Removal of ppb-level DDTs from aqueous solution using organo ...

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(p,p0-DDT) from water. It was found that the adsorption of dichlorodiphenyltrichloroethane (DDT) and its metabolites (DDTs) depended greatly on the type and ...

© IWA Publishing 2013 Water Quality Research Journal of Canada





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,


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






L. Tan et al.


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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-


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


deionized water and baked at 450 C for 4 h before use.


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-


(p,p0 -DDE),




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


Table 1

L. Tan et al.



Removal of DDTs using organo-diatomites

Water Quality Research Journal of Canada





The related physicochemical properties of DDT and surfactants (25 ± 0.5 C, pH 6.8)



Mol. weight (g/mol)

Water solubility (mg/L)

o,p0 -DDE




p,p0 -DDE




p,p0 -DDT
















Molecular formulas


Howard & William (1997).


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


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


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).


L. Tan et al.


Removal of DDTs using organo-diatomites

Physical characterization of diatomite

Water Quality Research Journal of Canada





; 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).


Measuring hydrophobic characteristics of modified

The equilibrium relationships between the adsorbents and


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


agitated at 25 C for 24 h.


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


L. Tan et al.


Removal of DDTs using organo-diatomites

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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



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.


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


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).


L. Tan et al.


Removal of DDTs using organo-diatomites

Water Quality Research Journal of Canada





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.


L. Tan et al.


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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


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


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



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


L. Tan et al.


Removal of DDTs using organo-diatomites

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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


β Freundlich: Cs ¼ KCw


Langmuir: Cs ¼ KCw M=(1 þ KCw )


β β Redlich  Peterson: Cs ¼ KMCw =(1 þ KCw )


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-


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


Table 2

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Removal of DDTs using organo-diatomites

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Adjustable parameters and correlation coefficients for various isotherms R2

K (ng/L)











o,p0 -DDE 1.015 10



4.437 × 10




2.561 × 104









4.233 × 109

9.367 × 108




3.584 × 104








8.958 × 10




p,p -DDE Linear

1.031 1.563

p,p0 -DDT 1.043



3.882 × 10






1.005 × 10

3.590 × 104


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


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


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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Removal of DDTs using organo-diatomites

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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


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


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.


Table 3

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Removal of DDTs using organo-diatomites

Water Quality Research Journal of Canada





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


(min1) 3

5.74 × 10

values of the isotherms and kinetic models of the DDTs, De (m2 s1)

R2 4


o,p -DDE

1.33 × 10


2.18 × 10

p,p0 -DDE

5.45 × 104

4.19 × 104


1.59 × 1014

p,p0 -DDT

2.20 × 103

1.34 × 104


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).


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







surfactants increased the surface area of the ODs and replaced hydrophilic tails with hydrophobic ones on the surface of the ODs. The removal efficiencies of the three pesticides followed the order: p,p0 -DDT > o,p0 -DDE > p,p0 DDE. The adsorption efficiencies of the diatomites for the pesticides followed the order: GZY > GZF > GZYI > GZN. The AIs of pesticides increased with the increase in adsorbent concentrations. When the concentration of

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First received 18 November 2012; accepted in revised form 10 March 2013