Biopolymer coated clay particles for the adsorption of tungsten from water

Biopolymer coated clay (i.e. chitosan coated montmorillonite clay called biosorbent) particles were synthesized. The effects of tungsten concentration in feed water (20-500 ppm) and water pH (4, 5.5 and 6.4) on the zeta potential of adsorbent particles, tungsten removal, and adsorption equilibrium were studied using biosorbent and natural clay. Coating clay particles with chitosan shifts the net surface charge of clay from negative to positive and the point of zero charge (PZC) of clay from 2.8 and 5.8. The net surface charge of biosorbent particles decreases with an increase in the tungsten concentration of feed water because the positively charged sites are consumed by the adsorption of tungsten anions. Biosorbent was found to be much more effective than natural clay for the removal of tungsten. The tungsten removal efficiencies of both biosorbent and natural clay decrease with an increase in the pH level and an increase in the tungsten concentration of the feed water. Adsorption equilibrium studies show that tungsten removal is the highest at pH 4. Adsorption equilibrium data were modeled with the Langmuir, Temkin, and Freundlich isotherms and the model parameters were obtained. Adsorption of tungsten species on both biosorbent and natural clay seemed to obey Langmuir isotherm within the range of concentrations and pH investigated. The maximum tungsten adsorption capacities of chitosan, biosorbent and natural clay were found to be 632 mg tungsten/g of chitosan, 23.9 mg tungsten/g of biosorbent, and 5.45 mg tungsten/g of natural clay at pH 4, respectively. The tungsten species adsorption on biosorbent was found to be governed by the ionic attraction between the protonated surface groups on chitosan and the negatively charged tungsten species. Whereas, the tungsten species adsorption on natural clay is governed by the positively charged clay particle edges formed by broken bonds of Al-O and Si-O.