Remediation of heavy metals from water using Modified Clay-Chitosan Composites

Abstract

Water contamination with pollutants such as heavy metals is of significant concern because the toxicity exhibited by these metals can pose a severe threat to human health and the environment. Adsorption with the combined use of geological materials (such as clays) and biodegradable polymers (such as chitosan) seems to be a promising technique for purification of water contaminated with toxic metals. These materials not only provide a cost-benefit but are also materials derived from sustainable sources and are thus environmentally friendly. This study focuses on the preparation of functional composites (from bentonite clay and chitosan biopolymer) as cheaper, and sustainable adsorbents for effective removal of heavy metals from water. The work carried out in this thesis investigated the efficiency of mixing bentonite with two different chitosans via different preparation methods. The different forms of bentonite-chitosan (Bt-Ch), composites and beads, were prepared in the weight ratios of 90%/10%, 70%/30% and 50%/50%, and via solution blending and precipitation methods, respectively. 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 thermogravimetric analysis (TGA), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR). Subsequently, the different forms of the prepared clay-chitosan composites were investigated for their ability to remove metal ions (Pb, Pb-Cu and As) from aqueous solutions. Batch adsorption procedures (via statistical design of experiments) were used to study the removal of these metal ions. Results showed that Pb (or Pb-Cu) ions can be removed from aqueous solutions effectively. The amount of chitosan present in the adsorbent and its distribution within or outside the interlayer space of the bentonite clay was shown to have pronounced effects on the Pb (or Pb-Cu) uptake by these clay-chitosan composites, and the adsorption of Pb was significantly affected by the presence of other multi-competing ions. However, these composites showed a poor loading capacity (very low removal efficiencies) towards As ions. This led to modification of bentonite clay with chitosan and Fe(III) cations; the Fe-modified composites have demonstrated effective removal of As ions from aqueous solution. The developed Bt-Ch and Fe-modified composites exhibited good potential for re-use after five cycles of regeneration, thus, indicating their potential as cost-effective adsorbents for removal of these metal ions from both drinking and wastewater. Overall, this study provides insights and new valuable knowledge for inexpensive remediation of metal-contaminated water, especially when using composites made from cheap and sustainable materials.