The study was focused on the simultaneous removal of Methylene blue (MB) and Safranin-O (SO), as most together hazardous dye in wastewater, from aqueous solution by ultra-sonic-assisted adsorption onto the developed adsorbent i.e. Fe3O4 NPs. Fe3O4 NPs were synthesized, by using central composite design (CCD), the effects of variables such as pH, adsorbent dosage, MB concentration, SO concentration and contact time on binary dyes removal-were examined and optimized values were found to be 6, 0.01 g, 10 and 20 mg/L and 2 min, respectively. The very rapid removal of dyes makes this novel adsorbent as a promising tool for wastewater management applications. Different adsorption isotherm model i.e. Langmuir, Freundlich, non-modified Langmuir, modified competitive Langmuir, modified extended Langmuir, extended Freundlich and sips models are studied out of which extended-Freundlich model proved to be the best fit to equilibrium data because of correlation value (R-2) is 0.99 which is very close to 1, additionally it depicts the high adsorption capacity around (89.2-91.9 mg g(-1)). An empirical extension of competitive extended-Freundlich model was proposed to predict the simultaneous adsorption behavior of MB and SO. It was observed that pseudo-second-order equation is suitable to fit the experimental data. (C) 2014 Elsevier B.V. All rights reserved.