Kinetic studies of dexamethasone degradation in aqueous solution via a photocatalytic UV/H2O2/MgO process

The characterization of the catalyst

Figure 2 presents the morphological properties of the nanoparticles as determined by the scanning electron microscopy (SEM) analysis, performed before starting the reaction. As can be seen, it is obvious that the magnesium oxide nanoparticles had a porous, spongy structure. Figure 3 illustrates, in the amorphous, shape, two peaks seen in 2θ = 43 and 62, illustrating the presence of cubic MgO, and the peaks can be assigned to a pure phase of MgO. The FTIR spectrum was also tested for investigation and identification of the catalyst surface’s functional groups. It is well known that MgO chemisorbs H2O and CO2 molecules from the atmosphere due to its surface acid–base properties20. The major peaks appearing in the FT-IR spectra may be assigned to the following modes: (i) a broad vibration band around 2800–3700 cm−1 can be associated to OH stretching vibrations of the surface-bonded (or) adsorbed water, which was introduced in precursor solution (ii) the peak around 1629 cm−1 is devoted to OH bending vibrations of water molecules. According to Fig. 4, a strong peak band was detected at a wavelength of 528 cm−1, due to asymmetric vibrations of the Mg–O band. Peaks observed at 850 cm−1 corresponded to C=O stretching vibrations. (Comparative Study of Microwave and Conventional Methods for the Preparation and Optical Properties of Novel MgO-Micro and Nano-Structures) (MgO Nanoparticles Prepared By Microwave-Irradiation Technique and Its Seed Germination Application). The surface hydroxyl groups have been recognized to play an important role in the photocatalytic reaction since they can inhibit the recombination of photogenerated charge carriers, and also interact with the photogenerated holes to produce active oxygen species20.

Figure 2figure 2

Scanning electron microscope (SEM) image of the MgO nanoparticles.

Figure 3figure 3

XRD analysis of the MgO nanoparticles.

Figure 4figure 4

FTIR spectra of the MgO nanoparticles.

Impact of initial pH

The literature review revealed that AOPs are completely pH-dependent21. Hence, in this study, at the fixed hydrogen peroxide content of 1 mM, the pH values were changed from 3 to 11 to investigate the changes in the removal efficiency. The maximum removal efficiency (73%) by the UV/H2O2/MgO method was attained at a pH of 3 (Fig. 5). These findings are attributed to the surface properties of the adsorbent and the ionization/degradation of the adsorbate. The number of hydrogen ion increases gradually with decreasing pH. When H+ is adsorbed, the positive charge on the nanoparticle’s surface increases and, in turn, the electrostatic force between the cationic charge on the surface of the nanoparticle and the negative DEX molecule enhances, increasing the adsorption rate. It was found that the performance declined sharply when pH was raised. For example, a 45% decrease was seen in removal efficiency at a pH value of 11 within 30 min. As can be seen, the degradation rate remained unchanged after 20 min and was insignificant after 30 min. Therefore, reaction times between 0 and 30 min were selected for the rest of the experiments. Furthermore, a decrease in the removal efficiency of the UV/H2O2 in alkaline conditions can be caused by a reaction between H2O2 and solution alkalinity; this causes hydroxyl radicals to go down. Moreover, in comparison with a neutral pH, the nanoparticles are accumulated in acidic conditions; as a result, the catalyst’s effective surface area was enhanced22.

Figure 5figure 5

Impact of pH on DEX degradation: hydrogen peroxide dosage = 1 mM, DEX content = 20 mg/L, and MgO dosage = 0.05 g/L).

Impact of H2O2 dosage

In this study, under the following conditions: pH 3, DEX content of 20 mg/L, and catalyst dosage of 0.05 g/L, different initial contents of hydrogen peroxide (1–8 mM) were tested. According to the results presented in Fig. 6, the removal efficiency increased to 87% when the concentration of H2O2 was raised to 5 mM. It should be noted that, when the H2O2 concentration exceeded 5 mM, the removal efficiency started to decline. An excessive increase in H2O2 concentration causes part of ·OH to be inhibited and then HO2 is produced, which has a lower oxidation potential than ·OH (Eq. (1))23. Also, this decrease in performance can be because of the continuous degradation of H2O2 into oxygen and water as shown in Eq. (1)24.

$${text{H}}_{{2}} {text{O}}_{{2}} + cdot{text{OH}} to {text{ H}}_{{2}} {text{O }} + {text{ HO}}_{{2}}cdot$$

(1)

Figure 6figure 6

Impact of H2O2 dosage on the removal efficiency of DEX under the following conditions: pH = 3, DEX concentration = 20 mg/L and MgO dosage = 0.05 g/L.

Impact of initial DEX concentration

In photocatalytic processes, how the initial concentration of the pollutant affects the removal efficiency is of great importance. Figure 7 shows the impact of the initial DEX content on the removal efficiency in UV/H2O2/MgO. As can be seen, with an increased DEX concentration from 5 to 30 mg/L, the removal efficiency declined. And, 65% of DEX was degraded at a concentration of 30 mg/L. Within 5 min of the reaction and an initial DEX content of 5 mg/L, a 90% removal efficiency was reached (Fig. 7). The decrease in the removal rate by increasing the concentration of DEX can be attributed to the fact that at all concentrations, the amount of nanoparticles, contact time, and pH are the same. As a result, the amount of radicals produced is similar at all concentrations. Naturally, it is expected to see lower DEX degradation at lower concentrations. By contrast, at a lower initial concentration, the number of active sites on the catalyst’s surface capable of degrading DEX increases. Furthermore, ultraviolet light cannot penetrate effectively into the solution when there are higher concentrations of DEX25.

Figure 7figure 7

Impact of initial DEX content on DEX removal rate: pH = 3, hydrogen peroxide dose = 5 mM and MgO dose = 0.05 g/L.

Impact of the dose of MgO

In Fig. 8, it is shown how the changes in magnesium oxide (0.01–0.2 g/L) affected the removal efficiency of the pollutant in photo-oxidation. As can be seen, the removal efficiency went up with the increase in the dose of MgO. Nevertheless, when the dosage exceeded 0.05 g/L, the removal rate declined. At higher doses, there are more active sites and free electrons in the conductor, resulting in the generation of more hydroxyl radicals that can take part in the degradation26. Also, the removal rate of DEX at higher dosages of this nanoparticle was marginal, because the nanoparticles stuck together, causing the intensity of the UV lamp to decrease. Sobana et al. reported that during the photocatalytic reactions, the removal efficiency of Red Direct 23 increased as an increase in the number of active sites, resulting from a rise in the dosage of titanium dioxide doped with silver27. It should be noted that the current study’s findings are consistent with those of other related studies16.

Figure 8figure 8

Impact of MgO dose on the rate of DEX removal: initial dexamethasone content = 20 mg/L, pH = 3, and hydrogen peroxide dosage = 5 mM.

Impact of radical scavengers

In this study, the main reactive species in DEX degradation were identified using radical scavenging experiments under optimal conditions. To investigate the effects of different scavengers on DEX degradation, AA (0.2 mol/L), EDTA (0.2 mol/L), and TBA (0.2 mol/L) were added to the DEX solution as superoxide anion (O2·−), hole (h+), and hydroxyl radical (·OH) scavengers, respectively28. The results show three types of inhibition, corresponding to the three active species in the UV/H2O2/MgO process. From Fig. 9 it can be see that 87% of DEX can be removed in 30 min without a scavenger (Control). However, with the addition of AA, EDTA, and TBA, DEX removal efficiency decreased to 73.5%, 64.6% and 34.8%, respectively (Fig. 9). The rate of DEX degradation during the reaction process was less affected by the addition of AA (a scavenger of O2·−). Since TBA is a known ·OH scavenger29, the DEX degradation in the established UV/H2O2/MgO system in the presence of TBA clearly shows that the reaction with ·OH was the predominant active specie contributing to DEX removal. Furthermore, the decrease in DEX degradation in the presence of EDTA as a hole scavenger confirms h+ photogeneration. The hole reactive species directly or indirectly oxidizes DEX compounds by generating hydroxyl radicals through the oxidation of water molecules. Therefore, the main mechanism was discovered to be in the form of ·OH-driven reactions, confirming those ·OH radicals were key species in the UV/H2O2/MgO process in DEX degradation, as described in the following Eqs. (2)–(6)28,30:

$${text{MgO }} + {text{ light }} to {text{ e}}^{ – } + {text{ h}}^{ + } ,$$

(2)

$${text{h}}^{ + } + {text{ H}}_{{2}} {text{O }} to cdot {text{OH }} + {text{ OH}}^{ + } ,$$

(3)

$${text{e}}^{ – } + , 0.{text{5 O}}_{{2}} + {text{ H}}_{{2}} {text{O }} to , cdot {text{OH }} + {text{ OH}}^{ – } ,$$

(4)

$${text{h}}^{ + } + {text{ OH}}^{ – } to cdot {text{OH}},$$

(5)

$${text{DEX }} + cdot {text{OH}} to {text{Degradation Products}}.$$

(6)

Figure 9figure 9

The degradation rate of DEX in the presence of different radical scavengers (initial DEX concentration = 20 mg/L, pH = 3, H2O2 dose = 5 mM, and MgO dose = 0.05 g/L).

This result corresponds with Akbari et al.30 study that stated hydroxyl radicals are the main mechanism in ciprofloxacin antibiotic removal using S, N-doped MgO nanoparticles under UVA-LED.

TOC analysis and mineralization

In this study, the content of TOC was determined because DEX is initially converted to other degradation byproducts that are still organic. Thus, we determined the mineralization of DEX by recording TOC concentrations during the process. The TOC and COD concentrations of the samples were determined under the selected conditions (Fig. 10). It was found that the initial TOC was determined at 53.8 mg/L, and it declined to 23.5 mg/L after the exertion of the UV/H2O2/MgO process for 30 min, illustrating a mineralization rate of 56%. Accordingly, COD was reduced by up to 65%. However, at the same time of contact, the rate of DEX removal was 87%. Thus, it is claimed that for more mineralization, more contact time is required. For instance, the TOC removal rate increased to 98% within 120 min. It should be pointed out that lower by-products can be generated when a suitable contact time is regarded for reaching the mineralization rate of interest by the UV/H2O2/MgO process. It should be noted that, in the application of photocatalytic reactions, intermediates must be detected and eco-toxicological examinations should be performed.

Figure 10figure 10

TOC and COD removal (initial DEX concentration = 20 mg/L, pH = 3, H2O2 dose = 5 mM, andMgO dose = 0.05 g/L).

Comparison of the processes

In this study, the UV/H2O2 process was run in the presence and absence of the MgO catalyst. Also, the results of the UV and UV/MgO processes were compared. As indicated in Fig. 11, only 8% of the pollutant was degraded via the UV application within 30 min. Moreover, the performance of the UV/MgO process was nearly 17%, which may be because of the low adsorption rate that occurred on the surface of magnesium oxide. It should be noted that there was a dramatic difference between the removal efficiency rates of the UV/H2O2 photo-oxidation and the UV/H2O2/MgO process, which were found to be 61% and 87%, respectively. The activity of magnesium oxide in catalyzing oxidation decay was relative to the surface acid–base properties of the oxide. Water molecules can be adsorbed on the magnesium oxide’s surface due to the unsaturated state of surface electrons. As a result, surface hydroxyl groups may be formed. These groups play a basic role in the acid–base characterizations of magnesium oxide. Therefore, the process can be catalyzed well due to the surface hydroxyl groups. Thus, it is expected to see more DEX removal in the presence of magnesium oxide.

Figure 11figure 11

Comparison of the UV, UV/H2O2, UV/MgO and UV/H2O2/MgO processes under the optimum conditions.

Investigation of process kinetics

The behavior of DEX removal was studied by both the linear forms of pseudo-first and second-order kinetic models31 as expressed in Eqs. (7) and (8).

$${ln}_{Ct}={ln}_{C0}times {e}^{-kt},$$

(7)

$$frac{1}{{ln}_{Ct}}=frac{1}{{ln}_{C0}}+{k}_{2}t.$$

(8)

Here, C0 and Ct show DEX concentration at times 0 and t (min), respectively. k1 (min−1) and k2 (mg/L.min) are assigned to the first and second-order kinetic constants, respectively. Figures 12 and 13 show pseudo-first and second-order kinetic models obtained by plotting Ln (ct/c0) and 1/ct–1/c0 against reaction time. The values of k1 and k2 obtained by the corresponding kinetic models are given in Table 2. In addition, the R2 values for all the single, binary, and ternary processes are better fitted to the pseudo-second-order kinetic model. The findings strongly indicate that the reaction constant for the UV/H2O2/MgO process was the highest among other methods of DEX removal. This illustrates that the combined UV/H2O2/MgO methods were more effective in DEX removal than MgO, UV, UV/H2O2, UV/MgO processes.

Figure 12figure 12

The degradation of DEX under MgO, UV, UV/H2O2, UV/MgO, and UV/H2O2/MgO processes based on a pseudo-first-order model.

Figure 13figure 13

The degradation of DEX under MgO, UV, UV/H2O2, UV/MgO, and UV/H2O2/MgO processes based on a pseudo-second-order model.

Table 2 The obtained coefficients of first and second order kinetic models for the removal of DEX by UV, MgO, UV/H2O2 and UV/H2O2/MgO processes were calculated.

Degradation pathway of DEX

The hydroxyl radical interacts with organic pollutants quickly and strengthens C-unsaturated bonds. Because DEX molecules contain many OH groups, they are unstable to oxidation with a ·OH radical 32. In the present study, the intermediates were determined using the LC–MS method to determine the degradation pathway of DEX in the UV/H2O2/MgO process. Based on degradation intermediates of DEX and previously published data32,33, two possible pathways of DEX degradation were proposed (Fig. 14). In the first pathway, the attack ·OH radical to DEX molecule causes create a new intermediate (A). According to the intermediate (A), the products of (B), (C), and (D) could be attained by the cleavages of the bonds, which was related to the direct degradation of DEX compounds via the process of photodegradation. In the second pathway, the hydroxyl radical attack on the methylene group results in the mineralization of two carbon atoms and the creation of a ketone group on the remaining structure. The preferred loss and degradation in DEX are HF, which are continued by the combined losses of the HF and H2O molecules. (Intermediate D)34. Two water molecules are released after the ring breaks up. On further attack by the hydroxyl radical and after the breaking of the benzene rings, DEX was degraded to less refractory intermediate compounds, and thereby these compounds mineralized into CO2 and H2O18.

Figure 14figure 14

The degradation pathway of DEX under UV/H2O2/MgO process.

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