The deficiencies of the recently reported improved Hummers method for the synthesis of graphene oxide (GO), such as high reaction temperature (60 degrees C) and long reaction time (10 h), were successfully solved using a low-intensity ultrasonic bath for 30 min at 40 degrees C. Furthermore, compared to its conventional synthesis counterpart, a facile and fast, one-step ultrasonic method that excluded hydrazine hydrate was developed to synthesize reduced GO (rGO) from graphite (10 min, 50 degrees C) in the presence of hydrazine hydrate (rGO-C, 12 h, 90 degrees C). The adsorption characteristics of 2-chlorophenol (2-CP) from an aqueous solution were investigated using rGOs and GOs prepared by ultrasonic (rGO-Us/GO-Us) and conventional (rGO-C/GO-C) methods. Whereas 2-CP was completely removed with rGO-Us after 50 min, only 40% of 2-CP was eliminated with rGO-C. The maximum adsorption capacity of 2-CP calculated by the Langmuir model onto rGO-Us (208.67 mg/g) was much higher than that onto GO-Us (134.49 mg/g). In addition, the ultrasonic graphene adsorption capacities were much higher than the corresponding values of rGO-C (49.9 mg/g) and GO-C (32.06 mg/g). The enhanced adsorption for rGO-Us and GO-Us is attributed to their greater surface areas, excellent oxygenated groups for GO-Us and superior it-electron-rich matrix for rGO-Us, compared to other adsorbents. The adsorption of 2-CP on the rGO materials increased with increasing solution pH to a maximum around its pKa (pKa = 8.85), while the adsorption for the GO materials increased with decreasing solution pH. The adsorption mechanism proceeded via hydrogen bonding in neutral and acidic media, but via pi-pi electron donor-accepter (EDA) interactions between 2-CP and graphene materials in basic medium. The FTIR spectrum of GO-Us after adsorption indicates that the position and intensity of many peaks of GO-Us were affected due to the adsorption of different 2-CP groups at different pHs. (C) 2016 Elsevier Inc. All rights reserved.