Effects of soil organic matter molecular conformation and substrate additions on the formation and release of xenobiotics bound residues

Principle investigator: Prof. Dr. B. Marschner

Co-worker: A. Shchegolikhina

Background and objective

Most organic chemicals entering the soil form non-extractable residues with time, either as the parent compounds or as their metabolites. This occurs mainly mainly through interactions with the native soil organic matter (SOM), but there is only limited knowledge about the formation and binding mechanisms of these residues. The objective of this study is to determine the effects of SOM structural conformation on the bioaccessibility and biodegradation of the two 14 C-labelled model compounds nonylphenol and phenanthrene. This is on three hypotheses:
1. The formation of bound residues is affected by the structural conformation of SOM. In particular, sorption of xenobiotics to SOM with a high degree of cross-linking induced by polyvalent cations will be less linear because fast partioning is reduced in favour of slow sorption processes.
2. Changes in the physico-chemical environment (pH, electrolytes, temperature) affect the bioavailability of bound residues because they induce changes in the structural conformation of SOM.
3. The biodegradation of xenobiotics in bound residues can be enhanced by providing organic substrates for the soil microbes, either through direct co-metabolic degradation of the xenobiotics or through enhanced degradation of the SOM matrix (priming effects).


The structural conformation of SOM will be altered by changing the cation composition of the exchange sites and by removing or adding specific cations from/to the SOM matrix. The chemical alterations will be conducted by equilibrating the samples with different salt solutions and subsequent washing to eliminate soluble salts. The effects of the treatments on SOM-conformation will be analyzed by Differential Scanning Calorimetry (DSC) and 1 H-NMR relaxometry in cooperation with G. Schaumann (Koblenz).
The irradiated sterilized samples are then used to perform short-term sorption studies and a long-term aging experiment with the 14 C-labelled chemicals nonylphenol and phenantrhene. The main focus of the sorption studies will be on kinetics, since the changes in SOM conformation are expected to mainly affect the rapid and slow sorption and desorption and the accessibility of the sorption sites.
For the long-term aging experiment, the sterile soil samples will be incubated with the 14 C-labelled compound in sealed containers at room temperature in the dark for up to 12 months. In different intervals, the samples are analyzed for rapidly desorbable, bioaccessible and biodegradable xenobiotics. As a measure for the bioaccessibility of hydrophobic xenobiotics, cyclodextrine and a surfactant will be used for extraction. The determination of the compound's biodegradability will be carried out using a Respicond apparatus where the release of 14 CO2 from re-inoculated samples is monitored.
Several treatments will be included in the biodegradation assays:
1. Addition of easily degradable substrates (i.e. glucose, alanine, oxalic acid) to investigate, if xenobiotic degradation is substrate-limited.
2. Addition of xenobiotic analogues to induce specific enzyme synthesis and activity.
3. Changes in cation saturation prior to incubation to alter xenobiotic accessibility within the SOM matrix.
4. If degradation appears to be desorption rate limited, surfactant enhanced desorption experiments will be performed.