1

Adsorption of Methomyl pesticide on HDL (layered double

hydroxides for environmental protection

N. BOUDAOUD (1),(2)

, A.E.K TAYEB(2)

, H. MILLOUDI(2)

.

(1) Research unit in renewable energy (EPST CDER).

(2) Oran University Department of Chemistry chemistry laboratory materials and environment

31000 – Oran – Algérie

E-Mail: boudaoud_2007@yahoo.fr

Abstract:

Anionic clay (layered double

hydroxides )was utilized as adsorbent to

remove insecticide Methomyl (méthylthio-

1éthylidéne-amine) , from aqueous

solutions by adsorption. Batch experiments

were conducted to study the effects of pH

(03–10) ,the Mg+2

/Al+3

ratio the anion

exchange capacity, the nature Of

intercalated anion X (CO3,NO3,Cl-) and the

effect of layer charge on insecticide

Methomyl adsorption. the adsorption

isotherme ,described by freundlich model,

are of S-type with tendency to L type for

high Methomyl equilibrium concentration

.the lamellar structure, exchange capacities

among the highest in ion exchangers, and a

hydroxylated surface very reactive

promoting chemisorption anionic clays are

very interesting materials for adsorption

and intercalation of anions or molecules

environmentally undesirable.

1. Introduction

The extended contamination of soils and

ground water from the widespread use of

pesticides in modern agriculture is a

current concern that is impelling research

looking forremedies.The interest of anionic

clays in this field steams for their exchange

properties that make them promising

materials for the elimnation of pollutants.

On the other hand, anionic clays may be

used as soil models for predicting the

behavior of pollutants in soils. Anionic

clays (also called Layered Double

Hydroxides or LDHs) are rare in nature but

simple to prepare; hydrotalcite [Mg-Al],

hydrocalumite [Ca-Al] and the green rust

are LDHs that occur in nature. Their

structure is derived from that of brucite

Mg(OH)2 [1]. A wide range of composition

can be obtained for these materials by

changing the nature of the metal cations

and the ratio MII/M

III as well as the type of

the intercalated anion Xm- in the general

formula : [M2+

1-xM3+

x(OH)2]x+

[X m-

x/m.nH2O] abbreviated as [M2+

M3+

X] [2,3].

Hence, their chemical and physical

properties can be varied and this has led to

intensive search for uses of LDHs in

catalysis and sorption processes. Their

highion exchange capacities make them

promising materials for the elimination of

ecologically undesirable inorganic and

organic anions. Indeed, the removal of

chromate [4,5], nitrate, phosphate, arsenate

or vanadate anions [6,7] from

contaminated water by LDHs have already

beendemonstrated. An other objective

would be the use of LDHs and modified

LDH materials as recyclable sorbents and

heterogeneous catalysts for the treatment

of contaminated waste waters. The aim of

the preon magnesium-aluminium

hydrotalcites in order to establish their

sorption capacity and mechanism. The

adsorption experiments being conducted at

pH = 7, sent work is to study the sorption

capacity of de pestecide Methomyl

(méthylthio1éthylidéne-amine).

2

HDL are lamellar mixed hydroxides and

trivalent metals. The structure can be

described by the stack of sheets of

composition M (OH) 2 similar to those of

brucite. The presence of trivalent metal

leads to positively charged sheets. The

electro neutrality is guaranteed by the

species anionic solvated by water

molecules present in the space interlayer

[8]. Such as mineral phases are:

Hydrotalcites [Mg-Al] hydro calumite [Ca-

Al], the "green rust" [Fe2 +

- Fe3 +

].

However, these minerals are never

abundant, in nature, materials clay are

mainly cationic clays (phyllosilicates).

Work of Miyata in the 70s [9], including

the synthesis "Soft chemistry" HDL,

helped their development. Given the

multiple possible combinations of the

constituent metals sheets and interlamellar

species and the many variables of

preparation, it is possible to vary the

chemical and physical properties of HDL

where the interest growing these materials

in particular in the field of catalysis [10].

On the other hand, with exchange

capacities among the strongest ion

exchanger’s 3-4 meq /g and a hydroxylated

surface highly reactive to promote

chemisorptions, HDLs also provide

opportunities for trapping chemical

pollutants. The efficacy of HDL in the

treatment of polluted waters anions

nitrates, phosphates or chromates has

already been demonstrated [6,11]. Many

studies also focus on the use HDL as

carrier or catalyst for the degradation of

chemical pollutants [12]. The present work

focuses first on the study of the trapping

phase [Mg1-x Al x (OH) 2] [X.nH2O]

pesticide.

2. Experimental

A. Synthesis

All compounds HDL was prepared by the

co precipitation method at constant pH [9].

It consists in the controlled precipitation

carried by a base with a

solution containing salts of the two cations

in suitable proportions. Thus, the phase

[Mg2AlCO3] was synthesized at 25 ° C by

gradual addition of a solution of Mg and M

1 NO36H2O AlNO3, 6H2O 0.5 M in a

reactor containing 100 ml of pre-ionized

water and stirring magnetic. The pH is kept

constant at the value of 10 ± 0.1 by

simultaneous addition of a solution of 2M

sodium hydroxide. After a ripening period

of 24 hours at 85 ° C, the solid is recovered

by three cycles of washing / centrifugation

and then allowed to dry at a temperature 80

° C.

Table.1. Anionic Exchange Capacity

(AEC) for different ratios of metal cations

hydrotalcites [Mg1-x (AI x (OH) 2] [CO3]

0.5x nH2O, [13].

MII/M

III

x = MIII

/

(MII+M

III)

C.E.A

(meq/100g)

2/1 0.33 401

3/1 0.25 325

4/1 0.20 273

B. Samples characterization:

All samples were examined by XRD; this

technique allows getting information on

the sheet and the space between the sheets.

Measures X-ray diffraction was performed

on a SIEMENS D501 powder diffract

meter equipped with a copper anticathode

tube and a monochromatic of graphite

blade. The sample is spread on a glass slide

sprinkled by sieving to limit the

phenomena of preferred orientations of the

crystallites.

Spectroscopy infrarouge IR, This

technique allows to highlight the presence

of the anion in the interlamellar space [14]

as well as the interactions that may exist

with the matrix. In addition, this technique

is very powerful to detect the presence of

carbonate anions side via the characteristic

vibration band of carbonates at around

1360cm-1

. In this study the infrared spectra

were obtained in 4000-400 cm-1

on a

3

Perkin Elmer SPECTRUM. Materials are

dispersed in the powder of potassium

bromide (KBr), oven-dried, and are then

analyzed. Morphological Characterization,

The scanning electron microscopy (SEM)

is another technique commonly used

enough. It provides information on the

texture of the grains, the latter being

important for application of the materials

in catalysis [14].

3. Results and Discussion

Figure 1 shows the RX patterns of the most representative LDHs synthesized by direct co-precipitation of materials [Mg-Al-CO3], [Mg -Al-NO3], [[Mg-Al-SDS]. They are typical of a pure layered double hydroxide structure with sharp and symmetric reflections for (003), (006), (110)and (113) planes and broad asymmetric for (012),(015) and (018). The diffract grams X precursors [Mg2-Al-

CO3], [Mg2-Al-NO3], [Mg2-Al-SDS],

[Mg2-Al-500] are shown in Figure 1.

Materials crystallize in a trigonal P3 with a

stack of oxygen atoms. Indexing of X-ray

spectra can be performed with a hexagonal

lattice, containing three copies per cell.

The distance between the line of lattice

003, is connected to the sheet spacing

(distance between the sheet), and allows an

approximate calculation of the lattice

parameter c by the following equation: c /

3 = d003. The line (110) in the vicinity of

60 ° in 2 θ determines the interatomic

distance of the metal-metal sheet and the

lattice parameter a: a / 2 = d110 [13,14]. The lattice parameters of the compounds HDL are summarized in Table 2. These results are consistent with those in the literature [11], for the same material

MgAlCO3 (R=2) synthesized by the same method, have found a value of a equal to 3.02 ° and a value of 21.69 A ° .[15] when to them, for the material found Mg2AlNO3 has a value of 3.018A ° and a value of c 23.7 A °. Table.2. Parameters mesh anionic clays.

compounds a (A°) c (A°) d003

(A°)

Mg2AlCO3 3.021 21.695 7.690

Zn2AlNO3 3.018 23.700 7.900

20 40 60 80

MgAl-500

Mg-Almethomyl

Mg-AlCO3

2theta

Fig.1. Schématique représentation of

HDL

Fig.2.XRD patterns of [MgAl-CO3],[[MgAl-methomyl],

[MgAl-500].

Fig.3. show the FT-IR includes the infrared

spectra of HDL phases [Mg2AlCO3], [Zn2Al CO3],

[Mg2Al500]and patterns of the most representative.

4

This technique allows highlighting the presence of the anion in the interlamellar space [14] as well as the interactions that may exist with the matrix. In addition, this technique is very powerful to detect the presence of carbonate anions side via the characteristic vibration band of carbonates at around 1360cm-1. In this study the infrared spectra were obtained in 4000-400 cm

-1 on a Perkin Elmer SPECTRUM.

Materials are dispersed in the powder of potassium bromide (KBr), oven-dried, and are then analyzed. Infrared spectra of HDL phases show three characteristic domains: From 3400 to 3600 cm-1, bands of vibration of the hydroxyl groups of the sheet (ν OH) and water molecules sorbed physical and interleaved. The intensity and position of the band depend on the hydration rate respectively, of the charge density of the layers and the nature of the metal cations. From 2500 to 1000 cm-1 bands are bending vibration (δ H2O) molecules intercalated and adsorbed water and the characteristic bands of the anion present in inter leaf, we note the bands between 1350 and 1480 cm-1 corresponding to the vibration ν (CO3

2-)

highlighting sample contamination by carbonates despite the precautions taken experimental. Several studies [16,17] Nagy showed that

HDL contamination by carbonates cannot

be avoided, even if you are working under

an inert atmosphere. For low frequencies,

that is to say ν <1000 cm-1, the vibrations

are attributed to the observed network. We

distinguish vibrations (νM-O valence

between the metal and oxygen atoms, and

the bending vibrations of the metal-oxygen

bonds, oxygen (δ O-M-O). Strips are strips of

a signature HDL [14].

A. Methomyl adsorption on [Mg2-

AlCO3],[Zn2-Al- CO3]

The dye adsorption isotherms of

pesticide Methomyl were recorded at 25 °

C in a closed reactor with magnetic

stirring. Typically, a mass of 10 mg of clay

is dispersed in 25 ml of a solution of

colorant concentration variable Ci (2-

6mmole/l); After a contact time of 24

hours, the suspensions were centrifuged.

Herbicide concentration in the supernatant

solution this in equilibrium with the

adsorbed by clay Cs is determined by UV

4000 3500 3000 2500 2000 1500 1000 500

0,0

0,2

0,4

0,6

0,8

1,0

1,2

**

**

*

% tra

nsm

itance

Wavelength(cm-1

Mg2AlCO

3

Mg2Almethomyl

methomyl

Fig.4.influence of the pH

5

spectrometry (Methomyl: λ = 254nm.

Isotherms relate the amount of pollutant Cs

adsorbed on clay (expressed per gram of

clay: Cs (mol / g) = (Ci- Ce). V / m) that in

equilibrium in solution Ce. This adsorption

is well described by the Freundlich

equation: Cs = K.Ce1 / n. The linear

transform (log Cs = log K 1 / n.log Ce)

allows the calculation of the constants K

and 1 / n that provide information on

respectively the adsorption capacity of the

clay and the affinity adsorbate / adsorbent

(or the degree of linearity between Cs and

Ce). Giles et al. thus distinguish several

types of isotherms that relate to different

type’s d'interactions adsorb ate-adsorbent

[18].

The influence of pH on the adsorption of

Methomyl by phase [Mg2-Al-CO3]was

initially examined. Thus, an acidic pH <5

causes partial dissolution when in a basic

medium de clay pH> 7, competition with

anions carbonates is very likely given the

high affinity of HDL for these anions. The

pH also changes the solubility of the

insecticide and the surface charge clay.

The measurements show an adsorption

maximum at pH between 6- 7 which

corresponds to natural waters as indicated

by the effect of pH presented fig.4

Preliminary adsorption experiments were

conducted in order to determine the

optimal conditions for adsorption of

methomyl on LDHS regarding the contact

time The kinetic study (fig.1)shows that

the adsorption equilibrium state is reached

after contact time 100min. adsorption on

LDHS is very fast .it is also the case for

2.4D on HDLS [13]and monobutyltin in

clays minerals [19]. Here, the shape of the

Kinetic curve indicates that we are dealing

with only one adsorption phenomenon

which must be a surface process related to

the slow diffusion of the organic

molecules. To be sure that the equilibrium

state is reached, Methomyl -HDLS contacte

time of 24 h was applied in the adsorption

experiments

Fig.6 displays the Methomyl adsorption

isotherms on [Mg-AlCO3], [Zn-

AlCO3],[Mg-Al500]and [Mg-Aledta]

phases for a sample mass of 10 mg. one

can distinguish three different parts on the

adsorption isotherms of [Mg-AlCO3], [Zn-

AlCO3]. Curves phase are S-type and L-

type tend towards a higher concentration

for. Type S isotherm indicates interaction

competitive adsorbate / adsorbent and

adsorbate / adsorbat.la matrix [Mg-

AlCO3], [Zn-AlCO3] and [Mg-Aledta],

Significant, not adsorbed at the 1.10

mmole / l. curve tends to L-type change in

S L in a same isotherm can be explained by

the passage of a saturation of external sites

has a surface adsorption process with

internal retention gap feuillets.les

interaction internal étanteaucoup stronger

than surface. At low equilibrium

concentration Ce This for> 1.5 mmol,

adsorb ate-adsorbent interactions are

necessary and isotherms resemble S type

,according to the classification [18].This

2 3 4 5 6 7 8 9 10 119,30

9,32

9,34

9,36

9,38

9,40

9,42

adsorb

ed p

este

cid

e(m

mole

/L)

pH

0 200 400 600 800 1000 1200 1400 1600

0

20

40

60

80

100

120

adsorb

ed c

oncentration (m

mole

/l)

time(min)

Fig.5. Kinetic study of adsorption of Methomyl (Ci=

1.2mmole/l)

on 10mg of at 25°C [Mg2AlCO3]

6

isotherm assignment is supported by

Freundlihe 1/n values [20].the maximum

of Methomyl is reahed at the plateau( for

Ce=1.10mmole/l) at curve tends to L-type .

Isotherm of the S-type Indicates

competitive interactions adsorbate /

adsorbent and adsorbate / adsorbate, with

the Possibility for the solute to be adsorbed

gold horizontally to the area of the

adsorbent or vertically reflecting a minimal

competition entre le adsorbate and the

solvent for the adsorption sites. The shape

of the adsorption curve is characteristic of

a process of chemisorption isotherm is then

Modeled by the Langmuir equation.

The scanning electron microscopy (SEM)

is another technique commonly used

enough. it provides information on the

texture of the grains, it is important for the

application of material sin catalysis[14].

Fig.6. contains snapshots of scanning

electron microscopy obtained for anionic

clays studied. Textures of phases HDLs

[Mg2AlCO3] presents a mixture of

dispersed particles from large blocks and

spherical particles. Similar results were

observed [13] our material [Mg2AlCO3].

For cons, the crystallites of the matrix

[Zn2AlCO3] are compact, large and much

more swollen than those observed in the

snapshots [Mg2A CO3] and [Zn2AlCO3]. It

justified the change in shape of the

crystallites by the moisture level of nitrate

compared to other stage phases.

The fig.6 some [Mg2-Al-CO3], [Zn2-Al-

CO3] micrographs are shown. In general,

the solid materials morphology analyzed

by SEM presents particles with irregular

grains. A texture of phases [Mg2AlCO3],

[MgAlNO3] presents a mixture of poly-

dispersed particles in the form of large

blocks and of small spherical particles and

highly aggregated. The plate formed on the

[Mg2AlCO3] indicates particles with

angular lines similar results were observed

by [13].

4. Conclusion

With high anion exchange capacities and

highly reactive hydroxyl surfaces, the

layered double hydroxides are interesting

materials for trapping molecules

environmentally undesirable Through their

exchange properties, the anionic clays

provide opportunities for trapping anionic

organic pollutants. we initiated the study

of absorption properties of a series of

materials HDL, HDL EDTA , the pesticide over looked Methomyl. For this purpose,

we used materials crystalline type anionic

clay chlorinated [MgRAlCO3], prepared by

the co precipitation method in basic

medium (pH = 9) constant. The products

obtained were characterized by X-ray

diffraction, infrared spectroscopy, thermal

analysis, and scanning electron

microscope. The results of these analyzes

have confirmed the structure of the

precursor. The adsorption isotherms of

Methomyl on difference synthesized

materials, type S, indicate moderate

interactions between organic pollutants and

clay. Adsorption proceeds in exchange for

first and then the entire surface of the solid.

It depends on many factors such as pH, the

charge density of the layers, the

morphology of the clay and the ratio of

solid / liquid phases.

1,00 1,05 1,10 1,15 1,20 1,25

0

2

4

6

8

10

d

cb a

qe(m

mole

/g)

ce(mmole/l)

Fig.6.SEM micrographs from (a) [Mg2AlCO3],

(b)[Mg2AlNO3].

Fig.6. Methomyl adsorption isotherms for (a)

[Mg2AlCO3]

, (b) [Zn2AlCO3],(c)[Mg2Al500], (d)[Mg2Aledta].

7

By their composition rich HDLs can also be considered as relatively simple soil models. The study of complex clay / pesticide shows the influence of properties hydrophilic / hydrophobic and surface charge of the various components the transfer of pesticides. 5. References

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