Fluoride removal from water using Zr-modified meso- and macroporous carbons: Effect of pore structure and adsorption conditions

Abstract

We investigated the effects of the pore structure and adsorption conditions (pH and coexisting anions) on the fluoride (F–) removal capacity of Zr-modified porous carbons. The porous carbons used in this study possessed well-defined pore structures created by the template technique with pore diameters of 10, 30, and 150 nm. These porous carbons were oxidized to functionalize them with oxygen-containing groups before being modified with Zr4+. Zr4+ modification improved the F– removal capacity of porous carbons. The maximum amount of F– adsorbed increased by approximately 3–10 times by Zr4+ modification. The F– adsorption capacity depends on the pore size. The samples with larger pore diameters (smaller specific surface areas) exhibited higher F– adsorption capacities than the samples with smaller pore diameters. This tendency can be explained using the theory of liquid imbibition in porous media. The adsorption capacities at pH 7.0 and 11.1 were lower than those at pH 3.1. This tendency can be interpreted based on competition between F– and OH– on adsorption sites and attractive/repulsive interactions between F– and protonated/deprotonated adsorbent surfaces. The coexisting carbonate anions notably suppress the F– adsorption. This suppressive effect can be explained by the competitive bonding of F– and CO32– to the adsorption sites.