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 Wm(-2) and a T = 21 degrees 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 (Iuv = 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.