| This thesis investigates the roles of humic substance (HS) adsorption on Fe-oxide (goethite, alpha-FeOOH), molecular weight (MW) fractionation upon adsorption, and the molecular weight distribution (MWD) model and its application. High pressure size exclusion chromatography (HPSEC) is applied to determine the MW average and MWD.Since certain applications and our model require better precision for MW of HS, one chapter of this thesis focuses on improving precision and reproducibility of HPSEC measurements. A small variation of standard calibration function results in large differences in MW averages (20 to 50%). Hence, using the same set of standards is crucial for intra- and/or inter-laboratory reproducibility and comparability. The choice of low MW (LMW) cutoff can greatly affect number average MW and polydispersity (Mn and rho). For FA, we recommend either 2% of the maximum chromatogram height or MW = 50 as the LMW cutoff. UV detection wavelengths between 230 and 280 run give reasonable results. 'Standardization' of HPSEC procedures can lead to excellent reproducibility of MW measurements for HS.A log normal distribution (LND) model is proposed for the MWD of aquatic fulvic acids to provide a conceptual framework for studying these molecular size effects. The normal curve parameters mu and sigma are readily calculated from measured Mn and weight average MW (Mw), and vary from 2.7 to 3 for mu and from 0.28 to 0.37 for sigma for typical aquatic fulvic acids. The model is consistent with several types of MW data, including the shapes of HPSEC peaks.Adsorption isotherms and adsorption kinetics of an aquatic fulvic acid (XAD-8 resin extract) onto goethite were studied. Adsorption isotherms of overall samples and of the dominant intermediate molecular weight (IMW) fraction (1250--3750) fit a Langmuir model well. However, this overall good fit masked non-Langmuirian behavior of high and low molecular weight fractions (HMW, LMW, respectively). Preferential adsorption of HMW fractions at low pH, and of IMW fractions at high pH was observed. |
