Hydration affected soil:water sorption processes of xenobiotics - experiments and modeling approaches

Principle investigator: Prof. Dr. S. Thiele-Bruhn

Co-worker: T. Schneckenburger

 

Objectives
The sorption of xenobiotics in soil is governed by the physicochemical properties of the sorbate. Likewise sorption depends on the properties of soil organic matter (SOM) as the main sorbent in soil. SOM has a high molecular variability resulting in an amorphous structure and different structural domains. SOM is strongly influenced by environmental factors; hydration status and salts can either increase rigidity or cause softening of SOM. Different SOM domains exhibit different sorption mechanisms. Linear partitioning is assigned to a 'rubbery' phase; non-linear or diffusion limited processes are related to 'glassy' domains. However, knowledge on the variability of SOM and its sorptive properties is still limited. The aims of this project are to elucidate (i) the influence of water content, water contact time and of crosslinking and swelling agents on structural properties of SOM, (ii) the resulting influence on xenobiotic sorption, (iii) which is investigated using selected model sorbates with gradually varying molecular properties, (iv) to analyze experimental data with different mechanism-based isotherm and kinetic models, (v) and to confirm postulated mechanisms by computational chemistry. To this end batch trials are done with a surplus of water, thus initiating kinetic changes in differently preconditioned soil samples. The resulting sorption kinetics and thermodynamics are determined.

Hypotheses
• The rigidity of SOM is influenced by the presence of water. At small water contents, a hydrogen-bond based crosslinking (HBCL) of SOM segments by water molecules results in a more rigid structure. At moderate to large water contents, water exhibits a plasticizing effect and causes a swelling of SOM. The resulting change in the structure of SOM requires a time period of weeks and changes the permeability and sorbent properties of SOM that can be determined in sorption experiments.
• The structural conformation of SOM is further influenced by salts and chelators, here termed as additives. Strongly hydrated ions and complexing agents promote the swelling of SOM and disrupt crosslinks, respectively. Consequently xenobiotic sorption, depending on the sorbate, will be either enhanced or reduced.
• With a selection of diverse model sorbates, the SOM and SOM sorptive properties are elucidated by the resulting sorption kinetics and thermodynamics: The swelling of nanopores will be decisive for the sorption of nonpolar molecules of different molecular size while it is assumed that for the polar compounds the solvent assisted sorption and crosslinking of SOM are more crucial. Sorption will increase and become more linear with increasing molecular weight of the sorbate, sorption of molecules with hydroxy-groups will be stronger to glassy SOM and weaker to rubbery SOM.

Scientific approach
• Assessment of the influence of additives, water content and water contact time on structural properties of SOM.
• Investigation of the effects of these structural properties on the sorption kinetics of xenobiotics with respect to specific molecular properties of the sorbates.
• Investigation of the effects of structural properties of SOM on the sorption thermodynamics of xenobiotics with respect to the specific molecular properties of the sorbates.
• Setup of a mechanism-based mathematical description of the sorption processes investigated, starting with different kinetic models, the EDMM, and a parametric Freundlich sorption isotherm.
• Performance of molecular modeling calculations in order to confirm specific sorption mechanisms determining sorption behavior under the investigated conditions.

Cooperation within the priority program:
Prof. Dr. G. Schaumann, Prof. Dr. B. Marschner, Prof. Dr. J. Bachmann