Organic-geochemical investigations on the mode of incorporation of a defined nonylphenol isomer and the herbicide MCPA in soil derived organo-clay complexes

摘要

During the last decades the scientific interest to understand the interactions of organic contaminates with naturally occurring organic material increased. Emphasis was laid on the toxicity, bioavailability and transport of xenobiotics within the environment. Numerous publications have shown that high portions of anthropogenic substances could not be released by conventional extraction procedures from soils. It was assumed, that these non-extractable residues are incorporated in the organic matter or constituents of soils as both, the applied parent compound or a transformation product. Incorporation of the so called "bound residues" depends on several factors such as system characteristics (e.g. soil composition, pH-value), microbial activity and even the performed extraction method. So far, publications had predominantly dealed with extractable and hydrolytically cleavable contaminants. However, for the understanding of the incorporation behavior, knowledge of the binding mode of non-extractable substances is required. In terms of the performed work, numerous authors revealed organo-clay complexes as one of the major compartments in soil responsible for the formation of non-extractable residues. To enhance the knowledge of the formation of non-extractable (bound) residues and to understand and explain the re-mobilization potential as well as the environmental behavior of low molecular weight, organic contaminates, distinct experiments were performed with 13C and 14C-labeled substances. Both applied compounds differed in their biochemical properties and degradation behavior in soil. The first approach showed, that both 353-Nonylphenol (4-(3,5-dimethylhept-3-yl)phenol) and MCPA (4-chloro-2-methylphenoxyacetic acid) rapidly (48 h) formed non-extractable residues, especially in the clay fraction. A separation of organo-clay complexes in distinct sub-fractions by means of the MIBK method showed, that nonylphenol (NP) was predominantly incorporated into humic acids and MCPA into fulvic acids. In the following experiments, the mode of incorporation of both xenobiotics into organo-clay complexes was investigated in more detail. The performance of a sequential chemical degradation procedure facilitated the selective cleavage of chemical bonds and thus, revealed information on the mode of incorporation and the resulting relevance of non-extractable residues for the environment. Invasive chemical degradation of humic sub-fractions and the identification and quantitation of liberated compounds by GC-MS, revealed only NP as non-extractable substance. Moreover, the preferred mode of incorporation was determined as ester (amide) linkages, whereas ether and C-C-bonds were only of minor importance for the incorporation. During the incubation period, portions of chemically releasable residues decreased, which pointed to ageing processes either of the incorporated compounds, the humic material or the incorporation mode. In case of the MCPA-approach, two different concentrations of MCPA were separately applied to soil samples. The high concentration (1,000 mg MCPA/kg soil) led to a considerable decrease of the soil microbial activity as compared to the control samples without application and the low level assay (8.5 mg MCPA/kg soil). With increasing microbial activity, the amount of releasable non-extractable residues decreased, indicating a microbially induced mode of incorporation which could not be traced by the executed degradation methods. Equal to the NP-approach, the highest amount of radioactivity was liberated after cleavage of ester (amide) bonds (alkaline hydrolysis) and only the parent compound, MCPA, was found as non-extractable substance. In summary, we showed that ester (amide) bonds were of significant importance in the formation of non-extractable NP and MCPA residues. However, among the distinct humic sub-fractions, radioactive balancing indicated different modes of incorporation. It could be assumed that both xenobiotics were on the one hand directly linked to functional groups of the humic matter via covalent bonds or on the other hand sequestrated (entrapped) in cavities of the macromolecular structure. In terms of the behavior in the environment, reversible ester (amide) bonds can be cleaved rather easily under natural conditions (e.g. enzymatically catalyzed hydrolysis) resulting in a potential risk to be re-mobilized and thus both the non-extractable parent compound and their transformation products are still of great importance for the environment. Additionally to the results of the NP-approach, incubation experiments under biotic and abiotic conditions revealed a stereoselective separation of both NP-diastereomers when shifting from the extractable to the non-extractable (bound) state. Besides already know stereoselective processes e.g. microbial transformation, this is the first evidence of such a process induced by the formation of non-extractable residues.