Optimization of photocatalytic degradation of wastewater using oxide and non-oxide photocatalysts

Abstract

Wastewater treatment is a global concern, especially in developing countries with limited access to safe and clean facilities, resulting in individuals practicing unsafe and unsustainable human practices. This poses challenges for South African wastewater treatment plants (WWTPs) due to the aging infrastructures and the use of conventional technologies. Also, recent population growth, urbanization, and industrial activities have given rise to contaminating water resources with recalcitrant organic micropollutants (OMPs). Organic micropollutants cause severe environmental pollution, imbalanced ecosystems (aquatic life), human health risks, and oxygen depletion due to accelerated chemical oxygen demand (COD). Apart from the detrimental effects of wastewater on human health and the ecosystem, the United Nations (UN) sustainability development goal of obtaining clean water and sanitation (SDG #6) by 2030 is continuously threatened. Therefore, treating wastewater for reuse in the environment with good quality comes in handy. Against this background, photocatalysis, such as the advanced oxidation process (AOP), is reported as a promising, eco-friendly, and cost-effective technology for degrading organic contaminants (COD) into harmless compounds. However, the TiO2-based photocatalytic process has setbacks, such as recoverability and treatability efficiency, limiting its industrial application. Therefore, this study explored oxide and non-oxide photocatalysts as alternatives to TiO2-based photocatalytic processes for a local South African wastewater treatment. The photocatalysts considered were Titanium dioxide (TiO2), Iron (III) oxide (Fe2O3), Zinc Sulphide (ZnS), and Copper Sulphide (CuS). Their applicability was conducted experimentally by evaluating and optimizing the performance of oxide (TiO2, Fe2O3) and non-oxide (ZnS, CuS) photocatalysts under UV, UV-visible, and natural sunlight irradiation. The One-Factor-at-a-Time (OFAT) approach was used on the photocatalytic system to identify the relationship between the variables that influence the photocatalytic degradation treatment of municipal wastewater. The water quality parameters considered were pH, turbidity (NTU), colour (Pt. Co), and COD (mg/L). By employing the oxides and non-oxides under a constant UV irradiation light source vi and OFAT approach, the catalyst load (0.5-2.5 g/L), mixing speed (30-150 rpm), and exposure time (10-60 minutes) were investigated. Among the photocatalysts, CuS displayed the best results overall for above 50% COD removal efficiency, whilst ZnS was also efficient in removing above 50% turbidity and colour at a catalyst load of 1.5 g/L, mixing speed of 90 rpm, and UV exposure time of 45 minutes. It was established that CuS was the cheapest at R2.01/1.5g as compared to TiO2 at R32.47/1.5g. Subsequently, the photocatalysts were investigated using three different light sources: UV, UV-visible, and sunlight irradiation. UV-visible was the most favourable at a catalyst load of 1.5 g/L, mixing speed of 90 rpm, and irradiation time of 60 minutes. Thus, the high light intensity of UV-visible, 191,000 Lux, enhanced the photocatalytic performance of the four photocatalysts under this study, with the optimum COD removal values at 72.25%, 70.87%, 70.20%, and 46.66% for Fe2O3, ZnS, CuS, and TiO2 respectively. Furthermore, response predictive models were developed as a function of the input factors of the photocatalytic system for the treatment of municipal wastewater. This was done utilizing the response surface methodology (RSM) via the Box Behnken design (BBD) with the best-performing catalyst (CuS) and the best light source (UV visible), at the optimal conditions of catalyst load of 2 g/L CuS, a mixing speed of 120 rpm, and an exposure time of 30 minutes with treatability desirability of 96%. The selected optimal condition was then validated experimentally, and the results obtained were agreeable with the model-predicted values at 95% confidence levels. Moreover, a comparative study with CuS and TiO2 was evaluated with synthetic wastewater (SW) and raw wastewater (RW) at the optimal conditions. The results by CuS demonstrated above 55% COD, turbidity, and colour removal from both the SW and RW compared to the TiO2, which obtained below 35% removal from both SW and RW. Therefore, under the conditions investigated in this study, CuS was found to be the most cost-effective and viable photocatalyst alternative to TiO2 for wastewater treatment. However, the techno-economic and life cycle assessment must be explored to encourage the prospects of the CuS in the water settings