249
Potential Application of Periwinkle Shell Ash as Photocatalyst for the Heterogeneous Photocatalytic Decolourisation of Congo Red Dye in Aqueous Solution
F.A. Aisien
(a), N.A. Amenaghawon
(a)*, T.G. Osayamen
(a)(a)Department of Chemical Engineering, Faculty of Engineering, University of Benin, PMB 1154, Benin City, Edo State, Nigeria
*Corresponding author. E-mail : andrew.amenaghawon@uniben.edu Received 05 Jan 2015, Revised 02, Fev 2015, Accepted 05 Mars 2015.
Abstract
The potential of locally sourced periwinkle shell ash (PSA) has been explored as an alternative photocatalyst for the photocatalytic decolourisation of Congo red dye. The effect of process parameters such as irradiation time, initial dye concentration, catalyst dosage, pH of the solution and the addition of hydrogen peroxide (H2O2) on the extent of photodegradation was investigated. The optimum values of the process parameters were: irradiation time, 50 minutes; initial dye concentration, 10 mg/L; PSA dosage, 7 g/L and pH, 4. The addition of H2O2 enhanced the photodegradation process with almost 100 percent decolourisation achieved.
The adsorption equilibrium was well described by the Langmuir isotherm equation (R2=0.999) indicating mono layer type adsorption while the diffusion mechanism and kinetics of the process were well described by the intra-particle diffusion and Langmuir-Hinshelwood kinetic models with high R2 values of 0.998 and 0.999 respectively.
Keywords: Periwinkle shell ash, Congo red, Kinetics, Isotherm, Equilibrium
1. Introduction
The removal of organic pollutants from wastewater is an important aspect of environmental protection [1].
Considerable amounts of dyes used in the textile and dyestuff industries are lost as effluents during manufacturing and processing operations and these coloured effluents can cause serious environmental pollution problems when released into the environment [2,3]. These effluents are typically characterised by high biochemical oxygen demand, chemical oxygen demand, alkalinity and total dissolved solids [4].
With public concern regarding the effect of these pollutants on the increase, international environmental regulations are becoming stricter. This has necessitated the need for effective treatment methods for the removal of recalcitrant organic dye substances from wastewater. Treatment of dye containing effluents can be accomplished by physical, chemical or biological methods [5-8]. Adsorption, biodegradation, chlorination and ozonation appear to be the most commonly used of these methods. However, the implementation of these methods is limited in some respects. Traditional physical methods are not destructive and they have the disadvantage of been mere phase transfer methods requiring further treatment
250 [9,10]. On the other hand, even though chemical methods appear to be effective, their implementation is usually not economically feasible as the chemicals are required in high dosages [8]. Biodegradation based treatment methods lead to the degradation of the pollutants but it usually takes a long time for the effluent to reach acceptable standards. Moreover, dyes are recalcitrant organic compounds and they are usually resistant to biodegradation and more toxic and carcinogenic compounds may be produced in the course of treatment [6,9]. Thus it is necessary to develop new and effective methods for the removal of recalcitrant dyes and coloured substances from wastewater. Advanced oxidation processes (AOPs) offer an alternative means for the oxidative degradation of dyes in wastewater and other industrial effluents. These processes typically involve the use of hydrogen peroxide (H2O2), ozone (O3) or Fenton’s reagent in the presence of ultraviolet (UV) radiation [11-13]. Heterogeneous photocatalytic degradation has emerged as a promising technique for the removal of most of the organic pollutants including organic reactive dyes in aqueous media. It works based on the principle of surface activation of semiconductors notably zinc oxide (ZnO) and titanium dioxide (TiO2) by ultraviolet (UV) radiation. The advantages of this technique include total mineralisation of the pollutant, degradation of most organic compounds which are not readily amenable to other conventional treatment processes, faster and cheaper than most bioprocesses and radiation based processes as it can be carried out under direct sunlight [14-16].
The use of semiconductor photocatalysts such as ZnO and TiO2 has been promoted as a result of their high activity, stability under irradiation, reliability, low cost and availability [17]. Despite these attractive characteristics, the commercial application of semiconductors for the photocatalytic degradation of liquid wastes is limited by the recovery potential of the catalyst and economic viability of the process with respect to the efficiency in the use of radiation. As a result of these limitations, researchers have focused attention on the development of local photocatalysts with better recovery and light absorption capacity. Therefore this study is aimed at investigating the potential use of local waste material (periwinkle shell ash) for the photocatalytic decolourisation of Congo red dye in aqueous solution. The effects of factors such as irradiation time, initial dye concentration, catalyst dosage, pH and amount of oxidant (H2O2) on the degradation process were investigated. The photocatalytic degradation of Congo red dye was further evaluated by carrying out kinetic and isotherm studies.
2. Materials and methods
2.1. Preparation and characterisation of photocatalyst (Periwinkle shell ash)
Periwinkle shells were obtained from Warri in Delta State of Nigeria. The shells were washed and dried in an oven at 110oC to constant mass, followed by crushing and calcination at 600oC in a muffle furnace and subsequent sieving to obtain fine particles (< 350µm) of periwinkle shell ash (PSA). The prepared PSA was characterised by determining the composition using X-Ray Fluorescence (XRF) analysis. X-ray diffraction (XRD) was used to determine the ultimate elemental composition of the PSA using a Philips X-ray diffractometer [18]. Fourier transform infrared spectrometry (FTIR) was also carried out on the PSA and the IR spectra were recorded using Perkin Elmer spectrum 100 FT–IR spectrometer in the frequency range 4000 to 400cm-1, operating in ATR (attenuated total reflectance) mode. The surface structure and other properties of the PSA were evaluated by nitrogen adsorption method at -196ºC. The surface area of the PSA was determined using the standard BET equation.
2.2. Dye
251 Congo red, a diazo dye with molecular formula C32H22N6Na2O6S2 and molecular weight 696.66 g/mol was obtained from Stanvac Laboratory in Benin City, Edo State, Nigeria. The dye structure is shown in Figure 1.
The commercially obtained sample of the dye was used without further purification.
Figure 1. Congo red 3,3'-([1,1'-biphenyl]-4,4'-diyl)bis(4-aminonaphthalene-1-sulfonic acid)
2.3. Experimental set up
The experimental set up consisting of a laboratory-scale quartz photoreactor was designed in the form of a 1.2 m quartz tube with an internal diameter of 25 mm. The radiation was provided by four UV lamps (40W, TUV G6T5, λmax = 254 nm, manufactured by Philips, Holland) which surrounded the reactor so as to ensure a homogenous radiation field inside the reactor. The reactor set up was covered by a sheet of reflective aluminium to shield against external UV radiation and to also concentrate the UV radiation from the lamps onto the reactor. The light intensity at the centre of photoreactor was measured by Lux-UV-IR meter (Leybold Co.). Heat effect from the lamps was eliminated by blowing cooled air between the lamps and the quartz reactor such that the temperature of the reaction medium was maintained constant at 25 ±0.2 oC. The mixture of the photocatalyst and dye solution to be decolourised was held in a 10 L substrate holding tank which was stirred at 600 rpm to ensure a complete suspension of catalyst particles. Silicone tubing was connected to both ends of the reactor with one end connected to an easy load Masterflex peristaltic pump which served to convey the photocatalyst-dye mixture to the reactor.
2.4. Experimental procedure and analysis
The photocatalytic decolourisation studies conducted using the continuous flow system in the presence of UV radiation were carried out using 4000 mL of dye solution and a weighed quantity of photocatalyst. The dye solution was prepared in appropriate concentrations using deionised water. Air at a predetermined flow rate was bubbled through the dye solution in the holding tank with the UV lamps OFF. The lamps were subsequently turned on and the pump was started to convey the dye suspension into the reactor. In order to explore the effect of the pH, the solution’s pH was adjusted initially by adding 0.01N NaOH or 0.01N H2SO4 as required. Samples of 25mL aliquots were withdrawn at regular intervals and were immediately centrifuged at 8000 rpm for 10 min to remove suspended catalyst particles. The progress of photocatalytic decolourisation was monitored by measuring the absorbance of the solution samples with UV–Visible spectrophotometer (Shimadzu UV 2101 PC) at a wavelength of 520 nm. A calibration curve based on Beer–
Lambert’s law was established by relating the absorbance to the concentration. The effect of irradiation time, initial dye concentration, photocatalyst dosage, amount of oxidant and pH of solution on the decolourisation efficiency was investigated. The percentage photocatalytic decolourisation of Congo red dye was calculated using Equation (1).
252
o t 100
o
C C
Decolourisation efficiency C
(1)
Co and Ct are the initial and the liquid phase concentration of Congo red dye at time t respectively. The decolourisation efficiency estimates the extent of the azo bond cleavage of the dye which is the first step of the photocatalytic degradation [2].
3. Results and Discussions
3.1. Characterisation of photocatalysts
Table 1 shows the results of X-Ray Fluorescence (XRF) analysis for determining the chemical composition of PSA. The results of X-ray diffraction (XRD) to determine the ultimate elemental composition of the PSA are also presented in Table 1.
Figure 2. FTIR spectra of periwinkle shell ash
Table 1. Chemical composition of PSA Chemical component and composition
Oxides Composition (wt %) Elements Composition (wt %)
MgO 1.2 Fe 19.20
SiO2 33.2 Cr 6.30
ZnO 3.2 V 1.50
Fe2O3 5.0 Ni 9.00
MnO2 1.0 Se 0.13
Al2O3 9.2 Pb 0.08
CaO 41.3 Al 12.30
CuO 1.3 Zn 16.50
K2O 1.4 Sn 8.00
Na2O 1.38 Cd 0.05
TiO2 0.02 Cu 2.40
The major constituent of the PSA used in this study was calcium oxide (CaO) which accounted for 41.3 % of the weight of PSA characterised. This was followed by silica, aluminium oxide and iron oxide which accounted for about 33.2, 9.2 and 5% respectively as shown in Table 1. Some other oxides such as K2O, Na2O, TiO2 and MnO2 were also found to be present in small amounts. Results presented in Table 1 indicate
253 that the major element found in PSA is iron (Fe) which accounted for about 19.2 % of the weight of PSA characterised. This was followed by Zinc (Zn) and Nickel (Ni) which accounted for about 16.5 and 9 % respectively. Some of the oxides and elements presented in Table 1 have been reported to possess photocatalytic properties thus supporting the choice of PSA for this study [1,19]. The surface area, bulk density, and porosity of the PSA used in this study are presented in Table 2. The results presented in Tables 1 and 2 are similar to those reported in the literature [20,21]. Figure 2 shows the results of FTIR analysis of the PSA in the range 350 to 4400 cm-1. Peaks in the range of 3100 to 3500 cm-1 indicates the presence of OH groups and the stretching of the N-H bond of the amino group [22]. Absorption bands in the range of 2700 to 1430 cm-1 and 900 to 1380 cm-1 indicate the presence of phenyl groups and the stretching of the C-O bond in carboxylic groups present in the PSA
Table 2. Physical properties of PSA
Property Value
Surface area (m2/g) 400
Bulk density (kg/m3) 2940
Porosity (-) 0.004
3.2. Effect of irradiation time
Figure 3 shows the effect of irradiation time on the photocatalytic decolourisation of Congo red by PSA in the presence of UV radiation. The trend observed indicates that there was a rapid decolourisation of the dye during the first 30 minutes of the process. This can be seen from the steep increase in the decolourisation efficiency within the first 30 minutes of the process. The rapid decolourisation of Congo red dye observed during this initial stage of the process could be as a result of the abundant active sites available on the photocatalyst surface. In the course of the decolourisation process, these sites are then occupied by the dye molecules which results in the decline in the decolourisation rate observed thereafter [23].
Figure 3. Effect of irradiation time on the photocatalytic degradation of Congo red by PSA (pH 4; PSA dose, 7 g/L; initial concentration, 10 mg/L; temperature, 25oC)
Figure 4. Effect of initial dye concentration on the photocatalytic degradation of Congo red by PSA (pH 4; PSA dose, 7 g/L; temperature, 25oC)
254 During the last 10 minutes of the decolourisation process, the rate of reaction was observed to level off indicating that equilibrium had been reached after 50 minutes of reaction. The condition of equilibrium means that the active sites on the surface of the photocatalyst have been occupied by the dye molecules. This explains the insignificant change observed in the rate of decolourisation at equilibrium. Chakrabarti and Dutta [24] reported equilibrium times in the range of 90 to 120 minutes under varying conditions of photocatalytic degradation of model textile dyes in wastewater using ZnO as photocatalyst. In another study on the photocatalytic decolourisation of Remazol Red RR in aqueous system, Akyol et al. [14] obtained an equilibrium time of 25 and 35 minutes respectively when ZnO and TiO2 where used as photocatalysts. These results alongside those obtained in the present study indicate that the photocatalytic decolourisation of dyes is a fast kinetic process
3.3. Effect of initial dye concentration
The effect of initial concentration of the dye solution on the photocatalytic decolourisation process is an important aspect of the study. The initial dye concentration was varied between 10 to 60 mg/L and the results are shown in Figure 4. The trend observed indicates an inverse relationship between the initial dye concentration and the decolourisation efficiency. The decolourisation efficiency decreased with increasing initial concentration of the dye solution. Colour removal decreased from 87 to about 12% when the initial dye concentration was increased from 10 to 60 mg/L. Similar trend have been reported by other researchers [14,24,25]. The hydroxyl radical (OH•) is an important species during photocatalytic degradation. It is the principal oxidant responsible for high decolourisation efficiency [1,14]. With increase in the dye concentration, the amount of dye adsorbed on the surface of the catalyst at equilibrium is increased resulting in a decrease in the competitive adsorption of hydroxyl ions (OH-) as there are fewer active sites available for the adsorption of the hydroxyl ions. This consequently results in a reduction in the formation of OH•
radicals. Furthermore, as the concentration of the dye increases, the path length of photons entering the solution decreases because they are intercepted before they get to the surface of the photocatalyst resulting in a reduction in the amount of radiation absorbed by the photocatalyst and consequently the decolourisation efficiency decreases [25,26].
3.4. Effect of photocatalyst dosage
The decolourisation efficiencies for various catalyst doses are presented in Figure 5. The results show that the decolourisation efficiency initially increased with increase in catalyst dosage up to a maximum value of about 87 % at a catalyst dosage of 7 g/L. Further increase in catalyst dosage resulted in a decrease in the decolourisation efficiency. The initial increase in decolourisation efficiency observed might be attributed to the fact that at lower doses of the catalyst, the catalyst surface and absorption of light by the catalyst surface are the limiting factors hence increasing the catalyst loading enhances the efficiency of the process as a result of the increase in the number of active sites on the photocatalyst surface which in turn increases the number of free radicals (•OH and O22-
) produced in solution [24,27]. The decrease in decolourisation efficiency observed beyond the optimum catalyst dosage might be attributed to the increase in turbidity of the solution. Increase in turbidity results in increased opacity of the aqueous medium and enhancement of the light reflectance and scattering as a result of the excess of catalyst particles. Agglomeration and sedimentation of the catalyst particles is also possible at high doses, thus making a fraction of the catalyst surface inaccessible for radiation absorption and consequently resulting in a decrease in the decolourisation
255 efficiency [27]. Suri et al. [28] investigated the degradation of toluene, trichloroethylene and methylethylketone using different initial concentration of the hydrocarbon pollutant and TiO2 dosage. They reported that the optimum catalyst dosage was dependent on the initial concentration of the hydrocarbon pollutant. They further reported that using a higher dosage of the catalyst might not be advisable as a result of possible aggregation and reduced irradiation field as a result of increase in light scattering.
3.5. Effect of pH
The effect of pH on the photocatalytic decolourisation of Congo red dye by PSA is presented in Figure 6.
Because of the amphoteric behaviour of most semiconductor oxides, the pH is considered an important parameter governing the rate of reaction occurring on the photocatalyst surface [25]. The pH of the aqueous medium is important in two regards; firstly it controls the surface charge properties of the photocatalyst and secondly it affects the production of hydroxyl radicals which are powerful oxidising agents [25,29]. Figure 6 shows that the decolourisation efficiency increased with increase in pH, reaching a maximum value of 71%
at pH 4. Further increase in pH resulted in a decrease in the decolourisation efficiency. Similar observations was reported Qamar et al. [26] for the photocatalysed degradation of two selected azo dye derivative in aqueous suspension as well as Abdollahi et al. [29] for the photodegradation of m-cresol by zinc oxide under visible light irradiation. The initial increase in percentage decolourisation efficiency could be attributed to the electrostatic interactions between the positive catalyst surface and dye anions which lead to an increase in the adsorption of the dye on the catalyst surface [30]. At higher pH values beyond the optimum, there is an excess of OH− anions. The presence of large amounts of OH− ions on the surface of the catalyst as well as in the reaction medium favours the formation of OH• radicals which are the principal oxidising agents responsible for the decolourisation of the dye [1]. However, at higher pH values, the rate at which the hydroxyl radicals (•OH) are used up is accelerated resulting in a decrease in the decolourisation efficiency [29]. Furthermore, the abundant hydroxyl ions (OH-) generated at high pH values will compete with the negatively charged dye molecules for adsorption on the catalyst surface. The hydroxyl ions (OH-) will consequently make the surface of the catalyst to be negatively charged and as a result the approach of the negatively charged dye molecules to the catalyst surface will be slowed because of the repulsive force between the hydroxyl ions (OH-) and the dye molecules thereby leading to a decrease in the decolourisation efficiency [31].
Figure 5. Effect of PSA dosage on the photocatalytic degradation of Congo red by PSA
Figure 6. Effect of pH on the photocatalytic degradation of Congo red by PSA (initial
256 (pH 4; initial concentration, 10 mg/L; temperature,
25oC)
concentration, 10 mg/L PSA dose, 7 g/L;
temperature, 25oC) 3.6. Effect of oxidant
The photocatalytic decolourisation of organic pollutants could be significantly improved either in the presence of oxygen or by the addition of hydrogen peroxide. The effect of hydrogen peroxide (H2O2) on the photocatalytic decolourisation of Congo red dye by PSA is shown in Figure 7. The decolourisation efficiency initially increased with increase in hydrogen peroxide loading and reached a maximum value of 98% when 4 cm3 of hydrogen peroxide per litre of solution was used. Increasing the amount of hydrogen peroxide beyond 4 cm3 resulted in a negative effect on the photocatalytic reaction. A similar trend was reported by Behnajady et al. [25] for the photocatalytic degradation of acid yellow 23 by ZnO photocatalyst.
The initial increase observed in the decolourisation efficiency may be attributed to the oxidative effect of hydrogen peroxide. Hydrogen peroxide which is an electron acceptor is known to generate hydroxyl radicals hence its addition to the reaction medium could accelerate the reaction. This is accomplished as a result of the ability of H2O2 to scavenge the electrons trapped on the surface of the catalyst thereby lowering the electron-hole recombination rate and consequently improving the utilisation of holes in the production of the hydroxyl radicals (•OH) according to Equation (2).
• 2 2
H O eOHOH (2)
• •
2 2 2 2
H O O OHOHO (3)
•
2 2 2
H O hv OH (4)
In the presence of oxygen, H2O2 can also enhance the rate of the photocatalytic reaction (Equation 3);
however, H2O2 is a better electron acceptor than molecular oxygen based on their respective one-electron reduction potentials [32]. Furthermore, H2O2 can also cleave in the presence of UV radiation to directly produce the hydroxyl radicals (•OH) according to Equation (4). The free radicals formed by H2O2 create a strong oxidative environment which favours the photocatalytic decolourisation of the dye. When excess of H2O2 is added, it acts as a hydroxyl radical or hole scavenger to form the perhydroxyl radical (HO2•) (Equations 5 and 6) which is a much weaker oxidant than the hydroxyl radicals [25].
• •
2 2 2 2
H O OHH OHO (5)
•
2 2 2
H O hHO
(6)
Figure 7. Effect of oxidant (H2O2) loading on the photocatalytic degradation of Congo red by PSA (pH, 4;
257 initial concentration, 10 mg/L PSA dose, 7 g/L; temperature, 25oC)
3.7. Kinetic modeling
For engineering purposes, it is important to determine a rate equation that fits the experimental data. The kinetics of the process was studied using four kinetic models namely first order, pseudo second order, intra particle diffusion and Langmuir-Hinshelwood kinetic models.
3.7.1 First order model
The first order equation could be applied to model the kinetics of the degradation process when the concentration of the dye is small. This equation is expressed as follows:
r dC kC
dt
(7)
The first order reaction rate constant, k can be obtained from the integrated linear form of Equation (7) as follows:
ln o
i
C k C
t
(8)
The plot of ln Co/Ci versus t resulted in a linear relationship from which the values of k were determined as shown in Figure 8. The first order rate constants calculated from the plot are given in Table 3. Straight line plots were obtained for the range of dye concentration investigated. This indicated that the first order equation was applicable for the range of dye concentration. It can also be observed that the values of k reduced with increase in initial dye concentration. This is in line with the inference made from the results presented in Figure 4 that increasing the initial dye concentration leads to a reduction in the rate of the degradation reaction.
3.7.2 Pseudo second order model
The pseudo second order kinetic model follows the assumption that chemisorption is the rate-limiting step.
Figure 8. First order model fitted to kinetic data for Congo red photodegradation by PSA (pH, 4; PSA dose, 7 g/L; temperature, 25 oC)
Figure 9. Pseudo second order model fitted to kinetic data for Congo red photodegradation by PSA (pH, 4; PSA dose, 7 g/L; temperature, 25 oC)
258 Figure 10. Intra particle diffusion model fitted to
kinetic data for Congo red photodegradation by PSA (pH, 4; PSA dose, 2 g/L; temperature, 25 oC)
Figure 11. Langmuir-Hinshelwood model fitted to kinetic data for Congo red photodegradation by PSA (pH, 4; PSA dose, 7 g/L; temperature, 25 oC)
The equation is expressed in its integrated linear form as follows:
2 2
1 1
t e e
t t
q k q q
(9)
The initial adsorption rate, h (mg.g-1.min-1) is expressed as follows:
2 2 e
hk q (10)
k2 is the rate constant of the pseudo second order process (g.mg-1.min-1). The plot of (t/qt) versus t is shown in Figure 9. The kinetic constants calculated from the plot at different initial dye concentrations are shown in Table 3. It can be observed that the model was able to describe the kinetics of the process for initial dye concentration values of 10 and 20 mg/L. Beyond that, the model failed to describe the mechanism of the process.
Table 3. Kinetic constant parameter values for the photocatalytic decolourisation of Congo red
Co
(mg/L)
First order Pseudo second order model Intra-particle diffusion model
Langmuir- Hinshelwood model
k R2 k2 qe R2 KP C R2
kr 222.222 10 0.0722 0.979 0.0136 2.465 0.986 0.221 0.0317 0.992
20 0.0527 0.990 0.0262 2.853 0.914 0.398 0.0046 0.997 K 0.051 30 0.0368 0.948 0.0019 8.271 0.662 0.513 0.0069 0.997
R2 0.999 40 0.0346 0.973 8.87e-10 398.406 0.166 0.730 0.0184 0.998
3.7.3 Intra particle diffusion model
The diffusion mechanism of the process was elucidated by fitting the equilibrium data to the intra particle diffusion kinetic model [33]. The model equation is presented as follows:
1/2
t p
q K t C
(11)
Kp is the intra particle diffusion rate constant (mgg-1 min-1/2) and C is a measure of boundary layer effect.
The value of C indicates the contribution of the surface sorption in the rate controlling step. The calculated values of the intra particle diffusion rate constant and the boundary layer thickness at different values of initial dye concentration are presented in Table 3. The plot of qt versus t1/2 is presented in Figure 10. The
259 plot indicates the existence (although not significant) of some boundary layer effect and further showed that intra particle diffusion was not the only rate limiting step. The Kp values generally increased with increase in initial dye concentration. The high R2 values obtained for the kinetic parameters estimated within the concentration range investigated indicates that the intra particle diffusion model was able to describe the diffusion mechanism of the photodegradation process.
3.7.4 Langmuir-Hinshelwood model
The photocatalytic degradation kinetics of many organic compounds has often been modelled with the Langmuir–Hinshelwood equation. The equation also accounts for the adsorption properties of the substrate on the photocatalyst surface. The equation is expressed as follows:
1
r eq
o
eq
dc k KC r dt KC
(12)
ro is the initial rate of reaction in mg/L.min, kr is the rate constant for photocatalysis in mg/L min, K is the rate constant for adsorption in L/mg, Ceq is the concentration of bulk solution in mg/L at adsorption equilibrium, c is the concentration of bulk solution at any time t, and t the time in minutes. This equation may be linearised as follows:
1 1 1 1
o r eq r
r k K C k
(13)
Experimental values of 1/ro were plotted against 1/Ceq for the dye as shown in Figure 11 and the values of the calculated constants are presented in Table 3. The Langmuir-Hinshelwood model resulted in the best fit with highest R2 value to describe the kinetics of Congo red dye photodegradation by PSA.
3.8. Isotherm studies
For The Langmuir and Freundlich adsorption isotherm models were used to analyse the equilibrium data for the photodegradation of Congo red dye. The curves of the related adsorption isotherms were regressed and the parameters of the equations were thus obtained.
3.8.1 Langmuir isotherm
The linear form of the Langmuir equation is given as:
1 1
e e
e o L o
C C
q q K q
(14)
qo is the maximum sorption capacity (mg/g) of the adsorbent while KL is the sorption constant (L/mg). A linear plot of Ce/qe against Ce as shown in Figure 12 was employed to obtain the values of qo and KL from the slope and intercept of the plot respectively. The values of the Langmuir isotherm parameters as well as the correlation coefficient (R2) of the Langmuir equation for the photodegradation of Congo red by PSA are given in Table 4.
Table 4. Kinetic parameters for Langmuir and Freundlich isotherms
Langmuir isotherm Freundlich isotherm
qo (mg/g) KL (L/mg) R2 Kf (mg/g) n R2
0.515 6.276 0.999 0.435 1.628 0.962
260 The essential characteristics of the Langmuir isotherm can also be explained in terms of the separation factor (RL) which is a dimensionless constant [34].
1
(1 )
L
L o
R K C
(15)
Co is the initial concentration of Congo red dye. The dependence of the nature of adsorption on the value of the separation factor is expressed as follows: RL > 1 unfavourable, RL = 1 linear, 0<RL<1 favourable and RL
= 0 irreversible. For this study, the values of RL given in Table 5 are between zero and one indicating that the adsorption was favourable.
Table 5. RL values and type of isotherm
Initial concentration (mg/L) RL Value
10 0.016
20 0.780
30 0.703
40 0.639
3.8.2 Freundlich isotherm
The Freundlich isotherm is an empirical equation employed to describe heterogeneous systems. The Freundlich equation is expressed as follows:
( )1/n
e f e
q K C
(16)
This equation can be expressed in linearised form as follows:
lnqelnKf1/ lnn Ce
(17)
Kf and n are the Freundlich constants related to the adsorption capacity and adsorption intensity respectively.
A linear plot of lnqe against lnCe as shown in Figure 13 was employed to obtain the values of the constants from the intercept and slope of the plot respectively.
Figure 12. Langmuir isotherm model linearised to equilibrium data for Congo red photodegradation by PSA (pH, 4; PSA dose, 7 g/L; Temperature, 25 oC)
Figure 13. Freundlich isotherm model linearised to equilibrium data for Congo red photodegradation by PSA (pH, 4; PSA dose, 7 g/L; Temperature, 25 oC) The values of these parameters as well as the correlation coefficient (R2) of the Freundlich equation for the adsorption of Congo red by PSA are given in Table 4. The values of n between 1 and 10 typically indicate
261 that the adsorption was beneficial. The values of the constants obtained in this study are similar to those reported by previous researchers. Chakrabarti and Dutta [24] reported Kf and n values of 0.431 and 1.343 for the photocatalytic degradation of Methylene Blue in wastewater using ZnO as semiconductor catalyst. The apparent difference between the values could be as a result of difference in the range of concentration, type of material used, pH, temperature and properties of the adsorbent such as functional groups present on the surface, surface area, pore structure, etc. [35]. The high values of the correlation coefficients as shown in Table 4 indicate that the data conformed well to both isotherm equations; nevertheless, the Langmuir isotherm equation resulted in a better fit as seen in the higher value of the correlation coefficient
4. Conclusion
The photocatalytic decolourisation of Congo red dye in aqueous medium using periwinkle shell ash in the presence of UV light was investigated. The following conclusions can be drawn from this study.
Photocatalytic decolourisation of Congo red dye by PSA is influenced by factors such as contact time, initial dye concentration, PSA dosage, presence of oxidant and solution pH.
For the conditions considered in this study, the optimum values obtained are as follows: contact time, 50 minutes; initial dye, 10 mg/L; PSA dosage, 7 g/L; pH, 4.
The addition of Hydrogen peroxide (H2O2) enhanced the photodegradation process with almost 100 percent degradation achieved.
The adsorption equilibrium, diffusion mechanism and photodegradation kinetics were well described by the Langmuir isotherm, intra-particle diffusion and the Langmuir-Hinshelwood kinetic model with high correlation coefficient values.
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