Kinetics and Mechanism Study of Chemical Treatment of Methylene Green by Urea

FULL PAPER

* E-mail: [email protected]; Fax: 0092-021-9261330 Received August 9, 2009; revised October 30, 2009; accepted December 2, 2009.

748 © 2010 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Chin. J. Chem. 2010, 28, 748—754

Kinetics and Mechanism Study of Chemical Treatment of Methylene Green by Urea

Ahmed, Tehseena Uddin, Fahim*,a Azmat, Rafiab a Department of Chemistry, University of Karachi, Karachi 75270, Pakistan

b Department of Chemistry, Jinnah University for Women, Nazimabad, Karachi, Pakistan

The kinetics and mechanism studies, for the reduction of methylene green (MG) by urea, in acidic and alkali media, were studied at λmax＝652.8 nm by monitoring the depletion in MG concentration. The reaction was carried out by UV radiation, with variable dye concentration, reducing agent (urea), acid and base under different additive ions that are very common in dye waste water. The reduction followed pseudo first-order kinetics with respect to different anions, cations, dye, reductant and OH－ ion concentrations. It was found that most of the cations tested showed the inhibitory effect on dye decoloration, due to the formation of insoluble precipitate and followed the order K＋

＞Na＋

＞Al3＋＞Ca2＋

≈Mg2＋. Tested anions showed that the dye decoloration was significantly accelerated and followed the order Cl－＞Br－＞I－＞ 3NO−

＞24SO − . A mechanistic model involving generation of a complex of

dye with ions was proposed.

Keywords methylene green (MG), urea, additive ion, precipitation, OH－ ion, decoloration

Introduction

The release of colored compounds with the effluents from different industrial activities such as paper and pulp manufacturing, dying of cloths, leather treatment, printing, etc. into the environment is undesirable. This is not only because their color may affect the photosynthe-sis of aquatic plants but also they contain a substantial amount of organic dye macromolecules. Their break-down products may be toxic in nature and their removal from industrial effluents is a major environmental prob-lem.1-5 In fact many of these dyes have shown hazardous properties and are toxic in nature due to aromatic rings present in them. Strong color is the most visual charac-teristic of textile waste water. Decoloration has become an integral part of the textile waste water treatment process.6 Decolorization by organic reductant or adding decoloring chemicals is a common practice in the textile industry, which includes photolytic, photocatalytic and bio decoloration and degradation of various dye solu-tions. Advanced oxidation processes based on UV/H2O2 and ozonation represent novel methods and show prom-ising results in degrading and reducing many organic dyes. Furthermore, these processes generally depend upon the generation of OH－ radical, which ultimately increases the oxidation potential. Therefore, a non-biodegradable molecule breaks down into smaller ones, resulting in better color removal. These radicals can react with the dye molecule to undergo a series of reactions and as a result the dye molecule is finally re-duced or converted into simple smaller fractions.7-9

A detailed literature survey has revealed that the oxidation of dyes with many oxidants was reported but no work has been carried out on aerobic reduction of MG with urea under the influence of different additive ions, which are the part of industrial waste effluent. Therefore in this study an attempt has been made to in-vestigate the reduction of MG with urea to decolorize the dye under different optimum conditions. Further-more, it also discusses the reduction kinetics and mechanism involving the role of different ions in bleaching of dye.

Materials and methods

All reagents obtained from E-Merck were used as received. The experiment was divided into different ses-sions, including preparation of solutions, kinetic meas-urements, data analysis and investigation of kinetic salt effect. All solutions were prepared in de-ionized water and diluted before use. Kinetics was monitored on a UV/visible Schimadzu 160A spectrophotometer.

Preparation of sample solutions

MG stock solution of 1×10－4 mol•L－1 was prepared in 250 mL of deionized water. Dilutions of this stock solution were made with deionized water to obtain a series of dye solutions with varying concentrations of reagents.

Kinetics measurements

Kinetics was monitored by preparing three sets of

Study of Chemical Treatment of Methylene Green by Urea

Chin. J. Chem. 2010, 28, 748—754 © 2010 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.cjc.wiley-vch.de 749

reaction mixtures in which one species was varied while other two were kept constant. The three contents were mixed together and the progress of the (inlet) reaction was monitored by recording the change in optical den-sity with time λmax ＝ 652.8 nm on the UV/visible spectrophotometer. The absorbance value monitored in each case was plotted against time to obtain rate constant. The orders of reaction and activation parameters were evaluated by measuring the specific reaction rate at various temperatures and ionic strengths.10,11 Potassium chloride (KCl) and potassium bromide (KBr) were used to maintain the ionic strength of the medium. Percent decrease in absorbance was calculated by using the formula:

f

i

%decrease in absorption 1 100A

A

⎛ ⎞⎜ ⎟⎝ ⎠

＝ ×－

where Af＝absorbance after 15 min or final absorbance. Ai＝absorbance at 0 min or initial absorbance.

HPLC analysis of the decolorized mixture

The HPLC analysis of a mixture of dye was carried out in the presence of urea before and after irradiation. 35 mL of a reaction sample was extracted using chloro-form and then evaporated on a rotary evaporator (Bu-chii). 35 mL of the reaction mixture was taken and irra-diated with a UV bulb for 30 min. An organic compo-nent was formed after irradiation of the reaction sample and also extracted using chloroform and then evaporated on the rotary evaporator (Buchii). The residue was dis-solved into pure methanol and then subjected to HPLC

(Schimadzu) analysis. The samples were run on C8-Eclips columns (Agilent) using 0—100% methanol gradient over 30 min at 1 mL/min and analyzed by measuring the optical density (OD) at the wavelength of 280 nm.7,8

Results and discussion

Kinetics investigation was perused by following the changes in optical density of methylene green at λmax＝

652.8 nm, which are reported in Tables 1—3. Results show that reduction of methylene green with urea obeys pseudo first order kinetics with respect to dye, reductant, H＋ and alkali. The redox reaction of dye was investi-gated under aerobic condition. It was found that initially the reduction proceeded very slowly and then sharply. Less decrease in optical density leads towards a slow reduction process. Each kinetic run was followed by taking protonated MG and urea as the principal reactants. These runs were carried out in the presence and absence of urea (reductant). The absorption spectrum of MG in the presence of HCl, H2SO4 and NaOH is shown in Fig-ure 1. It was observed that in the absence of reductant, MG showed no significant reduction. This reflects the significance of reductant12-15 at low and high pH in the reduction of MG. Furthermore, kinetic study of the re-duction of MG was investigated at different concentra-tions of reductant, dye, monoprotic acid, diprotic acid, alkali and additive ions as illustrated in Table 3. % Decoloration of dye reported in Table 1 showed sig-nificant decoloration in the presence of alkali as com-pared to acids.

Table 1 Effect of change in monoprotic and diprotic acid and alkali on decoloration of methylene green (MG)a

[HCl]×102/(mol•L－1) v×105/(mol•L－1•s－1) k×104/s－1 % decoloration

1.0 1.00 2.23 18.5

2.0 5.00 1.42 13.8

3.0 5.00 1.12 12.2

4.0 5.00 1.05 10.3

5.0 5.00 1.00 8.1

[H2SO4]×102/(mol•L－1)

1.0 0.67 1.10 10.9

2.0 0.67 0.87 8.4

3.0 5.00 0.90 9.8

4.0 5.00 0.73 7.6

5.0 0.33 0.43 5.0

[NaOH]×102/(mol•L－1)

1.0 8.00 2.45 22.1

3.0 6.50 2.15 20.3

5.0 7.00 2.52 21.6

7.0 8.17 3.42 27.3

9.0 8.50 3.48 27.2 a ν＝rate of the reaction, k＝specific rate constant.

Ahmed, Uddin & AzmatFULL PAPER

750 www.cjc.wiley-vch.de © 2010 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Chin. J. Chem. 2010, 28, 748—754

Table 2 Effect of change in anions/cations on rate of reduction of MG with urea

Anion v×104/(mol•L－1•s－1) k×104/s－1 % decoloration

Cl－ 2.77 1.62 14.2

Br－ 2.55 1.53 13.3

I－ 2.48 1.53 13.11

3NO− 2.48 1.48 12.1 24SO − 2.32 1.45 11.9

Cation

Na+ 2.60 1.55 13.5

K+ 2.77 1.62 14.2

Ca2+ 1.77 0.83 6.3

Mg2+ 1.40 0.67 6.3

Al3+ 1.50 0.80 8.1

Table 3 Effect of change in concentrations of dye and reductant

[MG]×106/(mol•L－1) [Urea]×102/(mol•L－1) ν×105/(mol•L－1•s－1) k×104/s－1 % decoloration

1 0.8 0.50 1.47 13.1

1 1 0.67 2.12 18.2

1 2 0.50 2.35 21.3

1 3 1.17 2.62 22.3

1 4 1.33 2.53 20.7

8 1 7.50 3.25 30.85

9 1 7.50 2.72 25.766

10 1 9.00 2.63 25.446

20 1 20.67 2.57 24.828

30 1 32.33 2.37 24.514

Figure 1 Spectral change of MG in acidic and alkaline media.

Reduction investigated under varied concentrations of the dye showed the decrease in percent decoloration of dye solution in Figure 2 and Table 3. This behavior is in coincidence with the work reported earlier.7-16 Inverse relation was observed between dye concentration and decoloration as shown in Figure 2, which may be due to the increase in the number of dye molecules with the same number of absorbed photons. Furthermore, the dye reduction was enhanced with the increase in the concen-tration of urea17-22 reflecting the significance of the re-ductant in oxidation-reduction reaction of dye molecule as given in Figure 3. The rates of reduction were inves-tigated by adding HCl and H2SO4 to ascertain the role of

H＋ ion in dye reduction. It was observed that H2SO4 inhibited the reduction rate as compared to HCl as shown in Figures 3 and 4. This indicates that the pres-ence of H＋ ions in the reaction mixture causes the shift in equilibrium towards backward direction, where oxi-dized form of MG was generated. It may be due to the fact that the urea in acidic medium abstracts proton ac-cording to following reaction:23

2 2 2 3CO(NH ) H NH CONH��⇀↽��

＋＋＋ (I)

The protonated form of urea (i.e. 2 3NH CONH+ ) does not undergo the reduction of dye according to equation (I). Consequently the oxidized form of MG will remain in the reaction mixture. The rate of reduc-tion favored at alkaline pH may be due to the generation of OH－ ion according to the following reaction:23-28

2H O OH Hhv• •⎯⎯→ ＋＋＋

2 2 4CO(NH ) OH NH OCN +OH⎯⎯→－＋－－＋＋

2 2 4(NH ) COH NH OCN H⎯⎯→＋＋－＋＋＋



Figure 2 Effect of dye on rate of decoloration.

Figure 3 Plots for the reduction of methylene green (MG) ver-sus concentration of urea.

Figure 4 Effect of monoprotic acid, diprotic acid and alkali on reduction of dye.

The reactive species of urea in alkaline medium [(NH2)2COH＋] after hydrolysis generates one proton as mentioned in above equation, which is believed to be abstracted by dye and therefore reduction of dye can easily take place in alkaline solution according to the following Eq. (1).

Furthermore, neutral medium was not supportive towards the dye reduction process and no decoloration or reduction was observed, which may be due to the

formation of ammonium carbamide (NH2COONH4), hydrolyzed species of urea, according to following reac-tion.24-28

NH2CONH2＋H2O→NH2COONH4

NH2COONH4→2NH3＋CO2

Effect of additive ions on dye bleaching

The reduction of MG was investigated with some cations and anions that are generally present in the dye waste water.6-8 These ions Na＋, K＋, Ca2＋, Mg2＋ and Al3＋ were selected as cations and Cl－, Br－, I－, 3NO－ and 2

4SO － as anions. These ions were added individu-ally at constant dye concentration, reductant along with the base or acid. Each of these selected ions caused a certain decrease in percent decoloration of MG with urea. In the absence of reductant, no effect on dye re-duction was observed alone, which indicates that reduc-tant is essential for redox reaction of dye.

Effect of additive cations on dye bleaching

The mechanism of dye reduction in the presence of cations is explained by the equations mentioned below, on the basis of their chemical reactions in the reaction mixture. Most likely, Na＋ and K＋ ions undergo the following chemical reactions in solution with OH• radi-cals.

OH•＋Na＋→NaOH

OH•＋K＋→KOH

Table 2 summarizes that the above reactions have an appreciably high percent decoloration as compared with the other cations, which may be attributed to the forma-tion of alkali hydroxide. This supports the dye reduction due to the production of more hydroxyl ions. Similarly, the presence of Ca2＋, Mg2＋ and Al3＋ showed decrease in percent decoloration, which indicates that these ions in dye reduction scavenge the OH• radical according to the following reactions.8 This may result in the precipi-tate formation of their hydroxide, due to which, less de-coloration of dye was observed.

Ca2＋＋2OH•→2OH－

＋Ca2＋ Ca(OH)2

Mg2＋＋2OH•→2OH－

＋Mg2＋ Mg(OH)2

Al3＋＋3OH•→3OH－

＋Al3＋ Al(OH)3

It was found that the precipitate formation in the



presence of Ca2＋ is more than those in Mg2＋ and Al3＋, which explains that Ca2＋ along with OH－ forms in-soluble hydroxide, which inhibits the process of dye reduction/decoloration. The percent decoloration in leuco dye formation along with cations followed the order:

K＋

＞Na＋

＞Al3＋＞Ca2＋

≈Mg2＋

Effect of additive anions on dye bleaching

The dye decoloration with the addition of anions showed that the decoloration proceeded more favorably in the presence of anions than that of cations as illus-trated in Table 2. The values of the rate constant also support the favorable assistance in dye reduction by the anions as illustrated in Table 2. The change in percent decoloration in the presence of halides (Cl－, Br－ and I－) is illustrated in Table 2. Cl－ ions show most effective percent decoloration as compared to its own group but it has also affinity towards hydroxyl group according to the following reactions:18-20

Cl－＋hv→Cl•

Cl－＋Cl•→Cl

•－

OH－

＋Cl－→HOCl•－

HOCl•－＋H＋

→Cl•＋H2O

Furthermore, the presence of Br－ ions also de-creased the percent decoloration due to the following reaction as reported by Kundu et al.:19

Br－＋OH•→Br

•＋OH－

Thus decreasing the concentration of OH－ radical decreases the dye reduction. Similarly I－ ion also acts as a scavenger of hydroxyl radical.

I－＋OH•→OH－

＋I•

Furthermore, the presence of 24SO － ions in neat dye

bath solution causes decrease in percent decoloration as compared to other ions. This may be due to that the re-action of these ions towards hydroxyl radical or absorp-tion of oxygen results in their depletion as follows:10-20

24SO －

＋OH•→ 4SO －·

＋OH－

The dye reduction effected in the presence of 3NO－ may be due to the reaction with OH• radical. This may result in the formation of oxidizing species (HNO3 and O•) due to which decoloration occurs or may be due to degradation of MG.18 These ions in the reaction mixture followed the order:

Cl－＞Br－＞I－＞ 3NO－

＞24SO －

This can be explained by the fact that these ions

readily react with the hydroxyl radical, which may be attributed to the less percent in bleaching of dye. A comparison of added anions and cations, in the dye re-action mixture, suggests that bleaching of dye takes place more rapidly in the presence of anions, which may be due to their role as an electron donor along with the urea.

Effect of temperature on dye bleaching

Usually in the reduction of dyes, temperature plays an effective role. Commonly dye decoloration increases with the increase in temperature. Reactions of MG with urea in both acidic media (i.e. monoprotic and diprotic acids) were studied from 293—313 K. The plots of ln k vs. 1/T were linear as shown in Figure 5. It was found that the rate of reaction was directly related with the temperature.

Figure 5 A plot of ln k vs. 1/T.

A comparison of rate constant in both acidic media (i.e. HCl and H2SO4) at elevated temperatures showed the temperature dependence of rate of reaction. It is proved from Table 4 that the values of rate constant were not significantly different as reported by Sneha-latha et al.22 The energy of activation for HCl is 46.217 kJ/mol, while for H2SO4 is 21.91 kJ/mol as illustrated in Table 5. The value of free energy of activation is ap-proximately the same for both media, indicating the formation of a rigid activation complex. The entropy of activation in H2SO4 appeared to be greater than in HCl, which suggests a loosely bound transition state medium in H2SO4 as compared to HCl, as illustrated in Table 5.22

HPLC analysis of bleached dye sample

HPLC was carried out to verify in the decoloration of dye whether the bleaching of dye owed to leuco dye formation or degradation may also occur in the reaction mixture. HPLC chromatogram showed the degraded dye solution after 30 min of photolytic treatment of dye. From Figure 6, it is clear that no absorption peak of dye was observed in the chromatogram, which indicates that dye is degraded also with the formation of leuco dye. Hence color loss was due to the degradation of the dye with the reduction and the chromatogram showed the



Table 4 Effect of change in temperature on rate constant in monoprotic and diprotic acid

HCl H2SO4 Temperature/K v×106/

(mol•L－1•s－1) k×104/s－1

Comparison with literature22 k×104/s－1 v×106/

(mol•L－1•s－1) k×104/s－1

Comparison with literature22 k×104/s－1

293 1.33 0.47 — 3.33 0.63 —

298 3.33 0.72 — 3.83 0.73 —

303 3.33 1.02 5.13 4.83 0.92 5.56

313 10.00 1.90 9.60 6.67 1.20 10.00

323 13.33 2.70 15.40 8.67 1.43 16.70

Table 5 Activation parameters of the degradation of dye in the presence of HCl

For HCl Temperature/K

Ea/(kJ•mol－1) ∆H/(kJ•mol－1) －∆S/(J•mol－1•K－1) ∆G/(kJ•mol－1)

293 43.78 169.23 93.36

298 43.74 169.37 94.21

303 43.70 169.51 95.06

313 43.61 169.78 96.76

323

46.217

43.53 170.04 98.45

Literature comparision22 — 45.40 34.60 46.60

For H2SO4

293 19.47 250.19 92.78

298 19.43 250.37 94.04

303 19.39 250.50 95.29

313 19.30 250.76 97.79

323

21.91

19.22 250.02 100.30

Literature comparision22 — 18.40 33.70 8.67

Figure 6 Before and after irradiation in UV for 30 min.

small peaks in UV region. It was found that the dye molecule was decomposed into smaller fragments, which conformed that color loss was also related to the degradation along with the reduction as shown in Figure 6.18-20

Mechanism of dye reduction

The kinetic findings indicated that a proton was re-leased in alkaline medium by the reactive species of urea. The released proton was abstracted by the dye molecule to yield MGH (leuco form of methylene green).

12 2 2 2

2

(NH ) CO (NH ) COHk

k��⇀↽�� (1)

32 2 4(NH ) COH MG MGH NH OCN

k⎯⎯→＋＋－＋＋＋ (2)

3 2 2d[MG]

[(NH ) COH ][MG]d

kt

＋－＝ (3)

The steady state for the rate of reaction is given as

1 3 2 2

2 3

[(NH ) COH ][MG]d[MG]

d [MG]

k k

t k k

＋

－＝＋

(4)

Substitute (NH2)2COH＋

＝Ka[(NH2)2CO][H＋]

1 3 a 2 2

2 3

[(NH ) CO][MG][H ]d[MG]

d [MG]

k k Kr

t k k

＋

＝－＝＋

(5)

By taking reciprocal



2

1 3 a 2 2

1 a 2 2

1

[(NH ) CO][MG][H ]

1

[(NH ) CO][H ]

k

r k k K

k K

＋

＋

＝＋

The plot of 1/r and 1/[MG] is a straight line whose

slope＝k2/k1k3Ka[NH2]2CO][H＋]

and

intercept＝1/k1Ka[(NH2)2CO][H＋]

Conclusion

It was concluded that reduction reaction of MG by urea at both acidic and alkaline pH with different added cations and anions led to bleaching of dye with degrada-tion, which followed the first order kinetics. The dye de-coloration was less in the presence of cations, espe-cially in the presence of alkaline earth metals (Ca and Mg). At alkaline pH, formation of precipitate instead of dye de-coloration was observed. This suggests that the presence of these ions may inhibit dye bleaching and their existence may be harmful for the dye waste water, which ultimately threatens aquatic life. The slow de-coloration was observed in the presence of anions due to the effective role of their excess of electrons. This may possibly involves in the dye reduction, resulting in the formation of semi MG and leuco MG. The outcomes of this investigation suggest that the dye waste water should properly be treated with the urea before dis-charging in the aquatic channels to safe the aquatic life because the presence of different radicals in water may retard the bleaching of the dye.

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Kinetics and Mechanism Study of Chemical Treatment of Methylene Green by Urea