Oxidation of small aromatic compounds in rainwater by UV/H₂O₂: Optimization by response surface methodology

The presence of aromatic compounds in rainwater is a matter of concern, mainly when the use of rainwater in buildings is intended. The present work aimed to assess the oxidation of a mixture of small aromatic compounds (benzoic, 3,5-dihydroxybenzoic and syringic acids) in rainwater by the UV/H₂O₂ process, and the possibility of its optimization by the response surface methodology. The extent of oxidation was assessed by ultraviolet-visible and molecular fluorescence spectroscopies. During the oxidation of the mixture new chromophoric compounds were formed at an initial stage, but they were then degraded at a later stage. The increase of the H₂O₂ concentration, resulted in a higher extent of oxidation, while the initial pH value showed no influence in the oxidation of the mixture. The optimization of the oxidation was performed using the uniform design with the factors: initial H₂O₂ concentration, initial pH, and reaction time. The response surface model found, through the best subsets regression, described the extent of oxidation as function of the following variables: initial H₂O₂ concentration and reaction time, interaction between them, and also their respective quadratic forms. The optimal conditions, the lowest H₂O₂ concentration (3.1 mM) for a selected maximum reaction time (4 h), were applied to rainwater samples spiked with the mixture of contaminants and resulted in an extent of oxidation higher than 99.5%, validating the application of the model to real samples. Therefore, the UV/H₂O₂ process coupled to its optimization via response surface methodology may be an alternative for rainwater treatment in buildings.