Environ. Sci. Technol. 2001, 35, 984-990
Highly Effective Adsorption of Heavy Metal Ions by a Thiol-Functionalized Magnesium Phyllosilicate Clay ISABELLE L. LAGADIC,* MOLLY K. MITCHELL, AND BRYAN D. PAYNE Department of Chemistry, Oklahoma State University, Stillwater, Oklahoma 74078
A thiol-functionalized layered magnesium phyllosilicate material (called Mg-MTMS), prepared by a direct and costeffective co-condensation synthesis, has been investigated as a high-capacity adsorbent for heavy metal ions. Structural characterization by powder X-ray diffraction, infrared spectroscopy, solid-state 13C and 29Si NMR spectroscopies, and elemental analyses confirms the smectite-type structure and the high organic moiety content of this material. Mg-MTMS was found to be highly effective for the adsorption of Hg(II), Pb(II), and Cd(II) ions, exhibiting unprecedented metal ion uptake capacities of 603, 365, and 210 mg of metal/g of adsorbent, respectively. Mg-MTMS shows an equivalent affinity for the three metal ions, removing them from mixed metal solutions with an equal ion uptake capacity (∼400 mg of metal/g of adsorbent). Metal-loaded Mg-MTMS can be regenerated by acid treatment without altering the adsorbent properties. The high effectiveness of Mg-MTMS for the capture of metal ions is attributed to both the high concentration of complexing thiol groups (6.4 mmol of SH/g of Mg-MTMS) and the expansion capability of the framework, which facilitates the accessibility of the binding sites.
Introduction Water pollution by toxic metals remains an important environmental issue having a major impact on the public health and the economy. It has been reported (1) that, in terms of the quantity of water needed to dilute such wastes to drinking-water standards, the annual toxicity of all metals mobilized exceeds the combined total toxicity of the radioactive and organic wastes generated each year. In response to these problems, there has recently been a growing interest in the development of materials capable of removing low concentrations of toxic metal ions from contaminated waters (2). Effective adsorbents with a strong affinity and, subsequently, a high loading capacity for targeted metal ions have been prepared by functionalizing the surface of various substrates, such as polymers (3-6) or silicates [e.g., silica gel (7-10), clays (11-14), mesoporous molecular sieves (1517)], with metal complexing groups [e.g., crown ethers (1820), amines (21, 22), thiols]. Recent research (23) has focused on the preparation of thiol-functionalized adsorbents since they were expected to exhibit a specific binding ability toward highly toxic heavy metal ions such as Hg2+, Pb2+, and Cd2+ as a consequence of a soft Lewis acid-soft Lewis base interaction (24). * Corresponding author phone: (405)744-5941; fax: (405)744-6007; e-mail:
[email protected]. 984
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 35, NO. 5, 2001
Among the recently investigated adsorbent materials, thiol-functionalized mesoporous molecular sieves have attracted particular attention (23, 25-29). It has been shown (30) that the large uniform pore structure of these materials greatly facilitates the access of the metal ions to the thiol binding sites, resulting in improved metal-loading capacities as compared to those of substrates exhibiting a more irregular porosity such as silica gels. Initially, thiol functionalization of the internal surface of mesoporous materials was achieved by grafting mercaptosilylpropyl moieties through the hydrolysis of mercaptopropylsiloxane chains followed by condensation reactions with the hydroxyl groups of the channel walls (23, 25). Adsorbents with high metal-loading capacities (i.e., up to 505 mg (23) of Hg2+/g of adsorbent) were obtained using this procedure. However, by this method, the quality of the functional layer attached to the substrate is greatly affected by several factors such as the concentration of the silanol anchoring groups and their accessibility to the organic chains as well as the amount of physically adsorbed water (16). Control over these parameters requires that special care be given to the preparation of the mesoporous substrates and to the drying of the solvents used in the functionalization process. In some cases (23), repeated hydrolysis and silylation cycles are necessary to obtain a uniform coverage of the channel surface with thiol functionalities. To circumvent these difficulties, another approach for the functionalization of mesoporous silicates has recently been proposed (15, 17, 31), which consists of co-condensing siloxane and organosiloxane precursors in a templating environment. This one-step synthesis allows for the preparation of functionalized mesoporous solids with higher loading of organic functional groups and a more homogeneous surface coverage (32). Using this procedure, thiolfunctionalized mesoporous materials containing more thiol groups [from 2.3 (33) to 4.7 (28) mmol of SH/g of material] than chemically grafted analogous materials (25) (1.5 mmol of SH/g of material) were prepared. Surprisingly, the increase in binding site concentration does not necessarily result in higher metal loading onto the adsorbent. For example, very recently a co-condensed thiol-functionalized mesoporous material (denoted MP(2)-MSU-2) (33) with a SH content of 2.3 mmol/g of material was found to adsorb Hg(II) ions with a capacity of up to 461 mg (2.3 mmol) of metal/g of adsorbent. Previously, a thiol-functionalized MCM-41 solid also synthesized by co-condensation (28) and containing 4.7 mmol of SH/g of material had been reported to exhibit a Hg(II) ion-loading capacity of 421 mg (2.1 mmol) of metal/g of adsorbent. These experiments demonstrated that the concentration of SH groups is not the only factor affecting the metal adsorption capacity of mesoporous adsorbents. Because of the uniform pore structure of these adsorbents, the immobilization of metal ions is also controlled by their accessibility to the binding sites (25). Consequently, porous structures that retain a significantly open framework after functionalization of their pore walls will likely be more effective for the capture of metal ions. As a second consequence of the uniform pore structure, thiol-functionalized mesoporous adsorbents were found to demonstrate a high selectivity for Hg(II) ions while they exhibited a negligible affinity for Pb(II) and Cd(II) ions (26, 33). To explain this remarkable selectivity for the Hg(II) ions, Mercier et al. (26) postulated that the restricted volumes of the pore channels might decrease the variation of entropy associated with the reaction between the metal ions and the SH sites. As a result, this reaction loses its spontaneity except in the case of Hg(II) ions. In addition, as previously reported (33), the high 10.1021/es001526m CCC: $20.00
2001 American Chemical Society Published on Web 01/25/2001
preparation cost of some of these mesostructured adsorbents may limit their extensive use and commercialization. On the basis of the results presented above for thiolfunctionalized mesostructures as metal adsorbents, we can anticipate that silicate materials with a more open framework would offer higher metal ion loadings as well as a better affinity toward Pb(II) and Cd(II) ions. Naturally occurring, clay minerals offer a pore structure more flexible than that of molecular sieves since usually the interlayer space can expand to accommodate the guest species. Thiol-functionalized montmorillonite clay (called thiomont) synthesized by chemical grafting (13) has been reported as an heavy metal adsorbent. Although this material was found to be able to immobilize Hg(II), Pb(II), and Cd(II) ions, the metal-loading capacity for each of the ions was rather low (65, 70, and 27 mg of metal/g of adsorbent, respectively). In this paper, a thiol-functionalized layered magnesium phyllosilicate was prepared from mercaptopropyltrimethoxysilane and MgCl2 under basic conditions using a rapid, environmentally friendly, and low-cost co-condensation synthesis method in order to achieve a high loading of SH functionalities. The effectiveness of this material (hereafter denoted Mg-MTMS) for the adsorption of Hg(II), Pb(II), and Cd(II) ions was investigated and compared to that of analogous adsorbents. Unprecedented metal-loading capacities were obtained that were explained by the high content of SH binding groups and their easy accessibility due to the expansion capability of the flexible framework. Regeneration and reusability of the adsorbent was also studied to evaluate the potential of such a material in heavy metal removal and environmental cleanup technology.
Experimental Section Preparation of the Adsorbent: Mg-MTMS. The thiolfunctionalized magnesium phyllosilicate material was prepared in a one-step silylation process as previously described (34). Typically, 22.0 mmol (4.11 mL) of mercaptopropyltrimethoxysilane (Gelest, MTMS) was mixed with a stirred solution of 16.5 mmol (3.4 g) of MgCl2‚6H2O in methanol (100 mL). Aqueous sodium hydroxide (0.05 M, 400 mL) was then added to the mixture. After being stirred overnight, a white precipitate formed, was isolated by centrifugation, thoroughly washed with deionized water (5 times 40 mL) and ethanol (2 times 40 mL), and then dried at 65 °C in air to yield a white powder (denoted Mg-MTMS). Material Characterization. Samples of Mg-MTMS were characterized by powder X-ray diffraction, infrared spectroscopy, 13C and 29Si solid-state NMR spectroscopies, and elemental analysis. The surface area was also measured by BET analysis, using a NOVA 2200 (Quantachrome, Inc.) instrument. Powder X-ray diffraction patterns were collected on a Bru ¨ ker D8 diffractometer, using Cu KR radiation (λ ) 1.5405 Å). Infrared spectra were recorded in the 4000-400 cm-1 range using a Nicolet FTIR Prote´ge´ 460 spectrophotometer equipped with a diffuse reflectance accessory (DRIFTS). The samples were diluted (3% w/w) in KBr. The 13C and 29Si CP MAS NMR spectra were obtained at room temperature on a Chemagnetics CMX-300 spectrometer, using tetramethylsilane (TMS) as a reference sample. A frequency of 75 MHz, a sample spinning speed of 8 kHz, and a pulse delay of 3 s were used for the 13C NMR spectra, while for the 29Si NMR spectra, these parameters were 60 MHz, 6 kHz, and 10 s, respectively. The organic content of the adsorbent was determined by microanalyses performed by Atlantic Microlab, Inc. (Norcross, GA). The analyses for the metals were carried out by Desert Analytics (Tucson, AZ) and by Galbraith Laboratories (Knoxville, TN). Adsorption of Heavy Metal Ions: Pb2+, Hg2+, and Cd2+. Homoionic aqueous solutions of Pb(II), Cd(II) and Hg(II) ions in known concentrations, ranging from 0 to 20 ppm,
were prepared from the nitrate salts of the metals. A total of 100 mL of each solution was treated with 10 mg of Mg-MTMS with vigorous stirring for 12 h at room temperature. The metal ion uptake capacity of Mg-MTMS was determined by analyzing the metal concentrations of the solutions before and after treatment. Separate experiments were carried out to determine the maximum loading capacity corresponding to the saturation of the adsorbent with each of the metal ions. This maximum was calculated from the metal content of the adsorbent after treatment of homoionic solutions, containing 1 g of the metal nitrate in 100 mL of deionized water, with 100 mg of Mg-MTMS for 18 h at room temperature. The solid samples were thoroughly washed with waterD, and then dried in air prior to their analysis for adsorbed metal content. The Hg(II)-loaded derivative was also characterized by powder X-ray diffraction. Reaction times of 12 and 18 h, respectively, were used in order to allow enough time for the equilibrium to be reached as well as to perform the adsorption reactions on Mg-MTMS in conditions similar to those used for analogous adsorbents. The pH of the Pb(NO3)2 and Cd(NO3)2 solutions was 4.7 and 5.2, respectively. In the case of the Hg(II) ion adsorption experiments, a few drops of nitric acid were added to each Hg(NO3)2 solution in order to prevent the formation of oxides by maintaining a pH around 3.0. Ion competitive adsorption studies were conducted by treating 100 mL of a mixed metal solution containing nearly equimolar amounts of Pb(II), Cd(II), and Hg(II) ions with 10 mg of the adsorbent for 18 h at room temperature. It is noteworthy to mention that MgMTMS undergoes a distinctive color change from white to yellow upon the adsorption of Pb(II) ions. This change is particularly dramatic when Mg-MTMS is treated with solutions containing Pb(II) ions only and may be used to monitor the progress of the Pb(II) ion immobilization process. No color change was observed upon adsorption of Hg(II) or Cd(II) ions. Additional experiments were carried out in order to obtain a preliminary description of the sorption kinetics. The 10-mg samples of Mg-MTMS were treated with 100 mL of an aqueous solution of lead nitrate containing 10 ppm of Pb(II) ions. The supernatant solutions were analyzed for their lead content after respectively 15 min, 30 min, 2 h, and 4 h of reaction. Regeneration of the Adsorbent. The regeneration of MgMTMS after metal ion adsorption was tested using a procedure adapted from the literature (13). A fully leadsaturated Mg-MTMS sample, obtained by mixing 100 mg of Mg-MTMS with 1 g of Pb(NO3)2 in 100 mL of deionized for 18 h, was stirred in 150 mL of aqueous 5 N HCl. The yellow lead-loaded adsorbent immediately turned white. The solution was stirred overnight, and then the white powder was collected by filtration, washed with deionized water, and dried in air. The lead contents of the solid samples were determined before and after acid treatment to evaluate the effectiveness of this regeneration process. The reusability of Mg-MTMS as a heavy metal adsorbent was assessed by retreating the leached material with a Pb(NO3)2 solution, using the procedure described above. The yellow powder collected after treatment with this solution was washed with deionized water and dried in air. Its lead content was determined by elemental analysis.
Results Structural Characterization of the Adsorbent Mg-MTMS. The powder X-ray diffraction pattern of Mg-MTMS, represented on Figure 1, confirms the layered structure of the material. Reflections consistent with, but broader than, those of a 2:1 trioctahedral phyllosilicate structure were observed. The broadness of the peaks has been explained (35) in terms of intralayer and/or stacking disorder due to the presence of the organic chains. Based on the 001 reflection, we VOL. 35, NO. 5, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
985
FIGURE 1. Powder X-ray diffraction pattern of Mg-MTMS. calculated an interlayer distance of 12.1 Å. This value corresponds to a basal spacing expansion of about 2.7 Å as compared to the related talc structure (d001 ) 9.34 Å), thus suggesting that the organic moieties are directed toward the interlayer space. Similar results have been reported in the case of the silylation of the interlayer space of montmorillonite (13). However, such an increase does not allow the accommodation of a bilayer arrangement of the -(CH2)3-SH chains, whose length can be estimated at 6.1 Å. Similarly, this basal spacing increase cannot be the result of interdigitated or alternated organic chains only, as suggested previously by Mann et al. (35). We found that to account for an interlayer increase of only 2.7 Å over the parent talc compound, the organic moieties are likely to be titled at an angle of approximately 60° into the interlayer space in addition to being interdigitated or alternated. This also explains the observed disorder. The specific surface area of Mg-MTMS was measured by the BET method and was found to be 58 m2/g of material. The 13C CP MAS NMR spectrum of Mg-MTMS is shown in Figure 2. Intense resonance peaks are observed, indicating a high loading of organic moieties. In addition, the sharpness of the signals suggests that the organic groups remain fairly mobile within the interlayer space. The signal appearing in the 12-13 ppm range was assigned to the carbon (C3) directly bound to the silicon, while the most intense resonance, in the 27-28 ppm range, was attributed to the other two methylene carbons (C2 and C1 next to SH) of the propyl chain. A minor resonance signal was also observed at 24 ppm. The origin of this peak is still unclear but may be due to the C1 and C2 carbons of dipropyl disulfide, which may be formed as the result of an oxidation reaction between two adjacent thiol groups (28). This interpretation is consistent with the presence of an unresolved small peak at 43 ppm, which could then be attributed to the C3 carbon of the disulfide. Solid-state 29Si CP MAS NMR spectroscopy of Mg-MTMS (see Supporting Information) indicates a partial condensation of the silicon species. Similarly to Mann et al. (34, 35) we found that, in addition to fully condensed T3 (-69 ppm) silicon species, partially condensed T1 (-50 ppm) and T2 (-58 ppm) species were also present in Mg-MTMS (Tn ) RSi(OMg)(OSi)n-1(OR′)3-n, where R′ ) H or alkyl depending on the degree of hydrolysis). This incomplete condensation of the silicon species was attributed to the geometric constraints occurring at the organic-inorganic interface. Figure 3 shows the FT-IR spectrum of Mg-MTMS. The presence of a significant amount of mercaptopropylsilyl moieties was confirmed by the observation of intense bands assigned to the C-H vibrations in the 2850-2930 and 12501500 cm-1 regions and of stretching bands attributed to the S-H and C-S vibrations at 2555 and 686 cm-1, respectively. 986
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 35, NO. 5, 2001
FIGURE 2.
13C
CP MAS NMR spectrum of Mg-MTMS.
Additional bands characteristic of the inorganic framework (MgO-H at 3702 cm-1 and Si-O-Si at 1024 cm-1) can also be observed. The amount of mercaptopropylsilyl moieties contained in Mg-MTMS was quantitatively determined from the C, H, and S elemental analyses of the adsorbent: 23.8 wt % C, 5.13 wt % H, 20.59 wt % S. From these results, we calculated that Mg-MTMS contains 6.4 mmol of SH groups/g of adsorbent. To the best of our knowledge, this value represents the highest amount of thiol functionalities per gram of adsorbent yet reported for this type of material, being from two to five times greater than those obtained for thiol-functionalized mesoporous materials synthesized by direct co-condensation. Compared to thiomont (13), another smectite-type material functionalized with mercaptosilane, Mg-MTMS contains twice as many thiol groups. From the complete set of elemental analyses (19.42 wt % Si, 13.56 wt % Mg, 23.8 wt % C, 5.13 wt % H, 20.59 wt % S, and 24.26 wt % O), we found that Mg-MTMS correlated closely with the 2:1 trioctahedral phyllosilicate structure composition: [Si8R8Mg6O16(OH)4, where R ) (CH2)3SH)] derived from the talc structure, as reported in the literature (34). Heavy Metal Ion Adsorption Studies. The results for the adsorption of Pb(II), Cd(II), and Hg(II) ions by Mg-MTMS from homoionic solutions of different concentrations are gathered in Table 1. The very efficient binding ability of the adsorbent for those ions reduces the concentration of metals by approximately 99% in nearly all the solutions, resulting in negligible residual metal concentrations [10 ppm), MgMTMS was found to be more effective in the removal of the metal ions than the analogous thiol-functionalized smectite, thiomont (13). The amounts (in milligrams) of lead, mercury, and cadmium adsorbed per gram of Mg-MTMS were respectively two, three, and five times greater than those
FIGURE 3. Diffuse reflectance FT-IR spectrum of Mg-MTMS.
TABLE 1. Adsorption of Hg(II), Pb(II), and Cd(II) Ions by Mg-MTMS from Homoionic Solutions metal ions Hg(II) Pb(II) Cd(II)
initial concn (ppm)
concn after treatment (ppm)
M(II) ion adsorbed (mg/g)a
19 (18.00)b 12 6 18 (15.21)b 11 4 17 (14.42)b 10 6
