Thermally Treated Zeolite as a Catalyst in Heterogeneous Catalytic Ozonation—Optimization of Experimental Conditions and Micropollutant Degradation ^†

Abstract

Raw and thermally pretreated zeolite, denoted zeolite-T, was examined as a catalyst in the heterogeneous catalytic ozonation process. The catalytic activity was evaluated by the degradation of p-chlorobenzoic acid (p-CBA) at an initial concentration 4 μM, a typical biorefractory organic model compound. The results showed that the thermally pretreated zeolite presented higher catalytic activity than the raw material. Rates of 99.3%, 98%, and 94.5% degradation of p-CBA were achieved within 3 min of reaction/oxidation time, by applying the zeolite-T/O₃, zeolite/O₃, and single ozonation (O₃) procedures, respectively, under comparable experimental conditions. After 15 min treatment time, the concentration of p-CBA was found to be below the respective detection limit (0.025 μM) of the used analytical method (HPLC) for both catalytic processes, while the application of single ozonation ed to a 0.16 μM residual concentration of p-CBA. The removal of p-CBA was also examined for different initial pH values commonly found in natural waters, as well as for different oxidation reaction temperatures. Optimum conditions were defined as a pH value of 8 and 25 °C, during which the production of hydroxyl radicals in the aqueous phase was found to be increased. Furthermore, the degradation of two other common micropollutants (i.e., benzotriazole and carbamazepine), which present different reaction rate constants with ozone (i.e., 20 M⁻¹·s⁻¹ and 3 × 10⁵ M⁻¹·s⁻¹, respectively) was additionally evaluated. It was found that all these micropollutants can be sufficiently removed by the catalytic ozonation system in the presence of zeolite-T. However, it is worth noting that carbamazepine can also be easily removed by the application of even simple ozonation, and that the presence of catalyst simply decreased the necessary oxidation time for sufficient removal. On the other hand, benzotriazole presented a lower ozone degradation rate than the other micropollutants, and, during the early stage (i.e., after the third minute) of the oxidation reaction, it was found to be removed by 96.5%.

1. Introduction

In recent years, the relevant research, regarding water quality, focused (among other issues) on the presence and removal of micropollutants (MPs). These organic compounds occur in the aquatic environment in very small concentrations (in the range of μg/L or even lower). However, currently, due to global socioeconomic development, the variety of such substances has increased with potential harmful effects for human health (food chain) and the overall ecosystem [1]. These substances are usually designed to be refractories (i.e., not easily biodegradable) and, as a result, the conventional wastewater treatment plants in most cases are not able to sufficiently eliminate them [2]. The applied micropollutant treatment technologies can be divided into three groups, i.e., phase-changing technologies, biological treatment, and advanced oxidation processes (AOPs) [1]. This study focuses on a common advanced oxidation process, specifically, the heterogeneous catalytic ozonation treatment technique. Heterogeneous catalytic ozonation, utilizing several minerals as catalysts, has previously been reported to efficiently remove different emerging organic pollutants via the acceleration of hydroxyl radical production [3]. Minerals, such as zeolite [4], anatase [5], kaolin [6], and calcite [7], have shown satisfactory performance as catalysts in this process.

The aim of this study was (1) to accelerate the catalytic activity of a commonly applied zeolite catalyst through appropriate modification, applying the simple and inexpensive method of thermal treatment, and (2) to optimize the experimental conditions of heterogeneous catalytic ozonation with the use of thermally treated zeolite as a catalyst. For that purpose, a compound that can be practically removed only by hydroxyl radicals (p-CBA) was used as a probe compound [8]. As a result, the enhancement of this material’s catalytic activity was indirectly evaluated by accelerating the production of hydroxyl radicals through the improved removal efficiency of p-CBA. Furthermore, the removal of two other micropollutants (benzotriazole, carbamazepine) with different oxidation-relevant properties was also evaluated.

2. Materials and Methods

Zeolite was used as an efficient catalyst during the application of heterogeneous catalytic ozonation before and after its thermal treatment at 600 °C. All used chemicals were of analytical grade, except for acetonitrile and phosphoric acid, which were used for micropollutant determination by HPLC and were HPLC-grade. Benzotriazole was purchased from Sigma-Aldrich (Burlington, MA, USA), while carbamazepine was purchased from HPC (Germany); they were used as model compounds. Their main physicochemical properties were previously reported by Psaltou et al. [7]. Dipotassium phosphate (K₂HPO₄) and potassium phosphate monobasic (KH₂PO₄) were used for pH adjustments, when needed. All aqueous solutions were prepared with distilled/deionized water. Batch adsorption, ozonation, and catalytic ozonation experiments were conducted by following the procedure previously described by Psaltou et al. [9]. The concentration of ozone was determined via the application of the indigo colorimetric method [10], while the concentration of micropollutants was determined by HPLC (Thermo, Waltham, MA, USA), according to the protocol also described by Psaltou et al. [7].

3. Results

3.1. Zeolite as a Catalyst in Heterogeneous Catalytic Ozonation and the Selection of Optimal Conditions

In this research, zeolite was studied as a potential catalyst before and after its thermal treatment at 600 °C (abbreviated as zeolite-T) for the removal of p-CBA. Relevant adsorption experiments were conducted for comparison to eliminate the possibility of zeolite or zeolite-T acting as adsorbents. Both of these materials showed rather low uptake capacity for p-CBA at pH 7. However, the thermally treated zeolite presented higher adsorption capacity than the raw material, i.e., 68.9 μg p-CBA/g vs. 59.5 μg p-CBA/g, respectively. Figure 1a shows the results of ozone decomposition during single or catalytic ozonation, using zeolite and zeolite-T as catalysts. The presence of both materials increased the decomposition of ozone, when compared to single ozonation, although they presented similar decomposition rates. On the contrary, the p-CBA removal presented certain differences (Figure 1b). The addition of catalysts was found to improve p-CBA degradation, but the highest catalytic activity was presented by zeolite-T, which removed up to 97.5% of the micropollutant after the first min of reaction/oxidation time. Although the decomposition of ozone by zeolite-T was similar to untreated zeolite, the contact of micropollutant with the surface of zeolite-T was enhanced (due to higher adsorption capacity), and the efficiency of p-CBA degradation was overall improved.

The influence of the experimental conditions was further studied with the qualified catalyst zeolite-T. The rate of ozone decomposition could be divided into three categories according to the respective reaction rate constant. At pH 6 and 15 °C, ozone molecules are more stable [11] and, hence, the decomposition rate of ozone was low (0.057 min⁻¹ and 0.059 min⁻¹, respectively), whereas, when the pH value and temperature were raised to 8 and 35 °C, respectively, ozone decomposition was favored and, thus, was significantly higher. In this study, in which three pH values and three temperature values were examined, the ozone decomposition at pH 7 and 25 °C could be characterized as moderate.

Table 1. First-order kinetic constants of ozone decomposition under different experimental conditions using zeolite T as catalyst.

Conditions	k (min−1)
pH 6 and 25 °C	0.057
pH 7 and 25 °C	0.093
pH 8 and 25 °C	0.252
15 °C and pH 7	0.059
25 °C and pH 7	0.093
35 °C and pH 7	0.112

shows the first-order kinetic constants of ozone decomposition in different experimental conditions, when zeolite-T was added as catalyst in a heterogeneous catalytic ozonation system.

Since the initially examined micropollutant (p-CBA) could not react directly with ozone molecules [8], it was not removed efficiently when the ozone decomposition was low, as shown in Figure 2. However, when the pH value and the temperature were increased to 7 and 25 °C, respectively, the micropollutant removal was increased and its residual concentration was 0.1 μM, even after the first minute of oxidation. When these two parameters were further increased, the ozone decomposition was enhanced, but the p-CBA removal remained steady. At pH 8 the efficiency of the oxidation system reached 98.3% after the first minute (residual concentration 0.068 μM), because the production of hydroxyl radicals was enhanced. However, at 35 °C, although ozone decomposition was promoted, ozone dissolution in the aqueous phase was simultaneously reduced [11]. Therefore, less oxidant was available in the ozonation system, and this is probably the reason why the respective kinetic constant in this case was only 0.112 min⁻¹.

3.2. Removal of Micropollutants by Catalytic Ozonation with the Addition of Zeolite-T

Figure 3 shows the evaluation of the catalytic activity of zeolite-T against other common micropollutants, presenting different properties at 25 °C. Due to the fact that the catalytic activity of zeolite-T was very high, even at pH 7, the micropollutant removal study was conducted at this pH value in order to detect any differences, instead of at pH 8, which was the optimum pH value. At pH 8, the micropollutant removal would have been too fast and the differences in removal rates may not have been observed. According to a relevant previous study [7], the removal of benzotriazole, carbamazepine, and p-CBA, examined under the same experimental conditions, was 66.3%, 97.3%, and 88.8%, respectively, after the first minute of single ozonation. As shown in Figure 3b, these removal rates were increased to 67%, 100%, and 97.5%, respectively, with the addition of zeolite-T to the ozonation system. Carbamazepine presents a reaction rate constant with ozone equal to 3 × 10⁵ M⁻¹·s⁻¹ [12] and reacts quickly with O₃ molecules; therefore, this compound can be easily removed, even via the application of single ozonation. However, the addition of zeolite-T was found to reduce the respective oxidation time. Under these experimental conditions, the residual concentration of carbamazepine was under 5.9 μg/L even after the first minute of the reaction. Furthermore, zeolite could not be characterized as an effective adsorbent, since it also presented low adsorption capacity against benzotriazole (114 μg/g) and carbamazepine (217 μg/g), corresponding to 12% and 11.5% removal, respectively, upon applying only the adsorption process. Consequently, zeolite-T was proven to be an efficient catalyst for the removal of all three examined micropollutants.

4. Discussion

The thermal treatment at 600 °C was found to increase the catalytic activity of raw zeolite. Since the p-CBA ozone reaction rate constant was lower than 0.15 M⁻¹·s⁻¹, the examined (“model”) compound could not be practically removed by oxidation with ozone molecules. The main oxidative species in an ozonation system is the (secondary) production of hydroxyl radicals. These species can react quickly and unselectively with the present organic molecules. The reaction rate constant of p-CBA with the hydroxyl radicals was 5 × 10⁹ M⁻¹·s⁻¹. Thus, the observed increase in p-CBA removal corresponded to an increase in hydroxyl radical production.

The two main factors that can influence the decomposition of ozone are the pH value and the temperature. The ozone decomposition in aqueous phases depends highly upon the present pH value. The kinetic constant of ozone degradation for pH values under 8 is 1.4 × 10⁵ s⁻¹, while this value increases to 10⁸ s⁻¹ when the pH value is equal to or higher than 8 [13]. Since the degradation of p-CBA is accelerated by the higher production of hydroxyl radicals, the presence of a catalyst can further improves the oxidation efficiency of the treatment system. This improvement was more pronounced at pH value, in which the micropollutant residual concentration was just 30% lower after the first minute of oxidation, whereas, throughout this reaction, the obtained percentage remained rather low. Although the pH study proved that higher pH values can result in better oxidation efficiency, this was not the case regarding the investigation of temperature. As shown in Figure 2, the optimum removal of p-CBA was achieved at 25 °C. At 15 °C, similarly to the pH 6 case, ozone molecules are more stable and the degradation of ozone into hydroxyl radicals is rather limited. Certain micropollutants, such as p-CBA, which practically cannot be degraded by ozone molecules, were found to present lower removal efficiencies under those conditions. Accordingly, upon raising the temperature, the dissolution of ozone in the aqueous phase and the oxidation action were both reduced [11].

The removal of micropollutants, presenting different properties and reaction rate constants with ozone and hydroxyl radicals, was examined using zeolite-T as a catalyst. In the case of benzotriazole, the addition of the catalyst did not further accelerate its removal from the first minute, due to the lower ozone degradation. As shown in Figure 3a, the behavior of ozone decomposition was also dependent on the specific target compound. For example, p-CBA, probably because H₂O₂ was coproduced during the reaction [8], could accelerate the decomposition of ozone even after the first minute of oxidation, similarly to carbamazepine, which presents a high ozone reaction rate constant (3 × 10⁵ M⁻¹·s⁻¹) [12], whereas benzotriazole did not accelerate the decomposition in the first stage of the oxidation reaction. After the first minute, the ozone concentration in the system in the presence of benzotriazole was 1.06 mg/L, whereas, at the same timepoint, this concentration in the case of p-CBA was 0.67 mg/L. This lower ozone degradation led to a lower removal efficiency of benzotriazole during the early stage of the reaction, whereas, after the third minute of the reaction, 96.5% removal efficiency was achieved.

5. Conclusions

Thermal treatment proved to be a successful method to enhance the catalytic activity of a solid material, such as zeolite, when applied as a catalyst to improve the efficiency of heterogeneous catalytic oxidation. The removal of p-CBA was increased by 33% and 78% in the zeolite-T/O₃ system, compared to untreated zeolite/O₃ and to the single ozonation process, respectively. The optimum conditions for the removal of p-CBA were found to be pH 8 and 25 °C. Zeolite-T could also be characterized as an efficient catalyst for the removal of benzotriazole and carbamazepine. The residual concentration of carbamazepine under the applied experimental conditions was under the respective analytical determination limit (0.025 μM), even after the first minute of reaction, whereas, at the same timepoint, the benzotriazole concentration was 1.32 μM. The low removal of benzotriazole after the first minute of oxidation was due to the slower ozone decomposition during the early stage of this oxidation reaction. This study revealed that the decomposition of ozone depends not only on the pH value and the temperature of the treatment system, but also on the specific type of examined micropollutant in the ozonation system.