Microporous and Mesoporous Materials 247 (2017) 145e157
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Adsorption of heavy metals on functionalized-mesoporous silica: A review Enshirah Da'na King Faisal University, Eastern Province, Saudi Arabia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 January 2017 Received in revised form 20 March 2017 Accepted 27 March 2017 Available online 2 April 2017
This work is summarizing the main researcher's contributions in the development of heavy metals adsorbent. Emphasis has been placed on the factors affecting the performance of the adsorbents such as support structural properties, functional groups chemical properties, and properties of the combined inorganic-organic structure. The review includes adsorbents synthesized by two major synthesis routes namely (i) grafting and (ii) co-condensation. This literature review is aiming to trace the main achievements toward the synthesis of an efﬁcient heavy metals adsorbent. © 2017 Elsevier Inc. All rights reserved.
Keywords: Adsorption Mesoporous silica Grafting Cocondensation Heavy metals Functionalization Water treatment
1. Introduction Heavy metal ions get into water system via several manufacturing processes such as reﬁning, fertilizer, and pesticides and pose a severe risk to the environment . Similar to other pollutants, exceeding a threshold concentration in water, heavy metals are harmful to human as well as other living organisms due to their toxicity and bio-accumulation. Nonetheless, trace elements are essential nutrients required by the human body, consumption of high doses can lead to health complications such as irritation of the central nervous system, and kidney and liver hurt . Thus, removing excess heavy metals from industrial efﬂuents before discharging is an important issue for health and environment safety. Many conventional approaches have been utilized for eliminating these cations from water system such as coagulationeﬂocculation, chemical precipitation, ﬂotation, reverse osmosis, reverse osmosis, ion exchange, and ultra-ﬁltration [3,4]. Nevertheless, these methods have their own restrictions such as low efﬁciency, sensitive working environments, and production of toxic slurry . Hence, there is an urgent need for more practical and
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http://dx.doi.org/10.1016/j.micromeso.2017.03.050 1387-1811/© 2017 Elsevier Inc. All rights reserved.
environment friendly technologies. Adsorption is now considered among the most effective, economic and selective methods for water treatment and analysis purposes . Activated carbon has been extensively employed to purify contaminated water  because it has very high surface area, chemically stable, and durable. Nevertheless, their random pore geometry restricts pollutant species reaching to the adsorption sites, which in sequence, lowers removal efﬁciency. Furthermore, activated carbon prices keeps going up , which encouraged many researchers to focus on developing cheaper and more effective adsorbents based on naturally occurring materials such as agricultural waste materials. In addition to being cost-effective and environment friendly, agricultural waste is plentiful, renewable, and cheap. These agricultural residuals can be used to adsorb metal cations directly without any modiﬁcation. However, Chemical or physical modiﬁcations were recently reported to improve adsorption capacity [3,6e8]. Other researchers have utilized bacteria, fungi, algae and yeast for adsorbing heavy metals from aqueous solutions and reported signiﬁcant capacities . Another approach was to utilize natural materials and residuals for the development of new adsorbents. Among these adsorbents, clays and soil constituents , red mud , chitosan , natural zeolites  and ﬂy ashes  have many advantages such as availability in large quantities and low cost. In addition, they can be
E. Da'na / Microporous and Mesoporous Materials 247 (2017) 145e157
chemically modiﬁed to increase their binding capacity. However, these adsorbents show many disadvantages because of their heterogeneous structure, irregular pore size distribution, low adsorption capacity and low metal selectivity. Thus, it is essential to develop a new adsorbent that meet speciﬁc requirements, including: (i) Well-designed pore size and geometry with open pore structure to achieve fast kinetics, (ii) Accessible adsorption sites and suitable surface properties to achieve high adsorption capacity, (iii) High target metal selectivity in the existence of other ions such as Naþ and Ca2þ, (iv) Simple synthesis, (v) Low cost, taking into consideration not only the material synthesis cost, but also equipment cost, which depends on adsorbent performance, (vi) Regeneration ability under mild conditions, (vii) Long term stability upon adsorption-desorption cycling. Mesoporous silica for instance MCM-48, MCM-41, HMS, and SBA-15 are considered superior in this type of applications due to large surface area, narrow pore-size distributions and controlled pore sizes . Also, its afﬁnity towards target metals can be improved by introducing suitable functional groups to the surface [16e18]. This review focus on latest developments in the synthesis and evaluation of mesoporous silica as heavy metals adsorbent. These materials will be divided into two categories based on the functionalization route, the advantages and disadvantages of each route in addition to the factors affecting the adsorbent efﬁciency and selectivity will be discussed qualitatively in details. 2. General properties of mesoporous materials Ordered structure mesoporous silica has attracted researchers attention due to their important applications such as catalysis [19e22], environmental protection via adsorption of CO2 [23e29], adsorption of heavy metals from aqueous solutions [30e56], adsorption of organic pollutants [57e60], adsorption of volatile organic compounds , separation [62e67], medical applications [68e75], and sensing applications [74e77]. The importance of mesoporous materials came from their pore sizes, which allow controlled accessibility for large molecules, depending on pore geometry. Furthermore, the ordered mesoporous materials have open pore structure, which allows improved access and transfer of target molecules compared to bottleneck pores of amorphous materials, which resist molecular diffusion to the adsorption sites . In 1990, Kuroda et al.  synthesized mesoporous silica with uniform pore size distribution known as Folded Sheet Materials (FSM-16). In 1992, scientists at Mobil developed a new mesoporous silica labeled as M41S and reported MCM-41 and MCM-48 as new members of this family [80,81]. In 1995, Tanev and Pinnavaia  synthesized Hexagonal Mesoporous Silica (HMS) by means of neutral templates. In 1998, Stucky et al. [83,84] described the synthesis of Santa Barbara Amorphous (SBA-15), which is one of the most promising mesoporous materials. This highly ordered material has hexagonal structure with thick pore walls and tunable pore diameter. Thus, it seems to be a good candidate for separation processes such as adsorption. Environmental applications of mesoporous silica for adsorption of heavy metals require the material to have speciﬁc adsorption sites for the target ions; however, silica does not have such sites. Therefore, immobilization of appropriate chemical groups in the mesopores has attracted much attention [16,17]. Generally,
mesoporous silica synthesis is performed by condensing a source of silica in the existence of a suitable template agent followed by template removal and eventually anchoring of special functional groups onto the silica surface. Functional groups anchoring can be achieved by three main routes: (i) using a functionalized silane, (ii) co-condensation of silane with the silica, and (iii) post grafting of the silica . The earliest reports on adsorption applications of functionalized mesoporous materials are dedicated to heavy metals in wastewater using propylthiol-modiﬁed MCM-41  and HMS  silica. Since then, adsorption of metal ions from wastewater on functionalized mesoporous silica has attracted researcher's attention. Among the many silica mesophases, SBA-15 attracted much attention for many applications such as water treatment [30e32,61e66], catalysis  adsorption of biomolecules  and CO2 . The strong interest in this material is related to its huge surface area (600e1000 m2/g), narrow pore size distribution, large and controlled pore diameter (5e30 nm), which facilitate metal ions diffusion to the internal pore structure, resulting in fast adsorption kinetics . Furthermore, it is considered hydrothermally stable because of its thick walls, about 4 nm in comparison to 1 nm for MCM-41 . In addition, enhancement of the mechanical stability due to increased pore wall thickness is important for some applications using compressed pellets . All these features make SBA-15 a promising support to introduce many functionalities (-NH2, -SH, -S-, etc.) for developing suitable adsorbents for the target application [39,44,48e50,53,55]. 3. Functionalization of mesoporous silica Co-condensation (one pot-synthesis) and grafting (post-synthesis) are the two main routes that have been extensively studied for adjusting the mesoporous materials via covalent bonding of the organic functionality and the silica. In addition, mesoporous materials having functional groups within the framework, called periodic mesoporous organosilicates, have also been used for the elimination of heavy metals [98,99]. The post-synthesis grafting methodology was used to integrate a wide range of functional groups to the pre-synthesized mesoporous silica's walls. The characteristics of the functionalized materials prepared by grafting are inﬂuenced by their structural properties and chemical composition . Another main route for introducing functionality is cocondensation where the organosilane and silica precursor are condensed together. Co-condensation shows many beneﬁts such as uniform spreading of the functional groups and faster synthesis. Furthermore, it is very efﬁcient in immobilizing huge amounts of functional groups onto the mesoporous silica surface. 3.1. Adsorption of heavy metals on mesoporous silica prepared by grafting Development of heavy metals adsorbents depends on combining the open pore structure of mesoporous silica with suitable organic compounds that exhibit high reactivity toward the target metal ions. Grafting route was used for mesoporous silica modiﬁcation by many different organic functionalities. One major factor affecting the performance of grafted adsorbents is the appropriate choice of the organic group. As an example, Liu et al.  synthesized thiolated and aminated SBA-15 adsorbents and tested their afﬁnity for heavy metal cations. They found that the thiolated SBA-15 have a negligible afﬁnity for copper, while the aminated ones have exceptional binding ability. Similarly, Lee et al.  studied adsorption of heavy metal ions on aminated and thiolated large-pores mesoporous silica and reported the same
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results. In contrast, some researchers reported excellent Hg removal efﬁciency on thiolated mesoporous silica compared to the aminated ones [36,46]. Lee and Yi  grafted 3-(2-aminoethylamino)propyltrimethoxysilane onto HMS mesoporous silica surface. Their functionalized silica had copper removal capacity 10 times more than that of silica gel. One main concern about grafting is that it reduces the pore size of the modiﬁed-materials, especially when large size or large amount of functional groups are introduced [32,102e111], resulting in restricted diffusion to the adsorption sites. Mureseanu et al.  reported a sharp decrease in pore volume and surface area of silica after grafting (Table 1). For example, pore volume and surface area of aminopropyl functionalized SBA-15 (NH2-SBA-15) with an average of 3.1 aminopropyl molecules per nm2, decreased by 52% and 53%, respectively, compared to the SBA-15 support . Shiraishi et al.  studied adsorption of copper on many inorganic adsorbents (MCM-41, silica gel, and aluminum oxide), functionalized with ethylenediaminetetraacetic acid (EDTA) and diethylenetriaminepentaacetic acid (DTPA). The EDTA loading on the adsorbents surface was lower than DTPA loading. However, it was found that the Cu2þ removal efﬁciency of DTPA-modiﬁed silica was signiﬁcantly lower than that for EDTA-modiﬁed material. This suggests that the binding ability of DTPA attached to the silica is lower than that of EDTA, despite the fact that copperDTPA complex stability constant is reported to be higher than that of copperEDTA complex in aqueous solution . This is mainly because DTPA molecule is bigger than that of EDTA, and since these molecules are attached to the surface of the silica, adsorption of the metals on DTPA is expected to be restricted. Furthermore, as more DTPA ligand was anchored, more reduction of the pore size and some pore blockage is expected. Since grafted organic groups on mesoporous silica is often not evenly distributed, the apparent density of functional groups on the solid does not tell the real average space between such groups. This difﬁculty in prediction of the real distribution resulted in a problem in adsorption of metal ions, which required the linking with two or more surface ligands. Manu et al.  investigated the impact of amine surface loading of mesoporous silica gel. They prepared propylamine-functionalized mesoporous silica gel with three different loadings, namely GN1, GN2, and GN3. They reported a nonlinear relation between the adsorption capacity and amine groups loading on the silica gel surface. This is mainly related to irregular distribution of functional groups obtained by grafting since large silane molecules suffer diffusion limitations to penetrate deep into the pores, and consequently, they may tend to react with the hydroxyl groups near the pore mouth, leading to some pore blockage, thus decreasing surface area, pore volume, and accessibility of functional groups (Table 1). Walcarius et al.  reported that a minimum pore volume of 0.5 cm3g-1 after grafting was necessary to have good accessibility to the adsorption sites. Pinnavaia and Mercier  tested accessibility of thiol groups grafted on mesoporous silica via adsorption of Hg. Their results showed improved accessibility of the active binding sites with well-deﬁned mesoporous channels, compared to disordered structure materials, such as amorphous silica. Yuan et al.  grafted amino groups onto large pore size (10.3 nm) and small pore size silica (2e4 nm). Their results conﬁrmed a great effect of the pore size on the adsorbent performance. The large pore size one was grafted with 447 mg/g amino groups and showed high adsorption capacity (Pb2þ: 880.6 mg/g, Cu2þ: 628.3 mg/g, Cd2þ: 492.4 mg/g) compared to the small pore size samples . Additional exploration of the effect of amine groups accessibility on adsorption performance was performed by Zhang et al. . They prepared mono-, di- and tri-amine-grafted on SBA-15 and showed that these multi-amine grafted silica have nearly identical
afﬁnity to Hg2þ, Pb2þ, Zn2þ, Cu2þ, Cd2þ in wastewater and can efﬁciently remove them totally. The introduction of multi-amine silane resulted in great density of N groups and great adsorption ability showing the high accessibility of amine groups in the larger pore channels of SBA-15 , thus the adsorption capacity was improved. Using smaller pore MCM-41 silica, Yokoi et al.  indicated that the aminosilane react with silane groups on the surface located at the pore mouth and/or on the external surface. Thus, the amine groups concentrated at the pores entrance would hindered the diffusion of additional aminosilane into the pore, resulting in a non-uniform distribution of amino groups within the pores. Furthermore, the accumulation of the amine-organic moieties at the pore entrance would reduce the effective pore size (Table 1), reducing target molecules binding with amine groups attached to the pores. In addition to easy access, the rate at which cations reach the active sites in the mesoporous material is also an important factor affecting its efﬁciency. Walcarius et al. [114,117] studied the speed at which copper ions reached active sites of aminopropyl-grafted silica. They found that the size and the charge of the target species to be adsorbed inﬂuence the kinetic of the process. Walcarius et al.  compared between Cu2þ complexation with the aminegrafted silica gel and the protonation process of the adsorbent using HCl. The calculated Cu2þ diffusion coefﬁcient was 3e4 times smaller than that of Hþ, which is mainly due to the larger size of Cu2þ compared to Hþ. Furthermore, adsorption of Cu2þ involves diffusion of two positive charges, thus suffering more repulsive force. The same group  studied the ability of metal ions to reach the adsorption sites of APTESMCM-41 and APTESSBA-15 by protonation with HCl. They indicated that the extents of consumed protons to reach the equivalent point for APTESMCM41and APTESSBA-15 corresponded to 79% and 85%, respectively of the overall quantities of amine groups in each sample. The incomplete protonation is due to some pore blocking in materials with narrow openings. Furthermore, they evaluated the speed at which copper ions reached the active sites of aminopropyl-grafted silica having a pore diameter in the range 4 and 15 nm and organic loads of 1.4e1.9 mmolg1. They found that the diffusion process was very limited compared to those detected in homogeneous solution, depending on a number of parameters such as adsorbent pore size, target metal size, functional groups surface density, and the structure of the silica used. The same group  grafted aminopropyl on different ordered mesoporous silica with different pore sizes and conﬁgurations (MCM-41 and MCM-48) and silica gel. They studied copper accessibility to the adsorption sites and the adsorption rate. They reported that both accessibility and rate of adsorption were greater using ordered mesoporous silica with average pore size of 6e7 nm, while the ordered mesoporous structures of pore size (3.5 nm) showed similar efﬁciency as that of large-pore silica gel. Furthermore, they demonstrated that the beneﬁt of ordered materials can be achieved by avoiding pore blocking during the grafting procedure, otherwise there would be no beneﬁt of uniform pore structure in comparison to the disordered materials. The ordered structure of the adsorbent is expected to improve its performance by improving accessibility to the adsorption sites and/or speeding up transport processes within the pores. However, this is not applicable under all conditions; Goubert-Renaudin  found that binding sites accessibility of cyclam-functionalized silica was not the most important factor affecting the maximum adsorption efﬁciency and that ordered structure of the adsorbent did not result in a faster kinetics of the copper adsorption. This is mainly because the rate-limiting step was complex formation itself and not diffusion. The rate of Cu2þ binding with cyclam groups in aqueous solution was faster in basic medium because of functional
E. Da'na / Microporous and Mesoporous Materials 247 (2017) 145e157
Table 1 Structural properties and functional group content. Adsorbent
Surface area (m2g1)
Pore size (nm)
Pore volume (cm3g1)
Functional group loading (mmolg1) or%
SBA-15 N-SBA-15 NN-SBA-15 NNN-SBA-15 MSU-H MSU-H-AM MSU-H-GM MSU-H-MM MSU-H-VM MSU-H-pM SBA-15-p NH2-SBA-15-p NH2-SBA-15-f NH2-SBA-15-r SH-SBA-15-p SH-SBA-15-f SH-SBA-15-r APTS-SBA-15-AB FSM-16 SBA-15 SBA-15(SH) SBA-15(NH2) Commercial silica HMS (Soxhelt extracted-36 h) HMS (microwave extracted) SBA-15 NH2-SBA-15 SA-SBA-15 K60 K60-mono K60-di K60-tetra SBA-15 SBA-15-mono SBA-15-di SBA-15-tetra SBA-15 SBA-15-N-C SBA-15-N-C-H SBA-15-N-E C-SBA-15-N C-SBA-15-NN C-SBA-15-NNN OSU-6-W OSU-6-W-TCSPBr-1 OSU-6-W-TCSPBr-2 MCM-41 NH2-MCM-41 NH2-MCM-41 SH-MCM-41 COONa-MCM-41 NH2-MCM-41 MSM-e 2N-MSM-e GN1 GN1 GN2 GN3 APS-MCM-41-A APS-MCM-41-B APS-MCM-41-C APS-SBA-15 APS-MCM-48 MPS-MCM-41-A MPS-MCM-41-B MPS-MCM-41-C MPS-SBA-15 MPS-MCM-48 MCM-41-AEDTC MCM-41 magMCM-41 NH2-magMCM-41 LDAPY-MCM-41
775 373 336 256 580.27 44.28 4.77 86.08 36.86 114.47 767 504 580 626 504 652 637 673 1000 814 416 279 294 1062 995 697 368 317 457 316 380 399 836 368 400 441 790 345 178 565 572 508 477 1283 1048 719 1070 772 774 990 679 750 672 513 575 444 363 360 87 428 411 357 662 162 818 706 467 390 632 1000 800 670 246
6.8 6.0 6.1 6.1
1.20 0.76 0.76 0.74 0.0689
e 1.41 2.59 4.00
7.2 5.2 5.2 5.5 5.6 3.8 3.4 7.6 2.9 7.6 e e 14.7 2.46 2.47 8.3 7.8 6.9 e e e e e e e e 9.1 7.2 6.4 9.0 8.2 8.3 8.1 5.11 4.30 3.65 3.09 2.82 2.92 3.02 2.85 2.92 2.9 2.0 4.72 4.03 3.95 3.58 e e e e e e e e e e 4.16 3.17 3.2 2.92 1.2
0.99 0.67 0.75 0.78 0.67 0.79 0.84 0.75 1.26
0% 8.7% 9.3% 8.2% 9.6% 10.2% 7.3% 1.77 e e e e e 0.22a 0.21a e 3.1b 3.2b e 0.52 0.37 0.25 e 0.63 0.46 0.31 e 1.9 2.8 3.1 0.9 1.9 1.8 e 2.55b 4.53b e 1.01 2.26 1.00 1.46 2.53 e 5.66c e 0.51 1.01 1.45 3.3 2.8 2.6 2.2 3.0 2.8 1.55 1.0 1.0 2.7 32a e e 2.8 0.30
                                           [105e107] [105e107] [105e107] [104e110]     [109,110]                     
0.66 0.45 1.16 1.15 1.12 1.49 0.77 0.53 0.81 0.40 0.46 0.47 1.13 0.6 0.6 0.59 1.44 0.58 0.28 1.02 0.87 0.78 0.74 1.24 0.94 0.76 e e e e e e 1.05 0.35 0.61 0.58 0.49 0.40 0.16 0.24 0.76 0.73 0.31 0.19 0.52 1.24 0.79 0.23 0.54 e e e 0.26
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Table 1 (continued ) Adsorbent
Surface area (m2g1)
Pore size (nm)
Pore volume (cm3g1)
NH2/SiO2/Fe3O4 SH-SBA-16-A SH-SBA-16-B SH-SBA-16-C SH-SBA-16-D SH-SBA-16-E Na50 Na25 Na5 NH2-SBA-15 TCPP/NH2-SBA-15 GC1.5N GC3N GC4.5N GC6N DMDDA-41A DDA-41A DMDDA-48A DDA-48A NNN/SiO2 NH2/SiO2/A NH2/SiO2/B NH2/SiO2/C S N-S NN-S NNN-S S-S MNN MGNN
73 427 320 198 188 186 869 881 649 908 453 314 360 339 347 63 89 124 59 889 336.3 128.4 421.9 687e712 62e65 120e220 245e314 875e888 663 614
e 4.7 4.1 3.9 3.7 3.6 4.3 4.8 4.8 8 7.73 4.5 4.7 5.0 4.2 e e e e 3.8 4.6 2.4 6.1 5.1 e e e 4.1 1.81 1.53
0.47 0.38 0.37 0.29 0.14 e e e 1.41 0.70 0.46 0.50 0.49 0.49 0.28 0.31 0.16 0.17 e 0.488 0.279 0.556 e e e e e e e
a b c
Functional group loading (mmolg1) or%
e 6.37 6.99 7.69 8.23 9.03 0.76c 0.94c 1.94c e e 2.34 2.10 1.85 1.65 e e e e 3.5 e e e 0 1.8 1.6 1.1 2.1 1.69 1.62
                             
Value given as wt %. Value given as group/nm2. Value given as N wt %.
group protonation under acidic conditions. At pH 3.2, GoubertRenaudin  reported no important differences between ordered and non-ordered adsorbents because the adsorption process was controlled by the slow complexation step. The case was completely different at pH 5.7, where adsorption on ordered SBA15 was much faster than that on amorphous silica. In this case, mass transfer to the active sites was the rate-limiting step instead of complexation step. Thus, the beneﬁt from fast mass transfer in uniform mesopore channels of ordered materials over the amorphous can be achieved only if the chemical reaction is fast. The functional groups do not always have to be directly grafted on mesoporous silica surfaces via a single step. Yantasee et al.  ﬁrst grafted MCM-41 with acetamide phosphonate silane (APHMCM-41) and propionamide phosphonate silane (PPH-MCM-41). Then, they converted the ester forms of functionalized materials to acid forms using trimethylsilyl iodide and water. They studied the two functionalized adsorbent as potential heavy metals adsorbent. Other two-steps grafting procedure was reported by Mureseanu et al. . They ﬁrst grafted 3-aminopropyl-triethoxysilane (APTS) followed by salicylaldehyde (SA). They investigated the efﬁciency of heavy metal ions adsorption on grafted solids and reported high selectivity and adsorption capacity for copper ions. They found also that functionalized SBA-15 is more efﬁcient than modiﬁed amorphous silica gel, due to higher surface area and ordered structure. Furthermore, they found that at maximum Cu2þ loading (0.92 mmolg1) the Cu/N molar ratio was 0.42, which is very close to the theoretical ratio of 0.5. Alothman and Apblett  prepared di-, tri-, or penta-amine functionalized mesoporous silica via earlier treatment of a mesoporous silica (OSU-6-W) with bromopropyl organic then displaced bromine atoms with di-, tri-, or penta-amine, respectively. They tested the efﬁciency of the adsorbents for heavy metal cations
adsorption from aqueous solutions and studied the effect of total nitrogen content on the adsorption capacities. Jiang et al.  used a multi steps process starting by grafting propyl amine onto SBA-15 followed by reaction with methylacrylate in the presence of methanol, then reaction with ethylenediamine, resulting in the ﬁrst generation NH2-SBA-15-G1. The second, third and fourth generations were similarly synthesized by repeating the same steps. Finally, they introduced EDTA functionality. The NH2-SBA-15-Gn-EDTA adsorbent showed high adsorption efﬁciency (>94%) for Cu2þ. Furthermore, they found that samples with EDTA were more efﬁcient than the other samples. For example, the copper removal efﬁciency using NH2-SBA-15-Gn did not exceed 75% compared to more than 94% using NH2-SBA-15-GnEDTA. Alothman and Apblett [106,107] modiﬁed hexagonal mesoporous silica (OSU-6-W) with 3-glycidoxypropyltrimethoxyesilane (GPTMS) functional groups through one and two steps postsynthesis procedures. First, they activated the material in the presence of trimethylamine then, reﬂuxed it with GPTMS leading to (OSU-6-W-GPTMS). The ﬁnal glycidoxypropyl functionalized mesoporous silica (OSU-6-W-GPTMS-2) was synthesized by repeating the silylation step. They found that the functional group loading on OSU-6-W-GPTMS-2 was higher than that of OSU-6-W-GPTMS with a maximum copper uptake of 5.3 mmolg1, forming a 1.7 Cu2þ: 1 ligand complex. Separation of ions mixture depends on choosing the proper functional group with higher afﬁnity towards one target ion than others. As reported by Liu et al. , thiol-functionalized SBA-15 exhibited a negligible tendency to form complex with copper, whereas the aminated ones showed exceptional binding ability. According to hard and soft acids and bases (HSAB) theory , soft metal ions are more likely to make stable complex with soft donor
E. Da'na / Microporous and Mesoporous Materials 247 (2017) 145e157
atoms. Hence, as sulfur is softer donor than nitrogen, the aminated adsorbents have a higher tendency to complex with the harder metal ion Cu2þ, whereas thiolated adsorbents display a higher complexation afﬁnity with the softer ion Pb2þ. Lam et al. [108,109] applied this strategy for the separation of silver and copper , and separation of Cr2O2 7 and copper  using MCM-41 grafted with suitable functional groups. For coppersilver separation , they compared thiolated (SHMCM-41) and aminated (NH2MCM-41) mesoporous silica. They found that SHMCM-41 selectively adsorbed silver ions while copper ions remained in the solution. Similarly, NH2MCM-41showed higher selectivity to adsorb copper from the same binary solution. However, the selectivity of NH2MCM-41 towards copper cations strongly depended on solution pH. Adsorbents with amine groups have the ability to adsorb speciﬁc cations or anions with different solutions pH. Neutral amine groups are efﬁcient cations adsorbent, while protonated amine groups can adsorb anions via electrostatic attraction [109,120]. Once the solution pH is lower than the pKb of the grafted amine (The pKb value corresponding to the RNHþ 3 /RNH2 couple is about 4) , the protonated form of amine (NHþ 3 MCM-41) loses its complexation ability for copper ions. Lam et al.  utilized this property to separate copper ions from þ Cr2O2 7 . They found that the selectivity of NH3 MCM-41 for 2 adsorption Cr2O7 reach 100% at low pH (<3.5) with high adsorption capacity reaching the maximum value of two amines for one Cr2O2 7 . Solution pH does not only affect the RNHþ 3 /RNH2 ratio, but it also affects the charge distribution on the adsorbent surface. Since the surface charge of functionalized-silica results from the protonation or deprotonation of hydroxyl and active surface functional groups, one expected effect of the point of zero charge (PZC), the solution pH at which the surface is neutral, is the dependence of metal ions adsorption on the solution pH. It is preferred to have low PZC since it leads to an efﬁcient cations adsorbent in a wide pH range as the surface will be negatively charged at pH > PZC. Furthermore, the pH value should be maintained lower than precipitation of metal hydroxide. Dimos et al.  reported the preparation of MCM-41 grafted with N-(2-aminoethyl)dithiocarbamate (AEDTC) with a PZC of 3.2, which is one of the lowest PZC reported. This suggests that in solution with pH value larger than 3.2, the ≡ХΟ species will formed and thus, the MCM-41 surface will have a negative charge. Chen et al.  reported a PZC of 3.5 for MCM-41 grafted with aminopropyl and obtained a maximum copper uptake of 1.3 mmolg1 at pH of 5. Jung et al.  reported PZC of 5.44 for FSM-16 whereas; Da'na and Sayari  obtained a PZC of 8.62 for amine-functionalized SBA-15 with a maximum capacity of 0.6 mmolg1 at pH of 6.5. Lam et al.  reported a PZC of 2.95 for NH2MCM-41 with maximum copper adsorption capacity of 0.3 mmolg1 at pH of 5. Solution pH effect on heavy metal ions adsorption was explored by many other researchers [31,35,37,40,41,43,100,103,105,108,125e130], and some of their results are presented in Fig. 1 and Fig. 2. While generally an increase in copper adsorption capacity with increasing pH was reported, Shiraishi et al.  reported that EDTA ligands attached to silica surface act effectively to form a complex with copper ions at lower pH and this complexation ability decreases with increasing pH and vanishes at pH > 4. Despite the fact that the chelating ability between copper and free EDTA increases with increasing pH. This is mainly because increasing the solution pH increases the negative charge of the silica surface. Subsequently, the Hþ of the carboxylic group of the grafted EDTA attaches to the negatively charged adsorbent surface suppressing the complexation between copper and EDTA. Another important application of adsorption of copper on functionalized mesoporous silica is the pre-concentration of copper
Fig. 1. Effect of pH on the adsorption capacity.
Fig. 2. Effect of pH on the removal efﬁciency.
for detection purposes. Ballesteros et al.  used a pyrimidinecontaining hybrid material (LDAPY-MCM-41) synthesized by reacting aminopropyl-grafted MCM-41 with 4-chloro-2,6diaminepyrimidine. The pyrimidine-derivated material had high selectivity toward copper, and the pre-concentration of copper from water was 100 folds. Similarly, Ganjali et al.  grafted MCM-41 with salophen and used it for pre-concentration of Cu2þ ions. They obtained a pre-concentration factor of 500 with a detection limit of 34 ngL1, and a maximum capacity of 29 mgg1. In batch applications, it is difﬁcult to recover functionalized mesoporous silica in its powder state. This problem can be solved by using adsorbent with magnetic properties. This method includes integration of magnetic materials into the silica to allow these agglomerates to be magnetically separated [35,37,44,45,51,54]. Chen et al.  grafted propylamine on MCM-41 in the presence of 10 wt% iron oxide to introduce magnetic properties to the adsorbent. Similarly, Lin et al.  synthesized amine-grafted silica magnetite (NH2/SiO2/Fe3O4) to use it for adsorbing anions or cations by controlling the aqueous solution pH. Kim et al.  grafted 3-(2-aminoethylamino)-propyltrimethoxysilane on the surface of HMS shell containing magnetite-core structure. They indicated that
E. Da'na / Microporous and Mesoporous Materials 247 (2017) 145e157
Cu2þ adsorption on the developed adsorbent ﬁts with the Langmuir isotherm with qm value of 0.5 mmolg1 and Cu: NH2 molar ratio of 0.049 (Table 2). Simonescu et al.  recently synthesized Fe3O4 magnetite with o-Vanillin functionalized mesoporous silica and found it a promising adsorbent for retention of Pb(II) from polluted water. The adsorbent reached 80e90% of the maximum adsorption capacity (155.71 mg/g) in 60 min. .SimilarlyYuan et al.  developed mesoporous silica with Fe3O4 magnetic core that exhibits large pore size, high adsorption capacity and fast adsorption kinetics . Not only the choice of the support and the active groups affect the performance and the selectivity of the adsorbent prepared by grafting, the type of the counter anions in the solution affected metal ions adsorption on NH2MCM-41, with enhanced efﬁciency in the presence of sulfate ions [106,107]. Lam et al.  investigated the anion effect on copper adsorption on NH2-MCM-41 using Cu(NO3)2 and CuSO4 solutions. They found that copper cations adsorbed faster and more efﬁciently in the presence of SO2 4 anions compared to NO 3 , with 70% of the adsorption sites accessible to Cu2þ adsorption, while the remaining 30% of the functional groups were accessible only in the presence of SO2 4 . These sites are either energetically unfavorable or inaccessible, such as H-bonded amines with the unreacted surface hydroxyls or insulated amine groups. 50% extra copper (i.e., 1.33 mmolg1) was adsorbed from the CuSO4 solution since SO2 4 may stabilize 1:1 amine: copper complexes. They found that the Freundlich isotherm ﬁts the Cu2þ adsorption from Cu(NO3)2 and CuSO4 better than the Langmuir (Table 2). Jeong et al.  grafted tetrakis (4-carboxyphenyl)porphyrin (TCPP) on SBA-15 silica, and found it to have better adsorption performance than NH2-SBA-15 silica with maximum adsorption capacity around 13 mmolg1.
Another limitation of introducing functional groups by grafting is that packing is usually limited by the amount of Si-OH anchored to the surface . To illustrate these limitations, Aguado et al.  followed different template removal procedures prior to the modiﬁcation of SBA-15 with APTES: calcination (C), extraction (E) and calcination-hydration (C-H). The purpose of calcinationhydration was to increase the amount of OH groups offered for grafting, leading to higher loadings of functional groups. They found that the amine content reduced according to the series: GSBA-15-N-E > G-SBA-15-N-C-H > G-SBA-15-N-C, due to surface dehydration during calcination, leading to a lower amine loading capacity. They also showed that the hydration step should be performed carefully to prevent water accumulation on the surface, which can support heterogeneous spreading of organic groups, and thus pore blockage. Table 2 provides evidence that the highest copper adsorption capacity on propyl amine-grafted SBA-15 is less than the amine loading theoretical ratio, because two or more amine groups are required to make stabilized Cu-amine complex. Furthermore, Table 2 shows that the Cu2þ uptake on calcined GSBA-15-N-C is less than G-SBA-15-N-E and G-SBA-15-N-C-H because silica prepared after extraction and calcination-hydration exhibit higher amine loadings (around 3 against 1.9 mmolg1). Nevertheless, monoamine-grafted SBA-15 adsorption capacities are mostly less than capacities obtained for other mesoporous silica [90,110,113,124,131]. They also showed that the copper adsorption on sample functionalized with diamine (G-SBA-15-NN-E) is signiﬁcantly improved compared to silica functionalized with monoamine, since the existence of two amine groups in the organic molecule improves the metal uptake. Similarly, Manu et al.  controlled the hydroxyl group surface density by controlling calcination temperatures (150, 300, 450, and 600 C). Then they
Table 2 Freundlich and Langmuir constants for copper adsorption on different adsorbents. Adsorbent
APTS-SBA-15-AB APTS-SBA-15-AB APTS-SBA-15-AB G-SBA-15-NN-E G-SBA-15-N-C-E G-SBA-15-N-E G-SBA-15-N-C SH-MCM-41 SH-MCM-41 NH2-MCM-41 NH2-MCM-41 NH2-MCM-41 NH2-MCM-41 COONa-MCM-41 NH2-MCM-41 NH2-MCM-41 2N-MSM-e GN1 GN2 GN3 EDA-SAMMS NH2/SiO2/Fe3O4 SH-SBA-16-C GC1.5N GC3N GC4.5N GC6N DDA-41A DMDDA-48A DDA-48A DMDDA-41A a b
Value given as mgg1. Value given as Lmg1.
293 K 313 K 333 K e e e e Cu2þ Agþ Cu2þ Agþ Cu2þ/pH ¼ 2 Cr2O2 7 /pH ¼ 2 Cu2þ/pH ¼ 5 Cu(NO3)2 CuSO4 e e e e e e e e e e e e e e e
0.43 0.73 1.15 e e e e 0.0259 0.9694 0.2453 0.1077 0.00 0.84 0.22 0.73 1.27 e 1.416 5.77 17.132 e 1.53 0.5237 e e e e 1.06 0.95 0.99 1.08
4.7 5.14 3.98 e e e e 0.01 0.12 0.05 0.06 0.00 0.30 0.08 0.09 0.16 e 1.87 3.82 4.67 e 2.63 6.24 e e e e 1.90 1.81 1.88 1.85
0.98 0.97 0.96 e e e e e e e e e e e e e e 0.97 0.93 0.94 e 0.89 0.86 e e e e 0.97 0.96 0.96 0.98
0.54 0.88 1.34 0.83 0.39 0.35 0.24 e e e e e e e 0.76 1.33 0.50 0.485 0.50 0.85 26.9a 10.41a 36.42a 1.53 1.43 1.14 0.9 3.93 4.08 3.51 4.39
10.62 17.10 20.34 e e e e e e e e e e e 290 120 1.1 0.074 0.020 0.002 26.6b 0.037b 49.27 0.0012 0.0018 0.0202 0.041 0.38 0.35 0.39 0.31
0.96 0.96 0.97 0.99 0.99 0.99 0.99 e e e e e e e e e e 0.99 0.99 0.99 e 0.99 0.98 0.99 0.99 0.99 0.99 0.99 0.99 0.99 0.99
                              
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functionalized these materials with APTES and used them for the adsorption of Cu2þ ions. They indicated that the thermal treatment of the silica gel affects the density of surface hydroxyl groups and thus the NH2 density after grafting, which decreased from 2.34 to 1.65 mmolg1 when the temperature increased from 150 to 600 C as shown in Table 1. They found that the Cu2þ adsorption capacity calculated from Sips model decreased from 1.56 to 0.95 mmolg1, with the reduction in amine surface density from 2.34 to 1.65 mmolg1. Kim et al.  removed the surfactant from hexagonal mesoporous silica by solvent extraction and by calcination. They demonstrated that, template removal by calcination was more efﬁcient than solvent extraction, with surface area and pore volume of the calcined samples (1003 m2g-1 and 1.05 cm3g-1) larger than that of the extracted ones (672 m2g-1 and 0.75 cm3g-1). However, surfactant removal at high calcination temperatures leads to severe dehydration of surface hydroxyl groups on the pore walls, which reduced the functional group density that could be grafted and consequently, copper uptake capacity. Immobilization of cyclic polyamines, which result in stable complexes with a large number of metal ions , has also been used to modify silica surfaces. Goubert-Renaudin et al.  grafted silylated cyclam molecules with one, two, or four silyl groups onto both ordered mesoporous silica (SBA-15) and amorphous silica gel via one, two, or four arms and explored their stability and reactivity towards copper ions. They always observed incomplete copper uptake even in excess of functional groups compared to Cu2þ solution concentration, suggesting that Cu2þ is more stable in aqueous state than in a complex state. Furthermore, the number of ligands attaching cyclam functunalities to the silica surface was found to affect the copper adsorption on these adsorbents, with much higher capacities obtained by minimizing the number of arms. Similarly, multiarm binding resulted in better chemical resistance toward degradation . Some applications of mesoporous silica required large pore sizes, which could be achieved by postsynthesis hydrothermal treatment of silica in a solution containing suitable organic expander. In addition to increasing the pore size, this treatment introduces organic functionality to the inorganic framework. Sayari et al.  considered adsorption of metal cations such as Cu2þ on the pore-expanded and aminated mesoporous silica MCM-41. This material exhibited high-capacity (1.67 mmol Cu2þ/g) and fast adsorption, because of its open pore structure and suitable surface properties. Similarly, Benhamou et al.  inspected the adsorption performance of the pore-expanded MCM-41 and MCM-48 toward Cu2þ. They expanded silica by post-synthesis treatment with N-N dimethyldodecylamine (DMDDA) and dodecylamine (DDA) and found the expanded materials to be fast adsorbents. As shown in Table 2, they also found that the pore-expanded MCM-41 had higher adsorption capacity than the pore-expanded MCM-48 and that MCM-41 and MCM-48 pore-expanded with DMDDA were more efﬁcient than those pore-expanded with DDA indicating that the adsorption capacity depends on both, support and kind of amine used for pore expansion. Although grafting route has been signiﬁcantly developed, it is known that homogeneous spreading of functional groups is not frequently succeeded by this route [116,135]. Ordered mesoporous silica offers three diverse areas for reaction: the external particle surface, the internal pore surface, and the surface near the pore mouth. Grafting results in accumulation of organic groups around the pore opening leading to some pore blocking, which happens more readily in small pores silica than in those with large pores. In addition to pores blockage, grafting has the following drawbacks: (i) reducing the pore size leading to enhanced diffusion resistance within the pores. (ii) Concentration and distribution of organic functionality is controlled by the surface hydroxyl group density
and by their accessibility, precursor reactivity, and diffusion limitations, (iii) and most important, the complexity of the procedure as it often requires several steps to achieve the ﬁnal material . 3.2. Adsorption of heavy metal ions on mesoporous silica prepared by co-condensation In the co-condensation route, organic functionalities are condensed together with silica source such as tetramethylorthosilicate (TMOS) and tetraethylorthosilicate (TEOS) . Therefore, organically functionalized mesoporous silica can be obtained directly in one-step. Thus, reducing the number of synthesis steps and time. It is assumed that direct synthesis of organically functionalized mesoporous silica leads to homogeneously distributed groups. While, grafting route forms silica with higher thermal stability of the networks and more order structure than cocondensation . It is preferred to have homogeneous distribution of the organic functionalities to achieve high surface coverage without altering the diffusion of molecules within the pores. To investigate the differences in distribution obtained by the two routes, Lim and Stein  introduced vinyl-functionality to MCM-41 via cocondensation and grafting. In co condensation of v-MCM-41, higher vinyl density with uniform distribution was achieved, while higher hydrothermal stability of v-MCM-41 was achieved via grafting. Furthermore, they found that vinyl groups in a grafted sample were mostly positioned on exterior surfaces and around pores openings. They concluded a heterogeneous distribution of the vinyl groups on the MCM-41 surface. On the contrary, uniform distribution of vinyl groups within pores channels and on external surface was detected in v-MCM-41 prepared by co-condensation. Similar observations were reported by Yokoi et al.  for mono(N), di- (NN), and tri-amine (NNN) MCM-41 prepared by cocondensation and grafting. Their results demonstrated that, the adsorption capacities of (NNN) MCM-41 prepared by cocondensation increased when the amount of amine groups integrated to the surface increase. Conversely, for the (NNN) MCM-41 prepared by grafting increasing amine surface density resulted in lower adsorption capacity suggesting that the amine groups distribution on the silica by the two methods are clearly different. It is likely that increasing functional groups density while maintaining open pore structure would result in a higher performance. However, organic groups, which may interfere with the silica condensation and disturb the micellar structure, have to be avoided in the co-condensation process . Furthermore, deterioration of the periodic order may occur if the loading of functional groups exceeds a certain limit. Chong et al.  investigated the effect of the nature and the quantity of organosilanes initial concentration in the synthesis mixtures of TEOS and nonionic triblock co-polymer P123 and acidic medium. They found that changing synthesis conditions leads to different ranks of mesostructure disorders after functionalization, depending on the organosilanes used and its quantity in the reaction mixture. Organosilanes molecular size and shape and its chemical reactivity in acidic medium affect the reaction of P123 with silica and P123 micellation and thus ^ te  preaffect the structural properties. Walcarius and Delaco pared a wide range of mercaptopropyl-functionalized MCM-41 (MPTMS-MCM-41) adsorbents by changing the ratio of MPTMS/ TEOS. They compared between three categories of adsorbents: (i) MCM-41 with an ordered structure obtained by using MPTMS concentration < 10%. All functional groups in this sample were accessible however; there was some mass transfer limitations to diffusing deep in the mesopore channels to reach the active sites located there. (ii) Wormhole-like with cylindrical mesopores and less-ordered structures, 100% accessibility of the binding sites was
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possible by using less than 30% MPTMS. Using higher MPTMS concentration resulted in higher surface loading leading to lower accessibility because of pore blockage and increasing surface hydrophobicity (iii) Amorphous materials with maximum functional group loadings and very high resistance to mass transfer rates and limited binding sites accessibility. Aguado et al. [87,88] prepared thiolated mesoporous silica via co-condensing MPTMS and TEOS with P123 as a template. They studied the effect of changing MPTMS/(MPTMS þ TEOS) ratio in the range 5e50% on both the textural properties and metal ions adsorption capacity. To maintain the order mesoporous structure in the synthesized material with high propylthiol loading it was necessary to minimize the amount of TEOS. When MPTMS molar ratios up to 10% were used, samples with a good structural order were obtained, while materials obtained with larger MPTMS amounts showed less order. Similarly, Xue and Li  synthesized thiol-functionalized SBA-16 mesoporous silica using onepot method by changing the TEOS/MPTMS molar ratio. Their results (Table 1) showed that maintaining TEOS/MPTMS molar ratio between 3 and 4 was a factor to get high structural order and high Cu2þ uptake with a maximum of 36.38 mgg1 in the pH range of 5e6. Markowitz et al.  synthesized functionalized-SBA-15 mesoporous silicates via co-condensing TEOS and ethyl-(ETES), carboxylate-(CTES), and ethylenediamine-triacetic acid (EDTA). They showed that, preparing samples with a maximum of 5% silane resulted in ordered mesoporous materials. However, when silane quantity reach 20% a signiﬁcant structural disorder of the silica was observed. High adsorption capacity of Cu2þ was observed only for the EDTA-SBA-15, with Cu/N around 0.15, which is less than the 1:1 theoretical ratio for Cu2þ and EDTA, suggesting incomplete EDTA groups binding with Cu2þ. Burleigh et al.  synthesized ordered mesoporous organosilicas by co-condensation of bis(triethoxysilyl) ethane and N-(2-aminoethyl)-3-aminopropyl-trimethoxy silane (AAPTS). They found that by increasing AAPTS, both surface area and the pore volumes decrease, without signiﬁcant change in the pore diameter (Table 1). Thus, more than 70% of the AAPTS were available for copper adsorption. Furthermore, as the amount of functional silane increases from 0.17 to 0.87 mmolg1, the adsorption sites accessibility decreases from 80 to 70%. Algarra et al.  investigated the efﬁciency of metal ions adsorption on aminopropyl-functionalized MCM-41 synthesized by cocondensing TEOS and APTES with different TEOS/APTES molar ratios of 5, 25 and 50. They showed that the copper adsorption capacity could be increased by decreasing the TEOS: APTES ratio with maximum copper adsorption capacities of 1.28, 0.75 and 0.54 mmolg1, respectively. Dey et al.  synthesized adsorbent by co-condensation of TEOS and the organosilane N-[3-(trimethoxysilyl)propyl]diethylenetriamine and evaluated its adsorption performance. They obtained a maximum copper adsorption capacity of 2.2 mmolg1 with copper/ligand molar ratio of 1:2, considering 1.2 mmol of functional groups per gram adsorbent. Yang et al.  used CTAB and TMAOH as a template and ethanol as a solvent for synthesis of mesoporous adsorbents. When a mixture of CTAB and TMAOH was used as a structure directing agent, the resulted sample denoted in Table 1 as (C) indicated an increase in pore size, pore volume, and surface area. This is mainly because TMAOH has accelerated the silica condensation and thus, reinforced the physical structure. On the other hand, when only CTAB was used as a template, the resulted sample (B) showed smaller pore size, pore volume and surface area compared to the sample prepared with P123 (A). This is mainly because P123 has a longer chain than CTAB. The three samples were used to adsorb metal cations and the best adsorbent among the three was sample (C), with maximum copper uptake of 0.39 mmolg1.
Bois et al.  synthesized functionalized mesoporous silica with aminopropyl (N-S), amino-ethylamino]propyl (NN-S), (2aminoethylamino)-ethylamino]propyl (NNN-S), and mercaptopropyl (S-S) groups using dodecylamine as a template. As shown in Table 1, speciﬁc surface area of N-S sample was lower than that of NN-S and NNN-S. In contrast, thiolated sample (S-S) showed an increase in the speciﬁc surface area. They also indicated that the aminated silica was less ordered and less porous than the thiolated samples. They related that to the development of hydrogen bonding between amine and silanol groups. According to Bois et al.  aminopropyl is expected to strongly interact with silica surface, SiO … þNH3-, creating a cyclic structure that occupy more space within the pores. Possibility of forming this cyclic structures decreased when longer amine chains (NN- and NNN-S) is used. On the other hand, thiolated materials, do not form hydrogen bonds between thiol and silica surface and thus, silica structure is conserved. Maximum copper ion uptake by NN-S and NNN-S samples were 0.5 mmolg1 with Cu/N ratios of 0.3 and of 0.4 respectively, indicating that copper ions were not able to reach all amine groups. Adsorbent morphology is another important factor affecting adsorption. In this area, Nasreen et al.  studied the effect of functional groups introduced to MSU-H silica by co-condensation on adsorption performance and surface morphology. The surface morphology and textural properties of these functionalized MSU-H (AM, GM, MM, VM, and PM) were different according to the functional groups in the channels. Thus, affected the adsorption efﬁciency of the adsorbent with removal efﬁciency within the range of 62e96%, 38e99%, 68e99%, 79e93%, and 67e98% on AM, GM, MM, VM, and PM, respectively. Lee et al.  prepared aminated and thiolated SBA-15 with different pore channel lengths and different morphologies. Their results showed that initial adsorption rate was rapid, and then started to slow down. Furthermore, they found that the platelet morphology rate was faster than that of ﬁber-like and rod-like morphologies, and that the Pb2þ equilibrium adsorption capacity on platelet morphology was greater than that of the other morphologies and its pore size was the largest among all as shown in Table 1. This is mainly because platelet morphology has shorter channel length than those of rod-like and ﬁbrous SBA-15. Consequently, the diffusion distance and the time ions need to diffuse from bulk solution to the internal functional groups were reduced. In addition, the larger pore size minimize pore blockage by functional groups attached to the pore walls. Fakhfakh et al.  prepared ethylenediamine functionalized mesoporous silica with groups via co-condensation, and studied the inﬂuence of (i) presence or absence of structure directing agent (template), (ii) the synthesis temperature, and (iii) the acid used (HCl, CH3COOH, or C2H5COOH). The best textural properties were obtained in the absence of surfactant and using carboxylic acids dissolved in propanol. The copper adsorption capacity was 0.53e0.90 mmolg1 for silica prepared with HCl or acetic acid as catalysts, while the silica prepared with propionic acid showed a lower uptake of 0.41 and 0.13 mmolg1 for the solid prepared at 35 and 60 C, respectively. Sales et al.  synthesized ethylenediamine modiﬁed mesoporous silica via co-condensation of TEOS with N-[3-(trimethoxysilyl)-propyl]-ethylenediamine and ethylenediamine incorporated into 3-glycidoxypropyltrimethoxysilane to produce MNN and MGNN with structural properties shown in Table 1. They found that MGNN and MNN had similar maximum copper adsorption capacity of 1.4 mmolg1. Aguado et al.  synthesized SBA-15 functionalized with mono- (C-SBA-15-N), di- (C-SBA-15-NN) or tri- (C-SBA-15-NNN) amine functional groups by co-condensation. Their materials showed negligible metal adsorption capacity even with the high nitrogen content and suitable textural properties as shown in
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Table 3 Copper adsorbents regeneration conditions. Adsorbent
Functionality loss %
Capacity loss %
APTS-SBA-15-AB APTS-SBA-15-AB SBA-15(SH) SBA-15(NH2) MCM-41(SH) MCM-41 (NH2) SA-SBA-15 OSU-6-W-TCSPBr-1 NH2-MCM-41 COONa-MCM-41 2N-MSM-e Silica ED OSU-6-W-TCSPBr-1 APH-MCM-41 EDTA-G4-PAMAM-SBA-15 LDAPY-MCM-41 NH2/SiO2/Fe3O4 SH-SBA-16-C TCPP -SBA-15 NH2/SiO2/C
0.1 M EDTA 0.1 M HCl/0.1 M NaHCO3 HCl HCl HCl HCl 0.3 M HCl/0.3 M NaHCO3 2.0 M HCl 5.2 M HCl 5.2 M HCl/NaHCO3 0.1 M HCl 1 M HCl 2.0 M HCl 20 wt% HCl 0.1 M EDTA 1 M HCl 0.1 M HNO3 1 M HCl 0.2 M HCl 1 M HCl
10 10 3 3 3 3 4 4 1 1 1 1 3 10 1 3 e 7 3 8
<10 <10 e e e e e e e e 5.4 0 e e e e e 11.6 e e
10 40 40 40 65 65 3.3 30 0 0 e 0 24 0 5e7 0 e 9.9 2.3 9.1
       [105e107]     [106,108]       
Table 1. Later on, Da'na and Sayari [90,91] suggested several reasons for the negligible adsorption capacity of this material including: (i) Poor extraction of P123 template from the pores. (ii) Since SBA-15 is prepared in acidic conditions, this caused amine groups protonation , thus diminishing their availability for adsorption, and therefore, basic treatment is needed to recover neutral amine groups. (iii) Zero point charge (ZPC) of the aminated SBA-15 synthesized via co-condensation was reported around 8.62 implying that as the solution pH get lower than 8.62, the surface hydroxyl groups will be protonated (^SiOHþ 2 ) and the aminated SBA-15-AB surface will be positively charged. This implies that in a wide range of pH (pH < 8.62) the material will not adsorb copper effectively. They found that changing solution pH from 4 to 6.5 resulted in gradual increase in Cu2þ adsorption efﬁciency with zero Cu/N ratio at pH of 4.0. Generally alkylamines pKb value is about 4 . Therefore, at this pH, most of the amine groups will be protonated with no ability to complex with cations. In addition, the surface becomes positively charged at low pH leading to high repulsion between cations and the binding sites. Therefore, the approach of copper cations to the active adsorption sites will be inhibited. (iv) It is possible that there is some hydrogen bonding between amine groups and hydroxyl groups on the surface reducing their adsorption efﬁciency . 4. Adsorbents regeneration For commercial applications of an adsorption system it is important for the adsorbent to be structurally stable with steady metal adsorption capacity over a multi adsorptionedesorption cycles. In addition to adsorbents recycling, desorption gives important information about the reversibility of the adsorption process. Many researchers reported recycling of the adsorbents they used mainly by washing with HCl solution [89,90,100,102,105e109,111,112,118,125,128,130,142], HNO3 solution [103,127] or treatment with complexing agent such as EDTA [64,95] to elute the metal ions and their results are summarized in Table 3. After metal desorption with acids, the neutral amine should be recovered using NaHCO3 solution [90,102,109]. Da'na and Sayari  showed that EDTA was more powerful than HCl/NaHCO3 for regeneration of amine loaded silica since it reacts with Cu2þ forming a very stable octahedral complex . Acid treatment results in the protonation of the adsorbent surface, which increased positive surface charge density [90,99,123]. This might result in re-
adsorption of negative-ligand enclosed Cu2þ species as reported by Wambu et al. . They showed that such re-adsorption may be followed by surface precipitation that would immobilize readsorbed copper on the adsorbent. Another possible reason for loss of adsorption capacity is structural instability of the adsorbent, which upon regeneration treatment may lose its structural order and adsorption sites accessibility within the pores. Nitrogen adsorption data obtained by Liu et al.  are in good agreement with this argument. They indicated that the surface area and pore volume for SBA-15(SH) and SBA-15(NH2) adsorbents decreased only slightly (5%), however the pore size did not change. While a huge reduction in surface area and pore volume of MCM-41(SH) and MCM-41(NH2) adsorbents were observed, the surface area and pore volume of the regenerated MCM-41(SH) are 465 (665) m2g1 and 0.41 (0.61) cm3g1; for MCM-41(NH2), 282 (460) m2g1 and 0.26 (0.41) cm3g1. This result agreed perfectly with the previously reported results that SBA-15 is more stable than MCM-41  and may explain the lower regeneration performance of MCM-41 based adsorbent compared to SBA-15 as presented in Table 3. In some circumstances, the adsorption capacity increases after the ﬁrst regeneration cycle especially for samples with high loadings of functional groups. This is mainly because of high steric hindrance effect and the lower accessibility of adsorption sites [102,112]. After regeneration, the functional groups content may decrease. Therefore, the adsorption sites become more accessible to target ions than in the starting samples. Thus, the adsorption capacity increases as reported by Zhang et al. . 5. Conclusions Organically-modiﬁed mesoporous silica is considered very promising adsorbent for the retention of heavy metal ions. This is primarily related to the attractive features such as structure order, large surface area, and open porous structure to facilitate access to the functional groups. Thus, it has attracted researcher's attention to be examined for heavy metal ions adsorption. Depending on the target ions to be adsorbed, modiﬁcations of organic-inorganic hybrid materials were performed to ensure strong binding and high selectivity toward target pollutant. Despite the numerous investigations and interesting results presented earlier, mesoporous silica functionalization is still not applicable in real environments. In fact, after years of research in
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