Dissolution of various sources of gypsum in aqueous solutions and in soil

Abstract The dissolution of six sources of gypsum in water and 0.01 M CaCl 2 was examined in the presence and absence of soil. The gypsum samples included: analytical grade (AR), three sources of flue‐gas desulphurisation (FGD) gypsum, phosphogypsum (PG), and mined gypsum (MG). Dissolution in aqueous solutions was monitored by measuring the concentration of calcium (Ca 2+ ) and sulphate (SO ) ions. In soils which adsorb small amounts of SO −2 4 , dissolution was estimated from the increase in the concentration of Ca 2+ and SO in gypsum‐treated soil over the control soil. However, in soils which adsorb significant amounts of SO , measurement of solution SO concentration underestimates the extent of dissolution. Gypsum dissolution was larger in water (15.20 mmol litre −1 ) than in 0.01 M CaCl 2 (11.12 mmol litre −1 ), and this was attributed to the Ca 2+ common‐ion effect. The rate of dissolution decreased in the order: AR > FGD > PG > MG. Dissolution was 2 to 10 times faster for powdered (< 500 .m) gypsum than for the discs obtained by pelletising. The differences in the rate of dissolution between the gypsum sources and between powder and disc samples were related to both surface area and the presence of CaCO 3 , impurity. The rate of dissolution was 3 to 8 times faster in the presence than in the absence of soil. Whereas the dissolution of gypsum in the presence of soil followed first order reaction kinetics, it followed second order kinetics in the absence of soil. This difference in reaction kinetics resulted from the continuous removal of Ca2+ and SO by the soil.