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A Log-Normal Distribution Model for the Molecular Weight of Aquatic Fulvic Acids

Stephen E. Cabaniss,*^† Qunhui Zhou,^‡ Patricia A. Maurice,^‡ Yu-Ping Chin,^§ and George R. Aiken^#

Department of Chemistry, Kent State University, Kent, Ohio 44242, Department of Geology, Kent State University, Kent, Ohio 44242, Department of Geological Sciences, The Ohio State University, Columbus, Ohio 43210, and United States Geological Survey, Marine Street, Boulder, Colorado 80303

Environ. Sci. Technol., 2000, 34 (6), pp 1103–1109

DOI: 10.1021/es990555y

Publication Date (Web): February 15, 2000

Corresponding author phone: (330)672-3731; fax: (330)672-3816; e-mail: scabanis@kent.edu.

†

Department of Chemistry, Kent State University.

‡

Department of Geology, Kent State University.

The Ohio State University.

United States Geological Survey.

Abstract

The molecular weight of humic substances influences their proton and metal binding, organic pollutant partitioning, adsorption onto minerals and activated carbon, and behavior during water treatment. We propose a log-normal model for the molecular weight distribution in aquatic fulvic acids to provide 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 standard deviations for typical aquatic fulvic acids. The model is consistent with several types of molecular weight data, including the shapes of high-pressure size-exclusion chromatography (HP-SEC) peaks. Applications of the model to electrostatic interactions, pollutant solubilization, and adsorption are explored in illustrative calculations.

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