Exploring the pathways of aromatic carboxylic acids in ozone solutions

Article abstract of DOI:10.1039/c7ra03039h

The reaction between ozone and natural organic matter (NOM) generates a certain amount of aromatic carboxylic acids (ACAs). Previous studies report that the pre-ozonation of fulvic acid can produce protocatechuic acid (PA), 3-hydroxybenzoic acid (3-OHBA), phthalic acid (PhA), and benzoic acid (BA). Ozonation of ACAs may result in diverse, low-molecular-weight carbonyl compounds, significantly affecting the formation of disinfection by-products (DBPs) following chlorination. The purpose of this study was to determine the degradation mechanisms, possible ozonation reaction pathways, ozonation intermediates and DBPs of four ACAs, and evaluate the ability of degrading micro-pollutants with different structures by ozonation, as well as to make up for the knowledge gap of DBPs generated. The results showed that two stages pseudo-first-order reaction for four ACAs and six intermediates accompanied the ozonation of ACAs, namely formaldehyde, glyoxal, methylglyoxal, fumaric acid, formic acid, and acetic acid. Whether the hydroxyl radical was involved or not, the ozonation of ACAs contained three steps; first step being the BA hydroxylation. Second, hydroxylation products of BA were oxidized to generate ring-opened compounds such as unsaturated carbonyl compounds. Third, short-chain aldehydes and carboxylic acids were formed. As for mineralization, the short-chain aldehydes and carboxylic acids were finally transformed into CO₂ and H₂O. In addition, not only ACAs themselves were the main DBPs precursors, but the intermediates of ACAs ozonation could also be the DBPs precursors, and they produced high yields of trihalomethanes (THMs) and haloacetic acids (HAAs). Therefore, the ACA levels should be kept under control in drinking water treatment plants.

Full text of DOI:10.1039/c7ra03039h

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Exploring the pathways of aromatic carboxylic
acids in ozone solutions
Cite this: RSC Adv., 2017, 7, 34339
a
a
Chongwei Cui and Shuili Yub
*
Xin Zhong,
The reaction between ozone and natural organic matter (NOM) generates a certain amount of aromatic
carboxylic acids (ACAs). Previous studies report that the pre-ozonation of fulvic acid can produce
protocatechuic acid (PA), 3-hydroxybenzoic acid (3-OHBA), phthalic acid (PhA), and benzoic acid (BA).
Ozonation of ACAs may result in diverse, low-molecular-weight carbonyl compounds, significantly
affecting the formation of disinfection by-products (DBPs) following chlorination. The purpose of this
study was to determine the degradation mechanisms, possible ozonation reaction pathways, ozonation
intermediates and DBPs of four ACAs, and evaluate the ability of degrading micro-pollutants with
different structures by ozonation, as well as to make up for the knowledge gap of DBPs generated. The
results showed that two stages pseudo-first-order reaction for four ACAs and six intermediates
accompanied the ozonation of ACAs, namely formaldehyde, glyoxal, methylglyoxal, fumaric acid, formic
acid, and acetic acid. Whether the hydroxyl radical was involved or not, the ozonation of ACAs contained
three steps; first step being the BA hydroxylation. Second, hydroxylation products of BA were oxidized to
generate ring-opened compounds such as unsaturated carbonyl compounds. Third, short-chain
aldehydes and carboxylic acids were formed. As for mineralization, the short-chain aldehydes and
carboxylic acids were finally transformed into CO2 and H2O. In addition, not only ACAs themselves were
the main DBPs precursors, but the intermediates of ACAs ozonation could also be the DBPs precursors,
and they produced high yields of trihalomethanes (THMs) and haloacetic acids (HAAs). Therefore, the
ACA levels should be kept under control in drinking water treatment plants.
Received 14th March 2017
Accepted 5th June 2017
DOI: 10.1039/c7ra03039h
rsc.li/rsc-advances
electron-rich functional groups (e.g. double bonds, activated
aromatic ring) are selectively attacked by molecular O3, while
cOH non-selectively reacts with organic compounds.10 O3
1. Introduction
Aromatic carboxylic acids (ACAs) represent a wide range of
chemicals used as raw materials or generated in various
industries such as printing and dyeing, pharmaceuticals, food,
cosmetics and paper mills.1 All compounds are toxic and display
low biodegradability.2,3 Benzoic acid (BA) is detected in the
effluents of the pharmaceutical industries.4 In printing and
dyeing industries, phthalic acid (PhA) is generally used as the
main raw material for acylation and esterication reactions.
Consequently, these ACAs can be found in diverse media, such
as surface water, sediments, landll leachate water and even in
atmospheric aerosols.5,6 BA (from 1.1 to 4.5 mg lꢀ1) has been
detected in surface water.7 PhA (12.2 ꢁ 0.8 mg lꢀ1) has been
found in tap water disinfected by ozonation combined with
chlorination.8
oxidation mechanism includes molecular O3 direct reactions
and cOH indirect reactions that are produced by O3 decompo-
sition. Currently, some electron-rich moieties and hydroxylation
products are intermediately formed during ozonation.11–14 On the
contrary, aromatics structures containing electron-withdrawing
functional groups how activated O3 as well as intermediate prod-
ucts received little attention. For one thing, aromatic compounds
containing electron-withdrawing functional groups (e.g. –Cl,
–COOH, –NO2, –CN) are generally O3 resistant. For another thing,
they could be effectively oxidized by cOH, with second-order rate
constants ranging from 108 to ꢂ1010 Mꢀ1 sꢀ1
.
15
Some researchers have reported the selectivity of compounds
with similar chemical structures during ozonation process.
´
Zhang and Croue selected six carboxylic acids, namely acetic
Ozone (O3), an electrophile with high selectivity, is
commonly used for degrading organic pollutants in water
treatment plants.9 However, organic compounds containing
acid, oxalic acid, citric acid, pyruvic acid, malonic acid and
succinic acid, to study their reactivity with molecular O3, and
the results demonstrated that their reactivity was low.16 Their
results clearly demonstrated that ozonation could not
completely degrade the carboxylic acids. Huang et al., reported
that though BA is recalcitrant to O3 attack, it undergoes efficient
degradation at an acidic pH (2.3).15 The degradation of BA and
aSchool of Municipal & Environmental Engineering, Harbin Institute of Technology,
Harbin 150090, China
bSchool of Environmental Science and Engineering, Tongji University, Shanghai
200433, China
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O3 decomposition were both enhanced on increasing the BA
concentration. Our previous studies showed that the pre-
ozonation of fulvic acid can produce protocatechuic acid (PA),
3-hydroxybenzoic acid (3OHBA), PhA, and BA, which could
continue to react with O3 during the main ozonation step in the
treatment of drinking water. It is noteworthy that their chemical
structures are closely similar (containing different numbers of
–COOH and –OH groups). However, considering the course of
degradation and mineralization, the knowledge about diverse
chemical structures of ACAs was not adequate, probably due to
the lack of a suitable analytical method. ACAs should be con-
verted into weaker polar and stable derivates so that they can be
applicable for GC determination. In order to avoid the complex
procedures of derivatization, our team employed solid phase
extraction-ultra high performance liquid chromatography (SPE-
UPLC) to determine the level of ACAs. Moreover, ozonation of
ACAs may result in diverse low-molecular-weight carbonyl by-
products, signicantly affecting the formation of disinfection
by-products (DBPs) following chlorination.
2.2 Experimental procedures
Stock solutions of 4 ACAs were prepared by dissolving 1.0 g
ACAs in 1000 ml puried water and mixing for 24 h, individu-
ally. Working solutions at 1.0 mg lꢀ1 concentration were ob-
tained by water dilution. All these solutions were stored at 4 ꢃC.
All ozonation experiments were operated on 500 ml SIMAX
bottles tted with a magnetic stirring bar. COM-AD-01 O3
generator (4 g hꢀ1, ANSEROS, Germany) produced O3 from pure
O2 ($99.2% purity). Subsequently, through a disperser placed at
the bottom of the reactor, O3 was scattered rapidly into ultra-
pure water. According to the direct UV absorbance method,17
the concentration of O3 stock solution (15–20 mg lꢀ1) was
measured. Phosphate buffer solution was added to ensure
a uniform solution pH of 7 (ꢁ0.1) in all experiments. The
ozonation reactions were terminated by sodium nitrite (200 ml,
6.9 g lꢀ1, J&K, Beijing, China).18 The ozonated samples were
stored at 4 ꢃC for not more than 24 h. The experimental scheme
included the following conditions: O3 contact time 1, 2, 5, 10,
20, 30 min, O3 1.0 mg lꢀ1 and pH 7.
Although O3 has long been used for disinfection and oxida-
tion in water treatments, there is a critical gap in information
related to the transformation of organic compounds. This has
become increasingly important in recent years because there is
a considerable concern about the formation of potentially
harmful degradation products and oxidation products from the
reaction with the matrix components. Thus, the purpose of this
study was to investigate the degradation mechanism, possible
ozonation reaction pathways, ozonation intermediates and
DBPs of 4 ACAs, evaluate the ability of degrading micro-
pollutants with different structures by ozonation, and make
up for the knowledge gap of the generated DBPs.
Chlorination was conducted on ozonated samples using
100 ml chlorine-free bottles. Chosen Cl2 dosage guaranteed that
a sufficient amount of Cl2 remained in solution aer incubation
for 24 h, and hence reactions were not limited by chlorine
concentration. Aer chlorine was added, samples were stored
ꢃ
headspace-free, at a pH 7 and 25 ꢁ 1 C in the dark for 24 h.
Ascorbic acid was used to quench the residual chlorine (100 ml,
10 g lꢀ1, Anpel, Shanghai, China).
2.3 Analytical methods
According to Standard Method 5310,17 the total organic carbon
analyzer (OI, Aurora1030) was used to measure the decay of total
organic carbon (TOC) during treatment.
The EPA 556 method and the EPA 552.2 method were indi-
vidually used for the analysis of aldehydes and dichloroacetic
acid (DCAA) in water samples (EPA, 1998, EPA, 2003). Aldehydes
and DCAA were analyzed by 7890B gas chromatograph tted
2. Materials and methods
2.1 Materials
The analytical standard including 15 carbonyl compounds with an electron capture detector (ECD) (Agilent Technologies,
(formaldehyde, acetaldehyde, propanal, butanal, pentanal, Palo Alto, USA). The DB-5MS capillary column (30 m ꢄ 0.25 mm
hexanal, cyclohexanone, crotonaldehyde, heptanal, octanal, I.D. ꢄ 0.25 mm lm thickness, Agilent Technologies, Bellefonte,
benzaldehyde, nonanal, decanal, glyoxal and methyl glyoxal) PA, USA) was applied for the separation. The carrier gas was He
and the derivatization reagent O-(2,3,4,5,6-pentauorobenzyl) (1 ml minꢀ1) and the detector make-up gas was N2 (30 ml
hydroxylamine (PFBOA) were purchased from Acc Standard minꢀ1). The aldehydes temperature program was as follows:
(New Haven, USA). HPLC grade n-hexane from Anpel (Shanghai, 50 ꢃC hold for 1 min,ꢀp1rogram at 4 ꢃC minꢀ1 up to 220 ꢃC,
ꢃ
China) was used as a solvent for the liquid–liquid extraction. program at 20 C min up to 250 ꢃC and hold at 250 ꢃC for
ꢃ
The Synergy UV-Ultrapure Water System (Millipore, Molsheim, 10 min. The DHAA temperature progꢀra1m was as follows: 35 C
France) provided organic-free water.
hold for 10 min, program at 2 ꢃC min up to 40 ꢃC, program at
Standards of the 7 carboxylic acids (>95% purity), formic acid, 5 ꢃC minꢀ1 up to 75 ꢃC and hold at 75 ꢃC for 15 min, program at
acetic acid, fumaric acid, PA, 3-OHBA, PhA and BA, were purchased 40 ꢃC minꢀ1 up to 1ꢀ010 ꢃC and hold at 100 ꢃC for 15 min,
from J&K (Beijing, China). Hydrochloric acid, sodium hypochlo- program at 40 C min up to 135 ꢃC.
ꢃ
rite, sodium hydroxide, potassium dihydrogenphosphate and
According to the purge and trap gas chromatographic
orthophosphoric acid were analytically pure and supplied by method, chloroform was measured using 4660-7890B-5077A gas
Shanghai (Shanghai, China). Silica-reverse phase sorbent (Supel- chromatograph (Agilent Technologies, Palo Alto, USA) equipped
clean ENVI-18) containing octadecyl functional groups, was with a mass spectrometer. The operating conditions were as
purchased from Supelco (Bellefonte, PA, USA). LiChrolut EN follows: magnetic mass analyzer scans from 35 to 200 m/z, 70 eV
(particle size 40–120 mm) was provided by Merck (Darmstadt, electron energy, ion source 250 ꢃC, carrier gas: He (1 ml minꢀ1),
ꢃ
ꢃ
Germany).
temperature program: 30 C hold for 10 min, program at 7 C
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minꢀ1 up to 72 ꢃC and hold at 72 ꢃC for 1 min, program at 40 ꢃC degradation in the ultrapure water. Hence, rst-order reactions
minꢀ1 up to 220 ꢃC and hold at 220 C for 1 min.
were not observed during the monitoring period. On the
ꢃ
The 4 ACAs and 1 aliphatic carboxylic acid (fumaric acid, PA, contrary, the reactions could be expressed in two steps pseudo-
3-OHBA, PhA and BA) were analyzed by SPE-UPLC.19 The SPE rst-order reactions, as shown in Fig. 1. It is clearly noticed that
system (Anpel, Shanghai, China) was assembled from a GAST the reactivity was in the order PA > 3-OHBA > BA > PhA. Previous
pump (MICH, USA). The SPE cartridges were conditioned using studies have indicated that O3 directed oxidation was effective
1 ml acetonitrile–methanol (1 : 1) and 1 ml ultrapure water. for degradation of organic pollutants bearing active groups,
Chromatographic analyses were carried out on a Waters H-class such as –OH, –NH2, and double bond.10,11 The existence of active
Ultra high performance liquid chromatography system (Waters, groups was advantageous for O3 for the electrophilic attack on
USA). The column was ACQUITY UPLC BEH C18 (2.1 ꢄ 50 mm the aromatic ring. Thus, this was more signicant for PA
I.D., particle size 1.7 mm, Waters, USA). A PHS-2C pH meter (bearing two electron-donating groups), resulting in a rapid
(Shanghai, China) was used to adjust the pH value. In brief, degradation. In addition, carboxylate group enhanced the
100 ml standard solution or ACAs water sample at pH ꢂ 1.3 nucleophilicity of reaction site, further activating the reaction
(adjusted with 5 M HCl) owed past the SPE cartridge at 2–3 ml with O3.
minꢀ1. The SPE cartridge was lled with 80 mg of the mixture of
These reaction rates are not only the function of the
LiChrolut EN/Supelclean ENVI-18 (1 : 1) sorbents. Later, ACAs concentration of O3 and cOH radicals, but are also dependent
were eluted by 1 ml methanol and collected in a 2 ml glass GC on other oxidative species, therefore these reaction rates cannot
vial.
be regarded as fundamental degradation mechanisms. It
Formic acid and acetic acid were analyzed by ICS-2100 Ion should be pointed out that although different aromatic groups
chromatograph (Thermo Scientic, USA) equipped with a Dio- result in different reaction rates, the degradation trends of ACAs
nex IonPac AS-19 capillary column (0.4 mm ꢄ 250 mm) and are found to be similar. It can be inferred that the ozonation
a Dionex IonPac AS-19 guard column (0.4 mm ꢄ 50 mm). Dio- pathway for degrading ACAs could form similar by-products
nex RFIC-EG eluent generator provided the mobile phase, with before the mineralization step.
the following conditions: ow 10 ml minꢀ1, 0–10 min 10 mM
KOH, 10–42 min linear ramp to 52 mM KOH, 42–45 min linear OHBA and 49.3% of PA degradation observed in 10 min. As the
There was about 39.7% of PhA, 43.3% of BA, 46.2% of 3-
ramp to 70 mM KOH, 45–50 min 10 mM KOH.
initial reaction stage proceeds rapidly, it could be advantageous
to regulate ozonation treatment in water treatment plants,
particularly for O3 active components.
2.4 Quality control
During ozonation, although ACAs were degraded, the
mineralization degree was not high, as evidenced by the TOC
measurement (Fig. 2). The mineralization degree of PA, 3-
OHBA, BA, and PhA was 28.8%, 24.2%, 20.9%, 16.2% at 30 min,
respectively. This implied the formation of ozonation by-
products. Although O3 is a strong oxidizer with selectivity, it
cannot mineralize the inert organics in a given time.20 The
transformation products detected during the ozonation
processes are discussed in the next section.
3.1.2 By-products of ozonation ACAs. Under the cycles of
continuous oxidation–decarboxylation, aromatic ring breakdown
results in the generation of a series of ring-opening compounds,
for example, the short-chain aldehydes and carboxylic acids. Six
intermediate compounds accompanying the ozonation of ACAs,
Calibration curves prepared for each compound were linear (R2
> 0.980). The method of aldehydes detection limits varied from
0.2 mg lꢀ1 to 4.0 mg lꢀ1. The method of carboxylic acids detection
limits varied from 0.8 mg lꢀ1 to 5.0 mg lꢀ1. The recovery rates of
each compound were greater than 80%. Experiments were
conducted in duplicates. Relative standard deviation of the two
measurements was below 15% in general. The SPSS soware
(IBM SPSS, version 17) was employed for statistical data
analysis.
3. Results and discussion
3.1 ACAs degradation by ozone
3.1.1 Degradation mechanisms of ACAs. Previous studies namely, formaldehyde, glyoxal, methyl glyoxal, formic acid, acetic
have shown that the ozonation kinetics are pseudo-rst-order or acid and fumaric acid were shown in Fig. 3, which were generally
pseudo-second-order reactions.11 There were three main detected in the ozonation of aromatic compounds.11,20–22 Since
considerations for ACAs concentrations in the degradation formic, acetic and fumaric acids were relatively inert to O3, they
mechanisms. First, the starting point of this study was based on were accumulated in the O3 treated water as being the end prod-
the actual water treatment plant (1.0 mg lꢀ1 O3); second, ACAs ucts. However, initially, the concentration of formaldehyde, glyoxal
were accurately analyzed by SPE-UPLC; nally, in order to and methyl glyoxal gradually increased with the contact time.
clearly reect the reaction course of O3 and ACAs, 1.0 mg lꢀ1 BA, Maximum concentrations were achieved at 5–10 min and
PA, PhA, and 3-OHBA were chosen. The residual concentrations decreased thereaer. On one hand, ozonation cleaves unsaturated
of four ACAs were detected at different contact times in order to aliphatic chain, opens rings, removes and oxidizes alkyl groups to
determine the reaction rates.
aldehydes; on the other hand, carboxylic acids are generated from
It is generally recognized that under acidic conditions, corresponding aldehydes that are oxidized. Moreover, Liu et al.,
molecule O3 is the primary active substance, whereas under reported that oxidation rates of aldehydes were typically lower than
basic conditions, cOH is the primary active substance. In addi- formation rates, resulting in the accumulation in the treated
tion, O3 not only depletes by reaction with ACAs, but also by self- water.23
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Fig. 1 Degradation of BA, PA, PhA and 3-OHBA at different contact times. Conditions: pH 7, 25 ꢃC, [O3]0 ¼ 1.0 mg lꢀ1, and [BA, PA, PhA, 3-
OHBA]0 ¼ 1.0 mg lꢀ1; (a) BA; (b) PhA; (c) PA; (d) 3-OHBA.
formic acid and acetic acid, the amount of formed formic and
acetic acids relates to the chemical structure of the ACAs. In
addition, it should be noted that, during the initial phase of O3
oxidation of BA, 3-OHBA was detected with a concentration
ranging from 6.2 mg lꢀ1 to 23.6 mg lꢀ1 and then quickly dis-
appeared. Since 3-OHBA could be further oxidized by excess O3,
it was not easily detected. Pillar et al., also reported that the
reaction rate constants of hydroxybenzoic acids with O3 and
cOH were 5.2 ꢄ 105 Mꢀ1 sꢀ1 and 1.1 ꢄ 1010 Mꢀ1 sꢀ1, respec-
tively.24 Moreover, Huang et al., found a signicant increase in
cOH production in the presence of hydroxybenzoic acids.15
Their results indicated that hydroxybenzoic acids could
promote O3 decomposition to generate cOH. These results
indicated that BA containing hydroxyl groups was easily elimi-
nated with O3. It can be concluded that hydroxylation products
were the main products of ozonation transformation organics,
Fig. 2 TOC decay during ozonation of ACAs. Conditions: pH 7, 25 ꢃC,
and the formation of formic acid, acetic acid and fumaric acid
may have resulted from further oxidation of these hydroxylation
products.
[O3]0 ¼ 1.0 mg lꢀ1, and [BA, PA, PhA, 3-OHBA]0 ¼ 1.0 mg lꢀ1
.
Carboxylic acids are an important type of newly formed
assimilable organic carbon (AOC), which are a signicant part
of organic DBPs in drinking water ozonation treatment
plants.7,25 Generally, compared to the precursors, the short-
For PhA and 3-OHBA, formic acid was the main by-product.
However, for BA and PA, the main by-products were formic acid
and acetic acid. Due to the different concentration levels of
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Fig. 3 The concentration of intermediates during ozonation of ACAs at different contact times. Conditions: pH 7, 25 ꢃC, [O3]0 ¼ 1.0 mg lꢀ1, and
[BA, PA, PhA, 3-OHBA]0 ¼ 1.0 mg lꢀ1
.
chain carboxylic acids are easily biodegradable, and they have ortho/para-position as compared to the meta-position, and there-
been proven to be associated with bacterial re-growth and bio- fore meta-position is the rst to undergo addition. Therefore, in the
lm formation in water distribution systems. Therefore, reaction between O3 and BA, the electrophilic group generally
certain measures should be adopted to control the source of attacks the meta-position, which leads to the formation of 3-OHBA.
carboxylic acids in drinking water treatment plants.
This could explain why 3-OHBA was detected in the process of BA
3.1.3 Possible pathway of ozonation of ACAs. Several ozonation. On the contrary, –OH results in a more electron-rich
studies have reported on O3 and cOH reacting with phenol, ortho/para-position compared to meta-position, at the same time
chlorophenol, alkene, salicylic acid and so on. Based on making the benzene ring more nucleophilic. In summary, the
previous research as well as taking the experimental results into combined action of –COOH and –OH on the electron distribution
consideration, the progress of ACAs ozonation and the forma- may lead to a more electron-rich at position 5 than position 4 and 6
tion mechanisms of intermediate products were speculated and in 3-OHBA. This could presumably result in the formation of
are described in Fig. 4. For example, the reaction mechanism of polyhydroxylated aromatic products. As described above, hydrox-
BA ozonation could contain three steps; the rst step being BA ylation aromatic intermediates react rapidly with O3. Therefore, PA
hydroxylation. Second, the hydroxylation products of BA are was not detected during 3-OHBA ozonation. Moreover, the effect of
oxidized to generate ring-opened compounds, and unsaturated –COOH steric hindrance should be taken into consideration.
carbonyl compounds are generated. In the third step, the short- Several researchers also reported that the rst step of the ozona-
chain aldehydes and carboxylic acids are formed. As for tion degradation pathway, the formation of polyhydroxylated
mineralization, the short-chain aldehydes and carboxylic acids aromatic products, was a predominant route.1,11,15,26–28
are nally transformed into CO2 and H2O.
At the same time, these hydroxylation aromatic intermediates
In O3 solution, the reaction between O3 and aromatic were further oxidized, leading to the ring opening by cycloaddition
compounds hinged largely on the electron-donating/withdrawing and formation of a compound containing two carboxylic groups.
properties of the functional groups. –COOH being a strong Through Criegee mechanism, this compound could be further
electron-withdrawing group, results in a more electron-decient decomposed into short-chain aldehydes (formaldehyde, glyoxal,
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Fig. 4 Possible reaction pathways of benzoic acid moieties during their reaction with O3.
methyl glyoxal) and carboxylic acids (formic acid and acetic).29 decompose hydroxylation aromatic products, resulting in the
Finally, aldehydes were constantly oxidized to form the corre- formation of ring-open products through cOH addition reac-
sponding carboxylic acid through a nucleophilic mechanism, and tions and Criegee mechanism that is similar to the reaction
some of them could be completely mineralized to CO2. Instead, illustrated in Fig. 4.
due to the stability of carboxylic acids in O3 solution, particularly
formic and acetic acids, they would be accumulated in the
solution.
3.2 DBPs formation by ozonation of ACAs
As shown in Fig. 6, direct chlorination disinfection and ozon-
ation–chlorination disinfection of ACAs generated a certain
amount of chloroform and DCAA, and the chlorination condi-
tions were as follows: pH 7, 25 ꢁ 1 ꢃC in the dark for 24 h. ACAs
ozonation promoted the production of DCAA, and the
increasing levels of BA and PA were found to be higher than 3-
OHBA and PhA. A consistent trend was observed for chloroform
formation; ACAs ozonation also promoted the production of
chloroform, whereas PA exhibited chloroform yield reductions.
This result was in agreement with the results of our previous
study on the raw water of Taihu Lake, where the formation
potential of THMs and HAAs was improved through pre-
ozonation step. Thus, this phenomenon suggested that the
amount of DOC was not a proper indicator for determining the
relative abundance of the precursors of chloroform and DCAA
in these experiments. In addition, the concentrations of chlo-
roform and DCAA reach a maximum when O3 reacts with ACAs
at 2–5 min and then decrease over time following O3
Beyond that, cOH radical, which was generated from O3
decomposition in water, may play an important role during the
ozonation processes; the reaction of cOH with BA was proposed
and is illustrated in Fig. 5. Most cOH reactions are analogous to
diffusion-controlled reactions and generally have the following
three mechanisms: H-atom abstraction, cOH addition to C–C,
and cOH interaction with compounds containing S, N, or P
atoms. In addition, cOH is more susceptible to add to C]C
rather than C]O, as the carbon atom is electron decient.
Since most radical reactions are composed of chain reactions,
the generation of other radicals and electron transfer reactions
occur rarely. Thus, these intermediates may not experience
addition and H-abstraction.11
BA mainly reacting with cOH may generate radical interme-
diates through addition reaction, resulting in the generation of
hydroxylation aromatic products such as 3-OHBA and PA.
Subsequently, O3, cOH and other oxidizers would further
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Fig. 5 Possible reaction pathways of benzoic acid moieties during their reaction with cOH.
application. The effect of ozonation on the reduction of chlo- precursors.32 Combining the results of the aforementioned
roform and DCAA was signicant initially and then slowed research studies and literature, it can be assumed that during
down. It also should be noted that chloroform was the main ACAs ozonation, the formed hydroxylation aromatic interme-
product of ozonation–chlorination reactions of ACAs; the more diates could be the main precursors. In addition, ACAs ozona-
reactive groups it contained, the greater the amount of DBPs tion leads to short-chain aldehydes and carboxylic acids
generated.
formation (e.g. formaldehyde, glyoxal, methyl glyoxal, formic
It is widely assumed that chlorine reacts with the electron- acid, acetic acid and fumaric acid). During decarboxylation, the
donating group of aromatic compounds through electrophilic structure of these short-chain aldehydes and carboxylic acids
substitution and ortho/para-position stepwise chlorination, tend to be converted into that of b-ketoacids and give high
resulting in the formation of THMs. However, in the presence of yields of THMs at solutions with pH around 7. As far as the
an electron-withdrawing group, meta-position is progressively DCAA formation was concerned, hydroxyl and carboxyl groups
substituted by chlorine, resulting in the formation of THMs. were found to be prone to generate DCAA.33
Besides, resorcinol structures are popularly regarded as the
Not only were the ACAs primary DBPs precursors, the inter-
most important THMs precursors.30,31 Although O3 can mediates of ACAs ozonation also could be DBPs precursors, and
decompose THMs or HAA precursors, O3 intermediate products they produced high yields of THMs and HAAs. Previous study
have a higher potential to form DBPs compared to the demonstrated the ubiquitous presence of fulvic acid in the
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Fig. 6 Formation of DBPs during ozonation–chlorination disinfection of ACAs.
terrestrial environment and that it generated a certain amount aldehydes and carboxylic acids were formed. As for minerali-
of ACAs and low-molecular-weight carbonyl compounds during zation, the short-chain aldehydes and carboxylic acids were
ozonation. Therefore, the ACA levels should be kept under nally transformed into CO2 and H2O.
control in drinking water treatment plants.
(4) Not only ACAs themselves were the main DBPs precur-
sors, but the intermediates of ACAs ozonation could also be the
DBPs precursors, and they produced high yields of THMs and
HAAs. Therefore, the ACA levels should be kept under control in
drinking water treatment plants.
4. Conclusions
Previous studies suggest that fulvic acid ozonation can produce
ACAs, such as protocatechuic acid, 3-hydroxybenzoic acid,
phthalic acid, and benzoic acid, to investigate ozonation
kinetics, possible ozonation reaction pathways, ozonation
intermediates and DBPs of 4 ACAs. The following conclusions
were obtained:
Acknowledgements
This study was performed with nancial support from the
National Water Pollution Control and Major Projects of Science
and Technology Management, Item No. 2012ZX07403001. The
authors gratefully acknowledge the assistance of Wujiang Hua-
Yan Water Co. Ltd.
(1) Two stages pseudo-rst-order reaction for 4 ACAs was
carried out at pH 7, 25 ꢃC, [O3]0 ¼ 1.0 mg lꢀ1 and [BA, PA, PhA, 3-
OHBA]0 ¼ 1.0 mg lꢀ1
.
(2) Six intermediates accompanied the ozonation of ACAs,
namely fumaric acid, formic acid, acetic acid, formaldehyde,
glyoxal and methyl glyoxal.
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