Electrogenerated iron-based adsorbents: A case study of an azo dye removal viewed from a fundamental physico-chemical perspective

Abstract

The aim of this manuscript was to investigate the electrogeneration of iron-based solids and their use in the adsorption of an azo dye. Experiments of electrochemical generation of iron hydr(oxides) were carried out in a Hoffman cell and in a well-stirred batch reactor at 25 °C. The oxidation of iron metal at the anode and the reduction of water at the cathode were the electrochemical reactions confirmed to occur. A 22 factorial design of experiments was applied to investigate the effect of initial pH and current on the current efficiency for the production of ferrous ion at an electrolyte concentration of 300 g L−1. The current efficiency for Fe2+ was much lower than unity (0.204 ± 0.009) and it was statistically independent of the examined factors (p > 0.05). An approximately 3:1 M mixture of amorphous ferric and ferrous hydroxides was electrogenerated before being possibly converted to dehydrated hydroxides such as magnetite. The iron hydroxides were formed by precipitation reactions of soluble iron species at a rate constant higher than 0.072 ± 0.013 s−1. Additional experiments of electrogeneration of these solid adsorbents at electrolyte concentrations between 250 and 1.5 g L−1 revealed a significant (p ≤ 0.05) decrease in the current efficiency for Fe2+ formation from 0.180 ± 0.006 to 0.118 ± 0.008. Experiments of tartrazine adsorption on the electrogenerated iron oxyhydroxides were carried out batchwise at different initial concentrations of the dye at 25 °C. Electrogeneration of oxyhydroxides and adsorption were described correctly by detailed kinetic models. The crystallinity of the electrogenerated adsorbent was examined by X-ray diffraction analysis, while its specific surface area, pore volume and average pore size were from BET analysis. SEM and EDX analyses were conducted to examine its morphology and elemental composition.