Zinc stannate modified graphitic carbon nitride for the photocatalytic reduction of Cr(VI) to Cr(III)

MSc (Chemistry), North-West University, Mahikeng Campus

Water is one of the most important natural resources for basic human life. Its pollution has become a serious threat to the survival of both humans and animals. Most water bodies have become dumping sites due to industrial growth, human population growth and urbanization. Over 30% of the world’s population still relies on these contaminated water bodies for drinking water. Among these contaminants, Cr(VI) is of great concern because of its ease of solubility and mobility therefore finding its way to the environment and water bodies, leaving all living organisms that use these resources exposed to the pollutant. Once it accumulates to a certain extent, Cr(VI) severely threatens human health, since it is carcinogenic and mutagenic. According to the World Health Organisation (WHO), the permissible limit for chromium in water is 0,1mg/L. To address the problems of water pollution, and specifically of Cr(VI) removal, there is ongoing research to explore more efficient, environmentally safe and cost effective methods of environmental remediation. Photocatalysis has proven to be a more promising method in wastewater treatment, due to its safe, cost effectiveness and environmental friendliness, more so as it uses abundant energy xii from the sunlight. Semiconductors are used as tools for the process of photocatalysis as they have a band gap which promotes generation of electrons and holes responsible for the production of radicals which are used in the process of pollutant degradation. However, to utilize the process to its fullest capabilities the right semiconductor needs to be employed for the process. Semiconductors such as ZnO and SnO have limitation of efficiency and practical application due to electron-hole recombination since most semiconductors absorb in the UV region. Therefore, the design of semiconductors that are able to overcome this limitation need to be revised, through the right band gap alignment. Through modification of traditional semiconductors ZnO and SnO by forming a heterojunction the bad gap will be reduced thus allowing the nanoparticle to absorb more in the UV region hence improving its optical properties and enhancing its photocatalytic properties as well. This research reports the modification of traditional semiconductor ZnO and SnO through the synthesis of heterojunction compound of Zn2SnO4-ZnO nanoparticles which were subsequently incorporated into graphitic carbon nitride using high temperature solid state method. The nanoparticles were then used for the application of the reduction of Cr(VI) to Cr(III) in water. In the process of the synthesis of the heterojunction, characterization revealed that at lower temperature of 600 ºC the Sn salt used acted as a dopant and did not form the heterojunction compound. Only at high temperature of about 1000 ºC and high concentration of Sn (15, 20 & 30%) was the heterojunction obtained as per XRD results. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) revealed that there was a variation in the size and shape with increase in concentration of Sn and also between the Sn doped samples and the Zn2SnO4-ZnO nanoparticles, with ZTO-ZnO showing two different morphologies. Optical properties were studied using photoluminescent (PL) and UV-Vis spectrophometer. All samples were found to exhibit absorption in the UV and visible region. Furthermore, the heterojunction at higher Sn concentration (20 & 30%) were selected to be incorporated into g-C3N4 as they revealed more positive results then the ones at lower concentration. The g-C3N4/ZTO-ZnO nanocomposite displayed peaks from the precursor materials: ZTO-ZnO and g-C3N4, which confirmed the incorporation of ZTO-ZnO nanoparticles into the g-C3N4. The photocatalytic properties of the two nanocomposite: g-C3N4/ZTO-ZnO@20% and gC3N4/ZTO-ZnO@30%, were evaluated using chromate(VI) salt. Photocatalytic reaction parameters including • the effects of solution pH, • effect of photocatalyst (g-C3N4/ZTO-ZnO@20) dosage, • effect of initial concentration of Cr(IV) on the rate of reduction of Cr(IV) were evaluated. The solution pH was varied from 2 to 8, and pH 2 displayed the highest removal of Cr(VI) at 99.2 and 72% for the g-C3N4/ZTO-ZnO@20% and gC3N4/ZTO-ZnO@30%, respectively. The effect of photocatalyst dosage was only performed for the (g-C3N4/ ZTO-ZnO@20) composite due to its efficiency within the solution pH. The highest percentage chromium removal was found to be 99.2% at photocatalyst loading of 2 g/L and the lowest was 64% for 0.5g/L. Finally, the kinetic study of the photocatalytic reaction showed an increase in the overall rate constant k with increase in initial Cr(VI) concentration.

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