Colloids dissolved organic matter

KEYWORDS Colloids, Dissolved organic matter, Fractionation, Fluorescence, Landfill leachate... 

Boon, J. J., Klap, V., and Eghnton, T. (1998). Molecular characterization of microgram amounts of colloidal dissolved organic matter (UDOM) in ocean water samples by direct temperamre resolved ammonia chemical ionization mass spectrometry. Org. Geochem 29, 1051—1061. 

Hence, in principle, we could explain the discrepancies between predicted and observed BAFt values by the reduction of the bioavailability of the compounds caused by sorption to colloidal organic matter present in the culture media. We should note, however, that depending on the nature of the dissolved organic matter (i.e., molecular size distribution, aromaticity, polarity, etc.), the KiDOC value of a compound may vary considerably (see Section 9.4). 

It would lie far beyond the aim of this chapter to introduce the state-of-the art concepts that have been developed to quantify the influence of colloids on transport and reaction of chemicals in an aquifer. Instead, a few effects will be discussed on a purely qualitative level. In general, the presence of colloidal particles, like dissolved organic matter (DOM), enhances the transport of chemicals in groundwater. Figure 25.8 gives a conceptual view of the relevant interaction mechanisms of colloids in saturated porous media. A simple model consists of just three phases, the dissolved (aqueous) phase, the colloid (carrier) phase, and the solid matrix (stationary) phase. The distribution of a chemical between the phases can be, as first step, described by an equilibrium relation as introduced in Section 23.2 to discuss the effect of colloids on the fate of polychlorinated biphenyls (PCBs) in Lake Superior (see Table 23.5). 

Another analytical constraint is the definition of dissolved organic matter. Filters in the 0. l-1.0- im size range pass colloids that are not truly dissolved. Studies discussed in this chapter will be limited to natural organic solutes that are either isolated by adsorption chromatography or ultrafiltered through 0.005- JLm filters. Although neither of these techniques will absolutely ex-... 

Published values of BAFs for HOCs, summarized by Swackhamer and Skoglund (10), range from 18 to 1,000,000. The variety of methods used in these studies prevents direct comparison, so it is unclear what factors are responsible for the large variation in BAFs. Several hypotheses have been proposed to explain these variations in accumulation and their deviations from KoW-based predictions. One hypothesis (II, 12) proposed that a lack of complete reversibility in the partitioning process is responsible for the deviations. A second (13-16) theory attributed the deviations to the presence of a third phase (colloids or dissolved organic matter) and the inability to accurately separate the dissolved and sorbed states. A third (17) proposal is that partitioning is dependent on sorbent concentration. And finally, a fourth (18, 19) hypothesis holds that this deviation is a function of the effects of molecular size and shape on cellular transport. 

Sorption coefficients quantitatively describe the extent to which an organic chemical is distributed at equilibrium between an environmental solid (i.e., soil, sediment, suspended sediment, wastewater solids) and the aqueous phase it is in contact with. Sorption coefficients depend on (1) the variety of interactions occurring between the solute and the solid and aqueous phases and (2) the effects of environmental and/or experimental variables such as organic matter quantity and type, clay mineral content and type, clay to organic matter ratio, particle size distribution and surface area of the sorbent, pH, ionic strength, suspended particulates or colloidal material, temperature, dissolved organic matter (DOM) concentration, solute and solid concentrations, and phase separation technique. 

Many factors potentially can affect the distribution of an organic chemical between an aqueous and solid phase. These include environmental variables, such as temperature, ionic strength, dissolved organic matter concentration, and the presence of colloidal material, and surfactants and cosolvents. In addition, factors related specifically to the experimental determination of sorption coefficients, such as sorbent and solid concentrations, equilibration time, and phase separation technique, can also be important. A brief discussion of several of the more important factors affecting sorption coefficients follows. 

Tranvik, L. J., and N. O. G. Jorgensen. 1995. Colloidal and dissolved organic matter in lake water Carbohydrate and amino acid composition, and ability to support bacterial growth. 

Direction 2. A large portion, usually >90%, of the organic matter imported from allochthonous and littoral/wetland sources to these aquatic ecosystems is predominantly in dissolved or colloidal form. Although a portion of the dissolved organic compounds may aggregate and shift to a particulate and hence gravitoidal form that may sediment out of the water, most of the imported dissolved organic matter is dispersed within the water... 

Detritus includes nonliving particulate, colloidal, and dissolved organic matter, and metabolically size affects only the rates of hydrolytic attack (Wetzel, 1995). Inland aquatic ecosystems collect organic matter, particularly... 

Boehme, J., and Wells, M. (2006). Fluorescence variability of marine and terrestrial colloids Examining size fractions of chromophoric dissolved organic matter in the Damariscotta River estuary. Mar. Chem. 101, 95-103. 

Coagulation processes in estuaries are affected by other factors such as clay composition, particle size, and concentration of dissolved organic matter, to mention a few. For example, early work has shown that metal hydroxides can flocculate from dis-solved/colloidal organic matter during the mixing of river-derived iron and seawater in the mixing zone of estuaries (Sholkovitz, 1976, 1978 Boyle et al, 1977 Mayer, 1982) (more details are provided on metal colloidal interactions in chapter 14). Surface sediments in... 

Wells, M.L. (2002) Marine colloids and trace metals. In Biogeochemistry of Marine Dissolved Organic Matter (Elansell, D.A., and Carlson, C.A., eds.), pp. 367-397, Academic Press, New York. 

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