Polyhaloaromatic compounds (XAr) have been found world-wide in pesticides, pharmaceuticals, flame retardants and personal care products. Most of these compounds are persistent and widely existing in the environment because of their recalcitrant properties in the soil and water. They are potentially carcinogenic to organisms and may induce serious risks to the ecosystem. Therefore, it is important to detect and quantify these ubiquitous XAr in the environment, and to monitor their degradation kinetics during the treatment of these recalcitrant pollutants. Chemiluminescence (CL) is a kind of light emission from complicated chemical reactions, during which high-energy excited-states can be generated and energy is released. CL analysis can be used to detect and quantify the specific compounds that determine the generation of CL emission. It has been recently discovered that unprecedent CL emission could be produced during the degradation of XAr mediated by advanced oxidation processes (AOPs), and a novel, rapid and sensitive CL-based analytical method was developed to not only detect XAr, but also monitor their degradation kinetics. For this distinct CL-based analytical method on the detection and measurement of XAr, it exhibits the excellent properties of relatively simple, rapid, sensitive, without complicated pretreatment.
Polyhaloaromatic compounds (XAr) have been found world-wide in pesticides, pharmaceuticals, flame retardants and personal care products
. Most of these compounds are persistent and widely existing in the environment because of their recalcitrant properties in the soil and water. More importantly, not only the oxidative DNA damage, but also protein and DNA adducts may be induced by these XAr compounds in vitro and in vivo systems
, which possibly makes them carcinogenic to mammalian organisms
. One typical group of XAr are polyhalophenols, some of which, such as 2,4,6-trichlorophenol and pentachlorophenol (PCP, the widely-used wood preservative) have been classified as priority pollutants by the U.S. Environmental Protection Agency (US-EPA)
. PCP was also classified as a group I human carcinogen by the International Agency for Research on Cancer (IARC)
. PCP is potentially carcinogenic to mammals. Hepatocellular carcinomas and hemangiosarcomas were observed from B6C3F1 mice under exposure to PCP
. In individuals with occupational exposure to PCP, malignant lymphoma and leukemia in humans were also found to relate to PCP
.
Therefore, detecting and quantifying the widespread XAr in the environment is crucial. The traditional analytical methods used to detect XAr, such as UV−Vis spectrophotometry, high-performance liquid chromatography (HPLC) and gas chromatography (GC)
, usually have many shortcomings: such as low sensitivity, time-consuming, requiring sample pretreatment, expensive apparatus and complicated operation. Therefore, a sensitive, simple, low-cost and effective analytical method to detect and quantify the ubiquitous XAr is urgently needed. Fortunately, chemiluminescence (CL) analytical method may partly meet such demand
A variety of methods and technologies have been used to degrade and treat recalcitrant XAr in the environment, including enzymatic biodegradation, physical adsorption and chemically advanced oxidation. Among them, advanced oxidation processes (AOPs) have been considered to be the most widely-used means for degrading and treating XAr, mainly because they are highly-effective and environmentally green
. Several alternative AOPs, such as Fenton and Fenton-like oxidation
, UV-photolysis
, and ozonation
, have also been developed to effectively degrade and treat the recalcitrant XAr. In these green AOP systems, the most reactive intermediate for degradation is the strong oxidative radical species hydroxyl radical (
OH)
.
The most well-known pathway for
OH generation is through the classic Fenton or Fenton-like reactions mediated by reactive transition metal ions
. Recently, an unprecedented metal-independent
OH-generating system (polyhaloquinones and H
O
) was discovered, and the molecular mechanism of typical nucleophilic substitution coupling with homolytic decomposition for
OH generation was proposed
. More interestingly, an unexpected intrinsic chemiluminescence (CL) emission can also be produced in this novel
OH-generating system, which was found to be specifically dependent on
OH production
. Taking the reaction of tetrachloro-1,4-benzoquinone (TCBQ, a carcinogenic quinone metabolite of PCP) with H
O
as an example, a typical two-step CL emission can be clearly observed, which is directly dependent on the two-step generating processes of
OH
. Moreover, not only for TCBQ, but also for other polyhaloquinones, such as other chloroquinones, fluoroquinones, bromoquinones and halonaphthoquinones, similar intrinsic
OH-dependent CL was produced. These results revealed an unprecedented
OH-generating and CL-producing system: polyhaloquinones (XQs) and H
O
.
It has been previously known that a variety of XAr, such as PCP, can be chemically-degraded into haloquinones during the AOPs with the generation and involvement of
OH. However, it is not clear whether
OH-dependent CL emission can also be generated in the degradation of PCP mediated by these AOPs. In addition, as one of the important final products of TCBQ after the interaction with H
O
, 2,5-dichloro-3,6-dihydroxyl-1,4-benzoquinone (DDBQ, an inert halohydroxyl quinoid compound) would not react with H
O
to generate
OH and produce CL
, but it is not clear whether the addition of extra
OH will induce the production of CL emission.
It has been previously known that the reaction between TCBQ and H
O
could unexpectedly produce highly-reactive
OH and specific
OH-dependent CL
. Moreover, PCP could be chemically degraded and converted to TCBQ in the classic
OH-generating Fe(II)-Fenton system
. Therefore, it is probable that CL mighte be generated in the PCP degradation process mediated by AOPs comsisting of the classic Fe(II)-Fenton system. As expected, neither
OH nor CL emission was detected when incubating PCP with H
O
, whereas a remarkable CL emission (510−580 nm) was generated when extra
OH was introduced by adding Fe(II)-EDTA (Fe(II)-ethylenediamine tetraacetic acid, a classic Fenton reagent) (
A), indicating that the degradation of PCP mediated by
OH-generating Fe(II)-Fenton system indeed produce intrinsic CL emissions
.
Intrinsic
OH-dependent CL was generated from PCP degradation mediated by Fe(II)-Fenton system, during which chloroquinones were formed as the critical intermediates
. (
) Intrinsic CL emission was produced by PCP/Fe(II)-Fenton system; (
)
OH scavenger DMSO can markedly inhibit CL emission; (
,
) several chloroquinone intermediates were generated from PCP during the CL emission.
Interestingly, similar to the CL generated from TCBQ/H
O
, the CL derived from PCP/Fe(II)-Fenton system was also directly dependent on
OH generation, as shown by the following line of evidence
: (1) The CL emission from the PCP/Fe(II)-Fenton system was significantly inhibited by dimethyl sulfoxide (DMSO), a typical
OH scavenger (
B); (2) both the yields of
OH and the intensity of CL emission increased with the increasing dosage of Fenton reagents; (3) CL was also produced from the other widely known
OH-generating Fenton agent Fe(II)-NTA (nitrilotriacetic acid)
, and a good correlation was observed between the CL emission and the kinetics of
OH formation.
Previous studies have reported that the degradation of chlorophenol could produce chloroquinone as intermediates
. So it is possible that the critical species to initiate the CL emission from PCP/Fe(II)-Fenton system might be such chloroquinone intermediates. As expected, five transient chloroquinone intermediates were identified (
C,D), they were TCBQ, tetrachloro-1,4-hydroquinone (TCHQ), tetrachloro-1,2-hydroquinone (tetrachlorocatechol, TCC), trichlorohydroxyl-1,4-benzoquine (TrCBQ-OH) and 2,5-dichloro-3,6-dihydroxyl-1,4-benzoquinone (DDBQ)
. It should be noted that besides DDBQ, the other four chloroquinones could undergo interactions with H
O
to generate CL, and the addition of an extra
OH-generating Fenton reagent can markedly enhance CL emission. These results verify that the CL production from PCP/Fe(II)-Fenton systems is originated from the generation of chloroquinones.
On the basis of the previously discovered mechanism for CL generation from the TCBQ/H
O
system
, and together with the above findings, the underlying molecular mechanism for
OH-dependent CL emission from the degradation of PCP mediated by the classic
OH-generating Fe(II)-Fenton system was proposed (
)
:
The molecular mechanism for the unexpected
OH-dependent CL emission from the degradation of PCP mediated by Fe(II)-Fenton system
. The classic Fenton system could produce large amounts of reactive
OH, which further attacks PCP via electrophilic addition and/or electron transfer pathways, forming pentachlorophenoxyl radical and tetrachlorosemiquinone radicals. The latter radicals then convert to tetrachloroquinones, which further react with H
O
to produce high-energy quinone-1,2-dioxetanes, and finally emit the intrinsic
OH-dependent CL as reported before.
The above findings suggest that it is possible to develop an undiscovered novel CL-based analytical method for the detection and quantification of PCP. Further studies proved it was indeed the case
: the unique CL-generating property of PCP was used to develop a novel analytical method for detecting and measuring trace amounts of PCP, and it was found that the LOD (limit of detection) value was 1.8 ppb and the linear range (LR) was 2.6–18,620 ppb for PCP as detected by this CL method. Both the LOD and LR values are lower than the concentration of PCP (40 ppb) in the body fluids of people under non-occupational exposure, and much lower than PCP (19,580 ppb) concentration under occupational exposure
. Interestingly, the kinetics of CL emission was found to correlate well with the kinetics of PCP degradation: when PCP degradation achieved the maximum, CL emission was no longer observed (
). These results indicate that this novel CL-based analytical method could also be used to monitor the degradation kinetics of PCP.
The above findings that remarkable CL emission can be produced from the PCP/Fe(II)-Fenton system suggest that CL may also be generated from the interactions of other chlorophenols (CPs) with the classic Fe(II)-Fenton system. If so, there might be a close relationship between the CP structures and their abilities to generate CL. As expected, similar
OH-dependent intrinsic CL could also be generated from the other 18 CPs (
). The intensity of CL emission induced by CPs was strongly dependent on both chlorination level and chlorine substitution position. An obvious SAR between CPs structures and CL emission was observed
: (1) In general, as the chlorination level increases, the intensity of CL emission increases; (2) for CPs congeners, the CL increased in the order of para- < ortho- < meta-chlorine substitution with respect to the −OH group of CPs. For example, 2,5-dichlorophenol (DCP), 2,3,5-trichlorophenol (TCP) and 2,3,5,6-tetrachlorophenol (TeCP) generated the strongest CL emission among all the DCPs, TCPs and TeCPs congeners, respectively.
Actually, the properties that CL could be generated from all nineteen CPs in Fe(II)-Fenton system were also used to detect and quantify these ubiquitous CPs. As shown in
, for those CPs that could produce an obvious CL emission, the LOD value could reach as low as 0.007 μM by this highly-sensitive CL analytical method.
Intrinsic
OH-dependent CL were also generated from all 19 chlorophenols in AOPs mediated by Fe(II)-Fenton system
.
CPs | LOD (μM) | LR (μM) |
---|
2,3,4-TCP | 0.3 | 0.3~100 |
2,4,6-TCP | 0.3 | 0.3~100 |
3,4,5-TCP | 0.07 | 0.1~100 |
2,4,5-TCP | 0.01 | 0.03~100 |
2,3,6-TCP | 0.07 | 0.07~100 |
2,3,5-TCP | 0.003 | 0.007~100 |
2,3,4,6-TeCP | 0.01 | 0.03~100 |
2,3,4,5-TeCP | 0.01 | 0.03~100 |
2,3,5,6-TeCP | 0.007 | 0.01~100 |
PCP | 0.007 | 0.01~100 |
Similar to the degradation of PCP, the degradation of other CPs by Fenton system-mediated AOPs also generated chloroquinones as the major intermediates, they were chloro-1,4-benzoquinones (CBQs), chloro-1,4-hydroquinones (CHQs) and chloro-1,2-hydroquinones (also called chlorocatechols, CCs)
. Moreover, all the above chloroquinones could react with H
O
to produce CL, and the addition of Fenton reagent can markedly enhance CL emission
. Interestingly, CL emission of all nineteen CPs was found to primarily depend on the yields and types of the corresponding chloroquinone intermediates generated from CPs
: a good relationship was observed between the CL intensity of CPs and the total yields of corresponding CBQs/CHQs and TCC/3,4,6-TCC (tetrachlorocatechol and 3,4,6-trichlorocatechol, the two CCs which emit stronger CL) (
A,B). More interestingly, not only chloroquinone intermediates, but chlorosemiquinone radicals (CSQs
) were also produced during the CL emission of CPs in Fenton-like systems, and the types and yields of which were also in good agreement with the emission of CL and the generation of chloroquinone intermediates (
).
The intensity of CL emission from CPs correlated well with the total yields of the formation of corresponding chloroquinone intermediates and chlorosemiquinone radicals (taking PCP and three TeCPs for example)
. (
) The intensity of CL emission; (
) the total yields of CHQs/CBQs and CCs; (
,
) the yields of chlorosemiquinones measured by ESR (
) and UV−Vis method (
).
In the tests of acute toxicity to
, a good relationship was observed between the chemical structures of 19 CPs and their acute toxicity
: (1) The higher the level of chlorine substitution, the stronger the toxicity of CPs; (2) for CPs congeners, their toxicity increased in the order of non- < mono- < di-ortho position-chlorophenols. Moreover, the rules for the relationship between the CPs structures and their degradation rates during
OH-generating AOPs have been reported
, which were listed as follows: (1) The higher the level of chlorine substitution, the slower the degradation rate of CPs; (2) for CPs congeners, the degradation rate decreased in the order of 3-/5- > 2-/4-/6-chlorine substitution CPs.
In summary, based on the above results, together with the previously reported studies on SAR
, good correlations between the CP structures and their chemical activities (CL emission, toxicity and degradation rate) was found, they were listed as following
: (1) The higher the level of chlorine substitution for CPs, stronger CL emission, higher toxicity and slower degradation could be observed; (2) for CPs congeners, CPs with position-3 or -5 chlorine substitution show stronger CL emission, higher toxicity and faster degradation; while those CPs congeners with position-6 chlorine substitution show much weak CL emission, lower toxicity, and slower degradation. These findings may suggest that, utilizing the distinct CL-generating property of CPs induced by classic Fe(II)-Fenton system, a novel CL-based method can be developed, to not only detect and quantify trace amounts of CPs in pure or real samples, but also provide valuable information for evaluating the toxicity or degradation rate of CPs.
More importantly, during the AOPs mediated by the
OH-generating Fe(II)-Fenton system, besides PCP and the other eighteen chlorophenols, similar
OH-dependent intrinsic CL was also generated from other halophenols and several other classes of XAr. These compounds include (
)
: other halophenols such as pentafluorophenol (PFP), 2,4,6-tribromophenol (2,4,6-TBP), the flame retardant 3,3′,5,5′-tetrabromobisphenol A (TBBPA), and the broad-spectrum antibacterial agent triclosan (TCS); halogenated naphthoquinone pesticides such as 2,3-dichloro-1,4-naphthoquinone (2,3-DCNQ); chlorophenoxyacetic acid herbicides such as the notorious 2,4,5-trichlorophenoxyacetic acid (2,4,5-T), one important component of Agent Orange; halogenated benzene biocides such as pentachlorobenzene (PCB); iodinated pharmaceuticals such as triiodothyronine (T3); These results indicate that most or even all XAr can generate
OH-dependent CL in the degradation mediated by the Fe(II)-Fenton system. Moreover, similar CL spectra were also observed from these XAr, which were found to be attributed to the analogous molecular mechanism and similarity in structures of light-emitting species.
Similarly, based on the CL emission properties of XAr, a novel and sensitive CL analytical method to detect and quantify these ubiquitous XAr was developed. As anticipated, we successfully detected and quantified traces of several typical XAr, including PFP, 2,4,6-TBP, TBBPA, TCS, 2,3-DCNQ, 2,4,5-T, PCB and T3. For example, using this novel CL analytical method, we can directly detect concentrations as low as 0.03 μM for TCS, 0.07 μM for TBBPA, and 0.03 μM for T3 (
)
.
Similar
OH-dependent intrinsic CL were also produced from the degradation of several typical XAr in classic Fe(II)-Fenton system
.
XAr | LOD (μM) | LR (μM) |
---|
PCP | 0.01~70 | 0.007 |
TCS | 0.07~30 | 0.03 |
TBBPA | 0.1~10 | 0.07 |
2,3-DCNQ | 0.1~100 | 0.07 |
PCB | 0.1~10 | 0.07 |
PCB | 0.1~10 | 0.07 |
T3 | 0.03~1 | 0.03 |
More importantly, this novel CL analytical method based on Fe(II)-Fenton system has been utilized to evaluate and detect whether XAr is contained in an actual environmental sample, the discharge from a paper mill
. As anticipated, obvious CL emission could be generated from the discharge of paper mill in the presence of Fenton agent, and further analysis suggested that the discharge contained 2,4-dichlorophenol and 4-chlorophenol. Furthermore, this newly-developed CL-based analytical method can also be used for monitoring the degradation kinetics of XAr in their treatment mediated by AOPs. In the PCP/Fe(II)-Fenton system, the kinetics of CL emission correlated well with the kinetics of PCP degradation
: the profiles of CL emission coincided with the kinetic curves of PCP degradation, and no further CL emission could be generated when PCP degradation finished.
As mentioned above, unexpected
OH-dependent intrinsic CL could be generated from TCBQ/H
O
, with DDBQ formed as an important final product, whereas neither
OH nor CL could be generated by DDBQ and H
O
. However, it was unexpectedly discovered that a remarkable CL emission could be induced by adding some redox-active transition metal ions like Fe(II) and cobalt(II) (Co(II)), particularly for Co(II), which induced a much stronger CL emission than Fe(II) (
A,B)
. These results suggest that not only the key reactive oxygen species (ROS) intermediates for CL emission, but also the underlying molecular mechanism of the unexpected strong CL emission from DDBQ in Co(II)-Fenton-like system might be different from the CL in the classic Fe(II)-Fenton system.
Distinct CL emission was generated from DDBQ in Co(II)-Fenton-like system, dependent on site specifically produced
OH
. (
) The CL emission generated from DDBQ/Co(II)-Fenton-like system; (
) the intensity of CL emission generated by Co(II) was higher than other active transition metal ions; (
) DMSO only slightly inhibited CL emission from DDBQ in Co(II)-Fenton-like system, but significantly inhibited CL in Fe(II)-Fenton system; (
) the effect of DMSO on the generation of
OH in DDBQ/Co(II)-Fenton-like system and DDBQ/Fe(II)-Fenton system.
It should be noted that, in the Co(II)-Fenton-like system, similar CL emissions were also observed when substituting DDBQ with other halohydroxyl quinones such as 2,5-dibromo-3,6-dihydroxyl-1,4-benzoquinone, chlorocatechols such as 3-chlorocatechol, 3,4-dichlorocatechol, 3,4,6-trichlorocatechol, 3,4,5-trichlorocatechol and tetrachlorocatechol (the latter two are typical effluents from bleached kraft pulp mills
), and other halocatechols such as tetrabromocatechol and tetrafluorocatechol (
A,B)
.
Analogous
OH-dependent CL emission were also generated from halocatechols with the typical Co(II)-Fenton-like system, which can be utilized to quantify trace amount of halocatechols and DDBQ
. (
) The structure of several halocatechols; (
) the total CL intensity for these halocatechols; (
,
) CL emission of 3,4,6-TrCC and TCC; (
) the LOD and LR for the quantification of DDBQ, 3,4,6-TrCC and TCC by the distinct CL analytical method mediated by Co(II)-Fenton-like system.
It has been previously known that the generation of free
OH was critical for CL generation by TCBQ (the precursor of DDBQ) and H
O
, and adding
OH scavenger could markedly inhibit CL emission