Synthesis, Characterization, and Application as a Chromium(VI

The kinetic constants of the metal adsorption, which could be used to optimize the .... Values of 0.1 < 1/n < 1.0 show the favorable adsorption of Cr(...
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Ind. Eng. Chem. Res. 2004, 43, 2247-2255

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Synthesis, Characterization, and Application as a Chromium(VI) Adsorbent of Amine-Modified Polyacrylamide-Grafted Coconut Coir Pith Maya R. Unnithan, V. P. Vinod, and T. S. Anirudhan* Department of Chemistry, University of Kerala, Kariavattom, Trivandrum 695 581, India

The amine-modified polyacrylamide-grafted coconut coir pith carrying -NH3+Cl- functional group at the chain end (PGCP-NH3+Cl-) was investigated as an adsorbent for its possible application for the removal of chromium(VI) from aqueous solution and wastewater. The infrared spectroscopy results were used to confirm the graft copolymer formation and -NH3+Cl- functional group. The grafting of polyacrylamide onto the coir pith improved the thermal stability of the adsorbent and enhanced the apparent activation energy for the thermal degradation of PGCPNH3+Cl-. X-ray diffraction pattern and scanning electron microscopy (SEM) studies were carried out to investigate the crystallinity and morphology of the adsorbent. The decrease in crystalline domains in PGCP-NH3+Cl- results in the loss of tensile strength of the grafted chain and consequently enhances the free mobility of the grafted chain. Batch adsorption technique using PGCP-NH3+Cl- was applied for the removal of chromium(VI) anion from aqueous solution and wastewater. The maximum adsorption of 99.4% (12.43 mg/g) took place from an initial concentration of 25.0 mg/L Cr(VI) at 30 °C, pH 3.0, and an adsorbent dosage of 2.0 g/L. The kinetics of sorption of Cr(VI) ions were described by a pseudo-second-order kinetic model. The temperature dependence indicates the exothermic nature of the process. Equilibrium isotherms were determined for different temperatures and the results are analyzed using the Langmuir and Freundlich isotherm equations. Adsorption isotherm experiments were also conducted for comparison using a commercial chloride form Dowex, a strong base (quaternary amine functionality) anion exchanger. Quantitative removal of 22.7 mg/L Cr(VI) in 50 mL of electroplating industry wastewater by 125 mg of PGCP-NH3+Cl- was observed at pH 3.0. Alkali regeneration was also tried for several cycles with a view to recover the adsorbed metal ions and also to restore the sorbent to its original state. Introduction The presence of hexavalent chromium [Cr(VI)] in water and wastewater has received much attention in recent years. Most often Cr(VI) enters the aquatic environment through the discharge of the effluents from electroplating, metal finishing, magnetic tapes, leather tanning, pigment, and chemical manufacturing industries. Cr(VI) is a well-known highly toxic metal, considered as a priority pollutant. The most widely used technique for removing Cr(VI) from wastewater is adsorption using activated carbon, which is ineffective at very low concentration.1 In this case, activated carbon requires complexing agents to improve the adsorption performance in removing metal ion from aqueous solutions. Ion-exchange treatment, which is the second most widely used technique, can be used for Cr(VI) removal at very low concentrations to the region of parts per million. Ion-exchange technique does not present a sludge disposal problem and has the advantage of recovery and regeneration of adsorbent; however, it does not appear to be economical and suffers from a lack of selectivity. Research has therefore been directed toward improving the selectivity. The cost problem might be solved by using naturally occurring low-cost polymers such as lignocellulosic materials. While their adsorption capacity is usually less than those of commercial ion* To whom correspondence should be addressed. E-mail: [email protected].

exchange resins, these materials could provide an inexpensive substitute for the treatment of wastewater. To enhance the adsorption capacity, the lignocellulosic materials are modified in various ways, such as treatment by inorganic and organic compounds, acids, and bases. For example, Ajmal et al.2 reported that phosphate-treated sawdust (lignocellulosic material) shows a remarkable increase in sorption capacity of Cr(VI) as compared to untreated sawdust. Nowadays, the use of lignocellulosic materials in many applications has increased considerably. Of them, utilizing sawdust coconut coir pith and coconut husk for the production of ion exchangers to remove heavy metals have revealed good potential in wastewater treatment.3,4 Grafting of synthetic polymers onto solids followed by functionalization is a well-known method for the modification of the physical and chemical properties of the adsorbent and to improve the adsorption capacity.5 Graft polymerization of lignocellulosic materials and waste biomass can also prevent the organic substances from leaching out of the materials. This modification may also increase the stability of the adsorbent materials, which is an important aspect of commercial development of biosorbent materials. Materials used as polymer support for the preparation of adsorbents having different functional groups include tea leaves,6 coconut husk,7 bagasse pith,8 sawdust,9 and cellulose.10 Recently, we proposed a convenient procedure for surface modification which involved the grafting of

10.1021/ie0302084 CCC: $27.50 © 2004 American Chemical Society Published on Web 04/02/2004

2248 Ind. Eng. Chem. Res., Vol. 43, No. 9, 2004 Scheme 1. Preparation of Polyacrylamide-Grafted Coconut Coir Pith Having -NH3+Cl- Functional Groups

(model QS/7). The apparent surface area (SN2/BET) was obtained from N2 adsorption isotherm data using the BET equation. Porosity measurements of the adsorbents were done using a 30000 psi mercury intrusion porosimeter (Micrometrics poresizer 9310). The anion-exchange capacity (AEC) of the adsorbent was determined by NO3- saturation method using a column operation. The pH of zero point charge (pHzpc) of the adsorbent at different ionic strength12 was determined by a potentiometric titration method. The surface charge density, σo (C/cm2), was calculated from the titration curve using the following equation

σo ) polyacrylamide onto lignocellulosic material using N,N′methylenebisacrylamide as a cross-linking agent and subsequent functionalization of the polymer network with desired reagent for treating Cr(VI) wastewater.11 The present work involves coconut coir pith as the lignocellulosic material, which is readily available aplenty, even without offering any cost. The present study concentrates on the use of polyacrylamide-grafted coconut coir pith having -NH3+Cl- functional group (as anion exchanger), PGCP-NH3+Cl-, for the effective removal of Cr(VI) from aqueous solutions. The recovery, regeneration and comparison test, and application studies were also conducted to assess the practical utility of the adsorbent. Experimental Section Adsorbent Preparation. Coconut coir pith (CP) was procured from a local coir industry and was subjected to washing with distilled water to remove the surfaceadhered particles and water-soluble materials. It was then dried at 80 °C. The particles of -80 + 230 mesh size were separated by using standard sieves. Grafting of polyacrylamide10 onto CP was carried out in a twonecked round-bottomed flask that was kept in a constanttemperature bath maintained at the required temperature. Scheme 1 represents the general procedure adopted for the preparation of polyacrylamide-grafted CP having -NH3+Cl- functional groups (PGCPNH3+Cl-). For this procedure, 20.0 g of CP (1) was soaked with 300 mL of a solution containing 5.0 g of N,N′- methylenebisacrylamide (2) and peroxydisulfate (2.0 g) for 15 min before graft copolymerization was started. To the mixture then 7.5 g of acrylamide (3) was added and this was stirred vigorously at 70 °C. The polyacrylamide-grafted CP (PGCP) was then washed with water and dried at 80 °C. To convert it into an anion exchanger, PGCP was refluxed with 25 mL of ethylenediamine (en)2 for 8 h. The product was then washed with toluene and dried. It was shaken with 0.1 M HCl for 4 h, washed well to remove excess chloride ions, and dried at 80 °C. The functionalized PGCP (PGCP-NH3+Cl-) was sieved and particles having the average diameter of 0.096 mm (-80 + 230 mesh) were used throughout the study. Characterization Methods. The FTIR spectra of CP and PGCP-NH3+Cl- were obtained using the pressed disk technique on a Shimadzu FTIR model 1801. The adsorbents were characterized by physical adsorption of N2 at 77 K using Quantasorb surface area analyzer

F(CA - CB + [OH-] - [H+]) A

(1)

where F is the Faraday constant (C/eq). CA and CB are the concentrations of acid and base (eq/L) after each addition during titration. [H+] and [OH-] are the equilibrium concentration of H+ and OH- bound to the suspension surface (eq/cm2). A is the surface area of the suspension (cm2/L). Thermal stability of the adsorbents was studied with a Metler Toledo Star thermogravimetric analyzer. The adsorbents were also examined by scanning electron microscopy (SEM). The samples were mounted on aluminum micro-studs, gold-coated, and analyzed with an S-2400 Hitachi scanning electron microscope (S-2400). X-ray diffraction patterns were obtained with a Siemens D 5005 X-ray Unit using Ni-filtered Cu KR radiation. Adsorption Experiments. Batch adsorption experiments were carried out on a temperature-controlled water bath shaker set at 200 rpm and maintained at the desired temperature using stoppered conical flasks (100 mL). One hundred milligrams of the adsorbent was thoroughly mixed with 50.0 mL of the Cr(VI) solutions, whose concentration, pH, and temperature were previously known. The ionic strength of the suspension was kept constant at 0.01 M NaCl. After the flasks were shaken for the desired time, the suspensions were filtered through Whatman filter paper No. 41 and the concentration of Cr(VI) in the filtrate was measured spectrophotometrically by developing a purple violet color in 1,5-diphenyl carbazide in an acid solution.13 Kinetic studies were carried out at constant pH (3.0) with an initial concentration range from 50.0 to 200.0 mg/L and adsorbent dose from 2 to 10 g/L. Samples were withdrawn at regular intervals to plot the removal percentage versus time. In the present research work, the effects of pH, contact time, initial concentration of Cr(VI), adsorbent dose, temperature, and modeling of adsorption data were studied. Similar batch experiments were carried out using a commercial chloride form Dowex strong base (quaternary amine functionality) anion exchanger for comparison. For this a commercial anion exchanger, Dowex supplied by Aldrich Wisconsin, USA, was used. No pretreatment was given to the Dowex and it was used as received in the experiments. Desorption and Regeneration Studies. Regeneration of the adsorbent and recovery of the adsorbed Cr(VI) were carried out by shaking the spent PGCPNH3+Cl- in 0.1 M NaOH for a period of 5 h. After equilibrium was attained, the sorbent was filtered and washed with distilled water. It was further dried at 80

Ind. Eng. Chem. Res., Vol. 43, No. 9, 2004 2249

Figure 2. Potentiometric titration curves depicting the surface charge as a function of solution pH. Figure 1. FTIR spectra of CP and PGCP-NH3+Cl-. Table 1. Surface and Physical Properties of CP and PGCP-NH3+Clmagnitude parameter

CP

PGCP-NH3+Cl-

surface area, SN2/BET (m2/g) zero point charge (pHzpc) apparent density (g/mL) anion-exchange capacity (meq/g) porosity (mL/g) particle size (mm)

81.4 6.9 0.82 0.31 0.43 0.096

92.5 7.6 0.91 1.62 0.57 0.096

°C for 2 h. The sorbent sample thus regenerated was reused for the adsorption purpose. The loading and regeneration cycles were repeated three times. After each cycle the sorbent was washed with distilled water and dried. Results and Discussion Adsorbent Characterization. The IR spectrum of the PGCP-NH3+Cl- was different from that of the starting material CP (Figure 1). The IR spectrum of CP shows an asymmetric absorption band at 3407 cm-1, which is attributable to the hydrogen-bonded O-H stretching vibration from the cellulose structure of the CP. The broad absorption band observed in PGCPNH3+Cl- at 3420 cm-1 represents the overlap of C-H, N-H, and CdO stretching vibrations. Spectra of CP and PGCP-NH3+Cl- showed peaks at 2928 and 2925 cm-1, respectively, indicative of the C-H stretching from the -CH2 group. The bands at 1782 and 584 cm-1 for CP and 1789 and 563 cm-1 for PGCP-NH3+Cl- arise from CdO stretching of hemicellulose and β-glucosidic linkage, respectively.14 The peaks at 1627 cm-1 (amide carboxyl group) and 1565 cm-1 for PGCP-NH3+Cl- are due to the presence of an aliphatic amide group. Additional peaks at 1452 and 1020 cm-1 in PGCPNH3+Cl- (which are absent in the CP) indicate the presence of aliphatic C-N vibration and -CH2NH3+ type nitrogen. These observations clearly indicate the formation of polymeric chain (backbone) and the presence of -NH3+Cl- functional group in PGCP-NH3+Cl-. The surface area calculated from the N2 adsorption isotherm along with salient properties of CP and PGCP-NH3+Cl- are shown in Table 1. Results also indicate that PGCP-NH3+Cl-exhibits higher surface area compared to that of CP and this is probably due to the enhanced adsorption of N2 in the wider micropores and mesopores. Porosity of PGCP-NH3+Cl- was also found to be high. The plots of σo vs pH for CP and

Figure 3. TG and DTG curves of CP and PGCP-NH3+Cl-.

PGCP-NH3+Cl- at different ionic strengths are shown in Figure 2. The points of intersection of σo vs pH plots yield the pHzpc value of 6.9 and 7.6 for CP and PGCPNH3+Cl-, respectively. The increase in pHzpc after chemical modification indicates that the surface becomes more positive and this helps to adsorb negatively charged species such as chromate through electrostatic interaction. A variation in density and AEC was also observed after chemical modification. Comparative TG and DTG curves for pure CP and PGCP-NH3+Cl- are shown in Figure 3. The TG curves indicate the better thermal stability of the grafted copolymer with respect to CP. The TGA curves obtained were characterized by determining the decomposition temperature, that is, the initial decomposition temperature, TDi, and the temperature at the maximum rate of weight loss, Tmax. In the case of original CP, TDi is approximately 200 °C, and in the case of PGCPNH3+Cl-, TDi is 280 °C. TG curves exhibit an initial direct weight loss of 3.5% for CP starting at 60 °C and ending at 120 °C, whereas for PGCP-NH3+Cl-, a weight loss of 2% due to the loss of physically adsorbed water starts at 80 °C and ends at 120 °C. The temperature for 10% weight loss (T10) and Tmax are the two main criteria used to indicate the thermal stability of polymers.15 The higher the values of T10 and Tmax, the higher the thermal stability of the system. The values of T10 were found to be 160 and 190 °C for CP and PGCP-NH3+Cl-, respectively. The DTG curves obtained from the original TGA are also shown in Figure 3. As can be seen in its DTG curve, the decomposition of CP is produced in a single step with a maximum rate temperature of weight loss at 336 °C,

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Figure 4. Plot of ln[ln(w0/wt)] versus θ for thermal degradation under nitrogen at a heating rate of 20 °C/min of CP and PGCPNH3+Cl-.

Figure 5. X-ray diffraction patterns of CP and PGCP-NH3+Cl-.

whereas that in the PGCP-NH3+Cl- decomposition appears at two decomposition maximums. The first one at 280 °C is associated with the degradation reaction by random chain scission in the backbone of the graft copolymer. The second maximum, in the range of 430 °C, is attributed to the degradation reaction by the chain scission in the branches. It is important to remark that when CP is grafted, a higher stability of the main chain is achieved. The Horowitz and Metzger16 method was used to calculate activation energy (Ea) of the thermal degradation for CP and PGCP-NH3+Cl-. Assuming that the order of reaction is the unit, the following equation is used to calculate Ea values

[ ( )]

ln ln

w0 wT

)

Eaθ RTmax2

(2)

where w0 is the initial weight of the material, wT is the residual weight of polymer at temperature T, and θ is T - Tmax. The Ea values are obtained from the slope of the plots of ln[ln(w0/wT)] versus θ (Figure 4) and were found to be 19.93 and 34.48 kJ/mol for CP and PGCPNH3+Cl-, respectively. The value of Ea for PGCPNH3+Cl- was higher than that in CP. This again indicates that graft copolymerization of CP gives higher stability in the PGCP-NH3+Cl-. The X-ray diffraction patterns of CP and PGCPNH3+Cl- are shown in Figure 5. The results suggest that CP and its grafted product PGCP-NH3+Cl- have corresponding crystalline structures. Native CP shows scattering at 2θ ) 10.8, 14.5, 18.1, and 34.3. XRD data for CP indicate that CP has a crystalline domain of cellulose structure. Diffraction maxima at 18.1 and 34.3 can be attributed to the crystalline region of cellulose. XRD studies have shown that, upon graft polymerization, a significant decrease in crystallinity occurs. The broad peak centered at 20 that appears on PGCP-

Figure 6. Scanning electron micrographs of CP and PGCPNH3+Cl-.

NH3+Cl- is due to the various lattice planes and is a combination of peaks of various intensities (lower ordered fraction).17 Thus, some rearrangement in the morphology of cellulose chains in PGCP-NH3+Cl- occurs as a result of grafting. The decrease in crystalline domains in PGCP-NH3+Cl- results in the loss of tensile strength of the grafted chain and consequently enhances the free mobility of the grafted chain. In this study, SEM is used to probe the change in morphological features of CP on grafting with polyacrylamide, PGCP-NH3+Cl- (Figure 6). The surface morphology of CP is different from that of PGCP-NH3+Cl-. The intercellular gaps in the form of longitudinal cavities in the CP can be clearly marked as the unit cells are partially exposed. To hold the unit cells firmly in the coir pith fibers, the intercellular gaps are filled by binder lignin and fatty substances. The size of the voids in the original CP is reduced after graft polymerization and some distortion of shape can be seen in the SEM of PGCP-NH3+Cl-. It has a rough surface caused by the rigid and hydrophobic nature of the N,N′methylene bis acrylamide cross-links. The SEM image of PGCP-NH3+Cl- clearly shows that polyacrylamide grafts have been deposited more on the surface of the unit cell than in the intercellular gaps. The polyacrylamide grafts reduce the degree of crystallinity, causing a reduction in tensile strength of the grafted chain. Effect of pH on Cr(VI) Adsorption. The effect of pH on Cr(VI) sorption by CP, PGCP-NH3+Cl-, and Dowex is shown in Figure 7. It can be seen that the percentage adsorption of Cr(VI) onto PGCP-NH3+Clis maximum at pH 3.0. For very acidic pH up to 3.0, the extent of removal increases with the increase of pH up to a maximum and then decreases as pH increases from 3.0 to 10.0. However, within the range of 6.0-8.0, the percentage adsorption of Cr(VI) onto PGCPNH3+Cl- was found to be almost constant. It is also clear

Ind. Eng. Chem. Res., Vol. 43, No. 9, 2004 2251

Figure 7. Effect of pH on the adsorption of Cr(VI) onto CP, PGCP-NH3+Cl-, and Dowex.

from Figure 7 that the optimum pH at which maximum removal of Cr(VI) by CP and Dowex occurred is observed over the pH range of 2.0-3.0. Above this pH range, adsorption gradually falls with increasing basicity of the medium. The extent of Cr(VI) adsorption onto PGCPNH3+Cl- at an initial concentration of 25.0 and 50.0 mg/L decreased from 99.1 to 45.6% and 89.9 to 22.6%, respectively, when the pH of the system varies from 3.0 to 10.0. The maximum adsorption of 50.2, 99.1, and 70.8% took place by CP, PGCP-NH3+Cl-, and Dowex, respectively, at pH 3.0 from an initial concentration of 25.0 mg/L. The data clearly show that PGCPNH3+Cl- is 1.4 and 2.0 times more effective than Dowex and CP, respectively. The effect of pH on the adsorption of Cr(VI) onto PGCP-NH3+Cl- can be interpreted with the help of the structure and surface charge of the adsorbent and the speciation of chromium. The perusal of the Pourbaix diagram18 clearly indicates that in the pH of highest sorption efficiency (pH 3.0) the dominant species were HCrO4- and Cr2O72-. The adsorbent having -NH3+Clfunctional group and acting as anion exchanger and hence the exchange of Cl- to anion take place in treatment with solution containing anions. PGCP-NH+ 3 Cl + HCrO4 h (3) PGCP-NH+ 3 ...HCrO4 + Cl 22PGCP-NH+ 3 Cl + Cr2O7 h 2(4) (PGCP-NH+ 3 )2...Cr2O7 + 2Cl

According to the speciation diagram of Cr(VI), the polymerized species18 such as Cr3O102- and Cr4O132- are formed at pH < 2.5 and these species exchanged Clions from the adsorbent surface with difficulty. This results in a lower adsorption through decreased anionexchange capability. The decrease in adsorption at higher pH may be due to competition from the OH- ions. Moreover, the pHzpc of PGCP-NH3+Cl- is 7.6 and hence the surface charge of the adsorbent is negative above 7.6. The adsorption of Cr(VI) oxy-anions is not possible on the negatively charged sorbent surface at very high pH values due to the electrostatic repulsion. Effect of Contact Time and Initial Concentration. The experimental results demonstrating the effect of initial Cr(VI) concentration (C0 ) 25.0-100.0 mg/L) with time is shown in Figure 8 for the adsorption of Cr(VI) on PGCP-NH3+Cl-. The percentage adsorption of Cr(VI) that increases very rapidly up to about 30 min occurred by surface reaction and slowly reaches the saturation point within 4 h, beyond which no further

Figure 8. Effect of contact time and initial concentration of sorbate on the adsorption of Cr(VI) onto PGCP-NH3+Cl-.

Figure 9. Effect of PGCP-NH3+Cl- dose on Cr(VI) adsorption at different Cr(VI) concentrations.

significant increase is observed. With the increase in the initial concentration of Cr(VI) from 25.0 to 100.0 mg/L, the percentage uptake of Cr(VI) decreases from 99.7 to 76.7%. At higher initial concentration, the available sites of adsorption become fewer and hence the percentage adsorption of Cr(VI) depends on the initial concentration. For fixed adsorbent dose, the total available adsorption sites are limited, thereby adsorbing almost the same amount of the sorbate, thus resulting in a decrease in percentage adsorption corresponding to an increase in initial sorbate concentration. Effect of Adsorbent Dose. Adsorption of Cr(VI) on PGCP-NH3+Cl- as a function of time at different adsorbent doses (Ws ) 2-10 g/L) and initial concentrations was also studied and in all cases the shape of the percentage adsorption versus time curves were similar with that observed in Figure 8. On the basis of studies conducted on the effect of contact time for the adsorption of Cr(VI) with adsorbent dose and initial concentration showed that an equilibrium time of 4 h is sufficient to reach equilibrium. The effect of variation in these parameters on Cr(VI) removal was quantified in terms of their plateau (4 h) values. Figure 9 shows the adsorption percentage of Cr(VI) as a function of initial concentration at different adsorbent doses. The Cr(VI) adsorption from its solution is influenced by adsorbent dose. The percentage adsorption of Cr(VI) increases from 90.5% (22.63 mg/g) to 100% (5 mg/g) by increasing the adsorbent dose from 2 to 10 g/L, at an initial concentration of 50.0 mg/L. At an initial concentration of 200.0 mg/L Cr(VI), the percentage removal increases from 53.5% (53.50 mg/g) to 94.4% (18.88 mg/g) by increasing the adsorbent dose from 2 to 10 g/L. This is because, at higher dose of adsorbent, due to increased surface area, more adsorption sites are available, causing higher removal of Cr(VI). The results also showed that the uptake per unit mass of the adsorbent was

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higher at lower dose of adsorbent. This is due to the fact that as the dose of adsorbent increases, there is less commensurate increase in adsorption resulting from the lower adsorption capacity utilization of the adsorbent. Observations of similar adsorbent concentration effect have also been reported by other workers19,20 who studied the adsorption of Cr(VI) on moss peat and sawdust carbon, respectively. Although direct comparison with other adsorbent materials is difficult due to the different applied experimental conditions, Lazaridis et al.21 observed that 110 mg of Cr(VI) was removed per gram of hydrotalcite (double-layered hydroxides) at a concentration of 10 mg/L at pH 6.0. In another publication Lazaridis and Asouhidou22 have reported removal of 120 mg of Cr(VI) per gram of hydrotalcite at an initial concentration of 100 mg/L at pH 6.0. The adsorption capacity was greater in hydrotalcite than in PGCP-NH3+Cl-. The difference in capacity may be due to the difference in the ionexchange behavior. Lazaridis and Asouhidou22 argued that hydrotalcite presents two kinds of anion retention sites: (i) within the interlayer and (ii) onto the external surfaces. The latter mechanism, even though constituting to a lesser extent, seems to affect the uptake capacity. Adsorption Dynamics. The kinetic constants of the metal adsorption, which could be used to optimize the residence time of industrial wastewater in a batch operation, were calculated using these data. The mechanism of Cr(VI) removal is thought to be complexation and ion exchange and in the absence of stoichiometric data it can be represented as K

S + M y\ z SM K′

(5)

where S is an available adsorption site (PGCP-NH3+) on the surface of the adsorbent, M is the dissolved Cr(VI) species, and SM is the adsorbed state. To quantify the extent of uptake in adsorption kinetics, a pseudo-second-order reaction kinetic model as suggested by Ho and Mckay23 is used. The kinetic rate equation for the above reaction is expressed as

d(qt) ) k(qe - qt)2 dt

(6)

where qe and qt are the amount of metal ions adsorbed (mg/g) at equilibrium and at time t, respectively. k is the rate constant of sorption [g/(mg min)]. Separating the variables in eq 6 and integrating for the boundary conditions t ) 0 to t ) t and qt ) 0 to qt ) qt gives23

1 t t ) + qt kq 2 qe

(7)

e

The straight line plots of t/qt versus t (Figure 10) observed at different concentrations and temperatures indicate the validity of the pseudo-second-order reversible kinetics for the present system of investigation. The values of k at different concentrations and temperatures calculated from the slope of the plots are given in Table 2. The data clearly show that the value of k decreased significantly with increasing initial concentration and temperature. Low temperature favors the adsorption of Cr(VI) by PGCP-NH3+Cl-. The thickness of the boundary layer decreases with the rise in solution temperature due to the increased tendency of Cr(VI) to escape from

Figure 10. Pseudo-second-order reversible kinetic plots for the adsorption of Cr(VI) onto PGCP-NH3+Cl- at different (A) concentrations and (B) temperatures. Table 2. Pseudo-Second-Order Rate Constants for the Adsorption of Cr(VI) onto PGCP-NH3+Clvariable initial concn (mg/L) 25 50 75 100 temp (°C) 20 30 40 50

k (g/mg min)

qe (mg/g)

r2

4.57 2.56 1.95 1.46

12.39 23.29 34.61 44.01

0.989 0.992 0.994 0.996

1.68 1.46 1.04 0.79

46.68 44.01 35.73 31.86

0.994 0.996 0.981 0.986

the solid phase to the liquid phase, and thus, as a result of an increase in the kinetic energy of the adsorbate species at high temperatures, a decrease in adsorption was observed.24 A linear relationship between k and the reciprocal of the temperature in the Arrhenius equation, for which the correlation coefficient was 0.99, can be represented in the form

17.906 (8.314T )

k ) 1.029 × 10-6 exp

(8)

The values of ko and Ea were found to be 1.029 × 10-6 g/mg min and -17.906 kJ/mol, respectively. The negative value of activation energy suggests that the rise in solution temperature does not favor the Cr(VI) adsorption by PGCP-NH3+Cl-. Adsorption Isotherms. The adsorption isotherm studies are of fundamental importance in determining the adsorption capacity of Cr(VI) onto PGCP-NH3+Cland to diagnose the nature of adsorption. Experimental isotherms determined at 20, 30, 40, and 50 °C are presented in Figure 11. It can be seen that the adsorption isotherms exhibit an H-shape, which corresponds to the classification of Giles.25 The H-type isotherms are characteristic of the cases of high affinity of solute for an adsorbent. Such isotherms indicate that there is no competition from the solvent for adsorption sites. The adsorption isotherm data at different temperatures were processed using the Langmuir and Freundlich isotherm models, which are represented by eqs 9 and 10, respectively.

Ind. Eng. Chem. Res., Vol. 43, No. 9, 2004 2253

Figure 12. Variation of ∆Hx with respect to surface loading. Table 3. Langmuir and Freundlich Isotherm Constants for the Adsorption of Cr(VI) onto PGCP-NH3+Cl- and Dowex (Adsorbent Dose 2.0 g/L) Figure 11. Comparison of the experimental (legends) and the model fits of Langmuir and Freundlich (lines) isotherms for the adsorption of Cr(VI) onto PGCP-NH3+Cl- at different temperatures and of Dowex at 30 °C.

Langmuir: Ce Ce 1 + ) qe Qob Qo

(9)

Freundlich: 1 log qe ) log KF + log Ce n

(10)

where Qo and b are Langmuir constants related to maximum monolayer adsorption capacity and energy of adsorption, respectively. KF and 1/n are Freundlich constants related to adsorption capacity and intensity of adsorption, respectively. The experimental results were analyzed using regression analysis to fit the Langmuir and Freundlich isotherm models. The Langmuir and Freundlich isotherm constants were obtained from the linearized plots of Ce/qe versus Ce and log qe versus log Ce, respectively, and are tabulated in Table 3. To compare the validity of the isotherm equations more definitely, the normalized standard deviation, ∆q, is calculated using eq 11

∆q(%) ) 100 ×

exp 2 - qcal ∑[(qexp e e )/qe ]

x

n-1

(11)

where the superscripts “exp” and “cal” are the experimental and calculated values and “n” is the number of measurements. The values of ∆q are given in Table 3. On the basis of ∆q values and correlation coefficients, it is found that the isotherm of Cr(VI) can be best described by the Freundlich isotherm equation. The Langmuir values fit well to experimental results only for a limited range of concentrations. It is due to the heterogeneity of the adsorbent material. Earlier workers22 also proposed the Freundlich isotherm model for describing the adsorption of Cr(VI) on hydrotalcite, where an ion-exchange mechanism was proposed. From Table 3, it can be seen that the maxima of adsorption, Qo and KF, increase with decreasing temperature, indicating the exothermic nature of the adsorption process. Values of 0.1 < 1/n < 1.0 show the favorable adsorption of Cr(VI) on PGCP-NH3+Cl-. The ultimate adsorption capacity of PGCP-NH3+Cl- can be calcu-

Langmuir constants temp (°C)

Q° (mg/g)

b

∆q (%)

20 30 40 50 Dowex (30 °C)

127.28 123.35 111.396 108.39 109.28

0.0052 0.0081 0.0114 0.0239 0.0024

30.1 15.2 12.2 10.4 9.5

Freundlich constants r2

KF

1/n

∆q (%)

r2

0.978 17.53 0.322 5.7 0.990 0.979 7.98 0.419 6.5 0.987 0.982 4.89 0.475 4.9 0.993 0.978 2.71 0.545 3.6 0.997 0.971 2.34 0.685 4.1 0.993

lated from the isothermal data by substituting the required equilibrium concentration in the Freundlich equation. Thus, for an equilibrium concentration of 1 mg/L, each gram of PGCP-NH3+Cl- can remove 2.71 mg of Cr(VI) at 50 °C, which is increased up to 17.53 mg at 20 °C. Information concerning the magnitude of the heat of adsorption and its variation with surface coverage can provide useful information concerning the nature of the surface and the adsorbed phase. The heat of adsorption determined for a constant amount of adsorbed adsorbate is known as isosteric heat of adsorption, (∆Hx), and is calculated using the Clausius-Clapeyron equation. For this purpose, the equilibrium concentration (Ce) at a constant amount of adsorbed Cr(VI) is obtained from the adsorption isotherm data at different temperatures (Figure 11). The values of ∆Hx were calculated from the plots of ln Ce versus l/T for different amounts of Cr(VI) adsorption. As shown in Figure 12, the isosteric heat of adsorption is varied with surface loading. This result indicates that the PGCP-NH3+Clused has an energetically heterogeneous surface. The variation of ∆Hx with surface loading is usually attributed to the presence of some lateral interactions between adsorbed molecules. In adsorption studies, it is valuable to calculate the ∆Hx at a limit of zero coverage since it has been used as a criterion of the adsorption affinity.26 The value of ∆Hx at a limit of zero coverage can be evaluated from the extrapolation of ∆Hx versus surface loading plot and its value is found to be -74.90 kJ/mol. Comparison with Other Adsorbents. To justify the validity of PGCP-NH3+Cl- as adsorbent for Cr(VI) adsorption, its adsorption potential must be compared with a commercial ion exchanger. Adsorption isotherm for a commercial chloride form anion exchanger Dowex at 30 °C is depicted in Figure 11. The Langmuir and Freundlich isotherm constants for the adsorption of Cr(VI) on Dowex were calculated using regressional analysis and are summarized in Table 3. The values of Langmuir (Qo) and Freundlich (KF) parameters for

2254 Ind. Eng. Chem. Res., Vol. 43, No. 9, 2004 Table 4. Freundlich Constants for the Adsorption of Cr(VI) onto Different Adsorbents at 30 °C adsorbents

KF

1/n

commercial activated carbon activated bagasse carbon activated jute carbon Amberlite IRA-900 Cl- form amine-modified coconut coir (Cl- form) layered double hydroxide leaf mould PGCP-NH3+Cl-

4.115 0.19 1.55 2.89 0.33

0.411 0.981 0.368 0.319 0.089

Ramos et al.27 Chand et al.28 Chand et al.28 Baes et al.29 Baes et al.29

references

1.15 3.8 7.98

0.185 0.58 0.419

Goswamee et al.30 Sharma et al.31 present work

Table 5. Compositions of Cr(VI) Electroplating Wastewater parameter

value

Cr(VI) (mg/L) CN- (mg/L) Cu(II) (mg/L) Zn(II) (mg/L) Cd(II) (mg/L) Ni(II) (mg/L) pH COD (mg/L) BOD (mg/L) suspended solids (mg/L)

22.7 12.5 28.7 4.2 3.8 5.1 4.3 160.4 49.7 412.5

Cr(VI) were 1.1 and 3.4 times, respectively, greater in PGCP-NH3+Cl- than in Dowex. The amount of adsorbed Cr(VI) is high enough for PGCP-NH3+Cl- to be able to effectively remove Cr(VI) from industrial wastewater even at low pH application. The values of Freundlich constants on different adsorbents reported in the literature with the adsorbent in the present study are summarized in Table 4. It may be observed that the uptake of Cr(VI) on PGCPNH3+Cl- is very much greater than the adsorbent materials reported in the literature. Tests with Electroplating Industrial Wastewater. The utility of the PGCP-NH3+Cl- was demonstrated by treating it with real industrial wastewater. Industrial wastewater sample collected from the local electroplating industry was characterized using standard methods13 and the composition is given in Table 5. Figure 13 demonstrates that the treatment of Cr(VI) in wastewater is not significantly different from the results predicted on the basis of batch experiments using Cr(VI) only. It can be observed that a minimum adsorbent dosage of 25 mg is sufficient for the removal of 43.2% of the total Cr(VI) from 50 mL of wastewater containing 22.7 mg/L of Cr(VI) in the presence of other ions. The complete removal of Cr(VI) from 50 mL of sample was achieved by 125 mg of the adsorbent. The adsorbed Cr(VI) was recovered with 0.1 M NaOH. The results are shown in Figure 13. Above 97.0% recovery was achieved using 0.1 M NaOH. Recovery of Cr(VI) by Adsorption-Desorption Cycle. To make the adsorption process more economical and also to obtain practical information on the recovery of Cr(VI) using PGCP-NH3+Cl-, it is necessary to regenerate the spent adsorbent. Regeneration of PGCPNH3+Cl- and recovery of Cr(VI) was carried out by treating spent adsorbent material with 0.1 M NaOH. The desorption and regeneration data are presented in Figure 14. An efficiency of 99.0% Cr(VI) was obtained by using 0.1 M NaOH in the first cycle and is therefore suitable for regeneration of adsorbent. Figure 14 clearly shows a gradual decrease in Cr(VI) adsorption with an increase in the number of cycles. After a sequence of four cycles, the Cr(VI) uptake capacity of the adsorbent

Figure 13. Effect of adsorbent dose for the removal of Cr(VI) from electroplating wastewater by PGCP-NH3+Cl-.

Figure 14. Four cycles of Cr(VI) adsorption-desorption with 0.1 M NaOH.

had been reduced from 99.0 to 93.4%. The desorption efficiency in all four cycles was very high; >97.0% recovery of Cr(VI) was observed in each cycle. The small fraction of sorbed metal not recoverable by regeneration presumably represents the metal that is bound through strong interaction and, as a result, sorption capacity is reduced in the subsequent cycle. The results indicate that the PGCP-NH3+Cl- can be used repeatedly in the adsorption-desorption cycle. Cost Estimation. The removal of Cr(VI) and other toxic metals by macroporous strong base anion-exchange resins with quaternary functionality (R-N+-(CH3)3) and polymer matrixes of either poly(styrene) divinylbenzene (DVB) or poly(acrylic) DVB in Cl- or OH(example, Amberlite IRA-900) has been reported in the literature. All these polymeric adsorbents are very expensive and are available for US $40-65 per kg of resin. The cost-effective and economical removal of toxic heavy metals from sewage, industrial, and mining wastewaters can be done only with low-cost and easily available adsorbents. The raw material used in the present study, coconut coir pith, is available from the coir industry free of cost, as a waste, and including expenses for transport, chemicals for surface modifications, electrical energy, etc., the final product PGCPNH3+Cl- would cost approximately $19.0 per kg. The cost of commercially available anion exchanger, Dowex, is $35.3 per kg. The overall cost of treatment with PGCP-NH3+Cl- is cheaper than commercially available polymeric resins. PGCP-NH3+Cl- may also be used for the removal of other metals from aqueous solutions as the Cl- ions in the adsorbent exchange with toxic anionic metal species and are expected to bring down the cost factor. Further work is now undergoing to determine the effectiveness of PGCP-NH3+Cl- at removing As(V), As(III), Se(VI), and V(V) from aqueous solutions.

Ind. Eng. Chem. Res., Vol. 43, No. 9, 2004 2255

Conclusions The present study clearly established that polyacrylamide-grafted coconut coir pith (PGCP) having -NH3+Cl- functional groups, (PGCP-NH3+Cl-), is an effective adsorbent for the removal of Cr(VI) from aqueous solutions. Sorption of Cr(VI) is pH-dependent and the best results are obtained at pH 3.0. The percentage removal was found to depend on the dose of adsorbent, contact time, and initial concentration of sorbate. The kinetics of Cr(VI) on PGCP-NH3+Clfollows a pseudo-second-order kinetic expression. The activation energy of adsorption can be calculated using rate constants. The equilibrium adsorption isotherm data fit the Freundlich model. The process is exothermic in nature. The isosteric heat of adsorption was also determined and was found to vary with surface loading. Quantitative removal of Cr(VI) from electroplating industry wastewater confirmed the validity of the results obtained in batchwise studies. The spent adsorbent can be regenerated and reused by alkali treatment. Acknowledgment The authors are thankful to the Head, Department of Chemistry, University of Kerala, Trivandrum, for providing Laboratory facilities. One of the authors (V.P.V.) is grateful to the Council of Scientific and Industrial Research, New Delhi, for the financial support in the form of SRF. Literature Cited (1) Huang, C.; Chung, Y. C.; Liou, M. R. Adsorption of Cu(II) and Ni(II) by pelletised biopolymer. J. Hazard. Mater. 1996, 45, 265-277. (2) Ajmal, M.; Rao, R. A.; Siddiqui, B. A. Studies on Removal and Recovery of Cr(VI) from Electroplating Wastes. Water Res. 1996, 30 (6), 1478-1482. (3) Raji, C.; Anirudhan, T. S. Removal of Hg(II) From Aqueous Solution by Sorption on Polymerized Sawdust. Indian J. Chem. Technol. 1996, 3, 49-54. (4) Sreedhar, M. K.; Anirudhan, T. S. Preparation of an Adsorbent by Graft Polymerization of Acrylamide onto Coconut Husk for Mercury(II) Removal from Aqueous Solution and Chloralkali Industry Wastewater. J. Appl. Polym. Sci. 2000, 75, 12611269. (5) Hegazy, E. H.; Kamal, H.; Khalifa, N. A.; Mahmoud, G. A. Separation and Extraction of Some Heavy and Toxic Metal Ions from their Wastes by Grafted Membranes. J. Appl. Polym. Sci. 2001, 81, 849-860. (6) Singh, D. K.; Tiwari, D. P.; Saksena, D. N. Removal of Lead from Aqueous Solution by Chemically Treated used Tea Leaves. Indian J. Environ. Health 1993, 35, 169-177. (7) Anirudhan, T. S.; Sreedhar, M. K. Modified Coconut Husk for Mercury Removal from Wastewater. Pollut. Res. 1998, 17 (4), 381-384. (8) Simkovic, I.; Laszlo, J. Preparation of Ion Exchanger from Bagasse by Cross-Linking with Epichlorohydrine - NH4OH or Epichlorohydrine - Imidazole. J. Appl. Polym. Sci. 1997, 69, 2561-2566. (9) Raji, C.; Anirudhan, T. S. Preparation and Metal Adsorption Properties of the Polyacrylamide-Grafted Sawdust having Carboxylate Functional Group. Indian J. Chem. Technol. 1996, 3, 345-350. (10) Navarro, R. R.; Sumi, K.; Matsumura, M. Improved Metal Affinity of Chelating Adsorbents through Graft Polymerization. Water Res. 1999, 33, 2037-2044. (11) Unnithan, M. R.; Anirudhan, T. S. The Kinetics and Thermodynamics of Sorption of Chromium(VI) onto the Iron(III)

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Received for review March 3, 2003 Revised manuscript received October 10, 2003 Accepted November 5, 2003 IE0302084