Hydrophobic Fe-Zeolites for Removal of MTBE from Water by

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Hydrophobic Fe-Zeolites for Removal of MTBE from Water by Combination of Adsorption and Oxidation Rafael Gonzalez-Olmos,†,‡ Frank-Dieter Kopinke,† Katrin Mackenzie,† and Anett Georgi*,† †

Helmholtz Centre for Environmental Research (UFZ), Department of Environmental Engineering, Permoserstrasse 15, D-04318 Leipzig, Germany ‡ LEQUIA, Institute of the Environment, University of Girona, Campus Montilivi, E-17071, Girona, Catalonia, Spain. S Supporting Information *

ABSTRACT: Several zeolites were evaluated as adsorbents for the removal of MTBE from water in a screening process. It was observed that the SiO2/Al2O3 molar ratio is a decisive factor for the adsorption properties, at least in the case of ZSM5 zeolites. ZSM5 zeolites with SiO2/Al2O3 ratios >200 were found to provide the best sorption properties for MTBE. To design a combined sorption/reaction method, regeneration of the loaded zeolites by selected advanced oxidation processes (AOP) was studied: (1) Fenton treatment using H2O2 with dissolved iron salts and (2) heterogeneous Fenton-like oxidation with Fe immobilized on the zeolites. The first was ineffective in regenerating loaded zeolites. However, heterogeneous catalysis using Fe species immobilized on the zeolite by liquid ion exchange was markedly more effective. Although these hydrophobic zeolites have a low ion exchange capacity, resulting in iron loadings of ≤0.09 wt %, it was possible to obtain sufficiently active catalysts. Hydrophobic Fe-zeolites can therefore be regarded as promising materials for the removal of MTBE from water, since they allow the combination of efficient adsorption and oxidative degradation of MTBE by H2O2. In contrast to the homogeneous catalysis by dissolved iron ions, these heterogeneous catalysts work at near-neutral pH and can be easily reused. Fe-zeolites as adsorbents/catalysts showed a good stability in both batch and column experiments.

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INTRODUCTION

Fe on the zeolite, regeneration of 0.2 g of Fe-zeolite with 30 mL of 30 wt % H2O2 solution), and the pH regime was not defined. The combination of pollutant adsorption and catalytic oxidation appears to be promising, especially for the treatment of water containing highly dilute contaminants, conditions engineers are often faced with in the case of groundwater remediation. MTBE is a typical example, since site remediation has to be performed even at contaminant concentrations in the microgram per liter range.4 Adsorption affinity and capacity of zeolites with respect to a certain compound depend on various parameters such as hydrophobicity of adsorbate and zeolite surface, as well as congruence of sorbate, pore sizes, and pore volume.4,5 Among the various adsorbents studied for removal of low concentrations of MTBE from contaminated water (several μg L−1 to about 10 mg L−1), zeolites of the MFI framework type (ZSM5, silicalite) having high SiO2/Al2O3 ratios showed excellent results, outcompeting even highperformance activated carbon (AC) samples.4,5,10

Zeolites are interesting candidates as adsorbents and catalyst carriers in water treatment processes because of their unique adsorption properties with respect to small organic molecules and because of their ion exchange capacity. In recent years, Fecontaining zeolites have been intensively studied as catalysts for wet peroxide oxidation (WPO) of various compounds, since they are able to activate H2O2 to generate highly reactive oxidant species at ambient conditions and near-neutral solution pH.1,2 In our previous study, we found indications that these reactive species consist of hydroxyl radicals and a second species, possibly of ferryl (FeIV) type.3 In addition, various hydrophobic zeolites, which are characterized by high SiO2/ Al2O3 ratio, were shown to be excellent adsorbents for small organic molecules, such as MTBE,4−6 which is an oftenoccurring groundwater pollutant.7−9 The combination of both approaches, adsorptive enrichment and catalytic oxidation, in an optimized hydrophobic Fe-zeolite material is highly desirable, but has found little attention in the scientific community so far. A work from Koryabkina et al.10 is one of the few attempts to regenerate a loaded hydrophobic zeolite with a heterogeneous Fenton-like reaction. However, the reaction conditions were very harsh (25 wt % of zerovalent © 2013 American Chemical Society

Received: Revised: Accepted: Published: 2353

September 26, 2012 January 22, 2013 January 24, 2013 January 24, 2013 dx.doi.org/10.1021/es303885y | Environ. Sci. Technol. 2013, 47, 2353−2360

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Table 1. Characteristic Properties of the Adsorbentsa adsorbent

supplier

maximum free pore diameter (Å)

Fe content (wt %)

BET surface area (m2 g−1)

particle size (μm)

ZSM5 (683) ZSM5 (400) ZSM5 (236) Fe-ZSM5 (26) Beta (200) Fe-Beta (35) Fe-Beta (25) DAY (>200) F 300 (AC)

Zeochem Zeochem Sued-Chemie Sued-Chemie Sued-Chemie Sued-Chemie Sued-Chemie Degussa Chemviron

5.6 5.6 5.6 5.6 7.5 7.5 7.5 7.4 Vmicro = 0.37 cm3 g−1 Vmeso = 0.16 cm3 g−1d

n.d. 0.37 0.03 2.2 n.d. 1.3 3.1 n.d. n.d.

330 265 385 370 580 600 600 910 905

1.0−17b 63−200c 4.4−7.1b 6.4−12b 5.6−15b 1.0−2.1b 250−630c 63−200c 630−1000c

a SiO2/Al2O3 molar ratios given in parentheses. bLower limit d50 − upper limit d90 determined by laser diffraction analysis. cSieved fractions of pelletized materials. dMicropore (Vmicro) and mesopore (Vmeso) volumes from BET measurements.

Batch experiments were conducted to compare the heterogeneous Fenton-like oxidation with conventional homogeneous Fenton and Fenton-like treatments as regeneration options for hydrophobic zeolites. In addition, some column experiments were conducted to test the stability of the zeolites under flow-through conditions during several adsorption cycles with successive heterogeneous Fenton-like regeneration steps.

However, inherent to adsorption processes is the problem of regeneration of the spent material. Regeneration of the adsorbent using advanced oxidation processes (AOP)10−12 is considered to offer a promising alternative to hydrothermal processes, since it can be performed on-site and in-situ within the reactor at near-ambient conditions. Moreover, zeolites are not subject to oxidative degradation as known for AC, even if hydroxyl radicals are involved, which would make zeolites a suitable candidate for long-lived recyclable catalysts. For regeneration, oxidation by Fenton-like processes would be a feasible strategy. To obtain heterogeneous Fenton-like catalysts, transition metals (mainly Fe, Cu) were introduced into zeolites by hydrothermal synthesis13,14 or ion exchange procedures15 in previous studies. Depending on the synthesis route and the iron content, iron can exist within zeolites in various states: as substituted iron atoms in the framework, as isolated iron ions or oxygen-bridged binuclear iron species at ion-exchange positions, as small clusters of iron species in the pores, and as large agglomerates of iron oxides on the external surface.16 Knowledge about the relationship between the iron speciation in Fe-zeolites and their catalytic activity for Fenton-like reactions is rather limited.3,17,18 However, from the broad knowledge about redox catalysis by Fe-zeolites in gasphase reactions (such as the selective catalytic reduction (SCR) of nitrogen oxides16), it can be anticipated that isolated mononuclear and binuclear iron species attached to ionexchange sites on the zeolite surface are mainly responsible for the catalytic activity, while large iron-oxide agglomerates could contribute to the parasitic decomposition of H2O2 into H2O + O2. To date, not much attention has been paid to zeolites with a high SiO2/Al2O3 ratio for the preparation of Fe-zeolite catalysts. This is due to their lower density of ion-exchange sites. However, the anticipated lower catalytic activity of hydrophobic Fe-zeolites with low iron content could be compensated for by the positive effect of adsorptive contaminant enrichment. In previous studies, we could show that adsorption of the contaminants on Fe-zeolites with moderate SiO2/Al2O3 ratio (200), F300 (AC), and ZSM5 (400), which were obtained as pellets. The latter were ground and sieved until the fractions indicated in Table 1 were obtained. Analytical Methods. For the analysis of MTBE, GC-MS was applied (for details see below). The dissolved organic carbon (DOC) content in aqueous samples was measured with a TOC analyzer (TOC-5050, Shimadzu). H2O2 content was determined photometrically using titanyl sulfate. Fe(II/III) in aqueous samples was analyzed with a photometric test (Spectroquant, Merck). Experimental details for X-ray fluorescence (Fe, Si, and Al content), UV−vis/DRS (Fe speciation), BET (textural properties) and XRD (crystallinity) analysis are given in the Supporting Information (SI).

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EXPERIMENTAL SETUP Preparation of Fe-Zeolite. The iron impregnation of 10 g zeolite ZSM5 (236) was carried out by means of liquid ion exchange in a stirred vessel for 24 h, using 1000 mL of a solution of 0.05 M FeSO4·7H2O (adjusted to pH = 3). In order to prevent oxidation of Fe(II), the solution was continuously purged with N2. Afterward, the synthesized Fe-ZSM5 (236) was separated from the solution by means of centrifugation, washed with deionized water (100 mL for each gram of Fezeolite) and dried at 100 °C overnight. The material denoted as Fe-ZSM5 (236)cal was additionally calcined at 550 °C in a muffle furnace for 5 h. Batch Experiments for Determination of Adsorption Isotherms and Degradation of MTBE. The equilibrium adsorption isotherms were measured at ambient temperature (25 ± 2 °C). They were obtained by preparing suspension samples with various concentrations of zeolite or AC (1−2 g L−1) and MTBE (1−100 mg L−1) in 250-mL flasks with 2354

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operated with tap water spiked with 4 mg L−1 MTBE, which was pumped through the column from bottom to top using a piston pump with a flow rate of 0.35 mL min−1. Several adsorption/regeneration steps were conducted whereby the conditions were varied. In the regeneration steps, H2O2 was recirculated through the Fe-zeolite bed at a flow rate of 0.8 mL min−1 over 48−72 h using a 100−250 mL reservoir with 2−5 g L−1 H2O2. Before and after each regeneration step, the column was flushed with about 10 pore volumes of deionized water.

Mininert valves. According to the adsorption kinetics of ZSM5 (236) (see Supporting Information, Figure S1) and published results,19 a mixing time of 24 h on a horizontal shaker was sufficient to reach the adsorption equilibrium. After the equilibration step, the concentration of the freely dissolved MTBE (Cfree) was determined by means of headspace sampling and GC-MS analysis. Sorption data reported were determined for as-received materials (not corrected for content of any binding materials if present). Batch degradation experiments were conducted in 120-mL flasks equipped with Mininert valves for headspace sampling and an additional valve for depressurizing the reactor and for adding reagent solutions. Homogeneous stoichiometric Fenton (with Fe(II)) and catalytic Fenton (with Fe(III)) reactions were carried out as follows: (I) preparation of Fe stock solutions using Fe(NO3)3 or FeSO4·7H2O2 (the latter was purged with N2), (II) addition of the iron stock solution to an aqueous solution of MTBE (100 mg L−1) adjusted to pH = 3 with HCl, and (III) start of the reaction by addition of H2O2. In the stoichiometric Fenton reaction, equimolar concentrations of Fe(II) and H2O2 were used (1−10 mM each), whereby H2O2 was added stepwise (0.1 mM/min). In the catalytic Fenton reaction, H2O2 (100 mM) was applied in excess with respect to Fe(III) (0.3 mM). The concentration of MTBE in these zeolite-free systems was analyzed by means of headspaceGC-MS analysis as described above. In experiments with zeolites and Fe-zeolites, MTBE adsorption was allowed to come to the equilibrium state, while the samples were shaken for at least 24 h. The concentrations of the freely dissolved MTBE fractions were determined by means of headspace-GC-MS. In the case of the homogeneous Fenton reaction, the suspension was adjusted to pH = 3 with HCl, iron stock solutions were added and the reaction was started by adding a defined amount of H2O2. Final chloride concentrations were ≤2 mM in all cases and thus clearly below the range for which inhibition of Fenton-like reactions because of OH-radical scavenging by Cl− was reported to be significant ([Cl−] ≥ 10 mM).22,23 Aliquots (1 mL) of the reaction suspension were mixed with 0.44 mmol Na2S2O3 (to quench the surplus of H2O2) and extracted for 24 h with dichloromethane, which contained toluene as internal standard. The extracts were analyzed by means of GC-MS. The total recovery of MTBE from zeolite suspensions (freely dissolved and adsorbed MTBE) in the absence of H2O2 was ≥90%. For experiments with Fe-zeolites, commonly the pH value of the suspension was adjusted to pH = 7, although in some reactions the pH was adjusted to other values. In each experiment, the H2O2 concentration was monitored and the iron leaching measured (analysis of dissolved iron after centrifugation of the sample). Catalyst concentrations are generally reported as Fe-zeolite mass per volume. In the experiments for catalyst recycling (6 cycles), the Fezeolite was separated by centrifugation after each cycle, fresh MTBE was added and after shaking for 24 h (allowing the MTBE adsorption to come to the equilibrium state) the reaction was restarted by adding H2O2. Column Experiment for MTBE Degradation (Intermittent Operation Mode20). Fe-ZSM5 (236)cal was used in column experiments. The powdered zeolite was pelletized using a disk press, gently crushed, and sieved. The 0.25−0.63 mm size fraction (0.2 g) was used as fixed bed in a small vertical glass column (length = 2 cm, diameter = 0.4 cm). The column was

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RESULTS AND DISCUSSION Adsorption Isotherms. The adsorption isotherms of MTBE on most of the zeolite samples and the AC can be reasonably well described by the Freundlich model (Figure S2 in Supporting Information). However, for the more hydrophobic zeolites the Freundlich fit can only be applied in the range of low concentrations (Cfree ≤ 1 mg L−1). At higher MTBE concentrations (Cfree ≥ 1 mg L−1), the slope of the isotherm is decreasing. In Table S1 (Supporting Information), the Freundlich parameters for the linear range of the doublelogarithmic plot are given. The zeolites differ markedly in their pore size: while ZSM5 has a 10-ring channel structure (minor and major axis dimensions of 5.1 × 5.5 Å and 5.4 × 5.6 Å for the sinusoidal and straight channels, respectively), the 12-ring channel system of Beta (6.5 × 5.6 and 7.5 × 5.7 Å) and Y (7.4 × 7.4 Å) zeolites have larger pore dimensions. On the basis of its kinetic diameter (6.2 Å), the MTBE molecule should be able to enter the pores of Beta, Y zeolites, and AC, whereas the compatibility with the pore opening is not self-evident in the case of the ZSM5 zeolite. In the concentration range studied (Cfree = 0.01−40 mg L−1), DAY is the most ineffective of all the adsorbents. This is illustrated by Figure 1a) with the plot of single-point adsorption coefficients (Kd = Csorb/Cfree) over Cfree. The zeolites with ZSM5 structure and high SiO2/Al2O3 ratio (>200) were the most effective adsorbents, even superior to AC in the range of low MTBE concentrations (Cfree = 0.01−1 mg L−1), while the differences among the various zeolites are less pronounced at higher MTBE concentrations. The variability in specific surface area of the materials (Table 1) obviously plays a minor role for MTBE adsorption compared to the effect of pore structure and hydrophobicity. In Figure 1 b), the adsorption coefficient at Cfree = 0.3 mg L−1 is plotted versus the molar SiO2/Al2O3 ratio. It is shown that increasing the SiO2/Al2O3 ratio in Beta zeolites does not significantly improve their adsorption affinity toward MTBE. However, for the ZSM5 zeolites, the adsorption coefficient Kd is improved with increasing SiO2/Al2O3 ratio until a plateau is reached at SiO2/Al2O3 = 400. The observed high sensitivity of the MTBE adsorption affinity of ZSM5 zeolites toward their alumina content is in agreement with other studies.4,5 Zeolites with high SiO2/Al2O3 ratios possess hydrophobic surfaces within their pores. Because of the incapability of water to form a condensed liquid phase in the narrow pores of very hydrophobic ZSM5 zeolites,18,24 the competition between adsorption of water and MTBE is strongly in favor of the organic compound for this type of zeolites. Another explanation for the higher adsorption affinity of MTBE toward MFI type zeolites might be higher MTBE-pore wall interaction energies with smaller pores.5 It should be mentioned that in the range of high MTBE concentrations in water (Cfree > 100 mg L−1), zeolites with larger pore dimensions (Beta, Y) become superior over MFI type zeolites (silicalite, ZSM5),5 since adsorption capacity, i.e. accessible pore volume, is of increasing importance at higher 2355

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Figure 2. Degradation of MTBE (C0,MTBE = 100 mg L−1) at pH = 3 by means of stoichiometric Fenton reaction (C0,Fe(II) = 1 mM; C0,H2O2 = 1 mM) without zeolite (▲) and with 25 g L−1 ZSM5 (400) (Δ); catalytic Fenton reaction (CFe(III) = 300 μM; C0,H2O2 = 100 mM) without zeolite (■) and with 25 g L−1 ZSM5 (400) (□); stoichiometric Fenton reaction (C0,Fe(II) = 10 mM; C0,H2O2 = 10 mM) without zeolite (●) and with 5 g L−1 ZSM5 (236) (○).

concentrations of H2O2 and dissolved Fe ions, the conversion of MTBE is vastly different in the absence and presence of the zeolite. The fastest reaction was achieved with a stoichiometric Fenton system (10 mM H2O2 and Fe(II)) where ≥99% of MTBE were degraded within 30 min. In contrast, in the presence of zeolites the highest conversion degree achieved was 37% within 24 h. This is due to the fact that the formation of hydroxyl radicals from H2O2 by dissolved Fe ions occurs predominantly in the bulk solution phase. The lifetime of these radicals is far too short to allow diffusion into the pore system of the zeolites. Regeneration by Heterogeneous Fenton-like Reaction. To obtain a material which can be regenerated by a heterogeneous Fenton-like reaction, Fe was introduced into ZSM5 (236) as explained above. The resulting material is denoted as Fe-ZSM5 (236). After the impregnation, the Fe content incorporated into the zeolite was 0.09 wt %. The iron speciation was studied by analyzing the UV−vis/DRS spectra of Fe-ZSM5 (236) (shown in Supporting Information Figure S3). According to the literature,15,21 absorption bands below 300 nm are caused by O → Fe ligand to metal charge transfer transitions of isolated Fe(III) ions and oxygen-bridged binuclear Fe(III) species. The band at λ ≈ 350 nm corresponds to oligomeric Fe(III)xOy clusters, whereas signals in the range λ > 400 nm are ascribed to large iron-oxide particles. For FeZSM5 (236), no iron-oxide particles and only a small fraction of Fe(III)xOy clusters (3.5% of the total Fe spectral band) were observed. The dominant fraction consists of iron species of low nuclearity (isolated or binuclear), which according to literature21 can be subdivided into Fe(III) in tetrahedral (band at λ ≈ 220 nm, 27.5%) and octahedral coordination (band at λ ≈ 285 nm, 69%). As has been described elsewhere,19 the decomposition of H2O2 with Fe-zeolites follows a pseudo-first-order kinetics: ln(C/C0) = −kH2O2 × t, with C/C0 as the relative residual concentration of H2O2 after a certain time (t) and kH2O2 as the pseudo-first-order rate constant. The catalyst activity for H2O2 decomposition, defined as AH2O2 = kH2O2/Ccat, where Ccat is the applied catalyst concentration, is 3.8 × 10−4 L g−1 min−1 for FeZSM5 (236). Even at nearly neutral conditions, Fe-ZSM5

Figure 1. (a) Single-point adsorption coefficients as function of equilibrium aqueous-phase concentration of MTBE for the various adsorbents used in this study (at native pH of adsorbent suspension in deion. water, that is, pH 5−6). Solid line for ZSM5 (236) zeolite and dashed line for AC were drawn as guides to the eye. (b) Adsorption coefficient Kd at Cfree = 0.3 mg L−1 for MTBE on (□) ZSM5 and (●) Beta zeolites as a function of SiO2/Al2O3 molar ratio of the zeolites. Note that Kd of ZSM5 zeolite with SiO2/Al2O3 = 400 could slightly vary due to an unknown amount of binding material.

loadings. In order to remain below the maximum permissible values for MTBE in the discharge, which are usually in the μg L−1 range,25 a sequence of “high capacity” and “high affinity” hydrophobic zeolites may be optimal. Zeolites with high SiO2/ Al2O3 ratios are less suitable for an Fe impregnation process due to their low density of ion-exchange sites. For this reason, ZSM5 (400) and ZSM5 (236) (SiO2/Al2O3 = 400 and 236, respectively) were selected for the following experiments, being a compromise between adsorption performance and suitability as catalyst support. Regeneration with Homogeneous Fenton/Fenton-like Reactions. Stoichiometric Fenton and catalytic Fenton reactions were carried out for the degradation of MTBE in the presence and absence of ZSM5 (400) and ZSM5 (236) under otherwise identical conditions. MTBE was almost completely adsorbed by the zeolites in these experiments (adsorbed fraction >99%). Figure 2 shows that adsorption of MTBE into the zeolites strongly inhibits its degradation when a homogeneous Fenton reaction is applied. At identical 2356

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Fe in the original material is probably largely framework iron (since it was present in trace amounts during hydrothermal zeolite synthesis) which is of minor activity in redox processes. In contrast, the ion exchange procedure resulted in isolated mononuclear and binuclear iron species attached to acidic sites at the zeolite surfaces, species which are generally attributed to high redox reactivity.18 With respect to iron speciation, the hydrophobic Fe-ZSM5 (236) is similar to the more hydrophilic Fe-ZSM5 (26) investigated in our previous study 3 , that is, isolated mononuclear and binuclear iron species were the dominant species in both Fe-zeolites. Thus, we suppose that the two FeZSM5 zeolites produce the same types of reactive species in heterogeneous Fenton-like reactions. From a detailed study with Fe-ZSM5 (26), we concluded that hydroxyl radicals and possibly a second oxidant of ferryl type are involved in heterogeneous Fenton-like reactions with Fe-zeolite catalysts obtained by iron ion-exchange procedures.3 The stability of Fe-ZSM5 (236) was investigated by means of recycling experiments. Fe-ZSM5 (236) and Fe-ZSM5 (236)cal samples were used to test the effect of calcination on the stability of the Fe-zeolites. Figure 4 shows the MTBE

(236) is able to produce reactive species from H2O2, driving the oxidation of MTBE as is shown in Figure 3. This is especially

Figure 3. Residual MTBE concentration (C/C0) in homogeneous Fenton reaction at pH = 3 with CFe(III) = 2.24 mg L−1 (○), heterogeneous Fenton-like reaction with 5 g L−1 Fe-ZSM5 (236) at pH0 = 7 (●), and homogeneous Fenton reaction due to leached iron (0.03 mg L−1 Fe) at pH = 3 (◊). In all reactions, C0,H2O2 = 3.5 g L−1, C0,MTBE = 50 mg L−1..

important for the treatment of contaminated groundwaters, which can have high buffer capacities when acidified to the optimum pH for homogeneous Fenton reactions (pH about 326). To evaluate the catalytic activity of the Fe bound to the zeolite, a homogeneous Fenton reaction at pH = 3 was carried out by treating a solution of 50 mg L−1 of MTBE using the same H2 O2 (3.5 g L−1) and Fe(III) (2.24 mg L−1 ) concentrations, either as dissolved iron or in form of 5 g L−1 Fe-ZSM5 (236) in the heterogeneous process. As Figure 3 shows, the MTBE removal rates in both cases are very similar. However, the heterogeneous reaction has the advantages of working at near-neutral conditions together with the possibility of recycling the Fe and avoiding the formation of sludge, which is inevitable for homogeneous Fenton reactions. The H2O2 consumption after reaction (24 h) was about 40% with both reaction types. The pseudo-first-order rate constant for MTBE oxidation in the heterogeneous Fenton-like reaction with 5 g L−1 Fe-ZSM5 (236) was kMTBE,Fe‑zeolite = 0.22 h−1. To estimate a possible contribution of the homogeneous Fenton reaction catalyzed by leached iron, the catalyst particles were removed after the experiment by centrifugation and fresh MTBE and H2O2 were added to the particle-free supernatant which contained Fe leached from the zeolite (0.03 mg L−1, corresponding to 1.5% of the total Fe). The pH value was adjusted to 3. The rate constant was found to be 13 times lower than that in the heterogeneous system (kMTBE,leached = 0.017 h−1). In addition, a significant contribution of zeolite-catalyzed MTBE hydrolysis at reduced pH,27,28 and any MTBE degradation catalyzed by the original ZSM5 (236) in the presence of H2O2, could also be excluded (Supporting Information Figure S4). Thus the catalytic activity observed can be exclusively assigned to the iron introduced into the zeolites. It is remarkable that the low increase in iron content from 0.03 wt % in the original ZSM5 (236) to 0.09 wt % after iron ion exchange, results in a tremendous increase in catalytic activity for heterogeneous Fenton-like reactions (see also Supporting Information for comparison of UV−vis/DRS spectra). This can be understood taking into account that the

Figure 4. Reduction in total MTBE and DOC concentrations in the aqueous phase in batch experiments with repeated use of Fe-ZSM5 (236) and Fe-ZSM5 (236)cal after 24 h of reaction using Ccat = 5 g L−1, C0,MTBE = 100 mg L−1, C0,H2O2 = 7.5 g L−1.

degradation and the DOC removal in the aqueous phase for the two catalysts, observed after 24 h in various single reaction cycles. In the case of Fe-ZSM5 (236), the catalyst activity is reduced during longer-term operation. The MTBE degradation after 24 h reaction time dropped from 97% in the first cycle to 53% in the sixth cycle. The loss in catalyst activity is also apparent in the remaining DOC concentration, which clearly increased. In the case of Fe-ZSM5 (236)cal, the catalyst stability increased significantly: no significant loss of catalytic activity was observed during the 6 reaction cycles. The DOC concentration after 24 h was lower than 10% of the initial values in each reaction cycle, which implies a high mineralization degree of MTBE and its intermediate products. The iron loss was around 5% after 6 reaction cycles for FeZSM5 (236)cal and 10% for Fe-ZSM5 (236). As shown in the Supporting Information (Figure S5), iron leaching occurred for both zeolites predominantly during the first two cycles (about 4% iron loss for Fe-ZSM5 (236)cal and 7% for Fe-ZSM5 2357

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(236)). According to the results of XRD analyses, the crystallinity of both zeolites was maintained despite processes such as Fe impregnation, calcination and repeated use as catalyst (see Supporting Information, Figure S6). UV−vis/DRS spectra of the calcined and noncalcined Fe-zeolite revealed only very minor changes (Figure S3, Supporting Information). Even though identification of the underlying processes is out of the scope of this study, it should be noted that calcination, possibly because of slight changes in iron speciation, has clearly a positive effect on the stability of the Fe-ZSM5 (236) catalyst. The influence of solution pH on the oxidation of MTBE was studied with Fe-ZSM5 (236) (shown in Table 2). The degree Table 2. MTBE Conversion Degree and H2O2 Consumption after 4 h during Heterogeneous Fenton-like Reaction with Fe-ZSM5 (236) under Various pH Conditions (C0,MTBE = 50 mg L−1, C0,H2O2 = 3.5 g L−1, Ccat = 2.5 g L−1) pHa 9 8 7 pH0 = 7/pHf = 5.1b 3

MTBE conversion (%) 8 25 38 76 66

± ± ± ± ±

2 3 4 5 3

H2O2 consumption (%) 2.3 1.4 13 21 27

± ± ± ± ±

0.2 0.5 1 2 5

a

pH kept constant during the reaction. bInitial pH = pH0/final pH after 4 h of reaction = pHf.

of MTBE conversion after 4 h was highest under neutral to slightly acidic conditions (i.e., in the experiment with initial pH = 7 and final pH = 5). When the initial pH was increased to 9, the rate of MTBE degradation decreased significantly, whereas it was only slightly reduced under acidic conditions (initial pH = 3). The consumption of H2O2 decreased continuously from acidic to alkaline conditions and was very low at pH = 9 (2.3% in 4 h), indicating that the Fe-zeolite is deactivated under these conditions. If the Fe-zeolite is applied in an intermittent mode, i.e. as adsorbent which is periodically regenerated by flushing with H2O2 solution, the pH conditions in the regeneration solution can be easily adapted to achieve optimal degradation rates and H 2 O 2 efficiencies. Continuous operation as adsorbent/catalyst with H2O2 on-stream would still be possible if the pH of the water can be adjusted to the neutral or slightly acidic range. To understand the influence of sorption on the rate of contaminant degradation, experiments were conducted where the concentrations of Fe-zeolite and MTBE were varied. The results are only briefly discussed here. Further details can be found in the Supporting Information. A high degree of adsorptive enrichment of the contaminant in the vicinity of the catalytically active centers is expected to have a positive influence on the probability of collisions between reactive species and contaminant molecules, and thus on the reaction rate. On the other hand, the achievable rate of contaminant degradation is limited by the rate at which reactive species are produced. Thus, increasing the concentration of adsorbed contaminants (i.e., increasing the rate of reactive species consumption) drives the system toward conditions where reactive species production becomes the rate-determining step. The most important question for the use of Fe-zeolites as adsorbent and catalyst in a fixed-bed reactor is whether the catalytic activity is still maintained at high contaminant loadings. In a batch experiment, we were able to vary the MTBE load on the Fe-zeolite in the range of 4−39 mg g−1

Figure 5. Influence of adsorbed MTBE concentration (Cads) on (a) adsorbed fraction (◊) and rate constant for MTBE decomposition (Δ) and (b) rate constant for H2O2 decomposition (Δ) and XH2O2 (■). CFe‑ZSM5(236) = 2.5 g L−1, C0,H2O2 = 3.5 g L−1, pH = 3.

while keeping the adsorbed fraction of MTBE close to 1 (Figure 5a). The corresponding equilibrium aqueous-phase concentrations of MTBE were between 0.07 and 0.9 mg L−1, which is reasonably representing the range for termination of adsorber operation. The first-order rate constant for MTBE degradation decreased with the 10-fold increase in MTBE load (for further mechanistic discussion of this finding see Supporting Information part). However, this decrease is only by a factor of 3, that is, the Fe-zeolite remained sufficiently active in order to degrade the adsorbed MTBE (Figure 5a). Remarkably, the efficiency of H2O2 utilization benefits from a high degree of contaminant enrichment in the zeolite. This is illustrated by the H2O2 consumption index (XH2O2), which was defined as moles of H2O2 consumed per mole of MTBE degraded until 50% conversion of MTBE are achieved. XH2O2 decreased from 82 to 8 when the concentration of adsorbed MTBE was increased from 4.0 to 63 mg g−1, respectively (Figure 5 b). A value of XH2O2 = 8 can be considered as proof of an efficient oxidation system. When varying the Fe-zeolite concentration, its catalytic activity for MTBE degradation had an optimum at conditions where MTBE was almost completely in the adsorbed state while its concentration on the Fe-zeolite was still high (Supporting Information Figure S7b). By increasing the H2O2 concentration, kMTBE can be increased. However, the correlation is not linear: a factor of 2358

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formate (TBF), tert-butyl alcohol (TBA), acetone, methyl acetate, alcohols < C4, carboxylic acids and formaldehyde are known intermediates of MTBE degradation.19 As Supporting Information Figure S9 shows, the concentrations of TBA and TBF monitored at the end of the third regeneration step were below the analytical detection limits (≤0.2 mg L−1 and ≤0.1 mg L−1, respectively), showing that they did not significantly contribute to the observed DOC value. Thus, the tert-butyl group in intermediates, which significantly affects their biodegradability, was successfully degraded, and the remaining DOC can be expected to consist mainly of short-chain organic acids. Environmental Implications. Hydrophobic Fe-containing ZSM5 zeolites are promising materials for the treatment of MTBE-contaminated water by means of adsorption/oxidation processes. Although these hydrophobic zeolites have a low density of acidic sites, it is possible to obtain sufficiently active Fenton-like catalysts by means of iron impregnation via liquid ion-exchange. Adsorption of MTBE by the Fe-zeolite has a positive effect on the efficiency of its degradation and H2O2 utilization. Compared to the rather hydrophilic Fe-zeolites previously tested for adsorption and oxidative degradation of MTBE (Fe-Beta and Fe-ZSM5 with SiO2/Al2O3 < 40)19,20 this hydrophobic Fe-zeolite shows strongly improved adsorption properties (factor >7 increase in Kd,MTBE at Cfree,MTBE = 0.3 mg L−1, Figure 2). The tested hydrophobic Fe-zeolites proved to be sufficiently stable, justifying further advanced testing for an application in flow-through, fixed-bed reactors. Regarding the comparison of the adsorbent properties of high-silica ZSM5 zeolites and AC for MTBE, previous studies based on laboratory-scale column tests have shown clear advantages for hydrophobic ZSM5 zeolites with respect to not only adsorber exhaustion rates but also interference of natural organic matter (NOM) on adsorbent performance.6,29 However, high-silica ZSM5 zeolites are more expensive than AC by a factor of about 10 in material cost per weight. Thus, competitiveness of zeolite adsorbents will depend on costefficient regeneration options. Off-site regeneration by thermal methods (including also transportation and replacement) as usually applied for AC would probably not be less expensive in the case of zeolites. However, on-site regeneration by means of wet oxidation in combination with the use of Fe-containing zeolites might be a cost-efficient alternative. In this case, treatment costs will be mainly dominated by the total useful lifetime of the zeolite, including repeated adsorption/ regeneration steps and the consumption of H2O2 for regeneration. A preliminary estimation of the relative costs for zeolite and oxidant is shown in the Supporting Information. However, more detailed studies, including long-term and field tests, are necessary in order to provide more realistic cost estimations for adsorption/oxidation processes based on Fezeolites.

10 in C0,H2O2 increases kMTBE by a factor of 3.5 (Supporting Information Figure S8). This is because H2O2 also acts as a consumer of the reactive species formed. Thus, although the use of higher amounts of H2O2 can lead to a slight increase in the rate of contaminant degradation, this increase is reached at the expense of a lower H2O2 utilization efficiency. Conversely, a higher H2O2 efficiency can be achieved by reducing its applied concentration, provided that sufficient reaction time, for example, for adsorbent regeneration, is available. Column Experiments. For column experiments, a sieved fraction (0.25−0.63 mm) of the pelletized zeolite Fe-ZSM5 (236)cal was used. During the adsorption steps, the residence time of the MTBE-containing water (C0 = 4 mg L−1) in the column (0.2 g catalyst) was 0.72 min and the approach velocity 2.8 cm min−1. Three adsorption steps were carried out (with approximately 38 000 bed volumes of water within 7 days per step), whereby the zeolite was regenerated in between by closed-circuit flushing with H2O2 solution. The MTBE concentration in the regeneration solution reservoir at the beginning of each regeneration step was in the range of 0.9−1.5 mg L−1 and decreased to ≤0.1 mg L−1 within two to three days in the regeneration mode. For comparison, the MTBE loading of the column was approximately 0.09 mmol (40 mg g−1). As Figure 6 shows, the Fe-zeolite was used with a very good

Figure 6. Breakthrough curves for MTBE in spiked tap water (C0,MTBE = 4 mg L−1) with intermittent regeneration. ◊, Cycle 1; □, Cycle 2; Δ, Cycle 3.

performance in three cycles. The breakthrough curve obtained is not ideal and could be improved by optimization of column geometry and flow conditions which, however, was not within the scope of the present work. The objective of this experiment was to examine the reproducibility of the adsorption behavior after the regeneration steps. Within the three cycles, the breakthrough curves did not change significantly. Based on an arbitrary termination criterion of 10% MTBE breakthrough (Cout/Cin), about 2000 bed volumes of water contaminated with 4 mg L−1 of MTBE could be treated before regeneration of the zeolite adsorber was necessary. Because of the nonlinear adsorption isotherms, the adsorber usage time is expected to increase at lower MTBE inflow concentrations. At the end of the third regeneration cycle, a DOC concentration of 11 mg L−1 was determined in the reservoir, representing 24% of the organic carbon originally adsorbed as MTBE in the previous adsorption step, thus indicating that a high degree of mineralization (76%) was achieved. tert-Butyl

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ASSOCIATED CONTENT

S Supporting Information *

Further information on experimental procedures; kinetics and isotherms of MTBE adsorption; zeolite characterization; influence of MTBE, zeolite and H2O2 concentration on reaction rate; intermediates in column regeneration; and consideration of cost aspects. This material is available free of charge via the Internet at http://pubs.acs.org/ 2359

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AUTHOR INFORMATION

Corresponding Author

*Tel: +49 341 2351760. Fax: +49 341 235451760. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This work was supported by PIEF-GA-2009-236583 (EU Marie Curie Fellowship, R. G.-O.) and JCI-2010-07104 (Juan de la Cierva Fellowship from Ministerio de Ciencia e Innovación (Spain), R.G.-O.).

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