Photocatalytic treatment of industrial wastewater containing citric acid and toxic heavy metals
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Date
2014-12
Authors
Baloyi, Siwela Jeffrey
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Abstract
The co-existence of organic acids and toxic heavy metals in natural water creates harmful effects on human, plants and animals. Therefore, it is necessary to treat organic acids and toxic heavy metal contaminated wastewater prior to its discharge to the environment. The aim of this study was to apply co-treatment of industrial wastewater containing citric acid and toxic heavy metals in single and binary systems using photocatalysis process. The hydrothermal method was used to synthesise dandelion-like TiO2 structures. Modifications of the dandelion-like TiO2 by deposition of gold nanoparticles and immobilisation on calcium alginate were done using deposition precipitation and one-step encapsulation methods, respectively. Dandelion-like TiO2 and dandelion-like TiO2 immobilised on calcium alginate (Alg/TiO2) were used as photocatalysts for Cr(VI), Hg(II) and citric acid removal from water.
The results showed that the production of dandelion-like TiO2 structures strongly depends on the reaction time and synthesis temperature as key process parameters. The characterisation of the dandelion-like TiO2 by X-ray diffraction (XRD), transmission electron microscope (TEM), scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) and Brunauer-Emmett-Teller (BET) revealed the crystal structure, morphology, chemical composition and surface area. It was found that the efficiency of photocatalytic process depends on the type of pollutants, initial pH of the solution, photocatalyst dosage, contact time, substrate initial concentration, UV wavelength and light intensity. The reduction efficiency of Cr(VI) ion and citric acid increased with decreasing the initial pH values and initial concentration. On the other hand, Hg(II) reduction efficiency increased with increasing the initial pH values and initial concentration. In a binary system, the reduction of Cr(VI) and Hg(II) was found to be faster than in the single and ternary systems. The relationship of the chemical reaction rate of Cr(VI), Hg(II) and citric acid were expressed by the pseudo-first-order kinetic equation. Addition of ferric ions to Cr(VI)-citric acid complex and Hg(II)-citric acid complex enhanced the reduction of Cr(VI) and Hg(II), a complete reduction was accomplished within 30 and 60 minutes (min) of irradiation time, respectively. The reduction efficiency of both Cr(VI) and Hg(II) in the presence of citric acid in a solution was still significant after four times of Alg/TiO2 reuse. These results indicated that the UV/TiO2 photocatalysis process can be considered as a suitable method to reach a complete reduction of Cr(VI) and Hg(II) in the presence of citric acid in a solution.
Description
M. Tech. (Chemical Engineering, Faculty of Engineering and Technology), Vaal University of Technology|
Keywords
Organic acids, Toxic heavy metals, Natural water, Industrial wastewater, Citric acid, Toxic heavy metals, Industrial wastewater treatment, Hydrothermal method