J Porous Mater DOI 10.1007/s10934-016-0245-5

Synthesis and application of amine functionalized silica mesoporous magnetite nanoparticles for removal of chromium(VI) from aqueous solutions Shahab Shariati1 • Maryam Khabazipour1 • Fariba Safa1

Ó Springer Science+Business Media New York 2016

Abstract In this research, novel nanoparticles of Kit-6 mesoporous silica magnetite were synthesized with 9.6 nm pore diameter and 241.68 m2 g-1 surface area. The synthesized mesoporous magnetite nanoparticles (MMNPs) were functionalized with amine groups. Scanning electron microscopy, powder X-ray diffraction, Fourier transform infrared spectroscopy and nitrogen adsorption–desorption method confirmed the morphology and structure of the synthesized nanoparticles. The amine functionalized MMNPs were used for sorption of toxic chromate ions from aqueous samples. The effect of various experimental parameters (four factors at three levels) on the sorption efficiency of Cr(VI) was studied and optimized via Taguchi L9 (34) orthogonal array experimental design. At optimum conditions, the sorption of the Cr(VI) was best described by a pseudo second-order kinetic model with R2 = 0.9999 and qeq = 129.8 mg g-1, suggesting chemisorption mechanism. Adsorption data were fitted well to the Langmuir isotherm and the synthesized sorbent showed complete ion removal with 185.2 mg g-1sorption capacity. Keywords Chromium(VI)  Kit-6 mesoporous silica  Magnetite nanoparticles  Amine functionalized nanoparticles

& Shahab Shariati [email protected] 1

Department of Chemistry, Rasht Branch, Islamic Azad University, Rasht, Iran

1 Introduction In recent years, with growth of civilization and industries, there is an increasing contamination of municipal and industrial wastewater by hazardous heavy metals [1]. Among the various heavy metals, chromium is one of the most toxic pollutants that is non-biodegradable, carcinogenic and mutagenic to living organisms. The toxicological and biological characteristics of chromium are related to its chemical forms. Chromium is usually available in two oxidation states of Cr(III) and Cr(VI) in soils and aqueous systems. Cr(VI) is highly toxic metal and causes health damages [2]. Maximum permitted levels of chromium for drinking and surface waters are 0.05 and 0.1 mg L-1, respectively [3]. A variety of methods have been used to treat Cr(VI) waste, such as reduction [4], photoreduction [5], ion-exchange [6], chemical precipitation [7], electrocoagulation [8], reverse osmosis [9], membrane separation [10] and adsorption [11]. Most of these methods require either high energy or large quantities of chemicals and disposal of the residual metal sludge. Adsorption is the most widely used treatment method due to its simplicity, sludge-free operation, and low cost [12]. Several investigators were used different adsorbents for the removal of Cr(VI) ions such as activated carbon [13], chitosan [14], biosorbents [15], and polymeric compounds [16]. However, these adsorbents possess a rather low capacity and selectivity toward Cr(VI). To solve this drawback and to improve the loading capacity, some functionalized mesoporous materials were used for adsorption of Cr(VI) due to their excellent textural parameters such as huge surface area, large pore volume and diameter, narrow pore sizes and interesting morphologies [17].

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A number of the modified small pore mesoporous, such as MCM-41 and SBA-1 have been reported for treatment of chromium compounds from wastewaters [18, 19]. Maximum adsorption capacities for different modified MCM-41 were about 0.91 and 0.97 mmol g-1, respectively [18, 20]. More recently, a modified SBA-15 mesoporous silica with large pores was developed, with adsorption capacity of about 113 mg g-1 for Cr(VI) removal. Large pores of the material facilitated the ion diffusion and improved the adsorption efficiency [21]. However, we need an adsorbent that not only shows high activity for Cr adsorption, but also possesses the ease of adsorbent separation and recovery. By using superparamagnetic NPs such as Fe3O4, we can solve the problem and fast separations can be achieved with high efficiency, and low cost. Also, the adsorbent can be easily separated from the waste and may be used several times. Inclusion of magnetic components in the adsorbent allows convenient and economical magnetic separation using an appropriate magnetic field instead of centrifugation and filtration steps. The aim of the present study was to develop a method for preparing amine functionalized Kit-6 silica mesoporous magnetite nanoparticles (Fe3O4@SiO2@Kit-6-NH2, NH2MMNPs) with high pore size in acidic condition as a new sorbent for Cr(VI) removal from aqueous solutions. For this purpose, Fe3O4 magnetic nanoparticles were synthesized via chemical coprecipitation of Fe2? and Fe3? solutions in basic media. For protection of the iron oxide core in the highly acidic conditions, it is necessary to coat the magnetite core with a silica layer before the synthesis of Kit-6 silica mesoporous shell, so surface of bare MNPs was coated with SiO2. Then, to increase the surface area and sorption ability of the synthesized nanoparticles, surface of the bare MNPs was coated by a highly ordered large pore mesoporous of KIT-6 (3-D Cubic Ia3d symmetry), for the first time. Functionalization of MNPs with amine groups produces different properties to sorbent and induced optimum interaction between sorbent and adsorbent. Therefore, amine groups were conveniently loaded on the surface of mesoporous MNPs-Kit-6 via chemical routes. New adsorbent exhibits high sorption capacity and fast sorption rate for Cr(VI) compared with other conventional sorbents.

2 Experimental 2.1 Materials Ferric chloride hexahydrate (FeCl36H2O), ferrous chloride tetrahydrate (FeCl24H2O), sodium hydroxide, tetraethyl orthosilicate (TEOS), 3-aminopropyl-triethoxysilane (H2NCH2CH2CH2Si(OC2H5)3, APTES) as organosilane, n-butanol, p-toluenesulfonic acid, potassium dichromate,

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absolute ethanol and hydrochloric acid (37 wt%) were purchased with high purity from Merck (Darmstadt, Germany). Pluronic P123 (EO20–PO70–EO20, MW = 5800) as a non-ionic surfactant was prepared from Aldrich (Milwaukee, WI, USA). All stock and working solutions were prepared using double distilled water. 2.2 Instrumentation The crystal phases and crystallinity of synthesized MMNPs were analyzed on X-PRTPRO (PANalitical, Netherlands) X-ray diffraction (XRD) instrument using Cu Ka radiation source with 2h range of 0.5°–70°. To investigate the chemical structure of synthesized NH2-MMNPs, Shimadzu Fourier transform infrared spectrophotometer (FT-IR-470, Japan) in the wave number range of 400–4000 cm-1 was used. Nitrogen adsorption–desorption experiments for determination of surface area and pore size of the nanoparticles were carried out at 77 K (Bel, Japan). Size and morphology of the modified nanoparticles were examined under a Philips XL 30 scanning electron microscope (SEM, Netherlands). For absorption measurements, a Shimadzu UV–Vis spectrophotometer (3100 pc series, Japan) was used. pH of the solutions was adjusted using a Crison pH meter (Basic 20, Spanish). For magnetic separation, a strong super magnet (1 9 3 9 5 cm) with 1.4 Tesla magnetic field was applied. 2.3 Synthesis of silica coated magnetite nanoparticles (Fe3O4@SiO2 MNPs) Fe3O4 MNPs were chemically synthesized with little modification in the methodology already described in the literature [22–24]. Briefly, 6.3 g of FeCl36H2O, 4.0 g of FeCl24H2O and 1.7 mL of HCl (12 mol L-1) were dissolved in 50 mL of deionized water in order to prepare stock solution of ferrous and ferric chloride. This solution was degassed with purging nitrogen gas (99 %) for 20 min. Simultaneously, 250 mL of ammonia solution (1.5 mol L-1) was degassed (for 15 min) and heated to 80 °C in a reactor. Then, the stock solution was slowly added to the ammonia solution using a dropping funnel during 60 min under nitrogen gas atmosphere and vigorous stirring (1000 rpm) by magnetic stirrer. During the whole process, the solution temperature was controlled at temperatures higher than 80 °C and nitrogen gas was purged to remove the dissolved oxygen. After completion of the reaction, the obtained black coloured MNPs were separated from the reaction medium by a supermagnet (1.4 Tesla), and then washed with 500 mL double distilled water four times. Finally, the obtained Fe3O4 MNPs were dried for 120 min at 90 °C. Due to instability of Fe3O4 MNPs under acidic condition, a silica layer was coated on the surface of the

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synthesized particles. For synthesis of Fe3O4@SiO2 MNPs, 1.0 g of the synthesized MNPs was homogeneously dispersed in 500 mL of ethanolic ammonia (25 mL, 25 wt%), under stirring at 80 °C followed by dropwise addition of ethanolic solution of TEOS (10.8 %v/v). After stirring at 80 °C for 2 h, the Fe3O4@SiO2 nanoparticles were obtained and washed several times with a mixture of water–ethanol (1:1). Then, the synthesized nanopartices were dried at 100 °C for 5 h. Color of the synthesized Fe3O4@SiO2 nanoparticles was dark brown.

was analyzed using UV–visible spectrophotometer by measuring the absorbance at kmax = 350 nm. All adsorption experiments were conducted at room temperature. The removal efficiency of chromium(VI) was calculated as:

2.4 Synthesis of Kit-6 silica mesopourous magnetite nanoparticles (Fe3O4@SiO2@Kit-6, MMNPs)

In order to obtain the optimum conditions and to achieve the maximum removal efficiency, minimum overall sorption time and the experimental costs, Taguchi method as an orthogonal array design (OAD) was employed [27]. In this work, an L9 (34) orthogonal array design was used to investigate the effects of four controllable factors including solution pH, ionic strength, contact time and sorbent mass, at three levels (Table 1). Table 2 shows the design matrix for examining adsorption behavior of the amine functionalized mesoporous magnetic adsorbent toward the chromium ion. All experiments were done using 75 mL solutions of 150 mg L-1 concentration of chromium.

The Kit-6 mesoporous silica with cubic Ia3d symmetry as shell on the magnetite core was synthesized with modification to the method described in the literature [25]. After dissolving 1.25 g P123 in 45 mL deionized water, 1 g of Fe3O4@SiO2, 2.4 mL of HCl solution (37 wt%) and 1.3 g of n-butanol (99.4 wt%) were added to the solution under vigorous stirring. After 1 h, 2.7 g of TEOS (as silica source) was added immediately, the mixture was left stirring at 35 °C for 24 h. Then, it was transferred into autoclave to crystallize at 100 °C for 24 h. The produced solid was filtered, washed, dried at 90 °C, and calcinated at 550 °C for 6 h. 2.5 Amine functionalization of Kit-6 silica mesopourous magnetite nanoparticles (NH2-MMNPs) Synthesis of amine functionalized MMNPs was carried out by the post-synthesis grafting method [26]. The method is based on the silylation reaction of organo alkoxysilane with surface silanol groups on the mesopores. A detailed experimental description for synthesis of Fe3O4@SiO2@Kit-6NH2 nanoparticles (NH2-MMNPs) is as follows. At first, 0.5 g of synthesized MMNPs was dispersed in 75 mL of toluene by stirring for 0.5 h at 50 °C. After that, 3.5 mg of ptoluenesulfonic acid and 1.0 mmol of organosilane (APTES) were added to the mixture. The mixture was heated up to 120 °C and stirred for 4 h. After refluxing for 4 h, the solid product was filtered and washed with absolute ethanol several times and was dried at 100 °C for 12 h. Figure 1 shows a schematic diagram for synthesis of NH2-MMNPs.

% Removal ¼

C0  Ct  100 C0

ð1Þ

where Co and Ct are initial and final (after treatment with adsorbent) concentrations of chromium, respectively. 2.7 Taguchi method

2.8 Adsorbent kinetic experiments A kinetic study was carried out to determine the rate of Cr(VI) removal by NH2-MMNPs. A series of solutions containing 150 mg L-1 Cr(VI) were treated by the adsorbent and the mixtures were stirred (600 rpm) for different times ranging 0–15 min. The concentration of residual Cr(VI) in the solution was monitored and the adsorption capacity qt at time t (mg g-1) was calculated by the following equation: qt ¼

ðC0  Ct ÞV W

ð2Þ

where Co and Ct are the initial and equilibrium concentrations (mg L-1) of Cr(VI) at a given time t, respectively. Also, V is the solution volume (L) and W is the weight of the adsorbent (g). The kinetics of Cr(VI) adsorption on the NH2MMNPs was analyzed using pseudo first-order, pseudo second-order and intra particle diffusion kinetic models. 2.9 Adsorption isotherm experiments

2.6 Adsorption studies A stock solution containing 150 mg L-1 Cr(VI) was prepared by dissolving a known quantity of potassium dichromate (K2Cr2O7) in double-distilled water. The pH value was adjusted by 1 M HCl and 1 M NaOH solutions. The concentration of the residual Cr(VI), in the solution

Adsorption isotherm study was done under optimum condition (0.08 g adsorbent, pH = 2, contact time 15 min, without salt addition) at Cr(VI) concentrations of 25–200 mg L-1. After completion of the reaction, nanoparticles were separated by a permanent magnet and the Cr(VI) content in the filtered solution was measured.

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Fig. 1 Schematic image for synthesis of Fe3O4@SiO2@Kit-6-NH2 core/shell structure (NH2-MMNPs)

3 Result and discussion

Table 1 Experimental factors and their levels Factor

Description

Level 1

Level 2

Level 3

A

Sorbent mass (g)

0.01

0.04

0.08

B

Ionic strength (mol L-1)

0.0

0.05

0.1

C

Contact time (min)

15

30

45

D

Solution pH

2

3

4

3.1 Charactrization of the sorbent FT-IR spectra of the synthesized NH2-MMNPs is shown in Fig. 2. The bands at *557 and 439 cm-1 are attributed to the Fe–O vibration of Fe3O4 in tetrahedral and octahedral

Table 2 Taguchi design matrix for adsorptive removal of Cr(VI) Run

Solution pH

Contact time (min)

Ionic strength (mol L-1) number

Sorbent mass (g)

Removal (%)

1

3

15

0.1

0.04

61.5

2

2

30

0.1

0.08

59.1

3

4

45

0.1

0.01

21.7

4

2

15

0.0

0.01

54.6

5

4

30

0.0

0.04

85.1

6

3

30

0.05

0.01

35.8

7

2

45

0.05

0.04

67.2

8 9

3 4

45 15

0.00 0.05

0.08 0.08

80.4 80.5

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J Porous Mater Fig. 2 FT-IR spectrum of the synthesized NH2-MMNPs

sites, respectively. Also, the peak at *1049 cm-1 is attributed to asymmetric stretching vibrations of Si–O–Si and band recorded in the region of 3429 cm-1, is related to bending vibration of N–H. Figure 3 shows the XRD patterns of bare Kit-6 (A) and NH2-MMNPs (B) in low and wide angels. Peaks with 2h of 1.0° and 1.8°, indicate well resolved (211) and (332) peaks which are typical for cubic order materials with la3d space group. Other peaks at 26.05°, 30.31°, 35.66°, 43.35°, 53.8°, 57.3°, 62.96° and 71.51° correspond to Fe3O4. As shown, the intensities of XRD patterns would also decrease and d spacing also shifts to small angles with increase mesopores coating on the iron oxide core. It seems that the absence of the prominent peaks revealed the mesostructure would collapse with iron oxid core, compared to that of the mesoporous Kit-6. Nitrogen adsorption–desorption isotherm of the MMNPs shows a characteristic type IV curve (Fig. 4a) with a distinct hysteresis loop in the p/p0 range of 0.6–0.9, indicating the presence of a narrow distribution of mesoporous pore size. The type IV isotherm (IUPAC classification) is typical for mesoporous systems. The typical BJH (Barrett–Joyner–Halenda) pore size distributions (Fig. 4b) indicate narrow pore size distributions for samples. The results are summarized in Table 3 and clearly indicate high surface area and large and uniform pores for the core/ shell structure of MMNPs. Therefore, it can be deduced that the pores of the silica mesoporous shell remain after loading on the surface of iron oxide nanoparticles. Figure 5 shows the SEM images of the synthesized MMNPs with particle size less than 17 nm. EDAX analysis of Fe3O4@SiO2@Kit-6 without amine functional groups

(Fig. 6) proves existence of Fe, O, Si in the mesoporous magnetic nanoparticles. 3.2 Experimental design and data analysis Results of the experiments according to L9 orthogonal array design (Table 2) were analysed using experimental design 8.0 software. The mean values of the three levels of each parameter revealed how the absorbance of the solutions changes with variation of the level of each factor (Fig. 7). The results of the OAD experiments can be treated by the analysis of variance (ANOVA). In ANOVA, the results of the sum of squares (SS) for different variables were calculated and effects of different factors on the response function were evaluated by computing F-ratio (variances ratio) and percent contribution (PC) values for each factor [28]. The ANOVA results (Table 4) showed that the sorbent amount was the most important parameter contributing to the adsorption efficiency. 3.2.1 Effects of ionic strength and contact time on the Cr(VI) removal efficiency The effect of ionic strength on the sorption of chromium (CCr = 150 mg L-1, V = 75 mL) was investigated by addition of NaCl to the solutions. The results showed that with the increase of NaCl concentration, the sorption capacity of the NH2-MMNPs and the removal efficiency decreased significantly (Fig. 7). Thus, further studies were carried out without salt addition. To study kinetics of the process, adsorption efficiency of Cr(VI) on NH2-MMNPs was followed as a function of

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according to Fig. 7. Thus, the optimum contact time was selected as 15 min. 3.2.2 Effect of pH and dose of adsorbent on the Cr(VI) removal efficiency

Fig. 3 X-ray diffraction pattern of (A) bare Kit-6, (B) synthesized NH2-MMNPs in small angle and (C) synthesized NH2-MMNPs in wide angle

Fig. 4 (A) Nitrogen adsorption–desorption isotherms measured at 77 K; (B) pore size distribution curves (inset) of core–shell structured synthesized and BET

Solution pH is one of the important parameters having considerable influence on the sorption of metal ions, because the surface charge density of the adsorbent and the metallic species depends on the pH [29]. In aqueous solutions, there is an equilibrium between different chromium species including H2CrO4, HCrO4-, Cr2O72-and CrO42-. Due to its anionic character, Cr(VI) is favorably adsorbed at lower pH values. But it shifts in favor of bivalent CrO42- ions as pH increases above 6.0 [30]. On the other hand, the surface of NH2-MMNPs has a positive charge and would be surrounded by the hydronium ions at acidic pHs which enhance the Cr(VI) interaction with binding sites of the sorbent via greater attractive forces. As pH increases, the positive charge density decreases. Therefore, electrostatic attraction between the negatively charged Cr(VI) and the positively charged NH2MMNPs decreases at pHs higher than 4, and adsorption decreased. Because the selected levels of pH were in optimum range, the percent contribution of pH factor was not significant (Table 4). Therefore, pH = 2 was selected for all further adsorption experiments. The effect of sorbent weight on the removal of Cr(VI) by NH2-MMNPs is shown in Fig. 7. As it can be seen, removal efficiency of Cr(VI) increased with increasing the amount of mesoporous magnetic nanoparticles. This can be attributed to increased adsorbent surface area and availability of more adsorption sites. The best adsorbent dosage was found to be 0.08 g for 75 mL of Cr(VI) solution (150 mg L-1), as the removal efficiency was near 100 % under optimal condition. In summary, the optimum conditions found for removal of Cr(VI) from 75 mL of 150 mg L-1 solution by new synthesized NH2-MMNPs sorbent were: 0.08 g sorbent weight, 15 min contact time, pH = 2 without salt addition. 3.3 Study of kinetic and adsorption isotherms

contact time (Fig. 7) from 15 to 45 min. The results (Fig. 8) showed that the removal efficiency of Cr(VI) increased with contact time up to 15 min and after that it was reduced

Figure 8 shows the removal efficiency of 150 mg L-1 concentration of Cr(VI) versus time after sorption by the

Table 3 BET surface area, pore volume, and pore size of the synthesized MMNPs Sample

SBET (m2 g-1)

d0 (nm)

Vtot (cm2 g-1)

Vp (cm2 g-1)

ap (m2 g-1)

˚) d100/d211 (A

a (nm)

W (nm)

Fe3O4@SiO2@Kit-6

241.68

9.26

0.583

0.566

224.84

99.20

24.29

2.84

BET surface area calculated in the range of relative pressure (p/p0) = 0 - 0.5 do mean pore dimeter (BJH), Vtot total pore volumes measured at (p/p0) = 0.98, Vp mean volume of the pores, ap surface of pores, d d-spacing, a unitcell parameter, w wall thickness

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Fig. 5 SEM photoghraph of MMNPs

In present study, the experimental data were evaluated using Lagergren’s pseudo-first order [31], pseudo-second order [32] and intra particle models [33] that are explained as follows: logðqe  qt Þ ¼ log qe 

Fig. 6 EDAX of Fe3O4@SiO2@Kit-6 (MMNPs)

k1 t 2:303

ð1Þ Pseudo - first order

Where k1 is the pseudo first-order rate constant (min-1), qe and qt (mg g-1) are the amounts of Cr(VI) ion adsorbed at equilibrium and time t, respectively. The rate constant can be calculated from the slope of the curve in Fig. 9.   t 1 1 ¼ þ t ð2Þ Pseudo - second order qt k2 q2e qe where k2 is the pseudo-second order adsorption rate constant (g mg-1 min-1). The kinetic parameters are determined from the linear plots of log (qe–qt) vs t for pseudo first order, or (t/qt) vs t for pseudo second order models (Fig. 9). The validity of each model is checked by the fitness of the straight line (R2) as well as the consistency between the experimental and calculated values of qe. Intra particular diffusion was characterized using the relationship between specific sorption (qt) and the square root of time (t1/2) according to the intra particle model as follows.

Fig. 7 Effect of four parameter in 3-levels in % removable of Cr(VI)

qt ¼ kp t1=2 þ C

synthesized NH2-MMNPs. As Fig. 8 shows, about 90 % of the Cr(VI) was removed by the adsorbent during the first minutes of the process while only a small part of the additional removal occurred during the rest contact time.

where qt is the amount of metal ions adsorbed at time t (mg g-1), kP is intraparticle diffusion rate (mg L-1 min-1/2), and C is the intercept. Kinetic parameters calculated from pseudo-first and -second order kinetics, as well as for intraparticle models, are summarized in Table 5.

ð3Þ Intra particle model

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J Porous Mater Table 4 ANOVA results for adsorption removal of chromium by NH2-MMNPs

Factor

SSa

DOFb

Variance

F

PCc (%)

Ionic strength (mol L-1)

0.1647

2

0.0823

53.1

27.5

Sorbent mass (g)

0.397

2

0.1985

128

67.2

Contact time (min)

0.0213

2

0.1065

68.7

3.1

pH

0.0034

2

0.0017

1.11

–

Error

0.0031

2

0.0015

–

0.53

a

Sum of Squares

b

Degrees of freedom

c

Percent of contribution

Fig. 8 Effect of time on removal efficiency of 150 mg L-1 concentration of Cr(VI) solution in optimum conditions

The pseudo-second order rate constant, k2, was determined as 0.0658 g mg-1 min-1 and the equilibrium adsorption capacity was obtained as 129.87 mg g-1. The best fit of the pseudo-second order kinetic model (squared correlation coefficient of 0.9999) shows chemisorption of Cr(VI) on the adsorbent via electrostatic attraction. 3.4 Adsorption isotherms Using appropriate correlation models for the experimental equilibrium data is of great importance to understand the mechanism of adsorption for Cr(VI). In this study, the two most common isotherms, Langmuir [34] and Freundlich models [35], were used to describe the adsorption data. Figure 10 shows the isotherms of Cr(VI) adsorption on the synthesized NH2-MMNPs under optimal condition. The Langmuir and Freundlich isotherm models are given by the following equations: 1 1 1 ¼ þ Qe Qmax kL Qmax Ce

Langmuir equation

Log qe ¼ log KF þ 1=n log Ce

123

Freundlich equation

where Ce is the equilibrium concentration of Cr(VI) anions in solution (mg L-1), qe the adsorbed value of chromate anions at equilibrium (mg g-1), Qmax is the maximum monolayer capacity of the adsorbent (mg g-1) and KL is the Langmuir binding constant which is related to the energy of adsorption (L mg-1). KF is the Freundlich constant (L mg-1) and n (dimensionless) is the heterogeneity factor. Plotting 1/Qe against 1/Ce gives a straight line with slope and intercept equal to 1/Qmax and 1/KLQmax Ce, respectively. Fitting of the data for Cr(VI) adsorption onto the synthesized NH2-MMNPs suggested that the Langmuir model gave a better fit (R2 = 0.997) than the Freundlich model. This indicates that the uptake of Cr(VI) ions occurs on a homogenous surface by monolayer adsorption without any interaction between adsorbed ions. The value of Qmax for adsorption of chromate anions was obtained from the Langmuir model as 185.2 mg g-1. 3.5 Study of reusability of the adsorbent A simple regeneration test was conducted to evaluate the reusability of the synthesized NH2-MMNPs. 0.08 g Cradsorbed NH2-MMNPs was mixed with 100 mL of water in pH = 11 for 30 min to remove the adsorbed Cr species. Then, the obtained magnetite was reused four times for the Cr(VI) adsorption test. The removal perecentages of Cr(VI) were found to be 96.8, 92.9, 91.5 and 90.5 %, respectively. The results showed effective regeneration of magnetite nanoparticles even after four cycles of elution/adsorption. 3.6 Comparison of chromium sorption capacity of the NH2-MMNPs with other sorbents Table 6 compares the performance of the synthesized NH2MMNPs with other reporeted adsorbents for Cr(VI) removel from aqueous solutions. As shown, the novel NH2MMNPs possess improved loading capacity and adsorb Cr(VI) slightly faster than the other adsorbents.

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Fig. 9 Pseudo first-order, Pseudo second-order and intra particle kinetic models for the uptake of Cr(VI) anions by NH2-MMNPs Table 5 Kinetic parameters and regression coefficients (R2) corresponding to different models

Intraparticle model R

2

0.57

Pseudo-second order model 2

KP

C

R

25.25

53.63

0.9999

Pseudo-first order model

K2

Qe

R2

K1

Qe

0.0658

129.87

0.7838

0.332

32.8

Fig. 10 Fitting of isotherm data to the Langmuir and Freundlich models (V = 75 mL, pH = 2.0, Wsorb = 0.08 g)

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J Porous Mater Table 6 Comparison of optimum conditions achieved by the proposed sorbent and the other reported sorbents for chromium removal Adsorbents

pH

Contact time (h)

Dose of adsorbent (g L-1)

Adsorption capacity (mg g-1)

References

NH2 functionalized Kit-6 mesoporous magnetite (NH2MMNPs)

2

0.25

1

185.2

This Work

Activated carbon-based iron containing adsorbents

2

48

0.6

68.49

[36]

Modified, cationic surfactant spent mushroom Chemically activated Neem Sawdust

3.39 4

1.15 3

5 6

43.86 24.63

[37] [38] [39]

Peanut shell

4

6

0.4

4.32

Poly- (methyl acrylate) fuctionalized guar gum

1

24

4

29.67

[40]

Immobilized mycelia in carboxy methyl-cellulose (CMC) of Lentinus sajor-caju

2

2

25

32.2

[41]

Mesoporous aluminosilicate

5.5

0.5

1.25

112

[]

Diatomite-supported magnetite nanoparticles

2.5

2

5

21.72

[42]

4 Conclusion Mesoporous magnetic Kit-6 was successfully synthesized and then, was functionalized with amine groups by a postgrafting method. Synthesis of magnetic silica mesoporous leads to increase the surface area and enhances the textural property of MNPs, that allows their use as a strong sorbent with high hydrothermal stability and ultra high adsorption capacity at short time compared with the conventional sorbents for Cr(VI) compounds. The amino-functionalized mesoporous magnetite exhibited high adsorption capacity of 185.2 mg g-1 in less than 15 min for adsorption of Cr(VI) from aqueous solutions. Finally, the equilibrium data were well fitted to the Langmuir adsorption isotherms. Acknowledgments Financial support by Rasht Branch, Islamic Azad University Grant No. 4.5830 is gratefully acknowledged.

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