g1)
a (mg g1 h1/2)
12.862 14.375 14.085 8.618 11.198
12.982 14.223 13.458 15.46 10.910
17.041 16.676 15.446 26.89 13.832
14.496 56.148 78.091 3.293 17.957
Fig. 8. Plot of intraparticle diffusion model at pH ¼ 4.3, 25 mg/L Ibuprofen solution with 1.6 g/L AC90T10. Fig. 7. Fitted profiles of Ibuprofen adsorption and photocatalytic kinetics of AC90T10 by pseudo-first-order, pseudo-second-order, and Elovich models (1.6 g/L AC90T10, 25 mg/L Ibuprofen initial solution, pH ¼ 4.3, 25 C).
the range of 0.2e0.3 g/mg. It also confirms that in 4 h reaction, 90% Ti coverage is too high for adsorption and degradation, long term experiments are recommended for this composite. 3.7. Adsorption and photo-catalytic mechanism of ACxTy The Weber-Morris intraparticle diffusion model was tested to investigate the mechanism. The intraparticle diffusion model rather refers to possible different mechanistic steps within the adsorption and photodegradation process. Weber-Morris intraparticle diffusion model is given by:
qt ¼ kp i t 0:5 þ Ci
(4)
where kp i (mg g1 h1/2) is the rate parameter of stage i, which is the slope of the straight line of qt versus t 0:5 , also named as intraparticle diffusion rate constant. C i (mg g1) represents the thickness of the boundary layer at the intercept of stage i. As can be seen from Fig. 8, there are three stages in the intraparticle diffusion plots. For pH ¼ 4.3, in the first stage the line looks like going through the origin, a small negative value was found because it's experimental data loaded with an error. In the first stage, there is an instantaneous adsorption and photodegradation. The second stage is less sharp and gradual adsorption and degradation take place with rate limiting intraparticle diffusion. The last stage is mild, and can be considered as the final equilibrium stage. As residual concentration of Ibuprofen is lower and lower, the intraparticle diffusion also occurs slower. From Fig. 8, it was confirmed that initially the intraparticle diffusion rate was quite
fast, but slowed down with contact time. The constant became larger as time increased, confirming that the boundary layer effect was more significant. The linear lines of second and third stage did not pass through the origin point, which might be caused by difference in mass transfer rate. Possible reason is that there are other rate limiting mechanisms in this process. For solution of pH ¼ 8.6 it was also tested, the intraparticle diffusion model was not in good agreement with the experimental kinetics, a linear relation was only found in third stage, no overall good fitting could be found.
4. Conclusions Different mixture ratios of AC and titanium dioxide were prepared. Adsorption and photo-catalytic experiments with different initial concentrations of Ibuprofen, at different temperatures, and at different pH were researched. Additionally morphological profiles were used to understand the mechanism of Ibuprofen removal from wastewater using these composites. The following conclusions can be drawn: (1) Due to synergy of adsorption and photodegradation, it is found that the composite AC90T10 gives the highest removal degree of 92% of Ibuprofen after UV light irradiation at 254 nm within 4 h. (2) Weight ratio of composite/Ibuprofen has limited effect on the removal degree within the concentration range of 5 mg/L and 25 mg/L. (3) Reaction time under UV light is very important for Ibuprofen removal. (4) The sorption capacity of AC90T10 for Ibuprofen removal is higher in acidic solution than in alkaline conditions.
Y. Gu et al. / Chemosphere 217 (2019) 724e731
(5) The pseudo-second order model gives the best fit for the kinetic data of AC90T10, however only minor differences are found with the other kinetic models. The combined adsorption and photodegradation effect could be responsible for that. Acknowledgements This work was supported by Hasselt University, Belgium(R8109), and Zhejiang Ocean University, China (21045012615). The Chinese author is grateful to Jenny Put for sample analysis, and Yvo Feytong for assistance during experiments. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.chemosphere.2018.11.068. References , M., Bouzid, J., Feki, M., Bla nquez, P., 2012. Removal of pharmaBaccar, R., Sarra ceutical compounds by activated carbon prepared from agricultural by-product. Chem. Eng. J. 211, 310e317. Behera, S.K., Oh, S.Y., Park, H.S., 2012. Sorptive removal of ibuprofen from water using selected soil minerals and activated carbon. Int. J. Environ. Sci. Technol. 9, 85e94. Boreen, A.L., Arnold, W.A., Mcneill, K., 2003. Photodegradation of pharmaceuticals in the aquatic environment: a review. Aquat. Sci. 65, 320e341. Castell, J.V., Gomez, M.J., Miranda, M.A., Morera, I.M., 2010. Photolytic degradation of ibuprofen. Toxicity of the isolated photoproducts on fibroblasts and erythrocytes. Photochem. Photobiol. 46, 991e996. Challis, J.K., Hanson, M.L., Friesen, K.J., Wong, C.S., 2014. A critical assessment of the photodegradation of pharmaceuticals in aquatic environments: defining our current understanding and identifying knowledge gaps. Environ. Sci. Process. Impacts 16, 672e696. Chien, S.H., Clayton, W.R., 1980. Application of Elovich equation to the kinetics of phosphate release and sorption in soils. Soil Sci. Soc. Am. J. 44, 265e268. Conradi, M., Junkers, T., 2013. Efficient [2þ2] photocycloadditions under equimolar conditions by employing a continuous UV-flow reactor. J. Photochem. Photobiol. Chem. 259, 41e46. €, M., Gopal, K., 2010. Artemisia vulgaris-derived Dubey, S.P., Dwivedi, A.D., Sillanp€ aa mesoporous honeycomb-shaped activated carbon for ibuprofen adsorption. Chem. Eng. J. 165, 537e544. Eskandarian, M.R., Choi, H., Fazli, M., Rasoulifard, M.H., 2016. Effect of UV-LED wavelengths on direct photolytic and TiO 2 photocatalytic degradation of emerging contaminants in water. Chem. Eng. J. 300, 414e422. Fatta-Kassinos, D., Vasquez, M.I., Kümmerer, K., 2011. Transformation products of pharmaceuticals in surface waters and wastewater formed during photolysis and advanced oxidation processes e degradation, elucidation of byproducts and assessment of their biological potency. Chemosphere 85, 693e709. Fu, H.L., Yao, K., Wen, Y.L., Guo, G.L., Ping, C., Hao, P.H., Kang, Y.P., 2015. Photodegradation of ibuprofen under UVeVis irradiation: mechanism and toxicity of photolysis products. Bull. Environ. Contam. Toxicol. 94, 479e483. Georgaki, I., Vasilaki, E., Katsarakis, N., 2014. A study on the degradation of carbamazepine and ibuprofen by TiO2&ZnO photocatalysis upon UV/Visible-Light irradiation. Am. J. Anal. Chem. 518e534. v e ^que, J.-M., Soneda, Y., Bellakhal, N., Duclaux, L., 2013. Guedidi, H., Reinert, L., Le
731
The effects of the surface oxidation of activated carbon, the solution pH and the temperature on adsorption of ibuprofen. Carbon 54, 432e443. ndez-Alba, A.R., Barcelo , D., 2006. Environmental Hernando, M.D., Mezcua, M., Ferna risk assessment of pharmaceutical residues in wastewater effluents, surface waters and sediments. Talanta 69, 334. Ho, Y.S., McKay, G., 1999. Pseudo-second order model for sorption processes. Process Biochem. 34, 451e465. Weber Jr., J., Morris, J.C., 1963. Kinetics of Adsorption on Carbon from Solution. Jones, O.A., Lester, J.N., Voulvoulis, N., 2005. Pharmaceuticals: a threat to drinking water? Trends Biotechnol. 23, 163e167. Junkers, T., 2016. Photo-induced Copper-mediated Acrylate Polymerization in Continuous-flow Reactors. Junkers, T., 2017. Precision polymer design in microstructured flow reactors: improved control and first upscale at once. Macromol. Chem. Phys. 218, 1600421en/a. Junkers, T., Hoogenboom, R., 2016. Advanced polymer flow synthesis. Eur. Polym. J. 80, 175e176. Junkers, T., Wenn, B., 2016. Continuous photoflow synthesis of precision polymers. React. Chem. Eng. 1, 60e64. Kaur, S., Rani, S., Mahajan, R.K., Asif, M., Gupta, V.K., 2015. Synthesis and adsorption properties of mesoporous material for the removal of dye safranin: kinetics, equilibrium, and thermodynamics. J. Ind. Eng. Chem. 22, 19e27. Kermagoret, A., Wenn, B., Debuigne, A., Jerome, C., Junkers, T., Detrembleur, C., 2015. Improved photo-induced cobalt-mediated radical polymerization in continuous flow photoreactors. Polym. Chem. 6, 3847e3857. Li, L., Quinlivan, P.A., Knappe, D.R.U., 2002. Effects of activated carbon surface chemistry and pore structure on the adsorption of organic contaminants from aqueous solution. Carbon 40, 2085e2100. Madhavan, J., Grieser, F., Ashokkumar, M., 2010. Combined advanced oxidation processes for the synergistic degradation of ibuprofen in aqueous environments. J. Hazard Mater. 178, 202e208. ndez-Arriaga, F., Esplugas, S., Gime nez, J., 2008. Photocatalytic degradation of Me non-steroidal anti-inflammatory drugs with TiO2 and simulated solar irradiation. Water Res. 42, 585e594. Mestre, A.S., Pires, J., Nogueira, J.M.F., Carvalho, A.P., 2007. Activated carbons for the adsorption of ibuprofen. Carbon 45, 1979e1988. Mestre, A.S., Pires, J., Nogueira, J.M.F., Parra, J.B., Carvalho, A.P., Ania, C.O., 2009. Waste-derived activated carbons for removal of ibuprofen from solution: role of surface chemistry and pore structure. Bioresour. Technol. 100, 1720e1726. Schuster, E.M., Wipf, P., 2014. Photochemical flow reactions. Isr. J. Chem. 54, 361e370. Snyder, S.A., Adham, S., Redding, A.M., Cannon, F.S., DeCarolis, J., Oppenheimer, J., Wert, E.C., Yoon, Y., 2007. Role of membranes and activated carbon in the removal of endocrine disruptors and pharmaceuticals. Desalination 202, 156e181. Stackelberg, P.E., Furlong, E.T., Meyer, M.T., Zaugg, S.D., Henderson, A.K., Reissman, D.B., 2004. Persistence of pharmaceutical compounds and other organic wastewater contaminants in a conventional drinking-water-treatment plant. Sci. Total Environ. 329, 99e113. Suchithra, P.S., Carleer, R., Ananthakumar, S., Yperman, J., 2015. A hybridization approach to efficient TiO 2 photodegradation of aqueous benzalkonium chloride. J. Hazard Mater. 293, 122e130. Tijani, J.O., Fatoba, O.O., Madzivire, G., Petrik, L.F., 2014. A review of combined advanced oxidation technologies for the removal of organic pollutants from water. Water Air Soil Pollut. 225, 1e30. Wang, X., Tang, Y., Chen, Z., Lim, T.-T., 2012. Highly stable heterostructured Ag-AgBr/ TiO2 composite: a bifunctional visible-light active photocatalyst for destruction of ibuprofen and bacteria. J. Mater. Chem. 22, 23149e23158. Wenn, B., Junkers, T., 2016. Continuous microflow PhotoRAFT polymerization. Macromolecules 49, 6888e6895. Worch, E., 2012. Adsorption Technology in Water Treatment, first ed. De Gruyter, Dresden.