Fate and Effects of Quaternary Ammonium Compounds in Soil -The role of microaggregation

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Quaternary alkylammonium compounds (QAAC) are cationic organic compounds used as surfactant and disinfectants in numerous applications, amongst others, in agriculture. Although only few concentration data are available for soils, findings for other environmental compartments point to their ubiquitous occurrence and accumulation with particular substance. A predicted environmental concentration for soils is 3.5 mg kg-1 and the occurrence of QAAC-resistant genes in soils has been demonstrated. Co-selection for antibiotic resistance would entail risks for humans and environment.In the preceding one-year project, granted within the first “Nachwuchsakademie Agarökosystemforschung”, we postulated that QAACs in agricultural soils will predominantly be retained in the interlayers of clay minerals, protecting them from microbial degradation and buffering their acute toxicity.With an experiment on the effect of clay minerals on the minimal inhibitory concentration (MIC) of QAAC for different model bacteria, we confirmed that addition of smectite shifts the MIC upwards, i.e. the acute QAAC toxicity is buffered. Sorption isotherms confirmed that smectites reduced free QAAC-concentrations. Transmission electron microscopy made the expansion of interlayer spaces upon QAAC addition visible. We observed, that QAAC exerts control over flocculation and aggregate stability. In the soil system the mobility of substances adsorbed to soil particles strongly depends on the size of the adsorbent. In case of aggregates with an internal surface, the size and microstructure of aggregates (soil microaggregate=SMA) affects the way a contaminant is accessed, released or sequestered. It is therefore the mission of the subsequent project phase, to understand the control stable aggregates, aggregate destabilization and formation exert on the bioavailability of QAAC and vice versa, to study the effect of the surfactant on microaggregation of soil particles.The research hypotheses of the proposed project are: (1) QAACs promote SMA formation and increase microaggregate stability, (2) a higher degree of SMA-formation increases QAAC sorption-desorption hysteresis, (3) microaggregation slows down QAAC mineralization and (4) microaggregation contributes to a measureable buffering of QAAC toxicity on soil microbes.Experiments will be performed with the help of artificially produced model SMA as well as with SMAs extracted from soil. Measurements of hydrodynamic radii and zetapotential and the determination of SMA geometry and porosity will serve to characterize SMA structure and properties. Kinetic QAAC degradation studies shall demonstrate the influence of SMAs. As a final step, artificial and “native” microaggregates will be employed in the microbial MIC-value test system and investigated for QAAC-toxicity buffering effect.