Chinese Chemical Letters  2019, Vol. 30 Issue (6): 1133-1136   PDF    
Facile surface modification of mesoporous silica with heterocyclic silanes for efficiently removing arsenic
Xiaohui Zhua, Wei Jianga, Weirong Cuia, Ruping Lianga, Li Zhanga, Jianding Qiua,b,*     
a College of Chemistry, Nanchang University, Nanchang 330031, China;
b Environmental Protection Materials and Equipment Engineering Technology Center of Jiangxi, Department of Materials and Chemical Engineering, Pingxiang University, Pingxiang 337055, China
Abstract: Arsenic being one of the most toxic heavy metals has long been a focus of concern due to its gravest threats to human health and environment. Herein, we employ a new method named ring-opening click reaction to synthesize thiol-and amino-bifunctionalized SBA-15 (denoted as bi-SBA-15) for directly remove As(Ⅲ) without preoxidation As(Ⅲ) to As(Ⅴ) from contaminated water. Quite different from commonly modified with trialkoxysilanes, ring-opening click reaction is substantially faster (0.5-2 h at 25 ℃) and more efficient. The synthesized bi-SBA-15 exhibits excellent adsorption performance of As(Ⅲ) and As(Ⅴ) in an aqueous solution with a great adsorption capacity (33.70 mg/g and 42.66 mg/g, respectively), a very short equilibrium time (< 30 min), which is much faster than those of other adsorbents reported previously. The excellent adsorption performance of bi-SBA-15 can be maintained with a wide pH range (3-8), various anions and natural organic matter, which provides an effective approach and a new insight to clean arsenic-contaminated water.
Keywords: Arsenite     Arsenate     SBA-15     Remove     Click reaction     Synergistic effect    

Arsenic is a ubiquitous trace element in the aquatic environment [1]. In natural water, arsenic is mostly found in two inorganic forms, namely, arsenate (As(Ⅴ)) and arsenite (As(Ⅲ)) [2]. As(Ⅲ) is more toxic, soluble, and mobile than As(Ⅴ) [3, 4]. A high level of arsenic in groundwater and surface water is a major concern worldwide, because of its high toxicity and carcinogenicity [5-7]. To minimize the possible health risks, the World Health Organization (WHO) has set a stringent limit of 10 ppb As for drinking water [8]. Nowadays, a variety of techniques are developed for arsenic removal from water [9-12]. Among these methods, adsorption is very promising due to its simplicity of operation, stability and low cost [13]. Common adsorbents include Al-based sorbents [14], chitosan complex [15], metal-organic frameworks (MOF) [16], Fe(Ⅲ) oxide [17]. Unfortunately, the negatively charged As(Ⅴ) can be efficiently removed by adsorption, neutrally charged As(Ⅲ) with higher toxicity may survive from these adsorbents [18]. Thereby, chlorine and permanganate are usually regarded as chemical oxidants to transform As(Ⅲ) to As(Ⅴ) [19]. But they will inevitably lead to water quality deteriorate [20]. Thus, developing the safe and effective adsorbents to simultaneously remove As(Ⅲ) and As(Ⅴ) from water without preoxidation of As(Ⅲ) into As(Ⅴ) is still challenging and imperative.

The ordered mesoporous silica materials with uniform and adjustable mesopores, large pore volumes and high surface areas are promising candidates for adsorption and separation applications [21-26]. Moreover, the surface chemistry of a mesoporous silica can be easily grafted with desirable organic groups by postmodification without any change of mesostructures [27, 28], tailoring their performance. The common method of modifying mesoporous silica is grafting trialkoxysilanes [29]. Although trialkoxysilanes can be successfully modify SBA-15, but it requires long-time reflux at higher temperatures to achieve effective graft, byproduct may be harmful to the properties of the final product, and the easily occurred cross-linking reaction leads to the blockage of the mesopores [30, 31]. Recently, Sailo et al. discovered an innovative yet simple ring-opening click reaction to modify the hydroxylated silicon using heterocyclic silanes [30]. The combination between heterocyclic silanes and hydroxylated silicon is mild, simple and timesaving, completion within 2 h at 25 ℃. Meanwhile, the reaction has high yield, without byproducts and the solvents is easy to remove, which opens a new avenue to modify Si-OH [30].

Herein, we chose SBA-15 as the matrix, which has the features of large pore volumes and high surface areas. Subsequently, the heterocyclic silanes were used to modify SBA-15 via ring-opening click reaction which contains silicon-sulfur (Si-S) or siliconnitrogen (Si-N) bond in the ring. Benefiting from the new method, thiol- and anime- bifunctional SBA-15 (bi-SBA-15) was successfully prepared in a simple process. Adsorption experiments show that the prepared bi-SBA-15 can effectively remove As(Ⅲ) and As(Ⅴ) from arsenic-contaminated water. The saturated adsorption capacities are 33.70 mg/g and 42.66 mg/g for As(Ⅲ) and As(Ⅴ), respectively. The result makes the materials promising for simple and efficient removal of arsenic from contaminated water, paving a way to their use in the treatment of large volumes of water contaminated with both arsenite and arsenate.

As depicted in Fig. 1, the common method of modifying SBA-15 is to graft trialkoxysilanes on the surface of SBA-15 by hydrolytic condensation. This method is complex and dangerous, requiring long time of reflux at 110 ℃. Here, we modify SBA-15 via ringopening click reaction, which is simple, safe and effective. This process is very mild, only needs to stir at 25 ℃ for 2 h. These textural properties of SBA-15 and bi-SBA-15 are determined by XRD and the results are shown in Fig. 2A. All samples exhibit three distinct scattering peaks assigned to (100), (110), and (200) reflections of the P6 mm space group, indicating a highly ordered 2D-hexagonal symmetry mesostructure. In addition, the (100) and (200) reflections of the bi-SBA-15 are weak and shift marginally to higher 2θ values, which could be due to a pore-filling effect of diaza-silane and thia-silane in the channels of SBA-15 [32].

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Fig. 1. (A) Schematic illustration of our method modified SBA-15 and compare with common method.(B) Structures of the thia-silane 2, 2-dimethoxy-1-thia-2-silacyclopentane and diaza-silane 2, 2-dimethoxy-1, 6-diaza-2-silacyclooctane(R = OMe; X = SH, NH2).

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Fig. 2. Small-angle XRD patterns (A), and nitrogen adsorption desorption isotherms (B) with inset the corresponding pore distribution curves of mesoporous silica SBA-15, and bi-SBA-15. Typical TEM images of mesoporous silica SBA-15 (C) and bi-SBA-15 (D).

The N2 adsorption-desorption isotherms of bi-SBA-15 show type Ⅳ isotherms with H1 hysteresis (Fig. 2B), incarnating the typical mesoporous structural characteristics similar to that of pure SBA-15. The surface areas and pore properties of SBA-15 and bi-SBA-15 are summarized in Table S1 (Supporting information). After modification, decreases in pore diameter (6.81–5.42 nm), pore volume (0.91–0.55 cm3/g) and surface area (613.59–289.13 m2/g) are observed. Compared with some other sorbents [33, 34], the specific surface area and pore volume of the resultant composites are still sufficient for arsenic removal. TEM images further clearly illustrate the structure of SBA-15 and bi-SBA-15 (Figs. 2C and D), both highlight the mesoporous structure of well-ordered hexagonal symmetry. Such structure is in line with the results of small-angle XRD and N2 adsorption measurements. These results indicate that the bi-SBA-15 nanocomposites are successfully fabricated by our innovative and simple approach and the basic structure of SBA-15 matrix is not damaged in the modification process.

The changes in surface composition of SBA-15 are further analyzed by FTIR spectra (Fig. S1A in Supporting information). For SBA-15, the bands at 808 cm-1, 1094 cm-1 and 460 cm-1 belong to the bond of Si-O-Si [35]. The peaks at 960 cm-1 and 3422 cm-1 are attributed to the vibration of Si-OH [35, 36]. After chemical modification, four new peaks appear at 680, 1465, 2400 and 2934 cm-1, originating from the N-H, C-N, S-H and C-H bonds, respectively [37-39]. Compared with the reported literature, the peak of S-H has a significant red shift, which may be closely related with the interaction between thiol and amino [40]. The thermal stability of bi-SBA-15 and pristine SBA-15 are studied by TGA (Fig. S1B in Supporting information). Compared with the 10% weigh loss of SBA-15, bi-SBA-15 exhibits weigh losses of 18% at 800 ℃. The larger weigh loss in bi-SBA-15 sample means not only the condensation of free silanol groups [41], but also thermal decomposition of the organic functional groups on the bi-SBA-15. All the results clearly demonstrate that heterocyclinc silanes were successfully incorporated with SBA-15.

As illustrated in Figs. 3A and B, the adsorption of arsenic by bi-SBA-15 is very fast, the removal efficiencies of As(Ⅲ) and As (Ⅴ) are 83.55% and 85.92% within 5 min, respectively. Furthermore, pseudo-first-order and pseudo-second-order kinetic models are employed to study adsorption kinetics of both types of arsenic species (Fig. S2 in Supporting information). The results show that the R2 of pseudo-second order model is higher than the R2 of pseudo-first order model (Table S2 in Supporting information), indicating that chemical adsorption is the ratecontrolling step for the arsenic being combined to the surface of bi-SBA-15.

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Fig. 3. Arsenic remove from water by bi-SBA-15. Adsorption kinetic of As(Ⅲ) and As(Ⅴ) (A, B) (initial As(Ⅲ)/As(Ⅴ) concentration, 25 mg/L; solid dosage, 1.0 g/L, 25 ℃), Adsorption isotherms of As(Ⅲ) and As(Ⅴ) (C, D) (solid dosage, 1.0 g/L; time, 1 h, 25 ℃).

To validate the adsorption capacity of the bi-SBA-15, the adsorption isotherms of bi-SBA-15 for As(Ⅲ) and As(Ⅴ) are examined (Figs. 3C and D). The adsorption data of As(Ⅲ) and As(Ⅴ) are fitted by Freundlich and Langmuir isotherm models (Fig. S3 in Supporting information). From the R2 values of these models, the Langmuir isotherm model is more suitable than Freundlich isotherm models (Table S3 in Supporting information). The results indicate that the adsorption of arsenic by bi-SBA-15 is monolayer sorption [20]. The corresponding saturated sorption capacities for As(Ⅲ) and As(Ⅴ) are 33.70 mg/g and 42.66 mg/g, respectively, which are comparable to other reported adsorbents (Table S4 in Supporting information), revealing that bi-SBA-15 is an effective arsenic decontaminant.

To explore the potential effect between amino and thiol in the biSBA-15 on the sorption of As(Ⅲ), a series of control experiments are carried out. As shown in Fig. 4, the adsorption capacity of the pure SBA-15 is negligible, even diaza-silane or thia-silane is used to modify SBA-15, only less than 3.78 mg/g of As(Ⅲ) is removed. The results indicate that the intrinsic adsorption capacity of these materials is extremely limited. The adsorption capacity of As(Ⅲ) is remarkably enhanced when diaza-silane and thia-silane are comodified in SBA-15 (Fig. S4 in Supporting information). Particularly, the best performance is achieved when the molar ratio of diazasilane and thia-silane is 4:6, almost all the As(Ⅲ) in the solution is absorbed. The results suggest that there might be a synergistic effect between amino and thiol in the sorption of As(Ⅲ).

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Fig. 4. As(Ⅲ) capture capacities on pure SBA-15 and different proportions of diazasilane and thia-silane co-modified SBA-15 (initial As(Ⅲ) concentration, 25 mg/L; solid dosage, 1.0 g/L; time, 1 h, 25 ℃).

The total survey and high-resolution XPS spectra (N 1s and S 2p) of the bi-SBA-15 before and after As(Ⅲ) uptake are shown in Fig. S5 (Supporting information). It can be found the bi-SBA-15 is mainly composed of Si, S, C, N, and O elements (Fig. S5A). After the uptake of As(Ⅲ), a new peak assigned to As 3d can be clearly observed (Fig S5B), indicating that As(Ⅲ) is successfully immobilized on the surface of the adsorbent. As revealed in Fig. S5C, high-resolution XPS spectrum of N 1s exhibits three prominent bands at 400.1, 399.5 and 398.6 eⅤ, corresponding to C-N-C, C-N and N-S, respectively [42-44]. After As(Ⅲ) treatment, the binding energy for the N-S peak is disappeared (Fig. S5D). The detailed S 2p spectrum of bi-SBA-15 presents three peaks that can be assigned to S-N (163.6 eⅤ), C-S (163.7 eⅤ) and C-S (164.1 eⅤ), respectively (Fig. S5E) [45, 46]. After the adsorption of As(Ⅲ), the peak at 163.6 eⅤ (S-N) disappears and a new peak appears at 162.8 eⅤ corresponding to As-S (Fig. S5F) [47]. The appearance of As-S accompanied with the absent of N-S and S-N indicates that As (Ⅲ) forms chelate with thiol which is capable of forming S-N bond with amino. As discussed above, we proposed a As(Ⅲ) removal mechanism: the synergistic effect between thiol and amino is identified in adsorb As(Ⅲ). Specifically, thiol is inferred to be the primary active site in the surface of bi-SBA-15 to adsorb As (Ⅲ) via formation chelate between the As(Ⅲ) and thiol [48]. Meanwhile, amino is found to play a key role in the synergism with thiol, which improves the affinity of thiol to As(Ⅲ), thereby greatly increasing the adsorption capacity of bi-SBA-15 for As(Ⅲ). At the same time, this assumption also well explains the red shift of S-H and the adsorption capacity of the co-modified SBA-15 for arsenite is increasing dramatically.

The pH of the solution determines the existed form of arsenic specie and surface properties of the bi-SBA-15, influencing the adsorption of arsenic onto the adsorbent. In order to assess the influence of initial pH value on sorption capacity, the adsorption experiments are carried out within the initial pH range of 1–10. biSBA-15 shows excellent adsorption performance to As(Ⅲ) with a wide pH range (Fig. S6A in Supporting information). As for As(Ⅴ) (Fig. S6B in Supporting information), it is interesting that the adsorption capacity of bi-SBA-15 is remarkably enhanced for As(Ⅴ) in an acidic solution than alkaline solution, which is up to 23.72 mg/g when pH is controlled at 3.0. This may arise from the fact that H2AsO4- is the dominant form of arsenate in water at pH 3.0 [41]. Inversely, the bi-SBA-15 is positively charged at pH 3.0 that attributes to the protonation of the amine groups. As a consequence, the bi-SBA-15 surface is protonated favoring binding of negatively charged arsenate through electrostatic interaction, therefore enhancing the adsorption capacity at pH 3.0 [40]. The results suggest its potential for practical applications to capture arsenic from wastewater.

Fig. S7 (Supporting information) shows the effect of some anions on the removal of As(Ⅲ) by bi-SBA-15. Generally, sulfate, chloride, hydrocarbonate and HA pose negligible effect on As(Ⅲ) removal in the studied concentration ranges. By contrast, the presence of phosphate suppresses As(Ⅲ) uptake significantly, this decrease can be explained by the chemical similarity of arsenite and phosphate, phosphate may compete for the active sorption sites on bi-SBA-15 surfaces, resulting in decreased adsorption capacity. Identical interference for arsenic uptake has also been reported in previous reports [49].The temperature of the adsorption system is another factor affecting the adsorption. As shown in Fig. S8 (Supporting information), the As(Ⅲ) adsorbing rate of biSBA-15 increases with increasing temperature due to higher temperature be favorable for the molecule diffusion [50], and more kinetic energy making As(Ⅲ) have a higher probability to contact the adsorbent.

In summary, bi-SBA-15 has been successfully fabricated via a simple and effective ring-opening click reaction. Due to introduction of abundant thiol and amino, bi-SBA-15 can rapidly capture As (Ⅲ) and As(Ⅴ) by chelation and electrostatic interaction, respectively. The calculated adsorption capacities are 33.70 mg/g for As (Ⅲ) and 42.66 mg/g for As(Ⅴ). Meanwhile, bi-SBA-15 realizes removing As(Ⅲ) from water without pre-oxidation of As(Ⅲ) into As (Ⅴ). In addition, the excellent adsorption performance of bi-SBA-15 can be maintained with various anions and natural organic matter, which shows great potential for practical application to efficiently remove arsenic contamination from water.

Acknowledgments

We greatly appreciate the supports of the National Natural Science Foundation of China (Nos. 21675078 and 21775065) and the Natural Science Foundation of Jiangxi Province (Nos. 20165BCB18022 and 2018ACB21008).

Appendix A. Supplementary data

Supplementary material related to this article canbefound, in the online version, at doi:https://doi.org/10.1016/j.cclet.2019.02.022.

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