Format

Existence of pharmaceuticals like antibiotics in the aquatic environment lead to long-term life threatening risks such as toxicity to aquatic animals and the disturbance of endocrine systems of higher organisms [1,2]. Antibiotics are of great demand as both human and veterinary medicines and their over-use increased their presence in water which is of concern due to the development of antibiotic resistant bacteria [3]. Ciprofloxacin (CIP), chemically is {1-cyclopropyl-6-fluoro1,4-dihydro-4-oxo-7-(piperazine-1-yl)-quinolone-3-carboxylic acid} (Fig.1). A number of bacterial infections including joint and bone infections, abdominal infections, respiratory tract infections, skin infections and urinary tract infections, to name few are being treated using this antibiotic. It is also preferred to treat people who have been exposed to anthrax or certain types of plague. It is a second generation fluoroquinolone antimicro-bial agent with a large spectrum of activity against many Gram-positive and Gram-negative aerobic and anaerobic bacteria. Also, a member of fluoroquinolone group is used world wide as a human and veterinary medicine [4]. A large number of antibacterial drugs are discharged in water bodies which directly reach into aquatic environment and Ru(III) Catalyzed Oxidation of Ciprofloxacin by Iron(III): A Kinetic and Mechanistic Approach

This is an open access journal, and articles are distributed under the terms of the Creative Commons Attribution-NonCommercial 4.0 International (CC BY-NC 4.0) License, which allows others to copy and redistribute the material in any medium or format, remix, transform, and build upon the material, as long as appropriate credit is given and the new creations are licensed under the identical terms. indirectly into animals and human being [5]. Wastewater analysis have detected the presence of fluoroquinolones in various range from dm -3 to ng dm -3 [6]. Due to their harmful effect on human health, these antibacterial compounds have been classified under the class of pollutants [7]. The oxidative transformation of fluoroquinolones antibacterial agents in a water treatment process plays an important role in this concern.
A powerful catalyst, ruthenium(III) is known to be in function in various redox reactions particularly in alkaline medium [8][9][10][11]. A complicated reaction mechanism for catalysis has been figured out. This is due to the formation of different intermediate complexes, free radicals and different oxidation states of ruthenium. An outer sphere mechanism is observed for kinetics of fast reactions between ruthenate(VII), RuO4 − , and manganate(VI) [12]. The rapid exchange between MnO4 2− and MnO4 − has been studied by a variety of techniques [13]. The catalyzed reaction between ciprofloxacin and hexacyanoferrate(III) in alkaline medium using copper as catalyst has been previously studied [14]. A microamount of ruthenium(III) is enough to catalyze the reaction in alkaline medium. Thus, Ru(III) catalyzed oxidation of ciprofloxacin by hexacyanoferrate(III) in aqueous alkaline medium mechanism finds to be more interesting thus we have selected the present study.
Hexacyanoferrate(III) oxidizes both inorganic and organic compounds in acidic, basic and neutral medium [15,16]. It has a redox potential of + 0.45V for [Fe(CN)6] 3− /[Fe(CN)6] 4− couple in alkaline medium leading to final product hexacyanoferrate (II), a stable product [17][18][19]. It acts as a hydrogen atom abstractor [20] and free radical generator [21]. There is a central iron ion at center which is surrounded by six negative CN − ions in an octahedral arrangement.
A literature survey revealed that ciprofloxacin oxidation was carried out using oxidants like KMnO4 both in acidic and alkaline medium, ozone free chlorine, ClO2 and HCF in presence of copper catalyst [14,[22][23][24][25][26]. However, oxidation of ciprofloxacin using hexacyanoferrate(III) using micro amounts of ruthenium catalyst showed a doubled increase in rate and hence the titled reaction is accepted to understand the mechanism of reaction and active species involved in ruthenium catalyzed oxidation of ciprofloxacin.

EXPERIMENTAL
All the chemicals used in the oxidation study were of analytical grade. All the reaction solutions were prepared using double distilled water. Solution of ciprofloxacin (m.w.: 331.346 g/mol) was always prepared freshly just before the experiment. To facilitate the dissolution of ciprofloxacin, a few drops of 1 M KOH were added to the stock solution [27].
The stock solution of oxidizing agent hexacyanoferrate(III) (Merck) was prepared by dissolving K 3 [Fe(CN) 6 ] in double distilled water and this solution was standardized iodometrically [28]. Accurate amount of RuCl 3 (S.D. Fine-Chem.) was weighed out, dissolved in 0.20 mol dm -3 HCl and made up to the mark and this ruthenium(III) solution was ascertained by EDTA titration [29].
Sodium hydroxide (Merck) were used to provide the required basicity and NaNO 3 were used to maintain the ionic strength. Sodium hydroxide is standardized with potassium hydrogen phthalate using phenolphthalein as indicator. Each and every time fresh solutions were used for kinetic run. Corning glass were used for storing the utilized reagents and for studying the kinetics of reaction unless otherwise specified. A double beam UV-visible spectrophotometer fitted with thermostatic compartment and a recorder was used for recording the disappearance of colour during the kinetic study.
Kinetic measurements: Kinetic studies were experimented under pseudo-first order conditions, where the drug concentration is excess over [HCF], at a constant ionic strength in alkaline medium at room temperature. The oxidant was pipetted out into the drug, which also contained the required quantities of NaNO 3 , NaOH and ruthenium. The reaction progress was understood by measuring the variation in colour using spectrophotometrically at 420 nm range where no other major absorption take place. The decrease in the concentration of HCF(III) with the decrease in intensity of colour was indicated by UV spectra (Fig. 2). The application of Beer′s law of HCF at 420 nm has been verified giving = 1050 dm 3 /mol/cm [30]. The k obs (pseudo-first order rate constant) was calculated from the slope of logarithm of absorbance versus time. The pseudofirst order plot was linear upto 80 % completion of oxidation study.

Stoichiometry:
The stoichiometric analysis was done by keeping the reaction mixtures mainly the drug and oxidant in various ratios for 24 h at room temperature in a closed vessel while keeping all other reactant concentration constant (0.1 N NaOH, 0.05 N NaNO3, 5 × 10 -6 mol/dm 3 ruthenium). The results indicated that 1 mol of ciprofloxacin reacts with 2 mol of HCF and following reaction is generated as: For product analysis, acid was added to the reaction mixture and extracted with ethyl acetate. The oxidized product 4-cyclopropyl-7-fluoro-2-hydroxy-6-(piperazin-1-yl)naphthalen-1(4H)-one was identified with the help of TLC and LC-MS (Fig. 3) analysis.

Dependence of ciprofloxacin:
The dependence of ciprofloxacin on the oxidation reaction was understood by changing the concentration of ciprofloxacin in ruthenium catalyzed study from 0.5 × 10 -4 to 4.5 × 10 -4 mol/dm 3 at constant concentration of HCF(III) 2.5 × 10 -4 mol/dm 3 , NaOH 0.1 N, NaNO3 at 0.05 N, Ru(III) 5 × 10 -6 mol/dm 3 at 25 ºC. A steady increase in rate was observed (Table-1). A plot of log kobs versus log [CIP] was linear and the slope was found to be 0.35, thus indicating fractional order dependence.

Dependence of medium:
The hydroxyl ion concentration effect on the present oxidation study on the rate was conducted by varying the concentration of NaOH from 0.01 to 0.25 N at fixed concentration of other reactant ciprofloxacin 2.5 × 10 -3 mol/dm 3 , HCF(III) 2.5 × 10 -3 mol/dm 3 , NaNO3 at 0.05 N, Ru(III) 5 × 10 -6 mol/dm 3 at 25 ºC. Effect of ionic strength and dielectric constant: Sodium nitrate concentration varied from 0.025 to 0.1 mol/dm -3 at 25 ºC. It was observed of having no substantial effect on rate of reaction with change of NaNO3 concentration.
Effect of added product: Ferrous sulphate was added to the reaction mixture from varying concentration keeping other reactants concentration constant and conditions same, but no change was observed in the kinetic reaction.
Presence for free radicals: Acrylonitrile was added to reaction mixture which contained an oxidant, HCF and reductant, ciprofloxacin along with alkali and ionic medium and was kept for 5 undisturbed. Then, it was diluted with methanol where a white precipitate was obtained, indicating the involvement of free radical in the reaction [31,32]. Effect of temperature: The temperature effect on ciprofloxacin oxidation with HCF using ruthenium as catalyst was studied at 308 to 323 K at fixed concentration of substrate, oxidant medium, ionic strength and catalyst (ciprofloxacin oxidation at 2.5 × 10 -3 mol/dm 3 , HCF(III) 2.5 × 10 -4 mol/dm 3 , NaOH at 0.1 N, NaNO3 at 0.05 N and Ru(III) 5 × 10 -6 mol/dm 3 ). The energy of activation is obtained from the slope of the linear Arrhenius plot log k vs. 1/T and the value was found to be 24.9 kJ/mol. Other thermodynamics parameters were calculated using Eyrings equation and tabulated in Table-2 [33][34][35]. Oxidation of ciprofloxacin using hexacyanoferrate(III) is a slow reaction which gets accelerated in the presence of micro amounts of ruthenium catalyst. The stoichiometry between ciprofloxacin and HCF was found to be 1:2, in the presence of Ru(III) catalyst and the orders were unit order with respect to HCF(III) and catalyst also fractional order with respect to substrate and alkali concentration. Ru(III) in alkaline medium exists as hydroxylated species [Ru(OH) × (H2O)6-x](3-x) where x < 6 [36][37][38]. The intermediate complex is formed by the interaction of anionic form of ciprofloxacin and Ru(III). This formed complex undergoes a slow reaction with HCF leading to the product. Hydroxyl ion concentration has a direct influence on reaction rate. Based on observation and experimental results mechanism is proposed for the scheme and rate law is derived. The non-zero intercept for plot between [Ru(III)]/kobs versus 1/[CIP], helps in the prediction of formation of complexes.
The following rate equation can be derived from Scheme-I.
The total concentration 'T' of CIP is given by K1K2[OH -]F[Ru(III)]F term is neglected as the ruthenium concentration is in micro level. Therefore, eqn. 9 is simplified: The total concentration of Ru(III) can be given as: The values of equilibrium constant, K 1 and K 2 are evaluated from the intercepts and slope of the plots of [Ru(III)]/k obs and 1/[CIP] and [Ru(III)]/k obs and 1/[OH -] and is found to be 15.3 dm 3 mol -1 and 9.2 dm 3 mol -1 , respectively (figure not shown).
The entropy of activation (∆S # ) tends to be more negative for reaction involving inner-sphere mechanism, whereas the reactions of positive ∆S # values proceed via an outer-sphere mechanism [39][40][41]. In the present study the high values of ∆S # (Table-2) expresses the mechanism is one electron transfer of inner-sphere nature confirming there is decrease in the randomness during the reaction process. Hence the formed intermediate complex is found to be more ordered than the reactants due to loss of degrees of freedom.

Conclusion
The oxidation of ciprofloaxin using hexacyanoferrate(III) in the presence of micro quantities of ruthenium helps in degradation of the antibiotic present in environment. The reaction was found to be unit order with respect to HCF(III) and ruthenium also fractional order with respect to CIP and OHconcentration. The reaction mechanism involve complex formation and free radical. Stoichiometry was found to be 1:2, that is 1 mol of CIP requires 2 mol of HCF. The major product was found to be 4-cyclopropyl-7-fluoro-2-hydroxy-6-(piperazin-1-yl)naphthalen-1(4H)-one which has detected by LC-MS. Activation parameters were evaluated with respect to slow step of reaction schemes.