Guar gum-grafted poly(acrylonitrile)-templated silica xerogel: nanoengineered material for lead ion removal

Background: Polysaccharides are renewable biodegradable natural materials and are accounted for to control the formation of hybrid silica nanocomposites by sol gel process. Methods: The synthesis of templated silica xerogel essentially includes two critical steps of hydrolysis and polycondensation reaction that are started by catalyst and silica precursor solution. Aside from this Saponification guar-graft-polyacrylonitrile (s-GG-g-PAN) as a copolymer are included in the precursor solution for providing a novel templating environment for silica matrix formation. The s-GG-g-PAN acts as a template for the silica produced in the blended framework because of the hydrogen bonding between the hydroxyl groups at silanols and hydroxyl group and carbonyl groups at the copolymer surface. Connected silanol groups can further hydrolyze and after that take part in the condensation reaction. Accordingly, the s-GG-g-PAN template gets trapped inside the resulting silica system, which on calcining at 900 °C is lost, producing pores of shape and size of the template. Results: The surface area and pore volume of the developed templated silica xerogel have been determined. The surface area and pore volume of the template silica xerogel (H900) were observed to be (240 m² g⁻¹) and (0.286 ccg⁻¹) respectively. The conditions for the adsorption by adsorbent had been enhanced, and kinetics and thermodynamic studies were performed. The best result in terms of lead adsorption was obtained with templated silica xerogel calcined at 900 °C. The % Pb²⁺ removal is observed to be 96% when H900 adsorbent was treated under ideal adsorption states of measurements 0.05g dose, 500mgL⁻¹ Pb²⁺ concentration, time 2 h, pH 5 at 30 °C. The adsorption information fitted satisfactorily to Langmuir isotherms, showing unilayer adsorption. In view of the Langmuir model, Qmax was calculated to be 2000 mgg⁻¹. Theadsorption demonstrated pseudo-second-order kinetics. The thermodynamic study revealed the endothermic and spontaneous nature of the adsorption. The adsorbent demonstrated good thermal stability and high reusability. Conclusions: The present study highlights the possibility of silica xerogel derived from saponified guar gum-grafted poly (acrylonitrile) toward its potential application as superior adsorbent for removal of Pb²⁺ from aqueous solution.