Acridine yellow as solar photocatalyst for enhancing biodegradability and eliminating ferulic acid as model pollutant

The possibility of using acridine yellow G (AYG) as solar photocatalyst for wastewater treatment has been examined in this paper. A phenolic compound, namely ferulic acid, has been employed as target pollutant. The effect of pH, concentration of the substrate and photocatalyst has been investigated. Control of pH is critical in the process, as rate constants obtained at pH 3 (k = 0.020 min−1) were one order of magnitude higher than in basic media (k = 0.002 min−1 at pH 9), due to differences in the absorption spectrum in the UVA–vis region. Under acidic conditions, 80% removal of the substrate was achieved after 3 h irradiation, although TOC decrease was moderate (around 20%). Nevertheless important detoxification of the solution was measured, and the remaining organic matter showed an enhanced biodegradability. For this reason, a combination of AYG-driven solar photocatalysis with biological treatment seems a good approach to deal with these effluents. Experimental data are consistent with an electron transfer mechanism between the excited photocatalyst and the substrate: involvement of hydroxyl radicals can be ruled out, and photophysical measurements indicate a quenching of the fluorescence of AYG in the presence of ferulic acid. The rate constant for this process was obtained from the Stern–Volmer equation (kq = 4.4 × 109 M−1 s−1). Finally, based on the Rehm–Weller equation, a ΔG = −22.8 kcal/mol was calculated, indicating that the process is thermodynamically favourable.