As early as in the 1910s, the Greek word “photocatalysis” was introduced: “photo” (phos) and “catalysis” (katalyo), which gives the meaning of breaking apart or decomposing using the light. In other words, it can be defined as the acceleration of a photoreaction with the presence of a catalyst. Semiconductor-mediated photocatalysis, also known as heterogeneous photocatalysis, is recognized as one of the best “green” technologies to confront the challenges of environmental pollution nowadays. Compared to other treatment techniques (e.g., flocculation, coagulation, membrane filtration, ion-exchange and adsorption), this advanced oxidation process has the excellent photocatalytic decomposition capacity to mineralize a variety of noxious pollutants completely and is free from the generation of secondary pollutants, thus no additional post-treatment of the sludge disposal is required.
Basically, the heterogeneous photocatalytic reaction involves oxidation and reduction in the presence of semiconductor photocatalyst, potential oxidizing agents (e.g., oxygen or air) and a light source. Within the last few decades, many metal oxide photocatalysts have been examined for their photocatalytic activity in the degradation of organic water pollutants. However, most of the practical applications and system design of commercial photocatalysts are mainly deteriorated by the enormous perpetual fatal limitation associated with the difficulty of recovery, and an additional separation process is required for their regeneration from the system. An efficient and ideal photocatalyst should be capable of maximizing the electron-hole transfer to the adsorbate, minimizing the recombination of photoexcited electron-hole pairs with high photo-stability, as well as is biologically and chemically inert, low-cost and good adsorptive behaviour to adsorb reactants under photonic activation.
The criteria for a novel photocatalyst have thus focused on improving the surface morphology with high surface area, crystalline structure, size and architectures for high carrier mobility, enhanced band gap with correct band gap positions and efficient visible light adsorption. Recent research was oriented towards immobilizing photocatalyst onto a new supporting surface. Considering the huge generation of agricultural biomass and their low economical value, aesthetic attention has been drawn to the innovative and sustainable utilization of ash as a renewable and low-cost precursor for catalyst preparation. Through a simple physical coating and hydrothermal process, the bio-ash (referred to as the solid residue produced by the combustion of biomass from a complete or incomplete burning process) supported nanocomposites could be developed. In judging the potential of a bio-ash as an efficient adsorbent, the specific surface area, compositions, and unburnt matters play an important role. To date, a different variety of bio-ash based photocatalysts, including rice husk ash, fly ash, volcanic ash, incense ash, durian shell ash and coffee residue ash based photocatalysts have been successfully developed (Debasish and Amitava, 2010; Surolia et al., 2010; Adam et al., 2013; Lum et al., 2020).
In line with the Green
Technology Master Plan Malaysia 2017-2030, the “Waste-to-Wealth” concept in
addressing the environmental problem has therefore created an alternative
strategy towards a sustainable zero-waste country.
Words by: Dr. Moon Wei Chek
References:
1. Adam, F., Appaturi,
J.N., Khanam, Z., Thankappan, R. and Nawi, M.A.M., 2013. Utilization of tin and
titanium incorporated rice husk silica nanocomposite as photocatalyst and
adsorbent for the removal of methylene blue in aqueous medium. Applied Surface
Science, 264, 718-726.
2. Debasish, S. and Amitava, B., 2010. Adsorptive mass transport of dye on rice husk ash. Journal of Water Resource and Protection, 2 (2010) 424.
3. Lum, P.T., Foo, K.Y., Zakaria, N.A. and Palaniandy, P., 2020. Ash based nanocomposites for photocatalytic degradation of textile dye pollutants: a review. Materials Chemistry and Physics, 241, 122405.
4. Surolia, P.K., Tayade,
R.J. and Jasra, R.V., 2010. TiO2-coated cenospheres as catalysts for
photocatalytic degradation of methylene blue, p-nitroaniline, n-decane, and
n-tridecane under solar irradiation. Industrial & Engineering Chemistry
Research, 49(19), 8908-8919.