Role of Alkyl Chain Length in Surfactant-Induced Precipitation of Reactive Brilliant Blue KN-R

To develop a cost-effective method for the effective removal of reactive brilliant blue KN-R (RBB KN-R) from wastewater, we investigated the interactions between RBB KN-R and three cationic surfactants with different alkyl chain lengths, namely dodecyltrimethylammonium bromide (DTAB), tetradecyltrimethylammonium bromide (TTAB), and cetyltrimethylammonium bromide (CTAB). Employing a conductivity analysis, surface tension analysis, ultraviolet-visible spectrophotometry, and molecular dynamics simulation, we ascertained that RBB KN-R formed a 1:1 molar ratio dye-surfactant complex with each surfactant through electrostatic attraction. Notably, an augmentation in alkyl chain length correlated with increased binding strength between RBB KN-R and the surfactant. The resulting dye-surfactant complex exhibited heightened surface activity, enabling interactions through hydrophobic forces to generate dye-surfactant aggregates when the molar ratio was below 1:1. Within these mixed aggregates, self-assembly of RBB KN-R molecules occurred, leading to the formation of dye aggregates. Due to the improved hydrophobicity with increased alkyl chain length, TTAB and CTAB could encapsulate dye aggregates within the mixed aggregates, but DTAB could not. The RBB KN-R aggregates tended to distribute on the surface of the RBB KN-R-DTAB mixed aggregates, resulting in low stability. Thus, at a DTAB concentration lower than CMC, insoluble particles readily formed and separated from surfactant aggregates at an RBB KN-R and DTAB molar ratio of 1:4. Analyzing the RBB KN-R precipitate through scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) and measuring the DTAB concentration in the supernate revealed that, at this molar ratio, all RBB KN-R precipitated from the dye-surfactant mixed solution, with only 7.5 ± 0.5% of DTAB present in the precipitate. Furthermore, the removal ratio of RBB KN-R reached nearly 100% within a pH range of 1.0 to 9.0 and standing time of 6 h. The salt type and concentration did not significantly affect the precipitation process. Therefore, this simultaneous achievement of successful RBB KN-R removal and effective separation from DTAB underscores the efficacy of the proposed approach.