Adsorptive removal of methyl orange dye in aqueous solutions using calix[4]arene-modified lead sulphide: insight into response surface methodology, isotherm and kinetic studies

The present study reports the adsorptive potential of an alternative and regenerative adsorbent, lead sulphide (PbS) modified with calix[4]arene towards a methyl orange (MO) dye solution. The chemical and morphological aspects of synthesised PbS nanoparticles and calix[4]arene-modified PbS were analysed by FESEM, EDX and FTIR, which indicated successful immobilisation of calix[4]arene on the PbS surface. The response surface methodology (RSM), based on central composite design (CCD), was conducted to evaluate the significant factors and to optimise the influence of various factors (adsorbent dosage, contact time and pH) on the adsorption of MO. Under optimal conditions (pH of 4, a dosage of 89.70 mg and a contact time of 129.76 min), the maximum adsorption of MO by calix[4]arene-modified PbS was observed. The MO uptake behaviour was well described by the Freundlich model and the pseudo-second-order kinetic model, indicating a multilayer chemical adsorption with a maximum adsorption capacity (q(max)) of 3.268 mg/g. The adsorption mechanism of MO dye on the surface of calix[4]arene-modified PbS can be attributed to various interactions such as dipole-dipole hydrogen bonding interactions, Yoshida hydrogen bonding, pi-pi interactions and electrostatic interaction. The results strongly demonstrated the use of novel adsorbents, calix[4]arene-modified PbS and a reusability strategy as an alternative adsorbent in MO dye removal.