Use of modified bauxite residue-based porous inorganic polymer monoliths as adsorbents of methylene blue

Abstract

Porous inorganic polymer (IP) monoliths were synthesised using a modified bauxite residue (BR) as the reactive fraction with the objective of using them as adsorbents for methylene blue (MB) from synthetic wastewater. First, unreactive, as-produced BR was transformed into a suitable glassy precursor material for IPs by blending BR with minor quantities of C and CaSiO3, thermally treating it at 1200 °C for 2 h, and quenching in water. The alkaline activation of this slag in combination with a pore foaming agent led to the formation of a highly porous microstructure with up to 85% in total porosity. The synthesised porous monoliths demonstrated high MB uptake (up to 17 mg of MB/g of IP with an initial MB concentration of 75 mg/L). A higher porosity of the monoliths, a higher pH, an increasing initial MB concentration as well as stirring of the testing solution have a positive effect on the adsorption capacity, while an optimum solution volume for adsorption was identified. Furthermore, the reuse of these novel monolithic adsorbents was demonstrated by repeating adsorption tests up to five cycles (adsorption-desorption), with a decrease in adsorption capacity of approximately 30%, but a promising cumulative uptake of about 40 mg of MB per g IP.