Surface-charging behavior of Zn-Cr layered double hydroxide

A Zn-Cr layered double hydroxide (LDH) having the formula Zn2Cr(OH)(6)Cl-0.7(CO3)(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 he 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. (C) 2004 Elsevier Inc. All rights reserved.