Adsorption of nitrate and phosphate ions from aqueous solutions using organically-functionalized silica materials: Kinetic modeling

Adsorption of nitrate and phosphate ions from aqueous solutions using organically-functionalized silica materials: Kinetic modeling

Fuel 110 (2013) 107–113 Contents lists available at SciVerse ScienceDirect Fuel journal homepage: www.elsevier.com/locate/fuel Adsorption of nitrat...

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Fuel 110 (2013) 107–113

Contents lists available at SciVerse ScienceDirect

Fuel journal homepage: www.elsevier.com/locate/fuel

Adsorption of nitrate and phosphate ions from aqueous solutions using organically-functionalized silica materials: Kinetic modeling Safia Hamoudi ⇑, Khaled Belkacemi Department of Soil Sciences and Agri-Food Engineering, Université Laval, Québec (Qc), Canada G1V 0A6 Centre de Recherche sur les Propriétés des Interfaces et la Catalyse (CERPIC), Université Laval, Québec (Qc), Canada G1V 0A6

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

þ

" C3 H6 —NH3 -mesoporous silicas were

"

"

"

a r t i c l e

i n f o

Article history: Received 9 April 2012 Received in revised form 12 September 2012 Accepted 17 September 2012 Available online 19 October 2012 Keywords: Nitrate Phosphate Adsorption Functionalized mesoporous adsorbents Kinetics

H2PO4NO3-

qt (mg/g)

"

high capacity adsorbents for nitrate and phosphate. A kinetic study was conducted for the first time for such adsorbents and adsorbates. The adsorption was very fast reaching equilibrium within 5 min of contact time. Pseudo-second-order model described adequately the experimental kinetic data. The activation energy was 6.91 kJ/ mol for nitrate and 8.59 kJ/mol for phosphate.

NH3+ NH3+

NH3+

NH3+

MCM-48-NH3+-G

Time (min)

a b s t r a c t A variety of propylammonium functionalized mesoporous silica materials (MS—NHþ 3 ) were synthesized via post-synthesis grafting and co-condensation and their efficiency to remove nitrate and phosphate anions in aqueous solution was investigated. Under the experimental conditions explored, the maximum removal for nitrate (36%) was obtained using MCM-48 functionalized via grafting. In the case of phosphate anions, SBA-15 functionalized via grafting exhibited the best removal of 56%. The corresponding adsorption capacities reached 45 and 57 mg/g-adsorbent for nitrate and phosphate, respectively. Using the MCM-48 functionalized via grafting, a kinetic investigation was conducted at temperatures ranging from 5 to 45 °C. For both anions, the adsorption was shown to be very fast reaching the equilibrium within the first 5 min of contact time between the adsorbent and the anions solutions. The pseudo-first order, pseudo-second order and intra-particle diffusion kinetic models were used to describe the obtained data. The pseudo-second-order model was found to describe adequately the experimental kinetic data. The activation energies of adsorption evaluated using the pseudo-second-order rate constants were 6.91 and 8.59 kJ/mol for nitrate and phosphate anions, respectively. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Water pollution by nutrients such as nitrogen and phosphorus is a serious environmental worldwide problem. Dissolved nutrient ⇑ Corresponding author at: Department of Soil Sciences and Agri-Food Engineering, Université Laval, Québec (Qc), Canada G1V 0A6. Tel.: +1 418 656 2131x8460; fax: +1 418 656 3723. E-mail address: safi[email protected] (S. Hamoudi). 0016-2361/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.fuel.2012.09.066

species such as ammonium, nitrate and phosphates are particularly important pollutants as they are involved in aquatic eutrophication, which is the abundance of aquatic plants, growth of algae, and depletion of dissolved oxygen [1]. Furthermore, the presence of nitrate ions in drinking water is a potential public health hazard including infant methemoglobinaemia ‘blue baby’ syndrome [2,3] and the potential formation of carcinogenic nitrosamines and nitrosamides [4]. Agricultural over-application of fertilizers, aquaculture, agri-food industries, municipal wastewaters and detergent

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manufacturing are the main sources of N and P release in the aquatic environment. Moreover, nitrate anions stem also from thermal power plants water discharges [5,6]. Several physico-chemical and biological processes have been investigated for the removal of dissolved nutrients in water and wastewaters. Among them, adsorption technologies are promising since they allow simple and economical operation resulting in less sludge production. Therefore, this field has attracted much interest and has been the subject of some reviews [7,8]. Very recently, several investigations were devoted to numerous adsorbents for the removal of nitrate and phosphates from water. Among these adsorbents, natural inorganic materials [9–12], activated carbon [13–15], agricultural residues [16–18] and synthetic inorganic [19–21] and a new class of materials, namely mesostructured materials [22–25] were reported to exhibit interesting adsorption performances. These materials received a great deal of interest from the scientific community since the early nineties and a huge number of reports described their application in catalysis [26–29], adsorption [7] and chemical sensing [30]. Besides, a close scrutiny of the literature data indicates a great lack of investigations on the kinetics of nutrient anions adsorption by mesostructured adsorbents. In contrast, an excess of kinetic information is available on the adsorption of these anions using natural inorganic and organic residues. Recently, we have successfully applied functionalized mesoporous silica SBA-15, MCM-48 and MCM-41 as adsorbents to the removal of nitrate and phosphate anions from aqueous solutions [23]. High adsorption capacities 46.5 mg NO 3 =g and 55.9 mg H2 PO 4 =g were reached. The aim of the present investigation is twofold. Firstly, propylammonium functionalized mesoporous silica materials were synthesized and applied for the adsorptive removal of nitrate and phosphate anions from water. Secondly, a kinetic investigation on the adsorption of the target anions was conducted on one of the most promising adsorbents investigated, i.e. propylammonium functionalized MCM-48 material. 2. Materials and methods 2.1. Chemicals Tetraethyl orthosilicate (TEOS, purity > 98%), 3-aminopropyltriethoxysilane (APTES, purity > 99%), cetyltrimethylammonium bromide (CTAB, purity > 99%), trimethylammonium hydroxide (TMAOH, 25%), Brij-56, Brij-76 and sodium nitrate were purchased from Aldrich. Triblock copolymer P123 (EO20PO70EO20) was provided by BASF. Sodium orthophosphate was obtained from BDH Inc. All chemicals were used as received without further purification. 2.2. Adsorbents synthesis Five different amino-functionalized mesostructured materials, i.e. MCM-41–NH2, MCM-48–NH2, SBA-15–NH2, MS-56–NH2 and MS-76–NH2 were synthesized either via co-condensation (labeled with the extension C) or post-synthesis grafting (labeled with the extension G). The synthesis of MCM-41, MCM-48 and SBA-15 pure and functionalized materials was performed as previously reported [23]. The MS-56 and MS-76 are novel materials synthesized in our laboratory for the first time. The strategy of their synthesis is based on the use of oligomeric surfactants Brij-56 and Brij-76. For the cocondensation procedure, the synthesis gel molar composition for 4 g of surfactant was: (1  x) TEOS: x APTES: 6.5 HCl: 0.0061 Brij-56 or 0.0034 Brij-76: 180 H2O. For instance, 4 g of Brij 76 or

Brij 56 was dissolved in a mixture solution of 80 g of 2.0 M HCl and 20 g of deionized water at 40 °C. TEOS was then added and the resultant solution was equilibrated at 40 °C for prehydrolysis during 3 h. Subsequently, the appropriate amount of APTES was slowly added and the resulting mixture was kept under stirring at 40 °C for 20 h. It was then transferred into a Pyrex bottle and kept at 100 °C under static conditions for 24 h. The obtained white precipitate was recovered by filtration, washed and dried. The extraction and calcination were performed as reported [23]. For all the functionalized materials, the molar ratio of the organosilane in the silica framework was fixed to 10%, i.e. x = 0.1 in the gel compositions. The post-synthesis grafting procedure leading to amino-functionalized materials was similar to all the materials investigated in the present work. The procedure consisted in refluxing 1 g (16 mmol SiO2) of each calcined and dehydrated material in 25 ml of dry toluene containing 1.6 mmol of APTES (TEOS/APTES ratio = 1/10) under vigorous stirring at 383 k for 10 h. The functionalized materials were collected by filtration, washed several times with isopropanol and finally dried at 373 K. Before any adsorption test, the amino-functionalized materials synthesized via either the co-condensation or post-synthesis grafting procedures were acidified in order to convert the amino groups into ammonium salts witch are responsible for the adsorption of anions. Thus, 100 mg of each material was stirred in 100 mL of 0.1 M HCl for 6 h without heating. The acidified materials were then recovered by filtration. The acidified materials are denoted þ hereafter with suffixes —NHþ 3 —C or —NH3 —G, for co-condensed and grafted materials, respectively. 2.3. Characterization Nitrogen adsorption–desorption experiments were performed at 77 K using a Quantachrome Autosorb 1Ò volumetric analyzer. Prior to analysis, the materials were degassed at 120 °C for 12 h. The specific surface area, SBET, was determined from the linear part of the BET plot (P/P0 = 0.05–0.30). The pore size distribution (PSD) was calculated from the desorption branch using the Barrett– Joyner–Hallenda (BJH) method, and the total pore volume was evaluated from the adsorbed amount at a relative pressure of about 0.99. 2.4. Adsorption tests Adsorption tests were performed batch wise in 100-mL Erlenmeyer flasks immersed in a temperature controlled water bath. Typically, 50 mg of acidified mesoporous silica was placed in a flask containing 50 ml of aqueous solution of either NaNO3 or NaH2PO4 used as nitrate or phosphate sources, respectively. The initial concentration was fixed to 130 mg/L for nitrate and 100 mg/L for phosphate. After 3 h of vigorous stirring (400 rpm) at room temperature, the suspensions were filtered and the obtained filtrate solutions were analyzed by high performance liquid chromatography (HPLC) using an ICS 2500 from Dionex equipped with an electrochemical detector EDA 50 A and an IonPacÒ AS18 (4  250 mm) column. The mobile phase was a solution of KOH 35 mM. The data processing was carried out by Chromeleon software from Dionex. The % removal of anions at equilibrium was calculated as:



ðC i  C e Þ  100 Ci

ð1Þ

The amount of anions retained on the adsorbent (mg/g) at equilibrium was calculated by:

  V qe ¼ C i  C e  m

ð2Þ

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The solid/water distribution ratios of anions were calculated by:

KD ¼

ðC i  C e Þ V  Ce m

ð3Þ

where Ci and Ce are the initial and final (equilibrium) concentrations of the anions in solution (mg/L), V the solution volume (L), and m is the mass of adsorbent (g). 2.5. Kinetic tests The kinetic investigation was performed using the MCM-48— NHþ 3 —G due to its good adsorption performances (see next section) and the ease and quickness of its synthesis [23]. Therefore, adsorption tests were also performed batch wise in the temperature range (5–45 °C) with an adsorbent loading of 5 g/L, an anion initial concentration of 300 mg/L and an agitation speed of 400 rpm. At preset adsorption times, aliquots of the suspensions were withdrawn, filtered with a micro-syringe (0.2 lm) and analyzed for nitrate or phosphate anions as described above. The amount of anions retained in the sorbent phase (mg/g) at time t was calculated as:

qt ¼ ðC i  C t Þ 

V m

ð4Þ

where Ci and Ct are the concentrations of the anions in solution at initial and time t (mg/L), V is the solution volume (L), and m is the mass of adsorbent (g). 2.6. Kinetic modeling To investigate the kinetics of adsorption of nitrate and monovalent phosphate anions on MCM-48—NHþ 3 —G, three models are considered, i.e. pseudo-first-order, pseudo-second-order and intraparticle diffusion models [31]. The pseudo-first order based on the assumption of proportionality between the adsorption rate and the number of free adsorption sites is expressed as:

dqt ¼ k1 ðqe  qt Þ dt

ð5Þ

where qe and qt are the amounts of nitrate or phosphate adsorbed (mg/g) at equilibrium and any time t (s) respectively, and k1 is the rate constant (s1). The pseudo-second order model based on the assumption that the adsorption rate is linearly related to the square of the number of unoccupied adsorption sites is expressed as:

dqt ¼ k2 ðqe  qt Þ2 dt

ð6Þ

where k2 is the rate constant (g/mg s). The intra-particle diffusion model refers to the theory proposed by Weber and Morris [32] stipulating that the adsorption process is influenced by several steps such as film or external diffusion, pore diffusion and adsorption on the pore surface. The intra-particle diffusion rate parameter is expressed in terms of the square root of time and adsorption capacity in the following form:

qt ¼ k3  t0:5

ð7Þ

where k3 is the rate constant of diffusion (mg/g s0.5). For the three kinetic models, the kinetic constants were determined by optimization using the Quasi-Newton constrained optimization method running a mixed quadratic and cubic line search procedure (MatlabÒ Software from MathWorks Inc.) for solving a least-squares problem, minimizing the quadratic criterion e2:

8 n X > < min e2 ¼ ðqit  qit Þ2 i¼1 > : k1 ; k2 ; k3 > 0

ð8Þ

where the bars stand for the experimental quantities. The mean deviation to the experimental data X is calculated as:

vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u n X 1u X ¼ t ðqit  qit Þ2 n i¼1

ð9Þ

where n is the number of experimental data for a given temperature. 3. Results and discussion 3.1. Nitrogen sorption measurements and BET area analysis Results of the N2 adsorption–desorption volumetric analysis of the synthesized materials are summarized in Table 1. For all the investigated materials, the pure silica sample displayed much higher BET surface areas (850–1430 m2/g) and total pore volumes (0.48–1.31 cm3/g) if compared to the amino-functionalized materials via either co-condensation or post-synthesis grafting methods. Moreover, the amino-functionalized materials via co-condensation exhibited the lowest BET surface areas, pore volumes and pore sizes in comparison to their counterparts obtained by postsynthesis grafting. This decrease in the textural parameters was previously reported to accompany the surface modification of mesoporous materials with organic functional groups [27,33] and was attributed to the filling of the pore framework by the terminal organic functional groups protruding into the surface of the pores. All the synthesized materials exhibited pore sizes belonging to the mesoporous domain except for the functionalized MCM-48 which denoted pore sizes of 1.1 and 1.8 nm (see Table 1) belonging to the microporous domain. This behavior was previously observed for ordered silica materials having pore sizes on the borderline between mesopore and micropore ranges [34,35]. 3.2. Adsorption of nitrate anions Data for the adsorption of nitrate anions are summarized in Table 2. The obtained results indicated that generally the adsorbents synthesized using the post-synthesis strategy exhibited higher % nitrate removals as well as adsorption capacities, in comparison to their counterparts synthesized via co-condensation.

Table 1 Textural properties of synthesized mesoporous silica. Adsorbent

BET surface area (m2/g)

Pore diameter (nm)

Total pore volume (cm3/g)

MCM-41 MCM-41–NH2–C MCM-41–NH2–G

1025 437 717

3.1 2.8 2.2

0.92 0.38 0.50

MCM-48 MCM-48–NH2–C MCM-48–NH2–G

1340 351 890

2.2 1.3 1.9

1.04 0.26 0.66

SBA-15 SBA-15–NH2–C SBA-15–NH2–G

910 396 408

6.6 5.4 5.4

1.31 0.82 0.97

MS-56 MS-56–NH2–C MS-56–NH2–G

982 428 570

3.6 3.5 3.7

1.15 0.34 0.77

MS-76 MS-76–NH2–C MS-76–NH2–G

850 634 363

2.3 2.0 1.8

0.48 0.40 0.20

S. Hamoudi, K. Belkacemi / Fuel 110 (2013) 107–113

Table 2 Nitrate adsorption on studied materials. Adsorbent

R (%)

qe (mg NO 3 /g adsorbent)

Kd (mL/g adsorbent)

MCM-41 MCM-41—NHþ 3 —C MCM-41—NHþ 3 —G

0.4 20.1 27.4

0.2 20.5 34.5

20 1970 2740

MCM-48 MCM-48—NHþ 3 —C MCM-48—NHþ 3 —G

0.0 34.0 35.8

0.0 44.4 44.8

0.0 5160 5530

SBA-15 SBA-15—NHþ 3 —C SBA-15—NHþ 3 —G

4.8 20.4 27.6

5.9 31.1 36.9

50 3350 4070

MS-56 MS-56—NHþ 3 —C MS-56—NHþ 3 —G

0.0 24.3 33.9

0.0 30.7 43.8

0.0 3180 5250

MS-76 MS-76—NHþ 3 —C MS-76—NHþ 3 —G

0.0 20.3 34.3

0.0 25.8 43.7

0.0 2550 5250

Therefore, nitrate removals were quite comparable for all the —NHþ 3 —G materials ranging from 27% to 34%. The corresponding adsorption capacities centered around 44 mg NO 3 =g adsorbent þ þ for MCM-48—NHþ —G, MS-56—NH —G and MS-76—NH 3 3 3 —G and  around 35 mg NO3 =g adsorbent for the MCM-41 and SBA-15 equivalent materials. The performances obtained over the different NHþ 3 —C materials were comprised between 20% and 34% removal and between 20 and 44 mg NO 3 =g adsorbent with the highest values being recorded for the MCM-48—NHþ 3 —C material. The distribution coefficients (Kd) which are a mass-weighted partition coefficient between the aqueous phase and the adsorbent solid phase ranged between 1970 and 5530 mL/g. The higher the Kd value, the more effective the adsorbent material is at capturing the target anion species. In our experiments, the chosen solutionto-solid ratio is 1000 (i.e., 0.1% sorbent loading), then a Kd of 100 indicates that 10 times as much of the target pollutant anion is captured on the adsorbent phase as is left in the surrounding solution (in other words, 90% capture). Kd values above 50 are considered acceptable, those above 500 are considered very good, and Kd values in excess of 5000 are considered outstanding [36].

using post-synthesis grafted materials were generally greater than those obtained with co-condensed counterparts. This leads to the conclusion that the studied anions seek more easily the ammonium groups, being on the external layers of mesoporous silica walls, than those occurring inside the pores. The adsorption capacities as well as the distribution coefficients between the solid and liquid phases Kd followed the same tendency as the % removal. It is noteworthy to point out the remarkably high distribution coeffiþ cients obtained using SBA-15—NHþ 3 —G and MS-76—NH3 —G materials (12,080 and 10,550), thus demonstrating the outstanding performances and selectivity of these adsorbents phosphate removal from solution. 3.4. Kinetic modeling The time course profiles for the adsorption of nitrate and phosphate using MCM-48—NHþ 3 —G at different temperatures (5–45 °C) are depicted in Fig. 1A and B. As seen, for either nitrate or phosphate anions, the adsorption proceeded in two steps: a very fast one occurring within the first 5 min of contact between the adsorbent and the anion solution followed by a slower one whilst the adsorbed amounts reached a plateau indicating that the equilibrium was attained. This behavior can be attributed to the ordered three dimensional open pore structure of the MCM-48—NHþ 3 —G

A 40

30

qt (mg/g)

110

20 T = 5 °C T = 15 °C T = 25 °C T = 35 °C T = 45 °C

10

3.3. Adsorption of phosphate anions

0 0

Data for the adsorption of phosphate anions are presented in Table 3. As seen, in this case also, the obtained anion removals

15

30

45

60

45

60

Time (min)

B 40

Table 3 Phosphate adsorption on studied materials. R (%)

qe (mg H2 PO 4 /g adsorbent)

Kd (mL/g adsorbent)

MCM-41 MCM41-NHþ 3 —C MCM41-NHþ 3 —G

0.0 20.2 20.0

0.0 20.5 20.5

0.0 2530 2500

MCM-48 MCM48-NHþ 3 —C MCM41-NHþ 3 —G

0.0 19.6 27.1

0.0 19.9 27.8

0.0 2440 3720

SBA-15 SBA-15—NHþ 3 —C SBA-15—NHþ 3 —G

0.0 45.6 55.9

0.0 46.7 56.7

0.0 8490 12080

MS-56 MS-56—NHþ 3 —C MS-56—NHþ 3 —G

0.0 29.2 36.8

0.0 30.2 38.1

0.0 4140 5860

MS-76 MS-76—NHþ 3 —C MS-56—NHþ 3 —G

0.0 33.8 48.6

0.0 34.8 53.2

0.0 5140 10550

30

qt (mg/g)

Adsorbent

20 T = 5 °C T = 15 °C T = 25 °C T = 35 °C T = 45 °C

10

0 0

15

30

Time (min) Fig. 1. Experimental adsorption kinetics on MCM-48—NHþ 3 —G for (A) nitrate and (B) phosphate with 5 g/L adsorbent loading.

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adsorbent facilitating greatly the transport of anions onto the surface adsorption sites. Furthermore, at equilibrium, the adsorption was negatively influenced by temperature as higher adsorption plateaus were reached at lower temperatures. In the case of nitrate, maximum adsorbed amount reached 32 and 26 mg/g at 5 and 45 °C, respectively. As for phosphate, 35 and 27 mg/g were obtained accordingly. The negative effect of temperature on the adsorption capacity was previously related to the exothermic character of the adsorption [37]. For the sake of clarity, Figs. 3 and 4 report only the experimental and the corresponding model calculated data obtained at 5 and 45 °C. As clearly evidenced in Fig. 2A and B, the intra-particle diffusion model failed to describe the experimental kinetic data for both nitrate and phosphate anions. Therefore, this model was ruled out. Using the pseudo-first order model, typical results of predicted profiles (solid lines) along with measured adsorbed amounts are depicted in Fig. 3A and B for nitrate and phosphate, respectively. There was a poor agreement between the model predicted and the experimental data at all the temperatures investigated for nitrate as well as phosphate anions, particularly in the fast step of adsorption. Indeed, the residual mean deviations presented in Table 4 were quite high (>5). However, the kinetic constants obtained followed a logical path as a function of temperature as to be dis-

cussed later (see Section 3.5). As presented in Table 4, the kinetic constants were quite similar for nitrate and phosphate anions. This somewhat discredits the pseudo-first order kinetic model since the chemical nature of two anions is different except for the negative charge. As depicted in Fig. 4A and B and Table 4, application of the pseudo-second order kinetic model indicated a pretty good agreement between the measured and the predicted data for both anions at all the temperatures investigated. The pseudo-second order model proved to describe adequately both the fast and the slow adsorption steps. Furthermore, the obtained residues were low ranging most often below three except for the data obtained at 15 °C with phosphate and nitrate anions. Several investigations reported that the pseudo-second order kinetic model represented well the experimental adsorption data for phosphate using a variety of adsorbents such as modified wheat residue [38]; ZnCl2 activated coir pith carbon [39]; anion exchange resin from lignocellulosic residues [40]; metal loaded orange waste [41]; dolomite [42]; akaganeite [43]; Lanthanum(III) doped mesoporous silicates [44]; calcined alunite [45] and MgO microspheres [46]. As for nitrate, the pseudo-second order kinetic model was found to adequately describe the adsorption kinetic data obtained with

A 35 30

50

25

qt (mg/g)

A 60

qt (mg/g)

40

30

20 15 10

20

T = 45 °C T = 45 °C

T = 5 °C

5

T = 5 °C

10 0 0

10

20

0

10

20

30

40

50

40

50

60

60

Time (min)

B 40

B 60

35

50

30 25

qt (mg/g)

40

qt (mg/g)

30

Time (min)

0

30

20 15

20

T = 45 °C

T = 5 °C

10

10

T = 45 °C

T = 5 °C

5

0

0 0

10

20

30

40

50

60

Time (min) Fig. 2. Experimental and theoretical adsorption kinetics on MCM-48—NHþ 3 —G for (A) nitrate and (B) phosphate with 5 g/L adsorbent loading. Solid lines show intraparticle diffusion model.

0

10

20

30

40

50

60

Time (min) Fig. 3. Experimental and theoretical adsorption kinetics on MCM-48—NHþ 3 —G for (A) nitrate and (B) phosphate with 5 g/L adsorbent loading. Solid lines show pseudo-first order kinetic model.

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A 35

Table 4 Kinetic constants for nitrate and phosphate adsorption on MCM-48—NHþ 3 —G. Nitrate

30

qt (mg/g)

25 20 15 10 T = 45 °C

T = 5 °C

5 0 0

10

20

30

40

50

Phosphate

Temp (K)

k1

X

k1

X

Pseudo-first order 278 288 298 308 318

1.5961 1.6801 1.7734 1.8777 1.9961

6.4986 5.4836 7.5358 6.6184 5.1552

1.5961 1.6801 1.7734 1.8777 1.9951

7.3713 7.2094 7.3691 5.7218 7.9485

Pseudo-second order k2

X

k2

X

278 288 298 308 318

2.6546 4.6694 2.3413 3.7075 2.5059

0.0473 0.0529 0.0568 0.0648 0.0771

3.0994 4.0631 1.8025 2.4765 2.5100

0.0505 0.0531 0.0649 0.0667 0.0721

60

Time (min) functionalized mesoporous silica as adsorbent for nitrate and phosphate in solution, the adsorption mechanism occurred via ion exchange. Explicitly, the MCM-48—NHþ adsorbent exhibiting 3 surface propylammonium groups together with their Cl counter-anions acts as an anion exchanger where the Cl are exchanged by the nitrate or phosphate anions in solution [37]. Furthermore, the obtained activation energies compare pretty well with values reported in the literature: from 5.44 kJ/mol for phosphate adsorption on dolomite [51] to 3.39 kJ/mol for modified wheat residue [38]. In the case of nitrate, the activation energy was previously reported to be around 21.03 kJ/mol [47].

B 40 35 30

q t (mg/g)

25 20 15

4. Conclusion

10

T = 45 °C

T = 5 °C

5 0 0

10

20

30

40

50

60

Time (min) Fig. 4. Experimental and theoretical adsorption kinetics on MCM-48—NHþ 3 —G for (A) nitrate and (B) phosphate with 5 g/L adsorbent loading. Solid lines show pseudo-second order kinetic model.

different adsorbents such as activated carbon and sepiolite [9]; Amberlite IRA 400 resin [47]; zinc chloride treated activated carbon [15]; pure and surfactant-modified sepiolite [48]; chitosan hydrogel beads [49] and modified wheat residue [50]. 3.5. Effect of temperature on kinetic parameters and evaluation of the activation energy Arrhenius plots of the obtained rate constants using the pseudofirst and pseudo-second order models for all the temperatures investigated were used to evaluate activation energies of the adsorption process. Using the pseudo-first order model kinetic constants, activation energies of 4.08 and 4.07 kJ/mol were established for nitrate and phosphate, respectively. These similar values indicating the same adsorption behavior of the nitrate and phosphate ions despite their different chemical nature makes the pseudo-first order kinetic model inadequate to describe the adsorption kinetics. Using the pseudo-second kinetic model, the obtained activation energies were 6.91 and 8.59 kJ/mol for nitrate and phosphate, respectively. These values fall in the range of physical adsorption (0–40 kJ/mol) indicating the physical nature of the adsorption process. Indeed, as reported previously, using the propylammonium

The adsorptive removal of nitrate and phosphate anions from aqueous solutions was investigated using several propylammonium funtionalized mesoporous silica materials. These materials demonstrated high adsorption performances in terms of anions removal, capacities and distribution coefficients. Moreover, the adsorption was shown to be very fast and can be accomplished in less than 5 min. The adsorption kinetic behavior was shown to be adequately described using the pseudo-second order kinetic model. Acknowledgments Fundings provided by the Natural Sciences and Engineering Research Council of Canada (NSERC) and the Canada Foundation for Innovation (CFI) are gratefully acknowledged. References [1] Levine SN, Schindler DW. Phosphorus, nitrogen, and carbon dynamics of experimental Lake 303 during recovery from eutrophication. Can J Fish Aquat Sci 1989;46:2–10. [2] WHO. Health hazards from nitrate in drinking water. In: Environ Health 1. Copenhagen: World Health Organisation; 1985. [3] Wright RO, Lewander WJ, Woolf AD. Methemoglobinemia: etiology, pharmacology, and clinical management. Ann Emerg Med 1999;34:646–56. [4] AWWA. Water quality and treatment: a handbook of community water supplies. 4th ed. New York: McGraw-Hill; 1990. [5] Selvin Pitchaikani J, Ananthan G, Sudhakar M. Studies on the effect of coolant water effluent of Tuticorin thermal power station on hydro biological characteristics of Tuticorin coastal waters, South East Coast of India. Curr Res, J Biol Sci 2010;2:118–23. [6] Mohsen MS. Treatment and reuse of industrial effluents: case study of a thermal power plant. Desalination 2004;167:75–86. [7] Walcarius A, Mercier L. Mesoporous organosilica adsorbents: nanoengineered materials for removal of organic and inorganic pollutants. J Mater Chem 2010;20:4478–511. [8] Bhatnagar A, Sillanpää M. A review of emerging adsorbents for nitrate removal from water. Chem Eng J 2011;168:493–504.

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