Magnesium binding by terrestrial humic acids

Environmental context The behaviour of magnesium, which is an essential element for all living organisms, in terrestrial environments is influenced by natural organic matter. This study shows that magnesium binding by terrestrial humic acids exhibits a pronounced ionic strength-dependence indicating a strong preference for electrostatic binding to humic acids. This interaction is expected to influence the mobility of humic substances and their associated trace elements. Abstract Magnesium binding by three terrestrial humic acids was investigated at pH 8 and 25 °C as a function of Mg 2+ activity and ionic strength using NaCl as the background electrolyte. The Mg 2+ activity in solution was directly measured with an Mg 2+ -selective electrode in the titration experiments. In addition, coagulation experiments using Ca 2+ and Mg 2+ as the coagulants were carried out at pH 8. For the titration data, the NICA-Donnan model was used to quantitatively describe Mg 2+ binding to the humic acids considering electrostatic and specific Mg 2+ binding. Mg 2+ binding to humic acids was found to be strongly affected by ionic strength variations indicating that Mg 2+ 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 Mg 2+ activity and was related to functional groups with low proton affinities. For all three humic acids studied, the fitted Mg 2+ affinity constants for specific binding were lower than the respective Ca 2+ affinities. Corresponding to the observed differences in cation binding and the known differences in ion hydration, Ca 2+ was observed to be the stronger coagulant as compared with Mg 2+. The results suggest that Mg 2+ may influence the mobility of trace elements that are strongly bound to humic acids such as mercury, although Mg 2+ is not expected to directly compete with strongly sorbing elements for specific binding.