Adsorption of cationic dyes onto biopolymer-bentonite composites: kinetics and isotherm studies

Two biopolymer-bentonite composites, namely, bentonite-alginate beads (BAB) and bentonite-alginate-double walled carbon nanotube beads (BACB) were prepared. The synthesised adsorbent beads were characterised using Brunauer-Emmet-Teller (BET) analysis, Scanning Electron Microscopy (SEM), Fourier Transform Infrared Spectroscopy (FT-IR), X-ray diffraction (XRD) and Thermogravimetric Analysis (TGA). Batch adsorption studies were performed to assess the dye removal capacity of both adsorbents. Three cationic dyes, namely, Methylene blue (MB), Crystal violet (CV) and Rhodamine B (RB) were used to evaluate the adsorption capacities of the prepared adsorbents. Thermodynamic studies were carried out at different temperatures to assess the adsorption behaviour of both adsorbents. Various kinetic models, i.e., pseudo-first-order (PFO), pseudo-second-order (PSO), Elovich and intra-particle diffusion (IPD) models were studied to determine the adsorption kinetics whereas Freundlich and Temkin isotherm models were utilised to study adsorption at equilibrium. Adsorption data of both BAB and BACB fitted well with Freundlich as well as Temkin isotherm for all dyes under investigation. Kinetics data of BAB and BACB followed PSO model and both adsorbents exhibited the highest adsorption capacity for MB (267.14 mg g(-1) and 400.55 mg g(-1), respectively) amongst the dyes under study. The dye removal efficiency of both the adsorbents followed the order MB > CV > RB. Regeneration studies revealed that the dye removal percentage ranged from 51% to 93% up to six adsorption-desorption cycles.