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 wat er treatment. Due to a variety of analytical limitations, these proper ties are frequently measured under conditions that are not typical of water treatment system (e.g., high ionic strength). In this paper a ma thematical model, similar to one described in Bartschat at al. (Enviro n. 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 fu lvic acid. The model incorporates molecular size information by assumi ng these molecules to be impenetrable spheres of some experimentally d etermined average size. It also incorporates electrostatics by using t he Poisson-Boltzmann equation to calculate the surface potential of th e average size molecules. The model described here can be calibrated w ith data from conventional methods used to characterize NOM. It can th en be used to extrapolate behavior with respect charge, proton, and ca lcium complexation to conditions typical of dilute drinking waters. Fu ll calibrations are reported for two aquatic fulvic acids: one based o n a new data set, and one based on the work of Dempsey.