Solar-driven photocatalytic degradation of gallic acid using TiO₂: kinetics, optimization, and catalyst regeneration

Abstract

This study explores the photocatalytic degradation of gallic acid (GA) using TiO₂ under natural sunlight. Initial adsorption tests conducted in the absence of light determined that a 30-min pre-adsorption phase was necessary prior to initiating photocatalysis. Sunlight alone demonstrated minimal degradation of GA, but the presence of TiO₂ significantly accelerated the process, achieving a 96% degradation rate within 120 min. The optimal TiO₂ concentration was identified as 1 g/L. The influence of pH was also investigated, with gallic acid’s natural pH (4.6) yielding the highest degradation efficiency. At initial GA concentrations of 0.1 and 0.2 mM, degradation was nearly complete within 60 and 120 min, respectively, while higher concentrations adversely impacted performance. The degradation process followed pseudo-first-order kinetics, with high correlation coefficients (R² > 0.95). A representative rate constant (k_obs) of 0.03 min⁻¹ was obtained at initial gallic acid concentration of 0.2 mM. Catalyst regeneration studies revealed that simple water washing retained 74% of catalytic efficiency after five cycles, outperforming HCl and NaOH treatments. XRD and SEM analyses confirmed the preservation of TiO₂’s structural integrity post-regeneration.

Graphical abstract