The degradation of carbamazepine (CBZ) and ibuprofen (IBP) in aqueous matrices was investigated by TiO<sub>2</sub> and ZnO photocatalysis initiated by UV-A and visible-light irradiation. Emphasis was given on the effect of operating parameters on the degradation effectiveness, such as catalyst type and loading (50 - 500 mg/L), initial drug concentration (10, 40, 80 mg/L) and wavelength of irradiation (200 - 600 nm). In an effort to understand the photocatalytic pathway for CBZ and IBP removal in terms of primary oxidants, the contribution of HO· was evaluated. With this scope, the radical-mediated process was suppressed by addition of an alcohol scavenger, isopropanol, (i-PrOH), described as the best free hydroxyl radical quencher. The photodegradation rate of the pharmaceuticals was monitored by high performance liquid chromatography (HPLC). According to the results, visible-light exposure, at λ<sub>exc</sub> > 390 nm, takes place as a pure photocatalytic degradation reaction for both compounds. IBP was found to have overall high conversion rates, compared to CBZ. IBP oxidized fast under photocatalytic conditions, regardless the adverse effect of the increase of initial drug concentration, or low catalyst load, irradiation upon visible-light, by either titania or zinc oxide. Finally, addition of isopropanol showed a significant inhibition effect on the CBZ degradation, taken as an evidence of a solution-phase mechanism. In the case though of IBP degradation, the hole mechanism may be prevailing, suggested by the negligible effect upon addition of isopropanol indicating a direct electron transfer between holes (h<sup>+</sup>) and surface-bound IBP molecules. A plausible mechanism of IBP and CBZ photocatalysis was proposed and described.
None can deny how significant is the continuous development and research on the area of synthesis and production of a variety of drugs of pharmaceutical importance for both mankind and animals. However, within the last few years, both the occurrence and fate of pharmaceutical residues and their metabolites in environmental matrices, have attracted scientific interest. These compounds are classified as emerging pollutants, while their main pathway into the environment is pharmaceutical industries, excretory products of medically treated humans and animals followed by their inefficient removal in wastewater treatment plants [
Non-steroidal anti-inflammatory drugs (NSAIDs) are some of the most frequently detected groups of pharmaceuticals in environmental samples, one of the most widely available drugs in the world. The main common characteristic in the NSAID group is the carboxylic aryl acid moiety that provides their acidic properties. Ibuprofen (IBP) belongs to this family of medicines, which is an analgetic drug mainly used for the treatment of rheumatoid arthritis, myoskeletal injuries and fever. Its presence in effluents of wastewater treatment plants in Greece has been reported: 0.05 μg/L were quantified in the effluent of the plant of Heraklion [
Carbamazepine (CBZ) is a neutral anticonvulsant pharmaceutical, used primarily in cases of epilepsy and bipolar disorder. It is also used as drug of first choice in situations of trigeminal neuralgia and in the treatment of bipolar disorder [
Even though their concentration in the environment is low (ng/L to μg/L), harmful effects may arise from their continuous input, their synergistic toxicity and additive effects because of their presence as mixtures [
TiO2 together with ZnO are two semiconductors broadly used in heterogeneous photocatalysis to degrade a broad range of pollutants due to their wide band gap, their spectral overlap with sunlight emission (about 5%), biological and chemical stability, low toxicity and reduced cost [
Mendez-Arriaga et al. [
AOPs are based on the oxidation of the target pollutant by reactive species. Upon irradiation, the first step in the heterogeneous TiO2 photocatalysis is the production of electrons
It is therefore accepted that in heterogeneous photocatalysis two oxidative agents can be considered: the photo-produced holes h+ (mainly involved in the de-carboxylation reaction (“photo-kolbe”) and/or the HO• radicals, free or surface-bound, which are known as strongly active and degrading but non-selective agents. Previous research [
The effect of alcohols [
The aim of this work was to study the degradation of two pharmaceuticals, CBZ and IBP, in an aqueous matrix by TiO2 (P-25) and ZnO photocatalysis, initiated by UV-A and, most importantly, visible-light irradiation. Emphasis was given on the effect of operating parameters on the degradation effectiveness, such as catalyst type and loading, initial drug concentration and wavelength of irradiation. Furthermore, the contribution of HO• to the photooxidation mechanism for the pharmaceutical removal was evaluated. To achieve this goal, we suppressed the HO• radical-mediated process by addition of an alcohol scavenger, isopropanol [i-PrOH]. Isopropanol has been described as the best hydroxyl radical quencher due to its high-rate constant reaction with the radical (1.9 × 109 L∙mol−1∙s−1) [
IBP and CBZ, both of 99% purity, were purchased from Sigma Aldrich and used as received. Their molecular structures are presented in Scheme 1, while their main properties are shown in
Aqueous solutions of each pharmaceutical were prepared by addition of the appropriate amount of the drug (10, 40 and 80 mg/L) to 500 mL deionised water. For detection and quantification purposes, the range of concentrations in this study is higher than those typically detected in the environment. In the experiments conducted for the evaluation of the contribution of hydroxyl radicals to the photocatalytic degradation, isopropanol, which is a well known hydroxyl radical scavenger, was added to the solution. An amount of isopropanol (1.62 mL for CBZ and 1.85 mL for IBP) was added at the beginning of the photocatalytic reaction, at a molar concentration 103 times higher than the initial concentration of the pharmaceuticals.
For the photocatalytic experiments, a metal oxide semiconductor catalyst, namely TiO2 or ZnO, was added to the solution and maintained in suspension by magnetic stirring. A series of experiments were run varying catalyst concentration from 50 to 500 mg/L for each pharmaceutical. Each time, the suspension was first stirred in the dark for ca. 40 min, to ensure establishment of adsorption/desorption equilibrium. For the initiation of the photocatalytic experiment, light was allowed to irradiate the reactor. In the case of photolysis, no catalyst was added to the solution. The progress of the photochemical and photocatalytic drug removal as a function of time was monitored periodically by withdrawing aliquot from solution/suspension with a help of a pipette. With-
Scheme 1. Chemical structures of (a) IBP and (b) CBZ.
(a) (b)
Scheme 1. Chemical structures of (a) IBP and (b) CBZ.
. Main properties of the pharmaceuticals used
Property | Ibuprofen | Carbamazepine |
---|---|---|
Therapeutic group | NSAIDs | Antiepileptic |
Molecular formula | C13H18O2 | C15H12N2O |
Molecular weight | 206.3 | 236.3 |
Solubility in water (mg/mL) | 0.041 (25˚C) | 0.17 (25˚C) |
pKa | 4.9 | 7 |
Type | Anionic | Cationic |
. Main properties of the applied catalysts (manufacturer data)
Property | Titania-P25 | Zinc oxide |
---|---|---|
Energy gap (eV) | 3.20 | 3.37 |
BET surface (m2/g) | 55 ± 15 | 20 ± 5 |
Mean particle size (nm) | 21 | 100 |
pHpzc | 6.8 | 9 ± 0.3 |
drawn samples as required were filtered with 0.22 mm filters to remove catalyst particles prior to analysis. The first sample was taken at the end of the dark adsorption period, just before light irradiation, in order to determine the concentration of the compound in solution, which was hereafter considered as the initial concentration [C0].
The photocatalytic degradation of the two pharmaceuticals was carried out in a specially designed photocatalytic reactor provided by Heraeus (Noblelight GmbH, Hanau-Germany), equipped with a light source (Scheme 2). The borosilicate glass reactor of diameter 1 - 1.5 cm and 500 mL capacity were made with ports for sampling and gas/air purge. The irradiation was provided by a medium pressure mercury lamp (TQ 150), keeping a constant power at 150 W, with an emission spectrum of 200 - 600 nm, and λmax at 365 nm. The lamp was mounted axially in the reactor inside a cylindrical, double walled lamp jacket. The UV-A experiments were run using a lamp jacket made of quartz, while the visible-light experiments were conducted with a M380 glass jacket, which filtered out the UV lines at λexc < 390 nm, limiting the irradiation near to the visible-light spectrum. Light intensity in the reactor was 58 - 60 mW/cm2 in the UV experiments, while for the visible-light transmission runs it was ~31 mW/cm2. Photocatalytic experiments were carried out at a constant stirring speed (600 rpm) insured by a magnetic stirrer at the reactor basis, at a constant temperature maintained by water circulating in the double walled lamp jacket.
The photodegradation of the pharmaceuticals was followed by HPLC, where the filtered transparent solution samples were analysed for the detection of IBP & CBZ compounds in solution, using a Hewlett Packard 1100 system, equipped with a G1315A diode array detector (DAD). The analytical column was a Hypersil BDS C8 (250 mm × 4 mm × 5 μm) from Thermo Electron, thermostated at 35˚C. Analytes were separated by gradient elution with ACN (A) and a 25 mM potassium dihydrogen phosphate solution (B) at a flow-rate of 1.2 ml/min. The gradient elution was as follows: 0 min, 15% A; 5 min, 15% A; 15 min, 70% A; 18 min, 15% A. The DAD signal was set at 288 nm for CBZ and 225 nm for IBP and peak areas were used for the quantification of each pharmaceutical. According to a review work by Munoz et al. [
Scheme 2. Photoreactor set up by Heraeus Noblelight GmbH, Hanau-Germany.
The photochemical and photocatalytic degradation of the two tested drugs, CBZ and IBP in aqueous matrices, was investigated using UV-A and visible-light irradiation sources. The type and catalyst load, together with the initial drug concentration effect were assessed as fundamental operational parameters in heterogeneous photocatalysis.
For CBZ aquatic solution, the pH was 6.0, less than the point of zero charge of the TiO2 (6.8 for P-25) and ZnO (9 ± 0.3), leaving the surface of the catalysts slightly electropositive. In the case though of aquatic IBP solution, pH recorded to be 4, meaning that the charge-character of the catalyst surface in this solution was strongly electropositive.
Control experiments under otherwise identical conditions showed that no degradation was observed when the experiments were conducted in the dark or in the absence of the semiconductor. Therefore, both UV light and catalyst were indispensable for the pharmaceuticals degradation. From the dark period measurements, the adsorption extent of approximately 10% was observed for CBZ, same as in the case of IBP, indicating that a fraction of nearly 10% of each drug was adsorbed on either TiO2 or ZnO surface because of the electrostatic attractions. Beginning with the assumption that the Langmuir model is strictly followed, that is the adsorption-de- sorption process approaches the equilibrium, the surface of the catalyst is homogeneous, the different active adsorption sites on the surface are equivalent, while a single layer of drug molecule is formed onto the surface, the kinetic would be accounted as a pseudo-first order model [
where, C is the drug concentration, k is the rate constant, and t is the reaction time.
By integrating the equality, the following equations were obtained:
where, Ct is the drug concentration at time t, and C0 is the initial drug concentration.
The logarithmic plots of the normalized drug concentration with time gave a straight line. The regression coefficient of the linear fitting, R2 was greater than 0.97 in all cases.
Drug photocatalytic efficiency, indicated by the decrease of initial concentration, in each case was assessed by HPLC measurements at the indicated irradiation times.
Heterogeneous photocatalysis depends on the initial concentration of the organic substrate [
In this work, in order to test the effect of the initial drug load on the photocatalytic rate, two different concentrations of CBZ (10 and 80 mg/L) and IBP (10 and 40 mg/L) were tested, irradiated under UV light in the presence of 100 mg/L catalyst. As pharmaceuticals have only been traced in environmental samples within a concentration range of μg/L, or even less in the case of ibuprofen, assessment of drug photodegradation at higher loadings is impractical and of no actual interest.
As seen in
In the case of IBP, the initial 10 mg/L load resulted in an overall fast-rate catalysis, i.e., in ca. 10 min of UV irradiation in the presence of 100 mg/L of TiO2, almost all drug was removed, with similar findings for ZnO assisted catalysis (
A series of experiments by many different works [
In this work, the concentration of TiO2 (P-25) and ZnO in the suspension was varied between 50 and 500 mg/L to test the catalyst load effect on the degradation of each pharmaceutical, keeping its initial concentration each time constant (10 mg/L). Figures 3-6 present the [CBZ]0 and [IBP]0 removal-time profiles of each drug with each catalyst at different loadings. In
As shown in Figures 3-6 for the same drug solute concentration (that of 10 ppm of either CBZ or IBP), increasing in each case the amount of catalyst in suspension from 50 to 500 mg/L, of both P-25 & ZnO, the rate of
Effect of the [CBZ]0 on the photocatalytic degradation, comparing two different initial concentrations (10 & 80 mg/L) of the drug, under UV irradiation, [TiO2] = [ZnO] = 100 mg/L
Effect of the [IBP]0 on the photocatalytic degradation, comparing two different initial concentrations (10 & 40 mg/L) of the drug, under UV irradiation, [TiO2] = [ZnO] = 100 mg/L
Effect of P-25 TiO2 loading on CBZ degradation upon UV irradiation: % removal-time profiles at [CBZ]0 = 10 mg/L and various catalyst loadings
. Apparent first-order rate constants for aqueous CBZ degradation upon UV-A irradiation at different initial drug concentrations [TiO2-P25] = [ZnO] = 100 mg/L
[CBZ] (mg/L) | k1 (min−1) × 10−3 (TiO2) | k2 (min−1) × 10−3 (ZnO) |
---|---|---|
10 | 97 | 81 |
80 | 14 | 11 |
. Apparent first-order rate constants for aqueous IBP degradation upon UV-A irradiation at different initial drug concentrations [TiO2-P25] = [ZnO] = 100 mg/L
[IBP] (mg/L) | k1 (min−1) × 10−3 (TiO2) | k2 (min−1) × 10−3 (ZnO) |
---|---|---|
10 | 382 | 326 |
40 | 139 | 122 |
the photocatalytic process increases, indicating the importance of available catalyst surface (higher number of active sites) for adsorption-degradation on the surface of the particle, upon UV illumination. However, the rate from 250 to 500 mg/L gradually slows down, pointing that the optimal adsorption of efficient photons has been nearly reached. Higher amount of the catalyst may not be useful both in view of possible aggregation, as well as reduced irradiation field. Above a limit value, the increase in turbidity of the solution reduces the light transmission through the solution. In addition to this, at high solid concentration, there is a loss in surface area available for light-harvesting for the generation of h+/e− pairs, occasioned by agglomeration (particle-particle interactions). Finally, part of the originally activated TiO2 may also be deactivated through collision [
Effect of ZnO loading on CBZ degradation upon UV irradiation: % removal-time profiles at [CBZ]0 = 10 mg/L and various catalyst loadings
Effect of P-25 TiO2 loading on IBP degradation upon UV irradiation: % removal-time profiles at [IBP]0 = 10 mg/L and various catalyst loadings
Effect of ZnO loading on IBP degradation upon UV irradiation: % removal-time profiles at [IBP]0 = 10 mg/L and various catalyst loadings
In the case of IBP photodegradation, the important observation is that the rates for both catalysts are significantly higher for all different catalyst loadings, with a complete conversion of the chemical to take place almost at half time than in the case of CBZ, i.e., within 15 min nearly all of the initial IBP concentration was already removed from solution under UV-A irradiation.
In
. Apparent first-order rate constants for aqueous CBZ degradation upon UV-A irradiation under different type and catalyst loadings ([CBZ]0 = 10 mg/L)
[TiO2] (mg/L) | k1 × 10−3 | [ZnO] (mg/L) | k2 × 10−3 |
---|---|---|---|
zero (photolysis) | 31 | zero (photolysis) | 31 |
50 | 78 | 50 | 75 |
100 | 97 | 100 | 81 |
250 | 133 | 250 | 94 |
500 | 155 | 500 | 113 |
. Apparent first-order rate constants for aqueous IBP degradation upon UV-A irradiation under different type and catalyst loadings ([IBP]0 =10 mg/L)
[TiO2] (mg/L) | k1 × 10−3 | [ZnO] (mg/L) | k2 × 10−3 |
---|---|---|---|
zero (photolysis) | 140 | zero (photolysis) | 140 |
50 | 175 | 50 | 181 |
100 | 382 | 100 | 326 |
250 | 390 | 250 | 366 |
500 | 422 | 500 | 390 |
Effect of TiO2-P25 compared with ZnO on CBZ degradation % removal-time profiles at [CBZ]0 = 10 mg/L and catalyst loading of 50 mg/L
Effect of TiO2-P25 compared with ZnO on IBP degradation % removal-time profiles at [IBP]0 = 10 mg/L and catalyst loading of 50 mg/L
study, at the same load, for the degradation of the same initial amount of the tested drugs. Even though a similar trend was overall observed for both P-25 and ZnO loadings,
However, after the high initial photocatalytic rate for TiO2, a steady state follows, with both catalysts reaching after 30 min the same level of CBZ photodegradation (that of 90%).
In the case of IBP photodegradation, as seen in
The effect of the irradiation wavelength on the rate of degradation of CBZ and IBP pharmaceuticals was studied, using UV-A and visible-light irradiation.
Having that UV-A assisted photodegradation (photolysis) of CBZ and IBP is significant, especially for the reactive IBP (k of 0.140 min−1), it may be assumed that conversion of the chemicals cannot work in a pure photocatalytic regime, meaning that the use of any catalyst works in addition to photolysis, so as to improve the rate
Comparison of CBZ degradation upon irradiation with UV versus Vis light, in the presence and absence of photocatalyst, [CBZ]0 = 10 mg/L; [TiO2-P25] = [ZnO] = 100 mg/L
. Apparent first-order rate constant for aqueous CBZ degradation upon different irradiation source (UV-A, Vis.) [CBZ]0 = 10 mg/L; [TiO2-P25] = [ZnO] = 100 mg/L
Lamp | Catalyst | k (min−1) × 10−3 |
---|---|---|
UV-A | none | 31 |
TiO2 | 97 | |
ZnO | 81 | |
Vis | none | 0 |
TiO2 | 57 | |
ZnO | 49 |
. Apparent first-order rate constant for aqueous IBP degradation upon different irradiation source (UV-A, Vis.) [IBP]0 = 10 mg/L; [TiO2-P25] = [ZnO] = 100 mg/L
Lamp | Catalyst | k (min−1) × 10−3 |
---|---|---|
UV-A | none | 140 |
TiO2 | 382 | |
ZnO | 326 | |
Vis | none | 0.73 |
TiO2 | 199 | |
ZnO | 144 |
Comparison of IBP degradation upon irradiation with UV versus Vis light, in the presence and absence of photocatalyst, [IBP]0 = 10 mg/L; [TiO2-P25] = [ZnO] = 100 mg/L
and the extent of the overall process. However, under visible-light exposure, where photolysis is negligible, the use of catalysts is most crucial for drug degradation.
In the photocatalytic degradation, one of the main uncertainty is whether oxidation proceeds via direct electron transfer between substrate and positive holes, or via an HO• radical-mediated pathway. As direct oxidation of short aliphatic alcohols by photogenerated holes may be considered negligible, having a very weak adsorption power on TiO2 surface in aqueous media, alcohols are usually used as a diagnostic tools of HO• radicals mediated mechanism [
The addition of isopropanol to the solution containing CBZ in the presence of TiO2, irradiated with simulated UV light, modifies the reaction course. Due to its low affinity to the TiO2 surface, isopropanol was expected to compete mainly for HO• radicals [
In the case where the reaction is catalysed by ZnO, the effect of HO• scavenger is also pronounced (
In the current work, after addition of the HO• scavenger (i-PrOH) the rate constant k, for the photodegradation of [CBZ]0 of 10 mg/L catalysed by TiO2, dropped from 0.097 to 0.016 min−1, while from 0.081 to 0.022 min−1 in the case of ZnO. Degradation kinetics are well described by a pseudo-first-order model (R2 = 0.98) (see
However, in the case of IBP degradation,
Comparing the above results observed for IBP versus CBZ degradation process, it could be suggested in a word that the degradation of IBP seems to be a result of a hole-dominated surface reaction, while in the case of CBZ the initial process is shifted to a homogeneous radical reaction in the bulk solution. In the following section a possible drug degradation mechanism based on this suggestion is described.
The hydroxyl radicals, strongly active and degrading, react very rapidly with aromatic ring compounds [
Effect of i-PrOH on degradation rates of CBZ in aqueous TiO2 and ZnO suspensions, [CBZ]0 = 10 mg/L; [TiO2] = [ZnO] = 100 mg/L
. Apparent first-order rate constant for aqueous CBZ and IBP degradation upon UV-A irradiation, under addition of isopropanol [CBZ]0 = [IBP]0 = 10 mg/L; [TiO2-P25] = [ZnO] = 100 mg/L
Aqueous matrix | k1 (min−1) × 10−3 (for TiO2) | k2 (min−1) × 10−3 (for ZnO) |
---|---|---|
CBZ without addition | 97 | 81 |
CBZ + i-PrOH | 16 | 22 |
IBP without addition | 382 | 326 |
IBP + i-PrOH | 160 | 215 |
Effect of i-PrOH on degradation rates of IBP in aqueous TiO2 and ZnO suspensions, [IBP]0 = 10 mg/L; [TiO2] = [ZnO] = 100 mg/L
UV irradiation. IBP having a more open chemical structure with only one aromatic ring and carboxyl in its structure, may be oxidised preferentially by the photogenerated holes, mainly involved in the decarboxylation reaction (“photo-kolbe”), rather than by the non selective HO• radicals. Arriaga et al. [
According to the overall experimental findings in this study and a general literature review on photocatalytic processes, a proposed mechanism of IBP-TiO2 catalysis may be described as follows: The anionic IBP molecules under acidic conditions (pH of solution ca. 4) are first adsorbed on the cationic TiO2 surface (the IBP molecule is linked to the Ti surface metallic cation through one oxygen atom of carboxyl group) where the degradation reaction is mostly initiated by the direct electron transfer reaction between a positive hole and a surface- bound IBP molecule. The Ti-O bond has a relatively high covalent character, and the oxygen atoms of IBP, being relatively strong electron donors, are able to direct interact with valence band photogenerated holes. As the valence band hole migrates to the surface, it is primarily captured by the adsorbed IBP molecules, rather than by the adsorbed water or hydroxyl groups, testified by the fast initial drug removal. Of course, the photo-produced HO• ads/free could not be excluded, attacking as well the drug molecule, but their role in the degradation process would not be the major one, and certainly not the one to initiate the oxidation reaction, as the process proceeds with a fast rate in their absence too (see
It must though be clear that the deduction to this theory is valid for an aqueous matrix, given the specific experimental conditions, where there is not: 1) any catalyst surface modification in process (either by the presence of a hole scavenger, such as iodide, and/or by the presence of surface site inhibitors, such as F− or
The mechanism proposed in this paper unifies literature findings for the photodegradation of organic pollutants by titania semiconductor photocatalysis [
The degradation of two typical pharmaceuticals, Carbamazepine (CBZ) and Ibuprofen (IBP), was studied by means of the two most common type commercial photocatalysts, TiO2 (P-25) and ZnO using both UV-A and visible-light irradiation. Operational parameters, such as type and catalyst loading and initial drug concentration were complementary assessed. To complete this study, the oxidative role of the photocatalytically generated hydroxyl radicals in the bulk solution was investigated by addition of isopropanol scavenger. The main remarks are summarized as follows:
• In the case of irradiation under visible-light, the contribution of the photochemical degradation for both pharmaceuticals tested is negligible and hence it can be assumed that catalysis in visible-light exposure takes place as a pure photocatalytic degradation reaction (photocatalytic regime).
• Regarding the pharmaceuticals, IBP photocatalytic conversion was found to be overall faster in relation to CBZ, in the presence of either P-25 or ZnO catalyst, under either UV or visible light, indicating that it is highly reactive especially under photocatalytic conditions.
• Comparing the catalysts, TiO2 (the type of P-25) showed generally better photocatalytic efficiency in the degradation of both pharmaceuticals compared to ZnO.
• Addition of isopropanol (HO• quencher) showed a significant inhibition effect on the CBZ degradation, taken as an evidence of a solution-phase mechanism. In the case though of IBP degradation, the negligible effect upon addition of isopropanol drives to the conclusion that a direct electron transfer between holes and surface-bound IBP molecules dominates the degradation pathway, i.e., the hole mechanism may be prevailing. A plausible photocatalytic mechanism was proposed and described in details.
This project is implemented through the Operational Program “Education and Lifelong Learning”, Action Archimedes III and is co-financed by the European Union (European Social Fund) and Greek national funds (National Strategic Reference Framework 2007-2013).