Synthesis and Characterization of Titanium Dioxide Modified With Molybdenum Trioxide and Tungsten Trioxide Composites for Point of Use Treatment of Water

Abstract

The availability of adequate and clean drinking water is a global topic affecting community health and socioeconomic development. Kenya like many developing countries has got many people in both rural and urban areas who lack access to safe piped drinking water. This study investigated the potential application of coupled TiO2 photocatalysts in point-of-use treatment of water by studying the degradation of methylene blue, reduction of heavy metals and disinfection of escherichia coli using five different types of water sources commonly applied for domestic use. A ceramic water filter coated with synthesized TiO2-WO3 nanoparticle composites was designed and developed from clay materials from Siaya, Kisumu, Homabay and Nyeri counties. Titanium (IV) oxide nanoparticle composites modified with varying amounts of tungsten (VI) oxide and molybdenum (VI) oxide were separately synthesized, characterized and evaluated for water purification potential. Photocatalytic degradation of recalcitrant organic compounds, inactivation of microbial pollutants and remediation of water polluted with heavy metals were investigated using methylene blue, escherichia coli and selected heavy metals as model pollutants. Effects of varying pH, catalyst loading, temperature, and concentration of contaminant of interest were applied to investigate optimal working conditions for each photocatalyst. The performance of the photocatalysts under visible light, sunlight and solar simulator radiations was tested at optimal conditions, while point zero charge of both catalysts were determined using pH drift method. Diffractometer, Attenuated Total Reflection Fourier Transform Infra-red, the Brunauer-Emmett-Teller analyzer, Ultra violet - visible spectrometer, Scanning electron microscope, Transmission electron microscope and the Energy dispersive X-ray were used to characterize synthesized photocatalysts. The physico-chemical water quality parameters of the water sources pH ranged from 6.60 ± 0.80 - 9.10 ± 0.10 and 6.80 ± 0.60 - 9.50 ± 1.00 in dry and wet seasons respectively; conductivity from 244 – 5,785 μS/cm and 141– 2,006 μS/cm, and TSS from 7.00 - 194.00 mg/L and 311.00 - 440.00 mg/L throughout the dry and rainy season. Chemical oxygen demand varied from 89.60 ± 0.14 - 153.60 ± 1.10 mg/L, and 112.00 ± 0.10 mg/L to 233.60 ± 0.05 mg/L with an average of 119.4 mg/L and 153.4 mg/L during the rainy and throughout the dry seasons respectively. Pb ranged between 0.48 ± 0.38 mg/L and 1.62 ± 0.28 mg/L with an average of 1.09 mg/L; Cd from 0.17 ± 0.00 mg/L to 2.45 ± 0.02 mg/L with an average of 1.52 mg/L, and Cr (VI) from 0.17 ± 0.00 mg/L to 2.41 ± 0.01 with an average of 1.54 mg/L. Nitrate varied from 2.5 mg/L to 19.5 mg/L, and phosphate from 0.05 mg/L – 2.25 mg/L, E. coli, from 55 – 4,080 cfu/mL. Pesticide residue analysis in water using Gas chromatography - mass spectroscopy identified 18 organochlorine pesticides. Optimal conditions for photocatalytic degradation of Methylene blue were pH 2, 20 ppm methylene blue, 0.25 g photocatalyst and nanocomposite in the ratio 1:1. Mineralization of methylene blue monitored by percentage reduction in chemical oxygen demand was 86% in 3 hours, while 2 g/L catalyst loading was the most effective at deactivating escherichia coli. Both lead and cadmium levels between 25 ppm – 200 ppm registered 100% removal at pH 4, 8 and 10, while levels between 100 – 200 ppm recorded 96% - 98% removal at pH 8 and 10. Photocatalytic degradation of methylene blue followed first order kinetics, with percentage degradation of 20 ppm methylene blue decreasing from 93% in cycle 1 to 87% in cycle 4. The zero-point charge for TiO2-WO3 recorded a vi value of pH 6.2, while that of TiO2-MoO3 recorded a value of pH 5.5. Band gap of unmodified TiO2 was 3.317eV, while TiO2-WO3 and TiO2-MoO3 recorded values of 3.213 eV and 3.00 eV respectively. The Transmission electron microscopy revealed that both modified catalysts had crystalline structure, while TiO2 exhibited anatase phase and tetragonal shape. The WO3 and MoO3 exhibited triclinic orthorhombic shapes respectively. The crystallite sizes of TiO2, TiO2-WO3 and TiO2-MoO3 were 22.2 nm, 56.7 nm and 29.9 nm respectively, and mesoporous in nature. The ceramic water filters developed registered flow rates from 18 mL/hr – 150 mL/hr, shrinkage tests carried out using ceramic bars ranged from 5% - 9%, and porosity from 29.1% - 49.9%. The Langmuir model had a correlation coefficient of 0.9972, showing a perfect fit with an adsorption capacity of 0.813 mg/g. Results from this study showed that different sources of water were polluted with pesticides, E. coli and heavy metals necessitating treatment before applied for drinking water purposes.