Adsorptive removal of heavy metals from wastewater using chitosan coated bentonite beads

Abstract

The contamination of water by heavy metals from anthropogenic and/or natural contamination is a world-wide environmental problem. Advanced wastewater treatments such as adsorption, are economically and environmentally friendly in removing non-biodegradable toxic heavy metals from the wastewater. The focus of recent studies has been specifically finding effective and low-cost adsorbents. Thus, the present study focuses on the use of chitosan coated bentonite beads prepared from low molecular weight chitosan and bentonite to remove heavy metals from wastewater. Chitosan and bentonite adsorbents are preferred due to natural occurrence, abundance, non-toxicity, potentially a lower cost compared to other adsorbents.

Chitosan coated bentonite beads were successfully prepared in this study using low molecular weight chitosan and bentonite. The prepared chitosan coated bentonite beads were characterised using Fourier transform infrared (FT-IR), Scanning electron microscopy (SEM), and swelling tests. The characterisation tests proved that the prepared chitosan coated bentonite beads possess very good physicochemical properties for adsorption. In addition, chitosan coated bentonite beads proved to have a complex chemical structure due to the presence of several functional groups such as O-H, NH2, C=O, C-O-H, Al-O, Si-O and Si-O-Al. Functional groups such as O-H and NH2, are cation binding groups which cause the chitosan coated bentonite beads to be suitable for adsorption of heavy metals.

Synthetic wastewater with Fe and Cu was prepared to evaluate the prepared chitosan coated bentonite beads. The Fe and Cu metals were chosen to simulate wastewater in this study based on the abundance in the acid mine drainage. A 23-factorial design of experiments was used to study various conditions on the adsorption capacity of the prepared chitosan coated bentonite beads. Experimental results proved that the chitosan coated bentonite beads could remove Fe and Cu from wastewater with a maximum adsorption capacity of 12mg/g resulting in a percentage removal up to 99.5% and 95.2% for Cu and Fe, respectively. The adsorption capacity was found to vary with initial pH. For a pH range between 2 and 4, an increase in pH significantly increased the removal of Fe and Cu from synthetic wastewater. The optimal pH for adsorbing Cu and Fe in this study was found to be pH 4, a further increase in pH from pH of 4 to a pH of 6 proved to have a lesser effect on the removal of Fe and Cu from wastewater. Furthermore, the study also proved that both chitosan and bentonite possess different physical and chemical properties. Through evaluation beads of different chitosan bentonite compositions, it was proven that the optimal bead should have a composition of 50% chitosan/bentonite composition.