Water treatment by photodegradation on orthorhombic antimony sulfide powder and effect of key operational parameters using methyl orange as a model pollutant

2.1 Synthesis of Sb₂S₃

Analytical grade antimony chloride SbCl₃, urea (NH₂)₂Co, ethylene glycol C₂H₆O₂ and sodium sulfide Na₂S were purchased from Merck. In the hydrothermal method (Zhu et al., 2008) two separate solutions were prepared. 0.5 M solution of SbCl₃ was prepared by dissolving 0.346 g in 3 ml of ethylene glycol (EG) and 10 ml distilled water was added to this solution. 1.5 M Na₂S solution was prepared by dissolving 1.103 g in 10 ml distilled water. The two solutions were mixed together to get yellow suspension to which 30 ml of 1.5 M solution of urea was added. The mixtures were maintained individually at a temperature of 105, 150 and 200 °C for 4, 6 and 12 h in a Teflon lined autoclave. The resultant yellowish white suspension was cooled at room temperature, filtered and washed with distilled water and finally dried at 37 °C for 3 h.

2.2 Characterization of the materials

Phase purity and crystallinity of the prepared powders were studied by powder X-ray diffractometer (PANalytical 3040/60 X’Pert PRO) with Cu Kα radiation source. Diffraction peaks were indexed according to the standard pattern of orthorhombic stibnite powder (ICSD ref. code No. 03-065-2434, a = 11.302 Å, b = 11.222, c = 3.834: V_cell = 4.64 ų). N₂ adsorption isotherms were used to determine the Brunauer–Emmett–Teller (BET) specific surface area using method employing Quantachrome NovaWin 2. Particle size and surface morphology were studied using scanning electron microscope (JEOL, JSM5910). Thermal analysis was performed on Perkin Elmer (TGA/DTA, Diamond Series) thermal analyser at heating rate of 5° per minute. The FTIR spectra were recorded in the mid-IR region i.e. 400–4000 cm⁻¹ using an Excalibur Series Bio-Rad (FTS 3000MX) spectrometer.

2.3 Photocatalytic studies

Methyl orange (MO) was used as a test chemical compound for the photocatalytic studies. 100 mg/L of stock solution was prepared and necessary dilutions were carried out in 50 ml of distilled water in order to prepare 1, 5, 19, 25 and 50 mg/L solutions to study the effect of concentration of MO solution. 0.1 g of catalyst was used in 50 ml of 5 mg/L solution for the study of the effect of pH of solution. Different pH (pH = 4, 7 and 10) were maintained using buffer tablets, the pH of MO solution being 8.5. Catalyst dosage studies were carried out by using 0.1, 0.2, 0.4 and 0.6 g of catalyst in 5 mg/L of solution of MO at room temperature which was maintained at pH 4. Effect of reaction time was studied by taking 50 ml of 5 mg/L MO solution maintained at pH 4 for the time intervals of 1–4 h. Effect of temperature was studied on 50 ml of 5 mg/L MO solution at pH of 4 and temperature was maintained at 35, 70 and 100 °C for 3 h. For the above mentioned studies, the reaction flask was well stirred in the dark for 1 h to maintain adsorption–desorption equilibrium. The absorption spectrum was taken after 1 h dark reaction. The UV response of the catalyst was studied after illuminating the sample with the UV lamp (3UV-38, 4 W) for 1 h. Visible light reactions were carried out in direct sunlight suing concentrating mirrors. The degradation behavior of methyl orange was studied at the λ_max of the methyl orange at 470 nm. Aliquots were removed after desired times and the absorption spectra were recorded using T80 UV/vis spectrophotometer. Initial and final absorbance was measured to calculate the concentrations of MO solution and degradation efficiency of the catalyst by using the following equation:

3.1 Characterization of the synthesized material

Phase purity of the synthesized powders under varying temperature and times of reactions in autoclave were studied by X-ray diffraction (XRD) analysis on well ground powders. The standard pattern of stibnite, Sb₂S₃ (ICSD ref. code No. 03-065-2434, a = 11.302 Å, b = 11.222, c = 3.834: V_cell = 4.64 ų) was taken as a reference pattern. The XRD patterns of the samples synthesized under various conditions are given in Fig. 1. Lattice constants “a, b and c”, unit cell volume “V_cell”, Scherrer crystallite size “D” and X-ray density “ρ_X-ray” were calculated using the following relations (Maghraoui-Meherzi et al., 2010):

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Figure 1

Comparison of the X-ray diffraction patterns of the Sb₂S₃ powders synthesized under variable conditions using hydrothermal method.

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The patterns match well with the standard pattern of stibnite indicating the single phase Sb₂S₃ powders synthesized in all the experimental conditions used for the synthesis as shown in Fig. 1. The calculated values of the lattice constants are given in Table 1 along with the corresponding values for the standard pattern. The values of lattice constants a and c are found to be greater than those of the standard pattern while those of b are lower compared to the standard pattern. The same applies to the increase in cell volumes which is more predominant in the sample synthesized at temperature of 200 °C for 12 h. The X-ray density is found to be lower than the value for the standard sample due to the increase in the cell volume according to Eq. (5). The values of Scherrer crystallite size range from 10 to 50 nm which increases with the increase in the autoclave temperature. The reason for the shift in lattice parameters and corresponding expansion in the lattice may be attributed to the strain in the lattice due to the temperature effects (Deshpande et al., 2005) and increase in grain size. The hydrostatic pressure on grain boundaries is lowered with the increase in grain size which results in lattice expansion (Rane et al., 2013). The comparison of the d-spacing of the most intense peaks in the patterns with the standard pattern is given in Table 2. The values match well with the standard pattern. The d-spacing values for the planes (2 1 1), (0 1 3), (1 1 3), (4 1 3) and (2 2 1) are slightly shifted to higher values. As no extra peaks are observed in the patterns so reason of shift due to the presence of impurity phase is nullified. Hence, the reason for shift in d-spacing may be due to the expansion of lattice as mentioned above. From the view point of temperature and time efficiency, the material synthesized at 105 °C for 4 h was selected for future use and further characterization.

The surface area of the material determined by the N₂ adsorption was found to be 64 m²/g. The high surface area Sb₂S₃ powder was successfully synthesized in the present study as opposed to other reports (Li et al., 2008; Sun et al., 2008; Han et al., 2010). The pore volume and the pore diameter were 0.16 cc/g and 79 Å respectively. Based on the pore diameter the material was characterized as mesoporous material. The surface morphology as seen under electron micrograph is shown in Fig. 2. The material is porous and smaller particles joined together to form large agglomerates. Almost a homogeneous particle size distribution is seen in the micrographs. The FTIR spectrum of the synthesized material at room temperature is given in Fig. 3. Peaks below 700 cm⁻¹ can be assigned to the S—S vibrations or to the S—Sb—S stretching vibrations (Pilapong et al., 2010). C⚌O and C—N peaks from unreacted urea are seen at ∼1700 and 1000–1200 cm⁻¹. The sample synthesized at 105 °C for 4 h was subjected to thermal analysis for the study of thermal stability and TGA pattern of the synthesized material is shown in Fig. 4. Thermal stability of the sample is evident at the studied temperature range of 25–1000 °C with only 12% weight loss in three steps attributed to the loss due to removal of adsorbed water and unreacted reactants.

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Figure 2

Scanning electron micrograph of the hydrothermally synthesized Sb₂S₃ powder in an autoclave maintained at a temperature of 105 °C for 4 h.

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Figure 3

Fourier transform infrared spectroscopy of the hydrothermally synthesized Sb₂S₃ powder in an autoclave maintained at a temperature of 105 °C for 4 h.

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Figure 4

Thermogravimetric analysis of hydrothermally synthesized Sb₂S₃ powder in an autoclave maintained at a temperature of 105 °C for 4 h.

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UV/vis absorption spectrum of the synthesized Sb₂S₃ material taken in the form of dispersion was studied in the range of 200–800 nm as shown in Fig. 5. The absorption spectrum showed three regions of absorption, one at ∼250–300 nm, a broad absorption peak in the visible region at 350–550 nm and a small absorption peak is observed at 950–1000 nm. The π–π^∗ transitions and the surface plasmons are found to be responsible for the absorbance in the mentioned regions of electromagnetic spectrum (Subramanian et al., 2010). The indirect and direct band gaps of Sb₂S₃ powders are 1.54 and 1.72 eV (Pilapong et al., 2010; Subramanian et al., 2010) respectively which correspond to energies of the absorbed light in the visible region and higher energy regions of electromagnetic spectrum as shown in Fig. 5.

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Figure 5

Absorption spectrum of the antimony stibnite powder synthesized at an autoclave temperature of 105 °C for 4 h. Inset: Band gap of the synthesized powder.

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3.2 Photocatalytic studies

The photocatalytic studies were carried out on the Sb₂S₃ powder synthesized at 105 °C for 4 h in autoclave. For the estimation of the activity of the synthesized material as a photocatalyst, the reaction flask containing 50 ml of 5 mg/L solution of MO and catalyst dosage of 0.1 g at room temperature was maintained for 1 h each in dark (to maintain the absorption–desorption equilibrium), UV light and sunlight. The corresponding UV/vis absorption spectra after each reaction step are shown in Fig. 6. The percent degradation under three conditions was calculated to be 7.4%, 22% and 47% respectively. The high surface area (62 m²/g) of the synthesized powder could be responsible for the 7.4% removal by the process of adsorption in the dark. Other reports (Li et al., 2008; Sun et al., 2008) achieved ∼72% degradation under illumination by Xe lamp and contributions to degradation by adsorption, UV and visible light were also not segregated. After the preliminary test studies, effects of following factors on the visible light degradation efficiency of the synthesized Sb₂S₃ powder have been studied: (a) concentration of the methyl orange solutions, (b) pH, (c) catalyst dosage, (d) time and (e) temperature.

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Figure 6

Degradation of 5 ppm solution of methyl orange under dark, UV and sunlight for 1 h.

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3.2.1

3.2.1 Effect of concentration of the solutions

Fig. 7 shows the degradation efficiency of 0.1 g Sb₂S₃ photocatalyst for 50 ml solution of different concentrations of methyl orange, i.e. 1, 5, 19, 25 and 50 mg/L at room temperature. Two lower concentrations are skipped for clarity of the figure. The % degradation of the 1, 5, 19, 25 and 50 mg/L solutions calculated at λ_max of MO were 90%, 81%, 77%, 74% and 36% respectively. The increase in dye concentration results in the lower degradation rates. The degradation at the surface of the catalyst is dependent upon (a) generation of ^•OH radicals by the absorption of sunlight by the catalyst, and (b) reaction of ^•OH and the dye. At low dye concentrations, the sunlight is absorbed by the catalyst and the generated ^•OH radicals react with the dye and higher degradation rates are observed. At high dye concentrations, most part of the surface of the catalyst is covered by the dye itself so light is not effectively absorbed by the catalyst. Similarly, the dye itself can absorb sunlight thus reducing the ^•OH generation; hence, lower degradation rates are observed at high dye concentrations (Konstantinou and Albanis, 2004).

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Figure 7

Degradation of 1–50 ppm solution of methyl orange using 0.1 g of catalyst in sunlight for 4 h at a pH of 4 at room temperatures.

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3.2.4

3.2.4 Kinetic and thermodynamic studies

The pseudo-first order model has been applied to the degradation reactions of 5 ppm solution of methyl orange kept at room temperature at a pH of 4. The time of reaction was varied from 1 to 4 h in the sunlight. The pseudo-first order kinetics is based on the assumption that the initial degradation reaction step is the attack of OH^• radical on the dye molecule, and the generation of OH^• radicals remains constant and after the passage of time the degradation by-products also compete with the reaction of OH^• and the dye (He et al., 2011).

The rate equation for the pseudo-first order kinetics model is given in Eq. (6) as follows:

(6)

$$\ln C_o/C_t=k_{p-1}t$$ where C_o is the initial dye concentration, C_t is the dye concentration at time t and k_p−1 is the pseudo-first order rate constant.

The plot of ln C_o/C_t against t gives a straight line as shown in Fig. 8. The slope is equal to pseudo first order rate constant, the value being 0.0012 min⁻¹.

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Figure 8

Pseudo first order kinetics followed by the degradation of MO in the presence of photocatalyst.

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Different reaction setups of 5 ppm solution of MO utilizing 0.1 g of Sb₂S₃ at pH of 4 were kept in sunlight for period of 3 h and were maintained at temperatures of 35, 70, and 105 °C. The results are plotted in Fig. 9. The increase in degradation is observed with the increase in temperature (Hu et al., 2010).

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Figure 9

Degradation of 5 ppm solution of methyl orange under different reaction temperatures.

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The summary of the characterization and the degradation reactions would help in proposing the mechanism of reaction. The single phase Sb₂S₃ is confirmed by the XRD and surface area is found to be appreciable for the adsorption of dye. The size of dye can be correlated to the pore diameter obtained from BET. The lower concentrations were degraded well while acidic pH was found to be suitable. The degradation was directly proportional to the amount of catalyst. The intensity of smaller peak below 300 nm increases indicating the formation of some by-products as a result of degradation. The diagrammatic representation of the above mentioned parameters is given in Fig. 10. The crystal structure of Sb₂S₃ is formed of 1-D ribbons of polymerized [Sb₄S₆]_n units. Each ribbon is linked to its four neighbors by weak Sb—S and S—S bonds (Caracas and Gonze, 2005). The absorption of UV and visible wavelengths corresponds to the band gaps that allow the absorption of UV/vis light simultaneously resulting in the generation of electron–hole pairs. The e and holes undergo secondary reactions with the H₂O and O₂ to produce ^•OH radicals which carry out the degradation reaction on the MO. Now as the reaction was carried out at varying pH conditions, so at pH of 4 both the protonated (azonium and ammonium) and deprotonated forms exist while and at pH 7–10 only deprotonated form exists (Pires et al., 2012a). The ^•OH radicals react with the dye molecules to produce the hydrazine derivatives as by-products.

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Figure 10

Reaction mechanism for the degradation of MO using Sb₂S₃ powder.

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