Distinctive sorption mechanisms of 4-chlorophenol with black carbons as elucidated by different pH

► Surface-area normalized sorption capacity of rice straw biochar is higher than soot. ► Rice straw biochar can sorb 4-chlorophenol via electrostatic attraction. ► Sorption capacity of rice straw biochar significantly decreased from pH 10 to pH 4. ► An endothermic sorption occurred at pH 10 due to the electrostatic attraction. ► The hydrophobic interactions dominate the sorption process at pH 4.