Kinetics of Photocatalytic Degradation of Textile Dyes using TiO2-Modified Biochar under Visible Light Irradiation

Photocatalytic advanced oxidation processes using titanium dioxide are constrained by UV light requirements and rapid electron-hole recombination, limiting their practical applicability for wastewater treatment in Nigeria where solar energy is abundant but UV intensity is variable. This study develops TiO2-modified biochar composites from palm kernel shell biochar sourced from Delta State and investigates the kinetics of photocatalytic degradation of methylene blue and reactive red 120 textile dyes under visible light irradiation. TiO2 was deposited onto biochar surfaces by a sol-gel method at loadings of 20, 40, and 60 weight percent and characterised by XRD, BET, DRS, FESEM, and XPS. Photocatalytic degradation experiments were conducted in a batch reactor under visible light at dye concentrations of 20 to 100 mg/L, catalyst doses of 0.5 to 3.0 g/L, pH 3 to 10, and irradiation time up to 180 minutes. The Langmuir-Hinshelwood kinetic model was fitted to concentration-time data to extract apparent rate constants. The 40 wt% composite achieved the highest degradation rates, attributed to improved visible light absorption through biochar-induced oxygen vacancy formation in TiO2 and reduced charge carrier recombination. Apparent rate constants for methylene blue and reactive red 120 were 0.0312 and 0.0218 per minute, representing 3.6-fold and 2.8-fold improvements over unmodified TiO2. TOC analysis confirmed 82.4% mineralisation of methylene blue at 180 minutes. The catalyst retained 89% activity over five cycles, confirming practical reusability. Keywords: TiO2-biochar composite, photocatalytic degradation, textile dye, visible light, kinetics