Resumo

VILLALOBOS, Mario  e  ANTELO, Juan. A unified surface stuctural model for ferrihydrite: proton charge, electrolyte binding, and arsenate adsorption. Rev. Int. Contam. Ambient [online]. 2011, vol.27, n.2, pp.139-151. ISSN 0188-4999.

Ferrihydrite (FH) is a common hydrous ferric oxide nanomineral in aqueous geo-chemical environments. Its small particle sizes (1.5-5 nm) expose a very high specific surface area at the mineral/water interface, and this may have considerable influence on the transport and fate of a variety of trace and major elements through diverse sorption processes. In particular, arsenate anions show a very high affinity for Fe(III) oxide surfaces, including FH, and their fate in contaminated environments is almost invariably associated to these. The extremely small FH nanoparticles, which show high particle aggregation when dried, preclude experimental determination of important surface parameters for the thermodynamic description of its adsorption behavior, such as available specific surface area in aqueous suspension. In the present work we have compiled eight sets of published acid-base surface titration data for synthetic preparations of FH across a wide range of particle sizes, and unified their description through a face-distribution site-density model developed previously for goethite. We show that the surface proton charge behavior of FH in conjunction with its As(V) adsorption behavior may be adequately described using the affinity constants derived for goethite, by assuming the FH surface to be composed predominantly of singly-coordinated >OH groups, with a site density equal to that of the (010) goethite face (Pnma space group). Also, through the applied model the available specific surface area of each FH preparation in aqueous suspension may be successfully derived, showing values between 330 and 1120 m²/g. The implications of the results reported here are highly relevant for predictive purposes of FH surface reactivity in general.

Palavras-chave : ferrihydrite; surface structure; point of zero charge; specific surface area.

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