Ind. Eng. Chem. Res. 2008, 47, 3401-3409
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Adsorption of Chromium(VI) from Aqueous Solutions Using an Imidazole Functionalized Adsorbent Hyung-Jun Park and Lawrence L. Tavlarides* Department of Biomedical and Chemical Engineering Syracuse UniVersity, Syracuse, New York 13244
Separation of chromium(VI) from aqueous solutions is investigated using an imidazole functionalized solgel adsorbent. This adsorbent has been formed by the sol-gel synthesis method. The speciation diagram of Cr(VI) in an aqueous system with varying pH is studied by analyses of the equilibria equations. Batch adsorption equilibrium studies show a decrease in chromium uptake capacity with increase in pH in the range from 2 to 9, and the uptake capacity at pH 2.5 is found to be 2.93 mmol/g (152 mg/g). The Langmuir adsorption isotherm gives a satisfactory fit of the adsorption data. A kinetics study conducted with different concentrations of chromium(VI) in a batch reactor shows a rapid rate of adsorption. The adsorbent shows a high selectivity toward Cr(VI) and negligible adsorption of Cu(II), Ni(II), and Zn(II). Adsorption tests in a fixed bed column show a sharp breakthrough curve. Stripping of the chromium-loaded column bed is achieved using 4 M HNO3. Twenty cycles of adsorption and desorption process are performed for SOL-IPS adsorbent. The adsorbent maintains 90% of original capacity during 15 cycles, and loss of 25% of capacity until 20 cycles of operation. The imidazole functional adsorbent is demonstrated to be an effective sorbent material for the separation of chromium(VI) from aqueous solutions. 1. Introduction The presence of hexavalent chromium in wastewater has received much attention in recent years. Chromium contamination is generated from various industries such as electroplating, pigment manufacture, leather tanning, metal finishing, etc. This metal ion has a significant impact on human health and other organisms.1 Chromium has two oxidation states in aqueous systems, Cr(III) and Cr(VI), and the mobility and toxicity of these ions are different. Cr(VI) is about 100 times more toxic than Cr(III).2 The risk of surface water contamination is greater than groundwater contamination.3 According to the United States Environmental Protection Agency (USEPA), the concentration of chromium in drinking water has been regulated with a maximum allowable level of 0.1 mg/L for total chromium.4 This problem of chromium ions in the environment is made more prominent by the increased chromium discharge from industrial wastes. It is reported that the USEPA estimates that 97379 lb of chromium is released from surface water discharges and 29 million pounds is released due to dissolution from rain.5 The common concentrations of hexavalent chromium found in typical wastewaters are 50-100 mg/L.6 The traditional means to separate Cr(VI) from wastewaters are chemical reduction, precipitation, evaporation and ion exchange. Although ion exchange resins can substantially remove metal ions, they do not show mechanical strength because of swelling of polymeric skeleton and low selectivity. In precipitation process, Cr(VI) is reduced to Cr(III), followed by precipitation of Cr(III) as Cr(OH)3. However, this process is high in costs and treated water still has high chromium ion concentrations and pollutes surface waters. Gradually, industry is seeking replacement methods from traditional metal recovery and separation techniques to overcome disadvantages. These methods have led to the development of new techniques like adsorption and membrane separation. Adsorption is an economically reasonable alternative, and by using functionalized adsor-
bents on backbones such as organoceramic materials, an alternative separation is proposed to overcome the drawbacks of the traditional techniques. Various adsorbents studied for chromium removal from aqueous solutions include activated carbon,7 activated alumina,8 coated silica gel,9 treated sawdust,10 peat moss,11 and raw rice bran.12 Accordingly, adsorption-based technologies have been reported for the removal of hexavalent chromium. Imidazole and its derivatives have been used to separate various metal ions from aquous solutions.13-15 It appears that the protonated form should have favorable chelation characteristics for the anionic forms of chromium(VI), and should be a promising ligand for separation of this species. Accordingly, the present work is focused on the adsorption equilibrium isotherm, the kinetics in batch and fixed-bed columns, the selectivity in the presence of other competing ions, and the operational stability of an imidazole functionalized sol-gel adsorbent (namely SOL-IPS13) as an alternative material for Cr(VI) removal from aqueous solutions. Because the surface properties of adsorbents to remove specific metal ions can vary with the solution conditions, the mechanisms of adsorption on the adsorbents can also vary significantly. The pH of the system determines the adsorption capacity due to its influence on the surface properties of the adsorbent and different ionic forms of the Cr(VI) solution. In industrial wastewater, the predominant forms of chromium are the HCrO4- and CrO42-. To achieve a quantitative understanding of the adsorption mechanism in this study, a series of experiments are conducted to obtain information about the adsorption phenomena. It is assumed that the number of protonated imidazole ligands reacting with Cr(VI) is the same as the number of charges of the Cr(VI). Therefore, studies include construction of adsorption equilibrium isotherms; kinetics of adsorption in batch and fixedbed column; selectivity of Cr(VI) in the presence of competing ions; and operational stability. 2. Experimental Section
* To whom correspondence should be addressed. Tel.: 1-315-4431883. Fax: 1-315-443-1243. E-mail:
[email protected].
2.1. Materials, Reagents, and Equipment. SOL-IPS is a sol-gel material composed of clusters of imidazole ligands on
10.1021/ie7017096 CCC: $40.75 © 2008 American Chemical Society Published on Web 03/21/2008
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the pore surface and silica backbone. Details of the adsorbent synthesis and structure are reported elsewhere and that synthesis procedure is employed here for SOL-IPS preparation.13 The SOL-IPS adsorbent is synthesized by the independently hydrolyzed and condensed N-(3-triethoxysilylpropyl)-4,5 dihydroimidazole (IPS, Gelest) and tetraethoxysilane (TEOS, Fluka). The molar ratio between IPS and TEOS is 1 to 2. IPS is hydrolyzed and homocondensed for 30 min at room temperature with the molar ratio of IPS:EtOH:water ) 1:3:2. TEOS is hydrolyzed and homocondensed for 30 min at room temperature with the molar ratio of TEOS:EtOH:water ) 1:4:1. These partally condensed silanes are mixed and co-condensed to produce a gel material. The gelled materials are aged for 24 h at 25 °C and dried for 24 h at 80 °C. The dried adsorbents are crushed into the desired particle size ranges of 75-125 µm and 125-180 µm for characterization. Average pore diameter and surface area of the adsorbents are measured by nitrogen adsorption at 77 K with Micromeritics ASAP 2000. A thermostatically controlled shaker bath (Precision Scientific model 50) is used in batch adsorption experiments. Analyses of total chromium concentration are measured with ICP-MS (Perkin-Elmer model Elan 6100) and atomic absorption spectrophotometer (Perkin-Elmer model 2380). Batch adsorption experiments are conducted in a temperature controlled shaker water bath. pH is measured using a Chemcadet pH meter. Fixedbed experiments are studied with a 0.7 cm i.d. column and cartridge pump (Master Flex model 7519-20) to control flow rate. A 5000 mg/L Cr(VI) stock chromate solution is prepared from chromium(VI) oxide (Sigma Aldrich). All other chemicals used in this study are of reagent grade. 2.2. Characterization. The effect of solution pH on metal ion adsorption at 25 °C is investigated using 50 mg/L of chromium concentration at pH in the range from 2 to 9. A 0.1 M acetate buffer is used. Equilibrium uptake capacities of SOLIPS-F22 are determined using 0.1 g of adsorbent with 50 mL of Cr(VI) solution buffered by 0.1 M NaAc at various initial concentrations (50-1250 mg/L) and pH (2.5, 4.5, 6.0) at room temperature. The amount of adsorbed chromium ions is calculated by a mass balance between the initial and equilibrium concentrations. Kinetics experiments are executed in a batch reactor with various initial metal ion concentrations (50, 200, 750 mg/L) at 25 °C, each buffered by 0.1 M acetate. The batch reactor is a 500 mL beaker, in which a solution of 250 mL and the introduced adsorbent particles are stirred rapidly with a floating magnetic stirrer to ensure complete mixing. One milliliter samples are taken at desired reaction times between 2 and 120 min to prepare plots of C/C0 vs time. Selectivity of SOL-IPS for various hazardous metals is determined by simultaneous adsorption of Cr(VI), Cd(II), Zn(II), and Fe(III) under excess molar quantities of sites. Equimolar quantities of each metal (0.0248 mmol) in a 100 mL solution at pH 3.1 is prepared for contact with 0.2 g of SOL-IPS-F22 for 24 h at room temperature. The salts of these metals used in the solution preparation are chromium oxide, cadmium oxide, zinc chloride, and iron (III) sulfate hydrate. The reaction mixture is filtered and the filtrate is collected for analyses. The composition provides 16% saturation of the functional ligand sites. This level of saturation minimizes competition between metals for sites and permits ligand to metal affinities to be determined. Stripping experiments are executed in a 0.7 cm i.d. column using three concentrations of nitric acid (1, 2, 4 M). Volumes of 100 mL of 750 mg/ L Cr(VI) solutions at pH 2.5 are passed through the column bed containing 0.2 g of adsorbent. The volume of stripping agents used is 50 mL. A 5 mL aliquot of
Figure 1. Speciation of chromium(VI) in an aqueous solution, total concentration 0.001 M.
DI water is used to wash the column before and after stripping steps to ensure discharge of remaining metal ions. The column adsorption performance is carried out using an initial chromium concentration (50 mg/L) at a flow rate of 1 mL/min through a 0.7 cm i.d. column packed with 0.5 g of adsorbent. Effluent concentrations are collected at volume intervals for analysis. The stability of the adsorbent is tested by repeated adsorption and desorption experiments in a 0.7 cm i.d. column packed with 0.2 g of adsorbent. Volumes of 100 mL of 750 mg/L Cr(VI) solution at pH 2.5 are used to saturate the column. 100 mL of 1 M nitric acid is selected as the stripping solution. A 10 mL volume of DI water is used to wash column before and after stripping steps to ensure discharge of remaining metal ions. 3. Background and Theory 3.1. Aqueous Phase Chemical Equilibrium and pH Isotherm. Aqueous phase equilibrium concentrations of chromium species are evaluated using appropriate chemical reaction equilibrium analyses to understand the distribution of various forms of chromium(VI) ions in solution. The anions of chromium are known to exist in the following forms depending on the pH of the aqueous solution.16
H2CrO4 T H+ + HCrO4-
(log K1 ) -0.8)
(1)
HCrO4- T H+ + CrO42-
(log K2 ) -6.5)
(2)
2HCrO4- T Cr2O72- + H2O HCr2O7- T H+ + Cr2O72-
(log K3 ) 1.52) (log K4 ) 0.07)
(3) (4)
Algebraic equations for the five chromate ion species are obtained in terms of the above equilibrium constants and solved for the individual concentrations at various pH values. Details are provided elsewhere.17 Chromate ion species in the aqueous solution evaluated using the analyses show that HCrO4- and CrO42- are the predominant species in total chromium concentration, but the distribution of each species depend on total concentration of chromium(VI) and the pH. From Figure 1, the predominant chromium species is HCrO4- at 2 < pH < 6.5, whereas at pH > 6.5, CrO42- is predominant. The dependency of the adsorption of chromium(VI) with solution pH using SOL-IPS-F22 is investigated because of the variations of solution pH in various applications. Figure 2 shows the removal of Cr(VI) with respect to the equilibrium pH over
Ind. Eng. Chem. Res., Vol. 47, No. 10, 2008 3403
sol-gel structure. The activity coefficients of these species in these reactions are calculated using the Gu¨ntelberg approximation,18 and in the case of these species, they are assumed to be 1 because of the dilute concentrations of interest. The multispecies Langmuir isotherm can be derived and written as a follows;
q ) q1 + q2 + q3
(12)
where
q1 )
Keq1[HCrO4-][RNH+]T
(13)
1 + Keq1[HCrO4-]
1 + 2Keq2[Cr2O72-][RNH+]T -
x1 + 4K q )
Figure 2. Effect of pH on the removal of Cr(VI): weight of SOL-IPSF22 ) 0.1 g; volume ) 50 mL; contact time ) 24 h; buffered by 0.1 M acetate.
the range of 2 - 9. The results show that chromium removal increases with decrease in pH values. The increase in the adsorption capacity at lower pH values is due to the availability of protonated imidazole sites for anionic species adsorption to take place. 3.2. Equilibrium Isotherm. The adsorption reactions of Cr(VI) with the imidazole functional group in an aqueous phase can be represented by the proton pairing reaction. The adsorption of various anionic complexes results from the variation of complex compositions because of the change of pH and amount of complexing anions. The speciation information is employed to describe the composition of the anionic solution. A three species equilibrium model is developed based on chemical reactions to describe the equilibrium isotherm. The following is a general adsorption mechanism describing the protonation of the imidazole site followed by the adsorption of charged chromate ions:
RN + H+ T RNH+
(5)
HCrO4- + RNH+ T RNH+HCrO4-
(6)
Cr2O72- + 2RNH+ T (RNH+)2Cr2O72-
(7)
CrO42- + 2RNH+ T (RNH+)2CrO42-
(8)
2
Keq2 )
{RNH+HCrO4-} {HCrO4-} {RNH+} {RNH+Cr2O72-} {Cr2O72-} {RNH+}2 +
Keq3 )
{(RNH )2CrO4 }
x
1 + 4Keq3[CrO42-][RNH+]T 2Keq3[CrO42-]
(15)
The above equation can be simplified because only two species (HCrO4-, Cr2O72-) exist and adsorb between pH 2.5 and 4.5. Concentration of each chromium species is determined from the speciation diagram (Figure 1).
q)
Keq1[HCrO4-][RNH+]T 1 + Keq1[HCrO4-]
+
1 + 2Keq2[Cr2O72-][RNH+]T -
x1 + 4K
2-
eq2[Cr2O7
][RNH+]T
2Keq2[Cr2O72-]
Keq1[HCrO4-][RNH+]T
for 2.5 e pH e 4.5 (16)
+
1 + Keq1[HCrO4-] 1 + 2Keq2[Cr2O72-][RNH+]T -
x
1 + 4Keq2[Cr2O72-][RNH+]T
+
2Keq2[Cr2O72-] (10)
1 + 2Keq3[CrO42-][RNH+]T -
x
1 + 4Keq3[CrO42-][RNH+]T
2-
{CrO42-} {RNH+}2
(14)
2Keq2[Cr2O72-]
q3 )
q)
(9)
][RNH+]T
1 + 2Keq3[CrO42-][RNH+]T -
The equilibrium constants for reaction mechanisms 6-8 are
Keq1 )
2-
eq2[Cr2O7
(11)
2Keq3[CrO42-]
for pH g 6.0 (17)
Here, q is the metal uptake per unit mass of adsorbent where RNH+ is a protonated imidazole moiety found in the
(mmol/g); [RNH+]T is the maximum capacity of the adsorb-
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ent (mmol/g); Keq1 (L/mmol), Keq2 (L‚g/mmol2), and Keq3 (L‚g/mmol2) are the equilibrium constants related to maximum adsorption capacity. Alternatively, the single species Langmuir adsorption isotherm model can also be applied to the adsorption data of the present work
KCT[RNH+]T q) 1 + KCT
(18)
where CT is the total equilibrium concentration (mmol/L) of the five chromate species. This model is compared to the above and used for simplicity if appropriate. The equilibrium constants Keq1, Keq2, Keq3, and K are determined by fitting experimental data using a nonlinear regression method of Levenberg-Marquardt in Origin by Microcal.19 3.3. Adsorption Kinetics. A batch reactor is employed for these studies. The reaction can be defined by a sequence of processes that affect the rate: mass transfer of ions through the liquid surrounding the particle, diffusion of ions through the particle pores, and chemical reaction of the ion with the ligand site on the pore. The slowest step is considered to be the ratedetermining step. The film-pore model is considered to describe the adsorption data. 3.3.1. Film)Pore Model. The film-pore model is developed by considering the transport of chromium(VI) through the film and pore to be rate controlling and that the particles are spherical in geometry. No surface diffusion is prevent; pore diffusivity is independent of temperature, chromium ion concentration, and solution pH; the concentration of imidazole groups is uniform within the pellet; and local equilibrium is maintained between the adsorbate in the liquid inside the pores and that in the adsorbed phase. According to this model, the following equations along with initial and boundary conditions are employed to describe the adsorption kinetics in the batch recycle reactor. The macroscopic conservation equations of the model are20
VT (cbo - cb) ) w q q)
3 R3
(19)
∫R0 qr2 dr
(20)
where qj is the chromium concentration in the pellet, which is averaged over the pellet volume. The particle is assumed to have randomly distributed uniform pores throughout. In the absence of potential gradients, assuming Fick’s law applies, the pore diffusion metal ion species equation for adsorption can be written as
Dp
[ + F ∂q∂c] ∂c∂t ) r ∂r∂ (r ∂c∂r) p
p
2
2
(21)
Initial and boundary conditions of the partial differential equation with consideration of film mass transfer are given by
c ) 0,
te00ereR
∂c ) 0, ∂r Dp
r)0
∂c ) kf (cb - c), r ) R ∂r
(22a) (22b) (22c)
Here, VT is the reservoir volume where changes in metal concentrations are measured; w is the mass of adsorbent; q is
the local equilibrium chromium concentration in the pellet; cb is the concentration of Cr(VI) in the reservoir; cbo is the initial concentration of Cr(VI) in the reservoir; c is the Cr(VI) concentration in the adsorbent pore solutions; r is the radial distance from the adsorbent particle center assuming a spherical geometry; p is particle porosity; kf is the external film-mass transfer coefficient; Fp is the density of the particle; Dp is the pore diffusion coefficient. These equations including the initial and boundary conditions are non-dimensionalized, transformed to a set of ordinary differential equations by taking radial finite differences, and solved by the technique of method of lines.21 In this computation, 100 radial grids for a particle are used for precise prediction of concentration profiles in the pores. Pore diffusion and external mass transfer coefficients are calculated by using the following conventional correlations:22
Dp )
pDM τ
kf ) JD
us Sc2/3
(23) (24)
where DM is the molecular diffusivity of Cr(VI), τ is the tortuosity of the adsorbent, us is the fluid superficial velocity, Sc is the Schmidt number, and JD is the Colburn coefficient. 3.4. Adsorption Kinetics in Fixed-Bed. The material balance over the fixed bed for this study neglects axial dispersion and assumes constant superficial velocity and bed porosity.25,26,27 The model for the particle includes mass transfer resistances through the external liquid film and pores of SOL-IPS-F22. The pore diffusion model (eq 21) with initial and boundary conditions (eq 22a-c), parameter correlations, and the Langmuir isotherm (eq 18) are coupled and solved with the mass balance equation for the column. The single species Langmuir isotherm model is justified for use from the comparison of the equilibrium studies shown below. The column mass balance equation is given by
∂cb ∂cb ∂qj + + Fb ) 0 ∂z ∂t ∂t
(25)
∂qj 3kf (c - c|r ) R) ) ∂t RFp b
(26)
cb ) 0, z g 0, and t e 0
(27a)
cb ) cb0, z ) 0, and t > 0
(27b)
us
where Fb is the bulk density of the bed, Fb ) (1- ) Fp ) 0.69 g/cm3, is the packed column void fraction ( ) 0.33), R is the average particle radius (1.53 × 10-2 cm), c|r)R is the Cr(VI) concentration at particle outer surface, and cb0 is the Cr(VI) concentration in the feed solution. 3.5. Numerical Solution. Numerical procedures are employed to solve the system of partial differential equations for the two sets of experimental results performed in this study: the batch reactor and the fixed bed reactor. The method of lines (MOL) developed previously25,26,27 is used to solve both sets of equations simultaneously. A Fortran program that has been developed for chromium adsorption is modified and applied for this study. Details on the original program code and the model algorithm can be found elsewhere.27 The program code for chromium adsorption is provided elsewhere.17 The MOL is
Ind. Eng. Chem. Res., Vol. 47, No. 10, 2008 3405 Table 1. Material Characteristics of SOL-IPS Adsorbents13 (ND ) Nondectectable) functional group adsorbent skeleton sr. no. pore vol pore diameter (av) surface area pellet densitya pellet porositya q(H)
imidazole silica SOL-IPS-F22b ND (0.11 cm3/g) 14 Å ND (0.44 m2/g) 0.986 g/cm3 0.109 3.01 mmol/g
a
c
SOL-IPS-FD-1c ND 56.8 Å 356.9 m2/g 2.48 mmol/g
b
Calculated using correlations from ref 33. Non-hydrothermally treated. Hydrothermally treated.
Figure 4. Adsorption equilibrium isotherm of Cr(VI) at pH 2.5, 4.5, 6.0: weight of SOL-IPS-F22 ) 0.1 g; volume ) 50 mL; contact time ) 24 h; buffered by 0.1 M acetate; solid lines represent prediction of eq 18; AARD ) 2.68. Table 2. Langmuir Isotherm Constants for Eqs 16, 17, and 18 Keqa
pH
Figure 3. Adsorption equilibrium isotherm of Cr(VI) at pH 2.5, 4.5, 6.0: weight of SOL-IPS-F22 ) 0.1 g; volume ) 50 mL; contact time ) 24 h; buffered by 0.1 M acetate; solid lines represent predictions of the eqs 16 and 17; AARD ) 2.43.
applied to eqs 19-24 for the batch kinetics and eqs 21-24 and eqs 25-27 for the fixed bed. 4. Results and Discussion 4.1. Adsorbent Characterization. The results of SOL-IPS characterization for are summarized in Table 1. SOL-IPS-F22 is observed to have small pore diameter (14 Å), pore volume (0.11 cm3/g), and pellet porosity (0.109). However, the average pore size and surface area of the hydrothermally treated adsorbent SOL-IPS-FD-1 increase to 56.8 Å and 357 m2/g, respectively. It is noted that the ASAP 2000 employed to obtain these characteristics did not have the capability to measure pore volumes and surface area in the micorpore range. It is expected that reasonable values of pore volume and surface area would be present to concur with the adsorbent capacity observed. Through this treatment, the hydrogen uptake capacity decreases from 3.01 to 2.48 mmol/g. The increases of pore diameter and surface area and the decrease of uptake capacity can be explaned by the refinement of part of the silica structure through dissolution and recombination. 4.2. Equilibrium Studies. Two models were developed to describe the equilibrium adsorption isotherm data as given in eq 16 for pH 2.5 to 4.5 and eq 17 for pH g 6.0, based on the assumed chemical equilibria; and the simple Langmuir model of eq 18 for adsorption. The equilibrium adsorption isotherm obtained from the batch experiments at three pH values (2.5, 4.5, and 6.0) are shown in Figures 3 and 4 for the respective models. It is observed that the maximum Cr(VI) adsorption capacity at pH 2.5 is 2.93 mmol/g, whereas at pH 4.5 and pH 6.0 the
2.5epH e6.0 Keq1 ) 1.007 L/mmol ( 15 % 2.5epH e6.0 Keq2 ) 0.067 L‚g/mmol2 ( 71% 6.0 Keq3 ) 0.791 L‚g/mmol2 ( 16% a
[RNH+]T pH (mmol/g)
Keqb 1.075 L/mmol ( 15%
2.5
2.93
4.5
2.65
6.0
1.80
Calculated by eqs 16 and 17. b Calculated by eq 18.
maximum capacities are 2.65 and 1.80 mmol/g. On the basis of the maximum capacity observed at pH 2.5 (2.93 mmol/g) and the ligand density of 2.98 mmol/g,13 the results suggest that the chromium ions are adsorbed by a 1:1 mechanism. The optimal Langmuir constants permit a reasonable agreement between the Langmuir adsorption isotherm and the experimental equilibrium isotherm data as shown in Figure 3 and 4. Equilibrium constants of two different models: multispecies and single species Langmuir isotherm model are shown in Table 2. The simple Langmuir isotherm model represents the data well although it does not reflect the chemistry of the chromium system. A structural comparison between the two models to represent the experimental data is made by comparing the absolute average relative deviation (AARD) between the model and experimental results.
AARD )
1 m‚n
∑1 ∑1 | m
n
qj,calc - qj,exp qj,exp
|
(28)
i,j
where m is the total number of data sets, n is the number of data points for one given set, qj,calc and qj,exp are the predicted and experimental amounts of chromium adsorbed for a given data point j. The AARD results are 2.43 and 2.68 for the multispecies and single species Langmuir model, respectively. There results are acceptable for equilibrium isotherm data. The single species Langmuir model is chosen for kinetics and column adsorption analyses as the optimal loading indicates a 1:1 mechanism and the statistical analysis for this mechanism, gives satisfactory results. 4.3. Adsorption Kinetics. Three 250 mL solutions of different initial concentrations of chromium (50, 200, 750 mg/L)
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Figure 5. Adsorption kinetics of chromium on SOL-IPS-F22 for different chromium concentrations: weight of SOL-IPS-F22 ) 0.5 g; volume ) 250 mL; pHi 4.5; particle size ) 125 µm-180 µm; solid lines represent predictions of eqs 19-22: Dp ) 6.54 × 10-6 cm2/s; τ ) 2; AARD ) 0.04.
at pH 4.5 are placed in contact in a batch mode with 0.5 g of the adsorbent. As shown in Figure 5, equilibrium is reached within 30 min of contact time. The rapid adsorption equilibrium is an indication that the surface of the SOL-IPS-F22 is readily available for adsorption. In addition, this may also be due to the available open pore channels accompanied by hydrophilic pore surfaces, and rapid kinetics. The film-pore model described by eqs 21 and 22 is used to describe these kinetics. An average particle diameter of 153 µm in the range 125-180 µm is used in these calculations. The fitting parameters of the pore diffusion model are the molecular diffusivity, DM, and tortuosity, τ. The best fit values are 1.207 × 10-4 cm2/s and 2, respectively. The pore diffusion coefficient (Dp) and external mass transfer coefficient (kf) were calculated using the relationships of eqs 23 and 24 are 6.54 × 10-6 and 2.61 × 10-2 cm/s, respectively. The pore diffusion model having the minimal AARD value of 0.04 predicts the experimental data well for this adsorbent as shown in Figure 5. The experimental results are also analyzed with the shrinking core model.17 The AARD value for this model is 0.14, for the best fit diffusion coefficient of 5.39 × 10-6 cm2/s. On the basis of a comparison of the correlation coefficient obtained after fitting the experimental data with the film-pore model and shrinking core model, it is concluded that the pore diffusion model describes better the diffusion process. 4.4. Breakthrough Curve. The breakthrough experiment is performed with a 0.7 cm i.d. column packed with 0.5 g of SOLIPS-F22 to evaluate the performance of this adsorbent in a fixed bed adsorption column. This experiment is conducted by using a chromium concentration of 50 mg/L, which falls within the concentration range of industrial wastewaters. As shown in Figure 6, the sharp effluent concentration of the breakthrough curve shows that the adsorption of chromate ions with imidazole is fast and that there exists minimal mass transfer resistance in the adsorbent pores. The effluent concentrations are observed to be below 0.1 mg/L for up to 560 bed volumes, the EPA regulation limit for total chromium concentration. Approximately 1-2 ppm of chromium from the feed concentration is leached from the column after approximately 560 bed volumes until the major breakthrough occurs at 1200 bed volumes. This concentration
Figure 6. Breakthrough curve of chromium on a SOL-IPS-F22 bed: 1 bed volume ) 0.9625 mL; [Cr]feed ) 50 mg/L at pH 2.6; volume ) 2000 mL; weight of SOL-IPS-F22 ) 0.5 g; Q ) 1.1 mL/min; capacity ) 102 mg/g; Dp ) 6.54 × 10-6 cm2/s; τ ) 2. The computational grids are shown in the inset.
Figure 7. Breakthrough curve of chromium on a SOL-IPS-F22 bed showing effluent concentrations.
value of 1-2 ppm exceeds the EPA limits. This earlier leaching of chromium(VI) may be due to the presence of chromic acid H2Cr2O7, or it may be due to a chemistry shift of the chromium(VI) ions from predominantly HCrO4- to a solution with less HCrO4- and increasing concentrations of H2CrO4 as the pH decreases to approximately 2.5. We note that the pH decreases from 7.0 to 2.5 due to the presence of DI water initially in the column. The effluent concentrations as a function of the bed volume shown in Figure 7 demonstrate the chromium removal efficiency of the column. From this figure, it is clear that the SOL-IPS column can maintain concentrations less than 0.1 mg/L (shown by horizontal dotted line) in the effluent for up to 560 bed volumes. This pore diffusion model represents the experimental breakthrough curve for its slope and the breakthrough capacity (C/C0 ) 1.0) well by using the diffusivity (DM) of Cr(VI) ions and the tortuosity (τ) of the adsorbent obtained from the batch kinetics modeling. The breakthrough capacities for Cr(VI) ions are 1.96 and 1.60 mmol/g as determined by the breakthrough experiment and by the adsorption equilibrium isotherm experiments, respectively. The results of column adsorption are similar to the adsorption isotherm results. A treatment facility which uses SOL-IPS-F22 to remove Cr(VI) ions can be designed with three beds in series to take advantage of the high capacity and avoid leakage of ions before
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Figure 8. Stripping test results of chromium using 4 M HNO3: weight of SOL-IPS-F22 ) 0.5 g; 4 M HNO3 250 mL; 1 bed volume ) 0.96 mL. Table 3. Results from Batch Contact Selectivity Experimenta metal ion
% extraction
metal ion
% extraction
Cr(VI) Fe(III)
55.6 27.8
Cd(II) Zn(II)
0.0 0.11
a Conditions: equimolar amount of each metal (0.0248 mmol) at pH 3.1; SOL-IPS-F22 ) 0.2 g; volume ) 100 mL; contact time ) 24 h.
saturation is achieved. A second polishing bed can capture the leaking Cr(VI) from the first bed. The third bed could be used for the stripping and regeneration cycle. This sequence was described elsewhere by Tavlarides and Deorkar.28 Another option is to consider the trade off of higher pH, where H2CrO4 is negligible, but the sorbent capacity is decreased. 4.5. Stripping. Although sodium hydroxide is typically used as the stripping agent in several chromium sorption studies,18,29 acid solutions are used as stripping agents in this work due to the instability of the adsorbent at high pH conditions (>pH 10). Strong acids are expected to strip the adsorbed ions as the nitrate ion replaces the chromate complexes30 under these conditions. Further, these ions can form the neutral H2CrO4 at these low pH values (see Figure 1). Further protonation of the ligand occurs. On the basis of preliminary stripping studies, 4 M HNO3 solutions have the best stripping efficiency, thus this solution is used as the stripping agent for chromium. The subsequent stripping of the loaded chromium using 4 M HNO3 is shown in Figure 8. A stripping efficiency of 94.1% at 30 bed volumes and 99.0% at 121 bed volumes is obtained. To complete the cycle and prepare for subsequent adsorption, the nitrate ions from the stripping solution are rinsed from the column by washing with DI water. 4.6. Selectivity. The selectivity of SOL-IPS-F22 for different metals [Cr(VI), Cd(II), Fe(III), Zn(II)] is investigated and the results are presented in Table 3. Equimolar amounts of each metal (0.0248mmol) in 100 mL solution at pH 3.1 are contacted with 0.2 g SOL-IPS-F22. The moles of the metal ions present in the solution can produce 16% saturation of the available
Figure 9. Stability test of SOL-IPS-F22 for multiple adsorption and desorption cycles; qi is the chromium uptake capacity after the ith cycle; q0 is the chromium uptake capacity after the first cycle; loading with 100 mL of a 750 mg/L Cr(VI) solution at pH 2.5; (a) stripping with 100 mL (BV ) 263) of 4 M HNO3 and washing with 5 mL of DI water; (b) stripping with 100 mL (BV ) 263) of 1 M HNO3 and washing with 10 mL of DI water.
capacity of the adsorbent at the capacity of 2.93 mmol/g. Results show high selectivity of the adsorbent for Cr(VI) and negligible uptake of Zn(II) and Cd(II). These latter metal ions can be separated from chromium containing solutions using adsorbents with thiol functionality, such as SOL-AD-IV.25,26 The adsorption of Fe(III) is approximately half of that of Cr(VI). It may be possible to separate the iron from the chromium with suitable stripping agents. 4.8. Stability. The adsorbent stability for multiple adsorption/ desorption operation is tested in the fixed bed scheme for 11 and 20 continuous cycles using 4 M HNO3 and 1 M HNO3, respectively. The stability is measured as qi/q0 between a subsequent ith loading qi and the initial chromium(VI) loading qo. The adsorbent retains 80% of the first chromium adsorption capacity throughout the test period, with stripping efficiencies greater than 75% (Figure 9). The small fraction of the adsorbent that is not regenerated is likely due to irreversible chromium binding through strong interaction and oxidation of the imidazole group. These results show stability of SOL-IPS-F22 in multi-
Table 4. Comparison to Other Materials for Cr(VI) Adsorption material
support material
Q (mg/g)
equilibrium time
pH
SOL-IPS-F22 sawdust layered double hydroxide aliquat 336 maghemite nanoparticles aminated polyacrylonitile fiber
silica/imidazole iron(III) complex/sawdust Al, Zr, Zn, Mg/hydrotalcite solid impregnated resin/aliquat 336 maghemite/iron polyacrylonitile fiber/amine
152 173 28 10 19.2 20.7
30 min 2h 2h 3h 14 min 1h
2.5 3 6 5 2 2.4
regeneration cycles 20
6 3
ref this work 2 31 32 23 24
3408
Ind. Eng. Chem. Res., Vol. 47, No. 10, 2008
cycle column operation. The adsorbent become gelatinous because of high acid concentration after 11 cycles of operation with stripping solution of 4 M HNO3. No attempt was made to reconstitute the material to the original particles. When lower concentration of stripping solution and relatively more washing solution (10 mL of DI water) is used, the adsorbent maintains adsorption capacity and stripping efficiency up to 20 cycles of operation. Also other stripping solutions such as HCl can be evaluated for greater reuse of the adsorbent. For comparison, Lee et al.13 reported on the stability of SOL-IPS-F22 in palladium adsorption. They report that the adsorbent maintains approximately 70% of its original loading capacity for 20 cycles. The results indicate that SOL-IPS-F22 adsorbents are chemically stable to 1 M HNO3 stripping solution and that effective regeneration is achieved. 4.9. Comparison to Other Adsorbents. The chromium extraction performance of SOL-IPS-F22 is compared to other adsorbents, as shown in Table 2, where maximum adsorption capacity, equilibrium time, and regeneration cycles are compared. Among these materials found in the literature, the iron (III) complex of the carboxylated polyacrylamide-grafted sawdust and the activated carbon materials show high adsorption capacities of chromium. However, the time required to reach equilibrium is in the order of hours, whereas that for SOL-IPSF22 is in the order of minutes. This implies that SOL-IPS-F22 shows relatively minimal mass transfer resistances and that more feasible operating conditions can be employed in column adsorption schemes. Further, sawdust and activated carbon materials do not appear to be regenerable. The most attractive characteristic of SOL-IPS-F22 is its ability to reuse for an extended number of cycles (20) without major loss of its original chromium uptake capacity (∼80%). Therefore, it shows potential application for the industrial treatment of chromium containing wastewaters. 5. Conclusion A high performance organo-ceramic functionalized adsorbent with imidazole has been prepared through the sol-gel synthesis method. SOL-IPS-F22 is characterized for chromium(VI) ions separation from aqueous streams. The adsorbent exhibits a high selectivity toward chromium and negligible adsorption of zinc and cadmium. A high adsorption capacity of 2.93 mmol/g is observed at pH 2.5; however, the capacity decreases with increase in solution pH. The adsorption equilibrium data are represented by the Langmuir isotherm model. The experimental values of single species Langmuir isotherm constant are 1.075 L/mmol in the equilibrium pH range of 2.5 to 6.0. In the multispecies Langmuir isotherm model, equilibrium constants are 1.007 L/mmol, 0.067 L‚g/mmol,2 and 0.791 L‚g/mmol2 for Keq1, Keq2, and Keq3, respectively. Analysis of chromium(VI) adsorption indicates a 1:1 complexation mechanism between imidazole sites and chromium species. Kinetic studies confirm that almost all chromium(VI) ion removal takes place within 30 min for all concentration ranges and conditions studied. In fixed bed adsorption, the effluent concentration satisfies EPA regulations of less than 100 ppb up to 560 bed volumes. An intermediately small breakthrough of one to two percent of feed chromium concentration occurs from 560 to 1200 bed volumes after which a sharp breakthrough occurs to inlet concentration. A three adsorption bed sequence could be used to take maximum advantage of the SOL-IPS-F22 capacity, while achieving EPA chromium(VI) discharge regulation. Chromium stripping is found viable for a fixed bed adsorption unit with
stripping efficiencies greater than 99% using 4 M HNO3. Moreover, stability of SOL-IPS-F22 over multiple adsorption/ desorption cycles is demonstrated with 90% of the original capacity retained after 15 cycles, and loss of 25% of capacity until 20 cycles of operation. However, using high acid concentration of stripping solution can interfere with the reuse of column. Nomenclature R ) ionization fraction c ) chromium(VI) concentration in the pore, mmol/L cb ) chromium(VI) concentration of the bulk, mmol/L cbo ) initial chromium(VI) concentration of the bulk, mmol/L cs ) chromium(VI) concentration at the pellet surface, mmol/L cT ) total chromium(VI) concentration, mmol/L Dp ) pore diffusion coefficient, cm2/s DM ) molecular diffusion coefficient, cm2/s K ) equilibrium constant, L/mmol Keq1 ) equilibrium constant, L/mmol Keq2 ) equilibrium constant, L‚g/mmol2 Keq3 ) equilibrium constant, L‚g/mmol2 kf ) film coefficient, cm/s q ) local concentration in the pellet, mmol of Cr/g of adsorbent r ) radial direction of the pellet, cm Rp ) radius of pellet, cm qmax ) max capacity of the adsorbent, mmol/g t ) time, min us ) superficial velocity, cm/s V ) volume of solution, L VR ) volume of reactor, L VT ) volume of tank, L z ) axial direction in the column, cm τ ) particle tortuosity p ) pellet porosity b ) bed porosity Fp ) pellet density, g/cm3 Fb ) bed density, g/cm3 Fs ) solid density of the adsorbent, g/cm3 θ ) corrected time of column calculations, t-z/ us; min, s Acknowledgment The financial support from the National Science Foundation through Grant CTS-0120204 is gratefully acknowledged. Literature Cited (1) Deng, S.; Bai, R. Removal of trivalent and hexavalent chromium with aminated polyacrylonitrile fibers: performance and mechanisms. Water Res. 2004, 38, 2423-2431. (2) Unnithan, M. R.; Anirudhan, T. S. The Kinetics and Thermodynamics of Sorption of Chromium(VI) onto the Iron(III) complex of a Carboxylated Polyacrylamide-Grafted Sawdust. Ind. Eng. Chem. Res. 2001, 40, 26932701. (3) Lazaridis, N. K.; Pandi, T. A.; Matis, K. A. Chromium(VI) Removal from Aqueous Solutions by Mg-Al-CO3 Hydrotalcite: Sorption-Desorption Kinetic and Equilibrium Studies. Ind. Eng. Chem. Res. 2004, 43, 22092215. (4) USEPA Analytical Feasibility Support Document for the Six-Year ReView of Existing National Primary Drinking Water Regulations, Office of Ground Water and Drinking Water; EPA 815-R-03-003; EPA: Washington, DC, March 2003. (5) USEPA Occurrence Summary and Use Support Document for the Six-Year ReView of National Primary Drinking Water Regulations, Office of Water; EPA-815-D-02-006; EPA: Washington, DC, March 2002. (6) Chun, L.; Hongzhang, C.; Zuohu, L. Adsorptive removal of Cr(VI) by Fe-modified steam exploded wheat straw. Process Biochem. 2004, 39, 541-545.
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ReceiVed for reView December 14, 2007 ReVised manuscript receiVed March 3, 2008 Accepted March 5, 2008 IE7017096
