title: Effect of polydispersity on natural organic matter transport.
abstract: The mobility of humic-substance dominated natural organic matter (NOM) concentrated from a freshwater wetland by reverse osmosis was examined in sand columns at pH 5-8, in 0.001 M and 0.01 M NaClO4. Greater mobility was observed at higher pH and lower ionic strength, although breakthrough curves (BTCs) for bulk NOM exhibited extensive tailing under all conditions examined. Based on observations from previous batch experiments indicating preferential adsorption of intermediate to high molecular weight (MW) NOM, we postulate that 'adsorptive fractionation' of the NOM pool leads to the observed tailing behavior, and develop a novel approach to assess the effects of polydispersity on transport of NOM and associated contaminants. BTCs for different NOM fractions were constructed by separating column effluent MW distributions determined by high-pressure size exclusion chromatography into five discrete intervals or 'bins' and calculating the mass of NOM within each bin at four sampling times. Observed retardation factors (Ro), reflecting median arrival time relative to that of a nonreactive tracer, ranged from 1.4 to 7.9 for the various bins and generally increased with MW. NOM retarded transport of the contaminant metal Cd (2.5 ppm, in 0.01 M NaClO4) slightly at pH 5 and more substantially at pH 8. Although Cd had little or no effect on bulk NOM transport, retention of the more aromatic, IMW-HMW NOM appeared to be slightly enhanced by Cd. Study results demonstrate that heterogeneity in retardation as a function of MW is likely a major factor contributing to bulk NOM BTC tailing and may have important implications for contaminant transport.
authors.affiliation: Department of Civil and Environmental Engineering and Earth Sciences, University of Notre Dame, 156 Fitzpatrick Hall, Notre Dame, IN 46556, USA.
authors.forename: Lindsay A
authors.initials: LA
authors.lastname: Seders Dietrich
authors.forename: Daniel P
authors.initials: DP
authors.lastname: McInnis
authors.forename: Diogo
authors.initials: D
authors.lastname: Bolster
authors.forename: Patricia A
authors.initials: PA
authors.lastname: Maurice
mesh_terms: D002104:Cadmium / Q000032:analysis; D004573:Electrolytes / Q000737:chemistry; D006812:Humic Substances / Q000032:analysis*; D006863:Hydrogen-Ion Concentration; D008962:Models, Theoretical; D008970:Molecular Weight; D009038:Motion; D012235:Rhodamines / Q000032:analysis; D012822:Silicon Dioxide / Q000737:chemistry; D014865:Waste Disposal, Fluid
chemical_list: D004573:Electrolytes; D006812:Humic Substances; D012235:Rhodamines; D002104:Cadmium; C022027:lissamine rhodamine B; D012822:Silicon Dioxide