Organic Adsorption onto Colloids

Fairhurst et al (1995) found that adsorption of HA at 5 mgL (as organic carbon) on 50 mgL hematite decreased with increasing pH from 65% at pH 2 to 10% at pH 10. The apparent point of zero charge (pHpZc) of the colloids was shifted from pH 8 (no HA) to a pH 2. Such charge inversion of colloids 

depending on the concentration of organics (Prasanthi et al. (1995)). Hematite particles in a natural environment are usually stable, as destabilised colloids settle rapidly. Song et al. (1994) addressed the masking of colloid surface heterogeneity by dissolved organic matter. This can enhance colloidal mobility. Kretzschmar et al (1997) showed that HSs also stabilise kaoUnite colloids. 

The mobility of geothite in water was measured as a function of HS concentration and the presence of calcium. The mobility is highly concentration dependent up to a calcium concentration of 2 rngL k The effect of bivalent ions is not great for high HS concentrations and low ion content (Tipping and Heaton (1983)). 

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Removal to sediments. Removal of surface-reactive trace elements from the oceans readily occurs by adsorption onto settling particles, and this process is most pronounced in the typically high-energy, particle-rich estuarine environment. Particles are supplied by rivers, augmented by additions of organic material generated within the estuary. Also, floes are created in estuaries from such components as humic acids and Fe. The interaction between dissolved and colloidal species is enhanced by the continuous resuspension of sediments in... 

Honeyman and Santschi 1989). Therefore, flocculation of colloids to form settling particles in estuaries is an important mechanism for trace element removal (Sholkovitz 1977). This is particularly true of Fe, which is a ubiquitous colloidal species and is removed at low salinities. Additional removal may occur by adsorption onto floes, as demonstrated by mixing of organic-rich waters with seawater in the laboratory (Sholkovitz 1977). 

The link between colloids and surfaces follows naturally from the fact that particulate matter has a high surface area to mass ratio. The surface area of a 1cm diameter sphere (4jtr ) is 3.14 cm, whereas the surface area of the same amount of material but in the form of 0.1 pm diameter spheres (i.e. the size of the particles in latex paint) is 314 000 cm. The enormous difference in surface area is one of the reasons why the properties of the surface become very important for colloidal solutions. One everyday example is that organic dye molecules or pollutants can be effectively removed from water by adsorption onto particulate activated charcoal because of its high surface area. This process is widely used for water purification and in the oral treatment of poison victims. 

The speciation, concentrations and residence times of dissolved substances in natural waters are dependent on many factors and processes. Important factors Include temperature, pH, redox potential, ionic strength and the concentrations of other dissolved species such as organic and Inorganic ligands as well as the presence of suspended particulate and colloidal matter. Important processes in addition to rate of input, and biochemical cycling include precipitation, complexatlon, coagulation and adsorption onto suspended particulate matter. 

Day, G.M. et al.. Adsorption of natural organic matter onto goethite. Colloids Siiif A, 89, 1,1994. 

Perdue et al. (1976) observed an excellent linear correlation between the combined concentrations of Fe and Al and DOM in the Satilla River system (southeast U.S.A.). Such a correlation is likely to result from the formation of dissolved organic complexes rather than the adsorption of DOM onto colloidal iron hydroxide particles. Furthermore, using vapour pressure osmometry, Perdue et al. (1976) determined the number-average molecular weight of the DOM to be 1296, thus enabling them to derive an (Fe + Al) DOM stoichiometric binding ratio of 0.8—1.1, in agreement with the values for soil... 

Adsorption Adsorption of namral organics on inorganic onto Colloids colloids decreases with pH. Natural organics adsorb to some extent under most conditions, resulting in a hydrodynamic lay er thickness of 1 to 3 nm and very negatively charged colloids. 

The adsorption of natural organics onto colloids may be used to enhance organics retention by membranes... 

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. 

Aloulou F, Boufi S, Belgacem MN, Gandini A. Adsorption of cationic surfactants and subsequent adsolubilization of organic compounds onto cellulosic fibers. Colloid Polym Sci 2004a 283 344-350. 

Data in Table V also show that, lipophilic substituents such as the phenyl group dramatically decrease the nematicidal activity. This behavior may be related to soil adsorption, since the movement of nematicides in soil is limited by the degree of adsorption onto soil colloids, and adsorption depends on the hydrophobicity of the chemicals. When the nematicides tend to be strongly adsorbed on the soil organic colloids, they are removed from the soil solution and are unavailable to kill nematodes. 

E. Dfaz, S. Orddnez, A. Vega, Adsorption of volatile organic compounds onto carbon nanotubes, carbon nanofibers, and high-surface-area graphites. J. Colloid Interface Sci. 305, 7-16 (2007). doi 10.1016/j.jcis.2006.09.036... 

The thermodynamic properties of U-Th series nuclides in solution are important parameters to take into account when explaining the U-Th-Ra mobility in surface environments. They are, however, not the only ones controlling radionuclide fractionations in surface waters and weathering profiles. These fractionations and the resulting radioactive disequilibria are also influenced by the adsorption of radionuclides onto mineral surfaces and their reactions with organic matter, micro-organisms and colloids. 

A study of iron, cadmium and lead mobility in remote mountain streams of California by Erel et al. (1990) showed that the excess of atmospheric pollution-derived lead and cadmium is rapidly removed downstream. The comparison of truly dissolved, colloidal, and surface particle concentrations measured in the stream with the results of a model of equilibrium adsorption indicates that the mechanism of removal in this organic-poor environment is essentially by uptake onto hydrous iron oxides. The experimentally determined partition coefficients (Dzomback and Morel, 1990) explain the behavior of lead however, they fail to explain the cadmium removal. It is proposed by the authors that cadmium is taken up by surfaces other than hydrous iron oxides. 

Adsorption is an important process in many industrial, biological, and environmental systems. One compelling reason to study adsorption phenomena is because an understanding of colloid stability depends on the availability of adequate theories of adsorption from solution and of the structure and behavior of adsorbed layers. Another example is the adsorption of pollutants, such as metals, toxic organic compounds, and nutrients, onto line particles and their consequent transport and fate, which has great environmental implications. Often, these systems are quite complex and it is often favorable to separate these into specific size for subsequent study. 

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