Introduction

Traditional physical techniques, such as flocculation, adsorption on activated carbon and ultra-filtration may be employed efficiently to remove such recalcitrant pollutants. However, they are non-destructive, because they just transfer the dye compounds from water to another phase which would cause a secondary environmental pollution [1,2,3,4,5,6]. This has led to the requirement of regeneration of the adsorbent materials and post-treatment of solid wastes and both of them are expensive [7].

In the course of recent years, elective techniques as advanced oxidation processes (AOPs) for organic materials treatment have been studied [8,9,10], incorporating chemical oxidation with reagents for example: ozone, hydrogen peroxide, ozone/UV, hydrogen peroxide/UV and Fenton’s reagent [hydrogen peroxide + Fe(II)]. A photocatalytic strategy for the oxidation of organic contaminants in wastewater by UV or solar irradiation and TiO2 has additionally been investigated [11,12,13,14,15,16]. TiO2 has very attractive properties such as environmentally friendly, low cost, low harmfulness and a wide band gap of 3.2 eV, which results in good stability and avoids photo-corrosion [17]. Generally, since TiO2 is used as powder, which brings about a few disadvantages, for example, trouble in catalyst separation. This issue was expelled with catalyst immobilization on the solid surfaces. But, this technique intrinsically diminishes quantum efficiency because of the lessening of active surface area [18, 19]. Nevertheless, the high probability of hole–electron recombination in TiO2 during the photocatalytic process has to be solved yet [20, 21].

The photoelectrochemical technology with external potential seems promising for research on photocatalytic degradation of organic pollutants, and many academic studies have been focused on this area [22,23,24,25,26,27,28,29,30]. Cresols are persistent compounds and are poisonous to a wide range of living beings. They may have adverse effects on ecosystems and additionally on human health, bringing about serious environmental contamination. m-Cresol is a methylated derivative of phenol and is a substance regarded as a priority contaminant by the United States Environmental Protection Agency (USEPA) [31, 32]. Cresols are utilized in the pharmaceutical and textile industries, and are produced as intermediates in the preparation of pesticides, to a great extent utilized in the formulation of antioxidants; they likewise have numerous uses in the fragrance industry [33]. This extensive range of industrial applications accounts for the frequent presence of cresols as contaminants in industrial wastewater. The Ministry of Environment and Forest (MOEF), Government of India, has set a greatest concentration level of 1.0 mg L−1 of cresol in industrial effluent for safe discharge in the surface waters [34]. Thus, the removal of m-cresol has received extensive consideration using different techniques [35,36,37,38]. Notwithstanding, based on current information, application of photoelectrocatalytic methods for removal of m-cresol has not been reported.

In this study, the photoelectrocatalytic degradation of m-cresol using the TiO2 thin film-modified carbon sheet photoelectrode was investigated. This electrode was provided by covering a carbon plate surface with a known quantity of the TiO2 slurry. Furthermore, in this research, the effect of different factors (such as initial m-cresol concentration, pH value of solution, and bias potential used) was studied on degradation of m-cresol.

Experimental

Reagents

The solutions were prepared with deionized water. Orthophosphoric acid and its salts in the pH ranges 3.0–11.0 were applied to prepare buffer solutions. For fabricating the carbon paste electrode, paraffin (density 0.88 g cm−3) and graphite powder (particle diameter 0.1 mm) from Fluka were utilized. The received m-cresol from Merck was used. Carbon electrodes were prepared from cutting a carbon sheet in the known size. TiO2 powder with product name P-25, particle size 30 nm and surface area 50 m2 g−1 was obtained from Degussa Corp. In TiO2 powder, there is anatase form of TiO2 as the significant portion compared to other forms.

Construction of the photoelectrode

Dip-coating strategy was employed for covering TiO2 films from a TiO2 suspension. To get ready of this suspension, a certain quantity of the Degussa P-25 powder was suspended in water (250 g L−1) and sonicated for 30 min. After sonicating, a carbon sheet (3.0 × 3.0 cm2) was dunked into TiO2 suspension for 15 s and then lifted up and preheated at 90 °C for fabrication TiO2 film. The technique from plunging to preheating was rehashed for six cycles. The TiO2 loading corresponds to 6.6 mg of TiO2 spread on an area of approximately 6.2 cm2 of the carbon sheet. The uncovered area of the carbon sheet was used to make the electrical contact. Finally, the TiO2-modified electrode was heated at 400 °C for 1 h. The TiO2 thin film prepared stuck well to graphite surface and had sufficient stability in the pH limit 1–13.

Physical and photoelectrochemical characterization

The photoelectrocatalytic oxidation of m-cresol was performed in a single photoelectrochemical compartment which was constructed of a cylindrical quartz cell (3.0 cm diameter × 8.0 cm height with a 1.8 mm wall), a TiO2-modified carbon sheet electrode as anode, a Pt rode serving as cathode, and an Ag/AgCl/KCl (3 M) serving as the reference electrode (Fig. 1). The reactor and the UV lamp were put in a black box to keep away from extraneous illumination. The photoreactor has three electrodes which were set in the center of the reactor in parallel and connected with the potentiostat (µ Autolab Type III). A carbon paste electrode (CPE) was utilized as a working electrode for electrochemical examination of m-cresol concentration variations in the different processes such as photoelectrocatalysis by measurements of the peak current (IP) decay at the peak potential (EP) oxidation of m-cresol in its differential pulse voltammograms over various times. The scan rate for recording DPVs was 10 mV s−1. The photoactive surface of the anode (TiO2) was illuminated by a 4 W medium pressure mercury lamp with a power intensity of 0.4 mW cm−2 as UV light source. Scanning electron microscopy (SEM) (Philips Corp., XL30 model) was used for seeing of the surface morphology of the TiO2-modified carbon electrode. Measurements of pH were made with a Denver Instrument Model 827 pH meter equipped with a Metrohm glass electrode. Zeta potential of the TiO2 was measured using a Zeta potential meter (Stabino, Particle Mertix Company, Germany).

Fig. 1
figure 1

Schematic diagram of the photoreactor system

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Results and discussion

Surface morphology of the photoelectrode

The TiO2-modified photoelectrode surface was studied utilizing SEM. Figure 2 indicates typically the top of SEM pictures of the unmodified (A) and TiO2-modified (B) carbon electrodes. It can be shown from the micrograph that spherical TiO2 particles were distributed uniformly on the surface of carbon, and that the mean size is nearly equal to that of the original Degussa P25 TiO2. The cross-sectional image of TiO2 film (Fig. 2c) clearly shows three different layers: (1) a porous transparent TiO2 layer ∼ 4.3 μm, and (2) a scattered TiO2 layer ∼ 1.0 μm.

Fig. 2
figure 2

The top of SEM micrographs of a unmodified and b TiO2-modified carbon sheet electrodes. c A cross-sectional photograph of TiO2 thin film on the carbon sheet

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Photoelectrochemical properties of the photoelectrode

The mechanisms underlying the heterogenous photocatalysis employing the semiconductor TiO2 catalyst have been intensively reported in many papers [39]. The key reactions involved in the photocatalysis are listed in Table 1. Upon irradiation of TiO2 with light energy equivalent to or greater than its band gap energy (anatase ~ 3.2 eV), the electron is excited from valence band (VB) to the conduction band (CB) (Eq. 1). Figure 3a(1) depicts the mechanism of the electron–hole pair formation when the TiO2 particle is irradiated with light of adequate energy. The light wavelength for such photon energy usually corresponds to λ < 400 nm. Owing to the reaction between the photogenerated vacancy and adsorbed water, a heterogeneous hydroxyl radical is generated and then this radical can be straightforwardly oxidized by the hole or by ·OH formed from Eq. (2). Likewise, the photoinjected electron can create other oxidizers such as O ·−2 , HO ·2 and H2O2 with less strength and more ·OH based on Eqs. (3–6). The recombination of electrons ascended to the valence band either with unreacted holes or with adsorbed hydroxyl radicals which is the notable obstacle of photocatalysis as observed in Fig. 3a(2) and Eqs. (7, 8).

Table 1 Some basic reactions involved in the photocatalysis process
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Fig. 3
figure 3

Schematic photoexcitation of a semiconductor particle (a) and separation of electron–hole using an anodic voltage (b)

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The application of an anodic bias to a TiO2-modified photoelectrode further provides a potential gradient within the film to drive away the photogenerated holes and electrons in different directions efficiently. The photogenerated holes could oxidize the organic mixtures at the anode surface [Fig. 3a(3)], while the photogenerated electrons were transferred to the receiver at the metallic cathode through the outside electrical circuit [Fig. 3a(4)]. As mentioned in Fig. 3b, thin semiconductor special films obtained from especial suspensions contains small particles which are in near contact with each other and are capable of showing photoelectrochemical characteristics similar to polycrystalline semiconductor films.

Hydrodynamic amperometry was utilized in buffer solution in the presence and absence of m-cresol solution under UV light to investigate photoelectrochemical answer of the TiO2-modified photoelectrode (Fig. 4). As appeared in Fig. 4, the ascent and fall of the photocurrent reacted well to the illumination being turned on and off. The photocurrent comes on expeditiously after the illumination, and afterward the photocurrent achieves a consistent state. This pattern of photocurrent can be highly reproduced for several on–off cycles of radiation. A certain increase seen in the photocurrent under illumination shows that photogenerated electrons on the TiO2-modified photoelectrode can be adequately directed to the auxiliary electrode utilizing applied positive potential, which would be crucial for banning of charge recombination. The presence of m-cresol in buffer solution gives a significantly easier mechanism for the exchange of the photogenerated holes in the TiO2-modified photoelectrode over the film/electrolyte interface against either adsorbed water molecules or hydroxyl groups. As a result, the higher photocurrent is created in the presence of m-cresol.

Fig. 4
figure 4

Photocurrent–time curves of the TiO2-modified carbon sheet electrode for several on–off cycles of radiation at an applied potential of 0.45 V in phosphate buffer solution (pH 7.0) in the a absence of m-cresol and b presence of 400.0 mg L−1 m-cresol

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Furthermore, to more assess the photoelectrochemical activity of the TiO2-modified photoelectrode and to make sure its ability to oxidize m-cresol by photoelectrocatalysis, cyclic voltammetry tests in the dark and under UV light illumination were carried out both with and without m-cresol (Fig. 5). As can be observed in Fig. 5, the electrochemical oxidation current value of the TiO2-modified photoelectrode in the presence of m-cresol in the dark environment and under irradiation is varying (curves c and d). This demonstrates that m-cresol as a hole scavenger by obtaining holes could be viably oxidized and as a result would be effective positively to electron–hole separation.

Fig. 5
figure 5

Cyclic voltammograms of the TiO2-modified carbon sheet electrode in phosphate buffer solution (pH 7.0) a under dark conditions and b under UV irradiation, c under dark conditions containing 400.0 mg L−1 m-cresol and d under UV irradiation containing 400.0 mg L−1 m-cresol

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Comparison of different methods for m-cresol removal

Experiments on photoelectrocatalytic, photocatalytic, electrochemical and direct photolytic degradation of m-cresol were conducted to compare m-cresol degradation efficiencies among four individual processes (Fig. 6). All experiments were carried out in phosphate buffer solution (pH 7.0) containing initial m-cresol concentration 7.0 mg L−1. Also, about photoelectrocatalytic and electrochemical degradation processes, the investigations were performed at the surface of TiO2-modified photoelectrode biased at E = 0.45 V. In Fig. 6 and other Figs. 7, 8 and 9, the relative concentration of m-cresol, C/Co, was obtained by evaluating the ratio of m-cresol concentration C at time t and the initial m-cresol concentration Co in the solution, at t = 0. It is easy to see that only 4.5% m-cresol was removed by electrochemical oxidation within 60 min, which indicates that the direct photolysis and photocatalysis processes played important roles in the removal of m-cresol. The removal efficiencies obtained by the two techniques were 28 and 42%, respectively, while the removal efficiency by photoelectrocatalysis was higher, to 70%, inside the similar time. As can be seen, the experimental results approved that the removal rate of m-cresol in the photoelectrocatalytic oxidation was more than that of the photocatalytic, direct photolytic and electrochemical oxidation.

Fig. 6
figure 6

Differential pulse voltammograms of 7.0 mg L−1 m-cresol at the TiO2-modified carbon sheet electrode surface before (a) and after (b) electrochemical, c direct photolytic, d photocatalytic and e photoelectrocatalytic degradation processes, in 0.1 M phosphate buffer solution (pH 7.0) and E = 0.45 V vs Ag/AgCl/KCl (3 M)

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Fig. 7
figure 7

Plot of lnCo/C as a function of time for degradation of m-cresol using a photoelectrocatalysis, b photocatalysis, c direct photolysis and d electrochemical oxidation methods (calculated based on the data of Fig. 6)

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Fig. 8
figure 8

Effect of different initial m-cresol concentration on m-cresol degradation: a 5.0, b 7.0, c 9.0 and d 11.0 mg L−1 on photoelectrocatalytic degradation of m-cresol in phosphate buffer solution (pH 3.0) on a TiO2-modified carbon sheet electrode biased at E = 0.45 V

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Fig. 9
figure 9

a Photoelectrocatalytic removal of m-cresol as affected by pH on photoelectrocatalytic degradation of m-cresol in solution containing initial m-cresol concentration 5.0 mg L−1 on a TiO2-modified carbon sheet electrode biased at E = 0.45 V. b The Plot of zeta potential for TiO2 nanoparticles suspended in phosphate buffers as a function of the pH of the suspension

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Figure 7 indicates differential pulse voltammograms of m-cresol in the various times for photoelectrocatalytic process with scan rate 10 mV s−1 for instance. For kinetic description of the photocatalytic and photoelectrocatalytic reactions, the Langmuir–Hinshelwood kinetic model formula has often been utilized. The experimental results can well fit the first-ordered reaction model equation, ln(Co/C) = f(t) = kt (k is rate constant) (Fig. 7). Utilizing the slope of the curve, the corresponding reaction rate constant, k, can be calculated for evaluation of the degradation efficiency of m-cresol in the electrochemically assisted photocatalysis, photocatalysis, direct photolysis and electrochemical oxidation processes. The experimental results summarized in Table 2 demonstrated that the reaction rate of m-cresol degradation in the electrochemically assisted photocatalysis process was faster than that of photocatalysis, direct photolysis and electrochemical oxidation.

Table 2 Rate constants (k) and electrical energy per order (EEo) compared PEC degradation of m-cresol with PC, DP and EC removal processes
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There are various important factors in choosing a waste-treatment technology, inducing economics, economy of scale, regulations, effluent quality aims, operation (maintenance, control, safety) and robustness (flexibility to change/upsets). Despite the fact that these factors are necessary, financial aspects are frequently paramount. Since photodegradation of aqueous organic pollutant is an electric energy-intensive process, and electric energy can represent a noteworthy portion of the operating costs, simple figures-of-merit based on electric energy consumption can be very useful and informative. Recently, the international union of pure and applied chemistry (IUPAC) has proposed two figures-of-merits for advanced oxidation processes (AOPs) on the utilization of electrical energy. In the zero-order range, the appropriate figure-of-merit is the electrical energy per mass (EEm) defined as the kW h of electrical energy required for degradation of one kg of the pollutant [40]. In the case of low pollutant concentrations, which applies here, the appropriate figure-of-merit is the electrical energy per order (EEo), defined as the number of kW h of electrical energy required to reduce the concentration of a pollutant by 1 order of magnitude (90%) in 1 m3 of contaminated water. The EEo (kW h m3 order−1) can be determined from the following equations:

$$E_{\text{Eo}} = P \times t \times \, 60 \, \times \, 1000/V \times \, \ln \left( {C_{\text{o}} /C} \right),$$
(9)
$$\ln \left( {C_{\text{o}} /C} \right) \, = k \times t,$$
(10)

where P is the rated power (kW) of the AOP system, t is the illumination time (min), V is the volume (L) of the water in the reactor, Co and C are the initial and final pollutant concentrations and k is the pseudo-first-order rate constant (min−1) for the decay of the pollutant concentration [41, 42]. From Eqs. (9) and (10), EEo can be written as follows:

$$E_{\text{Eo}} = \, \left( {38.4 \, \times P} \right)/\left( {V \times k} \right).$$
(11)

The calculated EEo values for photoelectrocatalytic, photocatalytic, electrochemical and direct photolytic degradation of m-cresol are shown in Table 2. As it is clear, the efficiency of photoelectrocatalysis process is higher than other degradation processes.

The supposed degradation reactions paths are shown in Scheme 1 [43]. Based on this scheme, the photoproducts of m-cresol in aqueous solution were 3-hydroxybenzaldehyde(1), 2-methylben-zene-1,4-diol(2), 4-methylbenzene-1,2-diol(3), 3-methylben-zene-1,2-diol(4), 2,2′-dimethylbiphenyl-4,4′-diol(5), 2′,4-di-methylbiphenyl-2,4′-diol(6), 4,4′-dimethylbiphenyl-2,2′-diol (7), 2′,6-dimethylbiphenyl-2,4′-diol(8), 3-methyl-4-(m-toly-loxy)phenol(9).

Scheme 1
scheme 1

Proposed degradation of m-cresol reactions and suggested products

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Effect of initial m-cresol concentration

Figure 8 demonstrates the profiles of photoelectrochemical degradation of m-cresol at its different initial concentrations. The experiments were performed in phosphate buffer solution (pH 7.0) containing 5.0, 7.0, 9.0 and 11.0 mg L−1 m-cresol on a TiO2-modified photoelectrode biased at E = 0.45 V vs. reference electrode. It can be seen that the amount of the photoelectrocatalytic degradation of m-cresol increased but the reaction rate diminished sharply with the increase of the initial concentration of m-cresol. Since the photo-assisted electrocatalytic oxidation occurred on the catalyst surface, not in the bulk of the solution [44, 45], the reduction in m-cresol degradation rate at higher initial m-cresol concentrations can be explained by the fact that at higher m-cresol concentrations, the light intensity reaching the TiO2 film surface is reduced due to the lower transparency of the solution.

Effect of pH amount

The pH value is an important factor in the photocatalytic and photoelectrocatalytic removal processes, because the pH value of the solution will alter the present configuration of degraded types and surface charges of catalysts. As a result, the species of the reaction mixtures and its adsorption balance on the TiO2 depend on the pH values of reaction solution. Moreover, it should be pointed out that the other parameters, e.g., the adsorption size of the active species similar to elemental oxygen and decreased potential of TiO2 valence band changes [46], affect the photoelectrocatalysis process according to the pH value. The effect of pH on the degradation rate of m-cresol is presented in Fig. 9a (initial m-cresol concentration 5.0 mg L−1 over 1 h illumination). For this aim, the effect of pH value was investigated through the experiments conducted with 5.0 mg L−1 m-cresol in buffer solutions with pH 3.0–11.0 on the TiO2-modified photoelectrode biased at E = 0.45 V. One can observe that the degradation rate is highest at pH 4.0. Also, we can easily see that the amount of m-cresol degraded decreases sharply to pH 7.0 but it is almost constant to pH 10.0 and then increases after that at pH 11.0.

Figure 9b shows the results of measuring the zeta potential for suspended TiO2 particles as function of the pH of the suspension. The pH of the iso-electric point for TiO2 was found to be almost 5.2. Hydroxyl groups on TiO2 surface under various pH amount conditions, undergo the following reactions [47]:

$${\text{pH}} < {\text{PZC: TiOH}} + {\text{H}}^{ + } \rightleftarrows {\text{TiOH}}_{2}^{ + } ,$$
(12)
$${\text{pH}} > {\text{PZC: TiOH}} + {\text{OH}}^{ - } \rightleftarrows {\text{TiO}}^{ - } + {\text{H}}_{2} {\text{O}}.$$
(13)

The catalyst surface is positively charged while at pH value above just about 6.0, henceforth, at more acidic pH, it is negatively charged [48]. Also, since m-cresol is as weak organic acid in water, therefore, depending on the pH value in the solution, m-cresol might be exhibited in various structures. In this way, the adsorption amount of m-cresol at the surface TiO2 most likely as the initial step in the process of photoelectrocatalysis and subsequently photoelcectrocatalytic degradation efficiency of m-cresol is related to the initial pH value of the solution. pH has doublet effect in reaction, one is pH effect in adsorption of substrate molecules and second is pH effect in the OH− and OH· concentration that affected degradation performance resulting from reaction of holes with H2O. Therefore, in some cases, these two effects are alien and reversely affected degradation performances. In these cases, adsorption of substrate molecules at the surface of electrode is more effective than OH· concentration. For photoelectrocatalytic and photocatalytic degradation of compounds, it is necessary that compounds are absorbed at the surface of TiO2. Studies showed that pH 4.0 was optimum pH for photoelectrocatalytic degradation of m-cresol (Fig. 9). At low pH values, m-cresol molecules presented in the neutral form might be effectively adsorbed onto the surface of TiO2 with positive charges. In pH > 8, the surface containing negative charge steadily is shaped onto TiO2 and due to the repulsion effect between the TiO2 surface and the ionized m-cresol with negative charge at high pH, adsorption of m-cresol at the surface TiO2 and accordingly its degradation turn out to be substantially more difficult than that at low pH values. Therefore, it can be expected that more acidic pH amounts would be desirable for the photoelectrocatalytic degradation of m-cresol. At pH 11.0, m-cresol oxidation is being easier due to the presence of large amounts of hydroxyl radicals with strong oxidizing capacity [49].

Effect of bias potential

It is obvious that the degradation rate of expanded increased significantly as the applied cell voltage increased. Applying a potential gradient over the titania film resulted in an electric field, which keeps photogenerated charges separated and forced the photogenerated holes and electrons to move in inverse directions. Therefore, bias potential is a key element that affects photoelectrocatalytic removal efficiency. Experiments were conducted in phosphate buffer solution (pH 4.0) containing m-cresol concentration 3.8 mg L−1 over time experiment 60 min using five levels of bias potential, 0.45, 0.65, 0.85, 1.05, 1.25 and 1.45 V (Fig. 10). These results obviously exhibit that the degradation rate increases as a function of applied potential up to E = 1.25 V. These outcomes suggest that the production and segregation of electron–hole pairs are accelerated under higher voltage situations. The majority of the photogenerated electrons were evacuated either by the electric field or by reaction with dissolved oxygen. Degradation prompted with further increase in potential. The reason of this subject might be the oxidation of more water by photogenerated holes [45]. As a result, potential 1.25 V was chosen as the best potential for investigating the degradation of m-cresol.

Fig. 10
figure 10

Influence of different applied potentials on degradation of m-cresol in phosphate buffer solution (pH 4.0) containing initial m-cresol concentration 5.0 mg L−1 with time experiment 60 min

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Conclusion

In this work, TiO2-modified carbon plate photoelectrode was used as an appropriate electrode for degradation of m-cresol. Results demonstrated that photo-assisted electrocatalysis degradation of m-cresol was more compelling than different procedures, for example, direct photolytic, electrochemical and photocatalytic removal. Likewise, it was observed that PEC degradation of m-cresol at the surface of the TiO2-modified photoelectrode significantly was dependent on buffer pH, initial m-cresol concentration and bias potential applied through electrode. The improvement of m-cresol removal in the photoelectrocatalysis process was credited to photoinduced electron and hole separation by the bias potential and in view of acquired results, direct electrochemical degradation was not involved. Such a helpful aspect of electrochemically assisted photocatalysis can apply its use in photocatalytic reactors with immobilized semiconductor particles.