Molecular size distribution, humic substances

Peuravuori, J., and K. Pihlaja. 1997. Molecular size distribution and spectroscopic properties of aquatic humic substances. Analytica Chimica Acta 337 133. 

Summers, R. S., Cornel, P. K., and Roberts, P. V. (1987). Molecular size distribution and spectroscopic characterization of humic substances. Sci. Total Environ. 65,27-37. 

The task, however, of determining molecular weights for humic substances has not been simple. Humic substances comprise one of the most widely distributed classes of natural products on Earth. Defining the chemistry and understanding the nature of humic substances have long been hampered because most humic substances are not discrete chemical entities, but are a complex mixture of organic substances (Kononova, 1966) with a wide range of molecular sizes (polydisperse). 

The influence of the pore size distribution of carbon on NOM uptake has been recognized by several researchers [57, 63]. Likewise, Karanfil and coworken [64] and Kilduffand coworkers [65] concluded that the adsorption of humic substances was largely governed by molecular size distribution in relation to pore size. Moreover, a good linear relationship (Fig. 25.6) was foundbetween the amount adsorbed by different carbons and their pore volume between 0.8 and 50 nm [61, 66] when the adsorption was carried out at pH 3. This is because electrostatic effects are minimized under these experimental conditions and nonelectrostatic interactions predominate. The adsorption mechanism would be due to hydrophobic and/or TT-TT-electron interactions, and in this case as with other electrolytes (see above). 

Shaw P.J., de Haan H , Jones R.I. (1994), Applicability and reliability of gel filtration to study aquatic humic substances revisited the effects of pH on molecular size distributions. Environmental Technologjq 15,753-764. 

Size exclusion chromatography (SEC) has been used to measure molecular weight (MW) distribution of humic substances (3, 6-9). Coupled with detection methods such as molecular fluorescence spectroscopy and dissolved organic carbon analysis (7), electrochemical detection (9), and atomic emission spectroscopy (5), SEC has been used extensively to study humic-metal complexes. A major disadvantage of SEC is that it does not provide adequate resolution for separating humic materials as they do not appear to be made up of distinct fractions with large differences in MW. 

Comparison of humic substances from different lakes indicates a high variation in concentration, composition, and molecular weight. The extent to which differences in methodology contribute to this variation has not been evaluated. Temporal and spatial distributions of dissolved humic substances and humic-associated organic substances are presented for five representative lakes. General parameters (UV absorbance, DOC measurements with or without fractionation on the basis of molecular size) do not adequately reflect the dynamic nature of various humic substances in lake ecosystems. 

Wrobel, K., Sadi, B. B. M., Wrobel, K., Castillo, J. R., and Caruso, J. A., Effect of metal ions on the molecular weight distribution of humic substances derived from municipal compost ultrafiltration and size exclusion chromatography with spectrophotometric and inductively coupled plasma-MS detection. Anal. Chem., 75, 761-767, 2003. 

The extent of competition may also be a function of the adsorbate molecular size, correlated with the activated carbon pore size distribution. Activated carbon fibers tliat are exclusively microporous (more than 96 % of micropore volume) present a selectivity property for pesticides or phenol in the presence of higher molecidar weight compounds like humic substances, due to the direct connection of micropores to their external surface [41]. Using granular activated carbon, which does not have this molKUilar sieve property, a 20 to 70 % reduction in adsorption is obtained for atrazine in raw water compared with equilibria in distilled water [42]. 

Pore size distribution Pore diameters tontrol which sizes of molecules are accessible to them. - micropores (c 2 nm) for microorganics, pesticides mesopores (2-50 nm) for higher molecular weight compounds like dyes, humic substances... 

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