Degradation mechanisms of chemicals in sediment must be known in order to permit more accurate assessment of aquatic pollutants. One possible degradation mechanism -- abiotic transformation -- has received little attention, however. In this study, the abiotic transformation of para- substituted benzonitriles in an extract prepared by protein extraction from sediment was compared with that in raw sediment and in water.
In water, the benzonitriles were hydrolyzed to benzoic acid through benzamides at elevated temperature. In anaerobic river sediment, the benzonitriles were transformed to the corresponding benzoic acids, except for iodo- and methoxy-derivatives. In the sediment extract, the benzonitriles, including iodo- and methoxy-derivatives, were transformed to benzoic acids. Benzonitrile transformation did not produce benzamides as intermediates in the latter two media. Transformation in sediment and in sediment extract must have been mediated by an enzyme whose activity is similar to that of nitrilase.
Analyses using quantitative structure activity relationships (QSAR) were carried out with the three sets of rate constants measured in the three media. The rate constants in sediment and sediment extract were correlated mainly with the hydrophobicity substituent parameter π. Rate constants for purely chemical hydrolysis in water were correlated with the electronic substituent parameter Hammett σp. These results showed that the reaction mechanisms in sediment and sediment extract resembled each other and indicate that the abiotic reaction mediated by the extracted sediment protein fraction was responsible for at least part of the reaction occurring in raw sediment.