Abstract
The chemical and thermal stability of ionic liquids (ILs) makes them interesting for a large variety of applications in nearly all areas of the chemical industry. However, this stability is often reflected in their recalcitrance towards biodegradation, which comes with the risk of persistence when they are released into the environment. In this study we carried out a systematic investigation of the biodegradability of pyrrolidinium, morpholinium, piperidinium, imidazolium and pyridinium-based IL cations substituted with different alkyl or functionalised side chains and using halide counterions. We examined their primary degradability by specific analysis and/or their ultimate biodegradability using biochemical oxygen demand tests according to OECD guideline 301F. Biological transformation products were investigated using mass spectrometry. A comparison of the biodegradation potential of these ILs shows that for all five head groups, representatives can be found that are readily or inherently biodegradable, thus permitting the structural design of ILs with a reduced environmental hazard.
Highlights
Ionic liquidsIonic liquids (ILs) are chemicals with an outstanding design potential
If primary biodegradation was observed, we used WTW OxiTop® devices measuring the biochemical oxygen demand (BOD) to determine the full mineralisation of the ionic liquids (ILs). In this full mineralisation test a compound was classified as readily biodegradable if it exhibited >60% degradation in relation to the theoretical oxygen demand (ThOD) within 28 d and an exponential growth phase of 10 d. In this study this result was attributable solely to the cation, because most of the IL anions investigated contained inorganic moieties (“halides”) (Tables 1–6) that were irrelevant to biodegradation tests based on the measurement of oxidisable carbon in the molecule
It is highly likely that chemicals classified as “readily biodegradable” are biodegradable in the environment with just a low risk of persistence, or even none at all
Summary
Ionic liquids (ILs) are chemicals with an outstanding design potential Their components – cationic and anionic molecular core structures with different side chains – can be combined in innumerable ways to create substances with tailor-made physico-chemical properties important for industrial applications. The variety of ILs available for an individual process together with their high operational safety is advantageous for the performance and handling of the process, and for sustainable development.[23] In this context the environmental compatibility of a chemical plays a major role. The production of substances that simultaneously possess low toxicity and environmental non-persistence is a demanding task for chemists dedicated to sustainable chemistry.[24] its fulfilment appears feasible in the context of the enormous design potential of ILs.[25,26,27] The structural aspects that 2174 | Green Chem., 2014, 16, 2174–2184
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