Colloidal phase

Babiarz CL, Hurley JP, Hoffinaim SR, Andren AW, Shafer MM, Armstrong DE. 2001. Partitioning of total mercury and methyhnercury to the colloidal phase in fresh waters. 

Initial °Th and Pa are generally considered to be associated with a detrital component that becomes cemented, or occluded, within the speleothem. This component may be composed of clays, alumino-silicates or Fe-oxyhydroxides (Fig. 3) with strongly adsorbed and Pa. Th and Pa incorporated in speleothems and similar deposits may also have been transported in colloidal phases (Short et al. 1998 Dearlove et al. 1991), attached to organic molecules (Langmuir and Herman 1980 Gaffney et al. 1992) or as carbonate complexes in solution (Dervin and Faucherre 1973a, b Joao et al. 1987). 

Influence of U colloidal transport in organic-poor surface waters has been far less studied. Riotte et al. (2003) reported U losses from 0 to 70% during ultrafiltration experiments for surface waters of Mount Cameroon without nearly any DOC. Even in the low concentration waters, U can be significantly fractionated from other soluble elements by the occurrence of a colloidal phase, probably inorganic in origin. However, such fractionations are not systematic because of the occurrence of various colloidal phases, characterised by different physical and chemical properties, and hence different sorption and/or complexation capacities (Section 2.1). 

Several studies have examined the partitioning of U on particles and colloids. Results from detailed sampling and particle separation in the Amazon estuary shows that most of the uranium at the Amazon River mouth is associated with particles (>0.4 im) and that >90% of the uranium in filtered water (<0.4 im) is transported in a colloidal phases (from a nominal molecular weight of 10 000 MW up to 0.4 im) (Swarzenski et al. 1995 Moore et al. 1996). Mixing diagrams for uranium in different size fractions in the Amazon estuary reveal that uranium in all size fractions clearly display both removal and substantial input during mixing. 

Figure 5. The in 0.2pm and 3 kD filtered water and colloids phase (3kD - 0.2pm) and particles (>0.2 pm) as well as material from sediment traps plotted versus conductivity in the low salinity zone (0-3) of the Kalix River estuary. The stippled area marks the reported annual range in at the Kalix river mouth, which show a substantial variation compared to the uranium concentration. Data from Andersson et al. (2001). Copyright 2001 Elsevier Science.

Although the abundance of silver in the Earth s crust is comparatively low (0.07 pgg-1), it is considered an environmental contaminant and is toxic at the nanomolar level. As an environmental pollutant it is derived from mining and smelting wastes and, because of its use in the electrical and photographic industries, there are considerable discharges into the aquatic environment. Consequently, there have been studies on the geochemistry and structure of silver-sulfur compounds [31]. Silver, either bound to large molecules or adsorbed on to particles, is found in the colloidal phase in freshwater. In anoxic sediments Ag(I) can bind to amorphous FeS, but dissolved silver compounds are not uncommon. A more detailed study of silver speciation in wastewater effluent, surface and pore waters concluded that 33-35% was colloidal and ca. 15-20% was in the dissolved phases [32]. 

The importance of the colloidal phase in the distribution of water pollutants is a relatively recent issue in the environmental literature [4,105,106]. The phenomenon of colloidal solubility enhancement was detected by workers in several fields and was largely unexplained. The concept was apparently developed and forwarded by working with partitioning behavior of water pollutants in water/ sediment systems. 

Hence, DHS will manifest the greatest solubility enhancement for those pollutants which are the least soluble in water or the most attracted to the solid phase. Organic pollutants, which are soluble in water, are less likely to be sorbed onto the solid or colloidal phase in the absence of specific bonding interactions. 

Chemical analyses of marine organic matter usually starts with a segregation into the solid, dissolved, or colloidal phase. As described in Chapter 3.2, this segregation often... 

Contaminants bound to colloids also may lead to an increase in the apparent solubility of the compounds. Most colloidal phases are effective sorbents of low-solubility contaminants, due to their large surface area. For example. Fig. 8.21 depicts the solubilization of p-nitrophenol into hydrophobic microdomains, which defines the trace metal level in the groundwater of a coastal watershed (Sanudo-Wilhelmy et al. 2002). The authors emphasize that the (heavy) metals contained in the colloidal size fraction in some instances may reach more than 50% of what is considered dissolved metal this should be considered to properly understand the cycling of metals and carbon in the subsurface water. 

Babiarz et al. (2001) examined total mercury (Hg) and methyhnercury (Me-Hg) concentrations in the colloidal phase of 15 freshwaters from the upper Midwest and Southern United States. On average, Hg and Me-Hg forms were distributed evenly between the particulate (0.4 jm), colloidal, and dissolved (lOkDa) phases. The amount of Hg in the colloidal phase decreased with increasing specific electric conductance. Furthermore, experiments on freshwater with artificially elevated electric conductance suggest that Hg and Me-Hg may partition to different subfractions of colloidal material. The two colloidal Hg phases act differently with the same type of adsorbent. For example, the colloidal phase Hg correlates poorly with organic carbon (OC) but a strong correlation between Me-Hg and OC was observed. 

Mrestani, Y., Neubert, R. (2000). Characterization of cephalosporin transfer between aqueous and colloidal phases by micellar electrokinetic chromatography. J. Chromatogr. A 871 439-448. 

A variation of the CD process for PbSe involved deposition of a basic lead carbonate followed by selenization of this film with selenosulphate [64]. White films of what was identified by XRD as 6PbC03-3Pb(0H)2-Pb0 (denoted here as Pb—OH—C) were slowly formed over a few days from selenosulphate-free solutions that contained a colloidal phase and that were open to air (they did not form in closed, degassed solutions). CO2 was necessary for film formation—other than sparse deposits, no film formation occurred of hydrated lead oxide under any conditions attempted in this study. Treatment of these films with selenosulphate solution resulted in complete conversion to PbSe at room temperature after 6 min. The selenization process of this film was followed by XRD, and it was seen to proceed by a breakdown of the large Pb—OH—C crystals to an essentially amorphous phase of PbSe with crystallization of this phase to give finally large (ca. 200 nm) PbSe crystals covered with smaller (15-20 nm) ones as well as some amorphous material. 

In Ref. 8, crystals ca. 5 mn in size were deposited from a nitrilotriacetate (NTA)-complexed bath (no ammonia) at 40°C (a lower temperature than most CdS depositions). The composition of the bath was such that Cd(OH)2 was present as a colloidal phase (cluster mechanism-see Chap. 3). Under conditions where no hydroxide phase was present and the reaction proceeded via an ion-byion mechanism, much larger crystals (>70 mn) and a red-shifted spectrum were found. See Section 10.2.2 for more detail on the dependence of crystal size on the deposition mechanism. 

Table 5.5 Distribution of salts (mg I 1 milk) between the soluble and colloidal phases of milk (from Davies and White, 1960)...

The partition of salts between the soluble and colloidal phases is summarized in Table 5.5. In general, most or all of the sodium, potassium, chloride and citrate, one-third of the calcium and two-thirds of the magnesium and about 40% of the inorganic phosphate are in the soluble phase. 

As shown in Table 5.5, all the major ionic species in milk, with the exception of Cl-, are distributed between the soluble and colloidal phases, but the... 

These calculations leave about 500 mg of calcium and about 350 mg of phosphate present in the colloidal phase per litre of milk to be accounted for. The available evidence suggests that the excess CCP is present largely as tricalcium phosphate, Ca3(P04)2, or some similar salt. 

Milk serum is supersaturated with calcium phosphate, the excess being present in the colloidal phase, as described above. The balance between the colloidal and soluble phases may be upset by various factors, including changes in temperature, dilution or concentration, addition of acid, alkali or salts. The solubility product for secondary calcium phosphate, [Ca2+][HPOr] is about 1.5 x 1(T5 or pKs = 4.85. 

Addition of alkali has the opposite effect, and at about pH 11 almost all the soluble calcium phosphate occurs in the colloidal phase. These changes are not reversible on subsequent dialysis against untreated milk. 

In contrast to the above-described kinetic stability, colloids may also be thermodynamically stable. A stable macromolecular solution is an example we have already discussed. Formation of micelles beyond the critical micelle concentration is another example of the formation of a thermodynamically stable colloidal phase. However, when the concentration of the (say, initially spherical) micelles increases with addition of surfactants to the system, the spherical micelles may become thermodynamically unstable and may form other forms of (thermodynamically stable) surfactant assemblies of more complex shapes (such as cylindrical micelles, liquid-crystalline phases, bilayers, etc.). 

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