Sites sorption

For any given compound, there is likely a continuum of mechanisms with differing energy relationships that is responsible for sorption onto sod. For example, an organic molecule may be sorbed initially by sites that provide the strongest mechanism, followed by progressively weaker sites as the stronger sorption sites become tided. 

Although in many. soils with tow P availability, significant desorption of sparingly soluble Pi forms requires at least millimolar concentration levels of specific carboxylates (e.g., citrate, oxalate) in the soil solution, much lower concentrations (0.1 mM) were necessary to reduce. soil adsorption of Pi, which was applied simultaneously with carboxylates (100). Thus, competition of carboxylates with Pi for P sorption sites in the soil matrix may be a mechanism that can. 

Results of adsorption experiments for butylate, alachlor, and metolachlor in Keeton soil at 10, 19, and 30°C were plotted using the Freundlich equation. A summary of the coefficients obtained from the Freundlich equation for these experiments is presented in TABLE IV. Excellent correlation using the Freundlich equation over the concentration ranges studied (four orders of magnitude) is indicated by the r values of 0.99. The n exponent from the Freundlich equation indicates the extent of linearity of the adsorption isotherm in the concentration range studied. If n = 1 then adsorption is constant at all concentrations studied (the adsorption isotherm is linear) and K is equivalent to the distribution coefficient between the soil and water (Kd), which is the ratio of the soil concentration (mole/kg) to the solution concentration (mole/L). A value of n > 1 indicates that as the solution concentration increases the sorption sites become saturated, resulting in a disproportionate amount of chemical being dissolved. Since n is nearly equal to 1 in these studies, the adsorption isotherms are nearly linear and the values for Kd (shown in TABLE IV) correspond closely to K. These Kd values were used to calculate heats of adsorption (AH). 

The enhancing effect of NaCl on Cd uptake was due to chloride complexation of Cd (Smolders et al., 1998). High salinity in soils increased the concentrations of chloride complexes of trace elements (such as CdCl or CdCl2°) in soil solution, which increased and correlated best with Cd uptake of both plant species as discussed above. In addition, salinity also affected plant root function, and Na competition with Cd for sorption sites in soil may be a possible contributor. 

Usually, the zeolite inner surface characteristics are rather complex as a consequence of the (3D) character of the porous topologies of most of the zeolite types. The porous framework is a (3D) organization of cavities connected by channels. Inner surfaces are composed of several sorption sites characterized by their local geometry and curvature. Illustrative examples of such inner surface complexity are represented on Figures 1 and 2 they concern the Faujasite and Silicalite-I inner surfaces respectively. 

Figure 1. Schematic representation of a faujasite super-cage. Its developed inner surface, which is composed of two types of sorption sites, has been also represented.

Developed inner surfaces are represented to give clever illustrations of sorption sites geometry. Depending on the temperature range and the molecular size of sorbed... 

Qads.(max) = 5.7 molecules by unit cell). Generally speaking, Qacis.(max) is closely related to the molecular size, as it is observed for the other molecular species. Secondly, as shown on Figure 5, sorption isotherm sub-step observation could be another signature of zeolite inner surface influence. Such isotherm sub-step reflects a phase transition existence between a fluid phase and a solid phase stabilized by the inner surface sorption sites. 

We have observed large variations in the sorption capacities of zeolite samples characterized by (ID) channel systems, as for instance AFI (AIPO4-5 zeolite) and MTW (ZSM-12 zeolite) architectural framework types. Indeed, for such unconnected micropore networks, point defects or chemisorbed impurities can annihilate a huge number of sorption sites. Detailed analysis, by neutron diffraction of the structural properties of the sorbed phase / host zeolite system, has pointed out clear evidence of closed porosity existence. Percentage of such an enclosed porosity has been determined. 

Figure 1. Predicted sorption site of diphenyl-butadiene in straight channel of H5 8ZSM-5 (1DPB/UC)...

Goldberg (2002) found no evidence of any competition in sorption of arsenate and arsenite on Al or Fe-oxides and montmorillonite, but only a small and apparent competitive effect of equimolar arsenate on arsenite sorption on kaolinite and illite. The minor competitive effect in this study was due to the small concentrations of arsenic which is very low for saturation site. Competition for sorption sites is evident by increasing the surface coverage of the sorbents. Arsenate prevents arsenite sorption on metal oxides when the surfaces of the sorbents are saturated by the anions (Jain and Loeppert 2000 Violante and Pigna 2002). 

The sorption data of Cd2+ and Pb2+ by B. subtilis and E. coli were well described by a one-site complexation model (r2 > 0.9) with Cd2+ showing somewhat lower sorption affinities than Pb2+ (Kulczycki et al. 2002). A two-site sorption model yielded an improved fit but only for the E. coli data. The stability constants for the high- and low-affinity sorption sites differed by several orders of magnitude. The total metal sorption capacity of E. coli increased, and moved closer to the value of B. subtilis when the presence of low-affinity sorption sites was allowed. Ngwenya et al. (2003) used potentiometric titrations to assess the different types of sites present... 

In summary, it is clear that water absorbs into amorphous polymers to a significant extent. Interaction of water molecules with available sorption sites likely occurs via hydrogen bonding such that the mobility of the sorbed water is reduced and the thermodynamic state of this water is significantly altered relative to bulk water. Yet accessibility of the water to all potential sorption sites appears to be dependent on the previous history and physical-chemical properties of the solid. In this regard, the water-solid interaction in amorphous polymer systems is a dynamic relationship depending quite strongly on water activity and temperature. 

Reactions between Fe(ll) in contaminated groundwater (5.8 mg/L) and oxic sediment also affected As mobility. Ferrous iron was oxidized by manganese oxides to ferric iron which precipitated as hydrous ferric oxide, creating additional sorption sites. Evidence for this reaction included an increase in ferric oxide concentrations in reacted column sediments and manganese concentrations in leachate that were greater than in the initial eluent. 

Almost 100 % of the Ni was sorbed on the fresh waste rock after 400 minutes, while Ni retention reaches only 50 % after 1500 minutes on the weathered waste rock. The resulting Ni sorption capacities from these batch tests are 0.34 mg/g for the fresh and 0.19 mg/g for the weathered waste rock. However, the Ni sorption capacity of the fresh waste rock is probably greater, since all the Ni was retained in the batch test and the sorption sites were not fully saturated. 

Several investigators [7,123] suggested the use of a Langmuir-type saturation model in addition to the electrostatic model to account for saturation effects. The Langmuir model implies that there are a finite number of localised sorption sites [15] ... 

Anyway, these disputes over number of sorption sites are purely academic. Such high concentrations are not likely to be encountered in the environment and are likely to be lethal for all biological organisms. 

There exists an inverse relationship between the pK and the dielectric constant of the medium [123], This relationship gives an indication that the dielectric constant at the sorption site in the membrane is smaller than in the aqueous phase. 

Herbette et al. compared the sorption site of the structurally similar tertiary amines propranolol and timolol [164]. Propranolol has a naphthalene substituent on the aliphatic chain, which is deeply incorporated into the hydrophobic core of the membrane. In contrast, timolol carries a partially charged morphine ring at the same place. This substituent, due to its polarity and partial charge, does not interact favourably with the membrane interior. Consequently, the AnW at pH 7.5 is 20 times higher for propranolol than for timolol, and timolol has less influence on the phase transition. 

Carrilho, E. N. V. M., Ferreira, A. G. and Gilbert, T. R. (2002). Characterization of sorption sites on Pilayella littoralis and metal binding assessment using 113Cd and 27A1 nuclear magnetic resonance, Environ. Sci. Technol., 36, 2003-2007. 

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