ANNOUNCEMENTS
Bare Titanium dioxide (TiO2) and modified composite nanomaterials (NMs) were synthesized using sol-gel chemical method and which were used as adsorbent and photocatalysts for wastewater treatment. The modification included two routes: (1) Chemical modification using surfactants such that Cetyl Trimethyl Ammonium Bromide (CTAB) and Sodium Dodecyl Sulphate (SDS) as particle growth templates and composites are termed as SDS-TiO2 and CTABTiO2 throughout the thesis (2) Biomass modification using Bagasse Fly Ash (BFA) as the composite material is termed as BFA-TiO2 throughout the thesis. Detailed characterization & analysis were carried out using highly sophisticated techniques like X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Fourier Transform Infrared Spectroscopy (FTIR) and BET (Brunauer-Emmet-Teller) surface area analysis. XRD and FTIR depicted identical spectra in all cases, which confirmed that Bare, surfactant modified and biomass modified nanomaterials were pure TiO2 with only anatase as the constituent phase. Surfactant and biomass modification led to improved morphology from irregular shape in Bare TiO2 to regular and spherical shape in both types of modified TiO2 NMs, as depicted and analyzed using SEM and TEM. Average particle size also reduced from 14 nm in Bare TiO2 to 11 nm in case of CTAB-TiO2, 8 nm in SDS-TiO2 and 7 nm in BFA-TiO2. These improvements are expected to enhance adsorption and photocatalytic performance of these materials, which is elaborated in subsequent sections.
In case of surfactant modified TiO2, BET surface area analysis provided type IV mesoporosity, with H2 hysteresis loop and increase in surface area to almost four times as the result of surfactant modification. The adsorption intensity of Cd increased from 0.2027 mg dm−3 in Bare TiO2 to 0.3996 mg dm−3 in SDS-TiO2, which resulted in enhanced cadmium removal efficiency from 75.05% in Bare TiO2 to 99.56% in SDS-TiO2. Photodegradation of methylene blue dye under sunlight was carried out and decolourization efficiency enhanced from 62% with Bare TiO2 to 72% with CTAB-TiO2 and 90% with SDS-TiO2. Under optimized conditions (pH=9.2, Contact time=30min, Amount of TiO2=0.1 g, Irradiation time 120 min at room temperature), when real textile industry effluent was treated using TADOX® technology, it led to complete decolorization and wastewater quality parameter of treated water suitable for process reuse. When evaluated towards possible toxicity and environmental safety, both Bare TiO2 and SDS-TiO2 showed safe seed germination (100% for all concentrations). Root length, shoot length and vigor index increased uptil 500 mgL−1 concentration of SDS-TiO2 with respect to control, above this concentration the trend reversed and plant growth decreased probably due to accumulation of these NPs. These growth parameters indicate that the modified titania accelerated the process of seed germination and significantly shortened the germination time as compared to the control.
In case of Biomass modified titania (BFA-TiO2), the TADOX® treatment of synthetic dye bath led to 25% higher COD removal, 41% lower EEO and 25% lower cost of treatment per kg COD reduction as compared to Bare-TiO2. In case of real textile wastewater treatment, BFA-TiO2 led to 8.2% COD removal efficiency, 14% lower EEO and 7% reduction in cost of per kg COD removal as compared to Bare-TiO2 and the overall cost of treatment was 1.08 USD/m3. This cost of treatment is 30% lower when used with surfactant modified titania towards treatment of real textile wastewater. This further justifies use of BFA, a waste material abundantly available in India, which could not find a better place in the waste-energy scenario than enhancing energy efficiency and economic feasibility of the photocatalytic treatment process integrated in the real wastewater treatment.
At the same time, it is important to assess eco-safety aspects associated with Bare and BFA-TiO2 NMs, hence there were two areas of study: (i) Direct exposure of Bare and BFA-TiO2 through seed soaking in wide concentration range (10 to 5000 mgL-1) and (ii) Assessment of quality of treated water using these NMs. Their effect is individually studied over both plant growth and Daphnia Magna Survival. It was observed that the direct exposure of Bare and BFA-TiO2 nanomaterials (NMs) in a wide concentration range (0, 10, 50, 1000 and 5000 mgL-1) towards growth of Pisum sativum (Garden pea) depicted 100% seed germination and enhanced plant growth, indicating ecosafe materials. The presence of these TiO2 based NMs (at 1000 mgL-1) in leaves was confirmed using DAB test with development of dark purple colour on leaves. The presence of TiO2 based NMs in leaves, lead to increased chlorophyll content (chlorophyll a, chlorophyll b and Total chlorophyll) as compared to control, which is further indicative of the growth and productivity status of the plants. When Daphnia magna was exposed to both these NMs during cultivation, it showed 100% survival until 100 mg L-1 and substantially decreased to 90% at 250 mg L-1 in BFA-TiO2, indicating safe exposure with till a certain limit.
The treated textile wastewater was regularly used for watering the plants grown from the sowing to seedling stage. The continuous increase in root and shoot lengths confirmed that the photocatalytically treated water is safe for irrigation and horticulture purposes. Similarly, the treated water was used to cultivate Daphnia magna for 48 h; 90- 100% survival in case of modified and Bare-TiO2 treated water complies with CPCB norms of safe surface water discharge and also justifies safe reuse of treated water.
