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C.Y. Chan et al. / Chemosphere 222 (2019) 371e380
improved by the photocatalytic pre-treatment, and this proposed photocatalytic-biological integrated
system can effectively treat various classes of triazine-containing pollutants.
© 2019 Elsevier Ltd. All rights reserved.
1. Introduction
degrade melamine into ammeline first and then K. pneumoniae can
further degrade ammeline into other products. Photocatalytic
Triazine ring containing compounds are widely used in agri-
culture and textile industries, for example, atrazine and simazine
are the most commonly used triazine herbicides while Irgarol®
1051 is used to replace organotins as an anti-fouling paints. Besides,
reactive Brilliant Red K-2G (K-2G, CI: Reactive Red 15) is extensively
used for dyeing nylon, silk and wool and past studies showed that
the concentration of a single dye in dyebath effluent is around
100 mg/L (Alaton et al., 2002; Kabdas¸ lı et al., 2009). They are
persistent and toxic in nature (Chan, 1998; Zhang et al., 2008;
Fernandez and Gardinali, 2016; Liu et al., 2016), considerable
amount of wastewater generated from agricultural runoff and
textile processing have caused serious environmental impacts.
Previous studies showed that some of their degradation in-
termediates are even more toxic and persistent compared to their
parental compounds (Chan, 1998; Gottlieb et al., 2003; Ralston-
Hooper et al., 2009).
Among different physical treatments of wastewater, adsorption
is widely used with its high flexibility and simplicity of design as
well as high efficiency in removing stable pollutants (Dabrowski,
2001; Robinson et al., 2001; Allen and Koumanova, 2005; Yagub
et al., 2014). However, adsorption only removes the pollutants
from wastewater without degradation or decomposition of the
pollutants (Rasalingam et al., 2014), further degradation of the
pollutants is still required for regeneration of the sorbent and post-
treatment steps. Moreover, activated carbon adsorption for
wastewater containing high concentration of synthetic dyes is not
cost-effective (Forgacs et al., 2004). To prevent reduction in
removal efficiency, reactivation is required, yet 10e15% loss of the
expensive activated carbon would be resulted (Robinson et al.,
2001). Chemical method such as coagulation and precipitation is
an alternative way to treat dye effluents. After coagulation and
precipitation process, mud and sludge containing dye molecules
would be generated, additional cost is needed for the further
treatment of the mud and sludge with pollutants (Vikas and Sandip,
2013; Ahmad et al., 2015). On the other hand, reductive dechlori-
nation of triazine-containing pollutants can be performed by using
acidic zero-valent iron powder (Dombek et al., 2001; Sun et al.,
2013). However, this technology requires strong acidic condition
and large amount of iron, so it is not cost-effective and favorable in
large-scale application. Conventional biological treatment is inef-
fective for triazine compounds because those enzymes used for
triazine degradation are not easily found in contaminated envi-
ronment, for example, half-life of atrazine have been reported from
107 to 187 days in non-adapted soil, while biodegradation of
atrazine found in adapted soil samples was greatly affected by
unfavourable physiochemical conditions like low temperature
(20 ꢁC) and soil moisture content (Gebendinger and Radosevich,
1999; Krutz et al., 2008), in addition, the microbial degradation
process was easily suppressed by co-contaminated heavy metals
like copper (Dewey et al., 2012). On the other hand, these con-
straints can be overcome by using advance oxidation processes
(AOPs) in which they can be served as a general pre-treatment step
prior to biodegradation process. For instance, Klebsiella pneumoniae
has a lack of triazine hydrolase (Mulbry, 1994) to transform mel-
amine into ammeline through deamination, yet AOPs can help to
oxidation (PCO) shows the highest degradation efficiency among
different kinds of AOPs compared to ozonation and photolysis
(Halmann, 1996; Herrmann, 1999). For example, degradation of
Procion H-exl dyes, which are also reactive azo dyes as K-2G, using
H2O2/UV, Fenton, UV/Fenton, TiO2/UV and TiO2/UV/H2O2 was re-
ported (Riga et al., 2007). TiO2/UV/H2O2 was found to have the
highest removal efficiency among these processes because more
hydroxyl radical (ꢀOH) can be produced for the degradation. Many
studies have shown that triazine pollutants can be degraded into
ꢀ
less substituted intermediates by OH produced from PCO system
using TiO2 and ultraviolet (UV) light at 365 nm (UV365 nm). Unlike
other kinds of organic pollutants, complete degradation and
mineralization of triazine ring cannot be achieved, instead, a PCO-
resistant product, cyanuric acid (CA), is formed and accumulated
after prolonged PCO treatment (Chan et al., 2004a). Past studies
indicated that certain bacteria can use CA as a good nitrogen source
for their growth (Ernst and Rehm, 1995; Chan et al., 2004b). As a
result, a promising treatment approach can be proposed to degrade
and detoxify triazine pollutants by integrating photocatalysis with
biodegradation.
In this study, simazine, the second most commonly used
monochloro-s-triazine herbicide with similar structure to atrazine,
Irgarol® 1051, a widely used methylthio-substituted s-triazine
antifouling agent with limited PCO degradation study, and K-2G, a
s-triazine containing azo dye with complex structure, were selected
for comparison to investigate the treatment efficiency and toxicity
of s-triazine with different substitution and also the intermediates.
The physicochemical properties of the three compounds are sum-
marized in S1 (Supplementary data). PCO experiment was con-
ducted in aqueous slurry with TiO2 suspension. The Microtox® test
(Vibrio fischeri), microalgae and Artemia growth inhibition test were
used to provide a fast and comprehensive detoxification profile for
different pollutants. Klebsiella pneumoniae with high CA degrading
ability was isolated to treat with CA accumulated from PCO spent
solution as a sole nitrogen source. Under the optimal conditions,
PCO pre-treatment shows significant improvement of the biode-
gradability of those selected triazine compounds.
2. Experimental
2.1. Photocatalytic oxidation (PCO)
2.1.1. Chemicals
Simazine (Supelco), Irgarol® 1051, ammeline (Riedel-de Haen),
€
ammelide (TCI), melamine (Fluka) and CA (Aldrich) were purchased
from respective commercial sources with purity greater than 98%.
The organic solvents, acetonitrile, ethyl acetate and methanol, were
obtained from Mallinckrodt (ChromAR®) while Milli-Q water was
used throughout the experiments. Sodium hydroxide (Ajax) and
hydrochloric acid (37%) (BDH) were used (0.2 M) for pH adjust-
ment. For the photocatalyst, a 10,000 mg/L stock solution of TiO2
P25 (Degussa) was prepared in Milli-Q water and kept in dark at
ꢁ
€
25 C. Hydrogen peroxide (H2O2) (35% w/v) (Riedel-de Haen) was
used to examine the effect towards PCO reaction. K-2G obtained
from Shanghai Chemical Co. was used without further purification.