CHAPTER 5 SUMMARY AND CONCLUSION

176 CHAPTER 5 SUMMARY AND CONCLUSION 5.1 GENERAL The depletion of non-renewable energy sources and increasing levels of environmental pollution ar...
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176

CHAPTER 5 SUMMARY AND CONCLUSION

5.1

GENERAL The depletion of non-renewable energy sources and increasing

levels of environmental pollution are both becoming serious global concerns. H2S is generated due to various industrial activities and sewage treatment plants. Sulfide wastewater was generated from anaerobic digestion of sewage sludge in STPs, tanning of hides, pulp and paper industries and caustic scrubbing of H2S. H2S and sulfide wastewater cause odor nuisance at low concentration. It is a toxic substance with characteristics rotten egg smell that can undergo a number of oxidation reactions. It may lead to SO2 emission and cause corrosion of pipelines. H2S causes health impacts to human beings. Hence the removal of sulfide ion from the wastewater is necessary. Many treatment schemes have been suggested for treating H2S gas. Present removal methods are the Claus process and Wet absorption methods. These methods have some drawbacks. Biological and chemical removal methods are employed for removal of sulfide from wastewater. In conventional biological treatment processes, microorganisms are used to oxidize H2S dissolved in liquid under aerobic conditions. Chemical removal methods are filtration of sulfide wastewater after liming and recycling and oxidation by oxidizers and air oxygen. Other methods are thermochemical, electrochemical and photochemical methods for the decomposition of H2S in an alkaline solution. Of all the methods, solar photocatalysis has received attention in view of solar energy utilization for the promotion of useful chemical reactions.

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The high desire to use renewable energy sources is not only for the limited amount of fossil energy, but also for environmental protection, particularly for reducing CO2 emissions. Solar energy is the primary source for clean and renewable energy alternatives. Photocatalytic decomposition of H2S in an alkaline solution is not only for the removal of sulfide ion and also for the generation of useful fuel Hydrogen (H2). Hydrogen is a product that is fully capable of sustaining the world’s energy needs now and in the future. If renewably produced, H2 would be a fuel used that does not contribute to environmental damage (pollution) and supports the human well being. ‘Solar photocatalytic generation of H2 from hydrogen sulfide (H2S) in an alkaline solution’ is a ‘Cleaner Production Method’ for generating H2 and also an environmentally benign process. It has triple advantages of the generation of H2, waste minimization and odor or pollution control. 5.2

SUMMARY In this research study, an attempt was made for synthesizing five

types of novel solar UV-visible light responsive nanophotocatalysts viz., CdS NPs, ZnS NPs, TiO2 NTs, CdS-ZnS NCs and CdS-ZnS/TiO2 NCs for the photocatalytic generation of H2 from H2S in an alkaline solution. The characterization of the synthesized novel solar UV-visible light responsive nanophotocatalysts for various parameters viz., particle size (XRD and TEM), molecular vibrations (FTIR), band gap energy (DRS UV-Vis

studies),

morphology (SEM), surface area (BET) and binding energy (XPS) were carried out.

The outdoor solar feasibility studies were conducted for evaluating the activity of the solar UV-visible light responsive nanophotocatalysts. The lab-scale (1 L) and bench-scale (5 L) tubular photocatalytic reactors (LTR and BTR) were developed and the performance of the photocatalytic reactors was

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studied by optimization of operating variables viz., concentration of sulfide ion, concentration of sulfite ion, pH, catalyst amount, lamp power, recycle flow rates and volume of wastewater under the batch recycle mode and influence of inlet flow rates on continuous mode for the photocatalytic generation of H2 from H2S in an alkaline solution. In order to study the applicability of the process, sulfide wastewater was collected from sewage treatment plant for the photocatalytic generation of H2. Based on the % conversion, suitable mode was selected and kinetic analysis of the data was performed. The pilot-plant reactor was designed and the cost of the process was estimated. 5.3

CONCLUSION The conclusion drawn from the present study is as follows. The

nanophotocatalysts

had

been

synthesized

by

coprecipitation and hydrothermal methods. The physical characterization of the nanophotocatalysts had been carried out by XRD, TEM, FTIR, DRS UV-Vis, SEM, BET and XPS. From the studies, it was observed that the synthesized nanoparticles sizes were less than 10 nm and active in UVvisible portion of sunlight. The H2 generation rate over CdS NPs, ZnS NPs, TiO2 NTs, CdS-ZnS NCs and CdS-ZnS/TiO2 NCS was compared. The H2 generation rate over CdS NPs, ZnS NPs, TiO 2 NTs, CdS-ZnS NCs and CdS-ZnS/TiO2 NCs were observed to be 419, 157, 209, 838 and 1100 µmol/h, respectively. From the results, it was concluded that CdS-ZnS/TiO2NCs was better than other nanophotocatalysts. The highly active CdS-ZnS/TiO2 NCs was selected and used for further studies.

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The feasibility studies had been carried out in order to optimize the operating variables such as concentration of sulfide ion, concentration of sulfite ion, pH, catalyst amount, lamp power, recycle flow rates and volume of wastewater on the batch recycle mode in LTR and BTR. The results are presented in the Table 5.1. In the continuous mode, the decrease in inlet flow rates from 1 L/h to 50 mL/h, the % conversion on H2 generation increased. In LTR, in batch recycle mode, the maximum conversion was 37.2 % whereas in the continuous mode, the maximum conversion was found to increase upto 50.6 %. In BTR, in batch recycle mode, the maximum conversion was 32.3 %, whereas in the continuous mode, the maximum conversion was found to increase upto 76 %. Table 5.1 Optimized values of operating variables

Sl.No. 1. 2. 3. 4. 5.

6. 7.

Parameters

Optimized Value LTR 0.05

BTR 0.05

Concentration of sulfide ion (M) Concentration of sulfite 0.2 0.2 ion (M) pH 11.3 11.3 Catalyst amount (g/L) 0.5 0.5 (3 sun (3 sun Lamp power (W) (Power of sun lamp = 100 lamps + 1 lamps + 1 UV UV W and the Power of UV lamp) lamp) lamp = 8 W) Recycle flow rates (L/h) 18 18 Volume of wastewater 200 1000 (mL)

Total volume % of H2 (mL) Conversion LTR 64

BTR LTR BTR 250 25.3 19.8

76

315

30

24.9

76 76 76

315 315 315

30 30 30

24.9 24.9 24.9

94 94

408 408

37.2 37.2

32.3 32.3

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Finally the process applicability was evaluated with sulfide wastewater from the sewage treatment plant (STP). From the results, it was found that, in batch recycle mode, 176 mL of H2 generated per litre of solution (90 min), whereas in the continuous mode of operation 655 mL of H2 generated per litre of solution (flow rate of 50 mL/h). The design of pilot-plant solar photocatalytic reactor was done for treating 1 m3 containing sulfide concentration of 898 mg/L. The treatment cost was estimated to be 284/m3 for the photocatalytic generation of H2 from sulfide wastewater and it capable to generate 655 L of H2 which equals to 6701 kJ of energy. Results obtained from this research indicated that the synthesized novel solar UV-visible light responsive nanocomposite (CdS-ZnS/TiO2 NCs) and developed photocatalytic reactors effectively decomposed the H2S in an alkaline solution along with generation of clean gas fuel (H2) under solar irradiation. This process can be feasible in sub-tropical countries like India where sunlight is available throughout the year. 5.4

SCOPE FOR FURTHER STUDY Developments of pilot-plant solar photocatalytic reactor for the

generation of H2 from industrial effluents.