Evaluation of soil analytical methods for the characterization of alkaline Technosols: II. Amorphous constituents and carbonates

Purpose: The aims of this study were to identify potential sources of error in common methods for determination of amorphous oxide concentrations and carbonate concentrations, as applied to a Technosolic material (bauxite residue), and where possible, suggest improvements to the methods. Materials and methods: An acid ammonium oxalate (AAO) extraction was applied to fresh and weathered bauxite residues, at soil to solution ratios varying from 1:100 to 1:800. Two methods for carbonate concentration were compared: the 'weight loss' method, and the 'difference in total C' method. These were applied to six weathered bauxite residue samples, with CaCO3 concentrations ranging from 0.1-2 % weight. Results and discussion: Chemically extractable amorphous content was underreported in bauxite residue at the standard 1:100 extraction ratio, likely due to Al and Si saturation of the oxalate complex. A 1:400 soil: AAO ratio extracted the highest amount of amorphous material. Some crystalline minerals such as sodalite, inherited from the Technosolic parent material, were soluble in the acid ammonium oxalate extractant. The difference in total C method was more precise than the weight loss method for the determination of carbonate concentration in bauxite residues. Conclusions: The high amorphous content of bauxite residues requires a wider soil to solution ratio (1:400) for acid ammonium oxalate extraction than is used for typical soil materials (1:100). The difference in total C method is recommended for the routine analysis of field samples where small variations in carbonate concentration need to be detected. © 2013 Springer-Verlag Berlin Heidelberg.