Visible light induced heterogeneous advanced oxidation ... - NOPR

Indian Journal of Chemistry Vol. 47, June 2008, pp. 830-835

Visible light induced heterogeneous advanced oxidation process to degrade pararosanilin dye in aqueous suspension of ZnO Brijesh Parea,*, Pardeep Singha & S B Jonnalagaddab a

Laboratory of Photocatalysis, Department of Chemistry, Govt. Madhav Science College (Vikram University), Ujjain (M. P.) 456 010, India Email: [email protected]/[email protected] b University of KwaZulu-Natal, Westville, Durban, South Africa Received 16 January 2008; revised 8 May 2008

The photocatalytic degradation of pararosanilin dye has been investigated under visible light in the presence of aqueous suspension of ZnO under different conditions. The reaction has been studied by monitoring the change in substrate concentration employing visible spectrophotometric analysis as a function of time. The absorbance of dye under investigation is found to decrease in the presence of ZnO and visible irradiation. The degradation of selected dye has been studied under different process variables such as reaction pH, catalyst loading, substrate concentration, light intensity and presence of electron acceptors, and hydroxyl radical scavengers. The rate degradation was strongly influenced by all the above-mentioned parameters. The estimated COD and CO2 values of treated dye sample indicate complete degradation of dye. A slight decrease in pH and gradual increase in conductivity has also been observed during the degradation process. ZnO has been found to be potentially efficient in solar irradiation for the degradation of dye. Kinetic analysis indicates the complex nature of heterogeneous photocatalytic process. IPC Code: Int. Cl.8 B01J23/06; C07B33/00

Large quantities of synthetic dye effluents may get discharged into the aquatic environment from three major sources viz., dye manufacturers, dye users (i.e. textile, paper, plastics industries, etc.) and diffuse or household discharge as a result of leaching of dyes from manufactured products1. As a consequence, dyes have become a major source of environmental pollution. Conventional physico-chemical methods (reverse osmosis adsorption and electro-flocculation) used to treat dye effluents have their own drawbacks. These methods are not destructive merely transfer dyes from liquid to solid phase causing secondary pollution. Chlorination and ozonation are not economically viable due to high dose process requirements2,3. Activated sludge process does not work efficiently due to high solubility4. Thus, new treatment methods are needed for the degradation of hazardous dye chemicals or converting them into innocuous compounds in aqueous system. In recent years, advanced oxidation processes (AOPs) have emerged as contemporary oxidative technique for degradation of detrimental organic compounds. UV/H2O2 or UV/Fenton processes have been found to be promising for the degradation of dyes5-8.

Heterogeneous photocatalysis has paved the way to destructive technology leading to mineralization of most of organic pollutants to CO2, water and mineral acids using atmospheric oxygen under ambient conditions utilizing semiconductor photocatalyst9,10. Recently, TiO211, 12 and ZnO13-15 have been used as effective and nontoxic semiconductor photocatalysts for the degradation of wide range of organic chemicals and synthetic dyes. Solar UV-light reaching the surface of the earth and available to excite TiO2, an UV active photocatalyst, is relatively small and also artificial UV light source are somewhat expensive16. ZnO appears to be a suitable alternative to TiO2. Upon exposure to visible radiation, the photocatalyst generates electron/ hole pairs (eCB-/hVB+) as shown in Fig. 1. The holes at ZnO valence band can oxidize adsorbed water molecules or hydroxide ions to produce hydroxyl radicals, a very strong oxidizing agent. Many studies reveal that OH. is the main oxidising species responsible for the mineralisation of organic pollutant in aqueous system17,13-15. It has been used in the advanced oxidation of wastewater in paper industry18. The photocatalytic efficiency of ZnO has been proved in

PARE et al.: PHOTOCATALYTIC DEGRADATION OF PARAROSANILIN

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Fig. 1– Photocatalytic degradation of dye pollutants using ZnO illuminated by visible radiation.

photocatalytic degradation of acid green 16 dye19, auramine O20, textile dyes (procion brilliant yellow, procion briiliant magenta yellow and procion brilliant)21 and phenol22. ZnO/UV system in combination with Fenton reagent (optimal concentration) is a highly efficient technology23. Pararosanilin dye (PR) has been used in schiff's reagent to detect cellular DNA, mucopolysccharides and proteins (ninhydrin + schiff's stain) in biological sytem24. According to Environmental Protection Act (U.S.), this dye has carcinogenic and mutagenic effect for mammalian cells25, 26. In the present study, we have investigated the photocatalytic degradation of triarylmethane dye, pararosaniln using ZnO and visible radiation source. The effect of various experimental parameters on the process performance has been investigated. Total degradation was assessed in term of chemical oxygen demand (COD) and CO2. Materials and Methods Pararosanilin was obtained from Aldrich Chemical Company (USA). The photocatalyst, ZnO used in the study was obtained from Merck (~99% pure having surface area of 10 m2/gm). All the solutions were prepared by dissolving the calculated amount of appropriate compound in doubly distilled water. The irradiation experiments were carried out in a pyrex vessel reactor with dimension of 7.5 cm x 6 cm (height x dia). The outer wall of pyrex vessel was surrounded with thermostatic water as shown in Fig. 2 to keep the temperature in the range of 25-30°C. The irradiation was carried out using 500 W (800×103 lux) halogen lamp. The intensity of irradiation was varied by changing the position lamp either upward or downward, as the case may be, using height adjustable stand from reaction vessel.

Fig. 2 – Schematic representation of photocatalytic reactor. Procedure

During the photocatalytic experiment, after stirring for ten minute, the slurry composed of dye solution and catalyst was placed in dark in the order to establish equilibrium between adsorption and desorption. Then slurry was agitated by stirring magnetically with simultaneous exposure to visible light. At specific time intervals aliquot samples (3 mL) were withdrawn and centrifuged to remove the ZnO particles. The aliquot samples were analyzed at 530 nm to assess the extent of decolorization spectrophotometrically. After analysis both centrifuged ZnO and solution were put back in reaction vessel. The intensity of visible radiation was measured by a digital lux-meter (Lutron LX-101). The pH of reaction medium was constantly monitored and not adjusted unless otherwise stated. COD and CO2 were determined by the methods reported eariler27,28. During all the photocatalytic experiment, 100 ml of dye solution was irradiated each time. Efficiency of photocatalytic process was investigated as in Eq. 1. Efficiency (%) = (C0 – C)/ C0 × 100

… (1)

where C0 is the initial COD value of dye solution at time t = 0 hr and C is final COD value of dye solution in different time intervals such as 2, 4, 6 and 8 hr of photocatalytic treatment in the presence of ZnO and visible irradiation.

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INDIAN J CHEM, SEC A, JUNE 2008

Results and Discussion Role of pH

The pH of the solution is an important parameter in the photocatalytic reactions taking place on the semiconductor particulate surface. Therefore, the decolorization of PR was studied at different pH values of reaction medium (in the pH range 4-11). The decolorization of PR as a function of reaction pH is shown in Fig. 3. The rate of decolorization was found to be maximal at pH 8.21. At this point, 99 % of the dye colour was removed in 30 minutes. Further increase in pH results in decrease in the rate of photodegradation. The pH of zero point charge for ZnO is about 9 (ref. 29). At pH 7-8 due to metalbound OH-, negatively charged active sites on the surface of catalyst are preferentially covered by positively charged dye molecule. At lower value of pH, (~4), positively charged active sites on the surface of catalyst result in low concentration of positively charged of dye molecule on the surface of catalyst. Also with increasing pH, surface concentration of dye molecules and hydroxyl radicals increases. But ZnO is amphoteric in nature and is dissolved at lower pH, forming salts. At higher pH, it forms zincates such as [Zn(OH)4]2- (refs 30, 31). All these factors are responsible for optimum value of photodegradation of pararosanilin at pH 8.2. Similar findings have been reported by Sakthivel and others using ZnO/UV for the photocatalytic degradation studies32-35.

(8×10-7mol dm-3) and K2S2O8 (8 × 10-7 mol dm-3) have very little effect on degradation rate even in the presence of visible irradiation (Fig. 3). The photocatalytic degradation of pararosanilin was conducted at different concentration of H2O2 and K2S2O8 as shown in Fig. 4. The results indicates that the rate of degradation of PR increased with increasing concentration up to 8.0×10-7 mol dm-3 but above this concentration, degradation efficiency was found to decrease. This is because hydrogen peroxide can inhibit the electron-hole recombination by accepting photogenerated electron from the conduction band of semiconductor and promote charge separation and also form OH. radicals according to Eqs. 2 and 3. -

e CB

+ H2O2 .H2O2 + O2

OH

.

+

2OH

OH

-

.

... (2) ... (3)

When H2O2 is in excess, it may act as a hole or OH. scavenger having detrimental effect on photocatalytic degradation. This explains the need for an optimal concentration H2O2 for the maximal effect36,17,33,35. . H 2O2 + OH

. HO2

+

H2O

... (4)

Oxidizing agents such as H2O2 and potassium peroxidisulphate (K2S2O8) play an important role of oxidizing agents in photocatalytic process. For blank photodegradation process without ZnO, both H2O2

K2S2O8 on the other hand, can also trap the photogenerated conduction band resulting in the formation of sulphate ion (SO4-.), a strong oxidising agent (standard reduction potential = 2.6V). In addition it can trap the photogenerated electrons and/or generated hydroxyl radicals.

Fig. 3 – Effect of solar irradiation on decolorization of pararosanilin dye. {[PR] = 8 × 10-5 mol dm-3, [ZnO] = 200 mg/ 100 mL, Irradiation intensity = 25000 lux, pH = 4.15, H2O2 = 8 × 10-7 mol dm-3 and K2S2O8 = 8 × 10-7 mol dm-3. 1, Solar light + ZnO (∆); 2, Vis. + ZnO(■); 3; K2S2O8 + Vis (○); 4, H2O2 + Vis(▲); 5, ZnO ()}.

Fig. 4 – Effect of oxidants (H2O2, K2S2O8) on rate constants of pararosanilin dye. {[PR] = 8 × 10-5 mol dm-3, [ZnO] = 200 mg/ 100 mL, Irradiation intensity = 5600 lux, pH = 4.15}. 1, H2O2 (▲); 2, K2S2O8 ()}.

Effect of oxidants


-. SO4 + e CB -. SO4 + H2O -

-

S2O8 + eCB

SO4

2-

... (5)

. 2+ OH + SO4 + H -. SO4

+

SO4

2-

... (6) ... (7)

The decrease in rate of photodegradation above optimal concentration 9.0×10-7 mol dm-3 is due to the adsorption of sulphate ions formed during the reaction on surface of ZnO deactivating a section of photocatslyst37,38,31 (Fig. 4). Effect of solar light

Use of solar light in place of artificial irradiation is preferred from the industrial point of view. Decolorization of PR dye was conducted under solar light in month of November between 10 am to 4 pm. During solar light experiment, halogen lamp was replaced by solar light. Under solar light, complete decolorization of dye was achieved in less than 45 min (Fig. 3). Solar light-assisted decolourisation is two times faster than artificial irradiation-assisted process. Solar light could be effective for degradation process if all the conditions are properly optimised. No observable loss of dye derivative takes when experiments were carried out in the absence of visible irradiation or ZnO (Fig. 3).

200 mg/100 mL (2g/L) and then decreases. This observation can be explained in terms of availability of active sites on the catalyst surface and the penetration of visible light into the suspension. The total active surface area increases with increasing catalyst dosage. At the same time, due to an increase in turbidity of the suspension, there is decrease in the visible light penetration as a result of increased scattering effect and hence photoactive volume of the suspension decreases40,17. Effect of chloride and carbonate ions

Direct or substantive dyeing is normally carried out in a neutral or slightly alkaline dyebath, at or near boiling point, with the addition of either sodium chloride (NaCl) or sodium carbonate (Na2CO3). The photocatlytic degradation rate constant decreases with the increase in Cl- and CO32- ions concentration from 3×10-6 mol dm-3 to 11×10-6 mol dm-3 (Fig. 5). The inhibition is undoubtedly due to their ability to act as a scavenger for hydroxyl radical (OH.). The mechanism of hydroxyl radical scavenging is given by following equation17,35,36. . Cl + OH . Cl -. Cl2

-

Cl + OH +

-

Cl + h VB . Cl + Cl

Effect of substrate concentration

The effect of dye concentration was investigated by varying the initial concentration from 2×10-5 mol dm-3 to 13×10-5 mol dm-3. The rate of photodegrdation was found to increase up to a concentration 8×10-5 mol dm-3. This is due to the fact that more dye molecules are available in photoactive volume for the photodegradation process. The rate of photodegradation was found to decrease with further increase in dye concentration above the optimal value. The decrease is attributed to fact that the dye itself will start acting as filter for incident irradiation and reduce the photoactive volume. Excessive adsorption of dye molecule on the catalyst surface hinders the competitive adsorption of OH- ions and lowers the formation rate of hydroxyl radicals due to which rate of photodegradation decreases39,35. Effect of catalyst loading

Experiments performed with different concentration of ZnO shows that the photodegradation efficiency increases with an increase in ZnO up to

833

CO3

2-

+

OH

.

.CO3 + OH

... (8) ... (9) .... (10) ... (11)

Chemical oxygen demand (COD) and free CO2 measurements during photodegradation process

In the order to study the extent of mineralization of PR over illuminated ZnO suspension, COD and free CO2 measurements were carried out. COD test allows the measurement of waste in terms of the total quantity of oxygen required for the degradation of organic matter to CO2 and inorganic ions. During 8 hours of irradiation, reduction in COD value from 96 mg/L to 1.6 mg/L, and increase in CO2 value from 22 mg/L to 990 mg/L, indicates the photodegradation of treated dye solution. A decrease in pH and increase in conductivity of solution is observed with increase in the extent of mineralization. From Table 1, it is clear that reduction in COD proceeds very slowly initially. In contrast to initial decolorization rate, this is very fast and decreases exponentially as seen in Fig. 3. After 80 min of

INDIAN J CHEM, SEC A, JUNE 2008

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Table 1 – COD and CO2 estimation at different intervals of time under optimum reaction conditions: {[PR] = 8 x 10-5 mol dm-3, [ZnO] = 200 mg/100 mL, Irradiation intensity = 25000 lux and pH = 8.2} Time (hr)

COD (mg/L)

CO2 (mg/L)

Efficiency (%)

pH

Conductivity (mS/cm)

0 2 4 6 8

96 64 44 16.4 1.6

22 110 220 660 990

0 33 54 83 98

7.1 6.9 6.8 6.6 6.5

0.435 0.701 0.85 0.964 1.23

irradiation, decolorization reaches up to 99%, while, COD decreases only by 33 % even after 120 minutes. Kinetics of dye disappearance

The kinetics of dye disappearance of pararosanilin is represented in Fig. 5. First order kinetics model can be utilized as follows: ln (C0/C) = kt

… (12)

where C0, C, t and k are the initial dye concentration, dye concentration in decolorization time 't' and apparent rate constant (time-1), respectively. The linear fit between ln (C0/ C) and irradiation time under different pH and 200 mg/100 mL (2g/L) of catalyst can be approximated as pseudo-first order kinetics. The decreasing trend of rate constant is as: pH 8.21 (11.1×10-4 s-1)> pH 9.1 (9.9×10-4 s-1) > pH 9.85(7.9 ×10-4 s-1)> pH 7.53(6.9×10-4 s-1) > pH 5.1 (4.1 ×10-4 s-1) > pH 4.1 (3.6 ×10-4 s-1). Here appropriate simple power law model was examined for the photocatalytic degradation. n

R = k1[PR] 1 + k2[PR]

n2

… (13)

where n1, n2, k1 and k2 are the appropriate orders of the reactions and the rate constants for photocatalysis (R1) and photolysis (R2). Several kinetic studies in photocatalysed degradation have been concerned with the power law models41-43. To obtain the appropriate parameters in Eq. 13, the differential method of analysis based on data of dye concentration versus time was used to get following equation log R 1= log k1 + n1log[PR]

… (14)

log R 2= log k2 + n2log[PR]

… (15)

where (R = R1 + R2). The kinetic parameters for photocatalysis and photolysis are as follows: n1 = 1.84; k1=2.9×10-1

Fig. 5 – Effect anion concentration on rate of photodegradation of pararosanilin dye. {[PR] = 8 × 10-5 mol dm-3, [ZnO] = 200 mg/ 100 mL, Irradiation intensity = 5600 lux and pH = 4.15. 1, Na2CO3 (); 2, NaCl, (■)}

mol dm-3 s-1; n2 = 0.84; k2= 2.7 × 10-5 mol dm-3)0.14 s-1. The order of reaction for photcatalysis is more than one indicating a rather complex nature of overall mechanism. For decomposition of direct red 71 dye41 and sodium dodecylbenzene sulfonate solution39, orders of reaction of 2.16 and 1.32 respectively have been reported. The net rate of degradation (mol dm-3 s-1) of pararosanilin under the experimental condition used can be given as: R = 2.9 × 10-1[PR]1.84 + 2.7 × 10-5[PR]0.84

… (16)

Conclusions In this study, photocatalytic degradation of a pararosanilin dye has been investigated using ZnO catalyst and visible irradiation. This study shows that the rate of degradation (11.1×10-4 s-1) is maximum in slightly basic medium (pH=8.2) due to higher adsorption of dye molecule on surface of catalyst. At 200 mg/100 mL (2g/L) of catalyst loading, color removal reaches maximum value (99%) in one hr of irradiation time. Catalyst loading exhibits conflicting effects on the photocatalytic process; at lower loading level, photonic adsorption controls the reaction rate. Light scattering by catalyst particles predominates over photonic adsorption above optimal concentration. Rate constants for photocatalytic decolorization has maximum value of 5×10-4 s-1, 6.6×10-4 s-1 and 6.95×10-4 s-1 at optimal concentration of dye (8×10-7 mol dm-3), H2O2 (8×10-7 mol dm-3) and K2S2O8 (9×10-7mol dm-3), respectively, and thereafter the reaction rate decreases with increase in concentration. At 11×10-6 mol dm-3 of NaCl and Na2CO3, rate constants have been found to be 1.5×10-4 s-1 and


2.2×10-4 s-1, respectively. This confirms their detrimental effect on rate of degradation. Solar light in the presence of ZnO decolorizes the dye in less than 45 min of irriadtion time. From the estimation of COD and free CO2, it is evident that under selected experimental condition, complete mineralization of pararosanilin takes place in near about 8 hours. The results obtained using Vis/ZnO photocatalytic system show that pararosiniln dye can be degraded using ZnO and visible irradiation, non-exhaustible source of energy. Under above-mentioned experimental conditions, visible irradiation may be replaced by solar light. Further studies on the complex nature of heterogeneous photocatalysis and better understanding of operative conditions shall give great opportunities for its application to carry out pilot scale wastewater treatment involving dye pollutants. References 1 Bouzdia I, Ferronato C, Chovelon J M, Rammah M E & Hermann J M, J Photochem Photobiol A, 168 (2004) 23. 2 Robbinson T, Mucmullan G, Marchant R, & Nigam P, Biores Technol, 77 (2001) 247. 3 Zamora P P, Kunz A, Moaraes R, Pelegrini R, Moleiro P C & Reyes, Chemoshere, 38 (1999) 835. 4 Ledakowicz S, Soleca M & Zylla R, J Biotechnol, 89 (2001) 175. 5 Georgiou D, Meldies P, Aivasidis A & Gimouhopoulos K, Dye Pigments, 52 (2002) 69. 6 Majcen L M, Slokar Y M & Taufer T, Dyes Pigments, (1997) 281. 7 Galidno C & Kalt A, Dyes Pigments, 40 (1999) 27. 8 Xie Y, Chen F, He J, Zhao J & Wang H, J Photochem Photobiol A ,136 (2000) 235. 9 Sauqib M & Muneer M, Dyes Pigments, 56 (2003) 37. 10 Konstantinou I K & Albanis T A, Appl Catal B Environ, 49 (2004) 1. 11 Curri M L, Comparelli R, Cozzoli P D, Mascolo G, & Agostiano A, Mater Sci Eng C 23( 2003) 285. 12 Poulios I, Micropoulou E, Panou R & Kostopoulou E, Appl Catal B Environ, 41 (2003) 345. 13 Akyol A & Bayramoglu M, J Haz Mat B, 124 (2005) 241. 14 Behnajady M A, Modirshala N & Hamazavi R, J Haz Mat B, 133 (2006) 226. 15 Daneshvar N, Rabani M, Modirshahla N & Behnajady M A Q, J Photochem Photobiolo A, 168 (2004) 857. 16 Anpo M, Pure Appl Chem, 72 (2000) 1787. 17 Daneshvar N, Salari D & Khataee A R, J Photochem Photobiol A, 162 (2004) 317. 18 Yeber M C, Rodrigrgeuz J, Freer J, Baeza J, Duran N & Mansilla H D, Chemosphere 39 (1999) 10.

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