DISSOLUTION AND SOLUBILITY OF NATURAL AND SYNTHETIC RHODOCHROSITES AT 25 degrees C

Natural and synthetic rhodochrosites (MnCO3) were used to investigate their dissolution in different waters at 25 C. For the dissolution of natural rhodochrosite in pure water, air-saturated water, CO2saturated water, solution of initial pH 3 and solution of initial pH 9, the aqueous Mn concentrations increased with time to a constant state after 2640 h with the final values of 0.079-0.086, 0.081-0.087, 0.086-0.087, 0.581 and 0.070 mmol/L, respectively. While for the dissolution of synthetic rhodochrosite in solution of pH 3, pure water of pH 6.2 and solution of pH 9, the aqueous Mn concentrations increased to a constant state after 2640 h with the final values of 0.732, 0.214 and 0.194 mmol/L, respectively. For the dissolution of natural rhodochrosite in natural waters, the Mn concentrations increased progressively and were finally in order of sea water (0.0089 mmol/L) > groundwater (0.0028-0.0036 mmol/L) > river water (0.0021 mmol/L), which were far below those in the artificial waters; the possible precipitation of calcite, aragonite, dolomite and magnesite could inhibit the dissolution of rhodochrosite and decreased the released Mn concentration in water. The solubility products (Ksp) were estimated to be 10-1096'05 10-11.09 0.05 and 10-10 43 0 00_1 0-10 48 0 02 with the Gibbs free energies of formation (AGf) of -818.45 0.42 -819.20 0.30 and -815.44 0.02-815.73 0.15 kJ/mol for the natural and synthetic rhodochrosites, respectively.