Total buffering capacity of CaCO3-undersaturated and saturated waters buffered with agricultural limestone or hydrated lime and its importance to the liming of shrimp culture ponds

Abstract

The pond's alkaline reserve (AR; particulate CaCO3) plus its total alkalinity (TA; dissolved HCO3−, CO3−2) would be equal to the pond's total buffering capacity (TBC). The present work aimed at determining the TBC of CaCO3-undersaturated and saturated waters buffered with CaCO3 and Ca(OH)2, and to discuss the TBC importance to the liming of shrimp culture ponds. Suitable volumes of seawater and distilled water were mixed up to prepare a batch of mesohaline water (CaCO3-saturated water; salinity = 11.2 g/L; LSI, Langelier Saturation Index ≈ 0). Besides, a batch of distilled water was used as a freshwater control (CaCO3-undersaturated water; LSI = −2.2). One single application of 1.00 g analytical-grade CaCO3 and Ca(OH)2 were carried out in 10-L gallons of each water type. For each water type and liming product, there were three applications of 1.0 mL HCl solution made at two-week intervals. The waters' pH, specific conductance, TA, total hardness, calcium hardness and Ca+2 were periodically monitored over a 55-day period. Three days after the 1st acidification, the pH of water increased for both liming products, mainly in the CaCO3-undersaturated water gallons. The grand TBC were as follows: −69.6% and − 80.1% for the CaCO3-undersaturated waters provided with CaCO3 and Ca(OH)2, respectively; −80.3% and − 78.7% for the CaCO3-saturated waters with CaCO3 and Ca(OH)2, respectively. It has been concluded that the application of agricultural limestone or hydrated lime in CaCO3-saturated waters, although pointless in the short term, might prove useful over time as an alkaline reserve of the shrimp's pond to be mobilized in cases of acidic inputs such as the occurrence of strong and persistent rains.