Study on degradation of diesel pollutants in seawater by composite photocatalyst MnO₂/ZrO₂

In recent years, visible photocatalysis, as a promising option, has become one of the hot spots in environmental remediation research (Ye et al. 2018; Peng et al. 2019; Reddy et al. 2019; Onkani et al. 2020; Waimbo et al. 2020). Compared with traditional technology, it has more prospects in the field of contaminant treatment for its low cost, no secondary pollution, and simple process. In different semiconductors, zirconium dioxide (ZrO₂) is widely used in numerous applications due to its excellent physio-chemical properties. It is a harmless element with good stability, and is a cheap and environmentally friendly material (Kim et al. 2010; Gao et al. 2017; Debnath et al. 2020). However, due to the wide band gap (∼5 eV) of ZrO₂, the capture ability in the visible region is weak. ZrO₂ has poor catalytic activity in the visible region, which weakens its use as an effective photocatalyst material.

To overcome the above-mentioned problems and to improve its photocatalytic performance, manganese dioxide (MnO₂), as a modifier, was introduced in this study to the surface of ZrO₂ by a simple and facile method. MnO₂ possesses a narrow direct band gap (∼0.25 eV) and high optical absorption characteristics that enable effective utilization of the visible light. Mn is therefore selected as a modified dopant to produce composite catalyst (Huang et al. 2020). Many experts and scholars have studied the modification of ZrO₂ and found that ZrO₂ has a high photocatalytic activity and efficiency (Agorku et al. 2015; Gaikwad et al. 2017; Thejaswini et al. 2017; Venkata Reddy et al. 2020). The present study focuses on the synthesis of highly efficient Mn-doped ZrO₂ catalysts using an inexpensive and eco-friendly co-precipitation method for degradation of diesel pollution in seawater. Furthermore, the degradation of diesel pollution in seawater was optimized by an orthogonal experiment.

Throughout the study, deionized water and analytically pure chemicals were used. The chemicals include zirconium oxychloride (ZrOCl₂·8H₂O), manganese sulfate (MnSO₄), sodium hydroxide (NaOH), N-hexane and absolute ethanol, etc. The instruments included a magnetic stirrer, an ultrasonic cleaner, a centrifuge, a drying oven, a muffle furnace, a UV photocatalytic reaction box and 752 UV spectrophotometer.

The MnO₂/ZrO₂ photocatalyst was prepared by co-precipitation with different molar ratios of Mn to Zr. The preparation process is shown in Figure 1.

The MnO₂/ZrO₂ photocatalysts were instrumentally characterized using scanning electron microscopy (SEM) for the surface morphology, X-ray powder diffraction (XRD) for phase analysis and crystallinity, energy-dispersive spectroscopy (EDS) for the elements and their relative content, and UV-Vis diffuse reflectance spectroscopy (UV-Vis-DRS) for the optical property.

A 50 mL sample of seawater polluted by diesel was taken at a certain concentrations. MnO₂/ZrO₂ composite photocatalyst was added and irradiated by visible light. The amount of residual diesel in seawater was determined using the UV-Vis spectrophotometry method after reaction. Finally, the degradation efficiency of photocatalysts was calculated.

Figure 2 shows the SEM image (magnification ×100,000) of photocatalysts for the different doping ratios. From the figure, the pure ZrO₂ particles reveal a regular globular structure. The MnO₂/ZrO₂ photocatalysts with different doping ratios have high crystallinity, which reveals irregular globular structures in comparison to a pure ZrO₂. With the increase in doping ratio, the photocatalysts show obvious reunion phenomena.

Figure 3 shows the XRD patterns of the pure ZrO₂ and the MnO₂/ZrO₂ with doping ratio of 30%. The XRD pattern for pure ZrO₂ signifies the existence of monoclinic form through the presence of a significant peak (2θ) at 30.731° (JCPDS:80–0784). Similarly, the synthesized MnO₂/ZrO₂ shows ZrO₂ diffraction peaks that correspond to the monoclinic form. The peaks at 30.423°, 35.318°, 50.313° and 60.181° indicate that the crystallinity of ZrO₂ is not likely to be affected by the doping of MnO₂. The XRD spectrum of MnO₂/ZrO₂ is analogous to that of pure ZrO₂, with no obvious peaks about the existence of Mn or MnO₂. This non-existence reflects on the low level of Mn ions doping and also signifies the possible efficient intercalation of Mn ions into the ZrO₂ crystal lattice sites.

The EDS pattern of the sample is shown in Figure 4. Based on the graph, the sample contains Zr, O and Mn, and the mass fraction of Zr, O and Mn elements is 47.04, 21.45 and 15.64% respectively, proving that MnO₂ has been well doped in ZrO₂.

Figure 5 shows the UV-Vis-DRS spectra for the pure ZrO₂ and the MnO₂/ZrO₂ with doping ratio of 30%. The spectral pattern reveals that the absorption edge of the MnO₂/ZrO₂ photocatalyst shifts to higher wavelength in comparison to the pure ZrO₂ and implies enhanced photon absorption under visible light. This red shift may be attributed to the decreased band gaps from the lattice defects produced by the trace level doping of MnO₂ into the ZrO₂ lattice or from the increased crystal domains. This observation is in agreement with the XRD data, which reveals variations in the crystallite sizes with varying doping ratio. Hence, it is obvious that the effect of MnO₂ doping has a great influence on the characteristic absorption of ZrO₂.

Figure 6(a) shows the effect of photocatalytic efficiency of MnO₂/ZrO₂ and its doping ratio variation on diesel degradation under visible light. The experiment was carried out using 50 mL of 0.2 g/L of diesel with the varying doping ratio (0, 10, 20, 30, 40 and 50%) of MnO₂/ZrO₂ photocatalysts. Based on the graph, it is clear that the removal rate of diesel would first increase, and reach the highest point when the doping ratio was 30%. The reason may be that when the doping ratio is over 30%, the separation efficiency of the dopant carriers on the composite photocatalyst decreases, reducing the activity of the photocatalyst and hindering the degradation rate.

Figure 6(b) shows the influence of photocatalytic efficiency of MnO₂/ZrO₂ and its calcination temperature variation on diesel degradation under visible light. The experiment was carried out using 50 mL of 0.2 g/L of diesel with the varying calcination temperature (350, 400, 450, 500, 550 and 600 °C) of MnO₂/ZrO₂ photocatalysts. From the graph, the order of diesel degradation efficiency increases linearly with the increase of calcination temperature from 350 °C to 500 °C and then after decreases significantly. The possible reason is that the calcination temperature affects the growth of the photocatalyst crystal phase. The crystal lattice of MnO₂/ZrO₂ photocatalyst tended to mature when the calcination temperature reached 500 °C, and the obvious diesel degradation efficiency could be obtained. It also revealed that the catalyst had excellent thermal stability and was difficult to be sintered at high temperature. However, when the calcination temperature was more than 500 °C, agglomeration occurred in the MnO₂/ZrO₂ photocatalyst, and effective contact with pollutants was decreased.

Figure 6(c) shows the effect of photocatalyst dosage on diesel degradation under visible light. The dosage of MnO₂/ZrO₂ material required for the diesel degradation, using different photocatalyst amount ranging from 0.1 to 0.6 g/L, was studied. From the graph, the diesel degradation rate is around 29.6% without any photocatalysts (only by evaporation). With the increase of MnO₂/ZrO₂ dosage, the removal rate shows a trend of rising first and then falling. The maximum is 72.13%. The reason is due to an efficient increase in the active electronic or hole pairs with an increase in the photocatalyst, which can improve the efficiency of the photocatalyst. However, when the photocatalyst reaches a certain amount, it is accompanied by the scattering of light, which would hinder the absorption of visible light and result in the decrease of degradation rate.

Figure 6(d) shows the influence of photocatalytic efficiency of MnO₂/ZrO₂ and its different initial concentration of diesel on diesel degradation under visible light. The experiment was carried out using the varying concentrations of diesel ranging from 0.1 to 0.6 g/L. From the graph, the removal rate is inversely proportional to the increase in concentration of diesel. This is because diesel has an influence on the incidence of visible light and it is difficult for the photons to enter into the system as the path length decreases. In addition, the large amount of diesel covers the surface of the photocatalyst. In this case, diesel molecules absorb more light compared to the photocatalysts and hinder the efficient photon absorption efficiency on the photocatalyst material, which results in the influence of photocatalytic efficiency on the degradation of diesel.

Figure 6(e) shows the influence of photocatalytic efficiency of MnO₂/ZrO₂ and its varying concentration of hydrogen peroxide (H₂O₂) solution on diesel degradation under visible light. The experiment was carried out using different concentrations of H₂O₂ solution ranging from 0.1 to 0.6 mg/L. From the graph, the removal efficiency rises and then falls slowly in pace with the concentration of H₂O₂ solution. The maximum removal rate is obtained when the concentration of H₂O₂ solution is 0.2 mg/L. Related studies have shown that H₂O₂ can facilitate photocatalytic degradation. As an electron acceptor, H₂O₂ can trap photoelectrons and prevent the recombination of electron-hole pairs on the surface of catalyst and produce strong oxidizing ·O₂²⁻ and ·OH. However, H₂O₂ also hinders the hydroxyl radicals and superoxides. Therefore, appropriate amounts of H₂O₂ can improve the removal rate, but excessive amounts of H₂O₂ can hinder the reaction.

Figure 6(f) shows the influence of photocatalytic efficiency of MnO₂/ZrO₂ and its varying illumination time on diesel degradation under visible light. The experiment was carried out using the different illumination time ranging from 1 to 6 h. Based on the graph, the degradation rate of diesel is directly proportional to the illumination time. The oxygen molecules captured a mass of electrons and the amount of hydroxyl radicals and superoxide radicals in water also increased significantly. Hydroxyl radicals have strong oxidation properties, which can oxidize organic pollutants into inorganic substances to attain the purpose of degradation, thus the photocatalytic degradation reaction rate increased.

In order to determine the interaction of factors affecting photocatalytic oxidation of MnO₂/ZrO₂, six factors were selected: the dose of MnO₂/ZrO₂, initial concentration of diesel, doping ratio, illumination time, calcination temperature, and concentration of H₂O₂ solution. The orthogonal experimental data are given in Table 1.

The optimal photocatalytic combination to reach the degradation rate of 92.92% was as follows: the dose of MnO₂/ZrO₂ photocatalyst was 0.5 g/L, the initial concentration of diesel was 0.7 g/L, the doping ratio was 40%, the calcination temperature was 400 °C, the illumination time was 5 h, the concentration of H₂O₂ solution was 10 mg/L. The order of factors affecting photocatalysis degradation rate was: illumination time > doping ratio of catalysts > calcination temperature > initial concentration of diesel > dose of photocatalysts > concentration of H₂O₂ solution. The orthogonal experimental data are shown in Table 2.

The MnO₂/ZrO₂ was successfully prepared by co-precipitation. Using XRD, SEM, EDS and UV-Vis-DRS characterization, the crystal form of the catalyst was confirmed as a monoclinic form and the MnO₂ was successfully doped into the bulk of ZrO₂. The response range of ZrO₂ to optical light was greatly broadened. This is the first report on a comprehensive analytical study on the effect of various physio-chemical parameters towards diesel degradation using the synthesized photocatalysts. The results showed that under optimum reaction conditions, the degradation rate can reach 92.92%. By experiments, it was confirmed that the MnO₂/ZrO₂ has an obvious photocatalytic performance under visible light and it can be used in the practical application on treatment of diesel pollution with the characteristics of high efficiency and no secondary pollution.

We sincerely thank the State Oceanic Administration People's Republic of China (201305002), Liaoning Science and Technology Public Welfare Fund (20170002), Science Foundation of Department of Ocean and Fisheries of Liaoning Province (201733), and Department of Science and Technology of Liaoning (2016LD0105) for their financial support of this study.

All relevant data are included in the paper or its Supplementary Information.