Competitive Adsorption of Anionic Dyes from Aqueous Single and Binary Solutions with CoAl Layered Double Hydroxide

Layered double hydroxides (LDH) have previously been extensively studied as high capacity adsorbents for the removal of dyes from water. However, a comprehensive understanding of why one dye is adsorbed more than another still remains unknown. In addition, a very little is known about how adsorption mechanisms scale when more than one dye is present in the solution. The adsorption capacity of cobalt-aluminum (CoAl) LDH was investigated for methyl orange (MO), remazol brilliant blue (RBBR) and allura red (AR), at different dye concentrations. The maximum mass of dye adsorbed was obtained for MO (2.267 mmol/g), followed by RBBR (0.258 mmol/g) and AR (0.195 mmol/g). X-Ray Diffraction and Fourier transform infrared analysis results demonstrated that surface adsorption and electrostatic interactions contributed to the adsorption while intercalation did not. In a binary solution of MO and RBBR, the highest mass of MO adsorbed was reduced to 1.521 mmol/g while the maximum RBBR mass adsorbed increased to 0.268 mmol/g. CoAl LDH had a preferential adsorption for RBBR within concentrations up to 0.026 mmol/L while MO was preferred at higher concentrations. Overall, the findings suggest that the adsorption capacity of an LDH is highly dependent on the number, characteristics and the equilibrium concentrations of dyes present in a solution.