Gypsum Precipitation – A Medium to Control Sulphate Pollution of Freshwater Sediment Leachates

To treat sulphate pollution resulting from freshwater sediments,an experimental study was done. This experimental study was carried out to determine the effects of pH values and calcium supplements (as CaCl₂) on the formation of gypsum in a representative wetland sediment that was aerated. It was foundthat the differences between dissolved sulphate in untreatedcontrol and the other treatments with CaCl₂ amendments canbe attributed to the precipitation of gypsum. At the same time, the system was highly affected by Ca adsorption/desorption thatwas controlled by pH values. ECOSAT equilibrium simulations showed that CaCl₂ additions (0.2–0.3 mol L^-1) to aerated freshwater sediment resulted in removal of 78–98% ofsulphate (from its total content) from the solution. It wascaused by favourable conditions for gypsum formation i.e. acidicpH values were not increased due to the neutral character of CaCl₂. On the contrary to CaCl₂ amendments, CaCO₃supplements (in the same Ca doses as applied in CaCl₂) caused the removal of only 5–25% of sulphate. The significantincrease of pH values after the application of CaCO₃ limitedgypsum formation due to high saturation of the system with CO₃ ^-2 and HCO₃ ^-. It resulted in the prevalence of CaCO₃ formation in this system. However, theaddition of Cl^- can facilitate the mobilisation of cationicmetals like Cd and Pb in the form of Cl^--complexes. Therefore, CaCl₂ treatment should be used in the sites thatdo not contain elevated amounts of these metals.