Zinc Removal from Water via EDTA-Modified Mesoporous SBA-16 and SBA-15

The removal of zinc ions from water was investigated using two types of ordered mesoporous silica (SBA-15 and SBA-16). Both materials were functionalized with APTES (3-aminopropyltriethoxy-silane) and EDTA (ethylenediaminetetraacetic acid) through post grafting methods. The modified adsorbents were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), X-ray diffraction (XRD), nitrogen (N2) adsorption–desorption analysis, Fourier transform infrared spectroscopy (FT-IR), and thermogravimetric analysis. The ordered structure of the adsorbents was conserved after modification. SBA-16 was found to be more efficient than SBA-15 owing to its structural characteristics. Different experimental conditions were examined (pH, contact time, and initial zinc concentration). The kinetic adsorption data followed the pseudo-second-order model indicating favorable adsorption conditions. The intra-particle diffusion model plot represented a two-stage adsorption process. The maximum adsorption capacities were calculated by the Langmuir model. The adsorbent can be regenerated and reused several times without a significant decline in adsorption efficiency.


Introduction
In the past few decades, water contamination has been considered a major problem worldwide. Effluents containing heavy metals such as copper, lead, zinc, and cadmium have been discharged without any treatment into the environment from several industries, such as from steel production, electroplating, and tanneries [1]. The discharged heavy-metal-contaminated wastewater is considered a serious threat to both human health and the ecosystem. Heavy metals are non-biodegradable and may be carcinogenic, thus their presence in water in high amounts could result in critical health issues to living organisms [2][3][4]. Zinc is largely spread in nature and is an essential trace metal for both humans and aquatic organisms [5]. However, if the zinc dosage exceeds a certain quantity, it becomes harmful to organisms [6] as it could interact with biological macromolecules, resulting in a change in their activity and poisoning [7]. Currently, various methods are available for heavy metal removal from water, including membrane filtration, coagulation, precipitation, ion-exchange, and adsorption [8]. The latter is very effective in eliminating heavy metals and is an attractive technique because it does not require complex and expensive installations [9,10]. The adsorption mechanism is defined by the physicochemical properties of the adsorbent and heavy metals and operating conditions (i.e., temperature, adsorbent amount, pH value, adsorption time, and initial concentration of metal ions) [11]. The efficiency of several low-cost adsorbents has been studied, including clay, chitosan, fly ash, zeolites, and activated carbon [12]. Moreover, adsorption is the best method to use when the metal ions' concentrations are below 100 mg L −1 [13]. The adsorption capacity is dependent on the pore size of adsorbents along with the active sites found on their surface [14] and, in recent years, many researchers have focused on developing new For the template of SBA-16 and SBA-15, Pluronic F127 (EO106PO70EO106) and Pluronic P123 (EO20PO70EO20) triblock copolymers, respectively, were used. Tetraethylorthosilicate (TEOS 98%) was used as the source of silica. For modification, 3-aminopropyltrimethoxysilane (APTES 99%) and ethylenediaminetetraaceticacid disodium salt (EDTA-Na2) were utilized. Hydrochloric acid (HCl, 37%), sodium hydroxide (NaOH), toluene, zinc nitrate, and sodium bicarbonate (NaHCO3) were also used in this study. All of the reagents were purchased from Sigma Aldrich (St. Louis, MO, USA) and utilized as received without any further purification. Ultrapure water was used throughout.

Adsorbents Characterization
For determining the textural properties, a Micromeritics TRISTAR sorptiometer (Micromeritics Instrument Corp., Norcross, GA, USA) was used and nitrogen adsorptiondesorption isotherms were obtained at −196 • C. The as-synthesized samples were out gassed at 350 • C under vacuum for at least 5 h before measurement and overnight at 150 • C for the modified samples. Low angle X-ray diffraction (XRD) patterns were obtained with an Empyrean X-ray diffractometer (Malvern Panalytical Ltd., Royston, UK) with Cu Kα (λ = 1.54 Å) radiation and a 0.008 o min −1 rate of scanning between 0.65 • and 5 • 2θ. SBA-16 and SBA-15 morphologies were determined by scanning electron microscopy (SEM, JEOL 7001 FEG, Tokyo, Japan) and transmission electron microscopy (TEM, JEOL 2100 UHR at 200 kV, Tokyo, Japan). Thermogravimetric analysis (TG) was conducted using SDT Q600 TA Instrument from 25 to 900 • C in air using SDT Q600 TA Instruments (New Castle, DE, USA). The functional groups were identified in the range of 4000-400 cm −1 by Fourier transform infrared spectroscopy (FT-IR-6300 JASCO, Oklahoma City, OK, USA) through mixing the samples with KBr and pressing them into pellets. The amount of immobilized carboxyl groups after EDTA modification was measured by back titration [35].

Batch Adsorption Experiments
The zinc nitrate salt was dissolved in ultrapure water and solutions with different zinc ion concentrations (between 10 and 500 ppm) were prepared. For batch adsorption studies, 20 mg of SBA-16-EDTA or SBA-15-EDTA was added to 20 mL of metal solution of concentration C and the flask was stirred at room temperature (RT) at 300 rpm for 180 min. At the end of each step, the zinc concentration was determined using an atomic adsorption spectrophotometer (AAS, Perkin Elmer AA200, Waltham, MA, USA). The removal efficiency was calculated by Equation (1) [36]: where C 0 and C t are the heavy metal initial concentration and at concentration at time t, respectively. The adsorption capacity (mg g −1 ) of the adsorbent at equilibrium was calculated by Equation (2) [36]: where C e is the concentration at equilibrium, (V) is the volume in L of metal solution, and m is the mass in g of the adsorbent. The adsorption isotherms were established by varying the initial metal ion concentrations between 10 mg L −1 and 500 mg L −1 . The solutions were stirred for 180 min at RT and then filtered and the remaining metal ions were measured by AAS in order to calculate C e and q e . The pH effect was studied by varying the solution pH between 2 and 8 using 0.1 M HCl or 0.1 M NaOH. Adsorbents' regeneration was performed with 1 M HCl solution.

Adsorbents' Characterization
The surface morphologies of the adsorbents were obtained by SEM ( Figure 2). SBA-16 appeared as fine cube particles while SBA-15 had rod-shaped particles. The crystallographic structure of both materials was investigated by TEM. SBA-16 images before and after EDTA modification ( Figure 3A,C) showed arrays of highly ordered uniform cages demonstrating the 3D cubic structure of SBA-16 that remained unaffected after functionalization. As for SBA-15, Figure 3B,D revealed the highly ordered 2D hexagonal structure (honeycomb structure), which also remained intact after modification.
XRD patterns are shown in Figure 4. The peaks in SBA-16 diffractograms corresponding to the (110), (211), and (220) planes, which are indexed at 2θ = 0.8, 1.1, and 1.8, respectively, are characteristics of the cubic body-centered structure (Im3m) [37]. The three diffraction peaks in the SBA-15 pattern, indexed at (100), (110), and (200) planes, are characteristic of the two-dimensional hexagonal symmetry (P6mm) [38]. The three obvious characteristic peaks for SBA-15 are at 2θ = 0.9, 1.7, and 2.2, referring to the (100), (110), and (200) planes, respectively. After EDTA modification, there was a slight pattern shift due to the decrease in pore size, but the symmetrical structure was conserved. For SBA-16-EDTA, the (110) plane shifted to a lower diffraction angle (110) compared with unmodified mesoporous silica SBA-16. Such a shift is due to the decrease in lattice parameters a due to pore filling with amino and EDTA functional groups.   XRD patterns are shown in Figure 4. The peaks in SBA-16 diffractograms corresponding to the (110), (211), and (220) planes, which are indexed at 2θ = 0.8, 1.1, and 1.8, respectively, are characteristics of the cubic body-centered structure (Im3m) [37]. The three diffraction peaks in the SBA-15 pattern, indexed at (100), (110), and (200) planes, are characteristic of the two-dimensional hexagonal symmetry (P6mm) [38]. The three obvious characteristic peaks for SBA-15 are at 2θ = 0.9, 1.7, and 2.2, referring to the (100), (110), and (200) planes, respectively. After EDTA modification, there was a slight pattern shift due to the decrease in pore size, but the symmetrical structure was conserved. For SBA-16-   Figure 5 illustrates the nitrogen adsorption-desorption isotherms, and the textural properties and pore size are included in Table 1. According to the IUPAC classification, classical type IV isotherms were made of both materials. SBA-15 exhibited an H1 hysteresis loop, affirming the presence of well-defined and cylindrical mesopores. On the other hand, SBA-16 exhibited an H2 hysteresis loop, which is characteristic of a cage-like mesoporous structure with narrower entrances than the cage itself [39]. For SBA-15, the capillary condensation occurred at a higher relative pressure than for SBA-16, showing that the mesopores of SBA-15 are larger than those of SBA-16. After modification with functional groups, the size of the mesopores along with the surface area and the mesoporous volume decreased for all of the samples. The large decrease in the surface area after modification is mainly the result of the micropores' blockage by amino propyl groups after modification. As for the shape of the mesopores, no important change was observed after modification and the structure remained intact, which was also proved by XRD and TEM.
Thermogravimetric analyses under air were also conducted for the samples before and after modification ( Figure 6). The weight loss that occurred below 200 °C is due to the desorption of water. Before modification, the weight losses between 200 and 900 °C are attributed to the silicate networks' dehydroxylation. For the modified samples, significant weight losses were observed between 200 and 900 °C. Aminopropyl groups were thermally degraded between 100 and 550 °C and the decomposition of EDTA occurred in the same temperature range as well.   Table 1. According to the IUPAC classification, classical type IV isotherms were made of both materials. SBA-15 exhibited an H1 hysteresis loop, affirming the presence of well-defined and cylindrical mesopores. On the other hand, SBA-16 exhibited an H2 hysteresis loop, which is characteristic of a cage-like mesoporous structure with narrower entrances than the cage itself [39]. For SBA-15, the capillary condensation occurred at a higher relative pressure than for SBA-16, showing that the mesopores of SBA-15 are larger than those of SBA-16. After modification with functional groups, the size of the mesopores along with the surface area and the mesoporous volume decreased for all of the samples. The large decrease in the surface area after modification is mainly the result of the micropores' blockage by amino propyl groups after modification. As for the shape of the mesopores, no important change was observed after modification and the structure remained intact, which was also proved by XRD and TEM. Thermogravimetric analyses under air were also conducted for the samples before and after modification ( Figure 6). The weight loss that occurred below 200 • C is due to the desorption of water. Before modification, the weight losses between 200 and 900 • C are attributed to the silicate networks' dehydroxylation. For the modified samples, significant weight losses were observed between 200 and 900 • C. Aminopropyl groups were thermally degraded between 100 and 550 • C and the decomposition of EDTA occurred in the same temperature range as well.  The infrared spectrum of calcined SBA-15 and SBA-16 ( Figure 7) shows typical bands of silanol groups at 3500-3750 cm −1 [40]. After modification with aminopropyl groups, the intensity of these bands decreased, while the bands characteristic of aminopropyl groups   The infrared spectrum of calcined SBA-15 and SBA-16 ( Figure 7) shows typical bands of silanol groups at 3500-3750 cm −1 [40]. After modification with aminopropyl groups, the intensity of these bands decreased, while the bands characteristic of aminopropyl groups appeared. These new bands are attributed to the symmetric and asymmetric stretching of CH 2 groups (v as (CH 2 ) = 2933 cm −1 , v s (CH 2 ) = 2876 cm −1 ), as well as NH 2 vibration (v as = 3372 cm −1 , v s = 3300 cm −1 ) [41]. The band at 1594 cm −1 corresponds to NH 2 bending. The anchoring of EDTA on amino groups resulted in the disappearance of NH 2 stretching vibration bands at 3372 and 3300 cm −1 . Moreover, the C-O asymmetrical carboxylate stretching vibration was observed at 1675 cm −1 . The band at 1744 cm −1 was attributed to the stretching vibration of the carboxylic group [42].

Effect of Contact Time and pH
For both SBA-16 and SBA-15, equilibrium was reached quickly (within the first 30 min) and the amount of Zn 2+ adsorbed was much higher for SBA-16 ( Figure 9). The obtained results may be due to the difference in structure between SBA-15 and SBA-16. The latter cage-like structure favors the diffusion of zinc ions.
The pH of the solution directly affects zinc ion adsorption because it controls its speciation as well as the adsorbent surface charge. The removal of Zn 2+ ions on both adsorbents increased as pH increased from 2 to 7 ( Figure 9). As the pH increased from 2 to 4, the adsorption efficiency increased from 35% to 78%. At pH 5, the adsorption further increased to 96.6% and reached 99% at pH 6 and 7. Above pH 7, the adsorption capacity slightly decreased (97%). For pH values higher than 3, EDTA molecules have a carboxylate form, thus increasing zinc complexation. Above pH 7, zinc starts to precipitate and form complexes with OH − (Zn(OH)2).

Effect of Contact Time and pH
For both SBA-16 and SBA-15, equilibrium was reached quickly (within the first 30 min) and the amount of Zn 2+ adsorbed was much higher for SBA-16 ( Figure 9). The obtained results may be due to the difference in structure between SBA-15 and SBA-16. The latter cage-like structure favors the diffusion of zinc ions.

Adsorption Kinetics
The two kinetic models, pseudo first-order and pseudo-second order, used to calculate the kinetic parameters are expressed in Equations (3) and (4), respectively [43]: where qt and qe are the quantity of metal ions adsorbed (mg g −1 ) at time t (min) and at equilibrium, respectively. k1 (min −1 ) and k2 (g mg −1 min −1 ) are the pseudo-first-and pseudosecond-order rate constants. The theoretical qe values obtained from the pseudo-second-order kinetic model were very close to the experimental ones ( Table 2). The obtained results indicated that zinc ion adsorption on both adsorbents followed the pseudo-second-order kinetic model ( Figure  10). This model is based on sorption equilibrium capacity and presumes that the sorption capacity is proportional to the number of active sites occupied on the sorbent [44]. This suggests that the adsorption rate mainly depends on the active adsorption site content on the adsorbent surface, and the rate-limiting step is chemisorption involving valence forces through sharing or exchange of electrons between specific sites on adsorbent and metal ions [45]. The chemical interaction between EDTA and metal ions is correlated and in accordance with the kinetic results obtained. The pH of the solution directly affects zinc ion adsorption because it controls its speciation as well as the adsorbent surface charge. The removal of Zn 2+ ions on both adsorbents increased as pH increased from 2 to 7 ( Figure 9). As the pH increased from 2 to 4, the adsorption efficiency increased from 35% to 78%. At pH 5, the adsorption further increased to 96.6% and reached 99% at pH 6 and 7. Above pH 7, the adsorption capacity slightly decreased (97%). For pH values higher than 3, EDTA molecules have a carboxylate form, thus increasing zinc complexation. Above pH 7, zinc starts to precipitate and form complexes with OH − (Zn(OH) 2 ).

Adsorption Kinetics
The two kinetic models, pseudo first-order and pseudo-second order, used to calculate the kinetic parameters are expressed in Equations (3) and (4), respectively [43]: where q t and q e are the quantity of metal ions adsorbed (mg g −1 ) at time t (min) and at equilibrium, respectively. k 1 (min −1 ) and k 2 (g mg −1 min −1 ) are the pseudo-first-and pseudo-second-order rate constants. The theoretical qe values obtained from the pseudo-second-order kinetic model were very close to the experimental ones ( Table 2). The obtained results indicated that zinc ion adsorption on both adsorbents followed the pseudo-second-order kinetic model ( Figure 10). This model is based on sorption equilibrium capacity and presumes that the sorption capacity is proportional to the number of active sites occupied on the sorbent [44]. This suggests that the adsorption rate mainly depends on the active adsorption site content on the adsorbent surface, and the rate-limiting step is chemisorption involving valence forces through sharing or exchange of electrons between specific sites on adsorbent and metal ions [45]. The chemical interaction between EDTA and metal ions is correlated and in accordance with the kinetic results obtained.  Figure 10. Pseudo-second-order model kinetic model (above) and intra-particle diffusion model (below) for Zn 2+ adsorption (pH = 6 at RT). To determine if the intra-particle diffusion is a rate-limiting step in the zinc adsorption on both adsorbents (SBA-15 and SBA-16), the intra-particle diffusion model proposed by Weber and Morris [46] was used to analyze the kinetic results. This model is expressed as follows: q t = K id t 1/2 + C (5) where k id is the rate constant of intra-particle diffusion (mg g −1 min −1/2 ) and C is the intercept (mg g −1 ). High C values propose that external diffusion has a greater role than the rate-limiting step, because the C value is related to the boundary layer thickness [47]. A plot of the zinc amount adsorbed (q t ) versus t 0.5 should be linear, and if the line passes through the origin, then intra-particle diffusion is the only rate-controlling step [48]. The obtained results are illustrated in Figure 9 and the parameters are displayed in Table 3. The plots present two linear parts indicating that two steps have occurred. The first sharp part corresponds to the external surface adsorption, while the second part represents the gradual adsorption step, such that the intra-particle diffusion is rate-limiting [49]. As the plot q t versus t 0.5 was not a straight line passing through the origin, the process of adsorption is not controlled only by the intra-particle diffusion where film diffusion might have an effect on the kinetics. Table 3. Parameters of the intra-particle diffusion model.

Adsorption Isotherms
The adsorption behavior for both adsorbents was analyzed by Langmuir and Freundlich isotherm models in order to model the amount of solute adsorbed per unit of adsorbent, q e , as a function of equilibrium concentration in the bulk solution, C e , at a constant temperature. The Langmuir and Freundlich, models, are expressed in Equations (6) and (7), respectively: C e q e = 1 K L q max + C e q max (6) logq e = logK f + 1 n logC e where C e and qmax denote the metal concentration (mg L −1 ) at the equilibrium state and the adsorption capacity (mg g −1 ), respectively. The value of n is the inverse of the heterogeneity factor of the adsorption process. Meanwhile, K L and K f are the Langmuir (L mg −1 ) and Freundlich (mg g −1 ) constants related to the mean free energy of adsorption, respectively [50,51]. The adsorption isotherms of the experimental data are shown in Figure 11 and the parameters of these two models are shown in Table 4. From the linear regression correlation coefficient R 2 , it can be deduced that the equilibrium data could be well described by the Freundlish isotherm, so the adsorption is reversible in a heterogeneous system that is not limited to the formation of monolayers [52]. The values of n were all between 1 and 10, indicating that the adsorption performance of zinc ions on both adsorbents was favorable under the studied conditions [53], so both adsorbents can be considered efficient for zinc metal ion removal, with the preference for SBA-16. Moreover, the Freundlich expression is an exponential equation and, therefore, assumes that, as the metal ions' concentration increases, their concentration on the adsorbent surface also increases, indicating a non-ideal adsorption, not limited to monolayer formation. not limited to the formation of monolayers [52]. The values of n were all between 1 and 10, indicating that the adsorption performance of zinc ions on both adsorbents was favorable under the studied conditions [53], so both adsorbents can be considered efficient for zinc metal ion removal, with the preference for SBA-16. Moreover, the Freundlich expression is an exponential equation and, therefore, assumes that, as the metal ions' concentration increases, their concentration on the adsorbent surface also increases, indicating a non-ideal adsorption, not limited to monolayer formation.  Contrary to the Langmuir isotherm model, which is commonly used for monolayer adsorption, where most of the adsorption sites have equal affinities toward the asorbate, the Freundlich isotherm model is used to describe a heterogeneous chemisorption process in which the surface is not energetically uniform [54]. Isotherms of this form have been observed for a wide range of heterogeneous surfaces, including activated carbon, silica, clays, metals, and polymers. In the case of SBA-16-EDTA and SBA-15-EDTA, the obtained results showed that the data fitted Freundlich, as previously mentioned, and this is mainly  Contrary to the Langmuir isotherm model, which is commonly used for monolayer adsorption, where most of the adsorption sites have equal affinities toward the asorbate, the Freundlich isotherm model is used to describe a heterogeneous chemisorption process in which the surface is not energetically uniform [54]. Isotherms of this form have been observed for a wide range of heterogeneous surfaces, including activated carbon, silica, clays, metals, and polymers. In the case of SBA-16-EDTA and SBA-15-EDTA, the obtained results showed that the data fitted Freundlich, as previously mentioned, and this is mainly because of the heterogeneous surface of both adsorbents, taking into consideration the presence of some unfunctionalized silanol groups, amino groups along the EDTA fixed on the majority of sites. Such heterogeneity will directly govern the adsorption process on both adsorbents and, as a result, the obtained isotherm as well. It can be concluded that Freundlich fits well over the entire concentration range; however, there is an obvious deviation at higher concentrations. So, in general, the Freundlich isotherm will be a more accurate approximation at lower concentrations [55]. Moreover, the Freundlich expression is an exponential equation and, therefore, assumes that, as the metal ions' concentration increases, their concentration on the adsorbent surface also increases, indicating a non-ideal adsorption, not limited to monolayer formation [56]. Concerning the chemisorption process, the presence of functional groups can be used as evidence in proposing the adsorption mechanism. The Zn-chemisorption mechanism onto EDTA-modified SBA-16 and SBA-15 can be proposed as follows: Zn 2+ ions bind to carboxylate groups of EDTA via ionic forces with carboxylic oxygen atoms. These oxygen atoms exhibit a negative charge in their structure as a result of the dissociation of carboxylic groups. The negatively charged oxygen atom in carboxylate anions will coordinate with zinc cations, resulting in the formation of metal-carboxylate complexes (COO-Zn) on the adsorbents' surface [57,58].

Adsorbents' Regeneration
Batch desorption experiments and the desorption efficiencies were compared for both adsorbents. In two beakers, each containing 40 mL of 1 M HCl and 20 mg of SBA-15-EDTA or SBA-16-EDTA at RT, the mixtures were stirred at 300 rpm for 2 h. Then, the mixtures were filtered and dried in order to be used again. Five regeneration cycles were performed. Figure 12 shows that, after the fifth regeneration cycle, both adsorbents conserved about 85% of their removal efficiency.
presence of some unfunctionalized silanol groups, amino groups along the EDTA fixed on the majority of sites. Such heterogeneity will directly govern the adsorption process on both adsorbents and, as a result, the obtained isotherm as well. It can be concluded that Freundlich fits well over the entire concentration range; however, there is an obvious deviation at higher concentrations. So, in general, the Freundlich isotherm will be a more accurate approximation at lower concentrations [55]. Moreover, the Freundlich expression is an exponential equation and, therefore, assumes that, as the metal ions' concentration increases, their concentration on the adsorbent surface also increases, indicating a nonideal adsorption, not limited to monolayer formation [56]. Concerning the chemisorption process, the presence of functional groups can be used as evidence in proposing the adsorption mechanism. The Zn-chemisorption mechanism onto EDTA-modified SBA-16 and SBA-15 can be proposed as follows: Zn 2+ ions bind to carboxylate groups of EDTA via ionic forces with carboxylic oxygen atoms. These oxygen atoms exhibit a negative charge in their structure as a result of the dissociation of carboxylic groups. The negatively charged oxygen atom in carboxylate anions will coordinate with zinc cations, resulting in the formation of metal-carboxylate complexes (COO-Zn) on the adsorbents' surface [57,58].

Adsorbents' Regeneration
Batch desorption experiments and the desorption efficiencies were compared for both adsorbents. In two beakers, each containing 40 mL of 1 M HCl and 20 mg of SBA-15-EDTA or SBA-16-EDTA at RT, the mixtures were stirred at 300 rpm for 2 h. Then, the mixtures were filtered and dried in order to be used again. Five regeneration cycles were performed. Figure 12 shows that, after the fifth regeneration cycle, both adsorbents conserved about 85% of their removal efficiency.

Conclusions
In this study, two adsorbents were prepared by EDTA immobilization on SBA-15 and SBA-16 mesoporous silica. The ordered mesostructures of the obtained hybrid organic/inorganic materials were well preserved after modification. The modified adsorbents were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), X-ray diffraction (XRD), nitrogen (N2) adsorption-desorption analysis, Fourier transform infrared spectroscopy (FT-IR), and thermogravimetric analysis. The effects of contact time and pH on zinc adsorption on both materials were studied. The adsorption

Conclusions
In this study, two adsorbents were prepared by EDTA immobilization on SBA-15 and SBA-16 mesoporous silica. The ordered mesostructures of the obtained hybrid organic/inorganic materials were well preserved after modification. The modified adsorbents were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), X-ray diffraction (XRD), nitrogen (N 2 ) adsorption-desorption analysis, Fourier transform infrared spectroscopy (FT-IR), and thermogravimetric analysis. The effects of contact time and pH on zinc adsorption on both materials were studied. The adsorption process was fast, indicating the high affinity of adsorbents to chelating zinc ions. The modified SBA-16 showed higher efficiency for eliminating Zn 2+ compared with SBA-15 owing to its favorable structure characteristics (pore structure). The kinetic data well fitted the pseudo-second-order model, where the rate adsorption process depends on the exchange kinetics between the ligand and zinc ions. The effect of intra-particle diffusion was also investigated. Equilibrium data were also fitted by Freundlich isotherm. Both adsorbents can be regenerated using HCl solution and reused for up to five cycles.  Institutional Review Board Statement: Not applicable.

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