Magnesium binding by terrestrial humic acids

Magnesium binding by three terrestrial humic acids was investigated at pH 8 and 25 °C as a function of Mg²⁺ activity and ionic strength using NaCl as the background electrolyte. The Mg²⁺ activity in solution was directly measured with an Mg²⁺-selective electrode in the titration experiments. In addition, coagulation experiments using Ca²⁺ and Mg²⁺ as the coagulants were carried out at pH 8. For the titration data, the NICA–Donnan model was used to quantitatively describe Mg²⁺ binding to the humic acids considering electrostatic and specific Mg²⁺ binding. Mg²⁺ binding to humic acids was found to be strongly affected by ionic strength variations indicating that Mg²⁺ binding largely arose from electrostatic (nonspecific) interactions with negatively charged functional groups of the humic acids. Data modelling suggested that the relative contribution of specific binding increased with decreasing Mg²⁺ activity and was related to functional groups with low proton affinities. For all three humic acids studied, the fitted Mg²⁺ affinity constants for specific binding were lower than the respective Ca²⁺ affinities. Corresponding to the observed differences in cation binding and the known differences in ion hydration, Ca²⁺ was observed to be the stronger coagulant as compared with Mg²⁺. The results suggest that Mg²⁺ may influence the mobility of trace elements that are strongly bound to humic acids such as mercury, although Mg²⁺ is not expected to directly compete with strongly sorbing elements for specific binding.

external page https://doi.org/10.1071/EN18084