Cation exchange properties of heat-treated Australian brown coal: influences of pre-exchanged calcium ions

Loy Yang brown coals, Australia, with pre-exchanged calcium ions (Ca-LY) or without pre-exchanged calcium ions (LY) were heat-treated at a desired temperature between 50 and 400°C and their exchanged amounts of Co2+ or Mg2+ were measured as a function of pH of the aqueous solution. The exchanged amount of divalent cations for Ca-LY heat-treated at 50°C was larger than that for LY heat-treated at the same temperature over the whole pH range. In the low pH region between 2 and 4.5, the exchanged amount of divalent cations for LY heat-treated above 250°C was almost zero, while that for Ca-LY heat-treated at the higher temperature such as 300°C was 0.1–0.3 mmol g−1. These results were discussed based on the amounts and the types of the carboxyl groups present on the heat-treated brown coal surfaces.