Original Investigation of Nanocomposite Photocatalyst Systems for Simultaneous Removal of Heavy Metals and Organic Dyes from Textile Effluent: Mechanistic Pathways and Scale-Up Framework

Simultaneous treatment of inorganic heavy metals and organic dye co-pollutants in textile effluent requires advanced photocatalytic systems capable of mediating both reductive and oxidative chemistry under practical operating conditions. This dissertation presents an original investigation of novel nanocomposite photocatalysts comprising graphitic carbon nitride coupled with bismuth vanadate and iron oxide nanoparticles for visible-light-driven simultaneous removal of chromium(VI) and reactive blue dye from simulated and real textile effluent. Nanocomposite synthesis was accomplished by hydrothermal method, and structural, optical, and electronic characterisation was performed by high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, UV-vis diffuse reflectance spectroscopy, photoluminescence spectroscopy, and Mott-Schottky analysis. Density functional theory calculations were applied to predict band alignment and charge transfer mechanisms at heterointerface junctions. Reactive oxygen species identification was achieved by electron paramagnetic resonance spectroscopy and quenching experiments. The photoreduction mechanism of chromium(VI) was traced by X-ray absorption near-edge structure spectroscopy. A continuous flow photoreactor was designed and fabricated, and scale-up kinetics were modelled using the Langmuir-Hinshelwood framework with mass transfer corrections. Real effluent from a textile mill in Kano State was used for final validation experiments. Results show that the ternary nanocomposite achieved 99.6 percent chromium(VI) reduction and 97.2 percent dye mineralisation simultaneously under simulated solar irradiation within 90 minutes. Continuous flow photoreactor throughput achieved 50 litres per hour at greater than 90 percent efficiency. This dissertation advances knowledge in photoredox catalysis and provides a practically validated scale-up framework for industrial adoption. Keywords: nanocomposite photocatalyst, carbon nitride, bismuth vanadate, chromium, textile effluent