JOURNAL OF ENVIRONMENTAL SCIENCE AND HEALTH Part A—Toxic/Hazardous Substances & Environmental Engineering Vol. A39, Nos. 11–12, pp. 2855–2866, 2004
Organo-Silicate Nanocomposites for the Removal of Chlorinated Phenols from Aqueous Media: Kinetics and Environmental Stability Tarek M. Abdel-Fattah* and Brian Bishop Department of Biology, Chemistry, and Environmental Science, Christopher Newport University, Newport News, VA
ABSTRACT Organosilicate nanocomposite hexagonal mesostructure (NHMS) was synthesized from dodecylamine and tetraethyl orthosilicate (TEOS). The nanocomposite (NMCM) of numbers 41, 48, and 50 were synthesized from cetyltrimethylammonium bromide (CTABr) and (TEOS). The rates, affinity, and stability of these synthetic nanocomposite materials to remove and retain chlorinated phenols from aqueous solution were investigated; all materials have the ability for sorption and retention of 2,4-dichlophenol (2,4-DCP). Batch absorption kinetics indicates that NHMS, NMCM-41, NMCM-48, and NMCM-50 are 1st order reactions, with rate constants of 0.412, 0.296, 0.112, and 0.0161 hr1, respectively. Average percent 2,4-DCP removal for NHMS, NMCM-41, NMCM-48, and NMCM-50 was 92, 98, 90, and 52%, respectively. Isothermic measurements fit Freundlich and Langmuir models. NHMS and NMCM-41 best fit Langmuir model. Stability of the adsorbents in 0.10 M CaCl2 for 56 days was in the order: NMCM41 > NHMS > NMCM-50 > NMCM-48.
*Correspondence: Tarek M. Abdel-Fattah, Department of Biology, Chemistry and Environmental Science, Christopher Newport University, Newport News, VA 23606; E-mail:
[email protected]. 2855 DOI: 10.1081/LESA-200034024 Copyright & 2004 by Marcel Dekker, Inc.
1093-4529 (Print); 1532-4117 (Online) www.dekker.com
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Abdel-Fattah and Bishop Key Words: Chlorinated Mesostructure; Adsorption.
phenols;
Remediation;
Nanocomposite;
INTRODUCTION Dichlorophenols (DCP) are aromatic compounds used as antiseptics, herbicides, wood preservatives, and in the paper and pulp industry. Additionally, they are intermediates in the synthesis of other chlorinated phenols. At significant levels, exposure to DCP can cause death, chronic exposure results in kidney and liver damage. The U.S. Environmental Protection Agency has placed 2,4-dichlorophenol on the Water Contaminant Candidate List.[1] In addition, 2,4-DCP is toxic to aquatic organisms.[2] Organosilicate nanocomposites (OCS) are crystalline lattice structures prepared from tetraethylorthosilicate (TEOS) and either an ionic (cetyltrimethylammonium) or neutral (dodecylamine) surfactant. As-synthesized nanoporous materials have orderly pore openings and specific crystalline phases: nanocomposite hexagonal mesostructure (NHMS)[3] and nanocomposite NMCM-41 have independent channels, NMCM-48 has cubic and interconnected channels, while NMCM-50 has layered structure.[4] All materials have large surface area per gram ratios making them excellent candidates for pollutant remediation.[5] Previous studies have been accomplished using both as-synthesized and calcined MCM-41 and MCM-48 for the removal of benzene from gas streams as well as the use of MCM-41 for the removal of trichloroethylene and tetrachloroethylene from water.[5,6] The Vartuli et al.[6] study only concentrated on sorption capacities while the Zhao et al.[5] focused on sorption capacity as well as stability. Additionally, several studies have examined the use of inorgano-organo-clays and clay-organic complexes as adsorbents for chlorophenol.[7,8] Wibulswas et al.[9] found that the addition of cetyltrimethylammonium to pillared clays change the hydrophilic surface properties and increase the clay’s sorption capacity for phenolic compounds. Boyd et al.[10] demonstrated that structural formula of the organic cation on smectite clay changes its sorptive properties. As the complexity of the organic surfactant increased, the surfactant-smectite hydrophobic properties increase, increasing sorption capacity. Other synthesized materials studied for the removal of chlorophenols are alkylmethylammonium surfactants.[11] The objective of this study is to screen the efficiency of different structured nanocomposite materials for the removal of chlorinated phenols from aqueous media in terms of reaction rate, capacity, stability in an oxidizing environment. These nanocomposite materials can be used as novel adsorbents in water treatment and barrier liners.
MATERIALS AND METHODS Materials Reagents used for the synthesis included NH4OH (30 wt. %), Tetraethylorthosilicate (TEOS), Si(OC2H5)4, and Cetyltrimethylammonium bromide (CTABr),
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CH3(CH2)15Nþ(CH3)3Br (Aldrich, as supplied). Reagent grade chemicals were used for all solutions. The synthesis of the silica nanocomposite materials was performed using the following reaction compositions: NHMS was prepared using TEOS, MW ¼ 208.33; C12 amine ‘‘dodecylamine’’ CH3(CH2)11NH2 MW ¼ 185.36; and EtOH ‘‘ethanol’’ C2H5OH MW ¼ 46. Molar ratios were: 30.3 mmol TEOS:6.74 mmol C12 amine:217 mmol EtOH:500 mmol H2O. NMCM-41 was prepared using TEOS MW ¼ 208.33; CTABr MW ¼ 364.46 in molar ratios of: 3.636 mmol CTABr:15.15 mmol NaOH:30.3 mmol TEOS: 3939 mmol H2O. NMCM-48 was prepared using TEOS MW ¼ 208.33; CTABr MW ¼ 364.46 in molar ratios of: 19.695 mmol CTABr:15.15 mmol NaOH:30.3 mmol TEOS:1878.6 mmol H2O. NMCM-50 was prepared using TEOS MW ¼ 208.33; CTABr MW ¼ 364.46 in molar ratios of: 104.95 mmol CTABr:28.65 mmol NH4OH:70.08 mmol TEOS: 11111.11 mmol H2O.
Method The resulting gel was aged for 3 days at 110 C in Teflon-lined stainless steel autoclaves. The product was filtered, washed with distilled water, and dried in air. X-ray powder diffraction (XRD) patterns were obtained on a Siemens diffractometer equipped with a rotating anode and Cu-K radiation (wavelength ¼ 0.15418 nm). All masses were weighed using a Sartorius balance, model BP210S (Cary, NC). Temperature experiments were completed with a Bigger Bill reciprocating shaker and water bath (Barnstead/Thermolyne, Dubuque, IA).
Kinetics Study Batch adsorption kinetics began with addition of 50.00 mL aqueous 2,4-DCP [50.00 mg L1] to 0.1000 g of adsorbent materials in glass bottles and agitated on a reciprocating shaker at 100 rpm for specific timeframes (1, 2, 4, 6, 8, 10, 12, 24, 48, and 72 h) at constant temperature (23.0 1 C). At each timeframe a 3.00 mL aliquot was removed and filtered through spun glass. Samples were analyzed on a Helios UV-Vis spectrophotometer ( ¼ 284.4 nm), Spectronic Instruments, Rochester, New York and concentration calculated from a standard curve. The integrated rate law models how reactants and product concentrations vary with time. The linearized integrated rate law for a first-order reaction is shown in Eq. (1). Linearized 1st Order Rate Law can be expressed as[12] log ½Ct ¼ ðk=2:303Þ t þ log½Co
ð1Þ
where: [C]t ¼ concentration of C at any time t, [C]o ¼ original concentration of C at some initial time, and k ¼ rate constant.
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Isotherms Study Batch adsorption isotherms began with addition of 25.00 mL aqueous 2,4-DCP [100.00 mg L1] to varying masses of adsorbent (0.0125, 0.0250, 0.0375, 0.0750, and 0.1250 g) in glass bottles and agitated at 100 rpm at constant temperature (23.0 1 C). After 72 h a 3.00 mL aliquot was removed from the solution, filtered through spun glass and analyzed for 2,4-DCP. Samples were analyzed on a Helios UV-Vis spectrophotometer ( ¼ 284.4 nm) and concentration calculated from a standard curve. The equilibrium removal of hydrophobic chlorinated phenols can be mathematically expressed in terms of adsorption isotherms. Comparison with Langmuir and Freundlich isotherm models were made using the following equation. Langmuir adsorption isotherm is expressed by Eq. (2)[13] qe ¼ Xm KL Ce =ð1 þ KL Ce Þ
ð2Þ
Equilibrium Uptake is shown by Eq. (3) qe ¼ ½ðCi Ce Þ VM
ð3Þ
where: qe ¼ mg of adsorbate per g of adsorbent at equilibrium; Xm ¼ mg of solute adsorbed per g of adsorbent; KL ¼ the Langmuir constant equal to liter of adsorbent per mg of adsorbate; Ce ¼ equilibrium concentration of adsorbate in solution (mg L1); Ci ¼ adsorbate initial concentration (mg L1); V ¼ volume of solution (L); and M ¼ Amount of adsorbent used (mg). The linear form of Langmuir isotherm is expressed by rearranging Eq. (2) to the following: Ce =qe ¼ 1=ðXm KL Þ þ ð1=Xm Þ Ce The Freundlich isotherm is expressed as depicted in Eq. (5) qe ¼ Kf Ce1=n
ð4Þ [13]
ð5Þ
where: qe ¼ mg of adsorbate per g of adsorbent at equilibrium; Kf ¼ the Freundlich constant; n ¼ A Freundlich constant, which is always greater than one; and Ce ¼ concentration of adsorbate in solution (mg L1). The linear form of Eq. (5) may be expressed as follows: lnðqe Þ ¼ lnðKf Þ þ ð1=nÞ lnðCe Þ
ð6Þ
Results of experimental data were compared to Eq. (4) and Eq. (6) to confirm Langmuir and Freundlich isotherm for adsorption.
Environmental Stability Study Environmental stability began by adding 0.5000 g of adsorbents to 50.00 mL aqueous 2,4-DCP [50.00 mg L1] in glass bottles and agitated on a reciprocating shaker at 100 rpm for 72 h at constant temperature (23.0 1 C). Adsorbents were filtered through spun glass. These adsorbents, along with the filter material,
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were placed in glass bottles in 50.00 mL of 0.10 M aqueous Calcium chloride, and placed on a reciprocating shaker at 100 rpm at constant temperature (23.0 1 C). At one hour a baseline concentration of 2,4-DCP was determined on a Helios UV-Vis spectrophotometer ( ¼ 284.4 nm). The amount of concentration of 2,4-DCP in the Calcium chloride solution was determined at 7, 14, 21, 28, 35, 42, and 56 days, and subtracted from the baseline concentration, to determine desorption amount.
Statistical Analysis Comparison between adsorbents and other parameters was completed by single and two-factor ANOVA. Percent relative standard deviations were computed for all replicate samples analyzed.
RESULTS AND DISCUSSION Materials Characterization Since the discovery of mesoporous molecular sieves, they were used as heterogeneous catalysts or as adsorbents materials.[6,14] Characterization of these mesoporous materials was investigated by their x-ray powder diffraction (XRD) patterns.[3,4] The XRD patterns for as-synthesized (composites) NMCM-41, NMCM-48, NMCM-50, and NHMS[3] are shown in Fig. 1. The observation of a high-intensity peak having a d spacing of approximately 4.2 nm and several higher angle peaks having d spacing consistent with hexagonal lattice is typical of NMCM-41.[4] The XRD pattern of NMCM-48 exhibited a high-intensity peak having d spacing of approximately 4.1 nm confirms a cubic phase.[4] The XRD pattern of NMCM-50 characteristically displays multiple peaks that are high orders of the first peak indicating a lamellar type material.[4] The powder diffraction (XRD) pattern of NHMS exhibited a single diffraction peak corresponding to a d spacing of 3.8 nm.[3] The XRD patterns of these nanocomposites agree with literature patterns.[3,4] The NHMS structure is hexagonal with ‘‘worm’’ shaped independent and irregular pore openings of 3.8 nm, with hydrophobic tail attached to amines head group aligned along the silicates framework. The NMCM-41 organic structure has hexagonal arrays of pore openings arranged in a uniform pattern; NMCM-48 has cubic phase structure with uniform pore openings and a complex internal structure; while NMCM-50 is lamellar with independent, uniform layered structure. Similar to the NHMS, these materials have hydrophobic tail attached to quaternary ammonium ion head group aligned along the silicates framework (Fig. 1). Removal efficiencies for all nanocomposites were greater than 90% except NMCM-50 was 52% (Fig. 2).
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a) NHMS
Relative Intensity
b) NMCM-50
c) NMCM-48
d) NMCM-41
1.00
2.00
3.00
4.00
5.00
6.00
7.00
8.00
9.00
10.00
2 2θ Figure 1. X-ray powder diffraction (XRD) of different organosilicate nanocomposite materials.
Kinetics Studies Rate constants of nanocomposites for adsorption of 2,4-DCP were determined using batch adsorption (Table 1, Fig. 3). The NHMS showed the fastest rate of 2,4DCP adsorption, with a 1st order rate constant of 0.412 h1, reaching maximum sorption of 98% removed after 6 h. This high rate of reaction may be attributable to the 2-dimension structure, thick walls and irregular pore openings of 3.8 nm found in NHMS,[3] providing large surface area and greater exposure of hydrophilic organic groups (Fig. 1a).[15] NMCM-41 showed the next highest rate constant of 0.296 h1 after 72 h, reaching maximum sorption of 98% after 12 h. While the NMCM-41 has
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100%
2,4-DCP removed at 72 hr
90% 80% 70% 60% 50% 40% 30% 20% 10% 0% NHMS
NMCM-41
NMCM-48
NMCM-50
Nanocomposite Material
Figure 2.
Percent DCP removal with 72 h exposure to nanocomposite materials.
Table 1. Sorption rate nanocomposite materials.
constants
for
organosilicate
Adsorbent
K (h1)
NHMS NMCM-41 NMCM-48 NMCM-50
0.4120 0.2962 0.1124 0.0161
a 2-dimensional structure, it has thinner walls[3,15] and greater uniformity in pore opening size and pattern than the NHMS (Fig. 1d), which also increases surface area and organic group exposure.[15] Both NMCM-48 and NMCM-50 neither had the rate nor sorption capability of the NHMS or NMCM-41. Vartuli et al. found during comparisons of as-synthesized MCM-41 and as-synthesized MCM-48 that as-synthesized MCM-41 had twice the sorption capacity than that of as-synthesized MCM-48.[6] They attributed the result to the ordered structure of as-synthesized MCM-41, which increased its capacity over that of the complicated channel system of as-synthesized MCM-48 (Fig. 1c).[6] Their findings can also be correlated to the performance of NHMS, also having unidirectional pores, over that of a cubic bicontinuous of NMCM-48. NMCM-50 is lamellar with independent, uniform layered structure. This motif may reduce surface area and obscure surfactant hydrophobic tail available for the removal of 2,4-DCP.
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50
[C]0-[C]t
40
30
20
10
0 0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
Time (h) NHMS
Figure 3.
NMCM-41
NMCM-48
NMCM-50
DCP removal vs. time by different nanocomposite materials.
Isotherm Study Results from experimental data were used in Eqs. (4) and (6) and compared to determine which model Freundlich or Langmuir most accurately described the sorption characteristics. Sorption is the term that describes when organic materials attach to an adsorbent (the chemical and physical adsorption processes which take place). When the adsorption rate equals the desorption rate, equilibrium is achieved and the capacity of the adsorbent material is reached. The theoretical adsorption capacity of the adsorbent, the hydrophobic organic chains in this study, can be determined by calculating the adsorption isotherm. The Langmuir model is the simplest model and assumes that the adsorption sites on the surface of an adsorbent are occupied by the adsorbate in the solution. Within the Langmuir model KL is an indicator of sorption intensity. The Langmuir constant KL represents the degree of sorption affinity that the adsorbate has to the adsorbent. The maximum adsorption Xm is the maximum adsorption capacity associated with complete monolayer cover and is typically expressed in mg g1. The Freundlich model and associated constant KF, which is also related to capacity, assumes an infinite supply of unreacted adsorbent sites and tends to represent heterogeneous materials better than other models. The higher the Freundlich KF constant there is a potential to characterize the adsorbent as more reactive, although the constant tends to be site and adsorbent specific. Within the 1/n constant, n is an associated empirical constant that is dependent on the heterogeneity of sorbing sites. When n > 1, interactions are binding at sorbing sites, whereas when
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Table 2. Langmuir and Freundlich models parameters and the corresponding correlation coefficients. a. Langmuir model r2
KL
XM (mg g1)
NHMS NMCM-41 NMCM-48 NMCM-50 b. Fruendlich model
0.9540 0.9441 0.1405 0.3922
0.0922 0.0787 0.0159 0.0075
104.1667 227.2727 312.5000 109.8901
Adsorbent
r2
KF
n
NHMS NMCM-41 NMCM-48 NMCM-50
0.9958 0.9221 0.8615 0.8876
Adsorbent
709.9046 881.4547 38.0365 0.0651
2.4355 1.4411 0.8972 0.7252
n < 1 suggests the model does not approximate the binding interactions for this process. Batch isotherm studies were conducted to determine Langmuir and Freundlich affinity constants for OCS materials with 2,4-DCP (Table 2). The Langmuir model states that all adsorbent sites are independent of neighboring sites. Both the NHMS and NMCM-41 had valid r2 values for the Langmuir model with adsorption capacities of 104.17 and 227.27 mg g1, respectively. The increased affinity of NMCM-41 over NHMS may be attributable to its larger pore size, yet more importantly in this case the larger surfactant amine group (C16 for NMCM-41 vs. C12 for NHMS) may create greater hydrophobic and organophilic properties. The uniformity of hexagonal and cubic shapes of these materials may lend themselves to equivalent and independent adsorption sites. Conversely, NMCM-48 and NMCM-50 did not fit the Langmuir models, both having low r2 values and indicating no independence of adsorption sites. The Freundlich model states that each adsorption site is nonuniform and may interfere with neighboring sites either through preexisting conditions or repulsive forces between adsorbed molecules. The NHMS indicated a better fit (r2 > 0.99) in the Freundlich than the Langmuir. This indicates the more heterogeneity of the irregularly shaped pore openings NHMS. Therefore, providing a greater number of sorption sites, although a small amount of these sites may have created or preexisting repulsive forces that limit adsorption. The relatively high Freundlich constant (KF ¼ 709.90) indicates a higher potential for this material to be characterized as reactive, yet this constant tends to be adsorbent and site specific. The NMCM-41 did not fit the Freundlich as well as the Langmuir, yet indicated a high Freundlich constant (KF ¼ 881.45). The n constant is an associated empirical constant that is dependent on the heterogeneity of sorbing sites.[16] For values greater than 1, interactions are binding at sorbing sites, whereas n values less that 1 suggest the model does not approximate the binding interactions. Based on this description,
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the NHMS indicates that the heterogeneity of the sorbing sites is adding to the binding interactions. The NMCM-48 and NMCM-50 materials had adequate r2 values for the Freundlich model, however, they had n values less than one, indicating that the Freundlich model does not approximate the binding interactions occurring with these materials.
Environmental Stability Batch adsorption studies were accomplished to determine the stability of the nanocomposites in an oxidative environment (Table 3, Fig. 4). The NHMS and NMCM-41 indicated some initial desorption and instability over the first 21 days; both having increases and decreases from the baseline concentration, the NMCM-41 more so than NHMS. This may be expected due to their 2-dimensional structure allowing increased movement of the calcium chloride through the structure. However, both adsorbents stabilized and decreased the concentration of 2,4-DCP below that of the baseline by the end of the trial. The lamellar structure of NMCM50 had the greatest stability, indicating little fluctuation from the baseline concentration, as well as very little adsorption or desorption over the trial period. Its layered motif restricts movement of the calcium chloride through the structure,
Table 3.
Desorption of 2,4-DCP from nanocomposite materials in 0.1 M CaCl2.
Adsorbent
Concentration baseline (s.d.) (mg L1)
Concentration 56 days ( s.d.) (mg L1)
NHMS NMCM-41 NMCM-48 NMCM-50
2.28 1.25 4.12 3.17 2.48 1.81 2.58 1.29
0.13 1.81 0.00 0.19 5.58 7.01 1.47 0.44
10.00
[C]0-[C]t
8.00 6.00 4.00 2.00 0.00 0
5
10
15
20
25
30
35
40
45
50
55
Time (d) NHMS
Figure 4.
NMCM-41
NMCM-48
NMCM-50
Environmental stability of organosilicate nanocomposite materials.
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giving it greater stability in this type of environment. The 3-dimensional structure of NMCM-48 made it the least stable; initially desorbing 8.06 0.18 mg L1 of DCP in the first 14 days of the trial. Throughout the remainder of the trail the NMCM-48 remained relatively stable, with concentrations of 2,4-DCP fluctuating between 5.58 and 7.02 mg L1.
CONCLUSION This study shows the effectiveness of organosilicate nanocomposites, primarily NHMS and NMCM-41 because their unidirectional pore structures, for the adsorption of chlorinated phenols from aqueous media. Both these adsorbents had 1st order rate constants of 0.413 and 0.296 h1, respectively. In addition, these two adsorbents had the greatest affinity; isotherm batch studies determined these materials have adsorption capacities for 2,4-dichlorphenol of 104.17 and 227.27 mg g1, respectively. When placed into an oxidative environment, these two adsorbents showed some initial instability, NHMS and NMCM-41 desorbing only 2.8 mg and 4.3 mg of 2,4-DCP, respectively. After 21 days, NHMS and NMCM-41 re-adsorbed all the 2,4-DCP that had initially been desorbed. These nanocomposite materials can be used as adsorbents for effective and rapid removal of chlorinated phenols from aqueous media.
REFERENCES 1. U.S. EPA. Announcement of the drinking water contaminant candidate list. Fed. Reg. 1998, 63, 10276. 2. Callahan, M.A.; Slimak, M.W.; Gabel, N.W.; May, I.P.; Fowler, C.F.; Freed, J.R.; Jennings, P.; Durfee, R.L.; Whitmore, F.C.; Maestri, B.; Mabey, W.R.; Holt, B.R.; Gould, C. Water-Related Environmental Fate of 129 Priority Pollutants; EPA-440/4-79-029b; 1979; Vol. II, 84-1-8. 3. Tanev, P.T.; Pinnavaia, T.J. A neutral route to mesoporous molecular sieves. Science 1995, 267, 865–867. 4. Beck, J.S.; Vartulli, J.C.; Roth, W.-J.; Leonowicz, M.E.; Kresge, C.T.; Schmitt, K.D.; Chu, C.T.; Olson, D.H.; Sheppard, E.W.; McCullen, S.B.; Higgins, J.B.; Schlenker, J.L. A new family of mesoporous molecular sieves prepared with liquid crystal templates. Am. Chem. Soc. 1992, 114, 10834–10843. 5. Zhao, H.; Nagy, K.L.; Waples, J.S.; Vance, G.F. Surfactant-templated mesoporous silicate materials as sorbents for organic pollutants. Environ. Sci. Technol. 2000, 34 (22), 4822–4827. 6. Vartuli, J.C.; Malek, A.; Roth, W.J.; Kresge, C.T.; McCullen, S.B. The sorption properties of as-synthesized and calcined MCM-41 and MCM-48. Micropor. Mater. 2001, 44–45, 691–695. 7. Mortland, M.M.; Shaobai, S.; Boyd, S.A. Clay-organic complexes as adsorbents for phenol and chlorophenols. Clays and Clay Minerals 1986, 34 (5), 581–585.
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8. Srinivasan, K.R.; Fogler, H.S. Use of inorgano-organo-clays in the removal of priority pollutants from industrial wastewaters: adsorption of benzo(a)pyrene and chlorophenols from aqueous solutions. Clays and Clay Minerals 1990, 38 (3), 287–293. 9. Wibulswas, R.; White, D.A.; Rautiu, R. Adsorption of phenolic compounds from water by surfactant-modified pillared clays. TransIChemE. 1999, 77, Part B, 88–92. 10. Boyd, S.A.; Shaobai, S.; Lee, J.-F.; Mortland, M.M. Pentachlorophenol sorption by organo-clays. Clays and Clay Minerals 1998, 36 (2), 125–130. 11. Jin, X.; Zhu, M.; Conte, E.D. Surfactant-mediated extraction technique using alkyltrimethylammonium surfactants: extraction of selected chlorophenols from river water. Anal. Chem. 1999, 71 (2), 514–517. 12. McMurry, J.; Fay, R.C. Chemistry, 2nd Ed.; Prentice Hall: New Jersey, 1998; 472–482, 838–843. 13. Muhammad, N.; Parr, J.; Smith, M.D.; Wheatley, A.D. Adsorption of Heavy Metals in Slow Sand Filters, Proceedings of the 24th WEDC International Conference on Water Supply and Sanitation, Islamabad, Pakistan, 1998, 346–349. 14. Corma, A. From microporous to mesoporous molecular sieve materials and their use in catalysis. Chemical Reviews 1997, 97, 2373–2419. 15. Abdel-Fattah, T.M.; Pinnavaia, T.J. Tin-substituted mesoporous silica molecular sieve (Sn-HMS): synthesis and properties as a heterogeneous catalyst for lactide ring-opening polymerization. Chemical Comm. 1996, 665–667. 16. Calace, N.; Dimuro, A.; Nardi, E.; Petronio, B.M.; Pietrolletti, M. Adsorption isotherms for describing heavy-metal retention in paper mill sludges. Ind. Eng. Chem. Res. 2002, 41, 5491–5497. Received May 7, 2004
