Influence of water matrix on copper-catalysed continuous ozonation and related ecotoxicity

• Copper-catalysed continuous ozonation significantly improve carboxylic acids mineralization degree.
• Operational copper concentration does not present remarkable aquatic toxicity.
• Water matrix exerts a huge influence in copper catalyst performance.
• Water matrix exerts a huge influence in copper aquatic toxicity.

The continuous ozonation of a mixture of carboxylic acids (formic, acetic, oxalic and maleic) has been performed under non-catalytic and copper-catalysed ozonation using a synthetic water matrix and a real sewage treatment plant (STP) effluent. The aim was to study the effect of water matrix on catalytic performance, particularly considering the toxicity of treated water to aquatic organisms. The non-catalytic ozonation of carboxylic acids in synthetic water resulted in a low reduction (36%) of the total organic carbon (TOC), the main feature being the accumulation oxalic acid due to the partial oxidation of maleic acid. Catalytic ozonation, adding copper concentration of 20 μg L−1, achieved a TOC reduction of 75%, mainly due to the total depletion of oxalic acid. In wastewater effluent, the same general pattern was found with oxalic acid as the main by-product and its almost complete removal in catalytic ozonation. However, to attain the latter it was necessary to use copper concentrations as high as 100 μg L−1. Copper proved to be a good catalyst for the oxidation of oxalic at near neutral pH, with short reaction times and matrix with high scavenging rate. The aquatic toxicity of treated mixtures was studied by means of five standard species placed on different trophic levels: Vibrio fischeri, Pseudomonas putida, Pseudokirchneriella subcapitata, Tetrahymena thermophila and Daphnia magna. The results showed that copper in STP effluent was less toxic than in synthetic water, an effect attributed to copper complexation with organic and inorganic compounds present in the matrix. The reduced biological availability could also explain the lower catalytic effect observed in real wastewater.

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