A Novel Amino and Carboxyl Functionalized Mesoporous Silica as an

Article Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

pubs.acs.org/jced

A Novel Amino and Carboxyl Functionalized Mesoporous Silica as an Efficient Adsorbent for Nickel(II) Li Zhang,† Fangxiang Song,† Yonggui Wu,‡ Liang Cheng,§ Jin Qian,§ Shuai Wang,∥ Qianlin Chen,⊥ and Yan Li*,⊥ †

School of Chemistry and Chemical Engineering, Guizhou University, Guiyang 550025, China School of Resources and Environmental Engineering, Guizhou University, Guiyang 550025, China § School of Electrical Engineering, Guizhou University, Guiyang 550025, China ∥ School of Pharmacy, Guizhou University, Guiyang 550025, China ⊥ Institute of Advanced Technology, Guizhou University, Guiyang 550025, China

Downloaded via UNIV OF CALIFORNIA SANTA BARBARA on January 6, 2019 at 18:17:01 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

S Supporting Information *

ABSTRACT: In this study, the as-prepared spherical mesoporous silica was modified by amino and cyano groups via grafting, and the amino and carboxyl functionalized adsorption materials (MSS@NH2@COOH) were obtained after cyano group hydrolysis. The introduction of amino and carboxyl groups increases the surface active sites of the materials. The materials were characterized by SEM, TEM, XRD, N2 adsorption/desorption, XPS, etc. BET analysis revealed the amino and carboxyl functionalized mesoporous silica maintained their mesostructure nature and its specific surface area was 113 m2·g−1. TG-DTA and FT-IR confirmed effective grafting on the surface of mesoporous silica with functionalized groups. The simulated wastewater of Ni(II) was carried out to test the adsorption properties of the materials, and several influential factors (pH, initial nickel ion concentration, adsorbent dose) were discussed. The saturated adsorption capacity increased clearly from 46.55 mg·g−1 of MSS to 125.57 mg·g−1 of MSS@NH2@COOH. Even recycling three times, the removal rate of Ni(II) ions using MSS@NH2@COOH as an adsorbent could still be over 75%. Adsorption isotherms, kinetics, and thermodynamics of adsorption were taken to clarify the adsorption process. This newly developed adsorbent material does exhibit good reusable performance and effective removal of the heavy metal ions of Ni(II).

1. INTRODUCTION In recent years, the development of industry and agriculture has caused severe heavy metal pollution in water, which makes the solution urgent. An extremely low concentration of heavy metal ions has a toxic effect on organisms in water. It cannot be degraded but enriched in organisms and then transferred to the advanced creature gradually through the food chain. The heavy metal ions taken into the human body will react with the physiological macromolecules such as enzymes, causing chronic poisoning and cumulative harm. Heavy metal ions such as mercury are not only a cause of the damage to the central nervous system but cause chest pain, dyspnea, and even lung and kidney failure with high concentrations.1,2 Lead may be harmful to kidney and brain tissue, the reproductive system, and cellular activity.3 A variety of syndromes, lung and kidney problems, even dermatitis, stomachache, and pulmonary fibrosis result from nickel toxicity.4 Heavy metal wastewater treatment technology mainly includes the ion exchange method,5 adsorption method,6,7 chemical precipitation,8,9 reverse osmosis,10 membrane filtration,11 etc. Adsorption is an accepted and effective method of © XXXX American Chemical Society

heavy metal treatment because of its low cost, good effect, and strong operability. Mesoporous materials have become the most typical functional materials for adsorption and catalysis due to their large specific surface area and pore volume, controllable size and morphology, strong activity, and adsorption capacity. Since the first synthesis of M41S mesoporous silica materials in 1992,12 such materials with rule channel structure have attracted people’s attention immediately and have obtained rapid development. However, there is no active center of mesoporous silica materials, which limits its application in many fields greatly. In order to solve this problem, the surface modification was researched.13−17 Lee et al.13 prepared mesoporous silica materials using different templates to remove the ions of Pb(II), Cu(II), and Ni(II) in water, and the metal uptake capacity was found to decrease following Pb(II) > Cu(II) > Ni(II). The saturated adsorption of Ni(II) was 2.85 mg·g−1, which showed the poor Received: August 4, 2018 Accepted: December 17, 2018

A

DOI: 10.1021/acs.jced.8b00689 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

Scheme 1. Functional Schematic Diagram of MSS

adsorption kinetics and isotherms of Ni(II) were investigated (see Scheme 1).

removal effect of unmodified mesoporous silica to Ni(II). Saha et al.14 investigated the ordered mesoporous carbon modified with sulfide and applied it to the water which contains Hg(II), Pb(II), Cd(II), and Ni(II) ions. It showed that the adsorption capacity followed the order of Hg(II) > Pb(II) > Cd(II) > Ni(II) and there is competition of adsorption between these metal ions. The maximum adsorption quantity of Ni(II) was only 1.2 mg·g−1, indicating the functional groups of sulfur are not beneficial for removing Ni(II) ions. Xu et al.15 evaluated the potential of removing Cu(II), Ni(II), and Zn(II) ions using magnetic functionalized mesoporous silica. The adsorption effect obeys the order of Cu(II) > Ni(II) > Zn(II), suggesting the amino groups are helpful for the adsorption of Ni(II) ions. The adsorption of mesoporous silica with functionalized thiol groups for the heavy metal ions in sewage were examined by Kang et al.16 There is good selectivity of Pt(II) and Pb(II), but it is opposite for Ni(II). Furthermore, the carboxyl groups can efficiently capture Ni(II) with excellent stability.17 According to the research information we have now, we first obtained a new mesoporous silica with amino and carboxyl groups simultaneously as an adsorbent for heavy metal ions. The introduction of amino and carboxyl groups endows mesoporous silica amphiphilic, which increases the complexation site of the material. This allows the prepared material to be used not only for the adsorption of heavy metal ions and organic dyes but also for the loading of acidic or basic drugs. In this study, the spherical mesoporous silica in a successful synthesis used hexadecyl trimethylammonium bromide (CTAB) as the template agent and gelatin as the co-template agent through the sol−gel method. The obtained materials were functionalized with polyamino and cyanogen first, and the product of carboxyl functionalization was taken by hydrolysis of cyanogen. The multifunctional mesoporous silica materials were used to adsorb the simulated wastewater containing nickel to explore the influence factors (solution pH, temperature, adsorbent dosages, and initial metal ion concentrations) on the adsorption properties, establish the adsorption model, and study the adsorption dynamics and thermodynamics. The

2. MATERIALS AND METHODS 2.1. Chemical Materials. Cetyl trimethylammonium bromide (CTAB), absolute ethyl alcohol, gelatin, methyl alcohol, methylbenzene, isopropanol, and high purity nickel were purchased from Tianjin KGM Chemical Reagent Co., Ltd. (China). Etraethoxysilane (TEOS), kalium chloratum, and ammonium hydroxide were purchased from Chengdu Jinsan Chemical Reagent Co., Ltd. (China). Concentrated hydrochloric acid was purchased from Chongqing Chuandong Chemical Co., Ltd. (China). 1,3,5-Trimethyl benzene was perchased from Aladdin Industrial Corporation (Shanghai, China). 2-Cyanoethyltriethoxysilane (CTES) was purchased from TCI (Shanghai, China). 3-[2-(2-Aminoethylamino) ethylamino]propyl-trimethoxysilane (NQ-62) was purchased from Shanghai Macklin Biochemical Co., Ltd. (China). Deionized water used in all experiments was made by laboratory. All chemical reagents were of analytical grade and purchased without further purification. 2.2. Adsorption Studies. For adsorption of heavy metal ions Ni(II), the 1000 mg·L−1 simulated sewage of Ni(II) ions was prepared by high purity nickel and mixed with MSS and MSS@NH2@COOH. The solution was centrifuged at 4000 rpm for 15 min, and the samples were measured at 530 nm wavelength using an UV spectrophotometer. Different values of pH were adjusted by 2 M NaOH or 2 M HCl. Several effect parameters, such as the solution pH, adsorbent dose, initial concentration, and contact time, were also investigated. The adsorption rate and adsorption capacity of metal ions by the adsorbents was calculated on the basis of the following equation19 R=

C0 − C t × 100 C0

Qe = B

(C0 − Ce)V W

(1)

(2) DOI: 10.1021/acs.jced.8b00689 J. Chem. Eng. Data XXXX, XXX, XXX−XXX


Article

Figure 1. SEM images of MSS (A) and MSS@NH2@CN (B); TEM images of MSS (C, D).

where R is the removal efficiency of the metal ions, C0 and Ct are the initial and final (equilibrium) concentrations of the metal ions in solution (mg·L−1), qe is the adsorption capacity at equilibrium, V is the volume of the solution, and W is the mass adsorbents. Kinetics can be used to explore the factors affecting the adsorption rate during the chemical process. The control mechanism of the adsorption process was studied according to the pseudo-first-order and pseudo-second-order kinetic equations and the intraparticle diffusion model, and the experimental data was tested in order to analyze the absorption rate of ions. The pseudo-first-order kinetic, pseudo-secondorder kinetic, and intraparticle diffusion models are expressed as follows18−20 log(qe − qt) = log qe −

k1t 2.303

Langmuir model assumed monolayer adsorption onto a surface containing a finite number of identical adsorption sites, and the adsorbed particles are completely independent. The Langmuir model corresponds to chemical adsorption, and it is given as21 Ce C 1 = + e Qe qmKL qm

where qe is the equilibrium adsorption capacity of adsorption in mg of metal/g of adsorbent, Ce is the equilibrium concentration of metal ions in mg·g−1, qm is the maximum amount of metal sorbent in mg of metal/g of adsorbent, and KL is the constant that refers to the bonding energy of adsorption in 1/mg. The empirical Freundlich equation is based on the adsorption on a heterogeneous surface, which is commonly described as22

(3)

Q e = K f Ce1/ n

t 1 t = 2 + qt qe qe k 2

(4)

qt = Kdt 1/2 + I

(5)

(6)

(7)

where qe is the equilibrium adsorption capacity of the adsorbent in mg of metal/g of adsorbent, Ce is the equilibrium concentration of heavy metal ions in mg·L−1, Kf is the constant related to the adsorption capacity of the adsorbent in mg·L−1, and n is the constant related to the adsorption intensity of the adsorbent. The study of adsorption thermodynamics can not only understand the degree and driving force of the adsorption process thoroughly but also analyze various factors, such as adsorbent structure, solvent properties, adsorption temperature, and adsorption time on the adsorption effect. The adsorption properties are usually determined by thermody-

where qe and qt are the amount of metal ions adsorbed on the adsorbent in mg·g−1 at equilibrium and at time t, respectively; K1 and K2 are the constants of first-order and second-order adsorption in min−1; Kd is the intraparticle diffusion rate constant (mg·g−1·min−1); and I is a constant that gives an idea about the thickness of the boundary layer (mg·g−1). The most common and widely used models of isothermal adsorption are the Langmuir and Freundlich models. The C



Article

Figure 2. TEM image of one single MSS@NH2@COOH nanoparticle and energy-dispersive X-ray spectroscopy element mapping of C, N, O, and Si elements in one MSS@NH2@COOH nanoparticle.

Figure 3. Nitrogen adsorption isotherm (A) and pore size and pore volume (B) of MSS and MSS@NH2@COOH.

in the eluting solution was determined by spectrophotometry, and the desorption efficiency were calculated as25

namic parameters, namely, free energy (ΔG°), entropy change (ΔS°), and heat of adsorption (ΔH°). It can be judged if the reaction process is spontaneous or not according to the value of ΔH°. By judging the value of ΔG°, it can be understood that the process of the adsorption reaction is exothermic or endothermic. The value of ΔS° reflects the affinity of the adsorbent to heavy metal ions. Thermodynamic equations used for these calculations are given as23,24 Kd =

Ce

(8)

ΔS° ΔH ° − R RT

(9)

ΔG° = ΔH ° − T ΔS°

(10)

1 1 + C0b

(Ce − C0)Ve × 100 qem

(12)

where C0 is the initial concentration of heavy metal ions in the eluent (mol·L−1), Ce is the equilibrium concentration (mol· L−1) after adsorption, qe is the adsorption capacity in the removal test, Ve is the volume of the eluent (L), and m is the mass of the adsorbent. 2.4. Silane Grafting. The grafting is calculated as26

qe

ln Kd =

RL =

α=

Grafting (%) = Organic composite (g)/Bare silica (g) × 100

(13)

where the content of organic components in the sample is calculated from the weight loss on the TG curve.

(11)

3. RESULTS AND DISCUSSION 3.1. Characterization of Adsorbents. Figure 1 shows the surface morphology of the sorbents by both scanning electron microscope (SEM) and transmission electron microscope (TEM). The electron micrograph shows that there is no change of morphology, but the agglomeration became serious. We can see the clear channel and small particle size presented in the TEM image. All of the expected elements including carbon, nitrogen, oxygen, and silicon can be detected in the element mapping (see Figure 2), and the element mapping after removal of Ni(II) is shown in Figure S1. In Table S1, the EDS data proves that the amino group was successfully modified to the surface of mesoporous silica for the presence of nitrogen and the increases of oxygen and silicon after modified. A large number of carbons exist before modification but decrease after functionalization, maybe attributed to the organic small molecular chains remaining in the pores during the process of template agent removal, but they dissolve in

−1

where Kd is the adsorption equilibrium constant (L·kg ), R is the molar gas constant (8.314 J·mol−1·K−1), T is the absolute temperature (K), RL is the separation factor, b is the Langmuir constant in L·mg−1, and C0 is the initial concentration of metal ions in mg·L−1. ΔS° (kJ·mol−1) and ΔH° (kJ·mol−1) are calculated by the slope and intercept of the linear fitting curve of ln Kd and 1/T function, taking the temperature T as the horizontal coordinate, ΔG° is plotted in vertical coordinates. After regression, the slope of the line is ΔS°, and the intercept is ΔH°. 2.3. Desorption Experiments. In this experiment, the adsorbent was added to 1 mol·L−1 hydrochloric acid which was used as a desorption agent. Then, the solution was placed in the thermostatic shaker with a rotation speed of 300 rpm for 24 h. The extracted materials are dried after repeated washing with deionized water. The materials were used for adsorption− desorption three times. The concentration of heavy metal ions D



Article

thermogram of MSS, with the three peaks corresponding to three decomposition processes at 100−600 °C and its weight loss being 8.9%. It is probably due to the loss of an alkyl chain from the surfactant and an amide from the gelatin. The small weight loss above 600 °C is due to the gradual dehydroxylation of silanol groups. Figure 4B shows the thermogram of MSS@ NH2@COOH, and there are mainly two decomposition processes: A weight loss of 7.82% is observed at the temperature of 100−250 °C, which is reasonably attributed to the loss and degradation of a carboxyl group. This was followed by a major weight loss of about 27.95% between 250 and 580 °C originating from the successful attachment of a polyamine chain and a carboxyl group to the surface or channel of the material. A small weight loss also can be observed between 580 and 1000 °C for the loss of the carbon chain on the surface and residual silanol. According to the DTG curve, MSS has three decomposition processes at 100−600 °C, and its weight loss is 8.9%. In addition, MSS@NH2@COOH has two main decomposition processes, which reach the maximum decomposition rate at 108 and 303 °C and correspond to 7.82 and 27.95% weight loss, respectively. The surface functional groups were identified using FTIR spectroscopy. NQ-62 exhibited several characteristics spectral bands in Figure 5A. The stretching vibrations of SiO and  CH2 can be seen at 458 and 1471 cm−1, respectively. The bands at 2843 and 2939 cm−1 are attributed to the stretching vibration of CH2. The bands at 3350 and 3284 cm−1 can be assigned to the stretching vibrations of NH at primary and secondary amines. The other band located at 1066 cm−1 can be attributed to the stretching vibrations of CO. In the FTIR spectrum of Figure 5B, several bands at 3500−3000, 1078 and 1230, 807, and 954 cm−1 are respectively ascribed to the stretching vibrations of NH, the stretching vibrations of Si OSi, the asymmetrica stretching vibrations of SiOSi, and the stretching vibrations of SiOH. However, the band at 954 cm−1 corresponds to the asymmetrical stretching vibrations of SiOH that disappeared, which indicates the interaction of NQ-62 and the hydrogen bond on SiOH. The other disappeared bands at 2810 and 2930 cm−1 corresponding to the CH2 of the template agent indicate the template agent is almost removed. The band at 459 cm−1 reflects the stretching vibrations of SiO. In the case of CTES and NQ62, the stretching vibrations of CH2 were presented at 2873, 2942, 2810, and 3019 cm−1. In addition, the new bands of  CN at 2248 cm−1 and COOH at 1725 cm−1 indicate the successful modification of cyan and carboxyl, respectively. The infrared spectra of MSS@NH2@COOH befor and after

organic solvent after reflux for 30 h in toluene or ethanol solution. Figure 3 shows that N2 adsorption−desorption isotherms for both samples are of typical type IV with H2(b) hysteresis loops27 according to IUPAC classification, suggesting their mesoporous nature. The capillary condensation of MSS occurred at a relatively higher pressure than that of MSS@ NH2@COOH samples, representing the larger pore size.28 Calculation indicates that the BET specific surface area and pore volume decrease from MSS (759.1534 m2·g−1, 1.1282 cm3·g−1) to MSS@NH2@COOH (113.8859 m2·g−1, 0.2526 cm3·g−1). The decrease of pore diameter, pore volume, and surface area is the reason that the presence of pendant organic chains covalently bonded to the inorganic network have the ability to partially block the entrance of nitrogen molecules28 and indicated that the polyamine and carboxyl chains were fastened inside mesopores. Since nickel ions occupy the active site of adsorption, the specific surface area and pore volume after adsorption are somewhat reduced. All of the structural parameters of the samples are shown in Table 1. The high steepness appeared at a relative pressure of 0.4−0.1 corresponding to H2(b) hysteresis loops. Table 1. Pore Texture Parameters of MSS and MSS@NH2@ COOH before and after Adsorption of Metal Ion

sample MSS MSS@NH2@COOH before adsorption MSS@NH2@COOH after adsorption

BET specific surface area SBET (m2·g−1)

BJH adsorption cumulative volume of pore, Va (cm3·g−1)

BJH adsorption average pore diameters Da (nm)

759.1534 113.8859

1.1282 0.2526

4.1 3.2

0.155

2.67

50

In Figure S2A, the XRD patterns of MSS and MSS@NH2@ COOH showed similar steamed bread peak and no other obvious peaks appeared, which revealed their amorphous nature. Figure S2B was the SAXS pattern of MSS and MSS@ NH2@COOH. There is a very strong diffraction peak at 2θ = 1.02°, d = 4.3 nm, showing MSS materials have better mesoporous order and after MSS@NH2@COOH of modification has no effect mesoporous order. The thermogravimetry analysis was performed to determine the amount of functional groups in the synthesized materials. The weight loss at 25−100 °C for both MSS and MSS@NH2@COOH may be due to the loss of physisorbed water. Figure 4A shows the

Figure 4. Thermal weight loss (TG), differential thermal gravity (DTG), and differential scanning calorimetry (DSC) for MSS (A) and MSS@ NH2@COOH (B). E



Article

Figure 5. Infrared analysis diagram of NQ-62 (A) and MSS amino and carboxyl groups (B).

in Figure 7C. These values of peaks are close to the binding energies of O 1s on SiO2 and COOH at 532.48 eV (SiO2), 533.18 eV (CO), and 531.38 eV (CO) in Figure 7D. The C 1s XPS spectrum of MSS@NH2@COOH can be curve fitted into six peaks at approximately 286.78 eV (CO), 286.38 eV (CO), 281.58 eV (C 1s), 284.48 eV (CC), 288.9 eV (OCO), and 285.18 eV (CN) in Figure 7E. According to the XPS results, it can be regarded that cyan was successfully transformed into the carboxyl. Meanwhile, we found that the multiamine chain was successfully grafted on the surface of mesoporous silica. 3.2. Adsorption Studies. 3.2.1. Effect of Solution pH. The solution pH is believed to be an important determining factor for metal ion adsorption into the adsorbent because hydrogen ions always act as a strong competing adsorbent to affect the surface charge of the adsorbent, the degree of ionization, and the speciation of the metal ion in solution. In brief, 80 mg of MSS and 40 mg of MSS@NH2@COOH were dispersed in 30 mL of solution of 200 mg·L−1 Ni(II) at different pH values (1.5, 2.5, 3.5, 5.0, 6.0, and 7.0) and stirred for 1.5 h at 25 °C. It can be seen in Figure 8 that the maximum removal rate and saturation adsorption capacity of MSS were 62.22% and 50.46 mg·g−1, respectively. For MSS@NH2@ COOH, the adsorption rate of metal ions increased with the augmentation of pH. Taking into account the pKa values of 4.88 for silanol, 8.48 for amino, and 2.86 for carboxyl, it is obvious that, at pH < 5, MSS@NH2@COOH will exist in its protonated form and can interact well with nickle ions. At pH 5, the maximum removal rate reached 83.3% and the maximum saturation adsorption capacity was 115.25 mg·g−1. At lower pH, the competition between hydrogen ions and heavy metal ions and the protonated amino groups increases under acidic conditions, resulting in the reduction of adsorption binding sites and the coordination ability of amino groups and heavy metal ions, which leads to a lower removal efficiency. The highest adsorption quantity arrived at pH 5.0 for the reduction of competition hydrogen ion and the increase of the nonprotonated amino group which form a negative potential.28−32 With the continued increase of the pH to 7.5, nickle ions precipitate as a result of the higher concentration of hydroxide ions in the solution forming hydroxide.31−33 It can be seen in Figure 9 that MSS@NH2@COOH exists as a buffer at pH 5−7, so the pHe can be regarded as pHpzc which was found to be 3.92. The low pHpzc value implies that MSS@ NH2@COOH is an effective nickle adsorbent as the pH increases above 3.92, and the surface of the modified

adsorption of nickle ion also can be seen in Figure S3. The peak of the CN stretching vibration of amine groups around 1183 cm−1 disappears, and the shifting peaks at 3438 and 1525 cm−1 on nickel(II) adsorption can reasonably conclude that the nitrogen atom of the amine group should be one of the adsorption sites for nickel binding on the MSS@NH2@ COOH. In addition, the deprotonated carboxyl group at 1390 cm−1 and the CO stretching vibration peak at 1652 cm−1 have a higher displacement, indicating the interactions of metal cations with the carboxyl group.29 According to the content of organic calculated from the weight loss on the TG curve, the calculated grafting of amino and carboxyl groups is 34 and 13.8%, respectively. It can be seen in Figure 6 that the zeta potential of MSS@ NH2@CN is 45 mV, but it is reduced to 12 mV after carboxyl

Figure 6. Zeta distribution data of MSS@NH2@CN and MSS@ NH2COOH.

modification, indicating that the negatively charged carboxyl group neutralizes most of the positively charged amino groups and both the amino group and the carboxyl group are successfully grafted onto the MSS. The XPS of data were used to analyze of the MSS@NH2@ COOH adsorbent, as shown in Figure 7. The survey of MSS@ NH2@COOH was shown in Figure 7A, including O 1s, N 1s, C 1s, Si 2s, and Si 2p whose detailed XPS survey of the regions and chemical bonding information was shown in Figure 7B−E. The peaks massed at 402.28, 399.98, and 401.38 eV were for NH, CNH2, and CNC bonds, respectively, for N 1s in Figure 7B, which belong to NQ-62 of the multiamine chain. The peaks at 104.18, 102.78, and 103.48 eV correspond to Si 2p on SiO2, SiO, and Si 2p3/2 bond, respectively, for Si 2p F



Article

Figure 7. XPS analysis diagram of MSS@NH2@COOH (A) for the binding energies of the N 1s (B), Si 2p(C), O 1s (D), and C 1s (E).

Figure 8. Effect of different pH values of solution on the Ni(II) ion removal rate (A) and adsorption quantity (B).

mesoporous silica is dominated by negative charge which is beneficial to adsorb anions. 3.2.2. Effect of Adsorbent Dose. Batch adsorption experiments were done at different amounts of MSS (20, 40, 80, and

120 mg) and MSS@NH2@COOH (5, 10, 20, 30, 40, 60, 80, and 120 mg) in 30 mL solution of 200 mg·L−1 Ni(II) at pH 5.0 and stirred for 1.5 h at 25 °C. As graphically represented in Figure 10, the adsorption of Ni(II) decreased with the amount G



Article

simulated sewage of 200 mg·L−1 Ni(II) and contacted at several times (5, 15, 30, 50, 70, 90, 120, and 180 min) at pH 5.0 and stirred for 1.5 h at 25 °C. The time dependence of the removal effect of Ni(II) was depicted in Figure 12. For the sample MSS@NH2@COOH, Ni(II) was instantaneously adsorbed with removal of 45% for the availability of many active sites. With the decrease in the number of chelating groups on the sorbent surface, the sorption process becomes slower and then reached a final equilibrium at 90 min after the adsorption active sites were occupied. The saturation adsorption capacity is 125.57 mg·g−1, and the corresponding removal rate is 81.37%. However, for the sample MSS, both the removal rate and the adsorption capacity were all lower than that on MSS@NH2@COOH with 62.08% removal and 46.55 mg·g−1 saturation adsorption capacity. It was related to the affinity of abundant groups of amino and carboxyl on MSS@NH2@COOH and the Ni(II) ions.34 The kinetics of the adsorption process is vital in wastewater treatment, as it provides essential information on the reaction pathway and the solute uptake rate.35 In order to clarify the sorption process of Ni(II) in the test sorbents, the adsorption kinetics data were fitted by pseudo-first-order, pseudo-secondorder, and intraparticle diffusion models (see Figure 13). The resulting parameters of the pseudo-first-order and pseudosecond-order models are listed in Table 2. It can be seen that the adsorption kinetics data for both adsorbents are in good agreement with the pseudo-second-order model with a correlation coefficient over 0.99. These facts clearly indicate that the chemical sorption is mainly a sorption process. According to the multilinear plots of the intraparticle diffusion model, it is not the sole rate-limiting step and the second gradual adsorption stage is rate-controlled.32 In order to evaluate the potential of MSS and MSS@NH2@ COOH in removal of Ni(II) from aqueous solution, the adsorption capacities on MSS and MSS@NH2@COOH were compared with other materials, as listed in Table 3. As can be seen from this table that the adsorption capacities obtained from MSS and MSS@NH2@COOH, especially for MSS@ NH2@COOH, are much higher than those reported elsewhere. Therefore, MSS@NH2@COOH can be considered as a promising adsorbent for removal of Ni(II). 3.4. Adsorption Isotherms. To identify the sorption behaviors of Ni(II) ions, 80 mg of MSS and 40 mg of MSS@ NH2@COOH were dispersed into 30 mL of aqueous solution at different initial concentrations (25, 50, 100, 200, 300, 400, and 500 mg·L−1) on the condition of different temperatures

Figure 9. pHe as a function of pHi for a solid to solution ratio of 1 g/ L, time of equilibration of 24 h, and background electrolyte of 0.1 M NaOH.

of adsorbents increased due to the number of active sites decreased. Five mg of MSS@NH2@COOH was able to remove 37.66% Ni(II) from aqueous solution, while MSS could remove only 35.15% at a dosage of 40 mg. At the same dosage of 120 mg samples, MSS can remove 66.77% Ni(II) and MSS@NH2@COOH reached 87.1%. The maximum amount of adsorption occurred at a dosage of 5 mg of MSS@NH2@COOH because of the amino and carboxyl groups on the MSS@NH2@COOH interacting with the heavy metal ions by a complexion mechanism. The optimized adsorbent dose was 40 mg of MSS@NH2@COOH, and the prepared adsorbent was compared with another studied adsorbent in Table S2. 3.2.3. Effect of Initial Ni(II) Concentration. Batch experiments were conducted by containing 80 mg of MSS and 40 mg of MSS@NH2@COOH with 30 mL of aqueous solution of Ni(II) ions at different initial concentrations (25, 50, 100, 200, 300, 400, and 500 mg·L−1) at pH 5.0 and stirred for 1.5 h at 25 °C. The results were provided in Figure 11. The adsorption capacity of two samples all increased with the initial Ni(II) ion concentrations increasing. For the sample of MSS@NH2@ COOH, the removal rate of Ni(II) was first increased and then decreased. At low concentrations, the removal rate was higher due to a larger amount of available amino and carboxyl chelating groups, whereas, at higher concentration, the opposite was observed due to limited adsorption sites. 3.3. Adsorption Kinetics. A total of 80 mg of MSS and 40 mg of MSS@NH2@COOH was dispersed into 30 mL of

Figure 10. Effect of different amounts of MSS and MSS@NH2@COOH on the Ni(II) ion removal rate (A) and adsorption quantity (B). H



Article

Figure 11. Effect of different initial Ni(II) ion concentrations on the Ni(II) ion removal rate (A) and adsorption quantity (B).

Figure 12. Effect of different contact times on the Ni(II) ion removal rate (A) and adsorption quantity (B).

indicated. And the minus value of ΔS° reflects the affinity of adsorbent to heavy metal ions and the structural changes of adsorbates in adsorbents. The RL classified as RL > 1, 0 < RL < 1, and RL = 0 suggests that adsorption is unfavorable, favorable, and irreversible, respectively. As shown in Figure 15, as the RL values in the figure lie between 0 and 1, the adsorption isotherm is favorable. The decrease of RL indicating the adsorption of nickel ions onto MSS@NH2@COOH was more favorable at a higher concentration of Ni(II). The higher the value of Kd is, the more favorable it adsorbed heavy metal ions. At the beginning, the distribution coefficient increases with the concentration of nickel ions increasing for there are many available amino and carboxyl chelating groups. The following decrease of Kd suggests the less favorable adsorption sites with an increase of nickel ion concentration.34,40 The kinetic, isotherm, and thermodynamic data and related relative standard deviations (RSDs) for MSS and MSS@NH2@ COOH are listed in Table S3. 3.6. Adsorption Selectivity. The effectivenes of MSS@ NH2@COOH for the removal of Ni(II) was carried out by introducing different cations like Zn2+ and Cr6+. It can be seen from Table 6 that, among the three coexisting ions (Ni, Zn, and Cr), MSS@NH2@COOH has the best adsorption effect on nickel with 81.33% removal rate, while the lowest removal rate of zinc is only 9.43%, which indicates the good selectivity to nickel of MSS@NH2@COOH. 3.7. Desorption Studies. From an economic perspective, for absorbents, regeneration of materials is an important characteristic to estimate the ability of reutilization for removing heavy metal ion from the environmental samples.

(298, 308, and 318 K) and stirred for 1.5 at 25 °C. The Langmuir and Freundlich equations are used to interpret the isotherm data. The linear plots of the Langmuir and Freundlich equations were shown in Figure 14, and the fitting parameters are listed in Table 4. The Freundlich model shows higher correlation coefficients than the Langmuir model, indicating that the sorption of Ni(II) in both sorbents follows the Freundlich sorption model well, which corresponds to multilayer adsorption of samples. It may be due to the Freundlich equation being based on a heterogeneous surface that causes the heterogeneous distribution of active sites on the sample’s surface. With higher adsorption energy of MSS@ NH2@COOH, the values of K from MSS@NH2@COOH are higher than those of MSS. Furthermore, Qm values evaluated for MSS@NH2@COOH are higher than those for MSS, indicating the adsorption capacity on MSS@NH2@COOH was enhanced via functionalization. 3.5. Adsorption Thermodynamics. In this experiment, MSS and MSS@NH2@COOH were designed to adsorb heavy metal nickel ions at different temperatures. The concentrations were between 25 and 500 mg·L−1, and the detailed operation procedure was shown in section 3.2.3. Figure S4 shows that, with the increase of the temperature of the solution system, the adsorption amount of the two materials to heavy metal Ni(II) decreases gradually, indicating that the MSS and MSS@NH2@ COOH adsorption of Ni(II) belonged to the exothermic process. The calculation results of three thermodynamic parameters are shown in Table 5. The negative of ΔG° shows the adsorption of heavy metal ions is spontaneous. When the value of ΔH° is negative, the exothermic process is I



Article

Figure 13. Pseudo-first-order model (A) and pseudo-second-order model (B) for Ni(II) ion adsorption on MSS; pseudo-first-order (C), pseudosecond-order (D), and intraparticle diffusion (E) for Ni(II) ion adsorption on MSS@NH2@COOH.

Table 2. Parameters of Pseudo-First-Order and Pseudo-Second-Order Kinetics of MSS and MSS@NH2@COOH for Adsorption of Ni(II) Ions pseudo-first-order kinetics

pseudo-second-order kinetics

sample

initial concentration C0 (mg·L−1)

saturated adsorption Qe (mg·g−1)

K1 (min−1)

Qe,c (mg·g−1)

R2

K2 (min−1)

Qe,c (mg·g−1)

R2

MSS MSS@NH2@COOH

200 200

46.75 125.65

0.07089 0.10508

34.92 82.11

0.741 0.966

0.00291 0.00118

48.71 130.89

0.999 0.999

Table 3. Comparison of the Ni(II) Adsorption Quantities Reported in This Work and the Literature adsorbent

adsorption quantity (mg·g−1)

ZC-16 Ni(II)-MIIP NiIMS2, NiIMS1 MCM-41/TMSPDETA MSN with cyano groups MSS MSS@NH2@COOH

2.58 87 20.8, 22.9 58.47 57.3−72.41 46.55 125.57

adsorption conditions pH pH pH pH pH pH pH

3, 7, 6, 6, 5, 5, 5,

m m m m m m m

= = = = = = =

100 mg, T = 28 °C, C0 = 4 mg/L 200 mg, C0 = 1000 mg/L 10 mg, T = 30 °C, C0 = 250 mg/L 50 mg, T = 25 °C, C0 = 30 mg/L 100 mg, T = 30 °C, C0 = 20 mg/L 80 mg, T = 25 °C, C0 = 200 mg/L 40 mg, T = 25 °C, C0 = 200 mg/L

J

adsorption equilibrium time (min)

reference

1440 5 20 60 150 90 90

13 36 37 38 39 this work this work



Article

Figure 14. Langmuir (A) and Freundich (B) isotherms of MSS; Langmuir (C) and Freundich (D) isotherms of MSS@NH2@COOH.

Table 4. Adsorption Isotherm Parameters of the Langmuir and Freundich Models of MSS and MSS@NH2@COOH for Adsorption of Ni(II) Ions Langmuir isotherm model sample MSS

MSSMSS@NH2@COOH

Qm (mg·g−1)

KL (L·mg−1)

R2

1/n

KF (L·mg−1)

R2

298 308 318 298 308 318

113.71 87.81 83.82 229.99 222.94 219.734

0.00396 0.00183 0.00161 0.00816 0.00669 0.00632

0.36953 0.32346 0.27934 0.96389 0.92995 0.91634

1.13253 1.48263 1.54082 1.04451 0.94239 1.23309

0.2981 0.0331 0.0227 1.7961 1.3734 0.2115

0.99013 0.95683 0.95193 0.99906 0.99969 0.99963

Table 5. Thermodynamic Parameters of MSS and MSS@ NH2@COOH for Adsorption of Ni(II) Ions MSS ΔG0 (kJ mol−1) ΔH0 (kJ mol−1) ΔS0 (kJ mol−1)

Table 6. Adsorption of Three Metal Ions (Ni, Zn, and Cr) on MSS@NH2@COOH, with an Initial Concentration of 200 mg·L−1, pH 5, 25 °C, and Adsorption Time of 90 min

MSS@NH2@COOH −20.7824 −25.61 −0.0162

Freundich isotherm model

temperature (K)

ΔG0 (kJ mol−1) ΔH0 (kJ mol−1) ΔS0 (kJ mol−1)

−0.4798 −4.5058 −0.0137

adsorption ions 2+

removal (%) adsorption capacity (mg·g−1)

Ni

Zn2+

Cr6+

81.33 122

9.43 12.11

67.6 98.18

Thus, the repeatability of the MSS@NH2@COOH used was studied using the processing described in section 2.3. At the first adsorption−desorption experience, the removal rate of Ni(II) was 81.56%. After recycling three times, the removal rate of Ni(II) dropped to 75.45% (see Table 7 and Figure S5). This may result from the adverse effects of the desorbing agents on the binding sites.13 It can be seen that the functionalized sample can be reused at least three times with Table 7. Parameter of Desorption and Cyclic Adsorption of MSS@NH2@COOH Figure 15. Values of RL and Kd for adsorption of Ni(II) ions onto MSS@NH2@COOH.

K

cycle times

1

2

3

desorption rate cyclic adsorption rate

90.14 81.56

88.22 78.32

80.64 75.45



Article

(4) Srivastava, N. K.; Majumder, C. B. Novel biofiltration methods for the treatment of heavy metals from industrial wastewater. J. Hazard. Mater. 2008, 151, 1−8. (5) Namasivayam, C.; Kadirvelu, K. Uptake of mercury (II) from wastewater by activated carbon from an unwanted agricultural solid by-product: coirpith. Carbon 1999, 37, 79−84. (6) Tofighy, M. A.; Mohammadi, T. Adsorption of divalent heavy metal ions from water using carbon nanotube sheets. J. Hazard. Mater. 2011, 185, 140−147. (7) Fu, F.; Xie, L.; Tang, B.; Wang, Q.; Jiang, S. Application of a novel strategy-Advanced Fenton-chemical precipitation to the treatment of strong stability chelated heavy metal containing wastewater. Chem. Eng. J. 2012, 189−190, 283−287. (8) Zhuang, X.; Zhao, Q.; Wan, Y. Multi-constituent co-assembling ordered mesoporous, thiol-functionalized hybrid materials: synthesis and adsorption properties. J. Mater. Chem. 2010, 20, 4715−4724. (9) Ju, X. J.; Zhang, S. B.; Zhou, M. Y.; Xie, R.; Yang, L. H.; Chu, L. Y. Novel heavy-metal adsorption material: ion-recognition P(NIPAMco-BCAm) hydrogels for removal of lead(II) ions. J. Hazard. Mater. 2009, 167, 114−118. (10) Mohsen-Nia, M.; Montazeri, P.; Modarress, H. Removal of Cu2+ and Ni2+ from wastewater with a chelating agent and reverse osmosis processes. Desalination 2007, 217, 276−281. (11) Mcmanamon, C.; Burke, A. M.; Holmes, J. D.; Morris, M. A. Amine-functionalised SBA-15 of tailored pore size for heavy metal adsorption. J. Colloid Interface Sci. 2012, 369, 330−337. (12) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Ordered mesoporous molecular sieves synthesized by a liquid-crystal template mechanism. Nature 1992, 359, 710−712. (13) Thirumavalavan, M.; Wang, Y. T.; Lin, L. C.; Lee, J. F. Monitoring of the Structure of Mesoporous Silica Materials Tailored Using Different Organic Templates and Their Effect on the Adsorption of Heavy Metal Ions. J. Phys. Chem. C 2011, 115, 8165−8174. (14) Saha, D.; Barakat, S.; Van Bramer, S. E.; Nelson, K. A.; Hensley, D. K.; Chen, J. H. Noncompetitive and Competitive Adsorption of Heavy Metals in Sulfur-Functionalized Ordered Mesoporous Carbon. ACS Appl. Mater. Interfaces 2016, 8, 34132−34142. (15) Wu, P.; Xu, Z. Silanation of nanostructured mesoporous magnetic particles for heavy metal recovery. Ind. Eng. Chem. Res. 2005, 44, 816−824. (16) Kang, T.; Park, Y.; Yi, J. Highly Selective Adsorption of Pt2+ and Pd2+ Using Thiol-Functionalized Mesoporous Silica. Ind. Eng. Chem. Res. 2004, 43, 1478−1484. (17) Bala, T.; Prasad, B. L.; Sastry, M.; Kahaly, M. U.; Waghmare, U. V. Interaction of different metal ions with carboxylic acid group: A quantitative study. J. Phys. Chem. A 2007, 111, 6183−90. (18) Ding, P.; Huang, K. L.; Li, G. Y.; Zeng, W. W. Mechanisms and kinetics of chelating reaction between novel chitosan derivatives and Zn(II). J. Hazard. Mater. 2007, 146, 58−64. (19) Ho, Y. S.; Mckay, G. Pseudo-second order model for sorption processes. Process Biochem. 1999, 34, 451−465. (20) Yao, Z. Y.; Qi, J. H.; Wang, L. H. Equilibrium, kinetic and thermodynamic studies on the biosorption of Cu (II) onto chestnut shell. J. Hazard. Mater. 2010, 174, 137−143. (21) Vasconcelos, H. L.; Camargo, T. P.; Gonçalves, N. S.; Neves, A.; Laranjeira, M. C. M.; Fávere, V. T. Chitosan crosslinked with a metal complexing agent: Synthesis, characterization and copper(II) ions adsorption. React. Funct. Polym. 2008, 68, 572−579. (22) Gervas, C.; Mubofu, E. B.; Mdoe, J. E. G.; Revaprasadu, N. Functionalized mesoporous organo-silica nanosorbents for removal of chromium (III) ions from tanneries wastewater. J. Porous Mater. 2016, 23, 83−93. (23) Maity, J. P.; Hsu, C. M.; Lin, T. J.; Lee, W. C.; Bhattacharya, P.; Bundschuh, J.; Chen, C. Y. Removal of fluoride water through bacterial-mediated novel hydroxyapatite nanoparticle and its efficiency assessment:Adsorption isother, adsorption kinetic and adsorption thermodynamics. Environment Nanotechnology, Monitoring & Mangement. 2018, 9, 18−28.

a good removal effect to Ni(II) ions, reflecting the good adsorption capacity of MSS@NH2@COOH.

4. CONCLUSIONS A new adsorbent with amino and carboxyl groups was first obtained by grafting amino and cyano groups onto as-prepared MSS and hydrolyzing cyano groups to carboxyl groups. It was found that the materials prepared were effective adsorbents of Ni(II) ions and the maximum adsorption capacity of MSS@ NH2@COOH is 125.57 mg·g−1. Analysis of experimental data demonstrated that the adsorption of Ni(II) was better fitted with a pseudo-second-order model, which indicates the adsorption process of MSS@NH2@COOH to Ni(II) was chemical adsorption. The adsorption isotherms were represented well by the Freundlich equation, and the thermodynamic parameters inferred that the adsorption process was spontaneous and endothermic. Through a low cost and easy preparation method, the dual functional mesoporous silica with large specific surface area, recyclability, and relatively high adsorption effect was synthesized. It could not only be a promising adsorption material for the heavy metal ions and organic pollutant, but its application in drug carriers should be taken seriously.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.8b00689.

■

Experimentally detailed procedures for synthesis, more characterization data, including nitrogen adsorption− desorption isotherms, SEM and EDS images, XRD patterns, element mapping, and FITR of MSS@NH2@ COOH before and after adsorption of nickle ion (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Fangxiang Song: 0000-0002-1670-8623 Funding

We acknowledge financial support from the Guizhou provincial science and technology department joint fund project (QKHPTRC[2017]7289 and QKHPTRC[2017]5788) and the Sci-Tech Cooperation Foundation of Gui-zhou Province ([2012]7007). Notes

The authors declare no competing financial interest.

■

REFERENCES

(1) Al-Othman, Z. A.; Inamuddin; Naushad, M. Forward (M2+−H+) and reverse (H+−M2+) ion exchange kinetics of the heavy metals on polyaniline Ce(IV) molybdate: A simple practical approach for the determination of regeneration and separation capability of ion exchanger. Chem. Eng. J. 2011, 171, 456−463. (2) Nanseu-Njiki, C. P.; Tchamango, S. R.; Ngom, P. C.; Darchen, A.; Ngameni, E. Mercury(II) removal from water by electrocoagulation using aluminium and iron electrodes. J. Hazard. Mater. 2009, 168, 1430−1436. (3) Khezami, L.; Capart, R. Removal of chromium(VI) from aqueous solution by activated carbons: Kinetic and equilibrium studies. J. Hazard. Mater. 2005, 123, 223−231. L



Article

polymerization approach and their application for removal of Cu2+ from aqueous solution. RSC Adv. 2016, 6, 11419−11429.

(24) Heidari, A.; Younesi, H.; Mehraban, Z. Removal of Ni (II), Cd (II), and Pb (II) from a ternary aqueous solution by amino functionalized mesoporous and nano mesoporous silica. Chem. Eng. J. 2009, 153, 70−79. (25) Awad, F. S.; AbouZeid, K. M.; El-Maaty, W. M. A.; El-Wakil, A. M.; El-Shall, M. S. Efficient Removal of Heavy Metals from Polluted Water with High Selectivity for Mercury(II) by 2-Imino-4-Thiobiuret Partially Reduced Graphene Oxide (IT-PRGO). ACS Appl. Mater. Interfaces 2017, 9, 34230−34242. (26) Guo, Z. X.; Yu, J. Grafting of dendritic polyethers onto nanometre silica surface. J. Mater. Chem. 2002, 12, 468−472. (27) Wang, L.; Duan, G.; Chen, S. M.; Liu, X. H. Particle Size and Dispersity Control by Means of Gelatin for High-Yield Mesoporous Silica Nanospheres. Ind. Eng. Chem. Res. 2015, 54, 12580−12586. (28) Silva, A. L. P.; Sousa, K. S.; Germano, A. F. S.; Oliveira, V. V.; Espínola, J. G. P.; Fonseca, M.; Airoldi, G. C.; Arakaki, T.; Arakaki, L. N. H. A new organofunctionalized silica containing thioglycolic acid incorporated for divalent cations removal-A thermodyamic cation/ basic center interaction. Colloids & Surfaces A Physicochem. Colloids Surf., A 2009, 332, 144−149. (29) Naskar, A.; Guha, A. K.; Mukherjee, M.; Ray, L. Adsorption of nickel onto bacillus cereus m116: a mechanistic approach. Sep. Sci. Technol. 2016, 51, 427−438. (30) Ghorbani, F.; Younesi, H.; Ghasempouri, S. M.; Zinatizadeh, A. A.; Amini, M.; Daneshi, A. Application of response surface methodology for optimization of cadmium biosorption in an aqueous solution by Saccharomyces cerevisiae. Chem. Eng. J. 2008, 145, 267− 275. (31) Amini, M.; Younesi, H.; Bahramifar, N.; Lorestani, A. A. Z.; Ghorbani, F.; Daneshi, A.; Sharifzadeh, M. Application of response surface methodology for optimization of lead biosorption in an aqueous solution by Aspergillus niger. J. Hazard. Mater. 2008, 154, 694−702. (32) Sari, A.; Tuzen, M.; Citak, D.; Soylak, M. Equilibrium, kinetic and thermodynamic studies of adsorption of Pb(II) from aqueous solution onto Turkish kaolinite clay. J. Hazard. Mater. 2007, 149, 283−291. (33) Altin, O.; Ozbelge, O. H.; Dogu, T. Effect of pH, flow rate and concentration on the sorption of Pb and Cd on montmorillonite: II. Modelling. J. Chem. Technol. Biotechnol. 1999, 74, 1139−1144. (34) Soltani, R.; Dinari, M.; Mohammadnezhad, G. Ultrasonicassisted synthesis of novel nanocomposite of poly (vinyl alcohol) and amino-modified MCM-41: a green adsorbent for Cd (II) removal. Ultrason. Sonochem. 2018, 40, 533−542. (35) Heidari, A.; Younesi, H.; Mehraban, Z. Removal of Ni(II), Cd(II), and Pb(II) from a ternary aqueous solution by amino functionalized mesoporous and nano mesoporous silica. Chem. Eng. J. 2009, 153, 70−79. (36) Faghihian, H.; Adibmehr, Z. Comparative performance of novel magnetic ion-imprinted adsorbents employed for Cd2+Cu2+ and Ni2+ removal from aqueous solutions. Environ. Sci. Pollut. Res. 2018, 25, 15068−15079. (37) He, R.; Wang, Z.; Tan, L.; Zhong, Y.; Li, W. M.; Xing, D.; Wei, C. H.; Tang, Y. W. Design and fabrication of highly ordered ion imprinted SBA-15 and MCM-41 mesoporous organosilicas for efficient removal of Ni2+ from different properties of wastewaters. Microporous Mesoporous Mater. 2018, 257, 212−221. (38) Ghorbani, M.; Nowee, S. M.; Ramezanian, N.; Raji, F. A new nanostructured material amino functionalized mesoporous silica synthesized via co-condensation method for Pb(II) and Ni(II) ion sorption from aqueous solution. Hydrometallurgy 2016, 161, 117− 126. (39) Wang, B.; Zhou, Y.; Li, L.; Xu, H.; Sun, Y.; Wang, Y. Novel synthesis of cyano-functionalized mesoporous silica nanospheres (MSN) from coal fly ash for removal of toxic metals from wastewater. J. Hazard. Mater. 2018, 345, 76−86. (40) Dinari, M.; Mohammadnezhad, G.; Soltani, R. Fabrication of poly (methyl methacrylate)/silica KIT-6 nanocomposites via in situ M


A Novel Amino and Carboxyl Functionalized Mesoporous Silica as an

Recommend Documents