Improved adsorption of Congo red from aqueous solution using alkali-treated goethite impregnated chitosan hydrogel capsule

Goethite impregnated chitosan hydrogel capsules (GS5) were treated with an alkali solution and the effect of alkali treatment on the adsorption performance of the treated capsules for Congo red (CR), and the changes in physico-chemical properties, morphology, and mechanical stability of the capsules were studied. The goethite particles were synthesized via the co-precipitation method from ferric chloride hexahydrate solution and NaOH. The as-obtained goethite nanoparticles were successfully impregnated into chitosan capsules via an anionic surfactant gelation approach. The alkali treatment of GS5 with 0.02 N alkali solution (GSN5) increased adsorption capacity from 1040.54 mg/g (GS5) to 1641.96 mg/g as obtained by Langmuir adsorption isotherm. The goethite impregnated chitosan capsules more closely followed the Freundlich adsorption isotherm than the Langmuir adsorption isotherm model suggesting heterogeneous adsorption sites on the capsules, and both varieties of the capsules showed a better fit to first-order-kinetic rate model. Elemental analysis, inductively coupled plasma optical emission spectroscopy (ICP-OES), and energy dispersive X-ray (EDX) spectroscopy clearly indicated that alkali treatment completely removed SDS from the capsules and also, caused some morphological changes in the structural network of capsules as obtained by scanning electron microscopy (SEM). The mechanical stability study indicated that alkali treatment caused slightly more weight loss of the capsules under mechanical agitation.