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Reuse of treated wastewater for irrigation of crops is growing in arid and semi-arid regions, whilst increasing amounts of biosolids are being applied to fields to improve agricultural outputs. Due to incomplete removal in the wastewater treatment processes, pharmaceuticals present in treated wastewater and biosolids can contaminate soil systems. Benzodiazepines are a widely used class of pharmaceuticals that are released following wastewater treatment. Benzodiazepines are represented by a class of compounds with a range of physicochemical properties and this study was therefore designed to evaluate the influence of soil properties on the sorption behaviour and subsequent uptake of seven benzodiazepines (chlordiazepoxide, clonazepam, diazepam, flurazepam, oxazepam, temazepam and triazolam) in two plant species. The sorption and desorption behaviour of benzodiazepines was strongly influenced by soil type and hydrophobicity of the chemical. The partitioning behaviour of these chemicals in soil was a key controller of the uptake and accumulation of benzodiazepines by radish (Raphanus sativus) and silverbeet (Beta vulgaris). Benzodiazepines such as oxazepam that were neutral, had low sorption coefficients (K) or had pH-adjusted log octanol-water partition coefficients (log D, pH6.3) values close to 2 had the greatest extent of uptake. Conversely, benzodiazepines such as flurazepam that had an ionised functional groups and greater K values had comparatively limited accumulation in the selected plant species. Results also revealed active in-plant metabolism of benzodiazepines, potentially analogous to the known metabolic transformation pathway of benzodiazepines in humans. Along with this observed biological transformation of benzodiazepines in exposed plants, previously work has established the widespread presence of the plant signalling molecule γ-amino butyric acid (GABA), which is specifically modulated by benzodiazepines in humans. This highlights the need for further assessment of the potential for biological activity of benzodiazepines following their plant uptake.

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The abiotic protection of low molecular weight organic compounds (LMWOC) in soils may be an important regulator of C cycling. The study of the protection of LMWOC through sorption typically employs soils shaken in solution, which may be compromised by biological activity. We used 14C-labelled glucose as a LMWOC in batch assays of four different soils with contrasting physico-chemical properties (Arenosol, Luvisol, Ferralsol and Andisol). The commonly used biocides NaN3 and HgCl2 were employed alone or in concert across a range of concentrations to assess their efficacy in inhibiting microbial degradation. For short (<1 day) sorption experiments, low NaN3 concentrations (50 µM) were sufficient to inhibit microbial activity, whereas for longer (2 week) equilibrium assays, only high HgCl2 concentrations (10 mM) were effective in all soils tested. All combinations of biocide were most effective in inhibiting microbial activity in the Arenosol and least effective in the Andisol. Thus, in some soils, particular care is required with biocide selection for complete inhibition of biological activity. The findings presented here are of relevance for the design of studies investigating sorption of labile organic compounds in soils.