Evaluating methods for quantifying cation exchange in mildly calcareous sediments in Northern Alberta

Cation exchange capacity (CEC) and exchangeable cation composition are important properties in defining water–sediment interactions in contaminant transport scenarios, such as the seepage of process-affected water from oil sands tailings impoundments. At present, detailed characterization of these properties has been overlooked in the surficial Pleistocene sediments of the Athabasca Oil Sands region in Canada. Although a great variety of methods for determining CEC exist and continue to be developed, there is a deficiency in detailed comparisons amongst these methods in the literature, and consequently uncertainty still remains as to which extractants/methods are most suitable for use with sediments containing partially soluble carbonate salts. The present study (1) conducts an up-to-date evaluation of several distinct methods for their suitability in characterizing cation exchange properties in mildly calcareous sediments; using them to (2) evaluate the sorptive capacity and easily displaceable cations of mildly calcareous, surficial sediments near Fort McMurray, Alberta. Four conceptually different CEC methods were chosen: (a) a summation method (Lithium Chloride extractant); (b) a method that displaces an index cation after intermediary washes (Sodium Acetate–Ammonium Acetate); (c) a method using a high affinity cation complex (Silver Thiourea); and (d) a titration method using an organic cationic dye (Methylene Blue). Exchangeable cations were characterized using the Lithium Chloride and Silver Thiourea methods. Analysis suggests that the Methylene Blue test is least impacted by the presence of carbonate mineral phases. Results from field sediments suggest that the exchangeable cations, in decreasing abundance, consist of Ca > Mg > K > NH₄ > Na; while the CEC was found to be 4.7 ± 2.7 meq 100 g⁻¹ in the clay-rich glacial till, 0.7 ± 0.0 meq 100 g⁻¹ in the underlying sand and 4.1 ± 2.3 meq 100 g⁻¹ in the till-sand transition zone.