Molecular size distribution, humic

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. 

Some characteristics of humic and fulvic acid samples. Unfortunately, we were not able to perform some desired analyses of the samples in the first place because of very small quantities in which some of the samples were available, and secondly, because some samples were not soluble enough to do molecular size distribution analyses. 

For most of the samples the molecular size distribution was not measured owing to the samples incomplete solubility, which shows their hydrophobic character. The molecular size distribution was determined ( 32) only for HALR humic acid sample (deposit of Ruppia maritima, Linnaeus) using size exclusion chromatography. The carbon content was measured by DOC and by a UV detector. Two peaks were... 

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). 

De Haan, H., 1972b. Molecular-size distribution of soluble humic compounds from different natural waters. Freshwater Biol., 2 235—241. 

Kainulainen T., Tuhkanen T., Varriainen T., Heinonen-Tanski H., Kalliokoski P. (1994), The effect of different oxidation and filtration processes on the molecular size distribution of humic material. Water Science Technology, 30, 9,169-174. 

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. 

Total protein assays have the advantage of being relatively straightforward compared to molecular-level analyses. Methods with fluorescence-based detection are also highly sensitive, and thus amenable direcdy to DON. Quantitative interpretation for environmental mixtures such as seawater, however, may be problematic for some samples. Most methods react with specific moieties (e.g., coomassie blue binds to lysine and arginine) and thus results obtained can depend on protein composition, size distribution, and even conformation (Sapan et ai, 1999), making the careful choice of calibration standards important. In addition, common components of natural samples, such as humic materials (e.g., Mayer et ai, 1986), carbohydrates (Sapan et ai, 1999), or NH3 may interfere with quantification. Overall, colorimetric methods can be very useful as quick, Hkely semi-quantitative estimates of total protein or peptide. However, potential biases inherent in the mechanism of a specific method should be considered before one is chosen, and appHcation of newer molecular assays (e.g., CBQCA) should be carefully examined in terms of natural sample matrix (Nunn et ai, 2003). 

The particle size distribution for the humic acid fraction is depicted in Figure 4. No material sedimented out until the most extreme conditions were applied (40,000 rpm for 24 hr), when some lightening of color at the top of the solution was observed. The sedimented particles had a Stokesian diameter of around 2 nm, which means that a particle size gap of three orders of magnitude exists between these and the next largest particles detected (5 xm). From the experimentally determined coal particle density of 1.43 g/cm, it was calculated that a solid sphere of diameter 2 nm would have a molecular mass of 4000. If the molecules were rod-shaped, even smaller molecular masses would be predicted. Literature values of the molecular mass of regenerated humic acids range between 800 and 20,000, with the values clustering around 1,000 and 10,000 (i5, 16, 17). 

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. 

Very little is known about if and when humic acid molecules are incorporated into stable residues. No information is available concerning aliphatic chains, aromatic structures, the distribution of the residual functional groups, or molecular size of the incorporated molecules. 

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). 

Hong, J. K. and Yamane, I. (1981). Distribution of inositol phosphate in the molecular size fractions of humic and fulvic acid fractions. Soil Sci. Plant Nutr. 27, 295-303. 

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]. 

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