Study of the iron(III)-modified clinoptilolite in the adsorption of phosphate from aqueous medium: Mechanism and kinetics

Abstract

Clinoptilolite-rich tuff (Z) enriched with Fe(III) was studied in the removal of phosphate ions present in aqueous medium at pH = 6.5. Fe(III) modification was performed by a simple wet impregnation giving the product (FeZ) with about 18 wt% Fe. Transmission electron microscopy showed the presence of a flaky Fe(III) amorphous precipitate on the clinoptilolite sheets and a preserved clinoptilolite crystallinity. The modification increased the specific surface area from 28.6 to 140.3 m2 g–1. FeZ effectively adsorbed phosphate, the removal rate at 298 K varying from 86% to 42.5% (for C0 = 50 mg dm–3 and C0 = 400 mg dm–3). The sorption isotherms were in accord with the Langmuir model, giving for the Langmuir constant (RL) values in the range 0–1 that are characteristic of a favourable adsorption. The data for adsorption kinetics were best described by the pseudo-second-order model suggesting chemisorption as the phosphate sorption mechanism. Intra-particle diffusion was present in the adsorption, but it was not the rate-limiting step. A31P static spin-echo mapping nuclear magnetic resonance (NMR) measurement was performed for studying the phosphate–FeZ interaction. The results showed that the phosphate adsorption on FeZ proceeds through electrostatic interactions and covalent bonding, the latter being more pronounced.