Dissolved Organic Matter from a colloidal perspective

Sammanfattning: Dissolved organic matter (DOM) is considered the most bioavailable fraction of soil, and thus play a key role in the cycling of carbon. Because of its mobility, DOM also forms the connection between terrestrial and aquatic systems, and constitutes an important vector for nutrients and contaminants. DOM is in this thesis operationally defined as the organic matter in an aqueous solution which is not retained upon filtration using a pore size of 0.2μm. This means that DOM is a heterogeneous mixture of different chemical components, ranging in size from small molecules up to colloidal particles of a few hundred nanometres.In this thesis, we have characterised DOM from the molecular to colloidal length scale, using a combination of spectroscopy, microscopy and scattering techniques. The DOM was obtained by extraction of soil from the organiclayer of a boreal spruce forest. We found that the dominant chemical component of DOM was carbohydrates, and that about half of the organic carbon was present in colloidal form. The structure of the DOM colloids depended on the extraction procedure used. Large dense aggregates observed at room temperature and below could be dispersed into smaller components by increasing the temperature or pH. Additionally, we studied the bacterial decomposition of DOM and found that the colloidal fraction remained intact, while small molecules were readily decomposed. We have also assessed interactions between DOM and hematite nanoparticles, using neutron scattering. Here, we found that the low molecular weight fraction of DOM induces charge reversal and aggregation of the hematite particles. Our combined results show that the colloidal fraction of DOM needs to be considered when assessing DOM bioavailability and mobility.