A set of core(ferrite, FeO)-shell(carbon, C) composites, [email protected], [email protected], [email protected], and carbon nanotubes (CNT) impregnated with 2% of Fe ([email protected]%Fe) were prepared. The different composites were tested as redox mediators (RM) in the biological reduction of the azo dye Acid Orange 10 (AO10). Materials were tested at different concentrations from 0.1 to 1.0 g L⁻¹. In the absence of RM, the AO10 decolourisation after 24 h of reaction was only 30% at a rate of 0.2 d⁻¹. In the presence of the core-shell composites, better results were obtained with [email protected] materials at the amount of 1.0 g L⁻¹. The extent of AO10 decolourisation was above 90% and rate improved circa 29-fold. With [email protected]%Fe, the best efficiency (98 ± 3%) was achieved with 0.5 g L⁻¹ leading to a 79-fold rate increase. In abiotic controls, though at lower extent, the reduction of the dyes also occurred likely due to the electron transfer from Fe²⁺ to carbon and then to the dye. To prove this, assays combining single CNT and FeO materials were performed, and FeO had effect in the reaction only when combined with CNT. In the biological assay, the rate was the double and the percentage of decolourisation increased from (88 ± 6)% to (97 ± 1)%, when both materials were present in solution as compared with results in the presence of CNT alone. Under abiotic conditions, decolourisation occurred only in the presence of both materials, with a final percentage of (54 ± 2)%. Owing to their magnetic character, materials were removed from the media and successfully applied in successive cycles.