HETEROVALENT CATION EXCHANGE EQUILIBRIA IN TWO SAVANNA SOILS
HETEROVALENT CATION EXCHANGE EQUILIBRIA IN TWO SAVANNA SOILS
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Date
2006-07
Authors
SAMUEL, YAKUBU
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Abstract
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ABSTRACT
Cation exchange equilibira involving K , Ca and Mg were studied in two
representative savanna soils. The study examined Ca-K and Mg–K exchange
equilibria with a view to establishing their preferential retention of the cations in
savanna soils. Soil samples were collected at two depth ranges (0-15cm) and (15-
30cm) from Zaria and Mokwa respectively. A solution of constant chloride salts
concentration (0.1M) containing variable concentrations of K, Ca and Mg was
prepared. The K concentration of the solution was prepared to vary from 0 to 200
mmol L-1 while the Ca and Mg solution were made to vary from 100 to 0 mmol L-1 in
each case. The Ca and Mg equivalents of the Ca-K and Mg-K mixtures were made to
increase from 0 to 1 while the equivalent fractions of K were made to decrease from 1
to 0 in the mixtures. For the Mokwa soil, the total adsorbed metal charge (Qi) for Ca –
K exchange was 22% greater than the total adsorbed metal charge (Qi) for Mg-K
exchange, thus indicating some preference for CaCl+ over MgCl+ by the surface
exchange sites. This was, however, in contrast to the relative consistent total
adsorbed metal charge (Qi) for both Ca-K and Mg-K exchange in Zaria soil. Several
cation exchange models were used to describe the Ca-K and Mg-K exchange
equilibira. The exchange models tested in the study were the Kerr selectivity
coefficient (Kk), Vanselow selectivity coefficient (Kv), Gapon selectivity coefficient
(KG), Gaines and Thomas selectivity coefficient (KGT), Davies selectivity coefficient
(KD) and the thermodynamic exchange constant (Kex).The trends in selectivity
coefficients as a function of exchanger phase composition for Ca-K exchange were
similar to Mg-K exchange. All selectivity coefficients determined for Mg-K and Ca-K for
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the soils were far greater than 1 at all exchanger phase composition, suggesting a
preference for K to either Ca or Mg by the soils. The Ca-K and Mg-K exchange
isotherms were thus compared with the thermodynamic non-preference isotherm
(TNI) line by plotting the charge fractions of adsorbed K versus the charge fraction of
K in soil solution in the soil solution. The result indicated that the experimental points
were below the TNI line for both Zaria and Mokwa, clearly suggesting non-preference
for either Ca or Mg in the presence of K. The K preference by the soil can be
explained by the relatively high hydration shell of Ca and Mg as compared to K that
tends to limit the rate of diffusion of Ca and Mg from the soil solution to the surface
exchange sites as compared to K. The thermodynamic exchange constant Kex for the
surface soil fron Zaria was 7.98 while the subsurface soil was 7.81, but Kex decreased
sharply to 3.12 and 4.60 for the surface and subsurface soil respectively for Mokwa.
The negative values of ΔGex determined for Ca-K and Mg-K exchange seemed to lend
additional credence to the notion that K was strongly preferred to Ca and Mg by the
soils. The selectivity for K in the Zaria soil was stronger than the Mokwa soil
apparently because of differences in mineralogical composition.
Description
DEPARTMENT OF SOIL SCIENCE
FACULTY OF AGRICULTURE
AHMADU BELLO UNIVERSITY, ZARIA.
Keywords
HETEROVALENT,, CATION,, EXCHANGE,, EQUILIBRIA,, SAVANNA,, SOILS.