BACKGROUND The contribution of persulfate oxidation to water treatment technologies based on advanced oxidation processes is nowadays widely accepted. However, the importance of the water matrix on the removal of micropollutants has been neglected during early screening of heterogeneous catalysts, which can hinder further developments in the field. RESULTS Three magnetic graphitic nanocomposites (MGNCs), obtained by carbon encapsulation of magnetite (Fe3O4), nickel ferrite (NiFe2O4) and cobalt ferrite (CoFe2O4), were prepared and employed as heterogeneous catalysts in the activation of persulfate for the degradation of antibiotic sulfamethoxazole (SMX). The importance of the water matrix and its constituents on the performance of the MGNC materials was assessed. It was found that their relative performance in terms of oxidation of SMX in pure water decreases in the order: NiFe2O4/MGNC > Fe3O4/MGNC > CoFe2O4/MGNC. However, the order changed (Fe3O4/MGNC > NiFe2O4/MGNC > CoFe2O4/MGNC) when drinking water or secondary treated wastewater were employed. The individual effects of the water matrix constituents, such as bicarbonate, chloride, pH, and humic acid, or the use of added radical scavengers, on the catalyst performance were investigated, while the heterogeneous nature of the process was confirmed through leaching tests. CONCLUSION A realistic water matrix should be considered throughout all the steps involved in the evaluation of new heterogeneous catalysts for the activated persulfate oxidation of SMX. Otherwise, misleading conclusions may be drawn, limiting potential improvements of this treatment technology. (c) 2019 Society of Chemical Industry