Chiang Mai Journal of Science

Oxytetracycline Removal via Manganese Sulfate/Sodium Periodate Process: Influencing Factors, Mechanism, and Predicted Toxicity

1. INTRODUCTION

Antibiotics are commonly used pharmaceuticals for treating bacterial diseases in both humans and livestock farming. Currently, due to high usage and some misuse, antibiotic residues in the environment are steadily increasing. Surface water and groundwater environments, in particular those receiving urban sewage, industrial wastewater, and landfill leachate, often contain various antibiotics [1]. Oxytetracycline (OTC) is a commonly used veterinary antibiotic with significant usage volumes. When wastewater containing this substance is discharged either centrally or dispersed without treatment, it causes pollution in receiving water bodies and antimicrobial resistance in bacteria [2]. The treatment methods of antibiotics (including OTC) have been continuously reported, including biological treatment processes (e.g. moving bed biofilm reactor [3], aerobic granular sludge sequencing batch reactor [4]), hydrothermal treatment [5], heterogeneous photocatalysis [6], gas phase dielectric barrier discharge plasma [7], and adsorption [8]. However, conventional biological methods are relatively mild and exhibit poor performance in the degradation of antibiotics. Additionally, physical methods such as adsorption show good effectiveness for heavy metals in water treatment [9, 10], but their adsorption efficiency for antibiotics remains limited. Notably, methods utilizing reactive species generated in situ to degrade OTC are more favored, as the high oxidative activity results in a greater conversion rate of OTC into inorganic products. For example, using UV/H₂O₂ to generate hydroxyl radicals (•OH) and using UV/peroxymonosulfate to generate sulfate radicals (SO₄^•-) for the degradation of OTC are effective methods [11, 12].

Nowadays, activating periodate (PI, IO₄^-) to produce iodate radicals (IO₃^•) and other reactive species has emerged as an alternative approach. The degradation efficiency of PI on OTC depends on the performance of various activation methods. Methods reported for PI activation include cryoactivation [13], ultraviolet radiation [14], ultrasonic cavitation [15], transition metal activation [16], and carbon material activation [17]. The first three activation methods require energy input, with electricity being the most fundamental energy source, and they incur relatively high operating costs. Carbon material activation is a mild activation method that may involve non-radical pathways, offering significant application potential [18]. However, material preparation is relatively cumbersome and difficult to scale up for mass production. Transition metal activation methods offer fast reaction rates, high processing efficiency, and readily available materials, highlighting their advantages [19]. Transition metal catalysts used for PI activation include iron [20], manganese [21], cobalt [22], nickel [23], copper [24], and others. Among these, manganese catalysts are relatively inexpensive, non-toxic, and highly efficient, making them quite valuable. Studies utilizing heterogeneous catalysts such as manganese oxides (MnO₂, Mn₂O₃, Mn₃O₄, Mn_xO_y,: α-MnO₂, β-MnO₂, γ-MnO₂, ε-MnO₂, δ-MnO₂.) in PI activation have achieved favorable results in removing organic pollutants (sulfanilamide [25], bisphenol A (BPA) [26], sulfamethoxazole [27]) and elucidated key mechanisms (free radical oxidation and non-free radical oxidation). Moreover, studies have also been conducted on the degradation of various organic compounds by PI activated by homogeneous manganese in different valence states. For example, the Mn⁷⁺/PI process was employed to remove bisphenol F, methotrexate, and tetracycline [28]. Specifically, the Mn²⁺/PI process could remove almost 100% BPA within 120 min (Mn²⁺=0.04 mM, PI=0.2 mM, BPA=0.1 mM, pH=9.5) [29] and 99.8% sulfadiazine (SDZ) within 2 min (Mn²⁺=0.015 mM, PI=0.5 mM, SDZ=5.0 mg/L, pH=5.5) [30]. Yu et al. [31] also achieved nearly 100% SDZ removal within 2 min by the Mn²⁺/PI system under the conditions of Mn²⁺=0.02 mM, PI=0.1 mM, SDZ=0.02 mM, pH=6.5. The aforementioned studies have made significant contributions to the activation of PI by manganese-based catalysts and have also confirmed the high efficiency of manganese catalysts.

Manganese sulfate (MnSO₄) is a manganese salt that readily dissolves in water, releasing divalent manganese ions (Mn²⁺). It can activate PI to generate reactive species, making MnSO₄ environmentally friendly and potentially promising. Despite the existing body of work on homogeneous manganese-based catalysts (e.g., Mn²⁺, Mn⁷⁺), research on their application in PI activation is still fragmented and in its early stages. This study employs MnSO₄ (Mn²⁺) as a catalyst to activate PI for the degradation of OTC. Having established the effectiveness of the MnSO₄/PI process for OTC removal, an evaluation of various influencing factors was conducted, including operational parameters and water quality conditions. Subsequently, radical quenching experiments were performed to identify reactive species generated during the reaction. Finally, based on LC-MS results and T.E.S.T. analysis, the OTC removal mechanism, degradation pathways, and product toxicity were determined. While Mn²⁺-activated periodate systems have been reported for various organic contaminants [29–31], molecular-level identification of reactive sites and systematic toxicity evaluation of intermediates remain largely unexplored. OTC, a structurally complex tetracyclic antibiotic with multiple electron-rich functional groups, has not been studied in this system, and its degradation behavior cannot be extrapolated from simpler pollutants. This study thus contributes novel mechanistic insights through Fukui function/DFT-based site identification, pathway elucidation, and comprehensive toxicity assessment of OTC intermediates.

2. MATERIALS AND METHODS

2.1 Chemical Reagents
All chemical reagents used (Table S1) were analytical grade and experimental solutions were prepared using chemical reagents in quantities required for each experiment.

2.2 Experimental Procedure
The reaction vessel for removing OTC was a 1000 mL beaker. We weighed appropriate amounts of OTC and PI separately, then prepared pollutant and oxidant solutions of specific concentrations using deionized water. During the reaction, we first added manganese sulfate powder to the target pollutant solution and mixed thoroughly. Then we added the sodium periodate solution to initiate the reaction and started timing. We took samples at 2, 5, 10, 20, and 30 min to measure the residual OTC concentration in the water samples. We adjusted the solution pH using 0.1 M and 1 M NaOH and HCl. The experimental steps for the influencing factors test and the radical quenching test were consistent with this section, except that the dosing of each chemical reagent was controlled according to the experimental protocol during the addition phase.

OTC concentrations were determined by measuring the absorbance of the samples at 356 nm using a spectrophotometer (722N, Shanghai Youke Instrument Co., Ltd., Shanghai, China). Samples filtered through a 0.22 μm polyethersulfone membrane were placed in the spectrophotometer to detect their absorbance (Abs). The OTC concentration (mg/L) was calculated using the linear equation: Absorbance = 0.00616 + 0.03001 × OTC concentration, R² = 0.99988.

The intermediate products of OTC were characterized and detected using LC-MS (Thermo Scientific Ultimate 3000 UHPLC-Q Exactive system). Chromatography was performed on an Atlantis T3 column (100 mm × 3.0 mm, 3.0 μm) at a column temperature of 30 °C, with gradient elution using a mobile phase of 0.1% formic acid in water and acetonitrile. The mass spectrometer operated with a sheath gas flow rate of 40 arb, auxiliary gas flow rate of 10 arb, spray voltage of 3.8 kV for positive ions and 3.0 kV for negative ions, capillary temperature of 320 °C, and auxiliary gas temperature of 300 °C. First- and second-stage scanning was performed using Full MS/MS² top 10 scanning mode.

3. RESULTS AND DISCUSSION

3.1 Comparison of MnSO₄/PI and MnSO₄/PMS in Degrading OTC
Peroxymonosulfate (PMS) is a popular oxidant in advanced oxidation processes. The performance of MnSO₄/PI and MnSO₄/PMS systems was compared. As shown in Fig. 1, the MnSO₄/PI system degraded 88.4% of OTC within 10 min and 90.1% within 30 min. In contrast, the MnSO₄/PMS system degraded only 21.7% of OTC within 30 min, reaching 71.9% degradation at 120 min. Although the Mn²⁺/PMS system might be further optimized by adjusting the Mn²⁺: PMS ratio or pH, the comparison under identical conditions was intentionally designed to evaluate the intrinsic reactivity of the two oxidants toward OTC degradation. This approach allows a fair assessment of their relative performance without introducing condition-specific biases. MnSO₄/PI exhibits superior degradation performance for OTC and can achieve rapid degradation within a shorter timeframe. Neither PI alone nor MnSO₄ alone exhibited highly satisfactory degradation effects on OTC. Furthermore, the pseudo-first-order rate constants for PI alone, MnSO₄ alone, and the MnSO₄/PI system were determined to be 0.00256, 0.00244, and 0.25178 min⁻¹ (Figure S1), respectively, further confirming the superior performance of the MnSO₄/PI system for OTC degradation. Mn²⁺ activates PMS to generate Mn³⁺ and SO₄•⁻, which degrade OTC via the non-selective SO₄• radicals [32]. However, Mn²⁺ activation of PI (IO₄^-) may produce IO₃• or even higher-valent metal oxide species (Mn(IV)=O or Mn(V)=O), which are typical electrophiles capable of effectively attacking electron-rich groups in the OTC molecule (e.g., phenolic rings, enol groups, amino groups) [33,34].

The experiment results also revealed that higher concentrations of Mn²⁺ catalyst do not necessarily yield better results. As shown in Fig. 2(b), adding 0.1 mM MnSO₄ removed 77.3% of OTC within 30 min. Increasing the Mn²⁺ dosage from 0.3 mM to 0.9 mM maintained the OTC removal rate at approximately 83%, with no further change observed. From the kinetic plots (Figure S3), the pseudo-first-order rate constant at 0.1 mM MnSO₄ was found to be 0.20007 min⁻¹, whereas the rate constants at the remaining MnSO₄ concentrations exhibited no significant differences (0.38744-0.40407 min^-1). Excessive Mn²⁺ catalyst may cover the active reaction sites on the oxidant, hindering further increases in reaction rate. Furthermore, this may stem from a specific matching relationship between the amount of catalyst and the amount of oxidant, governed by stoichiometric reactions. When the oxidant quantity is fixed, more catalyst does not necessarily yield better results. If the oxidant is consumed too rapidly, insufficient oxidant remains to supply active species.

Next, the effect of varying PI concentrations on OTC degradation was examined at a fixed Mn²⁺ dosage, as shown in Figure 2(c). When the Mn²⁺ concentration was fixed at 0.3 mM, increasing the PI concentration from 0.1 mM to 0.5 mM raised the OTC removal rate from 32.3% to 82.3%. Further increasing the PI concentration to 0.7 mM and 0.9 mM increased the OTC removal rate to 87.9% and 89.6%, respectively, with a slow increase. This indicated that further increasing the PI concentration did not enhance the degradation rate. When the PI concentration was raised from 0.1 to 0.9 mM, the pseudo-first-order rate constant correspondingly increased from 0.00488 to 0.24079 min⁻¹ (Figure S4). The changes in reaction rates were in good agreement with the corresponding trends in OTC removal efficiency. Considering the relatively high cost of PI, a concentration of 0.5 mM was the optimal concentration for the reaction. Moreover, the corresponding Mn²⁺ concentration of 0.3 mM was most compatible with PI activation at this point.

Previous experiments employed deionized water as the base medium, incorporating pollutants, catalysts, and oxidants to initiate chemical reactions. In actual water sources, certain common anions may also influence these reactions. Therefore, the effects of chloride ions (Cl^-), nitrate ions (NO₃^-), bicarbonate ions (HCO₃^-), and carbonate ions (CO₃^2-) on OTC removal were investigated within the Mn²⁺/PI system. In Fig. 2(d), the black bars represent the control reaction without any added anions, where the corresponding OTC removal rate was 90.1%. Different concentrations of Cl^-, NO₃^-, HCO₃^-, and CO₃^2- all inhibited the reaction where Mn²⁺ activated PI to degrade OTC. Moreover, as the concentration of added anions increased, the negative effect of each anion on OTC degradation became more pronounced. Cl⁻ (20 mM), NO₃⁻ (20 mM), HCO₃⁻ (1 mM), and CO₃²⁻ (1 mM) reduced the OTC removal efficiency from 90.1% to 80.5%, 81.8%, 88.6%, and 86.6%, respectively, which remained within an acceptable range. The underlying reason may be that these anions undergo quenching reactions with the reactive species generated during advanced oxidation processes, causing the reactive species to disappear or transform into other less reactive species, as shown in Eqs. (1)-(5) [36-38].

\[ \mathrm{Cl^- + \bullet OH \rightarrow HOCl^{\bullet-}} \tag{1} \]
\[ \mathrm{NO_3^- + \bullet OH \rightarrow OH^- + NO_3^\bullet} \tag{2} \]
\[ \mathrm{HCO_3^- + \bullet OH \rightarrow CO_3^{\bullet-} + H_2O} \tag{3} \]
\[ \mathrm{HCO_3^- + IO_3^- \rightarrow CO_3^{\bullet-} + IO_3^- + H^+} \tag{4} \]
\[ \mathrm{CO_3^- + CO_3^{\bullet-} \rightarrow CO_2 + CO_4^{2-}} \tag{5} \]

In addition to various anions, natural water also contains natural organic matter, with humic acid and fulvic acid constituting the majority. This experiment employed humic acid to simulate the presence of natural organic matter in water, thereby investigating the effect of humic acid (HA) dosage on the degradation of OTC within the Mn²⁺/PI system. The water quality characteristics of the river water employed in this study are summarized in Table S2. As shown in Fig. 2(e), the addition of HA had a slight effect on OTC degradation. The reaction without HA addition removed 90.1% of OTC, while 20 mg/L HA reduced the OTC degradation rate to 86.3%. This indicated that natural organic matter did not significantly inhibit the degradation of OTC in the Mn²⁺/PI system. Considering that deionized water was still used for solution preparation at this stage, it was necessary to further investigate catalytic reactions in actual aqueous media. Therefore, OTC solutions were prepared using tap water and river water as media for catalytic reactions, and the results were compared with those obtained in deionized water media. The specific experimental results are shown in Fig.2(f). At 30 min of reaction time, the OTC removal rates in the three water matrices of deionized (DI) water, tap water and river water were 90.1%, 79.8%, and 67.5%, respectively. Compared to DI water, the OTC removal rate in river water decreased by 22.6%. This process can be considered for use in source water pretreatment or advanced wastewater treatment.

It should be noted that measurements of TOC, DOC, COD, or carbon balance were not conducted in this study. Although LC-MS analysis confirmed the structural transformation of OTC into intermediates with progressively lower molecular weights (Fig. 5), and T.E.S.T. analysis indicated reduced toxicity of the final products (Fig. 6), these findings do not provide direct quantitative evidence of complete mineralization to CO₂ and H₂O. Therefore, the observed OTC removal in the Mn²⁺/PI system represents structural degradation and transformation rather than confirmed mineralization. Future studies incorporating TOC analysis and carbon mass balance calculations are necessary to quantify the extent of mineralization. Furthermore, the toxicity predictions are based solely on in silico modeling using T.E.S.T. and have not been experimentally confirmed. Bioassays (e.g., algal growth inhibition or bacterial luminescence tests) are needed to verify the actual toxicity of the degradation products in future studies.

\[ \mathrm{Mn^{2+} + IO_4^- + 2H^+ \rightarrow Mn^{3+} + IO_3^- + H_2O} \tag{6} \]
\[ \mathrm{Mn^{3+} + e^- \rightarrow Mn^{2+}} \tag{7} \]
\[ \mathrm{3IO_4^- + 2OH^- \rightarrow 2O_2^{\bullet-} + 3IO_3^- + H_2O} \tag{8} \]
\[ \mathrm{IO_4^- + 2O_2^{\bullet-} + H_2O \rightarrow IO_3^- + 2OH^- + 2\,{}^1O_2} \tag{9} \]
\[ \mathrm{IO_4^- + H_2O \rightarrow IO_3^- + 2\bullet OH} \tag{10} \]

In addition, all degradation experiments were performed at an initial OTC concentration of 40 mg/L, which was selected to facilitate UV–Vis quantification and to maintain comparability with previous AOP studies. However, this concentration is substantially higher than typical environmental residues of OTC (generally in the µg/L range). Whether the Mn²⁺/PI system remains equally effective at such environmentally relevant concentrations requires further investigation using more sensitive analytical techniques such as HPLC–MS/MS. Therefore, the environmental applicability of the system at realistic concentration levels should be interpreted with caution.

The optimal catalyst dosage and oxidant consumption determined herein (e.g., 0.3 mM Mn²⁺ and 0.5 mM PI) provide a useful reference for reagent dosing in practice. Based on the optimal reagent dosages (0.3 mM Mn²⁺ and 0.5 mM PI) and current industrial prices (ca. USD 0.6/kg for MnSO₄·H₂O and ca. USD 100/kg for periodate), the estimated chemical cost is approximately USD 11.5 per ton of water. While this is economically feasible for emergency remediation, further cost-benefit evaluation is needed for long-term operation.

In the Mn²⁺/PI system, the activation of periodate by Mn²⁺ is generally considered to proceed via a redox cycle: Mn²⁺ is initially oxidized by periodate to Mn³⁺ or Mn⁴⁺, which subsequently decompose to generate reactive species and are reduced back to Mn²⁺ [30]. Based on this literature-reported mechanism, the dominant manganese species in the final effluent is expected to be Mn²⁺. However, the initial Mn²⁺ dosage (0.1–0.5 mM, corresponding to 5.5–27.5 mg/L) exceeds the WHO drinking water guideline for manganese (0.1 mg/L). Thus, a post-treatment step would be required to remove residual manganese before discharge or reuse. Common approaches include alkaline precipitation (e.g., adjusting pH to 10–11 to precipitate Mn(OH)₂) or adsorption onto MnO₂-coated sand or zeolite.

Periodate (IO₄⁻) is expected to be consumed during the reaction and partially converted to iodate (IO₃⁻) as a reduction product. The potential accumulation of IO₃⁻ and other iodine-containing species derived from periodate conversion warrants consideration, particularly for applications involving treated water reuse or discharge. In the present study, the iodide concentration in the tested waters was low; however, the formation of IO₃⁻ from periodate itself is independent of initial iodide levels. Therefore, the possible accumulation of iodine-containing transformation products should not be overlooked. Future studies should monitor iodine species (e.g., IO₃⁻, I⁻, and organic iodine compounds) in the treated effluent to fully assess the potential environmental risks associated with the Mn²⁺/PI process.

Although the present study employed a batch reactor, continuous operation could be achieved by designing a plug-flow or packed-bed reactor with immobilized Mn catalyst. These aspects are critical for advancing the Mn²⁺/PI process toward practical water treatment and merit systematic investigation in subsequent work.

4. CONCLUSIONS

PI-AOPs are emerging methods that offer significant advantages in the environmentally friendly treatment of recalcitrant organic compounds. Manganese, as an activator for PI, is not only readily available and cost-effective but also highly efficient. In this study, sodium periodate was activated using manganese sulfate to degrade the commonly used veterinary antibiotic OTC in water, and the main findings are as follows:

(1) The Mn²⁺/PI process effectively removes OTC from water. It can remove about 90% of OTC within 30 min (with a pseudo-first-order kinetic rate constant of 0.25178 min⁻¹) under the conditions of 0.3 mM Mn²⁺, 0.5 mM PI, and pH 5–9. This also demonstrates the wide pH tolerance of the Mn²⁺/PI process.

(2) The impact of Cl^-, NO₃^-, HCO₃^-, CO₃^2-, and humic acid on OTC removal rates is within an acceptable range. Cl^-, NO₃^-, HCO₃^-, CO₃^2-, and humic acid reduce the OTC degradation rate from 90.1% to 80.5%, 81.8%, 88.6%, 86.6%, and 86.3%, respectively. This indicates that the process is highly adaptable to these anions and HA.

(3) Different water matrices have a significant impact on the removal efficiency of OTC. Compared to DI water, the OTC removal rate in river water and tap water decreases by 22.6% and 10.3%, respectively. Therefore, the Mn²⁺/PI process may be considered for use in combination with other methods, such as for the pretreatment of raw water or the advanced treatment of wastewater.

(4) The Mn²⁺/PI process for OTC removal is suggested to involve multiple reactive species. The scavenger experiments suggest that IO₃• may make an important contribution to OTC degradation, with possible additional roles played by ¹O₂, •O₂⁻, and •OH. These species may attack the aromatic ring and adjacent carbon atoms (e.g., C1, C6, C9, C10, C11), causing OTC to undergo ring-opening and chain-breaking. Although several intermediates were predicted to be more toxic than OTC for some endpoints, the later-stage products P10 and P11 generally showed lower predicted acute toxicity.

In summary, MnSO₄/PI process is effective at removing OTC from water and demonstrates strong applicability in the pretreatment of raw water and the advanced treatment of wastewater.

AUTHOR CONTRIBUTIONS

Hongyan Wei: Conceptualization, Funding acquisition, Writing-review & editing. Tianbing Xu:Methodology, Formal analysis, Resources. Nuowen Hu: Project administration, Visualization. Zongkai Han: Data curation, Investigation, Software. Tiehong Song: Conceptualization, Writing-review & editing, Visualization.

CONFLICT OF INTEREST STATEMENT

The authors declare that they hold no competing interests.

FUNDING

This project is funded by the Scientific Research and Cultivation Projects of Changchun University of Architecture Civil Engineering of China: Contract number CJKJ202610.

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