Adsorption of bisphenol A (BPA) from aqueous solutions by carbon nanotubes: kinetic and equilibrium studies

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Contamination of water resources by bisphenol A (BPA) is considered as a critical environmental problem. In this study, the feasibility of BPA adsorption from aqueous solutions by single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs) was investigated in a batch system. In order to define the kinetic and isotherm of the reaction, specific experiments were performed at BPA concentrations of 5–50 and 2–50 mg/L, respectively. The BPA uptake by both adsorbents was found to be rapid; also, uptake reached to equilibrium in 60 min for all of the cases. The kinetic data of both adsorbents were also described by the saturation model (R2 > 0.99) as well as the pseudo-second-order rate equation (R2 > 0.99). Based on the saturation rate model, the maximum reaction rates (km) of BPA adsorption by the SWCNTs and MWCNTs were in the ranges of 0.023–0.089 mg/L min and 0.013–0.060 mg/L min, respectively. In the pH range of 3–11, the optimized condition of pH for BPA adsorption by both the adsorbents was 9.0. The isotherm data of the SWCNTs and MWCNTs were found to be in the best fitness with the Freundlich–Langmuir and Langmuir isotherm models (R2 > 0.99), respectively. According to the Langmuir model, the maximum adsorption capacities of BPA by the SWCNTs and MWCNTs were, respectively, 71 and 111 mg/g. Therefore, it can be concluded that the MWCNTs were more efficient adsorbent for BPA than the SWCNTs.