O M A Khamaysa
email: khamaysa.oday@univ-oeb.dz

Ilhem Selatnia

Hadjer Zeghache

Hassane Lgaz
email: hlgaz@konkuk.ac.kr

Assia Sid

I.-M Chung

Merzoug Benahmed

Noureddine Gherraf

Paul Mosset

Mohammad Ahmad Oday

Khamaysa

Ill-Min Chung

Oday Mohammad

Ahmad Khamaysa

Enhanced Corrosion Inhibition of Carbon Steel in HCl Solution by a Newly Synthesized Hydrazone Derivative: Mechanism Exploration from Electrochemical, XPS, and Computational Studies

Keywords: Hydrazone derivative, Carbon steel, Corrosion inhibitor, XPS, Molecular dynamics simulation

published or not. The documents may come

Enhanced corrosion inhibition of carbon steel in HCl solution by a newly synthesized hydrazone derivative:

Mechanism exploration from electrochemical, XPS, and computational studies

Introduction

Metals and their alloys play an important role in our daily lives; they are extensively used in domestic life, numerous technological and industrial applications [START_REF] Sanaei | Application of green molecules from Chicory aqueous extract for steel corrosion mitigation against chloride ions attack; the experimental examinations and electronic/atomic level computational studies[END_REF][START_REF] Dehghani | A combined experimental and theoretical study of green corrosion inhibition of mild steel in HCl solution by aqueous Citrullus lanatus fruit (CLF) extract[END_REF].

However, these materials are very prone to undergo corrosion when exposed to aggressive environments, especially those containing SO [START_REF] Verma | A green and sustainable approach for mild steel acidic corrosion inhibition using leaves extract: experimental and DFT studies[END_REF] 2-and Cl -anions, leading to damage of equipment, deterioration of surface, and append a huge economic loss [START_REF] Gupta | Aminoazobenzene and diaminoazobenzene functionalized graphene oxides as novel class of corrosion inhibitors for mild steel: experimental and DFT studies[END_REF][START_REF] Verma | A green and sustainable approach for mild steel acidic corrosion inhibition using leaves extract: experimental and DFT studies[END_REF][START_REF] Ahmed | Synthesis and investigations of heterocyclic compounds as corrosion inhibitors for mild steel in hydrochloric acid[END_REF]. Numerous methods were used to moderate this problem, among them, the application of corrosion inhibitors has been the most effective processes to enhance the resistance of these materials and control the damage caused by corrosion [START_REF] Loto | Inhibition studies of the synergistic effect of chemical compounds on 3CR12 ferritic steel corrosion in acid solutions[END_REF].

The utmost frequently used corrosion inhibitors are organic compounds, comprising moieties with π and non-bonding electrons, heteroatoms (N, O, S,….) and/or polar functional groups [START_REF] Qiang | Experimental and theoretical studies on the corrosion inhibition of copper by two indazole derivatives in 3.0% NaCl solution[END_REF][START_REF] Ralkhal | Synthesis and construction of a highly potent hybrid organic/inorganic anti-corrosive pigment for effective corrosion control of mild steel in simulated seawater[END_REF], which are of paramount importance during adsorption process of inhibitors on metal surfaces [START_REF] Zhang | Synergistic inhibition effects of octadecylamine and tetradecyl trimethyl ammonium bromide on carbon steel corrosion in the H2S and CO2 brine solution[END_REF][START_REF] Usman | Inhibition of API 5L X60 steel corrosion in CO2-saturated 3.5% NaCl solution by tannic acid and synergistic effect of KI additive[END_REF]. Over the years, a vast variety of organic compounds has been tested, especially those including nitrogen "N" such as azine derivatives [START_REF] Abderrahim | 1, 2-bis (4-chlorobenzylidene) Azine as new and effective corrosion inhibitor for copper in 0.1 N HCl: a combined experimental and theoretical approach[END_REF], amino acids [START_REF] Khaled | Corrosion control of copper in nitric acid solutions using some amino acids-A combined experimental and theoretical study[END_REF], imines [START_REF] El-Lateef | Experimental and computational investigation on the corrosion inhibition characteristics of mild steel by some novel synthesized imines in hydrochloric acid solutions[END_REF], Schiff bases [START_REF] El-Lateef | Electrochemical and theoretical quantum approaches on the inhibition of C1018 carbon steel corrosion in acidic medium containing chloride using some newly synthesized phenolic Schiff bases compounds[END_REF],

aminobenzonitrile [START_REF] Sığırcık | Inhibition efficiency of aminobenzonitrile compounds on steel surface[END_REF] and hydrazine [START_REF] Fouda | Evolution of the corrosion-inhibiting efficiency of novel hydrazine derivatives against corrosion of stainless steel 201 in acidic medium[END_REF]. Hydrazone and its derivatives are azomethines characterized by the presence of the triatomic grouping >C=N-N<, which represent an important family of organic compounds possessing a wide variety of properties due to their both hard and soft base characters [START_REF] Rollas | Biological activities of hydrazone derivatives[END_REF]. The azomethine group -NHN=CHin hydrazones is highly reactive, where the two nitrogen atoms have nucleophilic nature and the carbon atom has both electrophilic and nucleophilic characters. Due to these promising features, hydrazones are recently getting more and more interest as corrosion inhibitors [START_REF] Lgaz | Synthesis and evaluation of some new hydrazones as corrosion inhibitors for mild steel in acidic media[END_REF][START_REF] Chafai | Synthesis, spectral analysis, anti-corrosive activity and theoretical study of an aromatic hydrazone derivative[END_REF][START_REF] Lgaz | Improved corrosion resistance of mild steel in acidic solution by hydrazone derivatives: an experimental and computational study[END_REF][START_REF] Chaitra | Inhibition activity of new thiazole hydrazones towards mild steel corrosion in acid media by thermodynamic, electrochemical and quantum chemical methods[END_REF]. Therefore, we have chosen to investigate the potential of a hydrazone derivative to be a potent inhibitor against corrosion of steel in acidic solution. Traditionally, the evaluation of inhibition performance is performed experimentally using several chemical and electrochemical methods. However, it is difficult to unveil the exact mechanism that takes place between metal surface and inhibitor molecule using only these experimental methods. Recently, computational chemistry methods have emerged as a powerful tool in the field of corrosion inhibition to provide more detailed information to investigate the mechanism of inhibition at the atomic scale [START_REF] Guo | Theoretical insight into an empirical rule about organic corrosion inhibitors containing nitrogen, oxygen, and sulfur atoms[END_REF][START_REF] Guo | Toward understanding the adsorption mechanism of large size organic corrosion inhibitors on an Fe(110) surface using the DFTB method[END_REF][START_REF] Lgaz | Corrosion Inhibition Behavior of 9-Hydroxyrisperidone as a Green Corrosion Inhibitor for Mild Steel in Hydrochloric Acid: Electrochemical, DFT and MD Simulations Studies[END_REF][START_REF] Akbarzadeh | Molecular/electronic/atomic-level simulation and experimental exploration of the corrosion inhibiting molecules attraction at the steel/chloride-containing solution interface[END_REF][START_REF] Bahlakeh | Highly effective mild steel corrosion inhibition in 1 M HCl solution by novel green aqueous Mustard seed extract: Experimental, electronic-scale DFT and atomicscale MC/MD explorations[END_REF].

In this effort, the corrosion inhibition performance of 1-(4-isopropylphenyl) -2-

(2,4-dinitrophenyl) hydrazone for CS in 1.0 M HCl was evaluated by means of several chemical and electrochemical techniques. The potential formation of a protective layer and chemical states of the CS surface were discussed through contact angle, X-ray photoelectron spectroscopy (XPS) and SEM analyses. Besides, important theoretical insights were generated from DFT computations and molecular dynamics (MD) simulations, which shed more light on inhibitor-iron interactions.

Experimental

1. Materials and sample preparation

Carbon steel (API 5L-X60) samples with chemical composition of (in wt.%): C (0.26),

Mn (1.35), P(0.03), S (0.03) and the rest Fe were obtained from petroleum storage tanks (Algeria). Rectangular pieces with the same size (1×1×1cm 3 ) were used for all measurements. The CS specimens were treated according to the ASTM G1-03 standard method [START_REF]American Society for Testing and Materials (Filadelfia, ASTM G1-03: Standard Practice for Preparing, Cleaning, and Evaluating Corrosion Test Specimens[END_REF]. The acidic environment is a 1.0 M HCl solution. After preliminary tests, the concentration range of the studied inhibitor was fixed between 10 -4 and 5×10 -3 M. 3) HYD (iso) used as a corrosion inhibitor was obtained as red crystals with 85 % yield through equimolar condensation between DNPH (1) and isopropyl benzaldehyde (2) in an acid medium (Ethanol / H 2 SO 4 (3 drops)) during 4 hours of stirring under reflux (Scheme 1). A full spectral analysis is represented in supplementary information.

Synthesis of inhibitor

(E) -1-(2,4-dinitrophenyl) -2-(4-isopropyl benzylidene) hydrazone (
Scheme 1. Schematic representation of the synthetic pathway for the tested inhibitor HYD (iso).

3. Weight loss and electrochemical measurements

ASTM G 31-72 standard laboratory methodology was adopted to realize the weight loss measurement [START_REF] Astm | Standard Practice for Laboratory Immersion Corrosion Testing of Metals[END_REF]. The CS specimens were immersed for 2 hours in blank and inhibited solution at different temperatures ranging from 298 to 328 K. After the required time, CS coupons were retrieved from test solutions, and corrosion products were removed by a pickling acid solution, then cleaned with distilled water, followed by an acetone wash, and dried before re-weighting them again. Following equations were used to estimate the corrosion inhibition performance of the used inhibitor [START_REF] Djeddi | Study on methylene dichloride and butanolic extracts of Reutera lutea (Desf.) Maire (Apiaceae) as effective corrosion inhibitions for carbon steel in HCl solution[END_REF][START_REF] Orubite | Inhibition of the corrosion of mild steel in hydrochloric acid solutions by the extracts of leaves of Nypa fruticans Wurmb[END_REF]:

Corrosion rate (C R ):

The inhibition efficiency ( :

The surface coverage ( Where is the average weight loss (g), is the surface area of CS in cm², and is Potentiodynamic polarization curves were obtained by sweeping the electrode potential ± 250 mV versus E ocp with a scan rate of 1 mV/s. Inhibition efficiency was defined as below [START_REF] Benahmed | Steel corrosion inhibition by Bupleurum lancifolium (Apiaceae) extract in acid solution[END_REF][START_REF] Boudiba | Experimental, theoretical and mathematical studies of Echium italicum L. extract as a corrosion inhibitor for carbon steel in acidic medium[END_REF]:

where and are the corrosion current densities in blank and inhibited solutions, respectively.

Each test was carried out in triplicate.

4. Computational details

4. 1. Quantum chemical calculations

For the DFT study, HYD (iso) was geometrically optimized using Generalized gradient approximation (GGA) with double numerical basis sets plus polarization (DNP) [START_REF] Perdew | Generalized gradient approximation made simple[END_REF] in aqueous solution (COSMO solvation model) [START_REF] Klamt | COSMO: a new approach to dielectric screening in solvents with explicit expressions for the screening energy and its gradient[END_REF]. All calculations were performed using the Material studio program [START_REF] Akkermans | Monte Carlo methods in materials studio[END_REF]. The convergence thresholds for energy, maximum force, and maximum atomic displacement were of 1×10 -5 Ha, 2×10 -3 Ha Å -1 , and 5×10 -3 Å, respectively, and the quality of optimization was set at fine. Various quantum chemical parameters such as E HOMO , E LUMO , the energy gap (

), the fraction of electron transferred (ΔN) and Fukui indices were obtained from different equations described in Supplementary Material.

4. Molecular dynamics simulation

Recently, molecular dynamics (MD) simulation became a popular technique in the field of corrosion inhibition, shading more light on the adsorption behavior of inhibitor molecules [START_REF] Lgaz | On the understanding of the adsorption of Fenugreek gum on mild steel in an acidic medium: Insights from experimental and computational studies[END_REF][START_REF] Guo | Toward understanding the anticorrosive mechanism of some thiourea derivatives for carbon steel corrosion: A combined DFT and molecular dynamics investigation[END_REF][START_REF] Guo | Anticorrosive Effects of Some Thiophene Derivatives Against the Corrosion of Iron: A Computational Study[END_REF]. The adsorption behavior of HYD (iso) on the iron surface and in the presence of water molecules and HCl was investigated by performing MD simulations using the visualizer, amorphous cell and discover modules implemented in Materials Studio from Accelrys Inc [START_REF] Akkermans | Monte Carlo methods in materials studio[END_REF]. The Fe (110) system is picked up in the present study because it is the most stable among three common Fe substrates [START_REF] Lgaz | On the understanding of the adsorption of Fenugreek gum on mild steel in an acidic medium: Insights from experimental and computational studies[END_REF].

Non-bonding, van der Waals and electrostatic interactions were set as atom-based summations using the Ewald summation method with a cutoff radius of 15.5 Å. More details about the simulation procedure and the calculation of interaction and binding energy between the inhibitor molecule and Fe (110) were reported in another works [START_REF] Abderrahim | 1, 2-bis (4-chlorobenzylidene) Azine as new and effective corrosion inhibitor for copper in 0.1 N HCl: a combined experimental and theoretical approach[END_REF][START_REF] Selatnia | Synthesis and Characterization of a Bis-Pyrazoline Derivative as Corrosion Inhibitor for A283 Carbon Steel in 1M HCl: Electrochemical, Surface, DFT and MD Simulation Studies[END_REF][START_REF] Aoufir | The effect of the alkyl chain length on corrosion inhibition performances of 1,2,4-triazole-based compounds for mild steel in 1.0 M HCl: Insights from experimental and theoretical studies[END_REF][START_REF] Lgaz | Exploring the potential role of pyrazoline derivatives in corrosion inhibition of mild steel in hydrochloric acid solution: Insights from experimental and computational studies[END_REF][START_REF] Dehghani | Detailed-level computer modeling explorations complemented with comprehensive experimental studies of Quercetin as a highly effective inhibitor for acid-induced steel corrosion[END_REF].

5. SEM and contact angle analyses

The surface analysis of corroded and inhibited CS samples was studied using QUANTA FEG250 scanning electron microscope instrument. The static contact angle values were recorded on the CS samples exposed to the uninhibited and inhibited solution by the DSA100 Kruss instrument.

6. XPS Measurements

The X-ray photoelectron spectroscopy is a sensitive tool to probe the chemistry of materials. To this end, elemental analysis and characterization of the CS surface were performed by XPS measurements. Measurements were done on a Kratos Axis Ultra DLD spectrometer employing the following conditions: an analyzer pass energy of 20 kV, with a total energy resolution of 0.9 eV, and Al-Kα X-ray source (hν = 1486.6 eV).

Results and discussion

Weight loss: Influence of inhibitor concentration and temperature

Table 1 and Figure 1 represent the variation in the effectiveness of the investigated inhibitor molecule towards CS acidic dissolution with their concentrations. It is clear from Figure 1a that the presence of HYD (iso) decreases the dissolution of the steel samples, meanwhile the inhibition performence reaches a maximum value of 96.32%

in the presence of 5×10 -3 M of HYD (iso) at 298 K, which reflects its excellent corrosion inhibition properties. This phenomenon is attributed to the coverage of the metal surface by the accumulation of more number of molecules leading to the formation of an adsorbed film from HYD (iso) compound, and therefore, isolating it from the corrosive solution [START_REF] Lgaz | On the understanding of the adsorption of Fenugreek gum on mild steel in an acidic medium: Insights from experimental and computational studies[END_REF]. Considering the effect of temperatures, results in Table 1 reveals an increase in the corrosion rate of CS samples immersed in blank and inhibited solutions with a rise in temperature, suggesting a decline in the strength of adsorption of HYD (iso) molecules at higher temperatures. We also note that the inhibition efficiency decreases from 96.32% (at 298 K) to 91.25 % (at 328 K) (via Figure 1b). This slight decrease confirms that the tested compound acts as an efficient inhibitor at higher temperatures. Activation parameters are provided with their discussion in Supplementary data.

2. Surface analysis by XPS

XPS is a useful tool to probe the chemistry of the organic inhibitor layer formed on the CS surface, which can be crucial in unveiling the adsorption mechanism of HYD (iso) on CS. Measurements were done on the carbon steel surface after its immersion in 1.0 M HCl solution containing 5×10 -3 M of the hydrazone derivative. The obtained XPS spectra and the high-resolution spectra of C 1s, O 1s, N 1s, and Fe 2p signals and their deconvolutions are given in Figures 2 and3. The appearance of peaks of N and O in protected sample surface spectra (Figure 2) confirms the adsorption of HYD (iso) on the iron surface.

The C 1s spectrum (Figure 3a) shows three peaks at 284, 286 and 288 eV. The first peak has the major influence and corresponds to C-C, C-H of aromatic bonds and C=C [START_REF] Li | Investigation of Losartan Potassium as an eco-friendly corrosion inhibitor for copper in 0.5 M H2SO4[END_REF][START_REF] Chugh | A comprehensive study about anti-corrosion behaviour of pyrazine carbohydrazide: Gravimetric, electrochemical, surface and theoretical study[END_REF]. The second peak confirms the presence of the C-N and C=N bonds [START_REF] Yadav | Carbohydrate compounds as green corrosion inhibitors: electrochemical, XPS, DFT and molecular dynamics simulation studies[END_REF]. The last peak may be ascribed to the carbon atom of the C=N + [START_REF] Singh | Electrochemical, surface and quantum chemical studies of novel imidazole derivatives as corrosion inhibitors for J55 steel in sweet corrosive environment[END_REF].

The deconvolution of O 1s high-resolution spectra in Figure 3b gives three components. The first peak located at lower binding energy (530.6 eV) can be attributed to O 2-, and could also be linked to oxygen atoms in Fe 2 O 3 and/or Fe 3 O 4 oxides bonded to Fe 3+ [START_REF] Kumar | Chemically modified biopolymer as an eco-friendly corrosion inhibitor for mild steel in a neutral chloride environment[END_REF]. The small peak located at around 532.2 eV possibly originates from hydroxyl oxygen -OH of iron oxides, such as FeOOH and/or Fe(OH) 3 [START_REF] Kharitonov | Corrosion inhibition of aluminium alloy AA6063-T5 by vanadates: Local surface chemical events elucidated by confocal Raman micro-spectroscopy[END_REF].

A single peak is observed in the N 1s spectra (Figure 3c) at binding energy 399 eV is attributed to N-Fe, due to the coordination of the nitrogen atom in the hydrazone moiety with the iron atom of the steel surface [START_REF] Chugh | A comprehensive study about anti-corrosion behaviour of pyrazine carbohydrazide: Gravimetric, electrochemical, surface and theoretical study[END_REF].

The deconvolution of Fe 2p may be fitted into four main peaks (Figure 3d). The first peak located at 711eV confirms the presence of complexes of ferric ion such as Fe 2 O 3 (i.e., Fe 3+ oxide) and/or Fe 3 O 4 (i.e., Fe 2+/ Fe 3+ mixed oxide) and FeOOH (i.e., oxy-hydroxide). Based on the oxidation state in the literature [START_REF] Ruttanapun | Effects of Pd substitution on the thermoelectric and electronic properties of delafossite Cu1-xPdxFeO2 (x=0.01, 0.03 and 0.05[END_REF], the second peak located at  712 eV is associated to ferryl ions Fe +4 , which can lead to the formation of an oxoiron (IV) complex (Scheme 2) [START_REF] Ruttanapun | Effects of Pd substitution on the thermoelectric and electronic properties of delafossite Cu1-xPdxFeO2 (x=0.01, 0.03 and 0.05[END_REF][START_REF] Kanatani | Magnetic properties of SrTiO3-buffered Ba(Fe0.2Zr0.8)O3-δ films on Si(001) substrates[END_REF], while the peak observed at  715.63

eV is assigned to the satellite of Fe(III). XPS results confirm the effective adsorption of HYD (iso) molecules and how they interact with CS surface.

Scheme 2. The formation of an oxoiron (IV) complex.

Electrochemical measurements

3.1. EIS measurments

The CS/ HCl medium interface in the absence and presence of HYD (iso) at different concentrations were investigated by EIS measurement at 298 K. The obtained results are presented in Figure 4 (a, b andc). In Nyquist plots (Figure 4a), a single depressed semi-circle similar in shape for both uninhibited and inhibited solution is observed, and its diameter increases with increasing inhibitor concentration, indicating that the HYD (iso) molecules adsorbed at the CS/electrolyte interface without changing the corrosion mechanism and the inhibition behavior is controlled by the charge transfer process [START_REF] Dehghani | Integrated modeling and electrochemical study of Myrobalan extract for mild steel corrosion retardation in acidizing media[END_REF]. On the other hand, the imperfection of semicircle refers to the frequency dispersion, which usually results from the roughness and heterogeneity of the metal surface [START_REF] Hanini | Influence of different polyphenol extracts of Taxus baccata on the corrosion process and their effect as additives in electrodeposition[END_REF]. In the bode plots (Figure 4b,c), it is observed that, over a wide spectrum of frequencies, only one-time constant is observed. Such behavior is interpreted as due to a relaxation effect resulting from inhibitor's adsorption on steel surface [START_REF] Benahmed | Adsorption and Corrosion Inhibition Properties of Butanolic Extract of Elaeoselinum thapsioides and Its Synergistic Effect with Reutera lutea (Desf.) Maires (Apiaceae) on A283 carbon Steel in Hydrochloric Acid Solution[END_REF]. The increase in the low-frequency impedance modulation values (Figure 4b) is also attributed to the adsorption of HYD (iso) molecules on the CS surface [START_REF] Hegazy | Three novel di-quaternary ammonium salts as corrosion inhibitors for API X65 steel pipeline in acidic solution. Part I: Experimental results[END_REF]. Moreover, the obvious increase in the phase angle value from 65° in the blank solution to 74° in the presence of 5×10 -3 M of HYD (iso) can be attributed to the formation of a protective layer on the CS surface.

In order to analyze the experimental results, an electrical equivalent circuit model (EEC) was employed (Figure 4d), which consists of a polarization resistance (R p ), a constant phase element (CPE), and a solution resistance (R s ). The parameters obtained by fitting the impedance spectra are listed in Table 2. The constant phase element (CPE) was employed instead of a pure capacitor, which is affected by imperfections of the surface, and its impedance is described by the following expression [START_REF] Selatnia | Synthesis and Characterization of a Bis-Pyrazoline Derivative as Corrosion Inhibitor for A283 Carbon Steel in 1M HCl: Electrochemical, Surface, DFT and MD Simulation Studies[END_REF]:

Where Q is the magnitude of the CPE; j is the imaginary number; ω is the angular frequency; n is the deviation parameter (-1≤ n≤1), which characterizes the properties of CPE. The equation 18 is used to calculate the value of the double layer capacitance (C dl ) [START_REF] Selatnia | Synthesis and Characterization of a Bis-Pyrazoline Derivative as Corrosion Inhibitor for A283 Carbon Steel in 1M HCl: Electrochemical, Surface, DFT and MD Simulation Studies[END_REF]:

According to results reported in Table 2, as the inhibitor concentration increase, its accumulated molecules at the metal/solution interface will adsorb at the inner Helmholtz plane, replacing water molecules. As a consequence, the double layer thickness increases and C dl values decrease, which help blocking the active sites for corrosive dissolution, thus forming a protective layer against acid attack.

Potentiodynamic polarization curves

The Potentiodynamic polarization curves for CS in hydrochloric acid in the absence and presence of different concentrations of HYD (iso) are shown in Figure 5. The corrosion parameters such as corrosion potential (E corr ), corrosion current density (i corr ), anodic and cathodic Tafel slopes (β a and β c ) and the inhibition efficiency ( %)

were listed in Table 3. As can be observed from Figure 5, the presence of the HYD (iso) shifts slightly the corrosion potential to the anodic direction and retarded both hydrogen evolution and metal dissolution reactions. Also, we noted that the values of E corr did not vary drastically and the displacement is less than 85mV relating to the potential of the blank solution (via Table 3), reflecting mixed cathodic/ anodic inhibition effect of the tested inhibitor [START_REF] Dehghani | Integrated modeling and electrochemical study of Myrobalan extract for mild steel corrosion retardation in acidizing media[END_REF]. Moreover, corrosion current density (i corr )

values decreased and the inhibition efficiency increased with the addition of HYD (iso), due to the adsorption of inhibitor's molecules on CS surface, which blocks the active sites [START_REF] Zhang | The inhibition mechanism of imidazoline phosphate inhibitor for Q235 steel in hydrochloric acid medium[END_REF]. The higher inhibition efficiency marked in the presence of different concentrations of HYD (iso), i.e., 90-95%, might be due to the high electron densities on the molecule resulting from the presence of -electrons in the aromatic ring and lone pair electrons.

Adsorption isotherm

For further information about the adsorption process of HYD (iso) molecules onto the metal surface as well as thermodynamic parameters, several adsorption isotherm models were used to fit the experimental data (Langmuir, Temkin, and Frumkin…etc.). Langmuir adsorption isotherm model (Eq. ( 19)) [START_REF] Chaouiki | Assessing the impact of electrondonating-substituted chalcones on inhibition of mild steel corrosion in HCl solution: Experimental results and molecular-level insights[END_REF] showed the best fit with correlation coefficients very close to unity (R²=0.999) at all studied temperatures.

(

) 19
where K ads and θ refer to Langmuir adsorption equilibrium constant and surface coverage. The K ads , which is obtained from the intercepts in Figure 6, is related to the standard Gibbs free energy of adsorption ΔG 0 ads as follow [START_REF] Bribri | The use of Euphorbia falcata extract as eco-friendly corrosion inhibitor of carbon steel in hydrochloric acid solution[END_REF]:

Where R, T, and C solvant denote the perfect gas constant (8.314 J K -1 mol -1 ), the absolute temperature (K), and the concentration of water in solution (55.5 mol/l).

Results are represented in Table 4, from which we can see that K ads values decrease with the increase in temperatures. Obviously, this could be due to the desorption of HYD (iso) molecules at higher temperatures [START_REF] Djeddi | Study on methylene dichloride and butanolic extracts of Reutera lutea (Desf.) Maire (Apiaceae) as effective corrosion inhibitions for carbon steel in HCl solution[END_REF]. The values of are tabulated in Table 4. The higher negative values of the standard free energy change of adsorption approve that adsorption of inhibitor's molecules is proceeding spontaneously; they are ranging from -39.55 to -37.07 KJ mol -1 , which signifies that the adsorption of HYD (iso) occurs via a combination of physical and chemical processes [START_REF] Verma | 5-(Phenylthio)-3Hpyrrole-4-carbonitriles as effective corrosion inhibitors for mild steel in 1M HCl: Experimental and theoretical investigation[END_REF][START_REF] Nwanonenyi | Hydroxypropyl Cellulose as an Efficient Corrosion Inhibitor for Aluminium in Acidic Environments: Experimental and Theoretical Approach[END_REF].

Surface morphology

The surface morphology of CS samples after 2h immersion in blank and inhibited (5×10 -3 M of HYD (iso)) solutions are displayed in Figure 7. The presence of a protective film on the steel surface and its physical properties can be surveyed by contact angle measurements. The obtained results are displayed in Figure 8(a, b), from which we could see that the lowest contact angle (55.0°) was measured for the sample immersed in acidic solution. This could be due to the hydrophilic nature of the surface that makes the drop of water easily speared over the surface. In contrast, a significant change in the contact angle was observed in the presence of HYD (iso) (102.4°), confirming the presence of the hydrophobic film on the surface [START_REF] Singh | Comprehensive investigation of steel corrosion inhibition at macro/micro level by ecofriendly green corrosion inhibitor in 15% HCl medium[END_REF].

Quantum chemical calculation

Global reactivity descriptors

To establish a relationship between the inhibitor structure and its corrosion inhibition performance, and to unveil the inhibition mechanism, density functional theory (DFT)

approach has been adopted. The optimized molecular structure, HOMOs, LUMOs, and molecular electrostatic potential maps were presented in Figure 9. It is obvious that the distribution of the HOMO density cover the entire molecular structure of HYD (iso). This could be attributed to the presence of a conjugation effect and a high electron density, which increases the electron-donating ability of the molecule, and thus an enhanced electron transfer interaction between inhibitor's reactive sites and vacant orbitals of the metal [START_REF] Selatnia | Synthesis and Characterization of a Bis-Pyrazoline Derivative as Corrosion Inhibitor for A283 Carbon Steel in 1M HCl: Electrochemical, Surface, DFT and MD Simulation Studies[END_REF]. On the other hand, the LUMO electron density is well distributed over the 2,4-dinitrophenyl cycle, which an indication of its high ability to receive electrons from the metal surface. Interestingly, the 2,4-dinitrophenyl moiety exhibits both characters, i.e., the ability to receive and dontae electrons, and therefore it is expected to play a crucial role in donor-acceptor interactions between inhibitor molecule and iron atoms. A similar conclusion could also be drawn from the the ESP map, where the red color, which stands for the negative region (nucleophilic attack) is located on the nitro groups (-NO 2 ), indicating its greatest ability to form a covalent bond with d-orbital of metal.

Some global reactivity descriptors calculated in the aqueous phase are listed in Table 5. It is well-known that a higher E HOMO values reveal a high electron-donating ability while a lower value of E LUMO signifies the propensity of a molecule to accept electrons. Moreover, the energy gap E is another index of reactivity of molecules. A lower value of E indicates that an inhibitor molecule is more polarizable and could strongly interact wih a metal surface [START_REF] Ogunyemi | Molecular modeling and quantitative structure-property relationships (QSPRs) of purine derivatives as corrosion inhibitor in acid medium[END_REF]. While it is not possible to discuss these parameters without comparing them with those of a similar compound, it is important to note that energy gap of the present molecule is very low, compared to many reported corrosion inhibitors [START_REF] Aoufir | The effect of the alkyl chain length on corrosion inhibition performances of 1,2,4-triazole-based compounds for mild steel in 1.0 M HCl: Insights from experimental and theoretical studies[END_REF][START_REF] Lgaz | Exploring the potential role of pyrazoline derivatives in corrosion inhibition of mild steel in hydrochloric acid solution: Insights from experimental and computational studies[END_REF][START_REF] Lgaz | Assessing corrosion inhibition characteristics of hydrazone derivatives on mild steel in HCl: Insights from electronic-scale DFT and atomic-scale molecular dynamics[END_REF]. Yet again, this observation confirms our finding that the investigated hydrazone derivative has outstanding corrosion inhibition effects. The obtained ∆N value is positive, suggesting a high electron sharing tendency between inhibitor's molecule and the metal surface [START_REF] Lgaz | Assessing corrosion inhibition characteristics of hydrazone derivatives on mild steel in HCl: Insights from electronic-scale DFT and atomic-scale molecular dynamics[END_REF].

Active sites

The active sites responsible for donor-acceptor (D-A) type interactions between inhibitor molecule and iron surface can be determined more precisely from neutral atomic charge distribution and Fukui indices [START_REF] Mi | Theoretical evaluation of corrosion inhibition performance of three antipyrine compounds[END_REF]. The Mulliken charges and Fukui indices of HYD (iso) are displayed in Figure S3. It is reported that a higher negative charge of heteroatom means a high electronic density, and indicates a high tendency for participation in the D-A types of interaction [START_REF] Kr | Molecular level insights for the corrosion inhibition effectiveness of three amine derivatives on the carbon steel surface in the adverse medium: A combined density functional theory and molecular dynamics simulation study[END_REF]. The analysis of Mulliken results represented in Figure S3 indicates that a high negative Mulliken atomic charge occur on N8, N9, O27, O28, O29, O30 atoms, and some carbon atoms as well. They are expected to be involved in interactions with the metal surface and are the most susceptible reactive sites for adsorption [START_REF] Lgaz | Exploring the potential role of pyrazoline derivatives in corrosion inhibition of mild steel in hydrochloric acid solution: Insights from experimental and computational studies[END_REF]. The widespread distribution of reactive sites is likely one of reasons why tested compound has a potent corrosion inhibition performance.

The Fukui indices are significant for analyzing the local reactive sites responsible for the nucleophilic and electrophilic behavior of an inhibitor. Based on literature, atoms with high values of indicate that these sites have a nucleophilic attack character, whereas those having higher values of are prone to electrophilic attack [START_REF] Lgaz | Improved corrosion resistance of mild steel in acidic solution by hydrazone derivatives: an experimental and computational study[END_REF].

From the calculated Fukui indices, the most preferable sites for the electrophilic From the above analysis, we could conclude that the high inhibitory power of HYD (iso) is a result of its widespread active sites, which play the key role in D-A interactions with the metal surface.

Molecular dynamics simulation

To assess the adsorption of the inhibitor molecule on metal surfaces in a mimic condition, MD simulations were performed. The obtained E interaction value of HYD (iso) on Fe (110) surface is -2402.94 Kcal/mol. This larger negative value suggests that the adsorption of the hydrazone molecule on the iron surface is spontaneous and strong. Radial distribution function (RDF) g(r) (or pair correlation function) is a mathematical tool that describes the shape and positions of a particle with respect to one another. It is defined as follows [START_REF] Hansen | Theory of Simple Liquids: with Applications to Soft Matter[END_REF]:

∑ ∑ (21)
where represents the particle density of B averaged over all shells around particle A.

In general, chemical interactions showing a peak at a position between 1 and 3.5 Å, while peaks located at more than 3.5 Å characterize physical interactions [START_REF] Lgaz | On the understanding of the adsorption of Fenugreek gum on mild steel in an acidic medium: Insights from experimental and computational studies[END_REF]. In present RDF results (via Figure 10(c)), the first prominent peak is located at 3.01 Å, which suggests a chemical bonds formation between the hydrazone molecule and iron atoms. Moreover, several other peaks located outside 3.5 Å may be attributed to physical interactions. Together, these results further support those from experimental and quantum chemical investigations.

Protection mechanism and comparison with other hydrazones

Generally, the mechanism of action of corrosion inhibitor on metal surface in acid medium is supposed to be influenced by the chemical structure of the inhibitor molecules, the nature and charge of the metal. Through the above results, the adsorption of HYD (iso) on CS surface can be explained as follows:

-Corrosion inhibition experiments were carried out in HCl medium, which leads to a positive charge of the metal surface [START_REF] Solmaz | Investigation of adsorption and corrosion inhibition of mild steel in hydrochloric acid solution by 5-(4-Dimethylaminobenzylidene)rhodanine[END_REF]. In addition, the tested inhibitors are expected to be protonated. In this form, the positive charge of the metal will favor the adsorption of chlorides (Cl -) on its surface, creating a bridge between charged metallic surface and protonated inhibitor molecules, and thus results in increases in the attractive forces between them. This suggestion is supported by XPS results, which indicated the presence of a charged nitrogen atom. At this stage, corrosion protection is driven primarily by physisorption, therefore, blocking reaction sites of the mild steel [START_REF] Aoufir | The effect of the alkyl chain length on corrosion inhibition performances of 1,2,4-triazole-based compounds for mild steel in 1.0 M HCl: Insights from experimental and theoretical studies[END_REF].

-As demonstrated in the results described above, the corrosion process could be also blocked by chemisorption. The presence of electron-rich aromatic rings and heteroatoms is expected to increase the transfer of loosely bound electrons from molecules to electron-deficient iron orbitals. Within this mechanism, the presence of two aromatic rings enables the inhibitor's molecule to donate π-electrons to unfilled iron orbitals [START_REF] Lgaz | Exploring the potential role of pyrazoline derivatives in corrosion inhibition of mild steel in hydrochloric acid solution: Insights from experimental and computational studies[END_REF].

-Along with these interactions, it should also be emphasized that the high molecular weight and the large size of the hydrazone derivative can also be considered as another factor that may influence its adsorption on the metal surface. Figure 11 represents all possible interactions between the hydrazone derivative and the iron surface.

In several recent papers, one can find several hydrazone derivatives tested for corrosion inhibition of different metal and alloys. This family of inhibitors has demonstrated good ability to inhibit metal corrosion. A comparison with other hydrazone derivatives in acid media is given in Table 6, and HYD (iso) exhibited better corrosion inhibition behavior.

Conclusion

In the present study, a new synthesized hydrazone derivative was evaluated as corrosion inhibitor for the carbon steel/1.0 M HCl system. The study revealed that HYD (iso) acted as a good corrosion inhibitor and its effectiveness increases by concentration. The adsorption mode of HYD (iso) on the CS surface obeys to Langmuir adsorption isotherm. The spontaneity of adsorption was justified by negative values of , and their value reveals a combination of physical and chemical interactions. Electrochemical results indicated that the hydrazone derivative acted as a mixed-type inhibitor, also, the increment in the values of R p with increasing concentration showed the effective adsorption of inhibitor on the surface.

SEM, contact angle and XPS analysis explained well enough why the investigated compound is an efficient corrosion inhibitor and how it exerts its corrosion inhibition effect. Computational calculations supported experimental findings by providing useful theoretical insights. The results indicated that the presence of nitro groups makes the tested compound an excellent corrosion inhibitor, thereby providing a highest inhibition efficiency compared to previously studied hydrazone derivatives.

Thus, this study could be a target for future research efforts in the corrosion inhibition field. Journal Pre-proof

"Declaration of competing interest

  the exposure time in hour. and are respectively the values of corrosion rate in blank and inhibited solutions. The electrochemical experiment were done a three-electrode system connected to Voltalab instrument (model PGZ 301) and monitored by Voltamaster 4 software. The three electrodes of the used electrochemical cell are: The CS as a working electrode (surface area of 1cm²), a platinum wire as auxiliary electrode, and saturated calomel electrode (SCE) as the reference electrode. To achieve the stabilization of the system, the CS working electrode was immersed in the test solution for 30 min at open circuit potential (E ocp ). After that, electrochemical impedance spectroscopy (EIS) was conducted in a frequency range from 100 KHz to 0.01 Hz with a signal amplitude perturbation of 10 mV. The EC-Lab software was used for simulation and fitting of EIS data, and polarization resistances were used to estimate the inhibition efficiency [31]: where and are polarization resistances in the absence and presence of the inhibitor, respectively.

Figure 7 (

 7 a) displays a large damaged surface area in the absence of inhibitor; the corrosion products covered the whole surface due to the metal dissolution. However, the influence of the addition of HYD (iso) to the acid solution on the morphology of the steel surface is evident, as observed in Figure7(b). It is apparent (via Figure7(b)) that the dissolution of the metal substrate is remarkably decreased, and almost a smoother surface is achieved, suggesting that the studied compound is adsorbed on the metal surface and creating a protective film.

  attack are located in the N (8), N (9), C (11), O (27), O (28), O (29) and O (30) atoms with the most positive part of , whereas the C (14), N (26), O (27), O (28), O (29) and O (30), are the most reactive sites for the nucleophilic attack. Both Fukui indices and Mullikan distribution results are in good agreement with the frontier orbital distribution of inhibitor.

Figure 10 (

 10 a)-(b) displays top and side views of the optimized adsorption configuration of HYD (iso) on the iron surface (in its most stable plane, i.e., (110)). A close examination of the figure revealed that the adsorption of the studied inhibitor on the iron surface is almost parallel, indicating a maximum coverage, which in turn leads to a high interaction with the iron surface.

Figure captions:Fig. 1 .

 1 Figure captions:

Fig. 2 .

 2 Fig. 2. XPS survey spectrum of HYD (iso) treated-carbon steel in 1.0 M HCl.

Fig. 3 .

 3 Fig. 3. XPS spectra of CS specimens in the presence of 5×10 -3 M of HYD (iso): (a) C

Fig 4 .

 4 Fig 4. EIS results: (a) Nyquist; (b) and (c) Bode and phase angle plots for CS in 1.0

Fig 5 .

 5 Fig 5. Potentiodynamic polarization curves for CS steel obtain in 1.0 M HCl in the

Fig. 6 .

 6 Fig. 6. Langmuir isotherm adsorption of HYD (iso) on the CS in 1.0 M HCl at

Fig. 7 .

 7 Fig. 7. SEM micrograph of CS surface (a) in the absence of inhibitor and (b) in the presence of 5×10 -3 M of inhibitor.

Fig. 8 .

 8 Fig. 8. Water contact angle results for the unprotected (a) and protected sample (b).

Fig. 9 .

 9 Fig. 9. The optimized structures, frontier orbital density distributions, and electrostatic

Fig. 10 .

 10 Fig. 10. Equilibrium adsorption configuration of the HYD (iso) on the Fe (110)

Fig. 11 .

 11 Fig. 11. Possible interactions between inhibitor molecule and carbon steel surface.

Figure 1 .

 1 Figure 1.

Figure 2 .

 2 Figure 2.

Figure 3 .

 3 Figure 3.

Figure 4 .

 4 Figure 4.

Figure 6 . 35 Figure 7 .

 6357 Figure 6.

Figure 8 .

 8 Figure 8.

Figure 9 .

 9 Figure 9.

Figure 10 .

 10 Figure 10.

Figure 11 .

 11 Figure 11.

Table 1 .

 1 Weight loss results of differents concentrations of HYD (iso) in 1.0 M HCl at different temperatures for 2 hours.

	Temperature (K)	C (M)	C R (mg cm -2 h -1 )	(%)		θ
		Blank	1.32100.1055	-		-
	298	1×10 -4 5×10 -4	0.12470.0082 0.09190.0670	90.56 93.12		0.9056 0.9312
		1×10 -3	0.07920.0032	94.00		0.9400
		5×10 -3	0.04860.0016	96.32		0.9632
		Blank	1.95210.1241	-		-
	308	1×10 -4 5×10 -4	0.23230.0099 0.15660.0029	88.10 91.98		0.8810 0.9198
		1×10 -3	0.14350.0033	92.65		0.9265
		5×10 -3	0.09570.0042	95.10		0.9510
	318	Blank 1×10 -4 5×10 -4	2.57440.0854 0.34030.0077 0.30070.0050	-86.78 88.32	-	0.8678 0.8832
		1×10 -3	0.22600.0155	91.22		0.9122
		5×10 -3	0.17380.0015	93.25		0.9325
		Blank	3.14230.0074	-		-
	328	1×10 -4 5×10 -4	0.47920.0036 0.43360.0051	84.75 86.20		0.8475 0.8620
		1×10 -3	0.37330.0011	88.12		0.8812
		5×10 -3	0.27500.0049	91.25		0.9125

Table 2 .

 2 Impedance parameters and inhibition efficiency values for carbon steel in 1.0 M HCl solution containing different concentrations of HYD (iso) at 298 K.

	Inhibitor C (M)	R p (Ω cm 2 )	R s (Ω cm 2 )	10 -3 Q (F S -1 )	n	C dl (μF cm -2 )
		Blank 24.371.62 0.2710.040 0.893	0.963 522.34.2 -
		1×10 -4 306.33.10 0.3250.017 0.619	0.990 519.51.7 92.04
	HYD (iso)	5×10 -4 335.02.74 0.1740.036 0.559	0.992 475.02.3 92.73
		1×10 -3 372.91.93 0.3450.010 0.520	0.993 426.71.3 93.46
		5×10 -3 767.51.65 0.2200.024 0.328	0.997 261.22.5 96.82

Table 3 .

 3 Polarization parameters and the inhibition efficiency of carbon steel

	corrosion in 1.0 M HCl medium without and with different concentrations of HYD (iso)
	at 298 K.
	Compound	C (M)	-E corr (mV/SCE)	i corr (mA cm -2 )	β a (mV dec -1 )	-β c (mV dec -1 )
		Blank	454.03.5 1.22680.0012 107.41.6 143.82.1	-
		1×10 -4	449.72.3 0.11580.0011 95.20.8	115.81.9	90.56
	HYD (iso)	5×10 -4	442.61.9 0.09270.0019 51.11.7	115.71.3	92.44
		1×10 -3	447.13.7 0.07190.0024 67.32.4	109.33.0	94.13
		5×10 -3	438.71.5 0.05120.0035 48.91.1	122.82.7	95.82

Table 4 .

 4 Standard thermodynamic parameters of the adsorption of HYD (iso) in 1.0 M

	HCl solution.
	Compound	T (K)	10 -3 (M -1 ) K ads	R²
		298	56.80	0.999	37.07
	HYD (iso)	308	52.55	0.999	38.12
		318	45.17	0.999	38.96
		328	35.80	0.999	39.55

Table 5 .

 5 Calculated quantum chemical indices of the studied compound.

	Descriptors	E HOMO	E LUMO	E	N
		(eV)	(eV)	(eV)
	HYD (iso)	-5.355	-3.725	1.630	0.086

Table 6 .

 6 Comparison of the inhibition efficiency of HYD (iso) with that of some hydrazone derivatives previously published.

	Inhibitor/ Concentration	Metal/ Medium	Inhibition efficiency (%)	Reference
	HZD 2	Mild steel/1 M HCl	94	[67]
	HZD 3	Mild steel/1 M HCl	92	[67]
	HZD 4	Mild steel/1 M HCl	86	[67]
	2-ABNH	Mild steel/1 M HCl	95	[68]
	HZD-1	Mild steel/1 M HCl	95	[20]
	HZD-2	Mild steel/1 M HCl	90	[20]
	HZD-3	Mild steel/1 M HCl	84	[20]
	TH-1	Mild steel/0.5 M HCl	86	[21]

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."

Conflict of 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.

Graphical abstract

Journal Pre-proof J o u r n a l P r e -p r o o f


Gnougon Joohyun Kim

Nina Coulibaly

Sunho Yoon

Aymen Amine Assadi

Khalil Hanna
email: khalil.hanna@ensc-rennes.fr

Sungjun Bae

Gnougon Nina Coulibaly

Aymen Amin

Red mud-activated peroxymonosulfate process for the removal of fluoroquinolones in hospital wastewater

Keywords: flumequine, ciprofloxacin, hospital wastewater, red mud, hydroxylamine, peroxymonosulfate

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

According to a report published by the World Health Organization in 2017 (World Health Organization, 2017), resistance to antibiotics is presently considered as one of the greatest threats to health, food security, and development globally. Antibiotics, which are commonly used to prevent and treat bacterial infections, are the most prescribed drugs worldwide and frequently encountered in hospital and urban wastewater [START_REF] Okeke | Socioeconomic and behavioral factors leading to acquired bacterial resistance to antibiotics in developing countries[END_REF][START_REF] Rodriguez-Mozaz | Occurrence of antibiotics and antibiotic resistance genes in hospital and urban wastewaters and their impact on the receiving river[END_REF]. Indeed, the presence of penicillins, macrolides, sulfonamides, fluoroquinolones, and tetracyclines in hospital wastewater (HW) has been reported with concentrations up to 15 µg/L [START_REF] Reemtsma | Organic Pollutants in the Water Cycle: Properties, Occurrence, Analysis and Environmental Relevance of Polar Compounds, Organic Pollutants in the Water Cycle: Properties, Occurrence, Analysis and Environmental Relevance of Polar Compounds[END_REF]. The amoxicillin concentration measured in HW of a large German hospital was between 28 and 82.7 µg/L, and the ciprofloxacin in concentrations between 3 and 89 µg/L [START_REF] Reemtsma | Organic Pollutants in the Water Cycle: Properties, Occurrence, Analysis and Environmental Relevance of Polar Compounds, Organic Pollutants in the Water Cycle: Properties, Occurrence, Analysis and Environmental Relevance of Polar Compounds[END_REF].

Fluoroquinolones (FQs) are the most potent antimicrobial agents used in human and veterinary medicine. Because their removal during wastewater treatment is incomplete and they are continually being released into the environment, they are frequently detected in aquatic systems [START_REF] Rodriguez-Mozaz | Occurrence of antibiotics and antibiotic resistance genes in hospital and urban wastewaters and their impact on the receiving river[END_REF][START_REF] Watkinson | The occurrence of antibiotics in an urban watershed: From wastewater to drinking water[END_REF]. Particularly, the presence of flumequine (FLU) and ciprofloxacin (CIP) in surface water, groundwater, and sediments at concentrations ranging from nanograms to micrograms per liter have been reported [START_REF] Zhang | Comprehensive evaluation of antibiotics emission and fate in the river basins of China: Source analysis, multimedia modeling, and linkage to bacterial resistance[END_REF].

To remove such antibiotics from wastewater, advanced oxidation processes (AOPs) have attracted considerable attention recently, owing to their ability to generate reactive oxygen species (ROS) that can efficiently remove refractory and non-biodegradable organic contaminants [START_REF] Boczkaj | Wastewater treatment by means of advanced oxidation processes at basic pH conditions: A review[END_REF][START_REF] Deng | Degradation of ciprofloxacin using Α-MnO2 activated peroxymonosulfate process: Effect of water constituents, degradation intermediates and toxicity evaluation[END_REF]. Recently, sulfate radical (SO 4

•-)-based AOPs have become a new focus for wastewater treatment owing to the better selectivity and longer half-life of SO 4

•-in comparison with the hydroxyl radical ( • OH).

Reportedly, peroxymonosulfate (PMS) can be activated by UV [START_REF] Qi | Oxidation of flumequine in aqueous solution by UV-activated peroxymonosulfate: Kinetics, water matrix effects, degradation products and reaction pathways[END_REF], heat [START_REF] Xu | The mechanism of degradation of bisphenol A using the magnetically separable CuFe2O4/peroxymonosulfate heterogeneous oxidation process[END_REF], or a transition metal [START_REF] Sang | Enhanced transition metal oxide based peroxymonosulfate activation by hydroxylamine for the degradation of sulfamethoxazole[END_REF]. However, its activation in UV and heat systems is associated with disadvantages, such as high cost and high energy consumption [START_REF] Zou | Rapid acceleration of ferrous iron/peroxymonosulfate oxidation of organic pollutants by promoting Fe(III)/Fe(II) cycle with hydroxylamine[END_REF]. Additionally, Co 2+ /PMS systems demonstrate the best performance with respect to SO 4 •-generation [START_REF] Anipsitakis | Radical generation by the interaction of transition metals with common oxidants[END_REF]; however, the toxicity of cobalt, as well as the resulting secondary pollution associated with its use, stand as a major setback to the application of Co 2+ /PMS systems [START_REF] Deng | Degradation of ciprofloxacin using Α-MnO2 activated peroxymonosulfate process: Effect of water constituents, degradation intermediates and toxicity evaluation[END_REF][START_REF] Ji | Efficient performance of porous Fe2O3 in heterogeneous activation of peroxymonosulfate for decolorization of Rhodamine B[END_REF]. More recently, it has been reported that replacing cobalt with a nontoxic metal, such as Mn 2+ or Fe 2+ , opens up new avenues for research into the development of more economical and efficient PMS activation techniques [START_REF] Cao | Degradation of tetracycline by peroxymonosulfate activated with zero-valent iron: Performance, intermediates, toxicity and mechanism[END_REF]. Furthermore, to activate PMS for the oxidative degradation of organic contaminants, Fe-containing minerals, including pyrite (FeS 2 ; [START_REF] Feng | Degradation of 1,4-dioxane via controlled generation of radicals by pyrite-activated oxidants: Synergistic effects, role of disulfides, and activation sites[END_REF], hematite (α-Fe 2 O 3 ; [START_REF] Jaafarzadeh | Catalytic degradation of 2,4dichlorophenoxyacetic acid (2,4-D) by nano-Fe2O3 activated peroxymonosulfate: Influential factors and mechanism determination[END_REF], and magnetite (Fe 3 O 4 )/akageneite(β-FeOOH) composites [START_REF] Li | Peroxymonosulfate activation for efficient sulfamethoxazole degradation by Fe3O4/Β-FeOOH nanocomposites: Coexistence of radical and non-radical reactions[END_REF], have been employed, indicating that heterogeneous Fe minerals can be effectively used for PMS activation.

Red mud (RM), which is a massive byproduct of the Bayer process that is used to manufacture alumina [START_REF] Hamid | Novel bimetallic catalyst supported by red mud for enhanced nitrate reduction[END_REF], is well known as a mixed metal composite, primarily composed of Fe 2 O 3 in combination with other species, such as Al 2 O 3 , SiO 2 , CaO, and TiO 2 . Given that it contains CaO, its disposal into the natural environment can cause environmental problems, such as an increase in the pH of the surrounding soil and groundwater [START_REF] Mymrin | Environment-friendly method of high alkaline bauxite's Red Mud and Ferrous Slag utilization as an example of green chemistry[END_REF]. Therefore, many attempts have been made for its reuse as an economical and eco-friendly alternative for environmental remediation technologies [START_REF] Bhatnagar | A review of the use of red mud as adsorbent for the removal of toxic pollutants from water and wastewater[END_REF]. Reportedly, it has been used as an adsorbent for dyes, phenols, and heavy metals [START_REF] Bhatnagar | A review of the use of red mud as adsorbent for the removal of toxic pollutants from water and wastewater[END_REF][START_REF] Sushil | Catalytic applications of red mud, an aluminium industry waste: A review[END_REF]. However, its ability to enhance the advanced oxidation of antibiotics, while eliminating the emerging contaminants from wastewater has not yet been clarified. Therefore, the primary objective of this study was to investigate the ability of RM to enhance the advanced oxidation of antibiotics, while removing antibiotic contamination from wastewater. Additionally, for the first time, its catalytic activity with respect to the removal of antibiotics from hospital wastewater (HW)

under mild conditions was examined, and the ability of hydroxylamine (NH 2 OH, HA), which is a reducing agent, to accelerate ROS generation in the presence of RM was investigated.

In this study, RM was characterized using various surface analysis methods, including Xray fluorescence (XRF), Brunauer-Emmett-Teller (BET) adsorption isotherms, X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS). The FLU removal capacity of the PMS/RM/HA system at an initial pH of 7 was examined, and the effect of different PMS, HA, and RM concentrations on the FLU removal was evaluated. Additionally, the performance of the PMS/RM/HA system was examined in the separated and combined removal of FLU and CIP in HW. Using liquid chromatography-mass spectroscopy (LC-MS), the FLU and CIP oxidation pathways were identified, and the mechanisms of their removal were investigated. 1.

Materials and methods

Chemicals and materials

Oxidation experiments and analytical methods

Sorption experiments were conducted in polypropylene tubes (15 mL) within a pH range of 2-10, under aerobic conditions. HCl and NaOH solutions (0.1 M each) were used to adjust the pH, and RM (0.1 g L -1 ) was added to 5 μM FLU in a final volume of 10 mL. The suspension was stirred for 24 h, and 1.5 mL of the final solution was filtered. Then, its concentration was determined using an Alliance UV controller high performance liquid chromatography (HPLC) system equipped with an autosampler (Waters 717 plus), a C18 column (250 mm × 4.6 mm i.d., 5 μm), and a UV detector (Alliance UV 2489). The FLU and CIP mobile phase, which was set at a flow rate of 1 mL min -1 in the isocratic mode, consisted of a mixture of acetonitrile and water (15/85 v/v and 45/55 v/v, respectively) containing 0.1% formic acid.

To avoid the influence of UV and visible light, all oxidation experiments were performed at ambient temperature (20 ± 2 °C) under dark conditions. Different amounts of RM were dispersed in a 200 mL aqueous solution with FQ (FLU or CIP) concentrations of 5 µM After thorough mixing, different concentrations of PMS and HA were added to initiate the reaction, and the mixtures were shaken at 300 rpm at a temperature of 20 ± 2 °C and a pH of 7.0 ± 0.2.

The pH adjustment was done within 5 minutes after adding RM and PMS by 0.1 M solutions of HCl and NaOH, respectively. For the experiment with HW, the suspended solid was filtered and 5 µM of FQ was spiked into the HW.

To identify CIP and FLU intermediates resulting from the oxidation by the PMS/RM/HA system, a Thermo ultimate 3000 HPLC with an LTQ ion trap mass spectrometer (Thermo Electron Corp., USA) equipped with an ESI source that can be operated in both the positive and negative ion modes was used. More details for LC-MS analysis are provided in supplementary data.

To perform quenching experiments using t-BuOH and i-PrOH, the desired quenchers were added into the reaction solutions prior to the addition of HA and PMS. At each sampling time, 1.5 mL of the treated solution was collected for the determination of the concentrations of FQs using HPLC. To provide the direct evidence of radical generation in the PMS/RM/HA system , we prepared two different systems using PMS/RM/HA and H 2 O 2 /RM/HA at pH 7 and analyzed the radical generation by electron spin resonance spectrometer (ESR, JES-FA200, JEOL). The suspension containing 100 mM 5,5-dimethyl-1pyrroline N-oxide (DMPO) was injected to ESR tube and analyzed at a resonance frequency of 9.45 GHz, microwave power of 0.998 mW, modulation frequency of 100 kHz, field center of 340 mT, modulation amplitude of 1.0 G, sweep width of 1.0 G, time constant of 30 ms and sweep time of 30 s.

The effects of the co-presence of inorganic (phosphate, sulfate, chloride, and nitrate anions) and organic (LHA) species on FLU degradation were investigated in UPW under the same conditions. An ion chromatography instrument (IC, DIONEX DX-120) equipped with a conductivity detector, and an IonPac AS19 analytical column (4 × 250 mm), with KOH solution as the mobile phase, was used to determine the concentration of the inorganic species.

The amount of Fe leached during the reaction was measured using a UV-vis spectrophotometer (GENESYS 10S, Thermo) at 510 nm following the 1,10-phenanthroline method [START_REF] Tamura | Spectrophotometric determination of iron(II) with 1,10-phenanthroline in the presence of large amounts of iron(III)[END_REF]. The concentration of residual PMS was measured by the iodometric titration method [START_REF] Liang | A rapid spectrophotometric determination of persulfate anion in ISCO[END_REF]. After 6 h of reaction, 0.1 mL of aliquot was filtered and transferred to a quartz cuvette containing 2.9 mL of KI and NaHCO 3 solution (600 mM and 70 mM, respectively) for PMS concentration measurement by UV-vis spectrophotometer at 352 nm [START_REF] Liang | A rapid spectrophotometric determination of persulfate anion in ISCO[END_REF]. The leaching of other metals after PMS sequential addition was also monitored using inductively coupled plasma-mass spectrometry (ICP-MS, 7800, Agilent, USA). The degradation products of HA (i.e., NO 2 -and NO 3 -) were quantified using IC. Mineralization, i.e., the removal of total organic carbon (TOC), was investigated using a TOC-meter (Shimadzu TOC-VCSH). All experiments were performed in triplicates, and demonstrated good reproducibility within an average standard deviation of less than 5%.

Results and discussion

Characterization of RM

XRF analysis demonstrated that RM predominantly consisted of iron oxide (37%), alumina (24%), and silica (10%), in combination with other mineral phases, including TiO 2 , Na 2 O, MgO, CaO, K 2 O, MnO, and P 2 O 5 (Table S1).

Enhanced removal of FLU using the PMS/RM/HA system

The effect of pH variation on FLU adsorption onto the RM surface is illustrated in Fig. S2.

As typically encountered in the adsorption of organic anions onto metal oxides, FLU adsorption was high under low pH conditions, and decreased as the pH increased. This observed sorption behavior can be attributed to the combined effect of pH-dependent FLU speciation (pK a = 6.5) and RM surface charge characteristics (PZC = 7.3). Under pH conditions higher than both pK a and PZC, the FLU and RM surfaces were negatively charged, leading to less favorable electrostatic interactions.

To determine the most efficient FLU removal system, the kinetics of FLU removal with and without PMS, RM, and HA were investigated (Fig. 2). Assuming that RM promotes the generation of ROS, leading to the degradation of the target compound, and that the concentration of these generated species are constant over the reaction time, the FLU removal kinetics can be described using Eq. ( 1), which is a pseudo-first-order equation.

[ ]( ) [ ] , (1)
where k app , which represents the rate constant (h -1 ) of the pseudo-first-order reaction, is obtained by considering the linear regression of ln ([FQs]/[FQs] 0 ) vs. time (t).

The control experiment using HA alone demonstrated that the addition of 0.05 mM HA did not have any significant effect on the FLU removal kinetics. Furthermore, using 1 mM of PMS alone or in combination with 0.05 mM HA resulted in an FLU removal of up to 40%.

The fast removal occurred within 1 h of reaction could be attributed to direct PMS action [START_REF] Yin | Enhanced peroxymonosulfate activation for sulfamethazine degradation by ultrasound irradiation: Performances and mechanisms[END_REF] and/or alkaline-activation by NaOH used for pH adjustment at the initial stage of reaction [START_REF] Wang | Activation of persulfate (PS) and peroxymonosulfate (PMS) and application for the degradation of emerging contaminants[END_REF]. When 0.05 g L -1 of RM and 0.05 mM HA were used, the FLU removal efficiency was 30%, showing the initial adsorption capacity of RM for the FLU removal. Contrarily, after 6 h of reaction in the RM/PMS system, the FLU removal efficiency observed was 75% (k app = 0.248 h -1 , Table S2), which might be caused by sum of FLU adsorption by RM alone and FLU removal by PMS alone. The FLU adsorptiondesorption equilibrium was attained after 1 h (see dotted line in Fig. 2), and finally, complete FLU removal was observed after 6 h of reaction using a system obtained by adding 0.05 mM HA to the PMS/RM system. This new system exhibited a k app value (0.577 h -1 ) that was 2.3 times higher than that of the PMS/RM-only system, suggesting that HA plays a crucial role in enhancing FLU removal.

The enhanced FLU removal in the PMS/RM/HA system possibly resulted from the reduction of Fe (III) to Fe (II) by HA on the surface of α-Fe 2 O 3 present in RM (Eq. ( 2) and

(3)), which activated PMS, generating reactive radicals, i.e., SO 4 •-and • OH (Eq. ( 4) and ( 5)) [START_REF] Zou | Rapid acceleration of ferrous iron/peroxymonosulfate oxidation of organic pollutants by promoting Fe(III)/Fe(II) cycle with hydroxylamine[END_REF].

NH 2 OH + ≡Fe III → ≡Fe II + NH 2 O • + H + (2) ≡Fe III + NH 2 O • → ≡Fe II + NHO + H + (3) HSO 5 -+ ≡Fe II → SO 4 •-+ ≡Fe III + OH - (4) HSO 5 -+ ≡Fe II → SO 4 2-+ ≡Fe III + • OH (5)
To investigate the Fe(III) reduction capacity of HA, the production of dissolved Fe(II) before and after HA addition under anaerobic conditions was investigated. Figure S3 illustrates that in the presence of HA, virtually no Fe(II) dissolution occurs. However, XPS analysis demonstrated that peaks corresponding to Si and Ti on RM surface were not significantly changed before and after HA addition (Figs. These results indicate that the proportions of the Fe(II) and Fe(III) surfaces were 10.4 and 89.6%, respectively. When HA was added to the RM suspension, an increase in the Fe(II)

surface content (26.8%) was observed, indicating that the Fe(III) surface had been reduced to Fe(II). These results imply that HA can reduce Fe(III) to Fe(II) on the RM surface, thus enhancing PMS activation. However, a significant decrease of the Fe(II) content (from 26.8 to 1.3%) was observed when we added PMS to HA/RM system, suggesting that the reduced Fe(II) surface can be oxidized to Fe(III) after PMS activation.

ESR analysis was conducted to provide more direct evidence of radical generation in the PMS/RM/HA system (Fig. 4). We have prepared two different systems using , with the second-order reaction constant = 1.9×10 9 M 1 s 1 and = 4-7.42 10 7 M 1 s 1 , while t-BuOH can be considered to be more selective toward • OH ( = 6.0 x 10 8 M -1 s -1 ) than SO 4

•-( = 4-8.4 x 10 5 M -1 s -1 ) [START_REF] Anbar | A compilation of specific bimolecular rate constants for the reactions of hydrated electrons, hydrogen atoms and hydroxyl radicals with inorganic and organic compounds in aqueous solution[END_REF][START_REF] Buxton | Critical Review of rate constants for reactions of hydrated electrons, hydrogen atoms and hydroxyl radicals (⋅ OH/⋅O-in Aqueous Solution[END_REF][START_REF] Neta | Rate Constants for Reactions of Inorganic Radicals in Aqueous Solution[END_REF]. The percentages of • OH and SO 4 •-that reacted with FLU and the scavenger could be estimated using the bimolecular rate constants and concentrations of each species [START_REF] Kamagate | Mineralization Enhancement of Pharmaceutical Contaminants by Radical-Based Oxidation Promoted by Oxide-Bound Metal Ions[END_REF]. According to our calculations (Table S3), t-BuOH (5 mM) is selective for • OH since it will react with more than 98% of • OH. i-PrOH was used at very high concentration (20 mM) to ensure effective and total scavenging of both radical species.

Addition of 5 mM of t-BuOH resulted in a slight decrease in FLU removal (Fig. S4),

suggesting that hydroxyl radical is not the main reactive species for FLU removal. The slight inhibition observed could be attributed to the reactivity of t-BuOH with SO 4

•-under our experimental conditions (Table S3). The addition of i-PrOH (20 mM) almost completely inhibited FLU degradation, confirming the predominance of the sulfate radical (Fig. S4).

In this study, the impact of light on the PMS/RM/HA system was also investigated by performing additional experiments under UV-A (λ max = 365 nm; intensity: 3.4 mW cm -2 ) and visible light (λ > 400 nm; intensity: 0.5 mW cm -2 ) irradiations (Fig. S5(a)). In comparison with the results obtained under dark conditions (k app = 0.577 h -1 , Table S2), UV-A irradiation resulted in a faster FLU removal rate (k app = 1.396 h -1 ), whereas visible light irradiation had no effect on the removal rate (k app = 0.495 h -1 ). This can be explained taking into consideration the inability of visible light to reduce Fe (III) to Fe (II) or activate PMS (Qu et al., 2019;[START_REF] Wang | Peroxymonosulfate enhanced visible light photocatalytic degradation bisphenol A by single-atom dispersed Ag mesoporous g-C3N4 hybrid[END_REF]. Despite the enhanced FLU degradation efficiency under UV-A irradiation due probably to UV-A assist PMS activation [START_REF] Khan | Kinetic and mechanism investigation on the photochemical degradation of atrazine with activated H2O2, S2O82-and HSO5[END_REF][START_REF] Yang | Degradation efficiencies of azo dye Acid Orange 7 by the interaction of heat, UV and anions with common oxidants: Persulfate, peroxymonosulfate and hydrogen peroxide[END_REF], further experiments using the PMS/RM/HA system were conducted in the dark.

Effects of experimental factors on FLU removal

The effects of the concentrations of PMS, HA, and RM on FLU removal using the PMS/RM/HA system were investigated under a range of PMS (0.5-1.5 mM), HA (0.05-2 mM), and RM (0.05-0.2 g L -1 ) concentrations. The results depicted in Fig. S6(a) and Fig.

5(a)

indicate that an increase in PMS concentration results in a higher and faster FLU removal rate (k app = 0.192 and 1.923 h -1 at 0.5 and 1.5 mM, respectively (Table S2). This can be attributed to the increased production of SO 4 •-and • OH, as shown in Eq. ( 4) and (5).

However, higher HA concentrations (> 0.05 mM) exhibit a negative effect on the FLU removal rate (k app = 0.577 and 0.041 h -1 at 0.05 mM and 2 mM, respectively (Fig. S6(b), Fig. S2)). These observations support the scavenging role of HA according to Eq. ( 6) and ( 7) [START_REF] Zou | Rapid acceleration of ferrous iron/peroxymonosulfate oxidation of organic pollutants by promoting Fe(III)/Fe(II) cycle with hydroxylamine[END_REF].

5(b), and Table

NH 2 OH + SO 4 •-→ SO 4 2-+ nitrogenous products (k < 1.5 × 10 7 M -1 S -1 ) (6)
NH 2 OH + • OH → OH -+ nitrogenous products (k < 5.0 × 10 8 M -1 S -1 ) (7) Indeed, nitrogenous products ( 0.5 mg L -1 of NO 3 -and NO 2 -), which possibly resulted from HA degradation as shown in Eq. ( 6) and (7), were observed at the end of the 6 h reaction. Additionally, increasing the RM dosage, particularly during the early stage of the reaction (Fig. S6(c) and Fig. 5(c)), enhanced the FLU removal (k app = 0.577 and 0.993 h -1 at 0.05 mg L -1 and 0.2 mg L -1 , respectively (Table S2).

The effect of HA on the Fe(III)/Fe(II) cycle on the RM surface was also evaluated using two other reductants, i.e., dithionite and sulfite (Fig. S6(d) and Fig. 5(d)). In the presence of dithionite, the FLU removal efficiency was 40%, including initial removal of 23% via adsorption while almost no further removal was observed in the presence of sulfite. These demonstrated the superior FLU removal performance of the PMS/RM/HA system, owing to the stronger scavenging properties of dithionite and sulfite toward the radicals ( = 5.5×10 9 M 1 s 1 [START_REF] Buxton | Critical Review of rate constants for reactions of hydrated electrons, hydrogen atoms and hydroxyl radicals (⋅ OH/⋅O-in Aqueous Solution[END_REF]) and = 5.0×10 8 M 1 s 1 [START_REF] Neta | Free-radical chemistry of sulfite[END_REF], not available for dithionite) than those toward the HA (Eq. ( 6) and ( 7)).

Figure S5(b) indicates that pH has a considerable effect on FLU sorption and oxidation in the PMS/RM/HA system. A significant decrease in FLU removal is observed when the initial pH was increased from 6 to 9. At pH > pH PZC-RM (i.e., 7.3), there may be a decrease in the electrostatic repulsion between RM and HSO 5 -, resulting in a decrease in the FLU removal efficiency. Conversely, at a pH of 6, the electrostatic attractions of both the negatively charged FLU -and HSO 5 -with the positively charged RM surface are favored.

These can enhance the initial adsorption of FLU and PMS activation on RM surface, and consequently, the increase in the FLU removal efficiency.

Effect of mixture on removal process and oxidation pathways

The effect of the co-existence of FQs (i.e., FLU and CIP) on their respective removal was investigated. Figure 6 depicts that in both single (Fig. 6(a)) and binary mixture systems (Fig. units and (ii) the strong interactions between SO 4

•-and the piperazine ring [START_REF] Guo | Enhanced degradation of aqueous norfloxacin and enrofloxacin by UV-activated persulfate: Kinetics, pathways and deactivation[END_REF][START_REF] Jiang | Sulfate radical-based oxidation of fluoroquinolone antibiotics: Kinetics, mechanisms and effects of natural water matrices[END_REF]. Furthermore, a slight decrease in FLU adsorption was observed in the binary system (Fig. 6(b)), possibly owing to the competition with CIP.

To identify CIP and FLU intermediates in single systems, HPLC-ESI-MS technology was employed; the mass spectra obtained are presented in Table S4. Figure S8 depicts the variation in the total ion current (TIC) chromatogram of the detected intermediates during CIP oxidation. Seven major intermediates with molecular ion masses of m/z 332, 348, 334, 316(1), 316(2), 245, and 202 were identified (Fig. S9) using the ESI-positive mode that enable to suggest a plausible degradation pathway (Scheme 1). After 30 min of reaction, the peak of CIP molecule (m/z 332 at RT 10.61) decreased with appearance of peaks at RT 6.39 and 2.32 corresponding to m/z 334 and 348, respectively. This indicates that (1) the cleavage of the piperazine ring (m/z 332 334) owing to attack by SO 4 •-and • OH [START_REF] Li | Highly efficient activation of peroxymonosulfate by cobalt sulfide hollow nanospheres for fast ciprofloxacin degradation[END_REF][START_REF] Shah | Hydroxyl and sulfate radical mediated degradation of ciprofloxacin using nano zerovalent manganese catalyzed S2O82[END_REF][START_REF] Zhao | Efficient removal of ciprofloxacin by peroxymonosulfate/Mn3O4-MnO2 catalytic oxidation system[END_REF] and ( 2) the hydroxylation reaction (m/z 332 348)

owing to the attack of the quinolone ring by • OH [START_REF] Deng | Degradation of ciprofloxacin using Α-MnO2 activated peroxymonosulfate process: Effect of water constituents, degradation intermediates and toxicity evaluation[END_REF][START_REF] Li | Highly efficient activation of peroxymonosulfate by cobalt sulfide hollow nanospheres for fast ciprofloxacin degradation[END_REF], occurred simultaneously at the initial oxidation of CIP in the PMS/RM/HA system. Then, we observed the decrease in peaks for m/z 334 and m/z 348 after 1 h of reaction with appearance of two peaks at RT 4.02 and 5.53 corresponding to m/z 316(1) or 316(2) (Table S4). These can be attributed to (1) the defluorination that resulted in the formation of m/z 316(1) from m/z 334 [START_REF] Zhao | Efficient removal of ciprofloxacin by peroxymonosulfate/Mn3O4-MnO2 catalytic oxidation system[END_REF] and ( 2) piperazine ring opening + decarboxylation that resulted in the formation of m/z 316(2) from m/z 348. Additionally, a peak at 1.06 RT corresponding to m/z 202 was first detected after 1 h of reaction (Fig. S8). After 3 h of reaction, most of the peaks had disappeared, and two distinct peaks at RT 6. (positive mode) appeared. Additionally, a relatively small signal appeared at RT 16.4 (m/z 242) in the negative mode. These findings indicate that both (i) the hydroxylation of the quinolone ring + the ring opening on the C C double bond (m/z 262 → 296) (Feng et al., 2017a(Feng et al., , 2017b[START_REF] Feng | Degradation of fluoroquinolone antibiotics by ferrate(VI): Effects of water constituents and oxidized products[END_REF][START_REF] Qi | Oxidation of flumequine in aqueous solution by UV-activated peroxymonosulfate: Kinetics, water matrix effects, degradation products and reaction pathways[END_REF] and (ii) the defluorination of FLU (m/z 262 242) [START_REF] Feng | Degradation of flumequine in aqueous solution by persulfate activated with common methods and polyhydroquinone-coated magnetite/multi-walled carbon nanotubes catalysts[END_REF][START_REF] Rodrigues-Silva | Degradation of flumequine by photocatalysis and evaluation of antimicrobial activity[END_REF] occurred simultaneously during the initial oxidation of FLU by the PMS/RM/HA system. After 2 h of reaction, a new peak at RT 15.76 corresponding to m/z 252 (positive mode), was observed. This possibly resulted from hydroxylation, ring cleavage, and FLU decarboxylation (m/z 262 252). However, as the reaction progressed (2 h 5 h), the intensity of the peak corresponding to m/z 296 decreased, while the intensity of those corresponding to m/z 252 (RT 15.76, positive mode) and m/z 244 (RT 16.55, negative mode) increased. These results indicate that pathway 2 may be the main degradation pathway through which the formation of m/z 252 from m/z 296 via decarboxylation can be induced. Additionally, via hydroxylation, defluorination, and hydrogen abstraction, m/z 252 could be further converted to m/z 244. This study is the first to report the identification of the intermediate, m/z 244, as a byproduct of FLU oxidation owing to PMS activation.

Impact of HW components on the removal performance

The kinetics of FLU and CIP removal from HW is illustrated in Fig. 7. No significant FLU removal was observed in HW (k app = 0.014 h -1 , Table S2; Fig. 7 (a)). The inhibitory effect in HW can be attributed to the major dissolved components in HW, including chlorides, phosphates, nitrates, sulfates, and dissolved organic matter. It should be noted that these wastewater components exist at very high loading with respect to the concentration of target compounds (e.g. molar ratio [phosphate]/[FLU] = 126; [chloride]/[FLU] = 3540), which may lead to strong competitive and/or radical scavenging effects.

To evaluate the effect of each of these ligands on FLU removal efficiency, the FLU removal by the PMS/RM/HA system in UPW was examined in the presence of inorganic ions and organic matter, similar to those present in HW (Fig. 8). The results obtained revealed that chlorides significantly improved the FLU removal as evidenced by an approximately 25-fold higher k app value (k app = 14.401 h -1 , Table S2), relative to that obtained in the absence of chlorides (k app = 0.577 h -1 ). Reportedly, Cl -in water can react with PMS, generating HOCl (Eq. 8; [START_REF] Yang | Significance of B-site cobalt on bisphenol A degradation by MOFs-templated CoxFe3-xO4 catalysts and its severe attenuation by excessive cobaltrich phase[END_REF]. Moreover, chlorine radicals (Cl•) can be formed in water when chlorides in water react with SO 4

•-(Eq. 9; [START_REF] Lei | Rate constants and mechanisms of the reactions of Cl• and Cl2•with Trace Organic Contaminants[END_REF]. Furthermore, Cl -in moderate concentration as in this study and at neutral pH could react with SO 4

•-, resulting in the production of • OH [START_REF] Lutze | Sulfate radical-based water treatment in presence of chloride: Formation of chlorate, inter-conversion of sulfate radicals into hydroxyl radicals and influence of bicarbonate[END_REF]. This indicates that there was an additional possibility for converting SO 4 •-into • OH. Therefore, the enhancement of the FLU removal in presence of Cl -can be attributed to the formation of reactive Cl species and/or • OH.

Cl -+ HSO 5 -→ SO 4 2-+ HOCl (k = 2.06 × 10 3 M -1 S -1 ) (8)
Cl -+ SO 4 •-→ SO 4 2-+ Cl• (k = 3.1 × 10 8 M -1 S -1 ) (9)
On the contrary, the presence of phosphates resulted in a strong inhibition of the FLU removal, whereas nitrate (k app = 0.267 h -1 ) and sulfate (k app = 0.245 h -1 ) exhibited lower FLU removal inhibitory effects (Fig. 8). This strong inhibitory effect of phosphates can be explained by considering its strong adsorption onto metal oxides, thereby reducing the interaction between the oxidant and solid surface. Indeed, we confirmed this strong binding of phosphate by conducting an independent adsorption set, which showed great affinity of phosphate to RM surfaces up to 5 mgP/g at pH 7. At molecular-level, phosphate binds strongly to Fe(III) oxides through monodentate and/or multidentate-mononuclear surface complexes, and a variety of surface complexation models has been proposed to describe the phosphate removal by mineral surfaces [START_REF] Khare | XANES investigation of phosphate sorption in single and binary systems of iron and aluminum oxide minerals[END_REF]. Furthermore, phosphate can also exhibit strong scavenging effect toward SO 4 •-and • OH (Eq. ( 10) and ( 11); [START_REF] Buxton | Critical Review of rate constants for reactions of hydrated electrons, hydrogen atoms and hydroxyl radicals (⋅ OH/⋅O-in Aqueous Solution[END_REF][START_REF] Neta | Rate Constants for Reactions of Inorganic Radicals in Aqueous Solution[END_REF].

SO 4 •-+ HPO 4 2-→ HPO 4 •-+ SO 4 2- (k = 1.2 × 10 6 M -1 s -1 ) (10)
• OH + HPO 4 2-→ HPO 4

•-+ OH - (k = 1.5 × 10 5 M -1 s -1 ) (11)

Reportedly, humic substances generally act as ROS scavengers given that some of their electron-rich moieties (e.g., phenolics) can react with radical species [START_REF] Deng | Degradation of ciprofloxacin using Α-MnO2 activated peroxymonosulfate process: Effect of water constituents, degradation intermediates and toxicity evaluation[END_REF][START_REF] Jiang | Sulfate radical-based oxidation of fluoroquinolone antibiotics: Kinetics, mechanisms and effects of natural water matrices[END_REF]. Additionally, other quinone moieties can possibly activate PMS to generate ROS, thereby accelerating the degradation of organic pollutants [START_REF] Deng | Degradation of ciprofloxacin using Α-MnO2 activated peroxymonosulfate process: Effect of water constituents, degradation intermediates and toxicity evaluation[END_REF][START_REF] Zhou | Activation of Peroxymonosulfate by Benzoquinone: A Novel Nonradical Oxidation Process[END_REF]. To investigate the effect of humic acids on FLU removal, LHA was added to the PMS/RM/HA system (Fig. 8). The results obtained demonstrated that LHA caused a slight decrease in FLU removal efficiency, indicating that humic acids have lesser effect on FLU removal using the PMS/RM/HA system. The strong inhibitory effect of phosphates was also observed in HW containing a binary FQ system, i.e., FLU + CIP (Fig. S12). Therefore, it can be concluded that the presence of phosphates in HW is primarily responsible for the degradation of FLU removal efficiency in the PMS/RM/HA system. Nevertheless, CIP degradation was not affected by the presence of these compounds (Fig. 7(b)) because of its fast reaction with SO 4 •-.

To overcome the inhibitory effect of phosphates in HW, the concentration of PMS was increased from 1 to 5 mM (Fig. 9(a)). Thus, there was a gradual increase in FLU removal efficiency as PMS concentration increased up to 5 mM, at which complete FLU removal was observed. A significant increase in the k app value was observed at a PMS concentration of 3 mM (k app = 0.177 h -1 , Table S2) and 5 mM (k app = 0.388 h -1 ), in comparison with that at 1 mM (k app = 0.014 h -1 ). This can be attributed to a higher production of SO 4 •-as PMS concentration increases. Regarding TOC removal, a removal efficiency of only 30% was obtained using 3 mM PMS for 24 h (Fig. 9(b)). However, the sequential addition of 3 mM PMS over seven times enhanced TOC removal, up to approximately 95% after the seventh addition (Fig. 9(b)), suggesting that controlling the PMS concentration, as well as its addition mode, could guarantee the successful mineralization of FQs using the PMS/RM/HA system.

During the sequential experiments, the amount of metals leached (i.e., Al, Fe, Si, and Ti) was insignificant (significantly low concentrations during the first reaction), as presented in Table S5. Additionally, no remarkable increase in metal leaching was observed over seven reaction cycles (Fig. S13), indicating that RM was fairly stable in the PMS/RM/HA system.

Conclusion

In this study, the effective activation of PMS by the RM/HA system for the oxidative degradation of FLU and CIP in HW was demonstrated, and the associated oxidation pathways were proposed. On the RM surface, HA acted as a reducing agent in the Fe III /Fe II cycle, and its addition to the PMS/RM suspension significantly enhanced FQ removal. The results also demonstrated that SO 4 •-possibly played a more crucial role in FQ removal in PMS/RM/HA systems than • OH. Additionally, the high reactivity between SO 4 •-and CIP, which resulted in the rapid removal of CIP, was observed under all the experimental conditions employed in this study. Furthermore, the effect of competition on FLU removal depended on the nature of the organic and inorganic compounds present in HW. Particularly, phosphates could significantly inhibit FLU removal owing to their strong adsorption onto the RM surface, as well as their scavenging effect toward SO 4 •-. However, this inhibitory effect could be successfully overcome by controlling both the PMS concentration and its addition mode.

Globally, approximately 95% of alumina is produced via the Bayer process, which yields 1-2 tons of RM for each ton of alumina that is produced [START_REF] Bray | Sustained Bauxite Residue Rehabilitation with Gypsum and Organic Matter 16 years after Initial Treatment[END_REF]

  Figure 1(a), which illustrates the XRD diffractogram of raw RM, indicates the presence of peaks corresponding to hematite, boehmite (γ-AlO(OH)), anatase (TiO 2 ), calcite (CaCO 3 ), and quartz (SiO 2 ). Based on the zeta potential measurements under different pH conditions (2, 4, 6, and 8), the point of zero charge (PZC) of RM (Fig. S1) was 7.3, and its BET surface area determined from the multipoint N 2 adsorption isotherm was 23 m 2 g -1 . Additionally, the characterization of the morphology and structural features of RM using TEM analysis at different optical magnifications revealed that it consisted of nanosized particles (< 100 nm) with different morphologies (Fig. 1(b)) that were aggregated to form bigger particles (Fig. 1(c)).

  3(a) and (c)), whereas main peak corresponding to Al(2p 3/2 ) shifted from 73.9 to 74.5 eV (Fig.3(d)), indicating the formation of Al hydroxides after the reaction[START_REF] Epa | NIST 2008 User Guide[END_REF]. These observations possibly resulted from the formation of Fe(II)-Al(III)-layered double hydroxides during the reaction between the reduced Fe(II) and Al 2 O 3 on the RM surface[START_REF] Elzinga | Formation of layered Fe(II)-Al(III)-hydroxides during reaction of Fe(II) with aluminum oxide[END_REF]. The portion of deconvoluted peak of O 1s at 532.1 eV (hydroxide species) certainly increased to 35.8% when it compares with RM alone (25.5%) (Fig.3(b)). In addition, H 2 O peak, "vacancy" peak (O vac ), and O 2 peak were observed at 533.8, 531.4, and 530 eV, respectively[START_REF] Liu | Unusual Fe-H bonding associated with oxygen vacancies at the (001) surface of Fe3O4[END_REF]. To confirm the surface reduction of Fe(III) to Fe(II) by HA, the Fe region of the XPS spectra was also analyzed. The Fe(2p 3/2 ) narrow-scan spectra of raw RM consisted of three main peaks at 709.5 (Fe(II) species), 710.7, and 712.8 eV (Fe(III) species; Fig.3(e);[START_REF] Sihn | Immobilization of uranium(VI) in a cementitious matrix with nanoscale zerovalent iron (NZVI)[END_REF].

6

  (b)), 5 μM of CIP was completely removed using PMS/RM/HA. Additionally, k app values approximately 11.7 and 11.0 times higher than those for FLU removal in both single (Fig.S7(a)) and binary mixture systems (Fig.S7(b)), respectively, were obtained for CIP, owing to the faster reaction between CIP and SO 4•-( = 1.2×10 9 M -1 s -1 ; Mahdi-Ahmed and Chiron, 2014). CIP, which contains cyclopropane and piperazine rings, exhibits high reactivity with ROS via (i) the cleavage of the cyclopropane moiety owing to the loss of CH 2

  57 and RT 1.06, corresponding to m/z 245 and m/z 202, respectively, were observed. After 5 h, when the reaction reached completion, the only peak that remained corresponded to m/z 202. This study is the first to report the intermediate, m/z 202, as a byproduct of CIP oxidation owing to PMS activation. These results indicate that the loss of a secondary amine nitrogen by m/z 316(1) owing to SO 4 •attack could lead to the formation of m/z 245 (Ao et al., 2018), from which m/z 202 could be obtained via C-N bond cleavage, decarboxylation, and hydroxylation. Similarly, m/z 202 could also be formed from m/z 316(2) via the further oxidation of the piperazine ring, coupled with C-N bond cleavage.

Figure

  Figure S10 depicts the variation in the TIC chromatogram of the detected intermediates

  Scheme 2. Proposed reaction pathways for the degradation of FLU in the PMS/RM/HA system.

Fig. 2 .

 2 Fig. 2. Removal kinetics of FLU in different reaction systems. Conditions: [FLU] 0 = 5 µM, [PMS] 0 = 1.0 mM, [HA] 0 = 0.05 mM, [RM] = 0.05 g L -1 , reaction time = 6 h (1 h for

Fig. 4 .

 4 Fig. 4. ESR spectra of the DMPO-OH and DMPO-SO4 •-adducts generated in RM/PMS/HA and RM/H 2 O 2 /HA systems. Conditions : [RM] = 0.05 g L -1 , [HA] = 0.05 mM, [PMS] 0 =

Fig. 5 .

 5 Fig. 5. Variation of kinetic rate constants at different (a) PMS, (b) HA, and (c) RM

Fig. 7 .

 7 Fig. 7. Removal of (a) FLU and (b) CIP in single system and different water matrices. Conditions: [FLU] 0 = [CIP] 0 = 5 µM, [PMS] 0 = 1.0 mM, [HA] 0 = 0.05 mM, [RM] = 0.05 g L - 1 , reaction time = 6 h (1 h for adsorption of FLU to RM surface), pH = 7.0 ± 0.1, V = 200 mL.

Fig. 8 .

 8 Fig. 8. Effect of (a) inorganic ions and (b) organic ligands on FLU removal. Conditions: [FLU] 0 = 5 µM, [PMS] 0 = 1.0 mM, [HA] 0 = 0.05 mM, [RM] = 0.05 g L -1 , reaction time = 6 h

Fig. 9 .

 9 Fig. 9. (a) Effect of PMS concentration (1, 3 and 5 mM) on FLU removal by PMS/RM/HA system in HW. Conditions: [FLU] 0 = 5 µM, [HA] 0 = 0.05 mM, [RM] = 0.05 g L -1 , reaction

  Humic Substances Society, and raw RM, which is obtained as a byproduct of the Bayer process, was obtained from the Korea Institute of Geoscience and Mineral Resources at Daejeon, South Korea. The RM was used as supplied, i.e., without further chemical treatment. It was only rinsed with ultra-pure water (UPW). The physical characteristics of RM were identified by various surface analyses such as XRF, TEM, XRD, BET, zeta potential analyzer, XPS. More details for the surface analyses are provided in supplementary data. To prepare standard solutions, UPW with a resistivity of 18.2 MΩ•cm, obtained from a

	Millipore Milli-Q system was used. HW was provided by the University Hospital Center of
	Rennes City (CHU Rennes -Hôpital Pontchaillou, France). CHU Rennes covers a broad
	range of clinical services and health needs and has a big hospitalization capacity (2K+ beds,
	150K+ admissions and 500K+ consultations in 2018) in different areas (medicine, surgery,
	obstetrics, and gynecology). As a result, the wastewater sample investigated in this work
	represents well the HW in Europe. Its physico-chemical characteristics are presented in Table
	FLU (C 14 H 12 FNO 3 ; > 98%), CIP (C 17 H 18 FN 3 O 3 ; > 98%), sodium persulfate (Na 2 S 2 O 8 ), PMS
	(available as Oxone (KHSO 5 • 0.5KHSO 4 • 0.5K 2 SO 4 )), isopropanol (i-PrOH, C 3 H 8 O), tert-
	butanol (t-BuOH, C 4 H 10 O), potassium iodide (KI), potassium hydroxide (KOH), sodium
	chloride (NaCl), sodium nitrate (NaNO 3 ), HA (NH 2 OH, 50% wt), sodium sulfate (Na 2 SO 4 ),
	sodium sulfite (Na 2 SO 3 ), and disodium phosphate (Na 2 HPO 4 ) were purchased from Sigma-
	Aldrich Inc., USA. Sodium dithionite (Na 2 S 2 O 4 ) was purchased from PubChem, whereas

sodium hydroxide (NaOH) and hydrochloric acid (HCl, 37% extra pure) were obtained from Acros Organics. Standard leonardite humic acid (LHA) was obtained from the International

  PMS/RM/HA and H 2 O 2 /RM/HA at pH 7. No signal was detected in RM/ H 2 O 2 /HA system, indicating that H 2 O 2 could not be activated in RM/HA suspension at pH 7. In comparison, apparent signals of DMPO-OH adduct with the intensity ratio of 1:2:2:1 and DMPO-SO 4

•adduct were observed in RM/PMS/HA system. The predominant reactive species was further investigated using t-BuOH and i-PrOH because of their different selectivity (i.e., secondorder rate constants) with  OH and SO 4  

. i-PrOH reacts with both  OH and SO 4  

Acknowledgements

We thank the Ministère de l'Enseignement Supérieur et de la Recherche Scientifique of Ivory Coast for a PhD grant (NC). This work was supported by the Campus France (PHC-STAR) program and the National Research Foundation of Korea (project number NRF-2017K1A3A1A21013653 and NRF-2019R1C1C1003316).

Conflicts of interest

There are no conflicts to declare.

Declaration of interests

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.

Appendix A. Supplementary data

The following are the supplementary data related to this study.