Chemical Modeling of Boron Adsorption by Humic Materials Using the Constant Capacitance Model

Abstract

The constant capacitance surface complexation model was used to describe B adsorption behavior on reference Aldrich humic acid, humic acids from various soil environments, and dissolved organic matter extracted from sewage effluents. The reactive surface functional groups on the humic materials were assumed to be a carboxyl site, XOH, and a phenol site, YOH. Initially, total concentrations of the sites, XOH_T and YOH_T and the proton dissociation constants for the carboxyl site, LogK_X-, and the phenol site, LogK_Y-, were optimized by fitting the constant capacitance model to potentiometric titration data on reference Aldrich humic acid obtained from the literature. Subsequently, the model was fit to experimental B adsorption data obtained from the literature by optimizing two tetrahedral B surface complexation constants: LogK_XB- for a carboxyl site and LogK_YB- for a phenol site. The model was well able to describe the experimental B adsorption data both as a function of solution B concentration (isotherm data) and solution pH (envelope data) for all humic materials. The ability to represent changes in B adsorption as a function of solution pH is the main advantage of the constant capacitance model over adsorption isotherm equations. Results from the current study can be used to describe B adsorption behavior on diverse humic materials of interest in environmental and agricultural situations.