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

Water-sediment interaction: surface properties of loessic, estuarine, lacustrine and fluvial sediments Garcia M.G.1, Borgnino L.2, Hidalgo M. del V.3, Avena M.4, De Pauli C.P.2, Blesa M.A5 (2) Depto de Fisicoquimica. Fac. Cs Químicas. Universidad Nacional Córdoba; (3) Universidad Nacional Tucumán; (4) Depto de Química. Universidad Nacional del Sur; (5) Universidad Nacional de San Martín Surface characteristics of minerals and organic particles assemblages in the sediments are known to influence the reactivity of these particles towards contaminants and their transport in aquatic environments. For that reason, sediment samples from different environments were analyzed in order to define their surface properties. Loess sample (LS-1) was collected from the bottom of an irrigation channel in Tucumán Province, while the fluvial sediment (SR-1) corresponds to the first 5 cm bed channel of the middle reaches of the Salí River (Tucumán, NW Argentina). Both, estuarine (LM-4) and lacustrine (LM-2) sediments were collected at the Los Molinos reservoir, province of Córdoba (Central Argentina). Chemical and mineralogical analyses were performed. The pH-dependent surface charge was determined by acid-base potentiometric titrations at constant temperature (25ºC) and at three different salt concentrations (10-1M, 10-2M, and 10-3M). Microelectrophoretic mobility was measured in suspensions of the dried sediments at I= 10-2 M and within a range of pH from 2 to 11. Regarding the chemical composition, major difference among the analyzed sediments is related to the content of organic matter and pH. As expected, the sediments from the bottom lake (LM-2) showed the highest content of organic matter (14%), followed by the estuarine deposits (7%). On the contrary, the content of organic matter in the fluvial sediment and in the loess is lower than 1.5%, which likely determines the predominant alkaline pH of these suspensions. XRD diagrams reveal that the silty-clayed fraction of the fluvial sediment mainly consists of phyllosilicates (chlorite and illite) and grains of quartz. Minerals identified in the loess were illite, lepidocrocite, quartz, smectite, feldspars, and gypsum. SEM images also revealed the occurrence of abundant grains of glass, aggregates of framboidal pyrite, and coatings of Fe oxy-hydroxides on the surface and filling fractures in the grains of glass. Illite, illite/smectite, kaolinite, kaolinite/smectite, chlorite, and palygorkite have been determined as the dominant minerals in the sediments of the Los Molinos reservoir. Potentiometric titration curves at different ionic strengths were parallel in the analyzed range of pH in the suspensions of the fluvial, estuarine and lacustrine sediments. Due to the occurrence of minerals of low point of zero charge (pzc) it has been considered that the pH-dependent charge in these sediments is negative at pH values higher than 4. The curves of the loess suspensions on the contrary, crossed at near pH 9.2, which would suggest that the sites in the Fe oxy-hydroxides coatings mainly control protonation/deprotonation reactions in this sediment. Microelectrophoretic mobility showed a negative net potential for all the sediments at pH higher than 2, which is attributed to the structural charges originated by isomorphic substitutions in the lattice of clays.             The pH-dependent surface charge was modelled with FITEQL4.0 using the DLM. Adjusted parameters were the protolitic constants of the identified sites; the number of surface sites was estimated from the potentiometric titration results.