A log-normal distribution model for the molecular weight of aquatic fulvicacids

The molecular weight of humic substances influences their proton and metal binding, organic pollutant partitioning, adsorption onto minerals and activ ated carbon, and behavior during water treatment. We propose a lognormal mo del for the molecular weight distribution in aquatic fulvic acids to provid e a conceptual framework for studying these size effects. The normal curve mean and standard deviation are readily calculated from measured M-n and M- w and vary from 2.7 to 3 for the means and from 0.28 to 0.37 for the standa rd deviations for typical aquatic fulvic acids. The model is consistent wit h several types of molecular weight data, including the shapes of high-pres sure size-exclusion chromatography (HP-SEC) peaks. Applications of the mode l to electrostatic interactions, pollutant solubilization, and adsorption a re explored in illustrative calculations.