Annals of Environmental Science
Volume 1, February 2007, Pages 45-56
Dissolved Organic Nitrogen Fractionation
J.A. Leenheer, A. Dotson and P. Westerhoff, U.S. Geological Survey, PO Box 25046, MS 405, Denver Federal Center, Denver, CO 80225, and Department of Civil and Environmental Engineering, Arizona State University, Engineering Research Center, Room ECG 252, G-Wing, Tempe, AZ 85287-5306, USA
Received August 29, 2006; in final form November 29, 2006; Accepted January 2, 2007
A preparative dissolved organic fractionation method was developed to concentrate isolated fractions in organic nitrogen for nitrogenous disinfection by-product research. The method was developed and tested on dissolved organic matter (DOM) derived from an algal culture, a bacterial culture, a meso-trophic lake during the beginning of an algae bloom and an effluent from a wastewater-treatment plant. The method was based on the assumption that most organic nitrogen in DOM is in proteins and amino acids. Proteins, peptides, and certain amino acids are retained on the macroporous styrene/ divinylbenzene resins XAD-1 and XAD-4 at pH > 10, whereas the macroporous methylmethacrylate resin XAD-8 used in standard DOM fractionation did not adsorb these compounds at pH > 10. Nitrogen-rich colloids were first isolated by dialysis of a roto-evaporated concentrate. The dialysis permeate was passed through a 1 liter (L) column of XAD-1, a 0.5 L column of XAD-4, and a 1 L column of MSC-1H cation-exchange resin in series. Hydrophobic acids were desorbed from the XAD-1 resin with 1 L of 0.1 M NaOH and amphiphilic acids were desorbed from the XAD-4 resin with 0.5 L of 0.1 M NaOH. After desorption of the acids, the XAD-1 and XAD-4 resins were immediately acidified with a 0.01 M HCl rinse, and the protein rich hydrophobic neutral fraction (XAD-1), and amphiphilic neutral (peptide and amino acid) fractions were desorbed with 75% acetonitrile/25% water. After evaporation and freeze drying, the hydrophobic neutral fraction was extracted with 50 milliliters (mL) of ethyl acetate to remove non-nitrogenous acids. The fractions were character-rized by elemental and spectral analyses. The bacterial and algal samples had large percentages of nitrogen-rich colloid and base and neutral fractions, and the lake sample and wastewater treatment plant effluent had large percentages of nitrogen-poor acids.