Lithium Extraction to Determine Ammonium in the Exchange Complex of Bentonite

ASTM D7503, Standard Test Method for Measuring the Exchange Complex and Cation Exchange Capacity of Inorganic Fine-Grained Soils, is used to determine soluble and bound cation concentrations as well as the cation exchange capacity of fine-grained soils used in barrier systems for waste containment, including bentonite in geosynthetic clay liners (GCLs). ASTM D7503 prescribes the use of 1 M ammonium acetate (NH₄OAc) as an extraction liquid for bound cations, which masks ammonium (NH₄⁺) that may be in the exchange complex in bentonite in contact with municipal solid waste (MSW) leachate. A 1 M lithium bromide (LiBr) extraction liquid is proposed as an alternative to the 1 M NH₄OAc in ASTM D7503 so that NH₄⁺ in the exchange complex can be quantified without interfering with measurement of other bound cations of interest for waste containment. Nearly the same distribution of bound major cations was obtained using the method in ASTM D7503 and the proposed alternative using 1 M LiBr for two bentonites from two new GCLs (not permeated). Using the 1 M LiBr method revealed substantial NH₄⁺ bound to eight bentonites obtained from GCLs that had been permeated with different MSW leachates. The bound NH₄⁺ concentration varied directly with NH₄⁺ concentration in the leachates, and this variance explained a charge imbalance obtained from tests conducted on the bentonites using the standard method in ASTM D7503.