Two-site Model for Aluminum Oxide with Mass Balanced Competitive pH+Salt/Salt Dependent Reactions

A backtitration technique was used to collect proton isotherm data for an Al oxide, and was found to be stoichiometrically related to the cation and anion isotherm behavior. The adsorption of various ions by the Al oxide surface was modeled based on: (i) two surface sites, (ii) pH+salt-dependent reactions (H and Cl~, 2OH~, or Na and OH"), (iii) competitive salt-dependent reactions (Na or Cl~), (iv) COj(aq) pH-dependent reactions with the surface-OH groups, and (v) presence of an unknown MC1salt (0.0005 M). The total number of Al sites was 1.7 pmol mr or 3.4 tano\ m~ of total available sites. The pH of point of zero salt effect (PZSE) represented the surface condition in which the negative charges (or cation surfaces) equaled the positive charges (or anion surfaces); the cation exchange capacity (CEC) equaled the anion exchange capacity < AEC) at this value. The CEC-AEC data and proton isotherm data were stoichiometrically correlated. The pH of PZSE values ranged from 7.SO to 7.76 depending on the electrolyte concentration present, with lower values as the concentration increased. The negative shifts in the PZSE values are due to anion impurities initially present on the surface and positive shifts are due to cation impurities. Additional Index Words: cation/anion/proton isotherms, competitive reactions, equilibrium constants, mass balanced equations, PZSE, PZNC, ZPC. Schulthess, C.P., and D.L. Sparks. 1987. Two-site model for aluminum oxide with mass balanced competitive pH+salt/salt dependent reactions. Soil Sci. Soc. Am. J. 51:1136-1144. T ADSORPTION OF DISSOLVED SUBSTANCES OH a solid phase (charcoal) was accidentally discovered in 1785 by J.T. Lowitz (Figurovsky, 1973). A.L. Lavoisier (1743-1794) advanced a chemical, rather than mechanical, theory for the phenomenon of adsorption (Figurovsky, 1973). Modeling of the surface chemistry of oxides, colloids, or soils, has long been recognized by researchers as essential in optimizing crop yields, soil productivity, or the efficiency of any system requiring an interaction with a solid phase. Adsorption models were initially intended to describe gas adsorption by solids, and have been applied with some success to aqueous adsorption. The most fa1 Published with the approval of the Director of the Delaware Agric. Exp. Stn. as Miscellaneous Paper no. 1172. Contribution no. 215 of the Dep. of Plant Science, Univ. of Delaware, Newark, DE 19717-1303. Received 17 Nov. 1986. 2 University Graduate Research Fellow and Professor of Soil Chemistry, respectively. mous contribution was made by Langmuir (1918), whose equation may be easily derived for cation adsorption onto an oxide surface site. Assuming the following surface reaction