Anion Adsorption by Allophanic Tropical Soils: I. Chloride Adsorption1
- H. Gebhardt and
- N. T. Coleman2
The adsorption of Cl by Andepts from Mexico, Colombia, and Hawaii was measured in solutions of HCl, HCl + NaCl, and AlCl3. Chloride adsorption varied from 0-8 meq/100 g at pH 6 to as much as 32 meq/100 g at pH 3.8. At given pH, adsorption was concentration-dependent in a manner consistent with the Langmuir adsorption equation. For a B-horizon sample from San Gregorio, Mexico, the Cl adsorption maxima, in meq/100 g, were 7.4 at pH 6; 10.7 at pH 4.8; 13.4 at pH 4.4; 17.3 at pH 4.2; and 31.6 at pH < 4. The average a from the Langmuir equation was 0.04 liter/meq. Adsorbed Cl was removed by leaching with water, and was exchanged by NO3.
Chloride adsorption from HCl or HCl-NaCl was accompanied by the consumption of protons; adsorption from AlCl3 resulted in the hydrolysis and precipitation of Al. Protons consumed or Al hydrolyzed exceeded Cl adsorbed by amounts corresponding closely to the effective CEC. The results suggest that protons are adsorbed to produce positively charged sites, which bind Cl nonspecifically.
Chloride adsorption capacity can be conveniently measured by shaking soil with AlCl3 solution and measuring Cl uptake. Chloride capacities of soils and clay minerals, determined by the AlCl3 procedure, were, in meq/100 g, 7–30 for Dystrandepts; 16–18 for Hydrandepts; 2–4 for acid soils containing crystalline clay and oxide minerals; 4 for kaolinite; zero for montmorillonite and illite.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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