CHAPTER-3 APPLICATION OF SYNTHESIZED NANOPARTICLES AS A PHOTOCATALYST: DEGRADATION OF DYES

Transcription

1 CHAPTER-3 APPLICATION OF SYNTHESIZED NANOPARTICLES AS A PHOTOCATALYST: DEGRADATION OF DYES The dyes are natural as well as synthetic that make the world more beautiful through the coloured products, ionizing, and aromatic organic compounds. They are classified as anionic, nonionic, and cationic dyes. The colour of industrial effluent is mainly due to the presence of waste dyes, pigments, and other coloured compounds. The major anionic dyes are direct, acid, and reactive dyes, among these the most complicated ones are the brightly coloured, water soluble reactive, and acid dyes 267. The residual dyes from different sources (e.g., textile industries, paper industries, pulp industries, dye intermediates industries, pharmaceutical industries, Kraft bleaching industries etc.) are considered a wide variety of organic pollutants introduced into the natural water resources. The discharge of dye containing effluents into the water is undesirable not only because of their colour but also because their breakdown products are toxic, carcinogenic, and mutagenic 268. A large amount of dyes in water bodies stop the reoxygenation capacity of the receiving water, cutoff sunlight, upsetting biological activity in aquatic life, and also the photosynthesis process of aquatic plants or algae 268. A single operation can use a number of dyes from different chemical resulting in a complex wastewater 269. The wastewater composition is mainly depending on the different organic based compounds, inorganic chemicals, and dyes used in the industrial processing steps. The azo, and nitro components are reduced in anoxic sediments with regeneration of toxic amines 270. The toxic compounds can be formed in the environment through transformation of dye precursors. 93

2 Majority of the known metals and metalloids are very toxic to living organisms, even those considered as essential can be toxic if present in excess and their concentration have been largely increased as a result of human activities. However, excess intake of trace metal ions is responsible for chronic inflammatory disease, Parkinson's disease, initiator of cancer, kidney damage, endocrine disruption, immunological or neurological effects, and other disorders Nanocrystalline materials are promising due to their unique properties such as small particle size, large surface to volume ratio, and the ease with which they can be anchored onto the solid matrices for enhanced treatment of wastewater 275. Majority of the nanoparticles have unique properties of small size, large surface area, stronger magnetism, well UV absorption, surface activity, heat conductance, and disperse property so they used to remove the organic dyes from waste water 276. The modified nanoparticles also used for the removal of dyes from polluted waste water 277. Chemical oxidation represents the conversion or transformation of pollutants by chemical oxidation agents other than oxygen/air or bacteria to similar but less harmful or hazardous compounds and easily biodegradable organic components. The modern textile dyes are resistant to mild oxidation condition which is existing in biological treatment systems. Therefore, it is an environmental, essential issue to remove such organic dyes and heavy metals from water by the application of adequate simple, cheaper, efficient materials, mechano-physico-chemical, and biological treatment procedures to achieve green, and clean environment. Photocatalysis has become more useful than the conventional oxidation method. Semiconductor photocatalyst is inexpensive, nontoxic, and capable of extended use without substantial loss of photocatalytic activity. 94

3 In present chapter, the effect of synthesized undoped nanoparticles, doped metal oxide, doped metal sulphide nanoparticles on the photodegradation of organic dyes, and removal of heavy metal like chromium was reported. Further the effect of different parameters including ph, amount of nanoparticles, amount of dyes on photodegradation, and removal of chromium have also been reported. The photodegradation of dyes and removal of chromium was studied by analytical techniques like UV-visible spectrophotometer. This chapter is further divided in four sections: Section A: Degradation of methyl blue dye using PbO, 3 %, and 7 % Ni doped PbO nanoparticles. Section B: I) Degradation of rose bengal dye using CdO, 3 %, and 7 % Cs doped CdO nanoparticles. II) Removal of chromium (VI) using CdO, 3 %, and 7 % Cs doped CdO nanoparticles. Section C: Degradation of methylene blue dye using PbS, 5 %, and 10 % Co doped PbS nanoparticles. Section D: Degradation of crystal violet dye using ZnS, 3 %, and 7 % Sm doped ZnS nanoparticles. 95

4 Section-A Degradation of Methyl Blue Dye Using PbO, 3 %, and 7 % Ni Doped PbO Nanoparticles 3.1A. Introduction: Dyes are produced mainly for industrial uses such as textile dyeing, so different dyes have been synthesized to give a wide range of colour. Triarylmethane dyes include the quininoid arrangement as the actual chromophore. The quininoid ring has three benzene rings is equivalent, there can be rearrangement of bonds and any of the benzene ring could take up this arrangement. Methyl blue belongs to triarylmethane dyes, widely used as antiseptic dyes in polychrome staining methods, and has applications in histological and microbiological staining solutions 278. For coating triarylmethane dyes are used on the surface of aluminum electrolytically 279. Due to mitochondrial effects of triarylmethane dyes, it has applications in photodynamic therapy 280. Triarylmethane dyes are used in a wide variety of application within the chemical, pharmaceutical, and life science industries. Some industrial uses of the triarylmethane dyes are as staining agents 281, ink dyes 282, and ph indicators. These dyes are also used in medicinal application as photochemotherapy agents 283, and binding agents 284. Triarylmethane dyes are applied extensively in nylon, cotton, wool, and silk. They are also used for colouring food, oils, fats, waxes, varnishes, cosmetics, paper, leather, and plastics 285. Number of physico-chemical methods viz. adsorption on activated carbon, electro coagulation, flocculation, froth flotation, ion exchange, membrane filtration, ozonation, and reverse osmosis have been used for decolourization of waste water

5 Fig.3.1. Structure of methyl blue dye But most of the methods are less efficient, costly, limited applicability, and produce wastes which are difficult to dispose 287. Therefore, attention has to be focused on techniques that lead to the complete degradation of pollutants 288. The advanced oxidation process uses the H 2 O 2 /UV system for the removal of methyl blue dye 289. The bacterial isolate aeromonas hydrophila was used for the decolourization of triarylmethane dyes 290. Agricultural waste material has been used as adsorbent for the removal, and recovery of a triarylmethane dye 291. The semiconducting PbO nanoparticles were used as a photocatalyst for the removal of various dyes 292. Therefore, in the present study the photocatalytic activity of the synthesized lead oxide (PbO) semiconducting nanoparticles was investigated by taking methyl blue dye (Fig.3.1) as model compound. The photocatalytic efficiency of transition metal (nickel) doped PbO semiconducting nanoparticles was also studied to demonstrate the effect of doping. The photocatalytic activity of PbO and nickel doped PbO nanoparticles for the degradation of methyl blue dye in aqueous solution at different ph has been studied. The course of photodegradation was monitored by UV-visible spectrophotometer. 97

6 3.2A. Experimental: 3.2.1A. Materials and methods: The synthesized PbO, 3 %, and 7 % Ni doped PbO nanoparticles (Chapter 2, Section-A) was used as photocatalyst. The methyl blue (Loba Chemie, 98.0 %), sulphuric acid (Loba Chemie, 91.0 %), and sodium hydroxide (Sigma Aldrich, 98.5 %) were used for the degradation study. Double distilled water was used throughout the experiment. A stock solution of methyl blue (6 x 10-4 M) was prepared in distilled water, and its degradation was carried out by PbO, 3 %, and 7 % Ni doped PbO nanoparticles. Irradiation was carried out keeping whole assembly exposed to a 200 W tungsten lamp (Philips, light intensity = 60.0 mwcm -2 ). The ph of the solution was measured by a digital ph meter (Elico model Ll 120). The desired ph of the solution was adjusted by the addition of previously standardized sulphuric acid and sodium hydroxide solution. The necessary condition for the correct measurement of absorbance is that the solution must be free from semiconducting nanoparticles, and impurity. The progress of photocatalytic reaction was followed by recording absorbance at regular time intervals using UV-visible spectrophotometer (Elico Model SL 159) A. Photocatalytic degradation of methyl blue: In photocatalytic degradation, methyl blue dye solution (50 ml), the catalyst (PbO or Ni doped PbO photocatalyst) were taken in a beaker, and exposed to light. The dye solutions were mixed properly with a magnetic stirrer during the reaction process. A dye solution of about 2-3 ml was taken out at regular interval and their absorbance was recorded at 607 nm using UV-visible spectrophotometer. The control experiment was also conducted under light without catalyst to measure any possible direct photocatalysis of this dye. The photocatalytic degradation of methyl blue was 98

7 evaluated at different ph, with different concentration of dye, and various amount of nanoparticles photocatalyst. 3.3A. Results and Discussion: Photodegradation process assisted by semiconducting PbO and Ni doped PbO nanoparticles depends on various parameters like nature and concentration of organic substance, concentration, and type of the semiconducting photocatalyst, light source and intensity, ph, and temperature 293. The absorbance of the prepared dye solution was measured before, and after degradation at different time interval. The colour removal of the dye solution was measured at 607 nm, and from the respective absorbance the percentage of photodegradation was calculated using the equation, Where, (Absorbance) o is the absorbance before degradation and (Absorbance) t is the absorbance at time t A. Effect of ph on photodegradation of dye: The ph of the solution is one of the important controlling parameter in the degradation of dye in presence of metal oxide. Since it is not only plays an important role in the characteristics of waste water but also determines the surface charge properties of nanocrystalline photocatalyst, the size of aggregates formed, the charge of dye molecules, adsorption of dyes on surface of nanocrystalline photocatalyst, and the concentration of hydroxyl radical. The photodegradation of methyl blue was carried out in ph 2.0 to 12.0 with photocatalyst PbO, 3 %, and 7 % Ni doped PbO with amount gm/50 ml, and keeping methyl blue dye concentration as 3 x 10-4 M (Fig. 3.2a). The ph of the dye solution was adjusted using sulphuric acid, 99

8 and sodium hydroxide. The rate of photodegradation of dye increases with increase in ph of the solution, and the maximum degradation of dye was observed at the ph 10. Further increase in ph shows decrease in the rate of the photodegradation of dye. The increase in the photocatalytic degradation with ph may be due to more generation of OH radicals which are produced by the interaction of OH - with hole (h + ) of the semiconducting photocatalyst. But after optimum ph the decrease in the rate of photodegradation of dye may be due to the fact that methyl blue in its anionic form will experience a force of repulsion with negatively charged surface of the photocatalyst due to adsorption of more OH - ions on the surface of the photocatalyst A. Effect of concentration of dye on photodegradation: The photodegradation of methyl blue dye was carried out with 1 x 10-4 to 6 x 10-4 M concentration of dye. The PbO, 3 %, and 7 % Ni doped PbO photocatalyst with gm/50 ml was used for degradation at ph 10.0 (Fig. 3.2b). It has been observed that the rate of photodegradation of dye increases with increase in the concentration of the methyl blue (MB) dye. It may be due to increase in concentration of methyl blue dye molecules which are available for excitation, energy transfer, and hence increase in the rate of photodegradation of the dye. The rate of photodegradation was found to be decrease with increase in the concentration of the dye above 3 x 10-4 M. This may be attributed due to dye itself will start acting as a filter for the incident light, and it will not permit the desired light intensity to reach the photocatalyst. As an effect of this only few photons reached to the catalyst surface. Due to this the production of hydroxyl radical and superoxide radicals are limited, and photodegradation was found to be negligible thus decrease the rate of photocatalytic degradation of methyl blue dye. 100

9 3.3.3A. Effect of amount of photocatalyst on photodegradation: The amount of semiconducting photocatalyst also affects the rate of photocatalytic degradation of methyl blue, hence different amount of photocatalyst were used (Fig. 3.2c). It has been observed that the rate of photodegradation of methyl blue dye increases with increase in the amount of photocatalyst but ultimately, it decreases to some extent after a certain amount (0.075 gm/50 ml). This may be attributed to the fact that as the amount of photocatalyst was increased the exposed surface area also increases but after a certain limit, if the amount of photocatalyst was further increased, then there will be no increase in the exposed surface area of the photocatalyst. It may be considered like a saturation point, the increase in the amount of photocatalyst after this saturation point will only increase the thickness of the layer at the bottom of the reaction vessel A. Effect of nature of photocatalyst on photodegradation: Experiments were carried out with catalyst of the type PbO, modified PbO, and keeping methyl blue dye concentration constant as 3 x 10-4 M to study the effect of nature of photocatalyst on photodegradation of dye. It was found that degradation of methyl blue dye was more with 3 %, and 7 % Ni doped PbO than PbO nanoparticles (Fig. 3.2c). The presence of nickel ion in PbO nanoparticles shifts the absorption of light towards visible range. The band gap energies of Ni doped PbO photocatalyst was minimum as compared to band gap energy of undoped PbO photocatalyst. The nickel doped PbO nanoparticles have large surface area as compared to undoped PbO nanoparticles therefore, these nanoparticles are more effective as compared to PbO photocatalyst. It reveals that the nickel doping reduce the band gap energy, retard the electron/ hole pair recombination rate, and promote the absorption of visible light. 101

10 Generally, the smaller the size of PbO and nickel doped PbO nanoparticles, the faster the holes and electrons generated by incident light will migrate toward the surface and more active centers are formed on the nanoparticle surface. Furthermore, the generated electrons do not combine conveniently with holes due to the very large area to volume ratio. The recombination of holes and electrons result in an increase in photocatalytic efficiency 294. The synthesized PbO, 3 %, and 7 % Ni doped PbO nanoparticles has more photocatalytic efficiency at ph 10.0, concentration of dye solution 3 x 10-4 M, and gm/50 ml of amount of synthesized PbO and Ni doped PbO nanoparticles photocatlyst. 102

11 % Degradation % Degradation %Degradation (a) 7 % Ni-PbO 3 % Ni-PbO PbO ph (b) 7 % Ni-PbO 3 % Ni-PbO PbO Concentration (M) x (c) 7 % Ni-PbO 3 % Ni-PbO PbO Amount (gm) Fig.3.2. Degradation of methyl blue a) Effect of ph, b) Effect of dye concentration, and c) Effect of nature and amount of photocatalyst 103

12 Section-B I) Degradation of Rose Bengal Dye Using CdO, 3 %, and 7 % Cs Doped CdO Nanoparticles 3.1B. Introduction: Rose bengal dye is 4, 5, 6, 7-tetrachloro 2, 4, 5, 7 -tetraiodo derivative of fluorescein is an organic anionic dye that has been clinically used to assess damage to the ocular surface epithelium in ocular surface disease 295. Application of rose bengal to the ocular surface of patients with dry eye results in patches of superficial punctate staining, the frequency, intensity of which has been used to characterize the disease, assess its severity, and monitor the clinical response to therapy 296. The rose bengal binds to several tear components, albumin, lactoferrin, transferring, and lysozyme 297. Rose bengal in vivo believed to visualize diseased or dead cells 298, has a strong stinging property implying a toxic component. The rose bengal has been found to be toxic to the cells, causing morphological changes such as detachment, separation, loss of motility, disruption, swelling, intracytoplasmic vacuole formation, and lysis 299. The rose bengal in low concentration predominantly stains cell junctions, and at higher concentrations causes cell separation 300. As a photosensitizer rose bengal can kill micro-organisms like viruses, bacteria, protozoa can induce photodynamic effects in vitro on red blood cells, and cardiomyocytes The various dyes found in industrial effluents ultimately enter the aquatic ecosystem, and can create various environmental hazards. Therefore, it is necessary to remove these toxic substances, coloured dyes, and try to make the waste water usable for industrial or domestic use. In the absence of light exposure the stained cells lose their vitality indicating that rose bengal is intrinsically toxic in the presence of light 104

13 there is an additional photodynamic damaging effect in biological systems 306. Rose bengal also causes photosensitized oxidation of sarcoplasmic reticulum vesicle membrane, inhibits calcium uptake, and ATPase activity 307. Rose bengal dye was photocatalytically degradated with semiconducting or metal doped semiconducting material 308. The photodegradation of rose bengal was carried out by MnO 2 309, TiO 2 membrane supported on a porous ceramic tube 310, and Cr substituted MnFe 2 O 4 ferrospine 311. Nanoparticles are expected to play a crucial role in water purification 312 because of their high reactivity. The photocatalytic purification of water has been proposed as an alternative for removing pollutants from water 313. Fig.3.3. Structure of rose bengal dye In the present work, the photocatalytic activity of the synthesized cadmium oxide (CdO) semiconducting nanoparticles was investigated by taking rose bengal dye (Fig.3.3) as model compound. The photocatalytic efficiency of cesium doped CdO semiconducting nanoparticles was also studied to demonstrate the effect of doping. The degradation of rose bengal dye in aqueous solution at different ph has been studied. The course of photodegradation of dye was monitored by UV-visible spectrophotometer. 105

14 3.2B. Experimental: 3.2.1B. Materials and methods: The rose bengal (Loba Chemie, 99.0 %), sulphuric acid (Loba Chemie, 91.0 %), sodium hydroxide (Sigma Aldrich, 98.5 %), synthesized CdO, 3 %, and 7 % Cs doped CdO nanoparticles (Chapter-2, Section-B) were used for the degradation study. The photocatalytic degradation of rose bengal was carried out using CdO, 3 %, and 7 % Cs doped CdO nanoparticles. For the irradiation of light a 200 W tungsten lamp (Philips, light intensity = 60.0 mwcm -2 ) was used. The ph of the dye solution was measured by a digital ph meter (Elico model Ll 120). The standardized sulphuric acid and sodium hydroxide solutions are used to obtain the desired ph of solution. The progress of photocatalytic reaction was monitored by taking absorbance at regular time intervals using an UV-visible spectrophotometer (Elico model SL 159) B. Photocatalytic degradation of rose bengal: A 50 ml of rose bengal dye solution with photocatalyst (CdO or Cs doped CdO nanoparticles) was exposed to light. The absorbance of homogenized dye solutions were recorded at 550 nm using UV-visible spectrophotometer at different time interval. 3.3B. Results and Discussion: The parameters like concentration of organic dye substance, amount and nature of the semiconductor photocatalyst and ph of dye solution was studied B. Effect of ph on photodegradation: In order to study the effect of ph on photodegradation of rose bengal dye was carried out at ph range 4.0 to 10.0 with photocatalyst such as CdO, 3 %, and 7 % Cs doped CdO with amount gm/50 ml and keeping rose bengal dye concentration 0.50 x 10-5 M (Fig. 3.4a). The ph of dye solution was adjusted using sulphuric acid 106

15 and sodium hydroxide. The rate of photodegradation of rose bengal dye increases with increasing ph up to 8.0 and above this there is a decrease in the rate of photodegradation of dye. At low ph the anionic dye molecules remains in protonated form, and semiconductor surface posses the positive charge due to adsorption of H + ions. So dye molecules repel the semiconducting photocatalyst molecules, and rate of photodegradation is decreased. As ph of solution increases, repulsion between dye and photocatalyst molecules decreases, and hence the rate of photodegradation increases. But after certain limit (ph-8.0) the photocatalyst surface becomes negatively charged and again shows repulsion towards dye molecules. Therefore, after this point the rate of photodegradation of dye was decreased B. Effect of dye concentration on photodegradation: The dye solution (0.25 x 10-5 to 1.50 x 10-5 M) and photocatalyst CdO, 3 %, and 7 % Cs doped CdO with amount gm/50 ml at ph was used to study photodegradation of dye (Fig. 3.4b). The rate of photocatalytic degradation increases with an increase in the concentration of the rose bengal dye. It may be due to the fact that as the concentration of rose bengal dye increases, more dye molecules are available for excitation and energy transfer, hence an increase in the rate of photodegradation of the dye was observed. The rate of photocatalytic degradation was found to be decrease with increase in the concentration of the dye above 0.5 x 10-5 M B. Effect of amount of photocatalyst on photodegradation: The rate of photodegradation depends on the amount of semiconducting photocatalyst therefore, different amount of photocatalyst were used ( gm/50 ml) for the photodegradation of dye (Fig. 3.4c). The rate of photocatalytic degradation of rose bengal dye increases with an increase in the amount of 107

16 photocatalyst but ultimately, it decreases to some extent after a gm/50 ml amount of CdO and Cs doped CdO photocatalyst B. Effect of nature of photocatalyst on photodegradation: The photodegradation of dye was carried out with different photocatalyst like CdO, 3 %, and 7 % Cs doped CdO nanoparticles. The results obtained shows that photodegradation of rose bengal dye was more with Cs doped CdO than undoped CdO nanoparticles (Fig. 3.4c). These may be due to cesium ion in CdO nanoparticles photocatalyst shifts the absorption of light towards visible range and the band gap energies of modified CdO nanoparticles was less as compared to band gap energy of CdO nanoparticles. The surface area of Cs doped CdO nanoparticles are large as compared to undoped CdO nanoparticles which also increase the rate of photodegradation. The synthesized CdO or modified CdO nanoparticles shows maximum photocatalytic activity at ph-8.0. The 0.50 x 10-5 M concentration of rose bengal dye, and gm/50 ml of amount of synthesized CdO or modified CdO nanoparticles was the optimum condition for the degradation of rose bengal dye. 108

17 % Degradation % Degradation % Degradation (a) 7 % Cs-CdO 3 % Cs-CdO CdO ph (b) 7 % Cs-CdO 3 % Cs-CdO CdO Concentration (M) x (c) 7 % Cs-CdO 3 % Cs-CdO CdO Amount (gm) Fig.3.4. Degradation of rose bengal dye a) Effect of ph, b) Effect of dye concentration, and c) Effect of nature, and amount of photocatalyst 109

18 II) Removal of Chromium Using CdO, 3 %, and 7 % Cs Doped CdO Nanoparticles 3.4B. Introduction: Industrial wastewater containing heavy metals are produced from different industries. Heavy metals can causes serious health effects including cancer, organ damage, nervous system damage at higher doses, it causes irreversible brain damage, and in extreme cases death 314. Electroplating and metal surface treatment processes generate significant quantities of waste water containing heavy metals. The pigment manufacturing industry produces pigments that contain chromium compounds. All of this process produces a large quantity of wastewaters, residues, and sludges that can be categorized as hazardous waste requiring extensive waste treatment 315. Chromium with Cr (VI) is considered harmful even in small intake quantity, whereas Cr (III) is considered essential for good health in moderate intake 316 and an essential nutrient. The chromium occurred naturally in the Earth s normal mineral soil 317 and human activity further contributes to chromium in the environment. The sources of Cr (VI) emissions are, i) chromium plating, ii) chemical manufacturing of chromium, and iii) evaporative cooling towers 317. While combustion of coal and oil also release large quantities of chromium. Due to the discharge of large amounts of metal contaminated waste water are the most hazardous among the chemical intensive industries. Because of their high solubility in the aquatic environments heavy metals can be absorbed by living organisms. They enter the food chain and large concentration of heavy metals may accumulate in the human body. If the metals are beyond the permitted concentration they can cause serious health disorders 318. Chromium (VI) will penetrate the skin 10,000 times faster than Cr (III) which causes 110

19 dermal disease it also causes damage to lung, and Cr (VI) is a carcinogenic 317. If chromium concentrations are high then inhalation by breathing through the mouth can cuases stomach ulcers and it also causes immunological effects. High concentrations of Cr (VI) fumes causes disease like dizziness, headache, and weakness. Inhalation of chromium in various forms may cause chromosome effects in human. However chromium (VI) is rarely naturally occurring, relatively soluble in aqueous systems, and is readily transformed in groundwater. Therefore, it is necessary to remove such heavy metal both to decrease the amount of wastewater produced and to improve the quality of the treated effluent. The removal of heavy metal from industrial effluents can be achieved by chemical oxidation, ion exchange, chemical precipitation, and etc. Different physico-chemical methods such as carbon adsorption, ion exchange, and reverse osmosis are used for advanced purification of polluted water. As an alternative to these methods recently the removal of heavy metal contaminant by means of nanoparticles has been focused due to its high surface area. Adsorption is rapid and efficient process for the removal of environmental pollutants. Nowadays, various nanomaterials have been used for the removal of heavy metals from industrial effluents 319, 320. The removal of chromium by the synthesized CdO, and Cs doped CdO semiconducting nanoparticles was investigated. The removal of chromium in aqueous solution at different ph, amount of CdO or Cs doped CdO nanoparticles, and different initial concentration of chromium has been studied. The course of removal of chromium was monitored by UV-visible spectrophotometer. 3.5B. Experimental: 3.5.1B. Materials and methods: The synthesized CdO, and modified CdO nanoparticles (Chapter-2, Section-B) was used for the removal of Cr (VI). The potassium dichromate (Loba Chemie, 111

20 99.5 %), sulphuric acid (Loba Chemie, 91 %), and sodium hydroxide (Sigma Aldrich, 98.5 %) were used for the study. Potassium chromate as a source of Cr (VI) as a heavy metal and synthesized CdO, 3 %, and 7 % Cs doped CdO nanoparticles as a adsorbent were used in this investigation. The ph of the solution was measured by a digital ph meter (Elico model Ll 120). The desired ph of the solution was maintained by the use of previously standardized sulphuric acid, and sodium hydroxide solution. The progress of removal of chromium was followed by taking absorbance using an UV-visible spectrophotometer (Elico model SL 159) B. Removal of Chromium (VI): For removal of chromium, 50 ml of Cr (VI) solution and the adsorbents (CdO or Cs doped CdO nanoparticles) were taken in a beaker. The chromium (VI) solutions were mixed properly. The absorbance of chromium solution was recorded at 435 nm using UV-visible spectrophotometer at different time interval. Adsorption of chromium (VI) was evaluated at different ph, different concentrations of chromium (VI), and various amount of CdO and Cs doped CdO nanoparticles adsorbent. 3.6B. Results and Discussion: Adsorption process assisted by CdO and Cs doped CdO nanoparticles adsorbent depends on various parameters like concentration of Cr (VI), concentration and type of the adsorbent, ph, and temperature 293. The absorbance of the Cr (VI) solution was measured before, and after removal at different time interval using UVvisible spectrophotometer. From the respective absorbance obtained, percentage removal of chromium was calculated using the equation, 112

21 Where, Co is the initial concentration of Cr (VI), and Ce is the equilibrium concentration of Cr (VI) B. Effect of ph: The removal of Cr (VI) was carried out in ph range 1.0 to 6.0 using adsorbent CdO and modified CdO (0.200 gm/50 ml) keeping chromium (VI) concentration constant as 200 mg/lit (Fig. 3.5a). The ph of the Cr (VI) solution was adjusted using sulphuric acid, and sodium hydroxide. The rate of removal of Cr (VI) increases with increasing ph up to 3.0 and above this it shows decrease in the rate of removal of chromium. The maximum removal of Cr (VI) occurs at ph 3.0. The influence of ph of the initial solution on the chromium (VI) adsorption is explained by the ionic state of the functional groups from CdO, and Cs doped CdO nanoparticles involved in metal binding as well as by the occurrence of the hydrated cation (Cr 6+ ) as a predominant ionic species. At low ph the competition between the H + ions, and Cr 6+ ions for the adsorption sites of CdO and Cs doped CdO causes low adsorption. By increasing initial ph, the dissociation degree of the hydroxyl groups and the negative charge density on the CdO, and Cs doped CdO surface are increasing resulting in a higher adsorption ratio by the electrostatic interaction with cations (Cr 6+ ). After the optimum ph, the negative charge density goes on increasing due to which decrease in adsorption of chromium (VI) B. Effect of concentration of Cr (VI): The removal of chromium (VI) was carried out with mg/lit initial concentration of Cr (VI) using different adsorbent such as CdO and modified CdO nanoparticles with amount of gm/50 ml and at ph 3.0 (Fig. 3.5b). The rate of removal of Cr (VI) is more up to 200 mg/lit, and further increase in concentration of Cr (VI) the rate of removal of Cr (VI) decreases. During the increase in initial 113

22 concentration of chromium (VI) the surface area of CdO and Cs doped CdO nanoparticles available for the adsorption is not sufficient therefore, the rate of removal of chromium decreases with increase in concentration of chromium (VI) B. Effect of amount of CdO nanoparticles: The amount of adsorbent affects the rate of removal of chromium (VI) hence different amounts of CdO and modified CdO nanoparticles were used for the removal of chromium (VI) metal ion (Fig. 3.5c). The results shows that the rate of removal of Cr (VI) increases with increase in the amount of CdO and Cs doped CdO nanoparticles but ultimately it remains constant after a gm/50 ml amount. This may be due to as the amount of CdO, and Cs doped CdO nanoparticles was increased the exposed surface area also increases but after a certain limit the increase in CdO and Cs doped CdO nanoparticles, there will be no increase in the exposed surface area of the adsorbent. It may be considered like a saturation point the increase in the amount of CdO, and Cs doped CdO nanoparticles after this saturation point will only increase the thickness of the layer at the bottom of the reaction vessel. The removal efficiency is directly related to the number of active sites available. After certain dose of adsorbent the maximum adsorption sets, and hence the amount of ions bound to the adsorbent and the free ions remains constant even with further addition of the adsorbent B. Effect of nature of CdO and modified CdO nanoparticles: Adsorption phenomenon was carried out with adsorbent such as CdO and modified CdO with Cr (VI) concentration solution 200 mg/lit to study the nature of adsorbent on removal of Cr (VI). The removal of Cr (VI) was more with Cs doped CdO than undoped CdO nanoparticles (Fig. 3.5c). The cesium doped CdO nanoparticles have large surface area as compared to undoped CdO nanoparticles 114

23 therefore, Cs doped CdO nanoparticles are more effective as compared to undoped CdO nanoparticles. In semiconductor-electrolyte interface with light energy greater than the semiconductor band gap, electron-hole pairs (e - /h + ) are formed in the conduction, and the valence band of the semiconductor respectively 321. These charge carriers which migrate to the semiconductor surface are capable of reducing or oxidizing species in solution having suitable redox potential. The synthesized CdO and Cs doped CdO semiconducting nanoparticles shows more efficiency at ph 3.0, 200 mg/lit of initial concentration of Cr (VI) and gm/50 ml of amount of synthesized nanoparticles. 3.7B. Adsorption isotherm: The synthesized nanoparticles CdO, and modified CdO are considered to have insignificant capability to adsorb heavy metals. From the study of adsorption at different condition the Langmuir and Freundlich adsorption isotherm parameters is to be evaluated. a) Langmuir isotherm: The validity of Langmuir adsorption isotherm is tested by plotting Ce/(x/m) against Ce. Where, x = the mass of solute adsorbed, m = mass of adsorbent, x/m = the mass of solute adsorbed per unit mass of adsorbent, and Ce = the equilibrium concentration of the adsorbed substance in the solution. The characteristic of Langmuir isotherm model can be described by separation factor (R L ) as follows, R L = 1/ (1 + b Co) (5) Where, b is the Langmuir constant (Fig. 3.6), and Co is initial concentration of Cr (VI) ion. The adsorption process is thermodynamically unfavorable, if R L > 1, linear if R L = 1, thermodynamically favorable if 0 < R L < 1 and irreversible if R L = 0. In these adsorption experiments calculated R L value is in the range to , 115

24 therefore the adsorption process is thermodynamically favorable. The adsorption capacity (a) of 7 % Cs doped CdO nanoparticles is more to that of CdO nanoparticles which is due to the surface area of 7 % Cs doped CdO nanoparticles is more to that of undoped CdO nanoparticles. b) Freundlich isotherm: The validity of Freundlich adsorption isotherm is tested by plotting log (x/m) against log Ce. Where, x = the mass of solute adsorbed, m = mass of adsorbent, x/m = the mass of solute adsorbed per unit mass of adsorbent, and Ce = the equilibrium concentration of the adsorbed substance in the solution. The Freundlich isotherm constants (K f and n) calculated from Fig. 3.6, and given in Table 3.1. The R 2 value for the adsorption of Cr is obtained from Fig. 3.6, fits for Freundlich isotherm very well. The n value for adsorption of the Cr is greater than 1, revealing that adsorption was a favorable process. 116

25 % Removal % Removal % Removal (a) % Cs-CdO 3 % Cs-CdO CdO ph (b) % Cs-CdO 3 % Cs-CdO CdO Concentration (mg/lit) (c) 7 % Cs-CdO 3 % Cs-CdO CdO Amount (gm) Fig.3.5. Removal of chromium a) Effect of ph, b) Effect of dye concentration, and c) Effect of nature and amount of adsorbent 117

26 log (x/m) Ce/(x/m) (a) 7 % Cs-CdO 3 % Cs-CdO CdO Ce (b) 7 % Cs-CdO 3 % Cs-CdO CdO log Ce Fig.3.6. a) Langmuir adsorption isotherm, and b) Freundlich adsorption isotherm 118

27 Table.3.1. Langmuir and Freundlich adsorption isotherm parameters Langmuir parameters Freundlich parameters Adsorbent b a R 2 n K f R 2 CdO % Cs-CdO % Cs-CdO

28 Section-C Degradation of Methylene Blue Dye Using PbS, 5 %, and 10 % Co Doped PbS Nanoparticles 3.1C. Introduction: The industrial effluents containing dyes are responsible for the environmental problems, and affect the aquatic life. Synthetic dyes have been used in textile, dyeing, paper pulp, plastic, leather, cosmetics, and food industries 322. Methylene blue is chemically inert, stable in the environment, and most common pollutants in the industrial effluents. The dyes have good solubility in water, and may be found in trace quantities in industrial water. Colour, and dye stuff discharge from these industries possess certain hazards. The effluents of industries contain dyes 323 and for the removal of these synthetic dyes the effluents are to be treated before discharging the effluents into the environment. Methylene blue is an important cationic dye and is used in many textile manufacturers it releases aromatic amines, and is a potential carcinogens 324. Methylene blue is used as model dye because, i) methylene blue is one of the commonly used dye and ii) methylene blue is well known for its adsorption characteristics 325. Methylene blue has many used in medicine it has been found to improve the hypotension associated with various clinical states 326, improves hypoxia, hyper dynamic circulation in cirrhosis of liver, and severe hepatopulmonary syndrome 327. It is also used in transient, and reproducible improvement in blood pressure, and cardiac function in septic shock 328. Methylene blue is being investigated for the photodynamic treatment of cancer 329. The adsorption of methylene blue is used to determine the specific surface area of natural cotton fibers

29 It can causes toxicity in high doses the features of toxicity being cardiac arrhythmias, coronary vasoconstriction, decreased cardiac output, renal blood flow, and mesenteric blood flow. It affects on skin and turns urine greenish blue 331. It also shows inhibiting property may precipitate potentially fatal serotonin toxicity due to high dose, and rarely can cause severe anaphylactic shock 332, 333. It can have various harmful effects, on inhalation it can give rise to short periods of rapid or difficult breathing while by ingestion through the mouth it produces a burning sensation, and may cause nausea, vomiting, diarrhea, and gastritis 334. Fig.3.7. Structure of methylene blue dye Dyes are more stable due to their complex aromatic molecular structure, and therefore difficult to biodegrade. Many dyes are toxic and may cause destruction or inhabiting of their catalytic capabilities 335. Photocatalytic degradation has a more tendency to control aqueous contaminants, and hazardous pollutants 336. The methylene blue dye was degradated by photocatalysis because it is fast, convenient, and less expensive method for degradation of the dye 337. The photocatalyst like TiO 2 caoting 338, anthraquinone sulfonate and cyclohexanol 339, UV/H 2 O 340 2, TiO 2 /UV 336, CeZrO 341 4, ZnO 342, and TiO was used for the degradation of methylene blue dye (Fig. 3.7). The semiconductor with strong oxidative power of photo excited holes is an important factor in photo oxidative degradation of organic pollutants

30 To study the photocatalytic activity of the synthesized lead sulphide (PbS) semiconducting nanoparticles methylene blue dye was used as model compound. The photocatalytic efficiency of cobalt doped PbS semiconducting nanoparticles was also studied to demonstrate the effect of doping on photocatalytic activity. The photocatalytic activity for the degradation of methylene blue dye in aqueous solution at different ph, different initial concentration of dye, and amount of photocatalyst has been studied. 3.2C. Experimental: 3.2.1C. Materials and methods: The synthesized PbS and modified PbS nanoparticles (Chapter-2, Section-C) were used as a photocatalyst. The methylene blue (Loba Chemie, 95.0 %), sulphuric acid (Loba Chemie, 91.0 %), and sodium hydroxide (Sigma Aldrich, 98.5 %) were used for the degradation study. The photocatalytic degradation of methylene blue was carried out with different amount of PbS and modified PbS nanoparticles. A 200 W tungsten lamp (Philips, light intensity = 60.0 mwcm -2 ) was used as a source of light. The ph of the solution was measured by a digital ph meter (Elico model Ll 120). The standardized sulphuric acid and sodium hydroxide was used to adjust the ph of dye solution. The rate of photocatalytic degradation was measured by using an UV-visible spectrophotometer (Elico model SL 159) C. Degradation of methylene blue: A 50 ml methylene blue dye with photocatalyst (PbS or Co doped PbS) were taken in a beaker, and exposed to light for the degradation study. The dye solutions were mixed properly, and their absorbance was recorded at 662 nm using UV-visible spectrophotometer at different time interval. The effect of ph, concentration of dye, 122

31 and amounts of photocatalyst on photocatalytic degradation of methylene blue was evaluated. 3.3C. Results and Discussion: The colour intensity of the dye solution was measured before, and after degradation at different time interval at 662 nm using UV-visible spectrophotometer. The photodegradation of dye was calculated using the absorbance values. The photocatalytic degradation of methylene blue (MB) dye believed to take place according to the following mechanism, PbS + hυ e - (conduction band) + h + (valence band) Where, e - (conduction band), and h + (valence band) are the electrons in the conduction band, and the electron vacancy in the valence band. Further, h + (valence band) + H 2 O OH + H + O 2 + e - (conduction band) O 2 Above reaction shows that recombination of electron and the hole is prevented. The ۰ OH and ۰ O 2 produced in above reaction can then react with the MB dye to other species, O 2 + H 2 O H 2 O 2 H 2 O 2 2 OH OH + MB MB ox. MB + e - (conduction band) MB red. The process of photocatalysis is possible due to the presence of dissolved oxygen, and water molecules C. Effect of ph on photodegradation: The photodegradation of methylene blue dye was carried out with ph range 2.0 to 12.0 with photocatalyst PbS, 5 %, and 10 % Co doped PbS with amount 123

32 0.100 gm/50 ml, and dye concentration 6 x 10-4 M (Fig. 3.8a). The maximum degradation of dye was observed at ph Further increase in ph after the optimum ph resulted into the decrease in the rate of photodegradation. The increase in the photocatalytic degradation with increase in ph may be due to more generation of OH radicals which are produced by the interaction of OH - with hole (h + ) of the semiconducting photocatalyst. But after optimum ph value the decrease in the photodegradation may be due to the fact that in more alkaline solution the methylene blue dye molecules are negatively charged and their absorption is also affected by an increase in the density of the semiconductor surface. Thus due to columbic repulsion substrate is scarcely adsorbed. At high ph values the hydroxyl radicals are so rapidly scavenged that they do not have the opportunity to react with the dyes therefore, the rate of photodegradation of dye decreases C. Effect of dye concentration on photodegradation: In order to evaluate the effect of initial dye concentration on the rate of photodegradation, the dye was degradated with gm/50 ml of photocatalyst, and at ph 10.0 within 2 x 10-4 M to 8 x 10-4 M dye concentration. The rate of photocatalytic degradation was found to decrease with increase in the concentration of the dye. The results show that, the initial dye concentration was increased then time required for the degradation of dye also increased. The rate of degradation of dye was observed more up to 6 x 10-4 M above this rate of degradation was less (Fig.3.8b). It is due to the available free radicals are not sufficient for degradation at high concentration of methylene blue dye. When concentration of methylene blue dye is increased then most of the light absorbed by the dye instead of photocatalyst 345 and formation of hydroxyl radical and superoxide radicals are limited therefore, photodegradation was found to be negligible. 124

33 3.3.3C. Effect of amount of photocatalyst on photodegradation: The photodegradation of dye was carried out in the gm/50 ml of synthesized nanoparticles photocatalyst. It has been observed that the photocatalytic degradation of methylene blue increases with an increase in the amount of photocatalyst but ultimately, it decreases to some extent after a certain amount i.e gm/50 ml (Fig. 3.8c). This may be attributed to the fact that as the amount of photocatalyst was increased, the exposed surface area also increases but after certain limit, and if the amount of semiconducting photocatalyst was further increased then there will be no increase in the exposed surface area of the photocatalyst. With an increase in catalyst the light penetration through the solution becomes difficult, and rate of photodegradation decreases. Increase in photocatalyst concentration decreases photo absorption which in turn reduces the dye absorption onto the photocatalyst surface thus reducing the reaction rates C. Effect of nature of photocatalyst on photodegradation: The photocatalyst like PbS and modified PbS was used to demonstrate the effect of nature of photocatalyst on photodegradation of dye. It was found that degradation of methylene blue dye was more with Co doped PbS than undoped PbS nanoparticles photocatalyst (Fig. 3.8c). The presence of cobalt ion in PbS nanoparticles photocatalyst shifts the absorption of light towards visible range. The band gap energy for Co doped PbS photocatalyst was less as compared to band gap energy of PbS nanoparticles photocatalyst. The cobalt doped PbS nanoparticles have large surface area as compared to PbS nanoparticles, therefore they are more effective as compared to undoped PbS nanoparticles photocatalyst. The synthesized PbS and Co doped PbS semiconducting nanoparticles shows maximum photocatalytic activity at ph-10.0, 6 x 10-4 M initial concentration of dye, 125

34 and gm/50 ml of amount of PbS, and Co doped PbS nanoparticles photocatalyst. 126

35 % Degradation % Degradation % Degradation (a) PbS 5 % Co-PbS 10 % Co-PbS ph (b) PbS 5 % Co-PbS 10 % Co-PbS Concentration (x 10-4 M) (c) Amount (gm) PbS 5 % Co-PbS 10 % Co-PbS Fig.3.8. Degradation of methylene blue dye a) Effect of ph, b) Effect of dye concentration, and c) Effect of nature and amount of photocatalyst 127

36 Section-D Degradation of Crystal Violet Dye Using ZnS, 3 %, and 7 % Sm Doped ZnS Nanoparticles 3.1D. Introduction: Crystal violet dye is a cationic triarylmethane dye which has an affinity for both cellulosic and proteinaceous materials 346. Crystal violet was used in the determination of salmonellae in dried food products 347. It has mutagenic and antimicrobial property therefore, used to prevent fungal growth in poultry feed 348. Due to its bacteriostatic property, it is used in medical solutions and skin infections 349. Crystal violet can be used in textiles, paints, printing ink, duct tape, masking tape, clear plastic tape, surgical tape, black electrical tape, and other tapes 350. Crystal violet is a protein dye in nature and finds application as enhance for bloody fingerprints. It causes moderate eye irritation, causing painful sensitization to light, permanent injury to the cornea, conjunctiva, and it is highly toxic to mammalian cells. In extreme cases it may also lead to respiratory, and kidney failures 350. Amount of dyes in effluents is usually lower but due to their strong colour they are visible even at very low concentrations thus causing serious aesthetic problems in wastewater disposal 351. Textile effluents enters into the food chain of aquatic organisms may cause various physiological disorders like hypertension, sporadic fever, renal damage, and cramps etc 352. The coloured textile effluents introduced into rivers, and lakes results in reduced dissolved oxygen concentration, and create toxic condition to aquatic flora 353. Crystal violet should be regarded as a biohazard substance and is a potent clastogenes, toxic to aquatic life which is responsible for promoting tumor growth in some species of fish, and also known as potent carcinogenic 354. When this 128

37 dye enters in environment impart colours to water sources, and damage living organisms by stopping the reoxygenation capacity of water, blocking sunlight, and therefore disturbing the natural growth activity of aquatic life thus the colour removal of textile waste water is a major environmental concern. These dye molecules are stable and so its removal is difficult. Fig.3.9. Structure of crystal violet dye The removal of crystal violet from effluents is essential to protect human health, and protection of water resources. Now a day s heterogeneous photocatalysis has emerged as an important destructive technology leading to the total mineralization of the organic pollutants including the synthetic dyes 355. There are various chemical and physical methods including adsorption, osmosis, flocculation, electrolysis, reduction, electrochemical treatment, and ion pair extraction were used to remove the dye. The crystal violet dye can be degradated by using nano TiO 356 2, semiconducting iron (III) oxide 357, nitrogen doped TiO 358 2, Ta 2 O 359 5, silver ion doped TiO 360 2, Fenton and Fenton like systems 354, and TiO 2 suspensions 361. The photocatalytic activity of the synthesized zinc sulphide (ZnS) semiconducting nanoparticles was investigated by taking crystal violet dye as model 129