Article
pubs.acs.org/ac
Cetyltrimethylammonium Bromide-Coated Fe3O4 Magnetic
Nanoparticles for Analysis of 15 Trace Polycyclic Aromatic
Hydrocarbons in Aquatic Environments by Ultraperformance, Liquid
Chromatography With Fluorescence Detection
Hao Wang,†,‡,⊥ Xiaoli Zhao,*,†,⊥ Wei Meng,*,† Peifang Wang,§ Fengchang Wu,† Zhi Tang,†
Xuejiao Han,† and John P. Giesy∥
†
State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of Environmental Sciences,
Beijing 100012, China
‡
College of Water Sciences, Beijing Normal University, Beijing 100875, China
§
Key Laboratory of Integrated Regulation and Resource Development on Shallow Lakes, Ministry of Education, College of
Environment, Hohai University, Nanjing 210098, China
∥
Department of Veterinary Biomedical Science and Toxicology Centre, University of Saskatchewan, 44 Campus Drive, Saskatoon,
Saskatchewan S7N 5B3, Canada
S Supporting Information
*
ABSTRACT: Accurate determination of polycyclic aromatic
hydrocarbons (PAHs) in surface waters is necessary for
protection of the environment from adverse effects that can
occur at concentrations which require preconcentration to be
detected. In this study, an effective solid phase extraction
(SPE) method based on cetyltrimethylammonium bromide
(CTAB)-coated Fe3O4 magnetic nanoparticles (MNPs) was
developed for extraction of trace quantities of PAHs from
natural waters. An enrichment factor of 800 was achieved
within 5 min by use of 100 mg of Fe3O4 MNPs and 50 mg of
CTAB. Compared with conventional liquid−liquid extraction
(LLE), C18 SPE cartridge and some newly developed
methods, the SPE to determine bioaccessible fraction was
more convenient, efficient, time-saving, and cost-effective. To evaluate the performance of this novel sorbent, five natural samples
including rainwater, river waters, wastewater, and tap water spiked with 15 PAHs were analyzed by use of ultraperformance,
liquid chromatography (UPLC) with fluorescence detection (FLD). Limits of determination (LOD) of PAHs (log Kow ≥ 4.46)
ranged from 0.4 to 10.3 ng/L, with mean recoveries of 87.95 ± 16.16, 85.92 ± 10.19, 82.89 ± 5.25, 78.90 ± 9.90, and 59.23 ±
3.10% for rainwater, upstream and downstream river water, wastewater, and tap water, respectively. However, the effect of
dissolved organic matter (DOM) on recovery of PAHs varied among matrixes. Because of electrostatic adsorption and
hydrophobicity, DOM promoted adsorption of Fe3O4 MNPs to PAHs from samples of water from the field. This result was
different than the effect of DOM under laboratory conditions. Because of competitive adsorption with the site of action on the
surface of Fe3O4 MNPs for CTAB, recoveries of PAHs were inversely proportional to concentrations of Ca2+ and Mg2+. This
novel sorbent based on nanomaterials was effective at removing PAHs at environmentally relevant concentrations from waters
containing relevant concentrations of both naturally occurring organic matter and hardness metals.
P
genic potential, PAHs in the environment have attracted
attention globally and some have been listed as priority
pollutants by the United States Environmental Protection
Agency (U.S. EPA).7−10
Concentrations of PAHs in ground and surface waters,
sediments, and the atmosphere are increasing due to activities
olycyclic aromatic hydrocarbons (PAHs), of which there
are thousands of possible variations in the environment,
consist of two or more fused rings without heteroatoms, with
some PAHs alkyl substituted.1,2 Most PAHs are released into
the environment during leaks or spills during extraction,
transport, and refinery of petroleum hydrocarbons or during
combustion of wood biofuels and fossil fuels such as coal and
petroleum and other paths, such as cooking, burning of
domestic wastes.2−6 Because of their ubiquitous presence,
chemical stability, potential for bioaccumulation, and carcino© 2015 American Chemical Society
Received: March 20, 2015
Accepted: July 8, 2015
Published: July 8, 2015
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Figure 1. Schematic representation of mechanism of adsorption of PAHs by Fe3O4−CTAB MNPs.
of humans.11−13 There is a need to monitor PAHs, but they can
occur at concentrations ranging from pg/L to ng/L, which, due
to their propensity to be bioaccumulated, have potential to
cause adverse effects, yet be less than the LOD of standard
analytical techniques. Moreover, various environmental factors,
such as chemical components, physical condition, can affect
performances of pretreatment techniques.14−16 Accurate
quantification of trace concentrations of PAHs in environmental matrixes, especially in water at environmentally and
toxicologically relevant concentrations is needed. To achieve
the required LOD, samples are concentrated and separated
from environmental matrixes by use of methods including
liquid−liquid extraction (LLE), solid phase extraction (SPE)
and solid phase microextraction (SPME) (Table S1 in the
Supporting Information). Each of these methods has
advantages as well as limitations. Some are time-consuming
and relatively expensive and result in large amounts of waste
solvents.17−23 Use of a solid adsorbent based on C18 cartridges,
to selectively preconcentrate PAHs from environmental
matrixes uses lesser amounts of organic solvents than does
LLE. An alternative to these more traditional approaches is the
use of adsorbents attached to nanoparticles that can be
separated by used of a magnetic field. One such process uses
cetyltrimethylammonium bromide (CTAB) coated onto
magnetic nanoparticles of iron oxide (Fe3O4) (Fe3O4−CTAB
MNPs). This method has excellent capacity to separate PAH
from environmental matrixes, especially water, and is less
expensive and quicker than the traditionally used methods that
employ C18 disks or cartridges as the solid phase.18,23 The fact
that larger volumes of water can be treated without breakthrough or interferences makes use of nanoparticles, such as
nanocarbon, C−Fe3O4 and Ag−Fe3O4 an attractive approach to
obtain lesser LODs for PAHs. Some of these solid phases might
be unsuitable for treatment of large volumes of sample and
could be time-consuming to separate with sufficient recoveries.24,19 The superparamagnetic properties of magnetic
nanoparticles (MNPs) contribute to their rapid magnetization
and separation from aqueous phases by use of external,
magnetic fields. When coated with appropriate functional
groups MNPs can enrich contaminants from large volumes of
water.25 Additionally, advantages of MNPs including Fe3O4 and
γ-Fe2O3 are their convenience, biocompatibility, and economical synthesis by use of chemical coprecipitation.26−28 While
other adsorbents such as stir bars or artificial fibers were
complicated to produce, Fe3O4−CTAB MNPs can easily be
synthesized. Because MNPs are magnetic, small particles with a
large total surface area, they are effective for rapid and
quantitative adsorption of PAHs and can easily be collected
into an organic solvent by use of a magnetic field.29,30 Once
separated from water, the organics trapped on the surface can
be extracted by use of an organic solvent. Thus, Fe3O4−CTAB
MNPs have promise as a solid phase for extraction of PAHs in
water.
Enrichment of analytes by use of MNPs is improved by
modification of surfaces of MNPs by addition of functional
groups, such as coupling agents, surfactants, or noble metals.31
Ionic surfactants can attach homogeneously onto charged
surfaces of MNPs by chemical self-assembly and due to their
hydrophilic groups, form hemimicelles, mixed hemimicelles or
admicelles.32 The mixed hemimicelles promote effective
adsorption of PAHs by hydrophobic interaction with hydrocarbon moieties (Figure 1).33−35 MNPs as a substratum for
sorbents successfully avoid time-consuming, blocking problems
during conventional SPE and, relative to LLE, also reduces the
amount of organic solvent used.
Those PAHs, which have been designated as priority
pollutants by the U.S. EPA, were quantified by use of
ultraperformance liquid chromatography in tandem with
fluoresence detection (UPLC-FLD), which can conviniently
quantify all 15 PAHs within 30 min, while maintaining
sufficient sensitivity, to attain LODs equivalent to the most
commonly used analytical procedures. To our knowledge, this
is the first report of utilization of MNPs for preconcentration of
trace concentrations of PAHs from natural water.
The objective of the present study was to develop a rapid,
simple, cost-effective SPE procedure using Fe3O4 MNPs
coupled with UPLC-FLD for quantification of trace concentrations of the 15 priority PAHs, designated by U.S. EPA, in
water. Several key factors that could influence recoveries and
accuracies and precision of determination of concentrations of
PAHs isolated from natural waters, such as pH, breakthrough
volume, type, and amounts of solvents used to elute analytes
from the solid phase were determined. Effects of DOM, such as
fulvic acid (FA) and humic acid (HA), and ions including Ca2+
and Mg2+ were investigated. Finally, the method was validated
by application to five environmental waters.
■
EXPERIMENTAL SECTION
Reagents and Chemicals. The standard solution containing Naphthalene (Nap), Acenaphthylene (Ace), Fluorene
(Flo), Phenanthrene (Phe), Anthracene (Ant), Fluoranthene
(Fla), Pyrene (Pyr), Chrysene (Chr), Benzo(a)anthracene
(Baa), Benzo(b)fluoranthene (Bbf), Benzo(k)fluoranthene
(Bkf), Benzo(a)pyrene (Bap), Dibenzo(a,h)anthracene
(DahA), Indeno(1,2,3-cd)pyrene (Icdp), and Benzo(ghi)perylene (BghiP) (2000 mg/L) was purchased from SigmaAldrich (St. Louis. MO) and diluted to 1 mg/L as the stock
solution for use in spiking waters. Samples were kept in the
dark at 4 °C until used. Acetonitrile (ACN), Dichloromethane
(DCM), and Acetone (DMK) were HPLC grade, and
purchased from Fisher Scientific Corporation (Fair Lawn,
NJ). Acetic Acid (AcOH, A.R. grade) and hydrochloric acid
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Table 1. Analytical Parameters of the Proposed Method
PAHs
naphthalene
acenaphthene
fluorene
phenanthrene
anthracene
fluoranthene
pyrene
chrysene
benzo[b]
fluoranthene
benzo(a)
anthracene
benzo(k)
fluoranthene
benzo[a]pyrene
dibenz[a,h]
anthracene
indeno[1,2,3-cd]
pyrene
benzo[ghi]perylene
a
slope ± SD [mV L
/ng]
correlation
coefficient (r2)
LODb
(ng/L)
RSD (%)
(n = 3)
103
104
103
104
103
103
104
104
0.782
0.888
0.992
0.999
1.000
1.000
0.998
0.993
20.6
0.7
3.9
10.3
0.5
3.5
6.4
1.0
0.4
18.0
10.2
3.9
6.9
4.4
2.4
2.4
5.2
0.1−400
(3.07 ± 0.12) × 104
0.996
1.7
4.3
6.2
0.1−400
(3.58 ± 0.11) × 105
0.995
1.2
7.5
0.0038
0.0005
6.35
6.75
0.1−400
0.1−400
(2.09 ± 0.11) × 105
(9.63 ± 0.20) × 104
0.995
0.990
1.7
3.3
6.0
4.2
IcdP
0.0005
6.51
0.1−400
(1.18 ± 0.06) × 105
0.996
3.5
4.3
BghiP
0.0003
6.9
0.1−400
(1.20 ± 0.02) × 103
0.999
2.3
8.2
water solubility
(g/m3)
log
Kowa
range of concn
(ng/L)
Nap
Ace
Flo
Phe
Ant
Fla
Pyr
Chr
Bbf
30.2
3.93
1.9
1.18
0.076
0.26
0.135
0.0019
0.014
3.45
4.22
4.38
4.46
4.54
5.2
5.3
5.61
5.78
0.1−400
0.1−400
0.1−400
0.1−400
0.1−400
0.1−400
0.1−400
0.1−400
0.1−400
(0.96
(1.91
(7.84
(1.25
(4.50
(1.63
(5.09
(4.49
Baa
0.011
5.91
Bkf
0.008
Bap
DahA
abbreviation
Water solubilities and log Kow of 15 PAHs are quoted from Huckins et al.
57 b
±
±
±
±
±
±
±
±
0.21)
0.19)
0.06)
0.03)
0.04)
0.02)
0.12)
0.15)
×
×
×
×
×
×
×
×
Detection limits were calculated by using S/N = 3.
and then sonicated for 1 min. After standing for 10 min on an
Nd−Fe−B magnet, Fe3O4 MNPs coated with CTAB were
isolated from solution and the supernatant decanted.
Preconcentrated PAHs associated with CTAB-coated Fe3O4
MNPs were eluted with 2 mL of ACN solution mixed with 5%
acetic acid (AcOH) (v/v) for 5 times. The eluent containing
PAHs was dried under a stream of nitrogen at 45 °C and
diluted to 1 mL with ACN.
Ultra performance liquid chromatography coupled with
fluorescence detection (UPLC-FLD, Waters, Massachusetts)
was employed to separate, identify, and quantify individual
PAHs. A CORTECS C18 column (100 mm × 2.1 mm i.d., with
particle diameter of 1.6 μm, Waters, Massachusetts) was used
to separate 15 EPA PAHs. The mobile phases were ACN and
ultrapure water at a flow rate of 0.4 mL/min, with an injection
volume of 2 μL. The mobile phase was an ACN/water gradient
program (45% ACN at start, 9.0 min hold, 15.0 min linear
gradient to 60%, 19.0 min linear gradient to 67%, 22.8 min
linear gradient to 77%, 26.5 min linear gradient to 100%, 28.0
min linear gradient to 45%). Excitation wavelengths were 221,
289, 252, 234, 265, and 300 nm, and emission wavelengths
were 337, 322, 377, 448, 390, and 412 nm for 0−5.5, 5.5−9.0,
9.0−12.0, 12.0−15.0, 15.0−18.0, 18.0−28.0 min, respectively.
Calculations for quantification of PAHs were accomplished by
use of Waters Power 2.0 software. Limits of detection (LOD)
for 15 PAHs were determined as being 3 times the signal-noise
ratio. PAHs were quantified by use of an external standard
curve with a linear working range of 0.1−400 ng/L. The
analytical parameters of the proposed method for PAHs are
shown (Table 1).
(A.R. grade) were purchased from Xilong Chemical Corporation (Guangdong, China).
Cetyltrimethylammonium bromide (CTAB, A.R. grade), (1hexadecy) pyridinium chloride monohydrate (CPC, A.R.
grade), ferric chloride (FeCl3·4H2O, A.R. grade), ferrous
chloride (FeCl2·6H2O, A. R. grade), NaOH (sodium hydroxide,
A.R. grade) were purchased from Sino-pharm Chemical
Reagent Co., Ltd. (Beijing, China). Fulvic Acid (Nordic
Aquatic Fulvic Acid Reference 1R105F) and Humic Acid
(Leonardite Humic Acid Standard 1S104H) were purchased
from the International Humic Substances Society (Colorado).
Synthetic, experimental ultrapure water were made from a
Millipore Integral 5 water purification system (Merck,
Germany).
The Multi N/C3100 TOC (Analytikjena, Germany) analyzer
was employed to determine the concentration of DOM in
samples, and concentrations of Ca2+, Mg2+ in samples were
determined by use of an Ion Chromatography System 1000
(Dionex Co.).
Collection of Samples. Samples of surface waters
investigated included two samples of river water collected
from the upstream stagnant pool (low-speed flow) and
downstream reach (high-speed flow) of the Qing River
(Chinese, Qinghe), one sample of rainwater (August, 2014),
one sample of wastewater collected from the Qing River
wastewater treatment plant (Haidian district, Beijing), and one
sample of tap water sample from our laboratory (Chaoyang
district, Beijing). The total volume of each sample was 10 L and
was collected with wide-mouth jars after being cleaned with
chromic acid and ultrapure water. Collected samples were
immediately filtered through a 0.45 μm glass fiber filter
(combusted at 450 °C for 4 h) combined with a filtration
device to remove suspended solids and stored at 4 °C. All
samples were analyzed within 5 days.
SPE Procedure and Sample Analysis. Fe3O4 MNPs were
synthesized by coprecipitation by use of a previously described
method.33 A 5 mL aliquant of Fe3O4 MNPs (20 mg/mL) and
10 mL of CTAB (5 mg/mL) were added to 800 mL of water,
either a synthetic or natural sample, and pH adjusted to 10.0
■
RESULTS AND DISCUSSION
Characterization of Fe3O4−CTAB MNPs. Fe3O4 MNPs
were characterized by use of transmission electronic microscopy (TEM) (Hitachi, Japan) at 80 kV. Particles were generally
uniform with a diameter of approximately 10 nm (Figure 2).
Hysteresis was not observed, and the largest saturation
magnetism of Fe3O4−CTAB MNPs was 58.7 emu/g,25
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Surfaces of Fe3O4 MNPs are negatively charged when the pH
was greater than the pH where the zeta potential of MNPs = 0,
which is defined as the point of zero charge (PZC).25,36
Cationic surfactants can attach to surfaces of nanoparticles by
strong electrostatic attraction to form hydrophobic hemimicelle, which creates a hydrophobic interaction with organic
pollutants, such as PAHs (Figure 1). Octanol−water partition
coefficients (Kow) of the 15 targeted PAHs were directly
proportional to molecular mass (Table 1), such that adsorption
of PAHs was inversely proportional to their solubilities in
water. However, recoveries of Nap, Ace, and Flo (log Kow ≤
4.46) were slightly less than those of other PAHs studied. This
result might be due to their greater solubilities in water and
greater volatility. For PAHs with log Kow values greater than
4.46, recoveries of greater than 80% were observed.
Effects of Amounts of Fe3O4 MNPs and Surfactant.
CTAB was employed as the surface modifier at a ratio 1:2 (w/
w) of CTAB and Fe3O4 MNPs, compared with CPC, and the
detailed information on this (Figure S1 in the Supporting
Information) and the effect of sample volume on recovery of
PAHs (Figure S2 in the Supporting Information) are provided
in the Supporting Information. In order to reduce consumption
of adsorbents, the effect of amount of Fe3O4 MNPs and CTAB
on recoveries of PAHs was determined. Recoveries of 15 PAHs
reached maxima separately as a function of the amount of
Fe3O4−CTAB MNPs added (Figure 3b). However, PAHs with
greater Kow reached the maxima faster than those with lesser
Kow. This result might be due to stronger affinities of Fe3O4−
CTAB MNPs for chemicals with greater hydrophobicity than
those with lesser Kow. The optimal amount of adsorbents used
was the mean of the additive amounts of adsorbents for the
greatest recovery of each PAH. On the basis of this analysis,
indicating their superparamagnetism excellence for rapid
separation.
Figure 2. Transmission electron microscopy (TEM) image of Fe3O4
MNPs.
Effect of Solution pH. pH is a key factor affecting
adsorption of PAHs by Fe3O4−CTAB MNPs and in this study
recoveries of PAHs were directly proportional to pH (Figure
3a), reaching a maximum at pH of approximately 10.0.
Figure 3. Recoveries of PAHs as functions of Fe3O4 MNPs as pH (a), ratio of 2:1 to CTAB (b), standing time (c), and 4 kinds of eluents (d) in
batch mode. Sample volume, 800 mL; volume of ACN, 10 mL. (a, c, and d) 100 mg of Fe3O4 MNPs; surfactant, 50 mg of CTAB; (b) pH 10.0.
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Figure 4. Recoveries of PAHs as functions of FA (a) and HA (b) in batch mode. Amount of metal oxide: 100 mg of Fe3O4 MNPs. Surfactant, 50 mg
CTAB; pH, 10.0; sample volume, 800 mL. Volume of ACN, 10 mL.
PAHs were assessed separately. FA and HA had similar effects
on recoveries of PAHs by Fe3O4−CTAB MNPs (Figure 4a,b).
The effect of DOM on recovery of PAHs could be divided into
three stages: the recovery of PAHs initially declined with the
addition of FA for 0−40 mg/L (HA for 0−50 mg/L), then
increased with the addition of FA for 40−80 mg/L (HA for
50−100 mg/L), and slightly decreased at the concentration of
FA for 80 mg/L (HA for 100 mg/L). The stages observed: (1)
competitive adsorption of DOM with Fe3O4 MNPs to bind
CTAB and PAHs, resulted in lesser recoveries of PAHs. DOM
is generally electronegative at pH 10.0, because there are more
negative charged functional groups than positive groups on
DOM.42−44 However, the majority of added DOM partitioned
into the aqueous phase, adsorbed CTAB to form hemimicelle
structure by electrostatic interactions and had a competition
with Fe3O4−CTAB MNPs for adsorbing PAHs by the
hydrophobic effect;36,43 and meanwhile, less DOM adsorbed
on the surface of Fe3O4−CTAB MNPs and competed with
PAHs for adsorption sites.45−47 The above both effects of
DOM resulted in the decrease of recoveries of PAHs by
Fe3O4−CTAB MNP. Thus, PAHs adsorbed by DOM or
DOM-CTAB would not have been extracted by Fe3O4 MNPs
due to the electrostatic repulsion between DOM or DOMCTAB and MNPs (Figure 5a,b). These combinations of pHdependent phenomena resulted in recoveries of PAHs being
inversely proportional to concentration of DOM until a
concentration of about 40 mg/L for FA and 50 mg/L for
HA. (2) Recoveries of PAHs were increasing until the
concentration of FA reached about 80 mg/L or for HA 100
mg/L. This phenomenon has been less reported. And at this
stage, the newly added FA and HA also adsorbed the CTAB
and PAHs, which might reduce electrostatic repulsion between
the DOM complex and Fe3O4−CTAB MNPs. As a result,
DOM complexes would be adsorbed onto surfaces of Fe3O4−
CTAB MNPs, and some polymers such as flocculation48,49
were gradually formed with addition of FA and HA due to
electrostatic and hydrophobic interactions. It has been reported
of removal of DOM from water with bentonite and
benzyltrimethylammonium bromide by the flocculation reaction (Figure 5c).50 (3) When more than 80 mg/L FA and 100
mg/L HA was added to the solution, newly added DOM would
also compete with previously added DOM and Fe3O4−CTAB
100 mg of Fe3O4 and 50 mg of CTAB were chosen as the
optimal amounts to use. Thus, in this study, amounts of
adsorbents were optimized to be more efficient and less
wasteful than is possible in studies using cartridges.
Optimization of Standing Time. Duration of separation is
a key factor for pretreatment methods. Shorter paths of
adsorption, which result in faster equilibrium are positive
characteristics of nanoadsorbents. A duration of approximately
5 min was determined to be sufficient to obtain maximum
enrichment of the 15 PAHs studied (Figure 3c). This was a
clear advantage compared with conventional pretreatment
methods, such as LLE and C18 SPE cartridge, which had
times to maxima of 72−1 080 and 125−333 min, respectively.17,18,37,38 Some other methods of preconcentration
require a minimum of 30 min to reach their maxima. A
detailed comparison is shown in Table S1 in the Supporting
Information.
Optimization of Desorption Conditions. PAHs were
eluted by mass action and destruction of hemimicelles with
organic solvents. ACN and DMK were used separately to elute
PAHs; 5% DCM was added to increase their capacity, and 5%
AcOH was also added to destruct the mixed hemimicelles
formed by CTAB under alkaline conditions to promote
desorption of PAHs. ACN was better for eluting PAHs than
was acetone (Figure 3d). This might have been due to greater
solubility of CTAB in ACN than acetone. In this study, 10 mL
(2 mL for 5 times) ACN was sufficient to ensure sufficient
recoveries of PAHs.
Effects of Fulvic Acid and Humic Acid. DOM is complex
and comprises a variety of organic substances including FA,
HA, carbohydrates, sugars, amino acids, proteins, inorganic
ions, among others.39 Since FA and HA are the main
components of DOM in the environment40 and carry a variety
of functional groups, they can interfere with adsorption of
PAHs by Fe3O4−CTAB MNPs, mainly through electrostatic
interaction or hydrophobic interaction due to their different
concentrations.41
Concentrations of total organic carbon (TOC) in aquatic
environments ranges for 20−100 mg/L, depending on soil
types in the watershed, climate, and hydrologic conditions,39
thus concentrations of FA and HA considered in this study
ranged from 0 to 120 mg/L and their effects on recoveries of
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hydrophobic organic pollutants, such as PAHs. However, lesser
recoveries of Nap, Ace, and Flo were likely due to their greater
volatilities and solubilities in water. Therefore, coefficients of
determination (r2) for Ace and Flo were 0.78 and 0.88, but an
adequate standard curve could not be obtained for Nap.
Analyses of Environmental Water Samples. Reproducibility of recoveries of PAHs by Fe3O4−CTAB MNPs was
investigated by spiking known amounts of the standard mixture
of PAHs into samples of rainwater, two samples of river water,
wastewater, and tap water. Figure 6 shows the chromatograms
of PAHs in the water samples of the Qing river by using UPLCFLD. Effects of physical-chemical factors were also investigated.
Total concentrations of 15 PAHs in the samples for rainwater,
upstream and downstream river water, wastewater, and tap
water were 924.3 ± 80.71, 1206.99 ± 89.20, 1669.91 ± 148.65,
2232.47 ± 38.12, and 305.54 ± 25.07 ng/L, respectively.
Recoveries of PAHs were about 90, 80, 70 and 60% in
rainwater, both samples of river water, wastewater, and tap
water, respectively (Table 2).
Concentrations of Ca2+, Mg2+, and DOM in natural water
affected performance of Fe3O4−CTAB MNPs on extraction of
PAHs. There could be two aspects of their interactions: (1)
Fe3O4 MNPs were electronegative at pH 10.0, and a
competitive adsorption of metal ions with CTAB on surfaces
of Fe3O4 MNPs prevented formation of mixed hemimicelles
and resulted in poorer recoveries of PAHs51,52 and (2) because
of strong complexation between Ca2+ and Mg2+ and some
functional groups (carboxyl, phenolic hydroxyl) of
DOM,42,46−48 which was also due in part to lesser adsorption
of CTAB to DOM. Thus, presence of DOM would indirectly
reduce the effect of metal irons on adsorption activity of MNPs
to PAHs, as well as increasing development of mixed
hemimicelles, which increased adsorption of PAHs by
Fe3O4−CTAB MNPs.53,54
Concentrations of TOC in five environmental waters are
23.22 ± 0.62, 8.41 ± 0.32 and 3.58 ± 0.44 mg/L for wastewater
and upstream and downstream river water, respectively, with no
DOC detected in tap water or rainwater (Table S2 in the
Supporting Information). Recoveries of PAHs from rainwater
were greater due to lesser concentrations of Ca2+, Mg2+, and
Figure 5. Schematic representation of interactions among CTAB,
DOM, PAHs, Fe3O4 MNPs (a) from adsorption of CTAB onto Fe3O4
MNPs, (b) from adsorption of CTAB onto Fe3O4 MNPs and less
DOM separately, (c) formed from DOM complex and Fe3O4 MNPs
by the bridging effect of CTAB, and (d) decrease recovery of PAHs by
sorption supersaturation of Fe3O4 MNPs to more DOM additives.
MNPs for adsorbing PAHs. One possible mechanism is that
Fe3O4−CTAB MNPs and DOM or DOM-CTAB would both
be more electronegative because the competitive adsorption of
the new added DOM to CTAB and PAHs from previously
formed polymers. Therefore, electrostatic repulsion between
DOM complexes and Fe3O4−CTAB MNPs was recovered due
to electrostatic repulsion regenerated by their electronegativity,
which decreased adsorption of PAHs by Fe3O4−CTAB MNPs
(Figure 5d).
Method Parameters. Calibration curves were run for 15
PAH in the range of 0.1−400 ng/L. Coefficients of
determination (r2) for PAHs (log Kow ≥ 4.46) were all greater
than 0.99, and LODs were calculated by using 3 times the
signal-to-noise and ranges from 0.4 to 10.3 ng/L, which
indicated suitability of MNPs for preconcentration of neutral,
Figure 6. Solid-phase extraction/UPLC-FLD chromatograms: (a) Qing river water sample and (b) Qing river water sample spiked with 60 ng/L of
PAHs.
7672
DOI: 10.1021/acs.analchem.5b01077
Anal. Chem. 2015, 87, 7667−7675
a
24.32
24.48
16.11
7.24
6.35
9.23
8.36
9.83
11.02
10.53
10.87
3.62
12.13
12.28
8.24
16.16c
181.94
492.39
319.28
92.33
13.63
10.97
14.10
8.69
10.33
10.69
9.26
13.17
10.01
13.22
6.98
1206.99
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
29.36
17.28
27.21
10.80
0.50
0.83
0.69
0.16
0.19
0.26
0.13
0.19
0.40
0.44
0.76
89.22
b
52.86
91.35
94.00
89.67
96.27
93.36
83.33
86.82
84.64
85.55
85.68
79.06
89.93
89.97
86.38
82.89
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
19.91
24.07
21.39
12.92
6.40
3.03
4.21
1.61
0.19
0.80
14.52
2.77
4.02
0.58
4.86
5.25c
recovery (%)
downstream water
detected (ng/L)
a
395.88
558.42
421.18
167.82
11.09
18.16
12.09
7.81
12.31
9.37
10.37
16.83
12.05
13.23
3.30
1669.91
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
12.96
71.68
49.12
8.70
0.50
2.24
2.57
0.04
0.09
0.06
0.07
0.11
0.09
0.33
0.09
148.65
b
86.34
79.69
86.71
84.60
96.11
87.74
82.46
81.06
82.21
78.87
77.80
74.15
80.76
79.22
85.66
85.92
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
21.25
18.31
13.50
10.20
7.32
7.07
1.66
1.74
3.13
1.12
1.98
1.53
1.90
2.49
1.76
10.19c
recovery (%)
upstream water
detected (ng/L)
a
Mean of three determinations. bStandard deviation for three determinations. cMean recovery of 15 PAHs.
45.99
63.53
85.22
80.04
82.78
87.44
85.10
92.44
94.80
96.86
97.40
102.99
101.53
92.04
111.04
87.95
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
6.24
37.39
22.75
6.02
0.98
0.75
1.11
0.21
0.21
0.26
0.31
0.45
0.83
2.43
0.77
80.71
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
97.12
270.47
329.57
168.75
18.25
12.54
8.77
2.81
1.87
2.73
1.44
0.77
1.54
4.25
3.42
924.3
Nap
Ace
Flo
Phe
Ant
Fla
Pyr
Chr
Baa
Bbf
Bkf
Bap
DahA
IcdP
BghiP
total
b
recovery (%)
rainwater
detected (ng/L)
a
PAHs
samples
Table 2. Results of Determination and Recoveries of Natural Water Samples Spiked with 60 ng/L of PAHs
360.00
1177.08
395.01
178.39
9.16
18.26
10.31
7.75
12.10
9.61
10.20
16.66
11.98
12.64
3.32
2232.47
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
13.74
19.55
2.39
0.96
0.16
0.15
0.14
0.02
0.08
0.02
0.01
0.01
0.03
0.57
0.29
38.12
b
62.64
67.06
80.79
95.19
72.52
85.47
88.25
77.83
76.89
69.88
75.21
72.92
82.17
78.29
98.42
78.90
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
16.95
15.25
11.44
5.05
1.05
3.35
2.45
3.59
3.24
3.30
3.46
2.87
4.32
3.79
9.80
9.90c
recovery (%)
wastewater
detected (ng/L)
a
128.6
65.01
69.75
30.41
2.40
2.69
0.06
0.75
1.15
0.15
0.48
0.41
0.47
1.98
1.23
305.54
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
14.00
3.21
4.46
2.19
0.12
0.17
0.08
0.05
0.03
0.01
0.01
0.00
0.23
0.29
0.22
25.07
19.40
47.71
56.89
63.60
66.55
63.65
63.25
63.50
62.94
63.95
62.34
61.02
65.60
63.57
64.50
59.23
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
1.71
3.55
8.04
0.19
1.91
0.04
6.06
3.62
2.11
2.58
2.97
5.34
2.89
3.26
2.21
3.10c
recoveryb(%)
tap water
detected (ng/L)
a
Analytical Chemistry
Article
7673
DOI: 10.1021/acs.analchem.5b01077
Anal. Chem. 2015, 87, 7667−7675
Analytical Chemistry
■
DOM. The relatively greater concentrations of Ca2+ and Mg2+
in tap water reduced adsorption of PAHs due to electrostatic
binding to Fe3O4 MNPs, which are competitive for CTAB, and
could prevent formation of mixed hemimicelles.55 DOM in
river water and wastewater reduced effects of Ca2+ and Mg2+ on
performance of Fe3O4 MNPs due to potential complexation of
the metal ions, which resulted in greater recoveries of PAHs.56
Some polymer micromolecules (PAHs-DOM-CTAB-Fe3O4)
might have formed due to the hydrophobic interaction of
DOM, CTAB, and Fe3O4 MNPs.50 Therefore, Fe3O4−CTAB
MNPs performed better in extracting stable hydrophobic
organic pollutions. Because of the negative and positive effects
of both DOM and metal ions, it is suggested that the most
accurate and reproducible method employing Fe3O4−CTAB
MNPs would be the use of internal standards using masslabeled PAHs.
CONCLUSIONS
The Fe3O4−CTAB MNPs, used in the present study, had
several advantages for extraction of PAHs from water. First,
their relatively large specific surface area provided more
adsorption sites for PAHs and their superparamagnetism also
benefited from rapid separation. Second, separation of PAHs
was completed within 5 min, and the preconcentration process
was convenient, which greatly shortened the duration required
for maximal extraction. Third, relatively small amounts of
organic solvents were needed, which avoided wasting of
solvents. Fourth, the Fe3O4−CTAB MNPs can easily be
synthesized with several low-cost chemicals, which might be
more suitable to industrialization for the determination of trace
organic pollutants. Last, the lesser biotoxicity of Fe3O4 MNPs
and CTAB might potentially reduce pollution of the environment, compared with other nanomaterials. In conclusion,
Fe3O4−CTAB MNPs as a solid adsorbent combined with
UPLC-FLD presented excellent performance in analyzing trace
PAHs in water.
ASSOCIATED CONTENT
S Supporting Information
*
Selection and additive amount of surfactants, selection of
sample volume and collected parameters of preconcentration
techniques for PAHs from water, Tables S1 and S2, and Figures
S1−S3. The Supporting Information is available free of charge
on the ACS Publications website at DOI: 10.1021/
acs.analchem.5b01077.
■
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Article
AUTHOR INFORMATION
Corresponding Authors
*E-mail: zhaoxiaoli_zxl@126.com. Phone: (+86)10-84931804.
Fax: (+86)10-84931804.
*E-mail: mengwei@craes.org.cn. Phone: (+86) 10-84915193.
Fax (+86) 10-84915194.
Author Contributions
⊥
H.W. and X.Z. contributed equally to this work
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
The research was supported by the National Natural Science
Foundation of China (Grants 41222026, 41130743, and
21007063).
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7675
DOI: 10.1021/acs.analchem.5b01077
Anal. Chem. 2015, 87, 7667−7675
1
Cetyltrimethylammonium Bromide-coated Fe3O4 Magnetic
2
Nanoparticles for Rapid Analysis of 15 Trace Polycyclic Aromatic
3
Hydrocarbons in Aquatic Environments by UPLC-FLD
4
5
6
Hao Wang,†,‡, Xiaoli Zhao,†,‡,* Wei Meng,‡,* Peifang Wang,§ Fengchang Wu,‡ Zhi Tang,‡ Xuejiao
7
Han,‡ John P. Giesy£
8
‡
9
Academy of Environmental Sciences, Beijing 100012, China;
#
State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research
10
#
11
§
12
of Education, College of Environment, Hohai University, Nanjing 210098, China;
13
£
14
Saskatchewan, 44 Campus Drive, Saskatoon, SK, Canada.
College of Water Sciences, Beijing Normal University, Beijing 100875, China;
Key Laboratory of Integrated Regulation and Resource Development on Shallow Lakes, Ministry
Department of Veterinary Biomedical Science and Toxicology Centre, University of
15
16
*Corresponding Authors: zhaoxiaoli_zxl@126.com, mengwei@craes.org.cn
17
Tel.: (+86)10-84931804; Fax: (+86)10-84931804;
18
Author Contributions: †These authors contributed equally to this work
19
This supporting information contains 2 Tables and 3 Figures. This document contains 10 pages,
20
including this cover page.
S-1
21
■ EXPRIMENTS
22
Ultra Performance Liquid Chromatography Tandem Fluorescence Detector. Samples were
23
analyzed at 45 °C. Mobile phases WERE ACN and ultrapure water (UP Water) at a flow rate of
24
0.4 mL/min, with an injection volume of 2 μL. Calculations for quantification of PAHs were
25
accomplished by use of Waters Power 2.0 software. Limits of detection for 15 PAHs were
26
calculated with 3 times the ratio of signal-noise (S/N). PAHs were quantified by use of an external
27
standard curve with coefficient of determination (r2) are all > 0.78 with a linear working range of 0,
28
0.1, 1, 10, 100, 200 and 400 ng/L.
29
■ RESULTS AND DISCUSSION
30
Selection of type and Amounts of Surfactants. CTAB and CPC were chosen as surface
31
modifiers for Fe3O4 MNPs. CPC contains a pyridine moiety, which can contribute to adsorption of
32
PAHs by π-π interaction of the aromatic portions of the two molecules. However, Greater
33
recoveries of PAHs were obtained by using CTAB than CPC (Figure S1a). Fluorescence
34
characteristics of pyridine interfered with quantification of some PAHs, such as Bkf and Bap. For
35
these reasons, CTAB was selected as the more appropriate surface amendment for Fe3O4 MNPs.
36
Recoveries of PAHs were directly proportional to the amount of CTAB added, with a
37
maximum recovery at 50 mg CTAB in the presence of 100 mg Fe3O4 MNPs (Figure S1b).
38
Addition of CTAB to Fe3O4 MNPs at pH 10.0, resulted in CTAB on surfaces of nanoparticles
39
attaching through their cationic ends by chemical self-assembly to form mixed hemi-micelles,
40
which resulted in greater concentrations of PAHs adsorbing to MNPs. However, when the amount
41
of CTAB was greater than 50 mg, hydrophobic interaction between the hydrophobic tails of CTAB
42
molecules occurred, instead of interactions between CTAB and PAHs. This then decreased
S-2
43
enrichment of PAHs from water by iron oxide nanoparticles. Therefore, to optimize subsequent
44
experiments CTAB was employed as the surface modifier at a ratio 1:2 (w/w) of CTAB and Fe3O4
45
MNPs.
46
Selection of Sample Volume. Breakthrough volume is a major parameter in preconcentration
47
of samples by use of SPE. A range of volume of samples ranging from 200 to 2000 mL was tested.
48
Recoveries of PAHs were inversely proportional to volume of the sample extracted by a fixed
49
amount of adsorbent. Due to their greater solubilities in water, the effect was greater for Nap, Ace,
50
and Flo. An optimal sample volume of 800 mL was selected for further studies (Figure S2).
51
Comparison between C18 cartridges and Fe3O4 MNPs. In this study, two kinds of commonly
52
used C18 cartridges, Supelclean LC-18 (PA., USA) and Waters Sep-Pak Vac C18 (Massachusetts,
53
USA), were employed to extract PAHs in downstream water of the Qing river (Beijing, China).
54
The experimental procedure of enrichment of PAHs by using both of C18 cartridges were as
55
follows: Samples of water were collected on May 25, 2015, and immediately filtered through glass
56
fiber filters (pore diameter 0.45μm), 800 mL water sample with and without 100 ng/L PAHs were
57
used, and three replicates were conducted for each SPE technique; Cartridges were activated with
58
5 mL methanol and 5 mL ultrapure water orderly before PAHs enrichment process, and then water
59
samples were filtered with a flow velocity of 5 mL/min, and later, 5 mL dichloromethane and 10
60
mL acetone were employed for eluting PAHs from cartridges.
61
Based on experimental results shown in Figure S3, recoveries of PAHs were generally similar
62
for the three methods. However, PAHs extraction process by Fe3O4 MNPs was complete in 0.5
63
hour, while more than 12 hours was spent by two kinds of C18 cartridges, even than all samples of
64
water had been filtered by 0.45 μm glass fiber filters. By comparing among methods of
S-3
65
pretreatment given in the literature and the method developed during this study (Table S1), the
66
advantages of Fe3O4 MNPs was demonstrated.
S-4
67
68
69
Table S1. Parameters for pre-concentration techniques for PAHs from water
Method
PAHs
LLE
SPE
SPE
SPE
SPE
SPE
SPE
SPE
SPE
SPE
SPE
SPME
SPME
SPME
SPE
SPE
16
16
15
16
12
10
10
16
16
8
15
12
16
5
15
15
adsorbent
sample volume
(mL)
Volume of consumed
Organic solvent (mL)
Time
(min)
the detective
limit (ng/L)
analyzer
hexane
C18 column
C18 column
C18 column
C18 column
C18 column
C18 column
C18 disk
nano carbon
C14- Fe3O4 NPs
Stir bar
Fiber
PDMS fiber
C-Fe3O4/C NPs
Fe3O4-CTAB NPs
C18 column7
300
1000
1000
1000
1000
200
1000
2500
500
350
10
25
10
25
800
800
77
10
15
10
90
14
13
115
15
3
0.15
0
0
2
10
15
72
125
333
167
333 –500
30
356
30~45
145
17
60
60
90
25
30
>720
0.033 – 0.13
1–5
NP
1
0.01 – 5.00
0.5 –25.0
10.0 – 166.9
0.01 – 11.3
2.0 – 8.5
0.1 – 0.25
0.2 – 2.0
0.03
30 – 590
0.7 – 49.6
0.13 –20.6
–
HPLC1-FLD
GC2-MS3
HPLC-FLD
GC-FID4
HPLC-UV5
GC-MS
HPLC-UV
HPLC-UV-FLD
GC-MS
HPLC-FLD
HPLC-FLD
GC-SIM6-MS
GC-MS
HPLC-FLD
UPLC-FLD
UPLC-FLD
reference
(S1)
(S2)
(S3)
(S4)
(S5)
(S6)
(S7)
(S8)
(S9)
(S10)
(S11)
(S12)
(S13)
(S14)
Our study
Our study
NP: No Reported; 1: high-performance liquid chromatography; 2: Gas Chromatography; 3: Mass Spectrometer; 4: Flame ionization detector; 5: ultraviolet detector;
6: selected ion monitoring; 7: the detail were presented in Figure S3
S-5
70
Table S2. Parameters of field samples.
Sample type
TOC (mg/L)
Ca2+ (mg/L)
Mg2+ (mg/L)
Sewage water
23.22±0.62
117±1.4
55±1.4
upstream water
8.41±0.32
67±1.4
19±2.8
downstream water
3.58±0.44
95.00±4.2
41±4.2
67±4.2
27±4.2
3±1.4
1±1.4
tap water
ND
rainwater
ND
a
a
: no detected
S-6
a
b
70
60
80
50
Recovery (%)
average recovery (%)
Nap
Ace
Flo
Phe
Ant
Fla
Pyr
Chr
Baa
Bbf
Bkf
Bap
DahA
IcdP
BghiP
100
CPC
CTAB
40
30
60
40
20
20
0
10
0
20
40
60
80
100
120
0
Surfactants concentration (mg/100 mg Fe3O4 MNPs)
20
40
60
80
100
120
140
160
Concentration of CTAB per 100 mg Fe3O4 MNPs
71
72
73
Figure S1. Recoveries of 15 PAHs as functions of several surfactants (a) and additive amount of CTAB (b), during batch mode. Amount of metal
oxide: 100 mg of Fe3O4 MNPs. pH:10.0, sample volume: 800 mL. Volume of ACN: 10 mL.
S-7
Nap
Ace
Flo
Phe
Ant
Fla
Pyr
Chr
Baa
Bbf
Bkf
Bap
DahA
IcdP
BghiP
100
Recovery (%)
80
60
40
20
0
400
800
1200
1600
2000
Volume of Samples (mL)
74
75
Figure S2. Effects of sample volume on recoveries of PAHs
S-8
Supelclean LC-18 cartridge
Waters Sep-Pak Vac C18 cartridge
Fe3O4 MNPs
100
Recovery (%)
80
60
40
20
76
77
78
Fl
o
Ph
e
An
t
Fl
a
Py
r
Ch
r
Ba
a
Bb
f
Bk
f
Ba
p
Da
hA
Ic
dP
Bg
hi
P
Na
p
Ac
e
0
PAHs
Figure S3. The recoveries of PAHs in river water sample by Supelclean LC-18
Cartridge, Waters Sep-Pak Vac C18 Cartridge and Fe3O4 MNPs
S-9
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
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