Dissolution rate of calcium carbonate and calcium hydroxide in saline waters and its relevance for aquaculture

Abstract

The solubilities of calcium carbonate and calcium hydroxide were determined in four waters of different initial salinity, pH, and alkalinity (in parentheses, respectively) as follows: freshwater (0.01gL−1, 6.26, 2.79mgL−1); diluted seawater (3.7gL−1, 7.37, 13mgL−1); diluted seawater (11.0gL−1, 7.97, 35.3mgL−1); seawater (31.7gL−1, 8.12, 109.1mgL−1). Calcium carbonate (97% pure) was added to four 2000-mL flasks of each water at 1.00g each. After 32days, the pH was between 8.1 and 8.3 in all flasks. Alkalinity reached approximately 60mgL−1 in waters initially at 0.01, 3.7, and 11.0gL−1 salinity. In the seawater, alkalinity decreased slightly after 32days. Analysis of results suggested that alkalinity would not increase at a Langelier saturation index of 0.62 or more. Flasks of seawater were treated with calcium hydroxide at rates of about 0, 12, 25, and 76mgL−1 of Ca(OH)2. The pH 24h after treatment ranged from around 8.2 to 8.6. Results of this study revealed that calcium carbonate does not dissolve in seawater and that the equilibrium alkalinity concentration in lower salinity water was about 60mgL−1. Calcium hydroxide – even at 75mgL−1 – did not cause a large increase in pH of seawater. These findings cast serious doubt on the effectiveness of the widespread use of agricultural limestone and lime in shrimp aquaculture ponds.