Stock solutions of ofloxacin (OFX) and norfloxacin (NFX), at a concentration of 300mg·L⁻¹, were prepared from high-purity standards (≥98%, Sigma Aldrich). The pH of the experimental solutions was adjusted using sodiumhydroxidepellets (NaOH, 97%) and hydrochloricacid (HCl, 37%), also supplied by Sigma Aldrich (Taufkirchen, Germany). All solutions and dilutions were prepared with ultrapurewater produced using a Milli-Q purification system. The main physicochemical properties of ofloxacin and norfloxacin are summarized in Table 1.

Table 1.Chemical properties of ofloxacin and norfloxacin.

Log Kow: octanol-water partition coefficient, Koc: organic carbon-water partition coefficient.

A 3.5 mL quartz rectangular cuvette was used for sample analysis with a Cary Eclipse VARIAN spectrofluorimeter connected to a microcomputer equipped with the Cary Scan software. The system covers a wavelength range from 200 to 900 nm, allowing the recording of excitation and emission spectra. Buffer solutions were prepared using a Consort C60010R pH meter (Belgium).

The reagents were weighed using a precision balance with an accuracy of 10⁻³ g. For UV-C irradiation, three 15 W germicidal lamps producing UV-C radiation were mounted in a triangular configuration inside the chamber. A Teflon coil was added to the system, allowing sufficient UV-C transmission to generate photoproducts more efficiently in flow conditions compared to the use of standard tubes.

Natural water samples were collected from the Senegal River in the Saint-Louis region using 1.5 L glass bottles according to standard procedures. The samples were filtered through a 25 mm PTFE syringe filter with a 0.2 μm pore size (Sigma-Aldrich) to remove suspended particulates.

Photodegradationexperiments were conducted using untreatedriverwater, leaving dissolved organic matter (DOM) intact to preserve natural conditions and account for its potential role in photosensitization.

Analyticalpreconcentrationforfluorescencemeasurements was performed separately using solid-phase extraction (SPE) with reverse-phase C18 cartridges. The cartridges were conditioned with 3 mL of methanol followed by 3 mL of ultrapure water. Then, 50 mL of the natural water samples were passed through the cartridges to concentrate the analytes and reduce residual organic contamination prior to fluorescence analysis.

This approach ensures that photodegradationkineticsreflectnaturalwatercomposition, while fluorescencemeasurementsareoptimizedforsensitivityandaccuracy.

The fluorescence excitation spectra of both antibiotics in water exhibit threedistinctexcitationbands (Figure 1). Both molecules share the first and third excitation bands, appearing at 230nm and 326nm, respectively. However, the second, most intense excitation band occurs at 287nmforOFX and 271nmforNFX. The emission spectra show a singleemissionband at 461nmforOFX and 415nmforNFX (Figure 1).

Lifetimes of the two antibiotics were measured in water at a pH of 4. The emission wavelength of 461 nm was used to measure the lifetime of OFX at a concentration of 0.72 μg∙mL⁻¹, and the lifetime of NFX was measured at 415 nm at a concentration of 0.8 μg∙mL⁻¹. Fluorescence lifetime (τ_F) measurements (Figure 2) give very different results. OFX has a long fluorescence lifetime of 7.08 ns, while NFX has a short one of 2.04 ns only, indicating that τ_F (OFX) = 3.5τ_F (NFX). The observed difference can be ascribed to the increased molecular rigidity imparted by the oxazine ring in ofloxacin, which bridges the N-1 and C-8 positions. This fused ring system constrains internal degrees of freedom and suppresses non-radiative deactivation pathways such as vibrational and torsional motions, thereby enhancing photophysical stability. In contrast, norfloxacin, which lacks this structural feature, exhibits greater conformational flexibility and a more dynamically accessible π-electron system. As a result, norfloxacin readily populates a triplet excited state with a lifetime on the order of ~1 μs, a process associated with photosensitization and subsequent photodegradation. Ofloxacin, on the other hand, displays a markedly low photodecomposition quantum yield (φ ≈ 0.001), indicative of a highly inefficient intersystem crossing from the singlet to the triplet manifold [13].

Figure 1. Excitation and emission spectra of OFX (—) and NFX (red curve) at a concentration of 2.5 μg∙mL⁻¹ in water river by FLUO method.

Figure 2. Lifetime measurements measured at a concentration of 2.5 μg∙mL⁻¹ in water pH = 4: OFX (+ experimental value; — fit) and NFX (x experimental value; red curve: fit).

Recent studies have shown that the fluorescence signal of organic compounds is highly sensitive to the pH of the surrounding medium [14][15]. Therefore, after preparing a solution containing OFX or NFX, a precise amount of acid or base was added to reach the desired pH using a pH meter. The resulting data were plotted (Figure 3) to generate a curve representing fluorescence intensity as a function of pH. For OFX, fluorescence intensity increaseswithpH, reaching a maximum at pH4, then gradually decreases up to pH10, and remains constant between pH10and13. In contrast, the fluorescence signal of NFX risesbetweenpH1and2, then remains nearly constant between pH2and5, with a slightly higher signal at pH4. Once the pH exceeds 5, the fluorescence signal progressively decreases until it becomes undetectable.

Figure 3. Evolution of the fluorescence intensity of OFX (■) and NFX (● (red )). Antibiotic concentration (2.5 μg∙mL⁻¹).

A comparative study of the photolysis of ofloxacin (OFX) and norfloxacin (NFX) was conducted to evaluate the influence of pH and the aqueous medium on the degradation of these fluoroquinolones under UV-C irradiation. Dilute solutions of both compounds were prepared and placed in UV-transparent quartz cuvettes at a fixed distance from the light source to ensure uniform exposure. Experimental conditions were maintained constant, with an ambient temperature of 25˚C and irradiation times adapted for each molecule. Degradation was monitored by spectrofluorimetry at their respective emission maxima: λ_em=461nmforOFX and λ_em=415nmforNFX (Figure 4(a) and Figure 4(b)). The percentage of photodegradation was calculated using the following equation: I 0 − I I × 100 , where I₀ is the initial fluorescence intensity and I is the intensity measured after 15minutesforOFX and 30minutesforNFX. During irradiation, the fluorescence intensity of both molecules decreasesgradually, indicating their transformation into photoproducts. After 15 minutes for OFX and 30 minutes for NFX, the observed photodegradation percentages were: OFX: 98% in river water, 41% at pH 4, 84% at pH 7, and 84% at pH 10; NFX: 94% in river water, 49% at pH 4, and 73% at pH 7. Photodegradation was particularly pronounced in natural river water, where dissolvedorganicmatter (DOM) andmetalionsactasphotosensitizers, generatingreactiveoxygenspecies (ROS) suchashydroxylradicals, singletoxygen, andorganicradicals, whichacceleratephotolysis. Under acidic conditions, molecules are predominantly protonated, reducing UV absorption and reactivity, which explains the low degradation percentages [16]. At neutral pH, fluoroquinolones exist mainly in zwitterionicforms, which favor high quantum yields and optimal UV absorption, thereby enhancing photolysis [17]. In basic media, OFX remains highly degraded (84%), suggesting that additional factors, such as radical formation at high pH, contribute to sustained photoreactivity [18]. These observations indicate that the photodegradation of fluoroquinolones strongly depends on molecularspeciation and the compositionofthemedium. Zwitterionic forms are the most reactive, whereas cationic and anionic forms exhibit relative stability. Photolysis is particularly efficient in natural water, confirming the role of organic photosensitizers in accelerating the photochemical degradation of OFX and NFX [14].

Figure 4. Evolution of the excitation and emission spectra of OFX and NFX in Senegal River water.

The analysis of the fluorescence intensity evolution curves as a function of irradiation time (Figure 5(a) and Figure 5(b)) allowed the determination of apparent rate constants and the assessment of pH effects on the photolytic stability of ofloxacin and norfloxacin. The photodegradation kinetic parameters of OFX and NFX in pure water and at different pH values are presented in Table 2.

(a)

(b)

Figure 5. Photodegradation kinetics of OFX (36.14 ng∙mL⁻¹) and NFX (31.94 ng∙mL⁻¹) in river water.

Table 2. Photodegradation kinetic parameters of OFX and NFX in river water and at different pH values.

^a Order = reaction kinetic order. ^br² = kinetic equation correlation coefficient. ^c t_1/2 = photodegradation reaction half-life time (min). ^dk = photodegradation rate constant (min^–1) and absolute error (±). ^e Conductivity of the water samples. ^f Photodegradation percentage.

The photodegradation kinetics of ofloxacin (OFX) were studied in river water and in buffered media at different pH values under UV-C irradiation. The kinetic parameters obtained (Table 2) show a significant variation in the degradation rate and photochemical behaviour of OFX depending on the pH of the medium. In river water and at neutral pH (pH = 7), degradation follows first-orderkinetics (r² ≈ 0.99), indicating that the photodegradation rate depends directly on the antibiotic concentration. This behaviour is consistent with previous reports [19], which showed that fluoroquinolones undergo first-order degradation under UV irradiation, reflecting a photoinduced process governed by direct absorption of radiation. In contrast, at acidic (pH = 4) and basic (pH = 10) conditions, the reaction follows zero-orderkinetics, suggesting that the rate becomes independent of concentration. A similar behavior has been reported for several fluoroquinolones under irradiation, particularly when the absorbance of the photoactive species reaches a saturation threshold that renders the reaction rate independent of the initial concentration. In such cases, zero-order kinetics typically arise from the saturation of specific chromophoric groups that absorb UV photons, from light-screening effects occurring under strongly acidic or basic conditions, or from the formation of protonated or deprotonated species that exhibit strong and constant absorbance in the irradiation range (Figure 6) [20]. In the present study, this mechanism provides a consistent explanation for the zero-order behavior observed for OFX at pH 4 and pH 10. At these pH values, OFX exists predominantly in its protonated form (pH 4) or deprotonated form (pH 10), both showing enhanced molar absorptivity in the UV-C region. The resulting saturation of photon absorption leads to constant photolysis rates during irradiation, thereby producing the observed zero-order kinetics.

The rateconstant (k) and half-life (t_1/2) confirm the pronounced effect of pH on molecular stability. In pure water (k = 0.247 min⁻¹, t_1/2 = 3 min) and at neutral pH (k = 0.116 min⁻¹, t_1/2 = 6 min), degradation is relatively fast, indicating efficient UV-C absorption. Under acidic conditions, the reaction is significantly slowed (k = 1.43 × 10⁻⁷ mol∙min⁻¹∙L⁻¹, t_1/2 = 35 min), reflecting greater chemical stability of protonated OFX. At low pH, protonation of the piperazinyl group decreases electron density on the aromatic ring, reducing photon absorption [21]. Conversely, under basic conditions (pH = 10), degradation is markedly accelerated (t_1/2 = 2.2 min), reflecting increased photo-instability. Deprotonation of carboxylic and ketonic groups favors the formation of photoexcited reactive species (•OH, O₂•⁻) capable of breaking C-N and C-F bonds within the molecule. This observation aligns with previous studies reporting rapid degradation of fluoroquinolones under alkaline conditions (Figure 6) [22]. High correlation values (0.95 ≤ r² ≤ 0.99) confirm the validity of the kinetic models applied and the reliability of the experimental data. Overall, the photolysis of OFX is strongly pH-dependent: the molecule is more stable in acidic media and more labile in basic media. These results highlight the importance of environmental pH in the photochemical fate of fluoroquinolones, a key parameter for evaluating their persistence and ecological impact in aquatic environments [23].

Figure 6. Protonation and deprotonation of fluoroquinolones.

The results show that the photodegradation of NFX follows first-orderkinetics (r² ≥ 0.992) under all experimental conditions, which is consistent with observations for other fluoroquinolones such as ciprofloxacin and ofloxacin [24]. This kinetics indicates that the degradation rate depends directly on the concentration of NOR in the reaction medium. The lowest rate constant (k = 0.045 min⁻¹) and the longest half-life (t_1/2 = 15 min) were observed at pH = 4, indicating greater stability of NFX under acidic conditions. This stabilization can be attributed to the protonation of basic functional groups, particularly those in the piperazinyl ring, which reduces the likelihood of forming reactive excited states and thus slows photodegradation [25]. Conversely, under neutral conditions (pH = 7) and in unbuffered water, NOR exhibits similar rate constants (k ≈ 0.087 - 0.088 min⁻¹) and half-lives of 8 minutes. These values indicate that NFX degrades more rapidly at this pH, likely due to the presence of zwitterionic or partially deprotonated forms that favors UV photon absorption and formation of photoactive species [26]. This pH-dependent behaviour has also been reported for other fluoroquinolones, such as levofloxacin and ciprofloxacin, whose photodegradation rates increase as pH approaches neutrality [17]. This suggests that the ionicformofthemolecule plays a key role in photochemical reactivity, influencing electron distribution and UV absorption efficiency [27]. These observations are consistent with the fact that fluoroquinolones exhibit their highest photoreactivity in the zwitterionic state. This protonation form combines opposite charges within the same molecule, generating strong internal electric fields that favor intramolecular charge-transfer transitions, generally more prone to photodegradation. The zwitterion also displays a higher dipole moment and a more planar conformation of the quinolone chromophore compared with the fully cationic or anionic species, both of which tend to experience electrostatic distortions. Such structural and electronic features enhance the efficiency of photon absorption and increase access to reactive excited states, thereby promoting the formation of photolytic intermediates. This provides a coherent physicochemical explanation for the enhanced photoreactivity observed near neutral pH, where the zwitterionic form predominates. These results show that NFX is more persistent in acidic media (pH < 7) and less stable in neutral waters, which are typical of surface waters. Thus, in the environment, its fate will strongly depend on local pH: faster degradation is expected in neutral to slightly basic waters, while greater accumulation could occur in more acidic environments [28]. Understanding this behavior is essential to predict the persistence and ecotoxicological potential of NFX in aquatic environments exposed to sunlight. Further studies, including the identification of photoproducts and the role of reactive oxygen species, would help clarify the underlying reaction mechanisms.

Although this study employed UV-C irradiation, which possesses higher photon energy than environmentally relevant solar UV-A and UV-B radiation, the results provide valuable insights into the relative photostability and pH-dependent photodegradation mechanisms of fluoroquinolones. UV-C irradiation allows for controlled and accelerated experiments that enhance the detection of subtle kinetic differences between ofloxacin (OFX) and norfloxacin (NFX) and facilitates mechanistic understanding of the roles of zwitterionic, cationic, and anionic species. While the absolute degradation rates under natural sunlight may be slower, the trends observed under UV-C including the influence of pH, the contribution of dissolved organic matter, and the relative susceptibility of different molecular forms remain informative for predicting environmental fate and designing water treatment strategies.

The results obtained for the photolysis of ofloxacin (OFX) and norfloxacin (NFX) in Senegal River water at different pH values show overall first-orderkinetics, with apparent rate constants ranging from 0.045 to 0.247 min⁻¹ (Table 2). Half-lives vary from 2 to 35 minutes depending on the pH and the compound, confirming rapid photodegradation under UV-C irradiation. These values are consistent with trends reported in recent literature for fluoroquinolones exposed to UV-C or UV-A radiation [29]. Under neutral or basic conditions, OFX exhibits significantly faster degradation (t_1/2≈3mininpurewater; 2.2minatpH10) than NFX (t_1/2≈8mininpurewater; 15minatpH4). This difference can be attributed to the chemical structure of OFX, whose piperazinyl group and carboxylic function promote stronger absorption at 254 nm and higher reactivity of anionic forms [30]. These observations are in agreement Wammer et al. [29] and Liu et al. [31], who showed that OFX photoreactivity strongly depends on its ionic form and the pH of the medium. The rate constant obtained in this study for OFX in pure water (k = 0.247 min⁻¹) is higher than those reported in previous studies. For example, Sebastian Snowberger et al. reported slower photolysis kinetics of fluoroquinolones under UV-C irradiation [32], while studies on ciprofloxacin degradation under UV-based processes also indicate lower apparent rate constants depending on experimental conditions [33]. These variations are likely due to differences in irradiation wavelength, photon flux, and solution composition, which are known to strongly influence photodegradation kinetics. This acceleration can be explained by the higher light intensity and the absence of interfering compounds in the experimental medium used. For NFX, the measured value in our study (k=0.088min⁻¹; t_1/2=8min) is of the same order of magnitude as that reported by Iqbal etal. [34], who found k=0.042min⁻¹ (t_1/2=16.4min) under UV alone. Differences may be attributed to lamp power, initial concentration, and water composition. Other recent studies, including Bobu et al. [35] and Cuarda-Correa et al. [36], also confirm that NFX exhibits slightly higher photostability than OFX, except in the presence of catalysts or oxidants such as H₂O₂. Zhan et al. [37] reviewed the effects of pH, ionic speciation, and experimental parameters on fluoroquinolone photolysis: the neutral or anionic form is the most reactive, whereas the cationic form (dominant at pH < 6) exhibits higher photostability. This trend is fully confirmed by our experimental results, where OFX degradation drastically decreases under acidic conditions (t_1/2=35minatpH4). Recent work by Niu and Busetti [38] and Zhang et al. [39] highlighted the role of singlet oxygen (1O₂) and dissolved organic matter (DOM) in indirect photolysis. These parameters can strongly influence observed kinetics in natural waters, making direct photolysis, as studied here, a useful reference for evaluating the fundamental behavior of fluoroquinolones in more complex systems. Overall, our results fit coherently within the range of kinetic constants reported in recent literature while providing additional insight into the effect of pH and a direct comparison between OFX and NFX in river water. The combined analysis of photolysis kinetic parameters and fluorescence lifetimes (τ_F) reveals a direct relationship between photochemicalstability and the dynamicsofexcitedstates of fluoroquinolones. OFX exhibits slower degradation and a longer fluorescence lifetime (τ_F=7.08ns) than NFX (τ_F=2.04ns), indicating a lower propensity for nonradiative decay and transition to the triplet state. This correlation between τ_F and kinetic constants (k, t_1/2) suggests that fluorescencelifetimeisarelevantphotophysicalindicator for predicting the photostability of fluoroquinolones in aquatic environments. It provides a complementary approach linking fundamental spectroscopic properties to the environmental reactivity of the studied compounds.