Qualitative and quantitative investigations of Cr (VI) uptake by amorphous nanoparticulate ferrites doped with organic chelating agents

Abstract

Current research focuses on the potential application of amorphous iron oxyhydroxides, specifically ferrihydrite (FHY) coprecipitated with and without ethylenediaminetetraacetic acid and ethylenediamine-N, N′-disuccinic acid as complexing agents, for the removal of chromium from aqous solutions. The Electron Probe Microanalysis, X-ray diffraction, SEM and TEM Microscopy, N₂ sorption/desorption, and FT-IR Spectroscopy allowed the assessment of the composition, mineralogy, crystallinity, morphology, size, and surface properties of the nanoparticulated samples. The incorporation of chelating agents resulted in differences in nanoparticle morphology, amorphous characteristics, and surface properties, but did not affect the Cr uptake by adsorption. Kinetic results showed fast (1 hour) Cr uptake capacity that increased with pH decrease and rising Cr concentration, approaching saturation at initial Cr concentrations exceeding 100 mg/L. Isothermal studies showed maximum uptake capacities of FHY of 37.79 mg Cr/g _FHY and the best fit to SIPS model indicates that Cr is chemically bound as mono and multiple layers at plain ferrites surface and under light acidic pH. Reuse tests revealed FHY capability of sustaining up to five cycles of adsorption and desorption, and possibly more. This underscores the potential of FHY as a promising adsorbent for the removal of Cr /at contaminated sites. Geochemical modelling was conducted to (i) assess the speciation of Cr (VI) in the solution and (ii) allow comparison of the theoretically simulated adsorption process with the experimentally obtained results. They indicate Cr species available to bind at varying pH and concentrations values, and that no Cr self-precipitation occurred within tested conditions. Interestingly, the adsorption models that were used to best simulate Cr uptake by FHY, did overestimate the experimental results. Hence, there is still a need to improve geochemical modelling software to simulate processes that occur in real settings.