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Effect of aging process on adsorption of diethyl phthalate in soils
amended with bamboo biochar
Xiaokai Zhang
a,b
, Ajit K. Sarmah
c
, Nanthi S. Bolan
d
, Lizhi He
a,b,
⇑
, Xiaoming Lin
e
, Lei Che
f
,
Caixian Tang
a,g
, Hailong Wang
a,e,
⇑
a
Key Laboratory of Soil Contamination Bioremediation of Zhejiang Province, Zhejiang A & F University, Lin’an, Hangzhou, Zhejiang 311300, China
b
School of Environmental and Resource Sciences, Zhejiang A & F University, Lin’an, Hangzhou, Zhejiang 311300, China
c
Department of Civil and Environmental Engineering, Faculty of Engineering, University of Auckland, Private Bag 92019, Auckland 1142, New Zealand
d
Centre for Environmental Risk Assessment and Remediation (CERAR), University of South Australia, South Australia 5095, Australia
e
Guangdong Dazhong Agriculture Science Co. Ltd., Hongmei Town, Dongguan, Guangdong 523169, China
f
School of Engineering, Huzhou University, Huzhou, Zhejiang 313000, China
g
Centre for AgriBioscience, La Trobe University, Kingsbury Drive, Melbourne, Victoria 3086, Australia
highlights
Biochar amendment significantly enhanced the soil adsorption of diethyl phthalate (DEP).
Aging can reduce the DEP adsorption to soils treated with biochar.
The reduction was greater under the alternating wet and dry than that under constantly moist aging process.
Soil organic carbon content can strongly affect the aging process of biochar.
article info
Article history:
Received 1 January 2015
Received in revised form 9 May 2015
Accepted 13 May 2015
Available online 23 May 2015
Keywords:
Adsorption
Aged biochar
Desorption
Diethyl phthalate
Soil contamination
abstract
Biochar is a carbonaceous sorbent and can be used as a potential material to reduce the bioavailability of
organic pollutants in contaminated soils. In the present study, the adsorption and desorption of diethyl
phthalate (DEP) onto soils amended with bamboo biochar was investigated with a special focus on the
effect of biochar application rates and aging conditions on the adsorption capacity of the soils. Biochar
amendment significantly enhanced the soil adsorption of DEP that increased with increasing application
rates of biochar. However, the adsorption capacity decreased by two aging processes (alternating wet and
dry, and constantly moist). In the soil with low organic carbon (OC) content, the addition of 0.5% biochar
(without aging) increased the adsorption by nearly 98 times compared to the control, and exhibited the
highest adsorption capacity among all the treatments. In the soil with high OC content, the adsorption
capacity in the treatment of 0.5% biochar without aging was 3.5 and 3 times greater than those of the
treatments of biochar aged by alternating wet and dry, and constantly moist, respectively. Moreover, con-
stantly moist resulted in a greater adsorption capacity than alternating wet and dry treatments regardless
of biochar addition. This study revealed that biochar application enhanced soil sorption of DEP, however,
the enhancement of the adsorption capacity was dependent on the soil organic carbon levels, and aging
processes of biochar.
Ó2015 Elsevier Ltd. All rights reserved.
1. Introduction
Biochar is a carbon-rich solid product produced from the pyrol-
ysis of biomass residues. Application of biochar to soil can improve
soil properties (Verheijen et al., 2010). As a recalcitrant carbon-rich
material, the application of biochar to soil has great potential to
mitigate greenhouse gas emission and sequester carbon
(Lehmann, 2007). Past studies have shown that biochar application
to soils can decrease the bioavailability of heavy metals and
organic pollutants in contaminated soils (Beesley et al., 2011;
Zhang et al., 2013). For example, Yang and Sheng (2003) observed
that the particulates produced from burning wheat and rice resi-
dues were 400–2500 times more effective than soil in sorbing
diuron over the concentration range of 0–6 mg L
1
in water. High
http://dx.doi.org/10.1016/j.chemosphere.2015.05.037
0045-6535/Ó2015 Elsevier Ltd. All rights reserved.
⇑
Corresponding authors at: Key Laboratory of Soil Contamination Bioremedia-
tion of Zhejiang Province, Zhejiang A & F University, Lin’an, Hangzhou, Zhejiang
311300, China.
E-mail addresses: helizhiyongyuan@163.com (L. He), hailong@zafu.edu.cn
(H. Wang).
Chemosphere 142 (2016) 28–34
Contents lists available at ScienceDirect
Chemosphere
journal homepage: www.elsevier.com/locate/chemosphere
specific surface areas and microporous structure make biochar a
sorbent for the adsorption of a range of organic and inorganic
chemicals. When biochar is applied to the soil, it undergoes a range
of biogeochemical interactions and their properties are likely to
change with time in soil, a process commonly referred to as ‘‘ag-
ing’’ and biochar properties may change during the process of
aging (Kookana, 2010). The process of aging can be either abiotic
or biotic which can lead to changes in the sorption characteristics
of the sorbent. For example, the presence of some metallic ions
(e.g. Cu
2+
and Ag
+
) and organic compounds with higher hydropho-
bicity and/or high molecular size can significantly alter surface
chemistry and pore network structure of biochar (Chen et al.,
2007; Wang et al., 2006). Also, aging processes may introduce
functional groups (i.e. carboxylic) to biochar surfaces, thereby
impacting its adsorption properties (Qian et al., 2015).
Phthalate acid esters (PAEs) are a class of organic compounds
widely used as plasticizers to improve the properties of plastics
(He et al., 2015; Fang et al., 2010). Surveys showed worldwide
production of PAEs is approximately 6 million tons per year
and a large amount of these compounds are released into the
air, sediment, natural water, wastewater, and soils (Bauer and
Herrmann, 1997; Mackintosh et al., 2006; Shailaja et al.,
2007). Recent years have witnessed widespread urbanization
around the world, causing a rapid expansion of the peri-urban
interface in many cities, especially in Asia (Zeng et al., 2008).
In many peri-urban areas in Asia (i.e., in China), land use has
been transformed from rice-based to vegetable-based systems
(Wang et al., 2013). Therefore, there is likely to be a greater
demand for plastic greenhouses and plastic mulch to grow veg-
etable crops to cope with the increasing demands. The PAEs in
the plastic greenhouses, plastic mulch and fertilizers can be
potentially released to the soil, resulting in unwanted conse-
quences of soil contamination. In recent years, high levels of
PAEs contaminated agricultural soils have already been detected
in the Pearl River Delta and northeast China (Xu et al., 2008;
Zeng et al., 2008).
Diethyl phthalate (DEP) is one of the most frequently used
phthalates possessing high aqueous solubility and is toxic at high
exposure levels as well as at low doses for a prolonged period
and it may accumulate in the soil and enter the human food chain
(Yang et al., 2005; Sun et al., 2012). Both the higher concentrations
of DEP and exposure to lower concentrations for a longer period of
time could impact human health (Sun et al., 2012). Therefore, it is
essential to find an effective way to remediate the soils contami-
nated with this pollutant.
In our previous study, we suggested that biochar can be used as
a sorbent to reduce the bioavailability of DEP in the contaminated
soils (Zhang et al., 2014). However, aging process may change the
adsorption capacity of biochar. Recently, most studies were
focused on the effect of constantly moist aging process on biochar’
adsorption capacity. Generally, the alternating wet and dry process
is more relevant to the field conditions. The objective of this study
was therefore to assess the effect of different aging processes on
the adsorption capacity of DEP to the soils amended with bamboo
biochar. We hypothesized that different aging processes altered
the adsorption ability of biochar for DEP.
2. Materials and methods
2.1. Chemicals, soils and biochar
Analytical standard of diethyl phthalate with a purity P99.5%
was obtained from Shanghai Lingfeng Chemical Reagent
(Shanghai, China). Stock solution of 400 mg L
1
DEP was prepared
in methanol and used for analytical purposes. High Performance
Liquid Chromatography (HPLC) grade of acetonitrile was obtained
from Tedia (Fairfield, Ohio, USA). Other chemicals were of analyti-
cal grade.
Two soils differing in organic carbon (OC) contents were col-
lected in November 2012 from a vegetable garden in the suburb
of Lin’an near Hu Tangxia village, in Hangzhou, China. The soil sam-
ples were collected from the surface with a depth of 0–0.2 m. After
air-drying, the soils were passed through a 2 mm sieve. Selected
physicochemical properties of soils are shown in Table 1.
Bamboo used to produce biochar was oven dried at 80 °C for
24 h. The stainless steel vessel was constantly purged with dry
nitrogen gas at 3.5 L min
1
and the vessel was heated over 3 h to
a final hold temperature of 820 °C. Physicochemical properties of
biochar are shown in Table 2.
2.2. Aging treatments
On the basis of our preliminary investigation, the application
rates of bamboo biochar were set at 0.1% and 0.5% (w/w) of soil
weight. Soils were mixed with biochars using a soil mixer and were
directly used in the sorption and desorption experiments. An addi-
tional set of samples, after adjusting the water content of soil to
about 70% of maximum water-holding capacity, were equilibrated
at 25 ± 1 °C for 30 d to evaluate the effect of aging. The samples
were checked daily for water loss by weighing, and appropriate
amount of deionized water was added to compensate for water
loss when required. Other well-mixed samples were incubated
under the same conditions and aging time, and added to compen-
sate for water loss every 10 d (alternating wet and dry).
2.3. Adsorption and desorption experiments
Sorption of DEP by the bamboo biochar was measured by the
batch equilibration technique following an earlier protocol (Yu
et al., 2006). Briefly, 4 g each of the treated soils was weighed onto
a glass centrifuge tube (40 mL) with Teflon-lined screw cap and
20 mL of 0.01 M CaCl
2
(the CaCl
2
was used to maintain the ionic
strength), containing 250 mg L
1
of NaN
3
to inhibit microbial activ-
ity, was added to obtain an initial solution concentration of 2, 4, 6,
8, and 10 mg L
1
of DEP (Yu et al., 2006; Wang et al., 2010).
Triplicate samples were prepared for each treatment. For the com-
parative assessment between different biochar treatments under
similar conditions, an equilibrium contact time of 24 h was found
to be adequate for the purposes of this study. Tubes were shaken
for 24 h, centrifuged at 2191 g for 10 min (Wang et al., 2010),
and the supernatant was analyzed for DEP concentrations using
High Performance Liquid Chromatography and UV detection. The
Table 1
Selected properties of soils (0–0.2 m).
Soils Organic carbon (OC) (g kg
1
) Total N(g kg
1
) pH Electrical conductivity
a
(dS m
1
) Clay (%) Silt (%) Sand (%)
Low OC soil 3.5 0.3 5.83 0.31 16.9 44.4 38.7
High OC soil 22 2.0 6.04 0.24 16.4 45.0 38.6
a
Electrical conductivity was measured in 1:5 water.
X. Zhang et al. / Chemosphere 142 (2016) 28–34 29
sorbed amount C
s
(mg kg
1
) was estimated by the mass difference
between the initial and final concentration as follows:
C
s
¼C
i
C
w
ðÞV
w
=m
s
ð1Þ
where C
i
is the initial solute concentration (mg L
1
); C
w
(mg L
1
)is
the equilibrium solution concentration, V
w
is the solute volume (L);
and m
s
is the soil mass (kg), respectively.
Those samples with the highest sorption loading were used for
desorption experiments (Yu et al., 2006). After 24 h of equilibra-
tion, the tubes were centrifuged and 10 mL of the supernatant in
each tube was taken out for analysis. Another 10 mL of 0.01 M
CaCl
2
(containing a concentration of 250 mg L
1
NaN
3
) was added
into each tube, shaken for 24 h, centrifuged, and the supernatants
were collected to analyze the DEP concentrations in the aqueous
phase as described above. The desorption process was repeated
four times and the supernatants were collected as separate four
desorption aliquots. The sorbed amount C
s
(mg kg
1
) in the desorp-
tion experiments was estimated as follows:
C
s
¼C
s
þC
i
C
w
ðÞV
w
=m
s
ð2Þ
where C
s
is the initial sorbed concentration (mg L
1
), C
i
is the half
percent of the initial solute concentration (mg L
1
); C
w
(mg L
1
)is
the equilibrium solution concentration, V
w
is the solute volume
(L); and m
s
is the soil mass (kg) (Yu et al., 2006).
To estimate the adsorption capacity for the biochar in the trea-
ted soil, K
f;biochar
was estimated by assuming that the adsorption of
biochar and soil organic matter has an additive effect.
K
f;amended soil
¼f
biochar
K
f;biochar
þð1f
biochar
ÞK
f;soil
ð3Þ
where K
f;amended soil
,K
f;biochar
, and K
f;soil
are the adsorption coeffi-
cients of amended soil, biochar, and soil, respectively; f
biochar
is
the fraction of the amended soil (Wang et al., 2010).
2.4. Chemical analysis
The concentration of DEP was measured using a reverse-phase
ACQUITY ultra performance liquid chromatography (UPLC) cou-
pled with a PDA detector. To determine DEP concentrations in
the equilibrium solution, the collected supernatant was passed
through a 0.45-
l
m Whatman and separation was carried out on
a ACQUITY UPLC BEH C
18
Column (1.0 100 mm, 1.7
l
m,
Waters, Milford, Massachusetts, USA). The isocratic mobile phase
consisted of 45% water and 55% acetonitrile and at a flow rate of
0.1 mL min
1
. The injection volume was fixed at 1
l
L. The UV
wavelength for detection of DEP was 225 nm and the retention
time for DEP in the UPLC system was 5.0 min.
2.5. Data analysis and sorption modeling
The adsorption and desorption isotherms of DEP in the soils
with or without biochar were modeled using the Freundlich equa-
tion: C
s
¼K
f
C
N
w
where C
s
and C
w
are the equilibrium sorbed and
aqueous phase concentrations respectively, K
f
(mg
1N
L
N
kg
1
)is
the Freundlich sorption coefficient and N(dimensionless) is the
measure of sorption non-linearity (N= 1 represents a linear iso-
therm). K
f
was quantified by the empirical Freundlich equation in
the log transformation form: LogC
s
= logK
f
+NlogC
w
and Nis the
exponent indicative of sorption mechanism associated with the
sorbent and sorbate interactions. The K
f
values and Nvalues in
the Freundlich equation are used for the comparison between the
sorbents. The larger value for K
f
indicates a larger adsorption
capacity of the sorbent. However, the larger value for Nindicates
a larger change in effectiveness over different equilibrium concen-
trations. When N> 1, the change in adsorbed concentration is
greater than the change in the solute concentration. A larger N
value indicates a stronger adsorption capacity of the sorbent with-
out reducing the K
f
value (Liu et al., 2007). The sorption–desorption
apparent hysteresis index (H) was determined by the equation:
H=N/N
fd
where, the hysteresis index (H) can be used to quantify
the degree of apparent hysteresis.
3. Result and discussion
3.1. Effect of soil organic carbon content and biochar on adsorption
Fig. 1 illustrates the sorption of DEP in all fresh biochar treated
soils. Biochar addition to soil significantly enhanced the sorption of
DEP, and the sorption capacity of the biochar-amended soils
increased with an increase in biochar content. The results of this
study show that the sorption data over the entire range of DEP con-
centrations were well described (R
2
P0.95) by the Freundlich
equation, and the sorption parameters are summarized in Tables
3 and 4. It can be observed that Nvalues for all biochar-amended
soils were generally smaller than those for the unamended soils,
indicating a decrease in the degree of isotherm linearity with bio-
char amendment. Because of the observed variability in
non-linearity as indicated by Nvalues, K
f
could be used to make
a comparison of sorption affinities among the biochar-amended
soils. The K
f;biochar
values in Tables 3 and 4 show that bamboo bio-
char has a high affinity to DEP. Previous studies showed that the
high surface area of biochar affected its adsorption ability
(Ahmad et al., 2014; Zhang et al., 2013). It can be found that bam-
boo biochar in this study has a high surface area (276 m
2
g
1
)
which contributes to its high adsorption capacity. In addition, the
p
–
p
interactions between DEP molecules and surface functional
groups of biochar could potentially contribute to the sorption pro-
cess (Ahmad et al., 2014). Another possible mechanism may be
that the bamboo biochar has a high amount of amorphous carbon,
thereby supporting the formation of micropores and increasing the
sorption capacity (Haghseresht et al., 1999). This is mainly due to
the high lignin content of bamboo which contributes to the high
content of aromatic carbon of biochar (Cesarino et al., 2012;
Deshpande et al., 2000).
In the biochar-unamended soils, the adsorption capacity of soils
for DEP correlated well with the OC content of soils (i.e., the soil
with low OC content exhibited lower values of K
f
as compared to
the soil with high OC content). Generally, the clay minerals and
OC content of soil affect soil’s adsorption capacity of organic pollu-
tants. The two soils used in this study have similar clay, silt and
sand contents, pH and electrical conductivity but differ in the OC
content by 6-fold (Table 1). Previous study demonstrated that soil
organic carbon was responsible for partitioning and sorption of the
organic pollutants in soil (Wang et al., 2010). Therefore, this study
Table 2
Physico-chemical characteristics of the biochar used.
Samples N (%) C (%) H (%) O (%) H/C O/C C/N Organic carbon (%) pH SSA
a
(m
2
g
1
) Ash content (%) Electrical conductivity
b
(dS m
1
)
Biochar 0.57 76.69 1.59 21.15 0.25 0.37 157 75.42 10.80 276 5.51 1.54
a
SSA: specific surface area.
b
Electrical conductivity was measured in 1:5 water.
30 X. Zhang et al. / Chemosphere 142 (2016) 28–34
indicates that the soil organic matter strongly affect the adsorption
capacity of DEP by the soils.
The addition of biochar at 0.1% and 0.5% dose rates significantly
enhanced the adsorption of DEP in both soils with low and high OC
content. This suggests that the presence of even small amounts of
biochar can strongly dominate the sorption of an organic compound
in soils (Zhang et al., 2014). Tables 3 and 4 show that the K
f
values
for DEP at the two application rates were about 16, nearly 98 times
higher than that of the control for the soil with low OC, and approx-
imately 4 and 10 times higher for the soil with high OC content. The
results thus demonstrate that biochar is more effective to enhance
the adsorption ability for soils with low carbon content than soils
with high OC content. This further suggests that the biochars exhib-
ited higher sorptivity to DEP than the soils with indigenous soil
organic carbon. Yu et al. (2009) reported that soils amended with
1.0% of Eucalyptus-derived biochars reduced the uptake of chlor-
pyrifos and carbofuran by Spring onion (Allium cepa). In the present
study, we observed that biochar had a strong adsorption capacity
for DEP, and this may effectively reduce the bioavailability of DEP
in soils and reduce the plant uptake.
It is important to note that for soils amended with biochar at 0.5%,
the K
f
values showed an opposite trend with the low OC soil exhibit-
ing a higher sorption capacity than the high OC soil. In addition, after
the application of biochar, the extent of increase in the adsorption
capacity on DEP of all the low OC content soil treatments was higher
than all the high OC content soil treatments. This may be attributed
to the attenuation of DEP adsorption of biochar by soil-derived dis-
solved organic carbon (DOC) that blocks the pores of biochars,
thereby reducing the overall accessibility to the sorption sites
(Zhang et al., 2010; Pignatello, 2013). However, Yang et al. (2013)
Fig. 1. Effect of bamboo biochar (BB) treatments on the adsorption (Ads.) and desorption (Des.) of diethyl phthalate (DEP) in soils (A: soil with low organic carbon; B: soil with
high organic carbon). Symbols are measured data and the lines are Freundlich model fits.
Table 3
Effect of bamboo biochar (BB) on parameters for the adsorption and desorption of diethyl phthalate in soil with low organic carbon.
Treatment
a
Adsorption parameters Desorption parameters H
K
f
(mg
1N
L
N
kg
1
)K
f;biochar
(mg
1N
L
N
kg
1
)
b
NR
2
K
fd
(mg
1N
L
N
kg
1
)N
fd
R
2
LS 0.46 ± 0.03 – 0.97 0.95 0.67 ± 0.02 1.04 0.96 0.93
LS + 0.1% BB 7.56 ± 0.03 7110 0.26 0.99 10.33 ± 0.09 0.03 0.99 8.67
LS + 0.1% AWD BB 3.84 ± 0.04 3411 0.57 0.97 8.99 ± 0.11 0.10 0.98 5.70
LS + 0.1% CM BB 5.23 ± 0.01 4779 0.35 0.99 9.50 ± 0.13 0.05 0.95 7.00
LS + 0.5% BB 45.02 ± 0.35 8914 0.67 0.99 44.90 ± 0.003 0.004 0.96 168
LS + 0.5% AWD BB 35.53 ± 0.31 7020 0.57 1 42.84 ± 0.04 0.005 0.98 114
LS + 0.5% CM BB 36.23 ± 0.22 7156 0.62 0.95 42.96 ± 0.12 0.007 0.98 89
a
LS is the soil with low organic carbon; AWD is alternating wet and dry aging process; CM is constantly moist aging process.
b
K
f;biochar
was calculated from Eq. (3) given in the text.
Table 4
Effect of bamboo biochar (BB) on parameters for the adsorption and desorption of diethyl phthalate in soil with high organic carbon.
Treatment
a
Adsorption parameters Desorption parameters H
K
f
(mg
1N
L
N
kg
1
)K
f;biochar
(mg
1N
L
N
kg
1
)
b
NR
2
K
fd
(mg
1N
L
N
kg
1
)N
fd
R
2
HS 2.12 ± 0.19 – 0.85 0.95 8.24 ± 0.13 0.31 0.93 2.74
HS + 0.1% BB 8.17 ± 0.02 5959 0.48 0.99 13.50 ± 0.06 0.11 0.94 4.36
HS + 0.1% AWD BB 4.51 ± 0.02 2528 0.69 0.96 10.24 ± 0.09 0.23 0.96 3.00
HS + 0.1% CM BB 5.14 ± 0.15 3115 0.59 1 11.42 ± 0.39 0.17 0.92 3.47
HS + 0.5% BB 21.88 ± 0.11 3936 0.80 0.98 39.79 ± 0.06 0.02 0.97 40.00
HS + 0.5% AWD BB 6.27 ± 0.03 859 0.71 1 20.19 ± 0.27 0.04 0.94 17.75
HS + 0.5% CM BB 7.10 ± 0.04 1017 0.62 0.99 20.99 ± 0.15 0.02 0.98 31.00
a
HS is the soil with high organic carbon; AWD is alternating wet and dry aging process; CM is constantly moist aging process.
b
K
f;biochar
was calculated from Eq. (3) given in the text.
X. Zhang et al. / Chemosphere 142 (2016) 28–34 31
showed that low DOC concentrations in soil enhanced but high DOC
concentrations decreased the sorption of DEP. Future research may
be warranted to examine the mechanisms contributing to the effect
of DOC on the adsorption of DEP in biochar-amended soil.
3.2. Effect of aging on adsorption
Upon the application of biochar to the soil, a range of biogeo-
chemical interactions can take place. One such process is aging of
biochar which will eventually affect the sorptive ability of biochar
for organic contaminants. The high specific surface area and micro-
porosity of biochar may change with time after biochar is applied
to soils (Zhang et al., 2010). In this study, we evaluated the effect
on the soils’ adsorption affinity for DEP of incubating
biochar-amended soils for 30-d under alternating wet and dry
and constantly moist conditions.
Like the other treatments, sorption isotherms of the aged treat-
ments were well described by the Freundlich model (R
2
P0.96)
(Fig. 2;Tables 3 and 4). There was a marked decrease in the K
f
val-
ues for the aged samples especially for the soil with high OC con-
tent as compared with the fresh biochar-amended soils. For
example, the K
f
value of the high OC soil freshly added with 0.5%
biochar was approximately 3.5 and 3 times higher than those of
the biochar-amended soil aged with alternating wet and dry, and
constantly moist, respectively. The results indicate that aging pro-
cess decreases the overall sorption of DEP in the biochar-amended
soils. This decrease may be attributed to the increased association
of DOC with biochar surfaces over time, resulting in the occupation
or blocking biochar sorption sites (Zhang et al., 2010), and thereby
making them less available to absorb DEP. The unique surface
characteristics of biochar make fresh biochar a very efficient sor-
bent for organic compounds, whereas after aging, these properties
change probably due to surface interactions such as surface cover-
age, pore blockage, and surface oxidation (Huang et al., 2003;
Cornelissen et al., 2004). In a previous study, Lou et al. (2012)
observed that aging resulted in reduced pentachlorophenol sorp-
tion in a soil amended with biochar. In contrast, after aging, the
DEP sorption capacity of the 0.5% biochar-amended low OC content
soil did not markedly change as compared to fresh biochar
amended soil. The lack of a significant effect may be due to the
much lower OC content (3.5 g kg
1
) in this soil, and hence the
30-d aging incubation did not result in enough additional coating
of biochar particles by DOC.
In this study, we have noted that aging also tend to decrease the
sorption capacity of DEP in the soils without biochar addition
(Table 5), although the effect is not significant. The results are con-
sistent with the finding of Yang et al. (2008) that aging of soil
resulted in a reduction in pyrethroid sorption. As we discussed
above, soil OC content can strongly affect its adsorption on the
organic pollutants. Therefore, the most likely reason of our result
might be that aging process decreased the content or sorption sites
of organic matter in the soils.
3.3. Effect of different aging conditions on adsorption
Previous studies had mainly focused on the effect of constantly
moist aging condition on biochar’s adsorption capacity (Yang et al.,
2008; Zhang et al., 2010). Under natural conditions, the irregular
rainfall and the often warm, dry climate generate rapid drying of
surface soils which can be treated as an alternating wet and dry
aging condition. Therefore, the study on the effect of alternating
wet and dry aging condition on biochar’s adsorption capacity is
more appropriate to direct the biochar application measures.
The sorption capacities of DEP to the aged biochar at different
aging conditions are shown in Fig. 2, and Tables 3 and 4. The sorp-
tion capacity of DEP varied between the aging treatments. The K
f
values followed an order of constantly moist > alternating wet
and dry. As discussed above, soil DOC can block the sorption sites
of biochar, which may result in the reduction of biochars adsorp-
tion capacity. Numerous studies have shown that both alternating
wet and dry, and constantly moist aging conditions can enhance
DOC concentration in soil (Lundquist et al., 1999; Zhang et al.,
2010), and the soil organic carbon mineralization rate determines
the DOC concentrations in the soil. For instance, Zengin et al.
(2008) demonstrated that constantly moist condition significantly
enhanced the carbon mineralization of soil. However, dry-wet
cycles increased moderate C mineralization compared with a con-
tinuously moist soil (Yemadje et al., 2014). In our study, the
organic carbon content of the high OC soil decreased from
22 g kg
1
to 19.5 g kg
1
for alternating wet and dry aged soil, and
down to 20.5 g kg
1
for constantly moist aged soil. After aging,
the organic carbon contents of the low OC soil also showed the
same decreasing trend. This lends further support to the argument
that the alternating wet and dry aging condition is more effective
for soil organic carbon mineralization, and thus increases DOC con-
centration. We postulate that the difference in K
f
values between
the aging conditions may be attributed to the differences in the
extent of blocking of sorption sites of biochar by DOC between
these two different aging conditions.
Fig. 2. Effect of aging processes (AWD: alternating wet and dry aging; CM: constantly moist aging) on the adsorption (Ads.) of diethyl phthalate (DEP) on bamboo biochar (BB)
in soils (A: soil with low organic carbon; B: soil with high organic carbon). Symbols are measured data and the lines are Freundlich model fits.
32 X. Zhang et al. / Chemosphere 142 (2016) 28–34
3.4. Effect of biochar treated and untreated soil on desorption
In this study, the sorption and desorption isotherms were com-
pared to assess the degree of reversibility of sorption reaction.
Desorption isotherms were well described by the Freundlich model
(see Fig. 1), with R
2
P0.92, and the desorption parameters are
summarized in Tables 3–5. The highly nonlinear sorption isotherm
and relatively flat desorption isotherm suggest that sorption/des-
orption of DEP in the soils were consistently hysteretic (Fig. 1).
The hysteresis index (H) was used to quantify the degree of appar-
ent hysteresis. Tables 2 and 3 show the Hvalues for all treatments.
The Hvalue of the original low OC soil was 0.93, indicating a min-
imal desorption hysteresis. As the OC content and the dose of bio-
char in the soil increased, the value of Halso increased. This is
mainly due to irreversible binding or sequestration of DEP on bio-
char or certain components of soil organic matter (Bhandari et al.,
1996).
Compared to the untreated control, the soils amended with bio-
char appeared to be more hysteretic for the desorption of DEP. For
the low OC soil, addition of 0.5% biochar enhanced the Hvalue by
more than 180-fold, leading to the highest adsorption capacity.
Therefore, the application of biochar markedly reduced the
bioavailability and bioaccessibility of organic pollutants in soil
(Yu et al., 2006).
Similar to the K
f
value, the Hvalues were higher in all the fresh
biochar-treated soils than the aged samples, and followed an order
of fresh biochar-treated soil > biochar-treated soil aged with con-
stantly moist > biochar-treated soil aged with alternating wet
and dry. All the aging treatments displayed a tendency to more
easily desorb the DEP into the soil again. However, the degree of
desorption hysteresis was quite different between the two aging
conditions. Compared with the alternating wet and dry cycles,
the constantly moist treatments showed a higher desorption hys-
teresis. However, the underlying mechanism of this difference is
not understood and needs further investigation.
In this study, the strong sorption followed by weak desorption
of DEP in biochar-amended soils indicates that biochar sequesters
DEP in soil. Even though desorption of DEP in the biochar-amended
soils shows consistent hysteresis, the experiments were conducted
in short term under laboratory conditions. Whether sorbed DEP
will release into the soil again is an issue that requires further
investigation. The long-term environmental fate of biochar and
DEP in contaminated soils has not been studied under realistic field
conditions. This aspect warrants some investigations given the
possibility of DEP release in case of extreme weather events such
as heavy rainfall extending for days.
4. Conclusions
The sorption capacity of DEP onto soils increased with soil OC
content and rates of biochar addition. It is generally accepted that
the higher adsorption capacity of DEP to biochar reduces the
bioavailability of DEP commonly released to agriculture soils during
vegetable cultivation and other management practices. The amount
of DEP adsorbed onto the biochar-amended soil decreased after dif-
ferent aging processes with the adsorption capacity of
biochar-amended soil following the order of without aging > aged
with constantly moist > aged with alternating wet and dry. The
low OC soil amended with 0.5% biochar showed the highest adsorp-
tion capacity and weakest desorption capacity. A decreased number
of effective sorption sites of biochar for DEP adsorption by high soil
DOC may explain why the high OC soil with 0.5% biochar had lower
adsorption compared with the low OC soil with the same treatment.
The application of biochars to DEP-contaminated soils may thus be
expected to change many ecotoxicological processes of DEP,
thereby reducing both bioavailability and subsequent risk of enter-
ing the food chain. However, how the different aging processes
affect the characteristics of biochar could not be ascertained.
Therefore, more work is needed to investigate the mechanisms on
how different aging conditions changes biochar properties and
their impact on sorption and desorption behavior of DEP.
Acknowledgments
This work was financially supported by the National Natural
Science Foundation of China (41271337), Zhejiang Provincial
Natural Science Foundation (Z15D010001), the Foreign Experts
Introduction Funds of the State Administration of Foreign Experts
Affairs of China (W20143300031), the Special Funding for the
Introduced Innovative R&D Team of Dongguan (2014607101003),
and Zhejiang A & F University Research and Development Fund
(2010FR097).
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