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

摘要

An ultraviolet light emitting diode (UV-A LED) system was built to test the capability of performing heterogeneous photocatalysis using TiO_2 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 TiO_2 concentration keeping constant the UV-A LEDs power and temperature. The photodegradation of different concentrations of pHBA with [TiO_2] = 500 mg L~(-1), I_(UV) = 85 W m~(12) 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 [TiO_2] = 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 [TiO_2] = 1000 mg L~(-1) and [pHBA] = 50 mg L~(-1) Analogous results were obtained ([TiO_2] = 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.