Metal adsorption selectivity

The concept of inner-sphere surface complexation as the basis for relative metal adsorption selectivity in soils was introduced in W. R. Heald, M. H. Frere, and C. T. deWit, Ion adsorption on charged surfaces. Soil Sci. Soc. Am. J. 28 622 (1964). Further quantitative development of this concept was given in... 

COLLECTION AND IDENTIFICATION OF BIOACTIVE ORGANIC COMPOUNDS OCCURRING IN RIVERS AND LAKES. ADSORPTION SELECTIVITY OF MONOSACCHARIDES ONTO HYDROUS METAL OXIDES... 

The presence of hydroxyaluminum- and hydroxyaluminosilicate polymer in interlayered montmorillonite greatly promotes the adsorption of Cd, Zn, and Pb (Saha et al., 2001). The adsorption selectivity sequences of montmorillonite (Pb > Zn > Cd) and interlayered montmorillonite (Pb Zn Cd) resemble the metal selectivity on amorphous Fe and Al hydroxides (Saha et al., 2001). On montmorillonite, the metals are predominantly adsorbed on the permanent charge sites in an easily replaceable state. However, a substantial involvement of the edge OH" groups of montmorillonite in specific adsorption of the metals is also observed, especially at higher pH (Saha et al., 2001). 

As conclusion, bifunctional fibers having both phosphonic acid and sulfonic acid groups exhibit the characteristic metal ion selectivity and high breakthrough capacities in addition to the extremely fast adsorption rates. Studies on behavior of FPS-f in adsorption of other heavy metals like Fe(III) are now in progress. The bifunctional fiber developed in this work is attractive to application to the protection of the environment because of its extremely rapid adsorption rates and characteristic metal ion selectivity. 

Selective and reversible adsorption of gaseous molecules such as dioxygen, carbon monoxide, ethylene, acetylene, and dinitrogen have been performed by the use of suitable macromolecule-metal complexes. Selective adsorption of metal ions such as UO has also been studied using polymeric ligands. 

The method of soil suspensions extracts is based on metal desorption/dissolution processes, which primarily depend on the physico-chemical characteristics of the metals, selected soil properties and environmental conditions. Metal adsorption/ desorption and solubility studies are important in the characterization of metal mobility and availability in soils. Metals are, in fact, present within the soil system in different pools and can follow either adsorption and precipitation reactions or desorption and dissolution reactions (Selim and Sparks, 2001). The main factors affecting the relationship between the soluble/mobile and immobile metal pools are soil pH, redox potential, adsorption and exchange capacity, the ionic strength of soil pore water, competing ions and kinetic effects (e.g. contact time) (Evans, 1989 Impelhtteri et al., 2001 McBride, 1994 Sparks, 1995). 

One method used adsorption to laboratory surrogate materials in an adsorption additivity model to estimate the relative contributions of Mn and Fe oxides and organic materials to Pb adsorption to natural biohlms. The second method made use of a novel selective extraction approach in which trace metal adsorption to surface coatings was measured before and after the selective removal of constituents. 

As elucidated from the adsorption experiment on BSA, adsorption of gallium ion by simple ion-exchange effect is negligible. The characteristics of metal ions such as ionic radius and hardness/softness, and the conformation of the metallothionein probably affect the selectivity of metal adsorption. The understanding of the mutual interactions among those factors would be a key dictor in designing the protein-based ligand suitable for a specific metal ion. 

TABLE 13 Summary of Physical and Chemical Surface Properties of Selected Activated Carbons and Their Metal Adsorption Capacities... 

In order to enhance or modify the chemical selectivity of an SERS substrate, it is possible to chemically derivatize the metal surface. For example, covalent bonding of a hydrocarbon to a silver island film should selectively adsorb nonpolar analytes from an aqueous solution. The general approach is shown schematically in Figure 13.24 for the case of metal ions detected by a surface-bound complexing agent (47). Field enhancement is provided by the substrate, while adsorption selectivity results from the chemistry of the derivatized... 

Just like metal adsorption onto activated carbons and biomaterials, different ions are absorbed on ion-exchange sites with different affinities. In other words, ion exchange resins are selective for ions. Multivalent ions normally are more strongly absorbed from dilute solutions than ions of lower valence (e.g., Ca " vs Na+). 

L. Delle Site, C. F. Abrams, A. Alavi, and K. Kremer (2002) Polymers near metal surfaces Selective adsorption and global conformations. Phys. Rev. Lett. 89, p. 156103 M. Praprotnik, L. DeUe Site, and K. Kremer (2005) J. Chem. Phys. 123, p. 224106... 

What are the absolute and relative abundances of important sorbent solids and what fraction of their surface areas are exposed to flowing water Any adsorption model we select that assumes a finite number of sorption sites, requires, as input, the area of a sorbing phase exposed to a given volume of water I.e.g., Cs(g/L) x 5 (m /g)] and a surface site density [ (sites/m-)] for that phase. Can we measure or estimate these values Such measurements and estimates are extremely difficult for metal adsorption by modern stream sediments, which may be mix-... 

Metal oxides selectively adsorb divalent cations even at solution pH values lower than the PZC of metal oxides. The mechanism of metal ion association with hydrous-oxide surfaces involves an ion-exchange process in which the adsorbed cations replace bound protons. Specifically, adsorbed cations raise the value of PZC of oxides. pH affects adsorption of metal cations, either by changing the number of sites available for adsorption or by changing the concentration of the cation species (Me +, MeOH+, Me(OH)2) that are preferentially adsorbed (Jackson, 1998). 

Sanchez J.M., Hidalgo M., Valiente M., Salvado V. New macroporous polymers for the selective adsorption of gold(III) and palladium(II). I. The S3mthesis, characterization, and effect of spacers on metal adsorption. Journal of Pol)nner Science Part A Pol3fmer Chemistry 2000 38 269-278. 

The synthesis of silicates that contain metal cations other than aluminium in framework positions results in solids with modified adsorptive and catalytic properties. The criteria for successful incorporation of cations into tetrahe-drally coordinated silicate frameworks are that they should exhibit solubility under synthesis conditions without the formation of an insoluble oxide or oxyhydroxide and that the substituting species should be able to adopt tetrahedral coordination. Preparation of a phase that is shown to contain other metals (by selected area chemical analysis in an electron microscope, for example) is no guarantee that the metal has adopted a framework site. Physicochemical methods must be used to determine whether this is the case. This is complicated when the metal adopts more than one site or is present at very low levels. Confirmatory evidence for framework substitution may be available from unit cell size determination (substitution of a cation larger than Si" will in general result in an increase in unit cell dimension), and determination of coordination geometry by NMR, UV-visible and EXAFS spectroscopies (Chapter 3), which are able to distinguish whether the metal is within the... 

Since the PZC is not far from neutral, either anionic or cationic precursors can be employed. The PTA cation was selected for the uptake-pH survey, and results are shown in figure 3.9b. Two surface loadings were selected, 1000 and 10000 m /1, to check for consistency. The pH shift results from the metal-containing and metal-free experiments (not shown) closely overlap and suggest that metal adsorption and proton transfer are independent and the adsorption is largely electrostatic. 

---