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Ind. Eng. Chem. Res. 2009, 48, 870–876
SEPARATIONS Adsorption of Ga(III) on Oxidized Coir Umesh S. Suryavanshi and Sanjeev R. Shukla* Department of Fibers and Textile Processing Technology, Institute of Chemical Technology, UniVersity of Mumbai, Nathalal Parekh Marg, Mumbai 400 019, India
Gallium is a strategic material due to its high-tech applications, but its availability in minerals is extremely poor to ensure economically feasible recovery. It is possible through the processing of other Ga-containing minerals, during which Ga gets concentrated. In the present work, Batch-wise adsorption of Ga(III) from the aqueous solution of gallium nitrate having different concentrations was attempted on unmodified (UC) and oxidized coir (OC). The SEM micrographs of OC show more porous structure due to the removal of pits from the coir surface by oxidation, leading to a 1.5× increase in the surface area. The effect of increasing pH from 1 to 3 greatly enhanced the adsorption. At pH 3, as high as a 70.53% adsorption was achieved on OC, beyond which Ga does not remain in solution form. Ga(III), being a trivalent ion, possesses substantial acidity, which increases with its concentration. The mechanism of Ga(III) adsorption on UC and OC has been put forward based on ion exchange. The adsorption fits to the Langmuir isotherm model and has been shown to obey pseudo second order kinetics. Considerably high adsorption capacity is gained by the coir upon simple oxidation treatment. The maximum adsorption capacity of OC was observed to be 19.42 mg g-1, as compared to 13.75 mg g-1 for UC. Desorption using 0.5 M HCl leads to nearly complete recovery of Ga(III) without affecting the physical characteristics of the adsorbent, like strength and crystallinity. Introduction Gallium is a relatively rare element that has found significant applications in the semiconductor industry.1-3 Gallium is classified as a “strategic” metal, because of its use in high technology and defense-related activities. It is not that rare of an element, being 30th in terms of abundance, at an average concentration of 19 mg g-1. As it is very uniformly distributed throughout the rocks at these concentrations, its extraction is not economical. There are no gallium-containing minerals of any economic significance. It is concentrated as a result of the processing of other minerals whose ores contain low concentrations of gallium. It is derived from wastes of industrial processes, such as flue dusts from the zinc industry, waste generated during the smelting of phosphate to produce elemental phosphorus, or sludge from the aluminum industry. For example, bauxite (the primary aluminum ore) typically contains 0.003 to 0.01% Ga.4 Concentrations in zinc ores (e.g., sphalerite) are comparable. Since primary sources are scarce, the general strategy is to recover gallium from intermediate industrial products. In the Bayer process, about 70% of the gallium content of bauxite is leached, along with aluminum, and about 30% is retained in the red mud. Gallium accumulates in the Bayer liquor with successive cycling, attaining concentrations of 100-200 ppm.3 Among the separation processes, the mercury amalgamation and electrolytic methods have been shown to be inefficient and expensive, besides posing environmental problems.5,6 Recent reviews by Demirbas7 and by Sud8 et al. about biosorption of heavy metals from wastewater highlight that the agricultural byproduct and their modified versions have a high capacity for adsorption of heavy metals such as Pb(II), Cd(II), * To whom correspondence should be addressed. Tel: +91 22 24145616. Fax: +91 22 24145614. E-mail: sanjeevrshukla@ rediffmail.com.
Hg(II), Cu(II), Ni(II), Cr(III), and Cr(IV), as well as some elements from lanthanide and actinides groups. The major advantages include the low cost, high efficiency, minimization of chemical or biological sludge, regeneration of biosorbents, and possibility of metal recovery.7-10 The recovery of heavy metals from aqueous solutions and/ or wastewater from various industries is an important issue for environmental protection and remediation.11 Biosorption studies have been mainly focused on the removal of heavy metal ions from industrial effluents, with detoxification prior to disposal being the primary goal.12 Functional groups responsible for metal binding in the biosorbents are carboxyl, phosphoryl, sulfhydryl, amino, sulfate, imidazol, thioether, phenol, amide, and hydroxyl in various biomolecules of peptide, protein, and polysaccharide moieties of cell walls. These functional groups bind the metal ions mainly by adsorption, ion exchange, and chelating effects.13 Coir, a natural lignocellulosic material, has been reported to be a very good adsorbent for many heavy metal ions.14-16 Very few references have been reported in literature for the removal/ recovery of Ga using natural adsorbent. In the present work, the adsorption of Ga(III) from the aqueous solution of gallium nitrate on coir, in its natural as well as oxidized form, has been reported. Coir is available as a cheap and abundantly available lignocellulosic fibrous material. The kinetics and order of reaction for Ga(III) adsorption have been reported. Desorption on oxidized coir and its effect on physical properties of coir has also been studied. 2. Materials and Methods 2.1. Adsorbate and Adsorbent. Gallium Nitrate was obtained from Sigma-Aldrich (UK). A 1000 mg L-1 stock solution of Ga(III) was prepared by dissolving 1.356 g Gallium nitrate salt in deionized water. The experimental solutions ranging from
10.1021/ie801259c CCC: $40.75 2009 American Chemical Society Published on Web 12/08/2008
Ind. Eng. Chem. Res., Vol. 48, No. 2, 2009 871 -1
10 to 200 mg L were prepared by appropriate dilution, and their accurate concentrations were determined using Atomic Absorption Spectrophotometer (AAS) (model GBC 932 plus, Australia). All chemicals used were of analytical reagent grade and obtained from Sigma Aldrich Chemicals, Mumbai. Waste coir fiber was procured from a local industry in Kerala, India. It was boiled in 1 g L-1 of nonionic soap solution for about 30 min followed by thorough washing with deionized water to remove soluble impurities and any dirt present. It was cut approximately in 1 cm long pieces and used as adsorbent. 2.2. Oxidation of Coir Fibers. Oxidation of coir was performed in small batches each of about 100 g, as per requirement of study. For oxidation, 100 g coir material was treated with 2 L solution containing 15 g (50%) hydrogen peroxide at pH 11.0 maintained using 1 g NaOH. The temperature of the bath was raised to 85 °C in about 20 min and oxidation was continued further for 2 h. After the treatment, coir fibers were filtered, washed thoroughly with hot deionized water, and then with cold water until neutral pH, followed by drying in air. Simultaneously, uncut coir fibers having a length of ∼6 cm were given similar oxidation treatment to evaluate their tensile strength for any possible degradation in fiber characteristics. The oxidation procedure employed was similar to the oxidative bleaching of cellulosic fibers in a textile processing house, except that the hydrogen peroxide concentration used was higher.17 The evaluation of oxidized coir in terms of percent crystallinity and breaking strength was performed for each batch separately, and the results were found to be in the range of (5% accuracy. Thus, repetition of processes and reproducibility of results were assured. 2.3. Characterization of Coir. 2.3.1. SEM. The change in surface morphology of coir samples due to oxidation was visualized by scanning electron microscope (Jeol JSM 6380 LA, Japan). The sample was first coated with platinum using coater (Jeol 1600) to make it conductive, and the micrographs were recorded at an accelerating voltage of 10 kV with various magnifications. 2.3.2. Surface Area Analysis. The surface area of unmodified coir (UC) and oxidized coir (OC) was determined by using a Micrometrics ASAP 2010 surface area analyzer (USA) using nitrogen adsorption-desorption method. Before obtaining the adsorption isotherm, the sample was purged with pure nitrogen gas for 6 h at a temperature of 150 °C to ensure the removal of any contaminants as well as any moisture that may be present on the surfaces. 2.3. Surface Charge. The effect of chemical modification of coir on the surface charge of UC and OC was studied by measuring the zeta potential at pH 3.0, using Anton Paar (Austria) Electro Kinetic Analyzer (EKA) based on streaming potential measurement. 2.3.4. Breaking Strength. The breaking strength of coir fibers was measured on a Universal Tensile Tester (Tinus Olsen H5KS, England) with a gauge length of 5 cm, employing the standard procedure ISO 5079.18 2.3.5. X-ray Diffraction. An X-ray Diffractometer (XRD) (Shimadzu XRD-6000, Japan) with a Cu KR source was used to detect the crystalline phases of the coir. The X-ray crystallinity values have been directly obtained. 2.4. Batch Adsorption Experiment. The kinetics of batch adsorption were studied for UC as well as OC at room temperature (32 °C). The coir material (0.1 g) was shaken with 40 mL of Ga(III) solution (50 mg L-1) for 180 min in a 250 mL Erlenmeyer flask in an orbital shaker machine (Rossari Biotech Ltd., Mumbai) at a constant speed of 100 rpm. The pH
of Ga solution was maintained at pH 3.0 ((0.02) using 1 M HCl throughout the experiment. The individual flasks were removed at a predetermined time and the solution, acidified with a drop of 1 M nitric acid, was filtered through Whatman Filter paper no. 41 to remove any coir particles. The filtrate was analyzed for residual Ga(III) using AAS at 294.4 nm. An “A” grade apparatus was used in all of the experiments. As a caution, the adsorption of Ga(III), if any, on the inner surface of flask was also estimated and found to be nil. The batch adsorption of Ga(III) on UC and OC was studied by varying the initial concentration from 20 to 200 mg L-1 for 120 min in a similar manner by maintaining the pH at 3. The pH of different concentrations of Ga(III) solution was measured. The effect of the initial solution pH on the adsorption of Ga ions on oxidized coir was examined by mixing 0.1 g coir with 40 mL of 100 mg L-1 Ga(III) solution and adjusting the pH using 1 M HCl between 1 and 3 with an interval of 0.5. The adsorption capacity was calculated using the following mass balance equation: (C0 - Ceq)V W Where, qeq is the equilibrium adsorption capacity (mg g-1), Co and Ceq are the initial and equilibrium liquid phase solute concentrations (mg L-1), respectively, V is the liquid phase volume (L), and W is the amount of adsorbent (g). Different concentrations of Ga(III) up to 200 ppm were used for adsorption on OC and UC without maintaining the pH, and the changes in pH after 120 min of adsorption were noted. The desorption study was carried out for metal recovery and reuse of the adsorbent. Fifty-mL portions of hydrochloric acid of varying concentrations (0.1 to 1 M) were used to desorb Ga(III) from 0.1 g coir. Ga(III) content in desorbed solution was determined using AAS. All of the experiments were performed three times and the average values have been reported. 2.5. Adsorption Isotherm Modeling. Out of a number of empirical adsorption isotherm models, the Langmuir and Freundlich are the most common. Although the Langmuir model does not take into account the importance of pH, nor does it shed light on the mechanism of the adsorption process, it provides information about the metal ion adsorption capability and provides a simple tool to compare two adsorbents under identical adsorption conditions.19 The linearized Langmuir equation is given by the following:20 qeq )
[
Ceq Ceq 1 ) + q qmaxb qmax
]
(1)
qmax and b can be determined from the linear plot of qe/q vs Ceq. The Langmuir isotherm can be expressed by means of a dimensionless constant, RL, referred to as separation factor or equilibrium factor, which provides information about spontaneity of adsorption process.21 It is given by the following: RL )
1 1 + bCo
(2)
Where, b is the Langmuir constant and Co is the highest initial Ga(III) ions concentration in mg L-1. The value of RL between 0 and 1 indicates favorable adsorption.22 The linearized Freundlich equation in logarithmic form is given bythe equation:23
872 Ind. Eng. Chem. Res., Vol. 48, No. 2, 2009
1 log q ) log K + log Ceq (3) n K and (1)/(n) can be determined by plotting logq vs logCeq. The Freundlich isotherm, although widely used, provides no information on the monolayer adsorption capacity, in contrast to the Langmuir model.24,25 2.6. Kinetic Modeling. The kinetics of sorption of heavy metals from wastewater has been studied mostly using pseudofirst order and pseudosecond order reaction models. The pseudofirst order Lagergren model, based on the assumption that the rate of adsorption is proportional to the number of free sites, is given by the equation:26 dq ) K1(qeq - qt) (4) dt where, qeq and qt are the adsorption capacities (mg g-1), at equilibrium and at time t, respectively and K1 is the Lagergren rate constant of the first order adsorption (min-1). Linearized version of this model was obtained after integration and by applying boundary conditions t ) 0 to t and qt ) 0 to qt as follows: K1t (5) 2.303 Linear plots of log(qeq - qt) versus t were plotted to evaluate this kinetic model and to determine rate constant and qeq from the slope and the intercept, respectively. The pseudosecond order reaction kinetic is expressed as follows: log(qeq - qt) ) log qeq -
dq ) K2(qeq - q)2 (6) dt where, K2 is the rate constant of pseudo second order adsorption (g mg-1 min-1). Applying boundary conditions t ) 0 to t and qt ) 0 to qt, the integrated and linearized form of eq 8 becomes: 1 t t ) + q K q2 qeq
(7)
2 eq
If the initial adsorption rate h (mg g-1 min-1) is as follows: h ) K2q2eq
(8)
t t 1 ) + q h qeq
(9)
then eq 8 becomes:
The pseudo second order plot was developed by plotting (t)/(q) vs t. The value of qeqand K2 were calculated from the slope and the intercept, respectively. 2.7. Statistical Analysis. To determine the model fit, mean squared errors (MSE) were calculated by taking square of the differences between the experimental metal uptake data (q) and the corresponding model prediction of the uptake (qm) and dividing the sum of those squared errors by the number of data points (p) for each set: p
∑ (q - q
2 m)
MSE )
1
p
(10)
3. Results and Discussion Lignocellulosic material like coir possesses both cellulose and lignin as major components, which have been shown to adsorb metal ions from their aqueous solutions.14,15 Chemical modifica-
Figure 1. Scanning electronic micrographs of unmodified coir. (a) Magnification 500× and (b) Magnification 1000×.
tion of lignocellulosic materials, mostly from agricultural waste, has been widely studied during the last 20 years to improve their metal adsorption capacities.27 The target functional groups are the hydroxyl, present in plentiful abundance, and the carboxyl groups. The oxidation of such materials results in the formation of a large number of carboxylic groups, which, at a suitable pH, ionize to form -COO-, thus becoming capable of binding the metal ions by various mechanisms.28 In our previous work, the chemical characterization of coir before and after oxidation could identify various factors responsible for the improved metal ion adsorption by oxidized coir.14,16 An SEM image showed the opening of pores on coir surface due to removal of tylose and other fatty acids resulting in more porous structure with enhanced surface area.29 The increased porosity was also confirmed earlier by a higher propanol-2 retention value. IR spectra and methylene blue (a cationic dye) absorption methods confirmed the presence of carboxylic groups in the oxidized coir which were shown to be responsible for increased Ni(II), Fe(II), Zn(II), and Cu(II) uptake upon oxidation of coir.14,16 3.1. Characterization of Coir. 3.1.1. SEM. SEM images of unmodified coir (UC) (Figure 1, parts a and b) show a number of pits all over the fiber surface. These are known to contain tylose and other fatty acids.29 The surface of UC may be observed to be quite non-uniform before the oxidation treatment. In the case of oxidized coir (OC), these pits are removed from the surface, resulting in the formation of open pores or voids (Figure 2a). Figure 2b shows highly porous structure inside the open pits of OC, consisting of several interlocking channels, which were absent in the UC. 3.1.2. Surface Area Analysis. The BET surface areas of UC and OC were found to be 0.455 and 0.675 m2 g-1, respectively. Thus, upon oxidation, the opening up of the pits has caused an almost 1.5× enhancement in the area available for adsorption.
Ind. Eng. Chem. Res., Vol. 48, No. 2, 2009 873 Table 1. X-ray Crystallinity and Breaking Strength adsorbent
crystallinity (%)
breaking strength (N)
RSD
89.0 85.0 84.6
3.57 2.86 2.68
1.29 1.37 2.03
UC OC OC after Ga(III) adsorption and desorption
Table 2. Effect of Ga(III) Sorption on Bath pH pH of bath after 120 min of adsorption
Figure 2. Scanning electronic micrographs of oxidized coir. (a) Magnification 500×and (b) Magnification 1000×.
This is in agreement with the observations of SEM images. 3.1.3. Surface Charge. Zeta potential values of UC and OC at pH 3 were -32mV and -78 mV, respectively, indicating generation of more negativity on the coir surface after oxidation treatement, which enhance the adsorption in case of OC. 3.1.3. Breaking Strength and X-ray Crystallinity. It is likely that severe oxidation treatment can cause damage to the physical form of coir. If so, then the possibility of its repeated use for adsorption-desorption (acidic) may decrease, particularly when used in columns at high hydraulic pressure. It is, therefore, necessary that a treatment given to the biomass to enhance its adsorption capacity should not decrease its physical strength to any significant extent. If the biomass is chemically modified to improve its adsorption capacity, then it is also necessary that the benefits achieved through enhanced adsorption are not wiped out by the limitations imposed on its reuse capabilities. This makes the use of an environmentally friendly adsorbent cost-effective. This factor has often been ignored in most of the biosorption studies and importance is given only to its ability to improve the uptake capacity of an adsorbent. One of the factors for consistent and repeated adsorption capacity is the unaltered physical form of the adsorbent, (viz., particle size). Hence, from the point of view of repeated use, the biomass with sufficient adsorption capacity and better physical durability (assessed in terms of breaking strength of the fibrous adsorbent) is superior to the one with higher capacity but unstable physical form. Table 1 shows that upon oxidation, the crystallinity and breaking strength of coir decreased, which may obviously be attributed to the severe oxidation treatment. The fibrous adsorbent has long polymeric chains that have alignment in the direction of the fiber axis, and due to the closeness of packing of the chains, crystallinity is imparted to the fiber. Severe oxidation causes partial breaking of the chains and damages the crystal structure of the cellulosic polymer in coir, thereby
Ga(III) conc. (mg L-1)
initial pH
UC
OC
0 10 20 40 60 80 100 150 200
6.96 3.41 3.08 2.91 2.80 2.73 2.68 2.58 2.52
6.94 5.12 5.14 5.26 5.31 5.38 5.47 5.56 5.60
8.26 6.52 6.63 6.72 6.75 6.76 6.85 6.87 6.92
reducing the breaking strength and the crystallinity. Also, as the pits are opened by oxidative treatment and coir becomes more porous, the strength decreases. The porous structure of OC can be clearly seen from the SEM images. The breaking strength and X-ray crystallinity data given in Table 1 for UC and OC at various stages of their use indicates that these values for OC are not significantly affected by desorption under acidic conditions. 3.2. Effect of pH. Cations in water are always surrounded by a progressively larger shell of water molecules because of electrostatic attraction. The smaller radius, greater positive charge, and electro negativity greater than 1.8 favor the electrostatic attraction. The water molecules in the innermost shell near the cation are most strongly attracted, and hence Ga(III) could be written as Ga(H2O)n3+, where n is the number of water molecules in the inner hydration shell varying from 1 to 6. Ga(III) has the atomic radius 1.25 Å and electro negativity 1.82. Hydrated metal ions can behave as acids and release H+ from their water ligands, which then become attached to the surrounding free water molecules, forming acidic protons, H3O+ and H5O2+. The stronger the electrostatic attraction between the water ligand and the metal cation, the more readily a proton is released to the surrounding water molecules and the more acidic is the hydrated metal cation. All metal cations with a charge of +3 or more are moderately strong acids. This process is one source of acidic water draining from mines.30 Thus, with increasing concentration of Ga(III) the solution pH becomes more and more acidic. This has been indicated in Table 2, where an increase in concentration of Ga(III) in bath from 10 mg L-1 to 200 mg L-1 decreases the pH to 2.52. Solution pH is of immense importance in metal ion adsorption studies, as it mainly decides the speciation of metal ions in solution, as well as the availability of various functional groups present in the adsorbent for adsorption depending on their respective pKa values. Figure 3 shows the effect of initial solution pH on the adsorption capacity of UC and OC for 100 mg L-1 Ga(III) solution. The adsorption dependency of Ga(III) on pH is similar to that observed for most of the divalent cations.10-12 Sorption of Ga(III) is found to be negligible up to pH 1.5, which thereafter reaches its maximum at pH 3. This is attributed to less competition of proton with metal ion at higher pH. Similar observation was reported by Pokrovsky et al. for adsorption of Ga(III) on amorphous silica.31 Beyond this pH,
874 Ind. Eng. Chem. Res., Vol. 48, No. 2, 2009 Table 3. Adsorption of Different Metal Ions on Coir qmax (mg g-1)
Figure 3. Effect of pH on uptake of Ga(III).
gallium either polymerizes or forms a hydroxide gel thereby loosing its solution characteristics.32 3.3. Mechanism of Ga(III) Adsorption. During batch studies, the values of pH were noted for the initial bath and the bath after sorption of Ga(III). As a control experiment, UC and OC were added to deionized water of pH 6.96 and agitated for 2 h. In the case of UC, no change in pH was observed, whereas OC caused an increase in pH to 8.26 (Table 2). This was attributed to the fact that oxidation of coir with hydrogen peroxide under alkaline conditions (pH 11.0) converts -COOH groups in coir into -COONa groups. When OC is added to deionized water, release of Na+ ions take place toward water by proton substitution on coir. Table 2 also indicates that at any initial concentration of Ga(III), the pH of the bath rise upon sorption of Ga by coir. The pH of precipitation of Ga(III) as gallium hydroxide is little beyond 3, and it increases slightly with an increase in initial concentration. The rise in pH upon adsorption may be attributed to two factors: (1) a decrease in Ga concentration in bath due to adsorption and (2) precipitation of Ga hydroxide which cannot be adsorbed by coir but reduces the soluble cation concentration contributing to the acidity of solution. A similar increase in pH from 3.5 (adjusted using HCl) to 6.0 was observed by Schiewer and Volesky33 after addition of biomass marine alga Sargassum fluitans to the metal ion-free solution. When OC is used as an adsorbent, both the release of Na+ from the -COONa on coir and the decrease in Ga(III) concentration in the bath due to adsorption enhance the pH to a level much higher than that found with UC. The increase in Ga(III) adsorption on coir upon increasing the pH from 1.0 to 3.0 may be attributed to the decrease in the competition between H+ and Ga(III) for the same binding sites. By increasing the pH, the degree of ionization of carboxylate groups (pKa ) 2.45) increases, and this facilitates the cation exchange (eq 12). R - COOH + Ga+3 f R - COOGa+2 + H+ +3
+2
metal ions
initial bath conc. (mg L-1)
UC
OC
ref
Ga(III) Cu(II) Zn(II) Fe(II) Ni(II)
203 467 212 444 456
13.75 2.54 1.83 2.84 2.51
19.42 6.99 7.88 7.49 4.33
14 14 14 16
3.4. Adsorption Isotherm Modeling. Figure 4 shows the relationship between equilibrium uptake (qeq) and equilibrium concentration (Ceq) of Ga(III), which clearly indicates that OC gives higher sorption than UC. With the increase in initial Ga(III) concentration, qeq increased and finally reached a plateau indicating saturation of the available binding sites with Ga(III). One important factor in evaluating the adsorbent performance is the initial gradient of the adsorption isotherm, as it depends on the affinity of the adsorbent at low metal concentration. The steep initial rise in the curve for OC suggests its better performance as adsorbent, than UC. The Langmuir model has shown better fit than the Freundlich model, as is apparent from the higher r2 value for both the adsorbents (Table 4). It is also apparent from the lower value of MSE error. A much higher value of b, given in Table 4, in case of OC than for UC shows better bonding of Ga(III) to OC. For Freundlich model, the value of Kf is higher in case of UC as compared to OC indicating the heterogeneous nature of unmodified coir, which is apparently clear from the differences observed in the SEM micrographs of the two types of adsorbents. 3.5. Kinetic Modeling. The effect of contact time up to 180 on qeq was studied (Figure 5). In the case of OC, the maximum adsorption took place within the first 30 min, with a steep rise in the adsorption; thereafter, it remained almost constant. However, for UC, this stage reached only after about 45 min. This has been attributed to more porous nature of OC, which allows a higher mass transfer rate, resulting in rapid adsorption of Ga(III) ions from aqueous solution. This rapid adsorption phenomenon is advantageous in process applications. All further batch adsorption experiments were conducted for 2 h for attaining the maximum possible sorption. Figure 6 shows the linearized plot for pseudo second order kinetic equations, for the adsorption of Ga(III) on OC. The Lagergren first-order rate equation shows poor fit to the experimental data (r2 ) 0.83). It may be observed that the qeq values estimated by first-order kinetic model differ substantially from those measured experimentally, suggesting that the adsorption of Ga(III) on OC is not a first-order reaction. This is because, in first order model, the uptake at lower initial concentration (which is much lower than the equilibrium concentration) strongly affects the slope of the straight line, and
(11)
+
(12) R - COONa + Ga f R - COOGa + Na From the data given in Table 3, it is clear that the adsorption of Ga(III) on UC or OC is much higher than that of any of the divalent metal ions studied earlier. This may be attributed to the formation of a 1:3 complex with Ga(III) compared to that of a 2:1 complex in the case of divalent metal ions.
Figure 4. Adsorption isotherm for Ga(III) on UC and OC.
Ind. Eng. Chem. Res., Vol. 48, No. 2, 2009 875 Table 4. Langmuir and Freundlich parameters for adsorption of Ga(III) Langmuir constants (statistical parameter)
Freundlich constants (statistical parameter)
adsorbent
qmax (mg g-1)
b (m g-1)
RL
r2
MSE
K
n
r2
MSE
UC OC
13.75 19.42
0.021 0.126
0.192 0.038
0.996 0.982
0.089 0.026
10.03 1.28
1.28 4.10
0.932 0.935
0.268 0.116
hence the y-intercept of linearized plot. It can be said that Lagergren first order rate equation is generally applicable if there is very rapid uptake, so that the equilibrium concentration is not much different from the initial concentration. The pseudo second order linearized plot is in very good agreement to the experimental data. Also, the value of experimental qeq is very close to that obtained by the model (Table 5). The correlation coefficients for the second-order kinetic model are nearly equal to 1, suggesting second-order kinetic model for adsorption of Ga(III) on OC. 3.6. Desorption Study. Table 6 shows the results of desorption using varying concentration of HCl as eluent. It may be
Table 6. Desorption of Ga(III) from OC Using Different Concentrations of HCl HCl conc. (M)
desorption of Ga(III) (%)
0.1 0.3 0.5 0.7 0.9 1.0
78.3 89.8 98.6 98.7 98.5 98.6
observed that 0.5 M HCl is sufficient to achieve the maximum desorption. Further increase in concentration does not change it significantly. At the same time, coir shows insignificant deterioration in strength and crystallinity upon adsorption followed by acid desorption (Table 1). Thus, the recovery of gallium can be effectively achieved. The study on column adsorption-desorption cycles is underway. 4. Conclusions Inexpensive and abundantly available biomass coir shows good potential for adsorbing Ga(III). Oxidation of coir enhanced the adsorption capacity from 13.75 mg g-1 to 19.42 mg g-1, which is substantially higher than that observed for the divalent cations studied earlier. Pseudo second order kinetics is obeyed with a perfect fit to the Langmuir isotherm. The adsorption takes place through an ion exchange mechanism, which is dependent on the pH of the Ga(III) solution. As the adsorbent does not significantly lose physical characteristics, the possibility of its reuse for repeated metal ion adsorption exists. Nearly complete recovery of Ga(III) has been achieved.
Figure 5. Effect of contact time on equilibrium adsorption capacity of coir.
Figure 6. Pseudo second order plot for adsorption of Ga(III) on OC. Table 5. Parameters for Psudo Second Order Kinetic Plot for Adsorption of Ga(III) on OC second-order model -1
qexpt (mg g ) 19.42
K2 (g mg
-1
6.73
min-1)
qcal (mg g-1)
r2
18.67
0.999
Literature Cited (1) Moskalyk, R. R. Gallium: the backbone of the electronics industry. Miner. Eng. 2003, 16, 921. (2) Grant, I. R. Gallium arsenides from mine to microcircuit. Trans. IMM 1988, 97, C48. (3) Selvi, P.; Ramasami, M.; Samual, M. H. P.; Adaikkalam, P.; Srinivas, G. N. Recovery of gallium from bayer liquor using chelating resins in fixedbed columns. Ind. Eng. Chem. Res. 2004, 43, 2216. (4) Abdul-Kadher, J. A. M.; Varadaraj, A.; Srinivas, G. N.; Shrivasan, R. Recovery of Gallium from sodium aluminate liquorssa review, chemical industry developments incorporating CP & E, January 1974, 22. (5) de Breteque, P. La. Method of recovering gallium from alkali aluminate lye. U.S. Patent 2,793,179, May 21, 1957. (6) Varadharaj, A.; Srinivasan, R.; Prabhakara, R. G. Effect of impurities on gallate reduction in alkaline solution sa linear stripping voltammetry study. J. Appl. Eletrochem. 1989, 19, 61. (7) Demirbas, A. Heavy metal adsorption onto agro-based waste materials: A review. J. Hazard. Mater. 2008, 157, 220. (8) Sud, D.; Mahajana, G.; Kaura, M. P. Agricultural waste material as potential adsorbent for sequestering heavy metal ions from aqueous solutionssA review. Biores. Technol. 2008, 99, 6017. (9) Qaiser, S.; Saleemi, A. R.; Ahmad, M. M. Heavy metal uptake by agro-based waste materials. EnViron. Biotechnol. 2007, 10, 409. (10) Fiola, N.; Villaescusaa, I.; Martı´nezb, M.; Mirallesb, N.; Pochc, J.; Serarols, J. Sorption of Pb(II), Ni(II), Cu(II), and Cd(II) from aqueous solution by olive stone waste. Sep. Purif. Technol. 2006, 150, 132. (11) Vieira, R. H.; Volesky, B. Biosorption: a solution to pollution. Int. Microbiol. 2000, 3, 17. (12) Volesky, B. Biosorption of HeaVy Metals; CRC press: Boca Raton, FL, 1990, 3. (13) Schiewer, S.; Volesky, B. Modeling of the proton-metal ion exchange in biosorption. EnViron. Sci. Technol. 1995, 29, 3049.
876 Ind. Eng. Chem. Res., Vol. 48, No. 2, 2009 (14) Shukla, S. R.; Pai, R. S.; Shendarkar, A. D. Adsorption of Ni(II), Zn(II), and Fe(II) on modified coir fibres. Sep. Purif. Technol. 2006, 47, 141. (15) Shukla, S. R.; Pai, R. S. Comparison of Pb(II) uptake by coir and dye loaded coir fibres in a fixed bed column. J. Hazar. Mater. 2005, 147. (16) Shukla, S. R.; Pai, R. S.; Gaikar, V. G.; Suryavanshi, U. S. Batch and column adsorption of Cu(II) on unmodified and oxidized coir. Sep. Sci. Technol., accepted for publication. (17) Trotman, E. R. Dyeing and Chemical Technology of Textile Fibres; Charles Griffin & Co Ltd.: Nottingham, UK, 1984, 196. (18) Sundaram, V.; Krishna Iyer, K. R.; Munshi, V. G.; Parthsarathy, M. S.; Ukidwe, A. V. Handbook of Methods of Tests for Cotton Fibres, Yarns and Fabrics; Cotton Technological Research Laboratory: ICAR, India, 1979, 61. (19) Volesky, B. Sorption and Biosorption; BV Sorbex: Montreal, 2003, 105. (20) Langmuir, I. The adsorption of gases on plane surfaces of glass, mica and platinum. J. Am. Chem. Soc. 1918, 40, 1361. (21) Hall, K. R.; Eagleton, L. C.; Acrivos, A.; Vermeulen, T. Pore and solid diffusion kinetics in fixed-bed adsorption under constant-pattern conditions. Ind. Eng. Chem. Fundam. 1966, 5, 212. (22) McKay, G.; Blair, H. S.; Garden, J. R. Adsorption of dyes on chitin. 1. Equilibrium studies. J. Appl. Polym. Sci. 1982, 27, 3043. (23) Freundlich, H. Ueber die adsorption in loesungen. Z. physic. Chem. 1907, 57, 385. (24) Aksu, Z.; Kutsal, T. A comparative study for biosorption characteristic of heavy metals ions with C. Vulgaris. EnViron. Technol. 1990, 11, 979.
(25) Do¨nmez, G. C.; Aksu, Z.; Ozturk, A.; Kutsal, T. A comparative study on heavy metal biosorption characteristics of some algae. Process Biochem. 1999, 34, 885. (26) Lagergren, S. Zur theoric dersogenannten adsorption geloster stoffe, kungliga svenska ventenskapsatemiens. Handlinger 1989, 24, 1. (27) Krishnani, K. K.; Ayyappan, S. Heavy metals remediation of water using plants and lignocellulosic agrowaste. ReV. EnViron. Contam. Toxicol. 2006, 188, 59. (28) Sadettin, S.; Do¨nmez, G. Bioaccumulation of reactive dyes by thermophilic cyanobacteria. Process Biochem. 2006, 41, 836. (29) Prasad, S. V.; Pavithran, C.; Rohatgi, P. K. Alkali treatment of coir fibres for coir-polyester composites. J. Mater. Sci. 1983, 18, 1443. (30) Eugene, R. W. Applications of EnVironmental Aquatic Chemistry; Taylor and Francis: Boca Raton, 2008, 109. (31) Pokrovsky, O. S.; Pokrovski, G. S.; Schott, J. Gallium(III) adsorption on carbonates and oxides: X-ray absorption fine structure spectroscopy study and surface complexation modeling J. J. Colloid Interface Sci. 2004, 279, 314. (32) Persson, P.; Zivkovic, K.; Sjoberg, S. Quantitative adsorption and local structure of gallium at the water-∝-FeOOH interface. Langmuir 2006, 22, 2096. (33) Schiewer, S.; Volesky, B. Modeling of the proton-metal ion exchange in biosorption. EnViron. Sci. Technol. 1995, 29, 3049.
ReceiVed for reView August 18, 2008 ReVised manuscript receiVed October 31, 2008 Accepted October 31, 2008 IE801259C
