Single and binary adsorption of reactive black 5 and Congo red on modified dolomite: Performance and mechanism

Abstract

Adsorption of reactive black 5 (RB5) and Congo red (CR) by dolomite treated at 900°C (D900) was investigated in single and binary solutions. D900 was first characterized by laser granulometry, SEM, FTIR, and pore-size distribution analysis. Both adsorption, desorption, and regeneration of the adsorbent were examined. A particular interest has been focused on the spectroscopic study for elucidating the mechanism of the interaction of RB5/CR with D900. The latter favors the uptake of Congo red at the expense of reactive black 5 regardless of time. Extended Langmuir equation gives a good estimation of our binary data. Also, the experimental points agree with the plotted 3D-surfaces. A comparison between the single and binary modes highlights an antagonism between the RB5 and CR molecules, i.e., the components of the mixture are mutually hindered during adsorption. At equilibrium, CR is more strongly coadsorbed than RB5, viz. 229.18 against 72.37mgg−1 at 40°C, respectively. In this context, methanol desorbs 84% of RB5 and 76% of CR of their respective adsorbed capacity. Knowing that RB5 and CR contain one and two amine functions, respectively, the preferential adsorption of CR is explained via a mechanism of inner-sphere complexation, implying amine groups. Dolomite heated at 900°C could prove to be a valuable adsorbent when a discrimination in the adsorption of azo dyes is required.