Effects of Surface Iron Hydroxyl Group Site Densities on Arsenate Adsorption by Iron Oxide Nanocomposites

초록

Nanocomposites containing maghemite (gamma-Fe2O3) and hematite (alpha-Fe2O3) were prepared by thermal decomposition (200 to 600 degrees C) of the products formed by reacting Fe-0 with aqueous oxalic acid. The iron oxide nanocomposites were characterized by TEM, XRD, VSM, BET, FTIR, XPS and surface potential titration. Batch-type sorption experiments showed that As(V) adsorption capacities of these nanocomposites decreased as the calcination temperature increased from 200 to 600 degrees C. The highest As(V) adsorption capacity (0.343 mmol/g) was achieved on the nanocomposite prepared at 200 degrees C, which had a particle size of about 10 nm and a pHpzc value of 6.1. The quantity of iron hydroxide surface sites on these nanocomposites were determined by fitting the surface titration data using a one site 2-pK constant capacitance model (CCM). The total quantities of surface sites correlated with the corresponding As(V) adsorption capacities, but this correlation was not linear. The surface hydroxyl oxygen to the total oxygen ratios were estimated by fitting the XPS O1s envelopes of these nanocomposites. These ratios were also positively correlated with the As(V) adsorption capacities. However, these correlations were also not linear. Evidence, including the amount of titratable proton surface sites and the percentages of hydroxyl oxygens, favored AsO43- adsorption occurring through an oxygen bridge between iron and arsenic at the surface. This occurs with the loss of water (or hydroxide) when surface Fe-OH groups react with AsO43- or HAsO42-. The nonlinear nature of the correlations between the adsorption capacities and both the surface site densities and the percentages of hydroxyl oxygen indicated that these two properties are not the only governing factors responsible for As(V) adsorption. Other possibly contributing factors are proposed. More research should be carried out to further investigate the surface-property effects on As(V) adsorption in future.

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