Surface Complexation Modeling: II. Strategy for Modeling Polymer and Precipitation Reactions at High Surface Coverage

Three triple-layer model (TLM) surface complexation models that allow for the formation of multinuclear surface complexes or precipitates are compared, based on their ability to simulate cobalt sorption on α-Al₂O₃. These models include: (1) a surface solid solution model, (2) a surface polymer model, and (3) a surface continuum model. The solid solution model accounts for high coverage data by invoking a coprecipitation surface reaction, while the polymer model accomplishes the same task with multinuclear surface complexation reactions. In the continuum model, two polymer reactions and one precipitation reaction are proposed. Modeling results indicate that all of the models work reasonably well at predicting sorption data from moderate to high surface coverage (0.1 to 100%). Because the continuum model is the only one presented which is consistent with spectroscopic data throughout the range of surface coverages examined, this model is suggested as the preferred one for modeling metal ion sorption data. Model predictions of pH-edge and isotherm data are discussed and used in the assessment of the merits of the three TLM models.