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 thehumicmaterialswere assumedto be a carboxylsite, XOH, and a phenol site, YOH. Initially, total concentrations of the sites, XOHTand YOHTand the proton dissociation constants for the carboxyl site, LogKX- ,a nd the phenol site, LogKY-, were optimized by fitting the constant capacitance model to potentiometric titration data on reference Aldrich humic acid ob- tained from the literature. Subsequently, the model was fit to experimental B adsorption data obtained from the literature by optimizing two tetrahe- dral B surface complexation constants: LogKXB- for a carboxyl site and LogKYB- for a phenol site. The model was well able to describe the exper- imental 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 adsorp- tion isotherm equations. Results from the current study can be used to de- scribe B adsorption behavior on diverse humic materials of interest in environmental and agricultural situations.