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| 5.0 Discussion |
5.1 Characteristics of the adsorbent
The approximate percentages of total carbon, nitrogen and hydrogen in the four adsorbents namely bengal gram husk (BGH); tur dal husk (TDH); coffee husk (CH) and tamarind husk (TH) are shown in Table 25. The relatively low percentage of nitrogen (0.86% for BGH, 1.13% in TDH; 0.63 % in CH and 0.94% in TH) in comparison to the carbon quantities, indicates that few nitrogen containing compounds are involved in the adsorption of dyes. A relatively larger percentage of hydrogen in comparison to nitrogen compounds indicates that carbon-hydrogen groups might be available for adsorption of dyes. The relatively low percentage of nitrogen shows that very less percentage of protein might be present in the husks. This is advantageous over protein rich adsorbents since proteinious materials are likely to putrefy under moist conditions (Ahalya et al., 2006).
5.2 Infrared spectroscopic studies
Unreacted samples of the four adsorbents used in the present study namely bengal gram husk (BGH), Tur dal husk (TDH), coffee husk (CH) and tamarind husk (TH) were subjected to Fourier transform infrared spectroscopy (FTIR). The spectra obtained are presented in Figures 3 to 6 for BGH, TDH, TH and CH respectively.
The spectra of BGH sample (Figure 2) reveal the presence of several functional groups on the surface which facilitates the adsorption of dyes. Wavenumber of 3000 and 3750 
for BGH indicates the presence of OH groups on the husk surface. The trough that is observed at 2918.18 and 893.25 indicates the presence of C-H groups. The 1634.34 band is a result of CO stretching mode, conjugated to a NH deformation mode and is indicative of amide 1 band. The trough at 1115.57 is due to CO or CN groups (Ahalya et al., 2005).
The absorption spectra of TDH (Figure 3) display a broad, intense --OH stretching absorption trough at 3431 , although the bands are dominated by the -OH stretch due to bonded water. Weaker ---CH stretch bands are superimposed onto the side of the broad -OH band at 3000–2800 . The strong peak at 1733 is caused by the C=O stretching band of the carboxyl group. The peak at approximately 1100 is due to either the C-O stretch of the -OH bend. However, the N-H stretch (3300 ) and the C-N stretch (1000 ) are not seen in this spectra due to the dominance of the ---OH stretch (Ahalya et al., 2006).
The spectrum of the pristine TH is complex due to the numerous and multifarious functional groups on the surface of the adsorbent (Figure 4). The broad and strong band ranging from 3200 to 3600 may be due to the overlapping of OH and NH stretching, which is consistent with the peak at 1115 and 1161 assigned to C–O and C–N stretching vibration, thus showing the presence of hydroxyl and amine groups on the adsorbent surface. The strong peak at 1674 can be assigned to a C=O stretching in carboxyl or amide groups. The bands at 2936 and 1558 are attributed to CH stretching and N–H bending, respectively.
The spectra of CH display a number of absorption peaks, indicating the complex nature of the material examined (Figure 5). The FTIR spectroscopic analysis indicated broad bands at 3412 , representing bonded –OH groups. The bands observed at about 2921–2851 could be assigned to the C–H stretch. The peaks around 1733 correspond to the C=H group and at 1652–1512 C=O. This C–O band absorption peak is observed to shift to 1035 .
5.3 Batch mode studies
In order to evaluate the feasibility and economics of adsorption, laboratory batch mode studies were conducted. In this study the optimum agitation speed i.e., good contact between the adsorbent and adsorbate was established at 120 rpm.
Parameters, which influence the extent of adsorption such as adsorbate concentration, agitation time, adsorbent dosage and pH were investigated. In addition to the above parameters, effect of pH on the desorption of dyes was investigated. The use of the adsorbent for continuous use was also determined by regeneration studies.
5.3.1 Effect of agitation time
Effects of agitation time and dye concentration on removal of rhodamine B, fast green, amaranth and methylene blue by the adsorbents shows that the amount of dyes adsorbed (mg/g) increased with increase in agitation time and reached equilibrium mostly after 60 minutes. But the time varied depending on the adsorbent and dye concentration. The percent dye removal at equilibrium time decreased with increase in dye concentration, although the amount of dye removed increased with increase in initial dye concentration. It is clear that the removal of dyes depends on the concentration of the dye.
Mameri et al (1999) reported that the available adsorption sites on the biosorbent are the limiting factor for dye uptake. The equilibrium time required by the adsorbents used in the present study is less, compared to others reported in literature. This is significant as equilibrium time is one of the important considerations for economical water and wastewater applications. In process application, this rapid (or instantaneous) biosorption phenomenon is advantageous since the shorter contact time effectively allows for a smaller size of the contact equipment, which in turn directly affects both the capacity and operation cost of the process.
5.3.2 Effect of adsorbent dosage on adsorption
The biosorption of dyes was studied at various biosorbent concentrations ranging from 0.5 to 5 mg/L. The percentage of dye removed increased with increase in adsorbent dosage due to increased adsorption surface area. For all the adsorbents studied adsorbent dosage of 1g – 2g/L was sufficient for adsorption of 90% of the initial dye concentration. Further increase in the adsorbent dosage did not show an increased removal of dye concentrations.
The percent removal of adsorbates increased with increase in adsorbent dosage and reached a particular constant value after a particular adsorbent dosage. This is also true for different pH values studied. A maximum removal of about 90% was obtained for all the adsorbates studied. However, the adsorbent dosage required for maximum percent removal varied with the concentration of initial dye ions. This is mainly due to the fact that a larger mass of adsorbent could adsorb larger amount of adsorbate due to the availability of more surface area of the adsorbent. But for each adsorbate (dye) studied the amount of adsorbate adsorbed after equilibrium per unit weight of adsorbent is different.
5.3.3 Effect of pH on the adosrption of dyes
(i) Methylene Blue (cationic):Effect of pH on methylene blue removal for different concentrations (10, 20, 50 and 100 mg/L) for the adsorbates namely bengal gram husk, tur dal husk, and tamarind husk, show that the percent removal was very efficient in the wide pH range of 6 to 11. However the percent removal decreased when the pH was reduced below 6. The decreased adsorption of methylene blue, a cationic dye at highly acidic pH is probably due to the presence of excess H+ ions competing with dye cations for the adsorption sites. As the pH of the system increases the number of negatively charged surface site decreases and the number of negatively charged surface site increases. The negatively charged surface sites on the adsorbent favours the adsorption of dye cation due to electrostatic attraction.
Increase in percent removal with increase in pH has been reported on the adsorption of methylene Blue and Basic Blue by Hydrilla (Low et al , 1993) and Chara aspera (Low et al , 1994). Nawar and Doma, 1989 have reported that the adsorption capacity was constant in the pH range of 2-10 for the adsorption of Sandocryl orange (basic dye) by peat, rice hulls and activated carbon. Coconut husk showed quantitative removal of Methylene blue at pH 12.00 (Low and Lee, 1990).
(ii) Amaranth (anionic) : The maximum removal of anionic dye, amaranth occurred at pH 2.0 for the different dye concentrations of 10, 20 and 50 mg/L and the percent removal decreased with increase in initial pH. The surface is positive at low pH where reaction (1) predominates, and is negative at higher pH when reaction (2) takes over.
As shown in the figures and Tables, it is found that the amount of amaranth adsorbed decrease with increasing solution pH.
At low pH, the following equation predominates:
With an increase in pH, positive charge at the oxide/solution interface decreases hence the adsorbent surface becomes negatively charged and will be associated with positively charged ions of the solution in the following manner :
Thus, there are no exchangeable anions on the outer surface of the adsorbent at higher pHs and consequently the adsorption decreases. Similar trends were observed in the adsorption of Congo red on red mud (Namasivayam and Arasi, 1997) and wollastonite (Singh, et al , 1994) and waste Fe (III)/Cr (III) hydroxide (Namasivayam et al , 1994)
Similar trend on pH effect has been reported by Namasivayam and co-workers for the adsorption of acid violet (acid dye) by residual biogas slurry (Yamuna, 1990) and waste banana pith (Namasivayam and Kanchana, 1992) Gupta et al, 1988 have reported that the adsorption of chrome dye (acid dye) on fly ash decreased with increase in pH. Yoshida et al, 1991 have found that the adsorption capacity of chitosan for acid orange II (acid dye) decreased with increase in pH.
Hwang and Chen, 1993 have reported that the removal efficiency of epichlorohydrin-cellulose polymer for acid blue 158 and acid orange 7 (both anionic dyes) decreased with increase in pH. Youssef , 1993 has reported that the acid dye stuffs – acid blue and acid violet showed maximum adsorption at acidic pH.
Bottom Ash and De-Oiled Soya was studied over a wide range of pH (2–7) for the adsorption of amaranth and it was found that in both the adsorbents the maximum uptake of dye occurred at pH 2.0 (Mittal et al , 2005). The anionic dyes namely amaranth, fast green and sunset yellow was maximally adsorbed by powdered peanut hull at pH 2.0 (Gong et al , 2005).
(iii) Rhodamine B (Cationic): The effect of solution pH on the adsorption of Rhodamine B by the different adsorbents are shown in Figures 88 to 90 and Tables 79 to 81. Adsorption of Rhodamine B increased with increase in pH. The maximum adsorption was found in the pH range of 7-11. The amount of the dye adsorbed decreased with the increase in the concentration of the dye molecules. As the pH of dye solution becomes higher, the association of dye cations with negatively charged functional groups in the adsorbent surface could be more easily take place as follows:
Lower adsorption of Rhodamine-B at acidic pH is due to the presence of excess H+ ions competing with the dye cation for the adsorption sites. When solution pH is low, the adsorbent surface and dye molecules both are highly in protonated form hence, there is electrostatic repulsion between cationic dye and protonated adsorbent surface which leads to lower removal of dye. Namasivayam and Kadirvelu (1994) have reported similar results. As the pH of the system increases, the number of positively charged sites decreases and the number of negatively charged sites increases. The negatively charged sites favor the adsorption of dye cation. Similar results have been reported for the removal of Rhodamine-B using agricultural solid wastes as adsorbents such as coir pith carbon (Namasivayam et al , 2001), silk cotton hull (Radhika et al , 2001) and pyrite (Demirbas et al , 2002).
(iv) Fast green (Anionic): At acidic pH quantitative removal of dye occurs for different initial dye concentrations (10, 20 and 50 mg/L) and the percent removal for all the adsorbents decreased with increase in the initial pH. In highly acidic media the adsorbent surfaces are highly protonated (positively charged) and favour the uptake of negatively charged (anionic) dyes like amranth and fast green. With increase in the initial pH of the dye, the degree of protonation of the adsorbent surfaces decreases gradually. Namasivayam and Yamuna, 1992, have reported that the percent adsorption of Congo red (anionic, direct dye) decreased with increase in pH. The anionic dyes namely amaranth, fast green and sunset yellow was maximally adsorbed by powdered peanut hull at pH 2.0 (Gong et al , 2005).
| 5.3.5 Adsorption isotherms: |
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Adsorption data for wide ranges of adsorbate concentrations and adsorbent doses have been treated by Langmuir (Langmuir, 1918) and Freundlich (Freundlich, 1907) isotherms, two widely used models. The Langmuir isotherm model is based on the assumption that maximum adsorption corresponds to a saturated monolayer of adsorbate molecules on the adsorbent surface, that the energy of adsorption is constant and that there is no transmigration of adsorbate in the plane of the surface. Langmuir isotherms were obtained by agitating the adsorbent of fixed dose and the adosrbate solution of different concnetrations for a contact time greater than equilibrium time. The Langmuir isotherm represents the equilibrium distribution of dye molecules between the solid and liquid phases. The following equation can be used to model the adsorption isotherm:
where q is milligrams of dye accumulated per gram of the biosorbent material; 
is the dye residual concentration in solution; 
is the maximum specific uptake corresponding to the site saturation and b is the ratio of adsorption and desorption rates (Chong and Volesky, 1995). When the initial dye concentration rises, adsorption increases while the binding sites are not saturated. The linearised Langmuir isotherm allows the calculation of adsorption capacities and the Langmuir constants and is equated by the following equation.
Thus a plot of 
vs should be linear if Langmuir adsorption were operative, permitting calculation of . The Langmuir isotherm model was followed by all the adsorbates and adsorbents in the present study. The comparison of sorption capacities of adsorbents (54 to 57) used in this study with those obtained in the literature shows that the four husks namely bengal gram husk, tur dal husk and tamarind husk are effective for the removal of dyes from aqueous solution.
The essential characteristics of a Langmuir isotherm can be expressed in terms of a dimensionless constant separation factor or equilibrium parameter 
, which is defined by
Where Co is the initial adsorbate concentration (mg/L) and b is the Langmuir constant (L/mg). The parameter indicates the shape of the isotherm as follows:
The values at different initial adsorbate concentrations indicate favorable adsorption for all the adsorbents and adsorbates studied.
The Freundlich equation is basically empirical, but is often useful as a means for data description. Freundlich isotherms were basically obtained by agitating the adsorbate solution of a fixed concentration and the adsorbent of different doses for a contact time greater than the equilibrium time. The Freundlich isotherm is represented by the equation (Freundlich, 1907):
where is the equilibrium concentration (mg/l), q is the amount adsorbed (mg/g) and 
and n are constants incorporating all parameters affecting the adsorption process, such as adsorption capacity and intensity respectively. The linearised forms of Freundlich adsorption isotherm was used to evaluate the sorption data and is represented as:
and n were calculated from the slopes of the Freundlich plots. The Freundlich isotherm basically indicates whether the adsorption proceeds with ease or difficulty.
Freundlich isotherm model was obeyed by all the adsorbates under the studied conditions. These results may be explained if adsorbent surface sites have a spectrum of different binding energies as suggested by Benjamin and Leckie, 1981.
The magnitude of the exponent ‘n’ gives the indication of favourability and , the capacity of the adsorbent/adsorbate system. The n values for the dyes were between 1 and 10 under the studied conditions, indicating beneficial adsorption (Yoshida, 1991).
The rate constant of adsorption is determined from the following first order rate expression given by Lagergren (1898)
where q and qe are amounts of adsorbate adsorbed (mg/g) at time, t (min) and at equilibrium, respectively, 
is the rate constant of adsorption (l/min). The linear plots of log10 (qe-q) vs t for all the dyes were studied at different concentration (Figures 147 to 172) shows the applicability of the above equation. Values of were calculated from the slope of the linear plots and are presented in Tables 105 to 116 for dyes. The rate constant for the dyes generally decreased with increase in adsorbate concentration. The rate constant for the adsorption of dyes is comparable with those in literature (Kadirvelu and Namasivayam, 2003; Periasamy and Namasivayam, 1994).
Both incineration and land disposal represent possible options for final disposition of spent adsorbent material. However, both methods directly or indirectly pollute the environment. If regeneration of dyes from the spent adsorbent were possible then it would not only protect the environment but also help recycle the adsorbate and adsorbent and hence contribute to the economy of wastewater treatment. Desorption studies help elucidating the mechanism adsorption.
Desorption experiments were carried out at different pH values. Desorption of the dyes with water was not significant. If the adsorption is by physical bonding then the loosely bound solute can be easily desorbed with distilled water in most of the cases (Agarwal et al., 2006). Hence, physical adsorption is ruled out. Among the three adsorbents used for adsorption experiments, the percentage dyes adsorbed on to tamarind husk were easily desorbed. Among the dyes, the percentage of amaranth desorbed was the highest with increase in pH. About 46.32% of the dye was desorbed from tamarind husk; 39.25% for bengal gram husk and 31.24% of amranth was desorbed from tur dal husk. The percentage of methylene blue desorbed did not exceed 2.45% for all the three adsorbents. Similarly low desorption percentages were observed in Rhodamine B and Fast green desorption.
From the desorption experiments, the following inference can be made. The negligible desorption of dyes with double distilled water indicates the predominance of chemical bonding between adsorbents and dyes. This implies that physical adsorption is not playing significant role in removal of dyes. This suggests either chemisorption or ion-exchange as the possible mechanism of dye removal. Since about 85% of dyes still remained on sorbents, which indicates that most of dyes are able to form strong bonds with the adsorbents.
5.4 Mechanism of adsorption
5.4.1 Dye adsorption
(i) Dye cation adsorption (Rhodamine B and Methylene Blue)
At acidic pH with an increasing concentration of the 
ion in dye solution, the surface 
ions would get neutralized by protonation, which facilitates the diffusion of dye molecules in the vicinity of the adsorbent. Consequently the positive charge density would be located more on the dye molecule at pH 4.0, and this accounts for the higher dye uptake on the negatively charged surface. Thus it is likely that the negative charge density on the surface will increase and will be associated with or 
ions according to the pH of the solution. These positively charged ions in the presence of dye solution could then be exchanged with dye cations as follows (Davis and Leckie, 1978).
Adsorption by chemisorption is represented by:
(ii) Dye anion adsorption (Fast green and Amaranth)
When dye anions are introduced in the system containing adsorbents, they may be adsorbed onto the positively charged surface in two ways (Sharma and Forster, 1994):
The first way :
This however, does not account for the change in pH of the solution after dye adsorption compared to the blank i.e. in the absence of dye anion.
The second way :
This accounts for the increase in pH of the solution after dye adsorption.
From the above observations, it is clear that the adsorption of Amaranth, Methylene blue, Fast green and Rhodamine B on bengal gram husk, tur dal husk and tamarind husk is predominantly chemisorption (Eq 5) and ion exchange is less operative (Eq 6). Moreover if ion exchange mechanism were operative, then the adsorption and desorption of the dyes would be reversible, when the operating pH ranges of these processses are reversed. As the binding is predominantly irreversible for all the dyes tested, it is concluded that ion exchange is not the predominant mechanism. Sometimes this kind of phenomenon is good, since the dye loaded adsorbent can be further employed for metal ion removal (Shukla and Sakherdande, 1990).
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