Rajeev Jain* and Shalini Sikarwar - Semantic Scholar€¦ · Jiwaji University, Gwalior 474011, India E-mail: [email protected] E-mail: [email protected] E-mail: [email protected]

158 Int. J. Environment and Pollution, Vol. 27, Nos. 1/2/3, 2006

Copyright © 2006 Inderscience Enterprises Ltd.

Photocatalytic and adsorption studies on the removal of dye Congo red from wastewater

Rajeev Jain* and Shalini Sikarwar Department of Environmental Chemistry, Jiwaji University, Gwalior 474011, India E-mail: [email protected] E-mail: [email protected] E-mail: [email protected] *Corresponding author

Abstract: Many industries such as paper, food, cosmetics, textiles etc. use dye in order to colour their products. The presence of these dyes in water even at very low concentration is highly visible and undesirable. Colour is the first contaminant to be recognised. Photocatalytic technique and adsorption methods offer a good potential to remove colour from wastewater. In the present paper these two methods were employed for removal of Congo red and both the techniques were found to be very useful and cost effective for a better removal of dye and the results were compared. The operating variables such as adsorbent dose, adsorbate concentration, pH etc were optimised.

Keywords: azo dyes; Congo red; photocatalytic treatment; adsorption; gram husk; charcoal.

Reference to this paper should be made as follows: Jain, R. and Sikarwar, S. (2006) ‘Photocatalytic and adsorption studies on the removal of dye Congo red from waste water’, Int. J. Environment and Pollution, Vol. 27, Nos. 1/2/3, pp.158–178.

Biographical notes: Rajeev Jain obtained his PhD in Chemistry from the University of Roorkee (now Indian Institute of Technology, Roorkee), India in 1978. He joined Jiwaji University at Gwalior, India as lecturer in 1982 and presently holds the position of Professor in Analytical Chemistry. He has published more than 150 research papers in various international journals. He is a widely travelled researcher and has visited many countries. His research interests include photochemistry, wastewater treatment, environmental and electro-analytical chemistry. He is a life member of Indian Science Congress Association, India, Indian Chemical Society and Electrochemical Society of India. Besides, he has also an expertise in water quality and wastewater treatment methodologies. At present he is writing a book in four volumes on instrumental methods of chemical analysis.

Shalini Sikarwar received an MSc in Environmental Science in 2000 from Jiwaji University at Gwalior, India. At present she is working as a Faculty and doing research in the Department of Environmental Chemistry, Jiwaji University at Gwalior, India. Her area of interest is photocatalytic and adsorption studies of dyes. She has two research papers to her credit.

Photocatalytic and adsorption studies on the removal of dye Congo red 159

1 Introduction

Azo dyes represent a major class of synthetic organic pigments that are manufactured worldwide and have a variety of applications such as textiles, paper, foodstuff, and cosmetics (Rys and Zollinger 1972). The toxicity and carcinogenic nature of these dyes and their precursors pose a threat to the environment (Reife and Freeman, 1996).

Moreover, their degradation often leads to the formation of highly carcinogenic aromatic amines. For example p-aminoazobenzene has been classified as a carcinogenic compound and there has been a restriction on the production of dyes based on this molecule (Rajeshwar and Ibanez, 1997). However azo dyes continue to be a source of pollution in industrial processes, which utilise dyes to colour paper, plastics as well as natural and artificial fibres. Wastewaters from dyeing industries are released into nearby land or rivers without any treatment because the conventional treatment methods are not cost effective.

The conventional methods for treating dye-containing wastewater are coagulation, flocculation, reverse osmosis and adsorption on various adsorbing materials (Rajeshwar et al., 1994). For removal of colour from industrial wastewater, adsorption has become one of the most effective and economical methods and activated carbon is the preferred adsorbent because of its efficiency, capacity and application on a large scale (Chermisionoff and Eller Bush, 1979; Mattson and Mark, 1971). Different workers have used various adsorbent materials for the removal of different pollutants (Cowan et al., 1991; Groffman et al., 1992; Gupta, 1998; Kesaoul-Qukel et al., 1993; Koeppenkastrop and De Careo, 1993; Lee and Low, 1989, 1991; Park et al., 1999; Periasamy and Namasivayam, 1994; Pollard et al., 1992; Rodda et al., 1993; Tan et al., 1985; Tyagi and Srivastava, 1995; Tyagi et al., 1997). Activated carbons have a large adsorption capacity for a variety of organic pollutants but are expensive due to difficult regeneration and higher disposal cost (Singh et al., 2003). In view of the high cost and tedious procedure for the preparation and regeneration of activated carbon, there is continuing search for the development of adsorbents using cheaper raw materials. Many researchers have studied the feasibility of less expensive activated carbons prepared from oil shake (Darwish et al., 1996), soyabean hulls (Flock et al., 1999), bagasse fly ash (Gupta et al., 1998), jute stick (Banerjee and Mathew, 1985), rice husk (Nawar and Doma, 1989), tamarind nut (Srinivasan et al., 1998), coconut coir (Hitchcock et al., 1983), salvinea molta (Sankaran and Anirudhan, 1999), coconut husk (Vinod and Anirudhan, 2002) for the removal of phenolic compounds. For any sorbent to be feasible, it must combine high adsorption capacity and fast adsorption capacity with inexpensive regeneration (Burleigh et al., 2002).

In recent years, photocatalytic degradation has attracted increasing attention as cleaner and greener technology for removal of toxic organic and inorganic pollutants in water and wastewater (Parsons, 2004). Semiconductor photocatalysis appears to be a promising technology that has a number of applications in environmental system such as air purification, water disinfection, water purification, and hazardous waste remediation. TiO2 catalysed photochemical degradation of organic pollutants in general, and a dye in particular in wastewater is a favoured and promising technique (Egerton, 1997). The organics are completely mineralised into water and CO2 without generating any harmful byproducts. This technique has been employed for the photomineralisation of large number of dyes such as methylene blue, direct acid dyes, azo dyes and reactive black (Gerixhac, 1993). The photocatalytic degradation of organic compounds on

160 R. Jain and S. Sikarwar

TiO2 has been analysed in terms of Langmuir–Hinshelwood kinetics for the compound

(Mathews, 1988; Pruden and Ollis, 1983; Hsiao et al., 1983; Ollis et al., 1984; Sabate et al., 1991; Hidaka et al., 1992). However, despite its promise, the development of a practical treatment system based on the heterogeneous photocatalysis has not yet been successfully achieved because of many operational parameters that must be considered. The initial step in the TiO2 mediated photocatalysed degradation is proposed to involve the generation of electron hole pair, which migrates to the photocatalyst surface to yield hydroxyl and super oxide radical anion. It has commonly accepted that the hydroxyl and super oxide radicals anions can initiate redox processes of the adsorbed substrate. Mechanistically, it is now commonly accepted that the photocatalyst TiO2 is first excited by UV light and subsequently initiates the photo degradation processes (Mills and Wang, 1999).

The present paper describes the practical applicability of photocatalytic and adsorption techniques for the removal of Congo red (Figure 1) a direct dye from the wastewater.

Figure 1 Structure of a Congo red dye

2 Experimental

2.1 Photochemical studies

2.1.1 Instrumentation

All experiments of photocatalytic degradation were conducted in a photocatalytic reactor (Figure 2). Irradiation was carried out using a 6W UV lamp, placed inside the well of quartz glass photo reactor of 150 mL capacity. The reactor set up was covered with dark black colour wooden box to prevent UV radiation leakage. The lamp emits predominantly UV radiation at a wavelength of 254 nm.

Figure 2 Schematic diagram of a photochemical reactor


2.1.2 Materials and methods

pH metric measurements were made on decibel DB 1011 digital pH metre fitted with a glass electrode, which was previously standardised with buffers of known pH in acidic and alkaline medium. The reaction kinetics was studied with the help of Spectronic 20 D+ thermospectronic spectrophotometer. The rate of decrease of colour with time was continuously monitored.

Anatase titanium oxide, 325 mesh, 99+% was obtained from Sigma Aldrich and used as such without further treatment. All laboratory reagents were of analytical grade used in this study. Double distilled water was used for necessary dilutions. For measuring pollutional load, COD determination were carried out by preparing a dye solution (1 × 10–4 M) for blank, thereafter a solution was used for photo degradation study and COD was determined after photochemical treatment. COD digestion apparatus (spectra lab-2015-s) was used for determining COD.

For photo catalytic degradation, 0.01M solution of Congo red in 100 mL of distilled water was prepared. Aqueous solution of Congo red of different concentrations were prepared from 0.01M stock solutions. In 100 mL of dye solution different catalyst concentration was added and irradiated with UV lamp to provide energy to excite TiO2 loading. To ensure efficient mixing of TiO2 catalyst in the reactor, oxygen was bubbled from the side of the reactor continuously throughout the reaction. At specific time intervals suitable aliquots of the sample were withdrawn and analysed after centrifugation. All the experiments were carried out at room temperature (30 ± 0.1°C) at an optimum pH 6.3, except for experiments where pH values were varied. Britton-Robinson buffers in the pH range 3.0–9.0 were prepared in distilled water.

2.1.3 Preliminary observations

Photodegradation of the dye was studied in the presence of illumination and oxygen but in absence of TiO2 catalyst to observe if any reaction occurred. A slight decrease in absorbance was seen probably due to irradiation.

The preliminary kinetics study indicated that during the first few minutes (2–5 minutes) after the lamp was turned on, the reaction was very slow; presumably, this is due to warming up of lamp to full power. A similar behaviour has been reported by others (Mathews, 1989), of this reason, in all subsequent treatments of the kinetics data, the initial slow reaction has been ignored.

2.2 Results and discussion

In view of the existence of several degradation pathways, the photo degradation was studied under the following experimental conditions in order to define the system completely.

• photo degradation of the dye in the absence of TiO2

• photo degradation of the dye in the presence of O2 and TiO2

• photo degradation of the dye in the presence of H2O2 and TiO2, but in the absence of air.


An aliquot of the solution under study was taken out and centrifuged and thereafter taken in the quartz cuvette and was put in the spectronic 20 D+ thermospectronic spectrophotometer with distilled water as reference. The decrease in absorbance value with time was noted. Congo red undergoes slow photo degradation on irradiation with UV light at pH 6.3 in the absence of TiO2 catalyst (Figure 3).

Figure 3 Effect of oxygen on photocatalytic degradation of Congo red (1 × 10–4M), in the absence of TiO2, at pH 6.3, temp (30 ± 0.1°C)

2.2.1 Effect of catalyst concentration

The rate of photo oxidation is, however, greatly enhanced when amorphous TiO2 is added to the solution. The photo degradation of dye in the presence of TiO2 is very sensitive to the presence of oxygen. It is clear from the (Figure 4) that with increase in the catalyst amount (in a fixed substrate concentration) the percentage of degradation increases. This is due to increase in the number of active sites to cause photo degradation process.

Figure 4 Effect of catalyst concentration on the rate of photocatalytic degradation of Congo red (1 × 10–4M), pH 6.3, temperature (30 ± 0.1°C)


The kinetics of dye degradation followed a first order course. The results indicate that the rate is dependent on TiO2 concentration. This indicates that in the presence of oxygen TiO2 exhibits catalytic activity. Oxygen was passed through the solution. At a TiO2 dose of 0.06 and 0.08 g/L the degradation rate almost becomes constant, thereafter it increases with increasing catalyst concentration. This shows that dose 0.08g/L are sufficient for optimum removal of congo red in the given concentration.

2.2.2 Effect of H2O2 concentration

It is observed that molar H2O2 concentration is a key factor that can significantly influence the degradation of Congo red because H2O2 concentration is directly related to the number of •OH radicals generated in the photo assisted reaction. The degradation rate of Congo red increases as the H2O2 concentration increases until a critical H2O2 concentration is achieved. However, when the H2O2 molar concentration is greater than 1.2 mM, the degradation of Congo red slightly slows down. This may be due to the scavenging effect. When using a higher H2O2 molar concentration, the further generation of •OH radicals in aqueous solution is expressed by the following equation: (Edwars and Curci, 1982; Halmann, 1996).

H2O2 + •OH → HO2 + H2O (1)

It is apparent from Figure 5 that the rate of degradation is markedly enhanced in the presence of H2O2 additive. H2O2 is not only known to inhibit the electron hole recombination process but also it generates hydroxyl radicals on abstraction of an electron from the conduction band (Gerixhac, 1993).

H2O2 + e–cb → •OH + OH– (2)

Figure 5 Photocatalytic degradation of Congo red (1 × 10–4M) with different concentrations of H2O2 in the presence of anatase TiO2 (0.08 g/L), pH 6.3, temperature (30 ± 0.1°C)

It is clear from Figure 5 that at 1.0 mM and 1.2 mM the degradation rate becomes constant and thereafter it increases. It shows that the critical H2O2 molar concentration for the degradation of 1 × 10–4M of Congo red is 1.4 mM. It can be inferred that an excess amount of H2O2 is needed to reach the maximum degradation of Congo red.


In the presence of oxygen and fixed amount of TiO2 and H2O2 the degradation rate of dye is reduced but the degradation is greatly enhanced in the absence of oxygen (Figure 6).

Figure 6 (a) Photodegradation of dye in the presence of H2O2, oxygen, TiO2 (0.08 g/L) and UV light (b) Photodegradation of dye in the presence of H2O2, TiO2 and UV light and (c) Photodegradation of dye in the presence of H2O2 and UV only

2.2.3 Effect of dye concentration

The photocatalytic degradation of Congo red decreases with increase in concentration, at a fixed amount of catalyst (0.08 g/L) (Figure 7). For a fixed catalyst dose active site remaining the same, the number of substrate ions accommodated in the inter layer space increases, so the rate of degradation decreases. This effect may be attributed to the fact that at higher concentration of the dye, the light is predominantly absorbed by the semiconductor photocatalyst. Light absorbed by the dye itself is not effective in bringing about its photodegradation. At 7 × 10–5, 8 × 10–5 and 9 × 10–5M concentration of dye the rate of photodegradation almost becomes constant.

Figure 7 Effect of adsorbate concentration on the photocatalytic degradation of Congo red, in presence of anatase TiO2 (0.08 g/L), pH 6.3, temperature (30 ± 0.1°C)


2.2.4 Effect of pH

The role of pH on the rate of photocatalytic degradation was studied in the pH range 3.0–9.0 at fixed loading of TiO2 0.08 g/L. Interestingly at lower pH colour of Congo red changed and the rate of degradation was slow and remains fairly constant at pH 3 and 4. In these pH values, the dye is predominantly in its anionic form. The higher rate of degradation may however, be attributed to the surface property of TiO2. Hence at pH values above 6.2 the surface will be negatively charged.

As the pH increases to 9, efficiency of the photodegradation of the dye increases (Figure 8). Enhancement in efficiency of dye degradation may be attributed to the reaction of hydroxyl radical with the Congo red. Hydroxyl radicals can react with the azo dyes containing amino functionalities both by electron transfer as well as by adduct formation. Thus at higher pH values, efficient deprotonation of the dye radical can result in more efficient degradation of the dye. At pH 6 and 7 the rate of degradation almost becomes constant and thereafter it increases further.

Figure 8 Effect of pH on the rate of photocatalytic degradation of Congo red (1 × 10–4M) in the presence of anatase TiO2 (0.08 g/L), temperature (30 ± 0.1°C)

Adsorption studies

Gram husk after converting to an adsorbent was used for the removal of dye from aqueous solutions and charcoal is used as a conventional adsorbent for comparing the results. All chemicals and reagents used were of analytical grade. Stock solution of Congo red (0.01M) was prepared in double distilled water. The concentration of dye was determined at maximum absorbance 498 nm. From the calibration curve the concentration of the test solution was determined at pH 6.7 and temperature 30 ± 0.1°C.

Adsorbent development

100 g of gram husk was treated with glacial acetic acid and then washed with double distilled water and again treated with hydrogen peroxide (30%) at 60°C for 24 hours to oxidise the adhering organic matter. The resulting material was washed with double distilled water, filtered and dried again to 100°C, powdered, ground and sieved to the desired particle size and characterised by XRD and SEM before use.


2.3 Results and discussion

Adsorption studies were carried out using charcoal and gram husk as adsorbents. The effects of adsorbents dose, pH, particle size, temperature, equilibrium time and concentration of Congo red were also studied.

2.3.1 Effect of adsorbent dose

The effect of adsorbent dose on removal of congo red was studied by varying the dose of adsorbent from (0.06–0.33 g/L) for charcoal and (0.375–4 g/L) for gram husk at fixed pH, temperature and adsorbate concentration. The results are presented in Figure 9(a) for charcoal and (b) for gram husk. The adsorption increases from 0.06 g/L to 0.2 g/L for charcoal while in case of gram husk the adsorption increases from 0.375 g/L to 3 g/L and then it becomes constant, indicating that a dose of 0.02 g/L for charcoal and 1.0 g/L for gram husk are sufficient for optimum removal of congo red in the given concentration.

Figure 9 Effect of adsorbent amount on the uptake of Congo red (a) charcoal and (b) gram husk, at different temperatures, pH 6.7, conc. = 6.0 × 10–5M

(a)

(b)


2.3.2 Effect of dye concentration

The dependence of concentration of the adsorbate on the rate of adsorption is used to define the rate-limiting step in the reaction. The adsorption studies were carried out in the concentration range 1 × 10–5 to 10 × 10–5M for charcoal and gram husk at optimum pH 6.7 and 20 minutes of contact time for charcoal and 30 minutes for gram husk. The results are depicted in Figure 10(a) for charcoal and (b) for gram husk. It is apparent from the figure that the adsorption increases from 1 × 10–5 to 8 × 10–5M and then it becomes constant for charcoal and in case of gram husk it increases from 1 × 10–5M to 6 × 10–5M and then it becomes constant at 7 × 10–5M and 8 × 10–5M and thereafter it further increases. The maximum adsorption capacity of the adsorbent is 8 × 10–5M for charcoal and 6 × 10–5M for gram husk.

Figure 10 Effect of initial adsorbate concentration on the uptake of Congo red (a) charcoal 0.2 g/L and (b) gram husk 1 g/L, pH 6.7, temperature (30 ± 0.1°C)

(a)

(b)


2.3.3 Effect of pH

The effect of pH on the adsorption of dyes has been studied by varying the pH of the medium from 2.0 to 12.0 at fixed concentrations of adsorbent (0.2 g/L) for 20 min for charcoal and 1 g/L for 30 minutes for gram husk. The pH of the medium was adjusted by using acetic acid and sodium hydroxide solutions. The pH effect profile on adsorption is given in Figure 11(a) for charcoal and (b) for gram husk. In the pH range 2.0–4.0 poor adsorption was observed with both adsorbents (Figure 11). The maximum uptake of Congo red takes place at pH 6.7 for charcoal with 99% colour removal after that it remains constant up to pH 12.0, whereas for gram husk maximum adsorption is 95.2% at pH 5.4 and then it becomes constant up to pH 10.0. At pH 10 dyes adsorb around 98.5% for charcoal and husk adsorbs around 92.3% indicating that charcoal and gram husk both remove colour very effectively in basic medium and adsorption increases and then remains constant in both cases with increasing pH.

Figure 11 Effect of pH on adsorption of Congo red (a) charcoal 0.2 g/L and (b) gram husk 1 g/L, temperature (30 ± 0.1°C), conc. = 6 × 10–5M

2.3.4 Effect of particle size

The adsorption rate is expected to vary as the reciprocal of the dm of the adsorbent particle for a given total weight of adsorbent. Thus the variation of the rate with particle size is another method, which is useful for the characterisation of the rate limiting mechanism of a particular system. In present investigations the adsorbent of different particle size <106, 106–125, 125–180, 180–212, 212–250, 250–300, >300 MIC were taken at dose (0.2 g/L for charcoal and 1 g/L for gram husk), pH, temperature and contact time (20 minutes for charcoal and 30 minutes for gram husk). Maximum adsorption for charcoal 94.86% and for wheat husk 66.6% was observed at mesh size <106 MIC. The rate of adsorption decreases with increases in mesh size of adsorbents Figure 12(a) for charcoal and (b) for gram husk.


Figure 12 Effect of particle size on adsorption of Congo red (a) charcoal 0.2 g/L and (b) gram husk 1 g/L, pH 6.7, temperature (30 ±0.1°C) conc. = 6.0 × 10–5M

(a)

(b)

2.3.5 Effect of contact time

The adsorption was also carried out for different contact time with a fixed adsorbent dose (0.2 g/L) for charcoal and (1.0 g/L) for gram husk, at optimum pH 6.7 and temperature 40°C. It is apparent from Figure 13(a) for charcoal and (b) for gram husk that increase in time of contact has a significant effect on the adsorption of Congo red. After attaining equilibrium time (20 minutes for charcoal and 30 minutes for gram husk) the rate of adsorption almost becomes constant in both the cases indicating that the optimum contact time for adsorption of charcoal is 20 minutes, and for wheat husk is 30 minutes.


Figure 13 Effect of equilibrium time on adsorption of Congo red (a) charcoal 0.2 g/L and (b) gram husk 1 g/L, pH 6.7, temperature (40°C), conc. = 6.0 × 10–5M

(a)

(b)

2.3.6 Effect of temperature

Adsorption studies were also carried out at different temperatures i.e., at 40°C, 50°C, and 60°C at a fixed dose of adsorbents 0.2 g/L for charcoal and 1.0 g/L for gram husk at pH 6.7. The results are presented in Figure 14(a) for charcoal and (b) for gram husk. It is clear from the figure that the rate of the uptake of dye increases with increasing temperature for both the adsorbents indicating the endothermic nature of the process. The adsorption follows the order of 40°C < 50°C < 60°C.


Figure 14 Effect of different temperature on adsorption of Congo red (a) charcoal 0.2 g/L and (b) gram husk 1 g/L, pH 6.7, conc. = 6.0 × 10–5M

(a)

(b)

2.4 Adsorption kinetics

The adsorption kinetics was evaluated at 40°C, 50°C, and 60°C with 0.2 g/L for charcoal and 1 g/L for gram husk at optimum pH with different contact times (Figure 15(a) and (b)) applying equation (3)

K = 2.303/t log10 Ci/Ce (3)

where

Ci: initial concentration (mg–l) Ce: equilibrium concentration (mg–l) t: time (min) K: rate constant (min–l).

A plot of log Ci/Ce vs. t results in a straight line with slope k/2.303 (Figure 15(a) for charcoal and (b) for gram husk) shows that the adsorption of dye follows first order rate equation. The values of adsorption kinetic constants at different temperatures are tabulated in Table 1.


Figure 15 Adsorption kinetics with charcoal at different temperatures (a) charcoal and (b) gram husk, pH 6.7, conc. = 6.0 × 10–5M

(a)

(b)

Table 1 Adsorption kinetic constants for Congo red

K × 10–3M, min–l Adsorbents 40°C 50°C 60°C Charcoal 28.09 35.46 32.70 Gram husk 6.90 8.06 8.06

Adsorption equilibrium isotherms are useful for the estimation of the amount of adsorbent needed for sorbing a required amount of sorbate from solution. The most widely used equation to represent the adsorption data is Langmuir and Freundlich.


2.5 Freundlich isotherm

The adsorption data of charcoal and wheat husk were fitted in to the linear form of Freundlich equation.

Log qe = log Kf + 1/n log Ce (4)

Where, qe is the amount adsorbed (mol/g), Ce is the equilibrium concentration of adsorbate (mol/g) and Kf and n are the Freundlich constants related to adsorption capacity and adsorption intensity, respectively. When log qe was plotted against the log Ce, straight line with slopes ‘1/n’ were obtained (Figure 16(a) for charcoal and (b) for gram husk). It is apparent from the figure that adsorption of dye on charcoal and gram husk follows the Freundlich isotherms. The Freundlich constants Kf and n are calculated from Figure 16(a) and (b) for charcoal and gram husk and the values of the constants at different temperatures are tabulated in Table 2.

Figure 16 Freundlich adsorption isotherms of (a) charcoal (b) gram husk, at different temperatures and pH 6.7

(a)

(b)


Table 2 Freundlich constants for charcoal and gram husk

Charcoal Gram husk Temperature (°C) Kf n R2 Kf n R2

40 1.0493 1.4017 0.9636 53.051 1.5506 0.9337 50 1.8369 1.5460 0.8818 321.588 2.1588 0.8623 60 6.1602 1.9120 0.926 254.917 2.0995 0.9038

2.6 Langmuir isotherm

Langmuir isotherm has been used by many workers for studying the sorption of a variety of compounds. The model assumes uniform energies of adsorption on to the surface and no transmigration of adsorbate in the plane of the surface. The linear form of this isotherm was analysed in the light of Langmuir model.

1/qe = 1/Q° = 1/b Q Ce (5)

Where, qe is the amount adsorbed (mol/g), Ce is the equilibrium concentration of the adsorbate (mol/g) and Q° and b are the Langmuir constants related to maximum adsorption capacity and energy of adsorption, respectively. When ‘1/qe’ is plotted against 1/Ce, a straight line with slopes 1/bQ° are obtained (Figure 17(a) for charcoal and (b) for gram husk) which shows that the adsorption of dye on charcoal and gram husk follows the Langmuir isotherm. The Langmuir constant ‘b’ and ‘Q°’ are calculated and the values at different temperatures are compiled in Table 3.

Figure 17 Langmuir adsorption isotherms of (a) charcoal and (b) gram husk, at different temperatures and pH 6.7

(a)


Figure 17 Langmuir adsorption isotherms of (a) charcoal and (b) gram husk, at different temperatures and pH 6.7 (continued)

(b)

Table 3 Langmuir constants for charcoal and gram husk

Charcoal Gram husk Temperature (°C) b (mol g–l) Q° (L mol–1) R2 b (mol g–l) Qo (L mol–l) R2

40 44.369 0.8668 0.9659 32.408 0.04531 0.9195

50 67.769 0.8107 0.8763 88.30 0.0329 0.8356

60 199.917 0.5884 0.9025 78.95 0.0382 0.9276

2.7 Thermo dynamic parameters

The thermodynamic parameters for the adsorption of dye on charcoal and gram husk were calculated using following equations and the values are given in Table 4.

∆G0 = –RT ln b’ (6)

ln b2/b1 = (∆H°/R) (T2–T1/T2T1) (7)

∆S° = –(∆G0– ∆H0)/T (8)

where b’, b1, b2 are the Langmuir constants at 40, 50 and 60°C respectively and obtained from the Langmuir isotherm, other terms have their usual meanings. The negative free energy values shown in Table 4 indicate the feasibility of the process and its spontaneous nature.


Table 4 Thermodynamic parameter of Congo red, for charcoal and gram husk

Adsorbent ∆G0 (kJ mol–l) ∆H0 (kJ mol–l) ∆S0 (JK–l mol–l)

Charcoal –9.86 96.73 0.340

Gram husk –9.05 –22.52 –0.043

2.8 Chemical oxygen demand (COD)

COD of initial coloured, photocatalysed and treated filtrate of charcoal and gram husk solutions was determined and it was observed that the COD value of the solutions obtained after photodegradation shows a significant decrease from 932–208.0 mg/L, indicating less toxicity of the photodegraded products thus formed in comparison to removal by charcoal (900–216.0 mg/L) and gram husk (900 –260 mg/L).

Table 9 COD values for Congo red, before and after treatment

Treatment method COD in mg/L

Congo red dye before treatment (1 × 10–4M) 932

Photocatalysed at pH 6.3 for 90 minutes 208

Congo red dye before treatment (6 × 10–5M) 900

Adsorption by gram husk (3.5 g/L) 260

Adsorption by activated charcoal (0.33 g/L) 216

3 Conclusion

The photocatalytic results of the study indicate that anatase titanium dioxide, as a catalyst is very efficient and effective to enhance the photocatalytic activity. Catalyst concentration and additives such as H2O2 were used to obtain a high degradation rate of Congo red.

Likewise the results presented on adsorption studies reveal the economic feasibility of the use of solid agro based waste gram husk, which is an inexpensive adsorbent, and available in abundance. Economically this material is cheaper than the cheapest variety of commercially available carbon.

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Rajeev Jain* and Shalini Sikarwar - Semantic Scholar€¦ · Jiwaji University, Gwalior 474011, India E-mail: [email protected] E-mail: [email protected] E-mail: [email protected]