Colloidal size fractionation

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. 

Fig. 6. Colloid size fractionation obtained by FFFF of (a) a Eu(III) humate solution ([HA] 30 mg/L Eu(III) ...

The particles that are picked up and carried along by streams and rivers, maintained in suspension by turbulence. The colloidal-sized fraction, particles of up to about 0.1 pm in diameter, that may be able to remain in suspension for considerable periods of time, even in quiescent waters, is called the wash load. 

Initially in this study, it was planned to critically evaluate AG data for complex clays, including chlorite, illite, and the smectites. However, there is much evidence that these clays dissolve Incongruently so that the apparent equilibria in solution are determined by secondary phases, such as gibbsite, boehmite, amorphous silica, and ferric oxyhydroxldes. The smectites are frequently the dominant clays in the colloidal size fraction in natural sediments. They have very large exchange capacities, and exhibit wide chemical variations. Usually, one or more of these factors have not been considered in the experimental solubility work. Even if appropriate corrections could be made, it is uncertain whether a AG value so obtained would have applicability to natural systems. 

HMW biopolymers are part of the colloid-sized fraction of marine organic matter that can be visualized and enumerated with transmission electron microscopy (TEM) (Wells and Goldberg, 1991, 1993). Colloids range in size from a few nanometers up to 1 p,m in size. Very small colloids (<30 nm) are irregular in shape, while larger colloids (—30-60 nm) are more spherical assemblies of 2-5 nm sized subparticles. Concentrations of small colloids (<200 nm) range from nondetectable (<10" colloids ml ) to... 

A variety of spectroscopic techniques have been applied to DOC isolated from seawater by cross-flow ultrafiltration or adsorption onto XAD resins. The two techniques isolate very different organic fractions from seawater. Hydrophobic fractions (such as marine humic material) are isolated on XAD resins [48], whereas the organic matter extracted by ultrafiltration is retained primarily on the basis of its molecular size and shape [49], resulting in isolates rich in nitrogen and carbohydrates (polysaccharides). Nuclear magnetic resonance (NMR) spectroscopy has proven successful in distinguishing between the specific structures of XAD-bound humics and the carbohydrates concentrated into colloidal size fractions. 

Some rivers, such as the Yellow River (China) can carry as much as several grams per litre of suspended solids [7]. Lakes and oceans can be considered to be huge colloidal suspensions, typically containing 0.5 -1.5 mg 1 of suspended fine solids. The near-colloidal-sized fraction, particles of up to about 0.1 pm in diameter, may be able to remain in suspension for considerable periods of time, days to weeks, even in quiescent waters. This fraction is called the wash load [6]. Table 9.2 shows the terminal sedimentation velocities of sand, silt and clay particles in seawater [8]. It can be seen that while the sand particles are likely to settle out near then-point of entrance to the ocean, the clay particles could, in principle, be transported almost anywhere in the world before sedimenting out. In addition to being suspensions, oceans can also comprise foam systems where they contain free gas bubbles (which can result from volcanic gases or the decomposition of organic matter). 

Water and wastewater treatment processes inevitably involve the removal of suspended solids (often referred to as turbidity), usually silt, clay, hydrous oxides and organic matter. Of these, the most difficult suspended solids to remove are the colloidal-sized fraction which, because of their small size, can easily escape both sedimentation and filtration. Examples of these would include spent protein and emulsions from domestic waters, bacterial cells, algae, viruses, amoeba, industrial waste colloids, silts, clays and organic matter from soil wash. Beyond drinking water treatment and industrial wastewater treatment, other application areas include mineral and petroleum processing, and pulp and papermaking, to name just a few. 

Buesseler KO, Bauer JE, Chen RF, Eglinton TI, Gustafsson O, Landing W, Mopper K, Moran SB, Santschi PH, Vernon Clark R, Wells ML (1996) An intercomparison of cross-flow filtration techniques used for sampling marine colloids overview and organic carbon results. Marine Chem 55 1-31 Buffle J, Perret D, Newman M (1992) The use of filtration and ultrafiltration for size fractionation of aquatic particles, colloids, and macromolecules. In Enviroiunental particles. Buffle J, van Leeuwen HP (eds) Lewis Publishers, Boca Raton FL, pl71-230... 

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. 

This method is used particularly for colloids. A colloidal dispersion is forced through a long column packed with nonporous beads with an approximate radius of 10pm. Particles of different particle size travel with different speeds around the beads and are thus collected in size fractions. 

The use of filters and membranes of different pore size to accomplish a sequential size fractionation is in principle, and under certain circumstances, possible it was proposed (for literature see Buffle, 1988 and 1991) to estimate the size of the various colloids and macromolecules and to determine to which extent trace elements (particularly metals) are associated with various size categories of colloids and macromolecules. Such sequential size fractionation techniques need to be applied with extreme caution we list some of the reasons why these techniques may yield errorous results (for details consult Buffle, 1991) ... 

Buffle, J., D. Perret and M. Newman (1992), The use of Filtration and Ultrafiltration for Size Fractionation of Aquatic Particles, Colloids and Macromolecules", in J. Buffle and H.P. v. Leeuwen, Eds., IUPAC Series in Environmental Physical and Analytical Chemistry, Lewis Publ., Chelsea, Ml. 

Geckeis, H., Ngo Manh, T. H., Bouby, M. Kim, J. I. 2003. Aquatic colloids relevant to radionuclide formation characterization by size fractionation and ICP-mass spectrometric detection. Colloids and Surfaces A Physicochemical and Engineering Aspects, 217, 101-108. 

Highly monodispersed, size-quantized semiconductor particles have been prepared by size fractionation of the colloidal samples by gel electrophoresis [591 ]. The method is illustrated by quoting the published recipe for CdS particle formation [591] ... 

By coupling flow field-flow fractionation (flow FFF) to ICP-MS it is possible to investigate trace metals bound to various size fractions of colloidal and particulate materials.55 This technique is employed for environmental applications,55-57 for example to study trace metals associated with sediments. FFF-ICP-MS is an ideal technique for obtaining information on particle size distribution and depth profiles in sediment cores in addition to the metal concentrations (e.g., of Cu, Fe, Mn, Pb, Sr, Ti and Zn with core depths ranging from 0-40 cm).55 Contaminated river sediments at various depths have been investigated by a combination of selective extraction and FFF-ICP-MS as described by Siripinyanond et al,55... 

Results and Discussion. Changes in concentrations of the actinides in solution in the blank containers are shown in Figure 9. A large fraction of the Pu, Am and Cm was removed from solution while only a small amount of the U and none of the Np was lost from solution. Since the starting concentration of Pu exceeds the solubility for the hydroxide (or hydrated oxide), the Pu probably precipitated as colloidal-size particles (2) the behavior of Am and Cm cannot be explained by a similar mechanism. However, there is recent experimental evidence that indicates the solubility products for Am and Cm carbonates may be of the order 10 ( ). If this is the case, there was sufficient... 

Studies into the distribution of trace elements in relation to the size fraction of stream sediments generally show that several elements including Mo, Cu, Zn, Mn and Fe are concentrated in the finest fractions of the sediment. The majority of stream sediment surveys have, therefore, been based on the collection of <0.200 mm material. The IGCP 259 and FOREGS standard sieve mesh is <0.150 mm as this is fine enough to only include the very fine sand, silt, clay and colloidal fractions, but is coarse enough to yield sufficient fine material in the majority of situations. 

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