Decontamination of bisphenol A and Congo red dye from solution by using CTAB functionalised walnut shell

In this research, the eco-friendly cationic surfactant modified walnut shell (WNS-CTAB) was synthesised to enhance the uptake for bisphenol A (BPA) and Congo red (CR) from aqueous solution. The characterisation of WNS-CTAB was performed using Fourier-transform infrared (FTIR), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), etc. to know its physiochemical properties. The adsorption equilibrium results were best described by the Langmuir isotherm model, which confirmed the monolayer adsorption of the pollutant molecules onto the adsorbent's surface. The maximum monolayer adsorption quantity of WNS-CTAB was established to be 38.5 mg g^-1 for BPA and 104.4 mg g^-1 for CR at 303 K, respectively. Pseudo-second-order kinetic models described the adsorption kinetics of both BPA and CR. Furthermore, the intra-particle diffusion was applied to analyse the kinetic results and was established that the rate was not solely controlled by diffusion. The mechanisms associated with BPA and CR adsorption onto the WNS-CTAB may include van der Waals interaction, hydrophobic interaction, and electrostatic force. WNS-CTAB demonstrated a good reusability potential with desorption through three successive adsorption-desorption cycles performed in both experiments. Moreover, in the binary system, the adsorption capacity of BPA witnessed a 66% decrease while CR saw marginal reduction of 8.0 %. This suggests that WNS-CTAB had a higher affinity for binding to CR with higher selectivity as compared with BPA. Therefore, WNS-CTAB has exhibited huge potential to serve as a functional material for practical use in the treatment of wastewater.