An ideal photocatalyst should be stable, inexpensive, non-toxic and, highly photoactive. Several semiconducting metal oxides and sulphides are used as photocatalysts because their band gap energies are suitable for absorption of UV and visible light. The most studied photocatalysts are: TiO₂, WO₃, Fe₂O₃, ZnO and ZnS. Recent research has reported that ZnO, which exhibits a direct band gap of about 3.2eV possesses higher photocatalytic efficiency than TiO₂. At the same time, the lower cost, biosafety and biocompatibility of ZnO indicate that it is suitable for large-scale water treatment operations. Therefore, different preparation methods have been used to synthesize ZnO and improve its photocatalytic activity. A significant draw back in the application of zinc sulphide and zinc oxide is their wide band gap of 3.6eV and 3.2eV respectively. Hence they cannot effectively absorb visible light. This has a consequent limitation for the use of zinc sulphide and zinc oxide as a solar light activated catalyst because about 50% of the solar spectrum is visible light, less than 5% is UV light [1].

Zinc ores are widely distributed throughout the world. Zinc is the 24^th most abundant element on earth crust. Natural Sphalerite (also Known as zinc blende) is the principal primary ore of zinc. Sphalerite is a zinc ore mineral found in many sedimentary basins around the world. Its texture is described as a polycrystalline aggregate which results from the precipitation of metal-rich brines in a carbonate host rock [2]. Sphalerite is mainly compose of ZnS with traces amount of Fe, S, Si, Mo etc. Zinc sulphide is a white to yellow colour powder or crystal. It is typically encountered in the more stable cubic form. Sphalerite composition varies widely depending on the origin, due to geochemical and environmental factors. Over 100 million tonnes of complex zinc sulphide minerals is available in Nigeria. Minor transition metals such as Fe, Pb, Cu, present in natural sphalerite can form polynary metal sulphide, which may lead to a visible light- driven photocatalytic activity. In this work, natural Sphalerite obtained from Abuni, Nasarawa state, Nigeria has been investigated in order to assess the effect of chemical composition (altered via acid leaching and calcination) on its photocatalytic activity using MB as the model pollutant. The natural Sphalerite was characterized using XRF, XRD and surface area analysis. It was leached with a mineral acid (hydrochloric acid) and organic acid (oxalic acid) [3,4].

Experimental Section

Materials: Natural Sphalerite sample from Abuni, Nassarawa State, Tap water, Distilled water, Oxalic acid AR (99.5%), Sodium chloride (99%), Sodium hydroxide (98%), Methylene blue (minimum assay 95%), Sieve (Mesh size 106 µm), Spatula, Beakers (100-500 ml), Measuring cylinders (50-250 ml), Crucibles, Laboratory mortar and pestle, Glass rod stirrer, Thermometer (0°C to 360°C), Sample bottles (glass and plastic), Petri dishes, Halogen Lamp (500 W, 200 W, 100 W, 60 W), Pipette, Burette, Retort stand, Pocket pH Indicator.

Instruments

Magnetic Stirrer, Electric furnace. (Nabertherm; 30-1400°C), Electric Oven (NYC-101; 30-400°C, Nabertherm; 30-650°C), Weighing Balances (Weda, T18; 0-100 kg, Metler Toledo; AB204; 0-210 g), Stop Watch, X-ray diffraction spectrometer (Shimadzu, 6000), X-ray fluorescence spectrometer (Pananalytical, Minipal 4), Hot plate/magnetic Stirrer (Trieb, 73-660 rpm).

Experimental Procedure

Raw Materials Collection: Natural Sphalerite also known as zinc blende which is the principal ore of zinc was collected from Abuni deposit of Nasarawa State Nigeria. The mineral was crushed to powder and a sieve of 106µm was used to sieve the sample to the required particle size ˂106 µm. The sample was analysed using XRF, XRD, UV-vis and surface area analysis techniques.

Leaching of Natural Sphalerite Oxalic Acid

The X-ray fluorescence result of the Sphalerite under investigation showed that Sphalerite compose mainly of zinc sulphide with traces amount of Fe, S, Si, Cu and Mo etc. Leaching experiments were performed in a 250 ml glass reactor equipped with a mechanical stirrer. The reactor was filled with 250 mL of 0.5 M oxalic acid which was heated to 80°C. For every leaching experiment, the solution mixture was freshly prepared by dissolving 10 g of the Sphalerite ore in 250 Ml of oxalic acid at 80°C. In all cases, the fraction of the Sphalerite dissolved was calculated from the initial difference in weight of the raw sample and the amount undissolved at various time intervals (10, 20, 40 and 90 mins), after oven-drying at about 60°C. The residue obtained at various leaching times is denoted by. S_OX10, S_OX20, S_OX30, S_OX40 and S_OX90, corresponding to leaching times of 10, 20, 40 and 90 mins, respectively.

The filtrate was dried at room temperature, calcined at a temperature of 1000°C for 4 hours and denoted with CL10, CL20, CL40 and CL90.

Calcination

The leachates obtained of air dried leachate were dried and then ground to powder form; the samples were then calcined at 1000°C for a constant time of 4 hours:

ZnS+2H₂C₂ O₄Zn(C₂ O₄ H)₂+H₂ S

(1)

FeS+2H₂C₂ O₄Fe(C₂ O₄ H)₂+H₂ S

(2)

The oxalate precursor was annealed at 1000°C for four hours to obtain ZnO powders. The decomposition of zinc oxalate dehydrate can be expressed as follows:

Zn(C₂ O₄H)₂ZnO+CO+CO₂

(3)

Fe(C₂ O₄H)₂FeO+CO+CO₂

(4)

FeO+O₂Fe₂O₃

(5)

Determination of Specific Surface Area of the Photocatalysts

The surface area for the natural Sphalerite was estimated according to Sears’ method by weighing 1.5 g of natural Sphalerite and acidifying with dilute hydrochloric acid to pH of 3-3.5. Then 30 g of sodium chloride was added, with stirring and the volume was brought to 150 mL with distilled water. The solution was titrated with 0.10 N sodium hydroxide. The Volume, V, needed to raise the pH from 4 to 9 was recorded. The surface Area was estimated from the equation:

S(m²/g) = 32V-25

(6)

Where, S = Surface Area, V = the volume needed to raise the pH from 4 to 9.

XRF Analysis

Table 1 presents the summary of the XRF results obtained. As earlier observed from the XRD results of the natural Sphalerite, the XRF results further confirmed the presence of iron as an impurity; with concentration as high as 38.16 wt% in the raw mineral. Traces of other metals such as Al, Si, Ca, Sc, Mn, Cu, Mo, Ag, Cs, Eu and Pb, were also detected by the XRF as shown in the results presented in the Table 1. The effect of the acids that was used to leach the samples to vary its chemical composition was observed from the XRF result of the leached samples, from the Table 1 as leaching time progress there is decrease in the percentage of iron, zinc, sulphur content which is as a result of the oxalic acids that was used for the leaching process.

XRD Analysis

Figure 1 shows the XRD pattern of the natural Sphalerite sample. As observed in the Figure 1, sharp peaks were obtained which shows that it is highly crystalline. The domination of the ZnS (Sphalerite) peaks is an indication that the mineral is mainly zinc sulphide based mineral. However, the prominent peak of FeS (siderite) indicated that the mineral contained reasonable amount of impurity.

Figure 1: XRD pattern for the natural Sphalerite

Table 1: Elemental Composition of the Developed Photocatalyst 

The three strongest characteristic peaks at 2θ of 28.4°, 47.3°, 56.1°, correspond to (6.823 nm), (5.777 nm) and (7.495 nm) crystallite size calculated. The average crystallite size of the raw sphalerite calculated using scherrers equation is 6 nm. In addition, XRD data revealed the presence of associated minerals such as α-SiO₂, FeS₂, FeTiO₃. The Sphalerite in the ore occurs as ferrous Sphalerite. The iron in the Sphalerite might be probably due to availability of iron in the hydrothermal fluid as the Sphalerite crystal growths. This process occurs by a reaction of the diffusing zinc ions transported in hydrothermal solutions with iron [5]. While X-ray diffraction can be very accurate in quantifying major components within a mixture, it is not very good at detecting constituents that are present in minor amounts (less than about 2%).

Surface Area Analysis

As mentioned earlier surface area for the sample were determined using sears method. Table 2 shows the results obtained.

The specific surface areas of the photocatalysts are listed in Table 2. As expected, the catalysts exhibited different surface areas.  

From the Table 2 there is a decrease in the specific surface area of the photocatalyst as leaching time increases which might be as a result of aggregation of the particles. The higher surface area of the calcine leachate could be explained in terms of the presence of small surface iron oxide particles whose core is the zinc ferrite. Typical literature values of surface areas for ZnO, ranges from 11-85. The relatively higher surface area as compared to related works might have resulted from combined effect of higher temperature of calcination and amount of surface iron added. Surface area is a strong function of the calcination temperature, particularly in the ranges of 800-1200°C from the result there was a sharp increase in surface area with the leachate that was calcined.

Table 2: Surface Area of the Developed Photocatalyst

Photocatalytic Degradation of MB using NS and Product of Its Leaching

The raw natural Sphalerite and product of its leaching (S10, S20, S40, S90) with hydrochloric and oxalic acids were tested for their photocatalytic activity under visible light with varying intensity of light (500 W, 200 W, 100 W, 60 W) at different time interval. Before then adsorption in the dark was done for each of the samples and it was discovered that there was mild removal of MB via adsorption in the dark (Table 3).

Table 3: Effect of Irradiation Time on the Percentage Degradation of MB at Various Lamp Powers

Effect of Irradiation Time on the Degradation of MB Using Oxalic Leached Samples

The different graphs showing the rate of degradation of MB using different composition of oxalic leached Sphalerite at different intensity [6,7].

From the graphs the raw sample gave a better degradation of the dye compared to different composition of oxalic leached sphalerite at different intensity which shows that natural sphalerite can be used as a visible light responsive photocatalysts. It was equally observed from the graph that sphalerite leaching using oxalic acid is very mild (Figure 2a-d).

Figure 2(a-d): Rate of degradation of MB using different composition of oxalic leached Sphalerite using (a) 500 W halogen lamp, (b) 200 W halogen lamp, (c) 100 W halogen lamp and (d) 60 W halogen lamp

The effect of photocatalyst activity of the leached samples of the oxalic leached samples gave a better degradation as shown from the graphs of their percentage degradation against irradiation time. The graph of their liner plots-Ln(C/C₀) Vs time were observed (with R² values higher than 0.9) which attested that photo-degradation on MB obeys first order kinetics. It can equally be seen that photocatalytic activity is a function of the intensity as the intensity increases a better photocatalytic activity was observed making the 500 W lamp the best intensity among every other lamp used for the experiment. 

Another observation is that as the time of leaching increases photocatalytic degradation decreases and degradation rate constant decreases thereby making the raw sample the best among all the samples. It was noticed that the leached samples from oxalic acid gave a better photo catalyst activity because of its mild nature and its gradual leaching process. When the solution of MB in the absence of catalyst was irradiated by sun light for different time interval, it was noticed that degradation was increasing as time of exposure increases. This continued until after 1 hour reaction time, 30.4% degradation was observed.

The Pseudo-First Order Kinetic Plots of MB Degradation of Oxalic Acid Leached 

The graphs showed a high activity for MB degradation with the different photocatalyst with the natural sphalerite having the best value of kapp and R² (Figure 3a-d, Table 4).

Figure 3(a-d): Pseudo-first order kinetic plots for MB photodegration using (a) 500 W halogen lamp, (b) 200 W halogen lamp, (c) 100 W halogen lamp and (d)60 W halogen lamp

The kinetic of photocatalytic degradation of MB was calculated using the first order equation:

Table 4: Derived Values of kapp and R² for Oxalic Leached Sample

Ln (C_O/C_t) = K_appt….42

Where,

K_appt   =    The pseudo-first order rate constant (min_¹)

C_O       =    Initial concentration

C_t          =    The concentration of MB at time t (min)

The Pseudo-First Order Kinetic Plots of MB Degradation of Dalcine Filtrate of Leached Sphalerite With oxalic Acid at 1000°C At Different Intensity

Since the focus is on the oxalic acid leached, the leachate was calcine at a temperature of 1000°C for four hours. The result of the photocatalytic experiment carried out showed that the Pseudo-first order kinetic plots of MB degradation of leachate that was calcined gave a higher values of Kapp and R² (with R² values higher than 0.9) which attested that photodegradation on MB obeys first order kinetics with leachate that was leached for 90min having the best compared to those of the oxalic acid residue leached samples (Figure 4a-d, Table 5).

Table 5: Derived Values of (kapp and R²) @1000°C Calcine

Figure 4(a-d): Pseudo-First Order Kinetic Plots for MB Photodegration Using (a) 500 W halogen lamp, (b) 200 W halogen lamp, (c) 100 W halogen lamp and (d) 60 W halogen lamp

The absorption edge of the pure Sphalerite sample is at 365 nm, corresponding to the band gap of 3.4 eV. This implies that the pure Sphalerite sample could not utilize visible light to generate electron-hole pairs [8,9]. However, the absorption spectra of the natural Sphalerite sample and calcine leachate shows both a steep absorption edge at about 450 nm and a broad absorption shoulder band in the vicinity of 400-600 nm. The absorption spectra of natural Sphalerite based photocatalyst suggest they can serve as a visible light-responsive photocatalytic reaction (Figure 5).

Figure 5: UV/VIS Absorption Spectra of the developed Photocatalyst

• A series of photocatalyst were prepared by treating raw Sphalerite with oxalic

• The prepared photocatalysts were characterized by XRF, XRD, UV-Vis spectroscopic techniques

• The leached samples (residues) exhibit lower photocatalytic activity than the natural Sphalerite

• The photocatalytic reactions obey pseudo - first order kinetics model

• The calcine leachate at 1000⁰C of the leached samples gave a better photocatalytic activity compared to the residue

• Pseudo-first order kinetic plots for MB photodegration showed a good regression because of the mild nature of oxalic acid during reaction

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