Phenol and its derivatives are present extensively in industrial wastewaters originating from resin manufacturing, petrochemical and oil refineries, paper making, coking, iron smelting, etc. [309]. High concentrations of phenolic compound are highly toxic, carcinogenic and persistent in the receiving aquatic environment. Therefore, it is essential to correctly treat wastewater containing phenolic compounds. Phenolic wastewaters are usually treated by biological methods, thermal decomposition and adsorption [310–314], but all of them have noticeable limitations. As an alternative, photocatalytic degradation of pollutants is able to mineralize the organic molecules completely into harmless carbon dioxide and water [315–319]. In addition to TiO2 (P25) and ZnO, BiPO4 recently has been discovered to have excellent photocatalytic activity for methylene blue (MB) degradation and mineralization and its efficiency is double than that of TiO2 (P25) [210,320]. Therefore, the mineralization of phenol by photocatalytic process with BiPO4 was accomplished here under UV-C lamp irradiation. The influence
Results and Discussion 80
of different experimental parameters such as catalyst loading, pH value, initial concentration and additives were also examined systematically.
The photocatalytic experiment shows that the concentration of phenol after UV light irradiation is obviously different in the absence and presence of BiPO4, as depicted in Figure 5.18. 10 ppm phenol solution was entirely degraded photocatalytically in the presence of BiPO4 after 5 hours reaction with the loading of 1 g/L. Meanwhile, the TOC results show that more than 95 % of organic carbon in phenol solution was oxidized to carbon dioxide. It clearly demonstrates that the mineralization is very efficient.
Figure 5.18 The residual amount of phenol (red lines) and TOC (black lines) in the process of photolysis and photocatalysis with BiPO4 (V).
In order to investigate the influence of operating parameters on the mineralization of phenol over BiPO4, catalyst loading, pH value, initial concentration and additives were examined systematically. The optimum photocatalyst loading amount was 1 g/L and both insufficient and overloaded photocatalyst had negative effect on the photocatalytic degradation of phenol. The histogram in Figure 5.19 clearly shows the relationship between loading amount and degradation ratio. For the effect of initial concentration of phenol, increasing the initial concentration decreased the degradation and mineralization
Results and Discussion 81 of reaction and hence could be considered as an unfavourable factor (Figure 5.20).
Moreover, pH value is also a critical parameter for photocatalytic degradation. In this case, five different initial pH values have been tested to study the influence of pH value.
It was found that the optimal pH value is 4, at which the mineralization ratio was more than 75% (Figure 5.21). Last but not least, additives like hydrogen peroxide and S2O8
have been shown to improve photocatalytic efficiency [321,322], so the effect of some common anion additives on the photocatalytic degradation of phenol over BiPO4 was also studied in this case. Figure 5.22 shows the influence of SO42-, Cl- and H2O2 on the TOCt/TOC0 of phenol. It could be clearly noticed that both SO42- and Cl- accelerated the mineralization of phenol solution. It is because that Sulphate ion can enhance the decomposition due to the formation of SO4•-, which acts as strong oxidizing agent or initiates the formation of hydroxyl radical. Additionally, halides can be thermodynamically oxidized by valence band holes and further accelerating the degradation. However, hydrogen peroxide has no significant effect on reaction rate.
Figure 5.19 Effect of catalyst loading on the degradation and mineralization ratio of phenol.
Initial concentration = 10 ppm and the reaction time 5 hours (V).
Results and Discussion 82
Figure 5.20 Effect of initial concentration on the A) photocatalytic degradation and B) TOC removal (Catalyst loading = 1 g/L). (V)
Figure 5.21 Effect of pH value on photocatalytic degradation of phenol (Initial concentration = 20 ppm; Catalyst loading = 1g/L). (V)
Results and Discussion 83
Figure 5.22 Influence of additives on phenol mineralization. (Initial concentrations = 20 ppm;
Catalyst loading = 1 g/L; additive concentrations = 20 ppm). (V)
Conclusions and Future Works 85
6 Conclusions and Future Works
Heterogeneous photocatalysis as an advanced oxidation processes (AOPs) has shown its great potential as a low cost, environmental friendly and sustainable wastewater treatment technology. The ability of this advanced oxidation technology has been widely demonstrated to remove persistent organic compounds and microorganism in water. In this work, some selected heterogeneous photocatalysts have been prepared and/or modified via different approaches in order to enhance their photocatalytic activities under light irradiation. Based on the principle of semiconductor photocatalysis and sufficient literature review, different methods such as solvothermal, hydrothermal, thermal decomposition with the assistant of additives have been used to prepare photocatalyst and improve their photocatalytic activity under light irradiation. A wide range of experiments was carried out at similar conditions to degrade Methylene Blue for evaluating the photocatalytic activity of modified semiconductor photocatalyst. The mechanisms have been proposed based on the characterization findings of as-prepared photocatalyst.
As the results, first of all, the photocatalytic activities of CaIn2O4 and BiPO4 have been successfully improved via novel preparation approaches. The photocatalytic degradation of MB over the prepared photocatalyst indicate that the semiconductors prepared via novel method have better photocatalytic activity. CaIn2O4 prepared by thermal decomposition have higher purity than that prepared by solid-state reaction and therefore, it is favorable for photocatalytic degradation of MB under visible light. A novel two-steps solvothermal approaches developed in this work could produce BiPO4 with small particles size. Comparing with BiPO4 prepared by hydrothermal method, BiPO4
prepared by this solvothermal method have higher surface areas and hence it shows higher photocatalytic activity. Meanwhile, this solvothermal method needs less time than hydrothermal method. All these findings indicate that proper preparation method could produce efficient semiconductors for photocatalysis.
Except to use novel synthetic route, another strategy, up-conversion luminescence agent loading, has been taken to improve the photocatalytic activity of BiPO4. Firstly,
Conclusions and Future Works 86
Er3+: YAlO3-loaded BiPO4 composite has been prepared for utilizing the visible light to generate UV light and further to improve the photocatalytic activity of BiPO4. It is obvious that the photocatalytic activity of Er3+: YAlO3-loaded BiPO4 had improved due to the fact that up-conversion luminescence agent increased the utilization of solar light.
Moreover, by directly doping lanthanide ions to BiPO4 is another routine was developed to improve its photocatalytic activity. The prepared both Er and Tm ions doped BiPO4
could apparently enhance the photocatalytic activity on MB degradation and O2 evolution.
Furthermore, the photo-stability of Ag3PO4 was improved by polyaniline coating.
The photocatalytic experiment results indicate that the photo-stability of silver phosphate have been significantly improved. This is attributed to remarkable delocalized conjugated structure of PANI.
Last but not least, the photocatalytic mineralization of phenol has been systematically studied in the presence of BiPO4. The influence of catalyst dosage, pH value, initial concentration and additives on mineralization behavior have been investigated and the findings confirmed that the mineralization was efficient under optimized conditions. It was also found that the intermediate process between photocatalytic degradation and mineralization is negligibly short.
As a conclusion, in this work, the photocatalytic activity of selected photocatalyst has been successfully improved by adopting different approach. This project appears to offer a great deal of hopes in handling hazardous and toxic chemical wastes into harmless end products at ambient temperature with minimum intermediates. For the future works, more approaches could be used to further enhance their photocatalytic activity, such as charge carrier separation, modification of energy band. The developed and optimized photocatalytic techniques evolved out of research may encourage the application of this technique to treat other industrial effluents.
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