Q

Inorganic compounds

Amorphous-SiO2

40-260 m2g-1

4.5-12 nm-2

pHzpc

= 3.0-3.5

Amorphous-FeOOH

160-700 m2g-1

0.1-0.9 mol/mol of Fe

pHzpc

= 7.9-8.1

Amorphous-Al2O3

260 m2 g-1

2-12 nm-2 (crystalline)

pHzpc

^ 9.4

Amorphous-MnO2

6-20 nm-2 (crystalline)

pHzpc

^ 2.3

Kaolinite Smectite Allophanes Illite

10-20 m2g-1 750-800 m2 g-1 500-700 m2g-1 90-130 m2g-1

0.6-3.6 nm-2 0.5-1.0 nm-2 0.4-1.2 nm-2 0.9-2.7 nm-2

Kaolinite Smectite Allophanes Illite

10-20 m2g-1 750-800 m2 g-1 500-700 m2g-1 90-130 m2g-1

0.6-3.6 nm-2 0.5-1.0 nm-2 0.4-1.2 nm-2 0.9-2.7 nm-2

Note: Values are representative (rather than exhaustive). Where possible, values are given for the amorphous phases of the inorganic colloids. For the inorganic colloids, the site densities refer to the maximum negative charge density for pH ^ pHZpc.

Source: (Modified from Buffle, J., et al., Environmental Science and Technology, 1998. 32(19): p. 2887-99 (and references therein) with permission from the American Chemical Society. Based in part on Buffle, J., Complexation Reactions in Aquatic Systems: An Analytical Approach. 1988, Chichester: Ellis Horwood; Thurman, Organic Geochemistry of Natural Waters. 1985, Dordrecht: Kluwer Academic Publishers Group; and Davis, J.A. and D.B. Kent, Reviews in Mineralogy, 1990, Mineralogical Society of America: Washington, D.C. p. 177-260.

It is therefore reasonable to represent the submicron inorganic colloids as compact, often negatively charged particles that cover the whole colloidal size range.

7.2.2 Allochthonous Macromolecules: Humic Substances

Humic substances (HS) generally represent the largest NOM fraction in freshwaters (typically 40 to 80%; ref. [9,19]. Because they are primarily ofpedogenic origin, they may also be present in the coastal waters of oceans but are generally absent from the pelagic zones. HS are chemically heterogeneous polymers with small molar masses (typically ~1000 Da; see ref. [23]), high charge densities at neutral or alkaline pH's (Figure 7.2A), and typical average lifetimes of several hundred years. Due to the significant degree of branching of HS, their high charge density (above pH 5; see ref. [24]) and degree of hydration (40% strongly bond water9), they are less flexible than linear biopolymers such as proteins. In dilute solutions (<10 mgl-1) at circumneutral pH and low ionic strength (<10-2 M), HS generally behave as small rigid globules, with diameters of 1 to 3 nm (Figure 7.2A and 7.2B; ref. [24-27]). HS aggregate formation is favored at high concentrations of electrolyte, protons (i.e., low pH), HS, or divalent cations.23,28,29 Although HS are sometimes represented as linear polyelectrolytes, the linear representation of the HS does not fit with either the experimental measurements discussed earlier in the chapter or the more modern models that consider HS as highly branched molecules.30 Nonetheless, because HS are both chemically heterogeneous and polydisperse, both within a given HS sample and with respect to samples isolated from different sources, there is no single useful structural model of a "typical" HS. Rather, most of the interesting properties of the HS result from their inherent

FIGURE 7.2 (A) Electrophoretic mobilities (EPM) and diffusion coefficients (D) of the International Humic Substances Society (IHSS) standard Suwannee River aquatic fulvic acid (SRFA) versus pH (A EPM; ■ D). Adapted from Hosse, M. and K.J. Wilkinson, Environmental Science and Technology, 2001. 35(21): p. 4301-6; Lead, J.R., et al. Environmental Science and Technology, 2000. 34(7): p. 1365-9 with permission from the American Chemical Society. (B) AFM image of the IHSS SRFA (10 mgl-1, 50 mM NaCl, pH 5.5) adsorbed to mica. Scan size is 600 nm x 600 nm. The image shows mainly isolated points (as opposed to aggregates) with adsorbed heights that averaged 0.8 ± 0.3 nm31. (Adapted from Balnois, E., et al. Environmental Science and Technology, 1999. 33(21): p. 3911-17 with permission from the American Chemical Society.)

FIGURE 7.2 (A) Electrophoretic mobilities (EPM) and diffusion coefficients (D) of the International Humic Substances Society (IHSS) standard Suwannee River aquatic fulvic acid (SRFA) versus pH (A EPM; ■ D). Adapted from Hosse, M. and K.J. Wilkinson, Environmental Science and Technology, 2001. 35(21): p. 4301-6; Lead, J.R., et al. Environmental Science and Technology, 2000. 34(7): p. 1365-9 with permission from the American Chemical Society. (B) AFM image of the IHSS SRFA (10 mgl-1, 50 mM NaCl, pH 5.5) adsorbed to mica. Scan size is 600 nm x 600 nm. The image shows mainly isolated points (as opposed to aggregates) with adsorbed heights that averaged 0.8 ± 0.3 nm31. (Adapted from Balnois, E., et al. Environmental Science and Technology, 1999. 33(21): p. 3911-17 with permission from the American Chemical Society.)

polydispersity and heterogeneity, implying that knowledge of property distributions will be more useful than mean values. For the above reasons, much still remains to be learned about the structure of HS, in particular, at low (i.e., environmentally relevant) concentrations.

7.2.3 Autochthonous Macromolecules: Proteins, Lipids, and Polysaccharides

In freshwaters, a large number of organic compounds, including polysaccharides, aminosugars, peptidoglycans, proteins, polyphenolic compounds, lignins, tannins, DNA, and polyhydroxybutyrates,28 are produced in the water column by the exudation or degradation of phytoplankton, aquatic bacteria, and aquatic macrophytes. Part of the autochthonous organic matter, especially structural components of plankton that are resistant to degradation, are subject to recombination and thus are similar to the HS except that they are generally more aliphatic and less hydrophilic, with slightly smaller charge densities and molar masses (~800 Da).9 Other classes of organic macromolecules, such as the reserve polysaccharides, are degraded within hours to days of their release into the water column.32 Aquatic proteins generally have a molar mass that is in the tens of thousands and a significant proportion of hydrophobic moieties, favoring the formation of globules with typical diameters of <3 to 4 nm.9 Nonetheless, the concentration of free protein in the water column is generally low due to its rapid degradation (hours to days) upon microbe death.33,34 Such observations are consistent with data showing that the amino acid contents of sediment traps decline rapidly with depth35 and that protease activity is high in surface water aggregates.36

Structural, fibrillar polysaccharides and peptidoglycans are also released from plankton as exudates or cell wall components.32,37-39 They can constitute a significant proportion of freshwater NOM, varying seasonally between ca. 5% and 30% in the surface waters of lakes (Figure 7.3; ref. [13,39]) and often accounting for an even larger proportion of the NOM in the surface waters of marine systems.40-42 In marine systems, fibrillar polysaccharides have been shown to be refractory enough to be found in the deep ocean where they may have lifetimes of hundreds of years4 while peptidoglycans appear to be degraded much faster (turnover time of weeks to months34). Freshwater polysaccharides are generally quite rigid due to a large quantity of strongly bound hydration water (up to 80%) and the association of the molecules into double or triple helices that can be stabilized by hydrogen or calcium bridges.43 TEM and AFM images of freshwater and marine biopolymers (e.g., Figure 7.3; ref. [40,44]) show that their total length may be larger than 1 ^m, whereas their thickness is often only a few nanometers. Furthermore, because they are often charged molecules (i.e., charge densities of the polysaccharides are typically in the range of 0 to -0.8 meqg-1, ref. [9]) their conformations can change as a function of the pH and ionic strength (Figure 7.3A,B). Indeed, in natural waters, the charge density of the natural organic polyelectrolytes is often lower than their maximum value due to partial protonation, complexation by metals, and electrolyte screening. In addition to the association of molecules into dimers and trimers, aggregation of those structures is thought to lead to the

FIGURE 7.3 (A) AFM image of the bacterial (Rhizobium meliloti) polysaccharide, suc-cinoglycan (10 mg l-1 ) in water. Scan size 500 nmx500 nm. (B) AFM image of succinoglycan (10 mgl-1) in 0.01 M KCl. Scan size 705 nm x 705 nm. (C) AFM image of succinoglycan (10 mg l-1) in 0.5 M KCl. Scan size 2.3 jm x 2.3 jm. (All three images were adapted from Balnois, E., etal. Macromolecules, 2000.33(20): p. 7440-7 with permission from the American Chemical Society.)

FIGURE 7.3 (A) AFM image of the bacterial (Rhizobium meliloti) polysaccharide, suc-cinoglycan (10 mg l-1 ) in water. Scan size 500 nmx500 nm. (B) AFM image of succinoglycan (10 mgl-1) in 0.01 M KCl. Scan size 705 nm x 705 nm. (C) AFM image of succinoglycan (10 mg l-1) in 0.5 M KCl. Scan size 2.3 jm x 2.3 jm. (All three images were adapted from Balnois, E., etal. Macromolecules, 2000.33(20): p. 7440-7 with permission from the American Chemical Society.)

formation of rigid, poorly defined structures including gels, flocs, and biofilms45 (Figure 7.3C).

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