Photocatalytic degradation of 2,4-dichlorophenol using silver halides catalysts

Abstract

Chlorophenols are classified as the most widespread and largest group of phenols. The presence of chlorophenols in the environment is related to the use and degradation of organic compounds including pesticides, phenoxyherbicides as well as phenolic biocides. The most popular chlorophenols such as 2,4-dichlorophenoxyacetic acid (2,4-D), 4-chloro-2-methylphenoxyacetic acid (MCPA) and 2,4,5-trichloro-phenoxyacetic acid (2,4,5-T) tend to produce phenol, 2-chlorophenol (2-CP) and 2,4-dichlorophebol (2,4-DCP) as byproducts. 2,4-DCP has been listed as a priority pollutant by the US Environmental Protection Agency (USEPA) due to its high toxicity, carcinogenicity, bioaccumulation and mutagenicity to living organisms. Conventional and advanced treatment strategies have been employed in preventing the spread of 2,4-DCP in the natural environment. However, conventional treatment methods generate large amounts of secondary pollutants, where as, advanced treatments tend to bear a very high operational and energy cost. There is therefore an urgent need for the scientific community to develop cost effective and environmentally friendly treatment processes for water and wastewater contaminated with 2,4-DCP and other aromatic organic co-pollutants. In one of the fastest growing technologies, photocatalysis is an advanced oxidation process that has the potential of degrading recalcitrant organic contaminants in water. This treatment process uses a semiconductor material as a catalyst which is irradiated using a light source (UV or visible light) to produce highly reactive •OH or O2• free radicals. The most commonly used experimental photocatalyst is titanium dioxide (TiO2) due to its semiconductor properties, chemical stability, non-toxic and low cost. The biggest limitation of TiO2 is its limited response to low output sources such solar light. Titanium dioxide is most effectively activated under ultraviolet (UV) light. The high energy consumption makes this process impracticable. Solar radiation is an integral part of renewable energy resources and a cheaper alternative energy source. On the other hand, processes that utilise the visible light range of solar radiation are desirable due the fact that solar radiation is abundant and cheap. In this study, a visible-light wavelength driven photocatalyst is developed based on the knowledge that inclusion of silver-halide complexes AgX (X=Cl, Br, I) can reduce the band-gap energy required to excite electrons and move them to hypothetical conduction band. Highly efficient silver halide (Ag/AgX where X= Cl, Br, I), photocatalysts were successfully synthesized through a hydrothermal method. The prepared samples were characterized using a range of techniques such as X-ray diffraction (XRD), X-ray fluorescence (XRF), scanning electron microscopy (SEM) and Brunauer-Emmett-Teller (BET). All compounds were confirmed to be pure with the associated planes being identified. XRD patterns of Ag/AgBr photocatalyst presented cubic phase AgBr. The presented diffraction peaks are sharp and intense, indicating the high degree of crystallinity of the Ag/AgX species. SEM presented agglomerated and non-uniformly distributed particles of the prepared catalysts, as a result of surfactant-free precipitation reactions in aqueous media. X-ray photoelectron spectroscopy (XPS) confirmed the presence element Ag on the surface of AgX (X = Cl, Br, I). The photocatalytic activity of these photocatalysts were evaluated through the degradation of 2,4-dichlorophenol (2,4-DCP) under UV and visible light irradiation. Variant photocatalytic efficiency was exhibited dependent on the material used. The Ag/AgBr photocatalysts exhibited the best efficiency among all three catalysts, resulting in an 83.37 % and 89.39 % photodegradation under UV and visible light after 5 h, respectively, at a catalyst loading of 0.5 g L⁻¹. Factors such as initial catalysts loading, pH effects and the initial organic contaminants concentration were also investigated. The reusability of the Ag/AgBr was evaluated and displayed a reduced stability after 5 cycles of irradiation. The photocatalytic capacity of Ag/AgBr decreased by 50 % after 5 cycles The 2,4-DCP degradation kinetics were determined to fit the pseudo-first order Langmuir-Hinshelwood model. The results from this study indicate the feasibility of utilizing Ag/AgX under visible light irradiation for the removal and mineralization of organic compounds for water.