9376
Ind. Eng. Chem. Res. 2008, 47, 9376–9383
Anion Effect on Cu2+ Adsorption on NH2-MCM-41 Koon Fung Lam, Xinqing Chen, Gordon McKay, and King Lun Yeung* Department of Chemical and Biomolecular Engineering, Hong Kong UniVersity of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, People’s Republic of China
The anion effect was investigated for the copper adsorption on NH2-MCM-41 from Cu(NO3)2 and CuSO4 solutions. The copper adsorption was higher and faster in the presence of SO42- anion compared to NO3-. The MCM-41 possesses well-ordered mesopores that are readily accessible and a uniform surface that is amenable to the attachment of the chemical moieties for creating tailored adsorption sites. The adsorption sites on NH2-MCM-41 were created by grafting aminopropyls on MCM-41, and the random deposition resulted in a site distribution best described by the Freundlich equation. The majority of the adsorption sites (i.e., up to 70%) are readily accessible to Cu2+ adsorption. The remaining sites were only accessible in the presence of SO42-. Evidence showed that the SO42- anion affects the adsorption by interacting with the dissolved copper to form [CuSO4]0 species, coadsorbing with Cu2+ to form stable complexes, and may even indirectly react with the weakly acidic silanol groups to liberate aminopropyls for more Cu2+ adsorption. 1. Introduction Adsorption is a popular and economical method for treating metal pollutions in natural, process, and wastewater.1 The common sorbents in metal adsorption, including activated carbons,2 chitosan,3 bone char,4 clays,5 and resins,6 are often structurally and chemically heterogeneous possessing multiple adsorption sites of varying accessibility. Modeling adsorptions on these complex surfaces is often a challenge, and the best efforts7-10 remained inadequate to fully describe the subtle interactions between the adsorbates and the surface. This may explain the differing reports in the literature. One example is the effects of anions on metal adsorptions, a well-documented but poorly understood phenomenon.11 The Cu2+ adsorption was investigated by Doula and Ioannou.12 They reported that the nitrate ion enhanced Cu2+ adsorption on clinoptilolite, while the presence of chloride and sulfate ions resulted in lower metal adsorption. However, a more recent report by the same group13 shows that copper surface complexes were formed on clinoptilolite at lower Cu adsorbed concentration in the presence of SO42- compared to the other two anions. Juang and coworkers14 also observed enhanced Cu2+ adsorption on goethite from solutions containing sulfate ions, but not from solutions with nitrate ions. The purpose of this work is to use MCM-41 to create an adsorbent that has well-defined pore channels and to populate the pore surface with adsorption sites by grafting organic functional moieties. Thus, the identity and quantity of the adsorption sites can be fully characterized and described. Such an adsorbent would be ideal for the study of the adsorption mechanisms and adsorbate-site interactions. The recent years have seen the emergence of selective adsorbents for metal ions adsorption prepared by incorporating hydrophilic Lewis bases and acids on the pore wall of mesoporous silicas (e.g., MCM41, SBA-15, and MCM-48). Fryxell’s group at the Pacific Northwest National Laboratory is among the pioneers in the design and use of selective mesoporous adsorbents.15-19 Their early work15 of grafting thiolpropyl groups on MCM-41 to produce a selective adsorbent for mercury adsorption opened a whole new research area. Since then, various functional groups * To whom correspondence should be addressed. Tel: 852-23587123. Fax: 852-2358-0054. E-mail:
[email protected].
including amino, diamino, triamino, ethylenediamine, malonamide, carboxy, 1-allyl, dithiocarbamate, and imidazole were successfully attached to the pore walls of MCM-48, MCM-41, and SBA-15 and were demonstrated for the adsorption of group VIII, VI, IB, and IIB metal ions as well as some of the lanthanide and actinide elements.15-26 Three different approaches including (1) manipulating the site chemistry according to Pearson’s hard-soft, acid-base (HSAB) principle,27 (2) controlling the spacing between neighboring sites, and (3) altering the adsorbates with chelates and counterions were successfully used to prepare highly selective adsorbents based on mesoporous MCM-41 for the adsorption of dyes,28 precious metals including gold, palladium, and silver,29-34 and toxic metals such as lead, chromate, and cadmium.35-37 This work investigates the role of anions on the adsorption of Cu2+ on aminopropyls grafted MCM-41 (NH2MCM-41). The effects of nitrate and sulfate anions on Cu2+ adsorption were investigated for different aminopropyl loadings, cation and anion concentrations, and pH. 2. Experiment 2.1. Mesoporous Adsorbents. The mesoporous MCM-41 was prepared from an alkaline synthesis solution containing cetyltrimethylammonium bromide (CTABr, 99.3%, Aldrich), tetraethyl orthosilicate (TEOS, 98%, Aldrich), and ammonium hydroxide (NH4OH, 28-30 wt %, Fisher Scientific) according to the procedure described in a recent work.28 The synthesis was allowed to proceed at room temperature for 2 h, and the powder product was filtered and washed before drying in an oven at 373 K. The CTA+ molecules were removed from MCM41 by air calcination at 823 K for 24 h, and the aminopropyls were grafted from 3-aminopropyltrimethoxysilane (APTS, 97%, Aldrich) solution in toluene by reflux. Different aminopropyl loadings were obtained by varying the APTS concentration in the reflux solution. The prepared adsorbents were characterized by X-ray diffraction (XRD, Philips 1830), nitrogen physisorption (Coulter SA 3100), scanning and transmission electron microscopies (SEM, JEOL 6300F; TEM JEOL JEM 2010), and zeta potential and particle size analyzer (Coulter Beckman Delsa 440SX). The composition and chemistry of the adsorbents were
10.1021/ie701748b CCC: $40.75 2008 American Chemical Society Published on Web 11/05/2008
Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008 9377
Figure 1. (a) Scanning electron microscopy and (b) nitrogen physisorption isotherm of NH2-MCM-41 (2.3 mmol/g). (c) Infrared peak shift at 1300 cm-1 attributed to the interaction between aminopropyl and silanol.52
analyzed by elemental analysis (Elementar Vario EL III) and Fourier transformed infrared spectroscopy (FTIR, Perkin-Elmer GX 2000). 2.2. Adsorption Experiments. The metal salts used in the adsorption study include copper nitrate trihydrate (>95%, Nacalai Tesque Inc.) and copper sulfate pentahydrate (99%, RDH). Sodium nitrate (99.5%, RDH) and sodium sulfate (99%, RDH) were added to obtain solutions with different anion concentrations, while the pH of the solutions were adjusted by adding a small amount of dilute acids (i.e., HNO3 or H2SO4) and base (i.e., sodium hydroxide). The batch adsorption experiments were conducted in a shaker bath kept at a constant temperature of 295 ( 2 K. The adsorption rate was determined by taking samples of the adsorption solution at fixed time intervals, while the equilibrium adsorption was measured at the end of the 5 day experiment. The initial and final concentrations of the metal in the solution were analyzed by inductively coupled plasma and atomic emission spectrometer (ICP-AES, PerkinElmer Optima 3000XL), while the nitrate and sulfate ion concentrations were measured by ion chromatography (Dionex, ED 40 detector). Three measurements were made for each sample, and the results were averaged. The instruments were calibrated against standards before each analysis. The equilibrium adsorption capacity was calculated from eq 1. qe )
(C0 - Ce)V m
(1)
where qe (mmol/g) is the adsorption capacity and C0 (mM) and Ce (mM) are respectively the initial and equilibrium metal concentrations. V (L) is the solution volume, and m (g) is the weight of the adsorbent. 2.3. Mathematical Modeling. The Langmuir and Freundlich adsorption equations were used to fit the adsorption data of Cu2+ on NH2-MCM-41. The Langmuir equation (eq 2) assumed a weak physisorption of the metal cations on a surface with homogeneous adsorption sites. In contrast, the Freundlich model
(eq 3) describes a more varied adsorption landscape.38 The fitting was done by minimizing the sum of square error (SSE) between the experimental data and model calculation. qL )
KLCe 1 + bLCe
qF ) KFCebF
(2) (3)
where q is the adsorption capacity, KL and bL are fitting parameters for the Langmuir equation, and KF and bF are fitting parameters of the Freundlich model. The pseudo-first-order (eq 4) and pseudo-second-order (eq 5) rate equations are often used to fit the adsorption rate data,39 while the Elovich and Ritchie equations (eqs 6 and 7) more accurately describe chemisorption.40,41 qt ) qe,max(1 - e-k1t)
(4)
qt )
k2qe,max2t 1 + k2qe,maxt
(5)
qt )
1 ln(1 + Rβt) β
(6)
qe,maxn-1 (qe,max - qt)n-1
) (n - 1)kt + 1
(7)
where qt is the adsorption capacity at given time t and qe,max is the maximum adsorption capacity; k1, k2, k, R, β, and n are the model fitting parameters. 3. Results and Discussion 3.1. NH2-MCM-41 Adsorbents. It is possible to prepare mesoporous silica in a variety of size and shapes including spheres, cylinders, and plates.42,43 Unger and co-workers44 were
9378 Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008 Table 1. NH2-MCM-41 Adsorbents with Different Aminopropyl Loadings
batch
aminopropyl loadinga (mmol/g)
specific surface area (m2/g)
aminopropyl per areab (group/nm2)
NH2-MCM-41 (a) NH2-MCM-41 (b) NH2-MCM-41 (c) NH2-MCM-41
0.46 1.09 1.49 2.26
1020 774 763 750
0.33 0.85 1.18 1.81
a The aminopropyl loading is determined by elemental analysis of carbon and nitrogen content. b Density of aminopropyl per area is calculated by dividing aminopropyl loading and the specific surface area.
among the first to report the synthesis of submicron spherical MCM-41. Spherical particles are often considered ideal for the adsorption study, but unfortunately the pores vary along the radius of the spherical MCM-4145 and formation of distinct zones of different pore symmetries is common.46-48 Often, the prepared spherical MCM-41 particles display a dense46 or porous core (e.g., disordered pores47 and cubic pore symmetry48) and an ordered mesoporous outer shell depending on the preparation. This introduces a greater complexity in the pore structure and transport. Elongated and cylindrical-shaped particles are common for SBA-15, but the long pore channels could prevent the uniform grafting of aminopropyls by the postsynthesis functionalization method. It is of course possible to directly anchor the organic moieties on the pore wall by co-condensation of the respective organosilanes with the silicon alkoxide precursor in the presence of structure-directing agents (SDAs), but the direct synthesis approach suffered from a narrow synthesis window and problems caused by the poorer mesoscopic order, homocondensation induced nonuniformity, and the need to remove the SDA without damaging the anchored organic residues. The use of this method for the adsorbent preparation was limited to a couple of examples.49,50 The disk-shaped NH2-MCM-41 shown in Figure 1a is a good compromise. Its regular shape is easily amenable to the mathematical description, and the 1-D cylindrical pores are uniformly accessible to both modification and adsorption. The NH2-MCM-41 has a mean particle diameter of 0.85 µm and a thickness of 0.2 µm according to SEM measurements, which is in good agreement with the dynamic light scattering experiment. The nitrogen physisorption isotherm for NH2-MCM-41 is shown in Figure 1b. The specific surface area of the NH2-MCM-41 is 750 m2/g, and has an average pore size of 2.92 nm. Table 1 lists the aminopropyl loading determined from elemental analysis of the nitrogen and carbon contents of the samples. FTIR analysis of the adsorbents detected infrared absorption peaks corresponding to the -NH2 (i.e., 3290 and 3359 cm-1) and alkyl chain (2863 and 2930 cm-1). There was a strong interaction between the terminal NH2 of the aminopropyl groups and the unreacted surface hydroxyls on the MCM-4151 at low loadings, which was indicated by a peak shift at around 1300 cm-1 in the FTIR spectra.52 Figure 1c plots the observed infrared peak shift as a function of the aminopropyl loading. 3.2. Cu2+ Adsorption and Aminopropyl Loading. Figure 2 plots the maximum copper adsorption on NH2-MCM-41 of different aminopropyl loadings. The amount of adsorbents was adjusted to keep the total aminopropyl sites fixed in the adsorption experiment at 2.3 mmol per liter solution. The plots clearly show the adsorptions from the copper sulfate solutions were consistently higher compared to the copper nitrate solutions. The MCM-41 does not adsorb Cu2+, and it is safe to presume that the original silanols have poor affinity for the Cu2+ adsorption. The aminopropyls grafted on the surface occupied
Figure 2. Adsorption of Cu2+ on NH2-MCM-41 with different aminopropyl loadings from Cu(NO3)2 and CuSO4 solutions. Note: the mass of adsorbent was adjusted to give the amount of aminopropyls constant at 2.3 mM; 100 mL of 3 mM copper salt solution; pH ) 5.0; t ) 5 days. Table 2. Model Fitting Parameters for the Adsorptions on NH2-MCM-41 from Cu(NO3)2 and CuSO4 Solutions Langmuir isotherm
species qe,maxa (mmol/g) KL 2+
Cu(NO3)2 Cu NO3CuSO4 Cu2+ SO42-
0.76 0.00 1.33 0.37
bL
SSE
Freundlich KF
bF
SSE
210 290 0.03 0.73 0.09 0.00 140 120 0.14 1.27 0.16 0.02
a The maximum adsorption capacity, qe, is determined by averaging the last three points of the isotherm data.
on the average 2.6 silanols, and about a third of the original surface hydroxyls (i.e., 3.8 mmol/g) were reacted when the aminoporpyl loading was 0.46 mmol/g. The adsorbent with this aminopropyl loading did not adsorb copper from the Cu(NO3)2 solution. This can be explained by considering that at this low loading most of the grafted aminopropyls are isolated from each other as illustrated in Figure 3[1]. The Cu2+ adsorption requires at least two aminopropyls and cannot adsorb on the isolated aminopropyl sites. In addition, the FTIR data in Figure 1c suggested that the grafted aminopropyls chemically interacted with the unreacted surface hydroxyls (Figure 3[2]), further diminishing their availability for adsorption. The same NH2-MCM-41 adsorbed 0.17 mmol/g Cu2+ from the CuSO4 solution, better than the Cu(NO3)2 solution but still less than the expected theoretical value of 0.23 mmol/g Cu2+. It is possible that the SO42- stabilized the Cu2+ adsorption on the individual aminopropyl sites as illustrated in Figure 3[3]. Another possibility is that the SO42- anions interacted with the weakly acidic surface silanols releasing the aminopropyls for adsorption as shown in Figure 3[4]. There are less unreacted surface hydroxyls at higher aminopropyl loadings, and consequently the peak shift in the FTIR related to aminopropylhydroxyl interactions decreases as shown in Figure 1c. Also, the hydrogen bond interactions between the APTS molecules increase the likelihood that the aminopropyls are grafted in close proximity to each other as illustrated in Figure 3[5]. The greater number of aminopropyls and their closer proximity favor the adsorption of Cu2+ as shown by the increase in copper adsorption from both Cu(NO3)2 and CuSO4 solutions (Figure 2). The adsorption from CuSO4 was higher, and it is believed that the SO42- stabilized the Cu2+ adsorption on the unfavorable sites, including the aminopropyls along the edges and isolated aminopropyls (Figure 3[3,6]). A transition to a higher adsorption was observed at an aminopropyl loading of 1.5 mmol/g. However, the adsorption from Cu(NO3)2 remained less than one Cu2+ for two aminopropyl sites even at the high loading of 2.9 mmol/g. This
Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008 9379
Figure 3. Schematic drawing of the proposed mechanisms of copper adsorption on NH2-MCM-41 at pH 5.
Figure 4. Adsorption isotherms of Cu2+ by NH2-MCM-41 from (a) Cu(NO3)2 and (b) CuSO4 solutions. Note the line was based on Freundlich equation (0.1 g adsorbent, 2.3 mmol/g aminopropyl loading, 100 mL copper salt solution, pH ) 4.7-5.2, t ) 5 days).
suggests that not all the grafted aminopropyls participated in the Cu2+ adsorption. In contrast, the same set of adsorbents display Cu2+ adsorption from CuSO4 better than the theoretical value of 1 Cu2+ per two aminopropyls. Figure 3[7-9] suggests various ways that SO42- can enhance the Cu2+ adsorption, including the adsorption [CuSO4]0 complex53 on the isolated aminopropyls shown in Figure 3[7]. A detailed adsorption study was performed to test the adsorption schemes proposed in Figure 3. 3.3. Adsorption Equilibria. Figure 4 plots the adsorption isotherms of Cu(NO3)2 and CuSO4 on NH2-MCM-41 at pH 5.0 and 293 K. Although a single chemical moiety was grafted on the adsorbent, the spatial distribution of the grafted aminopropyls dictates their interactions with the surrounding and defines their adsorption properties. It is expected that on the enormous surface of the MCM-41 both islands of aminopropyls and isolated aminopropyls will coexist, resulting in a heterogeneous surface. Indeed, the Freundlich adsorption equation fits the Cu2+ adsorption data better than the Langmuir adsorption equation (Table 2). The NH2-MCM-41 had an aminopropyl loading of 2.3 mmol/g and could adsorb up to 0.76 mmol/g Cu2+ from the Cu(NO3)2 solution, as shown in Figure 4a. The nitrate anion
was not adsorbed according to the analysis by ion chromatography. The data indicate that less than 70% of the grafted aminopropyls (i.e., 1.5 mmol/g) participated in the adsorption of Cu2+ from Cu(NO3)2. The remaining sites are either inaccessible or energetically unfavorable, such as the isolated aminopropyl sites and the peripheral aminopropyls sequestered by their interactions with the unreacted surface hydroxyls. Fifty percent more copper (i.e., 1.33 mmol/g) was adsorbed from the CuSO4 solution, and in this case, the SO42- adsorption was observed (Figure 4b). It is interesting to note that up to 0.8 mmol/g Cu2+ no SO42- was coadsorbed. This value is similar to the adsorption capacity of NH2-MCM-41 for Cu(NO3)2 solutions (Figure 4a), and it is reasonable to assume that the Cu2+ occupied similar sites on the adsorbents in both cases. The SO42- adsorption was only significant when the amount of Cu2+ adsorbed was above 0.8 mmol/g, and roughly one SO42- was adsorbed for every two Cu2+. It is believed that the SO42- frees the periphery aminopropyls for adsorption and could further participate by forming a stable complex with the adsorbed Cu2+ (Figure 3[8]). It is also possible that SO42induces a rearrangement in the Cu2+ adlayer to create a denser packing with three aminopropyl sites adsorbing two Cu2+ as
9380 Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008
Figure 5. (a) Copper adsorption on NH2-MCM-41 with the addition of NaNO3 and Na2SO4 salts and (b, c) the corresponding speciation diagrams calculated by Visual MINTEQ53 (0.1 g adsorbent, 2.3 mmol/g aminopropyl loading, 100 mL of 3 mM Cu(NO3)2, pH ) 5.0, t ) 5 days).
Figure 6. (a) Copper adsorption on NH2-MCM-41 from Cu(NO3)2 or CuSO4 solutions at different pH and (b) their corresponding surface zeta potential (0.1 g adsorbent, 2.3 mmol/g aminopropyl loading, 100 mL of 3 mM Cu(NO3)2, pH ) 5.0, t ) 5 days).
Figure 7. Plots of (a) copper adsorption and (b) q/qe × 100% as a function of time from Cu(NO3)2 or CuSO4 solutions (1.7 g adsorbent, 2.3 mmol/g aminopropyl loading, 1.7 L of 3 mM copper salt solution, pH 5 ( 0.3).
Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008 9381 Table 3. Model Parameters for the Adsorption Kinetics of Copper Salts pseudo-first order
CuSO4 Cu(NO3)2
pseudo-second order
Elovich model
Ritchie model
k1
qe
SSE
k2
qe
SSE
R
β
SSE
B
k2′
qe
SSE
1.40 0.60
1.16 0.60
0.39 0.18
1.50 1.30
1.20 0.63
0.20 0.12
1650 14.0
11.2 15.5
0.01 0.01
2.60 2.00
0.21 0.05
1.40 0.76
0.01 0.01
shown in Figure 3[9]. It was observed that one SO42- was adsorbed for every Cu2+ during the late stage adsorption near saturation. This could be due to the adsorption of [CuSO4]0 (Figure 3[7]). The experiment in Figure 5 was performed to further clarify the role of anions on Cu2+ adsorption. The study was carried out with Cu(NO3)2 with the addition of NaNO3 and Na2SO4 salts. Figure 5a shows that the Cu2+ adsorption from the 3 mM Cu(NO3)2 remained unchanged at 0.77 mmol/g even after adding 6 mM NaNO3. The corresponding speciation diagram for Cu(NO3)2 was calculated by Visual MINTEQ53 and plotted in Figure 5b. The nitrate ion has no effect on the dissociation of Cu(NO3)2 salt, and the copper exists mainly as divalent Cu2+ in the Cu(NO3)2/NaNO3 solutions. The model predicted less than 2% CuNO3+, and it is safe to ignore its effects on the adsorption. It can be seen in Figure 5a that addition of Na2SO4 salt to Cu(NO3)2 solution results in an immediate increase in copper adsorption. A maximum adsorption of 1.6 mmol/g Cu2+ was obtained after adding 4 mM Na2SO4. Visual MINTEQ shows the speciation of Cu(NO3)2 in Na2SO4 solution produces a [CuSO4]0 complex53 in addition to Cu2+, and its concentration is not negligible as shown in Figure 5c. Although it is tempting to attribute the enhanced copper adsorption solely to [CuSO4]0 adsorption, the adsorbed amount of SO42- suggested otherwise (Figure 4b). The zeta potential of the NH2-MCM-41 was measured during the adsorption of Cu(NO3)2 and CuSO4 at different solution pH. The pH was adjusted accordingly with HNO3 and H2SO4 acids. Figure 6a shows copper adsorption was negligible below pH 2.5 and reached a maximum, constant value above pH 4. It can be seen from the results that the adsorption from CuSO4 is consistently higher than Cu(NO3)2. Figure 6b plots the corresponding zeta potential of the adsorbent surface. The surface of the NH2-MCM-41 was positively charged after adsorbing Cu2+ from the Cu(NO3)2 solution. The surface charge increases at lower pH in spite of the lower Cu2+ adsorption due to the protonation of the aminopropyls to RNH3+. It is reasonable to expect that the adsorbent would be more positively charged due to a higher copper adsorption from the CuSO4 solutions. Even if the adsorption is solely from the neutral [CuSO4]0 complex, the surface charge is anticipated to be at least similar to that of Cu(NO3)2. The fact that the surface charge is lower and constant independent of the solution pH (Figure 6b) could only be explained by the coadsorption of negatively charged SO42-. The adsorption study indicates that NH2-MCM-41 possesses different adsorption sites, despite the fact that only a single chemical moiety was grafted on the adsorbent. The majority of the adsorption sites (i.e., up to 70%) are readily accessible to Cu2+ adsorption. The remaining sites were only accessible in the presence of SO42-. The results showed that the SO42- anion affects the adsorption by interacting with the dissolved copper to form [CuSO4]0 species, coadsorbed with Cu2+ to form stable complexes (e.g., Figure 3[8,9]), and may even indirectly react with the weakly acidic silanol groups to liberate aminopropyls for more Cu2+ adsorption (Figure 3[4]). 3.4. Adsorption Kinetics. Copper adsorption on NH2-MCM41 from Cu(NO3)2 and CuSO4 solutions was monitored with time and plotted in Figure 7. Table 3 shows that, among the
different kinetic models, the Elovich kinetic equation gave the best fit to the data (Figure 7). This suggests that Cu2+ ions were chemisorbed on the aminopropyls grafted on NH2-MCM-41. The data indicate that the adsorption from CuSO4 is roughly 6 times faster than that from the Cu(NO3)2 solution. It took 6 s to reach q/qe ) 0.5 and 30 min for q/qe ) 0.9 for CuSO4, while the adsorption from Cu(NO3)2 solution required 50 s and 180 min to reach the q/qe values of 0.5 and 0.9, as shown in Figure 7b. The 50 s that the adsorption took to reach q/qe of 0.5 for Cu(NO3)2 compares well with the 45 s reported by Walcarius and co-workers54 for Cu2+ adsorption from Cu(NO3)2 on MCM41 grafted with 2.3 mmol/g aminopropyltrimethoxysilane, although ethanol was used as a solvent in their experiments. Scheme 1 Cu2+ + ≡SiRNH2 f ≡SiRNH2-[Cu2+] ≡SiRNH2-[Cu2+] + ≡SiRNH2 f {≡SiRNH2}2-[Cu2+] The difference in the adsorption rate could be attributed to a difference in the adsorption mechanism. The copper adsorption involves two steps with the Cu2+ adsorbing on a single aminopropyl through an interaction with the lone electron pair in the amino headgroup to form an unstable adsorbed intermediate, followed by a reaction with a second aminopropyl to form a stable adsorbed Cu2+ on NH2-MCM-41 as depicted in Scheme 1. The first adsorption step in Scheme 1 is expected to be fast and reversible. One way that the sulfate anion could enhance the copper adsorption rate is by stabilizing the adsorbed intermediates (Scheme 2), thus increasing the intermediate concentration and therefore the overall adsorption rate. It is also possible that the adsorption from CuSO4 is mediated through the [Cu(SO4)]0 specie in the solution. The [Cu(SO4)]0 adsorbed on an aminopropyl forming a stable adsorbed intermediate of ≡SiRNH2-[Cu2+] · · · [SO42-] (Scheme 3) that could further react with another aminopropyl to form the expected 1 Cu2+ per 2 aminopropyl stoichiometery (Scheme 2b). Scheme 2 ≡SiRNH2-[Cu2+] + SO42- f ≡SiRNH2-[Cu2+] · · · [SO42-] (a) 2≡SiRNH2-[Cu2+] + SO42- f {≡SiRNH2-[Cu2+]}2[SO42-] (b) Scheme 3 [Cu(SO4)]0 + ≡SiRNH2 f ≡SiRNH2-[Cu(SO4)]0 ≡SiRNH2-[Cu(SO4)]0 f ≡SiRNH2-[Cu2+] · · · [SO42-] Scheme 4 ≡SiRNH2-[Cu2+] · · · [SO42-] + ≡SiRNH2 f {≡SiRNH2}2-[Cu2+] + SO42The proposed adsorption processes are consistent with the observed coadsorption of SO42-. At a low copper adsorption, the sulfate stabilized intermediates can readily react with the neighboring unreacted aminopropyls to form the aminopropyl bound Cu2+ and release the SO42- anion (Scheme 4). Indeed,
9382 Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008
the coadsorption of SO42- was negligible until q/qe > 0.8 (cf. Figure 4b). As the surface occupancy increased, there is less number of free aminopropyls and the likelihood for the reaction depicted in Scheme 4 diminishes; thus, the number of coadsorbed SO42- anion increases as shown in Figure 4b. 4. Concluding Remarks This work showed that the interactions of the anion with the solution specie, adsorbate, and adsorption sites could give raise to abnormal adsorption behavior referred to as the anion effects. Our task was made easier by eliminating the complexity associated with the tortuous pores and heterogeneous surface found in most adsorbents by using MCM-41 that possesses a high degree of pore symmetry and uniform surface properties. The adsorption sites were created by grafting aminopropyls on MCM-41, and the random deposition created a site distribution best described by the Freundlich adsorption equation. The higher and faster adsorption of copper by NH2-MCM-41 was obtained for SO42- compared to NO3-. The calculations indicated the presence of [CuSO4]0 complex, and the experiments showed the coadsorption of SO42-. The fast adsorption was attributed to the stabilizing effect of SO42- on the adsorbed intermediate. Acknowledgment The authors gratefully acknowledge the financial support from the Environmental Conservation Fund (ECWW05/06.EG01) and also thank the Materials Characterization and Preparation Facility (MCPF) for the use of their equipment. Literature Cited (1) Babel, S.; Kurniawan, T. A. Low-cost adsorbents for heavy metals uptake from contaminated water: a review. J. Hazard. Mater. 2003, B97, 219. (2) Mohan, D.; Pittman, C. U. Activated carbons and low cost adsorbents for remediation of tri- and hexavalent chromium from water. J. Hazard. Mater. 2006, B137, 762. (3) Gerente, C.; Lee, V. K. C.; Cloirec, P. L.; McKay, G. Application of chitosan for the removal of metals from wastewaters by adsorption Mechanisms and models review. Crit. ReV. EnViron. Sci. Technol. 2007, 37, 41. (4) Choy, K. K. H.; McKay, G. Sorption of metal ions from aqueous solution using bone char. EnViron. Int. 2005, 31, 845. (5) Bradl, H. B. Adsorption of heavy metal ions on soils and soil constituents. J. Colloid Interface Sci. 2004, 277, 1. (6) Losa Tanco, M. A.; Pacheco Tanaka, D. A.; Flores, V. C.; Nagase, T.; Suzuki, T. M. Preparation of porous chelating resin containing linear polymer ligand and the adsorption characteristics for harmful metal ions. React. Funct. Polym. 2002, 53, 91. (7) Liu, A.; Gonazalez, R. D. Modeling adsorption of copper(II), cadmium(II) and lead(II) on purified humic acid. Langmuir 2000, 16, 3902. (8) Chen, B.; Hui, C. W.; McKay, G. Film-pore diffusion modeling for the sorption of metal ions from aqueous effluents onto peat. Water Res. 2001, 35, 3345. (9) Stohr, C.; Horst, J.; Holl, W. H. Application of the surface complex formation model to ion exchange equilibria: Part V. Adsorption of heavy metal salts onto weakly basic anion exchangers. React. Funct. Polym. 2001, 49, 117. (10) Esposito, A.; Pagnanelli, F.; Veglio, F. pH-related equilibria models for biosorption in single metal systems. Chem. Eng. Sci. 2002, 57, 307. (11) Navarro, R. R.; Tatsumi, K.; Sumi, K.; Matsumura, M. Role of anions on heavy metal sorption of a cellulose modified with poly(glycidyl methacrylate) and polyethyleneimine. Water Res. 2001, 35, 2724. (12) Doula, M. K.; Iaonnou, A. The effect of electrolyte anion on Cu adsorption-desorption by clinoptilolite. Microporous Mesoporous Mater. 2003, 58, 115. (13) Doula, M. K.; Dimirkou, A. An EPR study of Cu adsorption on chlinoptililite from Cl-, NO3- and SO42- solutions. J. Porous Mater. 2008, 15, 457.
(14) Juang, R.-S.; Wu, W.-L. Adsorption of sulfate and copper(II) on goethite in relation to changes of zeta potentials. J. Colloid Interface Sci. 2002, 249, 22. (15) Feng, X.; Fryxell, G. E.; Wang, L.-Q.; Kim, A. Y.; Liu, J.; Kemmer, K. M. Functionalized monolayers on ordered mesoporous supports. Science 1997, 276, 923. (16) Fryxell, G. E.; Liu, J.; Hauser, T. A.; Nie, Z. M.; Ferris, K. F.; Mattigod, S.; Gong, M. L.; Hallen, R. T. Design and synthesis of selective mesoporous anion traps. Chem. Mater. 1999, 11, 2148. (17) Yantasee, W.; Lin, Y. H.; Fryxell, G. E.; Busche, B. J.; Birnbaum, J. C. Removal of heavy metals from aqueous solution using novel nanoengineered sorbents: Self-assembled carbamoylphosphonic acids on mesoporous silica. Sep. Sci. Technol. 2003, 38, 3809. (18) Fryxell, G. E.; Lin, Y. H.; Fiskum, S.; Birnbaum, J. C.; Wu, H.; Kemner, K.; Kelly, S. D. Actinide sequestration using self-assembled monolayers on mesoporous supports. EnViron. Sci. Technol. 2005, 39, 1324. (19) Fryxell, G. E. The synthesis of functional mesoporous materials. Inorg. Chem. Commun. 2006, 9, 1141. (20) Walcarius, A.; Etienne, M.; Lebeau, B. Rate of access to the binding sites in organically modified silicates-Part 2. Ordered mesoporous silicas grafted with amine or thiol groups. Chem. Mater. 2003, 15, 2161. (21) Matsumoto, A.; Tsutsumi, K.; Schumacher, K.; Unger, K. K. Surface functionalization and stabilization of mesoporous silica spheres by silanization and their adsorption characteristics. Langmuir 2002, 18, 4014. (22) Yoshitake, H.; Yokoi, T.; Tatsumi, T. Adsorption of chromate and arsenate by amino-functionalized MCM-41and SBA-1. Chem. Mater. 2002, 14, 4603. (23) Trens, P.; Russell, M. L.; Spjuth, L.; Hudson, M. J.; Liljenzin, J.O. Preparation of malonamide-MCM-41 materials for the heterogeneous extraction of radionuclides. Ind. Eng. Chem. Res. 2002, 41, 5220. (24) Mercier, L.; Pinnavaia, T. J. Heavy metal adsorbents formed by the grafting of a thiol functionality to mesoporous silica molecular sieves: Factors affecting Hg (II) uptake. EnViron. Sci. Technol. 1998, 32, 2749. (25) Antochshuk, V.; Olkhovyk, O.; Jaroniec, M.; Park, I.-S.; Ryoo, R. Benzoylthiourea-modified mesoporous silica for mercury (II) removal. Langmuir 2003, 19, 3031. (26) Kang, T.; Park, Y.; Choi, K.; Lee, J. S.; Yi, J. Ordered mesoporous silica (SBA-15) derivatized with imidazole-containing functionalities as a selective adsorbent of precious metal ions. J. Mater. Chem. 2004, 14, 1043. (27) Pearson, R. G. Hard and soft acids and bases. J. Am. Chem. Soc. 1963, 85, 3533. (28) Ho, K. Y.; Yeung, K. L.; McKay, G. Selective adsorbents from ordered mesoporous silica. Langmuir 2003, 19, 3019. (29) Lam, K. F.; Yeung, K. L.; McKay, G. An investigation of gold adsorption from a binary mixture with selective mesoporous silica adsorbents. J. Phys. Chem. B 2006, 110, 2187. (30) Lam, K. F.; C, M.; Fong, K. L.; Yeung, G. McKay, Recovery of high purity gold and silver using mesoporous adsorbents. Stud. Surf. Sci. Catal. 2007, 170B, 1969. (31) Lam, K. F.; Fong, C. M.; Yeung, K. L. Separation of precious metals using selective mesoporous adsorbents. Gold Bull. 2007, 40, 192. (32) Lam, K. F.; Fong, C. M.; Yeung, K. L.; McKay, G. Selective adsorption of gold from complex mixtures using mesoporous adsorbents. Chem. Eng. J. 2008, in press. (33) Lam, K. F.; Yeung, K. L.; McKay, G. A Rational approach in the design of selective mesoporous adsorbents. Langmuir 2006, 22, 9632. (34) Lam, K. F.; Yeung, K. L.; McKay, G. Selective mesoporous adsorbents for Ag+/Cu2+ separation. Chem. Commun. 2008, 2034. (35) Lam, K. F.; Ho, K. Y.; Yeung, K. L.; McKay, G. Selective adsorbents from chemically modified ordered mesoporous silica. Stud. Surf. Sci. Catal. 2004, 154C, 2981. (36) Lam, K. F.; Yeung, K. L.; McKay, G. Selective mesoporous adsorbents for Cr2O72- and Cu2+ separation. Microporous Mesoporous Mater. 2007, 100, 191. (37) Lam, K. F.; Yeung, K. L.; McKay, G. A new approach for Cd2+ and Ni2+ removal and recovery using mesoporous adsorbent with tunable selectivity. EnViron. Sci. Technol. 2007, 41, 3329. (38) Do, D. D. Adsorption Analysis: Equilibria and Kinetics; Imperial College Press:London, 1998. (39) Ho, Y. S.; McKay, G. A comparison of chemisorption kinetic models applied to pollutant removal on various sorbents. Trans. Inst. Chem. Eng. 1998, Part B, 332. (40) Taylor, H. A.; Thon, N. Kinetics of chemisorption. J. Am. Chem. Soc. 1952, 4169. (41) Ritchie, A. G. Alternative to the Elovich equation for the kinetics of adsorption of gases on solids. J. Chem. Soc., Faraday Trans. 1 1977, 73, 1650.
Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008 9383 (42) Schulz-Ekloff, G.; Rathousky, J.; Zukal, A. Controlling of morphology and characterization of pore structure of ordered mesoporous silicas. Microporous Mesoporous Mater. 1999, 27, 273. (43) Chan, H. B. S.; Budd, P. M.; Naylor, T. de V. Control of mesostructured silica particle morphology. J. Mater. Chem 2001, 11, 951. (44) Grun, M.; Lauer, I.; Unger, K. K. Homogeneous precipitation of siliceous MCM-41. AdV. Mater. 1997, 9, 254. (45) Pauwels, B.; Van Tendeloo, G.; Thoelen, C.; Van Rhijn, W.; Jacobs, P. A. Structure determination of spherical MCM-41 particles. AdV. Mater. 2001, 13, 1317. (46) Yoon, S. B.; Kim, J.-Y.; Kim, J. H.; Park, Y. J.; Yoon, K. R.; Park, S.-K.; Yu, J.-S. Synthesis of monodisperse spherical silica particles with solid core and mesoporous shell: mesopore channels perpendicular to the surface. J. Mater. Chem. 2007, 17, 1758. (47) Zhao, D.; Sun, J.; Li, Q.; Stucky, G. D. Morphological control of highly ordered mesoporous silica SBA-15. Chem. Mater. 2000, 12, 275. (48) Boissiere, C.; Van der Lee, A.; El Mansouri, A.; Larbot, A.; Prouzet, E. A double step synthesis of mesoporous micrometric spherical MSU-X silica particles. Chem. Commun. 1999, 2047. (49) Brown, J.; Richer, R.; Mercier, L. One-step synthesis of high capacity mesoporous Hg2+ adsorbents by non-ionic surfactant assembly. Microporous Mesoporous Mater. 2000, 37, 41.
(50) Guari, Y.; Thieuleux, C.; Mehdi, A.; Reye, C.; Corriu, R. J. P.; Gomez-Gallardo, S.; Phillippot, K.; Chaudret, B. In situ formation of gold nanoparticles within thiol functionalized HMS-C16 and SBA-15 type materials via an organometallic two-Step approach. Chem. Mater. 2003, 15, 2017. (51) Despas, C.; Walcarius, A.; Bessiere, J. Influence of the base size and strength on the acidic properties of silica gel and monodispersed silica beads: Interest of impedance measurements for the in situ monitoring of the ionization process. Langmuir 1999, 15, 3186. (52) Azour, H.; Derouault, J.; Lauroua, P.; Vezon, G. Spectrochim. Fourier transform infrared spectroscopic characterization of grafting of 3-aminopropyl silanol onto aluminum/alumina substrate. Spectrochim. Acta A 2000, 56, 1627. (53) Gustafsson, J. P. Visual MINTEQ, Version 2.40; KTH, Stockholm, Sweden, 2006. (54) Walcarius, A.; Etienne, M.; Bessiere, J. Rate of access to the binding sites in organically modified silicates. 1. Amorphous silica gels grafted with amine or thiol groups. Chem. Mater. 2002, 14, 2757.
ReceiVed for reView December 21, 2007 ReVised manuscript receiVed September 23, 2008 IE701748B
