Introduction

Phenolic compounds are widely used in various industries, generating strong demand for the manufacture of products such as dyes, pesticides and pharmaceuticals [1,2,3]. Phenol has been classified as one of the common organic pollutant present in wastewater, in large quantities [4]. Its source can be natural, arising from the decomposition of plants and animals in water, or from human activities such as industry, agriculture and households [5]. The need to eliminate and improve the means and strategies of treatment of phenols in order to break them down completely or into small molecules of low or zero toxicity, has become the concern of the world and researchers because of their adverse effects on human health and aquatic life [6]. These compounds are toxic to aquatic organisms and humans, acting as carcinogens and damaging to the red blood cells and the liver, even at low concentrations (5–25 mg L−1) [7, 8].

To avoid accumulation of the amount of these organic pollutants in the environment, there are various technologies in application such as adsorption [9] biological treatment [10], extraction [11], photocatalysis [12] and electrochemical processes [13, 14]. In recent years, electrochemical treatment has become a very reliable and widely used technology for pollution control and purification in a wide range of fields due to their efficiency and green performances [15,16,17]. In addition, this technology is particularly effective in removing a wide range of pollutants and allows efficient oxidation via electrochemical reactions [17, 18] with a high capacity to remove organic matter in a short time, without the need for additional chemical reagents or pre-treatment waste [19].

Many research have focused on the use of electrochemical methods either direct called anodic oxidation or indirect electro-oxidation of phenolic contaminants and other organic pollutants using different pretreated or modified electrodes in the presence of an appropriate electrolyte [13, 14, 17, 20]. Modified electrodes are more effective than unmodified substrates for oxidizing organic pollutants. The latter are less suitable for electrochemical treatment due to their low catalytic capacity and the production of intermediates that are difficult to treat [18]. The selected electrode materials must be highly reactive to organic oxidation and stable in an electrolytic environment, while being economical and environmentally friendly. They are generally classified into two categories: active and non-active anodes, depending on their ability to interact with hydroxyl radicals (•OH) on their surface. Active anodes, in particular, show a strong interaction with •OH, resulting in partial degradation of the targeted contaminants. On the other hand, non-active anodes have a low affinity for •OH groups, favoring complete mineralization on the substrate [17, 21].

Nanostructured materials are gaining interest in various fields of science and technology. The development of these nanomaterials appears very encouraging for water and wastewater treatment, due to their small size, large specific surface area, able to work at low concentration and porous structure, with affinity for various chemical groups [22,23,24].These characteristics contribute to their excellent performance in contaminant removal, giving them powerful adsorption and reactivity capabilities towards them. Nanostructured metal oxides are considered to be one of the most fascinating functional materials; they are nowadays an active research topic with extensive application that play a vital role in many areas of chemistry, physics, materials science and biotechnology [6].

For instance, nanostructured manganese dioxide has been popularized and received much attention for the remediation of wastewater containing organic pollutant [2, 16, 25] due to its remarkable properties that have led to many applications, especially in the fields of catalysis [26], adsorption [27] and cation exchange [28]. Due to its reactivity with contaminants under environmentally relevant conditions, manganese dioxide may oxidize contaminants in soils and/or be applied in water treatment applications, oxidant or catalyst important in the natural environment that can effectively degrade a range of organic contaminants such as phenol [25, 29], dye [1, 9, 26] and pesticides [2]. Wildly used in various applications in energy storage, it can be used as ideal electrochemical supercapacitor [30].

Therefore, a huge research interest and effort has been devoted to developing MnO2 nanoparticles with a nanoscale structure and a wide variety of oxides and a wide variety of oxides by means of diverse synthesis methods. It can therefore be produced by different methods such as sol–gel [31], hydrothermal [32], microwave-assisted reflux [33], precipitation [34] and by electrochemical route [16, 30, 35]. The electrochemical deposition adopted technique used to fabricate pure metals with homogeneous, adherent and well crystallized thin films and coatings, with simple electrodeposition, precise thickness and morphology at room temperature. MnO2 can electrodeposited by electrolysis method, cyclic voltammetry, etc. on different substrates such as SnO2 [1, 2, 16], graphite [25], stainless steel [26] and glassy carbon [35].

To our knowledge, few studies have focused on the preparation of MnO2 on a stainless steel substrate for the electrochemical degradation of phenol. In this context, the present study falls within the field of development of new low-cost and efficient electroactive electrode materials for the electrochemical oxidation of organic pollutants for wastewater treatment. Our contribution highlights: (i) synthesis of adherent nanostructured manganese dioxide, with high specific surface area, electrodeposited on a stainless steel plate (SS/MnO2) using a highly reproducible electrochemical synthesis technique, and (ii) evaluation of electrochemical performance of this electrode for the degradation of phenol in aqueous solution by varying experimental parameters such as the applied potential, pH and pollutant concentration, and an electrochemical degradation mechanism of phenol was proposed.

Experimental

Preparation of MnO2 thin film and characterization

The MnO2 thin films were electrodeposited at room Temperature by chronoamperometry at applied potential E = 0.65 V vs. SSE on a stainless steel electrode (SS) during about 1 h corresponding to an electric charge Q = 4 C (Coulombs), value set for the study. The electrodeposition of SS/MnO2 thin film was carried out in electrochemical cell with three electrode containing aqueous electrolytic solution monohydrate manganese (II) sulphate (MnSO4·H2O, 0.3 M, 99% Biochem) and its pH was adjusted to 1.8 using concentrated sulfuric acid solution (H2SO4, 98% Biochem). The reference electrode was saturated mercurous sulphate electrode (SSE, ESSE = 0.655 V vs. SHE (standard hydrogen electrode)). The counter electrode was a platinum plate and working electrode was a stainless steel plate (s = 4.5 cm2). The electrochemical cell was related to the Autolab Potentiostat/Galvanostat (PGSTAT 30) controlled by a computer using the GPES software. After electrodeposition, a black and adherent film was observed then rinsed with distilled water and dried in ambient air.

Electrochemical degradation of phenol

The degradation of phenol was performed by electrochemical route by chronoamperometry at different applied potential E ranging from 0.7 to 1 V vs. SSE on an electrode based on modified manganese dioxide thin film on a stainless steel plate (SS/MnO2). The experiments are performed at room temperature in an electrochemical cell containing three electrodes described above using the SS/MnO2 thin film as working electrode in 100 mL of aqueous phenol solution in the concentration range 5 to 40 mg L−1 (99.5%, Biochem) and under continuous stirring (300 rpm) for 4 h without deaeration. The pH of the solutions was adjusted with a pH-meter (Ohaus Starter 2100) by adding a few drops of the concentrated aqueous H2SO4 solution. The use of H2SO4 as a supporting electrolyte provides a conductive medium, minimizes electrochemical polymerization reactions causing fouling of the electrode [14, 18, 36]. Samples were taken during the experiment for analysis.

Characterization manganese dioxide thin film

The morphological and composition of MnO2 thin films electrodeposited on the SS electrode was performed by scanning electron spectroscopy (SEM) (Quanta FEG 250) coupled with an energy dispersive X-ray spectrometer (EDS). Its topography was examined by atomic force microscopy (AFM) (MFP-3D) and its crystallinity was characterized with XRD using (PANalytical Empyrean) diffractometer (CuKα radiation as the X-ray source, λ = 1.54056 Ǻ). The Brunauer–Emmett–Teller (BET) specific surface area and pore size distribution of this material was determined by measuring the nitrogen adsorption–desorption isotherms in a system at 77 K (Micromeritics ASAP 2420). For the purpose of XRD and BET analysis, electrochemically deposited MnO2 thin films on a stainless steel working electrode were scraped to collect sufficient MnO2 powder. The surface functional groups of MnO2 films were characterized by Fourier transform infrared spectroscopy (FTIR) in the range 400–4000 cm−1 (Thermo Scientific Nicolet IS5).

Analyses of solutions

Samples were analyzed by UV–Visible spectroscopy (Evolution 201) at λ = 270 nm and high-performance liquid chromatography (HPLC) (Dionex Ultimate ACC 3000) equipped with an autosampler column C18 (5 µm; 4.6 × 150 mm) reversed-phase column and a pump, as well as a detector with detector VWD RS (UV/Vis). Mobile phase consisting of methanol/water = 1/4 (V/V) was used for the analysis of phenol and its oxidation by-products, including hydroquinone, 1,4-benzoquinone, acetic acid and formic acid. The flow rate was fixed to 0.9 mL min−1 and the total time of analysis was 10 min. The identification of phenol and its intermediates by HPLC was achieved by comparing the retention time of the peak in the electrochemically oxidized sample with that of the authentic sample.

The percentage of phenol degradation was calculated using Eq. 1:

$$\mathrm{PD}\;\left({\%}\right)\;\mathrm=\;\frac{{\mathrm{C}}_{0}-\mathrm{C}}{{\mathrm{C}}_{0}}$$
(1)

C0 and C are phenol concentration at time zero and t, respectively at time t, PD Phenol Degradation.

Results and discussion

Synthesis of MnO2 film on stainless steel electrode

Figure 1 shows the cyclic voltammograms recorded in the absence and presence of Mn2+ ions. The absence of a peak on the cyclic voltamperogram recorded in the absence of Mn2+ indicates that the SS substrate used is inert in the potential range used to form manganese dioxide. In the presence of Mn2+ ions, an anodic peak was observed at 0,73 V vs. SSE. Studies carried out on the electrodeposition of MnO2 in acidic media and using different substrates (SnO2, SS, GC, Pt) [16, 26, 35, 37,38,39], have reported that the appearance of an anodic peak is attributed to the oxidation of Mn2+ to MnO2 at the electrode surface according to the overall electrochemical reaction Eq. 2:

Fig. 1
figure 1

Cyclic voltammograms on SS electrode1: without Mn2+ ions and 2: with Mn2+ ions, MnSO4·H2O (0.3 M), pH 1.8, scan rate = 10 mV s.−1

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$$\mathrm {Mn}^{2+}+\mathrm {2H}_{2}\mathrm O\to \mathrm {MnO}_{2}+\mathrm {4H}^{+}+\mathrm {2e}^{-}$$
(2)

The electrochemical synthesis of manganese dioxide thin films was performed by a simple and reproducible technique which is imposed potential chronoamperometry in MnSO4·H2O (0.3 M) solution at pH = 1.8 (Fig. 2). This technique can be used to reveal the nucleation and growth processes of manganese dioxide. A potential of 0.65 V vs. SSE, chosen from the range of manganese dioxide formation (Fig. 1), was applied to obtain homogeneous, thin and adherent films [1, 35]. It can be seen from the chronoamperometry curve that the current decreases rapidly after applying the potential corresponding to double layer charging followed by an increase of the current up to a maximum value corresponding to the nucleation-growth stage of the manganese dioxide thin film on the electrode surface and finally the current slowly decreases and reaches a non zero steady value corresponding to the diffusion-controlled stage with slow kinetic involving an increase in the mass of MnO2 deposited [40, 41]. After electrolysis, the SS electrode is covered with a black MnO2 film as shown in the figure.

Fig. 2
figure 2

Chronoamperometry curve on SS electrode at Eapp = 0.65 V vs. SSE

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Characterization of MnO2 thin film

Figure 3a shows that the morphology of the thin film deposit obtained by constant potential electrolysis is composed of sets of nanometric needles, randomly distributed with porous-like structures. The composition of this deposit identified by EDS analysis shown in Fig. 3b reveals the presence of manganese and oxygen in the deposit with the atomic ratio of about 1:2 indicating that the chemical formula of the film is MnO2. In addition to observed manganese and oxygen peaks, low levels of carbon and iron originated from the substrate were detected as well as sulfur impurities from the electrolyte solution.

Fig. 3
figure 3

a SEM image and b EDS spectrum of the deposited MnO2

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Atomic force microscopy provides information on material topography and surface roughness. A series of 2D and 3D AFM images of MnO2 films deposited at different electrolysis times (3, 5, 30 and 60 min) with a scan size of 1 × 1 μm are shown in Fig. 4. The surface morphology with a porous structure of MnO2 observed in the images Fig. 4a–d is in agreement with the SEM results. Analysis of AFM data shows nanoscale roughness estimated from RMS (root-mean-square) values of 4.714, 5.853, 7.235, 8.193 nm which is proportional to the electrolysis time. The surface skewness equaled to 0.190, 0.144, 0.005 for 3, 5 and 30 min respectively meaning the surface has more peaks than valleys deposition time and skewness surface of − 0.163 obtained at 60 min deposition time indicates the surface has more valleys than peaks. Surface kurtosis was 0.174, − 0.044, − 0.178 and 0.381 lower than 3 indicating a flat surface (Platykurtic) for all electrolysis times [42, 43].

Fig. 4
figure 4

AFM images 3D and 2D of SS/MnO2 at different deposition time: a 3 min, b 5 min, c 30 min and d 60 min

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Figure 5 shows the XRD patterns of two manganese dioxide samples. The first one corresponds to the manganese dioxide film covering the stainless steel substrate (SS/MnO2) by electrodeposition, according to the electrochemical conditions cited above. Several characteristic crystal peaks of SS and a single characteristic peak of MnO2 at 2θ = 37.187° were observed. This analysis clearly shows that the MnO2 film is thin and in order to reveal the crystal structure of this material, the second sample consisting of the MnO2 powder resulting from the synthesis of SS/MnO2 thin film was analyzed. The results revealed peaks at 2θ values of 37.187°, 42.045°, 55.701°, 66.625°, 79.099° and 93.778° attributed to the crystal planes of (131), (300), (160), (421) and (081), respectively, revealing the presence of orthorhombic γ-MnO2 with a good agreement with the standard reported data (JCPDS data sheet N° 14–0644, with lattice parameters a = 6.36 Å, b = 10.15 Å and c = 4.09 Å) [25, 44]. However, the presence of a low intensity peak around 11,268° suggests the presence of birnessite (δ-MnO2) [16, 27, 35] with a poor crystallinity [45]. It has been reported that electrolytic manganese dioxide has a more amorphous structure than that obtained by other synthesis methods. In addition, the use of an acidic electrolytic medium of manganese sulfate leads to a predominance of the γ-MnO2 variety which is formed by the random inter-growth of the ramsdellite (1 × 2) and pyrolusite (1 × 1) structures and exhibits excellent catalytic and electrochemical activities among the other crystallographic varieties of manganese dioxide [25, 37, 38].

Fig. 5
figure 5

XRD diagrams of: (1) SS/MnO2 thin film electrodeposited on SS substrate (black line) and (2) SS/MnO2 powder (red line)

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The Brunauer–Emmett–Teller (BET) surface area and the pore structure of MnO2 sample powder elaborated by the proposed electrochemical technique were investigated by measuring the nitrogen adsorption/desorption isotherms. Figure 6a shows the N2 adsorption/desorption isotherm. MnO2 shows type IV isotherm according to the IUPAC classification which is a characteristic of mesoporous materials with a pore diameter between 2 and 50 nm [46]. The analysis results revealed a large BET surface area of 368.390 m2 g−1, with average nanoparticle size of 16.287 nm which is in agreement with the SEM. The smaller the crystal size, the larger the specific surface area, which meant that there were more active sites to enhance the electro-generation efficiency of •OH and degrade the pollutants [47].

Fig. 6
figure 6

a N2 adsorption/desorption isotherms of γ-MnO2 and BJH and b pore size distribution curve

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Figure 6b shows the pore size distribution of the MnO2 determined using the Barrett–Joyner–Halenda (BJH) method from the desorption branch of the isotherm curve. According to the BJH model, the average pore diameter is 2.565 nm which is within the range of mesopore and the pore volume is 0.134 cm3 g−1. These results of BJH measurements suggested that the nanostructured γ-MnO2 electrodeposited on SS electrode was beneficial to the adsorption of organic pollutants [7].

Table 1 shows the BET surface area values of MnO2 nanostructures corresponding to different crystallographic varieties synthesized by different methods provided in the literature. The results indicate a wide range of specific surface area between 14.507 and 194 m2 g−1. These values depend not only on the crystallographic variety of MnO2 obtained by the synthesis technique used but especially on the controlled operating conditions. Mesoporous γ-MnO2 synthesized by the proposed electrochemical technique has a nanosize structure with higher large surface area than that obtained by other techniques and could be an ideal electrode material not only for energy applications as a cathode material in the batteries industry but also in the field of organic pollutants treatment.

Table 1 Comparative of surface area of MnO2 synthesized by different methods reported in literature
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Electrochemical degradation of phenol

Electrochemical behavior of MnO2 thin film

Figure 7 shows cyclic voltammograms recorded on both SS/MnO2 thin film and on the bare SS substrate for comparison, in aqueous acidic solution of phenol. It should be mentioned that the use of the acidic supporting electrolyte is suitable for the detection of phenolic compounds with high sensitivity [52], and prevents the formation of polymeric products [53]. Furthermore, active hydroxyl radicals have a stronger oxidative capacity toward organic molecules under acidic conditions, further enhancing their degradation [18, 25]. The cyclic voltammograms show that no peaks were observed using the bare substrate (SS) in the absence and presence of phenol, meaning that the substrate shows no electrocatalytic activity in the selected scan region. The same applies to the voltammogram recorded on the SS/MnO2 thin-film electrode in H2SO4 electrolyte. An oxygen evolution potential of around 1.08 V vs. SSE (1.73 vs. SHE) is observed, a value close to that of Pt (1.6 V vs. SHE in 0.5 M H2SO4) [54]. The oxygen evolution reaction occurs as a secondary reaction, which reduces the electrocatalytic degradation process of organic compounds [55]. So, the electrodes with high overpotential for oxygen evolution reaction have high electrocatalytic activity for the organics oxidation.

Fig. 7
figure 7

Cyclic votammograms of the bare SS electrode and SS/MnO2 thin film without and with phenol (10 mg L−1), 1: SS without phenol (green), 2: SS with phenol (blue), 3: SS/MnO2 without phenol (red) and 4: SS/MnO2 with phenol (black) pH = 2, scan rate = 10 mV s.−1

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Electrochemical wastewater treatment depends on the nature of the electrode material. Phenol can be mineralized on non-active anodes, known as high oxygen overpotential materials, such as PbO2 (1.9 V vs. SHE in 0.5 M H2SO4), SnO2 (1.9 V vs. SHE in 0.05 M H2SO4) and BDD (2.3 V vs. SHE). These electrodes have a high window of potential and low residual currents like the BDD electrode (more than 3 V vs. SHE in aqueous medium) [54] which give access to anodic potentials high enough for the generation of hydroxyl radicals. However, the possible toxicity of Pb2+ related to PbO2, the relatively short lifetime and low stability of SnO2 and the high cost of BDD limit their use. Active anodes with low oxygen evolution overpotential such as RuO2 (1.47 V vs. SHE), IrO2 (1.52 V vs. SHE) and Pt (1.6 V vs. SHE) electrodes, with restricted potential window and higher residual currents such as Pt (1.5 V vs. SHE) promote the partial and selective oxidation of phenol with the formation of aromatic intermediates such as hydroquinone, catechol and benzoquinone [18, 21, 54, 56,57,58]. It should be emphasized that the oxygen evolution potential of the SS/MnO2 electrode, synthesized by the proposed electrochemical technique, is higher than that active electrodes reported above.

The voltammogram recorded on SS/MnO2 in the presence of phenol shows a significantly higher anode current than that obtained in the solution without organic molecule. The SS/MnO2 thin film anode shows significant electrocatalytic activity for phenol oxidation, a broad peak was observed at about 0.9 V vs. SSE indicating that electron transfer occurs at the electrode surface corresponding to the direct oxidation of the organic molecule. In addition, a larger surface area of MnO2 can contributes to the adsorption of organics, so the rates of degradation increase [55]. Better adsorption of the organic molecule on the electrode surface would improve electron transfer for direct organic oxidation and allows for more efficient indirect oxidation by hydroxyl radicals generated on the electrode [18, 20]. Consequently, the SS/MnO2 electrode can be considered to have good electrocatalytic performance, which is desirable for the electrochemical oxidation of organic pollutants.

Effects of applied potential, pH and initial concentration of phenol

The influence of operating parameters on the electrochemical degradation of phenol onto thin films of γ-MnO2 and their kinetics was shown in Fig. 8. Assuming that phenol has been eliminated by large quantities of •OH generated on the surface of the MnO2 electrode during treatment, the electrochemical degradation kinetic of phenol can be given by Eq. 3 which can be solved by Eq. 4:

Fig. 8
figure 8

Electrochemical degradation of phenol using SS/MnO2 thin films electrode under different conditions: a applied potential, b pH and c initial concentration of phenol at Eapp = 0.8 V vs. SSE and their corresponding kinetics (d), (e) and (f)

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$$\mathrm v=-\frac{\mathrm{dC}}{\mathrm {dt}}=\mathrm k{\left[{}_{}^{\bullet }\mathrm {OH}\right]}^{\alpha }\mathrm C=\mathrm {k}_\mathrm {app}\mathrm C$$
(3)
$$\mathrm {ln}\left(\frac{\mathrm{C}}{{\mathrm {C}}_{0}}\right)=-\mathrm{k}_\mathrm {app}\mathrm t$$
(4)

α is the reaction order of •OH, k is the real rate constant, kapp is the pseudo-first-order kinetic apparent constant, and t is electrolysis time.

The effect of applied potential (Eapp) on the oxidation of phenol studied in the potential range of 0.7 to 1 V vs. SSE keeping the initial phenol concentration of 10 mg L−1 and pH of 5 constant. As shown in Fig. 8a, the rate of phenol degradation as a function of time at different Eapp shows that the rate of phenol degradation increases with increasing Eapp from 0.7 to 0.8 V vs. SSE and increases from 40% to a maximum rate of 87.37% after 4 h of electrolysis. The latter decreases to nearly 83% at Eapp = 0.9 V vs. SSE and finally drops sharply to 29.76% at Eapp = 1V vs. SSE. Therefore, the optimal applied potential is 0.8 V vs. SSE.

The electrochemical degradation of phenol is more favorable in acidic media than in neutral and alkaline media. The generation of oxygen, competing with the degradation of the organic compound, is limited in acidic medium, which favors the oxidation rate of the organic matter at the anode surface [14, 18, 20, 53]. Figure 8b shows the effect of pH carried out in pH range 2–5 at Eapp = 0.8 V vs. SSE and an initial phenol concentration of 10 mg L−1. The results show that the phenol degradation reaches a maximum rate of 87.37% at pH = 5 after electrolysis considered to be the optimum pH for phenol degradation.

The effect of the initial phenol concentration was studied by varying the initial phenol concentrations between 5 and 40 mg L−1 under at Eapp = 0.8 V vs. SSE and pH = 5 (Fig. 8c). It can be seen that the electrochemical degradation efficiency of phenol increased with decreasing initial concentration. Almost complete phenol degradation (98%) is achieved after electrolysis of 5 mg L−1 of phenol and reaches a degradation rate of 36.81% for an initial phenol concentration of 40 mg L−1. Electrochemical degradation of phenol is almost complete at low concentration, suggesting that phenol degradation is favored at low concentrations where the possibility of •OH attack of pollutant molecules is higher. However, an increase in the initial phenol concentration leads to greater production of intermediates on the electrode surface that can prevent contact between the phenol molecules and the active sites. As a result, these intermediates compete with the phenol by reacting with the active particles, thus affecting the efficiency of phenol degradation [59].

The kinetics of the phenol degradation reaction under the operating conditions follows pseudo-first-order kinetics as shown in Fig. 8d, e and f. The apparent rate constants (kapp) of phenol removal, corresponding to the slope of the plotted lines, increases from 0.075 to 0.478 h−1 with the increase of Eapp from 0.7 to 0.8 V vs. SSE then decreases beyond this potential to reach values of 0.343 h−1 at 0.9 V vs. SSE and 0.110 h−1 at 1V vs. SSE (Fig. 8d). kapp values increases with increasing pH with 0.047, 0.171, 0.183 and 0.478 h−1 for pH = 2, 3, 4 and 5, respectively (Fig. 8e). kapp decreased with increasing of initial phenol concentration with 0.970, 0.478, 0.223 and 0.115 h−1 for 5, 10, 20 and 40 mg L−1, respectively (Fig. 8f).

Mechanism of electrochemical oxidation of phenol

To better understand the reaction mechanism of phenol degradation on the SS/MnO2 active electrode, infrared spectroscopy (FTIR) characterization was performed on SS/MnO2 electrode before and after electrochemical treatment of phenol under optimum conditions of applied potential 0.8 V vs. SSE, pH 5 and initial phenol concentration of 5 mg L−1. During the reaction (4 h), samples are taken at regular intervals and analyzed by HPLC to monitor changes in pollutant concentration and the formation of reaction intermediates.

Figure 9 presents the infrared spectroscopy (FTIR) spectra in the wavenumber range from 400 to 4000 cm−1 of nanostructured γ-MnO2 thin film electrodeposited on SS electrode before and after 4 h of electrolysis at Eapp = 0.8 V vs. SSE of an aqueous solution containing 5 mg·L−1 of phenol at pH = 5. As can be seen, the same bands were observed in both spectra. The band at 512.01 cm−1 can be assigned to Mn–O and Mn–O–Mn stretching vibration which are related to the vibration mode of the MnO6 octahedra [51, 60, 61]. The absorption bands located at about 1617.98, 1384.14 and 1045.23 cm−1 are attributed to the bending vibrations of hydroxyl groups which are related to the Mn atoms (Mn-OH) [51, 61]. A broad band at about 3405.19 cm−1 corresponds to the stretching vibration of hydroxyl groups, mainly attached with traces of adsorbed moisture (H–OH) along with hydroxyl group present on Mn surface (Mn-OH) [60, 61]. These results are consistent with the literature and indicate the successful synthesis of nanostructured γ-MnO2. Furthermore, the FTIR spectra of both samples indicate the presence of the same functional groups on the MnO2 surface. The intensity of the peaks of all the functional groups detected did not increase after 4 h of electrolysis of the aqueous phenol solution, suggesting the absence of a fouling layer on the MnO2 film caused by the electrochemical polymerization reactions of the electrode [36].

Fig. 9
figure 9

FTIR spectra of MnO2 thin film electrode, 1: before and 2: after degradation of phenol

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Figure 10 reports HPLC chromatograms of the phenol solution recorded from 0 to 240 min under optimal experimental conditions. The results show that phenol is oxidized during electrochemical treatment due to the peak area of phenol decreasing during electrolysis and the appearance of other peaks (Fig. 10a). The enlargement of the chromatograms shown in Fig. 10b reveals three major peaks with retention times (tr) of 2.796, 3.991 and 4.621 min observed at early electrolysis. Based on the intermediates identified in the solutions, an electrochemical degradation pathway for phenol at the SS/MnO2 electrode surface is proposed as illustrated in Fig. 11. The intermediates identified are hydroquinone (tr = 2.796 min) and 1,4-benzoquinone (tr = 3.991 min).It was generally considered that the hydroxyl radicals attack benzene rings at ortho-position and para-position to produce catechol and hydroquinone. This suggests that the third peak observed at tr = 4.621 min can be attributed to catechol. This is followed by the oxidation of hydroquinone to 1,4-benzoquinone which redox couple. This is generally recognized as the first step in the electrochemical degradation of phenol after the formation of phenoxy radicals. Benzoquinone, which is considered an important intermediate in phenol oxidation, can be degraded with ring breakage to form various harmless carboxylic acids which finally yielded carbon dioxide and water [6, 18]. As shown in HPLC chromatograms, concentrations of these intermediates increased at the start of electrolysis and reached a maximum at times 30 min for hydroquinone, 180 min for 1,4-benzoquinone and 90 min for catechol, and then begin to decrease gradually during electrolysis. These intermediates are transformed into other by-products, giving rise to additional peaks. Acetic acid (tr = 2.112 min) and formic acid (tr = 2.269 min) were detected at times 180 min and 210 min respectively. Acetic acid reaches a maximum at 210 min before decreasing as it degrades to form formic acid, which increases until the end of electrolysis and then can be converted to CO2 and water.

Fig. 10
figure 10

a HPLC chromatograms of phenol degradation at different time and b enlarged HPLC chromatograms of phenol degradation

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Fig. 11
figure 11

Proposed pathway of phenol degradation on SS/MnO2 electrode

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Following the proposed phenol electrooxidation mechanism and the generalized scheme of the electrochemical conversion/combustion of organics on oxide anodes proposed by Comninellis [21], degradation of phenol at the SS/MnO2 electrode can take place through two states of “active oxygen”. Hydroxyl radical physically adsorbed •OH was generated by the discharge of water, according to Eq. 5:

$${\mathrm{MnO}}_{\mathrm{x}}+{\mathrm{H}}_{2}\mathrm{O}\to {\mathrm{MnO}}_{\mathrm{x}}\left({}_{}^{\bullet }\mathrm{OH}\right)+{\mathrm{H}}^{+}+{\mathrm{e}}^{-}$$
(5)

In the second step, the adsorbed hydroxyl radical can react with the oxygen present on the anodic oxide, producing a higher oxide or chemisorbed active oxygen MnOx+1 according to Eq. 6:

$${\mathrm{MnO}}_{\mathrm{x}}\left({}_{}^{\bullet}\mathrm{OH}\right)\to {\mathrm{MnO}}_{\mathrm{x}+1}+{\mathrm{H}}^{+}+{\mathrm{e}}^{-}$$
(6)

In the presence of organic radicals (R), it is possible to produce the complete mineralization, by the action of the physisorbed •OH, or selective oxidation products (RO) by the interaction with chemisorbed MOx+1 according to Eqs. 7 and 8:

$${\mathrm{MnO}}_{\mathrm{x}}\left({}_{}^{\bullet }\mathrm{OH}\right)+ R\to {\mathrm{MnO}}_{\mathrm{x}}+{\mathrm{CO}}_{2}+{\mathrm{H}}_{2}\mathrm{O}+{\mathrm{H}}^{+}+{\mathrm{e}}^{-}$$
(7)
$${\mathrm{MnO}}_{\mathrm{x}+1}+ R\to \mathrm{RO}+{\mathrm{MnO}}_{\mathrm{x}}$$
(8)

Both physisorbed and chemisorbed active oxygen also produces oxygen evolution Eqs. 9 and 10 respectively which presents a competing side reaction leading to a reduction in the efficiency of the anodic process [21, 55, 62].

$${\mathrm{MO}}_{\mathrm{x}}\left({}_{}^{\bullet }\mathrm{OH}\right)\to {\mathrm{MO}}_{\mathrm{x}}+{~}^{1}\!\left/ \!{~}_{2}\right. \mathrm{O}_{2}+{\mathrm{H}}^{+}+{\mathrm{e}}^{-}$$
(9)
$${\mathrm{MO}}_{\mathrm{x}+1}\to {\mathrm{MO}}_{\mathrm{x}}+{~}^{1}\!\left/ \!{~}_{2}\right. \mathrm{O}_{2}$$
(10)

Conclusion

A thin layer of mesoporous γ-MnO2 with a nanometric structure with a particle size of 16.287 nm and a specific surface area of 368.390 m2 g−1 was successfully deposited on a stainless steel substrate (SS/MnO2). The performance of this electrode for the electrochemical degradation of phenol was studied and the results showed almost complete phenol elimination under optimal experimental conditions of applied potential E = 0.8V, pH = 5 and a phenol concentration of 5 mg L−1 with pseudo-first-order kinetics. Based on the HPLC analysis, some degradation products such as hydroquinone, benzoquinone as well as acetic and formic acids were identified, suggesting that the SS/MnO2 electrode presents an electroactive surface beneficial for the oxidation of organic compounds in the wastewater treatment process. As a result, an electrochemical degradation mechanism of phenol on the SS/MnO2 electrode was proposed.