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Ozonation of parabens in aqueous solution: Degradation by-products and Mechanism of degradation
Kheng Soo Tay, Noorsaadah Abd Rahman and Mhd. Radzi B. AbasEnvironmental Research Group, Department of Chemistry, Faculty of Science, University of
Malaya, 50603 Kuala Lumpur, Malaysia.
AbstractCharacterization of degradation by-products (DBPs) formed during the ozonation of each selected parabens has been carried out using GCMS. 20 DBPs formedduring ozonation of selected parabens have been identified. Hydroxylation has been found to be the major reaction for the formation of the identified DBPs.Through the hydroxylation reaction, a variety of hydroxylated parabens was formed.
IntroductionParabens are commonly used as antimicrobial preservatives in cosmetics, food, pharmaceutical products and personal care products. These compounds havebeen reported to have estrogenic- (Terasaka et al., 2006) and antiandrogenic-like properties (Chen et al., 2007). Parabens are expected to be removed usingconventional water and sewage treatments (Canosa et al., 2006), however, their presence in river water (Benijts et al., 2004) and effluent of waste treatmentplants (Trenholm et al., 2008) has expelled this perception. Among various oxidation processes, ozone with higher standard oxidation potential is expected tooxidize organic pollutants more efficiently than chlorine and chlorine dioxide (Weinberg et al, 2008). During ozone oxidation, organic pollutants undergo a seriesof oxidation processes, and in some cases, toxic degradation by-products (DBPs) might be formed (Ikehata et al., 2006). Following the discharge of incompletelytreated effluents, these DBPs may emerge in the aquatic environment. Because ozonation is a common disinfection method used in water treatment, evaluationand determination of by-products produced from ozonation are important consideration for environmental protection purposes. The objectives of this work were (i)to identify the degradation by-products (DBPs) of parabens generated during ozonation, and (ii) to propose the degradation pathways of parabens during theozonation. Methylparaben (MeP), ethylparaben (EtP), propylparaben (PrP) and butylparaben (BuP) were selected for this study.
ResultsPresumably, formation of ester chain hydroxylation products began with the
ExperimentalOzonation was performed on the selected parabens individually in 1000mL cylindrical jacketed beaker at 25 oC. Ozone was continuously bubbledinto stirred parabens (100 μM) solution through a gas-dispersion tubeplaced at the bottom of the reactor. Ozone was produced from purifiedoxygen (99.8%) by an ozone generator. 20 mL of samples werewithdrawn every minute and nitrogen gas was used to remove the ozoneresidual. Sample extraction was carried out using solid phase extraction(SPE) method. Extracts obtained from extraction were silylated usingBSTFA and TMSCl (99:1) mixture for 4 h at 70 ºC. Silylated extracts weredried using nitrogen stream and re-dissolve in 100 µL of dichloromethane.A 1.0 μL aliquot of the solution was analyzed using GCMS. DBPs wereidentified based on its fragmentation pattern in mass spectrum.
Fig .1: Proposed reaction pathway for the hydroxylation of the aromatic ring of parabens via (a) ozone and (b) hydroxyl radical.
HO
O
OH O
O
HO
HO
HO
O
O OH
HO
O
OOH
OHO
O
O OH
O
HO
O
O OHHO
HO
O
O(HO)3
HO
OH
HO
O
O
OH
O
O
HO
HO
O
O
HO
(HO)2O
O
HO
(HO)3
HO
OHO
O
HO
HOO
O
HO
OH
O
O
HO
OHOH
O
O
HO
(HO)2
HO
O
OH O
O
HO
HOO
O
HO
OH
O
O
HO
(HO)2O
O
HO
(HO)3
HO
O
O
HO
O
O
HO
O
O
HO
O
O
Paraben DBPs
MeP
EtP
PrP
BuP
HO
O
O
R
HO
O
O
R
HH
HO
HO
O
O
R
H
S
O OOH
HO
O
O
R
OH
HO
O
O
R
OHOH
W
SeeScheme 1
V
HO
O
O
RH
OO
HO
O
O
R
H
O2-
T U
(I)
HO
O
O
HO
OH OH
HO
HO
OH
O
HO-
CO2-
Y
Z
HO
O
O
RHO
- R OH
(II)
HO
O
O H OH
X
or
ResultsPresumably, formation of ester chain hydroxylation products began with thehydrogen abstraction to produce the radical S which react with dissolved oxygen toform peroxyl radical T. T which could further dissociate into carbocation U (Sonntag,2006) which could further react with ·OH to form ester chain hydroxylation products,V. Reaction of V with ozone or ·OH formed aromatic ring and ester chainhydroxylation product, W. Parabens breakdown products could be formed frombreakdown of C – O bond by ·OH to form radical X which could directly react withwater to give Y. X could also rearrange to lose CO2 and further reacts with ·OH toform Z. DBPs can also further react with the highly reactive ·OH and ozone presentin the reaction mixture to form other products which are not identified in this study.
Table 1: DBPs of parabens.
Discussion
Hydroxylation of parabens was found to be the major reaction that occurred duringthe ozonation and a series of hydroxylation compounds were detected.Monohydroxylated parabens formed by hydroxylation of aromatic ring and thehydroxylation of ester chain of parabens were found to be the common DBPsdetected in the applied ozonation process. Hydroquinone and 4-hydroxybenzoicacid were two detected major parabens breakdown products in this experiment.
Hydroxylation was found to be a significant reaction in the ozonation ofparabens, mostly at the aromatic ring and the ester chain of parabens.Hydroxylation at the aromatic ring produced a series of aromatic ringhydroxylated products (P, Q and R) which can occur via a direct reactionwith ozone (Fig. 1a) or ·OH which was generated from the decompositionof ozone in water (Fig. 1b). Due to its electrophilic nature of ozone, ozonecan only react with the aromatic ring of parabens. Thus, ·OH is the onlyspecies available for reaction with the ester chain of parabens in theozonation. Pathway I shows the hydroxylation at the ester chain ofparabens and pathway II shows the formation of parabens breakdownproducts (Fig. 2).
Fig. 2: Proposed reaction pathway for the formation of ester chain hydroxylated DBPs and parabens breakdown products.
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of Pharmaceuticals in the Environment and In Water Treatment Systems. CRC Press, New York, 2008, pp. 217-227.
AcknowledgementThis project was financially supported by Ministry of Higher Education, University of Malaya and Malaysia Toray Science Foundation (MTSF).