The presence of sulfonamides (SAs) in aquatic environments has drawn increasing concern in recent years, but their environmental behaviors still remain unclear. This study investigated pH (1.0-11.0) effect on sulfadimethoxine (SDM), sulfamethoxazole (SMX) and sulfathiazole (ST) adsorption by multi-walled carbon nanotubes (CNTs) with various surface oxygen contents (C-o, 2.88-7.65%). The results showed that the adsorption was significantly affected by pH due to the change of SAs species. The calculated adsorption coefficients (K-d(+), K-d(0), and K-d(-)) for different SAs species (SA(+), SA(0), and SA(-)) followed the order of K-d(0) >> K-d(+) > K-d(-). The octanol-water partition coefficients (K-ow(pH)) of SAs at various pHs were calculated, and changed with pH significantly. A good linear relationship between the normalized adsorption coefficient (logK(oc)(pH)) and logK(ow)(pH) was observed (logK(oc)(pH) = a logK(ow)(pH) + b) for SDM (n = 54, R-2 = 0.970), SMX (n = 48, R-2 = 0.973) and ST (n = 48, R-2 = 0.968) adsorption by CNTs. Moreover, both a and b decreased linearly with increasing Co. The relationship can be validated externally using adsorption data of SMX in the literature (n = 46, R-2 = 0.935). The results clearly reveal that the hydrophobic interactions are the main mechanism responsible for the pH-dependent SAs adsorption. (C) 2015 Elsevier B.V. All rights reserved.