Synthesis and application of magnesium-based nanoparticles for photocatalytic degradation of methylene blue in aqueous solution: Optimization and kinetic modelling

Methylene blue (MB) is one of the main dyes consumed in the textile industry, and its release into the marine environment is accompanied by the deterioration of aquatic organisms. Subjecting humans to high dosages of MB causes severe health concerns, including toxic, mutagenic, and/or carcinogenic impacts. The treatment of wastewater containing dyes using conventional methods is becoming less efficient due to the complexity and stability of these synthesized dyes. This study focuses on the application of heterogeneous photocatalysis (semi-conductor photocatalysis), as an advanced treatment technique, for the degradation of MB dye in water. Herein, magnesium-based nanoparticles were synthesized using a simple co-precipitation technique. The characterization of the nanoparticles revealed that they had a relatively spherical structure with a particle size ranging from 20 to 70 nm. BET analysis showed mesoporous pore diameters in the 2.5-12 nm range. The nanoparticles had a relatively high surface area of about 116.55 m²/g. XRD analysis revealed a cubic crystalline structure of the photocatalyst material and a relatively high crystallinity index of about 63%. The band gap of the material was investigated and found to be about 4.43 eV which was sufficient for electron excitation under solar-simulated UV irradiation. Further, magnesium-based nanoparticles were applied for the photocatalytic degradation of MB at an initial concentration of 11 mg/L using a catalyst dosage of 0.5 g/L. The photocatalytic degradation experiments were carried out in a photo-reactor, emitting solar-simulated visible light with a wavelength of 510 nm. The degradation of MB was carried out at neutral pH, achieving 86.2±4.31% MB removal after 3 hours of photocatalytic activity. The degradation kinetics of the reaction were studied, and it was found that the degradation route followed pseudo-first-order kinetics with a rate constant of k₁ = 0.0104 min^-1. It’s supposed that MB was removed via both (i) adsorption onto magnesium-based nanoparticles followed by (ii) photodegradation into less toxic by-products as a result of the formation of super-oxide (O₂^•ˉ) and hydroxyl (OH^•) radicals responsible for the efficient degradation of MB dye. The electrical energy consumption was evaluated and achieved at 154 kWh/m³, equivalent to an operating cost of 14.3 USD/m³. The removal of MB by the manufactured magnesium-based nanoparticles showed good agreement with other photocatalytic degradation mechanisms reported in the literature.