Desalination 272 (2011) 259–264

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Desalination

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Adsorption thermodynamics studies of 2,4,5-trichlorophenoxy acetic acid onpoly-o-toluidine Zr(IV) phosphate, a nano-composite used as pesticide sensitivemembrane electrode

Asif Ali Khan ⁎, Tabassum AkhtarAnalytical and Polymer Research Laboratory, Department of Applied Chemistry, Faculty of Engineering and Technology, Aligarh Muslim University, Aligarh 202002, India

⁎ Corresponding author. Tel.: +91 5712720323.E-mail address: asifkhan42003@yahoo.com (A.A. Kh

0011-9164/$ – see front matter © 2011 Elsevier B.V. Aldoi:10.1016/j.desal.2011.01.033

a b s t r a c t

a r t i c l e i n f o

Article history:Received 12 August 2010Received in revised form 6 January 2011Accepted 12 January 2011Available online 22 February 2011

Keywords:Organic–inorganic composite material2,4,5-Trichlorophenoxy acetic acidPoly-o-toluidine Zr(IV)phosphateAdsorptionFreundlich and Langmuir isotherms

2,4,5-Trichlorophenoxy acetic acid (2,4,5-T) is used in forestry and in agriculture as a systemic herbicide (foreradication of undergrowth, etc). Adsorption behaviour of 2,4,5-T on the surface of poly-o-toluidine Zr(IV)phosphate was studied and some thermodynamic parameters such as Freundlich constants and Langmuirconstants were determined on the surface of cation-exchanger. In this work, the adsorption of 2,4,5-T oncation-exchanger from aqueous mediumwas studied with respect to pH and initial 2,4,5-T concentration. Theexperimental data were analyzed by the Freundlich isotherm, and the Langmuir isotherm. The resultsindicated that the cation-exchanger is very effective for the adsorption of 2,4,5-T from aqueous solutions.Pesticide sensitive membrane electrode was fabricated by using an electro-active composite material poly-o-toluidine Zr(IV) phosphate. The sensitivity towards the 2,4,5-T was monitored.

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© 2011 Elsevier B.V. All rights reserved.

1. Introduction

The contamination of surface and ground water by pesticides is animportant problem that the scientists are dealing with over the years.Pesticides are group of inorganic and organic compounds that maypollute water due to their extensive application in agriculture asrodenticides, insecticides, larvacides, miticides, mollucides, synergists,fumigants, fungicides, plant growth regulators and sterilants. Althoughmuch benefit is obtained from their uses, they have some undesirableside effects such as toxicity, carcinogenity and mutagenity [1,2].

2,4,5-Trichlorophenoxy acetic acid (2,4,5-T) is used in forestry andin agriculture as a systemic herbicide. It is considered moderatelytoxic and maximum allowable concentration in drinking water. Onthe other hand, 2,4,5-T is a poorly biodegradable pollutant. Conse-quently, it has been frequently detected in water bodies in variousregions of the world [3]. Therefore, the removal of pesticides fromwater is one of the major environmental concerns these days.

Several methods are available for pesticides removal such asphotocatalytic degradation [4,5], combined photo-Fenton and biolog-ical oxidation [6], advanced oxidation processes [7], aerobic degrada-tion [8], nanofiltration membranes [9], ozonation [10] and adsorption[11–19]. Adsorption on activated carbon is the most widespreadtechnology used to deal with purification of water contaminated bypesticides [20], dyes [21–23], phenols [24], etc.

It is interesting to use organic–inorganic composite materials inadsorption studies of pesticides [25–27]. Activated carbon, coal andbone char have been widely used in the adsorption of metal ions fromaqueous solution. Adsorption behaviour of some pesticides for theremoval of heavy metals from aqueous solution and industrialwastewater has been reported from our laboratory [28–31]. Suchadsorption studies of pesticides on composite materials can help us inmaking pesticide sensitive membrane electrode that can be used todetermine pesticide in trace amounts in the given sample of pollutedwater.

In the present study, poly-o-toluidine Zr(IV) phosphate, a nano-composite cation exchanger [32], is used for the adsorption studies of2,4,5-T and some important thermodynamic parameters were alsodetermined. On the basis of adsorption studies, a pesticide sensitiveelectrode was fabricated by using poly-o-toluidine Zr(IV) phosphatecomposite material.

Therefore, the purpose of this work was to evaluate the adsorptionpotential of poly-o-toluidine Zr(IV)phosphate cation-exchanger for2,4,5-trichlorophenoxyacetic acid. The equilibrium data of theadsorption process were then studied to understand the adsorptionmechanism of 2,4,5-T molecules onto the prepared cation-exchanger.

2. Experimental

2.1. Reagents and instruments

The main reagents used for the synthesis of the material wereobtained from CDH, Loba Chemie, E-merck and Qualigens (India Ltd.,

Table 1Characteristic properties of 2,4,5-trichlorophenoxy acetic acid.

Empirical formula C8H5Cl3O3

Rel. molecular mass 255.49 gDensity 1.803 g/cm3 at 20 °CBoiling point Above 200 °C decompositionMelting point 157–158 °C (acid); 113–115 °C (triethanolamine salt)Vapour pressure 0.7×10−6 Pa at 25 °CSolvolysis/solubility In water: 280 mg/l at 25 °C

In diethylether: 234 g/lIn toluene: 7.3 g/lIn xylene: 6.1 g/lSoluble in isopropyl alcoholThe alkali and amine salts of 2,4,5-T are readilysoluble in water, whereas the esters are virtuallyinsoluble in water but soluble in mineral oil.

260 A.A. Khan, T. Akhtar / Desalination 272 (2011) 259–264

used as received). Ortho-toluidine from CDH, 2,4,5-trichlorophenoxyacetic acid was obtained from Hindustan Ciba Geigy Ltd. (India). Allother reagents and chemicals were of analytical grade. An electronicbalance (digital, Sartorius-21OS, Japan) and an automatic tempera-ture controlled water bath incubator shaker (Elcon, India) were used.

2.2. Preparation of reagents solution

0.1 M solutions of zirconium oxychloride (ZrOCl2•8H2O) andphosphoric acid (H3PO4) of different molarities were prepared in4 M HCl and demineralised water (DMW) respectively. Solutions of20% (v/v) o-toluidine (C7H9N) and 0.4 M ammonium persulphate((NH4)2S2O8) were prepared in 2 M HCl.

The stock solution of 2,4,5-T (0.1 M) was prepared in Ethanol.Different concentrations (10−1–10−12 M) of 2,4,5-T were prepared bydiluting the stock solution with DMW, 0.1 M sodium hydroxide andphenolphthalein indicator.

2.3. Preparation of poly-o-toluidine Zr(IV) phosphate composite cation-exchange material (adsorbent)

The nano-composite cation-exchanger was prepared by sol–gelmixing of poly-o-toluidine, an organic polymer, into the inorganicprecipitate of zirconium(IV) phosphate. In this process, when the gels ofpoly-o-toluidine were added to the white inorganic precipitate ofzirconium(IV) phosphate with constant stirring, the resultant mixturewas turned slowly into a greenish black colored slurries. The resultantgreenish black colored slurries were kept for 24 h at room temperature.

Now the poly-o-toluidine based composite gels were filtered off,washed thoroughly with DMW to remove excess acid and any adheringtrace of ammonium persulphate. Thewashed gel dried over P2O5 at 40 °Cin an oven. The dried product was washed again with acetone to removeoligomers present in thematerial, and dried at 40 °C in an oven. The driedproduct was cracked into small granules and converted into H+ form bytreating with 1 M HNO3 for 24 h with occasional intermittent shaking,replacing the supernatant liquid with fresh acid 2 to 3 times. The excessacid was removed after several washings with DMW and finally dried at50 °C. The particle size of approximately 125 μmof the composite cation-exchanger was obtained by sieving and stored in desiccators. Hence anumber of poly-o-toluidine Zr(IV) phosphate nano-composite cation-exchanger samples were prepared, and on the basis of Na+ ion exchangecapacity (i.e.c.) that was found, 1.71 meq dry g−1 [33] was selected forfurther adsorption studies.

3. Characterization

3.1. Characterization of adsorbent

3.1.1. TEM (transmission electron microscopy) studiesTEM studies were carried out to know the particle size of the poly-

o-toluidine Zr(IV) phosphate composite cation-exchange material.

3.1.2. SEM (scanning electron microscopy) studiesMicrophotographs of the original form of poly-o-toluidine, (S-3);

inorganic precipitate of Zr(IV) phosphate, (S-2) and organic–inorganiccomposite materials poly-o-toluidine zirconium(IV) phosphate, (S-5)were obtained by a scanning electron microscope at variousmagnifications.

3.2. Characteristics of 2,4,5-trichlorophenoxy acetic acid

3.2.1. Origin/derivation2,4,5-T is produced synthetically by the reaction of 2,4,5-

trichlorophenol, chloroacetic acid and sodium hydroxide. Lindaneproduction residues are also useful for the 2,4,5-T synthesis.

3.2.2. Humans/mammals2,4,5-T considerably irritates the eyes and the skin. Skin resorption

is likely. Chronic exposure causes impairment of the liver function,changes in behaviour and nerve damage. 2,4,5-T is found to be slightlymutagenic and teratogenic. There are not enough facts to definitelyattest cancerogenic effects.

3.2.3. Plants2,4,5-T is an effective defoliant. Plants (and in particular

dicotyledons) absorb 2,4,5-T through their leaves and metabolisethe substance. Numerous metabolic processes are impaired.

3.2.4. Environmental behaviourThere is moderate sorption of 2,4,5-T on organic suspendedmatter

and on sediments. In soil, 2,4,5-T is either microbially degraded or itevaporates into the atmosphere.

Characteristic properties and toxicity or LD50 oral effects of 2,4,5-trichlorophenoxy acetic acid on different mammals are shown inTables 1 and 2.

4. Equilibrium studies

In 6 different 100 ml conical flasks, different volumes of 2,4,5-Tand water were added, as shown in Table 3. The solutions wereshaken well and 0.5 g of adsorbent (poly-o-toluidine Zr(IV) phos-phate) was added in each flask. The conical flasks were closed byrubber stoppers and let stand for about 24 h with intermittentshaking. Then the contents were filtered and titrated against N/10sodium hydroxide solution using phenolphthalein as indicator. Threereadings in each case were taken.

The adsorption percentage (Ads %) from standard solution wascalculated as:

Ads % = Cο−Ce.

Cο× 100 ð1Þ

where C0 and Ce are initial andfinal concentrations of 2,4,5-T respectively.

5. Fabrication of pesticide sensitive membrane electrode ofpoly-o-toluidine Zr(IV) phosphate cation exchanger

The membrane electrode was fabricated as reported in a previousstudy [32]. The whole arrangement is shown below:

Internal referenceelectrode (SCE)

Internalelectrolyte 0.1 M2,4,5-T

Membrane

Samplesolution

Externalreferenceelectrode (SCE)

The following parameters were evaluated to study the sensitivityof the electrode for the “2,4,5-T ” such as lower detection limit,electrode response curve and response time.

Table 2Toxicity or LD50 oral effect of 2,4,5-trichlorophenoxy acetic acid on some mammals andbirds.

MammalsRat LD50 500 mg/kg, oral acc. PERKOW, 1989Rat LD50 N5000 mg/kg, dermal acc. PERKOW, 1989Rat LD50 300–800 mg/m3, oral acc. RIPPEN, 1989Rat LC50 0.83 mg/l, inh. (4 h) acc. RIPPEN, 1989Mouse LD50 389 mg/kg, oral acc. RIPPEN, 1989Dog LD50 100 mg/kg, oral acc. RIPPEN, 1989Guinea pig LD50 380 mg/kg, oral acc. RIPPEN, 1989

BirdsChicken LD50 310 mg/kg, oral acc. RIPPEN, 1989

Fig. 1. Transmission electronmicroscopy (TEM) shows theparticle size of poly-o-toluidineZr(IV) phosphate cation-exchange material.

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5.1. Electrode response or membrane potential

A series of 2,4,5-T solutions of varying concentrations ranging from10−1 M to 10−12 M were prepared. External electrode and pesticideselective membrane electrode are plugged in digital potentiometerand the potentials were recorded. First, the electrode was soaked in1×10−4 M solution of 2,4,5-T for 2–3 days and for 1 h before use.When electrode was not in use it was kept in 1×10−4 M solution.Measured potential was plotted against selected concentration of thepesticide in solution.

5.2. The response time

The electrode is first dipped in a 1×10−5 M solution of 2,4,5-T andthen into 1×10−4 M concentration. The potential of the solution wasread at 0 s; just after dipping of the electrode in the second solution,the potential was recorded subsequently at the interval of 5 s. Thepotentials were then plotted vs. time.

6. Results and discussion

6.1. Characterization of adsorbent

From the TEM studies it is clear (Fig. 1) that the poly-o-toluidine Zr(IV) phosphate cation-exchange material shows particle size range of42.0 to 100.0 nm, thus the material particle size shows the nano-range.

SEM photograph of poly-o-toluidine Zr(IV) phosphate obtained atdifferent magnifications (Fig. 2) indicates the adhesion between twophases, i.e. inorganic ion-exchange material with organic polymer(poly-o-toluidine). The SEM pictures showed the difference in surfacemorphology of organic polymer, inorganic precipitate and compositematerial. It has been revealed that after binding of poly-o-toluidinewith Zr(IV) phosphate, the morphology has been changed.

6.2. Adsorption isotherms

The adsorption of 2,4,5-T on poly-o-toluidine Zr(IV) phosphatecation exchanger is represented by the adsorption isotherm. Out of

Table 3Different volumes of 2,4,5-trichlorophenoxy acetic acid and water use in the adsorptionprocess.

S. no. Volume of 0.1 M 2,4,5-trichlorophenoxyacetic acid

Volume of water(ml)

Total volume(ml)

1. 30 0 302. 25 5 303. 20 10 304. 15 15 305. 10 20 306. 5 25 30

the possible several isotherm equations, two have been applied forthis study, the Freundlich and Langmuir isotherms.

At a given temperature, the mass of a solute (or gas) adsorbed by asolid adsorbent at various concentrations (pressure in case of gases) isgiven by the linear form of Freundlich equation [34] expressed as

q = x =m = K: C1 = ne ð2Þ

In Eq. (2), x/m is the amount of 2,4,5-T adsorbed per unit weightof cation-exchanger(mg/g), and Ce is the equilibrium concentration of2,4,5-T (mg/l). Calculated and experimental q and Ce values obtained atdifferent initial 2,4,5-trichlorophenoxy acetic acid concentrations areshown in Table 4.

K and n are constants with K as the adsorption capacity of thesorbent and n giving an indication of how favorable the adsorptionprocess. Themagnitude of the exponent, 1/n, gives an indication of thefavorability of adsorption. Values of nN1 represent favourableadsorption condition [35,36]. K and n are Freundlich constants canbe calculated from the linear plot of log q vs. log Ce as shown in Fig. 3.The values of K and n can be determined from the intercept and slope,respectively of the logarithmic plot in Eq. (2).

log q = log K + 1 = n log Ce ð3Þ

The Langmuir adsorption model [37] is based on the assumptionthat maximum adsorption corresponds to a saturated monolayer of

Fig. 2. Scanning electron microphotographs (SEM) of chemically prepared poly-o-toluidineZr(IV) phosphate composite system at 2500× magnification.

Table 4Calculated and experimental q and Ce values obtained at different initial 2,4,5-trichlorophenoxy acetic acid concentrations.

S. no. Initial conc. of 2,4,5-T(C)

Equil. conc. of 2,4,5-T after adsorption(Ce)

Amount of 2,4,5-T adsorbed(x)

Weight of adsorbent(m)

q=x/m log q log Ce 1/q 1/Ce

1. 0.1 0.086 0.3924 0.5 0.7849 −0.105 −1.066 1.274 11.6282. 0.0833 0.0738 0.3368 0.5 0.6736 −0.172 −1.132 1.485 13.5503. 0.0667 0.0617 0.2815 0.5 0.563 −0.250 −1.210 1.776 16.2084. 0.05 0.042 0.1916 0.5 0.3832 −0.417 −1.377 2.610 23.8105. 0.0333 0.0273 0.1246 0.5 0.2492 −0.604 −1.564 4.013 36.6306. 0.01667 0.0137 0.0624 0.5 0.1248 −0.904 −1.863 8.013 72.993

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solute molecules on the adsorbent surfaces. The linear expression ofthe Langmuir model is given by Eq. (4)

q = abCe = 1 + bCeð Þ ð4Þ

or its linear form

Ce = q = 1= ab + Ce = a ð5Þ

where a and b are the Langmuir constants, related to the capacity andenergy of adsorption respectively, and can be determined from theslope and intercept of the Ce/(x/m) vs. Ce linear plot (Fig. 4). UsingLangmuir constant (b), a dimensionless constant termed as separationfactor RL could be calculated according to Eq. (6)

RL = 1 = 1 + bCoð Þ ð6Þ

where Co is the initial concentration of adsorbate in water. Depending onthe values of RL, the adsorption can be said favorable, unfavorable orirreversible. The values of RL from 0 to 1 are favorable. When a value isgreater than 1 the adsorption is unfavorable and zero signifies irreversibleadsorption [38].

From Figs. 3 and 4, the values of constants a and b in the Langmuirmodel and n and K in the Freundlich model and the linear regressioncoefficients (R2) had been calculated and are given in Table 5.

The values of constants a in Langmuir isotherm and K in Freundlichisotherm were 1.2 mg/g and 0.95 mg/g respectively, indicatingappreciable adsorption of 2,4,5-T on the surface of cation-exchanger.The value of n was 1.0 and signifies favorable adsorption. The higher

Fig. 3. Freundlich adsorption isotherm for 2,4,5-trichlorophenoxy acetic acid on poly-o-toluidine Zr(IV) phosphate.

value of KL calculated by Eq. (3) was 0.99 when the 0.1 mg/l concentration of 2,4,5-T was taken.

For 2,4,5-T, the values of a and b are found to be 1.2 mg/g and 0.1 l/mg(R2=0.999) respectively and the values of K and n are found to be0.95 mg/g and 1 (R2=0.998) respectively. Thus the Freundlich equationbecomes

q = 0:95C1e ð7Þ

and the Langmuir equation becomes

q = 0:12Ce = 1 + 0:1Ceð Þ ð8Þ

6.3. 2,4,5-Trichlorophenoxy acetic acid sensitive membrane electrode

Thepesticide (2,4,5-T) sensitive heterogeneous precipitatemembraneelectrode obtained from poly-o-toluidine Zr(IV) phosphate cationexchange material gave linear response in the range 1×10−1 M and1×10−6 M. Theworking concentration range is 1×10−1 M to 1×10−9 M(Fig. 5) for 2,4,5-T with a lower Nernstian slope of 28.1 mV per decadechange in 2,4,5-T concentration. The limit of detection determined fromthe intersection of the extrapolated segments of the calibrationgraph [39,40] was found to be 1×10−6 M. The potential response ofpoly-o-toluidine Zr(IV) phosphate cation exchange membrane electrodeof varying concentration from 1×10−1 M to 1×10−9 M of 2,4,5-T wasexamined in Fig. 5, which also depicts a linear range from 1×10−1 M to1×10−6 M with a lower Nernstian slope of about 28.1 mV per decadechange in 2,4,5-T concentration.

Fig. 4. Langmuir adsorption isotherm of 2,4,5-trichlorophenoxy acetic acid on poly-o-toluidine Zr(IV)phosphate.

Table 5Values of Langmuir and Freundlich constants and regression coefficients.

Freundlich constants Langmuir constants

K (mg/g) 0.95 a (mg/g) 1.2n 1 b (l/mg) 0.1R2 0.998 R2 0.999

Fig. 6. Time–response curve of 2,4,5-T sensitive membrane electrode.

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Promptness of the response of the pesticide sensitive electrodewas also determined. The average response time is defined as the timerequired for the electrode to reach a stable potential. The potentialwas also examined as a function of time over a concentration range of2,4,5-T and the results are summarized in Fig. 6. An average responsetime of about 15 s was found in the entire examined concentrationrange. The membrane could be successfully used up to one monthwithout any notable change in potential during which the potentialslope is reproducible within ±1 mV per concentration decade. If adrift in the potential is observed, the membrane is re-equilibratedwith 1×10−4 M solution for one week.

7. Conclusions

The present study shows that nano-composite cation-exchangeri.e. poly-o-toluidine Zr(IV) phosphate can be used as an adsorbent forthe removal of 2,4,5-T from aqueous solutions. The equilibrium datawere fitted to the Langmuir and Freundlich isotherm models, and theequilibrium data were best described by the Freundlich isothermmodel. The results indicated that the poly-o-toluidine Zr(IV) iseffective for the adsorption of 2,4,5-T from aqueous solutions. TEMphotograph shows the particle size of the composite material inbetween 42.0 and 100.0 nm and thus material can be considered asnano-composite. Sensitivity of 2,4,5-T towards poly-o-toluidine Zr(IV) phosphate nano-composite cation exchanger membrane elec-trode is indicated by change in potential with change in concentra-tion. Themembrane electrode possessed a quick response time of only10 s with a slope of 28.1 mV per decade change in the concentrationrange of 1×10−1–1×10−6 M. The membrane electrode showed good

Fig. 5. Calibration curve for 2,4,5- T sensitive membrane electrode in an aqueoussolution.

reproducibility detection limit and could be used successfully up toone month.

Acknowledgement

The authors are thankful to the Department of Applied Chemistry,Z.H. College of Engineering and Technology, Aligarh Muslim Univer-sity, A.M.U. (India) for providing research facilities. Special thanks toProf. H.S. Rathore for his guidance at various stages of the researchwork. The assistance providedby the R.S.I.C. Bombay, I.I.T. Delhi and I.I.T.Roorkee to carry some instrumental analysis are also acknowledged.Lastly, the authors are thankful to the Ministry of Environment andForest for providing facilities to carry the research work.

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