Effective removal of methylene blue from water using phosphoric acid based geopolymers: synthesis, characterizations and adsorption studies

Abstract

Phosphoric acid based geopolymers (PAGPs) are a class of geopolymers that are produced by phosphoric acid activation of metakaolin. In this work, two different PAGPs have been synthesized using phosphoric acid to alumina molar ratios of 1 : 1 and 1.2 : 1. The surface profile, chemical composition, micromorphology, and texture properties of the geopolymers were instrumentally determined. Both geopolymers have shown a mesoporous profile with the avg. pore size of 8.6 and 19.4 nm by GP-1M (P : Al = 1 : 1) and GP-2M (P : Al = 1.2 : 1), respectively. Thermogravimetric analysis revealed that these geopolymers were thermally stable up to 800 °C, although the formation of quartz, cristobalite and tridymite was observed in XRD analysis of the samples treated at 800 °C for two hours. The synthesized geopolymers were utilized for the adsorption of methylene blue (MB) by investigating the effect of the amount of adsorbent, pH of the solution and shaking period. The batch kinetics study fitted best into the pseudo second order (PSO) reaction kinetic model. In isotherm modelling studies, the Langmuir isotherm model was best fitted and was used to describe the mechanism of the adsorption. Experimental adsorption capacities (q_e) of 2.84 and 3.01 mg g⁻¹ were recorded for GP-1M and GP-2M, respectively. Used adsorbents were successfully regenerated by furnace treatment at 400 °C for two hours, and the regenerated adsorbents presented enhanced adsorption capacities in the range of 4.9–5.07 mg g⁻¹ for five repeat cycles, elucidating that the material is suitable for multiple time use.