Metal cations retention factors

Metal cations may be soluble, readily exchangeable, complexed with organic matter, or hydrous oxides, substituted in stoichiometric compounds, or occluded in mineral structures (see reviews by Brummer et al., 1986 Beckett, 1989 Forstner, 1991). The chemical factors that affect the retention of a specific chemical form of a trace metal (e.g. effects of pH and I on specific adsorption ) are well documented (Jones and Jarvis, 1981 Tiller, 1983 McBride, 1989 1991 Alloway, 1990 Forstner, 1991). When several components co-exist in a soil, the distribution of a trace metal among them will also depend on the type and relative quantities of the soil components how they change with pH, I, etc. and the extent of saturation of adsorption sites on soil adsorbents. 

Apart from radioactive compounds the radioactive wastes may contain non-active but chemically toxic substances. The national standards and regulations describe the discharge limits for these substances, too. Heavy metals are toxic compounds, most often present in liquid radioactive wastes. The experiments showed that most of these metals are removed by UF/complexation method. The method is inefficient for removal of monovalent ions, bivalent cations and anions are retained in 25%-50%, but high retention of the metals like Mn, Fe, Co, Cu, Pb, Cr was observed. The retention factors for those metals in two-stage experiment described above, were as follows ... 

The approach is to measure retention times of metal cations on columns containing low-capacity resins with eluents containing perchloric acid or various perchlorate salt.s. The perchlorate anion is used to eliminate any possible complexing of a metal ion by the eluent anion. Retention factors (k) are calculated from the retention data. 

Selectivity of Sulfonated Cation-Exchange Resin for Metal Cations Table 5.5. Retention factors (k) of various cations when sodium perchlorate eluents are used. 

Large retardation factors can mean that the leaching of metal cations from the soil surface into the subsoil is slow, even assuming that metal adsorption is by exchange processes only. As will be explained in Chapter 4, many metals adsorb in addition by strong forces, and this form of metal retention in soils is likely to be practically irreversible, leading to long-term immobilization. 

Another factor which can affect surfactant retention by these high surface area porous structures is. the charge relationships between the surfaces and the various chemical species. Since the surfactant is negatively charged, the more positive (or less negative) the surface becomes, the easier it is for adsorption to occur. Work by Tsai and Falcone (31) has shown that multivalent metal cations at relatively low levels can cause a surface to become more positive in certain pH ranges. The addition of silicates and certain other alkalis... 

Taking into account the two possible types of interactions in accordance with [1], the retention factor k of metal cation can be expressed as... 

The data listed in Tables 5.3-5.6 are simply observations concerning the effect of eluent concentration and resin exchange capacity on the retention factors of metal cations. A more fundamental approach is to examine the effect of physical and chemical variations in both the mobile and stationary phases on chromatographic behavior of ions. The factors affecting selectivity of ion chromatography have been reviewed in a recent publication [11]. 

As discussed in the early part of this chapter, macro-cyclic complexes are generally more stable in nonaqueous solvents than in aqueous solution. Therefore, it is expected that adding 18-crown-6 to a nonaqueous mobile phase would increase the retention time of cations to be separated. Fritz s group added 18-crown-6 to a nonaqueous IC mobile phase to study the retention of alkali metal cations and ammonium ion on a sulfonic acid cation-exchange resin. The retention factors of all the ions increased with increasing concentration of 18-crown-6 in acetonitrile eluent containing 1 mM methanesulfonic acid. Most notably. 

Solladie and coworkers545 confirmed the earlier result of Nishihata and Nishio546 that the carbonation of the a-sulphinyl carbanion proceeds under kinetic control with retention of configuration at the metallated carbon atom. However, they also found that the stereochemical outcome of this reaction depends on other factors. They observed that 90% of asymmetric induction may be achieved under kinetic control (reaction time < 0.5 min) by using a base with low content of lithium salts, a result consistent with an electrophilic assistance by the lithium cation (equation 286)545. 

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