Sulfonated humic acid (SHA) is a representative organic pollutant in oilfield wastewater. This study investigated the performance of modified fly ash in removal of SHA from aqueous solution. Two types of raw fly ash, including Shand fly ash (class C) and BD fly ash (class F) were used. X-ray diffraction and surface area analyses were also implemented to characterize the change of fly ash after modification. The results showed suitable acid-and microwave-assisted modification could enhance adsorption capacity of fly ash. The efficiency for SHA removal by treated Shand fly ash was higher than that by treated BD fly ash under the same conditions. Among different factors, pH was the most critical factor which could influence the adsorption performance of treated fly ash. The SHA removal efficiency increased as modified fly ash dosage increased up to 1 and 6 g/L for Shand and BD fly ash, respectively. Langmuir isotherm could better fit equilibrium data. Kinetic analysis results showed this process could be well described by pseudo-second-order model. There was higher adsorption capacity at higher temperature when temperature varied between 15 and 35 degrees C. The addition of NaCl also had influence on the adsorption performance of modified fly ash. The results of this study can help reveal the interactions of SHA pollutant and fly ash characteristics, which have important implications for the application of adsorption technologies in the treatment of oilfield wastewater. (C) 2015 Elsevier Ltd. All rights reserved.