Separation and Purification Technology 67 (2009) 251–261 Contents lists available at ScienceDirect Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur Removal of cadmium from aqueous solutions by adsorption using poly(acrylamide) modified guar gum–silica nanocomposites V. Singh a,∗ , S. Pandey a , S.K. Singh a , R. Sanghi b a Department of Chemistry, University of Allahabad, Allahabad 211002, India b 302 Southern Laboratories, Facility for Ecological and Analytical Testing, Indian Institute of Technology Kanpur, Kanpur 208016, India article info Article history: Received 31 August 2008 Received in revised form 20 February 2009 Accepted 28 February 2009 Keywords: Cd(II) adsorption Guar-graft-poly(acrylamide)-silica TEOS Nanocomposite abstract Novel nanocomposite adsorbent materials were synthesized by dehydroxylation condensation of tetraethoxy silane (TEOS) in the presence of guar gum-graft-poly(acrylamide) using ammonium hydrox- ide as catalyst and ethanol as co-solvent. The ratio of H 2 O: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 mate- rial thus obtained was calcinated (in air) in stages up to 1100 ◦ C where the binding ability of the material could be further tailored and materials of different performances were obtained. The material calcined at 600 ◦ C was found most efficient and its adsorbent behavior was studied in detail taking Cd(II) as rep- resentative 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 bioad- sorbent 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 × 10 -3 , 1.88 × 10 -4 and 2.05 × 10 -4 g mg -1 min -1 at 500, 700 and 900 mg L -1 initial Cd(II) concentrations, respec- tively. 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. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Cadmium is a natural, usually minor constituent of surface and groundwater. It may exist in water as a hydrated ion, as inorganic complexes such as carbonates, hydroxides, chlorides or sulfates, or as organic complexes with humic acids [1]. Cad- mium may enter aquatic systems through weathering and erosion of soils and bedrock, atmospheric deposition, direct discharge from industrial operations, leakage from landfalls and contam- inated sites, and the dispersive use of sludge and fertilizers in agriculture. Adverse health effects due to cadmium are well doc- umented and it has been reported to cause renal disturbances, ∗ Corresponding author. Tel.: +91 532 2461518; fax: +91 532 2540858. E-mail address: vndsng@yahoo.com (V. Singh). lung problems, bone lesions, cancer and hypertension in humans [2]. Several biosorbents [3–5] besides the polymeric materials like dithiocarbamate-anchored polymer/organosmectite [6] have been used for cadmium removal from aqueous solutions. An efficient metal ion adsorbent for wastewater treatment should consist of a stable and insoluble porous matrix having suitable active groups that interact with heavy metal ions. Because of large surface area, a high mass exchange characteristic, non swelling nature, excellent mechanical resistance and availability of reactive silanol groups, silica gel finds use as an adsorbent, both in amorphous [7] as well as in functionalized forms [8,9]. Functionalized porous silicas via templating method [11,12] have also been used effectively as heavy metal ion adsorbent. Polysaccharide–silca composites, due to the combined characteristics of silica and polysaccharides are poten- tially even more attractive candidates to be used as adsorbents as their combined properties represent an improvement upon the thermal, mechanical, and porosity properties compared with the 1383-5866/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2009.02.023