Adsorptive Removal of Phosphate Ions from Aqueous Solution Using

Jan 9, 2007 - solutions using synthetic HSZ 330 HUD Zeolite (Si/Al ratio: 2.75-3.25) and its ... affinity of HUD for phosphate ions was improved throu...
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Ind. Eng. Chem. Res. 2007, 46, 894-900

Adsorptive Removal of Phosphate Ions from Aqueous Solution Using Synthetic Zeolite Maurice S. Onyango,*,† Dalibor Kuchar,‡ Mitsuhiro Kubota,§ and Hitoki Matsuda‡,§ Centre for Process Engineering, Department of Process Engineering, UniVersity of Stellenbosch, Stellenbosch, PriVate Bag X1, Matieland, 7602, South Africa, and Department of Chemical Engineering and Departement of Energy Science and Engineering, Nagoya UniVersity, Nagoya 464-8603, Japan

Phosphate ions are responsible for the algal bloom in receiving water bodies and aesthetic problems in drinking water. From the environmental and economic points of view, management of such a contaminant and valuable resource is very important. The present paper deals with the removal of inorganic phosphate ions from aqueous solutions using synthetic HSZ 330 HUD Zeolite (Si/Al ratio: 2.75-3.25) and its Al3+-activated form (AlHUD) as adsorbents. Equilibrium and kinetic experiments were performed to study the effects of operating conditions such as adsorbent mass, solution pH, coexisting ions, and initial concentration on either the capacity or the rate of phosphate uptake by the zeolites. As a result, it was found that the efficiency of phosphate removal by the zeolites increased with an increase in adsorbent mass and a decrease in solution pH. Outersphere complex-forming ions such as nitrate, sulfate, and chloride improved slightly the phosphate removal efficiency while fluoride ions, which form inner-sphere complexes with binding sites, reduced the phosphate uptake capacity of the sorption sites. Equilibrium data were well-fitted with Langmuir, Freundlich, and Dubbinin-Radushkevitch isotherms. In the kinetic study, mass-transfer processes in the external and internal matrix of the sorbent were considered. The overall adsorption process was assumed to be controlled by intraparticle diffusion. Introduction Phosphate is found in water streams due to anthropogenic activities or geochemical processes. It is an essential nutrient for growth of microorganisms, and thus, the presence of phosphate in water streams has been known to cause algal bloom in receiving water bodies. This has led to deterioration of water quality of the receiving water bodies as a result of the depletion or reduction of oxygen levels. In water distribution systems, phosphate presence is associated with aesthetic problems. More attention has been devoted toward the reduction of phosphate levels in water streams as a consequence of severe legislation that has been adopted in many countries. In agricultural areas where phosphate fertilizers are used, the recovery of phosphate from waste streams is necessary since it is envisaged that this resource will be depleted in the near future.1 There are several treatment options for reducing the levels of phosphate in water streams with chemical precipitation and biological nutrient removal being the most commonly applied methods.2 The precipitation and biological treatment techniques, however, are sensitive to seasonal and diurnal variations in temperature and changes in feed concentrations.2 Moreover, achieving low phosphate concentrations is difficult. In view of these limitations, other techniques have been investigated. Adsorption is a robust and effective technique used in water and wastewater treatments and produces high quality product (water/wastewater).3 The success of an adsorption technology depends on the choice of an appropriate sorbent. In choosing a sorbent, the screening strategies commonly considered are capacity, regenerability/ reuse, local availability, compatibility, * To whom correspondence should be addressed. Fax: 27-21-8082059. E-mail: [email protected]. † University of Stellenbosch. ‡ Department of Chemical Engineering, Nagoya University. § Departement of Energy Science and Engineering, Nagoya University.

kinetics, and cost. Given the fact that no sorbent can singly be isolated that meets all the required traits, a lot of attention has, therefore, been devoted to the development of several potential sorbents4-8 that may suit given local conditions. In recent years, several researchers have shown that zeolites can be used in removing both health hazardous cations and anions from water.9-11 Previously, we have specifically shown that surface tailored synthetic zeolites are good sorbents for fluoride and arsenic.12-13 In this work, HUD, which is a kind of positively charged zeolite in the acidic and near neutral pH range, was applied in removing phosphate from water. The affinity of HUD for phosphate ions was improved through activation of the zeolite sorbent in an aluminum salt solution. The zeolite sorbents were then characterized and their phosphate adsorption behavior was studied under varying experimental conditions such as adsorbent mass, solution pH, coexisting ions, and initial concentration. The results are presented in terms of equilibrium isotherm and sorption kinetics. The analysis of the sorption kinetics considers diffusive transport only. To avoid the mathematical complexity of coupling the differential equations describing each step of the diffusion transport, the external and internal diffusions are analyzed separately. It is assumed that the overall uptake of phosphate ions is controlled by the intraparticle diffusion while external mass transfer is only dominant at near time zero. Materials and Methods Zeolite Adsorbent. In this work, HUD zeolite (H+ form; Si/ Al ratio: 2.75-3.25) manufactured by Tosoh Chemical, Japan, was used in batch equilibrium and kinetic studies. Separately, Al3+-activated HUD zeolite was prepared for batch studies by adding 50 g of HUD to 1 L of 0.075 M aluminum sulfate solution in accordance with a similar procedure presented elsewhere.12 The mixture was stirred for 2 days and then washed several times using demineralised water to lower the electrical

10.1021/ie060742m CCC: $37.00 © 2007 American Chemical Society Published on Web 01/09/2007

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conductivity. Finally, the Al3+-activated HUD zeolite was airdried at room temperature for 2 days. The zeolite prepared in this manner is hereafter referred to as Al-HUD. The electrokinetic properties of the HUD and Al-HUD zeolites were determined at various pH values by measuring the zeta potential of the zeolite samples using an electrophoretic light scattering spectrophotometer (ELS-7300K, Otsuka Electronics Co., Japan). From the zeta potential measurements, the pHpzc of each zeolite sorbent was determined. In addition, the pH of the sorbents in water was determined and the sorbents surface fractal dimension was evaluated from nitrogen adsorption isotherm. Effect of Adsorbent Mass. The effect of adsorbent mass was studied by varying the amount of each adsorbent from 0.05 to 0.30 g. The amount of each zeolite type was added to 50 mL of a 100-mg/L phosphate solution (pH 5.7) contained in plastic bottles. The bottles were placed in a thermostatic shaker and were agitated at 298 K for 7 days. At the end of the experiment, samples were withdrawn from the test bottles and filtered through a 0.2-µm syringe filter and the residual phosphate concentration was measured by an ion chromatograph with a column (Tosoh, TSKgel IC-Anion-PW) using an electroconductivity detector (Shimadzu, CDD-10AVP). Effect of Initial Solution pH. Experiments were carried out by varying initial solution pH values from 2.5 to 10 using 0.1 M NaOH or HCl, according to the same procedure and analytical technique used for the study of the effect of adsorbent mass. Accordingly, adsorption was done by adding 0.05 g of each zeolite adsorbent to 50 mL of a 100-mg/L phosphate solution contained in plastic bottles. The bottles were placed in a thermostat shaker and were agitated at 298 K for 7 days. Effect of Coexisting Ions. Experimental runs were carried out to assess the effect of coexisting ions such as sulfate, nitrate, chloride, and fluoride on phosphate uptake. In this case, the phosphate concentration was fixed at 100 mg/L while the concentrations of coexisting ions were varied. The experimental and analytical techniques were similar to those reported above. Equilibrium Experiments. To obtain data for the phosphate sorption at equilibrium, a fixed amount (0.05 g) of the sorbents was contacted with phosphate-containing aqueous solutions at 298 K. Samples of 50 mL of phosphate ion solutions at initial concentrations ranging from 50 to 300 mg/L were pipetted into 100-mL plastic bottles. The bottles were placed in a thermostatic shaker and shaken for 7 days at 298 K. At the end of the experiment, samples were withdrawn from the test bottles and filtered through a 0.2-µm syringe filter and the residual phosphate concentration was measured by an ion chromatograph. The equilibrium sorption capacity was determined from

qe )

Co - Ce F

(1)

where Co and Ce are the initial and equilibrium bulk-phase phosphate concentrations, respectively, qe is the amount adsorbed at equilibrium, and F is the ratio of adsorbent mass to adsorbent-free volume. Batch Kinetic Experiments. Short-term kinetic experiments were carried out in a 1-L stirred tank batch adsorber operated at about 298 K to analyze the mechanism of transport during phosphate uptake. The effect of the initial concentration on phosphate sorption kinetics using a uniform sorbent grain fraction of 0.15-0.30 mm was studied. At time zero and at selected time intervals thereafter (up to 8 h), 5-mL samples were taken and filtered through 0.2-µm syringe filters; the concentration of phosphate at any time was determined by an ion chromatograph. For the purpose of reproducibility of the results,

Table 1. Selected Zeolite Adsorbent Characteristics adsorbents

HUD

(before activation) silica/alumina ratioa pHpzc pH in water surface fractal dimension skeletal density (g/cm3) BET surface area (m2/g) net Al-exchanged a

Al-HUD

H+

forma

2.75-3.25 6.5 4.5 2.90 0.885 500a

7.5 4.3 2.86 0.885 642 negligible

Information supplied by the manufacturer.

the experiments were done in replicates and the results reported are averages of the experimental data. The amount of phosphate, q, adsorbed at any time, t, was calculated from

q)

Co - Ct F

(2)

where Ct is the bulk-phase phosphate concentration. Preliminary Desorption Studies. The desorption of phosphate loaded on HUD and Al-HUD zeolite particles was done using sodium hydroxide as a desorbing agent. A fixed amount (0.1 g) of each sorbent was contacted separately with 50 mL of sodium hydroxide solution contained in 100-mL plastic bottles. The bottles were placed in a thermostatic shaker and shaken for 1 day at 150 rpm. At the end of the experiment, samples were withdrawn from the test bottles and filtered through a 0.2µm syringe filter and the concentration of phosphate ions was determined by an ion chromatograph. Results and Discussion Adosrbents Physicochemical Properties. The H+-form synthetic HUD zeolite has irregularly shaped microparticles of size of about 1 µm. The Si/Al ratio is in the range 2.75-3.25, giving rise to high concentrations of terminal aluminol sites (d AlOH) when in contact with water. It is the terminal aluminol sites that are responsible for phosphate sorption. To further create more sites for sorption, the original zeolite material was activated in aluminum sulfate solution.12 The effect of activating the HUD zeolite was analyzed by measuring changes in electrokinetic properties, fractal dimension, BET surface area, pH in water, and skeletal density. Table 1 summarizes the HUD and Al-HUD sorbents’ physicochemical properties. It is observed that, upon surface activation, there were modulations in the electrokinetic properties and surface area of the HUD sorbent. Effect of Adsorbent Mass. In assessing a given technique for (waste)water purification, regulatory compliance is a key factor to consider. The ability of HUD and Al-HUD zeolites to remove phosphate to acceptable levels was thus tested by varying the masses of the zeolite adsorbents at a fixed initial phosphate concentration of 100 mg/L. The results of the residual phosphate concentration against amounts of different zeolites are shown in Figure 1. The residual phosphate concentration decreased with an increase in the amount of adsorbent due to an increase in the number of active sites. From an initial concentration of 100 mg/L, the zeolites were able to remove phosphate to concentrations below detectable levels. However, the removal efficiency was higher for Al-HUD zeolite. Meanwhile, the high efficiencies achieved in this work may be due to a higher concentration of terminal dAl-OH sites, both protonated and neutral, which leads to a greater capacity for a ligand exchange reaction.9 Effect of pH. An understanding of the effect of solution chemistry on an adsorption process is very important since the mechanism of adsorbate removal can be deduced from such a

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Figure 1. Effect of adsorbent mass on phosphate removal from water at 298 K: initial concentration, 100 mgPO43-/L; pHi, 5.7.

Figure 3. Effect of coexisting ions on phosphate uptake by HUD: adsorbent dose, 1 g/L; pHi, 5.7; initial concentration, 100 mgPO43-/L.

Figure 2. Effect of pH on phosphate uptake at 298 K: adsorbent dose, 1 g/L; initial concentration, 100 mgPO43-/L.

study. Moreover, the solution chemistry of water varies from one water source to another. It is therefore important to know how this variation may affect the removal of a target ion. The factors from solution chemistry that influence the adsorption process are the solution pH and coexisting ions. The effect of the latter is dealt with in the next section. A study was undertaken to ascertain to what extent the initial solution pH (varied from 2.5 to 10) affected the sorption of phosphate on HUD and Al-HUD zeolites. Figure 2 shows that the uptake of phosphate for each zeolite decreases with an increase in pH. However, Al-HUD shows a higher uptake ability. Reed et al.14 identified two forces that play a role in an adsorption process. These are the following: chemical interaction and electrostatic forces. The latter gives rise to Coulombic attraction or repulsion between binding sites and adsorbing ions. To understand how these forces affected the phosphate sorption behavior under different pH values, the following reactions are considered. Surface charge property of active sites (aluminol sites) on zeolite

dAlOH + H+ T dAlOH2+, pH < pHpzc

(3)

dAlOH + OH- T dAlO- + H2O, pH > pHpzc (4) Typical sorbent-phosphate reactions forming monodentates8

where dAlOH, dAlOH2+, and dAlO- are the neutral, protonated, and hydroxylyzed aluminol site, respectively. At solution

Figure 4. Effect of coexisting ions on phosphate uptake by Al-HUD: adsorbent dose, 1 g/L; pHi, 5.7; initial concentration, 100 mgPO43-/L.

pH values above 2, a large fraction of negative monovalent H2PO4- ions and a small fraction of undissociated H3PO4 dominate in solution.15 The binding sites are protonated (d AlOH2+) at such pH values. Consequently, a higher Coulombic attraction between the binding sites and H2PO4- ions (eq 5) in addition to chemical interaction (or molecular adsorption) leads to a higher phosphate uptake. As the solution pH is raised toward a neutral value, the magnitude of Coulombic attractive force reduces as the active sites become neutral. Subsequently, a typical adsorption reaction expressed by eq 6 that proceeds by chemical interaction involving ligand exchange dominates. In the absence of a Columbic component or in the presence of a reducing Coulombic component of the overall adsorption force, the quantities of phosphate adsorbed monotonously decrease with an increase in pH (see Figure 2). Above pHpzc, the binding sites have negative net charge (dAlO-). This enhances Coulombic repulsion between the sites and the phosphate ions leading further to a decrease in quantities of adsorbed phosphate ions. Equations 5 and 6 have only considered, for illustration purposes, the formation of monodentate complexes. However, details of the possibility of formation of such complexes as bidentate and binuclear during metal (hydr)oxide-phosphate interaction have been reported.8 Effect of Coexisting Ions. In this study, phosphate adsorption behavior is evaluated against two kinds of coexisting ions: those that form outer-sphere complexes (chloride, nitrate, and sulfate) and those that form inner-sphere complexes with binding surfaces, e.g., fluoride. The concentration of phosphate ions was fixed at 100 mg/L while the coexisting ions concentration was varied from 0 to 100 mg/L. Figures 3 and 4 present the results of phosphate uptake by HUD and Al-HUD as a function of the initial concentrations of the coexisting ions. On the one hand, the uptake of phosphate increased slightly, without any trend, in the presence of chloride, nitrate, or sulfate ions. A similar

Ind. Eng. Chem. Res., Vol. 46, No. 3, 2007 897 Table 2. Summary of Equilibrium Isotherm Parameters for Phosphate Uptake isotherm Langmuir HUD Al-HUD Freundlich HUD Al-HUD DR model Figure 5. Equilibrium isotherm plots at 298 K. Solid lines represent the simulation using the Freundlich isotherm.

trend was observed and discussed in detail by Chubar et al.16 On the other hand, the presence of fluoride significantly suppressed the phosphate adsorption. As the initial concentration of fluoride was increased, the quantities of phosphate adsorbed on HUD and Al-HUD decreased from 48.85 and 56.6 mg/g (in the absence of fluoride) to 35.9 and 29.1 mg/g (when the initial fluoride concentration was 100 mg/L), respectively. Since both phosphate and fluoride form inner-sphere complexes with metal oxide/hydroxide surfaces, the decrease in phosphate uptake in the presence of fluoride indicates that both ions competed for the same sorption sites. Adsorption Isotherms. Experimental data of phosphate adsorption onto HUD and Al-HUD zeolites at 298 K are shown in Figure 5. In the low concentration region, the amount of phosphate adsorbed on the zeolites sharply increased with an increase in equilibrium concentration, giving an indication of the high affinity of the binding sites for phosphate ions. This is a good attribute of the materials tested in this study since a high uptake at low equilibrium concentration will enable the treatment of a large volume of water before replacement or regeneration of the adsorbents. At high concentration, the increase in quantities adsorbed is gradual as a result of an almost full occupancy of the active sites. In modeling a fixed bed, equilibrium data are often required. Thus, it is important to describe equilibrium data with isotherms that give the minimum deviation between the model and experimental data. To achieve this, it is a common practice to test various isotherms, which in most cases are based on different premises. Moreover, the isotherms can be used to evaluate the capacity and nature of the interaction between a sorbent and a sorbate. This work considers the Langmuir, Freundlich, and Dubbinin-Radushkevitch isotherm models. The Langmuir isotherm is given by

qe )

qo,LbCe 1 + bCe

(7)

where qo,L is the Langmuir maximum adsorption capacity and b is a parameter indicative of adsorption affinity. The linearized form of eq 7 was used to fit the equilibrium data from which the Langmuir parameters, together with regression coefficients, were extracted and are summarized in Table 2. The Langmuir parameters b and qo,L are 0.034 L/mg and 79.4 mg/g for HUD zeolite and 0.085 L/mg and 75.8 mg/g for Al-HUD zeolite. The large difference in the affinity coefficient b is due to the fact that Al-HUD has a higher phosphate uptake ability in the lowconcentration range. The Freundlich isotherm model was applied in the nonlinear form expressed in eq 8

qe ) KFCe1/n

(8)

HUD Al-HUD

parameters qo,L (mg/g)

b (L/g)

R2

79.4 75.8

0.034 0.085

0.902 0.986

KF (mg/g)

1/n

R2

15.83 31.28

0.29 0.16

0.990 0.993

qo,DR (mg/g)

E (kJ/mol)

R2

130.5 92.9

13.4 (ion exchange) 19.6 (ion exchange)

0.981 0.980

where KF and 1/n are Freundlich isotherm parameters related to adsorption capacity and energetic heterogeneity, respectively. The Freundlich isotherm parameters together with regression coefficients are summarized in Table 2. The Freundlich isotherm gives the best representation to our data points. The 1/n values are less than 1 suggesting that if the equilibrium data truly follow the Freundlich mechanism then the actives sites were energetically heterogeneous. The Dubinin-Radushkevitch (DR) isotherm model was also applied. The model is expressed as follows

qe ) qo,DR exp(-β2)

(9)

where qo,DR is the DR maximum adsorption capacity, β is a constant related to sorption energy, and  is the Polanyi potential. The DR isotherm parameters, β, evaluated from eq 9 are 0.0028 and 0.0013 mol2/kJ2 for HUD and Al-HUD zeolites, respectively. The qo,DR values, on the other hand, are 130.5 and 92.9 mg /g for HUD and Al-HUD zeolites, respectively. The determined qo,DR values are unreliably large and inconsistent with the experimental results probably due to the shape of the isotherms. To evaluate the nature of interaction between phosphate and the binding sites, the mean free energy of sorption (E ) (2β)-0.5) per mole of the sorbate was determined. The free energies of adsorption of phosphate onto HUD and AlHUD zeolites are 13.4 and 19.6 kJ/mol, respectively, suggesting that the interaction between phosphate and the zeolites proceeded by ion exchange.12,17 This is in agreement with the premise that phosphate interaction with aluminol sites involves ligand exchange between phosphate and hydroxyl ions and complexation. Sorption Kinetics. Zeolites are bidispersed sorbents. In the current kinetic experimental setup, the zeolite sorbent particles are suspended in phosphate-containing aqueous solution of finite volume and sufficiently agitated to reduce the resistance to mass transfer across the laminar film boundary layer (represented by τ) surrounding the particles (Figure 6). As a result of mass transport (stages 1-3) and reaction kinetics (stage 4), the time course of phosphate sorption shows a two-stage phenomenon, typical of most sorption processes. The first stage is rapid and is evident during the initial period (Figures 7 and 8) while the second stage lags over the entire sorption period as the concentration gradient between the phosphate in the bulk and sorbed phase is attenuated. The kinetic analysis considers masstransfer processes outside and inside the sorbent matrix. It is assumed that the affinity of phosphate for the active sites is strong so that the mass-transfer process inside the particles is rate limiting. (A) Extraparticle Transport. The evaluation of the mass transfer of phosphate ions to the zeolite surface is done by

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Figure 9. Concentration decay curves for phosphate removal from water by HUD zeolite: adsorbent dose, 2 g/L; pHi, 5.7; temperature, 298 K.

Figure 6. Single zeolite adsorbent particle suspended in fluid.

Figure 7. Sorption kinetics of phosphate onto HUD zeolite: adsorbent dose, 2 g/L; pHi, 5.7; temperature, 298 K. Figure 10. Concentration decay curves for phosphate removal from water by Al-HUD zeolite: adsorbent dose, 2 g/L; pHi, 5.7; temperature, 298 K. Table 3. Summary of Diffusion Parameters diffusion kinetic parameters initial conc (mg/L)

kf ×10-3 (cm/s)

ro/kf (s)

50 100 150 200

1.86 1.25 1.10 0.90

6.08 9.04 10.27 12.56

50 100 150 200

2.99 2.15 1.78 1.48

De ×10-11 (cm2/s)

ro2/De (s)

NBi

1.19 4.20 9.51 8.39

1.07 × 107 3.04 × 106 1.34 × 106 1.46 × 106

>>100 >>100 >>100 >>100

Al-HUD 3.78 5.26 2.08 6.35 8.40 7.64 46.60

6.14 × 106 1.52 × 106 2.74 × 105

>>100 >>100 >>100

HUD

Figure 8. Sorption kinetics of phosphate onto Al-HUD zeolite: adsorbent dose, 2 g/L; pHi, 5.7; temperature, 298 K.

determining the film mass-transfer coefficients according to a simple method in which it is assumed that film diffusion resistance is dominant at or near time zero.18 Accordingly, the method involves the calculation of the initial slope of the concentration against time curve and substituting the obtained value into eq 10

d(Ct/Co) |t)0 ) -kfSA dt

(10)

where kf is the film mass transfer coefficient (cm/s) and SA is the specific surface area of the zeolites (cm-1). In order to apply eq 10, concentration decay curves were plotted (see Figures 9 and 10), and for the purposes of calculation, the slope was determined at t ) 1 min. The values of the obtained slopes were coupled into eq 10, in order to determine the film masstransfer coefficient for each sorbent and the initial concentration used. Table 3 summarizes the results obtained. The film masstransfer coefficients are on the order of magnitude of 10-3 cm/s and are observed to decrease with an increase in the initial concentration for both adsorbents. However, the magnitude of the film mass-transfer coefficient is higher for the case of Al-

HUD compared to HUD. This could be due to the differences in the initial attractive forces near the solid-liquid interface since Al-HUD had a stronger zeta potential at the solution pH used in this study. (B) Intraparticle Transport. When phosphate ions are transported passively in the internal matrix of zeolite adsorbent, two transport modes acting in parallel are involved. These are pore (molecular and Knudsen) and surface diffusions (Figure 6). Pore diffusion is dominant in macropores while surface diffusion, which involves hoping of adsorbed phosphate ions from site to site, dominates in the microparticles. In modeling, the two transport modes are combined in the form of an effective intraparticle diffusion. Thus, the intraparticle mass transfer for spherical geometry can be described by

∂q 1 ) De 2 ∂t r

( ∂q∂r)

∂ r2

∂r

(11)

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where De is the effective intraparticle diffusion coefficient, r is the radial dimension, and t is the time. The analytical solution of eq 11 for an adsorption in a bath of finite volume in which the concentration of the probe ion decays with time is expressed as19

{

6 exp(-βn2Det/ro2)

∞

q ) qe 1 -

∑

n)19Λ

∞

/(1 - Λ∞) + (1 - Λ∞)βn2

}

(12)

where βn represents the positive nonzero roots of eq 13

tan βn )

3βn 3 + (1/Λ∞ - 1)βn2

(13)

and Λ∞ ) (Co - Ce)/Co is the ultimate fraction of the solute (phosphate) adsorbed. In calculating the theoretical uptake curves according to eqs 12 and 13, a diffusion time constant θ ) De/ro2, which is an indication of the intraparticle sorption rate, was defined and used as a fitting parameter. Moreover, only 50 terms (n ) 50) were found to be sufficient and used in eq 12. The best-fit θ value was determined by minimizing the objective function φ expressed as

φ ) 100

x

Σ((qexp - qcal)/qexp)2 Ni - 1

(14)

where qexp is the experimental phosphate uptake, qcal is the calculated amount of phosphate ion sorbed, and Ni is the number of data points. The intraparticle diffusion parameters, the effective intraparticle diffusion coefficient (De), and the intraparticle diffusion time (1/θ ) ro2/De) are summarized in Table 3. The effective diffusion coefficients are in the range of 10-11-10 10 cm2/s and show significant dependence on initial concentration. Such values are common with chemisorption systems.20 The magnitude of the effective diffusion coefficients is on the same order of magnitude for both HUD and Al-HUD zeolite. Internal structural similarity may have been the cause for this observation. From nitrogen adsorption-desorption, both HUD and Al-HUD zeolite were characterized with large pores and surface area. When intraparticle diffusion time (1/θ) was compared with the extraparticle diffusion time (ro/kf) for both of the sorbents, the former was found to be much longer than the latter, indicating that intraparticle diffusion was slower. This is further shown by the values of the Biot number, which are >>100. Preliminary Desorption Studies. Adsorption is a wellestablished technology for water and wastewater purification. However, the success of this technology, for environmental and economic reasons, depends on the possibility of desorbing the target contaminant and reusing the adsorbent. For a valuable resource such as phosphate ions, desorbability is also important as a means of resource recovery. Thus, desorption of phosphate ions from HUD and Al-HUD adsorbents was carried out at various pH values. The pH of the solution was adjusted using either nitric acid or sodium hydroxide. Figure 11 shows the phosphate desorption results. The quantities of phosphate ions desorbed increased with an increase in pH of the desorbing solution and was higher for Al-HUD zeolite. Specifically, the percentage desorption increased from 7 to 38 and 9 to 56 for a change in pH from 5.8 to 11.83 for HUD and Al-HUD zeolites, respectively. The desorption results suggest that phosphate ions are tightly bound to the zeolite adsorbents. Therefore, an in situ

Figure 11. Effect of pH on phosphate ion desorption from zeolite sorbents.

column desorption employing various kinds of desorbing agents, desorption using other techniques, and testing phosphate-loaded zeolite as zeoponic are suggested for further studies. Conclusions The adsorptive removal of inorganic phosphate ions from aqueous solutions has been studied. A synthetic HUD zeolite with high internal surface area and its Al3+-activated (Al-HUD) form have been used. As a result, it was found that the efficiency of phosphate removal by the zeolites was reduced with an increase in solution pH and a decrease in sorbent mass. Ions such as nitrates, sulfates, and chlorides that form outer-sphere complexes with binding sites improved slightly the phosphate removal efficiency while fluoride ions, which form inner-sphere complexes with binding sites, reduced the phosphate capacity of the active sites. Equilibrium data were well-fitted with Langmuir, Freundlich, and Dubbinin-Radushkevitch isotherms. The kinetic data were analyzed using extraparticle and intraparticle diffusion models. From the models, values of the masstransfer coefficients were estimated and their relative importance was discussed. The zeolites have shown to be potential sorbents for the removal of phosphate ions from water. Preliminary desorption studies have given an indication that phosphate ions are tightly bound to the zeolites. An in situ desorption and regeneration in a column, the use of other desorption techniques, and testing of phosphate-loaded zeolite as zeoponic are proposed for further investigations. Nomenclature Cb ) bulk-phase concentration defined in Figure 6 (mg/L) Ce ) concentration of phosphate at equilibrium (mg/L) Co ) initial concentration of phosphate (mg/L) Cr)0 ) concentration of phosphate at the center of sorbent as defined in Figure 6 (mg/L) Cs ) concentration of phosphate at the sorbent surface (mg/L) Ct ) concentration of phosphate in the bulk phase at any time (mg/L) De ) effective intraparticle diffusion coefficient (cm2/s) E ) mean free energy (kJ/mol) KF ) Freundlich constant (L/g) kf ) film mass-transfer coefficient (cm/s) 1/n ) constant in Freundlich model Ni ) number of data points NBi ) Biot number pHi ) initial pH q )amount of phosphate sorbed at time t per mass of sorbent (mg/g) qcal ) calculated amount of phosphate sorbed per unit mass of sorbent (mgF/g)

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qeq ) amount of phosphate sorbed at equilibrium per mass of sorbent (mg/g) qexp ) experimental amount of phosphate sorbed per unit mass of sorbent (mgF/g) qo,L ) Langmuir maximum capacity per mass of sorbent (mg/ g) qo,DR ) DR maximum capacity per mass of sorbent (mg/g) qr)0 ) amount of phosphate sorbed at the center of sorbent (mg/g) qs ) amount of phosphate sorbed on the surface per mass of sorbent (mg/g) ro ) radius of the sorbent (cm) R2 ) regression coefficient SA ) specific surface area (1/cm) t ) time (min) Greek Letters β ) constant in DR model (mol2/kJ2) βn ) positive nonzero roots of eq 13  ) Polanyi potential (kJ/mol) F ) sorbent mass to particle-free liquid volume (g/L) φ ) objective function θ ) diffusional time constant (1/s) Λ∞ ) ultimate fraction of phosphate adsorbed at equilibrium Literature Cited (1) Levlin, B.; Hultman, B. Phosphorus Recovery from Phosphate Rich Sidestreams in Wastewater Treatment Plants. www.lwr.kth.se (accessed September 2006). (2) Zhao, D.; Sengupta, A. K. Ultimate Removal of Phosphate from Wastewater Using a New Class of Polymeric Ion Exchangers. Water Res. 1998, 32, 1613. (3) Choy, K. K. H.; Porter, J. F.; McKay, G. Film-Pore Diffusion ModelssAnalytical and Numerical Solutions. Chem. Eng. Sci. 2004, 59, 501. (4) Kuzawa, K.; Jung, Y.-J.; Kiso, Y.; Yamada, T.; Nagai, M.; Lee, T.-G. Phosphate Removal and Recovery with a Synthetic Hydrotalcite as an Adsorbent. Chemosphere 2006, 62, 45. (5) Namasivayam, C.; Sangeetha, D. Equilibrium and Kinetic Studies of Adsorption of Phosphate onto ZnCl2 Activated Coir Pith Carbon. J. Colloid Interface Sci. 2004, 280, 359. (6) Oguz, E. Removal of Phosphate from Aqueous Solution with Blast Furnace Slag. J. Hazard. Mater. 2004, 114, 131.

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ReceiVed for reView June 11, 2006 ReVised manuscript receiVed November 22, 2006 Accepted November 22, 2006 IE060742M