Characterization Of The Major Aquatic Colloids

In this chapter, colloids are defined as any organic or inorganic entity large enough to have supramolecular structure and properties (e.g., possibility of conformational changes for organic colloids or an electrical surface field for inorganic colloids), but small enough not to sediment quickly (hours to days) in the absence of aggregation. This definition implies that the colloidal size range will typically be between 1 nm and 1 ixm. In freshwaters, colloids include significant organic and inorganic components, with the proportions of each component depending upon the nature of the water body and surrounding watershed, the vertical and horizontal position in the water body, the climate, etc. Typical size distributions and some important physico-chemical characteristics of the principal freshwater colloids are given in Figure 7.1 and Table 7.1.

Solutes -10

; Colloids or

1 nm

Particles

log size /m

Viruses Bacteria Algae u go np

Hydroxy acids Amino acids Peptides

Cellular debris

Proteins

Polysaccharides

Fulvics

Humics

Hydrated anions organic compounds adsorbed on inorganicp articles

Fex(OH)y

Hydrated cations

FeOOH

MnO2

Clays

Carbonates

Metal sulfides Metal phosphates

FIGURE 7.1 Schematic representation, by size, of some of the important organic and inorganic components of freshwaters. (Modified from Buffle, J. and H.P. van Leeuwen, Environmental Particles. 1992, Chelsea, Michigan: Lewis Publishers, Inc. with permission from Lewis Publishers.)

7.2.1 Inorganic Colloids

The major inorganic colloids found in oxic freshwaters include alumino-silicates (clays), silica, and iron oxyhydroxyde particles.2,20,21 Other inorganic colloids can also be found, but they are usually minor components (e.g., aluminum or manganese oxides) or are only present in anoxic waters (e.g., elemental sulfur or FeS). Calcium carbonate is found in significant quantities in freshwaters, especially during periods of high primary productivity, but it is mostly particulate (i.e., >1 ^.m). Inorganic particles are generally electrically charged due to the isomorphic substitution of ions in the bulk solid phase, or to the reactions of surface functional groups with dissolved ions in the aqueous phase.22 Apart from the iron oxyhydroxides which are neutral or positively charged in the circumneutral pH range, the major inorganic colloids are generally negatively charged in water due to their low zero point of charge (Table 7.1; seeref. [6,21]).

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