Cation exchange capacity metal mobility

Humic substances constitute the bulk of the organic matter in most terrestrial soils. The functions they perform are multiple and varied and include the weathering of rocks and minerals, mobilization and transport of metal ions, and formation of stable aggregates by combination with clay minerals. Humic substances make a significant contribution to the cation-exchange capacity of the soil, and they are involved in the sorption of organic molecules applied to soils as pesticides. 

Meanwhile, the mobility of the metals is influenced by pH, redox potential, cation exchange capacity of the solid phase, competition with other metal ions, stability of metal complexes, and their concentration in the soil solution (Merian, 1991). Hence, in many cases, conditioning liquids are applied. 

Background. We have become particularly interested in the weathering of micas in soils. This weathering process is geochemically and environmentally important because it is a major source of potassium in the soil system (and therefore of paramount importance to plants), and the weathering products typically include other sheet silicates with high cation exchange capacities (CEC). These secondary minerals play an important role in the mobility of toxic metals and nutrients in soils. 

Lead within soils is distributed between solid and liquid phases, with the latter of major importance to the issue of lead bioavailability, for example, to plant roots where uptake can occur. Studies of lead species in this liquid mobile phase indicate that they exist as both complexed and ionic forms although the latter as simple ions are present in very low concentrations. The extent to which lead can move through soils, in turn, is the extent to which lead binds to insoluble organic and mineralogical inorganic species. The former are typically humic and fulvic acid derivatives, and the latter are surfaces of clays and metal oxides (U.S. EPA, 1986). The factors most important for lead movement within soils are pH, cation exchange capacity of... 

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]. 

The effect of multi-charged cationic species on the retention observed for different metals is illustrated in Table 6.3. Here, the ion-exchange capacity of a column (Cig) is varied by using different mobile phase concentrations of n-octanesulfonate. Equations (17) and (18) show the relationship between the ion-exchange capacity of a column and the retention of a metal species. The concentrations of complexing agents used throughout these experiments were constant 58.2 mM a-HIBA and 8.8 mM tartaric acid, pH 4.1. 

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