Mass action expressions for bidentate adsorption in surface complexation modeling: theory and practice

Area of Science:

• Geochemistry
• Environmental Science
• Surface Chemistry

Background:

• Surface complexation models (SCM) are crucial for predicting chemical adsorption to mineral surfaces.
• The incorporation of multidentate adsorption reactions has enhanced SCM predictive capabilities.
• However, inconsistencies in the mass action expression for multidentate reactions lead to ambiguity and errors.

Purpose of the Study:

• To synthesize the theoretical basis for modeling multidentate surface complexes.
• To address persistent ambiguity and mishandling in SCM applications involving multidentate reactions.
• To provide guidance for improving SCM practice and ensuring self-consistent model development.

Main Methods:

• Review and synthesis of theoretical approaches for multidentate surface complexation.
• Analysis of the mass action expression, focusing on the exponent (α) and site activity basis.
• Illustration of implications using a dataset on U(VI) adsorption to goethite.

Main Results:

• Variation in the exponent (α) and the basis for surface site activity (molar concentration vs. fraction) significantly impacts model outcomes.
• Incorrect application, such as setting α at two in a molar-based framework for bidentate complexes, causes critical errors.
• The choice of mass action expression directly affects the self-consistency and accuracy of SCM.

Conclusions:

• Standardizing the mass action expression for multidentate reactions is essential for accurate SCM.
• Clearer theoretical understanding and consistent application will improve predictions of adsorption phenomena.
• Recommendations are provided for handling multidentate reactions and publishing results to avoid confusion.