In situ and ex situ study of the enhanced modification with iron of clinoptilolite-rich zeolitic tuff for arsenic sorption from aqueous solutions

Adsorption methods have been developed for the removal of arsenic from solution motivated by the adverse health effects of this naturally occurring element. Iron exchanged natural zeolites are promising materials for this application. In this study we introduced iron species into a clinoptilolite-rich zeolitic tuff by the liquid exchange method using different organic and inorganic iron salts after pretreatment with NaCl and quantified the iron content in all trials by XRF spectroscopy. The materials were characterized by XRD, FTIR, FTIR-DR, UV-vis, cyclic voltammetry, ESR and Mössbauer spectroscopies before and after adsorption of arsenite and arsenate. The reached iron load in the sample T + Fe was %Fe₂O₃-2.462, n_Fe / n_Al = 0.19, n_Si / n_Fe = 30.9 using FeCl₃, whereby the iron leachability was 0.1-0.2%. The introduced iron corresponded to four coordinated species with tetrahedral geometry, primarily low spin ferric iron adsorbing almost 12 μg g^-1 arsenite (99% removal) from a 360 μg_As(III) L^-1 and 6 μg g^-1 arsenate from a 230 μg_As(V) L^-1. Adsorption of arsenite and arsenate reached practically a plateau at n_Fe / n_Si = 0.1 in the series of exchanged tuffs. The oxidation of arsenite to arsenate in the solution in contact with iron modified tuff during adsorption was observed by speciation. The reduction of ferric iron to ferrous iron could be detected in the electrochemical system comprising an iron-clinoptilolite impregnated electrode and was not observed in the dried tuff after adsorption.