A facile hydrothermal method is used to synthesize manganese ferrite nanoparticles with the addition of poly acrylic acid (PM). PM addition does not largely decrease the magnetic performance of MnFe2O4 nanoparticles, but highly enhance the adsorption properties for methylene blue (MB) dye. It demonstrates that the concentration of negative-COO- groups in the solution determines the adsorption capacity of MnFe2O4 nanoparticles for MB. The maximum MB adsorption capacity was 44.9 mg/g, and the adsorption kinetics of MB match well with the pseudo-second-order model. To reveal the influence mechanism of PAA addition on the properties of manganese ferrite nanoparticles, two different synthetic routes are designed by changing the adding method of PAA with the different mass ratio of PAA to MnFe2O4. Here, the second synthetic route is easier and simpler to operate than the first one. Moreover, with the PM addition, the adsorption capacity for the samples synthesized by synthetic route 2 increases by 43.4%, which is much higher than that (28.6%) of the samples synthesized by synthetic route 1. Synthetic route 1 reveals that PAA acting as a reducing agent participates in the reaction on the formation of MnFe2O4 ferrites. Herein, the Fe3+ ions bound by PM chains are reduced to Fe2+ ions. This work clearly points out that the PM addition contributes to enhancing the adsorption properties of ferrite nanoparticles for organic dye, which provides guidance to the adsorption study on the similar adsorbents for the removal of organic contaminant from the wastewater. (C) 2016 Elsevier B.V. All rights reserved.