Water purification by green photo-catalysts

22 Dec 2016. NUS chemists have developed photo-catalysts with enhanced catalytic properties for wastewater treatment.

With increasing water pollutants and the higher demand for water, water purification has become one of the key research focus areas in the environmental domain. Traditional water purification methods include filtration, adsorption, activated sludge treatment, coagulation and flocculation, etc. As these are not sufficient to cope with urban water treatment needs, more advanced water filtration techniques such as the reverse osmosis (RO) and membrane bioreactor (MBR) were developed. However, all these traditional and advanced water treatment methods remove contaminants and pollutants but are not able to eliminate them.

Advanced oxidation processes (AOPs) have been attracting research interest for wastewater treatment as they have the capability to eliminate contaminants and pollutants from water. The research team led by Prof Sam LI from the Department of Chemistry, NUS has developed TiO₂ compounds with sulfate chemical groups for use in AOPs. These chemical compounds function as catalysts with a much higher photocatalytic efficiency in breaking down the pollutants in wastewater when compared to pristine TiO₂. They found that the neutral sulfate species are responsible for promoting photocatalytic activity leading to the effective degradation of organic pollutants in reverse osmosis concentrate (ROC). ROC are the elevated levels of contaminants that remain during treatment of brackish water for potable water use.

AOPs generate highly reactive hydroxyl radicals (·OH) which are electrophilic (have a tendency to be attracted to electrons) and can oxidise most electron-rich organic molecules, converting them to harmless CO₂ gas and water. AOPs have been shown to be effective in treating hard to manage organic pollutants from a variety of sources, such as mature landfill leachate, anti-inflammatory and analgesic pharmaceuticals, antibiotics, chlorophenols, and several endocrine disrupting chemicals (EDCs). Ultra-violet (UV) irradiation used together with TiO₂ photo-catalyst is gaining growing acceptance as an effective AOP-based wastewater treatment method due to its high efficiency, low cost, non-toxicity and environmentally friendly characteristics.

TiO₂ is an UV active photo-catalyst which absorbs UV photons to generate negatively charged electrons in its conduction band (CB) and positively charged holes in its valence band (VB). These photo-generated electrons and holes react with oxygen and water to produce highly reactive hydroxyl radicals ·OH that are able to transform organic pollutants into harmless products (CO₂ and water). The research team synthesised sulfated SO₄^2-/TiO₂ photo-catalysts and established the correlation between their photocatalytic activities and the structures for six surface sulfate species. They found that acidic sulfate species are not as active as neutral ones, but acidic sulfate species could be converted to neutral ones when calcined (exposed to a high temperature) at 600 °C or neutralised. Brønsted acid amounts usually account for the catalytic activity of acid catalysts. But on TiO₂, Brønsted acid amounts do not correlate to the photocatalytic activity while Lewis acid sites promoted the activity. Their study provided insights on the mechanism involved in the enhanced photocatalytic activity of sulfated photocatalysts. It may also provide some clues to the reaction mechanisms of other superacid catalysts whose activities do not correlate to acid amounts well. The sulfated photocatalysts have the potential to be used for the effective treatment of wastewater from various sources, such as RO membrane concentrate, dyeing industries, farms, semiconductor industries, palm oil manufacturing plants, etc.

Figure shows a reverse osmosis membrane concentrate wastewater sample before and after photocatalytic treatment using the patented pending sulfated photocatalyts under a monochromatic UV lamp (365 nm; 100 W; 18 mW/cm²) for three hours.

Reference

Lin XH; Yin XJ; Liu JY; Li SFY*, “Elucidation of structures of surface sulfate species on sulfated titania and mechanism of improved activity” APPLIED CATALYSIS B” ENVIRONMENTAL, In Press, Corrected Proof, Available online 25 October 2016.