Journal of Hazardous Materials 163 (2009) 1254–1264
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The sorption of lead(II) ions on rice husk ash Tarun Kumar Naiya a , Ashim Kumar Bhattacharya a , Sailendranath Mandal b , Sudip Kumar Das a,∗ a b
Department of Chemical Engineering, University of Calcutta, 92 A P C Road, Kolkata 700009, India National Institute of Technical Teachers’ Training and Research, Block-FC, Sector-III, Salt Lake City, Kolkata 700106, India
a r t i c l e
i n f o
Article history: Received 6 March 2008 Received in revised form 21 June 2008 Accepted 22 July 2008 Available online 3 August 2008 Keywords: Rice husk ash Adsorption isotherm Effective diffusivity Sorption energy
a b s t r a c t Present study deals with the adsorption of Pb(II) from aqueous solution on rice husk ash. Rice husk is a by-product generally obtained from rice mill. Rice husk ash is a solid obtained after burning of rice husk. Batch studies were performed to evaluate the inﬂuences of various experimental parameters like pH, initial concentration, adsorbent dosage, contact time and the effect of temperature. Optimum conditions for Pb(II) removal were found to be pH 5, adsorbent dosage 5 g/L of solution and equilibrium time 1 h. Adsorption of Pb(II) followed pseudo-second-order kinetics. The effective diffusion coefﬁcient is of the order of 10−10 m2 /s. The equilibrium adsorption isotherm was better described by Freuindlich adsorption isotherm model. The adsorption capacity (qmax ) of rice husk ash for Pb(II) ions in terms of monolayer adsorption was 91.74 mg/g. The change of entropy (S0 ) and enthalpy (H0 ) were estimated at 0.132 kJ/(mol K) and 28.923 kJ/mol respectively. The negative value of Gibbs free energy (G0 ) indicates feasible and spontaneous adsorption of Pb(II) on rice husk ash. The value of the adsorption energy (E), calculated using Dubinin–Radushkevich isotherm, was 9.901 kJ/mol and it indicated that the adsorption process was chemical in nature. Application study was also carried out to ﬁnd the suitability of the process in waste water treatment operation. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Rapid industrialization has lead to an increased disposal of heavy metals into the environment. Environmentalists are primarily concerned with the presence of heavy metals due to their toxicity and impact on human health and environment. Lead poisoning in human causes severe damage to kidney, nervous system, reproductive system, liver and brain. Severe exposure to lead has been associated with sterility, abortion, stillbirths and neo-natal deaths [1,2]. Process industries, such as battery manufacturing, printing and pigment, metal plating and ﬁnishing, ammunition, soldering material, ceramic and glass industries, iron and steel manufacturing units generate large quantities of waste water contaminated with lead. In drinking water lead contamination occurs due to the corrosion and leaching of lead pipes and Pb/Sn solder joints associated with copper service lines used in household plumbing . The permissible level of lead in drinking water is 0.05 mg/L . The permissible limit of lead in waste water as set by Environment Protection Agency  is 0.05 mg/L and that of Bureau of Indian Standards (BIS) (IS: 10500 of 1992) is 0.1 mg/L . Keeping in view the importance of the situation, speciﬁcally toxicity in children, it has diverted the global
∗ Corresponding author. Tel.: +91 33 2350 8386; fax: +91 33 2351 9755. E-mail address: [email protected]
(S.K. Das). 0304-3894/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2008.07.119
attention towards understanding its behavioral pattern in ecosystem and metabolism for adopting measures for its effective removal from such industrial and municipal waste efﬂuents. The safe and effective disposal of Pb(II) containing waste water is a challenging objective for industries because cost effective treatment alternatives are not readily available. Common cleaning methods for the removal of heavy metal comprise membrane separation , electrochemical precipitation , emulsion per traction , ion exchange , preconcentration , fertilization  and adsorption [13–18]. These methods differ with respect to cost, complexity and efﬁciency. Among these technologies, adsorption is a user-friendly technique for the removal of heavy metal. This process seems to be most versatile and effective method for removal of heavy metal if combined with appropriate regeneration steps. This solves the problem of sludge disposal and renders the system more viable, especially if low cost adsorbents are used. Several recent publications utilized different inexpensive and locally abundantly available adsorbents like barley straw , waste tea leaves , sago waste , peanut hulls , hazel nut shell , saw dust [24,25], neem bark , chitin beads , thermally treated rice husk ash , waste banana , orange peels , cocoa shells , tree fern , coffee residue , rice husk , palm kernel ﬁbre , olive stone waste , orange peel , grape stalk , coir [37,38], tea waste , bagasse ﬂy ash [40,41], rice husk , etc.
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Nomenclature A AS b Ca Ce Cf Ct CS C0 De E E F(t)
G0 H0 kA kD Kad Kbq Kc Kd Kf Kid k m M n q qe qem qmax qt qtm r r2 RL S Ss S0 t V Xm XA XAe
adsorbate activated complex Langmuir constant (L/mg) concentration of Pb(II) on the adsorbent at equilibrium (mg/L) concentration of Pb(II) in solution at equilibrium (mg/L) ﬁnal concentration of Pb(II) in solution (mg/L) concentration of Pb(II) after time t (mg/L) adsorbent concentration in solution (g/L) initial concentration of Pb(II) in solution (mg/L) effective diffusivity (m2 /s) free energy transfer of 1 mole of Pb(II) from inﬁnity to the surface of the adsorbent mean sorption energy (kJ/mol) ratio of the amount adsorbed per g of adsorbent at time t to the amount adsorbed per g of adsorbent at equilibrium Gibbs free energy (kJ/mol) enthalpy of adsorption (kJ/mol) adsorption rate constant desorption rate constant Lagergren rate constant (min−1 ) constant obtained by multiplying qmax and b (L/g) thermodynamic equilibrium constant distribution coefﬁcient measure of adsorption capacity (mg/g) intraparticle rate constant ((mg/g) min1/2 ) pseudo-second-order rate constant of adsorption ((mg/g) min) amount of adsorbent added (g) mass of the adsorbent per unit volume (g/L) Freundlich constants, intensity of adsorption amount adsorbed per g of the adsorbent (mg/g) amount adsorbed per g of the adsorbent at equilibrium (mg/g) amount adsorbed per g of the adsorbent at equilibrium from the model (mg/g) maximum adsorption capacity (mg/g) amount adsorbed per g of adsorbent at time t min amount adsorbed per g of adsorbent at time t min using model mean radius of the adsorbent (m) correlation coefﬁcient separation factor active site on the adsorbent external surface area of the adsorbent per unit volume (m−1 ) entropy of adsorption (kJ/(mol K)) time (min) volume of the solution (mL) maximum adsorption capacity (mmol/g) fractional of adsorbate adsorbed on the adsorbent at any time, t fractional of adsorbate adsorbed on the adsorbent at equilibrium
Greek letters ˇ mass transfer coefﬁcient (m/s) ε Polanyi potential (kJ2 /mol2 ) constant related to energy (mol2 /kJ2 )
Rice husk is a by-product of milling process of rice crop. It accounts for about one-ﬁfth of the annual gross rice production of 545 million metric tons, of the world . Rice husk is mostly used as a fuel in the boiler furnaces of various industries to produce stream. The ash generated after burning the rice husk in the boiler is called rice husk ash. The rice husk was collected from the particulate collection equipment attached upstream to the stack of rice-ﬁred boilers. The ash generated got a severe disposal problem. Since rice husk ash is available in plenty and it has very potential as an adsorbent, the present study has been undertaken to report in detail the characteristics of Pb(II) adsorption in the batch process. Various kinetic models as well as isotherm models have been studied for their usefulness in correlating the experimental data. 2. Materials and methods Rice husk ash is a solid obtained from rice processing mill after burning of rice husk. Rice husk ash was collected from local rice mill, West Bengal, India. Rice husk ash, after collection it was homogenized and dried at 105 ± 5 ◦ C for 3 h and cooled to ambient temperature in a desiccators. All the necessary chemicals used in the study were of analytical grade and obtained from E. Merck India Limited, Mumbai, India. The physicochemical characterization of rice husk ash was performed using standard procedures. Characterization of the rice husk ash was carried out by surface area analysis, bulk density, particle size distribution analysis and scanning electron microscope (SEM). The surface area of the rice husk ash was measured by BET (Brunauer–Emmett–Teller nitrogen adsorption technique). The density of rice husk ash was determined by speciﬁc gravity bottle. The moisture content determination of adsorbent was carried out with a digital microprocessor-based moisture analyzer (Mettler LP16). The particle size distribution analysis was carried out using a Particle Size Distribution analyzer (Model 117.08, Malvern instruments, USA). The results of particle size distribution are shown in Table 1. To understand the morphology of adsorption of Pb(II) on rice husk ash, the samples were gold sputter coated and the scanning electron microscopic (SEM) micrograph were taken (Fig. 1) by using SEM (Model S3400, Hitachi, Japan). The structure of rice husk ash was studied using X-ray diffractograms (XRDs) obtained from an X-ray diffracto-meter (Model No. XRD 3000P, Seifert, Germany). SEM micrographs of the rice husk ash indicated that the surface was highly irregular and porous in nature. The X-ray diffraction analysis was done by using Cu K␣ as a source and Ni as a ﬁlter media, K radiation maintained at 1.52 Å (Fig. 2). The diffraction pattern conforms the peaks 2CaSiO2 , 2CaOFe2 O3 , 3CaOSiO2 , and some traces of complex phases of CaO·MgO·SiO2 with Al2 O3 and FeO. Table 2 shows the chemical composition of rice husk ash. Bulk density and surface area are reported in Table 3. The point of zero charge of the rice husk ash was determined by the solid addition method . To a series of 100 mL conical ﬂasks 45 mL of KNO3 solution (0.1 and 0.01 M) was transferred. The pH values of the solutions were adjusted by adding 0.1 M HCl or 0.1 M Table 1 Particle size distribution of the rice husk ash (250–350 m). Adsorbent (m)
Rice husk ash (%)
250–275 275–285 285–295 295–305 305–315 315–325 325–335 335–350
3.5 17.3 20.1 13.2 10.1 25.3 10.1 0.4
T.K. Naiya et al. / Journal of Hazardous Materials 163 (2009) 1254–1264 Table 3 Characteristics of rice husk ask. Adsorbent Surface area (m2 /g) Bulk density (g/cm3 ) Point of zero charge, pH Mean diameter (m)
Fig. 1. Scanning electron micrographs (SEM) of rice husk ash.
Rice husk ash 57.5 0.96 8.5 3.02 × 10−4
Atomic absorption spectrophotometer (VARIAN SPECTRA AA 55) was used to determine the Pb(II) ion content in standard and treated solutions. The pH of the solution was measured with a 5500 EUTECH pH meter using FET solid electrode calibrated with standard buffer solutions. The stock solution containing 1000 mg/L of standard Pb(II) were prepared by dissolving 1.61 g of AR grade Pb(NO3 )2 . Standard solution of particular Pb(II) concentration was prepared by proper dilution of stock standard with double distilled water. For optimization of pH, contact time, adsorbent dosage and initial concentration, batch experiments were performed at 30 ◦ C. Adsorption experiments for the removal of Pb(II) were conducted in the pH range of 2–7, adsorbent dosage 1–30 g/L, initial concentration from 3 to 100 mg/L and contact time from 0 to 300 min. pH of the solution monitored by adding 0.1 M HCl and 0.1 M NaOH solution as per required pH value. The kinetics and isotherms for the sorption data were studied under optimized condition of pH and contact time. All the experiments were done in triplicate and means of all the three are reported. Aliquots of the treated samples were ﬁltered though ﬁlter paper. The ﬁltrate was analyzed for remaining metal concentration in the sample using atomic absorption spectrophotometer as per procedure laid down in APHA, AWWA standard methods for examination of water and waste water, 1998 edition . The amount of metal adsorbed per unit mass of the adsorbent was calculated as q (mg/g) = (C0 − Cf )V/m and percent removal may be calculated as 100 × (C0 − Cf )/C0 . 3. Results and discussion 3.1. Effect of pH
Fig. 2. XRD study of rice husk ash.
NaOH solutions. The total volume of the solution in each ﬂask was made exactly to 50 mL by adding additional KNO3 solution (0.1 or 0.01 M as the case). The pH of the solutions was noted (pH0 ). 1 g of rice husk ash was added to each ﬂask. The suspensions were manually shaken and allowed to equilibrate for 48 h with occasionally shaken manually. The pH values of the supernatant liquid were noted. The difference between the initial (pH0 ) and ﬁnal pH (pHf ) values (pH = pH0 − pHf ) was plotted against pH0 . The point of intersection of the resulting curve will give the point of zero charge and reported in Table 3.
Table 2 Chemical composition of rice husk ash. Constituent
Percent by weight (%)
Loss on ignition Fe2 O3 Al2 O3 CaO MgO SiO2 Na2 O K2 O
12.2 0.6 0.3 1.4 0.5 84.3 0.4 0.2
The pH of the solution affects the charge on the surface of the adsorbents, so the change in pH also affects the adsorption process and the H+ ion concentration may react with the functional groups on the active sites on the adsorption surface. In general, adsorption of cations is favored at pH > pHPZC . The pH of the solutions has been identiﬁed as the most important variable governing metal adsorption. This is partly due to the fact that hydrogen ions themselves are strong competing ions and partly that the solution pH inﬂuences the chemical speciation of the metal ions as well as the ionization of the functional groups onto the adsorbent surfaces. In order to evaluate the inﬂuence of this parameter on the adsorption, the experiments were carried out at different initial pH values. The pH range was chosen as 2–6 in order to avoid metal hydroxides, which has been estimated to occur at pH > 6.5 for Pb(OH)2 . The effect of pH on adsorption efﬁciencies are shown in Fig. 3. Removal of Pb(II) increases with increasing solution pH and a maximum value was reached at an equilibrium pH of around 5. The same trend has also been reported in the removal of Pb(II) ions by other vegetable materials such as spent grain , Pinus sylvestris , and crop milling waste . The low degree of adsorption at low pH values can be explained by the fact that at low pH values the H+ ion concentration is high and therefore protons can compete with the lead cations for surface sites, since at low pH lead are present in solution as Pb2+ free cations. In addition when pH increases, there is a decrease
T.K. Naiya et al. / Journal of Hazardous Materials 163 (2009) 1254–1264
3.3. Effect of initial metal ion concentration The removal efﬁciency of Pb(II) ion by rice husk ash initially increased with increasing Pb(II) concentration. At lower concentrations, Pb(II) ion in the solution would interact with the binding sites and thus facilitated almost 100% adsorption. At higher concentrations, more Pb(II) ions are left un-adsorbed in the solution due to the saturation of the binding sites. This indicates that energetically less favorable sites become involved with increasing ion concentration in aqueous solution. During the ion-exchange process, the Pb(II) ion not only moved through the pores of the adsorbent mass, but also through the channels of the lattice. Diffusion was faster through pores and was retarded when the ion moves through the smaller diameter channels. The distribution coefﬁcient, Kd has been used to indicate the adsorption afﬁnity of a solid adsorbent towards a solute. In our present study, Kd may be written as Kd =
Fig. 3. Effect of pH on the adsorption of Pb(II): initial concentration, 10 mg/L; adsorbent concentration, 5 g/L; contact time, 2 h.
in positive surface charge (since the deprotonation of the sorbent functional groups could be occurs), which results in a lower electrostatic repulsion between the positively charged metal ion and the surface of rice husk ash, favoring adsorption. The process involved for Pb(II) adsorption are the following : Pb2+ + nH2 O = Pb(H2 O)n 2+
Pb(H2 O)n 2+ = Pb(H2 O)n−1 OH+ + H+ 2+
+ nH2 O = Pb(H2 O)n−1 OH + H
C0 − Ce X Ce
Here, the Pb(II) ion adsorption mainly be attributed to ionexchange reactions in the micropores of the adsorbents. Fig. 5 represents Kd as a function of Pb(II) ion concentration. The Kd values increased with the decreasing concentration of Pb(II) ion. In other words Kd values increased as the dilution of Pb(II) ion proceeds. 3.4. Effect of contact time The rate at which adsorption take place is of most important when designing batch adsorption experiments. Consequently, it is important to establish the time dependence of such systems under various process conditions. The experimental runs measuring the effect of contact time on the batch adsorption of metal solution containing 10 mg/L of Pb(II) at 30 ◦ C and initial pH value 5 is shown in Fig. 6. This result revealed that adsorption of Pb(II) is fast and the equilibrium was achieved by 1 h of contact time. Taking into
Perusal of the literature  on Pb(II) speciation shows that the dominant species is Pb(OH)2 at pH > 6.0 and Pb2+ and Pb(OH)+ at pH < 6.0. Maximum removal of Pb(II) was observed at pH 5. On further increase of pH > 6 adsorption decreases but the total lead removal increases due adsorption and also to the formation of hydroxide of lead which is precipitated [51,52]. The optimum pH value for adsorption was found to be 5 where Pb(OH)2 precipitation does not occur. 3.2. Effect of adsorbent concentration The effect of adsorbent dosage on the removal of Pb(II) ion at C0 = 10 mg/L was studied and results are represented in Fig. 4. The removal of metal ion was found to increase with an increase in adsorbent dosage from 1 to 25 g/L. The metal ion removed almost remain unchanged after adsorbent dosage 5 g/L. Increase in adsorption with increase in adsorbent dosage attributed to the availability of larger surface area and more adsorption sites. At very low adsorbent concentration, the absorbent surface become saturated with the metal ions and the residual metal ion concentration in the solution is large. With an increase in adsorbent dosage, the metal ion removal increases. For adsorbent dosage of 2.5 g/L, the incremental metal ion removal becomes very low as the surface metal ion concentration and the solution metal ion concentration comes to equilibrium with each other. For higher adsorbent dosage of 5.0 g/L, the removal efﬁciency becomes almost constant for the removal of Pb(II) ions onto rice husk ash.
Fig. 4. Effect of adsorbent concentration on adsorption of Pb(II): pH, 5; initial concentration, 10 mg/L; contact time, 2 h.
T.K. Naiya et al. / Journal of Hazardous Materials 163 (2009) 1254–1264
can be studied through the residual metal ion concentration in the solution. The study of adsorption kinetics describes the solute uptake rate and evidently these rate controls the residence time of adsorbate uptake at the solid–solution interface including the diffusion process. The mechanism of adsorption depends on the physical and chemical characteristics of the adsorbent as well as on the mass transfer process . The results obtained from the experiments were used to study the kinetics of metal ion adsorption. The rate kinetics of metal ion adsorption on rice husk ash was analyzed using pseudo-ﬁrst-order , pseudo-second-order , and intraparticle diffusion models . The conformity between experimental data and the model predicted values was expressed by correlation coefﬁcients (r2 ) and Chi-square (2 ). 3.5.1. Pseudo-ﬁrst-order model The adsorption of Pb(II) from a liquid phase to solid phase can be considered as a reversible process with equilibrium being established between the solution and solid phase. Adsorption phenomenon can be described as the diffusion control process, assuming a non-dissociation molecular adsorption of Pb(II) on rice husk particles as follows: A + S AS
Fig. 5. Effect of initial concentration on the adsorption of Pb(II): pH, 5; adsorbent concentration, 5 g/L; contact time, 2 h.
If initially no adsorbate present the adsorbent (i.e., CA S0 = 0 at t = 0), then assuming the ﬁrst-order rate kinetics the fractional uptake on the adsorbate by the adsorbent can be expressed as:
account these results, a contact time of 1 h was chosen for further experiments.
kA q = 1 − exp kA Cs + t qe ks
3.5. Adsorption kinetics study
Equation can be transformed as
Various kinetic models, namely pseudo-ﬁrst-order, pseudosecond-order and intraparticle diffusion models have been used for their validity with the experimental adsorption data for Pb(II) onto rice husk ash. With the average shaking speed of 120 rpm, it was assumed to offer no mass transfer (both external and internal external) resistance to the overall adsorption process. Therefore kinetic
log (qe − q) = log qe − where Kad =
kA Cs +
Kad t 2.303
q = XA and qe = XAe . Eq. (7) is so-called Lagergren equation . The results are depicted in Fig. 7. 3.5.2. Pseudo-second-order model The pseudo-second-order model is based on the assumption of chemisorption of the adsorbet on an adsorbent. The kinetic rate model can be represented as : dqt = k(q − qt )2 dt
Separating the variables in equation and integrating the boundary conditions, qt = 0 at time t = 0 and qt at time t, the following equation is obtained: 1 1 t = + t q qt kq2
This is a linear form equation for a pseudo-second-order reaction. The constant (k and q) can be experimentally determined from the plot (t/qt ) against t as shown in Fig. 8.
Fig. 6. Effect of contact time on the adsorption of Pb(II): initial concentration, 10 mg/L; adsorbent concentration, 5 g/L; pH, 5.
3.5.3. Intraparticle diffusion model The adsorbate transport from the solution phase to the surface of the adsorbent particles occur in several steps. The overall adsorption process may be controlled either by one or more steps, e.g. ﬁlm or external diffusion, pore diffusion, surface diffusion and the adsorption on the pore surface, or a combination of more than one steps. In a rapidly stirred batch adsorption, the diffusive mass
T.K. Naiya et al. / Journal of Hazardous Materials 163 (2009) 1254–1264
Fig. 7. Lagergren plot for the adsorption of Pb(II) by rice husk ash: pH, 5; initial concentration, 10 mg/L; adsorbent dosage, 5 g/L.
transfer can be related by an apparent diffusion coefﬁcient, which will ﬁt the experimental sorption-rate data. Generally, a process is diffusion controlled if its rate dependent upon the rate at which components diffuse towards one another. The possibility of intraparticle diffusion was explored by using the intraparticle diffusion model . qt = Kid t 0.5
If the Weber–Morris plot of qt versus t0.5 gives a straight line, then the adsorption process is controlled by intraparticle diffusion only. However, if the data exhibit multi-linear plots, then two or
Fig. 8. Pseudo-second-order plot for the adsorption of Pb(II) by selected adsorbents: pH, 5; initial concentration, 10 mg/L; adsorbent dosage, 5 g/L.
Fig. 9. Weber and Moris plot for the adsorption of Pb(II) by selected adsorbents: pH, 5; initial concentration, 10 mg/L; adsorbent dosage, 5 g/L.
more steps inﬂuence the adsorption–sorption processes. The mathematical dependence of fractional uptake of the adsorbate on t0.5 is obtained if the adsorption process is considered to be inﬂuenced by diffusion in the cylindrical and convective diffusion in the adsorbate solution. It is assumed that the external resistance to mass transfer surrounding the particles is signiﬁcant only in the early stage of the adsorption. The ﬁrst stripper portion represents this. The second linear portion is the gradual adsorption stage with intraparticle diffusion dominating. In Fig. 9 the data points are related by two straight lines—ﬁrst straight portion depicting the macropore diffusion and second representing the micropore diffusion. These show only the pore diffusion data. Extrapolation of the linear portion of the plots to the Y-axis gives the intercepts, which provide the boundary layer thickness. The deviation of straight lines from the origin (Fig. 9) may be due to difference in rate of mass transfer in the initial and ﬁnal stage of adsorption. Further, such deviation of straight line from the origin indicates that the pore diffusion is not sole rate-controlling step. The adsorption data for q versus t0.5 for the initial period usually attributed to boundary layer diffusion effects or external mass transfer effects . The slope of Weber and Morris plots q versus t0.5 are deﬁned as a rate parameter, characteristics of the rate of adsorption in the region where intraparticle diffusion is the rate controlling. In order to quantify the applicability of each model, the correlation coefﬁcients, r2 , was calculated from these plots. The linearity of these plots indicates the applicability of these three models. The values of rate constants and correlation coefﬁcients for each model are shown in Table 4. However, the correlation coefﬁcients, r2 , showed that the pseudo-second-order model, an indication of chemisorptions mechanism, ﬁts better with the experimental data than the pseudo-ﬁrst-order model. In addition, the Chi-square test was also done to support the best-ﬁt adsorption model. The equation for evaluating the best-ﬁt model is to be written as 2t =
(qt − qtm )2 qtm
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Table 4 Rate Kinetics for adsorption of Pb(II) by rice husk ash. Lagergren-ﬁrst-order Kad (min−1 ) r2 2
0.1144 0.989 0.464
higher compared to the volume of the rice husk ash particle and if the concentration of metal ions is assumed constant from the surface to the centre of the adsorbent particle the previous equation can be expressed as
Pseudo-second-order K2 (g mg−1 min−1 ) r2 2
0.2670 0.999 0.234
Weber and Moris Kid (mg g−1 min−0.5 ) r2 2
0.2043 0.9275 0.487
Bangham k0 (L/g) ˛ r2
19.6 2.45 0.795
3.5.4. Mass transfer analysis Mass transfer analysis for the removal of Pb(II) from aqueous solutions by rice husk ash were carried out using the following equation as proposed by McKay et al. :
Ct 1 − C0 1 + MKbq
MKbq 1 + MKbq
1 + MKbq MKbq
1 1 − F 2 (t)
2 De t r2
where plot of ln[1/(1 − F2 (t))] versus t provide a line from whose slope 2 De /r2 the diffusion coefﬁcient, De can be calculated. The value of diffusion coefﬁcient as calculated from the equation was found to be 2.509 × 10−10 m2 /s for the adsorption of Pb(II) onto rice husk ash. The pores of rice husk ash have different sizes along its length and adsorbent has a wide pore size distribution. The adsorbent–adsorbet and adsorbet–adsorbet interactions have their impact on the diffusion process and affect the value of De . The properties of the adsorbents pore size along the length of pore, orientation, electronic ﬁeld and the interaction of the adsorbet–van der Waals’ attractive forces, surface diffusion characteristics and adsorption mechanism, all affect the diffusion. Meso-pores are found to occupy most of the pore length of rice husk ash. The diffusion within the pores of wider path and weaker retarding forces of electrostatic interaction accounts for the greater De and one within the pore of narrower mesh widths and stronger retarding forces accounts for lower De . For the present system, the value of De , fall well within the values reported in the literature, specially for chemisorptions system (10−9 –10−17 m2 /s) . 3.5.6. Reichenberg model The rate of sorption is determined by applying well-known equation for the diffusion and mass transfer phenomena. For the fast reaction, the sorption may be due to ﬁlm diffusion  and occur within the micropores of the adsorbent. In that case Reicherberg equation is applied, i.e.:
ˇSs t (13)
3.5.5. Determination of diffusivity Kinetic data could be treated by the models given by Boyd et al.  which is valid for the experimental conditions used. Diffusion found to be rate controlling in the adsorption of Pb(II) onto the particles of spherical shape. As the volume of the solution is much
i2 De t2 r2
Applicability of Vermeulen’s approximation is limited and highly depend on the ratio of the initial metal ion concentration in rice husk ash (C0 )a and in the solution C0 and of the volume of adsorbent (Va ) and the volume of the solution (Vs ). For the criterion of ‘inﬁnite solution volume’ that is given by the ratio (C0 )a Va C0 (M)Vs , where the concentration of metal ion in the solution remains negligible throughout the process, and for the range 0 ≤ F(t) ≤ 1 in the solution of divalent exchangeable ions, Eq. (14) can be simpliﬁed  as
The plot of ln((Ct /C0 ) − (1/(1 + MKbq ))) versus t results a straight line of slope [((1 + MKbq )/MKbq )ˇSs ] and the values of mass transfer coefﬁcients (ˇ) calculated from the slopes of the plots was 4.45 × 10−5 cm/s with a high value, 0.9837, of co-relation coefﬁcient. The values of mass transfer coefﬁcients (ˇ) obtained from the study indicate that the velocity of the adsorbate transport from bulk to the solid phase was quite fast.
It has been found that 2 values are much less in pseudo-secondorder model than that of pseudo-ﬁrst-order and intraparticle diffusion model (Table 4). Thus based on the high co-relation coefﬁcient and low 2t value, it can be said that adsorption of Pb(II) onto rice husk ash follow pseudo-second-order model than that of intraparticle diffusion model. The intraparticle diffusion was also involved in the adsorption process of Pb(II) ions by rice husk ash. When pore diffusion limits the adsorption process, the relationship between the initial solute concentration and the rate of adsorption will not be linear . Besides for the adsorption on the outer surface of adsorbent, there is also possibility of transport of adsorbent ions from the solution to the pores of the adsorbent due to stirring on batch process. This possibility was tested in terms of intraparticle diffusion model. The linear portion of the plot for a wide range of contact time between the adsorbate and adsorbent does not pass through the origin. This deviation from the origin or near saturation may be perhaps due to difference in the rate of mass transfer in the initial and ﬁnal stages of adsorption. Further such deviation from the origin indicated that the pore diffusion is not the rate-limiting step. From Fig. 9 it may be seen that there are two distinct regions—the initial pore diffusion due to external mass transfer effects followed by the intraparticle diffusion. This indicates the mechanism of Pb(II) adsorption by rice husk ash is complex and both, the surface adsorption as well as intraparticle diffusion contribute to the rate determining step.
6 1 exp 2 i2 ˛
F(t) = 1 −
Above equation may be written as Bt = −0.4977 ln (1 − F(t))
The plot of Bt versus time is linear with a correlation factor of 0.9904 thereby indicating that sorption was controlled by ﬁlm diffusion. 3.5.7. Bangham’s equation Whether pore diffusion is the only rate-controlling step or not in the adsorption system is to be checked for kinetic data using Bangham’s equation :
C0 C0 − qt m
k m 0 2.303V
+ ˛ log (t)
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Table 5 Langmuir and Freundlich adsorption isotherm constants for Pb(II) on rice husk ash. Langmuir constants qmax (mg/g) b (L/mg) r2 2
91.74 0.109 0.9412 0.531
Freundlich constants Kf ((mg/g)/(mg/L)1/n ) n r2 2
8.366 1.331 0.9986 0.762
The RL value indicates the shape of the isotherm as follows:
Fig. 10. Langmuir plot for the adsorption of Pb(II) by selected adsorbents: pH, 5; adsorbent dosage, 7.5 g/L; contact time, 1 h.
where ˛ (<1) and k0 are constants. If the experimental data are represented by Eq. (18), then it is an indication that the adsorption kinetics is limited by pore diffusion. However above equation does not give a good ﬁt of the experimental data, indicating thereby that the diffusion of adsorbate into the pores of the adsorbent is not solely rate-limiting step. . With increase in contact time, the effect of diffusion process on overall adsorption could be ignored. Values of Bangham parameters, co-relation coefﬁcient and error function values are given in Table 4. 3.6. Adsorption isotherms The adsorption isotherm for the removal of metal ion was studied using initial concentration of between 10 and 300 mg/L at an adsorbent dosage level of 5.0 g/L for Pb(II) 30 ◦ C. The adsorption equilibrium data are conveniently represented by adsorption isotherms, which correspond to the relationship between the mass of the solute adsorbed per unit mass of adsorbent qe and the solute concentration for the solution at equilibrium Ce . 3.6.1. Langmuir adsorption isotherm The data obtained were then ﬁtted to the Langmuir adsorption isotherm  applied to equilibrium adsorption assuming monolayer adsorption onto a surface with a ﬁnite number of identical sites and is represented as follows: Ce Ce 1 + = qe qmax qmax b
Linear plot of Ce /qe versus Ce in Fig. 10 was employed to determine the value of qmax (mg/g) and b (L/mg). The data obtained with the correlation coefﬁcients (r2 ) was listed in Table 5. Weber and Chakraborti  expressed the essential characteristics and the feasibility of the Langmuir isotherm in terms of a dimensionless constant separation factor or equilibrium parameter, RL , which can be deﬁned as RL =
1 1 + bC0
Type of isotherm
RL > 1 RL = 1 0 < RL < 1 RL = 0
Unfavorable Linear Favorable Irreversible
According to McKay et al. , RL values between 0 and 1 indicate favorable adsorption. The RL value for the adsorption on rice husk ash at initial concentration of 10 mg/L (lowest concentration studied) and 300 mg/L (highest concentration studied) are 0.480 and 0.029, respectively. The data obtained represent a favorable adsorption. 3.6.2. Freundlich adsorption isotherm The adsorption data obtained were then ﬁtted to the Freundlich adsorption isotherm , which is the earliest relationship known describing the adsorption equilibrium and is expressed by the following equation, log qe = log Kf +
1 log Ce n
The Freundlich isotherm constants Kf and n are constants incorporating all factors affecting the adsorption process such as of adsorption capacity and intensity of adsorption. The constants Kf and n were calculated from Eq. (21) using Freundlich plots as shown in Fig. 11. The values for Freundlich constants and correlation coefﬁcients (r2 ) for the adsorption process are also presented in Table 5. The values of n between 1 and 10 (i.e., 1/n less than 1) represent a favorable adsorption. The values of n, which reﬂects the intensity of adsorption, also reﬂected the same trend. The n values obtained for the adsorption process represented a beneﬁcial adsorption. For the adsorption isotherm studies Chi-square, 2 test are also carried out 2e =
(qe − qem )2 qem
From Table 5, it is seen that experimental data are better ﬁtted to Langmuir (r2 = 0.9986, 2 = 0.762) than Freundlich (r2 = 0.9412, 2 = 0.531) adsorption isotherm. Therefore uptake of Pb(II) ion preferably follows the heterolayer adsorption process. 3.6.3. Dubinin–Radushkevich (D–R) isotherm The D–R isotherm  was employed in the following linear form: ln Cabs = ln Xm − ε2
The Polanyi potential , ε, can be expressed as, ε = RT ln
A plot of Cabs versus ε2 gave a straight line from which values of and Xm for Pb(II) was evaluated. Using the calculated value of ,
T.K. Naiya et al. / Journal of Hazardous Materials 163 (2009) 1254–1264 Table 6 Batch adsorption studies using rice husk ash as metal adsorbent for removal of Pb(II) from aqueous solution at different initial concentration: effect of temperature, experimental condition: pH 5, adsorbent concentration 5 g/L, and contact time 1 h. Temperature (◦ C)
% removal of Pb(II)
30 40 50
98.82 99.45 99.76
97.95 98.4 98.86
94.85 95.3 96.02
Fig. 11. Freundlich plot for the adsorption of Pb(II) by selected adsorbents: pH, 5; adsorbent dosage, 7.5 g/L; contact time, 1 h.
it was possible to evaluate the mean sorption energy, E, from 1 E= √ −2
Although the Freundlich isotherm provides the information about the surface heterogeneity and the exponential distribution of the active sites and their energies, it does not predict any saturation of the surface of the adsorbent by the adsorbate. Hence, inﬁnite surface coverage could be predicted mathematically. In contrast, D–R isotherm relates the heterogeneity of energies close to the adsorbent surface. If a very small sub-region of the sorption surface is chosen and assumed √ to be approximately by the Langmuir isotherm, the quantity can be related to the mean sorption energy, E, which is the free energy for the transfer of 1 mole of metal ions from the inﬁnity to the surface of the adsorbent. The estimated value of E was 9.901 kJ/mol, which is the range expected for chemisorptions (8–16 kJ/mol) .
Fig. 12. Dubinin–Radushkevich isotherm of Pb(II) onto selected adsorbents: pH, 5; adsorbent dosage, 7.5 g/L; contact time, 1 h; temperature, 30 ± 2 ◦ C.
3.7.2. Effect of temperature on thermodynamics parameter on adsorption of Pb(II) The variation in the extent of adsorption with respect to temperature has been explained based on thermodynamic parameters viz. changes in standard free energy, enthalpy and entropy. The dependence on temperature of adsorption of Pb(II) on the rice husk ash were evaluated using s, ln KC = −
S 0 H 0 + RT R
and 3.7. Adsorption thermodynamics 3.7.1. Effect of temperature on adsorption of Pb(II) To study the effect of temperature adsorption experiments are carried out at 30–50 ◦ C at optimum pH value of 5 and adsorbent dosage level of 5 g/L. The equilibrium contact time for adsorption was maintained at 1 h. The percentage of adsorption increases with rise of temperature from 30 to 50 ◦ C. The results were shown in Table 6 and it revealed the endothermic nature of the adsorption process which later utilized for determination of changes in Gibbs free energy (G0 ), heat of adsorption (H0 ) and entropy (S0 ) of the adsorption of Pb(II) from aqueous solutions. The increase in adsorption with rise in temperature may be due to the strengthening of adsorptive forces between the active sites of the adsorbents and adsorbate species and between the adjacent molecules of the adsorbed phase.
G0 = −RT ln KC
From the slope and intercept of the plot (Fig. 12), the values of H0 and S0 had been computed, while G0 was calculated using Eq. (27). The values of these parameters thus calculated are recorded in Table 7. It may be concluded from the positive values of H0 that the sorption process is endothermic while the positive value of S0 Table 7 Batch adsorption studies using rice husk ash as metal adsorbent for removal of Pb(II) from aqueous solution at different initial concentration: changes in Gibbs free energy(G0 ), heat of adsorption(H0 ) and entropy(S0 ). H0 (kJ/mol) 28.923
S0 (kJ/(mol K))
303 313 323
9.233 12.369 13.699
T.K. Naiya et al. / Journal of Hazardous Materials 163 (2009) 1254–1264 Table 8 Comparison of adsorption capacities of the adsorbents for the removal of Pb(II) with those other adsorbents. Sl. no. 1 2 3 4 5 6 7 8 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
Adsorption capacities, mg/g Pb(II)
Barley straw Waste tea leaves Sago waste Peanut Hulls Hazel nut shell Saw dust Saw dust Neem bark Rice husk ash (thermally treated at 300 ◦ C) Orange peels Cocoa shells Tree fern Coffee residue Rice husk Palm kernel ﬁber Olive stone waste Modiﬁed orange peels Grape stalk Coir Coir Tea waste Baggase ﬂy ash Rice husk ash
15.2 65.0 47.00 30.43 1.78 3.00 88.49 83.33 12.3
Reference         
4.0 33.0 40.0 20.0 11.0 49.9 9.15 252.78 49.89 18.6 48.83 65.1 2.50 91.74
             Present work
is an indicative of increased randomness at the adsorbent–adsorbet interface during the adsorption. The negative value of G0 conﬁrms the feasibility of the process and spontaneous nature of the adsorption process. 3.8. Comparison of adsorption capacity with different adsorbents reported in literature The adsorption capacities of the adsorbents for the removal of Pb(II) have been compared with those of other adsorbents reported in the literature and the values of adsorption capacities have been presented in Table 8. The values reported in the form of monolayer adsorption capacity. The experimental data of the present investigations are comparable with the reported values. 3.9. Application studies using industrial efﬂuents An efﬂuent sample from a battery manufacturing unit at Shyamnagar, near Kolkata, India was used for application studies for removal of Pb(II) in waste water matrix. The characteristics of efﬂuent samples were shown in Table 9. The batch adsorption study was carried under optimum conditions of pH 5, contact time 1 h and adsorbent dosage level of 5 g/L. Adsorption of Pb(II) on rice husk ash was found to 96.83% and it meets the IS 10500 of 1992 norms for discharge water . Table 9 Application study of the removal of Pb(II) from waste water using rice husk ash. Test parameter
pH Conductivity (mho s/cm) Pb (II) (mg/L)
2.7 1737 2.84
5 1695 0.09
1.2 214 64 18 26
0.87 194 60 15 24
– – Successfully meet the IS 10500 norms  – – –
Fe (mg/L) Ca (mg/L) Mg (mg/L) Chloride (mg/L) TSS (mg/L)
4. Conclusions In this study, batch adsorption experiments for the removal of Pb(II) from aqueous solutions have been carried out using rice husk ash as low cost, readily available adsorbent. The adsorption characteristics have been examined at different pH values, initial metal ion concentrations, contact time and adsorbent dosages. The obtained results can be summarized as follows: (1) The pH experiments showed that the governing factors affecting the adsorption characteristics of all adsorbents are competition of the H+ ions with metal ions at low pH values, maximum adsorption at pH 5 and at higher pH precipitation of hydroxyl species onto the adsorbents (pH 6–11). (2) Increase in mass of adsorbent leads to increase in metal ion adsorption due to increase in number of adsorption sites. Maximum uptake was obtained at adsorbent dose of 5 g/L, which may be considered as optimum adsorbent dosage level at the speciﬁed conditions. (3) The equilibrium time for adsorption of Pb(II) from aqueous solutions was achieved within 1 h of contact time. (4) At optimum conditions of pH, contact time and adsorbent dosage level removal efﬁciency of 99.3% for Pb(II) adsorption. (5) The experimental data were better described by pseudosecond-order model as evident from correlation coefﬁcient and Chi-square values (r2 and 2 ). (6) The effective diffusivity calculated using Vermeulen’s approximation was found to be 2.509 × 10−10 m2 /s which also indicate that the interaction between Pb(II) and rice husk ash is chemical in nature. (7) The Langmuir adsorption isotherm model was better used to represent the experimental data. The monolayer adsorption capacity was obtained 91.74 mg/g for rice husk ash. (8) Sorption energy for adsorption process were found to be 9.901 kJ/mol which is also indicated that it is the chemical adsorption phenomena responsible for the removal of Pb(II) from waste water. (9) Thermodynamic parameters studies showed that the Pb(II) adsorption were spontaneous in nature. The value of H0 and S0 were found to be 28.923 kJ/mol and 0.132 kJ/mol K, respectively. The positive value of H0 indicate the endothermic nature of the process while a positive value of S0 suggested an increase randomness at the solid–solution interface during the adsorption of Pb(II) onto rice husk ash. (10) Application study using waste water from a battery industry containing Pb(II) ion concentration along with other ions showed the removal efﬁciency of 96.83 ± 0.3%. Final Pb(II) concentration followed the IS 10500 norms for waste water discharge. (11) The study indicated the suitability of the rice husk ash for waste water treatment application.
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