Understanding the chemistry of crystalline polymethylenic carbon in soils as sinks for hydrophobic contaminants..
Patrick G. Hatcher and Ashish Deshmukh. The Ohio State University, 100 W 18th Ave, Columbus, OH 43210
While the traditional view is that sorption of hydrophobic organic contaminants (HOCs) in soils and sediments is associated with the aromatic components, recent evidence suggests that aliphatic components of soil can sorb significant amounts of these contaminants. Amorphous polymethylenic structures in soil organic matter (SOM) are believed to be sinks for HOCs. Alternatively, we demonstrate that crystalline components of soil organic matter can not only sorb HOCs but can retain them more readily. Diagenetically persistent plant biopolymers cutin, cutan and suberin that are rich in paraffinic structures, have been used as proxies for SOM in our studies to understand the nature of the sorption to aliphatic soil components. Data from 13C nuclear magnetic resonance (NMR) spectroscopy and differential scanning calorimetry (DSC) indicate that while tomato cutin is a rubbery polymer composed of amorphous polymethylenic structures, Agave americana cutan and potato suberin are glassy semi-crystalline polymers. Additional studies using two dimensional High Resolution Magic Angle Spinning NMR experiments allow us to investigate the molecular architecture of these biopolymers at a level of molecular resolution not previously possible. Sorption experiments demonstrate that, while HOC molecules undergo linear sorption with large log KOC values (> 5) in the rubbery biopolymers (cutin), they undergo non-linear sorption in the glassy biopolymers (cutan & suberin), which also have the ability to retain the pollutant better. Chemical shift anisotropy (CSA) NMR measurements show that HOCs have a high degree of mobility when sorbed to a rubbery biopolymer like tomato cutin in which the polymethylenic chains have higher molecular mobility, while there are constraints on their molecular motion when sorbed to less mobile glassy biopolymers, such as Agave cutan and potato suberin. 129Xe NMR shows that Xe atoms can occupy momentary voids in the amorphous regions of biopolymers, which are expected to have longer lifetimes in glassy biopolymers, leading to similar constraints on the motion of HOCs sorbed in such voids. This work elucidates the importance of the molecular mobility of SOM in the sorption process, and the significant role that glassy polymethylenic components of SOM play in the long-term sequestration of HOCs such as PAHs in the environment.