Photocatalytic degradation of an agro-industrial wastewater model compound using a UV LEDs system: kinetic study

An ultraviolet light emitting diode (UV-A LED) system was built to test the capability of performing heterogeneous photocatalysis using TiO₂ P25. The LEDs maximum wavelength is 365 nm with an irradiance power of 85 W m^-2. The device was tested in batch and continuous (CSTR) mode in a laboratorial scale reactor. The degradation of an agro-industrial wastewater model compound (p-hydroxybenzoic acid, pHBA) was investigated, assessing the effect of different experimental conditions such as pH, pHBA and TiO₂ concentration keeping constant the UV-A LEDs power and temperature. The photodegradation of different concentrations of pHBA with [TiO₂] = 500 mg L^-1, I_UV = 85 W m^-2 and a T = 21 °C were analysed by pseudo-first order kinetics. The results were applied to the Langmuir-Hinshelwood model yielding k_c = 0.885 mg L^-1 min^-1 and k_LH = 0.217 L mg^-1. In a comparative experiment the UV-A LEDs system showed faster kinetics (k = 0.0134 min^-1) than solar radiation (I_UV = 23 W m^-2; k = 0.0077 min^-1), with [pHBA] = 75 mg L^-1 and [TiO₂] = 500 mg L^-1. The values of the Electric Energy per Order (E_EO) = 115 kWh m^-3 order^-1 and the Specific Applied Energy (E_SAE) = 318 kWh mol^-1 order^-1 were obtained with [TiO₂] = 1000 mg L^-1 and [pHBA] = 50 mg L^-1. Analogous results were obtained ([TiO₂] = 500 mg L^-1) in a CSTR with a slight decrease in the first order kinetic constant due to the "non-ideal" reactor: from 0.0284 to 0.0158 min^-1 and from 0.0143 to 0.00825 min^-1 with [pHBA] = 50 mg L^-1 and 75 mg L^-1, respectively. This work shows that photocatalytic reactors with UV-A LEDs can advantageously replace conventional UV mercury lamps based reactors in the photodegradation of phenolic compounds.