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Selective Dissolution Effects on Cation-Exchange Capacity and Specific Surface of Some Tropical Soil Clays¹

ABSTRACT

Soil clays from eastern Panama and Wyoming bentonite changed in CEC and specific surface after sequential selective dissolution treatments. Dithionite-citrate treatment of the clays resulted in a small increase in CEC at both pH 4.8 and 8.2 and a decrease in ethylene glycol retention. Free Fe₂O₃ ranged from 2% to 9% of the clay weight and substantial Al₂O₃ and SiO₂ were also removed. Further treatment with hot 0.5N NaOH for 2 minutes again followed by citrate-dithionite resulted in 20% to 40% loss in clay weight as SiO₂ and lesser amounts of Al₂O₃ and Fe₂O₃. This treatment resulted in an increase of 7 meq in CEC at both pH ranges of soil clays and 20 meq for the bentonite. This was attributed to removal of interlayer material blocking exchange sites but there was a mean decrease in specific surface after this treatment. Clays treated with acid NH₄F-NH₄Cl reagent for 10 minutes decreased sharply in specific surface and loss in clay weight was from 9% to 17% with SiO₂ > Al₂O₃ > Fe₂O₃. After Na saturation and careful washing, Na replacement by 1N NH₄OAc at pH 4.8 also removed high amounts of F and Al making CEC estimation impractical because NaF and/or Na₃AlF₆ was occluded in the process of Na saturation.

¹ Florida Agr. Exp. Sta. Journal Series no. 3489. Contribution by Dept. of Soils, Univ. of Florida, Gainesville. This paper is part of a dissertation by funds from Battelle Memorial Institute for study of a proposed sea-level canal in Panama.

² Research Associate and Professor, respectively.

Received for publication December 22, 1969. Accepted for publication June 3, 1970.