Toward highly effective and easily separable halloysite-containing adsorbents: The effect of iron oxide particles impregnation and new insight into As(V) removal mechanisms

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Abstract Halloysite containing composites impregnated with Fe oxide particles (10 and 25% loading) were synthesized by co-precipitation method. The obtained uncalcined and calcined materials were tested for aqueous As(V) removal in different experimental conditions. The experiments were designed to test both the material’s adsorption capacity and the mechanisms responsible for As(V) removal. The XRD, Mossbauer spectroscopy, FTIR and SEM results revealed that Fe oxides formed during the synthesis were superparamagnetic maghemite (γ-Fe 2 O 3 ) nanoparticles with approximate size of 20 nm. Their presence as well as their homogenous distribution induced a strong response of the composites to the applied magnetic field, enabling magnetic separation of the adsorbents. The impregnation with Fe oxide particles resulted in an increase of surface area and changes of porosity. The As(V) adsorption studies revealed, that the material impregnated with Fe oxide exhibited higher adsorption capacity, than the starting mineral sample. Moreover the highest adsorption capacity of 180 mmol/kg was noticed for calcined composites. In order to better understand the As(V) adsorption mechanisms, the effect of pH and ionic strength was investigated as well as desorption, and the XPS studies were applied. The results revealed the formation of inner-sphere complexes between As(V) and composites surface as well as As(V) reduction coupled with Fe(II) oxidation as the main removal mechanisms. The chemisorption had a significant impact on the mineral support structure by increasing its stiffness as indicated by Mossbauer spectroscopy. The studied composites showed high stability in a broad pH range, measured as As, Si, Al and Fe release, which enables their use in environmental applications.