Hydrophilic acids

Figure 3. Continued. Excitation-emission spectra for hydrophobic neutral (e) and hydrophilic acid (f) fractions.

Sensitive dienol ether functionality in the diene carboxylate was shown to be compatible with the conditions of the aqueous Diels-Alder reaction (Eq. 12.26).84 The dienes in the Diels-Alder reactions can also bear other water-solubilizing groups such as the sodium salt of phosphoric acid and dienyl ammonium chloride (Eq. 12.27).85 The hydrophilic acid functionality can also be located at the dienophile.86... 

Table 1 presents the results of fractionations of the DOM. The result of mass balance calculation of the DOC system shows that more than 55 % of the total DOC was retained by XAD-8 resin column, involving the portions of Ho A and HbN/B, and DOC concentrations of the portion eluted by blackwashing (HoA) accounted for 47.4 % of total DOC, as compared with 26.25 % hydrophilic acids (HiA) of the total DOC. More than 11% of the total DOC passed through two resin columns, indicating that small molecular weight polar components were not absorbed onto by XAD-8 and XAD-4. The fractionation did cause potential loss of organic matter by permanent adsorption onto resin s polymers, which were 8.34 % for the XAD-8 resin and 6.41 % for the XAD-4 resin, respectively. 

Sample DOC (mg C/L) Volume of Water Processed (L) Hydrophobic and Hydrophilic Acids Extracted (g)... 

A more complex protocol is sometimes used, in which the acidified solution that has been passed through XAD-2 or XAD-8 resin is then passed through a column of XAD-4 resin. Back-elution of adsorbed DOM from the XAD-4 resin with NaOH yields an additional quantity of DOM that is variably known as hydrophilic acids, XAD-4 acids, transphilic acids, or amphiphilic acids. 

DOM can be fractionated into six fractions in terms of polarity by macrore-ticular exchange resins as described by Leenheer (1981) hydrophilic acid (HiA), base (HiB), and neutral (HiN), and hydrophobic acid (HoA), base (HoB), and neutral (HoN). The distribution of various fractions of DOM in the selected organic wastes was given in Table 10.1. 

As described above, immersion calorimetry constitutes a powerful technique for the textural and chemical characterization of porous solids. In the absence of specific adsorbate-adsorbent interactions, heats of immersion can be related to the surface area available for the molecules of the liquid. However, the use of polar molecules or molecules with functional groups produces specific adsorbent-adsorbate interactions related to the surface chemical properties of the solid. An adequate selection of the immersion liquid can be used to study hydrophilicity, acid-base character, etc. Table 2 reports the enthalpies of immersion (J/g) into different lineal and branched hydrocarbons (n-hexane, 2-methyl-pentane and 2,2-dimethyl-butane) for Zn exchanged NaX zeolites. 

Humic substances are a broad class of organic compounds operationaUy defined by their solubility at different pHs and retention on hydrophobic resins (Aiken, 1988 Thurman, 1985). There are three operational sub-categories of humic substances humic acids, which are soluble at a higher pH but become insoluble at a pH < 2 (isolated using XAD-8 resin) fulvic acids, which are hydrophilic acids soluble under aU pH conditions (isolated using XAD-4 resin), and humin, which is insoluble at any pH (Ishiwatari, 1992). For a review of humic substances in aquatic systems, see Hessen and Tranvik (1998), Benner (2002), and Chapter 3 by Aluwihare and Meador, this volume. 

The statistical summary in Figure 1 is organized in terms of the classification scheme of Leenheer and Huffman (1976), rather than in terms of HAs, FAs, and XAD-4 acids. Data for FAs and HAs are incorporated into the data set for hydrophobic acids, and data for XAD-4 acids are incorporated into the data set for hydrophilic acids. The statistical summary in Figure 1 is presented as percentages of DOC, because original data were generally expressed on that basis. It will be assumed in this discussion that all six fractions of DOM have similar carbon contents. It is evident that hydrophobic bases are rarely reported and, when they are measured. 

Several studies have coupled RO with fractionation methods for isolation and fractionation of DOM. Crum et al. (1996) used RO membranes to concentrate DOM, and then they used a series of UF membranes to fractionate the concentrated DOM into three size fractions. Ma et al. (2001) used RO membranes to concentrate DOM, and then they used XAD-8 resin (see earlier discussion) to fractionate the concentrated DOM into HAs, FAs, and hydrophilic acids. 

Resin-based methods of isolation and fractionation of DOM indicate that more than 80% of DOM is distributed in a 2 1 ratio of hydrophobic acids (56%) and hydrophilic acids (28%). The hydrophobic acids are further distributed in an -3 1 ratio of FAs (46%) and HAs (14%). Membrane-based methods of isolation and fractionation of DOM indicate that molecular weights of DOM fractions range from <1 kDa to more than 100 kDa however, these results are almost certainly too high. 

DOM is often fractionated into six fractions hydrophobic acids, bases, and neutral compounds, and hydrophilic acids, bases, and neutral compounds. In the median freshwater, more than 80% of DOC is distributed in a 2 1 ratio between hydrophobic acids and hydrophilic acids, and less than 20% of DOC is evenly distributed between hydrophilic bases and the two neutral fractions. Very little DOC is in the hydrophobic base fraction. In the median freshwater, —60% of DOC is generally distributed in a 3 1 ratio between FAs and HAs. 

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