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Arsenic is very harmful to most living organisms. The solubility data of As-containing compounds are significant in geoscience and environmental science. The arsenic-incorporated natroalunite precipitation has been proposed to eliminate arsenic from water, both for industrial practice and remediation of polluted areas. Unfortunately, only few works have been made on partial arsenic incorporation in natroalunite and the thermodynamic data for natroalunite and arsenic-incorporated natroalunite now are still lacking. Moreover, the dissolution mechanisms of arsenic-incorporated natroalunites have never been studied. In the present work, the dissolution of the synthetic natroalunite [Na0.93(H3O)0.61Al2.82(SO4)2(OH)6] and the synthetic arsenic-incorporated natroalunite [Na0.88(H3O)2.44Al2.35(AsO4)0.38(SO4)1.62(OH)6] at 25°C, 35°C, and 45°C was experimentally examined in HNO3 solution (pH of 2.00 and 4.00) and pure water. The characterizations confirmed that the solids showed no recognizable change after dissolution. All dissolutions underwent a pH variation, which was caused by a great depleting of H3O+/OH− ions, typically at the reaction beginning. The dissolution in H3O+ medium proved to be near-stoichiometric within the short beginning period, and the dissolved Na+, Al3+, SO42−, and AsO43− concentrations were stoichiometric according to the initial solids and then appeared to be nonstoichiometric with the Na/SO4 mole ratios higher and the Al/SO4 and AsO4/SO4 mole ratios lower than the stoichiometry until the experimental end, indicating that the components were released from solid to solution preferentially after the following order: Na+ (H3O+) > SO42− > AsO43− > Al3+. From the experimental results under the condition of initial pH 2.00 and 25°C, the solubility products [Ksp] and the Gibbs free energies of formation [ΔGf°] were calculated to be 10−81.02±0.33∼10−81.04±0.27 and −4713 ± 2 to −4714 ± 1 kJ/mol for the natroalunite and 10−92.30±0.30∼10−92.41±0.37 and −5078 ± 2 to −5079 ± 2 kJ/mol for the arsenic-incorporated natroalunite, respectively. The thermodynamic quantities, ΔG°, ΔH°, ΔS°, and ΔCp°, were determined to be 462303.43 J/K·mol, 122466.83 J/mol, −1140.39 J/K·mol, and 4280.13 J/K·mol for the natroalunite dissolution reaction at initial pH 2.00 and 25°C and to be 526925.48 J/K·mol, 159674.76 J/mol, −1232.38 J/K·mol, and 1061.12 J/K·mol for the dissolution of the arsenic-incorporated natroalunite at initial pH 2.00 and 25°C, respectively.

Natural magnesian siderite [(Fe0.8Mg0.2)CO'] was used to investigate its dissolution in different waters at ±25°C for 5520 h. For the dissolution in pure water, air-saturated water, CO2-saturated water and solution of initial pH 9, the aqueous Fe concentrations increased slowly to a steady state in 2160 h with the final concentrations of 0.0070-0.0072, 0.0060-0.0079, 0.0058-0.0067 and 0.0061 mmol/L, respectively. For the dissolution in natural waters after 4080h, the final aqueous Fe concentrations existed in the sequence of sea water (0.030-0.032 mmol/L) > river water and groundwaters (0.0058-0.0069 mmol/L). The potential formation of magnesite, calcite, aragonite and dolomite could hinder the dissolution of siderite and limited the Fe release into water. The average ion activity product (IAP) at the steady state (~Ksp) was computed to be 10-n.4H°.28 with the AG/value of -756.57±1.62 kJ/mol for the natural magnesian siderite [(Fe0.8Mg0.2)CO3], which dissolved congruently at the early stage, and then incongruently and moved progressively from right to upper-left of the solutus curve, the saturation curve for FeCO3 and the minimum stoichiometric saturation curve for (Fe0.8Mg0.2)CO3 in the Lippmann diagram. The aqueous Fe-poorer and Mg-richer solution was finally in equilibrium with the FeCO3-richer and MgCO3-poorer solid. [ABSTRACT FROM AUTHOR]

Natural and synthetic rhodochrosites (MnCOr) were used to investigate their dissolution in different waters at ±25°C. For the dissolution of natural rhodochrosite in pure water, air-saturated water, CO2-saturated 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 io-lo-96±0-08 - Iq-ii.o9±o.o5 an(j |q-io.43±o.oo|q-io.48±o.o2 with the Gibbs free energies of formation (AG/) 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. [ABSTRACT FROM AUTHOR]