Photocatalytic performance of PANI modified TiO₂: Degradation of refractory organic matter

Surface modification of TiO₂ with polyaniline (PANI) conducting polymer has been used to achieve visible light photoresponse, thereby increasing solar photocatalytic activity. In this study, photocatalytic performances of PANI-TiO₂ composites with different mole ratios were synthesized by the in situ chemical oxidation polymerization method and tested for the degradation of a model refractory organic matter (RfOM), namely humic acid in an aqueous medium under simulated solar irradiation in a comparative manner. Adsorptive interactions under dark conditions and interactions under irradiation were investigated as contributing factors to photocatalysis. Degradation of RfOM was monitored in terms of UV-vis parameters (Color₄₃₆, UV₃₆₅, UV₂₈₀, and UV₂₅₄) and fluorescence spectroscopic parameters as well as the mineralization extent by dissolved organic carbon contents. The presence of PANI exerted an enhancement in photocatalytic degradation efficiency compared to pristine TiO₂. The synergistic effect was more pronounced in lower PANI ratios whereas higher PANI ratios reflected a retardation effect. Degradation kinetics were assessed by pseudo-first-order kinetic model. For all UV-vis parameters analyzed, highest and lowest rate constants (k) were attained in the presence of PT-14 (2.093 × 10^-2 to 2.750 × 10^-2 min^-1) and PT-81 (5.47 × 10^-3 to 8.52 × 10^-3 min^-1), respectively. Variations in selected absorbance quotients, i.e., A₂₅₄/A₄₃₆, A₂₈₀/A₄₃₆, and A₂₅₃/A₂₀₃, were distinctive and compared with respect to irradiation time and photocatalyst type. Upon use of PT-14, a steady decreasing profile with respect to irradiation time was attained for A₂₅₃/A₂₀₃ quotient as 0.76-0.61, followed by a rapid decrease to 0.19 in 120 min. The incorporation effect of PANI into TiO₂ composite could be visualized in A₂₈₀/A₃₆₅ and A₂₅₄/A₃₆₅ quotients exhibiting an almost constant and parallel trend. As a general trend, decrease in the major fluorophoric intensity FI_syn,470 with photocatalysis was observed under extended irradiation conditions; however, an abrupt decline was remarkable in the presence of PT-14 and PT-18. Fluorescence intensity decrease correlated well with spectroscopic evaluation of rate constants. A thorough evaluation of spectroscopic parameters of UV-vis and fluorescence can provide significant information for practical applications in control of RfOM in water treatment.