Surface properties of sediments from two Argentinean reservoirs and the rate of phosphate release.

This article reports the surface properties (electrophoretic mobility, surface charge and specific surface area (SSA)), and phosphate release rates of sediments from two Argentinean reservoirs. Even though sediments are a heterogeneous mixture of minerals with different charging behavior, electrophoresis reveals that all sediment particles are negatively charged in the pH range 3-9. Iron and aluminum oxides, which usually carry a net positive charge at low pH, seem to be forming aggregates with negatively charged minerals and/or carrying a net negative charge due to the presence of organic matter. The phosphate release rate increased by increasing pH and temperature, and depended linearly on the SSA of the sediments. These data, together with the high activation energy of the process, indicate that the rate-controlling process is a chemical reaction occurring at the solid-water interface, and not a diffusion process. The rate-controlling step seems to be the breaking of bonds between phosphate and surface groups, breaking that should be preceded by one or more deprotonation steps at the surface and favored by electrostatic repulsion between the negative charge surface and the phosphate anion.

Surface-charging behavior of Zn-Cr layered double hydroxide.

A Zn-Cr layered double hydroxide (LDH) having the formula Zn(2)Cr(OH)(6)Cl(0.7)(CO(3))(0.15)2.1H(2)O was synthesized and characterized by powder X-ray diffraction, infrared spectroscopy, acid-base potentiometric titration, mass titration, electrophoretic mobility, and modeling of the electrical double layer. Adsorption of alizarin was also performed in order to show some particular features of the HDL. Net hydroxyl adsorption, which increases with increasing pH and decreasing supporting electrolyte concentration, takes place above pH 5. The electrophoretic mobility of the particles was always positive and it decreased when the pH was higher than 9. An isoelectric point of 12 could be estimated by extrapolating the data. The modified MUSIC model was used to estimate deprotonation constants of surface groups and different adsorption models were compared. Good fit of hydroxyl adsorption and electrophoresis could be achieved by considering both OH(-)/Cl(-) exchange at structural sites and proton desorption from surface hydroxyl groups. The modeling, in agreement with alizarin adsorption, indicates that most of the structural positive charge of the LDH is screened at the surface by exchanged anions and negatively charged surface groups. It also suggests that only structural charge sites initially neutralized by chloride ions are active for anion exchange. The remaining sites are blocked by carbonate and do not participate in the exchange.

Nucleus accumbens dopamine receptors in the consolidation of spatial memory.

Nucleus accumbens dopamine is known to play an important role in motor activity and in behaviours governed by drugs and natural reinforcers, as well as in non-associative forms of learning. At the same time, activation of D1 and D2 dopamine receptors has been suggested to promote intracellular events related to neural plasticity. Therefore, in this study we wished to investigate the role of the two classes of dopamine receptors within the nucleus accumbens on the consolidation of spatial information. On day 1, CD1 male mice were placed in an open field containing five different objects and, immediately after three sessions of habituation, the animals were focally injected within the nucleus accumbens with either the D1 antagonist SCH 23390 (12.5, 25 or 50 ng/side), or the D2 antagonist sulpiride (25, 50, 75 or 100 ng/side). Twenty-four hours later the ability of mice to discriminate an object displacement was assessed. Both the D1 and the D2 antagonists impaired the ability of mice to detect the spatial change. If the highest doses of the two antagonists were injected 2 h after the end of the last of the habituation sessions, no effect was observed in the reactivity to spatial change examined 24 h later. These data demonstrate that activation of both D1 and D2 receptors within the accumbens is necessary in the early stages of the consolidation of spatial information. The data are discussed in terms of involvement of nucleus accumbens dopamine in information processing in the absence of explicit reinforcers.