1. Introduction
Pharmaceuticals and personal care products (PPCPs) constitute a large and diverse group of organic compounds, including drugs, chemicals for medical diagnosis, sunscreens, cosmetics, soaps, and more [
1] that reach the environment and water sources from hospital and factory effluents, aquaculture facilities, animal excreta, and human excreta from sanitation system and from sewers. Additional PPCPs sources are disposal of expired drugs in landfills, poor storage of drugs in manufacturing plants, and fertilizers based on animal excreta [
2].
One of the most common pollutants belonging to this group is carbamazepine (CBZ). CBZ is approved by the US Food and Drug Administration (FDA) as a treatment for manic depression (bipolar disorder), trigeminal neuralgia and epilepsy [
3]. Since it is only partially removed by conventional wastewater treatment processes, it was suggested as a marker of anthropogenic activity in water sources [
4]. Its global consumption increased from 742 to 1214 tons per year between the years 1995-2015 [
5] and it was reported various at concentrations of up to 647 ng/L in surface water, 30 ng/ L in drinking water and up to 610 ng/L in groundwater [
6]. Although no significant health hazard were found upon exposure to carbamazepine residues in drinking water, however research performed in animals report possible health damage: for example, a study on its' influence on sperm production in young and adult rats reported that CBZ given before sexual development causes side effects on rat testes, resulting in more severe damage in the adult stage [
7]. Another study found increased damage to the DNA of rare Chinese minnows (
Gobiocypris rarus) with the increase in the concentrations of CBZ, together with a significant increase in the concentration of 8-OHdG free radicals and an accelerated process of apoptosis in the liver [
8].
Over the years, various methods for removing CBZ from water sources have been tested. Reverse osmosis (RO) and nano filtration (NF) membranes were found effective, but result in the accumulation of CBZ in the filtered brine, requiring complementary treatment [
9]. Membrane bio-reactor (MBR) in combination with activated carbon as an adsorbent substrate [
10], or adsorption on the surface of clay minerals/organoclays [
11], have also been shown to be effective, however the polluted matrix requires additional treatment to achieve CBZ complete removal.
Several studies adopted various Advanced Oxidation Processes (AOP) that have been proven to be effective in the degradation and mineralization of CBZ. Such processes are based on a variety of techniques aiming the production of oxidating agents as free radicals and other forms with a high oxidation potential, mineralizing resilient pollutants.
Several AOP processes found to be effective in the removal of CBZ. For example, the use of ozone (O
3), yielded efficient CBZ degradation, but large remains of degradation byproducts were found [
12]. Photo-Fenton reaction, which combines H
2O
2 and Fe
2+ has been shown effectiveness – but only in acidic conditions (pH=2.8-5.3), requiring additional treatment and pH adaptation [
13]. Several additional AOP treatments effective in CBZ degradation including various combinations of UV radiation, H
2O
2 and/or heterogeneous catalysts such as TiO
2 and ZnO [
12], or specifically engineered catalysts based on Fe
3O
4/SiO
2/ TiO
2 [
14] were described- but in most cases exposure times were 1 h or more. In experiments conducted in our research group (Azerrad & Shahar, 2023, unpublished results) a catalyst based on Fe
3O
4/SiO
2/ TiO
2/Ag exhibit efficient CBZ photo-degradation in a few minutes. The most widespread heterogeneous photocatalyst applied in AOP processes is catalytic grade TiO
2, since it is available commercially, cheap, non-toxic and chemically stable. Although in some cases results depended on the specific manufacturer [
15], its catalytic efficiency has been proven in the degradation of CBZ [
6,
16]. Homogeneous photo-reaction using H
2O
2 also has been proven efficient in CBZ degradation [
17]. Hetero-homogenenous photocatalysis combining TiO
2 and H
2O
2, has been shown to be effective in the degradation of a variety of various organic pollutants in water sources [
18,
19], including CBZ [
11].
Several mechanisms were proposed for the various stages of AOP processes. Absorption of UV radiation by H
2O
2, breaks bonds between the oxygen atoms and hydroxyl radicals (HO•) are formed, which are known as strong oxidizing agent [
20]. Hydrogen peroxide molecules absorb radiation in the range of 185-300 nm, where the highest hydroxyl radicals formation yield is achieved at wavelengths of 200-280 nm [
21]. In the UV/TiO
2 process, the photons absorbed by the catalyst cause excitation and an electron jump to a higher energy level, which creates a "hole" that acts as an oxidizer agent by attracting electrons. Such combination of excited electron-hole pairs can be applied to degrade specific chemicals [
22]. Several scenarios may arise: 1. The excited electron may return and fill the hole. 2. The electron-hole pair may oxidize a water molecule, resulting in the formation of a hydroxyl radical and a proton, which lowers the pH in the suspension. 3. The TiO
2 excited molecule may collide with a hydroxyl ion turning it into a hydroxyl radical. 4. The excited electron may attack an oxygen molecule and turn it into a superoxide [
23]. Due to the relatively broad energy gap of TiO
2 (3.2-4.0 eV) such processes are limited to ultraviolet radiation [
24] which is not common in solar radiation [
25].
To further elucidate the CBZ photodegradation process, this study presents a series of elementary steps, combined with the steady-state approximation "based on the assumption that intermediates in the reaction mechanism are consumed as quickly as generated", thus their concentration remain constant during the process [
26]. Such approach was adopted in several studies as degradation of caffeine [
27], the atmospheric degradation of N
2O
5 [
28] or the Lindemann-Hinshelwood process [
29,
30]. Although it is in some cases critized for over-simplification [
31,
32], it still “remains a powerful tool for the simplification of reaction structures” in the elucidation of kinetic processes.
The proposed mechanism may elucidate the physicochemical processes that occur throughout the CBZ degradation process and allow planning and implementation of specific and effective treatments for its degradation, depending on its concentration in the treated water source. A wide set of experiments was conducted, aiming to obtain circumstantial evidence of the proposed elementary steps.
5. Conclusions
This study presents a set of elementary steps aiming to elucidate the heterogeneous-homogeneous photocatalysis of carbamazepine, even though it should be emphasized that such approach suffers from over-simplification. The process presented here is based on the formation of three intermediate products (excited heterogeneous catalyst, excited heterogeneous catalyst/pollutant complex, and hydroxyl radicals). According to the preliminary proposed mechanism the different components should influence as follows:
Carbamazepine should be pseudo 1st order or pseudo 0th order at low or at high concentrations, respectively.
The homogeneous catalysts (H2O2) should be pseudo 1st order at all concentrations, without influencing CBZ pseudo-order.
The heterogeneous catalysts (TiO2) should be pseudo 1st order at all concentrations, without influencing CBZ pseudo-order.
The UVC irradiation intensity should be pseudo 1st order at all concentrations, without influencing CBZ pseudo-order.
A set of experiments was performed to test these outcomes, and indeed points [
1]-[
3] appear to behave according to the proposed model. As for point 4, since results indicate a different behavior, slight changes in the model were introduced to meet the measured results that indicated that UVC irradiation induces a pseudo 2
nd or pseudo 1
st CBZ order at low or high intensities, respectively. Additional experiments at other components conditions may be performed to further strengthen or correct and improve the proposed model, for example- point [
1] should be tested at higher CBZ concentrations, and point [
3] might require testing at lower hydrogen peroxide concentrations.
Assuming the model delivers a first approximation approach to the process, similar theoretical mechanisms may be useful to design more efficient AOP devices. For example, since high pollutants' pseudo-order processes are less effective in achieving complete removal at low pollutant concentrations [
33], in the case of CBZ degradation- low irradiation intensities should not be used to avoid pseudo 2
nd order processes.
Figure 1.
Degradation of 1 (squares), 5 (triangles), 15 (rhombus) and 20 (circles) mg L-1 CBZ with 0.5 mg L-1 TiO2, 5 mg L-1 of H2O2 and 8 UVC lamps. Panel (a) and panel (b) show the concentration related to its initial value (C/C0), and its natural logarithm, respectively. Lines indicate linear regression for each set.
Figure 1.
Degradation of 1 (squares), 5 (triangles), 15 (rhombus) and 20 (circles) mg L-1 CBZ with 0.5 mg L-1 TiO2, 5 mg L-1 of H2O2 and 8 UVC lamps. Panel (a) and panel (b) show the concentration related to its initial value (C/C0), and its natural logarithm, respectively. Lines indicate linear regression for each set.
Figure 2.
Degradation of 1 mg L-1 CBZ with 0.5 mg L-1 TiO2, 8 UVC lamps and 0.5 (rhombus), 1 (circles), 2 (triangles) or 5 mg L-1 (squares) of H2O2. Panel (a) and panel (b) show the concentration related to its initial value (C/C0), and its natural logarithm, respectively. Lines indicate linear regression for each set.
Figure 2.
Degradation of 1 mg L-1 CBZ with 0.5 mg L-1 TiO2, 8 UVC lamps and 0.5 (rhombus), 1 (circles), 2 (triangles) or 5 mg L-1 (squares) of H2O2. Panel (a) and panel (b) show the concentration related to its initial value (C/C0), and its natural logarithm, respectively. Lines indicate linear regression for each set.
Figure 3.
Degradation of 1 mg L-1 CBZ with 5 mg L-1 H2O2, 8 UVC lamps and 0.02 (squares), 0.05 (rhombus), 0.2 (circles) or 0.5 mg L-1 (triangles) of TiO2. Panel (a) and panel (b) show the concentration related to its initial value (C/C0), and its natural logarithm, respectively. Lines indicate linear regression for each set.
Figure 3.
Degradation of 1 mg L-1 CBZ with 5 mg L-1 H2O2, 8 UVC lamps and 0.02 (squares), 0.05 (rhombus), 0.2 (circles) or 0.5 mg L-1 (triangles) of TiO2. Panel (a) and panel (b) show the concentration related to its initial value (C/C0), and its natural logarithm, respectively. Lines indicate linear regression for each set.
Figure 4.
Degradation of 1 mg L-1 CBZ with 5 mg L-1 H2O2, 0.5 mg L-1 TiO2 and 2 (squares), 4 (circles), 6 (triangles) or 8 (rhombus) UVC lamps. Panels (a), (b) and (c) show the concentration related to its initial value (C/C0), its natural logarithm (ln(C/C0)), and its reciprocal (C0/C), respectively. Lines indicate linear regression for each set.
Figure 4.
Degradation of 1 mg L-1 CBZ with 5 mg L-1 H2O2, 0.5 mg L-1 TiO2 and 2 (squares), 4 (circles), 6 (triangles) or 8 (rhombus) UVC lamps. Panels (a), (b) and (c) show the concentration related to its initial value (C/C0), its natural logarithm (ln(C/C0)), and its reciprocal (C0/C), respectively. Lines indicate linear regression for each set.
Table 1.
CBZ Pseudo-orders, half-life times, root mean square errors (RMSE) and coefficient of determination (R2) for experiments with several CBZ concentrations, 0.5 mg L-1 TiO2, 5 mg L-1 (147 µM) of H2O2 and a UVC radiation intensity of 3605 W m-2 (corresponding to 8 UVC bulbs). .
Table 1.
CBZ Pseudo-orders, half-life times, root mean square errors (RMSE) and coefficient of determination (R2) for experiments with several CBZ concentrations, 0.5 mg L-1 TiO2, 5 mg L-1 (147 µM) of H2O2 and a UVC radiation intensity of 3605 W m-2 (corresponding to 8 UVC bulbs). .
CBZ concentration (mg L-1) |
CBZ pseudo-order na
|
Half life t1/2 (min) |
RMSE |
R2
|
1 |
0 |
2.77±17.8% |
0.346 |
0.683 |
5 |
0 |
5.09±4.26% |
0.328 |
0.765 |
15 |
0 |
6.05±7.41% |
0.142 |
0.925 |
20 |
0 |
7.37±4.68% |
0.046 |
0.977 |
1 |
1 |
1.08±12.02% |
0.077 |
0.921 |
5 |
1 |
2.22±3.64% |
0.037 |
0.995 |
15 |
1 |
3.96±1.31% |
0.035 |
0.999 |
20 |
1 |
6.27±1.31% |
0.028 |
0.999 |
1 |
1.11±4.90% |
0.95±6.05% |
0.072 |
0.955 |
5 |
0.83±2.04% |
2.33±1.10% |
0.030 |
0.996 |
15 |
0.76±3.25% |
4.18±1.28% |
0.021 |
0.998 |
20 |
0.61±8.71% |
6.55±1.15% |
0.018 |
0.998 |
Table 2.
CBZ pseudo-orders, half-life times, root mean square errors (RMSE) and coefficient of determination (R2) for degradation of 1 mg L-1 CBZ with 0.5 mg L-1 TiO2, UVC radiation intensity of 3605 W m-2 (corresponding to 8 UVC bulbs), and H2O2 ranging between 0.5-5 mg L-1 (14.7-147 µM).
Table 2.
CBZ pseudo-orders, half-life times, root mean square errors (RMSE) and coefficient of determination (R2) for degradation of 1 mg L-1 CBZ with 0.5 mg L-1 TiO2, UVC radiation intensity of 3605 W m-2 (corresponding to 8 UVC bulbs), and H2O2 ranging between 0.5-5 mg L-1 (14.7-147 µM).
H2O2 concentration (mg L-1) |
CBZ pseudo-order na
|
Half life t1/2 (min) |
RMSE |
R2
|
0.5 |
0 |
4.99±6.44% |
0.194 |
0.823 |
1 |
0 |
4.29±10.4% |
0.248 |
0.707 |
2 |
0 |
2.98±5.32% |
0.249 |
0.618 |
5 |
0 |
2.77±17.8% |
0.346 |
0.683 |
0.5 |
1 |
2.44±1.33% |
0.028 |
0.991 |
1 |
1 |
1.82±3.33% |
0.048 |
0.980 |
2 |
1 |
1.31±7.37% |
0.087 |
0.905 |
5 |
1 |
1.08±12.02% |
0.077 |
0.921 |
0.5 |
0.95±2.57% |
2.48±2.48% |
0.027 |
0.992 |
1 |
0.93±4.49% |
1.86±1.37% |
0.047 |
0.979 |
2 |
1.05±6.78% |
1.25±6.28% |
0.086 |
0.902 |
5 |
1.11±4.90% |
0.95±6.05% |
0.072 |
0.955 |
Table 3.
CBZ pseudo-orders, half-life times, root mean square errors (RMSE) and coefficient of determination (R2) for degradation of 1 mg L-1 CBZ with 5 mg L-1 (147 µM) H2O2, UVC radiation intensity of 3605 W m-2 (corresponding to 8 UVC bulbs), and TiO2 ranging between 0.02-0.5 mg L-1. .
Table 3.
CBZ pseudo-orders, half-life times, root mean square errors (RMSE) and coefficient of determination (R2) for degradation of 1 mg L-1 CBZ with 5 mg L-1 (147 µM) H2O2, UVC radiation intensity of 3605 W m-2 (corresponding to 8 UVC bulbs), and TiO2 ranging between 0.02-0.5 mg L-1. .
TiO2 concentration (mg L-1) |
CBZ pseudo-order na
|
Half life t1/2 (min) |
RMSE |
R2
|
0.02 |
0 |
2.19±16.3% |
0.302 |
0.518 |
0.05 |
0 |
2.47±7.39% |
0.293 |
0.578 |
0.2 |
0 |
2.84±13.84% |
0.262 |
0.663 |
0.5 |
0 |
2.77±17.8% |
0.346 |
0.683 |
0.02 |
1 |
0.83±5.29% |
0.096 |
0.889 |
0.05 |
1 |
0.90±4.48% |
0.073 |
0.933 |
0.2 |
1 |
1.06±4.55% |
0.086 |
0.902 |
0.5 |
1 |
1.08±12.02% |
0.077 |
0.921 |
0.02 |
0.81±15.6% |
0.88±4.22% |
0.093 |
0.890 |
0.05 |
0.88±11.8% |
0.91±5.70% |
0.070 |
0.936 |
0.2 |
1.07±8.56% |
1.02±7.34% |
0.085 |
0.907 |
0.5 |
1.11±4.90% |
0.95±6.05% |
0.072 |
0.955 |
Table 4.
CBZ pseudo-orders, half-life times, root mean square errors (RMSE) and coefficient of determination (R2) for degradation of 1 mg L-1 CBZ with 5 mg L-1 (147 µM) H2O2, 0.5 mg L-1 TiO2, and UVC irradiation intensity ranging between 901- 3605 W m-2 (corresponding to 2-8 UVC lamps).
Table 4.
CBZ pseudo-orders, half-life times, root mean square errors (RMSE) and coefficient of determination (R2) for degradation of 1 mg L-1 CBZ with 5 mg L-1 (147 µM) H2O2, 0.5 mg L-1 TiO2, and UVC irradiation intensity ranging between 901- 3605 W m-2 (corresponding to 2-8 UVC lamps).
UVC irradiation intensity (W m-2) |
CBZ pseudo-order na
|
Half life t1/2 (min) |
RMSE |
R2
|
901 (2 lamps) |
0 |
9.68±2.52% |
0.202 |
0.747 |
1803 (4 lamps) |
0 |
5.12±3.36% |
0.254 |
0.689 |
2704 (6 lamps) |
0 |
3.42±3.95% |
0.286 |
0.684 |
3605 (8 lamps) |
0 |
2.77±17.8% |
0.346 |
0.671 |
901 (2 lamps) |
1 |
5.02±5.63% |
0.096 |
0.938 |
1803 (4 lamps) |
1 |
1.78±6.06% |
0.068 |
0.947 |
2704 (6 lamps) |
1 |
1.18±4.03% |
0.051 |
0.966 |
3605 (8 lamps) |
1 |
0.98±17.8% |
0.083 |
0.968 |
901 (2 lamps) |
2 |
3.43±1.22% |
0.044 |
0.985 |
1803 (4 lamps) |
2 |
1.25±5.26% |
0.075 |
0.979 |
2704 (6 lamps) |
2 |
0.89±5.83% |
0.098 |
0.982 |
3605 (8 lamps) |
2 |
0.71±1.82% |
0.103 |
0.980 |
901 (2 lamps) |
1.92±3.41% |
3.46±4.97% |
0.041 |
0.955 |
1803 (4 lamps) |
1.41±2.65% |
1.50±3.53% |
0.057 |
0.950 |
2704 (6 lamps) |
1.03±2.86% |
1.16±2.93% |
0.050 |
0.967 |
3605 (8 lamps) |
1.11±4.90% |
0.95±6.05% |
0.072 |
0.955 |