Quaternary alkylammonium disinfectants in soil: Accumulation, sorption and its influence on toxicological effects towards bacteria

Abstract

The cationic surfactants of the group of quaternary alkyl ammonium compounds (QAACs) are used in numerous industrial and agricultural applications and are therefore released into the environment in large amounts. If these substances are present in the soil in non-toxic concentrations, soil microorganisms could develop resistance mechanisms that likewise increase their resistance to antibiotics (co-selection of antibiotic resistance). Against this background, this work aims at improving the understanding of the behavior and the ecotoxicological effects of QAACs in soil. Two hypotheses were tested: i) Continuous inputs of QAACs cause a long-term accumulation of these compounds in soils, and ii) Sorption of QAACs to expandable 3-layer clay minerals reduces their ecotoxicological effects. The first step was to develop methods for extracting QAACs from soils and sewage sludge and for quantifying their concentrations. Ultrasonic extraction (USE) followed by high performance liquid chromatography with tandem mass spectrometry (HPLC-MS/MS) turned out to be the best method for this purpose. Detection limits (LOD) for alkyltrimethylammonium compounds (ATMACs), benzylalkyldimethylethylammonium compounds (BACs) and dialkyldimethylammonium compounds (DADMACs) ranged from 0.1 μg kg-1 to 1.3 μg kg-1 and limits of quantification (LOQ) were between 0.2 μg kg-1 and 2.1 μg kg-1. The developed method allowed the investigation of the concentrations of QAACs in agricultural soils of the Mezquital Valley, Mexico, which were irrigated for different durations (0 to 88 years) with wastewater from Mexico City, to test hypothesis i). The most abundant QAAC homologues in the soils were BACs >ATMACs > DADMACs. Concentrations of QAACs increased linearly and slowly in the first years of irrigation (∑ QAAC: 2 - 23 µg kg-1), but after 40 years of wastewater irrigation, an exponential increase in QAAC concentrations (up to 155 µg kg-1) was observed. In contrast to pharmaceuticals, no apparent steady-state of concentration was reached after decades of wastewater irrigation. The long-term accumulation of QAACs in soil also suggested an effective sorptive preservation of the compounds from biodegradation. The reduction of toxicological effects on microorganisms due to sorption was tested by quantifying minimum inhibitory concentrations (MICs) of QAACs on selected bacteria (Escherichia coli, Acinetobacter, Enterococcus faecium, Enterococcus faecalis, Pseudomonas fluorescence) in the presence and absence of the clay minerals smectite and kaolinite. In the absence of smectite and kaolinite, the MICs ranged from 10 µg ml-1 to 30 µg ml-1 for BAC-C12 and from 1.0 µg ml-1 to 3.5 µg ml-1 for DADMAC-C10 for all strains except the more sensitive Acinetobacter strain. For all strains tested and both QAACs tested, the presence of smectite apparently increased the MIC values, while kaolinite had no effect on the MICs. Batch sorption experiments with the clay minerals proved that this shift in apparent MICs was related to a reduction of dissolved concentrations of the QAACs due to their sorption. The findings of this thesis thus prove the postulated long-term accumulation of QAACs in soils caused by continuous inputs and the key role of sorption for controlling their fate and ecotoxicological effects.