Manganese Dioxide – Based Photocatalytic Oxidation Processes for the Removal of Ciprofloxacin from Water: Materials, Mechanisms and Perspectives

Manganese Dioxide – Based Photocatalytic Oxidation Processes for the Removal of Ciprofloxacin from Water: Materials, Mechanisms and Perspectives

Padmaja Vempalli

Research Scholar, JNTU, Ananthapuram Andhra Pradesh, India

Dr. P. Sri Chandana

Professor, AITS, Kadapa Andhra Pradesh, India

Abstract: Ciprofloxacin (CIP), a broad-spectrum fluoroquinolone, is one of the most commonly detected antibiotics in surface waters and wastewater, contributing to ecological risks and the selection of antibiotic-resistant bacteria and genes. Various methods, including adsorption, photocatalysis, persulfate-based oxidation, and electrochemical oxidation, have been employed for its removal. Manganese dioxide (MnO2) is an earth-abundant, low-toxicity, polymorphic oxide characterized by a narrow band gap (typically 12 eV), a rich Mn(IV)/Mn(III)/Mn(II) redox chemistry, and strong surface reactivity towards phenolic, anilinic, and amine-containing micropollutants. This review synthesizes evidence on (i) the occurrence, risk, and resistance associated with CIP, (ii) adsorptive and photocatalytic benchmarks for CIP, (iii) the structureproperty relationships of -, -, -, -, and -MnO2, (iv) the dark and light-driven transformation of pharmaceuticals on MnO2, and (v) Mn-based catalysts that directly degrade CIP, including Mn/Co oxide photocatalysts and MnO2-activated peroxymonosulfate (PMS) systems. In the literature, -MnO2 and -MnO2 are identified as the most reactive polymorphs. The oxidation process, including piperazine-ring cleavage, hydroxylation, and F/OH substitution in CIP transformation, is driven by reactive oxygen species (h+, OH, O2 -, 1O2) and Mn(III) intermediates. However, there is a significant gap in research regarding direct, light-driven CIP degradation by pristine MnO2 polymorphs. We propose research priorities focusing on the mechanism, toxicity, real-water testing, and scale-up.

Keywords: ciprofloxacin, manganese dioxide, photocatalysis, peroxymonosulfate, reactive oxygen species, fluoroquinolone, water treatment

1. INTRODUCTION

   Antibiotics are extensively utilized in both human and veterinary medicine. Due to incomplete metabolism, they enter wastewater and receiving waters in their original or transformed forms. Monitoring of Hong Kongs Kai Tak River nullah, which receives treated effluent from two wastewater treatment plants, revealed median concentrations of approximately 1694 ng L1 for CIP and 632 ng L1 for clarithromycin. CIP and clarithromycin are identified as the primary contributors to ecological risk for aquatic organisms [1]. Even at these concentrations, residues maintain antibiotic-resistance genes (ARGs) and resistant bacteria [1]. In clinical settings,

   ciprofloxacin (CIP) has lost efficacy due to extensive resistance, prompting the use of machine-learning tools to predict resistant infections [2]. Thus, controlling CIP at its environmental source is integral to a comprehensive antimicrobial-resistance strategy. Conventional biological treatments do not completely remove fluoroquinolones, necessitating advanced technologies. Reviews on antibiotic abatement highlight semiconductor photocatalysis for its ability to utilize sunlight, operate under ambient conditions, and mineralize persistent organics [3,4]. Titanium dioxide (TiO2) remains the standard; however, it is constrained by a wide band gap and rapid charge recombination [4,5]. Manganese dioxide has garnered increasing interest as an alternative due to its abundance, low cost, environmental friendliness, natural occurrence in water and sediments, and its role in transforming micropollutants through adsorption and oxidation [6,7]. This review draws exclusively on the papers supplied in the uploaded compilation. Section 2 defines the scope. Sections 3 to 5 treat ciprofloxacin occurrence, adsorptive benchmarks and photocatalytic, electrochemical and singlet-oxygen alternatives. Sections 6 to 9 cover the structure, reactivity and CIP-specific performance of MnO2 based systems, including MnO2 carbon composites. Sections 10 to 14 compare mechanisms, kinetics, controlling factors, analytical methods and competing technologies, and Sections 15 to 18 discuss adjacent catalytic applications, practical considerations, a research roadmap and conclusions.

2. SCOPE AND APPROACH

   The review is built based on literature papers. They are categorized into six groups in (Table 1). The primary inquiry concerns the extent to which MnO2 based materials can facilitate the removal of ciprofloxacin from water, as well as the mechanisms involved. Investigations into other pollutants, such as acetaminophen, dyes, methotrexate, and nalidixic acid, are referenced when they provide insights into MnO2 surface chemistry and the formation of reactive species, as these processes are likely applicable to ciprofloxacin. Quantitative values are presented as reported by the original authors under their specific experimental conditions and should be considered indicative rather than directly comparable.

   Table 1. Map of the source documents by topic group.

   Group	Source papers	Contribution to this review
   A. CIP occurrence, risk, resistance	[1,2, 10]	Concentrations, ecological risk, ARGs, clinical resistance
   B. Adsorption of CIP and isotherm theory	[1014]	Benchmark capacities, mechanisms, modelling
   C. Antibiotic photocatalysis, AOPs, 1O2	[3,4, 1518]	Heterojunctions, reactive species, pathways, toxicity
   D. MnO2 materials and reactivity	[69, 1926]	Polymorphs, band gaps, dark and photo-enhanced oxidation
   E. Mn-based CIP degradation	[2730]	Direct CIP performance, PMS activation
   F. Adjacent catalysis	[5, 3138]	Design lessons: supports, doping, immobilisation

3. CIPROFLOXACIN IN THE ENVIRONMENT: OCCURRENCE, RISK AND RESISTANCE

   CIP is characterized by its persistence, stability, and widespread use, leading to its accumulation in aquatic environments. Incomplete degradation of CIP can facilitate the development of resistant strains [10, 11]. Data collected from Hong Kong indicate seasonal fluctuations in the levels of CIP and clarithromycin over a 16-month period (December 2019 April 2021), as well as the persistence of antibiotic resistance genes (ARGs) such as acrB-1, acrA-2, mexF, acrA-1, and mexE, in conjunction with CIP- and clarithromycin-resistant bacteria [1]. The same study demonstrated that a PbO2 thin-film anode effectively removed CIP and clarithromycin within fifteen minutes via electron transfer and reactive oxygen species, highlighting the potential of rapid oxidative treatment to reduce ARG pressure [1].

   The clinical aspect is just as significant. With ROC-AUC values of 0.737 (species-agnostic) and 0.837 (species-aware), an ensemble machine-learning model trained on 10,053 cultures from hospitalized patients predicted CIP resistance, identifying prior resistance, patient origin, and recent hospital resistance frequencies as the key predictors [2]. This paper highlights the importance of environmental release of fluoroquinolones because resistance already jeopardizes treatment, even though it does not address water treatment.

4. ADSORPTIVE REMOVAL: THE BENCHMARK

   Adsorption is the most intensively studied route for CIP. A systematic review and meta-analysis of studies published between 1990 and 2020 found that CIP adsorption data are generally best described by the Langmuir isotherm and pseudo- second-order kinetics, with optimum pH mostly between 6 and 8.5, a pooled mean removal efficiency of 59.32% (95% CI 44.6673.97), and a maximum capacity of 1575 mg g1 for carbon@silica core/shell nanoparticles derived from ZIF-8 [11].

   Selection of isotherm models (Langmuir, Freundlich and multi- parameter forms) and appropriate fitting criteria is itself an important methodological issue [12].

   The mechanisms are demonstrated by adsorbents based on carbon and clay. With maximal capacities of 379 and 240 mg g1, graphene oxide (GO) sorbed CIP and sulfamethoxazole. The uptake of CIP was primarily controlled by electrostatic attraction, whereas the uptake of sulfamethoxazole was controlled by electron-donoracceptor interaction; sorption was significantly influenced by pH and ionic strength (NaCl, CaCl2) [13]. After four cycles, a magnetic chitosan-grafted GO composite maintained roughly 72% of its capacity, reached

   282.9 mg g1 (pseudo-second-order kinetics), was repressed by Na+ and Ca2+, and was regulated by electrostatic and interactions [14]. According to a review of clay-based adsorbents, chemisorption predominates, removals up to roughly 90% are possible, and claymetal-oxide composites function better than virgin clays [10].

   Capacity is sensitive to pH and competing ions, and adsorption moves CIP to a solid phase without destroying it [11, 13, 14]. Destructive methods and hybrid adsorptioncatalysis materials, as the BiOX/GaMOF heterojunction, are motivated by this. Adsorption alone removed 50.42% of CIP in 60 minutes, and subsequent irradiation increased removal to 96.27% [16].

   The capacity of reported adsorption performance ranges from 47 ng g1 for birnessite to 1575 mg g1 for the best carbon@silica material, spanning over nine orders of magnitude [11]. Additionally, the meta-analysis finds typical enthalpy changes between 1212.6 and +170.21 kJ mol1 and operating temperatures between 273 and 388 K, demonstrating that both exothermic and endothermic absorption occur depending on the adsorbent [11]. To enhance performance, the authors suggest hybrid procedures, neural-network modelling, composite adsorbents, and adsorbent modification [11].

   Table 2. Representative adsorption benchmarks for ciprofloxacin.

   Adsorbent / dataset	Capacity or removal	Notes	Ref.
   Carbon@silica core/shell (from ZIF- 8)	1575 mg g1 (maximum reported)	Best performer in the meta-analysis	[11]
   Birnessite (-MnO2-type)	47 ng g1 (minimum reported)	Lowest capacity in the meta-analysis	[11]
   All adsorption studies (pooled)	59.32% mean removal (95% CI 44.6673.97)	Langmuir and pseudo-second-order fits; pH 68.5	[11]
   Graphene oxide	379 mg g1 (CIP)	Electrostatic attraction; sensitive to pH, Na+, Ca2+	[13]
   Magnetic chitosanGO	282.9 mg g1	Pseudo-second-order; ~72% capacity after four uses	[14]
   Claymetal-oxide composites	Up to ~90% removal	Chemisorption predominant	[10]
   BiOX/GaMOF (BG-15)	50.42% in 60 min (dark)	Lewis-acidic Ga, large MOF surface area	[16]

   Two aspects of these data are pertinent to the subsequent analysis of MnO2. Firstly, the surface-charge dependence of CIP uptake, characterized by electrostatic attraction in GO and suppression by competing cations, foreshadows the pronounced

   pH sensitivity of MnO2 oxidation, which similarly initiates with adsorption [13, 14, 24]. Secondly, even the least effective MnO2 type material in the adsorption dataset, birnessite, exhibits negligible capacity, indicating that MnO2 is not a prominent

   adsorbent for CIP; its significance lies in oxidative transformation rather than storage [11].

5. PHOTOCATALYTIC, ELECTROCHEMICAL AND SINGLET-OXYGEN ROUTES TO ANTIBIOTIC

   REMOVAL

   1. Semiconductor photocatalysis

      The application of visible-light photocatalysis to ciprofloxacin (CIP), norfloxacin, tetracyclines, and sulfonamides has been systematically reviewed, focusing on strategies such as doping, heterojunction construction, and surface plasmon resonance to enhance light absorption and inhibit recombination [3]. TiO2 based systems have been evaluated for the degradation of sulfamethoxazole, trimethoprim, and CIP under various conditions, including catalyst loading, pH, light source, inorganic ions, natural organic matter, and oxidants, with additional criteria being the mineralization and ecotoxicity of the resulting products [4]. The modification of TiO2 with carbon or transition metals demonstrates the potential of band-gap and charge-separation engineering to enhance photocatalytic activity. For instance, TiO2 graphene composites achieved over 90% decomposition of perfluorooctanoic acid (PFOA), compared to 16% with commercial TiO2 [5]. Additionally, Fe- and Ni-doped TiO2 have been synthesized for the degradation of dyes [37].

      Recent examples specific to CIP, as illustrated in Table 3, demonstrate the enhancement of performance through heterojunctions and carbon supports. ZnO nanorods, when decorated on GO, achieved a degradation of 99% of 10 ppm CIP within 75 minutes under sunlight, and 98% of methylene blue within 65 minutes. This outcome is attributed to the presence of oxygen vacancies and OH radicals [17]. A sandwich-like CuFe2O4/rGO/halloysite catalyst, with a surface area of 130 m² g¹ and a band gap of approximately 1.9 eV, facilitated nearly complete conversion of 20 ppm CIP in approximately 1 hour, following pseudo-first-order kinetics, with OH radicals and

      holes serving as the primary active species [18]. The BiOX/GaMOF S-scheme heterojunction integrated adsorption with OH, h, O2, and ¹O pathways, thereby reducing both the predicted and measured toxicity of the products [16].

   2. Singlet oxygen and fluoroquinolone selectivity

      A recent review of singlet oxygen (¹O) based oxidation indicates that fluoroquinolones generally undergo reactions with ¹O following second-order kinetics. The review highlights that ¹O provides selectivity and resistance to matrix interference. The primary reaction pathways include the oxidation of the piperazine ring, decarboxylation, and oxidation of the quinolone moiety, as well as the oxidation of cyclopropyl/oxazinyl substituents. Notably, products resulting from decarboxylation and quinolone oxidation exhibit the lowest predicted toxicity [15]. Given that MnO systems produce ¹O (as discussed in Section 10), this literature offers a framework for interpreting MnO-derived ciprofloxacin (CIP) products [6, 26, 29].

   3. Electrochemical oxidation as a comparator

      Electrochemical oxidation serves as a valuable non- photocatalytic benchmark. In the Hong Kong study, a PbO2 thin- film anode was electrodeposited from an electrolyte composed of 0.5 mol L1 Pb (NO), 0.1 mol L1 HNO, and 0.02 mol L1 NaF at a current density of 45 mA cm2 for 60 minutes. This anode was utilized in a 200 mL cell with an electrode spacing of 2 cm and a 0.05 mol L1 NaSO electrolyte. The stability of the system was assessed using SEM, EDS both before and after 2.5 hours and ten treatment cycles [1]. Hydroxyl and sulfate radicas, along with direct electron transfer, facilitated the removal of CIP and clarithromycin within fifteen minutes. The authors attribute the rapid elimination of antibiotics to the reduction of ARG contamination [1]. Although electrochemical methods achieve removal rates that surpass most photocatalytic systems, which typically require tens of minutes to hours, they necessitate the use of electrodes, an electrolyte, and electrical energy. Additionally, the long-term safety of lead-based anodes must be considered [1].

      Table 3. Representative ciprofloxacin (CIP) removal systems drawn from the source papers.

      System	Conditions / target	Main result	Dominant species	Ref.
      ZnO/GO (GOZ-10)	Sunlight; CIP 10 ppm	99% in 75 min	OH; O vacancies	[17]
      CuFe2O4/rGO/halloysite	Light; CIP 20 ppm; neutral pH	Near-complete in ~1 h; pseudo-first- order	OH, h+	[18]
      BiOX/GaMOF (BG-15)	Adsorption then light, 60 min	50.42% adsorbed; 96.27% total	OH, h+, O2, 1O2	[16]
      Mn/Co oxide (solgel)	Sunlight; pH 4; 80 mg catalyst; 0.01 mM CIP	k = 7.9 × 102 min1; photocatalysis > photolysis	Scavenging: ethanol and DMSO strongly inhibit	[27]
      -Fe2O3MnO2 / PMS	Visible light, PMS	98.3% in 30 min; k = 0.114 min1	Radical and non-radical	[30]
      -MnO2 / PMS	Hydrothermal nanoflowers, nanorods, nanoparticles	Activity: flowers > rods > particles > commercial	SO4, OH; 11 products	[28]
      -MnO2 / PMS	Multiple antibiotics incl. CIP; pH 3.5 9.0	> , , -MnO2	SO4, OH2, O, 1O2 ; Mn(III)/Mn(IV)	[29]

6. MANGANESE DIOXIDE: STRUCTURE, BAND GAP AND SYNTHESIS

   MnO2 is composed of MnO6 octahedra, which can connect to form various structural types: tunnel structures (-, -, – MnO2), layered structures (-MnO2, birnessite-type), and the three-dimensional -form. These one-, two-, and three- dimensional frameworks exhibit differences in surface area, defect density, and Mn (III)/Mn (IV) ratio, which can be

   adjusted through doping, morphology, and facet control [7, 9]. As a photocatalyst, MnO2 possesses a narrow band gap of approximately 12 eV, enabling response to visible and infrared light. However, its effectiveness is limited by agglomeration, rapid recombination, and low activity at low temperatures. These limitations can be mitigated by forming composites with carbon materials [8, 9]. Common synthesis methods include hydrothermal, solgel, precipitation, and reflux techniques [9].

   The reported optical properties of MnO2 exhibit significant variation depending on the polymorph and synthesis method employed (Table 5). Specifically, -MnO2 atomic sheets, when laser-crystallized and sonochemically exfoliated, demonstrate band gaps of 1.64 eV and 1.45 eV, respectively [19]. In contrast, -MnO2 nanostructures synthesized via the polyol method exhibit band gaps of 3.02 eV, 2.53 eV, and 2.45 eV, contingent upon the precursor ratio used [21]. Furthermore, the introduction of Ni doping reduces the band gap of MnO2 nanostructures from

   2.78 eV to 1.74 eV and transforms -phase flowers into / nanowires [22]. These variations underscore the necessity of considering MnO2 as a collection of distinct materials rather than a singular entity.

   Table 4 provides a summary of the structural classes. – MnO2 features (2 × 2) tunnels capable of accommodating K+, Na+, Ba2+, Mg2+, Ca2+, and water, which is believed to enhance its adsorption capacity. In contrast, -MnO2 possesses narrow (1

   × 1) tunnels that permit only small ions such as H+ or Li+ and exhibits relative stability. -MnO2 is characterized by an intergrowth of tunnel motifs, whereas -MnO2 is a layered material of the birnessite type [7, 23]. Additionally, morphology, including forms such as nanorods, nanotubes, nanowires, nanofibers, nanoribbons, nanosheets, and nanoflowers, influences accessible surface area and defect density. Notably, activity rankings may vary between morphologies even within the same phase [7].

   Table 4. Structural classes of MnO2 and reactivity evidence from the source papers.

   Polymorph	Dimensionality / structure	Features relevant to oxidation	Ref.
   -MnO2	1D, (2 × 2) tunnels, tetragonal	Cation- and water-hosting tunnels; strong PMS activation as nanoflowers; wire- like -MnO2 reported as most active SO4 generator	[7, 28]
   -MnO2	1D, (1 × 1) tunnels, tetragonal	Stable, narrow tunnels, limited ion diffusion; flower-like -MnO2 formed in situ is an effective UV photocatalyst	[6,7]
   -MnO2	Intergrown tunnel structure	Larger surface area and weaker electrostatic repulsion favour acidic-pH oxidation	[7, 23]
   -MnO2	2D layers (birnessite-type)	High catalytic potential at neutral pH; fastest in sunlight/MnO2 methotrexate and dye tests	[20, 23, 26]
   -MnO2	Hexagonal, cation-vacant (as reported)	High surface OH content and area; best PMS activator of four polymorphs	[7, 29]

   The provided studies demonstrate the variety of preparation techniques and the resulting diversity of materials. These include the hydrothermal growth of -, -, -, and -MnO2 [20, 28], the reduction of KMnO4 with ascorbic acid for -MnO2 and similar methods for – and -forms [26], thermal treatment for -MnO2 [29], polyol synthesis of -MnO2 nanostructures [21], co-precipitation for Ni-doped MnO2 [22], bottom-up laser

   crystallization of aqueous KMnO4 and top-down sonochemical exfoliation for -MnO2 atomic sheets [19] sol gel synthesis for Mn/Co oxide [27] and in-situ formation of colloidal MnO2 through permanganate reduction in water treatment [6]. As each method produces a distinct phase, surface area, defect content, and Mn (III)/Mn (IV) ratio, it is essential to consider the synthesis route when interpreting reported activities.

   Table 5. Structureproperty observations for MnO2 polymorphs and composites

   Material	Key property reported	Ref.
   -MnO2 atomic sheets	Band gap 1.64 / 1.45 eV (laser vs. sonochemical route); Raman peaks at 173 and 634 cm1	[19]
   -MnO2 nanostructures (polyol)	Band gap 3.02, 2.53, 2.45 eV; >50% Congo red degraded in 5 min	[21]
   -, -, -, -MnO2 (hydrothermal)	-MnO2 gives 90.4% methylene blue conversion in 180 min; high Mn4+/Mn3+, 2D morphology, lower Mn-O strength	[20]
   Ni-doped MnO2	Band gap 2.78 1.74 eV; / phase change	[22]
   MnO2-carbon composites	Carbon supports counter agglomeration and low activity	[8]

   A comparative analysis of dye photodegradation revealed tha -MnO2 exhibits the highest activity, attributed to its strong adsorption of methylene blue, distinctive two-dimensional morphology, weaker Mn-O bonds, and elevated surface Mn4+/Mn3+ ratio [20]. These characteristics adsorption, defects, and mixed valence are consistently observed in all MnO2 reactivity studies discussed herein.

7. MNO2 CARBON COMPOSITES FOR ANTIBIOTIC REMOVAL

   Historically, pristine MnO2 was regarded as an ineffective photocatalyst for water treatment due to its propensity to agglomerate and its low activity at reduced temperatures. However, its combination with carbon materials, such as graphene, reduced graphene oxide, graphitic carbon nitride, and activated carbon, addresses these limitations and has positioned MnO2 carbon photocatalysts as a focal point of research [8]. Hydrothermal synthesis is predominantly employed due to water’s cost-effectiveness and benign nature, alongside the

   closed, high-temperature, high-pressure conditions that facilitate intimate MnO2 carbon interaction and enhance electron transfer. Notable examples include -MnO2, RGO nanocomposites, where one-dimensional MnO2 rods are supported on reduced graphene oxide [8].

   The review, as summarized in Table 6, indicates that tetracycline removal rates range from 80% to 97% within 30 to 120 minutes under visible or UV irradiation. Notably, a pumice- supported rGO/MnO2 solid photocatalyst (PS@rGO@MnO2) achieved an 80% degradation of ciprofloxacin (CIP) within 6 hours under simulated sunlight. This photocatalyst maintained its performance over three cycles, demonstrated comparable efficacy under real sunlight and in natural water, and was effective against other antibiotics as well [8]. Additionally, the g-C3N4/MnO2/GO heterojunction removed up to 91.4% of tetracycline hydrochloride within 60 minutes and exhibited only a 10% reduction in activity after four cycles, without any structural changes [8]. Although these findings are secondary evidence, as they are reported by the review rather than the

   primary sources, they suggest that carbon MnO2 heterojunctions are a promising design for the removal of fluoroquinolones and tetracyclines.

   Table 6. MnO2 carbon photocatalysts for antibiotics, as summarized in the review of Fan et al.

   Photocatalyst	Target (initial conc.)	Light; dose; time	Removal	Ref.
   Porous Z-scheme MnO2/Mn-modified alkalinised g-C3N4	Tetracycline (10 mg L1)	Visible, 300 W Xe; 50 mg; 120 min	96.7%	[8]
   Carbon nanosheet/MnO2/BiOCl	Tetracycline HCl (20 mg L1)	UV, 300 W Hg; 20 mg; 30 min	80%	[8]
   g-C3N4/diatomite/MnO2	Tetracycline HCl (50 mg L1)	Visible; 30 mg; 60 min	87%	[8]
   g-C3N4/MnO2/GO (CMG-10)	Tetracycline HCl (10 mg L1)	Visible, 300 W Xe; 50 mg; 60 min	91.4%	[8]
   Pumice-supported rGO/MnO2	Ciprofloxacin	Simulated sunlight; 6 h	80%; reusable over 3 cycles	[8]

   The review acknowledges several limitations associated with MnO2 carbon composites. These composites typically achieve only 80 to 90% removal efficiency. Additionally, powdered catalysts present challenges in terms of even dispersion and recovery. There is also a risk of adsorbed contaminants being released into the environment during recycling. Furthermore, transitioning from laboratory conditions to real water bodies poses a significant challenge. The review suggests optimizing the MnO2 carbon ratio, incorporating metal or non-metal dopants, adding polymers or oxides, and developing higher-dimensional structures that are easily separable [8].

8. DARK AND LIGHT-DRIVEN TRANSFORMATION OF PHARMACEUTICALS ON MNO2

   1. Dark oxidation and the role of pH and phase

      Acetaminophen serves as an effective probe for assessing MnO2 reactivity. At neutral pH, layered -MnO2 exhibited a faster removal rate compared to tunnel -MnO2, attributed to its superior catalytic potential. Conversely, -MnO2 demonstrated a higher removal rate in acidic conditions due to its larger surface area and reduced electrostatic repulsion. The primary pathways involved were dimerization and hydrolysis oxidation, with acidic pH mitigating the environmental risk of the products [23]. Mechanistically, adsorption played a significant role under acidic conditions but was negligible at neutral and alkaline pH levels. Consequently, electron transfer was the rate-limiting step in acidic media, while precursor-complex formation limited the rate in neutral and alkaline conditions. The kinetics were characterized by surface-complexation, Michaelis Menten, or pseudo first order models, contingent on the loading [24]. For – MnO2, optimal removal was achieved at a high dose (800 mg L1), 50 °C, and pH 3.0 to 3.5, involving an endothermic reaction with dimer/trimer intermediates that were subsequently consumed [25].

   2. Photo-enhanced transformation

      Light alters the image. With pseudo-first-order constants of

      0.299 and 0.116 min1, freshly formed colloidal MnO2 (flower- like -MnO2) removed nalidixic acid and gemfibrozil under UV,

      pseudo-first-order kinetics under simulated sunlight, with rate constants of 4.6, 6.4, and 7.3 × 103 min1, greater than in the

      dark (1.6, 1.8, and 6.0 × 103 min1) [26]. Photolysis dominated for – (55.6%) and -MnO2 (40.3%), while MnO2 oxidation dominated for -MnO2 (about 70.1%), according to a contribution analysis that divided removal among photolysis, MnO2 oxidation, and reactive manganese species (RMnS); XPS identified Mn (III) as the primary RMnS, and EPR verified O and 1O2 in both dark and sunlight systems [26]. This partition serves as a helpful CIP template, which also undergoes direct photolysis.

9. MN-BASED SYSTEMS THAT DEGRADE CIPROFLOXACIN

   1. Photocatalysis with Mn/Co oxide

      A solgel Mn/Co oxide degraded CIP under sunlight faster than photolysis alone (k = 7.9 × 102 min1), with best activity at pH 4, high catalyst dose (80 mg) and low CIP concentration (0.01 mM); scavenger tests showed that ethanol and DMSO strongly suppressed the reaction, implicating radical species [27]. This is the only study in the compilation in which a pristine Mn oxide based photocatalyst is applied to CIP under sunlight.

   2. MnO2-activated peroxymonosulfate

      4

      More research has been done on MnO2’s ability to activate PMS. Tracking specific surface area and crystallinity, hydrothermal -MnO2 nanoflowers, nanorods, and nanoparticles triggered PMS for CIP degradation in the following order: flowers, rods, particles, commercial MnO2. Eleven oxidation products were found; piperazine-ring cleavage, hydroxylation, and F/OH substitution were the main pathways; removal was favored in acid and accelerated in alkaline media; bicarbonate and chloride had concentration- dependent dual effects; natural organic matter was harmful; and acute toxicity decreased over time [28]. Comparing -, -, -and -MnO2, the -phase activated PMS most efficiently across sulfonamides, CIP and azithromycin because of high surface hydroxyl content, unique structure and large surface area, operating at pH 3.5 to 9.0 through inner-sphere PMS complexation and Mn (III)/Mn (IV) cycling that produced SO ,

      2 2

      outperforming commercial MnO2 due to superior adsorption; OH, O and 1O EPR detected OH, O, and O – VB-XPS and UVvis results

      [29].

      showed that ROS arise solely from electron reaction with oxygen, with holes dominating nalidixic acid oxidation [6]. The closest analogue to CIP in the provided MnO2 photocatalysis literature is nalidixic acid, which is a quinolone in and of itself.

      Every polymorph was accelerated by light since methotrexate degradation by -, -, and -MnO2 followed

   3. Coupling light with PMS activation

      Magnetic -Fe2O3 MnO2, characterized by adjustable oxygen vacancies through varying the amorphous MnO2 content, achieved a 98.3% removal of CIP within 30 minutes under visible light/PMS conditions (k = 0.114 min1). An optimal vacancy density enhanced charge separation, while a Z scheme

      heterostructure maintained a robust redox potential. Both radical and non-radical pathways were active, and selectivity was preserved despite the presence of co-existing anions. The catalyst demonstrated stability, magnetic recoverability, and reusability [30]. This hybrid system exemplifies an optimal design strategy for MnO2, wherein the oxide is coupled with a secondary phase that facilitates charge separation or magnetic recovery, while maintaining Mn redox activity.

   4. Comparative CIP performance and secondary evidence

      In addition to the four principal Mn CIP studies, the provided reviews present further findings related to MnO2 and CIP degradation. Li et al.’s review indicates that -MnO2, when

      combined with dielectric-barrier discharge, achieved a 93.1% degradation of CIP within 50 minutes. Furthermore, copper based bimetallic oxides activated by H2O2 attained complete degradation, and wire-like -MnO2 was identified as the most active crystalline form for SO – generation [7]. The review on photocatalytic dyes highlights a MnO2/Mn2O3/Mn3O4 mixed valence photocatalyst, which removed 95.6% of CIP in 40 minutes under visible light, with an optimal pH of 7 within the tested pH range of 4 to 10 [9]. Table 7 places these alongside the primary studies.

      4

      Table 7. Mn based ciprofloxacin removal systems, primary and secondary evidence.

      Mn system	Process	Key conditions	Result	Ref.
      Mn/Co oxide (solgel)	Sunlight photocatalysis	pH 4; 80 mg; 0.01 mM CIP	k = 7.9 × 102 min1	[27]
      -Fe2O3MnO2	Visible light + PMS	Oxygen-vacancy tuned	98.3% / 30 min; k = 0.114 min1	[30]
      -MnO2 nanoflowers	PMS activation	Acidic or alkaline pH	Best of four -morphologies; 11 products	[28]
      -MnO2	PMS activation	pH 3.59.0	Best of , , ,	[29]
      MnO2/Mn2O3/Mn3O4	Visible photocatalysis	pH 7 optimum	95.6% / 40 min (secondary)	[9]
      -MnO2 + DBD	Plasma		93.1% / 50 min (secondary)	[7]
      PS@rGO@MnO2	Simulated sunlight	3 reuse cycles	80% / 6 h (secondary)	[8]

      The table shows that light, oxidant or plasma coupling is generally needed to reach greater than 90% CIP removal within an hour; none of the supplied studies reports a stand-alone, dark MnO2 oxidation of CIP of comparable speed. A fair interpretation is that MnO2 alone is a modest CIP oxidant at near-neutral pH, whereas its role as a PMS activator, as a visible- light photocatalyst in a heterostructure, or as a source of reactive Mn (III) in sunlight provides the strong effects.

   5. Case-study synopses of the primary MnCIP studies

      3

      -MnO2/PMS (Deng et al.). Three -MnO2 morphologies were synthesized hydrothermally and evaluated against commercial MnO2. The activity hierarchy observed was nanoflowers, nanorods, nanoparticles, commercial, primarily influenced by specific surface area and crystallinity. This study is distinguished by its comprehensive examination of water chemistry, including Cl, HCO , and NOM. It employs UPLC MS/MS to monitor eleven oxidation products, with five exhibiting a consistent increase and four declining post- formation. Additionally, the study assesses acute toxicity, which decreased with treatment progression and increased PMS dosage, and provides fluorescence EEM evidence of functionality within an actual water matrix [28].

      -MnO2/PMS (Khan et al.). -MnO2, synthesized through a straightforward thermal treatment, demonstrated superior efficiency in eliminating sulfamethoxazole, sulfachloropyridazine, sulfamethazine, ciprofloxacin, and azithromycin compared to -, -, and -MnO2. This enhanced performance is attributed to its high surface hydroxyl content, structural characteristics, and surface area. The process generated sulfate, hydroxyl, and superoxide radicals, as well as singlet oxygen, with superoxide (O ) serving as the 1O

      minimal interference from inorganic anions or natural organic matter (NOM) [29].

      -Fe2O3MnO2 (Zhao et al.).The proportion of amorphous- MnO2 was adjusted to optimize the oxygen-vacancy density, with the 2:1 composite demonstrating superior performance. The Z scheme heterostructure preserved a high redox potential, while the vacancies facilitated charge separation. The synergy between photocatalysis and PMS activation, along with the coexistence of radical and non-radical pathways, accounted for the 98.3% removal efficiency achieved within 30 minutes. Additionally, the catalyst was magnetically recoverable and maintained high selectivity in the presence of anions [30].

      Mn/Co oxide (Alam et al.). The solgel Mn/Co oxide was characterized using FESEM, XRD, BET, TGA, and diffuse reflectance techniques, and its performance was evaluated under sunlight. The photocatalytic process demonstrated superior efficiency compared to photolysis, with the reaction rate being influenced by pH (optimal at 4), catalyst dosage (80 mg), and CIP concentration (optimal at 0.01 mM). The presence of scavengers such as ethanol and DMSO significantly inhibited degradation. The primary advantage of this system is its reliance solely on sunlight and a simple oxide; however, the study lacks data on product identification, toxicity, and reuse, as indicated in the provided pages [27].

10. Mechanisms and transformation pathways

    2

    Figure 1 illustrates a composite mechanism derived from the source studies. Upon photo-excitation, MnO2 generates electronhole pairs, where electrons reduce O2 to O (and, through subsequent steps, to 1O2 and OH), while holes oxidize CIP directly or water to OH [6,9]. In the presence of PMS, the

    2 2

    cycling of surface Mn (III)/Mn (IV) produces SO and OH, as

    precursor. The addition of carbonate (10 mM) reduced sulfamethoxazole removal efficiency from 99% to 45%, indicating a significant role for superoxide. The proposed mechanism involves inner-sphere PMS MnO2 complexation and electron transfer between Mn (III) and Mn (IV). The system operated effectively across a pH range of 3.5 to 9.0, with

    4

    well as non-radical 1O2 [29,30]. In the absence of light, inner- sphere complexation followed by electron transfer to Mn (IV) results in Mn (III) intermediates, which function as reactive manganese species [24, 26].

    Figure 1. Composite mechanism for MnO2-based photocatalytic and PMS assisted CIP oxidation, synthesized from [6, 26, 2830].

    Fluoroquinolones undergo transformation through similar pathways across various systems, involving processes such as oxidation or the opening of the piperazine rig, hydroxylation, F/OH substitution, and decarboxylation or oxidation of the quinolone core [15, 28]. The toxicity results are promising but vary depending on the pathway: acute toxicity decreased during -MnO2/PMS treatment, with a higher PMS dose enhancing detoxification [28]; photocatalytic treatment using BiOX/GaMOF reduced both predicted and bacterial toxicity

    [16]; and oxidation products of the quinolone moiety from 1O2 were found to be the least toxic [15]. In the dark oxidation of acetaminophen by MnO2, an acidic pH lowered the risk associated with the products, although dimerization can lead to larger species that require monitoring [23, 25].

11. KINETICS AND MODELLING

    Photocatalytic and oxidative kinetics: Nearly all CIP degradation studies in the literature adhere to pseudo-first-order kinetics, including those involving CuFe2O4/rGO/halloysite [18], Mn/Co oxide [27], and -Fe2O3 MnO2/PMS [30]. This model is also applied to the removal of micropollutants using UV/MnO2 [6], methotrexate removal with sunlight/MnO2 [26], and humic acid coated TiO2 [38]. Table 8 collects the reported constants. Because pseudo-first-order constants depend on catalyst dose, pollutant concentration, light intensity and reactor geometry, they should be compared only qualitatively, or after normalization by catalyst mass, surface area or photon flux.

    Table 8. Reported pseudo-first-order rate constants.

    System	Pollutant	k (min1)	Ref.
    -Fe2O3MnO2 / visible light / PMS	Ciprofloxacin	0.114	[30]
    Mn/Co oxide / sunlight	Ciprofloxacin	7.9 × 102	[27]
    UV / colloidal MnO2	Nalidixic acid	0.299	[6]
    UV / colloidal MnO2	Gemfibrozil	0.116	[6]
    Humic-acid-coated TiO2 / LED white light	Bisphenol A	1.7 × 102	[38]
    Sunlight / -MnO2	Methotrexate	7.3 × 103	[26]
    Sunlight / -MnO2	Methotrexate	6.4 × 103	[26]
    Sunlight / -MnO2	Methotrexate	4.6 × 103	[26]
    Dark / -MnO2	Methotrexate	6.0 × 103	[26]

    Figure 2. Reported pseudo-first-order rate constants from Table 8 (dark bars: Mn-based systems). Values are from different experimental set-ups and are for qualitative comparison only [6, 26, 27, 30, 38].

    Surface-limited kinetics on MnO2: The kinetics of dark oxidation on MnO2 do not consistently follow first-order behavior. When acetaminophen is present in amounts greater than the available reactive surface sites, a model based on surface complexation and transformation is applicable. In contrast, acidic conditions are best described by a Michaelis Menten model, while neutral and alkaline conditions align with a pseudo-first-order model [24]. Analyzing -MnO2 using the Arrhenius equation revealed a positive activation energy, suggesting the reaction is endothermic. The data fit two assumptions regarding the rate-limiting step: either electron transfer or complex formation [25]. This implies that rate constants determined at a specific MnO2-to-pollutant ratio cannot be generalized to other ratios, explaining why initial concentration effects vary across different polymorphs [26]. For singlet oxygen, fluoroquinolones generally undergo reactions characterized by second-order kinetics, and it has been suggested that listing second-order constants along with reaction pathways could serve as a method for comparing oxidants [15].

    Adsorption models: CIP adsorption is typically modeled using Langmuir or Freundlich isotherms and follows pseudo- second-order kinetics [11, 14]. An examination of isotherm

    theory categorizes models based on their parameter count, advises on careful selection criteria, and highlights that while the widely used linear-regression method is straightforward, it can skew parameters. Therefore, non-linear fitting and model- selection statistics are recommended [12]. These considerations are equally relevant to MnO2 studies, where adsorption initiates surface oxidation.

12. FACTORS GOVERNING PERFORMANCE

    pH: Acidic conditions generally enhance MnO2 oxidation of organics by improving adsorption and limiting electron-transfer oxidation [2325]. Mn/Co photocatalysis of CIP reached its maximum efficiency at pH 4 [27], whereas -MnO2/PMS was effective across a pH range of 3.5 to 9.0 [29], and -MnO2/PMS showed increased activity in alkaline environments [28]. Since CIP is zwitterionic, its speciation affects interactions with the MnO2 surface charge; the pH-dependent net charge of -MnO2 was found to influence dye degradation [21].

    Catalyst dose and pollutant concentration: An increased dose of MnO2 enhanced the removal of methotrexate across all polymorphs, while the initial concentration of the pollutant had varying impacts on the polymorphs, indicating different mechanisms [26]. A high Mn/Co dose combined with a low CIP concentration promoted CIP removal [27]. However, an excess of catalyst can hinder the degradation of certain dyes, as seen with Congo red, where the trend reversed beyond a 10 mg dose [21].

    Water matrix: Chloride and bicarbonate exhibited dual effects dependent on concentration, and natural organic matter inhibited -MnO2/PMS [28]. In contrast, -MnO2/PMS was only slightly influenced by inorganic anions and NOM in the case of sulfamethoxazole [29], and -Fe2O3 MnO2 maintained high selectivity even with co-existing anions [30]. Fluorescence EEM results showed that -MnO2/PMS remained effective in actual water conditions [28].

    Crystal phase and morphology: The polymorphs are ranked by surface area, crystallinity, 2D morphology, and defect/vacancy chemistry [20, 28, 30]. Overall, -MnO2 and – MnO2 consistently outperform – and -MnO2 in neutral systems [20, 23, 26, 29], although -MnO2 nanoflowers can be potent

    PMS activators [28].

    Table 9. Summary of factors controlling CIP and related removal on Mn-based systems.

    Factor	Observed effect	Ref.
    Solution pH	Acid favours MnO2 oxidation of acetaminophen (-MnO2 optimum 3.03.5); Mn/CoCIP optimum pH 4; -MnO2/PMS works at pH 3.59.0; -MnO2/PMS accelerated in alkali; CuFe2O4/rGO/HNT effective at neutral pH	[18, 25, 2729]
    Catalyst dose	Higher dose raises rate (methotrexate, Mn/CoCIP); reversed for Congo red above 10 mg	[21, 26,27]
    Pollutant concentration	Low CIP concentration favoured removal; polymorph-specific effects of initial concentration	[26,27]
    Anions (Cl, HCO3)	Concentration-dependent dual effects on -MnO2/PMS; minor effects on -MnO2/PMS	[28,29]
    Natural organic matter	Detrimental for -MnO2/PMS; small effect on -MnO2/PMS	[28,29]
    Light source and intensity	Sunlight, visible and UV all enhance MnO2 reactions; photolysis contributes strongly for – and -MnO2	[6, 26,27]
    Temperature	Positive activation energy; endothermic on -MnO2	[25]
    Ionic strength (adsorption)	Na+ and Ca2+ suppress CIP uptake on GO-ype sorbents	[13,14]

13. ANALYTICAL AND CHARACTERIZATION METHODS

    Credible conclusions regarding MnO2 CIP chemistry necessitate the integration of materials, analytical, and biological

    methods, as outlined in Table 10. The characterization of materials through techniques such as XRD, Raman spectroscopy, SEM/TEM, BET, XPS, and UV vis diffuse reflectance with Tauc analysis determines the phase,

    morphology, surface area, oxidation state, and band gap [19

    O , and 1O in both UV/MnO and sunlight/dark MnO

    systems

    2 2 2 2

    21]. Reactive species are identified via scavenger experiments, which have implicated OH and species sensitive to DMSO or ethanol in Mn/CoCIP photocatalysis, and OH in ZnO/GO systems [17, 27]. Additionally, EPR analysis has detected OH,

    [6, 26]. Given that scavengers may react with Mn species and oxidants, it is essential to corroborate findings with both EPR and probe-compound evidence.

    Table 10. Analytical and characterization techniques used in the source studies.

    Technique	Purpose	Examples in source papers
    HPLC;UPLCMS/MS; RPLCQToF	Quantify CIP; identify intermediates	Environmental sampling [1]; 11 CIP products [28]; acetaminophen dimers/trimers [25]
    XRD, Raman, SEM/TEM, BET	Phase, morphology, surface area	MnO2 polymorphs [20]; -MnO2 sheets [19]; Mn/Co oxide [27]
    XPS, valence-band XPS	Mn oxidation state; band edges	Mn(III) as RMnS [26]; ROS formation pathway [6]
    EPR	Detect OH, O2- , 1O2	[6, 16, 26]
    UVvis, Tauc plot	Band gap	[21,22]
    Scavenger tests	Identify dominant species	[17, 27]
    Fluorescence EEM	Behavior in natural water background	[28]
    Toxicity prediction; bacterial assays	Product toxicity	[16, 28]
    16S rRNA sequencing; ARG profiling	Resistance and microbiome context	[1]
    Voltammetry (LSV/CV)	Electrocatalytic performance of Mn oxides	[22, 32]

    In the Hong Kong monitoring study, 0.45 µm-filtered water samples were analyzed by HPLC, and the bacterial community was characterized by amplifying the V3V4 region of the 16S rRNA gene with primers 341F and 806R [1]. Pairing chemical removal data with ARG and community data of this kind is rare in the catalyst literature and would add considerable value to MnO2CIP studies.

14. COMPARATIVE ASSESSMENT OF TREATMENT STRATEGIES

    Table 11 compares the strategies discussed. Adsorption is robust and simple but non-destructive and pH-sensitive [11, 13].

    TiO2 photocatalysis is mature but limited by a wide band gap and recombination [4, 5]. Heterojunction photocatalysts (ZnO/GO, CuFe2O4/rGO/HNT, BiOX/GaMOF) give excellent laboratory removal in an hour or less [1618]. MnO2 based routes add earth-abundant redox chemistry and a versatile role as PMS activator, and avoid precious metals [28, 29]. Electrochemical oxidation is fast but electrode- and energy- dependent [1], whereas singlet-oxygen oxidation offers selectivity and toxicity reduction but its generation by practical catalysts is still developing [15].

    Table 11. Strengths and limitations of CIP treatment strategies.

    Strategy	Strengths	Limitations	Ref.
    Adsorption	Simple; wide range of sorbents; high capacities	No destruction; pH and ionic-strength sensitive; regeneration loss	[11, 14]
    TiO2-based photocatalysis	Mature; stable; solar-capable	Wide band gap; recombination; catalyst recovery	[4,5]
    Heterojunction photocatalysis (ZnO/GO, BiOX/GaMOF)	Near-complete CIP removal in ~1 h; reduced toxicity	Complex synthesis; Bi/Ga cost; lab-scale	[16,17]
    MnO2 dark oxidation	Earth-abundant; strong at acidic pH	Mn leaching; pH-limited; slow for CIP at neutral pH	[23,24]
    MnO2 photocatalysis / sunlight	Visible response; Mn(III) boost; solar energy	Few CIP-specific studies; recombination; agglomeration	[9, 26,27]
    MnO2/PMS and light/PMS	Radical and non-radical routes; >98% in 30 min	Oxidant cost; sulfate residue; matrix effects	[2830]
    Electrochemical (PbO2)	CIP removed in 15 min; ARG mitigation	Electrodes, energy, lead-based anode	[1]
    1O2 -based AOPs	Selective; robust to matrix; less toxic products	Generation and detection challenges	[15]

    1. Integrating the evidence: a framework for selecting MnO2 systems

       Upon reviewing the source literature, three distinct regimes of MnO2 behavior are identifiable. In the surface-oxidation regime, which occurs under dark or weak illumination and in acidic to neutral pH conditions, reactivity is primarily influenced by adsorption, the reducibility of Mn (IV), and the availability of surface sites. Under these conditions, layered -MnO2 and high-surface-area – or -forms exhibit optimal performance, and the kinetics are saturable [2325]. In the photo-enhanced regime, which involves exposure to UV, sunlight, or visible light, photo-generated carriers and Mn (III) augment surface oxidation, and for certain polymorphs, direct photolysis of the

       pollutant may predominate. The contribution of each pathway in this regime is contingent upon the specific polymorph and pollutant involved [6, 26]. In the oxidant-activation regime, MnO2 functions as a redox mediator for PMS, with surface hydroxyl groups, Mn (III)/Mn (IV) cycling, and oxygen vacancies playing crucial roles in the production of both radical and non-radical species [2830].

       These regimes suggest a practical decision logic for CIP. Where acidic effluents or pretreatment steps are acceptable, dark MnO2 may be a low-cost polishing step but should be tested for Mn release. Where sunlight is available, -MnO2 or Mn (III)- rich heterostructures are the most promising candidates, but need CIP-specific verification. Where rapid, matrix-tolerant

       removal is the goal, MnO2/PMS or visible-light/PMS systems deliver greater than 95% removal in 30 min in the laboratory, at the price of an added oxidant [29,30].

       Discrepancies and open questions. Several results in the corpus are not fully reconciled. -MnO2 is the weakest sunlight/MnO2 polymorph for methotrexate but a strong PMS activator for CIP [26, 28], -MnO2 outperforms -MnO2 for PMS activation while -MnO2 is the most active in photocatalytic dye degradation [20, 29] and acid improves most MnO2 oxidations whereas alkaline conditions accelerate – MnO2/PMS [24, 28]. These differences probably reflect different rate-limiting steps (adsorption, electron transfer, oxidant activation, light absorption) and different synthesis routes, rather than inconsistency, and they argue strongly for pollutant-specific testing.

15. LESSONS FROM ADJACENT MNO2, TIO2 AND CARBON-COMPOSITE CATALYSIS

    The compilation encompasses studies byond water-phase antibiotic removal, whose design principles are transferable. Pt- loaded MnO2 nanoarrays on honeycomb cordierite achieved a T50 of 189 °C for acetone oxidation with 42 hours of water resistance, indicating that structured, monolithic MnO2 supports can enhance surface lattice oxygen activity [31]. This finding suggests that immobilized MnO2 arrays could serve as a pathway to recoverable water catalysts. In the field of electrocatalysis, Ni-doped MnO2 demonstrated improved oxygen reduction [22]. Additionally, Ni-MnO2/TiO2 reached a current density of 10 mA cm2 at 1.29 V for urea oxidation [32], while PtCu/RGOTiO2 exhibited a higher ORR limiting current density (6.14 mA cm2) compared to Pt/C [33]. Collectively, these results demonstrate that interfacial engineering between MnO2, TiO2, and graphene-type supports enhances charge transport.

    In the field of water treatment, GOTiO2 photoanodes were able to nearly completely decompose 0.5 mg L¹ of PFOA within 4 hours, resulting in the identification of 18 transformation products [34]. Similarly, Fe/sulfonated-ZrO2 achieved full conversion of clofibric acid in 1 hour and 57% mineralization over 6 hours through Fenton-like catalysis, notably without iron leaching [35]. Furthermore, TiO2 supported on nickel slag increased methylene blue removal efficiency from 48% (via photolysis) to 87.5% (via photocatalysis), highlighting the efficacy of waste-derived supports [36]. These examples underscore three critical design considerations pertinent to MnO2/CIP: the use of supports or heterojunction partners for effective charge separation, the importance of immobilization for recovery, and the necessity of benchmarking against direct photolysis.

16. PRACTICAL CONSIDERATIONS FOR IMPLEMENTATION

    Catalyst separation and reuse: Powder catalysts exhibit uneven dispersion and pose challenges in recovery, with quantitative losses during recycling potentially affecting the fate of adsorbed contaminants [8]. Potential solutions include magnetic separation (-Fe2O3 MnO2) [30], immobilization on structured supports such as MnO2 nanoarrays on cordierite honeycombs and pumice-supported rGO/MnO2 [8, 31], and the use of waste-derived supports like nickel slag [36].

    Stability, leaching and safety: A review of visible-light antibiotic photocatalysts highlights that most research is conducted at the laboratory scale, the removal of photocatalysts from treated water remains insufficiently studied, material stability and reusability at larger scales present significant challenges, and only a limited number of studies evaluate material safety in animal models [3]. For MnO2, the reduction of Mn (IV) during oxidation necessitates monitoring of dissolved Mn and the stability of reduced phases in every study.

    Energy and light: Sunlight-driven demonstrations, such as those involving Mn/Co oxide, ZnO/GO, and pumice-supported rGO/MnO2, facilitate low-energy operations [8, 17, 27]. Conversely, LED white light presents an energy-efficient alternative for catalysts responsive to visible light [38]. Photoreactors utilizing MnO2 must address the challenge of light penetration in colored or turbid water, a well-documented limitation of heterogeneous photocatalysts [16].

    Water matrix and mixtures: Real wastewater comprises chloride, bicarbonate, natural organic matter, and numerous co- contaminants that compete for oxidants or surface sites [4, 28]. Studies focusing on dyes often employ single, relatively easy- to-remove model pollutants, which may result in high efficiencies that do not necessarily translate to complex effluents [9].

    Resistance endpoints: Given that the motivation for ciprofloxacin (CIP) removal includes resistance control, treatment studies should report not only the removal of the parent compound but also the residual antibacterial activity and, where feasible, the reduction of antibiotic resistance genes (ARG) and resistant bacteria [1, 16].

17. CHALLENGES AND RESEARCH ROADMAP

    Evidence gap for pristine MnO2 photocatalysis of CIP: Among the studies provided, only one employs a Mn-oxide photocatalyst directly on CIP under light conditions without an oxidant [27]; the others utilize PMS, UV on different quinolones, or other pollutants [6, 2830]. It is necessary to conduct controlled comparisons of -, -, -, -, and -MnO2 for CIP under identical light, pH, and matrix conditions, ideally employing the photolysis/MnO2 oxidation/RMnS partitioning method used for methotrexate [26].

    Stability and Mn leaching: The reduction of Mn(IV) to soluble Mn(II) during oxidation limits the catalyst’s lifespan and raises concerns regarding effluent quality; therefore, stability, reuse, and metal leaching should be routinely reported, as demonstrated in the magnetic -Fe2O3MnO2 study [30]. Carbon and magnetic supports present promising solutions [8, 30].

    Real waters, mixtures and toxicity: Most studies on MnO2 employ model dyes or single pollutants at concentrations significantly exceeding environmental levels (ng L1 to µg L1 [1]). It is crucial to conduct tests in actual effluent, incorporating mixtures and endpoints such as antibiotic resistance genes (ARG) and antibiotic-resistant bacteria [1, 9]. Standard procedures should include product identification, mineralization, and ecotoxicity assessment [4, 15].

    Hybrid and scalable designs: Promising research directions encompass MnO2 carbon composites [8], S-scheme or Z-scheme heterojunctions with metal-organic frameworks (MOFs) or oxides [16, 30], doping to reduce the band gap [22], immobilized nanoarrays [31], and the integration of adsorption

    and catalysis to concentrate ciprofloxacin (CIP) at the active surface [11, 16]. Machine-learning tools, which are already employed to predict CIP resistance in clinical settings [2], could potentially be utilized in the future to screen catalysts and determine optimal operating conditions.

    1. Materials design

       Design priorities are derived from structureactivity evidence, emphasizing the control of polymorph and defect chemistry, specifically – and -MnO2 with controlled oxygen vacancies [20, 29, 30]. Additionally, narrowing the band gap through doping or heterostructure formation is crucial, as demonstrated by Ni doping, which reduced the gap from 2.78 to

       1.74 eV [22]. Furthermore, coupling MnO2 with conductive or charge-separating partners, such as rGO, g-C3N4, ferrites, or MOFs, in Z-scheme or S-scheme arrangements, is recommended [8, 16, 30]. The design of heterojunctions in the BiOX/GaMOF and -Fe2O3MnO2 systems serves as templates for the necessary interfaces [16, 30].

    2. Mechanistic understanding

       2

       Mechanistic studies should quantify the contributions of direct photolysis, surface MnO2 oxidation, Mn(III)-mediated oxidation, and reactive oxygen species (ROS) for CIP, as has been done for methotrexate [26]. It is also essential to clarify the origin of singlet oxygen (whether as a precursor of O or through direct energy transfer) [6, 29], and to map CIP transformation pathways with product toxicity, linking them to the pathway types identified for singlet-oxygen oxidation [15, 28].

    3. Process engineering and risk

       Key priorities encompass the utilization of continuous-flow or monolithic reactors employing supported MnO2 [31], alongside conducting life-cycle and cost assessments in comparison to TiO2 and adsorption alternatives [4, 11]. Additionally, the integration with biological treatment or adsorption processes is considered. Machine-learning methodologies, which have been successfully applied to predict clinical CIP resistance, may be adapted to forecast catalyst performance based on synthesis and operational variables [2].

       Table 12. Proposed research roadmap derived from the gaps identified in this review.

       Priority	Ga identified	Suggested study	Basis
       1	Few light-driven CIP studies on pristine MnO2 polymorphs	Compare -, -, -, -, -MnO2 for CIP under identical light, pH and dose; partition photolysis, MnO2 oxidation and RMnS	[26,27]
       2	Mechanism of Mn (III) and ROS formation with CIP	XPS, EPR and probe experiments; isotopic or competition tests	[6, 26]
       3	Stability, Mn leaching and reuse	Multi-cycle tests with dissolved-Mn and phase-stability analysis	[3, 30]
       4	Catalyst recovery and scale-up	Magnetic, supported or monolithic MnO2; pilot flow reactors	[8, 30,31]
       5	Real water and mixtures	Test in effluent, river water and with co-contaminants at ngµg L1	[1, 9]
       6	Toxicity and resistance outcomes	Product toxicity, antibacterial activity, ARG tracking	[1, 16, 28]
       7	Standardized reporting	Normalized rate constants, photon flux, control experiments, non-linear model fitting	[12, 26]

18. CONCLUSIONS

CIP is a persistent micropollutant associated with resistance, which cannot be effectively removed through adsorption alone. MnO2 serves as an appealing, cost-effective oxidant and photocatalyst, with its reactivity being influenced by its polymorph, defect state, and pH-dependent surface chemistry. Among these, – and -MnO2 are often the most reactive. The presence of light, PMS, or

both induces the formation of a mixture of h+, OH, SO , O , 1O , and Mn (III) species. These species facilitate the opening of the

4 2 2

piperazine ring, hydroxylation of the molecule, and displacement of fluorine, typically resulting in reduced toxicity. Future advancements require systematic studies of polymorphs for CIP under light, the development of durable and recoverable composite designs, and validation in real water environments with a focus on toxicity and resistance endpoints.

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