# Ligand-limited oxidation of ciprofloxacin by Mn(III)

Tra My Bui Thi, Tao Chen, Tao Luo, Yann R. Leroux, Khalil Hanna,
Jean-François Boily

► To cite this version:

Tra My Bui Thi, Tao Chen, Tao Luo, Yann R. Leroux, Khalil Hanna, et al. Ligand-limited oxidation of ciprofloxacin by Mn(III). Journal of Hazardous Materials, 2025, pp.138373-138373. 10.1016/j.jhazmat.2025.138373. hal-05093436

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Distributed under a Creative Commons Attribution 4.0 International License
eResearch Paper

Ligand-limited oxidation of ciprofloxacin by Mn(III)

Tra My Bui Thi<sup>a,1</sup>, Tao Chen<sup>a,1</sup> ID, Tao Luo<sup>b</sup> ID, Yann Leroux<sup>c</sup> ID, Khalil Hanna<sup>a</sup>,
Jean-François Boily<sup>b,*</sup>

HIGHLIGHTS

* Mn(III) species influence the environmental fate of antibiotics.

* Mn(III) complexation with pyrophosphate inhibits ciprofloxacin degradation.

* Mn(III)-PP complexes dissociate into aqueous Mn(III) species and form MnO₂ colloids.

* pH altered the redox potential of the Mn (III)/Mn(II) couple, affecting CIP degradation.

GRAPHICAL ABSTRACT

<figure><img src="image_2.png" /></figure>

ARTICLE INFO

Keywords:

Manganese

Ciprofloxacin

Oxidation

Aqueous speciation

Spectroscopy

A B S T R A C T

Mn(III) species play critical roles in determining the environmental fate of antibiotics released into natural systems. However, their reactivity is strongly influenced by complexation reactions with (in)organic ligands. This study investigates the impact of Mn(III) complexation with pyrophosphate (PP), a model environmental ligand, on the redox-driven degradation of ciprofloxacin (CIP), a widely used antibiotic and environmental contaminant. Spectroscopic analysis and thermodynamic modeling revealed that Mn(III)-PP complexes initially dissociate into MnOH²⁺ species, which can then disproportionate and form MnO₂ colloids. Both dissociation and disproportionation reactions had comparable trends at pH 4 and 7, with reactivities that were strongly dependent on Mn (III):PP ratios. The progress of CIP oxidation following direct coordination with Mn compounds over time was sigmoidal, with an initial lag phase attributed to Mn(III)-PP complex dissociation and disproportionation. CIP degradation was predominantly governed by pH, with maximal rate constants decreasing from k = 0.390 h⁻<fnref n="1" /> at pH 3 to k = 0.065 h⁻<fnref n="1" /> at pH 5, and no CIP removal under circumneutral to alkaline conditions. Cyclic voltammetry also confirmed that the strongly pH-dependent redox potential of the Mn(III)/Mn(II) couple aligned with facile CIP oxidation under acidic conditions. These collective findings indicated that ligand complexation, such as with PP, enhanced Mn(III) stability and mitigated dissociation and disproportionation reactions. The new

<footnote><sup>a</sup> Univ. Rennes, Ecole Nationale Supérieure de Chimie de Rennes, CNRS, ISCR-UMR 6226, Rennes F-35000, France</footnote>

<footnote><sup>b</sup> Department of Chemistry, Umeå University, Umeå SE-901 87, Sweden</footnote>

<footnote><sup>c</sup> Univ. Rennes, CNRS, ISCR-UMR 6226, Rennes F-35000, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: jean-francois.boily@umu.se (J.-F. Boily).</footnote>

<footnote n="1">¹ These authors contribute equally to this work and are designated as co-first authors.</footnote>
insight provided by this work on the speciation and redox activity of Mn(III) should thereby be considered for understanding ciprofloxacin degradation in contaminated water systems.

# 1. Introduction

Mn(III) is an abundant and highly reactive component in the natural pool of manganese. It serves both as an electron donor and acceptor [1, 2], playing an important role in various biogeochemical and water treatment processes [3–6]. Mn(III) appears to make up the majority of the (µM-level) dissolved pool of Mn in suboxic water columns of the Black [1] and Baltic [7] Seas. In England, soluble Mn(III) was also produced in a water treatment plant by reacting natural organic matter (NOM) with Mn oxides [4]. Although seemingly overlooked in the past decades, the complexation of soluble Mn(III) to (in)organic ligands could affect manganese redox cycling in natural waters [1,7–10]. For instance, Kostka et al. [11] highlighted the possibility that complexation with pyrophosphate (PP) affects the ability of Mn(III) to act as a terminal reactant in aquatic environments. Variations in Mn(III)-ligand (L) speciation could also play important roles in the fate of redox-active organic pollutants of great concern to society, such as antibiotics.

Mn(III)-L complexes are well-known intermediates in the degradation of organic contaminants. Reactions can occur through permanganate reduction with bisulfite [12,13], abiotic and biotic Mn(II) oxidation [14–18], and through Mn (IV) minerals [16,18] exposed to complexing ligands. For instance, Gao et al. [13] found that Mn(III)-L generation by Mn(VII)/Mn(II) reactions in the presence of PP or humic acid accelerated triclosan oxidation rates. Chen et al. [16] also reported that complexation with Mn(III)-oxalate complexes affected carbadox stability. Additionally, work on UO₂ dissolution by Mn(III) suggested that coordination structure impacted the ability to control binding capacity and heterogeneous electron transfer [19].

Present in naturally occurring ligands (e.g., NOM [4,5], biogenic siderophores [14,20], carboxylate [21]), phosphonate groups can act as an important stabilizer for soluble Mn(III) [21–23] in environmental and industrial settings (e.g., agricultural areas, domestic sources, and industrial facilities) [24–26]. Previous studies revealed that aqueous Mn (III)-PP complexes, which are stabilized by direct Mn-phosphate interactions, could greatly enhance the oxidative reactivity of MnO₂ [13, 18,27], yet the same species inhibited phenol oxidation [28]. Although ligand exchange should be the chief mechanism for organic pollutant oxidation by Mn(III)-L complexes [12–14,29–31], uncertainties remain over Mn(III) coordination structures, speciation, and redox activities.

In this study, we evaluated the ability of PP to regulate the structure and redox behaviors of Mn(III) complexes towards the degradation of ciprofloxacin (CIP). The interest for the fate of this important fluoroquinolone antibiotic has spiked in recent years, given its release to the environment [32,33] from activities in the pharmaceutical and livestock sectors [34–37]. The concern becomes even greater considering that metal-driven oxidation of CIP produces other compounds [38–40] of yet unknown antibacterial properties. To address these concerns, we developed a kinetic model describing the autocatalytic nature of the Mn (III)-driven oxidation of CIP, and explained how Mn(III)-PP complexes inhibit degradation. This study contributes to the growing body of literature on the reactivity of dissolved Mn(III) towards organic pollutants [6,41,42], while at the same time adding new insight needed to reframe our understanding of Mn redox cycling in nature.

# 2. Methods

## 2.1. Materials

Manganese(III) acetate dihydrate ((CH₃COO)₃Mn-2H₂O) and sodium pyrophosphate decahydrate (Na₄P₂O₇·10H₂O) were purchased from Sigma-Aldrich, and used as received. A thermodynamically stable Mn

(III)-PP stock solution was prepared following a procedure from the literature [22,23]. All Mn(III)-L stock solutions were made in N₂(g)-sparged anoxic ultrapure water (18.2 MΩ cm). These were prepared by mixing 10 mM Mn(III) acetate powder with alkaline solutions of 20–100 mM sodium pyrophosphate decahydrate, while stirring with a magnetic stirrer. This procedure produced pH ~ 7 solutions of Mn (III)-PP complexes at Mn(III):PP ratios in the 1:2–1:10 range. The pH of the original PP solutions was thereafter adjusted using 0.1 M HCl or 0.1 M NaOH.

Freshly-prepared solutions of Mn(III):PP ratios lower than 1:4 were immediately filtered (0.22 µm hydrophilic polytetrafluoroethylene, H-PTFE-20/13, Duren-Germany) to minimize rapid Mn(III) disproportionation, and to avoid the influence of interfering solid substances on the reaction process. Those at Mn(III):PP ratios of 1:4 and higher were filtered and stored in the dark in a refrigerator at 4 °C. The half-lives of those solutions ranged from about 25 to 530 days, depending on Mn:PP ratios [22,23]. Standard solutions of Mn(III)-PP complexes at concentrations of 10, 50, 100, 200, 500, and 1000 µM were then prepared in triplicates by dilution of a 10 mM Mn(III)-PP stock solution at a Mn:PP ratio of 1:5. The stock solution concentration was measured before each experiment to ensure that no decomposition occurred. These values were measured using an ultraviolet-visible (UV-Vis) spectrophotometer (Cary-50, Varian).

## 2.2. CIP oxidation

The time-resolved oxidation of CIP by Mn(III) was studied in 200 mL solutions, which were stirred at 25 °C in 250 mL Nalgene bottles open to the atmosphere in the absence of light. The solutions were prepared by adding aliquots of the CIP and Mn(III)-PP stock solutions to the reaction vessel. To minimize Mn(III) disproportionation, solution pH was adjusted to pre-selected values by adding aliquots of 0.1 M HCl or 0.1 M NaOH. The resulting solutions contained 20 µM CIP, 200–800 µM Mn (III), and PP at Mn:PP ratios of 1:2–1:10. The pH was not controlled during the course of the reactions, as excess PP buffered the solutions ($pK_a = 0.85, 1.96, 6.60, \text{and } 9.41$) [43]. Aliquots of 2 mL were periodically collected from the reaction vessel, and the reactions were promptly quenched by adding 10 µL of a 1 M hydroxylamine hydrochloride solution.

To examine the reactivity of colloidal MnO₂ from Mn(III) dissociation, the disproportionation of 100 µM Mn(III)-PP complexes was studied in a 7-h reaction at pH 4 and 7. Samples were taken at specific time intervals, and concentrations of Mn(III)-PP and MnO₂ colloids were analyzed immediately without filtration or quenching. A 20 µM CIP was thereafter added to evaluate the oxidation capacities of Mn(III) and MnO₂ colloids. Note that pH was not altered by Mn(III) disproportionation or CIP addition due to PP buffering. The CIP-containing suspensions were then periodically sampled, filtered, and quenched prior to CIP concentration measurements. As a control experiment, the oxidation of a 20 µM CIP solution mixed in a suspension containing 100 µM acid birnessite ($δ-MnO_2$) suspensions was studied at pH 4 and 7 at 0 and 1 mM PP. This $δ-MnO_2$ (specific surface area 85.0 ± 3.0 m²/g) was prepared and characterized by Li et al. [44]. All batch experiments were conducted under an open atmosphere and in the dark by covering the Nalgene bottles with aluminum foil. All the kinetic experiments were performed in triplicates at room temperature (25 ± 2 °C).

## 2.3. Aqueous species determination

Aqueous CIP concentrations were determined using a high-performance liquid chromatograph (Waters 600 controller) equipped
with a reversed-phase C18 column (250 mm × 4.6 mm i.d., 5 µm) and a UV detector (Waters 2489). The mobile phase was a mixture of acetonitrile/water (25/75 v/v) containing 0.1 % formic acid. The UV detector was set to 275 nm for CIP. CIP oxidation byproducts were thereafter analyzed using a Waters Ultra High-Pressure Liquid Chromatography-Mass Spectrometer (HPLC-MS; Acquity UPLC) system equipped with a Waters BEH C18 column (100 mm × 2.1 mm, 1.7 µm) and a mass spectrometer. The mobile phase consisted of acetonitrile containing 0.1 % formic acid (eluant A) and a 10%/90% acetonitrile/water mixture containing 0.1 % formic acid (eluant B), with a working gradient of 0 min/0 % A–1 min/0 % A–9 min/100 % A–12 min/0 % A, and a flow rate of 400 µL min⁻¹. An electrospray interface (ESI) was used for the MS measurements in positive ionization mode and full scan acquisition.

The disappearance of Mn(III)-PP complexes and the formation of colloidal MnO₂ in Mn(III) disproportionation experiments were monitored by UV-Vis spectrophotometry using a 10 mm light path quartz cuvette. Full-range scans in the 200–800 nm range were conducted to exclude background shifts from light scattering that could have possibly occurred by the formation of colloidal MnO₂ [45,46]. All sample absorbances were corrected by subtracting the blank absorbance of distilled water.

## 2.4. Electrochemical potentials

Cyclic voltammetry (CV) was used to determine the half-wave potentials (E₁/₂) of Mn(III)-PP complexes. Experiments were carried out in 100 mM NaCl solutions containing 1 mM Mn(III)-PP at pH 3–7, and with Mn(III):PP ratios from 1:2 to 1:10. All data were collected with a scan range between –1.0 V and +1.0 V at a scan rate of 100 mV/s using a three-electrode system consisting of a glassy carbon as the working electrode, a platinum electrode as the counter electrode, and a saturated calomel electrode (SCE) or Ag/AgCl/KCl as the reference electrode. The half-wave potential (E₁/₂) was used as an approximation of the formal potential (E⁰) of the redox system:

$$E_{1/2} = \frac{1}{2}(E_{pa} + E_{pc}) \quad (1)$$

and where Eₚₐ and Eₚₐc are respectively the anodic and cathodic peak potentials.

## 2.5. Vibrational spectroscopy

Fourier Transform Infrared (FTIR) spectroscopy was used to collect the vibrational spectra of Mn(III)-PP complexes in freshly prepared and filtered solutions of 20 mM Mn(III)-PP at Mn(III):PP ratios of 1:2–1:10. All spectra were acquired immediately after adjusting pH between 3 and 7. All measurements were made on 10 µL aliquots of freshly-prepared solutions using an Attenuated Total Reflectance (ATR) accessory (single-reflection diamond window, Golden Gate by Specac). The spectra were collected at a resolution 4 cm⁻¹ in the 600–4500 cm⁻¹ range, and at a forward/reverse scanning rate of 10 kHz using a Bruker Vertex 70/v FTIR spectrometer equipped with a DLA TGs detector.

Spectral analyses were subsequently focused on the 800–1400 cm⁻¹ range, which covers the main P-O(H) vibrational bands of PP [47]. All resulting spectra were offset by a linear baseline anchored at 800 and 1400 cm⁻¹, but were not normalized for absorbance. The spectral contributions of the water bending region (~1630 cm⁻¹) were thereafter subtracted using the spectra of matrix-matched samples without PP. The resulting matrix (A) of spectra collected over several pH and/or Mn:PP ratios where then expressed in terms of a linear combination of the product of spectral (e) and concentration (C) profiles, namely through the Beer-Lambert law (A = eₙCₙ). This was achieved using the program MCR-ALS [48] in the computational environment of MATLAB (The Mathworks, Inc.). The approximate number (n) of spectral components to be considered by MCR-ALS was obtained by evaluating the

dimensionality of A using the Factor Indicator Function [49] and by inspecting the lengths of linearly-independent components obtained by singular value decomposition (SVD). This number of component was then used for MCR-ALS. In support for the validity of the results, MCR-ALS analyses conducted at greater values of n produced redundant spectral components e, while analyses at smaller values of n did not reproduce the variance in the data. These efforts extracted the salient spectral profiles needed to interpret of plausible complexation structures of Mn(III)-PP complexes.

## 2.6. MCR analysis of UV-Vis spectra

Concentration profiles of Mn(III)-PP and MnO₂ colloids in Mn(III) disproportionation experiments were also extracted by MCR. The MCR analysis on the time-resolved UV-Vis spectra produced spectral fingerprints (Fig. S9c) and absorbance intensity profiles (Fig. S10) for both Mn:PP ratios of 1:5 and/or 1:2, at pH 4 and pH 7. Concentration profiles were then obtained using molar absorptivities at 258 nm for Mn(III)-PP (ε₂₅₈ nm = 6100 M⁻¹ cm⁻¹) [45,46] and at 368 nm for MnO₂ colloids (ε₃₇₀ nm = 9180 M⁻¹ cm⁻¹) [45,50]. In particular, MnO₂ concentrations were calculated with [MnO₂] = A₃₇₀ nm/ε₃₇₀ nm, and residual Mn (III)-PP concentrations with [Mn(III)-PP] = A₂₅₈ nm/ε₂₅₈ nm (cf. Fig. S10 for A₃₇₀ nm and A₂₅₈ nm).

## 2.7. Thermodynamic calculations

Speciation calculations of Mn(III)-PP solutions at Mn:PP ratios of 1:2 and 1:5 studied by FTIR (20 mM Mn(III)) and in CIP degradation experiments (200 µM Mn(III)) were performed using critically selected and experimentally calibrated equilibrium constants (Table S3) [23]. Geochemical speciation calculations were made using PHREEQC (version 2) [51]. Mn redox and PP hydrolysis reactions were not considered in these calculations to focus on the initial conditions and possible reactive intermediates. Note that the calculations were conducted for a simplified simulated system in the absence of CIP, and information on reaction kinetics was not included. The speciation calculations therefore only represented the final equilibrated states of each component. The kinetic behaviors of Mn(III) disproportionation were, additionally, studied in supplement to our thermodynamic calculations.

# 3. Results and discussion

## 3.1. Reactivity of soluble Mn(III)-PP towards CIP oxidation

Variations in Mn(III) concentration, pH, and Mn(III):PP ratio played the following key roles in the kinetics of CIP removal from aqueous solutions (Fig. 1). CIP concentrations were unaffected by Mn(III):PP ratios at and above 1:5, regardless of concentration or pH (Fig. 1(a,b)). This was confirmed further by experiments at pH 4 (Fig. 1c), showing that CIP removal was effective at lower ratios. Time-resolved batch experiments also revealed that Mn(III)-PP reactivity was facilitated only in the pH 3–5 range, while no reactivity took place at pH 7 and 9 (Fig. 1b).

The sigmoidal shape of the time-resolved removal of CIP, marked by an initial lag phase followed by an exponential degradation phase, was suggestive of an autocatalytic process also seen in previous Mn(III)-driven organic removal studies [41,42,52]. The data were modeled using a reparametrized Gompertz kinetic model by non-linear regression fitting of the data [53]:

$$\frac{C}{C_0} = 1 - e^{-e^{k(\lambda-t)} + 1} \quad (2)$$

where k (h⁻¹) is the maximum reaction rate, λ (h) is the lag phase duration, and C₀ is the initial and C the final CIP concentration. The
<figure><img src="image_3.png" /><figcaption>**Fig. 1.** CIP removal kinetics from solutions of C₀ = 20 µM CIP and dependence of CIP removal rates (k of Eq. (2)) in the presence of (a,d) 100–800 µM Mn(III) at pH 4 at Mn:PP ratios of 1:2 (red) and 1:5 (blue), (b,e) 200 µM Mn(III) at pH 3–9 at Mn:PP ratios of 1:2 (red) and 1:5 (blue), (c,f) 200 µM Mn(III) at Mn:PP ratios from 1:2–1:10 at pH 4. Dashed lines are predictions from the Gompertz model (Eq. (2)). (g) CIP oxidation by-products after a 7-h of reaction with 20 µM CIP, 200 µM Mn (III) at a Mn:PP ratio of 1:2 at pH 4.</figcaption></figure>

statistical indicator of the quality of the fit, the regression coefficient of determinations (R²), was reported in Table S1. The resulting model parameters captured the concept of CIP decomposition in relation to concentration, pH, and Mn:PP ratios (Fig. 1d–f and Table S1). Notably, decomposition rates were sufficiently high at Mn:PP ratios only below 1:5, with values scaling with Mn(III) concentration. These values plateaued at ~ 400 µM (k = 0.4 h⁻¹) and had shorter lag durations (Fig. S1). Removal rates were, additionally, only significant at pH values below ~ 6 (Fig. 1e).

This pH dependence also aligns with cyclic voltammetry measurements (cf. Fig. S2 and Table S2), which revealed half-wave potentials (E₁/₂) from 0.75 V at pH 5 down to 0.4 V at pH 9. As higher E₁/₂ values signaled solutions of greater electron transfer capability [54], these

findings add support to our kinetic results and models showing that acidic solutions (pH 3–6) had a greater propensity for CIP oxidation than more alkaline solutions. Also, from speciation calculations (Fig. S3) [22] showing that acidic conditions favored free PP species, these findings support the concept that CIP oxidation was driven by un-complexed Mn species.

CIP degradation products, which we identified by HPLC-MS, included four compounds resulting from the dealkylation of the piperazine group (Fig. 1g). As previously shown in CIP degradation studies [38,55], direct coordination between CIP functional (carboxyl and ketone) groups and a redox-active metal ion center or surficial high-valent metal [39,55,56], were needed to initiate electron transfer reactions.

Our kinetic analysis thus offered insightful observations into the
effects of Mn:PP ratio and pH on CIP oxidation. In the following section, we will turn to vibrational spectroscopy to resolve the molecular-level mechanisms controlling Mn(III)-PP stability. This will provide clues on the role of PP complexation in controlling the availability of redox-active Mn species in relation to CIP removal.

## 3.2. Stoichiometry and coordination structure

To assess how Mn:L ratios and pH controlled Mn(III) speciations, we probed functional groups of Mn(III)-PP complexes using ATR-FTIR spectroscopy (Fig. 2). These efforts were focused on fresh solutions at Mn:PP ratios from 1:2 to 1:10 at pH 7, where Mn(III)-PP complexes are stable [22]. From thermodynamic predictions of multiple equilibrium constants (Table S3, Fig. S3) and experiments [23,57], these solutions contained various concentrations of uncomplexed PP (PP<sub>free</sub>) species, alongside the dominant Mn(PP)<sub>2</sub><sup>5-</sup> complex.

Using the P-O stretching region as a primary marker for coordination, we found that the major band intensities (907, 954, and 1108 cm<sup>-1</sup>) scaled with Mn:PP ratios (Fig. 2a). MCR showed that the variance in these band intensities could be explained using three spectral components (Fig. 2b) [48]. Component MCR<sub>a</sub> I remained relatively constant over Mn:PP ratios while MCR<sub>a</sub> II and MCR<sub>a</sub> III scaled with ligand concentrations. Comparison with thermodynamic predictions (Fig. S3) supported the concept that MCR<sub>a</sub> I was from MnPP<sub>2</sub><sup>5-</sup> and that MCR<sub>a</sub> II and MCR<sub>a</sub> III were from PP<sub>free</sub> species. This assignment aligns with (i) a ~1060 cm<sup>-1</sup> band that was absent in free PP species and representing P-O-Mn(III) formation (Fig. 2, S4, Table S4), and (ii) the lack of response to deuteration (Fig. S5) which indicated that the species

was deprotonated. By comparing the spectra of free PP species at pH 7 and 8, we assigned MCR<sub>a</sub> II to H<sub>2</sub>PP<sub>2</sub><sup>2-</sup> and MCR<sub>a</sub> III to HPP<sup>3-</sup> (Table S4 and Figs. S4-S5).

This assignment was supported further by linear correlations between MCR absorbances and species concentrations (Fig. S6), as well as from the response of their bands to protonation (Fig. S4). In line with their concentration profile, the redox potentials of Mn(III)-PP complexes remained unchanged with variations in Mn(III):PP ratios (Fig. S2, and Table S2). This consequently strengthened our assignment of a 1:2 stoichiometry for Mn(III)-PP complex, even in the presence of excess PP. This also implied that all Mn(III) were in the form of the MnPP<sub>2</sub><sup>5-</sup> complex (Fig. S3).

In light of these findings, we propose that MnPP<sub>2</sub><sup>5-</sup> was the chief Mn (III)-PP complex [23] limiting the availability of the redox-active Mn species in aqueous solutions. Judging from the expectation that the Jahn-Teller distortion of the Mn(III)O<sub>6</sub> coordination environment involves an elongation of two axial Mn-O bonds and the shrinkage of four equatorial ones [58], the Mn<sup>3+</sup> center of the MnPP<sub>2</sub><sup>5-</sup> complex is likely chelated by four terminal phosphate oxygens (Fig. 2e). The coordination structure of MnPP<sub>2</sub><sup>5-</sup>, agrees well with numerical simulations of mineral surface-bound PP in a preferred mononuclear bidentate configuration [59] and Mn(III)-oxalate/uranium-PP structures [60,61]. Although the molecular orbitals of Mn(III) should not be fully occupied, elongated Mn-O bonds could only be accessible for monodentate water molecules. If this is the case, then the MnPP<sub>2</sub><sup>5-</sup> complex should be inactive towards CIP oxidation when the Mn<sup>3+</sup> center is unavailable for coordination with CIP.

With these concepts in mind, we can now gain information on CIP

<figure><img src="image_4.png" /><figcaption>**Fig. 2.** (a) ATR-FTIR spectra of the P-O stretching region of 20 mM Mn(III) solutions at Mn:PP ratios from 1:2 to 1:10 at pH 7. (b-c) MCR analyses of these spectra generated MCR<sub>a</sub> (b) spectral and (c) concentration profiles. Reference spectra of the free H<sub>2</sub>PP<sub>2</sub><sup>2-</sup> and HPP<sup>3-</sup> and of the complexed MnPP<sub>2</sub><sup>5-</sup> (Mn:PP ratio of 1:5) species are shown in (b) for comparison. (d) Concentrations profiles of Mn(III)-PP species generated by thermodynamic calculations (Table S3) for the same solution compositions in (a) are directly comparable to MCR concentration profiles in (c). (e) A proposed possible coordination structure of the MnPP<sub>2</sub><sup>5-</sup> complex.</figcaption></figure>
oxidation pathways made possible in solutions of lower Mn:PP ratios and lower pH (Fig. 1). MCR analyses of FTIR spectra of solutions (Fig. 3) captured this by showing a strong dependence of pH and Mn:PP ratios on MnPP<sub>2</sub><sup>5-</sup> stability. This was seen through the loss of the ~1060 cm<sup>-1</sup> band from the asymmetric stretch of the P-O-Mn(III) bond in acidic media (Fig. 3a, d), which signaled the presence of the uncomplexed MnOH<sup>2+</sup> species (Fig. S3). Of note, MCR extracted two components at Mn:PP = 1:2 (MCR<sub>c</sub> I = MnPP<sub>2</sub><sup>5-</sup>; MCR<sub>c</sub> II = PP<sub>free</sub>), yet three components were needed for Mn:PP = 1:5 (MCR<sub>b</sub> I = MnPP<sub>2</sub><sup>5-</sup> + PP<sub>free</sub>; MCR<sub>b</sub> II = MnPP<sub>2</sub><sup>5-</sup>; MCR<sub>b</sub> III = PP<sub>free</sub>) because of an excess of uncomplexed PP in those solutions.

The resulting MCR concentration profiles (Fig. 3c, f) revealed speciation changes at different pH values. Alongside our thermodynamic predictions (Fig. S3), these analyses confirmed that CIP oxidation reactions were only possible outside the stability field of the MnPP<sub>2</sub><sup>5-</sup> complex. This is a stability field where Mn speciation is dominated by hydrolyzed MnOH<sup>2+</sup> and other Mn products from MnOH<sup>2+</sup> disproportionation.

These results consequently indicated that shifting pH destabilized Mn(III)-PP complexes, and that solutions of lower Mn:PP ratio were more favorable to Mn(III) dissociation. Clear indications for speciation changes facilitating CIP oxidation will be discussed in the following section by linking speciation calculations to dissociation kinetics.

### 3.3. Kinetic of reactive Mn species formation

To identify the reactive Mn species driving CIP decomposition, we compared the chemical speciation of solutions used for our decomposition (200 µM Mn(III); Fig. S7b) with those used in our FTIR work

(20 mM Mn(III); Fig. S7a). These calculations revealed that the Mn(III)-PP complex was replaced by a hydrolyzed species MnOH<sup>2+</sup> under acidic but to Mn(OH)<sub>3</sub> under alkaline conditions (Fig. S3). As CIP decomposition occurred only below pH ~ 5.5 (Fig. 1e, S7b), we propose that CIP removal could be partly driven by interactions with MnOH<sup>2+</sup>. This species facilitated decomposition as the Mn(III) ion can be directly coordinated by CIP functional (carboxyl and ketone) groups, in contrast to the Mn(III)-PP species where Mn(III) is completely surrounded by PP functional groups (Fig. 2e). Coordination with CIP functional groups is thus expected to facilitate the electron transfer reaction (Mn(III) → Mn(II)) needed for CIP oxidation (CIP → CIP<sub>ox</sub>, Fig. 1g). The redox reactivity of MnOH<sup>2+</sup> originated from the greater delocalization of electrons in Mn<sup>3+</sup> caused by complexation with the polarizable OH<sup>-</sup> ligand. This strong redox capacity can also be understood from the higher redox potential of Mn(III) (E<sup>°</sup><sub>H</sub> (Mn<sup>3+</sup>/Mn<sup>2+</sup>) = 1.51 V) compared to that of solid Mn(IV) (E<sup>°</sup><sub>H</sub> (MnO<sub>2</sub>/Mn<sup>2+</sup>) = 1.23 V) [11,62]. Our interpretation that Mn(III)-PP conversion to MnOH<sup>2+</sup> leads to CIP degradation aligns with a previously held assumption that weakly-complexed Mn(III) species can be reactive oxidants [19].

In addition, we must stress that because Mn<sup>3+</sup>/MnOH<sup>2+</sup> species were unstable, colloidal MnO<sub>2</sub> and Mn(II) should also be formed via a reaction of the type [22,23,57,63]:

$$2 \text{Mn}^{(\text{III})}\text{OH}^{2+} \rightarrow \text{Mn}^{(\text{IV})}\text{O}_2 \text{(s)} + \text{Mn}^{(\text{II})} + 2\text{H}^+ \quad (3)$$

This was tested by tracking the time-resolved disproportionation of Mn (III) using UV-Vis spectrophotometry (Fig. 4a, b). From the characteristic absorbances of Mn(III)-PP complexes (λ<sub>max</sub> = 258 nm) [9,14,23], MnO<sub>2</sub> colloids (λ<sub>max</sub> = ~370 nm) [45,64–67], we find that disproportionation was considerably slower at high (1:5) than at low (1:2) Mn

<figure><img src="image_5.png" /><figcaption>Fig. 3. Vibrational spectral response of 20 mM Mn(III) solutions at Mn:PP ratios of 1:5 (a) and 1:2 (a) from pH 3 to pH 7. MCR analyses for n components of (a) generated MCR<sub>b</sub> spectral components in (b) and concentration profiles in (c). Those of (d) generated MCR<sub>c</sub> spectral components of (e) and concentration profiles of (f).</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>Fig. 4. Variations in UV-Vis spectra of Mn(III)-PP complexes at Mn(III):PP ratio of 1:5 (a) or 1:2 (b) at pH 4. Time-resolved concentrations showing the disappearance of Mn(III)-PP complexes (c) and the formation of MnO₂ colloids (d) at Mn:PP ratio of 1:2 or/and 1:5 and at pH 4 and 7.</figcaption></figure>

(III)-PP ratios (Figs. S9-S10 and Fig. 4c, d). This can especially be appreciated from the contrasting changes in Mn(III)-PP concentration at these two ratios, and in the rise in the concentration of MnO₂ colloids. We also note that pH affected the reactivity of both MnOH²⁺ and colloidal MnO₂, as lower pH favored CIP oxidation, a result that is in accordance with the increased redox potential of Mn(III)-PP solutions at lower pH (Fig. S2, Table S2). In addition to this, newly formed MnO₂ particles could efficiently contribute to CIP removal via surface complexation, and subsequently electron transfer [44,55,68,69]. This effect should be comparable to the results of an additional experiment (Fig. S8a) in which PP complexation had a slight inhibitory effect on CIP removal by synthetic δ-MnO₂.

Collectively, our findings revealed that Mn(III)-PP stability was central to CIP oxidation in mixed Mn(III)-PP-CIP solutions. Mn(III)-PP stability and its dissociation kinetics were largely controlled by Mn:PP ratios, while lower pH favored the participation of the redox-active MnOH²⁺ species, as well as the subsequently produced MnO₂ colloids. In summary, the weakening of PP-Mn³⁺ bonds by changes in solution composition can trigger CIP oxidation.

## 4. Conclusions

By identifying the impact of PP complexation on the redox activity of Mn(III) species, our work identified stability regimes for CIP and potentially other related fluoroquinolone antibiotic contaminants in water. This knowledge was developed by recognizing that PP complexation limits the accessibility of reactive Mn to CIP functional groups, which is necessary for electron transfer.

Chelation with natural (in)organic ligands can promote Mn(III) mobility in nature, contrasting with colloidal oxides that tend to hinder transport. It should, however, be an essential aspect to consider along proton and ligand concentration gradients experienced by migrating

bodies in aquatic and terrestrial environments. Notable settings include transitions across acid mine drainage, or acid rain run-off from high-lands to rivers and lakes. As ideas on the occurrence of dissolved Mn(III) and its formation mechanisms are developing in environmental geochemistry [9] and pollution remediation [10], this work highlights that complexed Mn(III) species tend to destabilize when moving across oxic-anoxic interfaces in water columns. This phenomenon could even provide new insight into working with acidic wastewater by allowing dissolved Mn(III) species and MnO₂ colloids to remove contaminants in environmental engineering applications [70,71]. Changes in concentrations of Mn(III) stabilizing ligands and protons could help explain the challenges associated in detecting low concentrations of dissolved Mn (III) in natural waters [1,72]. From a broader aspect, local Mn cycling should not only impact carbon cycles via organic compounds oxidation/mineralization [73,74] but also the cycling of other elements, such as lead [75], uranium [19], and mercury [76].

These findings should also be readily extended to a wide range of naturally occurring ligands that can extend the lifetime of Mn(III). Examples include organic acids [7,22], biogenic siderophores [7,77], or dissolved organic matter [5]. This diversity of chelating organic ligands on stabilizing Mn(III) could be a challenge to shed light on Mn(III) reactivity in the oxidation process. Accounting for mechanisms of intermolecular electron transfer should also be considered, as differences in ligand affinities to Mn(III) may lead to diverse stability regimes for organic contaminants. As more remains to be known on the roles of Mn(III)-ligand complexes in natural environments, we should expect more detailed studies on their structure and reactivity. Developing this knowledge will also be increasingly vital for developing strategies for managing the environmental fate of antibiotics, a task that will ultimately contribute to better water quality and public health. Work along this line should, at the same time, contribute in advancing knowledge on the coupled biogeochemical cycles of manganese and carbon.
## Environmental Implication

Aqueous Mn(III) species play important roles in degrading antibiotics released to the environment. This study demonstrates that ciprofloxacin, a predominant antibiotic contaminant, can be degraded when directly coordinated to Mn(III). However, degradation is inhibited if Mn (III) is complexed with strongly-binding ligands, such as pyrophosphate. Our research identified pH ranges where this competitive complexation stabilizes ciprofloxacin in water. These findings emphasize the importance of considering the inhibiting effects of pyrophosphate-like ligands on Mn(III)-driven oxidation of ciprofloxacin. This is especially vital for antibiotic transport monitoring programs aimed at improving water quality and public health.

## CRediT authorship contribution statement

**Jean-François Boily:** Writing – review & editing, Writing – original draft, Supervision, Project administration, Funding acquisition, Conceptualization. **Tao Luo:** Data curation, Formal analysis, Methodology. **Tao Chen:** Writing – original draft, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. **Yann Leroux:** Writing – original draft, Methodology, Formal analysis, Data curation. **Tra My Bui Thi:** Writing – review & editing, Writing – original draft, Methodology, Formal analysis, Data curation, Conceptualization. **Khailil Hanna:** Writing – review & editing, Writing – original draft, Supervision, Funding acquisition, Conceptualization.

## Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgments

The authors wish to thank support from Swedish Research Council (2020-04853, 2024-04694) and FORMAS (2022-01246) to JFB, the French National Research Agency via the INFLOW project (ANR-21-CE29-0008-01) to KH and JFB, the CNRS (IRP CHEMICY 2022-2027) to KH and JFB, and a Région Bretagne for a mobility grant for an extended research stay at Umeå University to TMBT.

## Appendix A. Supporting information

Supplementary data associated with this article can be found in the online version at doi:10.1016/j.jhazmat.2025.138373.

## Data availability

Data will be made available on request.

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