Facile synthesis of Ag/AgCI/BiOCI composite Z-scheme photocatalyst for visible-light-driven pollutant removal

Abstract

Degradation of organic contaminants in wastewaters emanating from industrial processing plants could render the water streams reusable for the purpose of reducing water consumption while protecting the environment from harmful pollutants. Organic pollutants can be removed from water using biological processes that mineralise the organics to H2O and CO2. However, mineralisation by biological processes take a long time and in many cases, total mineralisation is impossible to achieve. Alternatively, organics can be completely degraded and mineralised rapidly using chemical and/or photocatalytic advanced oxidation processes (AOP). Both systems have some short comings. In chemical AOP such as Fenton and photo-Fenton reagents, the chemical agents used remain in the water as pollutants requiring further removal. In photocatalytic oxidation processes, most current technologies use UV light as an energy source. The chemical processes are environmentally incompatible, whereas, the “green” photocatalysis is extremely expensive due to the consumption of electricity by high pressure UV light. Forerunner investigators of photocatalysis utilised TiO2 as the photocatalyst of choice. It has major drawbacks of which the most important one is that it is only activated under ultraviolet (UV) light irradiation. This high energy consumption made the process practically unfeasible. Solar energy (natural light and heat from sun) has great prospects with regards to acting as a substitute for UV since it is a renewable and cheaper energy source. This work therefore investigated the development of a heterogeneous all-solid-state Z-scheme silver/ silver chloride/ bismuth oxychloride (Ag/AgCl/BiOCl) photocatalyst that is able to utilise natural light through being activated by visible light irradiation. This will successfully serve as a green alternative in the use of renewable energy for pollution reduction while saving energy. The synthesised photocatalysts were characterised using various techniques. The purity and crystallinity of the synthesised photocatalysts were determined using x-ray diffraction (XRD) while x-ray photoelectron spectroscopy (XPS) was used to determine the elemental composition and chemical states present in the synthesised catalysts as well as confirm the presence of elemental Ag. Fourier-transform infrared spectroscopy (FTIR) specified the functional groups present while the morphology and chemical composition were determined on a scanning electron microscopy (SEM)/ energy dispersive x-ray spectroscopy (EDS). The surface area and pore size were measured on a Brunauer-Emmett-Teller (BET) and thermogravimetric analysis (TGA) was done to determine the thermal degradation of synthesised particles. Ultraviolet-visible spectroscopy (UV-VIS) was done to determine the photoabsorption range and bandgap of the particles as efficiency of photocatalysis is dependent on the properties and morphology of the semiconductor material. Degradation studies were carried out under both visible and UV light irradiation in a batch reactor. The activity of the synthesised Ag/AgCl/BiOCl photocatalyst was compared to that of commonly used TiO2. Specifically, while 60% degradation was achieved under UV light irradiation by both TiO2 and Ag/AgCl/BiOCl photocatalyst, in visible light irradiation, TiO2 measures only 14% in 4 h while Ag/AgCl/BiOCl measures a photodegradation efficiency of 53%. Other factors such as initial organic contaminants concentration, initial catalyst concentration, pH effects and individual compounds effect were also investigated. The reusability of the catalyst was also reported showing stability of the synthesised catalyst as after a total irradiation time of 48 h, 65% phenol degradation was measured. The phenol degradation kinetics were found to fit the widely used first-order Langmuir-Hinshelwood model. The result from the current study proves the feasibility of a novel process for mineralisation of organic compounds in water under cost effective visible light irradiation for the removal of recalcitrant and refractory organics from water.