THE INFLUENCE OF CHELATING-AGENTS ON THE KINETICS OF CALCITE DISSOLUTION

The kinetics of calcite dissolution in the presence of calcium chelati ng agents was investigated over the pH range of 3.3-12 using a rotatin g disk apparatus. The results show that the rate of dissolution is inc reased significantly by the presence of chelating agents such as CDTA, DTPA, and EDTA. The rate of dissolution is influenced by the kinetics of the chelation reactions and varies considerably with pH and type o f chelating agent. A surface chelation mechanism was introduced to des cribe the dissolution. The mechanism involves the adsorption of the ch elating agent onto the calcite surface and follows Langmuir-Hinshelwoo d kinetics. The dissolution is different from conventional hydrogen io n attack in that the chelating agent attacks the calcium component of the lattice rather than the carbonate component. Therefore, the rate o f dissolution is enhanced by the influence of hydrogen ion attack at l ow pH. In addition, the various ionic forms of the chelating agents re act with the calcite surface at different rates depending on the numbe r of hydrogen ions associated with the species. In general, the rate o f dissolution increases with increasing protonation. The surface compl exation mechanism was shown to describe the rate of calcite dissolutio n in the presence of chelating agents over the pH range of 4-12. (C) 1 998 Academic Press