Adsorption of Freshwater Dissolved Organic Matter to Clay and Polyethylene Particles

Abstract

Organic matter (OM), especially dissolved organic matter (DOM), plays several integral roles in aquatic systems. OM acts as a short-term sink of carbon and a food source for heterotrophs and shields biota from harmful UV radiation. It also facilitates the transport of nutrients, trace metals and pollutants in the environment. The uptake and transport of these compounds are related to the fate of the DOM to which they are bound. Suspended solids such as clays or microplastics can adsorb DOM into their interlayer spaces or onto their surfaces leading to: 1. Possible physical protection of OM that would have been mineralized or degraded by biota; 2. Increased transport of OM through the water column to the sediments (for sinking particles) or increased time for OM in the surface water (for less-dense microplastic particles); 3. Increased uptake by larger aquatic organisms. Increased particle-associated mobility (either by sinking through the water column or being transported at the surface via wind-driven processes) also increases associated nutrient, trace metal and pollutant transport, which in sufficient quantities, may perturb the aquatic system’s equilibrium and affect its chemistry. Microplastics, a new particle-type in aquatic systems, have been observed and documented in the world’s oceans since the 1970s, but their presence in the Laurentian Great Lakes was first recorded in 2013. The roles that microplastics and other particulates (both naturally occurring and anthropogenically impacted) play in aquatic environments need to be thoroughly studied so that a better understanding of their fate and environmental impact can be gained. The goal of this study was to qualitatively examine and compare the adsorption of open water Lake Superior DOM and DOM from a tributary stream to polyethylene microplastic spheres and to clays (kaolinite and montmorillonite). UV-VIS optical proxies were used to monitor changes in aromaticity (A254, SUVA) and molecular weight (E2:E3, S250-400, SR) within the remaining dissolved phase. Aromaticity proxies suggest that clays preferentially adsorbed aromatic species, while polyethylene had no significant effect on DOM composition. Changes in the amount of carbon remaining in the dissolved phase were measured using dissolved organic carbon (DOC) analysis while the amount adsorbed to the surface of the particulates was measured using elemental analysis (EA). DOC analysis did not show significant changes in the amount of dissolved organic carbon after sorption testing. EA was unable to provide a definitive answer for carbon adsorbed by polyethylene but suggests that kaolinite and montmorillonite adsorb similar amounts of carbon in both environments.