Ionic strength- and pH-dependence of calcium binding by terrestrial humic acids

Calcium binding by two terrestrial humic acids was investigated at 25 degrees C as a function of pH, ionic strength and Ca2+ activity with calcium titration experiments. A Ca2+- selective electrode was used for Ca2+ measurements to cover a wide range of Ca2+ activities (10(-8.5)-10(-2.5)). Experimental data were quantitatively described with the NICA-Donnan model accounting for electrostatic and specific calcium binding. The results showed that calcium binding as a function of Ca2+ activity was strongly affected by variations of pH and ionic strength indicating that electrostatic binding is an important mechanism for calcium binding by humic acids. Data modelling providing a good description of experimental data for both humic acids suggested that electrostatic binding was the dominant calcium binding mechanisms at high Ca2+ activities often observed in terrestrial environments. Specific calcium binding being quantitatively predominant only at low Ca2+ activities was exclusively attributed to binding sites exhibiting a weak affinity for protons considered to represent mainly carboxylic groups. Since the negative charge of the humic acids being prerequisite for electrostatic calcium binding was found to be mainly due to deprotonation of carboxylic groups except under alkaline conditions, carboxylic groups were identified to primarily control calcium binding of humic acids.