Potential interactions between sedimentary dissolved organic matter and mineral surfaces

The sorptive properties of pore water organic matter and organic matter easily extracted from marine sediments were examined in a series of batch adsorption and desorption experiments. Pore water natural organic matter (pNOM) and easily extracted natural organic matter (eNOM) were isolated from Sinclair Inlet, WA (USA) sediments. The NOM fractions were concentrated 9 times via rotoevaporation and then sorbed to, and desorbed from, organic-free sediment, sediment size fractions (63–250, 38–63 and <38 μm), montmorillonite and iron oxide. Sorption isotherms of pNOM varied little with changing sediment grain size or mineralogy, and Freundlich linearity coefficients (n) were all near 1 (range 0.65–1.15). eNOM was more heterogeneous in its sorption to the various surfaces, having a wider range of linearity coefficients (0.38–1.36) and a greater tendency to sorb to the montmorillonite. Average partition coefficients (Kd) for the pNOM and eNOM were small (0.1±0.04 and 0.08±0.03 l g−1), illustrating only a low affinity for the surfaces. Hystereses observed in the desorption isotherms relative to the adsorption isotherms extrapolate back to the origin, suggesting that the sorption of NOM was reversible. A multiple-component model fit to the sorption and desorption data suggests that both the pNOM and eNOM fractions can be described as 1:2 mixtures of two components with average Kdʹs of 3.2 and 0.02 l g−1, respectively. This result was confirmed in a cumulative adsorption experiment, where less than 40% of the pNOM was found to be surface-reactive. The overall similarity of the sorptive properties of pNOM and eNOM suggest that the two isolates are subgroups of a single sedimentary NOM pool. At natural sediment porosities between 0.5 and 0.95, pNOM is buffered by a surface-bound eNOM pool that is many times larger than the pNOM pool with which it is undergoing exchange.