Adsorptive Removal of Phosphate and Nitrate Anions from Aqueous

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Ind. Eng. Chem. Res. 2007, 46, 8806-8812

Adsorptive Removal of Phosphate and Nitrate Anions from Aqueous Solutions Using Ammonium-Functionalized Mesoporous Silica Safia Hamoudi,* Rabih Saad, and Khaled Belkacemi Department of Soil Sciences and Agri-Food Engineering; UniVersite´ LaVal, Que´ bec, Canada, G1K 7P4

Adsorption of nitrate and monovalent phosphate anions from aqueous solutions on ammonium-functionalized mesoporous MCM-48 silica was investigated. The adsorbent was prepared via a post-synthesis grafting method, using aminopropyltriethoxysilane, followed by acidification in HCl solution to convert the attached surface amino groups to ammonium moieties. The adsorbent was determined to be effective for the removal of both anions. The effects of pH, temperature, initial concentration of anions, and adsorbent loading on both anions adsorption were examined. At ambient temperature, the removal of nitrate was maximum at pH 8, with the increasing basicity of the adsorption medium, the equilibrium anion uptake was negatively affected by the pH. As for phosphate anions, the pH dependency profile denoted an optimal zone in the pH range of 4-6. According to the titration curve of H3PO4 with NaOH,19 phosphate anions occur either as monovalent H2PO4-, divalent HPO42-, or trivalent PO43- forms, depending on the solution pH (pK1 ) 2.2, pK2 ) 7.2, pK3 ) 12.4). Therefore, a pH range of 4-6 corresponds to the occurrence of monobasic phosphate anions (H2PO4-). Furthermore, pH influences the charge of the grafted propylamine groups on the mesoporous silica walls. Indeed, the pKa value for propylamine groups grafted on different mesoporous silica materials, as well as on amorphous silica gels, was evaluated to be ∼9,16,20 while the pKa value of propylamine is 10.6.21 Hence, when pH < pKa, the amino groups are charged positively upon protonation, which leads to ammonium moieties capable of attracting anionic species by electrostatic forces. In contrast, when pH > pKa, the alkyl-ammonium species are

grafted on the silica surface. Therefore, the actual amount of grafted propylamine groups was 1.55 mmol/g MCM-48 material. Moreover, nitrogen adsorption performed on the MCM-48NH3+ revealed a subsequent decrease in its textural properties. This may be attributed to the progressive pore filling of the material by the counteranion Cl-, which balances the positive charge formed by the protonation during reaction with HCl solution. Powder XRD patterns are depicted in Figure 3. Both MCM48 and MCM-48-NH2, as well as MCM-48-NH3+, exhibited XRD patterns typical of cubic Ia3d mesophase with characteristic Bragg peaks at ∼2.8° and 3.0° 2θ attributed to the (211) and (220) diffraction lines, respectively. This indicates that the modified materials retained the ordered cubic structure, although the MCM-48-NH3+ diffractogram denoted a slight shift toward higher values for the two main peaks, thus confirming the shrinking in the pore diameters, in agreement with the nitrogen adsorption data. 3.2. Adsorption Tests. 3.2.1. Effect of pH. The MCM-48NH3+ adsorbent exhibiting surface propyl-ammonium groups, together with their Cl- counter-anions acts as an anion

Ind. Eng. Chem. Res., Vol. 46, No. 25, 2007 8809

Figure 5. Effect of anion initial concentration and adsorbent loading on the adsorption for (a) the nitrate and (b) the phosphate at 25 °C. Lines show trends. Bars show standard deviations on measurements.

deprotonated and recover their free amine form, unable to exert any electrostatic effect toward anionic species present in solution. 3.2.2. Effect of the Initial Anion Concentration and Adsorbent Loading. As mentioned in the Experimental Section, calcined MCM-48 was acidified under the same conditions as the MCM-48-NH2 material to probe its adsorption performances in the removal of nitrate and phosphate anions from water. Tested separately using 130 mg/L nitrate solutions and 100 mg/L phosphate solutions for an adsorbent loading of 2 g/L, the obtained anion removals were 0.7% ( 0.4% and 1.8% ( 1.1%, respectively. Because the blank material failed to adsorb both nitrate and phosphate anions significantly, it was concluded that the adsorption occurred at the surface propyl ammonium functional sites of the MCM-48-NH3+ adsorbent. The effect of the initial anion concentration on nitrate and phosphate adsorption on MCM-48-NH3+ in the range of 100700 mg/L, using an adsorbent loading in the range of 2-10 g/L at ambient temperature is depicted in Figures 5a and 5b for nitrate and phosphate anions, respectively. For all the adsorbent loadings investigated, an increase in the initial concentration of either nitrate or phosphate from 100 mg/L to 700 mg/L translated in a decrease in the percentage uptake. The nitrate and phosphate adsorption from their respective solutions was significantly influenced by the adsorbent loading. For instance, the percentage adsorption of nitrate increases from 38% (19 mg/g) to 71% (7 mg/g) by increasing the adsorbent loading from 2 g/L to 10 g/L, at an initial concentration of 100 mg/L. At an initial concentration of 700 mg/L nitrate, the percentage removal increases from 7% (25 mg/g) to 30% (20 mg/g) by increasing the adsorbent loading from 2 g/L to 10 g/L. Generally, the same tendency was

Figure 6. Experimental and theoretical adsorption isotherms on MCM48-NH3+ for (a) the nitrate anion and (b) the phosphate anion with 5 g/L adsorbent loading. Solid lines represent the Langmuir model (denoted as “L” in the figure). For the sake of clarity, dotted lines are shown only for isotherms at 5 and 45 °C; these dotted lines refer to the Freundlich (“F”), Langmuir-Freundlich (“LF”), and Redlich-Peterson (“RP”) models.

observed for phosphate anions as depicted in Figure 5b. This is attributed to the fact that, at higher adsorbent loadings, because of increased surface area, more adsorption sites are available, leading to a higher removal of nitrate or phosphate anions. The results showed also that the uptakes per unit mass of the adsorbent (Qe) were higher at lower adsorbent loadings. This may be attributed to the fact that, at higher adsorbent loadings, some of the adsorption sites remain unsaturated during the adsorption process. However, it is worthy of mention that, even if the initial concentrations were intentionally varied to up to 700 mg/L for phosphate and nitrate, typical actual effluents heavily charged with nutrient compounds stemming for instance from agricultural pig farms contain, at most, 200 mg/L dissolved mineral and organic phosphorus species and 2 mg/L nitrate.22,23 In the case of aquaculture wastewaters, the concentrations of phosphate and nitrate are ∼80 mg/L and ∼20 mg/L, respectively.24 This makes the proposed adsorbent adequate for the treatment of such wastewaters. 3.2.3. Adsorption Isotherms. 3.2.3.1. Experimental Adsorption Isotherms. The adsorption isotherm studies are conducted to investigate the effect of temperature on the adsorption of nitrate and phosphate anions on MCM-48-NH3+ and to determine the maximum adsorption capacities. Hence, the adsorption isotherms for nitrate and phosphate anions on MCM-48-NH3+ at different temperatures are depicted in Figures 6a and 6b. As the figure shows, by increasing the temperature, the equilibrium uptake for both anions decreased, indicating the

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exothermic character of the adsorption process. Maximum adsorption capacities of ∼43 mg/g (0.7 mmol/g) and 46 (0.5 mmol/g) were obtained at the lowest temperature investigated (5 °C) for nitrate and phosphate anions, respectively. Comparison of the obtained capacities to the amount of grafted propylamine groups on the MCM-48 silica surface (1.55 mmol/ g) indicates that, at most, only 45% of the grafted groups were involved in the adsorption process. This may be attributed to the incomplete protonation of the propylamine groups grafted on the mesoporous silica material, as previously reported by Walcarius and co-workers,16,20 who observed a ratio of 40%: 60% between propylamine and propylammonium grafted species on amorphous silica gels, as well as on three different mesoporous silicas (i.e., MCM-41, MCM-48, and SBA-15). Furthermore, in the presence of water, aminopropyl-functionalized silica materials undergo proton transfer from silanol to amine surface groups, leading to the formation of zwitterion-like moieties (≡SiO-, +H3N≡), as proposed first by Golub et al.25 and later by Walcarius and co-workers.16,20 Because these zwitterion-like surface moieties are already charge balanced, they do not represent ion exchange sites for the anionic species investigated, thus diminishing the effective adsorption capacity. 3.2.3.2. Adsorption Isotherms Modeling. To date, thermodynamic parameters such as the enthalpy and entropy of adsorption of dissolved nitrate and phosphate anions onto the present mesoporous adsorbent (MCM-48-NH3+) were not reported in the open literature; therefore, it is proposed in this section to estimate these parameters based on known adsorption models. The adsorption isotherms data at different temperatures were restricted to the Langmuir, Freundlich, LangmuirFreundlich, and Redlich-Peterson isotherm models, which are represented by the following equations:

Langmuir equation: Qe )

KadQmaxCe 1 + KadCe

Freundlich equation: Qe ) KFCe1/n Langmuir-Freundlich equation: Qe )

KadQmaxCe1/n

Redlich-Peterson equation: Qe )

1 + KadCe1/n aCe 1 + KadCen

(3) (4) (5)

(6)

The isotherm constants were determined by optimization, using the Quasi-Newton constrained optimization method running a mixed quadratic and cubic line search procedure (Matlab software, from MathWorks Inc.) to solve a least-squares problem, minimizing the quadratic criterion :2

min 2 )

∑ (Qe - Qe)2

(7)

a, Qmax, Kad, n > 0 where the bars represent the experimental quantities. The relative mean deviation to the experimental data (Ω) is calculated as

Ω)

x( ) ∑

Qe - Qe

2

(8)

Qe

The model predicted isotherms are also presented in Figures 6a and 6b. Reasonable agreement was obtained between the

Table 2. Model Isotherm Constants for Nitrate and Phosphate Adsorption on MCM-48-NH3+ Langmuir Constants temperature (K)

Kad

Langmuir-Freundlich Constants

Qmax

Ω

temperature (K)

278 288 298 308 318

Nitrate 0.012 0.011 0.011 0.012 0.009

51.8 44.6 39.4 34.8 32.0

2.0 2.5 2.5 3.9 2.7

278 288 298 308 318

278 288 298 308 318

Phosphate 0.043 0.029 0.027 0.019 0.018

47.8 44.6 40.8 39.7 36.5

3.7 2.3 1.7 1.7 2.0

278 288 298 308 318

Freundlich Constants temperature (K) 278 288 298 308 318 278 288 298 308 318

KF

Nitrate 2.77 2.92 3.03 3.32 2.91

n

Kad

Qmax

n

Ω

Nitrate 0.012 0.006 0.003 0.001 0.001

52.2 41.6 35.1 30.2 27.4

1.02 0.84 0.72 0.58 0.63

1.9 2.2 1.3 2.2 1.4

Phosphate 0.090 54.5 0.052 48.6 0.045 43.4 0.009 37.5 0.007 34.1

1.45 1.27 1.20 0.82 0.78

1.5 1.4 1.2 1.1 1.4

Redlich-Peterson Constants Ω

temperature (K)

a

Kad

n

Ω

Nitrate 0.011 0.003 0.002 0.001 0.000

0.60 0.39 0.35 0.27 0.24

0.99 0.84 0.85 0.69 0.86

1.9 1.7 1.1 0.7 1.7

Phosphate 0.117 3.18 0.058 1.66 0.034 1.32 0.015 0.59 0.007 0.51

1.11 1.08 1.05 0.90 0.89

2.0 1.5 1.4 0.7 1.3

4.9 4.6 4.4 4.7 3.1

5.5 6.1 5.9 6.9 5.2

278 288 298 308 318

Phosphate 4.40 12.1 3.99 9.4 4.11 8.8 3.72 7.0 3.71 6.3

5.5 4.9 4.7 5.9 5.5

278 288 298 308 318

experimental data and the different models predicted isotherms for nitrate and phosphate anions. The estimated isotherm parameters are summarized in Table 2. Clearly, the LangmuirFreundlich and Redlich-Peterson models, which are characterized by three parameters, resulted in the lowest residues, which suggests the numerical adequacy of these models. The enthalpies and entropies of adsorption that are reported in Table 3 were evaluated from the temperature dependence of the adsorption constants according to the van’t Hoff law. As observed, the negative enthalpies obtained for all the models considered confirm the exothermicity of the adsorption process investigated. Enthalpies and entropies evaluated from the Langmuir-Freundlich and Redlich-Peterson models seemed to be excessively high. In contrast, energies determined from the Langmuir and Freundlich constants were determined to be reasonably comparable with those scarcely reported in the literature for nitrate and phosphate anion adsorption. For instance, Sahai determined enthalpies of -5 and -8 kJ/mol and entropies of -75 and -92 J/(mol K) for the adsorption of NO3- and H2PO4- anions, respectively, on R-silica.26 Discrimination between the four models was based on the goodness of fit (2 or Ω) and the physicochemical significance of the parameter estimates, according to the following Boudart criteria:27

∆S < 0

(9a)

10 e -∆S e 12.2 - 0.0014∆H

(9b)

Therefore, the ultimate model to be soundly retained must fulfill both conditions (eqs 7 or 8 and 9) simultaneously. Among the four models evaluated in the present investigation, only the Freundlich model fulfilled these conditions and was thus retained, although it presented the less-satisfactory fit to the experimental data. This model, which is the most popular for a

Ind. Eng. Chem. Res., Vol. 46, No. 25, 2007 8811 Table 3. Enthalpies and Entropies of Adsorption for Nitrate and Phosphate Anions on MCM-48-NH3+ for Various Models Nitrate Anion

Phosphate Anion

model

enthalpy, -∆H (kJ/mol)

entropy, -∆S (J/(mol K))

Boudart criteria fulfillment?

enthalpy, -∆H (kJ/mol)

entropy, -∆S (J/(mol K))

Boudart criteria fulfillment?

Langmuir Freundlich Langmuir-Freundlich Redlich-Peterson

2.5 8.4 70.6 62.4

45.7 16.6 291.9 260.4

no yes no no

16.1 11.8 51.6 51.2

84.4 21.6 202.5 202.3

no yes no no

single solute system, is known to be adequate to describe adsorption data obtained for concentrations within the intermediate range.28 For instance, it is reported that this model exhibits some discrepancies toward experimental data for dilute solutions attributed to its weak description of the Henry’s law.28 On the other hand, the Freundlich model was also reported to well describe equilibrium adsorption data obtained for adsorbents that have heterogeneous surfaces consisting of sites with different adsorption potentials.28 In contrast, the Langmuir model, which was originally developed to describe the chemisorption equilibrium of gases, is intended to describe homogeneous surfaces and monolayer adsorption.29 In the case of the MCM-48-NH3+ adsorbent, the surface heterogeneity may be related to the grafting procedure, which uses the silanol groups distributed randomly on the internal and external surfaces, as well as on pore apertures of mesostructured silica material. Silanol groups are classified into three different types, i.e., single (≡SiOH), geminal (dSi(OH)2), and hydrogen-bonded hydroxyl groups.11 Only single and geminal groups are involved in the grafting.11 3.2.4. Desorption and Regeneration. Because the success and adequacy of adsorption are dependent on the possibility of, on one hand, desorbing the target nutrient species for their eventual recovery and reuse as valuable fertilizers and, on the other hand, reusing the adsorbent for economic and environmental motivations, the desorption and adsorbent recycling possibility have been investigated. Therefore, Figure 7 shows the desorption kinetics, relative to the spent adsorbents used for nitrate and phosphate adsorption. As observed, the desorption was rapid and complete for both anions within 10 min. This leads to the conclusion that the adsorption of anions on MCM48-NH3+ is completely reversible. On the other hand, consecutive adsorption-desorption cycles data are shown in Table 4. As clearly demonstrated, no significant loss of adsorbent capacity was observed during five cycles of adsorption-

Table 4. Regeneration of the Adsorbenta NO3-

H2PO4-

cycle

% removal

amount adsorbed (mg/g)

% removal

amount adsorbed (mg/g)

1 2 3 4 5

46.0 ( 3.7 41.5 ( 8.7 40.5 ( 4.0 40.0 ( 9.4 43.3 ( 4.7

27.6 ( 2.2 24.9 ( 5.2 24.3 ( 2.4 24.0 ( 5.6 26.0 ( 2.8

59.0 ( 6.1 60.6 ( 3.3 52.5 ( 3.5 57.5 ( 5.4 56.6 ( 1.8

35.4 ( 3.7 36.4 ( 1.9 31.5 ( 2.1 34.5 ( 3.2 33.4 ( 1.0

a Operating conditions: nitrate or phosphate initial concentration, 300 mg/L; adsorbent loading, 5 g/L; temperature, 25 °C; adsorption time, 60 min.

desorption, making the adsorbent very suitable for the design of a continuous sorption process. 4. Conclusion A mesoporous silica functionalized with ammonium groups (MCM-48-NH3+) was synthesized and proved to be an effective high-capacity adsorbent for the removal of NO3- and H2PO4anions from aqueous solutions. The adsorption was influenced by pH, initial anion concentration, adsorbent loading, and temperature. The removal of nitrates was optimal at pH