Adsorption of Lead (II) Ion from Aqueous Solution Using Rice Hull Ash

Jun 18, 2008 - M. Ahmaruzzaman and Vinod K. Gupta. Industrial & Engineering Chemistry Research 2011 50 (24), 13589-13613. Abstract | Full Text HTML ...
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Ind. Eng. Chem. Res. 2008, 47, 4891–4897

4891

Adsorption of Lead(II) Ion from Aqueous Solution Using Rice Hull Ash Li-Hua Wang and Chun-I Lin* Department of Chemical Engineering, National Taiwan UniVersity of Science and Technology, Taipei 106, Taiwan

Adsorption of lead(II) ion from aqueous solution using rice hull ash (RHA) was explored in this work. The RHA prepared was found to be an efficient adsorbent. Experimental results indicated the rate of removal and the removal at equilibrium could be increased by increasing the initial lead concentration, pH, stroke speed, or adsorption temperature. They were also found to be increased by decreasing RHA dosage. The effects of RHA dosage and initial lead concentration were found to be pronounced, while those of pH, stroke speed, and adsorption temperature were less significant. The data of adsorption kinetics indicated the process was physisorption controlled and the pseudo-second-order rate equation suitably interpreted the overall process. An empirical relationship between lead adsorption and time was also determined. 1. Introduction Wastewater from electroplating industry contains many heavy metals such as copper(II), nickel(II), chromium (total), zinc(II), cadmium(II), and lead(II). The concentrations of these metals in the effluent of this industry are in the following ranges: 0.032-272.5, 0.019-2,954, 0.088-525.9, 0.112-252.0, 0.007-21.60, and 0.663-25.39 mg/L, respectively.1 The guideline values of these metals for drinking water set by World Health Organization are 2, 0.07, 0.05, 0.01, 0.003, and 0.01 mg/L, respectively.2 Hence, the concentrations of these metals in the effluent should be reduced to the values below them. Lead is one of the metals to be removed, since it is harmful to central and peripheral nervous systems.3 It is generally known that adsorption with activated carbon is a good method to remove heavy metals.4,5 However, the high cost of activated carbon induces investigators to search for alternatives to replace it. Raw rice hull has been employed to remove heavy metals from aqueous solution. The removal efficiencies were found to be high6 and low7 for different adsorption conditions. The adsorption of heavy metals by activated carbon made from rice hull,8,9 activated rice hull,7,10–12 and activated rice hull ash13 has also been studied. However, drawbacks including long preparation time,7,10,11,13 complex preparation process,3,10,12 lengthy removal time,7,10,11 low removal efficiency,9 only two heavy metals being removed by adsorbent,9 and the need to treat wastes generated from preparation of adsorbent9,10,12,13 have caused it to be less widely employed. Rice hull is a byproduct of the rice milling industry. The amount produced in Taiwan is far more than any local uses and, thus, has posed disposal problems. Hence, developing a new usage of rice hull is encouraged in order to reduce environmental pollution. Rice hull ash (RHA), prepared by calcinating rice hull under an air stream at 500 °C for 50 min, has been found to be an effective agent for bleaching sesame oil.14 This finding reveals the high potential of RHA in removing heavy metals from wastewater. Unfortunately, the performance of RHA prepared under similar conditions has not yet been investigated. To make up for such deficiency, this research aims to explore the feasibility and the kinetics of using RHA as an adsorbent to remove lead ion from aqueous solution. A survey of the relevant literature on the adsorption of lead(II) ion on rice hull or its derivatives shows this subject has not * To whom correspondence should be addressed. Tel.: +886-2-27376614. Fax: +886-2-2737-6644. E-mail: [email protected].

received reasonable attention in the past.6,9,10,12 Kim and Choi9 employed rice hull activated carbon as the adsorbent and found it can successfully remove lead. Khalid et al.6 also found that untreated rice hull was an efficient scavenger of lead from aqueous solutions. They found that the removal at equilibrium was a function of rice hull dosage, initial lead concentration, and pH value. Two kinds of rice hull have been used by Daifullah et al.10 One was soaked in KOH and treated with HCl, while the other was steam pyrolized at 650 °C for 1 h. The rice hulls thus prepared were observed to remove all lead ions from the wastewater. The adsorbent used by Wong et al.12 was tartaric acid treated rice hull. The effects of the following factors on the removal of Pb(II) ion at equilibrium have been studied: initial lead concentration, pH, stroke speed, and adsorption temperature. A rate equation for the removal has also been determined. The rice hull ash used by Feng et al.13 was prepared by immersing the rice hull in HCl for 4 h and then heating at 700 °C for 4 h. Thus, treated RHA has been found to be a suitable adsorbent to remove lead ion. Their experimental data indicated that the adsorption of lead by the RHA was contact time, ionic strength, particle size, and pH dependent. A simple adsorption rate equation has also been correlated in their report. In this study, the adsorption efficiency of the RHA, prepared in an easier way and calcined at a lower temperature, was examined. The effects of RHA dosage, initial lead concentration, pH, stroke speed, and adsorption temperature have also been studied. Finally, an adsorption rate equation incorporating the influence factors mentioned above was determined. 2. Materials and Methods 2.1. Materials. The aqueous solution containing lead(II) ion was prepared by dissolving reagent grade Pb(NO3)2 · 3H2O (Shimakyu’s Pure Chemicals, Osaka, Japan) in deionized water. The ionic strength of it was adjusted to 0.1 M by reagent NaNO3 (Shimakyu’s Pure Chemicals, Osaka, Japan), while the pH was regulated by 0.1 M HNO3 and 0.1 M NaOH. Both were prepared from reagent grade chemicals which were purchased from Kojima Chemicals (Tokyo, Japan). 2.2. Ashing of Rice Hull. Ashing of rice hull was performed in a box furnace (VT-10, Kinghwang, Taipei, Taiwan). Rice hull was washed three times by distilled water and then dried in an oven at 105 °C for 24 h. A 7 g portion of it was then placed in an alumina boat (16 cm × 4 cm × 2 cm). The furnace was heated up from room temperature with flowing air (20 mL/

10.1021/ie071521z CCC: $40.75  2008 American Chemical Society Published on Web 06/18/2008

4892 Ind. Eng. Chem. Res., Vol. 47, No. 14, 2008 Table 1. Content of Silicon and Metallic Ingredients of RHA (ppm) Si

Mg

Ca

Al

Fe

Cu

Ni

Ti

Ba

35 450

1942

1200

235

213

20.6

16.1

12.8

7.3

Table 2. Experimental Conditions for the Adsorption Experimenta variable RHA dosage (g/L) initial lead concentration (mg/L) pH (-) stroke speed (strokes/min) adsorption temperature (°C) a

value 1 100 2.4 30 30

2 207 4.2 60 35

3 350 5.3 120 40

4 500 6.0 180 50

5 700

20

Italic values are standard operation variables.

s), and after the furnace temperature was maintained at 500 °C for several minutes, the boat with rice hull was placed in the furnace. After 50 min, the boat was removed from the furnace and cooled in air. The composition of silicon and metals of the RHA was determined by an inductively coupled plasma-mass spectrometer, ICP-MS (Sciex Elan 5000, Perkin-Elmer, Shelton, MA), and their morphologies were observed using a scanning electron microscope (S360, Cambridge, U.K.). The average particle diameter of the RHA was measured by a laser diffraction particle size analyzer (LS-230, Beckman Coulter, CA). The specific surface area was obtained from nitrogen adsorption isotherms produced by a surface area analyzer (ASIMP-LP-VP2, Quantachrome, Boynton Beach, FL). The sample was heated to 170 °C and evacuated until the pressure was lower than 10-4 Torr. After that, liquid nitrogen (-196 °C) was adsorbed on the surface of the sample and then desorbed. The relation between the change in mass of the sample and partial pressure of nitrogen during adsorption and desorption was recorded. The manufacturer’s software provided the total volume and the pore size distribution by the BJH theory.15 The micropore volume and external surface area were obtained by employing the t-plot method,16,17 and the surface area due to micropores was obtained by the value difference between the overall and the external.18 The pH of the RHA was determined by a pH meter (PHB-9902, Ehun, Taipei, Taiwan). The composition of the RHA determined by ICP-MS is listed in Table 1, and the results of other measurements are summarized below. The RHA was an irregular granule with an average diameter of 68.0 × 10-6 m. The specific surface area, specific pore volume, average pore diameter, and pH as well as the pHpzc of RHA were 47.47 m2/ g, 0.137 × 10-6 cm3/g, 9.22 × 10-9 m, 10.2, and 8.3, respectively. 2.3. Procedure for Adsorption Experiments. Adsorption experiments were carried out in a 250 mL high density polyethylene bottle immersed in a reciprocated shaking bath (B603D, Hsin An, Taipei, Taiwan). A 50 mL aqueous solution was first loaded in the bottle and heated up from room temperature. After the temperature of the solution was maintained at the preset one for 20 min, the predetermined amount of RHA was dropped into the bottle and the adsorption began. After a certain time, the adsorption was stopped and the slurry was filtered through a piece of 1 µm Advantec filter paper. The lead concentration in the filtrate was measured by an atomic absorption spectrometer (AAS6 Vario, Analytic jenaAG, Jena, Germany). The relevant experimental conditions are listed in Table 2. Three runs were performed under the same experimental conditions, and the average value was employed.

3. Results and Discussion 3.1. Adsorption Kinetics. Five series of experiments were carried out to study the effects of RHA dosage, initial lead concentration, pH, stroke speed, and adsorption temperature on the removal of lead from aqueous solution. The obtained data are presented as the plots of removal of lead (mg/g), q, against bleaching time, t (min), as shown in Figures 1–5. The removal of Pb2+ at equilibrium, qe, and the percentage removal of Pb2+ at equilibrium, PRe, which are also important, for different adsorption conditions are documented in Table 3. The removal of Pb2+, q, and percentage removal of Pb2+, PR, used are defined as follows. Removal of Pb2+: q≡

(C0 - C)V w

(1)

Percentage removal of Pb2+: (C0 - C) PR ≡ × 100% C0 qw × 100% ) VC0

(2)

where C and C0 are the Pb2+ concentrations (mg/g) at t ) t and t ) 0, respectively; V is the volume of aqueous solution (L); and w is the mass of RHA used (mg). It is seen in these figures that the removal of Pb2+ is very fast in the initial stage and levels off, reaching equilibrium, after 30 min of adsorption. The equilibration times found in the literature are varied: 5,13 10,6 and 120 min.9,12 The present finding is in the range reported. Besides, Table 3 indicates the percentage removals of Pb2+ at equilibrium for all conditions are over 78.61% and most of them are over 85.79%. The fast rate of removal and the high percentage removal at equilibrium found proves RHA is an efficient adsorbent to remove Pb2+ from aqueous solution. Almost all reports6,9,10,12,13 found that rice hull and its derivatives were good adsorbents. However, Table 3. Removals of Pb2+ at Equilibrium, qe, and Percentage Removals of Pb2+ at Equilibrium PRe, t ) 180 min adsorption condition

qe (mg/g)

RHA Dosage 165.52 86.16 69.22 52.91 42.58 10.79 Initial Lead Concentration 100 mg/L 27.61 207 mg/L 68.15 350 mg/L 96.91 500 mg/L 148.30 700 mg/L 207.57 pH 2.4 47.22 4.2 55.11 5.3 56.54 6.0 57.22 Stroke Speed 30 strokes/min 61.50 60 strokes/min 62.06 120 strokes/min 65.74 180 strokes/min 65.41 Adsorption Temperature 30 °C 63.47 35 °C 63.91 40 °C 66.25 50 °C 66.53 1 g/L 2 g/L 3 g/L 4 g/L 5 g/L 20 g/L

PRe (%) 78.73 80.56 96.40 98.21 98.93 99.65 99.75 96.36 88.11 91.50 93.66 78.61 89.33 91.36 93.06 85.79 86.83 92.06 91.54 90.34 91.15 93.36 94.36

Ind. Eng. Chem. Res., Vol. 47, No. 14, 2008 4893

Figure 1. Plot of removal of Pb2+ against time. Effect of RHA dosage. Initial lead concentration, 207 mg/L; pH, 4.2; stroke speed, 120 strokes/min; adsorption temperature, 30 °C.

Figure 2. Plot of removal of Pb2+ against time. Effect of initial lead concentration. RHA dosage, 3.0 g/L; pH, 4.2; stroke speed, 120 strokes/min; adsorption temperature, 30 °C.

compared to their materials, the RHA used in this work is easier to prepare than theirs. Therefore, the cost of production of adsorbent is lower. The other observation from Table 2 is that the maximum percentage of removal at equilibrium is 99.75%, which was obtained under the following conditions: RHA dosage, 3 g/L; initial lead concentration, 100 mg/L; pH, 4.2; stroke speed, 120 strokes/min; and adsorption temperature, 30 °C. The maximum percentages of removal at equilibrium found by Kim and Choi,9 Daifullah et al.,10 Wong et al.,12 Khalid et al.,6 and Feng et al.13 were 92.5, 94.4, 98, 99, and 100%, respectively. The result obtained in this study is close to theirs. If the maximum removal of Pb2+ at equilibrium is of interest, the value of it is 207.5 mg/g, which occurred at RHA dosage, 3 g/L; initial lead concentration, 700 mg/L; pH, 4.2; stroke speed, 120 strokes/min; and adsorption temperature, 30 °C.

Relationships between the removal of Pb2+ and adsorption time with varying RHA dosage are shown in Figure 1. It is indicated that the higher the RHA dosage, the slower the removal of lead ion and the lower the removal of Pb2+ at equilibrium. The effects are found to be pronounced. No similar report has been found in the system of Pb2+/rice hull or its derivatives. However, the present results are found to agree with other systems.19,20 This finding can be interpreted as follows. The high RHA dosage renders a greater pore surface area available for adsorbing lead ion. Therefore, the total removal of lead is high, which causes the lead concentration in the solution, C, to be low. Thus, the value of the numerator of eq 1 is high. On the other hand, high RHA dosage also makes the value of the denominator of eq 2 high. The effect of the latter is more significant than the former. Thus, the net result is the removal of Pb2+, q, is low. If the ordinate of Figure 1 is changed

4894 Ind. Eng. Chem. Res., Vol. 47, No. 14, 2008

Figure 3. Plot of removal of Pb2+ against time. Effect of pH. RHA dosage, 3.0 g/L; initial lead concentration, 207 mg/L; stroke speed, 120 strokes/min; adsorption temperature, 30 °C.

Figure 4. Plot of removal of Pb2+ against time. Effect of stroke speed. RHA dosage, 3.0 g/L; initial lead concentration, 207 mg/L; pH, 4.2; adsorption temperature, 30 °C.

to the total removal of Pb2+ in milligrams, the result will be opposite, which coincides with literature reports.8,9 If the effect of RHA dosage on the percentage removal at equilibrium, PRe, is of interest, it is observed from Table 3 that the higher the RHA dosage used, the higher the PRe value. This result agrees with that reported by Khalid et al.6 Figure 2 shows the effect of initial lead concentration in the solution on the removal of Pb2+, q. It is found that the higher the lead concentration, the faster the removal and the higher the removal at equilibrium. Again, the effects are very significant. The results found coincide with those of other systems.21,22 It is due to the high concentration gradient between the bulk liquid and the pore surface. If the influence of initial lead concentration on the percentage removal at equilibrium is studied, one finds from Table 3 that, as the initial lead concentration is increased, the percentage removal is reduced

and then levels off. Similar trends were also observed by Wong et al.12 and Khalid et al.6 The obtained results of the effect of pH shown in Figure 3 are close to those of other reports.6,12,13 The trend is that the removal of Pb2+ and the removal at equilibrium are increased with an increase of pH while pH shows no effect on the removal when pH is around 6.0. Compared with the degree of influence, the effect of pH is less than those of RHA dosage and initial lead concentration. The effect of pH observed here can probably be explained by the ion-exchange mechanism proposed by Daifullah et al.10 and Khalid et al.:6 Pb2+ + 2(-SiOH) S Pb(-SiO)2 + 2H+ +

(3)

where Pb , -SiOH, and H are lead ion, silanol group, and proton, respectively. 2+

Ind. Eng. Chem. Res., Vol. 47, No. 14, 2008 4895

Figure 5. Plot of removal of Pb2+ against time. Effect of adsorption temperature. RHA dosage, 3.0 g/L; initial lead concentration, 207 mg/L; pH, 4.2; stroke speed, 120 strokes/min. Table 4. Average Values of the Determination Coefficient, r2 equations

average r2

C/C0 ) kdt0.5, eq 7 q ) (1/b) ln(ab) + (1/b) ln(t + (1/ab)), eq 8 ln(qe - q) ) ln qe - k1t, eq 9 t/q ) (1/k2qe2) + (1/qe)t, eq 10

0.9494 0.9393 0.9763 0.9998

The silanol group on the surface of RHA was produced through the hydrolysis of silicon oxide on RHA6 and the lead ion in solution substituted the proton of the silanol group.10 At low pH, the surface of the RHA was surrounded by proton (H+), which prevented the Pb2+ from approaching the silanol group and, consequently, reduced the adsorption of lead ion. With increasing pH value, electrostatic repulsion decreased due to the reduction of positive charge density around the silanol group, thus resulting in enhancement of lead adsorption.12 The results of the influence of the stroke speed on the removal of Pb2+ are plotted in Figure 4, from which it may be seen that the higher the stroke speed, the faster the removal and the higher the removal at equilibrium. Wong et al.12 reported the same tendency for the system of Pb2+/tartaric acid modified rice hull. The reason for this is that when the stroke speed is increased, the rate of mass transfer of lead ion through an artificial liquid film outside RHA particles is increased. It is also noted from Figure 4 that when the stroke speed is over 120 strokes/min, the effect of stroke speed becomes insignificant. This implies the resistance of the mass transfer of lead ion through the liquid film becomes null. Experiments were conducted at four temperatures within the range 30-50 °C, and the results are depicted in Figure 5. As predicted, the higher the temperature, the faster the removal. However, the degree of the effect is very low. This finding indirectly indicates the adsorption of the present system is an endothermic process and the heat of adsorption is very low. The present result agrees with that of Khalid et al.6 but disagrees with those of Wong et al.12 and Feng et al.13 This may be due to the fact that the adsorbents employed were manufactured by different processes. 3.2. Rate Equation. The lead ion in aqueous solution should pass through three consecutive steps to be adsorbed onto the surface of the pore inside the RHA particle: mass transfer of

ion through the boundary layer outside the RHA particle, diffusion of ion from the external surface of RHA particle to the neighborhood of the adsorption site through the pore of the RHA particle, and adsorption of the ion onto the adsorption site. The slowest step controls the overall process, and the overall rate equals the rate of that step. If the overall process is controlled by the film mass transfer, the overall rate equation is shown as follows23

(

ln

) (

)

1 + mkf mkf C 1 ) ln βSst C0 1 + mkf 1 + mkf mkf

(4)

The values of m and Ss are calculated according to the following two equations, respectively. w V 6m Ss ) dpFp(1 - εp) m)

(5) (6)

where dp ) RHA particle diameter (cm), kf ) adsorption rate constant (1/s · number of site), t ) adsorption time (s), V ) volume of RHA free solution (L), β ) mass transfer coefficient (cm/s), εp ) porosity of RHA particle (-), and Fp ) density of RHA particle (mg/L). If the pore diffusion step is the slowest one, the overall rate equation is24 C ) kdt0.5 C0

(7)

where kd is the rate constant (1/s0.5). When the adsorption is the controlling step, three kinds of equations have been employed corresponding to different adsorption mechanisms. Elovich equation for chemisorption:25 1 1 1 ln(ab) + ln t + b b ab where a ) constant (mg/g · s) and b ) constant (mg/g). Pseudo-first-order equation:26 q)

(

ln(qe - q) ) ln qe - k1t

)

(8)

(9)

4896 Ind. Eng. Chem. Res., Vol. 47, No. 14, 2008

where qe ) amount of Pb2+ adsorbed per gram of RHA at equilibrium (mg/g) and k1 ) pseudo-first-order rate constant (1/s). Pseudo-second-order equation:27 1 t 1 ) + t q k q 2 qe

(10)

2 e

where k2 ) pseudo-second-order rate constant (g/mg · s). Normally, the obtained kinetic data are regressed with eqs 4 and 7–10 and the appropriateness of the fittings can be known from the values of the determination coefficients calculated. Figure 4 shows when the stroke speed is over 120 strokes/min, the influence of film mass transfer can be neglected. Further, the stroke speeds employed for gaining the data shown in Figures 1–3 and 5 are 120 strokes/min. Therefore, all experimental runs with data reported in Figures 1–5 except those with stroke speeds of 30 and 60 strokes/min, shown in Figure 4, are not in the regime of film mass transfer control. Thus, data regression with eq 4 is not necessary and the expressions used for correlation are eqs 7–10. The average values of the determination coefficient, r2, were also calculated accompanying the correlation and are listed in Table 4. The results show the regression with eq 10 has the highest value. Therefore, it is believed the physisorption is the rate determining step. Thus, from now on, the pseudo-secondorder-type equation, eq 10, will be employed for further correlation. According to eq 10, a linear plot of t/q against t gives the values of slope 1/qe and intercept 1/k2qe2. From these two values, k2 and qe can be calculated for each experimental run. It is shown in Figures 1–3 and 5 that k2 and qe are RHA dosage, initial lead concentration, pH, and adsorption temperature dependent. These dependencies can be incorporated into the expressions of k2 and qe as follows

( ) ( )

E1 n1 n2 n3 d C0 pH (11) RT E2 m1 m2 m3 d C0 pH (12) qe ) qe0 exp RT where E1 and E2are the activation energies (J/mol); d is the RHA dosage (g/L); k20 is the constant (L/s · mg2); m1, m2, m3, n1, n2, and n3 are the dimensionless exponents; pH is the pH value of the solution (-); qe0 is the constant (mg/g); R is the universal gas constant (8.314 J/mol · K); and T is the absolute temperature (K). Owing to their monotonic dependencies, the power-law-type expressions have been chosen for the dependencies of the RHA dosage, initial lead concentration, and pH. It should be mentioned here that since the influence of the film mass transfer for the experimental runs with a stroke speed of 30 and 60 strokes/min cannot be excluded from the rate of the overall process, the data of these two runs were not included in the following regression. The method of determining the values of n1, n2, and n3 is presented as follows. When RHA dosage is varied while the initial lead concentration, pH, stroke speed, and adsorption temperature keep constant, eq 11 can be reconstructed to give k2 ) k20 exp -

( )

E1 n2 n3 n1 C pH d (13) RT 0 On taking the logarithm of both sides of the above equation, one obtains k2 ) k20 exp -

(

( )

)

E1 n2 n3 C pH + n1 ln d ln k2 ) ln k20 exp RT 0

(14)

If ln k2 is plotted against ln d, the exponent of d, n1, can be determined from the slope of the straight line obtained. The experimental data shown in Figure 1 were used for this purpose. Similarly, the values of n2 and n3 can be calculated from the data of Figures 2 and 3, respectively. If the adsorption temperature is varied while other variables are kept constant, eq 11 can be written in the following form

( )

E1 (15) RT After taking the logarithm of both sides, eq 15 can be rewritten as k2 ) k20dn1Cn02pHn3 exp -

( )( )

ln k2 ) ln(k20dn1Cn02pHn3) + -

E1 1 R T

(16)

As ln k2 is plotted against 1/T, the slope and the intercept of the obtained straight line are -E1/R and ln(k20dn1C0n2(pH)n3), respectively. The data shown in Figure 5 were employed for the regression and the activation energy E1 and constant k20 can be determined. Similarly, the values of m1, m2, m3, E2, and q20 can be determined using the values of qe of different operating conditions obtained from Figures 1–5. The final equation determined utilizing the Origin 7.0 subroutine28 is shown as t ) q -5

6.8413 × 10

1 + -9627 -1568 2 0.68 1.69 0.28 exp + d C0 pH RT RT t (17) -1568 -0.91 1.01 0.17 0.0685 × exp d C0 pH RT

[(

)]

) ( (

)

or in the differential form -9627 -0.14 0.66 0.26 dq ) 0.0146 × exp d C0 pH × dt RT 2 -1568 -0.91 1.01 0.17 0.0685 × exp d C0 pH - q (18) RT Wong et al.12 also correlated a pseudo-second-order equation for the system of Pb2+/tartaric acid modified rice hull without incorporating operating variables in the equation. Equation 18 reveals the rate of removal of Pb2+ and the removal of Pb2+ at equilibrium is proportional to the initial lead concentration and pH as well as inversely proportional to RHA dosage as represented by the positive exponents of C0 and pH as well as the negative exponent of d, respectively. The removal of Pb2+ being faster and the removal of Pb2+ at equilibrium being higher at higher temperatures are fully represented by the exponential terms. The solid lines shown in Figures 1–5 are drawn corresponding to the best values of k2 and qe during regressions. Figure 6 is prepared to compare the calculated and measured values of q which shows the calculated results and experimental data are in good agreement. A standard deviation, σ, of 3.78 in q has been calculated. The rate expression of eq 17 or 18 is thought to be helpful for designing and running a wastewater treatment vessel of the system of Pb2+/RHA.

(

[

)

(

)

]

4. Conclusions The efficiency of adsorption of Pb2+ on RHA and the effects of operating variables on the adsorption have been investigated

Ind. Eng. Chem. Res., Vol. 47, No. 14, 2008 4897

Figure 6. Comparison of calculated and experimental removal of Pb2+, q (mg/g).

in this work. Furthermore, a rate equation of adsorption has been regressed. Low cost rice hull ash prepared by calcinating rice hull at 500 °C under 20 mL of air/s for 50 min could remove lead(II) ion from aqueous solution effectively. The rate of removal of Pb2+ and the removal of Pb2+ at equilibrium were increased upon increasing the initial lead concentration, pH, stroke speed, or adsorption temperature. They could also be increased by decreasing RHA dosage. The influences of RHA dosage and initial lead concentration were found to be pronounced, while those of pH, stroke speed, and adsorption temperature were less significant. The overall adsorption process was seen to be controlled by the physisorption step, and the pseudo-second-order rate equation described the overall process properly. An empirical relationship between the lead removal and adsorption time was also determined. Literature Cited (1) Yang, W. F. Galvanizes Factory Waste Water Pollution Preventing and Controlling. In Industry Pollution PreVenting and Controlling Technical Manual; Yang, W. F., Ed.; Industrial Development Bureau, Ministry of Economic Affairs: Taipei, Taiwan, 1994; Vol. 1, p 10. (2) World Health Organization. Guidelines for Drinking-Water Quality, 3rd ed.; World Health Organization: Geneva, Switzerland, 2006; p 54. (3) Manahan, S. E. EnVironmental Chemistry, 6th ed.; CRC Press: Boca Raton, FL, 1994. (4) Aggarwal, D.; Goyal, M.; Bansal, R. C. Adsorption of Chromium by Activated Carbon from Aqueous Solution. Carbon 1999, 37, 1989. (5) Marzal, P.; Seco, A.; Gabaldo´n, C. Cadmium and Zinc Adsorption onto Activated Carbon: Influence of Temperature, pH and Metal/Carbon Ratio. J. Chem. Technol. Biotechnol. 1996, 66, 279.

(6) Khalid, N.; Ahmad, S. Removal of Lead from Aqueous Solutions using Rice Husk. Sep. Sci. Technol. 1998, 33, 2349. (7) Bhattacharya, A. K.; Venkobochar, C. Removal of Cadmium (II) by Low Cost Adsorbents. J. EnViron. Eng. 1984, 110, 110. (8) Srinivasan, K.; Balasubramanian, N.; Ramakrishna, T. V. Studies on Chromium Removal by Rice Husk Carbon. Indian J. EnViron. Health 1988, 30, 376. (9) Kim, K. S.; Choi, H. C. Characteristics of Adsorption of Rice Hull Activated Carbon. Water Sci. Technol. 1998, 38, 95. (10) Daifullah, A. A. M.; Girgis, B. S.; Gad, H. M. H. Utilization of Agro-residues(Rice husk) in Small Waste Water Treatment Plants. Mater. Lett. 2003, 57, 1723. (11) Ou, H. C.; Ke, M. S.; Jang, T. E. Method for Processing and Recycling Heavy Metals by Means of Synthesized Rice Hull Xanthate. Taiwan Patent 1226843, 2005. (12) Wong, K. K.; Lee, C. K.; Low, K. S.; Haron, M. J. Removal of Cu and Pb by Tartaric Acid Modified Rice Husk from Aqueous Solutions. Chemosphere 2003, 50, 23. (13) Feng, Q.; Lin, Q.; Gong, F.; Sugita, S.; Shoya, M. Adsorption of Lead and Mercury by Rice Husk Ash. Colloid Interface Sci. 2004, 278, 1. (14) Chang, Y. Y.; Lin, C. I.; Chen, H. K. Rice Hull Ash Structure and Bleaching Performance Produced by Ashing at Various Times and Temperatures. J. Am. Oil Chem. Soc. 2001, 78, 657. (15) Barrett, E. P.; Joyner, J. G.; Halenda, P. P. The Determination of Pore Volume and Area Distributions in Porous Substance. Computations from Nitrogen Isotherm. J. Am. Chem. Soc. 1951, 73, 373. (16) Lecloux, A. J. Texture of Catalysts. In Catalysis: Science and Technology; Anderson, J. R., Boudart, M., Eds.; Springer-Verlag: New York, 1981; Vol. 2, p 171. (17) Greff, S. J.; Sing, K. S. W. Adsorption Surface Area and Porosity, 2nd ed.; Academic Press: New York, 1982. (18) Sousa-Aguilar, E. F.; Liebsch, A.; Choves, B. C.; Costa, A. F. Influence of the External Surface Area of Small Crystallite Zeolites on the Micropore Volume Determination. Microporous Mesoporous Mater. 1998, 25, 185. (19) Gupta, V. K.; Rastogi, A.; Dwivedi, M. K. Process Development for the Removal of Zinc and Cadmium from Wastewater using Slag -a Blast Furnace Waste Material. Sep. Sci. Technol. 1997, 32, 2883. (20) Eligwe, C. A.; Okolue, N. B.; Nwambu, C. O.; Nwoko, C. I. A. Adsorption Thermodynamics and Kinetics of Mercury (II), Cadmium (II) and Lead (II) on Lignite. Chem. Eng. Technol. 1999, 22, 45. (21) Lopez, F. A.; Perez, C.; Sainz, E.; Alonso, M. Adsorption of Pb2+ on Blast Furnace Sludge. J. Chem. Technol. Biotechnol. 1995, 62, 200. (22) Ho, Y. S.; Mckay, G. Kinetic Model for Lead (II) Sorption on to Peat. Adsorpt. Sci. Technol. 1998, 77, 243–255. (23) Choy, K. K. H.; Ko, D. C. K.; Cheung, C. W.; Porter, J. F.; Mckay, G. Film and Intraparticle Mass Transfer during the Adsorption of Metal Ions onto Bone Char. J. Colloid Interface Sci. 2004, 271, 284. (24) Morris, C. J.; Weber, W. J. Proceedings of the 1st International Conference on Water Pollution Research. Water Pollut. Res.; Pergamon Press: New York, 1962; Vol. 2, p 213. (25) Aharoni, C.; Tompkins, F. C. Kinetics of Adsorption and Desorption and Elovich Equation. In AdVances in Catalysis and Related Subjects; Elex, D. D., Pines, H., Weisz, P. B., Eds.; Academic Press: New York, 1970; Vol. 21, pp 1-49. (26) Lagergren, S. About the Theory of So-called Adsorption of Soluble Substances. Kungliga SVenska Vetenskapsakademiens. Handlingar, Band 1898, 24, 1. (27) Ho, Y. S.; Mckay, G. Comparative Sorption Kinetic Studies of Dye and Aromatic Compounds on to Fly Ash. J. EnViron. Sci. Health 1999, A34, 1179. (28) OriginLab. Origin Help, version 7; OriginLab Corporation: Northampton, MA, 2000.

ReceiVed for reView November 8, 2007 ReVised manuscript receiVed March 3, 2008 Accepted May 3, 2008 IE071521Z