Channel flow cell studies of the inhibiting action of gypsum on the dissolution kinetics of calcite: A laboratory approach with implications for field monitoring

The rate of dissolution of surface-treated calcite crystals in aqueous acid ic solution has been studied using an adaptation of the channel how cell me thod with microdisc electrode detection. Surface treatments of calcite with sulfuric acid lead to the nucleation of gypsum overgrowths, which reduce t he rate of dissolution of calcite. Rate constants for untreated calcite and calcite pretreated with sulfuric acid conditions of 0.01 M for 1 h, 0.05 M for 5 h, and 0.1 M for 21 h are found to be 0.035, 0.018, 0.006, and 0.004 cm s(-1), respectively, Deterioration of calcite materials caused by acid deposition was investigated by held exposure of untreated and sulfate pretr eated calcite racks under urban conditions for 12 months. The rate constant for both pretreated and untreated calcite exposed to weathering is 0.003 c m s(-1). This suggests that calcite self-passivates the surface from furthe r reaction when exposed to acid deposition. However, surface studies indica te that the surface undergoes erosion and dissolution before passivation, P retreatment of the surface with sulfate protects the surface from acid depo sition so it remains less reactive toward acid compared with untreated calc ite. (C) 2001 Academic Press.