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 CaCO 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

Evaluation of soil analytical methods for the characterization of alkaline Technosols: I. Moisture content, pH, and electrical conductivity

Purpose: Applying standard soil analytical methods to novel soil materials, such as tailings or soils with unusual properties, should be done with caution and with special consideration of potential interferents and possible pretreatments. The aim of this study was to investigate the effects of common variations in methods on calculated total moisture content, pH, and electrical conductivity (EC) of saline alkaline soil materials. Materials and methods: Bauxite residue (an alkaline, saline-sodic Technosolic material) as well as two saline alkaline soils from coastal and lacustrine environments were dried under various temperatures and atmospheres, and then analyzed for pH and EC at various soil-solution ratios over time. Results and discussion: Calculated moisture content of all samples increased with drying temperature. Dehydration of gypsum elevated calculated moisture content. Decreases in soil-solution ratio decreased suspension EC and pH in highly alkaline samples. The pH and EC of soil/water suspensions generally rose with equilibration time for bauxite residue; stable values were attained within 24-120 h. Atmospheric carbonation substantially decreased the pH of samples dried at lower temperatures. Conclusions: Variations in temperature, time, and atmosphere during drying of highly alkaline and saline soil materials influenced calculated moisture content as well as chemical properties such as pH and EC. A drying temperature of 40 °C and drying to constant weight is recommended to minimize these effects. Soil-solution ratio, equilibration time, and sample preparation conditions influenced observed pH and EC, and should be standardized if attempting to compare results between studies