Photocatalytic Degradation of Water Soluble and Insoluble Dyes

Narahari Prasanthi† , O.SuryaNarayana and Giridhar Madras*

†Department of Chemical Engineering, Vignan’s Engineering College, Vadlamudi, Email:prashantinarahari@gmail.com

†Department of Chemical Engineering, Vignan’s Engineering College, Vadlamudi, Email:Surya_mango@yahoo.com

*Department of Chemical Engineering, Indian Institute of Science

To whom correspondence should be addressed, Tel: +91-80-309-2321; fax: +91-80-309-2321, Email address: giridhar@chemeng.iisc.ernet.in (Giridhar Madras)

Abstract

The degradation of two commercial dyes heterocyclic (azur pair)

and lysochromic azo dyes (sudan pair) has been studied by using

photocatalytic process under UV iiradiation. Sudan pair is lysochromic

azo dye. Solution combustion synthesized nano-TiO2 was used as the

catalyst.The effect of initial concentration of dye, different catalysts

and solvent on the degradation of dye was studied. It was found that

solution combustion synthesized nano TiO2 was more efficient than

Degussa P-25. A modified Langmuir–Hinshelwood approach was used

to study the kinetics and to determine the adsorption equilibrium

constant and the reaction rate constant.

Keywords: Photocatalysis, Azure, Sudan, dyes, TiO2, dye degradation,

combustion synthesis

Introduction A dye can generally be described as a coloured substance that

has an affinity to the substrate to which it is being applied. The dye is

usually used as an aqueous solution and may require a mordant to

improve the fastness of the dye on the fibre. Dyes are mainly used in

textile industries. In a typical dyeing plant, the major chemical usage

includes dyes or pigments, finishing agents, acids and alkali,

surfactants and auxiliaries. Textile effluents in one way or other way

released into forms of water. Dye effluents leads to acute effects.

Acute effects are the adverse effects that result from a single or short

term exposure to a chemical substance. The symptoms of these effects

generally develop rapidly and may manifest themselves during or

within a few minutes after exposure, or, as in the case of delayed

sensitization, several hours after exposure. Examples of acute effects

that may occur from exposure to some dyes are skin irritation, contact

dermatitis, eye irritation and respiratory tract irritation [2-8]. Some

dyes and dye formulations may be corrosive or can cause eye or skin

burns, and some reactive dyes may produce respiratory sensitization.

In order to overcome all these effects the degradation of dyes should

be done before they are dumped into water forms. Many methods like

adsorption, electrocoagulation, ultrasonic decomposition, advanced

chemical oxidation, nanofiltration, chemical coagulation and so forth

are used for the treatment of textile effluent[2-8]. But among all these

methods photocatalytic degradation using TiO2 in the UV illumination

was found to be more efficient and economical method [2-8].

In this study we have determined the photocatalytic degradation

rate of heterocyclic dyes (azur pair) and azo dyes (sudan pair) [14-15].

Azur pair is a heterocyclic dye. Azur dyes are soluble in water (slightly

in ethanol). Azur is used for staining semi-thin sections of plant tissue

and in nuclear staining. Azur A and B have same functional group but

differ in their structure as shown in figure 1.

Azur A

Azur B

Figure 1: Structural representation of azur A and azur B

.

Sudan III and IV are lysochromic azo dyes. These dyes are

insoluble in water where as fat soluble. This pair is used as colouring

agents, waxes, shoe polishes, petrol and so on. The variation in the

structure of sudan III and sudan IV is shown in figure 2.

Sudan III Sudan IV

Figure 2: Structural representation of sudan III and sudan IV

Photocatalysis

The heterogeneous photocatalysis was found to be best method

for the treatment of textile effluents. The term heterogeneous

photocatalysis refers to the phenomena in which the photosensitizer is

a solid material such as semiconductor or a supported metal oxide.

Photosensitization is a process by which a photochemical reaction

occurs as result of initial absorption of photon with suitable energy by

another chemical species called the photosensitizer.

Semiconductor is mainly characterized by its filled valence band and empty

conduction band. When semiconductor is iiradiated with the light energy of hv which

matches or exceeds the band energy gap, Eg, the electron, ecb-, gets excited from valence

band to conduction band leaving a hole hvb+ in the valency band. The input energy is

dissipated as heat when the positive hole of valence band and the conduction band

negative electron. Conduction band electron will be trapped in the metastable surface

states or react with electrons donor or electron acceptors adsorbed on the surface of the

semiconductor or within the electrical double layer of charged particles surrounding the

surface. The stored energy is dissipated within nanoseconds due to recombination in the

absence of electron and hole. The recombination can be prevented if there is a suitable

scavenger or surface detect space is available to trap electron or hole. The mechanism of

photocatalysis is shown in figure 3.

Figure 3: Primary steps in photochemical mechanism (1) charge carrier formation due to photon; (2) charge carrier recombination to liberate heat; (3) initiation of oxidation path by valency band hole; (4) initiation of reduction path by conduction band electron; (5)

further thermal or photocatalytic reactions to yield mineralization products; (6) trapping of conduction band electron in a dangling surficial bond to yield TiIII; (7) trapping of

valence band hole at surficial titaniol group.

The oxidizing power of the holes either directly or indirectly is utilized for the

organic photodegradation. In order to prevent the buildup of charge one must also

provide a reducible species to react with electron. In bulk semiconductor electrode only

one species either electron or hole is available for reaction due band bending where as in

small semiconductor suspensions both the spices are present on the surface. Therefore

careful consideration for oxidation and reduction should be followed.

During past two decades TiO2 semiconductor was used as a

photocatalyst for degradation of dyes. Kinetics showed that anatase

TiO2 was a superior photocatalyst to rutile TiO2 [5-15]. The metal

substituted nanocrystalline anatase TiO2 has more photocatalytic

activity [6-13]. The mechanism of dye degradation through direct hole

attack and hydroxyl radicals is explained in following reactions.

The electron is transferred from filled valency band to the empty conduction band

leaving a hole behind when TiO2 is iiradiated with UV light. This process occurs in

femtoseconds.

Charge carrier generation

(1)

After the excitation occurs across the band gap the formed electron-hole pair undergoes

charge transfer to the adsorbate on the TiO2 surface. The semiconductor remains intact

and the charge transfer continuous and the decomposition of adsorbate takes place.

Charge trapping of hole is slower (10ns) while trapping of conduction band electron is

much faster (100ps). The undesirable side reactions occur due to the recombination of

electron and hole either on surface or in the bulk of TiO2.

Recombination

(2)

TiO2 exists in hydroxylated form in aqueous suspensions with 5-15 mol/nm2 of

maximum surface coverage of hydroxyl group. The trapped holes may directly oxidise

adsorbate molecules or may react with surface hydroxyl groups or water to produce

hydroxide radicals which are known to be strong oxidizing agents.

Hydroxyl radical generation

(3)

(4)

where S represents the adsorption site on catalyst. The above reactions may also take

place in bulk phase. The surface bounded reactions are more thermodynamically feasible

if the oxidation potential of surface bound H2O or OH- lie above the band gap energy of

TiO2 compared to the bulk hole-capturing process. The concentration of the dye

molecules on the catalyst and the mobile hydroxyl radicals can be obtained from the

following reactions:

(5)

(6)

The reaction between dye and the hydroxyl radical can occur in four possible way as

they are present either on catalyst or in the bulk. The reaction can occur between (a)

bounded dye and bounded hydroxyl radical (b) Unbounded dye and bounded hydroxyl

radical (c) bounded dye and unbounded hydroxyl radical and (d) unbounded dye and

unbounded hydroxyl radical. Since the rate controlling step is not known all the four

possible mechanisms of hydroxyl radicals attack are considered to be occurring in

parallel. The direct attack by valence band holes can occur either with bounded or

unbounded dye. The oxidation of the dyes by direct hole attack and hydroxyl radicals is

given by following reactions.

Direct hole capturing

(7a)

(7b)

Hydroxyl radical attack

Case (a):

(8a)

Case (b):

(8b)

(8c)

(8d)

Where D0+ , D0- are the cation radicals formed by direct hole attack and hydroxyl attack

respectively, and M+ is the cation released from the molecule due to direct hole attack or

hydroxyl radical attack. The kinetics of dye degradation can be determined by applying

elementary stoichiometric balances with the quasi steady state assumptions for

intermediates (details are provided in Appendix A). The L-H parameter can be evaluated

by usual method of linearizing the rate expression the Eq. (A.11) of Appendix A.

(9)

Thus, a plot of inverse of initial rates (1/rDo) and inverse of initial concentration (1/[D]o)

would be linear with intercept and slope corresponding to ((koh/ko)+ko)-1 and ko-1 ((koh/ko)

+ko)-1, respectively.

In this study we have used the catalyst combustion synthesized

TiO2 [4, 7]. The kinetics of the degradation of the dyes using

combustion synthesized TiO2 was studied.

Experimental section

Materials

Azur A and B (S.D Fine Chemicals, India), sudan III and IV dyes

(Rolex Laboratary Reagents), ethanol, methanol, acrylonitrile, glycine,

(Merck, India) and titanium isoperoxide (Lancaster Chemicals, UK)

were obtained. We used double distilled water filtered through a

Millipore membrane filter.

Catalyst preparation and characteristics

Catalyst preparation

The solution combustion method was used to prepare nano-sized anatase TiO2.

The precusor titanyl nitrate [TiO(NO3)2] and the fuel glycine (H2N-CH2-COOH) were

used in this method. The titanyl nitrate was synthesized as follows: Titanyl hydroxide

[TiO(OH)2] was obtained by the hydrolysis of titanium isopropoxide [Ti(i-OPr)4]. Titanyl

nitrate was obtained by the reaction of titanyl hydroxide with nitric acid. In a typical

combustion synthesis, a Pyrex dish (with a volume of 300 cm3) containing an aqueous

redox mixture of stoichiometric amounts of titanyl nitrate and glycine in 30 mL water

was introduced into a muffle furnace that was preheated at 350°C. The solution initially

undergoes dehydration and a spark appears at one corner, which spreads throughout the

mass, finally yielding anatase titania. Thus TiO2 was formed by the complete combustion

of the titanyl-glycine redox mixture. The liberation of the large volumes of the gases in

this method, leads to the high porosity and high surface area of the material.

Catalyst characterization

The catalyst has been characterized by various other techniques such as X-ray

diffraction (XRD), Transmission electron microscope (TEM), BET, Gravity differential

thermal analysis (TG-DTA), X-ray photoelectron spectroscopy (XPS), IR, and UV

spectroscopy and the details are presented elsewhere [14,15].

Photochemical reactor

The photochemical reactor used in this degradation consists of a

jacketed quartz tube of 3.4 cm inner diameter (i.d), 4 cm outer

diameter (o.d), and 21 cm length and an outer pyrex glass reactor of

5.7 cm i.d., and 16 cm length. A ballast and capacitor were connected

the shell less 125 W high pressure mercury lamp (HPML) to avoid

fluctuations in the input supply. HPML is placed inside the jacketed

quartz tube. The heating of the solution due to the dissipative loss of

UV energy is controlled by circulating water through the annulus of the

quartz tube. The solution taken in the outer is stirred continuously to

ensure the uniform suspension of the catalyst in the solution.

Degradation experiment

A known concentration of each dye along with the catalyst of 1

g/L is taken in the outer reactor and subjected to the UV irradiations.

The reaction temperature was kept constant by circulating water

through the annulus of the quartz tube. Samples were collected at

regular intervals, centrifuged to remove the catalyst particles prior to

analysis.

Sample analysis

The extent of degradation of dyes was determined using UV-

Visible spectrophotometer. The UV-Visible spectrophotometer uses two

light sources, a deuterium (D2) lamp for ultraviolet light and a tungsten

(W) lamp for visible light. After bouncing off a mirror (mirror 1), the

light beam passes through a slit and hits a diffraction grating. The

grating can be rotated allowing for a specific wavelength to be

selected. At orientation of the grating, only monochromatic (single

wavelength) successfully passes through a slit. A filter is used to

remove unwanted higher orders of diffraction. The light beam hits a

second mirror before it gets split by a half mirror (half of the light is

reflected, the other half passes through). One of the beams is allowed

to pass through a reference cuvette (which contains the solvent only),

the other passes through the sample cuvette. This working principle is

shown in figure 4. The intensities of the light beams are then measured

at the end. The absorbancy is converted into concentration using

calibration curves.

Figure 4: Schematic representation of working principle of UV-Visible

spectrophotometer

Results and discussion

The photocatalytic degradation of sudan and azur pair was

investigated. No appreciable degradation of either of the dyes was

observed either in the absence of UV light or catalyst. TiO2 prepared by

solution combustion technique was employed for the study.

Effect of initial concentration

The initial concentration of the dye is important both in mechanistics

and from the application point of view. So, the effect of initial

concentration of dye was studied.

Figure 5: Photocatalytic degradation profiles of sudan III (ethanol as solvent) of different initial concentrations

The effect of initial concentration of sudan III with ethanol as

solvent on the photocatalytic degradation rate was investigated over

the concentration range of 10-20 ppm, since the pollutant

concentration is an important parameter in water treatment.

Experimental results are presented in Figure 5. It can be seen from the

figure that the concentration has a significant effect on the

degradation rates and rate of decrease in the dye concentration is

faster when the initial concentration is less. The degradation rate is

lower for higher initial concentration as the order decreases and,

therefore, the Langmuir–Hinshelwood rate form was proposed to model

the experimental data. The initial rates increases with increase in initial

concentrations and saturates at higher concentrations.

The external diffusion is negligible in the concentration range of

our investigation. Adsorption and surface reaction was assumed to be

the rate controlling steps and these parameters were determined using

L–H rate form.

Figure 6: Photocatalytic degradation of different initial concentration of

sudan IV (with ethanol as solvent).

Figure 6 shows the degradation profile of different initial

concentrations of sudan IV (with ethanol as solvent) with combustion

synthesized TiO2. This profile follows the same trend as that of sudan

III. The rate increases with increase in the initial concentration of the

dye. The degradation of 10 ppm sudan IV was almost complete in 15

min.

Figure 7: Photocatalytic degradation profiles of Azur I of different initial concentrations.

Figure 8: Photocatalytic degradation of Azur II of different initial

concentrations

The rate of photocatalytic degradation of azur pair is found

decrease with decrease in the initial concentrations of the range 10-20

ppm. This is shown in figures 7 and 8. 10 ppm initial concentration of

azur II degraded in 15 min where as 20 ppm degraded in 30 min.

Compared to the other dyes like Rhodamine B, Rhodamine Blue, RBBR,

and Orange G it was found that sudan pair and azur pair degraded fast.

Figure 9: Variation of inverse of initial rate with inverse of initial concentration of Sudan III with ethanol as solvent.

The degradation profiles of Sudan pair and azur pair for various

concentrations are shown in the figures 9, 10, 11, and 12. The initial

rate of degradation of dye was determined from the initial slope of the

concentration profile. The initial rate of the dye was found to increase

with increase in its concentration at fixed initial concentration. All the

parameters were determined using the initial rates and initial

concentration data where the effect of the intermediates on the

degradation rate can be conveniently neglected. The intercept

determines the kinetic coefficient that varies directly with the rate of

degradation and the parameter Ko represents the equivalent of

adsorption coefficient. The higher value of Ko implies the blocking of

the active sites of the catalyst due to strong adsorption. The higher

value of (koh/Ko + ko) indicates the higher degradation rates of the dye.

The intercept and slope of the sudan IV was found to be more than that

of sudan III.

Figure 10: Variation of inverse of initial rate with inverse of initial concentration of Sudan IV with ethanol as solvent.

The profile shows the lower intercept i.e., higher kinetic

coefficient and the higher slope indicates the lower adsorption

coefficient.

Figure 11: Variation of inverse of initial rate with inverse of concentration of azur I.

Figure 12: Variation of inverse of initial rate with the inverse of Initial concentration of azur II.

Effect of catalyst

Titanium dioxide is the semiconductor which has high

photocatalytic activity, is non-toxic, relatively inexpensive and stable in

aqueous solution. Several reviews have been written, regarding the

mechanistic and kinetic details as well as the influence of experimental

parameters. It has been demonstrated that degradation by

photocatalysis can be more efficient than by other wet-oxidation

techniques. We have tested the photocatalytic activity of two different

commercially available TiO2 powders (namely P-25 and solution

combustion synthesized TiO2) and also without catalyst on the

degradation kinetics of sudan pair and azur pair.

Figure 13: Degradation profile for combustion synthesized, Degussa catalyst and no catalyst for Sudan III of 21 mg/L initial concentration

and 1g/L catalyst loading and ethanol as solvent.

The effect of different catalyst on the 20ppm sudan III and sudan

IV were shown in the figure 13 and 14. It is found that the degradation

rate of dye with combustion synthesized TiO2 (CST) is faster than

Degussa P-25. Degussa P-25 degraded the dye in 40 min where as

combustion synthesized TiO2 degraded the same initial concentration

of dye in nearly 20 min. It may be concluded that, the higher

photocatalytic activity of the combustion synthesized TiO2 over

commercial catalysts may be due to higher surface area, high

crystallinity and pure anatase crystal structure. There are numerous

reports that show the addition of transition metal ions will increase the

photocatalytic degradation of organic molecules. To realize the higher

photocatalytic activity, transition metals such as Pt, Cu and Mn

substituted on TiO2 was employed.

Figure 14: Degradation profile for combustion synthesized, Degussa catalyst and without catalyst for sudan IV of 21 mg/L initial

concentration and 1 g/L catalyst loading and ethanol as solvent.

Figure 15: Degradation profile for combustion synthesized, Degussa catalyst and no catalyst for azur I of 21 mg/L initial concentration and 1g/L catalyst loading

Figures 15 and 16 shows the effect of different catalysts on azur pair of

dyes. It was determined that combustion synthesized TiO2 has more

photoactivity than Degussa P-25.

Figure 16: Degradation profile for combustion synthesized, Degussa catalyst and no catalyst for azur II of 21 mg/L initial concentration and 1 g/L catalyst loading

Effect of solvent

Figure 17: Profile describing the effect of solvent on sudan III of 21 ppm with catalyst loading of 1 g/L in the presence of UV light

The effect of solvent on lysochrome dye was determined. The

constant initial concentration i.e., 21 ppm of sudan III solution was

made was using different solvents like ethanol, methanol, acrylonitrile,

and different ratios of ethanol, methanol mixture. It determined that

methanol as a solvent degraded the dye fast compared to the other

solvents. The effect of different solvent on the dye is shown in the

figure 17.

Conclusion

The photocatalytic degradation of lysochrome (fat soluble dye)

and heterocyclic dyes was determined using combustion synthesized

nano-TiO2 as catalyst. The effect of different catalysts, initial

concentration of each dye and solvent were studied. It was found that

with the increase in the initial concentration of each dye the rate of

degradation also increases. Combustion synthesized nano-TiO2 has

more photolytic activity than Degussa P-25. The effect of solvents like

ethanol, methanol, acrylonitrile, mixture of ethanol and methanol in

the ratio of 50:50, 25:75, and 75:25 by volume on lysochromic dyes

was studied. Methanol as a solvent degraded very fast compared to

other solvents. Transition metal ion substitution on TiOs has negative

effect on the activity. This can be attributed to the metals being in

ionic state in combustion synthesized materials as evidenced by XPS

study. The Langmuir–Hinshelwood (L–H) model based on the hydroxyl

radicals attack and direct hole attack rate was developed and could

explain the experimental data.

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Appendix A

Based on the mechanism presented in the section, the rate of disappearance of the

dye by both direct hole attack and hydroxyl radical attack can be given as

where the bracketed term refers to the concentration of the species. The surface

concentration of the hydroxyl radical and the dye can be obtained from the adsorption

equilibrium, determined from reactions (5) and (6),

(A.2)

(A.3)

(A.4)

The hole concentration balance can be written as

(A.5)

By applying the quasi steady state assumption (QSSA), wherein it is assumed that the

concentration of the radical is small and invariant in time, for the total hydroxyl radical

and hole concentration, Eq. (A.1) can be written in terms of measurable variables. The

bulk hydroxyl radical concentration can be obtained by applying the QSSA to the Eq.

(A.4) is

(A.6)

Since the rate of recombination (Eq. (2)) is much faster than any other trapping steps [7],

the hole concentration can be obtained from Eq. (A.5) as

(A.7)

using the Eqs. (A.2), (A.3), (A.6), and (A.7) in Eq. (A.1), the rate of degradation is

(A.8)

where koh is the overall rate constant for direct hole attack ko the rate constant for

hydroxyl radical attack solely dependent on reaction conditions and catalyst properties

and is independent of the nature of dye used.Ko is adsordtion equilibrium constant

dependent on the characteristics of the dye and the rate-determining step involved. The

expressions for each constant are,

(A.9)

(A.10)

Based on the limiting steps, Ko can take any of the special forms like

for the hydroxyl radical attack cases a, b, c, and d respectively.

The superscripts represent the controlling mechanisms and expressions for the rate

constants are given in Table 1. Rearranging Eq. (A.*) and omitting the quadratic term as

D is very small, the rate of disappearance can be written in the Langmuir-Hinshelwood

form as

(A.11)

Different parameters used in the rate expressions

Table 1

The final expression is similar to the empirical expressions frequently used for

describing the photochemical reactions. The koh/Ko+ko is the Langmuir-Hinshelwood rate

constant for combined hydroxyl radical attack and direct hole attack, solely dependent on

reaction conditions and catalyst properties and is independent of the type of dye used. Ko

is another L-H parameter (adsorption equilibrium constant equivalent) dependent on the

characteristics of the rate-determining step.