It is necessary to provide sufficient contact time for the analytes and adsorbents to reach the adsorption equilibrium. As can be seen in Fig. 4d, the extraction efficiency was significantly enhanced with the extension of shaking time from 2 to 6 min, and then leveled off when further extends the shaking time. Satisfactory extraction recoveries of colorants in the range 88.3–93.0% were obtained at 6 min. These experimental data indicate that the adsorption equilibriums were achieved quickly, largely due to the high specific surface area and easy dispersibility of Fe3O4@PDA NPs in aqueous solution