Fe³⁺-bis-ethylenediamine complex bridged periodic mesoporous organosilica for the efficient removal of arsenate and chromate

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 [23], [24]. 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).

Scheme 1:

Preparation of Fe-EDPMO-20.

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.

Characterization

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).

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 [23], [24]. 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 [25].

Fig. 1:

XRD patterns of (a) as-synthesized Fe-EDPMO-20 and (b) Fe-EDPMO-20.

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 [26], [27].

Fig. 2:

SEM images of as-synthesized Fe-EDPMO-20 (a and b) and Fe-EDPMO-20 (e and f) and TEM images of as-synthesized Fe-EDPMO-20 (c and d) and Fe-EDPMO-20 (g and h).

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 [28]. 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.

Fig. 3:

N₂ adsorption-desorption isotherms and pore size distributions (Inset) of Fe-EDPMO-20.

FTIR analysis

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 [23], [27], [29]. 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 [30]. 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 [31], [32]. 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.

Fig. 4:

FT-IR spectra of (a) as-synthesized Fe-EDPMO-20 and (b) 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 [27]. 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 [27].

Fig. 5:

(a) ²⁹Si MAS and (b) ¹³C CP-MAS NMR spectra of Fe-EDPMO-20.

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 [33]. 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.

DRUV-vis spectrum

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 [34]. Such a tetrahedral environment is typical for the framework Fe³⁺ ions in Fe-substituted micro and mesoporous molecular sieves [35], [36].

Fig. 6:

DRUV-vis spectra of (a) EDPMO-20 and (b) Fe-EDPMO-20.

XPS analysis

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 2p_3/2 and Fe 2p_1/2 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 [37].

Fig. 7:

XP spectrum of Fe-EDPMO-20.

Thermogravimetric analysis

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 [19], [26].

Fig. 8:

TGA curve of Fe-EDPMO-20.

Adsorption studies

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); C_o the initial concentration of metal ion (mmol L⁻¹); and C_e 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 [38]. 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.

Fig. 9:

Adsorption of AsO₄³⁻/CrO₄²⁻ from aqueous solutions at various pH: Fe-EDPMO-20=10 mg, anion solution=10 mL (2 mM), time=24 h, pH=3–8.

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 [7], [16]. 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 [39].

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 [16]. 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 [7], [39].

Fig. 10:

Adsorption of AsO₄³⁻/CrO₄²⁻ from aqueous solutions with respect to time. Condition: Fe-EDPMO-20=10 mg, anion solution=10 mL (2 mM), pH=4, time=0.25–48 h.

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 [7], [39].

Fig. 11:

Adsorption of AsO₄³⁻/CrO₄²⁻ from aqueous solutions with respect to the initial amount of adsorbent. Condition: Fe-EDPMO-20=5–20 mg, anion solution=10 mL (2 mM), pH=4.

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 [7], [38], [39]. 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.

Fig. 12:

Adsorption of AsO₄³⁻/CrO₄²⁻ from aqueous solutions with respect to the initial solution concentration. Condition: Fe-EDPMO-20=10 mg, anion solution=10 mL, metal ion concentration=1–5 mM, pH=4.