Calcium and inorganic carbon speciation in Spanish Mediterranean brackish waters

Abstract

The concentrations of free ions and major inorganic complexes of calcium and inorganic carbon (i.e. free Ca >, free HCO3, free CO2- H3 , 2CO3 , CaSO°4 , CaCO°3 , CaHC0+ 3 , NaHCO°3 , MgHC0+3 , NaC0-3 and MgCO°3 ) have been calculated according to an ion-pair model, on the basis of their relative abundance in total concentrations, in Spanish Mediterranean brackish waters. Calcium complexes amount to 25% of total calcium, whereas bicarbonate and carbonate complexes reach 10 and 47% of total alkalinity respectively. NaHCO° 3 and MgHC0+ 3 expressed as percentages of total alkalinity, and NaC0- 3 , MgCO°3 and free CO2-3 expressed as percentages of total carbonate are dependent on ionic strength. NaC0-3 , MgCO°3 referred to total carbonate and free HCO3 , and free CO2- 3 as percentages of total alkalinity are strongly dependent on pH. Inorganic carbon complexes amount to an important fraction of total alkalinity when I (ionic strength)> 0.05 and pH> 8.5, when I> 0.10 and pH> 8.3, and when I> 0.20 and pH> 8.1. Calcium complexes cannot be expressed as single function of pH or ionic strength. Their maximum percentages appear at high pH values and moderate or high ionic strength, when CaS0° 4 and CaCO° 3 coexist at significant levels.