Soil, anion-exchange capacity

Anion A negatively charged ion in an electrolyte solution, attracted to the anode under the influence of a difference in electrical potential. Chloride is an anion. Anion exchange capacity Sum total of exchangeable anions that a soil can adsorb. Expressed as centimoles of negative charge per kilogram of soil. 

Some soils, particularly those in the tropics, have significant anion exchange capacity. For these soils, there is an attraction between soil colloids and the simple halogen and nitrate anions. Bringing these anions into solution for analysis requires an extraction, or replacing anions, just as does the analysis of exchangeable cations. 

Anion exchange capacity can be investigated in a similar manner except that the soil is first leached with an anion [4],... 

Figure 11.9 Sorption isotherms for some charged organic compounds interacting with natural solids (a) quinolinium cation on a subsoil of /oc = 0.024 and cation exchange capacity of 84 mmol/kg (Zachara et al., 1986), ( >) anilinium cation on a surface soil with /oc = 0.013 and cation exchange capacity of 112 mmol/kg (Lee et al., 1997), and (c) sorption of 4-(2,4-dichloro-phe-noxy)-butyrate anion on a sediment with/oc = 0.015 and unknown anion exchange capacity (Jafvert, 1990).

Figure 3.27 demonstrates that soil systems are mixtures of variable and constant charge minerals. It appeals that as pH rises above 3, the positive charge or anion exchange capacity (AEC) of the minerals decrease, but the CEC remains unaffected up to pH 5 and increases significantly above pH 5. Therefore, above pH 5 the potential of soil to adsorb cations (e.g., Ca2+, Mg2+, Mn2+, and Fe2+), increases, whereas the potential of soil to adsorb anions (e.g., SO2-) decreases. Since natural soils are mixed systems with respect to their charge, that is they contain both variable plus constant charge minerals,... 

The term pHznc in Equation 3.24 represents the point of zero net charge (PZNC). It is the pH value at which the cation exchange capacity equals the anion exchange capacity (Fig. 3.28). Equation 3.24 shows that pHznc varies with ionic strength (n), whereas pH0 or PZC is an intrinsic property of the mineralogically heterogeneous soil (Uehara and Gillman, 1980). 

Sorption in most soils attains a maximum when the neutral hydroxy complex of uranium is at a maximum. However, at pH 6 and above, and in the presence of high carbonate or hydroxide concentrations, uranium may form anionic complexes such as [U02(0H)4]. The mobility of anionic uranium complexes in soil is dependent upon the nature of the soil. For example, the decrease in sorption in soil with little anion-exchange capacity may result in increased mobility however, increased sorption in soil with high anion-exchange may result in decreased mobility (Allard et al. 1982 Ames et al. 1982 Brookins et al. 1993 Ho and Doern 1985 Hsi and Langmuir 1985 Tichnor 1994). 

Hydroxyoxide surfaces often appear to have concentration-dependent anion exchange capacities, but such behavior can also be explained by the collapse of the double layer at high salt concentrations. Under these conditions, positively charged sites are no longer masked by the DDLs of the predominantly negatively charged soil matrix. 

Soils possjess anion exchange capacities for certain anions that increase as the pH decreases. This property of anion exchange has been shown to be greatest in soils that consist largely of 1 1 clay minerals and hydrous oxides of iron and aluminum the 2 1 clays have a lower anion-retaining capacity. 

If it is assumed that the cation and anion exchange capacities (CEC and AEC) of a soil clay refer only to those ions in the interfacial region that can be displaced easily by a leaching solution, then the difference CEC — AEC is proportional to the sum ctos + crj. In mathematical terms,... 

Thus, ion retention and exchange by charged soil colloids is important for plant nutrient retention and has many environmental implications as an example, NOj is important for plant growth, but if it leaches, it will move below the plant root zone and leach into groundwater, where it is deleterious to human health. If a soil has a significant anion exchange capacity, nitrate can be held so it is available to plants. 

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