pH-dependent sulfonamides adsorption by carbon nanotubes with different surface oxygen contents

• The pH effect on SDM, SMX and ST adsorption by CNTs was investigated.
• The neutral species (SA0) had great contribution to SAs adsorption.
• Kow of SAs changed with pH.
• A significant linear relationship was found between log Koc(pH) and log Kow(pH).

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 (Co, 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 (Kd+, Kd0, and Kd−) for different SAs species (SA+, SA0, and SA−) followed the order of Kd0 ≫ Kd+ > Kd−. The octanol-water partition coefficients (Kow(pH)) of SAs at various pHs were calculated, and changed with pH significantly. A good linear relationship between the normalized adsorption coefficient (log Koc(pH)) and log Kow(pH) was observed (log Koc(pH) = a log Kow(pH) + b) for SDM (n = 54, R2 = 0.970), SMX (n = 48, R2 = 0.973) and ST (n = 48, R2 = 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, R2 = 0.935). The results clearly reveal that the hydrophobic interactions are the main mechanism responsible for the pH-dependent SAs adsorption.

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