EFFECT OF INITIAL PH

In the industrial wastewater treatment, regulation of pH value is important as it is a measure of how acidic or basic water is. The regulation of pH before treating the wastewater is crucial because the treatment process can be harmed by the dangerous level of acidic and basic condition. It affects the charge of the photocatalysts particles, the point at which the conductance and valence bands are located [12]. The value of pH is an important indicator of water that is changing chemically as stated by Idris et al. [13] and crucial characteristics of wastewater because it determines degradation efficiency. The photocatalytic activity of the pH at different pH condition (pH 4, 6.5, and 9) according to the RSM range has been studied to observe the effect of the different pH in the photocatalytic activity. The other parameters which are concentration of POMSE and catalyst loading were fixed at 100% concentration and 0.08 g/L, respectively. Figure 2.5(a) shows the pattern of the POMSE degradation percentage on the color removal under different pH in 60 minutes of duration.

Based on the results obtained horn Figure 2.5(a), the photocatalytic activity under pH 6.5 for 100% concentration of POMSE shows the highest percentage of degradation which is 34.5% in contrast to pH 4 (19.1%) and pH 9 (19.2%) in tenns of color removal. Figure 2.5(b) presents the percentage of degradation in turbidity during the pliotocatalytic activity in presence of ZnO-PEG (0.015 g/L of PEG) under different pH. The turbidity of the 100% concentration of POMSE was found to be 543 NTU before treatment and 283 NTU after the treatment respectively. The photocatalytic activity under pH 6.5 shows the highest percentage of turbidity removal (48%) and becomes constant after the 2 minutes of the process. Meanwhile, pH 9 shows the lowest percentage of turbidity removal (19.8%).

FIGURE 2.5 Percentage of degradation of a) color removal versus time b) turbidity versus time for photocatalysis process in presence of ZnO-PEG under different pH.

Figure 2.6 and Table 2.1 shows the concentration of DO under the influence of different pH. DO refers to the level of free, non-compound oxygen present in water or other liquids and one of the important parameters in assessing water quality because of its influence on the organisms living within the water. ADO level that is too high or too low can harm the aquatic life and affect water quality [12]. Based on Figure 2.5 and Table 2.3, the reading of DO for the samples under different pH is observed to be at 5.78 ppm at pH 4 and 4.93 ppm at pH 9, while the DO value for the optimum pH is at 6.09 ppm. DO value under the optimum pH 6.5 had the highest reading which is 6.09 ppm which shows the most desired reading. DO value under 5.0 ppm consider as harmful to the aquatic life so, 6.09 ppm is the safe value for the water to be discharged to the river. From the analyzed results, it can be concluded that the treated POMSE is safe to be discharged to the environment since the DO value is within the acceptable range.

FIGURE 2.6 Value of dissolved oxygen (DO) for photocatalysis process in presence of ZnO-PEG (0.015 g/L) under different pH for highest degradation time.

TABLE 2.1 Characteristics of 100% Concentration of POMSE After Photocatalytic Degradation in the Presence of 0.08 g/L ZnO-PEG Under Different pH

The results clearly show that the pH significantly affects the photocatalytic activity efficiency. According to the previous study of POMSE by Alhaj et al. [12], POMSE have been degraded efficiently at the pH near to the neutral condition. It has been established that at lower pH, the functional groups are protonated, thus raising the positive charge of the photocatalyst surface which decreases the degradation of organic molecules. While high value of pH of a solution shows the reduction on the degradation of organic molecules as the hydroxyl ions compete with the organic molecules for the adsorption on the surface of the catalysts. Thus, degradation efficiency declined at the lower pH or acidic condition and it can be deduced that the optimum pH for the photocatalytic activity of ZnO-PEG (0.015 g/L) is at pH 6.5.

EFFECT OF CATALYST LOADING

Effect of ZnO-PEG loading from 0.08 to 0.90 g/L for photocatalytic degradation of POMSE was investigated under pH 6.5 with a fixed concentration of POMSE (25%). It can be observed that in Figure 2.6(a) the color intensity of the treated POMSE decreased significantly when the loading of ZnO-PEG at its lowest. The amount of ZnO-PEG needed to affect a photocatalytic reaction grossly affects the overall process of photo-degradation just as its concentration is equally important to ensure a true heterogeneous photocatalytic system [14]. The high amount of loading increasing the photocatalytic activity, however, once a saturation phase had been obtained; the increase in the loading of the catalyst causes cloudiness in the POMSE. There has been a radial corresponding decrease in the efficiency of light photon adsorption due to the cloudiness caused by the additional catalyst and this further leads to the decrease in a surface area exposed to irradiation and thus decreases the photocatalytic effectiveness of the process [15].

The effect of the different catalyst loading can be observed where 0.08 g/L shows the highest degradation percentage (79%) in contrast to 0.50 g/L and 0.90 g/L (78% and 77%) though there are only small differences. Based on Figure 2.7(a) and Table 2.2, it can be analyzed that the most effective loading of a catalyst in the photocatalytic activity is 0.08 g/L. Figure 2.7(b) shows the trend of the turbidity removal percentage under different loading of photocatalysts (0.08 g/L, 0.50 g/L and 0.90 g/L). The percentage of turbidity removal of POMSE for different loading of ZnO-PEG (0.08 g/L, 0.50 g/L and 0.90 g/L) are 82.5%, 68.7%, and 73.1%. The highest turbidity removal obtained was for the loading of 0.08 g/L and this is corresponding to the percentage of color removal. Therefore, the greatest turbidity removal of POMSE obtained was for 0.08 g/L of ZnO-PEG.

Figure 2.8 demonstrates the pattern of DO value obtained for different loading of ZnO-PEG (0.08 g/L, 0.50 g/L and 0.90 g/L). based on Figure 2.8, the DO value of different loading of photocatalysts (0.08 g/L, 0.50 g/L and 0.90 g/L) obtained for the highest degradation time are 6.64 ppm, 6.98 ppm, and 6.35 ppm. All the obtained values are under the desired value which is higher than 5 ppm in order to provide a better living environment to the aquatic life. This support the finding on the previous research by Chong et al. [14], where increasing the amount of catalyst reduce the degradation efficiency as there are decrease in the electron-hole produced by the light photon. Hence, the optimum amount of ZnO-PEG loading is found to be 0.08 g/L.

FIGURE 2.7 Percentage of degradation on a) color removal versus time b) Turbidity versus time for photocatalysis process in presence ofZnO-PEG (0.015 g/L) under different loading of ZnO-PEG.

TABLE 2.2 Characteristics of 25% Concentration of POMSE After Photocatalytic Degradation in the Presence of Different Loading of ZnO-PEG

EFFECT OF POMSE CONCENTRATION

The concentration of POMSE is one of the important factors which affecting the effectiveness of the photocatalytic degradation process. This is due to the nature and concentration of organic constituents present in the POMSE where the higher the concentration of POMSE, the higher the concentration of organic constituents in the POMSE. If the surface of the photocatalyst is highly saturated with the concentration of the pollutants, there will be a reduction on the photonic efficiency and the catalyst becomes deactivated [16]. For the 25% and 50% of the POMSE concentration, the water was added to dilute the POMSE. This experiment was done under optimum pH (pH 6.5) and optimum loading (0.08 g/L) based on previous findings. Figure 2.9(a) shows the percentage of dye degradation under different concentration of POMSE. The results obtained for the three different concentrations (25%, 50%, and 100%) are 79%, 55%, and 34.5%, respectively. This shows that at the lowest concentration of POMSE, the highest percentage of POMSE degradation is obtained. While at 100% concentration of POMSE, the percentage of degradation shows the lowest result. Turbidity level of the POMSE was also been investigated to observe the effect of the POMSE concentration on the turbidity level. Figure 2.9(b) shows the percentage of degradation of POMSE for the turbidity under different concentration of the photocatalyst. The results obtained for the three concentrations (25%, 50%, and 100%) are 82% 55.2% and 47.9% respectively. The highest concentration of the POMSE gives the lowest percentage degradation and the lowest concentration of POMSE gives the highest percentage of degradation. These are correlated to the nature of the concentration of organic pollutants where the existence of more organic pollutants can lead to the deactivation of the catalyst. Hence, it can be concluded that at the lowest concentration of POMSE, the highest degradation can be obtained.

The DO value obtained for the different concentration of POMSE was shown in the Figure 2.10 where the value obtained for the three different concentrations (25%, 50%, and 100%) are 6.64 ppm, 5.95 ppm, and 5.6 ppm. The DO value for the lowest concentration is 6.64 ppm which is desirable. This is because of most of the aquatic life need to have DO above 5 ppm according to type of aquatic life. Lower than the said value will cause greater stress to the aquatic life. In order to mimic the ideal environmental systems, the freshwater ideally need around 8 mg/L of DO for optimum growth of the aquatic organism. Therefore, is can be deduced that at 25% concentration of POMSE, the DO obtained is nearest to the ideal condition which indicates that lowest POMSE concentration is safe to be discharged to the environment. This is corresponding to the percentage of color removal and turbidity removal where both indicate the 25% of POMSE concentration as the most efficient (Table 2.3).

FIGURE 2.8 Value of dissolved oxygen (DO) versus different loading of ZnO-PEG.

FIGURE 2.9 Percentage of degradation of (a) color removal versus time; (b) turbidity versus time for photocatalysis process in presence ofZnO-PEG (0.015 g/L) under different concentration of POMSE.

FIGURE 2.10 Value of dissolved oxygen (DO) versus different concentration of POMSE.

TABLE 2.3 Characteristics of POMSE After Photocatalytic Degradation in Presence of 0.08 g/L ZnO-PEG Under Different Initial Concentration of POMSE