Evaluation of biotreatability of ionic liquids in aerobic and
anaerobic conditions
Andreja Žgajnar Gotvajn, Elizabeta Tratar-Pirc, Peter Bukovec
and Polona Žnidaršic
̌
Plazl
ABSTRACT
The aim of our study was to set up an approach for reliable biotreatability assessment of ionic liquids
(ILs). As a case study, two different ILs were selected: pyridinium-based 1-butyl-3-methylpyridinium
dicyanamide ([bmpyr][dca]) and imidazolium-based 1-butyl-3-methylimidazole tetrafluoroborate
([bmim][BF
4
]). Toxicity in aerobic conditions was determined by measurement of inhibition of oxygen
consumption by activated sludge, while their biodegradability was calculated from measurements of
oxygen consumption and dissolved organic carbon elimination. For their biotreatability in anaerobic
conditions, the method with measurement of biogas production has been employed comparing
flocculent and granular sludge. Both ILs were less toxic and more biodegradable in anaerobic
conditions. IL [bmpyr][dca] was not toxic to granular sludge up to 742 mg L
1
and it even has been
degraded at this concentration in the presence of easily degradable glucose. Flocculent sludge was
completely inhibited at the lower concentration of 318 mg L
1
, but it degraded by 44% at 106 mg L
1
in the presence of glucose, indicating the appearance of cometabolism. IL [bmim][BF
4
] was less toxic
but also resistant to biodegradation in anaerobic conditions. It degraded via cometabolism 21% at
1,452 mg L
1
. It has been concluded that any assessment of biotreatability of ILs should include parallel
determination in aerobic and anaerobic conditions.
Andreja Žgajnar Gotvajn (corresponding author)
Elizabeta Tratar-Pirc
Peter Bukovec
Polona Žnidaršič Plazl
University of Ljubljana,
Faculty of Chemistry and Chemical Technology,
Aškerč eva 5,
SI-1000,
Ljubljana,
Slovenia
E-mail: andreja.zgajnar@fkkt.uni-lj.si
Key words | activated sludge, anaerobic processes, biodegradability, biotreatability, ionic liquid,
toxicity
INTRODUCTION
Ionic liquids (ILs) have attached great interest lately in wide
variety of small scale to industrial applications due to their
low volatility, non-flammability, adjustable miscibility and
reusability (Romero et al. ). Increasing application
possibilities and usage of ILs also result in higher amount
of final waste containing ILs and their potential appearance
in wastewaters ended in aerobic or anaerobic biological
wastewater treatment plant (Neumann et al. ). In the
past years numerous studies on biodegradability, bioaccu-
mulation and distribution of ILs in different environmental
compartments were accomplished to overcome drawbacks
for their wider industrial application (Zhao et al. ;
Pham et al. ).
The studies of environmental fate and toxicity of ILs
show that they are generally toxic, but their toxicity is strongly
dependent upon their composition and it vary across organ-
isms and trophic levels (Pham et al. ; Zagorc-Končan
et al. ). Initial studies confirmed poor biodegradability
of ILs and lately the biodegradable side chains were applied,
mainly ester groups derived from C
2
acid or C
4
or higher alco-
hol to increase biodegradability of imidazole-based ILs
(Gatherhood et al. ). Introduction of polar functional
groups to the alkyl chain has been shown as a promising
approach for increasing biodegradability of ILs to some
extent, but many authors proved, that increase length of
alkyl chain increased the rate of biodegradability but unfortu-
nately also toxicity (Pham et al. ). However, there is still a
lack of information about their treatability in anaerobic
conditions (Markiewicz et al. ). The anaerobic biodegra-
dation plays important role in different environmental
compartments, such as eutrophic lakes, soils or sediments,
while anaerobic digestion technology for waste and waste-
water treatment as well as soil remediation is growing
worldwide because of its economic and environmental
698 © IWA Publishing 2014 Water Science & Technology | 70.4 | 2014
doi: 10.2166/wst.2014.283