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Removal of ppb-level DDTs from aqueous solution using organo-diatomites Lingzhi Tan, Shihua Qi, Jiaquan Zhang, Xinli Xing, Wei Chen, Yuan Zhang and Chenxi Wu
ABSTRACT Three modified organo-diatomites (ODs) were used for removal of o,p0 dichlorodiphenyldichloroethylene (o,p0 -DDE), p,p0 dichlorodiphenyldichloroethylene (p,p0 -DDE) and p,p0 dichlorodiphenyltrichloroethane (p,p0 -DDT) from water. It was found that the adsorption of dichlorodiphenyltrichloroethane (DDT) and its metabolites (DDTs) depended greatly on the type and concentration of modifying agent, the concentration of adsorbent and the initial concentration of DDTs. The hydrophobic characteristics of ODs–DDTs interactions were verified by measuring the amounts of DDTs adsorbed on ODs. The analysis of contact angle and total organic carbon (TOC) measurements revealed that the hydrophilic tails on the ODs surface were replaced with hydrophobic ones by surfactants. The following conditions were strongly suggested to provide the optimum performance for adsorption of DDTs: raw diatomite is modified by cetyltrimethylammonium bromide (CTMAB); dosing quantity of OD is no more than 3.0 g/L. The removal efficiencies of the three pesticides on ODs followed the order: p,p0 DDT > o,p0 -DDE > p,p0 -DDE. The adsorption efficiencies of ODs for the pesticides followed the order: GZY > GZF > GZYI > GZN. This experiment showed that the fittest models for the experimental data were given by the Redlich–Peterson and homogeneous particle diffusion models. Key words
| adsorption, DDTs, organo-diatomite (OD), surfactants
Lingzhi Tan Shihua Qi (corresponding author) Jiaquan Zhang Xinli Xing Wei Chen Yuan Zhang Chenxi Wu State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Wuhan 430074, China E-mail:
[email protected] Lingzhi Tan Water Environment Monitoring Center of Yangtze River Basin, Yangtze River Valley Water Resources Protection Bureau, Wuhan 430010, China Jiaquan Zhang School of Environmental Science and Engineering, Hubei Polytechnic University, Huangshi 435003, China and Key Laboratory of Aquatic Botany and Watershed Ecology, Wuhan Botanical Garden, Chinese Academy of Sciences, Wuhan 430074, China
INTRODUCTION Organochlorine pesticides (OCPs) are composed of a class
et al. ; Chen et al. ; Yang et al. ; Zhang et al.
of man-made chemicals with pronounced persistence
). Because of their hydrophobic properties, concen-
against chemical/biological degradation. They are highly
trations commonly vary from several nanograms per liter
mobile in the environment and have a tendency to bioaccu-
to hundreds of nanograms per liter in micro-polluted surface
mulate and biomagnify in organisms (Katsoyiannis &
waters (Liu et al. ), which makes them difficult to treat
Samara ). Dichlorodiphenyltrichloroethane (DDT)
by conventional water treatment methods.
was the first synthetic OCP to be used on a large scale
The adsorption technique is by far the most versatile and
throughout the world. Due to the toxicity and adverse effects
widely used method to solve the water pollution. It has
upon humans and biota, its production and use has been
already become the favored method for small/medium-
banned for decades in many countries (Zhang et al. ).
scale operations to deal with organic pollution such as
However, recent reports have indicated that DDT and its
phenol, 17β-estradiol, and dexamethasone in the aquatic
metabolites (DDTs) were still detectable in surface water,
environment.
soil and sediment both in developed and developing
carbon (Fuerhacker et al. ; Efremenko & Sheintuch
countries (Zhou et al. ; Poolpak et al. ; Ferrante
; Lu & Sorial ), modified silicates, and porous
doi: 10.2166/wqrjc.2013.056
Various
adsorbents,
including
activated
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glasses (Mishael & Dubin ; Zadaka et al. ), have
for the enhanced remediation of hydrophobic organic
been reported to remove organic pollutants from water.
chemicals (Xu & Boyd ; Xu a, b). Therefore, a suit-
Because of the high expense and difficulty in recycling,
able surface modification technique is essential to improve
many adsorbents were limited in their use. The porous min-
the organic adsorption ability of the raw diatomite. Mean-
erals, which are cheap and widely available materials, were
while, diatomite is collected from seas and lakes and has
commonly used as adsorbents for treating organic water pol-
been approved as a food-grade material by the US Food
lution, but the hydrophilic layer on their surface decreased
and Drug Administration (FDA). Thus, organic-diatomite
their capacity for adsorbing hydrophobic organic com-
can be safely used to remove pesticides in the process of
pounds. Therefore, organo-clays were mainly designed to
wastewater treatment and drinking water purification.
promote the adsorption of neutral and hydrophobic pesti-
Other researchers also showed that organic-clay could
cides and slowed their releasing speed (Murakami et al.
be used as an effective adsorbent for removing organic pol-
). Recent studies have focused on using surfactant-modi-
lutants from water (Daifullah & Girgis ; Demirbas ;
fied minerals to remove organic contaminants from water
Dubey et al. ). However, to our knowledge, this is the
(Huttenloch et al. ; Lee et al. ). Many investigators
first study to use organic-diatomites as adsorbents for
have studied the adsorption of phenols and chlorinated
adsorbing the DDTs in ng-level concentration from contami-
phenols (Rytwo et al. ), naphthalene (Lee & Kim
nated water and to analyze the small-concentration scale
), and polyamine (Blachier et al. ) onto organo-
adsorption behavior.
clay. The mechanism of the surface modification is based
The purpose of this paper is to study the adsorption
on enlarging the surface area and enhancing the cationic
properties of DDTs on natural diatomite and the organo-
exchange. In addition, some researchers reported that both
diatomites (ODs). The adsorption isotherms of the ODs
ion exchange and hydrophobic bonding played important
were also studied. Special attention was focused on deter-
roles in the adsorption of organic compounds (benzene-
mining the effects of the surfactant loading, solid content
hexol, mesoxalic acid, and mellitic acid) onto cationic
in the suspension, and initial pesticide concentration
surfactant-modified minerals (Xu & Boyd , a, b).
during the adsorption process.
Li et al. () studied the long-term chemical and biological stability of surfactant-modified zeolite. The results showed that the organo-zeolite was completely stable in the investi-
MATERIALS AND METHODS
gated pH region (from pH 3 to pH 10), as well as under aerobic and anaerobic conditions. All these results demon-
Materials and reagents
strated that the surfactant-coated minerals were suitable to be designed as effective, safe, and economical environ-
All glassware was soaked in a potassium dichromate-sulfu-
mental materials for long-term in situ remediation of water
ric acid solution and washed in tap water and then
pollution.
deionized water and baked at 450 C for 4 h before use.
W
Diatomite is a mineral found in natural sediments, and
All organic solvents (such as acetone, n-hexane, and
is formed from the accumulation and compaction of dead
dichloromethane) are gas chromatography (GC) grade
diatoms that have sunk to the bottom of water bodies
reagents, and the other reagents in this experiment
(Goren et al. ). It has many excellent properties, such
were of analytical grade. All solutions were prepared
as high porosity, and excellent chemical and thermal resist-
with deionized water. The standard substances of o,p0
ance. Due to these properties, diatomite can be safely
dichlorodiphenyldichloroethylene (o,p0 -DDE), p,p0 dichloro-
used in the process of wastewater treatment and ground-
diphenyldichloroethylene
(p,p0 -DDE),
and
p,p0
0
water pollution control. Similar to the synthetic process of
dichlorodiphenyltrichloroethane (p,p -DDT), as shown in
amorphous silica, the reaction of diatomite is closely related
Table 1, were purchased from the Agro-Environment
to the presence of reactive sites on its surface (Yuan et al.
Protection Institute of the Ministry of Agriculture of
). Many surfactant-modified soils have been studied
China. The surfactants (Table 1), which were used as
268
Table 1
L. Tan et al.
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The related physicochemical properties of DDT and surfactants (25 ± 0.5 C, pH 6.8)
Compound
W
Mol. weight (g/mol)
Water solubility (mg/L)
o,p0 -DDE
318.03
0.14
6.00a
p,p0 -DDE
318.03
0.12
5.70b
p,p0 -DDT
354.49
0.025
6.36b
CTMAB
364.45
13,000
0.48
SDBS
348.48
20,000
0.45
PEG-4000
3,600–4,400
50
a
Molecular formulas
LogKow
–
Howard & William (1997).
b
Schwarzenbach et al. (2003).
cationic, anionic, and nonionic surfactants, were cetyltri-
dried and baked at 500 C for 8 h (Murakami et al. )
methylammonium bromide (CTMAB), sodium dodecyl
to produce thermally treated diatomite (GZN). After
benzenesulfonate (SDBS), and polyethylene glycol-4000
crushing and grinding, the GZN was sieved to particle
(PEG-4000) and were purchased from Sinopharm Chemi-
sizes of 0.063–0.075 mm and stored in tightly stoppered
cal Reagent Co. Ltd, China. The raw diatomite (88.0% of
glass bottles for later use. In order to investigate the influ-
SiO2, 4.5% of Al2O3, 3.0% of Fe2O3, and 4.5% other
ence of surfactants on the adsorption of DDTs, the same
oxides) was purchased from Zhengzhou Donghua Celite
quantity (500 g) of GZN was added to 0.5 L of the three
Product Co. Ltd, China.
surfactant solutions with the initial concentrations varying
W
The DDTs aqueous solutions were prepared by directly
from 1 to 20 mmol/L, respectively. The suspensions were
adding these standard substances to deionized water in
then shaken on a gyratory shaker at 150 rpm for 24 h to
sealed conical flasks. The mixtures were then agitated for
reach equilibrium (Lemic´ et al. ). After equilibrium
W
8 h using a horizontal shaker at 170 rpm and 25 C in
had been established, the solid and aqueous phases were
order to dissolve the organics in water homogeneously.
separated by silicon membrane and yielded a solid as ODs. The excess surfactants on the surface of the ODs
Preparation of the ODs
were removed by repeatedly washing with deionized water. According to the types of surfactants, these ODs
The raw diatomite (GZ) was washed in deionized water to
were named GZY (modified by CTMAB), GZYI (modified
remove fines and other adherent impurities. Then it was
by SDBS), and GZF (modified by PEG-4000).
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Physical characterization of diatomite
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; Zhang et al. ). All the analyses were done in duplicate.
The total organic carbon (TOC) contents of ODs were obtained with the Leco RC-412 multiphase C determinator
Effect of the concentration of OD in the solution
(St Joseph, MI, USA) in O2 atmosphere after correcting the total carbon from the inorganic-C content. The BET
Measured quantities of the GZY from 0.4 to 4.0 g (six
(Brunauer–Emmett–Teller) surface areas of ODs were deter-
samples in total) were added to 500 mL of solution with
mined by N2 physisorption at liquid N2 temperature on a
the same initial DDT concentration of 13.5 ng/L. The
Miraesi KICT-SPA 3000 Instrument (Miraesi, Korea). All
removal efficiencies of the DDTs onto the GZYI and the
pH measurements were measured with the Beckman
GZF were also examined following the same procedure
model 1009 pH Meter (Beckman, CA, USA). The contact
described above.
angles of the ODs were measured using the sessile drop method on a CAM 101 series contact angle goniometer
Isotherm adsorption models and kinetic adsorption
(KSV Company, Linthicum Heights, MD, USA).
models
Measuring hydrophobic characteristics of modified
The equilibrium relationships between the adsorbents and
diatomite
adsorbates were described by adsorption isotherms, which were determined by the residual amount of DDTs in the sol-
The hydrophobic characteristics of ODs–DDTs interactions
ution. Further, a standard agitated reactor experimental set-
were verified by measuring the amounts of DDTs adsorbed
up was used to determine the kinetic data. Dynamic contact
on the ODs. The adsorption efficiencies for DDTs were
between the ODs and the pesticide solutions were carried
determined by using a series of different surfactant-loaded
out on a mechanical shaker at its minimum speed. A fixed
OD samples. Blank and control experiments were also con-
amount of 0.5 g GZY adsorbents, which were meshed to
sidered. All solutions were adjusted at pH 7 with standard
0.15–0.2 mm, were shaken with 500 mL of DDTs with the
buffer with 30 mM MES (pH 6), 30 mM MOPS (pH 7), or
concentrations varying from 4 to 200 ng/L at 25 C for 5–
30 mM HEPES (pH 8) (Sinopharm Chemicals, China) and
200 min. In addition, the deionized water without the
W
agitated at 25 C for 24 h.
W
DDTs served as blank and batches without the ODs
The adsorption uptakes of the ODs have been investi-
served as control. The amounts of DDTs adsorbed on the
gated at the same initial DDT concentration of 20 ng/L.
ODs were determined according to the differences between
The ODs, which were modified by the CTMAB solutions
the initial and the equilibrium pesticide concentrations in
with five concentrations (1, 2, 5, 10, 15, and 20 mmol/L),
the solution. The reproducibility of the results was greater
were named as GZY (1), GZY (2), GZY (5), GZY (10),
than 95% after three replicates for each experiment.
GZY (15) and GZY (20), respectively. Measured quantities from 0.4 to 4.0 g of ODs, which were meshed to 0.15 mm,
Small-scale column tests
were added to 500 mL of the pesticide solutions. The suspensions were then vigorously agitated on a reciprocating W
A pilot test of the ODs as a sub-surface permeable reactive
shaker at 25 C for 24 h to reach equilibration (Flores-
barrier for remediation of contaminated groundwater was
Céspedes et al. ). The mixtures were then centrifuged
conducted at small-scale columns. Each column (2.5 cm
at 4,000 rpm for 30 min to discard the OD particles and
internal radius and 30 cm length) was carefully packed
produce 400 mL supernatant for each sample. The concen-
with 3.0 g of the GZY or GZN. Both of the adsorbents
trations of DDTs in the supernatants were extracted and
were meshed to 100 BBS (average 0.15 mm), ensuring that
then measured by gas chromatography–electron capture
the mixture was homogeneously distributed. The ground-
detector (GC–ECD), according to a modified method
water influent pH and initial concentrations of the DDTs
based on the USEPA 8080A method (Chen et al. ,
in this experiment were 6.5 and 23.7 ng/L. The influent
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water sample was collected at 0.5 m below the water surface
TOC of the GZY was highest with 8.24%, which was
from a river in Nan’an, Chongqing in July 2008. The geo-
roughly 274 times greater than that of the GZ. Compared
W
0
graphic information for the sampling site is E:106 35 1.3″,
to CTMAB, SDBS and PEG-4000 have a lower affinity to
N:29 250 23.7″. These samples were obtained using clean
the diatomite surface, although the TOC content of the
1.5 L amber glass bottles. The bottles were rinsed three
GZYI and GZF were increased up to 0.08 and 0.31%,
times with samples, and carefully filled without passing air
respectively. This phenomenon was caused by the negative
bubbles through the samples. In this test, the empty bed con-
charge on the diatomite surface, which produced an attractive
tact time (EBCT) was set at 5 min. By attaching 10-mL glass
force to cationic surfactant and a repulsive force to anionic
gas-tight syringes to the three-way valves, effluent samples
surfactant (Cheng & Wong ). With increasing added sur-
(1 L) were collected periodically from effluent out of the
factants, the electrical properties and available adsorption
column. Meanwhile, the influent with the same concen-
area of the ODs diminished. Hence, when the amounts of
tration was continuously injected into the column. The
surfactants were over 15 mmol/L, the TOC decreased.
W
columns were initially flushed with several pore volumes of distilled water before the DDTs solution was introduced.
On the other hand, the similar tendency of the ODs surface area and TOC can be seen in Figure 1. The GZY had the highest BET surface area of 7.13 m2/g when it was modified by 15 mmol/L of CTMAB. GZYI and GZF, which were
RESULTS AND DISCUSSION
modified by the surfactants at 15 mmol/L, also resulted in higher BET surface areas. This observation was related to the
BET surface area and TOC of the OD particles
improvement of the dispersal state of diatomites when the surfactants were added to the diatomites (Nunes et al. ).
As illustrated in Figure 1, all the TOC contents of the ODs
When the amounts of surfactant were over 15 mmol/L
increased with the GZ, whereas the TOC of the GZN
the surfactants could be adsorbed on both the inner surface
declined from 0.03 to 0%. Furthermore, the TOC contents
of particles and micropores, which led to the decrease of
of ODs increased with added amounts of surfactants. It is
BET surface areas (Miao et al. ; Reddy et al. ).
clear that the surfactants could be attracted onto the diatomite surface and the thermal treatment could effectively
Contact angles
remove the organic compound in the raw diatomite. However, different increasing tendencies of TOC contents were
The contact angles of all three ODs, shown in Figure 2,
shown among the various ODs, in particular for GZY. The
increased after the GZN was modified by the three
Figure 2 Figure 1
|
TOC and BET surface area of the diatomites.
|
Contact angles of the modified diatomites (these values represent an average of testing at least four locations using a droplet size of 10 L).
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surfactants. These results indicated that the modification technique could replace hydrophilic tails with hydrophobic ones on the ODs surfaces. The possible types of interactions between the headgroups of surfactants and organic contaminants can be of hydrophobic nature (van der Waals forces) or can be enhanced further by a stable covalent attachment of the organosilanes to the diatomite surface. Removal efficiency of the DDTs by the three adsorbents The data obtained from the adsorption tests were used to calculate the adsorption indexes (AIs) (%): AI ¼ ðC0 Ce Þ=C0 where C0 and Ce are the concentrations of the DDTs in solution (ng/L) at time t ¼ 0 and t ¼ teq, respectively. The AIs of the DDTs adsorbed by different ODs with different loading amounts and surfactant types are presented in Figures 3 and 4. The appearances of the three parts of Figure 3 follow a similar trend, showing that the AIs of the DDTs rose with the increase in loading amounts of surfactants in all investigated solutions. At a certain amount of diatomite, the more surfactants were added the more surfactants were adsorbed onto the diatomite before the surfactant saturation adsorption was reached. When the loading amounts of the surfactants reached 15 mmol/L, the AIs of the DDTs reached their maximums. The maximum AIs of o,p0 -DDE (Figure 3(a)) adsorbed by GZY, GZYI, and GZF were 77.38, 54.66, and 69.51%, respectively. In the case of p,p0 DDE (Figure 3(b)), the maximum AIs were 68.90% (GZY), 37.11% (GZYI), and 43.91% (GZF). The maximum AIs of p,p0 -DDT (Figure 3(c)) were 81.66% (GZY), 68.26% (GZYI), and 75.98% (GZF). These results were consistent with the TOC contents, which were thought to be sensitive to the adsorption capability of these OCPs. The results also revealed that GZN had little effect on the removal of DDTs (Figure 4). The AIs of the DDTs adsorbed by the GZN were 4.85% of o,p0 -DDE, 0.57% of p,p0 -DDE, and 2.37% of p,p0 -DDT. Compared with the GZN, the three ODs showed much better removal efficiencies. The capacities and efficiencies of the ODs on pesticide adsorption enhanced significantly. The removal
Figure 3
|
Adsorption indexes of the three pesticides adsorbed on ODs with different loading surfactant concentrations.
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diatomite surface which repelled the anionic surfactant. The results of TOC also provided evidence for this conclusion. Meanwhile, it was also observed that the AIs of o,p0 DDE and p,p0 -DDT were higher than that of p,p0 -DDE. This was because these two chemicals (o,p0 -DDE and p,p0 DDT) had more hydrophobic properties and higher octanol-water partition coefficients (Kow, as shown in Table 1) than p,p0 -DDE. The Kow was also closely related to the accumulation abilities of organic compounds on the OD surface. The increasing rate of AI of p,p0 -DDT adsorbed by the GZY was 1.18 times that of p,p0 -DDE and 1.05 times that of o,p0 -DDE. In the case of the GZYI, at the same concenFigure 4
|
Adsorption indexes of three pesticides adsorbed on four modified diatomites
trations, the increasing rate of AI of p,p0 -DDT was 1.84
with the same surfactant loading concentration (10 mmol/L).
times that of p,p0 -DDE and 1.25 times that of o,p0 -DDE. The increasing rate of AI of p,p0 -DDT adsorbed by GZF
0
efficiencies of the GZY were 69.38% (o,p -DDE), 64.90%
was 1.18 times that of p,p0 -DDE and 1.73 times that of o,
(p,p0 -DDE), and 75.66% (p,p0 -DDT), respectively. When using
p0 -DDE. The smaller amounts of p,p0 -DDE adsorbed on
0
GZYI as the adsorbent, the AIs were 51.66% (o,p -DDE),
the ODs were also probably the result of the repulsive
37.11% (p,p0 -DDE), and 58.98% (p,p0 -DDT). In the case of
forces between the adsorbate molecules and the weakly
0
0
GZF, the AIs were 67.51% (o,p -DDE), 38.91.% (p,p -DDE),
bound counterions.
and 72.37% (p,p0 -DDT). The results presented in Figures 3 and 4 show that the adsorption efficiencies of all three pesticides on the ODs fol-
Removal efficiency of the concentration of the ODs in solution
lowed the order: GZY > GZF > GZYI > GZN. Hence, the GZY was the most efficient material for adsorbing all
Previous studies of the adsorption behavior by organo-min-
three pesticides. As well, the TOC of the three ODs followed
erals reported that the relationship between the adsorption
the same order. It could be concluded that the higher TOC
efficiency and the concentration of the adsorbent was not
values of the ODs produced the better adsorption capacity
linear (Bouffard & Duff ; Dakovic´ et al. ). A similar
of organic compounds.
result was also obtained from the adsorption of DDTs onto
It also should be emphasized that the differences in
the GZY in this experiment.
removal efficiencies of DDTs were caused by the types of
The three curves (Figure 5) of the AIs showed that there
surfactants and solutes. The increase in AIs of the three pes-
was a small decrease at the suspension concentrations of
ticides, at the given concentrations, could be ascribed to the
1 g/L in each curve. The AIs of the DDTs reached maxi-
intensification of the hydrophobic properties on the surface
mums when the concentrations of the ODs increased to
of the ODs. Other studies also showed that the removal
3 g/L. When the concentrations reached 4 g/L, another
efficiency of surfactant-modified materials significantly
small declining tendency appeared in every curve. However,
increased compared with the raw minerals (Ko et al. ;
the AIs of the DDTs were still higher than those at the initial
Richards & Bouazza ). They indicated that the increase
suspensions (0.2 g/L) for each pesticide at this time. This
of the adsorption capacity may be ascribed to the ligand
appearance might indicate that the OD particles agglomer-
exchange on the mineral surface. The fact that the DDTs
ated with the large particles. The rate of adsorption was
removal efficiency of GZY and GZF were higher than that
due to the availability of the specific surface area on the
of GZYI was amazing. The lesser amount of DDTs adsorbed
adsorbent when the adsorption process was dependent on
on the GZYI may be caused by the negative charge on the
the surface morphology. Because the tiny cracks and
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equilibrium uptake isotherms (Equations (1)–(4)) were used to correlate the equilibrium data (Redlich & Peterson ; Chu & Chan ; Wang & Keller ).
Figure 5
|
Linear: Cs ¼ M þ KCw
(1)
β Freundlich: Cs ¼ KCw
(2)
Langmuir: Cs ¼ KCw M=(1 þ KCw )
(3)
β β Redlich Peterson: Cs ¼ KMCw =(1 þ KCw )
(4)
Amount of DDTs adsorbed versus the concentration of modified diatomite (GZY).
where Cs is the amount of the DDTs adsorbed per unit mass of adsorbent, Cw is the equilibrium concentration of the
channels on the surface of the particles provided added sur-
DDTs, K and β are the surface adsorption equilibrium con-
face area for small particles, the DDTs were more easily
stant in each equation, M is the constant that is
adsorbed onto small particles compared with large particles.
determined by heat content during this adsorption accord-
These results indicated that using OD small particles could
ing to the Redlich–Peterson model or the maximum
give more interfacial areas and better removal effects,
amounts adsorbed per gram of the ODs according to
which provided a potential method for the scientific appli-
Langmuir.
cation and the practical arrangements necessary for avoiding organic water pollution.
According to the K of the Langmuir model, the adsorption capacities may be 8.958, 9.367, and 10.05 L/ng for o,p0 -
In contrast to the AIs of the DDTs, the actual amount of
DDE, p,p0 -DDE, and p,p0 -DDT (Table 2), respectively. How-
the DDTs adsorbed per unit mass of adsorbent decreased
ever, the limiting factor in the adsorption experiments was
with the increase in suspension concentration (Figure 5).
the low water solubility of the DDTs in this research
At lower concentrations, the ratio of available surface
(Table 1). The Langmuir model might not exactly interpret
areas of the unit mass for adsorbing the DDTs was larger.
the adsorption behavior of DDTs with the ODs. Examining
The amounts of DDTs adsorbed were 107 and 5 ng/g of
correlation coefficients (R 2), we found that the fittest model
o,p0 -DDE, 63 and 4 ng/g of p,p0 -DDE, 112 and 7 ng/g of
for the experimental data was the Redlich–Peterson model
p,p0 -DDT, for suspension concentrations of 0.2 and 4 g/L,
(Table 2). According to the assumption of the Redlich–Peter-
respectively. The results showed that the adsorption efficien-
son isotherm theory, there is a homogeneous monolayer and
cies of the pesticides were also related to the Kow of each
bilayer coverage on the surface of the adsorbate. The results
pesticide. According to Table 1, the logKow of the three
indicated that the interaction happened both in the mono-
DDTs followed the order: logKow p,p0 -DDT was more than
layer and bilayer between adsorbate and the ODs surface.
6.3, while the logKow of o,p0 -DDE and p,p0 -DDE were 6.00
The adsorption more likely depended on the Redlich–Peter-
and 5.70. The adsorption efficiencies of the DDTs using
son model for the equilibrium concentration of the pesticide
the three adsorbents followed the same order: p,p0 -DDT >
in solution (Figure 6).
o,p0 -DDE > p,p0 -DDE. Batch kinetic model Adsorption isotherms The adsorption of non-polar organic compounds has been To describe the adsorption behavior of the OD adsorption,
reported to be a complex process (Murakami et al. ),
the isotherms were prepared for the DDTs. The four
in which the properties of the adsorbent and solute play a
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Table 2
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Adjustable parameters and correlation coefficients for various isotherms R2
K (ng/L)
M
Linear
0.992
4.066
12.054
Freundlich
0.991
3.690
Parameters
β
o,p0 -DDE 1.015 10
Langmuir
0.991
4.437 × 10
Redlich–Peterson
0.996
1,228.378
2.561 × 104
0.991
4.115
19.169
Freundlich
0.989
3.382
Langmuir
0.989
4.233 × 109
9.367 × 108
Redlich–Peterson
0.997
1,301.771
3.584 × 104
Linear
0.992
4.074
20.853
Freundlich
0.990
3.151
8.958 × 10
9
1.652
0
p,p -DDE Linear
1.031 1.563
p,p0 -DDT 1.043
Langmuir
0.989
3.882 × 10
Redlich–Peterson
0.997
1,240.684
10
10
1.005 × 10
3.590 × 104
1.572
diatomite; (3) the chemical reaction with the functional groups attached to the matrix (Warshawsky ). Previous studies have reported that the adsorbent phase could effectively control the diffusion of organic contamination from solution onto adsorbent particles (Boyd et al. ). In this case, the differential and algebraic equation (Valderrama et al. ) is given below:
XðtÞ ¼ 1
2 2 ∞ 6X 1 z π De t exp π 2 z¼1 z2 r2
(5)
where X(t) is the fractional attainment of equilibrium Figure 6
|
Adsorption equilibrium data modeled by the Redlich–Peterson isotherm.
at time t, De the effective diffusion coefficient of pesticides in the diatomite phase (m2/s), r the radius of the diatomite particle assumed to be spherical (m), and z is
significant role. The adsorption process occurs within the surfactant organic layer in the surface and liquid-filled pores inside the diatomite. The kinetic model selected to describe this adsorption is the homogeneous particle diffu-
an integer. X(t) can be calculated by the equation: XðtÞ ¼
ct ce
(6)
sion model. The rate of the reaction was assumed through the following processes: (1) the diffusion of pesticide
where ct (ng/g)and ce (ng/g) are the concentrations of the
through the surfactant phase surrounding the diatomite;
solutes loading on the diatomite phase at time t and when
(2) the diffusion of the pesticides through the matrix of the
equilibrium is attained (ng/g), respectively.
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When Equation (1) fits the whole range 0 < X(t) < 1, it can be simplified as the following expression: π 2 De In 1 X2 ðtÞ ¼ 2Kt, where K ¼ 2 r
(7)
If the surfactant film diffusion controls the rate of this adsorption, Equation (1) can be expressed as:
Inð1 XðtÞÞ ¼ 2Kli t, where Kli ¼
3De C rCr
(8)
Figure 7 shows the results of the DDTs adsorption kinetics in the form of Equations (7) and (8) for the model mentioned above. It can be observed that the model fits the data satisfactorily in almost all the range for the adsorbent-phase diffusion, with some fluctuation in the initial time for the three pesticides. The evidence that the trends of Equation (8) did not give a linear dependence indicated that the film diffusion did not happen as the controlling step. The results of the linear regression analysis for Equation (7) are shown in Table 3. The results indicated that the reaction layer was very thin and the surfactant film can be adjacent to the particles. The linear correlation coefficients (R 2) indicated a good fit for this model. Small-scale column tests Column tests were conducted to determine p,p0 -DDE or p,p0 DDT removal efficiencies for both GZY and GZN. The bulk densities of the two materials were measured to be 0.23 and 0.20 g/mL for GZY and GZN, respectively. The measuring bulk density method is described in the Soil Quality Test Kit Guide, Section І, Chapter 4, pp. 9–13 (United States Department of Agriculture ). Different flow rates were applied to have the same EBCT (5 min) for each material: 4.3 (GZY) and 5.0 mL/min (GZN). Thus, effluent samples of GZY and GZN were collected every 3.8 and 3.3 h. The time-series plots of p,p0 DDE and p,p0 -DDT concentrations in the effluents are shown in Figure 8. It showed that the GZY could effectively remove both p,p0 -DDE or p,p0 -DDT, whose concentrations were reduced to around 5 ng/L (p,p0 -DDT) and 6.5 ng/L (p,p0 -DDE), respectively. In contrast, neither of the two
Figure 7
|
Test of the kinetic model equations f (x) vs time (t) defined by the homogeneous particle diffusion model for pesticides adsorption on GZY.
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Table 3
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Linear regression analysis of functions –ln (1-X 2) vs time (t) for the homo-
adsorbent increased to 3.0 g/L, the AIs of the ODs reached
geneous diffusion model
the maximum values in the experiment. Comparing the R 2
Parameters 0
F(X ) ¼ f(t)
F(X ) ¼ f (t) Slope
intercept
(min1) 3
5.74 × 10
values of the isotherms and kinetic models of the DDTs, De (m2 s1)
R2 4
14
o,p -DDE
1.33 × 10
0.983
2.18 × 10
p,p0 -DDE
5.45 × 104
4.19 × 104
0.989
1.59 × 1014
p,p0 -DDT
2.20 × 103
1.34 × 104
0.946
5.06 × 1014
the fittest models for the experimental adsorption data were given by the Redlich–Peterson and homogeneous particle diffusion models.
ACKNOWLEDGMENTS This research was financially supported by the National Nature Science Foundation of China (No. 41073070) and the New Teachers Supporting Foundation of China University of Geosciences (No. CUG110816).
REFERENCES
Figure 8
|
DDE and DDT breakthrough curves for the GZY and GZN reactive barrier filter pack in the columns.
pesticides was effectively removed by the GZN. These results indicated that the GZY produced a much better performance than the GZN.
CONCLUSION In this study, the surfactant-modified diatomites were developed and applied for removal of the DDTs from contaminated water. The results of BET surface area, contact
angle,
and
TOC
measurements
revealed
that
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First received 18 November 2012; accepted in revised form 10 March 2013