Pb(II) Sorption under Batch and Continuous Mode ... - ACS Publications

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Ind. Eng. Chem. Res. 2008, 47, 1026-1034

Pb(II) Sorption under Batch and Continuous Mode Using Natural, Pretreated, and Amino-Modified Ectodermis of Opuntia L. A. Bernal-Martıńez,† S. Herna´ ndez-Lo´ pez,† C. Barrera-Dıáz,*,† F. Uren˜ a-Nu´ n˜ ez,‡ and B. Bilyeu§ Facultad de Quı´mica, UniVersidad Auto´ noma del Estado de Me´ xico, Paseo Colo´ n Interseccio´ n Paseo Tollocan S/N. C.P. 50120, Toluca, Estado de Me´ xico, Me´ xico, Instituto Nacional de InVestigaciones Nucleares, A.P.18-1027, Col. Escando´ n, Delegacio´ n Miguel Hidalgo, C.P. 11801, D.F., Me´ xico, and Department of Materials Science and Engineering, UniVersity of North Texas, P.O. Box 305310, Denton, Texas 76203-5310

This work presents the conditions for Pb(II) removal from aqueous solution using natural, pretreated, and amino-modified Ectodermis of Opuntia. The sorbent materials were characterized using Scanning Electron Microscopy, Fourier Transformed Infrared spectroscopy, Thermogravimetric Analysis (TGA) and UV-vis spectrometry, before and after contact with aqueous Pb(II) solutions. Pretreated Ectodermis of Opuntia with formaldehyde was the best material for the sorption of Pb(II). The Pb(II) uptake process was at a maximum at pH 5.0, which showed an adsoption capacity that was adequately described by a Langmuir adsorption isotherm. The Metcalf-Eddy model was used to describe the adsorption data from column studies; the sorption capacity was 58.46 mg Pb(II)/g for pretreated Ectodermis of Opuntia with 96% removal. 1. Introduction Heavy metal contamination of water is a serious threat to the global ecosystem. Strict environmental protection legislation and public environmental concerns drive the search for novel techniques to remove heavy metals from industrial wastewater. Although many techniques have been developed, most are expensive or difficult to implement. Adsorption is a traditional technique for water treatment,1 but the ion exchange resins tend to be expensive, so recent interest has developed in biosorption,2 using cellulose-based plant material to adsorb metal ions. Preliminary economic feasibility studies indicate these natural adsorbents have several advantages over existing technologies. Biosorbents are inexpensive,3 easy to chemically modify for increased efficiency,4 suffer fewer environmental interferences, can be used to recover valuable metals, and can easily be adapted to existing filtration systems. Agricultural waste is one of the richest sources for low-cost biosorbents. Agricultural wastes such as Opuntia (prickly pear cactus) peel possess little economic value and create disposal problems. Mexico is the largest producer of Opuntia in the world, with an annual production of 500 000 tons.5 Many plant materials can leach organic compounds like chlorophyll and carotene into aqueous solutions.6 In using these materials for metal sorption, this organic leachate produces a secondary pollutant in the water. Therefore, the material must be chemically treated to remove the organic compounds.7 Generally, acid-treatment has been used for replacing the natural mix of ionic species bound on the wall with protons and sulfates. The alkali-treatment causes breakage of the cellulose polymers, thereby hindering the operational stability of the biomass.8 Favorable effects of alkali-treatment have been reported for cation adsorption,9 such as the enhanced adsorption of Pb(II) by Saccharomyces uVarun biomass after boiling with NaOH.10 Studies show that CaCl2 pretreatment is the most suitable and * To whom correspondence should be addressed. Tel.: + (52)(722)-2173890. Fax: + (52)-(722)-2175109. E-mail: cbarrera@ uaemex.mx. † Facultad de Quı´mica, Universidad Auto ´ noma del Estado de Me´xico. ‡ Instituto Nacional de Investigaciones Nucleares. § Department of Materials Science and Engineering, University of North Texas.

economic method for the activation of algal biomass. CaCl2 pretreatment increases the Pb sorption capacity of Spirulina maxima by 84-92%.11,12 Metal ions have been shown to bind to carboxylate, hydroxyl, sulfate, phosphate, amide, and amino functional groups.13,14 Lignocellulosic materials have been chemically modified in different ways to increase the density of the effective functional groups for sorption.15 Grafting amine groups to substrates increases the capacity for metal sorption.16,17 Phosphate-treated sawdust shows a remarkable increase in metal-ion sorption capacity as compared to untreated sawdust.18 Polyacrylamidegrafted coconut coir pith, having -NH3+-Cl- as a functional group, works as an anion exchanger for removing metal ions.16 Polyethylenimine-modified fungal biomass shows a high sorption capacity for heavy metals.17 It has also been observed that the amination of the carboxylic groups of Ecklonia sp. significantly increased the metal ion removal rate.19 Natural, thermally treated, and protonated Ectodermis of Opuntia sp. have been previously used for the removal of ions from synthetic solutions.20 In this work, the utility of natural, formaldehyde-modified, and copolymer-grafted Ectodermis of Opuntia for the removal of lead ions from aqueous solution has been explored. The Ectodermis of Opuntia contains cellulose and hemicellulose fibers. Cellulose and hemicellulose contain carboxyl groups capable of coordinately binding with transition metal cations. However, the adsorbent can be modified to enhance metal adsorption. The modification of sorbents by grafting has been proposed as a way to improve sorption properties: for example Denizli et al. (1997), incorporate a dye-ligand onto a synthetic polymer to enhance its metal sorption capacity.21 Alternatively, raw biosorbents tend to leach large amounts of organic compounds during water treatment, as indicated by the green or yellow color of the water after biosorption. In order to prevent this phenomenon, a biosorbent should be conditioned before its use.22 The effectiveness of the sorbents for lead (II) adsorption was evaluated, along with the effect of pH and initial metal ion concentration. The kinetics of the isothermal rate of adsorption was measured and compared to Langmuir and Freundlich models.

10.1021/ie070861h CCC: $40.75 © 2008 American Chemical Society Published on Web 01/17/2008

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Figure 1. Chemical modification: the acrylamide with amine groups.

Figure 2. Synthesis of copolymer Poly-(NAAm-co-MAA).

To determine the practical applicability of the adsorbent for column operations, some dynamic parameters necessary for a factual design model must be determined. Laboratory fixedbed reactors were used to investigate the effect of a contaminantsorbent mixture on the breakthrough characteristics.23 Finally, the materials were characterized using scanning electron microscopy (SEM) for morphology, Fourier Transform Infrared (FT-IR) spectroscopy for chemical functional groups, and Thermogravimetric Analysis (TGA) for thermal reduction. 2. Materials and Methods 2.1. Biosorbent Preparation. The Ectodermis of Opuntia sp. was collected in the northern part of the Estado de Me´xico. It was rinsed with distilled water to remove dirt and then sun dried for 7 d. The dry biomass was crushed, milled, and sieved through a No. 20 mesh and then stored in a desiccator. This material is designated as the natural biomass (NB). 2.2. Biosorbent Pretreatment. A 10-g portion of NB was treated in 1 L of 0.2% (v/v) formaldehyde solution for 24 h with continuous stirring. The solid biomass was filtered, rinsed with distilled water, and then dried at 18 °C for 48 h, then stored in a desiccator. The resulting formaldehyde-treated biomass is designated as the pretreated biomass (PB). 2.3. Chemical Modified of Biosorbent. The chemical modification of the Ectodermis of Opuntia sp. consisted of four steps: (1) acidification of the biomass, (2) introduction of amino groups to the acrylamide monomer, (3) copolymerization of the modified acrylamide with methacrylic acid (MAA), and (4) the biomass-copolymer grafting. The acidified biomass (step 1) was prepared by mixing 10 g of NB with 10 mL of concentrated (98%) acetic acid in an ultrasonic processor Ultrasonik 28X at 50-60 Hz for 30 min. Afterward, the solid biomass was separated from the acid solution by decanting and dried at 50 °C for 24 h. To functionalize the acrylamide monomer (step 2), 0.1 mol (6 g) of the monomer (AAm) was reacted with 0.1 mol (6 g) of ethylenediamine (en)2, using 0.3 mL of concentrated HCl as a catalyst and 15 mL of ethanol as the solvent. The reaction mixture was refluxed for 4 h with vigorous stirring. The solid product (Figure 1) was then washed with ethanol, filtered, and dried under vacuum for 24 h. The copolymer Poly-(acrylamide-co-methacrylic acid) was synthesized (step 3) by reacting 0.7 mol (5 g) of N-(2-

aminoethyl)-acrylamide (NAAm) and 0.3 mol (1.6 g) of methacrylic acid (MAA) with 2 mL of N-methylpirrolidone (NMP) as the solvent and 0.2 mL of a 0.2M solution of benzoyl peroxide (BPO) in acetone as the initiator. The reaction was heated at 70 °C under a nitrogen atmosphere for 24 h (Figure 2). The precipitate was washed in acetone, filtered, and dried under vacuum for 24 h. The reaction yield was 90%. The grafting process (step 4) of Poly-(NAAm-co-MAA) onto the biomass was performed via a condensation reaction between the hydroxyl groups of the biomass and the carboxylic acid groups of the copolymer, as shown in Figure 3. For this, 10 g of acidified biomass (from step 1) was mixed with a solution of 5 g of copolymer (from step 3) in 25 mL of NMP with 0.5 mL of sulfuric acid catalyst. The mixture was refluxed at 60 °C for 24 h, then washed with warm water to remove the unreacted copolymer. The grafted product was filtered and dried under vacuum for 24 h. The reaction yield was 85%. The resulting grafted biomass is designated as the modified biomass (MB) in this article. 2.4. Biosorbent Characterization. The NB, PB, and MB were characterized using the following techniques: 2.4.1. Scanning Electron Microscopy (SEM): Topography and elemental analysis of certain features were performed on a Philips XL-30 microscope with energy dispersive X-ray spectroscopy (EDS). Samples were mounted onto metal holders using a conducting substrate. 2.4.2. FTIR: The FTIR spectra were collected using a Nicolet AVATAR 360 performing 64 scans with a resolution of 4 cm-1. The biomass samples were pressed in KBr disks. 2.4.3. Thermogravimetric Analysis (TGA): The thermogravimetric analyses were carried out in an SDT Q600 TA Instruments analyzer, which was operated under a nitrogen atmosphere at a heating rate of 25 °C/min from 25 to 1000 °C. 2.4.4. UV-vis Spectrometry: UV-vis spectra were obtained from samples of raw and treated biomass using a double-beam Perkin-Elmer 25 spectrophotometer. The samples were scanned at 960 nm/s within the 800-200 nm wavelength range in quartz cells with a 1-cm optical path. 2.5. Metal Detection in Aqueous Solution. Pb(II) aqueous solutions were prepared using a standard Pb(NO3)2 salt. The concentration of Pb(II) before and after the sorption process was determined by atomic absorption (AA) spectrometry using

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Ind. Eng. Chem. Res., Vol. 47, No. 4, 2008

Figure 3. Graft copolymer Poly-(NAAm-co-MAA) onto natural biomass (NB).

a SOLAAR AA S GE710751v, following the AWWA standard method.24 All concentration measurements were duplicated. 2.6. Batch Biosorption Study. Adsorption studies were carried out by batch technique to obtain rate and equilibrium date. The batch equilibrium tests were conducted at constant temperature (18 °C). A series of test tubes with 120 mL of 30 mg/L Pb(II) solution and 120 mg of sorbent were agitated for specific times ranging from 10 min to 3 h, to determine the kinetics. The pH was maintained at 5.0 ( 0.5 during the process. At specific time intervals, the solutions were centrifuged to separate the solid adsorbent from the supernatant. The adsorbent was dried and characterized using SEM, whereas the supernatant was analyzed for aqueous lead concentration using the atomic absorption method. 2.7. Kinetic Models. Kinetic data were analyzed with the pseudo-first-order and pseudo-second-order equations.25 2.8. Adsorption Isotherms. Experimental data were fit to Langmuir and Freundlich models. The adsorption isotherms for the biomasses were obtained using 5, 10, 20, 30, 40, and 50 mg/L of Pb(II) concentrations. When the optimal biomass was found, high concentrations of Pb(II) aqueous solution were also tested (100, 150, 200, 250, 300, 350, 400) as the lead-removal capacity was high. 2.9. Column Studies. A glass column (1-cm diameter) was filled with 0.5 g (dry weight) of hydrated biomass PB for each experiment. The sorbent was hydrated by adding 0.5 g of dry PB to a beaker of distilled water and mixing for 10 min. In each column study, the aqueous lead solution of specific concentration was percolated through the hydrated sorbent column by gravity at a flow rate of 1 mL/min. Metal concentrations of 250 and 400 mg/L were used at pH 5.0. Pb(II) concentration was monitored by atomic absorption at the exit of the column and the process was stopped when the exit concentration equaled the influent concentration, which indicated exhaustion of the sorbent. The column capacity and the capacity at complete exhaustion were determined by reported procedures.26,27 All experiments were conducted in duplicate. The exhausted sorbent was characterized using SEM. 3. Results and Discussion 3.1. Pretreatment of Ectodermis of Opuntia sp. The Ectodermis of Opuntia sp. contains organic compounds, such as carbohydrates and pigments, which inevitably leach into the aqueous solution during the biosorption. In this study, NB turned the aqueous solution brown or yellow due to

leaching, so the biomass was treated with formaldehyde to extract the organics. The pretreatment with formaldehyde solution (0.2% v/v) resulted in an 8% weight loss due to removal of organic compounds, but increased the effectiveness toward Pb(II) removal. UV-vis spectra results are explained in section 3.2.4. 3.2. Biosorbent Characterization. 3.2.1. Scanning Electron Microscopy (SEM): SEM provides information about the morphology, the surface texture and elemental composition of the Ectodermis of Opuntia in its natural, pretreatment, modified biosorbent, and post-sorption states. The micrographs in Figure 4 show the surface morphologies and features of the (a) NB, (b) PB, and (c) MB. The surface of the NB in Figure 4a shows a heterogeneous structure with inorganic particles irregularly dispersed in the organic matrix. After treatment with formaldehyde, the morphology of the PB in Figure 4b is more homogeneous as a consequence of the reaction between the hydroxyl groups of the cellulose with formaldehyde. The MB in Figure 4c shows a regular pattern of inorganic components bound at the active sites of the amino-copolymer grafted onto the biomass, giving a homogeneous structure. Elemental analysis can also be performed during SEM by EDS analysis. As shown in Figure 4a, carbon, oxygen, calcium, and potassium are the main elements present in the natural Ectodermis of Opuntia. After the pretreatment, the peak intensities of Ca and K decrease, while Al and S are detected, as shown in Figure 4b. Figure 4c shows the modified biosorbent has Al, S, and Ca. Aluminum appears because the tape of the sample holder is made of aluminum. 3.2.2. Infrared Spectroscopy Analysis (FTIR): Spectra are shown in Figure 5 in the range of 4000-400 cm-1 for the different types of biomass. The frequencies for the most important peaks are presented in Table 1. The FTIR spectra is a useful tool to identify functional groups in a molecule, as each specific chemical bond often has a unique energy absorption band, this technique also gives structural and bond information on a complex.28 For NB (Figure 5), peaks at 3432 and 1000-1100 cm-1, are indicative of the -OH stretching (primary and secondary) and C-OH bonding stretching vibrations from cellulose and lignin, 2922 and 1447-1384 cm-1 correspond to -CH2- from cellulose; 1320 cm-1 is very characteristic of carbohydrates, 1613 cm-1 corresponds to CdC from the lignin and 1750 cm-1 was assigned to a carbonyl group from ester due to others organic components beside lignin and cellulose.


Figure 4. Scanning electron microscopy micrographs (500× magnification) of the (a) NB, (b) PB, and (c) MB.

In the spectrum for PB, the peak in 3432 signal decreased considerably. This indicates that the formaldehyde in the conditioning process reacts with the hydroxyl and aldehyde groups in the biomass-producing acetal groups.4 The presence of acetal groups is confirmed in Figure 5, peaks 1050-1150 cm-1. However, this signal is difficult to note because there are contributions of C-OH vibrations from unreacted hydroxyl groups from cellulose and lignin. Nevertheless, a small change in intensity and splitting of the signals in this range indicates the chemical changes in the biomass. This reaction increases the stability the biomass and the homogeneity in the morphology (Figure 4). The FTIR spectra of Poly-(NAAm-co-MAA) and MB is also presented in Figure 5. The signal of 3394-2450 cm-1 is indicative of the -COOH in the copolymer. It is possible to distinguish two bonds: one at 3382 cm-1 and a smaller at 3196

cm-1. The first one includes the carboxylic acid -OH vibration, the amine and amide N-H groups. The smaller is an evidence of the primary amine (NH2) group in the copolymer. The signal at 1660 cm-1 was assigned to the carbonyl group from the amide and carboxylic acid. The signal at 1552 cm-1 corresponds to N-CdO stretching and H-bonded vibration from amide group. The spectrum of MB (Figure 5) is a combination of signals from both 1458 and 1407 cm-1, which corresponds to -CH2and -CH3 groups from the biomass andcopolymer. It is possible to distinguish those signals corresponding to grafting. First, the band at 3429 cm-1 is assigned to -OH and NH vibrations, carboxylic acid groups are not evident in this range. The signal at 1674 cm-1 corresponds to the carbonyl from the amide group and the shoulder at 1732 cm-1 corresponds to an ester group resulting from the condensation reaction (Figure 3).

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Figure 6. Thermogravimetric analysis of the biomasses NB, PB, and MB.

Figure 5. Infrared spectroscopy analysis (FTIR) spectra of NB, PB, MB, and poly-(NAAm-co-MAA). Table 1. Functional Groups of the Ectodermis of Opuntia (NB, PB, and MB) and the Corresponding Infrared Absorption Frequenciesa frequency (cm-1)

assignment

biosorbent

3400-3450 3300-3500 2840-3000 1725-1755 1590-1650 1450-1475 1400-1470 1320 1200-1400 1000-1200

-OH N-H C-H CdO -NH2 -CH2 -CH3 carbohydrate groups C-OH C-O

NB,PB,MB MB NB, PB, MB MB MB MB MB NB, PB, MB NB,PB,MB NB, PB, MB

a Note: The data of Infrared Absorption Frequencies were obtained from Pretsch E. et al.29

3.2.3. Thermogravimetric Analysis (TGA): In Figure 6, the thermogravimetric behavior of the Ectodermis of Opuntia NB, PB, and MB is shown. The TGA curves indicate that the thermal

stability of MB and PB is higher than that of NB. The decomposition temperature of NB was 218 °C, whereas that of PB was 226 °C, and that of MB was the highest at 234 °C. All samples showed a moisture loss below 150 °C. PB and NB had weight losses of 15% and 18%, while MB only had 4.3% due to the effect of the chemical treatment during grafting. 3.2.4. UV-Vis Spectroscopy: To determine the degree of leaching, the biomasses were mixed with deionized water for 60 min, and the resulting aqueous phase was analyzed for organic molecules by UV-vis spectroscopy (Figure not shown). There is a continuous signal curve in the region around 200800 nm in the spectra. It is interesting to note that the intensity of the curves decreases for PB in 220 nm and MB in 225 nm, in NB, the peak is around the 269 nm. The peak around 225 nm decreases 75% (absorbance decreases from 2.0 to 1.5) for MB, and for PB, the peak around 225 nm decreases 10% of the absorbance (absorbance decreases from 2.0 to 0.2). These results indicate that there is a significant color reduction of the Ectodermis of Opuntia with pretreatment with formaldehyde solution (0.2% v/v). One parameter which allows for the feasibility of industrial use of the biosorbents is the absence of organic leaching from the biosorbent to the aqueous solution. 3.3. Kinetic Studies. The sorption kinetic is one of the most promising characteristics to be responsible for the rate of biosorption and is a very important factor for design and process optimization in the industry. The effects of contact time on the adsorption of Pb(II) using NB, PB, and MB biosorbents are shown in Figure 7. The sorption kinetics have been extensively studied, and it has been commonly observed that the sorption rate is very rapid at the beginning of the process and then becomes slower as equilibrium is approached. In this study, for NB, the time equilibrium was 60 min. In the case of PB and MB, the time equilibrium for the biosorption of Pb(II) was attained within the first 10 min of contact. It is observed that in all cases, no desorption takes place. The kinetics models of pseudo-first order and pseudo-second order were applied, and kinetics constants for the biosorption of Pb(II) are presented in Table 2. The correlation coefficients obtained for the pseudosecond-order kinetic model are 1 for NB and PB 0.99 for MB. For the pseudo-first-order kinetic, the correlation coefficients were 0.86 for NB, 0.85 for PB. and 0.91 for MB. With those results, it was determined that the biosorption rate for the PB biomass has the highest rate constants: K2 ) 16.698 g mg/min and h ) 126.5884 mg g/min, provide the optimum biomass for continuous processing. For this reason, the subsequent tests were carried out only for PB biomass.


Figure 7. Kinetics of Pb(II) biosorption from aqueous solution as function of time. The Pb(II) concentration in the aqueous solution was 30 mg/L, the pH of the solution was 5.0, and it was conducted at room temperature.

Figure 8. Effect of pH on lead biosorption by biosorbent PB, Pb(II) lead concentration ) 30 mg/L, biosorbent dose ) 120 mg, contact time ) 10 min. Table 2. Kinetics Constants for the Biosorption of Pb(II), Pseudo-first and Second-order Kinetic Models pseudo-first-order sorbent materials NB PB MB

pseudo-second-order

K1 (L/ min)

qe (mg/g)

r2

K2 (g mg/ min)

qe (mg/g)

h (mg g/min)

r2

0.0689 0.8148 0.3887

1.2485 1.2820 1.9994

0.8638 0.8596 0.9126

0.1903 16.6980 1.4199

2.7571 2.7573 2.6918

1.4466 126.5822 10.2810

1 1 0.9998

3.4. Effect of pH on Adsorption. Results of earlier metal biosorption studies show that solution pH significantly influences biosorption.30,31 The pH of the aqueous solution is an important controlling parameter in the metal adsorption process. In this study, the effect of pH on lead adsorption by the PB was studied with initial lead concentration fixed at 30 mg/L, with the initial solution pH adjusted with nitric acid. The amount of lead adsorbed as a function of pH is shown in Figure 8. Under highly acidic conditions (pH 1.0-2.0) there was no biosorption of lead because metal binding sites on the biosorbent were closely associated with H+ which restricts the approach of metal cations due to repulsive forces. Furthermore, at low pH, there is increased competition between H+ and Pb+2 ions.32 As pH increases (3.0-5.0), the amount of biosorption increases with the maximum lead adsorption at pH 5.0. Biosorption increases with increasing pH because more electron-rich binding sites are exposed, which attract the metal cations to the biosorbent

surface.33 Experiments were not conducted beyond pH 6.0 to avoid heavy metal precipitation and interferences from biomass deterioration.34-36 3.5. Adsorption Isotherms. The adsorption isotherms are the most important data for determining the mechanism of sorption. Several isotherm equations are available, with the Langmuir and Freundlich models being the most commonly accepted. The Langmuir sorption isotherm assumes that biosorption takes place at specific homogeneous sites within the sorbent and has found successful application in many processes of monolayer sorption. The Freundlich isotherm is an empirical equation employed to describe heterogeneous systems. The adsorption isotherm studies are of fundamental importance in determining the adsorption capacity of Pb(II) onto the biomass and to determine the mechanistic parameters associated with Pb(II) adsorption. The results of the experiments were fitted according to the Langmuir and Freundlich models. Table 3

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Figure 9. Breakthrough curves of Co ) 250 mg/L and Co ) 400 mg/L. Table 3. Langmuir and Freundlich Constants for the Removal of Pb(II) Langmuir sorbent materials NB PB MB

Freundlich

KL (L/mg)

qe max (mg/ g)

r2

KF (mg/g)

1/n

n

r2

0.0354 0.2619 0.0390

75.0611 51.8430 69.5968

0.8105 0.9716 0.8385

2.0465 11.5452 2.1569

2.2667 0.5340 2.0496

0.4412 1.8725 0.4879

0.9615 0.9669 0.9591

shows the adsorption parameters. When the lead concentrations increased from 5 mg/L to 400 mg/L, the uptake of lead also increased. Table 3 shows the Langmuir and Freundlich constants and correlation regression coefficients of lead adsorption for all biosorbents used in this work. The adsorption isotherms of Pb(II) uptake by PB at pH 5 were fit to the Langmuir and Freundlich models, with the Langmuir model better fitting the experimental data. In previous work, natural Ectodermis of Opuntia showed a chromium adsorption capacity that was adequately described by the Langmuir adsorption isotherms. The authors conclude that these phenomena can be due to the heterogeneity of the adsorbent material.37 When Cd(II) ions were tested, the adsorption isotherms fit the Freundlich model. The removal of chemical oxygen demand (COD) from wastewater also follows the Langmuir model.22 3.6. Column Studies. In column experiments, 99% removal of metals from solution can be obtained before the breakthrough point is reached. Packed-bed reactors should be optimized for operational conditions like flow rate, bed height, matrix type, biomass loading, and metal concentration prior to large-scale application. The operational parameters, such as pH, influent metal concentration, size of biosorbent particles, and column length, are important in controlling metal removal in packedbed columns.38 The use of packed-bed column reactors for biosorption has the major advantage of an optimal exploitation of the sorption capacity, achieving very low effluent concentrations. The biosorbent is equilibrated at relatively high incoming concentration of the solution metal so that high uptake values are obtained, whereas the low concentration effluent encounters fresh and powerful sorbent material. Figure 9 shows the breakthrough curves of Pb(II) as a function of effluent volume (L). In this work, two initial concentrations in the influent were used to evaluate the performance in a continuous system. In Table 4, the breakthrough point and service time at breakthrough (min) are given. The capacity at complete exhaus-

Table 4. Parameters of the Melcalf-Eddy Equation adsorption break flow capacity renoval weight point % g Co mg/L (Ts) min CTs mg/L (Q) L/min (q) mg/g 250 400

120 120

8.8571 50.7619

0.0010 0.0010

58.4571 90.6285

96.4286 87.4040

0.5 0.5

tion was determined by the Metcalf-Eddy method (where the effluent plot joins the effluent in the breakthrough curve and dividing these values by the weight of adsorbent in the column). The column capacities of Pb(II) systems were found to be greater than their batch capacities. These are significant results because column studies had not been carried out with pretreated Ectodermis of Opuntia sp. (PB), and are encouraging for the use of this material in industrial processes. 3.7. Scaning Electron Microscopy (SEM) after Pb(II) Contact. SEM and EDS provide information about the morphology, surface texture, and elemental composition of the PB after Pb(II) contact. The micrograph in Figure 10 shows different morphological images of the PB. This technique also allows elemental analysis of the sample, as shown in the peaks which indicate that carbon, oxygen, and Pb(II) are the main elements present in the PB. 3.8. FTIR after Pb(II) Contact. The infrared spectra (FTIR) spectra are shown in Figure 11 in the range from 4000 to 400 cm-1 of the control biomass before and after the contact with Pb(II). Figure 11 shows the spectra of the PB biomass before and after the contact with Pb(II) ions in aqueous solution. It can be seen that after lead is adsorbed, the absorbance peak of the hydroxyl groups (stretching vibration) shifted from ∼3441 cm-1 to ∼3425 cm-1. The absorbance peaks of carbohydrate groups in 1320 cm-1 and in 1200-1400 cm-1 had been displaced to a certain extent as well. The 1050 cm-1 band corresponded to both the C-O stretching of carboxyl groups and the bending vibrations band of the hydroxyl groups. This band is observed in both spectra, but is


Figure 10. Scanning electron microscopy micrographs (500× magnification) of the PB after Pb(II) contact.

around 52 mg of Pb(II)/g, which is comparable with other sorbents as can be observed in Table 5. 4. Conclusions

Figure 11. Infrared spectroscopy analysis (FTIR) spectra of PB before and after the Pb(II) contact. Table 5. Comparison of Pb(II) Sorption Capacity of Different Sorbent Materials sorbent materials

qe max (mg/g)

ref

Cu(II)PMA APANF Pleurotus ostreatus palm kernel fiber PGHySnO-COOH sugar beet pulp coffee coarse tea green tea aloe PB

6.22 76.12 121.21 49.90 66.66 73.8 16.5 21.1 24.0 17.8 51.82

40 41 42 43 44 45 46 46 46 46 present work

The materials (NB, PB, and MB) were characterized using SEM, FTIR, TGA and UV-vis spectrometry, where the data from FTIR spectra confirmed the chemical modification of Ectodermis of Opuntia and its possible participation in the Pb(II) biosorption. The preatment of Ectodermis of Opuntia by formaldehyde solution (PB) prevented leaching from the biomass and facilitated the separation of the solution and biosorbent. The biosorption rate of Pb(II) was rapid, and equilibrium was reached at 10 min for PB and MB. In the case of NB, the equilibrium of the Pb(II) biosorption was reached at 1 h. The kinetics of Pb(II) biosorption onto PB followed a pseudo-secondorder equation. The biosorption rate for the PB biomass had the highest rate constants (K2 ) 16.698 g mg/min and h ) 126.5884 mg g/min) and provided us with the optimal biomass for continuous processing. The pH significantly influenced Pb(II) biosorption with the maximum adsorption observed at pH 5.0. The biosorption process of biomass (PB) was described better by the Langmuir model than with the other isotherm models. The Langmuir constants were KL ) 0.2619 L/mg and qemax ) 51.843 mg/g, with a correlation regression of 0.9716. These results demonstrate the great potential of using plant biomaterial residues as low-cost heavy-metal adsorbents. Acknowledgment The authors wish to acknowledge the support given by the Universidad Autońoma del Estado de Mexico, specifically the Facultad de Quı´mica (Project UAEM 2425/ 2007U). Support from CONACYT and supporting research by SNI are greatly appreciated. Literature Cited

shifted from 1050 cm-1 to 1063 cm-1 due to the Pb(II)-loaded biomass. The band at 1380 cm-1 in the spectrum of PB turns sharper and separately shifts to 1377 cm-1 in the Pb(II)-loaded biomass. This behavior reflects the interaction between the C-OH cellulose and lignin groups and the lead ions.7,37,39 3.9. Comparison with Other Sorbents. Although direct comparison of these biosorbents with other sorbent materials is not feasible, for the sake of comparison and owing to different applied experimental conditions, it was found, in general, that the PB adsorption capacity, using equilibrium experiments, was

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ReceiVed for reView June 22, 2007 ReVised manuscript receiVed October 16, 2007 Accepted October 29, 2007 IE070861H

Pb(II) Sorption under Batch and Continuous Mode ... - ACS Publications

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