Size-Based Fractionation and Quantification of Mobile Colloids and Colloidal Organic Carbon in Field Samples.

Natural colloids, often defined as entities with sizes < 1.0 μm, have attracted much research attention because of their ability to facilitate the transport of contaminants in the subsurface environment. However, due to their small size and generally low concentrations in field samples, quantification of mobile colloids, especially the smaller fractions (< 0.45 µm), which are operationally defined as dissolved, has been largely impeded. As a result,  the natural colloidal pool is likely overlooked or underestimated. The main objectives of this study are to: (1) develop an experimentally and economically efficient methodology to quantify natural colloids in different size fractions (0.1-0.45 and 0.45-1 µm) and (2) quantify mobile colloids including small size fractions, < 0.45 µm particularly, and colloidal organic carbon in different natural aquatic samples. Methodology based on the correlation between turbidity and mass concentration of soil colloid was developed, which enabled quick quantification of colloid concentrations of a large number of field samples collected from freshwater, wetland and estuaries in different size fractions. As a general trend, we observed high concentrations of colloids in the < 0.45 µm fraction, which constitutes a significant percentage of the total mobile colloidal pool (< 1 µm). Additionally, the correlation between organic carbon and colloids indicates that substantial amounts of carbon can be associated with < 0.45 µm colloids. This observation suggests that the operationally defined cut-off size for “dissolved” phase can greatly underestimate colloid concentration and colloidal organic carbon therefore the role that colloids play in carbon cycling or the transport of associated contaminants.