Colloidal fraction

Initial tests of Brownian pumping required the measurement of Th in colloids separated from seawater samples. " Th proved to be an especially useful tracer of colloidal uptake of metal species because of its constant source and relative abundance. Baskaran et al. (1992) and Moran and Buesseler (1992) used cross-flow filtration to separate the colloidal fraction and both studies reported significant (up to 78% of total) " Th in this fraction. Subsequent work largely supported these observations (Moran and Buesseler 1993 Huh and Prahl 1995) and suggested the importance of colloidal organic matter in scavenging Th (Niven et al. 1995). 

The operational separation of colloids does not directly identify the nature of the colloid fraction, which may consist of clays, Fe and Mn oxyhydroxides, or... 

Schell et al. [ 57] have described a sorption technique for sampling plutonium and americium, from up to 4000 litres of water in 3 h. Battelle large-volume water samples consisting of 0.3 xm Millipore filters and sorption beds of aluminium oxide were used. Particulate, soluble, and presumed colloidal fractions are collected and analysed separately. The technique has been used in fresh and saline waters, and has proved to be reliable and comparatively simple. 

In a study involving several contaminated freshwater streams in New Jersey Pinelands, Ross and Sherrell [8] have used CFF, with a 10 kDa (ca. 3 nm) cutoff, to separate the filtrate (<0.45 pm) into colloidal and truly dissolved fractions in freshwater systems. The colloidal fraction,/cou, was calculated by difference ... 

Microelectrophoresis (electrophoretic mobility) . This involves the measurement of particle charge in an applied field. For paper furnishes, the supernatant solution—which contains finely divided colloidal matter, is usually removed and used to conduct the measurement. It must be questioned therefore as to how reflective this is of the charge characteristics of the larger particles and fibres which settle. However, as it is the colloidal fraction which requires to be flocculated to assist retention during drainage, it is still a useful measurement. 

A/ -oxide, 16-hydroxystrychnine, and/or 2-hydroxystrychnine) and unidentified polar compounds were identified. The polar compounds were likely adsorbed strongly onto soil colloidal fractions. 

The suspended solid particle size and the volume of effluent also must be considered in examining deposition in the subsurface. For example, under leaching of a waste disposal site or following irrigation with sewage effluent, the coarse fraction of suspended solids is retained in the upper layer, while the finer colloidal fraction is more mobile, and its transport is controlled by the porosity of the subsurface solid phase. 

Environmental applications of FIFFF have been carefully collected in a review by Gimbert et al. [35]. Separations of nanoparticles belong to the FIFFF tradition and this sector has recently found new, fully deserved impulse for microparticle separations. The FIFFF technique has been applied to analyze humic material and submicron Fe colloids. Coupled with ICP-MS, FIFFF has been applied to detect the major and trace element chemistry of aquatic colloids in groundwaters and to determine the trace element distribution in soil and compost-derived humic and colloidal fractions in municipal wastewater. Recently, the ICP-AES has also been proposed as a specific detector for FIFFF to analyze inorganic nanoparticles (Figure 12.12). 

The discovery happened by accident. Lewis and Anders were frustrated by their failure to find the carrier of anomalous xenon in carbonaceous chondrites. They decided to try an extreme treatment to see if they could dissolve the carrier. They treated a sample of the colloidal fraction of an Allende residue with the harshest chemical oxidant known, hot perchloric acid. The black residue turned white, and to their surprise, when they measured it, the anomalous xenon was still there The residue consisted entirely of carbon, and when they performed electron diffraction measurements on it, they found that it consisted of tiny (nanometer sized) diamonds. After a detailed characterization that included chemical, structural, and isotopic studies, they reported the discovery of presolar diamond in early 1987 (Lewis et al., 1987). The 23-year search for the carrier of CCFXe (Xe-HL) was over, and the study of presolar grains had begun. 

Bulk techniques still have a place in the search for presolar components. Although they cannot identify the presolar grain directly, they can measure anomalous isotopic compositions, which can then be used as a tracer for separation procedures to identify the carrier. There are several isotopically anomalous components whose carriers have not been identified. For example, an anomalous chromium component enriched in 54Cr appears in acid residues of the most primitive chondrites. The carrier is soluble in hydrochloric acid and goes with the colloidal fraction of the residue, which means it is likely to be submicron in size (Podosck el al., 1997). Measurements of molybdenum and ruthenium in bulk primitive meteorites and leachates from primitive chondrites show isotopic anomalies that can be attributed to the -process on the one hand and to the r- and /7-processes on the other. The s-process anomalies in molybdenum and ruthenium correlate with one another, while the r- and /7-process anomalies do not. The amounts of -process molybdenum and ruthenium are consistent with their being carried in presolar silicon carbide, but they are released from bulk samples with treatments that should not dissolve that mineral. Thus, additional carriers of s-, r-, and/ -process elements are suggested (Dauphas et al., 2002). 

For the case of the PCBs it has been shown that sorption on colloids enhances the dissolved concentration which is usually determined on filtered samples in which most colloids are still present. Since the colloidal fraction contributes neither to air/ water exchange nor to sedimentation (only the large particles, not the colloids, sink to the bottom), the effect of the colloids can result in an increase of the residence time of the chemical in the lake. 

Selective diagenesis and recycling of P during the summer months resulted in the replenishment of the colloidal fraction to enrichment levels above spring values (see mid-July data). However, colloid mass concentrations were lower in midsummer than in spring. The size of the colloidal-P pool actually declined in the main water column during midsummer. The colloidal-P pool represents a major component of the P cycle. However,... 

ESR examination of nonchemically isolated fulvic acids showed that Mn2+ was the primary paramagnetic species observable (60, 61). Most likely, the soluble-colloidal fraction we identified in the speciation studies consisted primarily of such complexes. Because the ESR spectral characteristics of Mn in fulvic acid complexes are quite similar to Mn(H20)62+, Alberts et al. (62) suggested that the metal-fulvate interaction was weak. Stronger interaction would be expected to lead to changes in peak shape. This view leaves unexplained the ability of the complexes to survive the isolation procedure s long ultrafiltration steps, because weak interactions are usually associated with reversible complexation. 

In the (aquatic) environment elements occur in particulate-, colloidal- and dissolved forms. These forms are usually distinguished by filtration or centrifugation. Traditionally, a 0.45 um (membrane)- filter separates the particulate from the dissolved forms. This may result in the passage of colloidal fractions through the filter, classifying colloidal matter incorrectly within the dissolved fraction. Although the interaction between dissolved and particulate (surface) fractions cannot be neglected, it is common in speciation studies to consider the "dissolved" fraction. The dissolved forms of trace elements are mainly present as ... 

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. 

Relatively few biochemicals can be measured directly in natural waters because concentrations of individual compounds are low (nanomolar) and salts and other components often interfere with these analyses. DOM can be concentrated and isolated from natural waters for more thorough chemical characterization, and two approaches for DOM isolation, adsorption onto solid phases and ultrafiltration are now widely used. The adsorption of DOM onto XAD resins is used to isolate a fraction of DOM that is operationally defined as humic substances (Thurman, 1985). More recently, tangential-flow ultrafiltration with 1000 Da cutoff membranes has been used to isolate the high-molecular-weight or colloidal fraction of DOM (Benner et al., 1992, 1997). 

Literature data on the distribution of trace elements amongst the different size fractions in natural freshwaters are summarised in Table 8.1. There is a striking paucity of data for the environmentally important elements Cu, Pb and Cd. Elements which have a low affinity for complexing sites on NOM are present predominandy in the low molecular weight fraction, e.g. Cs, Sr, V while those which specifically coordinate to particular functional groups in NOM, e.g. Ag, Cd, tend to be present in the particulate and colloidal fractions. A major difficulty when trying to compare the literature data is that operational and variable size limits are used, and that filtration, which may lead to drastic artefacts, is still largely used for size fractionation. 

Berdie, L., J.O. Grimalt, and E.T. Gjessing. 1995. Combined fatty acids and amino acids in the dissolved + colloidal fractions of the waters from a dystrophic lake. Org. Geochem. 23 343-353. 

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