Ind. Eng. Chem. Res. 2004, 43, 2209-2215
2209
SEPARATIONS Chromium(VI) Removal from Aqueous Solutions by Mg-Al-CO3 Hydrotalcite: Sorption-Desorption Kinetic and Equilibrium Studies N. K. Lazaridis,* T. A. Pandi, and K. A. Matis Division of Chemical Technology, School of Chemistry, Aristotle University, GR-54124, Thessaloniki, Greece
In this study, the sorptive removal of hexavalent chromium from aqueous solutions by Mg-Al-CO3 hydrotalcite was investigated in a batch mode. The influence of solution pH, conditioning duration, initial chromium concentration, sorbent concentration, sorbent particle size, and temperature was tested at sorption kinetic runs. The influence of eluant concentration and volume was tested at desorption kinetic runs. Desorption experiments showed that the loaded material can be fully regenerated and reused. Four kinetic models have been evaluated if they fit the experimental data: the pseudo-first-order, a modified second-order, and two ion-exchange models. It showed that the second-order model could best describe the sorption and desorption kinetics. The Freundlich isotherm was used to fit equilibrium experiments. Hydrotalcite presented a sorption capacity of ∼17 mg Cr/g under the studied conditions. The calculated activation energy for the studied process was 24 ( 2 kJ/mol. 1. Introduction Chromium has been identified both as an essential micronutrient and as a chemical carcinogen. The carcinogenicity of chromium is a function of its chemical form. In general, compounds of trivalent chromium are less toxic than those of hexavalent chromium with respect to acute and chronic toxicity. The current maximum contaminant level for total chromium in drinking water is 100 µg/L, according to EPA. The potential for surface water contamination is greater than that for groundwater. Extensive studies have shown that metal finishing and leather tanning activities contributed significant amounts of chromium to surface waters.1 Sorption is one of the most common methods for the removal of hexavalent chromium from wastewaters. In several recent reports, various authors have documented the use of different sorbents, which can be separated into two classes, inorganic and organic/ biomasses. In the first class belong the following materials: zeolite,2 layered double hydroxides,3 and activated carbons.4-6 In the second class belong the following materials: chitin,7 Microcystis,8 Sargassum seaweed,9 Chlorella vulgaris,10 Casurina equisetifolia,11 Rhizopus arrhizus,12 Rhizopus nigricans,13 low-cost available adsorbents,14 poly(4-vinylpyridine)-coated silica,15 resinimmobilized activated sludge,16 carboxylated polyacrylamide-grafted sawdust,17 and biogas residual slurry.18 In this research, Mg-Al-CO3 hydrotalcite (denoted as HT), which is a member of the family of doublelayered mixed metal hydroxides, was studied as a sorbent material. The crystal structure of HT consists of positively charged brucite-type main layers with a * To whom all correspondence should be addressed. Tel: +32310 997807. Fax: +32310 997859. E-mail: nlazarid@ chem.auth.gr.
negatively charged interlayer composed of carbonates and water molecules, which can be exchanged by organic and inorganic anions.19 Hydrotalcite, when heated at 500 °C (HT500), is decomposed into magnesium and aluminum oxide solid solution. The latter can rehydrate and incorporate anions to rebuild the initial hydrotalcite structure. Hydrotalcite (HT) shows limited exchange properties, because carbonate is preferentially sorbed and prevents significant anion exchange.20 This work primarily focuses on the sorption and desorption kinetics of uncalcined hydrotalcite. Although sorption of hexavalent Cr by uncalcined and calcined hydrotalcite has already been reported, the investigation of the uncalcined material as a potential sorbent has not been extensively carried out.3,20 Generally, calcined hydrotalcite has superior anion sorption capacity in comparison to the uncalcined material, but it presents two shortcomings as: (i) a complicated regeneration stage (the exhausted calcined material must be desorbed and recalcined at 500 °C for reuse) and (ii) high basicity, which increases highly the pH of solutions.20 For these reasons, the present study aimed at proving that hydrotalcite could be an effective and cheaper alternative to the calcined material. 2. Material and Methods Materials. The sorbent was prepared in the laboratory by coprecipitating mixed metal solutions of magnesium and aluminum nitrates {Mg(NO3)2‚6H2O and Al(NO3)3‚9H2O} by raising the solution pH with the addition of sodium hydroxide and sodium carbonate solutions under intense mixing for 4 h at 35 °C, heating (at ∼65 °C for 18 h) to crystallize, then filtering and washing with water to remove sodium below 0.1% level on a dry weight basis. The filter cake was dried, pulverized in a ball mill (HT) and sieved to various particle
10.1021/ie030735n CCC: $27.50 © 2004 American Chemical Society Published on Web 04/01/2004
2210 Ind. Eng. Chem. Res., Vol. 43, No. 9, 2004
sizes. Specific surface area (83.95 m2/g), pore volume (0.449 cm3/g) and mean pore diameter (30 nm) were determined by nitrogen adsorption-desorption isotherms at liquid nitrogen temperature using a Quantachrom Autosorb-1 apparatus. The crystal structure of the material was examined by X-ray diffraction. Powder XRD patterns were received with a Siemens D500 X-ray diffractometer. Experimental Procedure for Sorption. Sorption kinetics was conducted in a continuously stirred plexiglass beaker immersed into a thermostated water bath and holding a volume of 2.0 L solution. Mixing was provided by a disk turbine furnished with six 45°inclined blades, using a Heidolph variable speed motor. An identical stirring rate of 250 rpm was used for all experimental runs, which ensures satisfactory homogeneity in the bulk and also eliminates the risk of possible attrition of the adsorbent particles at higher agitation speed. The influence of chromium concentration, hydrotalcite concentration, particle size, solution pH, temperature, and conditioning time was tested at these runs. For the equilibrium experiments, mixing was continued for 24 h, to allow hydrotalcite to adsorb chromium, until the solution reached equilibrium.21 Experimental Procedure for Desorption. Desorption experiments were perfomed in the same configuration by mixing 1 g of Cr(VI)-loaded hydrotalcite with the appropriate aqueous solution of eluant (Na2CO3) under 250-rpm agitation speed. The influence of concentration and volume of eluant was tested at batch kinetic desorption runs. At selected time intervals, samples of 2 mL (each) were extracted, and the metal ion content was determined spectrophotometrically at 543 nm, following the 1,5-diphenylcarbazide method.22 All the experiments were done in triplicate, and only the average results are reported in this paper. The accuracy of temperature and concentration measurements were (0.5° C and (0.05 mg/L, respectively. Chromium loading by the sorbent was calculated employing the following equation
qt )
(C0 - Ct)V m
(1)
3. Results and Discussion The main issue when searching for an appropriate sorption mechanism is to select a mathematical model that not only fits the data with satisfactory accuracy but also complies with a reasonable sorption mechanism. Generally, several steps are involved during the sorption process by porous sorbent particles: (i) bulk diffusion; (ii) external mass transfer (boundary layer or film diffusion), between the external surface of the sorbent particles and the surrounding fluid phase; (iii) intraparticle transport within the particle; and (iv) reaction kinetics at phase boundaries. In the case of chemisorption, the rate of sorption is generally controlled by the kinetics of bond formation.23 Modeling Sorption-Desorption Kinetics. To identify the correct mechanism, several models must be checked for suitability and consistency in a broad range of the system parameters. Providing sufficient agitation to avoid particle and solute gradients in the batch reactor allows the bulk diffusion to be neglected. Since in well-agitated sorption studies, film diffusion is usually only rate-controlling for the first few minutes, two
Table 1. Linearized Forms of the Homogeneous Diffusion Model (HDM) and Shrinking Core Model (SCM)a model HDM SCM
a
rate-controlling step
F(X)
independent variable
film diffusion particle diffusion film diffusion particle diffusion chemical reaction
-ln(1 - X) -ln(1 - X2) X 3 - 3(1 - X)2/3 - 2X 1 - (1 - X)1/3
t t ∫ t0C(t) dt ∫ t0C(t) dt ∫ t0C(t) dt
X is the fractional uptake to equilibrium (qt/qe)
Figure 1. Time variation of dimensionless chromium(VI) in the bulk at various pH values: [Cr] ) 10 mg/L, [HT] ) 1 g/L, N ) 240 rpm, T ) 30 °C, dp ) -250 + 125 µm.
simple-order kinetic models and two ion exchange models were tested. First-Order Kinetic Model. The first-order sorption model (Lagergren) can be expressed in integrated form as23,24
mqe Ct )1[1 - e-kS1t] C0 C0V
(2)
The first-order desorption model, as a function of the sorption loading, is given by eq 3.
qt ) q0e-kD1t
(3)
Second-Order Kinetic Model. The second-order sorption model in integrated form can be expressed as23,24
(
mqe Ct 1 )11C0 C0V β2 + kS2t
)
(4)
The second-order desorption model, as a function of the sorption loading, is given by eq 5.
(
qt ) q0
)
1 β2 + kD2t
(5)
Ion-Exchange Models. Two models were selected to fit the data: the homogeneous diffusion model (HDM) based on Fick’s law and the shrinking core model (SCM) based on modified Levenspiel’s theory.25-27 Table 1 presents the linearized forms of the above models with film diffusion, particle diffusion, and chemical reaction. Effect of pH on Sorption Kinetics. Figure 1 shows chromium concentration profiles versus time at three pH values (6.0, 8.0, 10.0). It is clear that chromium is much more highly removed at the lower pH. This fact can be attributed to two reasons: (i) The presence of
Ind. Eng. Chem. Res., Vol. 43, No. 9, 2004 2211
Figure 2. Time variation of dimensionless chromium(VI) in the bulk at various conditioning durations: [Cr] ) 10 mg/L, [HT] ) 1 g/L, pH ) 6.0, N ) 240 rpm, T ) 30 °C, dp ) -250 + 125 µm.
Figure 4. Time variation of dimensionless chromium(VI) in the bulk at various sorbent concentrations: [Cr] ) 10 mg/L, pH ) 6.0, N ) 240 rpm, T ) 30 °C, dp ) -250 + 125 µm.
Figure 3. Time variation of dimensionless chromium(VI) in the bulk at various initial chromium concentrations: [HT] ) 1 g/L, pH ) 6.0, N ) 240 rpm, T ) 30 °C, dp ) -250 + 125 µm.
Figure 5. Time variation of dimensionless chromium(VI) in the bulk at various particle sizes: [HT] ) 1 g/L, [Cr] ) 10 mg/L, pH ) 6.0, N ) 240 rpm, T ) 30 °C.
chromium anions strongly affects the ζ-potential of hydrotalcite. The isoelectric point of HT is reduced from its initial value, ∼11.1, in deionized water to pH ∼6.7. This suggests that some adsorption of anions occurs on the external surface, which inhibits the attraction of chromates at higher pH values. (ii) The reduction of pH decreases the number of hydroxide ions (OH-), which can compete with chromium anions.20 Similarly, for the same reason,3 chromium uptake by calcined hydrotalcite was increased as the pH of the solution was decreased from 10 to 6. Effect of Conditioning Time on Sorption Kinetics. Figure 2 presents the influence of the conditioning (wetting) time on chromium sorption by hydrotalcite. Two kinetic runs were perfomed after conditioning the sorbent for 1 or 14 h in deionized water and the results were compared with the reference conducted with zero conditioning time. The scope of these experiments was to investigate the significance of filling the pores of the sorbent with water and eliminating the time needed for chromium to be diffused into the solid. The experimental results showed only a small influence on the sorption rate. There was no significant change between 1 and 14 h of conditioning. For the rest of the work, all runs were conducted without any pretreatment. Effect of Chromium Concentration on Sorption Kinetics. Figure 3 depicts the dimensionless chromium concentration versus time by varying the initial metal concentration. For all the employed concentrations, there is a monotonic decreasing trend during the first minutes (∼25 min), and then nearly a plateau is reached. It is readily apparent that the residual
chromium concentration increased as the initial metal concentration increased. For example, hydrotalcite reduced chromium from 30 to 90% approximately when the metal concentration varied from 30 to 5 mg/L after 4 h of operation. Effect of Hydrotalcite Concentration on Sorption Kinetics. An attempt to enhance chromium removal was evaluated by examining the effect of a solid dose. Figure 4 shows the residual concentration at different sorbent doses. Employing 10 mg/L chromium, the percentage removal increased from 36 to 91% by increasing hydrotalcite concentration from 0.5 to 2 g/L. This is due to the fact that at higher sorbent doses, more surface area is available, which means higher numbers of adsorption sites. Effect of Hydrotalcite Particle Size on Sorption Kinetics. Adsorbent particle size has significant effect on the kinetics of adsorption due to the change of easily available adsorption sites. Figure 5 presents the time profiles of such a study. Decreasing the size of the sorbent has a dramatic effect on sorption rate but not on the sorption loading. The relatively higher sorption rate with smaller adsorbate particle may be attributed to the fact that smaller particles yield larger surface area. Generally, there is a tendency that smaller particles produce shorter time to equilibration.5 Effect of Temperature on Sorption Kinetics. A parameter with great significance in the sorption process is temperature. Figure 6 displays the isotherm time profiles of chromium depletion. Sorption was slightly accelerated as the temperature of the bulk was raised, while the sorption loading was slightly increased.
2212 Ind. Eng. Chem. Res., Vol. 43, No. 9, 2004
Figure 6. Time variation of dimensionless chromium(VI) in the bulk at various temperatures: [HT] ) 1 g/L, [Cr] ) 10 mg/L, pH ) 6.0, N ) 240 rpm, dp ) -250 + 125 µm.
Figure 7. Time variation of chromium(VI) loading on hydrotalcite at various eluant concentrations: [HT] ) 1 g/L, V ) 2 L, N ) 240 rpm, T ) 30 °C, dp ) -250 + 125 µm.
Whether this is due to a chemical reaction or diffusion control mechanism will be discussed later. In a similar manner, the sorption rate of chromium by calcined hydrotalcite was increased with temperature but at a more significant level.3 Effect of Eluant Concentration on Desorption Kinetics. With the purpose of reusing the chromium and the sorbent, desorption tests were performed. Sodium carbonate was tried as eluant due to the fact that the exchange affinity of hydrotalcite for carbonates is the highest. Figure 7 presents the kinetics of chromium(VI) desorption from loaded hydrotalcite under various eluant concentrations. Four different concentrations varying from 1 × 10-2 to 1 × 10-4 mol/L were tested. It is clear that desorption rate was increased by increasing the eluant concentration. Except for the lower concentration, all of the others can effectively extract chromium ions. For example, by employing 0.01 mol/L eluant concentration, the chromium loading was decreased from 5.4 to 0.1 mg Cr/g HT in almost 30 min. Effect of Eluant Volume on Desorption Kinetics. Figure 8 presents the kinetics of chromium(VI) desorption from loaded hydrotalcite under different eluant volumes. The eluant concentation was 0.01 mol/L, and the volume varied from 0.25 to 2 L. The rate of desorption was obviously increased by increasing the volume of eluant, for example, in 30 min the percentage of desorption was increased from 61.7 to 98.1% by applying 0.25 and 2 L of solution, respectively. To elucidate any modification in the nature of the sorbent material due to successive elution stages,
Figure 8. Time variation of chromium(VI) loading on hydrotalcite at various eluant volumes: [HT] ) 1 g/L, [Na2CO3] ) 0.01 mol/L, N ) 240 rpm, T ) 30 °C, dp ) -250 + 125 µm.
surface analysis was performed. An X-ray diffractogram of the laboratory-prepared hydrotalcite (Figure 9a), shows very sharp peaks, indicating a very crystalline ordered material. After five complete operation cycles of sorption-regeneration, the sorbent continued to exhibit the same pattern (Figure 9b); therefore these observations confirm the potential use of hydrotalcite as a sorbent material. Hydrotalcite performance continued for five more cycles, exhibiting the same results. Kinetic Mechanism Selection. Figure 10 illustrates the experimental data for 5 mg/L chromium from Figure 3 expressed as F(X) versus ∫ t0C(t) dt (shrinking core model) and F(X) versus time (homogeneous diffusion model), respectively. It is apparent that these models do not fit the experimental data over the total time range. Repeating the same procedure for all the other kinetic runs gave the same nature of the plots (data not presented). In Tables 2 and 3, the kinetic parameters and the corresponding regression coefficients (R2) for the sorption and desorption experiments, employing first and second-order models, are presented. The regression analysis provided satisfactory correlation coefficients for the two models. In most cases, the second-order kinetic presented better R2 values. First- and second-order kinetics have been used to describe chromium sorption by numerous sorbent materials.14,18,23 Hamadi et al., 2001, showed that the second-order chemical reaction model provides the best correlation of data for the sorption of chromium(VI) by adsorbents derived from used tires and sawdust.5 First- and second-order kinetic models were successfully used to fit the experimental data for chromium uptake by calcined hydrotalcite with the most appropriate one.3 We propose the following ion exchange mechanism for the sorption of chromium(VI) by hydrotalcite
Mg6Al2(OH)16CO3‚4H2O + 2HCrO4- f Mg6Al2(OH)16(HCrO4)2‚xH2O + CO32- (6) Mg6Al2(OH)16CO3‚4H2O + CrO42- f Mg6Al2(OH)16CrO4‚yH2O + CO32- (7) A plot of the sorption rate against the reciprocal temperature was performed for the kinetic data of Figure 6 (Arrhenius plot, not shown), giving a reasonably straight line (R2 ) 0.993), the gradient of which is - (E/R) and the activation energy of which can be
Ind. Eng. Chem. Res., Vol. 43, No. 9, 2004 2213
Figure 9. Powder X-ray patterns of (a) hydrotalcite as prepared and (b) after 5 cycles of operation-regenaration.
corresponding one (40 kJ/mol) for chromium uptake by calcined hydrotalcite.3 Modeling Sorption Isotherm. Two general-purpose equilibrium models were used to fit the experimental data: (i) Langmuir (eq 9) and (ii) Freundlich (eq 10).28
qe )
qmKLCe 1 + KLCe
(9)
qe ) KFCe1/n
Figure 10. Kinetic modeling using the shrinking core model (SCM) and homogeneous diffusion model (HDM), inset figure, for sorption of chromium. [HT] ) 1 g/L, [Cr] ) 5 mg/L, pH ) 6.0, N ) 240 rpm, T ) 30 °C, dp ) -250 + 125 µm.
calculated according to the following equation.23
ks2 ) Ae-E/RT
(8)
For diffusion-controlled processes, the activation energy of adsorption is
