Effects of some carboxylic acids on the Fe(III)/UVA photocatalytic oxidation of muconic acid in water

Aqueous solutions of muconic acid, taken as model compound, have been exposed to UVA radiation in the presence of Fe(III) and some carboxylic acids: citric, maleic, malic, malonic, succinic, tartaric and oxalic acids. Both black light lamps and solar light were used as a source of UVA radiation. Carboxylic acids studied can be classified into three groups according to the formation or not of photocatalytic or non-photocatalytic active ferricarboxylate complexes. Only carboxylic acids forming ferricarboxylate complexes that absorb in the UVA region are able to improve the photocatalytic oxidation rate of muconic acid since their photolysis likely yields free radicals that act as oxidants. Thus, citric, tartaric, malic and oxalic acids form photoactive ferricarboxylate complexes in the UVA region while malonic acid forms complexes that do not absorb light above 320 nm. Finally, succinic and maleic acids do not form complexes and, as a consequence, their presence do not improve muconic acid oxidation rate. Also, the presence of photoactive ferricarboxylate complexes improves the mineralization rate. Thus, working with black light, after 2 h reaction, TOC conversion increases from about 25% in the absence of carboxylate complexes up to approx. 48 and 75% in the presence of tartrate and oxalate complexes, respectively. The results are even better when solar light is used since total mineralization is reached after 2 h treatment. In this case, a linear correlation between muconic acid half life and 365 nm average intensity of solar radiance was observed. Fe(III)/UVA photocatalytic oxidation resulted to be more effective than TiO2/UVA photocatalytic oxidation at the conditions applied in this work.