Enhancing Levofloxacin Degradation in Contaminated Water: Catalytic Performance of Pegmatite in a Sodium Percarbonate/Ultrasound System

Abstract

Despite ongoing research into technologies for the removal of antibiotics from contaminated water, there is a necessity to provide a cost-effective water treatment solution. In this study, the activation of sodium percarbonate (SPC) by pegmatite soils ultrasonicated (US) has been used to degrade fluoroquinolone antibiotics, such as levofloxacin (LVFX), in contaminated water. The soil has been characterized before application using instruments such as XRF, SEM-EDX, and XRD, where, according to the XRF and XRD analyses, the pegmatite contained mainly Fe^III (59.8 g.kg⁻¹) and iron oxide (goethite). After characterization, the material was used for water treatment application, where the removal efficiency of LVFX using the SPC/US/Pegmatite system under the optimal conditions (i.e., [LVFX] = 60 µmol L⁻¹, [Pegmatite] = 0.75 g. L⁻¹, [SPC] = 60 µmol L⁻¹) was 95% with 57% mineralization, while the degradation efficiency was 90.7%. Scavengers’ experiments showed the involvement of OH^•, O₂^•−, and CO₃^•− in the degradation of LVFX, with the strongest implication of OH^• (i.e., 57.33%) in the process. The system’s reusability and catalytic capability have been determined to be satisfactory, through water washing and drying operations used before reuse in a fresh oxidation cycle. This cost-effective remediation method has shown to be a promising approach to removing antibiotics from contaminated water.

1. Introduction

Antibiotics are used in humans, aquaculture, livestock, and agriculture for their bactericidal and bacteriostatic effects [1]. Due to the overuse of antibiotics and their high rate of persistence, antibiotic-resistant genes and bacteria have been detected in the aquatic environment, causing serious damage to human health and the ecosystem [2]. These compounds have also been shown to severely impair the survival, growth, and reproduction of aquatic organisms [3]. They can accumulate in the body and then destroy the nervous system and other vital organs (e.g., liver, kidneys) [4]. Given the magnitude of these hazards, several remediation methods have been considered and tested, namely, photocatalysis and advanced oxidation processes (AOPs) (i.e., homogeneous Fenton [5]; direct or indirect photolysis [6]; electrochemical [7]; and the oxidation process using peroxydisulfate or peroximonosulfate, etc. [8]). These offer advantages over conventional remediation techniques (i.e., adsorption, biological processes, membrane filtration, and ion exchange), which do not completely remove these types of micropollutants [9]. The proposed processes use strong oxidants to generate radicals to break down large toxic molecules [10].

However, there are still several concerns to be addressed. For instance, in AOPs using Fe^III, radicals can be generated by the reactions between ferreous iron (Fe^II) obtained under the ultrasonic-assisted reduction system of Fe^III and strong oxidants [11]. However, the radical yield would be limited to the Fe^II amount in solution and would be influenced by the pH of the solution [8]. In addition, an excess of Fe^II would also quench the radicals. Hence, some of the AOPs still need to be improved. Over the last decade, the use of a solid H₂O₂ carrier (i.e., sodium percarbonate (SPC)) in AOPs has been widely promoted. This is because it releases H₂O₂ on contact with water, as shown in Equation (1), which is then used in the oxidation process [12].

2Na₂CO₃.3H₂O₂ → 2Na₂CO₃ + 3H₂O₂

(1)

Unlike conventional H₂O₂, SPC offers a number of benefits, including a wider pH range, easier handling, and safer transportation, thus avoiding any risk of explosion [13]. Moreover, SPC has an alkaline nature that inhibits the unwanted acidification of water, and it is potentially more stable and economical in its concentrated form [14], thus avoiding significant risks and issues in the implementation of in situ oxidation technologies. The results obtained by some researchers using the ferrous/SPC system showed that a large number of chemicals are required to achieve complete benzene and trichloroethylene degradation [15]. Previous studies used complexing agents to conserve iron solubility and improve the Fe^II/SPC system performance [16]. However, chelators can also react with HO^• radicals and thus reduce the oxidative performance of the system. Fu et al., 2017 also tried Fe^III-sulfate to activate percarbonate to degrade benzene, but Fe^III-sulfate is very acidic, corrosive, and toxic to humans [17]. The use of sodium percarbonate in water treatment is increasing considerably in today’s advanced oxidation processes [18,19,20,21,22].

In this study, an Ultrasound Iron Soil/SPC system for the degradation and mineralization of fluoro(quinolone) antibiotics has been developed, which has never been reported to our knowledge and is environmentally friendly.

The study aimed to investigate the degradation and mineralization of fluoro(quinolone) antibiotic, which is levofloxacin (LVFX), using this innovative system Ultrasound Iron-rich Soil/SPC. Then, the operating conditions were optimized, the percentage of the different radical species involved in the process was determined, and the catalytic ability of the pegmatite over several cycles was evaluated.

2. Results and Discussion

2.1. Pegmatite Soil Characterization

2.1.1. Morphology and Chemical Characterizations

XRF analysis showed that pegmatite elemental composition mainly consisted of Fe (59.8 g kg⁻¹), Al (269.2 g kg⁻¹), Ca (47.3 g kg⁻¹), Si (36.5 g kg⁻¹), and Ti (20.5 g kg⁻¹), thereby underscoring the predominance of alumina (23.1%), quartz (18.7%), iron oxide (4.77%), calcium oxide (4%), and titanium oxide (2%), in combination with other mineral phases such as Mn₂O₃, K₂O, MoO_3, and V₂O₅. SEM and EDX analysis confirmed this chemical composition of the soil, which remained identical after the oxidation experiments, despite the reduction in particle size from 100 µm to 50 µm (see Figure 1 and Figure 2).

XRD diffractogram spectra revealed mainly quartz (SiO₂), goethite (α-FeO(OH)), kaolinite (Al₈Si₈O₃₂), and muscovite (KAl₂(AlSi₃O₁₀) (see Figure 3). No structural changes were observed before or after the oxidation cycle within any of these characterization experiments.

2.1.2. Surface Characteristics

Depending on the various pH conditions (i.e., 2, 4, 6, 8, 10, and 12), the zeta potential measurements showed that the isoelectric point (IEP) of the pegmatite was 6.9, as shown in Figure 4A. Based on the multipoint adsorption isotherm of N₂, the specific surface area, micropore volume, and total pore volume were found as 26 m² g⁻¹, 5 m³ g^−1, and 28.40 m³ g⁻¹, respectively (see Figure 4B). Then, BJH Adsorption Pore Distribution showed that the average pore diameter was 1.49 nm (see Figure 4C).

2.2. Removal Capacity of Pegmatite

LVFX removal was evaluated under different oxidation systems with and without pegmatite (see Figure 5). The best yield with 95% removal was obtained from the heterogeneous process (i.e., SPC/US/pegmatite/[LVFX]_aq). Levofloxacin has two different pKa values (pKa1 = 5.7 and pKa2 = 7.9). At pH values between 5.7 and 7.9, levofloxacin exists mainly in the zwitterionic form in solution. Since pegmatite has an isoelectric pH in the order of 6.9 and therefore possesses more negative charge in solution, levofloxacin exists in greater concentration at the cavitation bubble interface and is therefore more easily attacked by oxidizing species.

The ultrasound-assisted system is less than 5%. To account for the adsorption on pegmatite during the oxidation reaction, desorption tests (an addition of NaOH up to pH = 11) were conducted, and the LVFX desorbed in reactional solution were plotted versus time (i.e., SPC/US/pegmatite/[LVFX]_Des). The resultant figure demonstrated a sorption of 4.3%, thereby indicating by the discrepancy between the aqueux (Aq) and desorption (Des) systems. Thus, the amount of LVFX degraded after desorption was 90.7% after 300 min of reaction. It should be noted that the prior equilibration of the LVFX with the pegmatite suspension for 5 h in the dark ahead of ultrasonication did not noticeably alter the kinetic performance. COD measurements over 300 min confirmed oxidation degradation in the SPC/US/Pegmatite/[LVFX]_Des system, with 57% of the LVFX mineralized under optimum conditions (see Figure 5B). After the addition of pegmatite into the SPC/US system, the mineralization was improved by almost 42% compared to those of the SPC/US system (See Figure 5B). This improvement may be due to the enhanced production of reactive oxygen species (ROS), such as hydroxyl radicals, as well as ferrous iron (Fe^II) [23,24]. In fact, SPC under ultrasound can generate higher radical species (i.e., OH^•, CO₃^•−) and H₂O₂ according to Equation (1). In addition, ultrasound can also increase Fenton-like action on the surface, thereby favoring the reduction of Fe^III to Fe^II, which then reacts with H₂O₂ supplied by the SPC to produce OH^• [25]. The addition of SPC resulted in an acceleration of the reaction rate compared with the US/Pegmatite system, thereby confirming the synergistic effect of SPC and Pegmatite. Qi et al. [26] achieved 96.53 ± 1.43% and 59.32 ± 2.31% of the degradation and mineralization rates, respectively, when removing LVFX using Fenton-ultrasonic process. The eventual content of trace elements in the treated water (e.g., Cr, Cu, Ni, Zn, Co, Pb, and Fe) was also checked using the DR 3900 spectrophotometer, which denoted that these elements were below the limits of quantification, except Fe (4 µg L⁻¹) for our experimental conditions.

2.3. Optimization of Operating Process Conditions

The effects of SPC concentrations (i.e., oxidant), pegmatite doses (i.e., catalyst), and initial LVFX concentration (i.e., pollutant) were studied for 300 min at pH 7 ± 0.2. The experiments were performed in duplicate. They showed excellent repeatability with a mean standard error of 3%.

2.3.1. Effect of Oxidant SPC Concentrations

The removal kinetics and second-order rate constant evolution of LVFX were determined at different concentrations of SPC (40 to 70 µM) at pH 7 ± 0.2. The degradation kinetics improved as the concentration of SPC increased from 40 to 60 µM and started to decrease above 60 µM. By following the degradation kinetics, the higher degradation was obtained with 60 µM of SPC with 95%, as shown in Figure 6A. In fact, this degradation of the floroquinolone compounds by ROS is described as a reaction of the second order [8,25]:

d [LVFX]/dt= −k [LVFX].[ROS]

(2)

where [ROS] is the equilibrium concentration of hydroxyl radical, [LVFX] is the levofloxacin concentration in water, k is the rate second order constant, and t is the reaction time. It is hypothesized that the instantaneous ROS concentration remains constant. In this instance, the degradation kinetics of LVFX in water can be described by the pseudo-first order equation as follows [8]:

$$[\mathrm{LVFX}{]}_{\mathrm{t}}=[\mathrm{LVFX}{]}_0 e^{-kapp. t}$$

(3)

where kapp is the pseudo-first apparent rate constant and was determined through the linear regression of ln (C_t/C₀) against reaction time t. It was subsequently plotted versus the concentration of SPC in Figure 6B. This plot clearly shows the phenomenon observed in the degradation kinetics with the increase in kapp from 40 to 60 µM. However, kapp decreased at higher SPC concentrations, thereby underscoring a trapping effect between hydroxyl radicals and hydrogen peroxide and/or the recombination of hydroxyl radicals [27]. Thus, at higher SPC concentrations, a competitive reaction occurs between LVFX and H₂O₂ towards the radical species.

In addition, the inhibition effect of SPC could also be ascribed to the role of OH^• scavenging through excess H₂O₂ and HCO₃⁻ in the solution, as illustrated by Equation (5). H₂O₂ should thus be regarded as an OH^• scavenger. Furthermore, excess HCO₃⁻ resulting from SPC at neutral pH may consume some of the OH^• (Equation (6)), thereby hindering the LVFX degradation [28].

H₂O₂ + OH^•→ HO₂^•− + H₂O, k_OH^•_{∙/H2 O2} = 1.6 × 10⁹ M⁻¹∙min⁻¹

(4)

HCO₃⁻ + OH^• → CO₃^•− + H₂O, k_OH^•_/HCO3⁻= 5.2 × 10⁸ M⁻¹∙min⁻¹

(5)

Yu et al. [29] demonstrated that excessive SPC concentration have an negative impact on the degradation of Dichlorvos.

2.3.2. Effect of Catalyst (Pegmatite) Dosage

The degradation kinetics of LVFX were followed as a function of catalyst dose, ranging from 0.2 to 1 g L⁻¹ (see Figure 7A). The determination of the pseudo-first-order apparent constant (kapp) was conducted in accordance with the methodology that had been previously established, and the resultant data were plotted against pegmatite doses (0.2 to 1 g L⁻¹) (see Figure 7B). As demonstrated in the preceding experiments, the apparent constant, as determined from the degradation kinetics, increased with the catalyst dose up to 0.75 g L⁻¹, at which point it then decreased. This occurrence can be attributed to the barrier formation that occurs when the pegmatite is highly loaded in an aqueous solution [26,29]. Moreover, the product radicals may be adsorbed by the metal oxide surface at a rate of k _oxide/OH• = 8 × 10¹¹ (g mL)⁻¹ S⁻¹ [30]. It is worth noting that the optimum loading pegmatite for LVFX removal is close to 0.75 g L⁻¹, with kapp = 0.0075 ± 0.0003 min⁻¹.

2.3.3. Effect of Levofloxacin Concentrations

The apparent constants (kapp) were evaluated by varying the initial concentrations of LVFX under the optimum conditions of pegmatite loading and oxidant concentrations, as shown in Figure 8. From the degradation kinetics curves in Figure 8A, the apparent rate constants were generated and plotted as a function of initial LVFX concentrations in Figure 8B. In fact, the apparent rate constant increased from kapp = 0.0031 ± 0.0003 to 0.0075 ± 0.0003 min⁻¹ and then decreased to 0.0022 min⁻¹ with increasing LVFX concentrations. This outcome may be ascribed to the competition between LVFX and H₂O₂ for interactions with a constant number of available sites on catalyst surfaces. As the LVFX concentration increases, the adsorption rate rises, leading to a decline in the number of pegmatite sites available for the generation of Fe^II during ultrasonication, as well as for interaction with H₂O₂. This finding is consistent with the conclusions of earlier research, which posited that the adsorption frequency of the catalyst, or the ultrasonication-assisted reduction of Fe^III to Fe^II, was considered to be the rate-limiting factor in the oxidation process [31]. In addition, higher concentrations of LVFX may reduce the number of photons entering the solution, thereby decreasing the efficiency of the ultrasound assisted reaction, as prior reported by [32].

2.4. Identification of Radical Species in the Degradation Process

Several radical species such as OH^•, HO₂^•/O₂^•−, and CO₃^•− can be formed during the heterogeneous ultrasound-assisted process [33]. To gain a better insight into the radical species implication in the degradation of LVFX, phenol (PhOH, k _OH•/PhOH = 6.0 × 10⁸ M⁻¹s⁻¹ [31], k _{CO3•−/PhOH} = 2.2 × 10⁷ M⁻¹s⁻¹ [32], and tert-butanol (t-BuOH, k _OH•/t-BuOH = 6.0 × 10⁸ M⁻¹s⁻¹ [33] were used as scavengers of OH^• and CO₃^•− in the SPC/US/Pegmatite system. BQ was used as a scavenger of the HO₂^•/O₂^•− radical, with a high constant rate towards the superoxide radical anions (k _O2•−/BQ = 9.6 × 10⁸ M⁻¹s⁻¹) and a fairly low rate towards the OH^• radical (k _OH•/BQ = 7.4 × 10⁶ M⁻¹s⁻¹) [34,35]. According to the pKa of HO₂^•/O₂^•−, which is 4.8, the hydroperoxide radical (HO₂^•) predominates in solution at pH below 4.8, whereas the superoxide radical predominates at pH above 4.8. In the presence of t-BuOH, only 38.7% of LVFX was degraded and 52% was inhibited, whereas 5.2% of LVFX was degraded and 85.5% was inhibited after the addition of PhOH (see Figure 9A), highlighting that 33.5% was due to CO₃°. The complete inhibition of LVFX degradation was observed after the addition of BQ + PhOH. Otherwise, the difference between PhOH and BQ + PhOH was 5.2% because 5% of the LVFX remained in suspension. Our study was carried out at pH = 7 ± 2, so the superoxide radical is important in the solution. This showed that OH^• and CO₃^•− were the primary reactive species for LVFX degradation using the SPC/US/Pegmatite system. Yu et al. [28] showed the presence of OH^• and CO₃^•− during the degradation of Dichlorvos. Since 52%, 33.5%, and 5.2% of the LVFX degradation were due to OH^•, CO₃^•−, and O₂^•−, respectively, the relative contributions of OH^•, CO₃^•−, and O₂^•− to the removal of LVFX (90.7%) were estimated to be 57.33%, 36.93%, and 5.74%, respectively (see Figure 9B).

2.5. Assessment of Pegmatite Catalytic Ability

The reusability and the catalytic capacity of the pegmatite were evaluated over five consecutive oxidation cycles, under the same optimum conditions at pH 7 ± 0.2, as shown in Figure 10. At the close of each oxidation cycle, the catalyst was effortlessly extracted from the reactor, thoroughly rinsed with ultrapure water, and then subjected to an overnight drying process at 50 °C prior to its reuse in subsequent experiments. As a result, the removal efficiency remained identical at the end of the fifth cycle (i.e., 95%), with the same mineralization (i.e., 57%), thereby underscoring the good catalytic capacity of the catalyst. The excellent stability of pegmatite during oxidation processes is due to two key factors. Firstly, it exhibits the minimal iron leaching upon the oxidation cycles. Secondly, it does not undergo the structural deformation of the catalyst [30,35]. As demonstrated in Figure 3, the X-ray diffractometer test conducted at the end of the fifth oxidation cycle revealed no discernible alterations in the structure when compared to the initial structure obtained prior to the oxidation processes.

3. Materials and Methods

3.1. Chemicals

All chemicals were not purified before use in this study due to their analytical grade. Levofloxacin (C₁₈H₂₀FN₃O₄, >99% purity) and sodium percarbonate (Na₂CO₃.1.5H₂O₂) were provided by Sigma-Aldrich (Saint-Quentin-Fallavier, France). Other chemicals used such as hydrochloric acid (HCl, 37% v/v), phenol (PhOH, C₆H₆O), tert-butanol (t-BuOH, C₄H₁₀O), and β-benzoquinone (BQ, C₆H₄O₂) were also purchased from Sigma-Aldrich (France). PubChem supplied the remaining chemicals such as sodium hydroxide (NaOH), acetonitrile (C₂H₃N), and formic acid (CH₂O₂). Finally, FerroVer^R iron reagent power pillows and LCK 380 kit were provided by Hach company (Loveland, CO, USA).

3.2. The Soil and Characterization Methods

The soil was sampled in the town of Aboisso (5°44′06″ N and 3°24′12″ W) in south-eastern Côte d’Ivoire. The sample was crushed and sieved at 50 mm. The fraction obtained was dried at 50 °C after being washed several times with ultrapure water (UPW). The physical and surface properties of the soil were determined using several instruments, such as X-ray fluorescence (XRF) instrument (Horiba, MESA-50, Kyoto, Japan), Scanning Electron Microscope coupled with Energy Dispersive X-ray (SEM-EDX) (Hirox, SH 4000 M, Tokyo, Japan), the X-Ray Diffraction instrument (GBC Emma, Hampshire, IL, USA), the specific surface area Brunauer–Emmett–Telle (BET) analyzer (V-Sorb 2800, Gold APP Instruments, Beijing, China), and the Zeta potential analyzer (Zeta Compact CAD, Naucelle, France). UPW with a resistivity of 18.2 MΩ cm, obtained from a Millipore Milli-Q system, was used to prepare standard solutions and synthetic wastewater.

3.3. Ultrasonic Degradation Tests and Analytical Methods

The reactor was constructed in a stainless-steel ultrasonic tank with a volume of 10 L. The dimensions of the tank are 15 cm high, 24 cm wide, and 30 cm long. The emitted power and frequency are 240 W and 40 kHz, respectively. The intensity produced was 15 mW cm², as indicated by a PMA2100 radiometer. All experiments (i.e., sorption and oxidation) were performed within a 250 mL capacity batch reactor at ambient temperature (28 ± 2 °C) and pH 7 ± 0.2. The solution containing the catalyst, oxidant, and pollutant was shaken continuously by ultrasound at a speed of 200 shakes per min. The pH of the mixture was measured using a PHS-38W microprocessor pH/mV/temperature meter. No significant variation in the temperature of the suspension was observed during the experiment. Two experiments were conducted at ambient temperature. In the initial experiment, the pollutant solutions (i.e., LVFX) and the catalyst (i.e., pegmatite) were stirred in the dark for 4 h to achieve adsorption saturation. In the subsequent experiment, the pollutants, the oxidants, and the catalyst were concurrently mixed under ultrasound. In both experiments, 250 mL of LVFX solution of an established concentration and a suitable dose of catalyst were added. Then, the required volume of oxidizing reagents (SPC) was added to the solution (Figure 11).

In order to ensure the stability of the pH of the solution, HCl and NaOH were used, with different concentrations (0.1; 0.01; 1 M). During the experiments, 3 mL of the solution was withdrawn every 30 min and analyzed.

LVFX concentrations were measured using direct-injection high-performance liquid chromatography (Waters 600 Controller, binary pump, Milford, MA, USA) combined with an autosampler (Waters 717 plus), a reverse-phase C18 column (250 mm × 4.6 mm i.d., 5 µm), and a UV detector (Waters 2489). The detector was settled at 292 nm. The experiments were performed with a binary mobile phase (i.e., acetonitrile/water (40/60, v/v)) containing 0.1% formic acid at a flow rate of 1 mL min⁻¹ in isocratic mode. The degradation efficiencies for different treatment times were determined according to the following equation [5]:

$$\mathrm{D}\mathrm{e}\mathrm{g}\mathrm{r}\mathrm{a}\mathrm{d}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n} \mathrm{e}\mathrm{f}\mathrm{f}\mathrm{i}\mathrm{c}\mathrm{i}\mathrm{e}\mathrm{n}\mathrm{c}\mathrm{y}(\mathrm{\%})={\displaystyle \frac{{\mathrm{C}}_0-{\mathrm{C}}_{\mathrm{t}}}{{\mathrm{C}}_0}}\times 100$$

(6)

where C₀ and C_t are the pollutant concentrations at the initial and a fixed-time interval t, respectively.

COD and amounts of Fe-dissolved iron leached residual during the reaction were measured using a DR 3900^TM spectrophotometer (Hach^R) using 10 mL of solution with pre-programmed method. The feasible leaching of some other heavy metals (i.e., Cr, Mn, Cu, Co, Ni, and Zn) from catalyst surfaces to the treated solution was also monitored using the XRF instrument. In order to ascertain the implications of CO₃^•−, OH^•, and HO₂^•/O₂^•− radicals in the elimination of LVFX, BQ, i-PrOH, and t-BuOH were used to carry out the scavenging test with 100 mM of each product. Indeed, the optimum concentration of scavengers for carrying out the above experiments was determined by varying their concentrations from 10 to 100 mM [8]. All these experiments were carried out in triplicate and exhibited good reproducibility, with a standard deviation of less than 3%.

4. Conclusions

This work has shown that pegmatite soils can catalyze the activation of SPC using ultrasound to produce mainly OH^• and CO₃^•− and effectively remove FQs such as LVFX. The initial characterization of the pegmatite showed the predominance of Fe (59.8 g kg⁻¹) and iron oxide (goethite). It is worth noting that the removal efficiency in the optimal conditions (i.e., [LVFX] = 60 µmol L⁻¹, [Pegmatite] = 0.75 g L⁻¹, and [SPC] = 60 µmol L⁻¹) was 95%, with 57% mineralization. On the other hand, the degradation efficiency was 90.7% after the desorption test. The scavenging test showed the involvement of OH^•, HO₂^•/O₂^•−, and CO₃^•− in the LVFX degradation, with the strongest involvement of OH^• (i.e., 57.33%) in the process. The pegmatite has been demonstrated to be reusable for various oxidation cycles without undergoing structural changes or the deactivation of its surface sites. However, it is important to note that water washing and drying processes are applied prior to reuse in a new oxidation cycle. This specific process of SPC activation has the potential to result in the creation of an effective yet cost-efficient water treatment approach that is capable of removing emerging compounds from contaminated systems.