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2175 © IWA Publishing 2012 Water Science & Technology | 65.12 | 2012

Degradation of azo dye with dirhodium(II) caprolactamate

as heterogeneous catalyst

Abeer S. Elsherbiny, Sahar H. El-Khalafy and Michael P. Doyle

ABSTRACT

The kinetics of the oxidative degradation of an azo dye Metanil Yellow (MY) was investigated in

aqueous solution using dirhodium(II) caprolactamate, Rh2(cap)4, as a catalyst in the presence of H2O2

as oxidizing agent. The reaction process was followed by UV/Vis spectrophotometer. The

decolorization and degradation kinetics were investigated and both followed a pseudo-first-order

kinetic with respect to the [MY]. The effects of various parameters such as H2O2 and dye

concentrations, the amount of catalyst and temperature have been studied. The studies show that

Rh2(cap)4 is a very effective catalyst for the formation of hydroxyl radicals HO• which oxidized and

degraded about 92% of MY into CO2 and H2O after 24 h as measured by total carbon analyzer.

doi: 10.2166/wst.2012.136

Abeer S. Elsherbiny (corresponding author)Sahar H. El-KhalafyChemistry Department, Faculty of Science,Tanta University, Tanta 31527,EgyptE-mail: abeer.elsherbiny@yahoo.de

Abeer S. ElsherbinyDepartment of Chemistry, Science and Art College,King Abdul Aziz University,Rabigh Campus, Rabigh 21911,Kingdom of Saudi Arabia

Michael P. DoyleDepartment of Chemistry and Biochemistry,University of Maryland,College Park, Maryland 20742,USA

Key words | catalytic oxidation, kinetic, Metanil Yellow

INTRODUCTION

Azo dyes constitute an important class of synthetic color,organic compounds, which are characterized by the pres-ence of one or more azo bonds (–N¼N–). They represent

about 50% of the worldwide dye production and arewidely used in a number of industries, such as textiledying, food, cosmetics, paper printing, with the textile indus-

try as the largest consumer (Ollgaard et al. ; Stolz ).It was reported that, during manufacturing or processingoperations, dye loss in wastewaters could vary from 2% for

basic dyes to as high as 50% for reactive dyes (Neill et al.). This leads to severe contamination of surface andground water as many of these compounds are not readily

biodegradable, suspected to be carcinogenic and can pro-duce toxic aromatic amines (Chung & Cerniglia ;Brown & De Vito ; Biano-Prevot et al. ; Neppolianet al. ). Up to now, the treatment methods for azo dyes

can be divided into three categories: (1) physical methods(Tan et al. ), such as flocculation, adsorption, nanofiltra-tion and ion exchange; (2) chemical methods (Feng et al.), such as the oxidative process with Fenton’s reagent,H2O2-ozone, H2O2-UV radiation, NaOCl and O3, and soon; and (3) biological degradation (Khan & Husain ),

including aerobic and anaerobic degradation. Physicalmethods are unable to absolutely decompose azo dyes,and chemical methods with high cost are rarely used inthe actual treatment process. Biodegradation processes are

the most widely used methods, are suffered from lowdegradation efficiency or even no degradation for somedyes; and the requirements of a large volume of treatment

equipment, so the combination of pretreatment processes,which transform the azo dyes into easier bio-decompositioncompounds, with biodegradation process is preferred

(Scott & Ollis ).Hydroxyl radicals generated in sufficient quantities by

advanced oxidation processes (AOPs), have the ability to oxi-

dize the majority of organic pollutants present in thewastewater effluents (Shu & Chang ; Kusic et al. ).Chemical oxidation technologies using oxidizing agents

such as H2O2 exhibited lower degradation rates comparedwith the processes based on the generation of free radicals(Echigo et al. ; Salem ; Bacioglu et al. ).Addition of catalysts to the systems that contain H2O2

leads to the generation of HO• which has a redox potentialof 2.8 eV (Wardman ). These radicals exhibited fasterrates for the degradation reaction compared with those of

conventional oxidants (Aplin & Waite ; Gemeay et al., ; Bali et al. ; Sonune & Ghate ).

In the present study, we reported a highly efficient

heterogeneous oxidation system using dirhodium(II)caprolactamate Rh2(cap)4 as a catalyst in aqueous mediumusing diluted hydrogen peroxide as oxidizing agent fordegradation of a monoazo dye, Metanil Yellow (MY) (see

2176 A. S. Elsherbiny et al. | Catalytic decomposition of azo dye in aqueous medium Water Science & Technology | 65.12 | 2012

Figure 1). MY was selected due to its toxicity and carcino-

genic nature as well as its presence in the wastewater ofseveral industries such as textile, tannery, soap, cosmetics,polishes, etc. The kinetics of the oxidative degradation of

MY dye using dirhodium(II) caprolactamate Rh2(cap)4with hydrogen peroxide were studied. The Langmuir–Hinshelwood (L–H) kinetic treatment is known to be agood model for the description of solid–gas reactions. Extra-

polation of this model to solid–liquid reactions requiressome modifications. The (L–H) kinetics, could well explainthe decrease of reaction constant with increasing MY con-

centration. Many reports have used the L–H model todescribe the degradation characteristic (Lu et al. ;Kumar & Davis ; Peil & Hofmann ; Wang et al.; Zhang et al. ). All the kinetics variables wereinvestigated, such as dye concentration, catalyst amount,hydrogen peroxide concentration, and temperature. Identifi-cation of possible degradation products during the process

was done.

EXPERIMENTAL

Materials

Reactive MY (98% purity) was purchased from B.D.H.Chemicals Ltd and used as received. The structural formula

of MY is shown in Figure 1. The stock solution of MY dyewas prepared in double-distilled water. The preparation ofdirhodium(II) caprolactamate has been previously described(Doyle et al. ). Hydrogen peroxide (30%W/V, Merck)

was used. The initial concentration of H2O2 was determinediodometrically using standard KMnO4 solution (Scott &Ollis ).

Kinetics measurements

In a typical kinetics experiment, a number of conicals(100 mL) containing a definite amount of the catalysttogether with the appropriate volume of MY solution(4.6 × 10�6 mol L�1) were placed in a water shaker

Figure 1 | Chemical structure of Metanil Yellow (MY).

thermostat at 25 WC± 0.2 in order to attain the desired temp-

erature. To each reactor an appropriate volume ofthermostatted 1.66 mol L�1 H2O2 solution was added andzero time was noted. The conicals were removed at regular

time intervals. The solid catalyst was quickly separated fromthe dye solution by filtration to quench the reaction. The fil-trate was then transferred into a 1 cm quartz cell to measureits optical density at λmax¼ 498 nm. The effect of reaction

conditions, namely dye concentration, H2O2 concentration,temperature, and amount of catalyst was studied to optimizethe reaction parameters for which the reaction was per-

formed by changing one of the parameters and keepingothers constant. Generally, the dye/H2O2 mixture wasstable for several hours without changes in the absorbance.

This means that no reaction takes place between MY andH2O2 in the absence of the catalyst.

Instruments

The kinetics of oxidation and decolonization of MY werefollowed using a UV/Vis Shimadazu-2001S double beamrecording Spectrophotometer linked to an electronic temp-

erature controller (TCC-260). A shaker water thermostat(Julabo SW20C) was used to shake the heterogeneous reac-tion mixture at 120 rpm at a fixed temperature ±0.2 WC.

In order to determine the extent of the mineralization,

total organic carbon (TOC) measurement was performedon filtered suspension sample using a Total Carbon Analy-zer Sevier 5310C. HNMR spectra were recorded on

Bruker DRX-500 MHz spectrometers.

RESULTS AND DISCUSSION

Kinetics measurements of the oxidation

A preliminary experiment was carried out to evaluate the

extent of the adsorption process of MY at the catalyst sur-face. As shown in Figure 2, there is a very slow decreaseof the MY concentration (only ≈4%) which is due to a

small adsorption of MY at the catalyst surface. This loweringin the MY concentration occurred without a color changefrom yellow to pink.

All the kinetics runs were measured under pseudo-first-

order conditions, in which the concentration of H2O2 was at10-fold excess that of the MY dye. Figure 3 presents thechange of the absorbance of MY with time when H2O2

and the catalyst were added to it. It is clear in thisfigure that the absorbance of MY at λmax¼ 434 nm

Figure 2 | Concentration trace with time for MY dye when brought into contact with

0.003 g Rh (IV) caprolactamate catalyst at 25W

C.

Figure 3 | Time-resolved absorption spectraof the reaction ofMYdye (4.45 × 10�6mol L�1)

with H2O2 (0.66 mol L�1) in the presence of 0.003 g Rh(IV) caprolactamate

catalyst and at 25W

C.

Figure 4 | First-order plot for the oxidation of MY dye with H2O2 in the presence

of 0.003 g of Rh(IV) catalyst at 25W

C. [MY]0¼ 4.45 × 10�6 mol L�1 and

[H2O2]0 ¼0.66 mol L�1.

2177 A. S. Elsherbiny et al. | Catalytic decomposition of azo dye in aqueous medium Water Science & Technology | 65.12 | 2012

disappeared and a new peak at λmax¼ 498 nm appeared.The value of this new peak at λmax¼ 498 nm increased

with time until it reached a maximum value at about

Table 1 | Specific rate constant of oxidation k, specific rate constant of decolorization k’ and

Temp (WC) k (10–3 L mol�1s–1) k’ (kJ/mol) E (kJ/mol

20 3.78

25 9.60 4.37 88.02

30 16.08

35 22.37

40 min from the reaction starting time. Then, it decreasedagain with time and the solution became colorless. Thismeans that the degradation of MY produces some colored

compounds (intermediate) that took a longer time to beremoved. This was further confirmed by characterizing sev-eral intermediates using 1HNMR techniques.

The plot ln(Ao/At) vs time, according to the first-orderequation: ln(Ao /At)¼ kobst for the increasing in the absor-bance at λmax¼ 498 nm, was linear in the first 10 min ofthe reaction, Figure 4, and after that it became curved in

the last 30 min of the reaction due to the small changein the absorbance of the product. This indicates the first-order kinetics of the reaction with respect to the MY con-

centration. This behaviour is rationalized in terms of amodified form of the L–H kinetic treatment, which hasbeen used successfully as a qualitative model to describe

solid–liquid reactions, where Ao is the absorbance of MYat t¼ 0 and is equal to the absorbance of the product att! ∞. At is the absorbance at time intervals, and kobs is

the observed rate constant. Values of k (specific rate con-stant) were determined from the relation k¼ kobs/[H2O2]and are listed in Table 1.

activation parameters for the oxidation of MY

) ΔH# (kJ/mol) ΔG# (kJ/mol) ΔS# (Jmol–1deg–1)

85.55 �74.22 536.14

2178 A. S. Elsherbiny et al. | Catalytic decomposition of azo dye in aqueous medium Water Science & Technology | 65.12 | 2012

A modified form of the L–H kinetic model is:

r ¼ �dC=dt ¼ kθ ¼ kKC0=1þ KC0 (1)

where r is the reaction rate for the oxidation of MY (molL�1 min�1), t is the reaction time, C is the concentration of

MY (mol L�1), θ is the fraction of surface covered, k is thespecific reaction rate constant for the oxidation of MY, C0

is the initial concentration of MY (mol L�1), and parameter

K represents the equivalent adsorption coefficient. The plotof the reciprocal reaction rate (1/r) with the reciprocal of theinitial dye concentration (1/C0) was linear (Figure 5) with

R2¼ 0.973.

Figure 6 | Absorbance spectra for the decolorization of MY dye with H2O2 in the pres-

ence of 0.003 g Rh(IV) catalyst at 25W

C. [MY]0¼ 4.45 × 10�6 mol L�1 with

[H2O2]0¼ 0.66 M. The inset shows the decrease of absorbance at λmax¼498 nm with time.

Kinetics measurements of the decolorization

The absorbance at λmax ¼ 498 nm started to decrease after

reaching its maximum value at about 40 min from thereaction starting time as shown in (Figure 6). Thisdecrease was fast in the beginning and became slower

and slower until the solution was colorless after 24 and/or 48 h depending on the initial concentration of theMY, [H2O2] and the amount of catalyst. The kinetics of

the decolorization of the intermediate was followed byrecording the decrease in the absorbance with time andplotting ln At vs time (Figure 7) according to the first-

order equation ln(At)¼ ln(Ao)–k0obs t. The plot was linear

with a slope equal to, k0obs which is the observed rate con-

stant for the decolorization. The specific rate constant

Figure 5 | Variation of the degradation rate of MY with its initial concentration according

to H-L kinetic model.

Figure 7 | First-order plot for the decolorization of MY dye with H2O2 in the presence

of 0.003 g of Rh(IV) catalyst at 25W

C. [MY]0¼ 4.45 × 10�6 mol L�1 and

[H2O2]0 ¼0.66 mol L�1.

of decolorization, k0, was calculated from the relationk0 ¼ k0

obs/[H2O2] and listed in Table 1. As can be seen,

the k0 value was much smaller compared with k, whichmeans the solution decolorization was much slower thanthe degradation of MY. From the above results, it can be

concluded that the oxidation of MY with H2O2 in thepresence of the Rh-complex catalyst is involved in twosteps: the first one is the appearance of the colored inter-

mediate which absorbs at λmax¼ 498 and the second oneis the decolorization of this intermediate.

2179 A. S. Elsherbiny et al. | Catalytic decomposition of azo dye in aqueous medium Water Science & Technology | 65.12 | 2012

In the absence of the catalyst, the dye/H2O2

mixture was stable for several hours without any noticeablechange in the absorbance of the MY, which indicates that noreaction had taken place between the dye and H2O2.

After 24 h about 92% of the dye was degraded comple-tely into its mineral components and this was identifiedby measuring with Total Carbon Analyzer and 1HNMRanalysis. 1HNMR spectra of MY in deuterated water at

the beginning and at the end of the reaction are given inFigure 8(a) and 8(b). All these measurements indicatedthat the catalytic decolorization of MY may be attributed

to breakdown of the azo bond, hydroxylation of the aro-matic ring leading at the end to ring opening andformation of carboxylic acids which undergo oxidation to

form CO2 and water (Sleiman et al. ).

Figure 8 | (a)1HNMR of MY in D2O at the beginning of the reaction. (b)1HNMR of MY in D2O

at the end of the reaction.

Factors affecting the oxidation rate

Effect of [H2O2]

The effect of the initial concentration of [H2O2], on the reac-tion rate was investigated at constant [MY]¼ 2.98 × 10�6

mol L�1, and constant amount of the catalyst (wt¼0.001 g) at 25 WC. The relation between the observed rate

constant and the [H2O2] is presented in Figure 8; in thisfigure, Increasing H2O2 concentration from 0.5 to2.25 mol L�1 led to increase in the reaction performance

because more hydroxyl radicals were formed. Also,(Figure 9) indicates that the reaction follows first-order kin-etics with respect to the [H2O2]. The slope of this straight

line is equal to the specific rate constant k, since kobs¼ k[H2O2].

Effect of [Metanil Yellow]

The influence of the dye concentration is important from theapplication point of view. Thus [H2O2] and the mass of the

catalyst were kept constant at 0.664 mol L�1 and 0.001 g,respectively. The [dye] was varied in the range 2 × 10�6 to7 × 10�6 mol L�1. Under these conditions, the reaction

obeyed pseudo-first-order kinetics with respect to [MY]. Asshown in (Figure 10), the values of the observed rate con-stant decreased with increasing concentration of MY. The

presumed reason is that when the [MY] increases, more mol-ecules are adsorbed on the surface of the catalyst. The largeamount of adsorbed MY dye is thought to have an inhibiting

effect on the generation of free radicals species, due to the

Figure 9 | Dependence of the observed rate constant kobs. on the initial concentration of

H2O2. [MY]0¼ 4.45 × 10�6 mol L�1, amount of Rh(IV) catalyst¼ 0.003 g at 25W

C.

Figure 10 | Dependence of the observed rate constant upon the initial dye concen-

tration, [H2O2]0¼ 0.66 mol L�1, amount of Rh(IV) catalyst¼ 0.003 g at 25W

C.

2180 A. S. Elsherbiny et al. | Catalytic decomposition of azo dye in aqueous medium Water Science & Technology | 65.12 | 2012

retardation of the interaction between H2O2 and the catalyst(Gemeay et al. ).

Now, from the above-mentioned results of the individual

orders, the reaction rate law can be suggested as:

d[MY]=dt ¼ k[MY][H2O2] (2)

Effect of the amount of catalyst

As can be seen in Figure 11 the variation of the observed

rate constant ko is linear as a function of the amount ofthe catalyst and no quadratic effect was associated with it.This linear relation in (Figure 11) confirms the first-order

kinetics with respect to the amount of the catalyst. Thus

Figure 11 | Relationship between the observed rate constant and the amount of Rh(IV)

catalyst, [H2O2]¼ 0.66 mol L�1, [MY]0¼ 4.45 × 10�6 mol L�1 at 25W

C.

the rate equation can be written down in the following form:

d[MY]=dt ¼ k[MY][H2O2][catalyst] (3)

Effect of temperature

Temperature is a crucial parameter in most of the catalyzedreactions, particularly those used in environmental and

industrial applications. The effect of temperature on therate of oxidation of MY was studied by varying the tempera-ture in the range 293–308 K and the data are given in

Table 1. The results indicate that the extent of oxidation ofMY increases with increase in temperature. According tothe apparent kinetic rate constants of oxidation at differenttemperatures, the apparent activation energy E, was com-

puted with the Arrhenius equation (Equation (4)):

k ¼ A exp(� E=RT) (4)

where k is the specific rate constant (in L mol�1 s�1), A isthe preexponential (or frequency) factor, E is the apparentactivation energy (kJ mol�1), R is the ideal gas constant(8.314 J mol�1 K�1); T is the reaction absolute temperature.

The Arrhenius plot of ln k vs I/T was linear (not shown)with correlation coefficient R2¼ 0.987. The apparent acti-vation energy E, obtained from the Arrhenius type plot

was found to be 88.02 kJ/mol (see Table 1). The change inthe free energy of activation, ΔG#, was evaluated from Eyr-ing’s equation (Yeremin ) using the data in Table 1:

k ¼ KbT=hð ÞeΔG#=RT ð5Þ

where Kb is Boltzmann’s constant, T is the temperature in

Kelvin, R is the gas constant and h is Planck’s constant.The change in the enthalpy of activation, ΔH#, was

deduced from the relationship (Yeremin ):

ΔH# ¼ E� RT ð6Þ

The change in the entropy of activation was determinedfrom the relationship (Yeremin ):

ΔG# ¼ ΔH# � TΔS# ð7Þ

All the activation parameters of the oxidation of MY dye

are depicted in Table 1. The higher value of E, than those ofa diffusion controlled reaction (10–21 kJ mol�1) (Pilling &

2181 A. S. Elsherbiny et al. | Catalytic decomposition of azo dye in aqueous medium Water Science & Technology | 65.12 | 2012

Seakins ) indicated a chemical reaction control. Also,

the higher value of, E (88.02 kJ mol�1) was evidence thatthe oxidation reaction of MY dye is entropy controlled(El-Daly et al. ), as the greater the value of ΔS#, the

greater is the probability of the formation of the activatedcomplex.

Mechanism

In view of the above experimental findings, the following

reaction mechanism was proposed. The first step of the reac-tion mechanism is the adsorption of H2O2 at the catalystsurface and formation of electron transfer set up complexin the pre-equilibrium stage (Equation (7)). This step is fol-

lowed by oxidation of the catalyst since Rh2(IV) wastransferred to Rh2(V) and the released electrons producethe radical species •OH and OH� from H2O2 (Equation (8))

(Catino et al. ). The association of the �OH withRh2(V) could change the oxidation state of Rh2(V) to yieldRh2(IV) (Equation (9)) (Catino et al. ). The generated

radical species •OH will attack rapidly the MY molecules(Equation (10)) to form the reaction intermediate. In thefinal step, the intermediate is decomposed into CO2 andwater.

Rh2ðcapÞ4 þH2O2 �����→←����� ½Rh2ðcapÞ4H2O2�electron transfer set up complex ð8Þ

[Rh2(cap)4H2O2] ! Rh 5þ2 þ �OHþ �OH (9)

Rh 5þ2 þ �OH ! RH 4þ

2 þ �OH (10)

dyeþ �OH ! oxidation products (11)

CONCLUSION

Addition of dirhodium(II) caprolactamate to theMY solutionin the presence of H2O2 led to a complete mineralization ofthe dye. The reaction takes place in two steps: the first step

is the appearance of the colored intermediate which absorbsat λmax¼ 498. The second one is the decolorization of thisintermediate. Investigation of the kinetic variables showed

that the maximum rates were obtained at [MY]0¼ 2.44 ×10�6 mol L�1, [H2O2]0¼ 0.9 mol L�1 and catalyst amount¼

0.007 g. These optimum conditions may render the current

method useful in treatment of polluted water with azo dyes.

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First received 25 October 2011; accepted in revised form 2 February 2012