Adsorption Science & Technology

Adsorption of Heavy Metal Ions on Pomegranate (Punica granatum) Peel: Removal and Recovery of Cr(VI) Ions from a Multi-metal Ion System Rifaqat A.K. Rao* and Fouzia Rehman Reprinted from Adsorption Science & Technology 21 Volume 28 Number 3 Multi-Science Publishing Co. Ltd. 5 Wates Way, Brentwood, Essex CM15 9TB, United Kingdom

195 Adsorption of Heavy Metal Ions on Pomegranate (Punica granatum) Peel: Removal and Recovery of Cr(VI) Ions from a Multi-metal Ion System Rifaqat A.K. Rao * and Fouzia Rehman Environmental Research Laboratory, Department of Applied Chemistry, Faculty of Engineering and Technology, Aligarh Muslim University, Aligarh 222, UP, India. (Received 22 January 29; revised form accepted 4 July 21) ABSTRACT: The fruit peel of pomegranate (Punica granatum) exhibits a high affinity for Cu(II), Ni(II), Cd(II) and Zn(II) ions. The maximum adsorption observed was that of Cu(II) ions, followed by Zn(II), Cd(II), Ni(II) and Cr(VI) ions. The adsorption of Cu(II) ions was dependent on the initial Cu(II) ion concentration, ph, adsorbent dose, temperature and contact time. The adsorption isotherms could be described by the Langmuir and Freundlich equations. Application of the χ 2 -test indicated that both these models were best obeyed at 2 ºC. Thermodynamic parameters such as H, S and G were evaluated, the adsorption process being endothermic and spontaneous in nature. The value of the mean free energy indicated that the adsorption process was chemical in nature. Kinetic data showed that the pseudo-second-order model provided the best fit for the experimental data. Attempts were made using batch and column methods to desorb Cu(II), Ni(II), Zn(II), Cd(II) and Cr(VI) ions from synthetic wastewater as well as from wastewater derived from electroplating activities. The breakthrough capacities of Cu(II), Ni(II), Zn(II), Cd(II) and Cr(VI) ions were 6, 2, 2, 2 and mg/g, respectively, when a mixture of these metal ions was treated. The adsorbent was utilized to recover Cr(VI) ions from electroplating wastewater. INTRODUCTION The contamination of water by toxic heavy metal ions through the discharge of industrial wastewater is a worldwide environmental problem. The environmental impact of such toxic metal ions has led to the enforcement of stringent standards for the maximum allowable limits of their discharge into open landscape and water bodies (UNEP 1989). Authorities enforcing these standards further require the treatment procedure to be environmentally friendly (Aderhold et al. 1996). Different methods used for the removal of metal ions are filtration, chemical precipitation, coagulation, solvent extraction, electrodialysis, ion-exchange, membrane processes and adsorption (Patterson 1997). Ion-exchange methods and adsorption are the most common and effective processes for this purpose. Activated carbon and different types of ion-exchange resins are very often used. However, their high price and regeneration costs have encouraged research to find low-cost adsorbing materials for the removal of heavy metal ions. Several materials in this category have been used successfully for the removal of heavy metal ions from industrial wastewater (Ajmal et al. 1996, 1998, 2, 21, 23; Gurbuz 29; Senthil Kumar et al. 28; Bhattacharya and Sen Gupta 29). * Author to whom all correspondence should be addressed. E-mail: rakrao1@rediffmail.com (Rakrao).

196 R.A.K. Rao and F. Rehman/Adsorption Science & Technology Vol. 28 No. 3 21 Copper is generally considered to be toxic to man at concentrations exceeding 5 mg/, imparting colour and an undesirable taste to water. The World Health Organization s guideline for drinking water based on its staining properties is 1 mg/ (WHO 1984). Above this level, copper causes acute and chronic disorders in human beings, such as gastrointestinal catarrh, cramp in the calves, haemochromatosis and skin dermatitis brasschills usually accompanied by high fever (Camp and Meserve 1964; Mukherji 1986). Industries discharging copper ions in their wastewater include those involved with electroplating, pulp and paper mills, fertilizer plants, steel work foundries, petroleum refineries, aircraft finishing, motor vehicle and non-ferrous metalwork (Fostner and Wittman 1981; Dalang et al. 1984; Jenkins 1976). Pomegranate peel, a by-product of the pomegranate juice industry, is an inexpensive material which is composed of several constituents including polyphenols, ellagic tannin, and gallic and ellagic acids (Seeram et al. 25; Ben Nasr et al. 1996). El-Ashtoukhy et al. (28) have used pomegranate peel as an adsorbent for Pb(II) and Cu(II) ions and compared its properties with activated carbon. The present paper describes the adsorption properties of pomegranate peel towards Cu(II), Cd(II), Ni(II), Zn(II) and Cr(VI) ions. EXPERIMENTAL Adsorbent Fruit peel of pomegranate was dried, crushed and washed several times with doubly distilled water until the filtrate was clear of all colouration. After such washing, the peel was dried in an air oven at 6 ºC for 24 h, then sieved to 15 3 µm mesh size and used as such. Adsorbate solution Astock solution of Cu(II) ions (1 mg/ ) was prepared by dissolving the desired quantity of copper nitrate (A.R. grade) in doubly distilled water (DDW). Solutions of other metal ions (1 mg/ ) were also prepared by dissolving their chlorides or nitrates in DDW, while the Cr(VI) ion solution was prepared using K 2 Cr 2 O 7. The ph values of all the stock solutions were in the range 3 5. Determination of the point of zero charge (ph PZC ) The zero surface charge characteristics of the pomegranate peel were determined using the solid addition method (Lataye et al. 26). Thus, 4 m of.1 N KNO 3 solution was placed in each of a series of 1 m conical flasks. The initial ph (ph ) of the solutions was roughly adjusted to between 2 to 8 by the addition of either.1 N HCl or.1 N NaOH. The total volume of the solution in each flask was adjusted exactly to 5 m by adding KNO 3 solution of the same strength. The initial ph values (ph i ) of the solutions were accurately measured by means of a ph meter. Then.5 g of adsorbent was added to each flask and the systems allowed to equilibrate for 24 h with intermittent manual shaking. The final ph (ph f ) of the supernatant liquid was noted. The difference between the initial ph (ph i ) and final ph (ph f ) values ( ph = ph i ph f ) was plotted against ph i. The point of intersection of the resulting curve with the abscissa, at which ph =, gave the ph PZC value.

Adsorption of Heavy Metal Ions on Pomegranate (Punica granatum) Peel 197 Determination of surface active sites The concentration of acidic sites on the pomegranate peel was determined via the acid base titration method (Boehm 1994). Thus,.5 g adsorbent was treated separately with 5 m of.1 N NaOH,.1 N Na 2 CO 3 or.1 N NaHCO 3 solution in 25 m conical flasks. The flasks were agitated at constant temperature (2 ºC) for 5 d. Subsequently, a 1 m sample from each flask was titrated with.1 N HCl solution and the resulting ph monitored via a ph meter. Adsorption studies Adsorption studies were carried out using a batch method. Thus,.5 g adsorbent was placed in a conical flask, 5 m of a solution of metal ions of desired concentration added and the mixture shaken in the shaker incubator for 24 h. The mixture was then filtered employing Whatman No. 41 filter papers and the final concentration of metal ions in the filtrate determined by atomic absorption spectrometry (AAS) using a GBC 92 instrument. The amount of metal ions adsorbed was calculated by subtracting the final concentration from the initial concentration. Effect of ph The effect of ph on the adsorption of Cu(II) ions was studied by taking 1 m of the Cu(II) ion stock solution in a beaker and adjusting the ph through the addition of either.1 N HCl or.1 N NaOH solution. The initial concentration of Cu(II) ions in the solution was determined and then 5 m of this solution was taken in a conical flask and treated with.5 g adsorbent for 24 h in the shaker incubator. The final concentration of Cu(II) ions present in the mixture thus obtained was determined as described above. Effect of time A series of 25 m conical flasks, each containing.5 g adsorbent and 5 m of a solution of known Cu(II) ion concentration, were shaken in a shaker incubator and samples of the solution removed and filtered at pre-determined intervals. The concentration of Cu(II) ions in each filtrate was determined by AAS and the amount of Cu(II) ions adsorbed in each case determined as described above. Effect of adsorbent dose Varying amounts of adsorbent (.1 1. g) were added at 2 ºC to a series of 25 m conical flasks, each containing 5 m of a Cu(II) ion solution of 5 mg/ concentration. The conical flasks were shaken in a shaker incubator for 5 min since adsorption was completed within this time span. The solutions were then filtered and the amount of Cu(II) ions adsorbed in each case calculated as described above. The same procedure was repeated at 3 ºC and 4 ºC, respectively. Desorption of Cu(II) ions as determined by batch methods Attempts were made to study the desorption of Cu(II) ions using a batch method. Thus,.5 g adsorbent was treated with 5 m of a Cu(II) ion solution (5 mg/ ) for 5 min. The adsorbent was washed several times with DDW to remove any excess of Cu(II) ions and then treated with 5 m

198 R.A.K. Rao and F. Rehman/Adsorption Science & Technology Vol. 28 No. 3 21 of a.1 N HCl solution. After shaking the system for 24 h, the amount of Cu(II) ions desorbed was determined. The same procedure was repeated employing different solution concentrations. Treatment of electroplating wastewater by batch methods In another experiment,.5 g adsorbent was treated with 5 m of electroplating wastewater in a conical flask and the concentrations of Cu(II), Ni(II), Zn(II) and Cr(VI) ions in the supernatant liquid determined after 5 min. The adsorbent was then washed with DDW several times, treated with 5 m.1 N HCl solution and the system shaken for 24 h. The concentration of metal ions desorbed was then determined by AAS methods. Desorption of Cu(II) ions as determined by column methods In this experiment,.5 g adsorbent was placed on a glass wool support in a glass column (.6 cm i.d.) and 5 m of a solution of Cu(II) ions (5 mg/ initial concentration) was passed through the column at a flow rate of 1 m /min. Subsequently, the column was washed several times with DDW to remove any unadsorbed Cu(II) ions, following which.1 N HCl solution was passed through the column at a flow rate of 1 m /min. The effluent was collected in 1 m fractions and the amount of Cu(II) ions desorbed determined in each fraction by AAS methods. Breakthrough capacity To measure the breakthrough capacity,.5 g adsorbent was placed on a glass wool support in a glass column (.6 cm i.d.). Then 1 m of a Cu(II) ion solution with an initial concentration (C ) of 5 mg/ was passed through the column at a flow rate of 1 m /min. The effluent was collected in 5 m fractions and the amount of Cu(II) ions in each fraction determined by AAS methods. The breakthrough curve was obtained by plotting C/C against the volume of effluent. The same procedure was applied to determine the breakthrough capacities of Zn(II), Cd(II), Ni(II) and Cr(VI) ions by passing 1 m of a solution containing a mixture of Zn(II), Cd(II), Ni(II) and Cr(VI) ions (each at an initial concentration of 1 mg/ ) through the column. Treatment of electroplating wastewater by column methods In another experiment, 1 m of electroplating wastewater was passed through.5 g adsorbent contained in the column employing identical conditions to those used above. The flow rate was maintained at 1 m /min throughout the experiment. Effluent was collected in 1 m fractions and the metal ion content in each fraction determined by AAS methods. RESULTS AND DISCUSSION Pomegranate peel showed a high affinity towards Cu(II), Cd(II), Ni(II) and Zn(II) ions at a ph value of 4.5. The data depicted in Figure 1 show that Cu(II) and Cd(II) ions were adsorbed to the same extent, followed by Zn(II) and Ni(II) ions. However, at this ph value, the extent to which Cr(VI) ions were adsorbed was the smallest. The adsorption of Cu(II) ions was also high in the presence of Cd(II), Ni(II), Zn(II) and Cr(VI) ions at ph 4.5.

Adsorption of Heavy Metal Ions on Pomegranate (Punica granatum) Peel 199.8 Amount adsorbed (mmol/g).7.6.5.4.3.2.1.74 mmol/g.68 mmol/g.73 mmol/g.75 mmol/g.42 mmol/g Cu(II) Ni(II) Zn(II) Cd(II) Cr(VI) Metal ions Figure 1. Adsorption of various metal ions from aqueous solution onto pomegranate peel. Experimental conditions: amt. of adsorbent employed =.5 g; ph of solution = 4.5. The data depicted in the bars in the figure correspond to the maximum amount adsorbed of the ion concerned. Effect of concentration and contact time The effect of ion concentration on the adsorption of Cu(II) ions was studied employing initial concentrations of.157,.314,.472,.629 and.787 mmol/, respectively. The results obtained are depicted in Figure 2. It will be seen from the figure that the adsorption capacity increased with increasing initial Cu(II) ion concentration; this may be attributed to the increase in the.8.7.6 q e (mmol/g).5.4.3.2.1 5 1 Time (min) 15 2 Figure 2. Effect of contact time on the adsorption of Cu(II) ions at different initial concentrations from aqueous solution onto pomegranate peel. Data points correspond to the following initial Cu(II) ion concentrations:,.157 mmol/ ;,.314 mmol/ ;,.472 mmol/ ;, 629 mmol/ ;,.787 mmol/.

2 R.A.K. Rao and F. Rehman/Adsorption Science & Technology Vol. 28 No. 3 21 concentration gradient between the bulk solution and the adsorbent surface, leading to an increase in the driving force responsible for the uptake of metal ions at high initial concentrations. The maximum adsorption of Cu(II) ions was.677 mmol/g at an initial Cu(II) ion concentration of.787 mmol/. The adsorption of Cu(II) ions increased rapidly and sharply with time over the initial stage of the adsorption process ( 2 min range) and then increased gradually to attain equilibrium within 5 min. Any further increase in contact time had no significant effect on the amount adsorbed. For this reason, the shaking time was maintained at 5 min in subsequent batch experiments. Effect of ph The adsorption of Cu(II) ions increased as the initial ph value (ph i ) of the aqueous solution increased. The maximum extent of adsorption occurred at ph 4 (Figure 3), with any further increase ph (final) 8 7 6 5 4 3 2 1.8.7.6.5.4.3.2.1 Amount adsorbed (mmol/g) 2 3 4 6 ph (initial) 7 8 Figure 3. Effect of ph on the adsorption of Cu(II) ions from aqueous solution onto pomegranate peel. Data points correspond to the following:, amt. of Cu(II) ions adsorbed (mmol/g);, final ph of the solution. in ph having a negligible effect. In acidic solution (low ph values), the uptake of Cu(II) ions is inhibited because of the high concentration of H + ions competing with Cu(II) ions for the available adsorption sites on the surface. As a consequence, protonation of the various functional groups (carboxylic and phenolic) on the surface of the adsorbent occurs. The final ph (ph f ) recorded in the figure was the ph of the solution at equilibrium. This is also an important factor since many sorption reactions have been considered under equilibrium conditions. Thus, when the initial ph of the system was 2, the final ph was virtually the same (ph f = 2.2), showing that the solution was acidic and contained fairly high concentrations of H + ions; thus, H + ions compete for the adsorption sites and the adsorption of Cu(II) ions was a minimum (.17 µmol/g). When the initial ph was adjusted to 3, the final ph values increased sharply (ph f = 5.8) and, at the same time, the adsorption of Cu(II) ions increased (.72 mmol/g). The adsorption of such a large quantity of Cu(II) ions together with H + ions led to a decrease in the H + ion concentration in the solution and this resulted

Adsorption of Heavy Metal Ions on Pomegranate (Punica granatum) Peel 21 in an increase in final equilibrium ph. The adsorption of Cu(II) ions continued to increase with increasing ph and became constant at and above a ph value of 4. The point of zero charge (PZC) has an important influence on such adsorption processes. Thus, the surface of the adsorbent was positive when ph < PZC, neutral when ph = PZC and negative at ph > PZC. The data depicted in Figure 4 indicate that the PZC value of pomegranate peel was 7, showing that the surface was negatively charged above this ph value and hence fairly large amounts of Cu(II) ions were adsorbed. The total amount of acidic sites (carboxylic, phenolic and lactonic) on the surface of the adsorbent was.28 mequiv/g. These acidic sites are protonated in acidic medium resulting in the low adsorption of metal ions by pomegranate peel at low ph values. 3.5 3. 2.5 2. ph f ph i 1.5 1..5.5 ph i 2 4 6 8 1 1. Figure 4. Determination of the point of zero charge (PZC). Adsorption isotherms The adsorption isotherm data were analyzed employing both the Langmuir and Freundlich isotherm equations (Ajmal et al. 2). The linear form of the Langmuir isotherm may be written as: 1/q e = 1/q m 1/b 1/C e + 1/q m (1) where q e is the amount of metal ions adsorbed per unit weight of adsorbent, q m is the maximum sorption capacity (mg/g) determined by the number of reactive surface sites in an ideal monolayer system, C e is the concentration of metal ions (mg/ ) at equilibrium and b is a constant related to the bonding energy associated with the ph-dependent equilibrium constant. Plotting 1/q e versus 1/C e for the data obtained experimentally at 2 ºC led to a straight line. However, such concurrence was not observed at the other temperatures employed. The values of b and q m obtained at all the various temperatures are listed in Table 1, while Figure 5 depicts the corresponding Langmuir isotherms. It will be seen that the q e (calc.) values were very close to the q e (exp.) values at 2 ºC but deviated appreciably when the temperature was increased. This indicates that the Langmuir isotherm fitted the experimental data at 2 ºC but deviated from the

22 R.A.K. Rao and F. Rehman/Adsorption Science & Technology Vol. 28 No. 3 21 TABLE 1. Langmuir and Freundlich Isotherm Parameters for the Adsorption of Cu(II) Ions by Pomegranate Peel Temp. (ºC) q m (mmol/g) Langmuir parameters Freundlich parameters b r 2 χ 2 R L K F 1/n r 2 χ 2 2.466.69.997.25.224 1.9.764.997.11 3.552.79.92.83.22 2.973.677.88.283 4.2662.59.92.544.253 3.14.618.889.723.25.2 q e (mmol/g).15.1.5.5.1 C e (mmol/ ).15.2 Figure 5. Langmuir isotherms for the adsorption of Cu(II) ions onto pomegranate peel. Data points correspond to the values of q e (exp.) at the following temperatures: ( ) 2 ºC; ( ) 3 ºC; ( ) 4 ºC. The corresponding values of q e (calc.) at these temperatures are shown by the plots depicted in the figure. same with increasing temperature. A χ 2 -test was also applied to this model. The advantage of such a test is that the values of q e (calc.) obtained from the model and the q e (exp.) values can be compared directly employing the same abscissa and ordinate (Ho 24). If the data from the model are similar to the experimental data, χ 2 is small and vice versa. The values of χ 2 were calculated using the following relationship: [ ] q (exp.) q ( calc.) / q ( calc.) χ 2 2 = e e e (2) The data listed in Table 1 show that the value of χ 2 was smallest at 2 ºC and also that the regression coefficient (r 2 ) was high at this temperature, thereby indicating that the Langmuir isotherm gave a good fit at 2 ºC. The linear form of the Freundlich isotherm may be written as: log q = log K + 1 log C e F n e (3) where K F is the Freundlich constant and n is another constant that provides information about the degree of heterogeneity of the surface sites. When n approaches zero, the surface site

Adsorption of Heavy Metal Ions on Pomegranate (Punica granatum) Peel 23 heterogeneity increases. Figure 6 depicts the Freundlich isotherms obtained for the experimental data at the various temperatures studied. Again, the plot of log q e verses log C e at 2 ºC was linear whereas as those for data obtained at higher temperatures deviated from linearity. The values of the corresponding Freundlich constants are also listed in Table 1 together with the values of χ 2 at the different temperatures. The smallest value of χ 2 was obtained at 2 ºC together with a high correlation coefficient (r 2 ), thereby indicating that the data could be fitted by the Freundlich model at 2 ºC but increasing deviations occurred at higher temperatures. However, the increase in the value of K F with temperature showed that the adsorption process was favoured at higher temperatures. When the Langmuir and Freundlich isotherms at 2 ºC are compared, it will be seen that the χ 2 value for the Freundlich isotherm was slightly lower (.11) than that obtained q e (mmol/g).2.18.16.14.12.1.8.6.4.2.5.1.15 C e (mmol/ ) Figure 6. Freundlich isotherms for the adsorption of Cu(II) ions onto pomegranate peel. Data points correspond to the values of q e (exp.) at the following temperatures: ( ) 2 ºC; ( ) 3 ºC; ( ) 4 ºC. The corresponding values of q e (calc.) at these temperatures are shown by the plots depicted in the figure. for the Langmuir isotherm (.25). Hence, it may be concluded that although both the Langmuir and the Freundlich models fitted the data obtained at 2 ºC, the Freundlich model provided the better fit. The essential characteristics of the Langmuir isotherm can be expressed in terms of a dimensionless constant separation factor or equilibrium parameter, R L, given by: R L = 1 ( + bc ) 1 (4) The magnitude of R L predicts the shape of the isotherm. A value of R L > 1 corresponds to an unfavourable isotherm, R L = 1 corresponds to a linear isotherm, < R L < 1 corresponds to a favourable isotherm, while R L = corresponds to irreversible adsorption (Ghodbane et al. 28). The values of R L obtained at 2 ºC, 3 ºC and 4 ºC, respectively, in the present work are also listed in Table 1 and indicate that, at all the various temperatures, the adsorption of Cu(II) ions onto on pomegranate peel was favourable.

24 R.A.K. Rao and F. Rehman/Adsorption Science & Technology Vol. 28 No. 3 21 Thermodynamic studies The equilibrium constants (K c ) at 3 ºC, 4 ºC and 5 ºC were calculated from the following relationship (Namasivayam and Ranganatham 1995): K c = C C AC e (5) where C AC and C e are the equilibrium concentrations (mg/ ) of Cu(II) ions on the adsorbent and in solution, respectively. Similarly, the standard free energy change, G, may be written as: G = RT ln K c (6) where T is the temperature in degrees Kelvin and R is the gas constant, while values of the standard enthalpy change ( H ) and standard entropy change ( S ) were calculated from the following relationship: S H log K c = 2. 33R 2. 33RT (7) The values of S and H were calculated from the slope and intercept of the linear plot of log K c versus 1/T (figure not shown), with the corresponding values together with those for K c and G being reported in Table 2. The positive value of H indicates that the process was endothermic, while the negative values of G indicate that the process was spontaneous and increased with increasing temperature. The positive values of S suggest increasing randomness at the solid/liquid interface during the adsorption process. TABLE 2. Thermodynamic Parameters for the Adsorption of Cu(II) Ions by Pomegranate Peel Temp (ºC) K c G (kj/mol) H (kj/mol) S [kj/(mol K)] 3 21.7 7.51 4 32.3 8.75 25.84.1138 5 4.6 9.64 The Dubinin Radushkevich (D R) isotherm (1947), which takes into account the heterogeneous nature of the adsorbent surface, was used to predict whether the adsorption occurred via a physical or chemical process. The D R equation may be written as: ln q e = ln q m βε 2 (8) where q e is the adsorption capacity (mol/g) at equilibrium, q m is the maximum adsorption capacity (mol/g), β is the activity coefficient and ε is the Polanyi potential which is given by: ε = RT ln + 1 1 C e (9)

Adsorption of Heavy Metal Ions on Pomegranate (Punica granatum) Peel 25 where R is the gas constant [J/(K mol)], T is the temperature (K) while C e is the equilibrium concentration (mol/ ) of Cu(II) ions in solution. The values of q m and β 2 can be obtained from the intercept and slope of the plot of ln q e versus ε 2 (figure not shown), while the mean free energy of adsorption, E, can be calculated from the following equation: E = 1 2 12 β / (1) The D R parameters and the values of the mean free energy are listed in Table 3. The values of E obtained were between 9.6 and 1 kj/mol, which are in the energy range expected for a chemisorption reaction (Helfferich 1962). TABLE 3. D R Isotherm Parameters for the Adsorption of Cu(II) Ions by Pomegranate Peel Temp. (ºC) β q m (mol/g) E (kj/mol) r 2 2.54.175 9.6.9967 3.53.169 9.7.994 4.5.251 1..9993 Adsorption kinetics The rate constants were calculated by using the pseudo-first-order (O Shannessy and Winzor 1996) and the pseudo-second-order (Ho and McKay 2) kinetic equations. Both equations are based on the solution concentration. The pseudo-first-order kinetic expression may be written as: log( q q ) = log q e t e k1 2. 33R t (11) where q e is the amount adsorbed at equilibrium, q t is the amount adsorbed at time t and k 1 is the pseudo-first-order adsorption rate constant. If pseudo-first-order kinetics apply, the plot of the experimental data in terms of log(q e q t ) versus t should be linear. The corresponding regression coefficients (r 2 ) and rate constants for pseudo-first-order kinetics at the various concentrations employed in the present study are listed in Table 4. Similarly, the pseudo-second-order kinetic equation may be written as: t q t 1 = + h t q e (12) where h = k 2 q 2, with k being the adsorption rate constant for the pseudo-second-order reaction and e 2 h being the initial adsorption rate. The values of k 2 were calculated from the slope of the linear plots of t/q t versus t (figure not shown). These values are also reported in Table 4. A comparison of the experimental adsorption capacities [q e (exp.)] and the calculated values [q e (calc.)] as obtained from equations (11) and (12) shows that q e (calc.) values obtained from the pseudo-first-order kinetic

26 R.A.K. Rao and F. Rehman/Adsorption Science & Technology Vol. 28 No. 3 21 TABLE 4. Kinetic Parameters for the Adsorption of Cu(II) Ions by Pomegranate Peel Conc. Pseudo-first-order parameters Pseudo-second-order parameters (mmol/ ) k 1 q e (exp.) q e (calc.) r 2 k 2 h q e (calc.) r 2 (mmol/g) (mmol/g) (mmol/g).157.46.116.11.958.182 1..116.999.314.499.259.18.87.123 29.41.261.999.472 2.89.425.137 1..5 18.51.58.979.629.693.566.125 1..3 25.83.572 1..787.54.677.61.86.1 46.72.686.999 model exhibited large deviations from the q e (exp.) values. However, when the pseudo-secondorder kinetic model was applied, the q e (calc.) values were very close to the q e (exp.) values. Such concurrence, together the high values of the correlation coefficient (r 2 ), confirm the applicability of the pseudo-second-order kinetic model to the experimental data. Adsorption/desorption studies Batch process In order to make the process more economical, desorption studies were carried out by batch as well as by column methods. Table 5 lists the data for the desorption of Cu(II) ions from pomegranate peel in various salt solutions as obtained by the batch process. The results indicate that Cu(II) ions were strongly adsorbed onto pomegranate peel and could not be recovered employing different salt solutions. However, 79.5% of the Cu(II) ions could be recovered when.1 N HCl was employed for desorption. This shows that Cu(II) ions may be removed by adsorption from industrial wastewaters containing other dissolved salts, since their presence will not affect the adsorption process. Attempts were also made to desorb various metal ions from electroplating wastewater by treating 5 m of wastewater with.5 g of pomegranate peel adsorbent under identical conditions. TABLE 5. Adsorption/Desorption of Cu(II) Ions as Determined by Batch Methods a Expt. No. Eluent Amt. adsorbed (mg) % Desorption 1 Distilled water 2.2. 2.5 M Acetic acid 2.2 29.5 3.1 M Acetic acid 2.2 36.3 4.5 M HCl 2.2 77.3 5.1 M HCl 2.2 79.5 6.5 M NaCl 2.35 2.1 7 1. M NaCl 2.35 6.4 8.1 M NaHCO 3 2.35 2.1 a Initial loading of Cu(II) ions = 2.5 mg.

Adsorption of Heavy Metal Ions on Pomegranate (Punica granatum) Peel 27 The corresponding analytical data for electroplating wastewater collected from one of the lock factories in Aligarh city are reported in Table 6. These indicate that such electroplating waste contained a high concentration of Cr(VI) ions (39 mg/g). Table 7 shows the results obtained for the adsorption/desorption of various metals from electroplating wastewater by the batch process. TABLE 6. Analysis of Electroplating Wastewater Metal ion Concentration (mg/ ) Cu(II) 21. Ni(II) 3.2 Cd(II).22 Zn(II).97 Cr(VI) 39. ph 5.6 TABLE 7. Desorption of Metal Ions from Electroplating Wastewater by Batch Methods a Metal ion Amount loaded (mg/ ) % Adsorption % Desorption Cu(II) 21. 9. 89.9 Ni(II) 3.2 81.2 77. Cd(II).22 9.9 8. Zn(II).97 67. 69.2 Cr(VI) 39. 18. n.d. a Eluent =.1 N HCl. Column process The results of desorbing Cu(II) ions from pomegranate peel by column methods employing.1 N HCl as the eluent are shown overleaf in Figure 7. These data show that of the 2.35 mg Cu(II) initially adsorbed, 1.87 mg were recovered via the use of.1 N HCl solution. The desorption process was rapid and, as shown in Figure 7, the maximum amount of Cu(II) ions recovered was achieved by the application of 1 m of the eluent. Continuous flow fixed bed column studies Breakthrough methods provide the most effective column process by making optimum use of the concentration gradient between the solute adsorbed by the adsorbent and that remaining in the solution. The process is continued until the metal ions in the effluent start appearing after passage through the column; this point, which marks the end of the working life of the column for practical purposes, is called the breakthrough point. This is important in process design because it directly

28 R.A.K. Rao and F. Rehman/Adsorption Science & Technology Vol. 28 No. 3 21 1.4 Amt. of Cu(II) ions desorbed (mg) 1.2 1..8.6.4.2 2 4 6 8 1 12 Volume of eluent (m ) Figure 7. Desorption of Cu(II) ions from pomegranate peel employing.1 N HCl as the eluent. Experimental conditions: initial amt. of Cu(II) ions loaded = 2.5 mg; amt. of adsorbent employed =.5 g; flow rate of eluent = 1 m /min affects the feasibility and economics of the process (Volesky 23).The breakthrough capacity of a column of pomegranate peel towards Cu(II) ions is shown in Figure 8. It is clear from the figure that 1 m of water containing 5 mg/ of Cu(II) ions could be passed through the column without Cu(II) ions being detected in the effluent. Hence, the breakthrough capacity was 1 mg/g. However, the breakthrough capacity towards Cu(II) ions was slightly reduced when a mixture of.7.6.5 C/C.4.3.2.1 2 4 Volume of effluent (m ) 6 8 Figure 8. Breakthrough behaviour of a column packed with pomegranate peel towards the desorption of Cu(II) ions. Experimental conditions: amt. of adsorbent employed =.5 g; flow rate of effluent = 1 m /min.

Adsorption of Heavy Metal Ions on Pomegranate (Punica granatum) Peel 29 Cu(II), Ni(II), Cd(II), Zn(II) and Cr(VI) ions was passed through the column. The breakthrough capacity curves for Cu(II), Ni(II), Zn(II), Cd(II) and Cr(VI) ions are shown in Figure 9 from which the breakthrough capacities for these ions at ph 4.5 were calculated as 6, 2, 2, 2 and ca. mg/g, respectively. It will be noted that the adsorption of Cr(VI) ions was virtually zero and more than 5% of the Cr(VI) ions were present in the effluent. It may therefore be concluded that Cu(II), Ni(II), Cd(II) and Zn(II) ions may be completely removed from a Cr(VI) ion solution containing these ions by the use of pomegranate peel as an adsorbent. 1.2 1..8 C/C.6.4.2 5 1 15 2 25 3 35 4 45 5 55 6 65 7 75 8 85 9 Volume of effluent (m ) Figure 9. Breakthrough capacities of different metal ions on a column packed with pomegranate peel. Data points correspond to the following metal ions:, Cu(II);, Ni(II);, Cd(II);, Zn(II);, Cr(VI). Experimental conditions: conc. of each metal ion = 1 mg/ ; amt. of adsorbent employed =.5 g; flow rate of eluent = 1 m /min. The behaviour of this adsorbent has been utilized for the purification and recovery of Cr(VI) ion solution from electroplating wastewater. Figure 1 shows the breakthrough curves for Cu(II), Ni(II), Zn(II) and Cr(VI) ions when 1 m of electroplating wastewater was passed through the column under identical conditions. It was found that, at a maximum, 1 m of wastewater could be treated without Cu(II), Ni(II) and Zn(II) ions being detected in the effluent. The amount of Cr(VI) ions determined in the first 1 m fraction was 16 mg/. The corresponding effluent which is free from Cu(II), Ni(II) and Zn(II) ions and rich in Cr(VI) ions may be re-used in the electroplating plant.

21 R.A.K. Rao and F. Rehman/Adsorption Science & Technology Vol. 28 No. 3 21 1.2 1..8 C/C.6.4.2 2 4 6 8 1 12 Volume of effluent (m ) Figure 1. Breakthrough curves of metal ions in electroplating wastewater on a column packed with pomegranate peel. Data points correspond to the following metal ions:, Cu(II);, Ni(II);, Cd(II);, Zn(II);, Cr(VI). Experimental conditions: amt. of adsorbent employed =.5 g; ph of wastewater = 4.5; flow rate of eluent = 1 m /min. CONCLUSIONS Pomegranate peel is a by-product of the pomegranate juice industry. It has been shown to have a good adsorption capacity for Cu(II), Cd(II), Ni(II) and Zn(II) ions at a ph value of 4.5, with the adsorption of Cr(VI) ions being a minimum at this ph. The order of adsorption of the metal ions studied was Cu(II) = Cd(II) > Zn(II) > Ni(II) > Cr(VI) at ph 4.5.The maximum adsorption capacity towards Cu(II) ions was.677 mmol/g at an initial Cu(II) ion concentration of.787 mmol/. Both the Langmuir and Freundlich isotherm models could be employed to fit the experimental data obtained at 2 ºC, but the Freundlich isotherm provided a better fit as indicated by the χ 2 -test. The adsorption process was endothermic, with the values of the mean free energy at the temperatures studied indicating the chemical nature of the process. Both the pseudo-firstorder and pseudo-second-order kinetic equations could be used to fit the kinetic data for the adsorption process, although the pseudo-second order model provided a better fit since the theoretical values of q e predicted by the model agreed well with the experimental values. Batch and column methods for the desorption of Cu(II) ions gave values of 79.4% and 8%, respectively. Attempts were made to desorb Cu(II), Ni(II), Zn(II) and Cr(VI) ions from electroplating wastewater. The breakthrough capacities of pomegranate peel towards these metal ions in a synthetic mixture as well as in actual electroplating wastewater were determined, from which it was concluded that 1 m of electroplating wastewater could be passed through a column containing.5 g adsorbent without any Cu(II), Ni(II) or Zn(II) ions being detected in the effluent at ph 4.5. In addition, the minimal adsorption of Cr(VI) ions at this ph together with the corresponding high adsorption extents of Cu(II), Ni(II), Cd(II) and Zn(II) ions allowed the recovery of Cr(VI) ions from electroplating wastewater, thereby enabling the purified solution to be re-used in the electroplating plant.

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