Silica-based mesoporous materials; emerging designer adsorbents for aqueous pollutants removal and water treatment

Microporous and Mesoporous Materials 266 (2018) 252–267

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Silica-based mesoporous materials; emerging designer adsorbents for aqueous pollutants removal and water treatment

T

Paul N.E. Diagboya∗, Ezekiel D. Dikio Department of Chemistry, Vaal University of Technology, Vanderbijlpark, Gauteng, South Africa

A R T I C LE I N FO

A B S T R A C T

Keywords: Silica Adsorbents Functionalization Water pollutants Water treatment

This is a review of recent literature on pristine and specifically functionalized mesoporous silica-based materials or ‘designer silicates’ used as adsorbents for aqueous pollutants removal and water treatment. Ordered mesoporous silicates, their synthesis, surfactants removal, and preparation of the designer silicates were briefly introduced before discussing their applications in the removal of inorganic and organic pollutants. Designer silicates, such as the nitrogen/thiol-containing, the magnetics, and the composites, are highlighted with their various applications for the removal of toxic metal cations, anionic species, dyes, pesticides, industrial organics, pharmaceuticals and other emerging pollutants. These applications were briefly discussed indicating salient features and using illustrating figures where possible.

1. Introduction Pollution of water sources by organic and inorganic chemical toxins is a priority concern worldwide. The magnitude of this concern is enormous, and remediation of contaminated water sources is seemingly intractable because of the diverse nature of pollution sources [1–9]. Most current water treatment technologies are either expensive or lack the effectiveness for complete toxins removal; hence, the search for more efficient technologies is a continuing process. Several techniques have been employed for pollutants removal from water, such as filtration, chemical precipitation, ion exchange, reverse osmosis, electrochemical treatment, membrane technology, evaporation recovery, photocatalysis, solvent extraction, and adsorption. However, most of these are limited by techno-economic and environmental considerations [2–4,10–17]. Advances in materials science have triggered large interests in specifically-designed adsorbents for water treatments. The concept of a synergistic combination of two or more molecules into a larger one has become a very successful approach in materials science for achieving highly efficient and specific goals in materials' property, such as adding new structural and functional properties superior to those of the individual pure components [1,2,8,14,18–24]. This kind of focus has resulted in the discovery of a plethora of designer adsorbents such as the mesoporous silica-based adsorbents [2,14,18,19,23]. Ordered mesoporous silica is eliciting a growing interest in water treatment chemistry due to its various remarkable properties. These properties include high specific surface area, large pore–size, chemical ∗

inertness, an abundant repertory of surface functional groups that can be tailored for excellent selectivity towards specific pollutant, good thermal stability and low cost of manufacture [18,25]. Highly porous adsorbents, which are of particular demand in aqueous separations involving large molecules, can be synthesized from ordered mesoporous silica in other to include a wide variety of functionally active chemical components by exploiting the structure-directing functions of electrostatic, hydrogen-bonding, and van-der-Waals interactions associated with amphiphilic molecules [26–28]. Since mesoporous silica meet most of the criteria for selection of adsorbents such as high specific surface area, large pore–size, chemical inertness and easy addition of abundant repertory of surface functional groups, pristine and functionalized silica [26–28] have been successfully synthesized and studied for adsorption of both organic and inorganic aqueous pollutants [29–34] Many recent references demonstrate the versatility of ordered mesoporous silica-based adsorbents. The objectives here are to focus on the advances in ordered mesoporous silica-based adsorbents used for aqueous inorganic and organic pollutants removal. Brief highlights of the types, synthesis routes and modifications/functionalization of the ordered mesoporous silica-based adsorbents are given. This is followed by an overview of several studies involved with the application of ordered mesoporous silica-based adsorbents in the specific adsorption of inorganic and organic pollutants from aqueous solutions. In summary, the review highlights the recent reports (mainly from 2009 to 2017) on the use of ordered mesoporous silica-based adsorbents for the removal of inorganic and organic pollutants from aqueous solutions, and presents a brief description of these

Corresponding author. E-mail address: [email protected] (P.N.E. Diagboya).

https://doi.org/10.1016/j.micromeso.2018.03.008 Received 20 November 2017; Received in revised form 3 March 2018; Accepted 10 March 2018 Available online 10 March 2018 1387-1811/ © 2018 Elsevier Inc. All rights reserved.

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Fig. 1. Morphologies of (a) MCM-41, (b) MCM-48, (c) MCM-50. Reproduced from Ref. [39] with permission from The Royal Society of Chemistry.

Usually, a single surfactant (such as the Pluronic-type surfactant; poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide)Pluronic P123 (EO20PO70EO20) or Pluronic F127, (EO106PO60EO106), and cetyltrimethylammonium bromide (CTMAB)) is used as the organic structure directing agent or template for the synthesis of surfactanttemplated silica materials [18,41–44]. However, binary surfactants of different molecular weights has been used resulting in the formation of dual-mesoporous silica materials [41].

adsorbents, advantages, and limitations; and reports on regeneration methods where available. To give insight into the adsorbents effectiveness, reported adsorption capacities are noted. 2. Ordered mesoporous silicates This class of mesoporous materials consists of extensive inorganic or inorganic–organic hybrid arrays with the exceptional long-range order, highly tunable textural and surface properties and controlled pore size and geometry [35]. These properties, in addition to their usually large surface areas, make them excellent adsorbent materials providing higher possibilities for adsorbent–adsorbate interactions. It is worthy to state here that silica framework is not crystalline but amorphous; it is the mesopores that are arranged periodically within the structure. Mesoporous silicates include the Mobile Composition of Matter (MCM) family (such as the MCM-41, MCM-48, and MCM-50; Fig. 1), the Santa Barbara Amorphous (SBA) family (such as the SBA-1, SBA-2, SBA-6, SBA-8, SBA-11, SBA-12, SBA-15, and SBA-16), KIT-5 and KIT-6 [36–38]. The MCM, like most silicates, are classified according to the order of the pores: hexagonal, cubic or mesolamellar phases for the MCM-41, MCM-48, and MCM-50, respectively.

2.2. Post-synthesis removal of surfactants To access the pores within the ‘as-synthesized’ mesoporous silica composites, it is necessary to remove the surfactants used in the selfassembly. Mesoporous silica synthesized via the surfactant-templated sol–gel reaction is carefully packed and shrink significantly and irreversibly when directly dried from solution thus losing its characteristics [41]. Thus, it is necessary to be cautious of template removal methods. Calcination is mostly used for the removal of surfactant from within the pores [45–49] though in some cases solvent extraction is preferred to calcination [18,42,50] and microwave digestion using UV [51]. The typical calcination temperature range is 550–800 °C in flowing air or nitrogen, while the optimized solvent extraction process is carried out by refluxing for 24 h using a high ratio ethanol to water mixture. Without high-temperature calcination, the extent of the silane modification can be relatively high, thus making the surface properties to be easily adjusted.

2.1. Synthesis of ordered mesoporous silicates Mesoporous silicates can be synthesized under varying range of pH conditions (from strongly acidic to highly basic – Fig. 2a) and temperatures (from below room temperature to ≈150 °C) using different templates or surfactants (cationic, anionic, neutral, zwitterionic, etc.) [36]. Under appropriate synthesis condition, the silicate's surface area, pore size, and volume can be controlled from the mesopore dimensions to macropore. Typically, mesoporous silica synthesis comprises the addition of a silicate source to the surface of ordered surfactant micelles in an aqueous solution to produce the ‘as-synthesized’ mesoporous silica. The discovery of template-assisted mesoporous silica synthesis using surfactants has led to the synthesis of mesoporous silica with high surface areas, tunable pore sizes, large pore volumes and a plethora of different kinds of functionalization [36,40]. Mesoporous silica materials are synthesized under conditions where surfactant–silica assembly occurs simultaneously with condensation of the inorganic species to produce the composites (Fig. 2b). The dimensions and morphologies of the resulting materials are dependent on the sol–gel chemistry conditions such as temperature, water content, pH, characteristics of surfactants or copolymers, and the concentrations and the sources of silica used. Hence, it is possible to design the silica size, mesostructure, and morphology by controlling of these parameters [41]. Ideally, mesoporous silica to be applied as an adsorbent for aqueous pollutant removal should possess large surface area, large pore size, and pore volume. These external and internal properties provide large surfaces for independent functionalization, as well as allow for a small amount of the adsorbent to have contact with a large volume of the aqueous solution and consequently a significant amount of the pollutant, resulting in possibly high removal efficiency.

2.3. Functionalization Though remarkable properties of ordered mesoporous silicates are highlighted for water treatment purposes, these properties are sometimes not accessible. A major challenge with mesoporous silicate upon template removal is its hydrophilicity due to the high surface silanol density (Fig. 3), consequently resulting in low adsorption for aqueous pollutants especially the hydrophobic ones. Functionalization, or attachment of highly active groups to the surface of the silicate material, has become a highly successful method of accessing these remarkable properties of ordered mesoporous silicates for water treatment purposes. This has resulted in a plethora of functional designer silicates with high efficiency and excellent selectivity towards specific aqueous pollutants. Typically, an organic functionality can be introduced post-synthesis by grafting or chemical attachment onto the silanol groups in the mesoporous silica or by co-condensation during the template-directed synthesis of the mesoporous material. The chemical attachment of organic functionalities onto the silanol groups changes the silica wall polarity and introduces the desired functionality which enhances the surface adsorption properties (Fig. 3). One advantage of post-synthesis chemical attachment is that the mesoporous structure is usually retained, though this depends on the size and loading of organic functionality added, as these could result in pore blockage. Post-synthesis functionalization of the silicate surface may be achieved by the 253

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Fig. 2. (a) The inorganic silica species (tetraethoxysilanes) can hydrolyze under acidic or basic conditions producing triethoxysilanol which condenses as shown. Hydrolysis occurs by the nucleophilic attack in water on the silicon atom on tetraethoxysilanes which produces alcohol (ROH), which is not shown in the reaction. (b) schematic diagram of surfactant-templated mesoporous silica showing typical synthesis route for MCM-41 or SBA-15. Reproduced from Ref. [39] with permission from The Royal Society of Chemistry. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

undesired polymerization reactions. With co-condensation, the final silica morphology can be controlled, and the above disadvantages of post-synthesis by chemical attachment are eliminated. However, the mesoporosity could be distorted if synthesis conditions are not carefully controlled [35,36,41,55].

Fig. 3. Functionalization typical silicate (SBA–15) with 3–aminopropylethoxysilane group. Reprinted from Ref. [18], p. 44, Copyright (2014), with permission from Elsevier Inc.

2.4. Characterization of silicates The aim of the routine characterization is to determine the physical (morphology, structure, and texture) and chemical (functional groups, nature of interactions between atoms and possibly their orientation) properties of the final synthesized meso-structured silicate materials. This implies that meso-structured silicates' characterization is classified into two groups: the physical and chemical characterizations. The physical characterizations include determination of; (a) the specific surface areas, pore volume and diameter, and porosimetry using gas physisorption method, (b) the organization of the silicate morphology up to the three-dimensional order and nanoscale using electron (both scanning and transmission) microscopic techniques, and (c) the types of meso-structures or any structural damage arising from the functionalization process using the diffraction (X-ray and electron) techniques [38,55,56]. The chemical characterizations include determination of; (a) the effectiveness of the desired functionalization, and possible nature and form of the chemical constituents using spectroscopic techniques such as the Infra-red, Raman and ultraviolet–visible spectroscopy, (b) the amounts of organic groups incorporated as well as their stability using thermogravimetry, (c) the quantities of some basic elements using elemental chemical analysis, and (d) the effectiveness of covalent bonding of the organosilanes to the silica framework and the extent of siloxane condensations using the nuclear magnetic resonance [38,55–57].

following approaches [36]: (a) Direct grafting (Primary modification) through reaction of suitable organosilane, in an appropriate solvent, with the silica surface under reflux [52,53]. (b) Reacting a previously grafted functionality with another functional species (Secondary modification) [47,54]. (c) Transformation of the grafted functionalities by additional treatments. Co-condensation during the template-directed synthesis of the mesoporous material involves the simultaneous condensation of the desired organic functionality (such as 3–aminopropyltriethoxysilane) alongside the primary silica source (usually the alkoxides such as tetramethyl (or tetraethyl) orthosilicate) in the presence of a surfactant. The desired organic functionality is covalently linked through a nonhydrolysable SieC bond to a siloxane species, which then hydrolyzes to form a silica network. Most organic functionalities are usually hydrophobic and tend to align with the hydrophobic end of the surfactant; hence, modifying the silica walls. Common organosilanes used in functionalization include the trialkoxy-organosilanes and chloro-organosilanes though the later is usually avoided because they give rise to 254

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3. Removal of aqueous pollutants

(II), Pb(II), Ni(II) and Zn(II) ions that the co-condensed amino-functionalization (they reported virtually no adsorption). However, the report of Da'na et al. [64–66] contradicts Aguado et al. [63] regarding the method of functionalization of SBA-15 surface and its effect on the adsorption performance. Their co-condensed amino-functionalized SBA-15 was used to study removal of Cd(II), Co(II), Cu(II), Zn(II), Pb (II), Ni(II), Al(III) and Cr(III) from single and multi-metal aqueous solutions. Amino-functionalized SBA-15 was able to adsorb the various metals to varying degrees with up to 95% adsorption observed at low aqueous single metal solutions (10 mg/L) indicating high sensitivity except for Co(II) and Cr(III). The adsorbent capacity decreased for coexisting metal cations in solution because of competition between cations for the amine groups. They also reported that application of their adsorbent for removal of Cu(II) in tap water, river water, and electroplating wastewater was a success. Bois et al. [67] grafted different functional (aminopropyl–, [aminoethylamino]propyl–, (2-aminoethylamino)-ethylamino]propyl–, and mercaptopropyl– groups) materials to MCM-41 for aqueous heavy metals adsorption. They observed that MCM-41 silica functionalized with multiple nitrogens ([amino-ethylamino]propyl- and (2-aminoethylamino)ethylamino]propyl-) groups exhibited higher loading capacity for Cu(II), Ni(II), Co(II) and the anion Cr(VI), while the mercapto functionalized silica exhibited higher loading capacity Cd(II). Kang et al. [68] showed that imidazole (N-(3-triethoxysilylpropyl)4,5-dihydroimidazole) functionalized SBA-15 could be used for precious metal-chelating and recovery. They observed that the amino-rich functionalized SBA-15 silica exhibited excellent selectivity for Pt(II) and Pd(II) ions over other metal ions such as Ni(II), Cu(II), and Cd(II), even with an excess of the latter cations. Adsorbed precious metal ions could be better recovered by simple HNO3 acid treatment compared to HCl or NaOH treatments. They concluded that the ligand-functionalized mesostructured silica could be used not only in toxic heavy metal removal but also in the effective separation of precious metal ions. Tapaswi et al. [69] also showed that SBA-15 functionalized with 1,4,7-triazacyclononane (another rich nitrogen compound), synthesized via post-synthesis grafting, exhibited fast and selective Cu(II) adsorption from aqueous mixed metal ions solution containing Cu(II), Cr(III), Ni(II), Co(II) and Li(I). The triaza-cyclononane functionalized SBA-15 had a maximum Cu(II) adsorption capacity of 42.6 mg/g and can be readily regenerated by HNO3eNH3 treatment. Amino-functionalized mesoporous and nano-mesoporous MCM-41 silica have also been synthesized using 3-aminopropyltrimethoxisilane and used for removal of Ni(II), Cd(II) and Pb(II) from aqueous solution [48]. It was shown that the amino-functionalized MCM-41 silica adsorbents exhibited preference for the metals in the order Ni(II) < Cd (II) < Pb(II) with the highest adsorption capacity for Pb(II) of 57.74 mg/g. Similarly, amino-functionalized SBA-15 (using N-3-trimethoxysilyl-propyldiethylenetriamine) of tailored pore sizes have been reported with good adsorption of 39 and 41 mg/g for Pb(II) and Cd(II), respectively [53]. One observation about the reported adsorption capacities for Heidari et al. [48] and McManamon et al. [53] is that they were low compared to others reported earlier. Aminopropyl (NH2) and melamine-based dendrimer amines (MDA) were separately used to functionalize SBA-15 mesoporous silica [46], and the resulting designer adsorbents were used to study the separate and ternary sorption of Pb(II), Cu(II) and Cd(II) ions onto SBA-15, NH2–SBA-15 and MDA–SBA-15 (Fig. 4). The NH2–SBA-15 and MDA–SBA-15 exhibited enhanced adsorption of more than 150 and 400%, respectively, increase over the pristine SBA-15. The Langmuir maximum adsorption capacities of MDA–SBA-15 was 130, 126 and 98 mg/g for Pb(II), Cu(II) and Cd(II), respectively. These results were remarkably high compared to others in the literature. Column experiments showed that metal ions removal percentage increased by 21.8%, 10.4% and 11.6% for Pb(II), Cu(II) and Cd(II), respectively, but adsorption efficiency was dependent on flow rate. The MDA–SBA-15 adsorbent showed good potential for reuse though slight decreases in

3.1. Inorganic pollutants In order to meet the evergrowing stricter water regulatory standards placed by governments and other regulatory bodies, numerous advances in material science are emerging for enhancing water purification process. Functional designer silicates with high efficiency and excellent selectivity towards specific aqueous pollutants are particularly interesting due to the recorded successes of these materials for aqueous inorganic species adsorption. Nitrogen/thiol and magnetic silica-based designer materials have been at the forefront of these advances. 3.1.1. Nitrogen and thiol designer silicates for inorganic pollutants Though pristine silicates have various remarkable properties which may be beneficial in the water treatment process, they have been outclassed by their designer derivatives. For instance, ordered mesoporous MCM-41 and MCM-48 silica were synthesized, amine functionalized with NeN dimethyl dodecyl amine and dodecyl amine, and used for adsorption of Cd(II), Co(II), Cu(II) and Pb(II) in aqueous solution [58]. The functionalized MCM-41 and MCM-48 silicates was over three-fold better than the pristine in the adsorption of these cations and were observed to be fast cationic adsorbents with higher affinity for Cu(II) and Pb(II) than for Cd(II) and Co(II) in single and mixed cations solutions, though adsorption capacity was reduced in the mixed solutions. These adsorbents exhibited a higher preference for Cu(II) in both single and mixed solutions while adsorption capacities in a single solution for amine-MCM-41 and amine-MCM-48 ranged from 227.9 to 279.6 mg/g and 186.5–260.5 mg/g, respectively. The adsorption capacities in mixed solutions were not significantly different for both the amineMCM-48 and amine-MCM-48 and ranged from 62.2 to 101.7 mg/g. Lee et al. [59] had earlier shown it is possible to incorporate different functional groups into one silica material. They designed bi-functionalized porous MCM-41 silica via the introduction of chelating ligands (mercaptopropyl and aminopropyl groups) which was used for the removal of heavy metal ions (Hg(II), Cu(II), Ni(II), Cd(II), and Cr(III)) from aqueous solutions [59]. These adsorbents showed improved efficiency for Hg(II), Cd(II), and Cu(II) removal but not for Ca(II), Mg(II), Ni(II), and Cr(III). The adsorption capacities of the mercapto- and amino-functionalized adsorbents ranged from 76.2 to 302.9 mg/g with a very high preference for Hg(II). Similarly, Burke et al. [60] showed that post-synthesis bi-functionalization, using aminopropyl and mercaptopropyl-silanes, can be achieved for mesoporous silica, and they attempted the removal of Ni(II), Cr(VI), Pd(II), Fe(II) and Mn(II) from water. The bi-functionalized mesoporous silica was highly effective in reducing a broad range of metal ions (from high concentration mixed metal ion solutions) to trace (> ppb) levels, while the mono-functionalized materials had specific responses to the metal ions. The maximum adsorption capacities were determined to be approximately equal (20 mg/g) for Cr(VI), Ni(II), Fe(II), Mn(II) and Pd(II) using the bifunctionalized mesoporous silica, and this was low in comparison to the report of Lee et al. [59]. Zhang et al. [61] have shown that in addition to aminopropyltriethoxysilane, multi-amines (such as N-[3-(triethoxysilyl)-propylethylene]diamine and triethoxysilyl propyl diethylenetriamine) can be grafted onto mesoporous SBA-15 silica. These multi-amine-SBA-15 adsorbents had excellent adsorption capacities for aqueous Hg(II) removal up to 726 mg/g. Interestingly, they also showed that the adsorbents could simultaneously remove Hg(II), Pb(II), Cd(II), Cu(II) and Zn(II) from wastewater. Liu et al. [62] had earlier reported high complexation affinity of Hg(II) by thiolated SBA-15 adsorbent while the aminated analog exhibited exceptional binding ability for metal ions such as Cu (II), Zn(II), Cr(III) and Ni(II). Aguado et al. [63] suggested that the method of functionalization of the SBA-15 surface may affect the performance of the final adsorbent. Post-synthesis grafted amino-functional groups exhibited far better adsorption efficiency for Cu(II), Cd 255

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aminopropyl SBA-15 in the synthesis of N-Propylsalicylaldimino-functionalized SBA-15 mesoporous silica (Fig. 6) [47]. This designer silicates showed high adsorption capacity and selectivity for Cu(II) ions (up to 46 mg/g), but these were lower for Ni(II), Zn(II) and Co(II) ions in both single and simultaneous adsorption and the adsorbent could be regenerated by acid treatment without altering its properties. N-Propylsalicylaldimino-functionalized SBA-15 reported by Dindar et al. [72] exhibited adsorption capacity of 64.2, 16.3 and 7.0 mg/g for Cr(VI), As (V), and Hg(II), respectively, though this was slightly lower than their aminopropyl-functionalized SBA-15 which had 79.6, 20.6 and 7.6 mg/g for Cr(VI), As(V), and Hg(II), respectively. Similarly, aminopropylfunctionalized manganese-loaded SBA-15 has been reported for Cu(II) adsorption from aqueous solution [54]. The study showed that the adsorbent could be regenerated and reused, exhibited high selectivity for Cu(II), and had maximum adsorption capacity over twice (≈40 mg/ g) that of the unfunctionalized manganese-loaded SBA-15. Another study using iminodiacetic acid for post-synthesis modification of mesoporous SBA-15 showed the modified adsorbent had Cd(II) uptake of 36.4 mg/g [73]. The foregoing has shown that amino functionalizing of silica may be essential for removal of aqueous pollutant cations. In that case, it may also be possible to load a high amount of amino or nitrogen groups on siliceous materials such as SBA-15 for higher cations removal. In line with this logic, Zhao et al. [74] employed toluene diisocyanate as ‘bridge ’ molecule between SBA-15 and ethylenediamine. One NCO group of the toluene diisocyanate was linked to the SBA-15 surface silanols while the other was linked to ethylenediamine (Fig. 7). Application of this high amino-load SBA-15 for removal toxic metal ions from aqueous solution showed greater than 98% removal efficiency for Cu (II), Zn(II), Cr(III), Ni(II) and Cd(II), and the authors attributed this to the strong complexation reactions between the metal ions and the grafted amine groups.

Fig. 4. Structure of the MDA–SBA-15. Reprinted from Ref. [46], p. 507, Copyright (2011), with permission from Elsevier Inc.

adsorption capacity were observed with subsequent cycles. Comparable to melamine-based dendrimer amines [46], polyamidoamine (PAMAM) dendrimers, another high-density nitrogen ligand, have been explored for designer SBA-15 [70]. Polyamidoamine is a unique chelating agent because of its high density of nitrogen ligands and the possibility of attaching functionalities such as primary amines, carboxylates, and hydroxymates, which can result in a substantial increase in bonding capacity for a variety of toxic metal ions. The PAMAM functionalized SBA-15 and EDTA-modified PAMAM-SBA-15 (Fig. 5) were used as adsorbents for the aqueous Ni(II), Pb(II), Cr(III), Cu(II), and Zn(II) ions. Adsorption results of both adsorbents were as high as 90% in most cases (using initial solution as high as 250 mg/L) with fast rate (15 min). The regenerated and reused adsorbents showed similar efficiency as the original. Sayari et al. [40] expanded the pore size of MCM-41 silica postsynthesis using NeN dimethyldecylamine. The material was found to be a fast, sensitive and high-capacity recyclable adsorbent for metallic cations and organic pollutants in aqueous solution. Adsorption capacities of 61, 54 and 106 mg/g were recorded for Co(II), Ni(II) and Cu (II), while 96 and 115 mg/g were observed for chloro-guaiacol and dinitrophenol, respectively. Recently, Sayari and co-workers [71] have shown that polyethylenimine-functionalized mesocellular silica foam could exhibit high Cd(II) adsorption capacity of up to 625 mg/g with stable reusability after three uses. This Cd(II) adsorption capacity value is fascinating considering several low values reported in literature. Coupling of other active functional groups to already functionalized silica materials has also been explored by coupling salicylaldehyde to

3.1.2. Magnetic silica-based adsorbents for inorganic pollutants In order to ease the separation process of the silica adsorbents from the aqueous solution after the pollutants adsorption, some workers have magnetized the silicates. One major advantage of such designer silicates is that they can be separated from aqueous solution after the adsorption process by an external magnetic force (Fig. 8a). For instance, Wang et al. [75] synthesized amino-functionalized core–shell magnetic mesoporous SBA-15 silica composite which showed excellent adsorptive capability (243.9 mg/g) toward Pb(II) ions. An interesting aspect of this adsorbent was that it could be easily removed from solution by an external magnetic field and regenerated easily by acid treatment; since most magnetic materials like magnetite dissolve in acidic media, protecting the magnetic material within the silica core-shell is an essential

Fig. 5. Schematic preparation of (a) PAMAM-SBA-15 and (b) EDTA modified PAMAM-SBA-15. Reprinted from Ref. [70], p. 317, Copyright (2007), with permission from Elsevier Inc.

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Fig. 6. Schematic of synthesis of N-Propylsalicylaldimino-functionalized SBA-15. Reprinted from Ref. [47], p. 1501, Copyright (2008), with permission from Elsevier Inc.

relative to those reported in literature here. Some researchers have argued that it is vital to make the adsorbents' preparation process eco-friendly by a substantial reduction of secondary pollutants. In line with this, Alotaibi et al. [82] synthesized iron supported on bio-inspired green silica. The eco-friendly synthesis mimics the bio-mineralization process and it is an easy process under mild conditions with less toxic reagents. The process considerably reduces the need for lengthy multistep procedures (≤72 h), high temperatures (≤120 °C) and extreme pH conditions. The iron supported bio-inspired green silica exhibited high arsenate ions removal capacity of 69 mg/g as well as good reusability. Similarly, biodegradable nano-composite of anionically modified guar gum with in-situ deposited silica nano-particles was synthesized through the sol-gel method [83]. The guar gumsilica nano-composite exhibited remarkable adsorption capacity for cationic dyes and metal cations of 781.3, 281.7, 645.2, and 709.2 mg/g for malachite green dye, safranin dye, Pb(II), and Cd(II), respectively, as well as good reusability. This adsorbent was also selective towards cations as opposed to anions. An attempt has been made at combing porous silica and graphene nanosheets decorated with α-FeOOH and functionalized with thiol groups (Fig. 9) [2]. Though the processes involved were quite cumbersome compared to other functionalized silicates discussed here, Hg (II) ion adsorption performance was outstanding: > 800 mg/g at 400 mg/L Hg(II), and this significantly exceeded currently available benchmark adsorbents. The adsorbent also exhibited efficient adsorption performance (∼100%) for removal of low (4 mg/L) and high (120 mg/L) concentration of Hg in real water samples using this composite in the form of membranes. Wang et al. [84] have also advanced the concept of combining multiple adsorbents by designing luminescent adsorbent that can be visually monitored during the adsorption process instead of using the usual instrumental methods to monitor pollutants concentration. Using a simple microplasma-assisted method, they prepared luminescent amino-functionalized ordered mesoporous silica-carbon dots composite with high specific surface area, high adsorption efficiency and selectivity for uranium even in the presence of various cations, as well as the ability for simultaneous in situ monitoring of the adsorption process. The combined individual properties of the pristine materials gave rise to this composite. Adsorption using the luminescent adsorbent could be monitored by monitoring the intensity of the composite's fluorescence which decreased as the uranium uptake increased.

process. Similarly, Yuan et al. [76] synthesized large pore (≈10 nm) amine-functionalized mesoporous silica shell with the Fe3O4 magnetic core (Fig. 8b). In comparison to smaller pore size shells (< 4 nm), the large pore size magnetic shell grafted far more amino functionalization and consequently had higher heavy metals adsorption capacity for Pb (II) (880 mg/g), Cu(II) (623 mg/g), and Cd(II) (492 mg/g). The adsorbent was also easily regenerated using simple acid treatment. Li et al. [77] had earlier synthesized and used thiol-functionalized magnetic spherical silica adsorbents for aqueous Hg(II) and Pb(II) adsorption. Though their adsorbent was not as efficient as that of Yuan et al. [76], the reported adsorption capacity for Hg(II) (260 mg/g) was higher than that of Pb(II) (91.5 mg/g), the adsorbent was reusable after regeneration in acid solution and stable in water matrices from strong acid and alkaline sources. Kumari et al. [78] have shown it is also possible to synthesize magnetite nanospheres with hollow interiors in a simple, one-pot, and template-free solvothermal method with ferric chloride as the iron precursor. However, a major drawback of the template-free solvothermal method is that it results in silicates with an unusually small surface area, in this case, 11.3 m2/g. This led to an extremely low aqueous metals adsorption capacity for Cr(VI) and Pb(II) of ∼9 and ∼19 mg/g, respectively. Similarly, magnetic amino (NH2) and melamine-based dendrimer amine (MDA) functionalized MCM-48 mesoporous silicates were used for the adsorption of aqueous Pb(II), Cu (II), Cr(VI) and Cd(II) ions [79]. The MDA-magMCM-48 exhibited adsorption capacity of 127.2, 125.8, 115.6 and 114.1 mg/g for the ions, respectively, and was reusable for up to three times without significant loss in adsorption capacity. Apart from the fact that incorporating magnetic materials within or on mesoporous silicate adsorbents eases the separation process of the adsorbent from the aqueous solution after the pollutants adsorption, the report of El-Toni [80] indicated that the incorporated magnetic materials enhanced the silicate adsorption properties. They synthesized un-magnetized amino-functionalized hollow core–mesoporous shell silica spheres using 3-aminopropyltrimethoxysilane. However, unlike other reports [75,76], their adsorbent exhibited lower adsorption capacity notably for aqueous Pb(II) and Cd(II) ions of 194 and 190 mg/g, respectively, the high surface area (718 m2/g) notwithstanding. Recently, cubic spinel crystallites of Zn−silica modified cobalt ferrite magnetic nano-structured composite (CoFe2O4) was synthesized and used for aqueous cationic pollutants from water [81]. The silica modified CoFe2O4 exhibited better adsorption efficiency for removal of both methylene blue and metal cations (Cr(III), Cu(II), and Pb(II)) over the Zn(II) modified CoFe2O4, though the adsorption capacity was low

Fig. 7. Schematics of the high NH2 loading on the SBA-15. Reprinted from Ref. [74], p. 1046, Copyright (2011), with permission from Elsevier Inc.

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Fig. 8. (a) The separation of amino-functionalized magnetic SBA-15 particles by an external magnet. Reprinted from Ref. [75] p. 900, Copyright (2015), with permission from Elsevier Inc.; (b) scheme showing the synthesis of large pore amino-functionalized mesoporous silica shell with Fe3O4 magnetic core and heavy metal adsorption process. Reprinted from Ref. [76] p. 160, Copyright (2013), with permission from Elsevier Inc.

mesoporous silica had high surface area was 762 m2/g with an adsorption capacity up to 103 mg/g at pH 2.0 in 7 min; a better result than that of the iminodiacetic acid-functionalized SBA-15 at pH 4.5. The imprinted material also exhibited high selectivity among six competing metal ions (Zn(II), Cd(II), Cu(II), Mg(II), Ca(II) and Fe(III)) with the adsorption efficiency of above 93% after six regeneration cycles. The

3.1.3. Other designer silica-based adsorbents for inorganic pollutants Inorganic functionalities have also been incorporated or imprinted on silicate surfaces to improve the metals' adsorption process. For instance, in order improve adsorption efficiency of Pb(II) from acidic wastewater (pH 2.0), Pb(II) imprinted mesoporous silica functionalized with iminodiacetic acid was fabricated [85]. The Pb(II) imprinted

Fig. 9. Schematics of synthesized graphene-DE-αFeOOHSH composite. (a) Natural graphite rock, (b) diatom silica from diatomaceous earth (DE), (c) dispersed graphene oxide sheets and 3-aminopropyltriethoxysilane modified DE, (d) graphene-DE composite (GN-DE) decorated with αFeOOH nanoparticles, (e) thiol-functionalized grapheneDE-αFeOOH-SH composite obtained using chemical vapor deposition from step e to step f. Reproduced from Ref. [2] with permission from The Royal Society of Chemistry.

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phosphate-modified SBA-15 had high surface area (≈670 m2/g), large pore size (9.1 nm), abundant surface phosphate groups, and the adsorbent exhibited ultrafast adsorption rate (2 min) with adsorption capacity of up to 204.4 mg/g and could be reused multiple times without significant loss in binding capacity. 3.2. Organic pollutants Studies of adsorption of organic pollutants on pristine and mesoporous silicates seem to be a less traveled part compared to adsorption of inorganic pollutants like the toxic metals. Here, we shall discuss some of the recent studies dealing with adsorption of organic pollutants on silicates. One major drawback in using pristine silicates for organics adsorption is that the surface silanol groups are hydrophilic and easily forms hydrogen bonds with water thus limiting the adsorption process. However, Huang et al. [88] have shown that this may not be the case always because pristine ordered mesoporous SBA-15 molecular sieves could be used for cationic dyes adsorption. The study suggested that electrostatic attraction between the silanol groups and cationic dyes is the primary removal process and SBA-15 exhibited excellent adsorptive capability for cationic dyes (methylene blue and Janus Green B) but almost no adsorption for anionic (Reactive black 5) and neutral dyes (dimethyl phthalate). The SBA-15 adsorption capacity for methylene blue and Janus Green B may reach 49.3 and 66.4 mg/g, respectively. Aside from pristine silicates, designer silicates have been employed in studies on aqueous organic pollutants removal. Here, we review recent reports.

Fig. 10. Schematics of phosphate-functionalized mesoporous SBA-15. Reprinted from Ref. [87], p. 198, Copyright (2017), with permission from Elsevier Inc.

authors also showed that the imprinted silicate was effective for real samples of strongly acidic wastewater. Other organic molecules, such as the biologically active dopamine, may be useful in the adsorption process. Uranium ions have been adsorbed in aqueous solution using post-synthesis dopamine grafted mesoporous silica [86], and the results showed a spontaneously exothermic process with fast adsorption rate (20 min), and a maximum adsorption capacity of 196 mg/g at pH 6.0. The influences of K+ and Na+ ions concentrations and solid-to-liquid ratio on the adsorption process were negligible. Coupling of inorganic phosphate to silicates is eliciting interest in aqueous pollutants removal [18,87]. Gao et al. [87] synthesized phosphate modified SBA-15 by post-graft approach (Fig. 10) and used this for adsorption of the rare earth ion Gd(III). The study showed that the

3.2.1. Nitrogen/thiol designer silicates for organic pollutants In order to improve the efficiency of the silica-based materials, several nitrogen-containing designer silicates have been synthesized and studied. Aminopropyl- and tripolyphosphate-functionalized SBA-15 have been studied for pesticide (pentachlorophenol– PCP) removal from aqueous solutions [18]. Fig. 11 depicts the scheme for the preparation of the tripolyphosphate functionalized SBA-15. Compared to pristine SBA-15, the functionalized derivatives exhibited higher PCP removal following the trend SBA-15 (0.6 mg/g) < tripolyphosphatefunctionalized SBA-15 (2.5 mg/g) < aminopropyl-functionalized SBAFig. 11. (a) cross section of ordered SBA–15 mesoporous silica; schematics of the synthesis of SBA–15 materials showing reactions with (b) 3–aminopropylethoxysilane and (c) tripolyphosphate; (d) interaction of tripolyphosphate with 3–aminopropylethoxysilane showing a flip electrostatic interaction/covalent bond between the phosphate and amine group. Reprinted from Ref. [18], p. 44, Copyright (2014), with permission from Elsevier Inc.

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15 (3.3 mg/g). The study attributed ≥75% increase in sorption upon functionalization to the electrostatic interaction between the PCP species and amino group. Aminopropyl-functionalized SBA-15 adsorbents with varied aminopropyl contents synthesized by co-condensation [89] have also been studied as adsorbents for humic acid, an ubiquitous pollutants in surface and groundwater. In contrast to humic acid adsorption on SBA-15, substantially enhanced adsorption was observed over the functionalized SBA-15 adsorbents with an increase in solution pH resulting in decreased adsorption over the aminopropyl functionalized SBA-15. Adsorption capacities of 8.5, 72.5 and 117.6 mg/g were recorded for SBA-15, NH2-SBA-15-5% and NH2-SBA-15-10%, respectively, where the percentage represents aminopropyl loading. Aminopropyl-functionalized SBA-15 has also been shown to exhibit a good capacity for removal of tannic acid from aqueous solution with adsorption capacity of 272.4 mg/g [90]. Bi-functionalized SBA-15 based designer adsorbents have been synthesized to target specific molecular features of pollutants [91]. While using any two of N-(2-aminoethyl)-3-aminopropyl-methyl-dimethoxy-silane, 3-phenoxy-propyl-dimethylchloro-silane and n-hexadecyl-triethoxysilane as functionalizing organo-silane, diamine/ phenyl-SBA-15, diamine/cetyl-SBA-15 and phenyl/cetyl-SBA-15 were specifically tailored for adsorption of eosin, 4-nonylphenol (4-NP) and di-n-butyl-phthalate (DBP), respectively, from aqueous solutions. Compared with the mono-functionalized adsorbents, bi-functionalized SBA-15 adsorbents exhibited better adsorption efficiency for pollutants at low concentration. The adsorption capacities for eosin, 4-NP, and DBP were approximately 382.3, 207.3 and 434.2 mg/g on the adsorbents, respectively. Considering the structural relationship between the adsorbents and the adsorbates (Figs. 12 and 13), Zhang et al. [91] attributed the success of pollutants removal to cooperative effects of hydrogen bonding, electrostatic attractions, hydrophobic interactions, and π–π stacking interactions. This cooperative effect could result from two functional groups on the same solid surface leading to better

Fig. 13. Chemical structures of eosin, NP and DBP. Reproduced from Ref. [91] with permission from The Royal Society of Chemistry.

adsorption performance. Recently, Fan et al. [92] showed that up to 40 mg/g adsorption capacity could be achieved from post-synthesis prepared phenyl-functionalized SBA-15 in the adsorption of aqueous DBP. Their result was lower than reported earlier [91] and it highlights the effect of cooperative adsorption. In line with the concept that cooperative effect [91] could result in enhanced adsorption process, thiol-functionalized mesoporous polyvinyl alcohol/silica-based nano-fiber prepared by sol-gel and electrospinning methods was used for simultaneous but sequential adsorption of Bisphenol A (BPA) and Cu(II) from their binary solutions [93]. It is interesting to note here that the sequential adsorption of one pollutant may lead to the enhanced synergistic adsorption of a second pollutant. Adsorption of Cu(II) was attributed to chelating interaction by thiol groups, while BPA was removed from solution due to intermolecular attraction and hydrogen bonding. The BPA if adsorbed on the thiol group first, could act as a molecular bridge for Cu(II) adsorption resulting in the synergistic adsorption. However, the co-existence of both pollutants resulted in decreased BPA uptake due to competition for the thiol groups. The authors also showed that co-existence of Cl− and NO3− ions enhanced Cu(II) adsorption but not BPA, and the adsorbent was reusable after sequential desorption of BPA and Cu(II) using ethanol and HCl solutions. Similarly, rhodamine 6G assisted adsorption of metanil yellow over succinamic acid-functionalized MCM-41 has also been reported [94]. Here, it was assumed that multilayer adsorption between rhodamine 6G and metanil yellow might have resulted in this. Another nitrogen-containing silicate is the polyacrylic acid-grafted mono-disperse silica nanoparticles [95] which has been shown to be a good adsorbent for methylene blue (≈305 mg/g). The polyacrylic acidgrafted silica nanoparticles were synthesized through a combination of mussel-inspired chemistry and Michael addition reaction. Firstly, the silica nanoparticles were coated with poly-dopamine through selfpolymerization under mild conditions, and this was then conjugated with amino-terminated polyacrylic acid (Fig. 14). The good adsorption capacity of this adsorbent is worthy of note. 3.2.2. Magnetic silica-based adsorbents for organic pollutants Various studies have reported that it is possible to magnetize designer silicate adsorbents to be used for organics removal from aqueous solution. However, unlike the adsorption of aqueous cations, less attention has been given to designer magnetic-silicates for removal of aqueous organic pollutants. Amino-functionalized magnetic mesoporous MCM-41 silica (magMCM-41-NH2) was synthesized and used for adsorption of tannic acid from aqueous solution [96]. The magMCM41-NH2 had an adsorption capacity of 510 mg/g, the adsorption was favorable in the pH range of 4.5–7.0 indicating the adsorbent usability over this pH range, and the process was enhanced by the presence of aqueous coexisting cations (Na, K, and Ca). The magMCM-41-NH2 (surface area 668 m2/g; pore size 0.53 cm3/g) was a better adsorbent for adsorption of tannic acid from aqueous solution when compared to the un-magnetized amino-functionalized SBA-15 which showed an

Fig. 12. Si solid-state NMR spectra of the three respective bi-functionalized adsorbents. Reproduced from Ref. [91] with permission from The Royal Society of Chemistry.

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Fig. 14. Schematics of polyacrylic acid-grafted mono-disperse silica nanoparticles. Reprinted from Ref. [95], p. 287, Copyright (2016), with permission from Elsevier Inc.

adsorption capacity of 272.4 mg/g (surface area 246 m2/g; pore size 0.43 cm3/g) [90]. This, as well as the study of Teo et al. [97], showed that magnetizing the adsorbent, in addition to easing adsorbent separation from solution, enhances its adsorption capacity. Magnetic core-shell silicates have also been recently used for removal of Congo red dye from aqueous solution [98]. Congo red dye adsorption on this magnetic silicate was mainly by electrostatic interaction and adsorption capacity was about 54.6 mg/g. Similarly, hybrid shells of iron oxide nanoparticles coated with organic κ-carrageenan and inorganic silica have also been used for the magnetically assisted removal of methylene blue from aqueous solution [99]. The hybrid shells exhibited good adsorption capacity (530 mg/g) which was attributed to the sulphate-rich surface of the κ-carrageenan. The hybrid shells were reusable with adsorption efficiency over six consecutive adsorption/desorption cycles relatively stable (> 97%). Both studies showed that in addition to magnetizing the silicate, functionalizing significantly enhances adsorbent efficiency as well.

ciprofloxacin; an indication that electrostatic interaction is more important than hydrophobic interactions between sulfonic-SBA-15 and aqueous ciprofloxacin species. The sulfonic-SBA-15 adsorbent could be reused for thrice for adsorbing ciprofloxacin. The same research group also used azide-functionalized hollow silica nanospheres synthesized from tetraethoxysilane and azidopropyltrimethoxysilane on F127 single micelle template as an adsorbent for ciprofloxacin adsorption [100]. They concluded that functional groups such as phenyl-acetylene, propargyl alcohol, 1-heptyne and 2-butyne-1,4-diol linked to these hollow silica nanospheres played important roles in their adsorption properties. Gao et al. [101] have also shown that SBA-15 functionalized with aminopropyl triethoxysilane, propyltrimethoxysilane, or phenyltrimethoxysilane may be used as an adsorbent for ciprofloxacin adsorption with possible adsorption capacities up to 2.2, 4.4, and 11.2 mg/g at 0.1 mg/L of ciprofloxacin. A comparative adsorption study of Boldine (2,9-dihydroxy-1,10-dimethoxyaporphine) was carried out using pristine and propyl-sulfonic acid-functionalized SBA-15, SBA-16, and mesocellular foam (MCF) [102]. Like other reports, functionalization enhanced adsorption while SBA-15 and its functionalized propyl-sulfonic derivative showed higher adsorption than SBA-16 and MCF, though no reason was proffered for this trend. Apart from incorporation of organic functionalities on the silica surface, inclusion of transition metal (such as Co(II), Ni(II) and Cu(II)) functionality onto the surface of MCM-41 [103] and SBA-15 [104] via an already attached organic amino-moiety enhanced the silica

3.2.3. Other designer silica-based adsorbents for organic pollutants In addition to the nitrogen-containing and magnetic functionalities of designer silicates, others have been studied recently. For instance, a comparison has been made of the effect of multiple roles of similar organic functional groups in designer silicate adsorbents specifically using mercapto- and sulphonic acid functionalization of SBA-15 for adsorption of the antibiotics ciprofloxacin [3]. It was observed that the introduced sulfonic functionality enhanced the adsorption of

Fig. 15. Modification of SBA-15 using the grafting technique. Reprinted from Ref. [104], p. 472, Copyright (2010), with permission from Elsevier Inc.

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Fig. 16. Suggested adsorptive mechanism for naturally obtained silicates. Reprinted from Ref. [114], p. 324, Copyright (2016), with permission from Elsevier Inc.

volume (0.85 cm3/g) in comparison to Al-MCM-41. Suraja et al. [110] also showed that nanosized cobalt oxide (Co3O4) particles incorporated (maximum at 10%) onto SBA-15 mesoporous silica showed adsorption potential (16 mg/g) for aqueous methylene blue. However, this was far less efficient when compared with Al-MCM-41 obtained from coal fly ash source [108]. Recently, Zhang et al. [111] showed that porous metal silicates could also be obtained from other waste sources. They prepared various three-dimensional metal (magnesium, zinc, nickel, and cobalt) silicates from waste rice husks and the magnesium silicate exhibited high aqueous adsorption capacities of 557.9, 381.3 and 482.8 mg/g for Pb(II), tetracycline, and UO22+, respectively. The rice husk porous silica doped with Ni nanoparticles also exhibited high catalytic activity as well as good stability for reduction of aqueous 4nitrophenol. In the same vein, Wang and co-workers have shown that metal ionmodified mesoporous silicates obtained from naturally occurring palygorskite or Illite clay may be used for efficient removal of both organic pollutants and metals cations [57,112–115]. Their reports were quite interesting because it highlighted that mesoporous silicates can be obtained from relatively abundant clay sources. Their Mg-modified palygorskite clay exhibited 218.9 and 107.9% increase in adsorption of the antibiotics chlortetracycline (329.84 mg/g) and oxytetracycline (207.47 mg/g), respectively, compared to the pure palygorskite clay [112]. The same adsorbent exhibited adsorption capacity of 527.2 and 210.6 mg/g for methylene blue and Cu(II) ions, respectively [57]. Similarly, Mg-modified mesoporous silicate obtained from illite clay showed higher chlortetracycline adsorption capacity of 408 mg/g compared with the pure Illite clay (159.7 mg/g) [115]. However, Znmodified red palygorskite clay was slightly less effective for removal of aureomycin and tetracycline with adsorption capacity of 384 and 337 mg/g, respectively, in contrast with the 154 and 140 mg/g of the pure red palygorskite clay [114]. The large specific surface areas, as well as large pore structures of these naturally obtained silicates, are noteworthy. The proposed mechanism for the removal of these pollutants from solution involves monolayer electrostatic interaction with charged surfaces, hydrogen bonding as well as complex formation

adsorption of Naproxen from aqueous medium. These transition metals grafting technique [104] on the silicates were similar (Fig. 15), and the authors concluded that similar strategies might hold promise for removal Pharmaceutical and Personal Care Products (PPCPs) from aqueous solution, especially those present at ppm levels. Recently inorganically modified spherical mesoporous MCM-41 silica using CuO exhibited better adsorption capacity for the cationic dyes– crystal violet and methylene blue (52.9 and 87.8 mg/g, respectively) compared to the pristine spherical mesoporous MCM-41 (46.2 and 65.7 mg/g, respectively) [105]. The same research group also showed that similarly, iron oxide-modified MCM-41 exhibited enhanced adsorption for aqueous aniline [106]. The enhancements in adsorption were attributed to electrostatic interactions between the mesoporous MCM-41 loaded metals (Cu and Fe) and the aqueous cations (pollutants: dyes and aniline), as well as, to a less degree, the enhanced pore structure of the adsorbent. Similar to the inclusion of transition metal functionality onto the silicate surface, coal fly ash (containing primarily Si, Al, and Fe at concentrations of 239.5, 108.0 and 60.3 mg/g, respectively) was used to modify 3-aminopropyltriethoxysilane functionalized mesoporous MCM-41 [107]. The synthesized material exhibited high affinity for aqueous Cu(II) ions with up to 95% removal at 45 mg/L. However, using coal fly ash as the source of amorphous SiO2 and Al2O3 components rather than as modifying agents, Zhou et al. [108] synthesized spherical Al-MCM-41. This was quite interesting taking into consideration that (i) the primary raw material source of the Al-MCM-41 was a supposed waste material-coal fly ash, (ii) the final BET surface area was up to 525 m2/g with a pore volume of 0.71 cm3/g, and (iii) methylene blue adsorption capacity of 277 mg/g. Their study suggested that Al-MCM-41 obtained from industrial solid waste of coal fly ash could be an economic adsorbent for treatment of methylene blue in aqueous solution. This has been supported by another study showing that fly ash Al-MCM-41 [108] is better than Al-SBA-15 [109] for aqueous methylene blue adsorption. The Al-SBA-15 [109] synthesized by the conventional method [18] exhibited a maximum adsorption capacity of 83 mg/g even with a larger surface area (693 m2/g) and pore 262

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Fig. 17. Schematics of the aerogel synthesis. Reproduced from Ref. [117] with permission from The Royal Society of Chemistry.

(Fig. 16) and possibly pore filling. The combination of adsorbents has also been studied as a method of increasing pollutants adsorption capacity. In this regard, mesoporous silica has been coated on micrometer-sized and nanometer sized zeolite A and these were compared with the pristine zeolite A for adsorption of butyraldehyde [116]. Adsorption on both composite materials almost doubled those of the pristine zeolites A (nano and micro; 151 and 146 mg/g, respectively); while the nano-zeolite A composite (314 mg/ g) exhibited better adsorption than micro-zeolite A composite (266 mg/ g). Lately, silica aerogels [117] reinforced with polyacrylonitrile fibers (Fig. 17) have been synthesized for use in adsorbing diesel oil from water and oil mixtures. This adsorbent exhibits high porosity and surface area, low density, and three-dimensional reticular structures with excellent adsorption capacity (Fig. 18) for oil (9.56 g/g). Interestingly, authors stated the adsorbent could be reused up to 100 times without

diminished property. This type of designer adsorbent is quite fascinating considering the very high adsorption capacity, and its benign, straightforward, and economic preparation. Another recently attempted synthesis of designer silicate with the simultaneous ability for fast removal of organic, inorganic, and microbial contaminants from aqueous solution [118]. Herrmann et al. [118] synthesized a polyoxometalate-ionic liquid (POM-IL) complex immobilized on porous silica (POM-SILP). Each component of this complex has its specific contribution which targets a particular type of contaminant resulting in the water-insoluble POM-IL composed of antimicrobial alkyl-ammonium cations and lacunary polyoxometalate anions with heavy-metal binding sites. The silica support also has radionuclides binding capacity, and this, in addition to the complex's lipophilicity enables adsorption of organic contaminants (Fig. 19). The use of POM-SILP in filtration columns could result in one-step multicontaminant portable water purification. Though various mesoporous silica materials have recently been investigated for pollutants removal from aqueous solution, SBA-15 and MCM-41 have received the greatest attention. Comparison of some recent designer silicates and pollutants adsorbed from aqueous solutions are shown in Tables 1 and 2. Several of these designer silicates have shown promising adsorption capacity for both organic and inorganic pollutants.

3.3. Some other environmental remediation applications Apart from the use of these silicates for direct adsorption of aqueous pollutants from water, designer mesoporous silica and its functionalized derivatives have been applied in areas such as photocatalysis. Titanium substituted SBA-15 has been prepared by several methods and used as support for TiO2 [49,121,122]. The photocatalytic activity of this material increased proportionally as the Ti content and TiO2 loading. Dou et al. [123] and Qin et al. [124] have also shown differently that mesoporous silica can be useful for removal of volatile organic compounds (VOCs). Dou et al. [123] functionalized SBA-15, MCM-41, MCM-48 and KIT-6 (Fig. 20) with phenyltriethoxysilane and investigated the static adsorption of benzene. The adsorption performance was observed to be related to the pore diameter: the larger the pore diameter, the better the adsorption performance. Hence, phenylgrafted KIT-6 offered the best adsorption performance while phenylgrafted MCM-41 exhibited the lowest adsorption efficiency than the other functionalized materials owing to the restricted size of the

Fig. 18. Demonstration of diesel oil adsorption by the aerogel with 0.3% fibers. Reproduced from Ref. [117] with permission from The Royal Society of Chemistry.

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Fig. 19. Purification of water using POM-SILPs: the POMSILP column filter removes toxic heavy metals (e.g. Ni(II), Pb(II), UO2(II)), aromatic organic pollutants (e.g. trityl dyes), and microbes (E.coli) due to the presence of antimicrobial tetra-alkyl ammonium cations and lacunary polyoxometalate anions with specific metal binding sites (orange arrow). Reproduced from Ref. [118], p. 1668, © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

mesopores. Qin et al. [124] reported a similar result. The success of using methyl and phenyl functionalized SBA-15 for removal VOCs has also been reported [125].

Table 1 Comparison of some designer silicates and some inorganic pollutants adsorbed from aqueous solutions. Designer Silicate

Pollutant

qe (mg/g)

Solution

Reference

Bio-inspired Fe-supported on silica a APTS-SBA-15 N-propylsalicylaldimine-SBA-15 Magnetic melamine-MCM-48 a NH2-HCMSSs Large pore magnetic amino-silica Imino-di-acetic acid- SBA-15 Magnetic core-shell APTS-silica Guar gum-Silica composite Polyethylenimine silica foam Co(II) ion-imprinted triglycine SBA-15 a SBA-TACN a SBA-TACN Magnetic melamine-MCM-48 APTS-SBA-15 N-propylsalicylaldimine-SBA-15 Magnetic melamine-MCM-48 a SBA-TACN Large pore magnetic amino-silica Mn-loaded APTS-SBA-15 Mn-loaded-SBA-15 Magnetic core-shell APTS-silica Phosphorous acid-SBA-15 a Magnetic MPTS-silica APTS-SBA-15 N-propylsalicylaldimine-SBA-15 SBA-TACN a Magnetic APTS-SBA-15 (microspheres) Magnetic melamine-MCM-48 a NH2-HCMSSs MPTS-silica Large pore magnetic amino-silica Magnetic core-shell APTS-silica Iminodiacetic acid-SBA-15 Pb(II)-imprinted iminodiacetic acid-SBA-15 Cobalt Ferrite Magnetic Zn(II)Silica Mg-Silicate Guar gum-Silica composite Dopamine-SBA-15 APTS-SBA-15 APTS-SBA-15-Carbon dot composite Mg-Silicate NH2-HCMSSs

As(V) As(V) As(V) Cd(II) Cd(II) Cd(II) Cd(II) Cd(II) Cd(II) Cd(II) Co(II)

69.6 20.6 16.3 114.1 190.5 492.4 36.4 22.5 709.2 625 181.7

Single Mixed Mixed Mixed Mixed Single Single Single Single Single Mixed

[82] [72] [72] [79] [80] [76] [73] [119] [83] [71] [120]

Co(II) Cr(III) Cr(VI) Cr(VI) Cr(VI) Cu(II) Cu(II) Cu(II) Cu(II) Cu(II) Cu(II) Gd(III) Hg(II) Hg(II) Hg(II) Ni(II) Pb(II)

0.9 3.6 115.6 79.6 64.2 125.8 42.4 628.3 40.7 19.9 29.9 204.4 1.31 7.6 7.0 6.7 243.8

Mixed Mixed Mixed Mixed Mixed Mixed Mixed Single Mixed Mixed Single Single Mixed Mixed Mixed Mixed Single

[69] [69] [79] [72] [72] [79] [69] [76] [54] [54] [119] [87] [77] [72] [72] [69] [75]

Pb(II) Pb(II) Pb(II) Pb(II) Pb(II) Pb(II) Pb(II)

127.2 194.4 0.44 880.6 76.7 ≈20 ≈144

Mixed Mixed Mixed Single Single Mixed Mixed

[79] [80] [77] [76] [119] [85] [85]

Pb(II)

≈20

Single

[81]

Pb(II) Pb(II) U(VI) U(VI) U(VI)

557.9 645.2 196 ≈90 ≈170

Single Single Single Single Single

[111] [83] [86] [84] [84]

UO22+ Zn(II)

482.8 193.0

Single Mixed

[111] [80]

4. Conclusion and outlook In order to meet the evergrowing stricter water regulatory standards placed by governments and other regulatory bodies, numerous advances in material science are emerging for enhancing the water purification process. One such advance includes the functionalized ordered mesoporous silica or designer silicates. In the last eight years, designer silicates have gained wide interest as adsorbents for aqueous pollutants due to the plethora of designer opportunities available. The nitrogen/ thiol-containing silicates and the magnetized derivatives or magnetics (surface or core-shell) have been extensively explored for the removal of toxic cationic and anionic species, dyes, pesticides, industrial organics, pharmaceuticals and other emerging pollutants. Recently, higher definition designer silicates, comprising of functionalized mesoporous silicates and other adsorbents synergistically combined, are proving far more successful materials than the lone functionalized silicates. In conclusion, the present review emphasizes the following: i. Though pristine silicates have various remarkable properties which may be beneficial in the water treatment process, they have been outclassed by their designer derivatives. ii. It is possible to synthesize designer silicates having single or multifunctionalizations, by co-condensation or post-synthesis grafting, using various molecules ranging from organic, inorganic to biological molecules. Inclusions of transition metals functionality onto the silicate surface via an already attached organic-moiety resulting in enhanced adsorption have also been achieved. Coupling of other active functional groups to already functionalized silica materials is an emerging trend that is proving successful for enhanced adsorption efficiency. iii. Recent studies have shown that some ‘supposed waste materials’, such as fly ash and rice husk, could be used as major raw material sources for designer silicates instead of the commercially-available organosilanes. This could drastically reduce the cost of designer silicates production. It is also possible to synthesize eco-friendly designer silicates which exclude the extreme conditions of the popular classical method of preparation such as extreme pH, high temperature and toxic chemicals. iv. Co-condensation or post-synthesis grafting of functional groups does not significantly affect adsorption efficiency, but the type and density of the functionality. For instance, multi-nitrogen-functionalized silicates exhibit higher pollutant loading capacity than the

a Modifying agents – APTS: Aminopropyltriethoxysilane; TACN: 1,4,7-triazacyclononane; NH2-HCMSSs: APTS hollow core-mesoporous shell silica sphere; MPTS: 3-mercaptopropyl)-trimethoxysilane.

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Table 2 Comparison of some designer silicates and organic pollutants adsorbed from aqueous solutions. Designer Silicate

Pollutant

qe (mg/g)

Solution

Reference

α-Fe2O3-MCM-41 SBA-15 Mercaptopropyltrimethoxysilane-SBA-15 Sulfonic acid-SBA-15 Silica coated magnetic nanospheres MCM-41 CuO/MCM-41 Phenyl-SBA-15 SBA-15 APTS-SBA-15 Guar gum-Silica composite Co3O4 doped SBA 15 Al-MCM-41 (spherical) Cobalt Ferrite Magnetic Zn(II)-Silica MCM-41 CuO/MCM-41 Magnetic carrageenan-silica hybrid shells SBA-15 Al-SBA-15 CH3-MCM -41 MCM -41 Silica aerogel SBA-15 APTS eSBA-15 Tripolyphosphate APTS eSBA-15 Guar gum-Silica composite Magnetic APTS-MCM-41 APTS eSBA-15 Mg-Silicate Zn-Silicate Zn-Silicate Mg-Silicate Mg-Silicate

acetylsalicylic acid Ciprofloxain Ciprofloxain Ciprofloxain Congo red Crystal violet Crystal violet di-n-butyl phthalate Humic acid Humic acid Malachite green Methylene blue Methylene blue Methylene blue Methylene blue Methylene blue Methylene blue Methylene blue Methylene blue Methylene blue Nitrobenzene Oil Pentachlorophenol Pentachlorophenol Pentachlorophenol Safranin Tannic acid Tannic acid Tetracycline Tetracycline Aureomycin Methylene blue Crystal violet

≈6.5 ≈100 ≈125 ≈250 54.6 46.2 52.9 ≈39 8.5 117.6 781.3 16.7 277.8 ≈24 65.7 87.8 530 45.1 79.8 375.5 25.8 9560 2.1 4.2 3.5 281.7 510.2 272.4 381.3 337 384 408.0 397.2

Single Single Single Single Single Single Single Single Single Single Single Single Single Single Single Single Single Single Single Single Single Single Single Single Single Single Mixed Single Single Single Single Single Single

[97] [3] [3] [3] [98] [105] [105] [92] [89] [89] [83] [110] [108] [81] [105] [105] [99] [109] [109] [8] [8] [117] [18] [18] [18] [83] [96] [90] [111] [114] [114] [113] [113]

*Modifying agents – APTS: Aminopropyltriethoxysilane; TACN: 1,4,7-triazacyclononane; NH2-HCMSSs: APTS hollow core-mesoporous shell silica sphere; MPTS: 3-mercaptopropyl)trimethoxysilane.

Fig. 20. Schematics of the surface functionalization of SBA-15, MCM-41, MCM-48 and KIT-6. Reprinted from Ref. [123], p. 1617, Copyright (2011), with permission from Elsevier Inc.

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sparse analogues– mono-nitrogen-functionalized silicates. v. The magnetics, apart from easing the adsorbent separation process by using an external magnetic force, enhances adsorption capacity. vi. Higher definition designer silicates have been synthesized by coupling ordered mesoporous silicates with other adsorbents. These kind of materials have resulted in a. Extra-ordinary adsorption capacity, b. Luminescent adsorbent that can be used to visually monitor the adsorption process instead of using the usual instrumental methods, and c. Designer adsorbents with the simultaneous ability for fast removal of organic, inorganic, and microbial contaminants from aqueous solution. vii. Designer silicates have shown high adsorption and excellent selectivity towards specific pollutant, as well as the capacity for simultaneous removal of aqueous pollutants. It is worthy to note that these materials exhibited excellent reusability. vii. The success of aqueous pollutants removal by designer silicates could be attributed to hydrogen bonding, electrostatic attractions, hydrophobic interactions, or π–π stacking interactions between the silicates and pollutant. In some cases, it has been attributed to the cooperative effects among these, or even sequential adsorption of one pollutant may lead to the enhanced synergistic adsorption of a second pollutant.

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With the large literature available and still being published on designer silicates, perhaps some of the biggest challenges to overcome now are: actual field trials, synthesis of greener designer silicates, and their large-scale production. (a) Available literature are mostly idealized laboratory studies and cannot be used to predict the usefulness of designer silicates in actual field situations due to the presence of interfering agents. The applicability of these materials might be another challenge since most laboratory studies are batch processes while the column or fixed bed is usually preferred in the industries. (b) The byproducts of the sol-gel synthesis methods are not environmentally friendly; greener synthesis methods for designer silicates is of high necessity so as not to cause a bigger environmental challenge will endeavoring to solve another. (c) Despite the promising potential of designer silicates, difficulty in large-scale production is still a stumbling block for their industrial applications because the currents synthesis methods are complicated, difficult to control, not eco-friendly, and results in small yields. It is envisaged that with the rapid development of new and facile synthesis methods, the listed challenges will result in exciting research outcomes, and designer silicates are bound to play a vital role in water treatment technologies. Acknowledgements We acknowledge the supports of Vaal University of Technology Research Directorate, the World Academy of Sciences (TWAS) Trieste Italy and the Chinese Academy of Sciences (CAS) China, for the award of CAS-TWAS Postgraduate Fellowship (FR number: 3240255024) to PNE Diagboya. References [1] P.N. Diagboya, B.I. Olu-Owolabi, D. Zhou, B.-H. Han, Carbon 79 (2014) 174–182. [2] S. Kabiri, D.N.H. Tran, M.A. Cole, D. Losic, Environ. Sci.: Water Res. Technol. 2 (2016) 390–402. [3] J. Gao, Y. Lu, X. Zhang, J. Chen, S. Xu, X. Li, X. Li, F. Tan, Appl. Surf. Sci. 349 (2015) 224–229. [4] B.I. Olu-Owolabi, A.H. Alabi, P.N. Diagboya, E.I. Unuabonah, R.-A. Düring, J. Environ. Manag. 192 (2017) 94–99. [5] P.N. Diagboya, B.I. Olu-Owolabi, K.O. Adebowale, J. Contam. Hydrol. 191 (2016)

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