The direct removal of arsenate (AsO43−) and chromate (CrO42−) from water were achieved using a Fe3+-bis-ethylenediamine complex-bridged periodic mesoporous organosilica with a 20% organosilane content (Fe-EDPMO-20). The bridged Fe3+-bis-ethylenediamine complex was introduced to the pore wall of the PMO by combining the pre-complexation and co-condensation processes. N,N′-bis[3-(triethoxysilyl)propyl]ethylenediamine (TESEN) and tetramethyl orthosilicate (TMOS) as silica precursors were used with cetyltrimethylammonium bromide (CTABr) as a surfactant under basic conditions for the preparation of highly ordered Fe-EDPMO-20. Transmission electron microscopy, X-ray diffraction, and N2 adsorption-desorption measurements confirmed that the Fe-EDPMO-20 had an ordered hexagonal p6mm mesostructure. The material had a Brunauer-Emmett-Teller surface area of 734 m2 g−1, pore diameter of 2.6 nm, and pore volume of 0.61 cm3 g−1. UV-vis and X-ray photoelectron spectroscopy confirmed that Fe3+ was embedded in the coordination site by the nitrogen atoms from ethylenediamine. The adsorption efficiencies of arsenate and chromate ions by Fe-EDPMO-20 were examined as a function of pH, stirring time, amount of adsorbent, and initial concentration of metal ion solution. The maximum adsorption for arsenate and chromate were 156 and 102 mg g−1 within 6 and 24 h, respectively, at pH 4.
Contamination of water by heavy metal pollutants, particularly arsenate (AsO43−) and chromate (CrO42−) anions, is an important environmental concern because of their high toxicity , . The negative impacts of arsenate and chromate on human health, which range from acute toxicity to chronic and carcinogenic effects, have led to the regulation of such anions and prompted research into new technologies. Arsenic is a ubiquitous element in the environment that has resulted in inevitable exposure to human beings . Arsenic exists in both organic and inorganic forms in nature; inorganic arsenic is found mostly in natural water systems. Generally, inorganic arsenic has two different oxidation states in natural aqueous systems: trivalent and pentavalent. Arsenic mostly exists as oxyanions in two major oxidation states, arsenite [As(III)] and arsenate [As(V)], depending on the redox conditions. Chromium is also a common pollutant found in industrial effluents because chromium salts are used extensively in several industrial processes, such as tanneries, electroplating, textile, dyeing, and metal finishing industries , . Chromium is found in various oxidation states, ranging from −II to +VI. Trivalent [Cr(III)] and hexavalent [Cr(VI)] chromium are of major environmental significance depending on the pH and redox conditions. The hexavalent form is 100–1000 times more toxic than the trivalent form, and its accumulation in the environment is of great concern , . Chromates (Cr(VI)) are soluble in water almost over the entire pH range and are mobile in the natural environment. Chromate forms several species, the relative proportions of which depend on both pH and total chromate concentration. Conventional methods for the removal of arsenate and chromate ions from various water sources include adsorption, ion exchange, electrocoagulation, and photoreduction. Most of these methods require either high energy or large quantities of chemicals . Compared to other water treatment technologies, adsorption is a promising method for metal removal having high removal efficiency, simplicity for operation, low cost, and high recycling rate without harmful by-products . The removal of these metal ions from industrial effluents, water supplies, and mine waters is an important challenge to mitigate one of the major causes of water and soil pollution. Under this circumstance, it is essential to develop novel and efficient adsorbents for arsenate and chromate removal from drinking water. Various kinds of adsorbents, such as traditional active carbon materials, metal oxides or bimetal oxides, surface modified materials, and some nanoparticle materials have been studied to remove arsenate and chromate from contaminated water. Among these materials, iron is particularly popular for its high affinity to both arsenate and chromate ions . Several iron(III) oxides, such as amorphous hydrous ferric oxide and crystalline hydrous ferric oxide can remove both arsenate and chromate from aqueous solutions , .
The characteristics of mesoporous materials, such as large uniform pore structure, high surface area, thermal stability, and ease of surface modification, are attractive to researchers seeking a suitable adsorbent for various ions present at low concentrations in the environment , , , , , . Recent studies have shown that functionalized mesoporous materials are very efficient in the removal of arsenate and chromate anions. Periodic mesoporous organosilica (PMO) materials are considered derivative materials of mesoporous silica, which can be obtained by the condensation of organo-bridged silsesquioxane precursors in the presence of organic soft templates. Using this approach, a broad spectrum of functional moieties can be incorporated into the stable framework. The flexibility and tunability in modification of the pore walls or the bulk of PMO allows for their use in a wide range of applications, including heterogeneous catalysis, encapsulation of transition metal complexes, selective recognition, adsorption of metal ions and drugs, separation, and sensor devices , , , , . By choosing the appropriate linker molecules, it is also possible to synthesize PMO materials with new and interesting properties, which are unknown in pure siliceous mesostructures. The possibility of incorporating various functional groups in the wall structure of PMO offers the preparation of metal complex-embedded PMO materials for future perspectives . To the best of the authors’ knowledge, however, there are no reports of the use of PMO with a metal complex in the wall structure as an adsorbent for arsenate and chromate ions.
In this study, Fe3+-bis-ethylenediamine complex-bridged PMO with a highly ordered structure was synthesized using a co-condensation pathway. The Fe3+ ion was used as the complex center, and is a well-known metal ion for adsorbing arsenate and chromate anions in water. The adsorbent synthesis included the preparation of a homogeneous solution of Fe3+-bis-TESEN complex separately in water that was then combined with TMOS before being added to the surfactant solution. The hydrolysis and distribution of the silane precursors in the surfactant solution led to the formation of a highly ordered mesoporous material in good yield. Surfactant removal was carried out in a mildly acidic ethanol solution for 3 h. The existence of uniformly distributed Fe3+ ion complexes in the framework of PMO offered good porosity and a well-ordered structure. The efficiency of the synthesized material in the removal of arsenate and chromate anions in water was examined under various pH, stirring times, amount of adsorbent, and initial metal ion concentration.
Materials and methods
N,N′-bis-[3-(triethoxysilyl)propyl]ethylenediamine (TESEN) was synthesized using a published procedure. Tetramethyl orthosilicate (TMOS, 98%), cetyltrimethylammoniumbromide (CTABr), ferric chloride hexahydrate (FeCl3·6H2O), sodium arsenate dibasic heptahydrate (Na2HAsO4·7H2O), potassium chromate (K2CrO4), and anhydrous toluene were purchased from Sigma-Aldrich. All chemicals were used as received. Doubly distilled water was used where necessary.
Synthesis of Fe3+-bis-ethylenediamine complex bridged periodic mesoporous organosilica (Fe-EDPMO-20)
A homogeneous solution of CTABr (2.62 g, 7.2 mM) and NaOH (0.576 g, 14.4 mM) in distilled water (108 mL, 6 M) was prepared initially at room temperature and allowed to stir for 2 h. A mixture of TESEN (4 mmol) and FeCl3·6H2O (2 mmol) in 20 mL of water was stirred at 60°C for 1 h. TMOS (32 mmol) was then added to this mixture and stirring was continued for another 0.5 h (The total silane content was limited to 40 mmol). The red blood colored homogeneous Fe3+-complex solution was added quickly to the surfactant solution with vigorous stirring at room temperature. After stirring for 2 h at room temperature, the mixture was kept at 90°C for 48 h under static conditions , . The as-synthesized Fe-EDPMO-20 was collected by filtration, washed with deionized water and methanol, and dried at 60°C. Surfactant extraction was carried out using 0.5 g of concentrated HCl (36%) in 150 mL of ethanol per gram of as-synthesized Fe-EDPMO-20 and stirred continuously at 60°C for 3 h. The material was filtered again, washed with water and ethanol, and then dried under vacuum at 60°C overnight. The surfactant-removed material was called Fe-EDPMO-20 (Scheme 1).
Adsorption of arsenate and chromate using Fe-EDPMO-20
A stock solution of arsenate or chromate (2 mM) was prepared by dissolving 62.4 and 38.8 mg of sodium arsenate dibasic heptahydrate (Na2HAsO4·7H2O) and potassium chromate (K2CrO4), respectively, in 100 mL of deionized water. Batch experiments were employed to examine the adsorption of arsenate and chromate from aqueous solutions onto Fe-EDPMO-20 at 25°C. Fe-EDPMO-20 (10 mg) was added to the arsenate and chromate solutions (10 mL, 2 mM) and shaken for a pre-determined time under ambient conditions. Subsequently, the adsorbent was filtered through a polypropylene micro filter, and the amount of metal adsorbed by Fe-EDPMO-20 was determined by inductively coupled plasma-atomic emission spectroscopy (ICP-OES). The effects of the parameters, such as pH, stirring time, amount of adsorbent, and initial concentration of metal ion solution, on the amount of arsenate and chromate adsorption were also studied under similar conditions.
Powder X-ray diffraction (XRD Rigaku D/Max 2500, 40 kV, 40 mA) was performed using Cu-Kα radiation. The XRD patterns were collected in the low-angle range from 1.2° to 10° 2θ. Transmission electron microscopy (TEM, JEOL 2010) was conducted at an accelerating voltage of 200 kV. To prepare the samples, a tiny piece was dispersed in ethanol using an ultrasonic dispenser. A droplet was placed onto a copper grid coated with an amorphous polymer or carbon film and dried under a warm air stream.
The scanning electron microscopy (SEM, JEOL 6400) images were collected at an operating voltage of 20 kV. Prior to imaging, the samples were coated with a nanometer thick gold layer using a HITACHI E-1010 sputter coater.
The N2 adsorption-desorption isotherms were measured using a Nova 4000e surface area and pore size analyzer. The samples were degassed at 120°C for 12 h before the measurements. The Brunauer-Emmet-Teller (BET) method was used to calculate the specific surface area. The pore size distribution curve was obtained from an analysis of the adsorption branch using the Barrett-Joyner-Halenda (BJH) method.
Fourier transform infrared (FTIR, JASCO FTIR 4100) spectroscopy was performed using KBr pellets over the frequency range, 4000–400 cm−1. Thermogravimetric analysis (TGA, Perkin Elmer Pyris Diamond) was carried out at a heating rate of 10°C min−1 in air.
13C-cross-polarized (CP) and 29Si magic angle spinning (MAS) nuclear magnetic resonance (NMR, Bruker DSX 400) spectroscopy was performed with a 4 mm zirconia rotor spinning at 6 kHz (resonance frequencies of 79.5 and 100.6 MHz for 29Si and 13C CP MAS NMR, respectively; 90° pulse width of 5 ms, contact time of 2 ms, recycle delay of 3 s for both 29Si MAS and 13C CP MAS NMR).
The absorption spectra of Fe-EDPMO-20 was measured using a SHIMADZU UV-1650 PC spectrometer optimized with a spectral width of 200–800 nm, a resolution of 0.5 nm, and a scanning rate of 200 nm min−1. Before scanning the UV-vis spectrum, a powder material was made into a disc using a hydraulic pelletizer.
To examine the surface elements of the materials and their oxidation state, X-ray photoelectron spectroscopy (XPS, HI-1600ESCA, Perkin-Elmer, City, State, USA) was carried out using non-monochromatic Mg Kα radiation operated at 15 kV under 10−7 Pa pressure, and with a photoelectron energy set to 1254 eV (Korea Basic Science Institute Daegu Center, Korea).
The Fe3+ content in the Fe-EDPMO-20 and the amount of adsorbed metal ions from solution were determined by inductively coupled plasma-optical emission spectrometry (ICP-OES, ACTIVA, JYHORIVA, Japan).
Results and discussion
Powder X-ray diffraction patterns
Figure 1 shows XRD patterns of (a) as-synthesized Fe-EDPMO-20 and (b) Fe-EDPMO-20. Both samples exhibited a single intense peak in the low-angle Bragg diffraction region, which was indexed to the d100 plane, indicating the characteristics of mesoporous M41S type materials , . The presence of three more reflections corresponding to d110, d200, and d210 suggests that the PMOs contained good quality hexagonal uniform pores with long range order. The d100 reflection for Fe-EDPMO-20 at 2.19° 2θ was observed in a slightly lower range compared to the same reflection of the as-synthesized Fe-EDPMO-20 (2.27° 2θ). The slight decrease in 2θ value corresponding to the d100 plane might be due to the negligible expansion of the pore diameter as a function of the removal of surfactant and heat treatment. The other three reflections, d110, d200, and d210, also showed the same trend in the peak position and persisted with the intensities decreasing after surfactant removal, indicating a well-ordered and good quality mesoporous material .
SEM and TEM images
Figure 2 presents representative SEM (a and b) and TEM (c and d) images of the as-synthesized Fe-EDPMO-20 and SEM (e and f) and TEM (g and h) images of Fe-EDPMO-20. The images show a spherical particle morphology with a mean particle size of 400 nm. A small amount of particle aggregation was also observed from the SEM images in Fe-EDPMO-20 (Fig. 2a and b), which is responsible for the formation of interparticle voids. Fe-EDPMO-20 also exhibited a partial spherical morphology (Fig. 2e and f), which indicated the persistence of undamaged material after surfactant extraction. The interparticle aggregation in these particles was comparatively less than the previous one due to the ethanol treatment during surfactant removal. The pore structure and well-defined mesoporous channels can be visible from the TEM images of the as-synthesized Fe-EDPMO-20 (Fig. 2c and d) and Fe-EDPMO-20 (Fig. 2g and h), which is consistent with the information provided by the XRD patterns of both materials. Both samples showed good long range order, confirming the 2D mesostructure. The results suggest that the Fe-EDPMO-20 matrices retained their morphological integrity (both shape and size) after extracting the surfactant , .
N2 adsorption-desorption analysis
Figure 3 presents the nitrogen sorption isotherms and their corresponding pore size distribution (Inset) for Fe-EDPMO-20. The interactive relationships among the surface properties and pore structural features of adsorption can be reflected through the adsorption isotherm. The isotherm displayed a type IV pattern with steep capillary condensation/evaporation steps and an obvious H1 hysteresis loop, which is characteristic of mesoporous materials according to the IUPAC classification . Regarding the textural properties of the synthesized material, the specific surface area of Fe-EDPMO-20 was calculated to be 734 m2 g−1. The pore diameter and pore volume of Fe-EDPMO-20 was 2.6 nm and 0.61 cm3 g−1, respectively. The total nitrogen (N) and Fe3+ content (M) in the material determined by elemental and ICP-OES analyses were 1.21 mmol g−1 and 0.77 mmol g−1, respectively, with a N/M value of 1.57.
Figure 4 presents the FT-IR spectra of (a) as-synthesized Fe-EDPMO-20 and (b) Fe-EDPMO-20. The spectra revealed the presence of all the peaks of TESEN in Fe-EDPMO-20. Imine (-NH), silanol (Si-OH), aliphatic -CH, -Si-C, N-C, Fe-N, and -Si-O-Si are the major functional moieties in the Fe-EDPMO materials. The spectra clearly revealed the difference between as-synthesized and surfactant removed Fe-EDPMO-20. Both surfactant and the TESEN molecule contain the -CH group. The vibrational peaks between 2999 and 2850 cm−1 was assigned to the -CH stretching vibrations in both materials. But in the as-synthesized Fe-EDPMO-20 (Fig. 4a), the intensity of FTIR peaks corresponding to -CH group was more intense than in the surfactant removed Fe-EDPMO-20 (Fig. 4b) due to the existence of surfactant molecules in its pore structure. The reduced intensity to the peaks assigned the successful removal of surfactants from the pores of the material by solvent extraction. The large band centered at 3439 cm−1 was attributed to the OH stretching frequency of the silanol groups in the inorganic framework. The band at approximately 1634 cm−1 was associated with the angular vibration of water molecules bonded to the inorganic framework , , . N-H is the ligand moiety in the TESEN molecule and the FT-IR spectra show the presence of vibrational bands at 1652 and 3410 cm−1. In addition, the relatively broad peaks at 789 and 1156 cm−1 indicate the presence of an N-H wagging vibration and the anti-symmetric stretch of C-N-C, respectively . The Si-O-Si entity is the major building block of the PMO framework, which can be observed from the stretching vibration in between 1100 and 1000 cm−1. To support the above observation, the O-Si-O bond showed a bending vibrational band at 670 cm−1. Moreover, a strong but broad band at 1210 cm−1 in both samples was observed due to the Si-C vibrational frequency, suggesting that the Si-C bond was not broken even after functionalization and subsequent surfactant removal , . In addition to the above peaks, the existence of a small but intense peak at 586 cm−1, corresponding to the Fe-N stretching vibration, indicates the presence of the Fe3+-bis-ethylenediamine complex in Fe-EDPMO-20.
29Si MAS and 13C CP-MAS NMR spectral analyses
The successful incorporation of the covalently anchored organic groups in the framework was better understood from the solid-state 29Si MAS NMR spectrum (Fig. 5a) of the Fe-EDPMO-20 material. In the spectrum, the strong resonances at δ≈−89, −100, and −109 ppm were assigned to Q2 [(SiO)2=Si-(OH)2], Q3 [(SiO)3≡Si-OH], and Q4 [(SiO)4≡Si-O-Si≡] species, respectively, which are present in the silicate framework of the Fe-EDPMO-20 material . These peaks were grouped into Q signals from silicon atoms in the framework without organic substitution. A second set of peaks were observed at δ≈−56 and −65 ppm, which corresponded to two T signals related to organo-substituted silicon. These peaks were assigned to the isolated terminal [T2, (SiO)2(OH)SiC] and cross-linked [T3, (SiO)3SiC] siloxanes, respectively. The intensity of the T3 signal was higher than that of the T2 species, which indicates better cross-linking in the framework. Moreover, the signal for T1 species was not observed in this sample .
The presence of a TESEN molecule in the bulk of the PMO material was confirmed from the 13C CP-MAS NMR spectrum of Fe-EDPMO-20 (Fig. 5b). Three signals were observed in the chemical shift values between (0–50 ppm) corresponding to Si-CH2CH2CH2- groups from TESEN, demonstrating the presence of an uncleaved Si-C bond. These three distinct resonances at δ=9.7, 21.4, and 43 ppm were assigned to C1, C2 and C3 carbon atoms, respectively, of the propylsilane segment in the organosilanes . The peak at 50 ppm was assigned to the C4 carbon atom of the ethylenediamine moiety in the TESEN molecule. No additional peaks were observed in the spectrum, indicating the complete hydrolysis of the ethoxy groups of the bissylilated molecule in the wall structure.
Figure 6 shows the DRUV-vis spectra of the (a) EDPMO-20 and (b) Fe-EDPMO-20. [EDPMO-20 was synthesized employing a similar procedure used for Fe-EDPMO-20 without the addition of FeCl3. 6H2O along with the silane precursors]. Si-MCM-41 showed no absorption bands over the wavelength range, 200–800 nm, while the results of the Fe-EDPMO-20 samples showed a very strong absorption in the wavelength range of 200–300 nm centered at approximately 256 nm, corresponding to the low-energy charge-transfer transitions between the tetrahedral nitrogen ligands and the central Fe3+ ion in the complex . Such a tetrahedral environment is typical for the framework Fe3+ ions in Fe-substituted micro and mesoporous molecular sieves , .
The XP spectrum of Fe-EDPMO-20 (Fig. 7) provided good support to the complexation and incorporation of Fe3+ in the framework of the PMO material. The appropriate peaks for the main elements were observed in the wide spectrum. The ethylenediamine functionality was also observed in the XP spectrum at a binding energy of 399.9 eV, corresponding to the N 1s binding energy. The Fe 2p spectra were measured to determine the oxidation state of complexed iron with ethylenediamine in the framework of the material. The Fe 2p3/2 and Fe 2p1/2 peaks at 712.0 eV and 725.5 eV, respectively, were assigned to Fe3+. The shakeup satellite peak at 719.9 eV revealed Fe in the Fe3+ ionic state. The inset shows the corresponding binding energies of Fe 3s and Fe 3p. The peaks at ca. 99.1, 282.69, and 529.5 eV were assigned to Si 2p, C 1s, and O 1s, respectively .
Figure 8 shows the TGA profile of Fe-EDPMO-20. Upon heating in air, Fe-EDPMO-20 undergoes total weight losses of 21% over the entire temperature range. The initial weight loss (~2%) below 120°C corresponded to the desorption and removal of physisorbed water from the pores of Fe-EDPMO-20. Normally, the decomposition temperature of organic moieties from organosilanes will be higher than functionalized M41S materials. In PMO, the organosilane has a strong interaction with the silane moieties in the framework not only due to the hydrogen bonded network interaction, but also because the deeply buried organosilane forms a strong chemical bond by hydrolysis during hydrothermal synthesis. In the case of Fe-EDPMO-20, an additional binding interaction was also expected due to the Fe3+-complex formation between the two TESEN molecules. A second and major weight loss accompanied by several exotherms occurred between 210 and 700°C due to oxidative decomposition of the organic content in two steps. This was observed in the TG curve Fe-EDPMO-20 as an initial weight loss between 210 and 400°C and a second weight loss between 400 and 700°C. The organic components of the material decomposed between the above temperature ranges, leading to a total weight loss of 19%. The rather extended decomposition up to 800°C was caused by silanol condensation and channel metamorphosis via proton transfer from the silanols to methylene groups , .
The amount of As(V) and Cr(VI) adsorbed by the Fe-EDPMO-20 from the aqueous solution at various pH and with respect to the stirring time was determined using the following equation:
where Q is the amount of the metal ion on the adsorbents (mmol g−1); V is the volume (L) of the aqueous solution; W is the weight of the adsorbent (g); Co the initial concentration of metal ion (mmol L−1); and Ce the equilibrium metal ion concentration in solution (mmol L−1). Adsorption analysis was performed in triplicate for each sample and only the mean data is reported.
Stability of Fe3+ ions in Fe-EDPMO-20 with respect to pH
To determine the leaching behavior of Fe3+ from the Fe3+-ethylenediamine complex, 10 mg of the Fe-EDPMO-20 in 10 mL of an aqueous solution with a pH of 1, 2, 3, 4, 5, or 6 was stirred for 24 h. The suspension was filtered through a polypropylene micro filter and the amount of Fe3+ leached out was determined by ICP-OES analysis. Regarding the release of total iron from Fe-EDPMO-20 into solution, the maximum total iron concentration in the solution was 2.6 and 1.1 mg L−1 with a leaching ratio of 0.011 and 0.005% at pH 1 and 2, respectively. The solution color became light yellow and then slightly cloudy. For the other pH values, the concentration of iron released was almost zero at pH 3 and zero for pH 4, 5 and 6, which indicated the formation of a stable Fe3+ complex by the two ethylenediamine molecules. The dissolution of iron from the complex molecule at a particular pH depends greatly on the strength of chelation and the type of ligand. This tendency is similar to previous reports that a lower pH favors iron dissolution . Fe3+ in the Fe-EDPMO-20 could work properly in the adsorption isotherm because the stability of the Fe3+ complex in PMO is greater at pH 4, 5 and 6.
Effect of pH on the adsorption of arsenate and chromate adsorption by Fe-EDPMO-20
To determine the effects of pH on the removal ability of Fe-EDPMO-20 for arsenate and chromate, the experiment was carried out using 10 mg of the adsorbent and 10 mL of the metal ion solution (2 mM) at different pH (pH 3–8) and the results are shown in Fig. 9. Both ions adsorbed in a similar manner with respect to pH. A smaller amount of adsorption was observed at pH 3. At pH 4, the amount of adsorption reached a maximum of 156 and 102 mg g−1 for arsenate and chromate, respectively. Subsequently, a gradual decrease in the amount of adsorption was reflected for both ions. pH 4 was the optimal pH for the maximum removal of arsenate and chromate from solution. The total Fe3+ content in the Fe-EDPMO-20 material was 0.77 mmol. Theoretically, one Fe3+ ion can adsorb three monovalent arsenate anions. Experimentally, this is not possible due to repulsion between the anion species due to steric hindrance. If 1:1 (active site: mono valent anion) adsorption is considered, a maximum of 107.8 mg g−1 arsenate can be adsorbed by the adsorbent. In addition, a maximum of 89.32 mg g−1 chromate ion can adsorb on the adsorbent. On the other hand, the observed amount of adsorption was 156 and 102 mg g−1 for arsenate and chromate, respectively. These values suggest the possible adsorption of more than one anion species or the multilayer adsorption of arsenate and chromate on the active site.
Arsenate exists mainly as H2AsO4− at pH 4. The divalent or trivalent anionic species of arsenate increase with increasing pH. This is unfavorable for arsenic removal in both situations. In addition, the ferric ion can dissolve in water at low pH, which will decrease the number of active sites in the adsorbent and reduce the removal ability. The pH of the adsorption medium will have a solid effect on the removal efficiency of the adsorbent. The negative charge on the surface of the silica material increases with increasing pH when pH>4, so the repulsive force between the arsenate and absorbents strengthens , . As shown in Fig. 9, the highest removal ability for chromate also appeared at pH=4 and can be explained by the ionic state in solution combining with the electrostatic attraction between the chromate ion and the active site of the adsorbent. Chromate exists mainly as HCrO4− in the pH range from 0 to 5.8. Above this pH, Cr(VI) exists as Cr2O72− and beyond pH 8, CrO42− species dominate. Therefore, below pH 5.8, Cr(VI) would be present in the mono-anionic form, while above pH 5.8, it is in its divalent anionic form. Therefore, when the solution pH was near neutral or basic, the extra negative charge of Cr(VI) might prevent further interactions with the ferric hydroxide formed in the Fe3+-bis-ethylenediamine complex at the PMO wall structure. In conclusion, anionic forms, such as Cr(VI) oxo-species, tend to be more attracted to positively charged FeOH2+ centers that form at lower pH, while the adsorption capacity decreases at high pH .
Effect of time on the adsorption of arsenate and chromate adsorption by Fe-EDPMO-20
Figure 10 shows the adsorption of arsenate and chromate with respect to time. A 10 mg sample of adsorbent was used in 10 mL of the metal ion solution (2 mM) at pH 4 by varying the adsorption time up to 48 h. A faster adsorption rate was observed for arsenate but a slower rate of adsorption was observed for chromate. Arsenate adsorption reached equilibrium (156 mg g−1) within 6 h but a longer time (24 h) was needed for chromate ions to attain equilibrium adsorption (102 mg g−1). Approximately, 78 and 92% of arsenate was adsorbed from the solution within the first 1 and 3 h, respectively, with respect to the equilibrium adsorption by Fe-EDPMO-20. After 6 h, the adsorption leveled off. The rapid adsorption in the beginning can be attributed to the higher concentration gradient and more active sites available for adsorption . This is a common behavior with adsorption processes and might be due to the rapid hydrolysis of Fe3+ in the adsorbents as result of the rapid diffusion of solvent as well as arsenate ions in the silica pores of Fe-EDPMO-20. On the other hand, only 43 and 57% of chromate ions were adsorbed with respect to the total chromate adsorbed by Fe-EDPMO-20 within the first 1 and 3 h, respectively, and adsorption leveled off after 24 h. The experiment demonstrated the higher affinity of the adsorption center for arsenate than chromate under identical adsorption conditions , .
Effect of the amount of Fe-EDPMO-20 on the adsorption of arsenate and chromate
The amount of adsorbent has a strong effect on the adsorption of metal ions from solution under particular pH conditions. When the amount of adsorbent in a fixed volume of adsorption medium is increased, a corresponding increase in the number of active sites is expected. An adsorbent dosage of 5, 10, 15, and 20 mg were used to analyze the effects of the amount of Fe-EDPMO-20 on the adsorption of arsenate and chromate. Figure 11 shows the effects of the amount of Fe-EDPMO-20 on the adsorption of arsenate and chromate from the adsorption medium (10 mL, 2 mM) at pH 4. The mixture was stirred mechanically at ambient temperature for 6 and 24 h, respectively, for arsenate and chromate. Subsequently, the mixture was filtered, and the concentration of anion remaining in solution was determined by ICP-AES. Plots of the amount of adsorbent versus the amount adsorbed suggest that the adsorption of both anions increased with increasing adsorbent dosage. The amount of adsorption at adsorbent amounts of 5, 10, 15 and 20 mg was 88, 156, 222 and 268 mg L−1 for arsenate and 54, 102, 147 and 189 mg L−1 for chromate, respectively. An increase in surface area and active sites are the major reasons for the observed increase in adsorption , .
Effect of the initial concentration of the metal ion solution on the adsorption of arsenate and chromate by Fe-EDPMO-20
The adsorption efficiency depends greatly on the effectiveness of binding between the adsorbent and adsorbate. In addition, the adsorption equilibrium can be altered in the adsorbent or adsorbate by merely altering the concentration of metal ions. To examine the effects of the initial metal ion concentration, a 10 mg sample of Fe-EDPMO-20 was suspended in 10 mL of the arsenate or chromate solution with a concentration in the range, 1–5 mM, at pH 4. The mixture was stirred mechanically at ambient temperature for 6 and 24 h, respectively, for arsenate and chromate. The mixture was filtered, and the concentration of anions remaining in solution was determined by ICP-OES. Figure 12 shows the amount of arsenate and chromate ions adsorbed as a function of the initial anion concentration in solution. A steady increase in adsorption for both arsenate and chromate with increasing metal ion concentration was observed. The amount of adsorption at metal ion concentrations of 1, 2, 3, 4 and 5 mM was 136, 156, 162, 169, and 172 mg g−1 for arsenate and 82, 102, 116, 121, and 124 mg g−1 for chromate, respectively. At low metal ion concentrations, the ratio of surface active sites to the total metal ions in the solution was high; hence, a major portion of the metal ions in solution may interact with the adsorbent and be removed from solution , , . At higher initial metal ion concentrations, a slight increase in the amount of adsorption was observed. A close examination of Fig. 12 shows that the rate of the increase in adsorption was reduced at very high initial concentrations. The decrease in the rate of increase in the amount adsorbed is the result of the adsorption/desorption equilibrium achieved between the metal ions in solution and the adsorbent.
In this study, the synthesis and adsorption ability of a Fe3+-bis-ethylenediamine complex bridged periodic mesoporous organosilica, Fe-EDPMO-20, was studied. XRD of Fe-EDPMO-20 before and after extraction of the surfactant indicated the formation of highly ordered 2D mesoporous organosilica. An organosilane incorporation of 20% was achieved on the PMO with respect to the total silane content. TGA confirmed the incorporation of organic groups in the bulk of the PMO material. 13C CP MAS NMR and FT-IR spectroscopy revealed the persistence of major bonds, such as -C-H, -Si-C, and N-C, in the TESEN molecule incorporated in the wall structure of Fe-EDPMO-20 without the cleavage of Si-C bonds. The 29Si MAS NMR spectrum of Fe-EDPMO-20 reflects the existence of two groups of signals, the three Q signals and two T signals related to silicon atoms without and with organic substitution, respectively, which reinforces the above statement. The very strong absorption in the wavelength range of 200–300 nm centered at approximately 256 nm in the UV-vis spectrum and the Fe 2p3/2 and Fe 2p1/2 peaks at 712.0 eV and 725.5 eV, respectively, in the XP spectrum were assigned to the complexation and persistence of Fe3+ ions in the bulk of the PMO as ethylenediamine complex. The adsorption experiments of Fe-EDPMO-20 revealed pH 4 to be the optimal pH and the time of adsorption for achieving the maximum intake was 6 and 24 h for arsenate and chromate, respectively. The detailed characterization and adsorption experiments confirmed the successful preparation of Fe3+-bis-ethylenediamine complex-bridged periodic mesoporous organosilica material with a large surface area and pore volume, abundant amino functionality, and complexation of Fe3+, which have high adsorption capacity for both arsenate and chromate anions.
A collection of invited papers based on presentations at 8th International IUPAC Symposium on Macro- and Supramolecular Architectures and Materials: Multifunctional Materials and Structures (MAM-17) held in Sochi, Russia, 6–10 June 2017.
The authors thank to the National Research Foundation of Korea through the Ministry of Science and ICT, Korea (NRF-2017R1A2B3012961); Brain Korea 21 Plus Program (21A2013800002) for financial support.
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