2. C as sorption domains for HOCs, Chefetz and Xing2
collected
a large and diverse set of published data on Phen Koc values,
aromaticity and aliphaticity of organic sorbents covering natural
and engineered sorbents. They found that when a large data set
was plotted, no specific correlation was presented between
Phen Koc values and aromaticity of natural sorbents, including
humic substances from different sources, biopolymers (such as
cellulose, chitin, lignin, cutin, and cutan), diagenesized samples
like kerogen, and biological samples such as algae, cuticles, and
leaves. Interestingly, when the data for engineered sorbents was
added to this data set, a general trend of increasing Koc with
increasing aromaticity was recorded although a significant linear
relationship between them was not obtained. Conversely, only
for natural sorbents, a general trend of increasing phanthrene
Koc values with increasing aliphaticity was displayed. If data for
engineered sorbents was included, no relationship was exhibited
between binding coefficients and aliphaticity.
The contribution of pore-filling mechanism to the sorption
of HOCs by SOM has been previously identified.21−23
Especially, it has been showed that the nanopore-filling is the
dominant mechanism for sorption of Phen and benzene by
NHC and coals.20
It has been mentioned above that a general
trend of increasing Phen Koc values with increasing aliphaticity
of only natural sorbents and a similar trend between Phen Koc
values and aromaticity of engineered samples were reported by
Chefetz and Xing.2
If pore-filling mechanism governs the
sorption of HOCs by sorbents, it is very reasonable to
hypothesized that microporosity of natural sorbents and
engineered sorbents should, respectively, be derived from
their aliphatic and aromatic moieties. However, how the
structure and microporosity of natural and engineered sorbents
are related to sorption of HOCs is not well understood.
One chemical degradation technique, referred to as
“bleaching”, has been previously employed to selectively
remove noncondensed aromatic moieties such as lignin-like
and polyphenols units in SOM, and simultaneously retain char-
derived aromatic C.24
Based on our above hypotheses,
bleaching treatment would influence nanopore characteristics,
in turn, affect sorption properties of natural and engineered
sorbents in a different pattern. Thus, this technique would aid
to test our hypothesis.
The major works of this study were therefore to (1) remove
the aromatic components of natural and engineered sorbents by
bleaching treatment; (2) determine the nanopore properties of
original natural and engineered samples (OR) as well as their
corresponding bleached samples (BL) using CO2 isotherms at
273 K; (3) obtain the aliphatic and aromatic C characteristics of
these OR and BL using cross-polarization magic angle spinning
C-13 nuclear magnetic resonance (CPMAS 13
C NMR); (4)
quantify the sorption affinity of HOCs to these OR and BL. In
this study, natural organic matter fractions (NOM), including
HA, HM, and NHC, were selected as natural sorbents; biochars
produced from rice straw and pine wood were used as
engineered samples.
■ MATERIALS AND METHODS
Sorbate and Sorbents. Phen was used as a sorbate and
purchased from Sigma-Aldrich Chemical Co. One river
sediment sample (bulk 1) was collected using a stainless steel
grab sampler in July 2008 from a river in the Tongzhou district
of Beijing.25
Three soil samples (bulk 5, bulk 7, and bulk 8)
were also collected to a depth of 20 cm in July 2007 from the
surface soils in the vicinity area of Tianjin near Bohai Bay,
China.25
Albic (A) and black (B) soils were sampled from
Sanjiang Plain, Heilongjiang province, China.26
The collected
samples were subjected to a series of treatment to obtain
different organic matter fractions including HA, HM, and
NHC, whose extraction along with their purification and
homogenization methods were described elsewhere.25,26
Briefly,
HA1 fraction was obtained from mixing extractions with 0.1 M
Na4P2O7 seven times.27
The soil residue after HAs extraction
was demineralized with 1 M HCl and 10% (v/v) HF at 1:5
solid/liquid ratio and shaking at 40 °C for 5 days continuously.
Finally the supernatant was removed by centrifugation at 4500
rpm for 30 min. The same treatment was repeated for six times
in order to get HM fraction containing adequate amount of
organic carbon (OC) and low mineral content. NHC fraction
was extracted from the whole soil using a HCl/HF/trifluoro-
acetic acid (TFA) method described elsewhere.19,28
Black
carbon (BC) in this study was obtained by heating an aliquot of
the NHC sample at 375 °C for 24 h with sufficient air.29
The
six biochars were produced from two kinds of feedstock
materials, rice straw, and pine wood, respectively. After washing
and grinding to obtain a particle size of less than 1.5 mm, these
feedstocks were charred at 300, 450, and 600 °C, respectively,
for 1 h in a closed container under oxygen-limited conditions in
a muffle furnace. Then the biochars were washed with 0.1 M
HCl followed by deionized (DI) water flushing until neutral
pH,30
subsequently oven-dried at 105 °C, and gently milled to
pass a 0.25 mm sieve (60 mesh) prior to further analysis. These
biochar samples were hereafter abbreviated and referred as to
their individual two initial capitals of feedstock source (rice
straw and pine wood) (i.e., RI and PI) and heat treatment
temperatures (HTT) (300, 450, and 600 °C) (i.e., RI300,
RI450, RI600, PI300, PI450, and PI600)
The details of bleaching procedures were described else-
where.24
Briefly, bleaching involved treating 10 g of each
sorbent (HA1, NHC1, NHC5, NHC7, NHC8, A-HM, A-NHC,
B-HM, B-NHC, RI300, RI450, RI600, PI300, PI450, and
PI600) three times with 100 g of sodium chlorite (NaClO2),
100 mL of acetic acid (CH3COOH), and 1000 mL of DI water
for 7 h for each time. All BL were freeze-dried, ground, and
stored for their characterization and sorption work.
Sorbent Characterization. The C, H, N, and O contents
of all samples were measured using an Elementar Vario ELIII
elemental analyzer (Germany). Solid-state cross-polarization
magic-angle-spinning 13
C NMR spectroscopy analysis was
performed on a Bruker Avance 300 NMR spectrometer
(Karlsruhe, Germany) operated at 13
C frequency of 75 MHz
to get structural information on all studied samples. The NMR
running parameters are available in the Supporting Information
(SI) and the chemical shift assignments are depicted else-
where.31
Surface area (CO2−SA) was calculated using nonlocal
density functional theory (NLDFT) and grand canonical
Monte Carlo simulation (GCMC) using CO2 isotherms at
273 K (Quantachrome Instrument Corp, Boynton Beach, FL)
(SI Figure S1) because previous studies show that N2 at 77 K
was unable to detect BC microporosity while CO2 at 273 K can
enter the micropores (0−1.4 nm).26,32
Sorption Experiment. All sorption isotherms were
obtained using a batch equilibration technique at 23 ± 1 °C.
Appropriate amount of investigated samples (0.1−8.0 mg) were
added to the background solution containing 0.01 M CaCl2 in
DI water with 200 mg/L NaN3 to minimize biodegradation.
The amount of sorbents was controlled to result in 20−80%
uptake of initially added Phen. The initial aqueous-phase Phen
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3. concentrations (C0, 2−1000 μg/L), which was chosen to cover
the range between detection limit and aqueous solubility (1.12
mg/L), were added into the vials and shaken for 10 days.
Preliminary tests showed that the apparent sorption equili-
brium was reached before 10 days. The blanks consisted of
Phen solution without sorbents. Headspace was kept minimal
to reduce solute vapor loss. After being shaken on the rotary
shaker for 10 days, all vials were placed upright for 24 h.30
The
supernatant was then withdrawn from each vial and was
transferred to a 2 mL vial for analyzing solution-phase sorbate
concentration with HPLC (HP model 1100, reversed phase
C18, 15 cm × 4.6 mm × 4.6 μm, Supelco, PA) with a diode
array detector for concentrations ranging from 2 to 1000 μg/L
and a fluorescence detector for concentrations from approx-
imately 0.2 to 50 μg/L.27
Isocratic elution was used at a flow
rate of 0.8 mL/min with a mobile phase: 90:10 (v:v) of
methanol and DI water. All samples, along with blanks, were
measured in duplicate.
Data Analysis. The sorption data were fitted to the
logarithmic form of Freundlich isotherm model:
= +q K n CLog log loge F e (1)
where qe [μg/g] is the equilibrium sorbed concentration; Ce
[μg/L] is the equilibrium aqueous concentration; KF [(μg/g)/
(μg/L)n
] is the Freundlich affinity coefficient; and parameter n
is the Freundlich exponential coefficient. The investigated
correlations among properties of sorbents as well as their
sorption coefficients of Phen (Pearson correlation coefficients:
r, and significant level: p) were obtained from the Pearson
correlation analysis by SPSS 16.0 software (SPSS Inc.).
■ RESULTS AND DISCUSSION
Characteristics of NOM Fractions and Biochars. The
elemental composition, atomic ratio, ash content, and surface
area of original and bleached samples (NOM fractions and
biochars) are shown in Table 1. The appreciable differences in
bulk compositions among various original NOM fractions
revealed their heterogeneous structures. Moreover, obviously
different chemical compositions detected in NHCs from
different soil/sediment sources (Table 1) were consistent
with the previous literature which postulated that the
Table 1. Yields by Bleaching Treatment, Elemental Compositions and Surface Area Analysis of NOM Fractions and Biochars
mass OC
recovery recovery C H N O (O+N) CO2−SA CO2−SA/OC Ash
samples (%)a
(%)b
(%) (%) (%) (%) /C (m2
/g) (m2
/g) (%)
NOM Fractions (Natural Sorbents)
HA1 54.2 4.0 2.9 26.5 0.41 24.9 45.9 12.4
NHC1 22.4 2.3 1.0 3.6 0.15 57.0 254.7 70.7
NHC5 15.7 1.2 1.2 6.7 0.39 44.4 282.8 75.1
NHC7 21.4 1.5 1.0 4.8 0.21 40.7 190.4 71.3
NHC8 12.1 1.6 0.9 4.4 0.13 9.5 78.1 81.1
A-NHC 42.2 4.3 0.7 19.2 0.36 31.4 74.4 33.6
A-HM 20.5 2.6 1.0 19.8 0.77 42.5 207.3 56.1
B-NHC 50.8 4.0 1.2 25.7 0.40 100.2 197.2 18.3
B-HM 19.3 2.4 1.3 19.9 0.83 61.0 316.1 57.1
HA1-BL 25.3 9.3 19.9 2.1 1.1 17.7 0.70 17.1 85.8 59.2
NHC1-BL 42.5 52.2 27.5 3.3 0.4 16.4 0.46 45.0 163.4 52.4
NHC5-BL 18.3 27.3 23.4 1.9 0.8 16.7 0.56 36.2 154.5 57.2
NHC7-BL 51.6 49.2 20.4 2.3 0.2 15.5 0.58 25.6 125.6 61.6
NHC8-BL 43.2 26.4 7.4 0.9 0.2 13.4 1.38 15.0 203.7 78.2
A-NHC-BL 29.9 19.1 27.0 2.1 0.6 15.0 0.43 8.0 29.7 55.3
A-HM-BL 30.2 41.7 28.3 2.9 1.7 20.0 0.57 27.1 95.9 47.1
B-NHC-BL 20.5 17.2 42.6 3.1 0.9 18.5 0.34 175.9 413.1 34.9
B-HM-BL 71.8 43.9 11.8 1.8 1.0 16.8 1.13 33.4 282.2 68.5
Biochars (Engineered Sorbents)
RI300 53.2 3.9 1.1 24.2 0.36 188.5 354.3 17.6
RI450 57.0 2.6 1.2 15.6 0.22 293.4 514.5 23.6
RI600 60.4 1.7 1.1 8.9 0.13 390.6 647.1 27.9
PI300 64.7 4.8 0.0 28.6 0.33 155.0 239.6 1.9
PI450 73.1 2.8 0.1 20.1 0.21 408.1 558.3 3.9
PI600 81.4 2.3 0.1 11.7 0.11 544.6 668.7 4.4
RI300-BL 24.4 12.3 26.9 3.0 0.4 24.9 0.70 85.2 316.3 44.9
RI450-BL 52.6 36.2 39.2 2.2 0.7 27.8 0.55 130.7 333.5 30.1
RI600-BL 66.1 54.9 50.2 1.6 0.8 19.6 0.31 257.2 512.8 27.8
PI300-BL 12.4 8.3 43.1 5.0 0.1 45.4 0.79 16.6 38.6 6.4
PI450-BL 57.1 40.3 51.6 2.4 0.0 36.6 0.53 110.9 215.0 9.4
PI600-BL 69.2 55.4 65.1 2.2 0.0 25.8 0.30 402.1 617.5 6.8
a
Mass Recovery (%) = M(BL)/M(OR) × 100, b
OC Recovery (%) = OC(BL) × M(BL)/[ OC(OR) × M(OR)] × 100, where M is the weight of original or
bleached sample (HA, HM, NHC, biochars); Mass Recovery denotes the bleaching treatment yields humic acids (HA), humins (HM),
nonhydrolyzable carbons (NHC), pine wood (PI), rice straw (RI), Original samples:OR; Bleached samples: BL.
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4. physicochemical nature of SOM can vary greatly as a function
of the origin, age, weathering, maturation, and soil depth.11,33,34
As for biochars, with increasing HTT, C content increased,
while H and O contents as well as bulk polarity decreased as
reported elsewhere (Table 1).35
The removal of aromatic C by
bleaching greatly altered bulk composition of all samples,
including natural and engineered sorbents (Figure 1 and Table
1). From the OC recovery (%) of the tested samples after
bleaching (Table 1 and Figure 1a), most of the C of HA was
removed because its recovery of OC was very low (9.3%),
suggesting that the HA contained small amounts of BC, which
is resistant to bleaching. Additionally, the OC recovery of
biochars reduced with the increasing HTT (Figure 1b),
indicating that the high-temperature biochars contained more
resistant C compared to the low-temperature biochars. After
bleaching, the C content of investigated samples generally
declined except for three NOM fractions covering NHC1,
NHC5, and A-HM (Table 1), which had high abundance of ash
contents (>55%). The ash contents of these three fractions
consistently decreased (Table 1), indicating that the increase of
bulk C contents in these three samples after bleaching could be
partly explained by the fact that NaClO2 used in bleaching
treatment can remove a portion of minerals under acidic
conditions.24
Furthermore, such a treatment led to the general
increase in the polarity (e.g., (N+O)/C) except for A-HM and
B-NHC (Figure 1c and d) as a portion of aromatic C and their
functional groups had been oxidized during the treatment,
suggesting that a fraction of hydrophobic aromatic components
was successfully removed.
The 13
C NMR spectra also illustrated that bleaching caused
structural modification (SI Figure S2 and Table S1). According
to the distribution of C functional groups, the reduction of the
relative content of aromatic C was noted in both NOM
fractions and biochars after bleaching (SI Table S1). Among
them, a regular alternation was observed in aromatic C content
of biochars after bleaching that the decreased content of
aromatic C after bleaching declined with the increasing of HTT
(SI Table S1 and Figure 2b), indicating that more condensed
aromatic C of the biochars produced at high HTT possibly is
more difficult to be bleached compared to the biochars at low
HTTs. As a result of the reduction of the aromatic C, the
relative intensity of aliphatic C (0−108 ppm) was enhanced
(Figure 2c and d). Nonetheless, it should be mentioned that
most bleached samples still contained a considerable portion of
aromatic C (SI Table S1). For instance, NHC1 still had 25.8%
of aromatic C after bleaching, which could be attributed to that
large percentages of aromatic moieties of the tested samples
were resistant to bleaching. The percentage of the remaining
aromatic C after bleaching to the total aromatic C of their
untreated counterparts was further calculated (SI Table S1). It
was found that regarding NOM fractions, the contribution of
bleaching-resistant aromatic C accounted for 6.8%, 25.8−
40.6%, and 10.3−49.9% to the total aromatic C of HA, HM,
and NHC fractions, respectively. Chefetz et al.24
demonstrated
clearly that bleaching, in the case of aromatic substrates, is
effective for decomposing noncondensed aromatic structures
such as lignin-like and polyphenols units detected in HAs, while
condensed moieties were not susceptible to be bleached. They
also proposed that the residual aromatic C after bleaching likely
originated from charcoal and/or charred plant materials,
collectively referred to as BC. Moreover, as shown in SI
Figure 1. Organic carbon (OC) recovery % (a and b) of natural
organic matter (NOM) fractions (left) and biochars (right) after
bleaching; comparison of bulk polarity (c and d), CO2-derivded
calculative surface area (CO2−SA) (e and f) and OC-normalized
CO2−SA (CO2−SA/OC) (g and h) between NOM fractions (left) or
biochars (right) and their corresponding bleached fractions.
Figure 2. Comparison of aromatic C (a and b) and aliphatic C (c and
d) between natural organic matter (NOM) fractions (left) or biochars
(right) and their corresponding bleached fractions.
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5. Table S2, the contribution of BC obtained from combustion at
375 °C of each NHC sample represented more than 10% to the
NHC fractions except of BC5 (7.6%). Therefore, it could be
concluded that the NOM fractions contained a certain amount
of BC, which could be also supported by ubiquitous occurrence
of BC in soils/sediments: median BC contents as a fraction of
total OC are 4% for 90 soils, 9% for 300 sediments and are up
to 30−45% in fire-impacted soils.36
The Relationship between Micropore Properties of
NOM Fractions and Biochars and Their Aromatic and
Aliphatic C. The microporosity and surface characteristics of
organic sorbents in soils/sediments are pivotal for the
mechanistic evaluation of sorption. It has been shown that
the traditionally recommended N2 sorption techniques would
underestimate the SA of OM with pores less than 0.5 nm.37−40
Since CO2 at 273 K can enter the micropores (0−1.4 nm),41
the application of CO2−SA helps us to gain a better insight into
nanoporosity and SA of SOM. The CO2−SA of the natural
sorbents ranged from 9.5 to 100.2 m2
/g and CO2−SA values of
the tested biochars was in the range of 155.0−544.6 m2
/g,
which was comparable to the CO2−SA of a temperature series
of wood biochars reported recently.40
Obviously, biochars
exhibit higher CO2−SA than natural sorbents (Table 1). The
CO2−SA values of the NOM fractions obtained in this study
were lower than those of the eight American Argonne Premium
coals (113−225 m2
/g)42−44
and comparable or lower than the
CO2−SA values of SOM and coals reported by Ran et al.44
In
this study, the CO2−SA of all samples generally decreased after
bleaching, except for NHC8, B-NHC (Figure 1e and f). It was
reported that CO2−SA of biochars is positively correlated with
their OC contents30
and the similar linear correlations were
also observed for NOM fractions in other investiga-
tion,39,41,44,45
which was consistent with our data (Figure 3a).
This suggests that OC is very likely a major contributor to
CO2−SA of sorbents. Therefore, to better compare the impact
of the removal of aromatic C on the SA of samples, OC-
normalized CO2−SA (CO2−SA/OC) was employed instead of
CO2−SA. The range of CO2−SA/OC values of the NOM
fractions and biochars investigated in this study was 45.9−316.1
m2
/g and 239.6−668.7 m2
/g (Table 1), respectively, suggesting
that besides C content of sorbents, other properties of OM
within these investigated sorbents, such as chemical composi-
tions, molecular structure, configuration and maturation as well
as geochemical alteration, should exert an influence on the
microporosity and SA. It was noted that the CO2−SA/OC
values of HA, NHC8, and A-NHC were less than 100 m2
/g
(Table 1), which is different from the previous results that the
range of CO2−SA/OC values (113.3−610.5 m2
/g) for a wide
range of NOM fractions and their average CO2−SA/OC is 185
m2
/g.39,41,44,45
As presented in Figure 1g and h, the bleaching
treatment, to a dissimilar extent, exerted an influence on CO2−
SA/OC of NOM fractions and biochars. With respect to NOM
fractions, CO2−SA/OC of six samples (NHC1, NHC5, NHC7,
A-NHC, A-HM, and B-HM) decreased after treatment,
whereas that of 1HA, NHC8, and B-NHC increased; in
contrast, CO2−SA/OC of biochars consistently declined after
the removal of aromatic C, implying that the micropores of
engineered sorbents were probably derived from aromatic
matrix, while those of natural sorbents were not necessarily
derived from aromatic moieties. In order to further examine the
molecular structure of NOM and its relationship with the
micropores of OM within natural and engineered sorbents, the
correlations between CO2−SA/OC and the contents of
functional groups as indicated by 13
C NMR were conducted
(Figure 4 and SI Figure S3). It was noted that CO2−SA/OC
values of both original and bleached biochars were significantly
and positively correlated with their aromaticity (Figure 4a) and
negative relationships between CO2−SA/OC values of biochars
and their aliphaticity were also detected (Figure 4b), providing
the robust evidence to support that nanopores of engineered
sorbents were majorly contributed by their aromatic moieties.
On the other hand, no specific correlations were obtained
between aromaticity as well as aliphaticity of only original
NOM fractions and their CO2−SA/OC values (SI Figure S3a
Figure 3. Correlations between CO2-derivded calculative surface area
(CO2−SA) of original and bleached natural organic matter (NOM)
fractions and biochars and their bulk C content (a); correlations
between logKoc values (mL/g) of Phen by original and bleached NOM
fractions and biochars and their organic carbon (OC)-normalized
CO2−SA (CO2−SA/OC) (b); correlations between logKoc values
(mL/g) of Phen by original and bleached biochars and their
aromaticity (c) and aliphaticity (d).
Figure 4. Correlations between CO2−SA/OC of original and bleached
biochars and their aromaticity (a) or aliphaticity (b); correlations
between calibrated organic carbon (OC)-normalized calculative
surface area (SA) (CO2−SA/OC) of original natural organic matter
(NOM) fractions excluding HA1, A-NHC, and NHC8 and their
calibrated aliphaticity (c) or calibrated aromaticity (d).
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6. and b). However, it was interesting to find that when the data
for bleached NOM samples, whose aromatic C was mainly
derived from BC, were added to the data set of SI Figure S3a
and b, the general trend of CO2−SA/OC values with
aromaticity or aliphaticity was changed (SI Figure S3c and
d). Although this change was not very remarkable, it was
assumed to be of significance since it seems to indicate that
aromatic C of BC which coexists with natural sorbents would,
to some degree, affect the relationship between aromaticity or
aliphaticity of natural samples and their CO2−SA/OC.
Consequently, to eliminate the effect of BC-derived aromatic
C as much as possible, the contents of both natural aromatic
and aliphatic C of original NOM fractions were obtained by
deducting contribution of bleaching-resistant aromatic C to the
total OC of each NOM fraction. Additionally, CO2−SA/OC
values of original natural samples were also calibrated by
deducting the contribution of CO2−SA/OC of bleached
counterparts. The calibrated aromaticity, aliphaticity, and
CO2−SA/OC values are listed in SI Table S3. Interestingly,
the calibrated CO2−SA/OC values of natural sorbents were
closely related to the calibrated aliphaticity, but negatively
related to the calibrated aromaticity excluding HA, A-NHC, and
NHC8 because their abnormally low CO2−SA/OC values
(Figure 4c and d). The above findings not only suggest that
aromatic moieties of BC, which coexists with NOM, could
affect the structure and microporosity but also demonstrate that
the microporosity of NOM was closely associated with their
aliphatic matrix, as we hypothesized.
The Role of Nanopores, Aromatic and Aliphatic C in
Sorption of Phen by Both NOM Fractions and Biochars.
The Freundlich isotherms are shown in SI Figure S4 and S5,
and the fitting parameters are listed in SI Table S4. The
sorption isotherms of Phen by original NOM and biochars
were nonlinear with n values being in the range of 0.50−0.89
and 0.38−0.71, respectively, and well fitted with the Freundlich
model (SI Table S4). The isotherms for biochars were all highly
nonlinear (n < 0.71), similar results were reported by Lattao et
al.,40
reflecting the predominance of adsorption/pore-filling
mechanisms. The removal of certain aromatic moieties by
bleaching resulted in the rise of n values as compared to that of
the untreated samples except for RI600. Especially for NOM,
the bleached samples nearly exhibited a linear and partition-
type sorption behavior (SI Table S4), which implies that a
more expanded sorbent was produced due to the removal of
aromatic moieties and also supports that aromatic moieties
should be the predominant components responsible for
nonlinear sorption process as reviewed by Chefetz and Xing.2
Bleaching exercised a great effect upon Koc (OC content-
normalized sorption coefficient) (SI Table S4). Except for
NHC8, the Koc of NOM fractions and biochars all decreased
compared with their untreated samples (SI Table S4), which
was similar to the results presented by Huang et al.46
Additionally, after removal of aromatic moieties, bulk polarity
(e.g., (O+N)/C) of the NOM fractrions and biochars generally
increased, which may be responsible for their decreasing Phen
Koc values because the polarity of SOMs can significantly affect
sorption capacity of HOCs and the SOMs with relatively low
polarity show the higher sorption capacity than those with high
ploarity.27,47,48
The significant and negative correlation of
logKoc values of Phen by the original and bleached biochars to
their bulk polarity (e.g., (N+O)/C) (SI Figure S6) supports
our hypothesis. However, recently, Lattao et al.40
found that no
simple relationship stands out between logKoc values and O/C
ratio, surface area (N2 and CO2), and porosity and they
demonstrated that sorption is a complex function of biochar
properties and solute molecular structure, and not very
predictable on the basis of readily determined char properties.
It has been widely documented that pore-filling mechanism
plays a key role in HOCs sorption by microporous solids of
SOM.21,22
For example, Ran et al.21
reported that sorption
behaviors of Phen and dichlorobenzene (DCB) by kerogen
were satisfactorily explained by hole-filling mechanism. Like
these studies, the significantly positive correlation between Koc
values of Phen by all original and bleached sorbents and their
CO2−SA/OC obtained in our case (Figure 3b) implied that
pore-filling could be a major mechanism regulating sorption
interactions of HOCs-SOM. Moreover, the slope of the linear
regression line for the NOM fractions was higher than that of
the biochars (Figure 3b), implying that although the biochars
generally have higher CO2−SA per unit mass of their OC than
the NOM fractions (Table 1), the sorption capacity of CO2−
SA per unit mass of OC within NOM fractions could be
remarkably higher than that within the biochars in this study.
Therefore, it can be assumed that the sorption capacity of
sorbents depends on not only their CO2−SA per unit mass of
OC but also on other factors such as the chemical composition,
structure and configuration of the contributor to CO2−SA.
Meanwhile, as we demonstrated before, nanopores of natural
sorbents and biochars were perhaps mainly derived from their
aliphatic and aromatic moieties, respectively. Thus, CO2−SA
associated with the aliphatic moieties within NOM fractions
should have higher sorption capacity compared to the CO2−SA
derived from the aromatic matrix within the biochars. As a
result, we must not think only of how much CO2−SA a sorbent
has, but also of its chemical composition (e.g., aliphatic and
aromatic moieties) to evaluate its sorption capacity for HOCs.
Furthermore, our data showed that the Phen Koc by both
original and bleached biochars was strikingly and positively
related to their aromaticity but negatively correlated to their
aliphaticity (Figure 3c and d). This was exactly the same as the
findings by Chefetz and Xing,2
who observed a general trend of
increasing Phen Koc values with increasing aromaticity of
engineered samples. However, in our work, there was no
significant correlation between Phen Koc of these tested NOM
fractions and their aromaticity or aliphaticity (SI Figure S3e and
f). Similar conclusions were previously reported by Yang et al.49
They performed experiments with sorption of Phen by HA and
HM fractions isolated from a single soil sample and showed
that neither aromatic nor aliphatic components of HAs and
HMs could serve as predictors of the soil’s ability to sorb Phen.
It has been above-mentioned that the aromatic C in NOM
fractions might partly originate from BC materials, which would
interfere in exploring where (aromatic or aliphatic C) the
nanopores of NOM originate from. Additionally, it was noted
that BC appeared particularly higher sorption affinity to Phen
with logKoc (Ce = 0.01Sw) ranging from 5.67 to 6.51 than NHC
because of high CO2−SA/OC (150.0−887.7 m2
/g) resulted by
ubiquitous micropores (SI Table S2). As long as BC materials
enter into soils and sediments, they would therefore influence
the sorption properties of HOCs by NOM and strengthen the
importance of aromatic C of NOM in HOCs sorption by soils
and sediments contaminated by BC, thus, the role of aliphatic
C within NOM in HOCs sorption could be correspondingly
masked. Therefore, we propose that the “pollution” of NOM by
BC materials could, to a large degree, account for no clear
relationship between Phen Koc values by NOM fractions and
Environmental Science & Technology Article
dx.doi.org/10.1021/es5022087 | Environ. Sci. Technol. 2014, 48, 11227−1123411232
7. their aliphaticity, consequently, influence on investigating the
role of aliphatic moieties within NOM fractions.
Environmental Implications. This study demonstrated
that the nanopores of natural (NOM) and engineered sorbents
(biochars) are closely related to their aliphatic and aromatic
matrices, respectively. Significant and positive correlations
between Phen Koc values by the NOM fractions or biochars
and their CO2−SA/OC in this study suggest that nanopore-
filling mechanism plays a dominant role in the sorption of
HOCs by these sorbents, which are found to be microporous
solids. In addition, aliphatic C of the NOM fractions and
aromatic C of the investigated biochars, respectively, were
demonstrated to be key factors affecting their microporosity
and sorption behaviors of HOCs. Moreover, BC is almost
composed of aromatic moieties and is characterized by
structural stability and high sorption capacity. It inevitably
changes the structures of NOM. Hence, the importance of
aliphatic C within NOM in the sorption of HOCs has often
been masked. We used a novel approach by combining
fractionation, bleaching, and 13
C NMR to estimate the effect of
BC. The findings of this work can explain the ongoing debate
on the relative role of aromatic and aliphatic C in the sorption
of HOCs by SOM and uncover that how the aliphatic and
aromatic C within both natural and engineered sorbents play
the role in the sorption of HOCs, which is important for
correctly predicting the fate of HOCs in soils and sediments.
The results described in this study provide important
implications for the interpretation of sorption mechanisms of
organic contaminants in SOM.
■ ASSOCIATED CONTENT
*S Supporting Information
Figure of Carbon dioxide (CO2) adsorption isotherm on the
various NOM factions and biochars, figure of 13
C NMR spectra
of original and bleached NOM fractions and biochars, figure of
correlations between CO2−SA/OC of original NOM fractions
and their aromaticity and aliphaticity, between CO2−SA/OC of
original and bleached NOM fractions and their aromaticity and
aliphaticity as well as between logKoc values of Phen by original
NOM fractions and their aromaticity and aliphaticity, figure of
sorption isotherms of Phen by NOM fractions; figure of
sorption isotherms of Phen by biochars, figure of correlation of
logKoc values of Phen by sorbent to their bulk polarity; table of
Functional Groups from the 13C NMR Spectra, table of
properties of BC obtained from combustion of NHC at 375 °C,
table of the calibrated aromaticity, aliphaticity and CO2−SA/
OC values of NOM fractions, table of Freundlich isotherm
parameters. This material is available free of charge via the
Internet at http://pubs.acs.org.
■ AUTHOR INFORMATION
Corresponding Author
*Phone: 86-10-58807493; fax: 86-10-58807493; e-mail:
sunke@bnu.edu.cn.
Notes
The authors declare no competing financial interest.
■ ACKNOWLEDGMENTS
This research was supported by National Natural Science
Foundation of China (41273106), Beijing Higher Education
Young Elite Teacher Project (YETP0273), and the Scientific
Research Foundation for the Returned Overseas Chinese
Scholars, State Education Ministry.
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