In this study, we attempt to quantify the roles of pH and cation charge density on the size and polydispersity of Suwannee River NOM aggregates suspended in water using Dynamic Light Scattering (DLS). Unraveling the details of NOM-NOM interactions in solution are a critical precursor to understanding the mechanisms of NOM-mineral interactions and how NOM-mineral composites influence the transport of organic and inorganic contaminants in the environment or the role of organo-mineral interactions in the global carbon cycle. Recent molecular dynamics computer simulations have suggested that cation bridging amongst NOM molecules and between NOM molecules and mineral surfaces are an important interaction mechanism in water-rich systems under basic conditions. They also suggest that such bridging is more important for systems bearing Ca2+ than for other alkaline earth metals and alkali metals. We tested these ideas in the laboratory by performing systematic DLS studies of NOM suspensions as a function of pH and time in the presence of Cs+, K+, Na+, Ca2+, Mg2+, and Sr2+. Our DLS instrument showed no statistically significant variation in mean equivalent hydrodynamic radius of suspended NOM aggregates with the cation charge density or time among the metals examined here with the exception of the Cs+ solution at pH 2. In this sample, we observe an increase in the mean aggregate size with increasing time over one month of observations. However, we do see trends in the number of unique hydrodynamic radii observed in the DLS results. Alkali metals at pH 2 tend to be monodisperse, single-maxima while increasing pH leads to increasing number of maxima, with mean hydrodynamic radii appearing between 100 nm and 1.5 mm. In contrast, the alkaline earth metal-NOM solutions exhibit multiple unique maxima at pH 2 and decrease in number of maxima with increasing pH. In alkaline earth metal-NOM solutions, the protonation state of NOM seems to be the dominant feature in determining the number of maxima with more deprotonation leading to increasingly monodisperse NOM aggregates. In contrast, charge density seems to be the dominant feature controlling the number of maxima among the alkali metals. From our data set, we hypothesize a model in which the percent saturation of available ion bridges between NOM molecules affects the degree of polydispersity and number of unique particle sizes.

Influence of cation charge density, ionic strength, and pH on NOM particle size distributions in aqueous solution

249th National Meeting (2015)

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