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 m(2) g(-1). FeZ effectively adsorbed phosphate, the removal rate at 298 K varying from 86% to 42.5% (for C-0 = 50 mg dm(-3) and C-0 = 400 mg dm(-3)). The sorption isotherms were in accord with the Langmuir model, giving for the Langmuir constant (R-L) 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. A P-31 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.