Water-sediment interaction: surface properties of natural sediments

GARCIA M.G.; BORGNINO, L.,; HIDALGO M. DEL V.,; AVENA, M.; DE PAULI, C.P.,; BLESA, M.A.

Mar del Plata

Congreso; V Congreso Iberoamericano de Física y Química Ambiental; 2008

CNEA - UNSAM

The surface acid-base properties of natural sediments from different reservoirs were analyzed via potentiometric titration and electrophoretic mobility determinations. The chemical and mineralogical characterization of sediments was also performed, with the aim of finding a relationship between these properties and the parameters that describe the pH-dependent surface charges of sediments. Modelling of the proton-binding surface charge in the studied sediments was performed using two types of sites following only deprotonation reactions. In the case of the loess two sites were considered, one following cation exchange and the other following a deprotonation reaction. The adjusted parameters were the intrinsic deprotonation (or cation exchange) constants and the site density. Results indicated that increasing content of organic matter lead to increasing site density. The total concentration of sites is highly dependent on the content of OM, but when we analyze the two proposed sites separately, we observe that the site in minor proportion (less than 40% of the total concentration of sites) and its corresponding intrinsic stability constant is better correlated with the content of Fe2O3, whereas the predominating site is better controlled by OM.  The net negative surface charge determined through electrophoretic mobility experiments is likely originated by isomorphic substitutions in the clays lattice and by the surface groups of organic matter. The negative surface charge remains almost constant at pH from 4 to 9 in all sediments except in the loess and the fluvial sediment from the middle reaches. In these two sediments the negative character of the net surface charge decrease under neutral to acidic conditions, indicating that under such conditions, H+-binding at the mineral edges (probably Fe oxy-hydroxide coats) become increasingly important. Key words: pH-dependent charge, sediments, potentiometric titrations