EFFECT OF CALCINATION TEMPERATURE ON THE COMPOSITION AND PHOTOCATALYTIC ACTIVITY OF NIGERIAN NATURAL ILMENITE, SPHALERITE AND WOLFRAMITE

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Department of Engineering

ABSTRACT
The growing concern for environmental preservation and the search for new energy sources to replace the fossil fuels have received much attention over the last four decades. Photocatalysis is a promising solution to these challenges. It is therefore, pertinent to investigate local indigenous photocatalysts and to increase their efficiency under visible light irradiation. Although visible light active photocatalysts require chemical modifications, their overall efficiencies can be significantly enhanced by controlling the structure and morphology. This structural and morphological modification can be achieved by calcination. Ilmenite, a natural occurring mineral ore of titanium as a visible light induced photocatalyst was characterized using XRF, XRD and specific surface area analysis (Sear’s method). Elemental analysis of the raw ore indicates elemental titanium and iron constitute 69.0% and 22.2%, respectively, as the major constituent. XRD patterns of the raw and calcined ilmenite depicts that the rutile phase was the dominant polymorphs. The Specific Surface Area of ilmenite was found to increase from 33.40m2/g to 66.92m2/g as a result of increase in porosity and then decrease to 24.60m2/g due to increase in crystal size as the calcination temperature increases. Photocatalytic activity of the samples was investigated for the degradation of methyl orange (MO) as a model pollutant under visible light irradiation. Calcination at 500oC for two hours gave a better degradation rate constant of 1.3x10-3min-1 and a degradation percentage of 19.07%. The structure and morphology of sphalerite, a mineral ore of ZnS was characterized by XRF, XRD, SEM and surface area analysis. The elemental composition of the sphalerite indicates that the main constituents are elemental zinc and sulphur of 58.6 and 16.0% respectively. XRD patterns of the raw and calcined samples depict a stepwise conversion of sphalerite into a ZnO composite photocatalyst. Photocatalytic test for the various samples were examined by measuring the degradation of MO. The result 7 indicates that sphalerite calcined at 700oC exhibit a higher degradation efficiency of 90.93% and a rate constant of 3.58x10-3min-1 due to phase change, decrease in crystal size, high specific surface area and oxygen vacancy defects of the calcined samples. Elemental composition of wolfram using XRF analysis indicated that the raw sample of the wolfram contains 64.45% tungsten and 15.10% iron. There XRD patterns did not show any remarkable differences in the number of peaks in the raw and calcined sample at 800oC; however, the intensity of the peaks increases due to crystal growth. The specific surface area of the samples decreases gradually as calcination increases. This is attributed to the slight increase in crystal size from 35.45nm to 39.40nm. Photocatalytic activity of raw and calcined wolframite investigated shows that calcination at 600oC for two hours gave a better degradation rate constant of 1.3x10-3min-1 and a degradation percentage of 32.53%.