No attempts were made to further adjust the acid/base model Acid/base properties and phenylphosphonic acid (H 2 L) comduring the evaluation of the three-component model. The acid/ plexation at the boehmite ( g-AlOOH)/water interface have been base and phenylphosphonic acid complexation features of the studied by means of potentiometric titrations and batch experiboehmite surface have been compared to those of the aged gments. All measurements have been performed in 0.100 M Na(Cl) Al 2 O 3 . ᭧ 1996 Academic Press, Inc. ionic medium at 25ЊC. The experimental data have been evaluated Key Words: boehmite; acid/base properties; alumina; aluminium on the basis of the surface complexation concept with the use of oxide; constant-capacitance model; adsorption; equilibrium conthe constant capacitance model to account for electrostatic contristant; oxide/water interface; phenylphosphonic acid; potentiometbutions. It was found that the boehmite surface possesses 65% ric titration; surface complexation. higher proton exchange capacity per surface area unit than previously recorded for an aged g-Al 2 O 3 surface. This high surface concentration (0.51 mol/kg g-AlOOH; site density Å 1.71 sites/ INTRODUCTION nm 2 ) was reflected in the acid/base model as a large contribution from electrostatic forces, and accordingly as a low specific capaci-Aluminium (hydr)oxide, Al 2 O 3 rxH 2 O, can exist in a varitance value. The model describing the acid/base reactions at the ety of modifications depending on a number of factors such boehmite surface is given by the following reactions, intrinsic conas temperature, moisture content, particle size and impurity stants, and specific capacitance: content. There is only one thermodynamically stable anhy-GAlOH / H / S GAlOH / 2 ; log b ( int ) Å 7.47 { 0.06 drous alumina phase, a-Al 2 O 3 , but several metastable transition aluminas, such as g-, u-, and d-Al 2 O 3 , exist. Moreover GAlOH S GAlO 0 / H / ; log b ( int ) Å 09.81 { 0.18 there are several important hydrated forms of alumina corre-C Å 0.92 F/m 2 sponding to the compositions AlOOH and Al(OH) 3 (1). In a previous work by the authors (2), it was shown The maximum adsorption of phenylphosphonic acid was observed that g-Al 2 O 3 is not a stable phase in aqueous medium but at 0log [H / ] õ 7 and it was found to equal 50% of the proton undergoes a phase transformation. It was observed that durexchange capacity. Within the approximate range of 7.7 õ ing this process, a bayerite (b-Al(OH) 3 ) layer was devel-0log[H / ] õ 9.5, a strong decrease in adsorption affinity occurs. Data for the three-component system H / -g-AlOOH surface-phe-oped on the surface of the g-Al 2 O 3 particles. It was also nylphosphonic acid can be explained with a model assuming a found that considerable changes with regard to the surface formation of two 1:2 (H 2 L/GAlOH) complexes according to the acid/base properties of the material occurred during this reactions surface transformation. For this reason, the partially phase transformed g-Al 2 O 3 phase was referred to as aged g-Al 2 O 3 . This observation implies that different modifications of log b ( int ) Å 2.56 { 0.04 forms of alumina stable in the presence of water at tempera-1 To whom correspondence should be addressed.
tures below 300ЊC.