The cation‐ and anion exchange characteristics of soils with a large sesquioxide surface area

Abstract

The theory on colloids with a pH‐dependent surface potential indicates that the anion exchange capacity of soils containing hematite or similar oxides depends on pH, electrolyte concentration and valency of counterions. These relationships were tested for the salinity‐ and pH‐range of importance to agriculture, using a percolation technique. MgSO4 was chosen as impregnating salt to minimize the overall error caused by salt exclusion. The assumptions on the anion exchange behaviour proved to be correct; moreover they applied also to a considerable fraction of the cation exchange. The results were formulated in terms of multiple linear regression equations relating the exchange capacities to pH and the square root of the electrolyte concentration. Consequently, the adsorption behaviour of ferruginous soils can not be characterized by the determination of CEC and AEC at one level of pH‐ and salt concentration in solution. Due to the inefficiency of the ethanol washing step, two standard methods overestimated CEC by a factor 2 to 4.

Role of organic matter and sesquioxides on variable charge of three soils from Galicia, Spain

“…According to Koenigs et al (1981), there is a linear relationship between CEC and the square root of the electrolyte concentration, in soils with variable charge. We found similar relationships in our samples ( Figure 5).…”

“…This experimental point might deviate from the CZSE mentioned in the Introduction section, especially if dissolution occurs. The KCl was used as saturating electrolyte because it is a known reagent in exchange studies (Koenigs et al, 1981), although K + may not fulfil the requirement that the background electrolyte must be indifferent (Sposito, 1981).…”

“…The method, which is described in detail hereafter, was modified from that used by Koenigs et al (1981). The samples were buffered at a desired pH value with ammonium acetate/acetic acid mixtures.…”

“…Standard methods of CEC determination use pHs and ionic strengths different from those in natural soil solutions. Consequently, they are not suitable to determine the CEC of a soil with variable charge at natural field conditions (Gillman and Bell, 1976;Fox, 1980;Koenigs et al, 1981).…”

“…Gillman (1979) proposed a simultaneous determination of CEC and AEC in an unbuffered dilute saturating solution. Koenigs et al (1981) According to Sposito (1981) the surface charge is the sum of various contributions: (1) permanent charge; (2) charge arising from net adsorption or desorption of potential-determining ions (PDI); (3) charge from inner sphere complexes (specifically adsorbed ions other than H + and OH); and (4) charge from outer sphere complexes (non-specifically adsorbed ions). This surface charge is counterbalanced by the 'dissociated charge', that is the excess charge of ions in the double layer.…”