Modeling pH and ionic strength effects on proton and calcium complexation of fulvic acid: A tool for drinking water-NOM studies

It is widely recognized that organic charge and metal complexation by natural organic matter (NOM) is critical to a wide range of adsorption and precipitation technologies used for the removal of NOM during water treatment. Due to a variety of analytical limitations, these properties are frequently measured under conditions that are not typical of water treatment system (e.g., high ionic strength). In this paper a mathematical model, similar to one described in Bartschat at al. (Environ. Sci. Technol. 1992, 26, 284-294) has been used to describe (A) the effect of ionic strength on proton complexation of fulvic acid and (B) the effect of pH and ionic strength on the calcium complexation of fulvic acid. The model incorporates molecular size information by assuming these molecules to be impenetrable spheres of some experimentally determined average size. It also incorporates electrostatics by using the Poisson-Boltzmann equation to calculate the surface potential of the average size molecules. The model described here can be calibrated with data from conventional methods used to characterize NOM. It can then be used to extrapolate behavior with respect charge, proton, and calcium complexation to conditions typical of dilute drinking waters. Full calibrations are reported for two aquatic fulvic acids: one based on a new data set, and one based on the work of Dempsey.