Kapok fibers modified with cationic surfactants: Structural insights and efficient removal of Cr(VI) and bisphenol A

In this study, kapok fiber (KF) a hollow and hydrophobic fiber, was modified with cetyltrimethylammonium bromide (CTAB) or cetylpyridinium chloride (CPC), rendering adsorbed amount of ∼0.75 × 10^-3 mol/g. Small-angle X-ray scattering (SAXS) measurements of dry KF/CTAB and KF/CPC evidenced a periodic distance of ∼2.6 nm and 2.8 nm, respectively, suggesting the presence of hemimicelles on the surface. KF/CTAB and KF/CPC were used as adsorbents in batch and column adsorption experiments to remove Cr(VI) ions, Bisphenol A (BPA), and their binary mixtures from synthetic solution and fresh water. The adsorbed amounts of Cr(VI) ions on KF/CTAB and KF/CPC, as determined from batch experiments, were 48.62 mg/g and 34.17 mg/g, respectively. X-ray photoelectron spectroscopy (XPS) analysis showed that Cr(VI) adsorption on KF/CTAB involved bromide displacement, while chloride remained on KF/CPC. Moreover, Cr(VI) ions were reduced to Cr(III) ions due to a possible oxidation of γ-sitosterol, one component of the KF wax. Density Functional Theory (DFT) calculations indicated that the interaction energy of CTAB- Cr(VI) pair (-167.8 kcal/mol) is more favorable than that of the CPC-Cr(VI) pair (-147.8 kcal/mol). The adsorbed amounts of BPA on KF/CTAB and KF/CPC were 41.66 mg/g and 22.62 mg/g, respectively. XPS analysis indicated the appearance of an OH peak at 533 eV after the adsorption of BPA, aligning with DFT calculations that predicted interactions between the counter-ions (Br or Cl) and BPA hydroxy groups. In column adsorption experiments, Cr(VI) ions were more effectively adsorbed onto KF/CTAB in the presence of BPA, demonstrating the potential of KF/CTAB for the simultaneous remediation of mixed contaminants in water treatment.