Fractal Character of Humin and its Components

Abstract

Natural organic matter in soils is usually found in intimate association with inorganic materials, primarily aluminosilicates, during the early stages of diagenesis to form an organo-mineral composite known as humin. Such composites typically represent more than 50% of the organic carbon present. Because of humin's insolubility and the surface reactivity of its organic components it is recognized as an important adsorbent of many anthropogenic organic compounds introduced into natural soil systems. Fractal analysis of small-angle X-ray scattering data was used to characterize the surface roughness of four humin samples following sequential removal of organic matter. The surface fractal dimensions were observed to decrease with removal of organic matter which also resulted in a decrease in surface pore size. These results suggest that the mineral components of humin have smooth surfaces over length scales of ~ 1 to ~ 15 nm, and that it is the organic matter coatings that are responsible for their surface roughness. Models clay-organic compounds exhibited results that are consistent with the observation made for natural samples. The surfaces of all the components of humin were found to be dominated by micro- and mesopores that could explain humin's high sorptive uptake of organic chemicals in the environment.