Bentonite-Chitosan composites or beads for lead (Pb) adsorption: Design, preparation, and characterisation

This study investigated the efficiency of mixing bentonite with chitosan via different preparation methods, subsequently the different forms were investigated for their ability to remove Pb(II) ions from solution. The different forms of bentonite-chitosan (Bt-Ch), composites and beads, were prepared via solution blending and precipitation methods, respectively and in the weight ratios of 90%/10%, 70%/30% and 50%/50%. The beads were further subdivided, identified as “beads-A" and “beads-B", and were formed by adding either bentonite suspension or bentonite powder, respectively, to solubilised chitosan solution. The composites and beads were characterised by X-ray fluorescence (XRF), thermogravimetric analysis (TGA), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and pH at zero point charge. XRF analysis showed a cation exchange mechanism occurred when chitosan was initially mixed with the bentonite. TGA results confirmed that beads contained more chitosan compared to their corresponding weight ratio equivalent composites. XRD results showed that chitosan was intercalated within the interlayer space of the bentonite for Bt-Ch composites and Bt-Ch beads-A and that the interlayer spacings increased with increasing chitosan loading. Though similar amounts of chitosan were present in both Bt-Ch beads-A and beads-B, there were fewer reflection shifts for beads B suggesting less intercalation of chitosan when the bentonite was added as a powder. FTIR spectra from the Bt-Ch composites and beads confirmed the presence of both chitosan and bentonite, and the NH bands of chitosan shifted to lower frequencies demonstrating their involvement in the bonding mechanism of chitosan with bentonite. The experimental adsorption data correlated well with both non-linear Langmuir and Freundlich isotherm models, and in which both chemisorption and physisorption processes played crucial roles. The Langmuir-maximum adsorption capacities of Pb(II) ions for all the analysed Bt-Ch composites and beads was found to range from 42.48 ± 4.22 to 94.60 ± 5.63 mg/g. The amount of chitosan present in the adsorbent and its distribution within or outside the interlayer space of the bentonite was shown to have pronounced effects on the Pb(II) uptake by the different Bt-Ch composites/beads, and although the chitosan greatly enhanced the adsorption of Pb(II) a cation exchange mechanism with the clay was still a dominant process. The adsorption of Pb(II) was also significantly affected by the presence of other multi-competing ions. Moreover, the developed Bt-Ch composites/beads exhibited good potential for re-use after five cycles of regeneration, thus, indicating their potential as cost-effective adsorbents for removal of Pb(II) ions from both drinking and wastewater.