Adsorption of acid green and procion red on a magnetic geopolymer based adsorbent: Experiments, characterization and theoretical treatment

Abstract

A magnetic geopolymer (MG) was used as an effective adsorbent for water decolorization, in particular, for the removal of acid green (AG) and procion red (PR) from aqueous solutions. AG and PR dyes adsorption mechanism for this adsorbent was experimentally and theoretically characterized via the analysis and description of eight adsorption isotherms obtained at 298–328 K. Results showed that the AG dye was strongly adsorbed in comparison to PR dye. It was also concluded that the temperature showed a minor effect on the variation of the adsorption capacity for both dyes. An analytical double layer adsorption model was chosen to explain these experimental findings. This model contained theoretical parameters that can be utilized to understand the dyes adsorption mechanism. The temperature effect on all model parameters was analyzed concluding that this operating parameter had a minor effect on the number of accepted amount of AG and PR dyes per receptor site of MG adsorbent (n) and the density of receptor sites (Dm). The trends of parameters n and Dm led to a slightly variation of the adsorption capacity at saturation (Qsat = 2.n.Dm) as a function of temperature. Particularly, it was also concluded that AG and PR dyes retained their adsorption orientations on the magnetic geopolymer surface. Overall, it was established that the adsorption performance for the removal of AG dye could be associated to its short molecular size. Calculated adsorption energies varied from 18.55 to 29.14 kJ/mol and from 8.55 to 14.77 kJ/mol for the AG-GP and PR-GP systems, respectively. These adsorption energies indicated that physical interactions were involved during the adsorption mechanism of these water pollutants.