Removal of cadmium from aqueous solutions by adsorption using poly(acrylamide) modified guar gum-silica nanocomposites

Novel nanocomposite adsorbent materials were synthesized by dehydroxylation condensation of tetraethoxy silane (TEOS) in the presence of guar gum-graft-poly(acrylamide) using ammonium hydroxide as catalyst and ethanol as co-solvent. The ratio of H2O:TEOS:EtOH was varied at fixed concentration of copolymer and catalyst to obtain a series of materials which were evaluated for their ability to bind cadmium from the aqueous solution in a preliminary investigation. The most efficient adsorbent material thus obtained was calcinated (in air) in stages up to 1100 degrees C where the binding ability of the material could be further tailored and materials of different performances were obtained. The material calcined at 600 degrees C was found most efficient and its adsorbent behavior was studied in detail taking Cd(II) as representative ion. The chemical, structural and textural characteristics of the material were determined by FTIR, XRD, TGA-DTA, PL, SEM and EDAX analysis. BET (Brunauer-Emmett-Teller) specific surface area and pore structure of the adsorbent was also examined. The adsorption behavior of the bioadsorbent was investigated by performing both kinetics and equilibrium studies in batch conditions. The adsorption conditions for the adsorbent were optimized by varying several experimental parameters i.e. contact time, initial cadmium concentration, temperature, adsorbent dose, electrolyte amount and pH of the solution. The adsorption showed pseudo-second-order kinetics with a rate constant of 2.85 x 10(-3), 1.88 x 10(-4) and 2.05 x 10(-4) g mg(-1) min(-1) at 500,700 and 900 mg L-1 initial Cd(II) concentrations, respectively. The adsorption data were modeled using both the Langmuir and Freundlich isotherms. The data fitted better to Langmuir isotherm indicating unilayer adsorption. The maximum adsorption capacity (Q(max)) for the composite was found to be significantly very high (2000 mg g(-1)). The thermodynamic study revealed the endothermic and spontaneous nature of the sorption. The composite exhibited very high reusability for more than six cycles. Thereafter its efficiency slowly declined and reached 56% by the 10th cycle. The porous composite sorbent was easy to prepare and was also found to be highly stable and photoluminescent making this biosorption approach quite attractive from the industrial point of view. (C) 2009 Elsevier B.V. All rights reserved.