In-Situ Investigation of Interactions between Magnesium Ion and Natural Organic Matter

Natural organic matter (NOM) generated in all niches of the environment constitutes a large fraction of the global pool of organic carbon while magnesium is one of the most abundant elements that has multiple roles in both biotic and abiotic processes. Although interactions between Mg2+ and NOM have been recognized to affect many environmental processes, little is understood about relevant mechanisms and equilibria. This study addressed this deficiency and quantified Mg2+–NOM interactions using differential absorbance spectroscopy (DAS) in combination with the NICA–Donnan speciation model. DAS data were obtained for varying total Mg concentrations, pHs from 5.0 to 11.0 and ionic strengths from 0.001 to 0.3 mol L–1. DAS results demonstrated the existence of strong interactions between magnesium and NOM at all examined conditions and demonstrated that the binding of Mg2+ by NOM was accompanied by the replacement of protons in the protonation-active phenolic and carboxylic groups. The slope of the log-transformed absorbance spectra of NOM in the range of wavelength 350–400 nm was found to be indicative of the extent of Mg2+–NOM binding. The differential and absolute values of the spectral slopes were strongly correlated with the amount of NOM-bound Mg2+ ions and with the concentrations of NOM-bound protons.