The Sorption Mechanism

In fact, one of the major applications of chitosan and some of its many derivatives is based on its ability to bind precious, heavy and toxic metal ions. Another article reviews the various classes of chitosan derivatives and compares their ion-binding abihties under varying conditions, as well as the analytical methods to analyze them, the sorption mechanism, and structural analysis of the metal complexes. Data are also presented exhaustively in tabular form with reference to each individual metal ion and the types of compounds that complex with it under various conditions, to help reach conclusions regarding the comparative efficacy of various classes of compounds [112]. 

Note The exact mechanism by which the respective bonded phases actually alter the nature of the sorption mechanism is still not yet clear. 

In conclusion, the different shapes of isotherms describing equilibrium distributions of a contaminant, between geosorbents and aqueous or gaseous phases, depend on the sorption mechanism involved and the associated sorption energy. At low contaminant concentration, all models reduce to essentially linear correlation. At higher contaminant concentration, when sorption isotherms deviate from linearity, an appropriate isotherm model should be used to describe the retention process. 

Here again we are concerned with the equilibrium sorption characteristics for all available sorption processes. Also, we need to have sufficient knowledge of the sorption mechanisms and kinetics to relate geologic to laboratory conditions. Likewise we require a comparable understanding of the displacement and release processes which permit migration to proceed. 

Ziegler, F., Giere, R. Johnson, C. A. 2001a. The sorption mechanisms of zinc to calcium silicate hydrate Sorption studies and microscopic investigations. Environmental Science and Technology, 35, 4556-4561. 

Measurements were also performed (with Cs, Sr and Am) on granite particles of various surface to volume ratios which would give some qualitative information on the sorption mechanisms involved. 

What is a sorption isotherm Which types of sorption isotherms may be encountered when dealing with sorption of organic compounds to natural sorbents Does the shape of a sorption isotherm tell you anything about the sorption mechanism(s) If yes, what If no, why not ... 

Further comparison with benzene sorption on this sample yields striking results if the two isotherms are plotted on the same axis, as shown in Figure 1. The fact that these two isotherms may be completely superimposed suggests that the mechanism of tert-butyl alcohol sorption is similar to that observed in benzene sorption. This indicates that the sorption mechanism of tert-butyl alcohol on this material is more influenced by organic interactions with the surface phenyl groups than by polar interaction with surface hydroxyls. (It should be noted, however, that a small amount of rehydroxylation is indicated by the low pressure hysteresis.)... 

The system methane-krypton 5A was selected for study because previous pure component studies for each of these sorbates on Linde 5A zeolite indicate that the sorption mechanisms are significantly different. 

The effect of temperature on sorption equilibrium is a direct indication of the strength of the sorption process. The weaker the interaction between sorbent and sorbate, the less the effect of temperature (Hamaker and Thompson, 1972). While temperature can influence sorption, the strength and direction of the effect depends on the properties of the sorbent and sorbate and on the sorption mechanism. Adsorption processes are generally exothermic, so the higher the temperature, the less the adsorption (Hamaker and Thompson, 1972). Hydrophobic sorption, however, has been shown to be relatively independent of temperature (Chiou et al., 1979). 

Qualitative and quantitative identification of the sugars in the hydrolysates of these humin isolates may give indications about their origins (plant or microbial), and identification of the configurations of the sugar linkages could lead to deductions about the sorption mechanisms. To achieve the latter objective will require isolation of the polysaccharide and/or mucopolysaccharide components. That will not be an easy task, but it is doable. 

Changes in the erythrocytes osmotic resistance were not observed. Adsorption of total plasma proteins on modified MC was lower than 12 %, but it was about 60 - 70 % on unmodified particles. Table 6 summarizes the results obtained of MC sorption efficiency to substances of different molecular mass in donor plasma. The sorption mechanism of low and middle molecular weight substances (phenobarbital and cyanocobalamin) on iron-carbon and restored-iron MC is apparently connected with absorption of molecules into the sorbent s pores. Iron-carbon composites have a more porous structure than restored-iron, therefore the... 

Bibler and Marson 1992). Retention of uranium by the soil may be due to adsorption, chemisorption, ion exchange, or a combination of mechanisms (Allard et al. 1982). Any soil property that alters the sorption mechanism will also alter the mobility of uranium in the soil. The sorption of uranium in most soils is such that it may not leach readily from soil surface to groundwater, particularly in soils containing clay and iron oxide (Sheppard et al. 1987), although other geological materials such as silica, shale, and granite have poor sorption characteristics (Bibler and Marson 1992 Erdal et al. 1979 Silva et al. 1979 Tichnor 1994). 

Although no detailed theory on the sorption mechanism of heavy metals on three-layer clays is available, many inferences imply the specific coordinative interaction of end-standing hydroxo groups with nonhydrolyzed metal ions. 

Most subsequent sorption theories, including those discussed here, have followed this general approach and postulate two forms of sorbed water. These theories may be classified into at least two general types based on the sorption mechanism assumed. One type assumes sorption on internal surfaces and is represented by the Dent theory (52), which is a modification of the classic Brunauer, Emmett, and Teller (BET) theory (53). The second type assumes that the wood-water system forms a solution, exemplified by the Hailwood-Horrobin theory. There have been other theories, not discussed here, that have also been applied to explain water sorption by hygroscopic materials (JO, 54, 55). 

Summary. Ion beam techniques such as KBS are a good analytical tool for studying interface phenomena due to their multielement capability and the possibility of working on whole rock sections. Interesting information on the sorption mechanisms can be obtained from these studies the colloid surface coverage is low (less than one monolayer) the retention mechanisms are partly controlled by the electric charges developed at the surfaces (colloid, mineral) the colloid detachment rate is very low indicating an irreversible character with... 

Empirical equations are simple and easy to use however, they fail to describe the sorption mechanisms and are not able to provide the concentrations of ions released from the IXR to the solution. Therefore, IX theory has been used to overcome the problems. 

Several hydrous oxides, such as those of aluminum, siTicon and, iron have been used to extract traces ions. Nevertheless, the sorption mechanism is not definitively established. Those oxides probably exhibit some ion exchange capacity among their properties and they can act as anionic or cationic exchangers and sometimes both. The separation of plutonium traces in the presence of HF by sorption onto an alumina column is based on its chemical similarities with thorium and lantanide elements reported by Abrao (2) In this case only thorium and rare earths are sorbed onto alumina from nitric acid-fluoride solutions while uranium remains in the effluent. 

The sucessful experiments for the retention of plutonium onto alumina from TTN0 -HF solution gave enough confidence to recomend the proposed method to separate traces of plutonium from waste solutions in the presence of macroamounts of uranium (VI). Of course, only macroamounts of thorium, uranium (IV) and rare earths are serious interfering ions, since they precipitate with HF. The behavior expected for neptunium in the same system should be similar to plutonium, thorium and rare earths. The retention of neptunium from HNO - HF solutions is in progress. The sorption yield for Pu was around 95%. The sorption mechanism is not well established. Figure 3 shows the proposed flowsheet for recovery of Pu traces from reprocessing waste solutions. 

The chemical environment of adsorbed paramagnetic ions [Cu(II), Fe(III), Mn(II)] has been studied by comparison of ESR (electron spin resonance) spectra of wet adsorbents treated with solutions of salts of these metals on the one hand and of some model compounds vide Section G) on the other. For instance, clustering or surface precipitation can be verified as the sorption mechanism. 

The sorption mechanism of chromate is unclear. Zachara et al. (1989) suggested that chromate forms an outer-sphere complex on the surfaces of Fe and Al oxides. However, spectroscopic studies have shown that chromate forms inner-sphere complexes (both bidentate and monodentate) on goethite (Fendorf et al., 1997). This anion has a smaller shared charge than do arsenite and arsenate. 

Ion exchange consists of the replacement of one adsorbed, readily exchangeable ion by another. The involvement of oxygen surfiice groups in the sorption mechanism by ion exchange was confirmed hy an improvement in adsorption onto oxidized adsorbents [10]. Carboxylic surface groups have been shown to be especially involved in the adsorption process [8], but this phenomenon may also occur with other ions such as [11] Ca, K, and Na " ". 

Many parameters as, e.g, pH, the ionic strength or the metal saturation affect the p/Ca s of the functional groups and thus the interactions between the metal ions and the humic material. Fortunately, however, detailed information on these interactions is not required, as long as film diffusion is the rate determining step of the sorption process. All we have to know in this connection is according to Eq. [8] or [11] the separation factor a (or the and the ion-exchange capacity C. These quantities depend of course on the above interactions, but they can be determined independently and without any detailed information on the sorption mechanism. 

Pace and Datyner ( 5, 5) have also proposed a model for the absorption (solution) of small molecules in polymers applicable at temperatures above and below Tg, which incorporates the dual-mode sorption model for the glassy region. The presence of microvoids is assumed for rubbery polymers as well as for polymer glasses. "Hole filling" is suggested as an important sorption mode above as well as below Tg, with one crucial difference between the sorption mechanism in the rubbery and glassy regions hole saturation does not occur in the rubbery state because new microvoids are formed to replace those filled with penetrant molecules. 

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