Thiol-functionalized Mesoporous Silica Research Articles

Activated Carbon Mitigates Mercury and Methylmercury Bioavailability in Contaminated Sediments

There are few available in situ remediation options for Hg contaminated sediments, short of capping. Here we present the first tests of activated carbon and other sorbents as potential in situ amendments for remediation of mercury and methylmercury (MeHg), using a study design that combined 2 L sediment/water microcosms with 14 day bioaccumulation assays. Our key end points were pore water concentrations, and bioaccumulation of total Hg and MeHg by a deposit-feeding oligochaete Lumbriculus variegatus. Four amendments were tested: an activated carbon (AC); CETCO Organoclay MRM (MRM); Thiol-SAMMS (TS), a thiol-functionalized mesoporous silica; and AMBERSEP GT74, an ion-exchange resin. Amendments were tested in four separate microcosm assays using Hg-contaminated sediments from two freshwater and two estuarine sites. AC and TS amendments, added at 2-7% of the dry weight of sediments significantly reduced both MeHg concentrations in pore waters, relative to unamended controls (by 45-95%) and bioaccumulation of MeHg by Lumbriculus (by between 30 and 90%). Both amendments had only small impacts on microcosm surface water, sediment and pore water chemistry, with the exception of significant reductions in pore water dissolved organic matter. The effectiveness of amendments in reducing bioaccumulation was well-correlated with their effectiveness in increasing sediment:water partitioning, especially of MeHg. Sediments with low native sediment:water MeHg partition coefficients were most effectively treated. Thus, in situ sediment sorbent amendments may be able to reduce the risk of biotic Hg and MeHg uptake in contaminated sediments, and subsequent contamination of food webs.

Silver chloride loaded mesoporous silica particles and their application in the antibacterial coatings on denture base

Thiol-functionalized MCM-41 type mesoporous silica particles(MSPs) were prepared and loaded with silver chloride to act as antibacterial agents. The antibacterial activity of the silver chloride loaded MSPs(AgCl-MSPs) was evaluated by the minimum bactericidal concentration(MBC) against Candida albicans(ATCC 10231). The AgCl-MSPs with the highest antibacterial activity were then dispersed in hybrid coatings with different mass ratios to fabricate antibacterial coatings. The antibacterial activity of the coatings was tested against Candida albicans(ATCC 10231) and Streptococcus mutans(ATCC 25175). The resulting antibacterial coatings exhibited high antibacterial activity, good adhesion to the substrate and high hardness.

Synthesis of novel thiol-functionalized mesoporous silica nanorods and their sorbent properties on heavy metals

Novel thiol-functionalized mesoporous silica nanorods (MSNRs) were synthesized through a base co-condensation method, in which two organoalkoxysilanes, tetraethoxylsilane (TEOS) and bis[3-(triethoxysilyl)propyl]tetrasulfide (TESPT), were used as silica precursors simultaneously. TESPT was firstly used for both morphology control and inner surface functionalization of mesoporous silica hybrid materials. The microstructures as well as porous character of the MSNRs were characterized by means of SEM, XRD, TEM and N2 sorption measurements. Infrared spectrum analysis and heavy metal ions (Ag+ and Cd2+) adsorption measurements were carried out to confirm the functionalized framework of MSNRs.

Thiol functionalized mesoporous silicas for selective adsorption of precious metals

Thiol functionalized mesoporous silica (TFMS) with ordered hexagonal pore structure was fabricated by one-step synthesis pathway. The selective adsorption for precious metals was investigated using single component and binary adsorption solutions. The TFMS displayed strong affinity for gold or platinum in specific conditions with a large adsorption capacity, and showed a high selectivity in the binary solutions within 30min. Furthermore, gold and platinum could be totally recovered by elution with 5M HCl and 0.7M thiourea–2M HCl, respectively.

Characteristics of Thiol-Functionalized Mesoporous Silica and its Application to Silver and Cadmium Ion Removal

Characteristics of Thiol-Functionalized Mesoporous Silica and its Application to Silver and Cadmium Ion Removal

Aerosol-Assisted Synthesis of Thiol-Functionalized Mesoporous Silica Spheres with Fe3O4 Nanoparticles

Thiol-functionalized mesoporous silica spheres having Fe3O4 nanoparticles are fabricated in one-pot by aerosol-assisted synthesis. A TEM image shows that Fe3O4 nanoparticles are successfully embedded within the mesoporous silica spheres. SEM images and SAXS profiles reveals that the encapsulating Fe3O4 nanoparticles do not affect the ordering of a mesoporous structure. The spherical morphologies are also well retained. The presence of cage-type mesopores with uniform size is confirmed by N2 adsorption-desorption isotherms and TEM observations. The spray-dried thiol-functionalized particles with Fe3O4 nanoparticles effectively adsorb mercury (II) ions due to their strong interaction to thiol groups embedded in the framework. The particles with the amount of Fe3O4 nanoparticles (3.5 wt%) show a saturated magnetization (over 1.0 emu/g). This magnetic property is useful for practical collection with magnet.

Adsorption of lead from water by thiol-functionalized SBA-15 silicas

Thiol-functionalized mesoporous silicas have been synthesized by co-condensation of tetraethoxysilane and varying contents of 3-mercaptopropyltrimethoxysilane in acidic medium with the block copolymer Pluronic 123 as a structure directing agent. Adsorption of lead in aqueous solution on the synthesized materials has been investigated. The adsorption data are fitted to Langmuir isotherms and a maximum adsorption capacity calculated from the Langmuir equation can reach 0.19 mmol of Pb/g. The stoichiometric ratio of S: Pb being 1:1 has been obtained for every adsorbent. The effect of the pH on lead adsorption, and desorption of lead on the lead-loaded materials have been studied.

Active Mercury(II) Ion Removal: Stoichiometrically Controlled Thiol-Functionalized Mesoporous Silica by a Mass Production Spray Dry System

Abstract Stoichiometrically controlled thiol-functionalized mesoporous silica particles are synthesized by a spray dry technique in one step. The EDS (energy dispersive X-ray spectrometer) mapping shows that thiol groups are well dispersed throughout the particles without phase separation at micrometer scale. From TEM (transmission electron microscope) observation, SAXS (small-angle X-ray scattering) measurements, and N2 adsorption–desorption isotherms, it is proven that the thiol groups can be successfully embedded in the frameworks up to 30% without any collapse of ordered mesoporous structures. The amount of the embedded thiol groups in the final products can be stoichiometrically controlled by changing the compositions of the initial precursor solutions, as is confirmed by 29Si NMR measurements. The total adsorption capacity of mercury(II) ions was increased in proportion to the amount of the thiol groups embedded in the frameworks.

Factors affecting the reactivity of thiol-functionalized mesoporous silica adsorbents toward mercury(II)

Numerous mercaptopropyl-functionalized silica spheres have been prepared by either post-synthesis grafting of MCM-41 and MCM-48 or self-assembly co-condensation of mercaptopropyltrimethoxysilane (MPTMS) or mercaptopropyltriethoxysilane (MPTES) and tetraethoxysilane (TEOS) precursors in hydroalcoholic medium in the presence of a cationic surfactant as templating agent and ammonia as catalyst. These materials of approximately the same particle size and morphology featured different functionalization levels, various degrees of structural order, and variable distribution of thiol groups in the mesopores. Their reactivity in solution has been studied using Hg(II) as model analyte. Total accessibility (on a 1:1 S:Hg stoichiometry basis) was demonstrated and quantified for well-ordered materials whereas less open and less organized structures with high degrees of functionalization were subject to less-than-complete loadings. Capacities measured at pH 2 were lower than at pH 4 because of distinct mercury-binding mechanisms. Kinetics associated to the uptake process were studied by in situ electrochemical monitoring of Hg(II) consumption from aqueous suspensions containing the various adsorbents. They indicate only little difference between materials of the MCM-41 and MCM-48 series at similar functionalization levels, fast mass transport in well-ordered mesostructures in comparison to the poorly or non-ordered ones (except at pH 2 where charge formation induced some restriction in materials characterized by long-range structural order), and even faster processes in the wormlike frameworks (characterized by shorter range structural order). Hg(II) binding to thiol-functionalized materials obtained by post-synthesis grafting was found to occur more rapidly in the early beginning of the uptake process as a result of a higher concentration of binding sites at the pore entrance in comparison to the more homogeneous distribution of these groups in the mesochannels of materials obtained by co-condensation.

Preparation and Adsorption Properties of Thiol-Functionalized Mesoporous Silica Microspheres

Mesoporous silica microspheres (MSM) approximately 1.0 μm in diameter were prepared by the hydrolysis and condensation of tetraethoxysilane (TEOS) under the presence of dodecylamine (DDA) as a molecular template and catalyst in ethanol−water solution. Isolated MSM were obtained in a narrow range of experimental conditions, which were experimentally correlated by the linear relationship between the molar ratio of ethanol and TEOS and that of DDA and TEOS. Isolated MSM modified with thiol groups were also prepared under similar experimental conditions, by co-condensation of TEOS and 3-mercaptopropyltrimethoxysilane (MPTMS). The thiol-functionalized MSM (SH-MSM) adsorbed silver ions from an aqueous solution. Asymmetrical polystyrene film, in which the SH-MSM were self-assembled on the surface of the water phase side of the film, was obtained by spreading and polymerizing of styrene monomer with dispersed SH-MSM on the water surface. The polymer film containing SH-MSM also effectively adsorbed silver ions.

Preparation and Characterization of Sulpho-Functionalized Ordered Mesoporous Silica

Sulpho groups have been successfully embedded in mesoporous silica with an ordered two-dimensional hexagonal structure. The sulpho-functionalized mesoporous silica was prepared by the oxidation of a thiol-functionalized mesoporous silica which was obtained via the hydrolysis and co-condensation of tetramethoxysilane (TMOS) and mercaptopropyl trimethoxysilane (MPTMS). The samples exhibited an ordered two-dimensional hexagonal pore array up to 30 mol% MPTMS loading. The large surface area of MCM-41 (ca. 1000 m2/g) was maintained after incorporation of the mercaptopropyl group and oxidation of its thiol group. Solid-state 29Si NMR spectra showed the existence a mercaptopropyl group in a silica framework and 13C NMR studies further proved that treatment with hydrogen peroxide led to oxidation of the sulpho groups. High ammonia uptakes with high differential heats of adsorption revealed the strong acidity of sulpho-functionalized mesoporous silica. The amounts of acid sites may be controlled by regulating the amount of MPTMS employed in the synthesis. The sulpho-functionalized mesoporous silica behaved as an acid catalysis for the isomerization reaction of but-1-ene to cis- and trans-but-2-ene.

Bifunctionalized Mesoporous Silicas for Cr(VI) Reduction and Concomitant Cr(III) Immobilization

Surfactant-templated thiol-functionalized mesoporous silica adsorbents have been prepared by cocondensation of mercaptopropyltrimethoxysilane and tetraethoxysilane in the presence of cetyltrimethylammonium bromide, which were then partially oxidized to get bifunctionalized materials containing both thiol and sulfonic acid moieties (MCM-41-SH/SO3H). The resulting organic-inorganic hybrid was applied to the uptake of chromium species according to a reduction-sorption mechanism involving reduction of Cr(VI) by thiol groups and immobilization of Cr(III) onto sulfonic acid moieties. These processes were strongly affected by pH, and the optimal conditions for effective chromium sequestration resulted from a compromise between pH values low enough to ensure quantitative reduction of Cr(VI) and not too low to enable Cr(III) binding to sulfonate groups, which was best achieved at pH 2-3. The effect of the solid-to-solution ratio and the relative amounts of -SH and -SO3H groups was also discussed. Even if Cr(VI) reduction by thiol groups resulted in the formation of sulfonic acid moieties, their contentwas not high enough to ensure quantitative Cr(III) immobilization, which was only attained with materials containing already some sulfonic acid groups prior to contacting Cr(VI) solutions. Redox speciation of sulfur and chromium species was analyzed by X-ray photoelectron spectroscopy (XPS) and used to support the proposed mechanism.

Ordered thiol-functionalized mesoporous silica with macrostructure by true liquid crystal templating route

Ordered thiol-functionalized mesoporous silica has successfully been synthesized via co-condensation of tetramethyl orthosilicate (TMOS) and 3-mercaptopropyltrimethoxysilane (MPTMS) in lyotropic liquid crystalline phase of Brij 56 surfactact (C 16EO 10). In contrast to the common synthesis in micellar solution, true liquid crystal templating synthesis can easily fabricate organic-functionalized mesoporous silica with desirable macro-morphology and the loading amount of organic moieties can also be controlled well. The resultant materials were characterized by X-ray diffraction (XRD), transmission electron microscope (TEM), nitrogen sorption, polarized optical microscopy, Fourier transform infrared (FT-IR) spectra, differential scanning calorimetry and thermogravimetric analysis (DSC–TGA), and elemental analyses (EA). Direct evidence of the presence of chemically attached thiol groups was provided by FT-IR, DSC–TGA and EA. Adsorption measurement indicates that the materials are effective for the removal of heavy metal ions from aqueous solutions. The Hg 2+ adsorption isotherm fits Langmuir behavior, but the Type-IV adsorption isotherm for Cd 2+ was unexpectedly observed, which is probably attributed to significantly different Cd 2+ affinity ability for the thiol groups on the external surface and those lining the framework pore channels of the nanoporous adsorbent.

13C CPMAS NMR Spectroscopy as a Versatile and Quantitative Tool for Determination of Mercury Adsorption Capacity in Thiol-Functionalized Mesoporous Silica SBA-1

13C cross-polarization magic angle spinning (CPMAS) NMR spectroscopy is demonstrated to be a valuable characterization tool for quantitative measurements of Hg2+ adsorption capacity in thiol-functionalized mesoporous silica SBA-1 (Santa Barbara Amorphous-1). This is the first report on the investigation of the spectral change in the 13C CP signals for mercaptopropyl-functionalized mesoporous materials doped with different Hg2+ concentrations. The chemical shift of the carbon atom adjacent to the thiol group is sensitive to the binding of the mercury ion, and its peak intensity can be used as a quantitative sensor for the amount of the mercury ion adsorbed. The 13C CPMAS NMR results are in good agreement with inductively coupled plasma-atomic emission spectroscopy (ICP-AES) analysis.

Synthesis of Mercaptopropyl Functionalized Nanosized Mesoporous Silica

Thiol-functionalized nanosized mesoporous silica particles with a diameter of 50-200 nm were prepared and characterized by TEM, TG, and DTA. (3-mercaptopropyl) trimethoxy silane (MPTMS) was introduced into the silica networks through three synthesis routes. The mercapto group induces significant changes in the particle morphology, transforming highly ordered hexagonal particles into nanoscale bars. The size of mesoporous silica can be adjusted by controlling reacting time. The addition of triethanolamine instead of NaOH leads to the formation of functionalized spherical particles having a diameter of ca 100 nm. The template can be extracted by HCl-ethanol solution for protecting thiol functional group. In addition, the formation mechanism of the mesoporous silica particles was also discussed.

Influence of synthesis conditions on mercury adsorption capacity of propylthiol functionalized SBA-15 obtained by co-condensation

Thiol-functionalized mesoporous silicas for aqueous mercury removal by adsorption have been synthesized by co-condensation of 3-mercaptopropyltrimethoxysilane (MPTMS) and tetraethoxysilane (TEOS) in acidic medium with the block copolymer Pluronic 123 as structure directing agent. The influence of two synthesis parameters, the time of TEOS prehydrolysis and the molar ratio x = MPTMS/(MPTMS + TEOS), on both the textural properties and mercury adsorption capacities have been investigated. Materials were characterized by low-angle XRD, nitrogen adsorption–desorption, 29Si NMR, TGA–DTA, transmission electron microscopy and elemental chemical analysis. Adsorbents are mostly mesostructured SBA-15 type silicas with suitable textural properties for mercury adsorption and exhibit thermal stability up to at least 285 °C. The amount of propylthiol incorporated to the materials is in accordance with the MPTMS/TEOS proportion in the synthesis medium (up to 4.1 mmol of S per gram of adsorbent). We have found that it is neccessary to employ a minimal amount of TEOS in order to preserve the mesoporous structure in the resulting material when a high quantity of propylthiol groups is incorporated. Prehydrolysis of silica precursor (TEOS) for 45 min before MPTMS addition leads to successful results. Longer prehydrolysis times can improve the textural and structural properties of materials but the amount of propylthiol incorporated and the mercury adsorption capacity are not superior. Experimental isotherms show that materials are efficient adsorbents of mercury at low aqueous concentration with maximum adsorption capacity as high as 4.1 mmol of Hg(II) per gram of adsorbent.

The preparation of Ag/mesoporous silica by direct silver reduction and Ag/functionalized mesoporous silica by in situ formation of adsorbed silver

Mesoporous silica (MS) with large pores and thiol functionalized mesoporous silica (TFMS) were synthesized. 29Si MAS NMR confirmed the functionalization of MPTMS on the surface of mesoporous silica. Silver nanoparticles were prepared by two methods: (1) direct reduction of Ag + ions with NaBH 4 in aqueous AgNO 3 solution containing MS, (2) in situ reduction of Ag + ions adsorbed on TFMS with NaBH 4. The characteristics of products from both methods were compared using SAXRD, TEM, and N 2 adsorption–desorption. Ag nanoclusters were mostly confined and dispersed in the channels of the TFMS and their sizes were under 6 nm. However, Ag nanoparticles on the MS formed outside the mesoporous channels rather than within them.

Effect of spin configuration on the reactivity of cytochrome c immobilized in mesoporous silica

Cytochrome c (cytc), a heme protein with a positive electric charge, is immobilized in the nanochannels of mesoporous silica (MPS) by either electrostatic forces or covalent bonding. The electrostatic interaction between cytc and MPS arises from the introduction of aluminum into the framework of MPS to produce a negative charge on the porous surface (Al-MPS). The covalent bonding arises from the binding between the heme iron center and the –SH group of mercaptotriethoxysilane in thiol-functionalized MPS (MPS-SH). The nanochannels of MPS provide a confining space that can prevent cytc from protein unfolding and preserve its activity. Cytc immobilized in Al-MPS exists in a high-spin state, as inferred from ESR and UV–Vis studies. This is different from native cytc, which shows primarily the low-spin state. The high-spin state arises from the replacement of the Met-80 ligands of heme Fe(III) by water or silanol groups on the silica surface, which can open up the heme groove for ready access of oxidants to the iron center and facilitate catalytic activity. MPS-SH-supported cytc can exist in both high- and low-spin states. The low-spin state arises from the replacement of the axial ligands of heme Fe(III) by the –SH group, which can result in poisoning of the active site and reduce the catalytic activity with respect to the decomposition of hydroperoxides. In ESR spin trapping experiments, we show that cytc catalyzes homolytic cleavage of the O–O bond of hydroperoxide and generates a protein cation radical (g = 2.00). A possible mechanism for the MPS-cytc catalytic oxidation of hydroperoxide is proposed based on a spectroscopic characterization of the system.

Mercury(II) binding to thiol-functionalized mesoporous silicas: critical effect of pH and sorbent properties on capacity and selectivity

The binding properties of mesoporous thiol-functionalized silica sorbents towards mercury(II) species were studied as a function of pH in a wide range (0–8), in the absence or in the presence of competing metal ions, from batch equilibration experiments. To this end, a series of thiol-functionalized adsorbents characterized by different structures (from completely disordered amorphous solids to highly ordered mesostructures), variable density of organic ligands (from 1 to 4 mmol g −1), and various degrees of porosity, have been prepared either by post-synthesis grafting or by the co-condensation route. Hg(II) binding to these thiol-functionalized silica samples is strongly dependent on pH, especially in acidic medium (pH < 4) where non-hydrolyzed Hg 2+ species become dominant. This behavior was found to be significantly affected by the degree of structural organization of the materials (amorphous or ordered mesoporous solids, short-range versus long-range structural order) and the adsorbent composition (density of functional groups). A beneficial effect of high structural order was observed in both the capacity (access to a high number of binding sites) and selectivity (towards other metal ions) for the ordered mesoporous sorbents in comparison to the amorphous gels, but this was only true for pH values down to 4, where Hg(II) species are mainly in the form of Hg(OH) 2. In more acidic medium, however, the sorption of the non-hydrolyzed Hg 2+ species underwent dramatic loss of effectiveness, which resulted in both lower capacities and worse selectivity. These restrictions were more marked when increasing the density of functional groups in the materials and, to lesser extent, when decreasing their level of structural ordering. They were interpreted on the basis of electrostatic considerations as the binding of Hg 2+ to thiol groups leads to the generation of positively charged complexes in the host material while that of Hg(OH) 2 involves the formation of neutral moieties. Possible regeneration of sorbents and re-use were also discussed.

Aqueous and vapor phase mercury sorption by inorganic oxide materials functionalized with thiols and poly-thiols

The objective of the study is the development of sorbents where the sorption sites are highly accessible for the capture of mercury from aqueous and vapor streams. Only a small fraction of the equilibrium capacity is utilized for a sorbent in applications involving short residence times (e.g., vapor phase capture of mercury from coal-fired power plant flue gases). So, dynamic capacity rather than equilibrium capacity is more relevant for these kinds of situations. Rapid sorption rates and higher dynamic capacity can be achieved by increasing the accessibility of active sites and decreasing the diffusional resistance to mass transport for the adsorbing species. This requires the use of open structured sorbent materials and attachment of functional groups on the external surface area of supports. The strong interaction of sulfur containing ligands (e.g., thiol) with mercury makes them suitable candidates for immobilization on these types of materials. In this study, inorganic oxide supports like alumina and silica are functionalized with thiol moieties like mercapto silane, cysteine and poly-cysteine for capturing mercury from aqueous and vapor phase. Aqueous phase Hg (II) sorption studies with cysteine/poly-cysteine functionalized silica showed that high dynamic capacity can be achieved by attaching active sites (thiol) on the external area of supports. Vapor phase Hg capture studies with thiol-functionalized mesoporous silica (Hg0 concentration = 3.37 mg/m3 with N2 as the carrier, gas temperature = 70 °C) yielded a capacity of 143 μg Hg/g for the sorbent. Although the sulfur content for the sorbent was low (0.80 wt. %) the molar ratio of Hg captured to sulfur was comparatively high (2.86×10−3) pointing to the high accessibility of sulfur sites.