Iron-enriched natural zeolite modified carbon paste electrode for H2O2 detection

Electrochimica Acta 55 (2010) 4050–4056

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Iron-enriched natural zeolite modified carbon paste electrode for H2 O2 detection Delia Gligor a,∗ , Andrada Maicaneanu b , Alain Walcarius c a

Department of Environmental Physics, Chemistry and Technology, “Babes-Bolyai” University, 30 Fantanele St., 400294 Cluj-Napoca, Romania Department of Chemical Technology, “Babes-Bolyai” University, 11 Arany Janos St., 400028 Cluj-Napoca, Romania c Laboratoire de Chimie Physique et Microbiologie pour l’Environnement, UMR 7564, CNRS – Nancy-Université, 405 rue de Vandoeuvre, 54600 Villers-les-Nancy, France b

a r t i c l e

i n f o

Article history: Received 24 November 2009 Received in revised form 18 February 2010 Accepted 18 February 2010 Available online 25 February 2010 Keywords: Zeolite modified carbon paste electrode H2 O2 electrocatalysis Amperometry

a b s t r a c t This work demonstrates that iron-enriched natural zeolitic volcanic tuff (Paglisa deposit, Cluj county, Transilvania, Romania) resulting from a previous use as adsorbent in wastewater treatment can be recycled into effective electrode modifier applied to the electrocatalytic detection of hydrogen peroxide. After physico-chemical characterization of tuff samples using various techniques such as chemical analysis, X-ray diffraction, scanning electron microscopy, infrared spectroscopy, BET analysis and X-ray photoelectron spectroscopy, the electrochemical response of the iron-enriched zeolites was studied on the basis of solid carbon paste electrodes modified with these samples. The results indicate that iron centers in the zeolite are electroactive and that they act as electrocatalysts in the voltammetric and amperometric detection of H2 O2 . Best performance was achieved in phosphate buffer at pH 7, showing a sensitivity of 0.57 mA M−1 cm−2 , a detection limit down to 60 ␮M, and a linear domain up to 100 mM H2 O2 . © 2010 Elsevier Ltd. All rights reserved.

1. Introduction Zeolites are known for their combined size selectivity and ion exchange capacity, properties that arise from their crystalline structure and aluminosilicate composition [1–3]. Due to their low cost and easy accessibility, in some regions, natural zeolites (zeolitic volcanic tuffs) became an attractive material for removal of heavy metals (i.e. iron, zinc, lead, etc.) from wastewaters (i.e. electroplating industry). Once they were used in a wastewater treatment process, exhausted natural zeolites can be regenerated and reintroduced in the process or can become part of other processes (i.e. catalytic or electrochemical processes). Iron-enriched natural zeolites cropped out from Transilvania region (Romania) were successfully used in the catalytic wet air oxidation or advanced oxidation (heterogenenous Fenton) of phenol from wastewaters [3,4]. In order to extend the scope of their applications, the present work aims at investigating their electrochemical response when incorporated into carbon paste electrodes. Zeolite modified electrodes (ZMEs) have found numerous applications in various fields and most of those developed in electroanalytical chemistry were based on zeolite modified carbon paste electrodes (ZMCPEs) [5–7]. Applications of ZMCPEs in electroanalysis can be classified into 4 categories, as briefly described hereafter.

∗ Corresponding author. Tel.: +40 723 758 100; fax: +40 264 307 032. E-mail address: [email protected] (D. Gligor). 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2010.02.058

(1) Voltammetric detection subsequent to open-circuit accumulation, where the target analyte, usually a redox-active cation, is accumulated by ion exchange in the zeolite (charge selectivity) prior to its voltammetric quantification [8–13]; molecular sieving at the molecular level also induces size selectivity [14]. (2) Direct amperometric or voltammetric detection by electrocatalysis, involving mainly redox mediators encapsulated in the zeolite cages [15–22]. (3) Indirect amperometric detection of non-electroactive species is probably the most elegant application of zeolite modified electrodes as it exploits both ion exchange and size selectivity properties [23]; it involves the use of a zeolite doped with a redox cation and a supporting electrolyte made of size-excluded cations (i.e., cations bigger than the zeolite pore aperture) so that the redox probe can be only exchanged and diffuse to the electrode surface when a small (nonsize-excluded) cation is injected, giving rise to an indirect amperometric response to these non-electroactive species [24,25]. This concept was also extended to the development of a detector for ion chromatography [26]. (4) Electrochemical biosensors for which the hydrophilic character of zeolites (enabling the exposition of higher active enzyme quantities when dispersed in CPEs [27]) and the possibility to concentrate suitable mediators by cation exchange [16,28,29] have been exploited. One has finally to note that a recurrent limitation of ZMCPEs is related to possible impregnation of the surrounding solution in the bulk paste in case of prolonged use in aqueous medium due to the

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hydrophilic character of zeolite particles [30]. To avoid any associated memory effects in successive experiments with the same electrode surface, it is necessary to increase the overall hydrophobicity of the electrode, which was reported to be possible by using solid paraffin instead of mineral oil as the binder [31]. On the other hand, the electrochemistry of iron-containing zeolites has not been so widely investigated, most works being directed to iron-doped zeolite samples (i.e., zeolite containing exchanged iron(III) ions) [32–35]. In such cases, the electrochemical response was mostly due to extrazeolite charge transfer reactions for which the electron transfer occurred subsequently to ion exchange of the probes for electrolyte cations, whereas intraframework iron species were mostly electrochemically silent except those confined to boundary sites of the zeolite matrix [34,35]. Some iron-doped ZMEs have been exploited for electrocatalytic applications (ascorbic acid or uric acid oxidation [36–40]) but all of them were based on synthetic zeolites in which Fe(III) species have been introduced by ion exchange. In this work we have thus examined the voltammetric behavior and electrocatalytic properties of a zeolitic volcanic tuff originating from Romania, which had been used beforehand as ion exchanger in the process of iron and zinc removal from a wastewater resulting from an electrochemical plating process. After basic characterization of the zeolite samples (crude unmodified tuff (Z) and iron-enriched sample (Z–Fe)) using various physico-chemical techniques, the materials were incorporated into solid carbon paste electrodes and further characterized by cyclic voltammetry at two distinct pH values. Then, their electrocatalytic properties towards hydrogen peroxide reduction were investigated by amperometry under various experimental conditions (pH, applied potential) and the analytical performance of the new ZMCPE were determined. 2. Experimental 2.1. Chemicals, reagents and materials The zeolitic volcanic tuff was cropped out from Paglisa (Z) deposit (Cluj county, Transylvania, Romania). The sample was first used as ion exchanger in the process of iron and zinc removal from a wastewater resulting from an electrochemical plating process. This was performed in either batch (1:10 and 1:20 solid:liquid ratio) or column mode (5 g zeolitic volcanic tuff, 0.055 ml/s, di = 15 mm) as previously described [3,41]. Prior to this process, the zeolitic rock was brought to a granulation of 0.2–0.6 mm by grinding and size separation, then washed with distilled water and dried at 105 ◦ C. The modified zeolitic volcanic tuff obtained by mixing all the exhausted samples from the ion exchange experiments was then calcined in air at 400 ◦ C for 4 h, when the Z–Fe form was obtained. Both unmodified (Z) and modified (Z–Fe) zeolite samples were characterized using various physico-chemical techniques (see below) before use. All solutions were prepared with high purity water (18 M cm−1 ) from a Millipore milli-Q water purification system. The supporting electrolyte used for electrochemical experiments was usually a 0.1 M phosphate buffer solution, prepared from Na2 HPO4 ·2H2 O and NaH2 PO4 ·2H2 O (Merck), adjusted at pH 7. More acidic solutions (i.e., pH 3) were obtained using H3 PO4 . Hydrogen peroxide was purchased from ACROS Organics. All other reagents were of analytical grade and used as received. 2.2. Electrode preparation Zeolite modified solid carbon paste electrodes have been prepared according to published procedure [31]. Two ZMCPEs with distinct zeolite contents were typically obtained as follow: (i) by

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thoroughly mixing 80 mg zeolite with 160 mg carbon graphite powder (<325 mesh, Alfa), for getting ZMCPE with 20% zeolite; and (ii) 120 mg zeolite and 120 mg carbon (30% zeolite); the resulting solids being then mixed with 160 mg of solid paraffin wax (Fluka) acting as an inert binder. To ensure proper dispersion of the zeolite and graphite particles into the composite, the mixture has to be heated above 60 ◦ C (i.e., the paraffin melting point) before being packed into a home-made PMMA (polymethylmethacrylate) cylindrical tube (geometric area: 0.13 cm2 ) with a copper wire providing an inner electrical contact. The electrode surface was mechanically smoothed on a paper sheet. Due to the hydrophobic character of the paraffin, the electrode was immersed 10 min in ethanol under stirring to remove the excess of wax onto the electrode surface prior to use, as commonly applied for this kind of solid carbon paste electrode [42]. Another kind of solid ZMCPE, based on the physical immobilization of zeolite particles onto the surface of the composite [43], was also prepared but it was rapidly discarded because of its lack of sensitivity in comparison to the classical “bulk” ZMCPE. The surface-confined zeolite modified electrode was prepared by mixing 240 mg graphite powder with 160 mg paraffin wax and spreading onto the electrode surface 20 ␮l of a zeolite suspension (2 g l−1 ) in ethanol, which was left for 20 min at room temperature to evaporate the solvent and washed with ethanol before use. 2.3. Apparatus The zeolitic volcanic tuff samples, in unmodified (Z) and modified (Z–Fe) forms, were characterized by means of elemental analysis, X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transformed infrared spectroscopy (FTIR), N2 adsorption–desorption (BET analysis) and X-ray photoelectron spectroscopy (XPS). The analyses of whole-rock chemistry were performed using usual analytical methods for silicate materials (wet chemistry) and atomic adsorption spectroscopy (for zinc determination). XRD measurements were performed on random powders using a Siemens Bruker unit with Cu K␣ anticathode. The diffractograms were recorded in the range of 2 values from 20◦ to 50◦ . The analytic conditions are 40 mA, 20 kV, step of 2◦ . A semi-quantitative XRD method was used to determine mineral composition. The micromorphological features of zeolite tuffs were examined in silver-coated, fresh surfaces of the selected samples with a JEOL JSM 5510LV scanning electron microscope (SEM). FTIR spectra were recorded using a Jasco 615 spectrophotometer, in the 400–4000 cm−1 range, with a resolution of 2 cm−1 . Specific surface areas and pore size distribution of the zeolitic samples were determined from N2 adsorption–desorption experiments using a Coulter SA 3100 Surface Area and Pore Size Analyzer (BET and BJH methods). Prior to adsorption measurement the zeolitic volcanic tuff was outgassed in N2 at 423 K for 13 h. The adsorption–desorption isotherms were recorded at 77 K in the relative pressure range from about 10−5 to 0.99. XPS analyses were carried out on an Axis Ultra spectrometer (Kratos Analytical, UK) with a hemispherical energy analyzer using a monochromatic Al K␣ source (1486.6 eV). Spectra were analyzed using Vision 2.2.0 software. Samples were prepared as powders dusted onto double-sided sticky tape. The XPS binding energies (BE) were measured with a precision of 0.1 eV and were correlated with BE from standard materials [44]. All electrochemical measurements were made at room temperature using an Autolab PGSTAT100 potentiostat and GPES electrochemical analysis system (Eco Chemie), equipped with a three electrode cell: the homemade modified carbon paste electrode, a platinum wire as auxiliary electrode, and a KCl saturated Ag/AgCl reference electrode. Prior to all experiments, the electrolyte solutions were deoxygenated by bubbling argon for 10 min.

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Table 1 Chemical composition of the crude zeolitic volcanic tuff sample used in this study. SiO2 , %

TiO2 , %

Al2 O3 , %

Fe2 O3 , %

CaO, %

MgO, %

Na2 O, %

K2 O, %

LOI 1000 ◦ C

66.25

0.21

12.16

1.35

4.48

0.20

1.60

1.60

12.15

2.4. Voltammetric and amperometric procedures The basic electrochemical behavior of ZMCPEs was examined by cyclic voltammetry in 0.1 M phosphate media (buffer solution at pH 7 or with added phosphoric acid to reach pH 3), typically at a potential scan rate of 50 mV s−1 . The electrocatalytic reduction of H2 O2 was studied by both cyclic voltammetry and constantpotential amperometry, from solutions prepared through additions of a freshly prepared H2 O2 solution to the supporting electrolyte. Amperometric experiments were carried out by applying suitable constant potentials in the range between −0.4 and 0.0 V vs. Ag/AgCl/KClsat . The experimental results presented here are the average of at least 3 identically prepared electrodes, if not otherwise mentioned. 3. Results and discussion 3.1. Physico-chemical characteristics of natural zeolite samples The pristine zeolitic material (Z) was first characterized by XRD (Fig. 1A), SEM (Fig. 1B) and chemical analysis (Table 1). The XRD diagram obtained on random powder of the whole material indicates the massive presence of clinoptilolite as the main zeolite species.

The semi-quantitative estimation from the X-ray diffractograms indicates that the zeolite content reached values around 70% from the crystallized fractions of the tuff. Other minerals identified are quartz, albite (feldspar) and montmorillonite. The bulk (wholerock) chemical analysis (Table 1) reveals the existence of significant amounts of secondary and/or hydrated components, as indicated by the high values of lost at ignition (LOI = 12.15%). Considering an average 15% LOI for pure clinoptilolite, the amount of this mineral in the sample from Paglisa can be estimated around 70%, in agreement with XRD data. Table 1 also indicates the presence of iron in this starting material. As no iron oxide can be evidenced in the XRD spectra, this suggests that iron is probably present as intraframework species (or in the form of oxide but in too small quantity to be detected). SEM image of the crude zeolitic volcanic tuff sample shows that the zeolite is present as tabular clinoptilolite crystals, in the form of either micron- and submicron-sized particles in the bulk matrix, or as larger crystals in the pores or voids (Fig. 1B). These zeolite crystals are about 2–10 ␮m in size (sometimes they reached 40–50 ␮m). These data agree well with the fact that clinoptilolite is the most frequent mineral species present in all the Transylvanian Depression zeolitic volcanic tuffs. After iron and zinc removal from electrochemical plating wastewaters by ion exchange [3,41], the amount of iron in the Z–Fe sample was slightly higher (from the initial 1.35 to 1.63%, as expressed in Fe2 O3 content) and the material contained 1.52 mg Zn2+ g−1 . The iron quantity present in the bulk sample is originated from the accompanying minerals in the zeolitic volcanic tuff sample, especially biotite (fillosilicate) mineral which has no exchange capacity properties [45]. Therefore only the iron brought in the zeolitic structure after the ionic exchange process with the electrochemical plating wastewater iron, will be available as active species on the electrode surface. Both Z and Z–Fe samples have been further characterized using FTIR and N2 adsorption–desorption (Fig. 2) in order to evidence any eventual change in the material properties. FTIR spectra of the Z sample (Fig. 2A) confirms the presence of specific zeolite peaks for which Table 2 Main FTIR peaks of the crude (Z) and modified (Z–Fe) zeolite samples from Paglisa (Cluj county) by comparison with literature data for the same quarry. IR signal attributiona

Sample Z

Wavenumber (cm−1 )

Fig. 1. (A) Powder XRD diagram and (B) SEM picture of the crude zeolitic volcanic tuff sample (Z) from Paglisa (Cluj county) used in this study. In part (A) of the figure, Cl represents Clinoptilolite, M montmorillonite, and Ab albite.

a b

Z–Fe

b

Paglisa

3620 3447 1636

3620 3447 1636

3610 – 1630

1208

Shoulder 1210

1064

1072

1080

796

793

795

725

721

735

669

669

675

608

606

610

469

468

465

T indicates the tetrahedrical position of Si and Al. Literature data [2].

O–H bond stretching O–H bond stretching H–O–H angular deformation T–O asymmetric internal stretching T–O asymmetric external stretching T–O external symmetric stretching T–O external symmetric stretching T–O external symmetric stretching Ring-coupled T–O external vibration O–T–O angular deformation

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identification has been provided in Table 2, in agreement with literature data [2,46–50]. In case of the modified zeolitic volcanic tuff sample (Z–Fe), some minor modifications in the intensity of the peaks (Fig. 2A) have been observed, which can be due to a partial destruction of the zeolite structure during the calcination process [3]. N2 adsorption–desorption isotherms (Fig. 2B) are of type II with a H3 type loop, typical of slit-shaped pores [51], showing a typical “kneecurve” which is characteristic of supermicropores around 20–30 Å [52], in agreement with pore size distribution (see inset in Fig. 2B). An additional, yet small, step in desorption was also observed at p/p0 = 0.47, which indicates the presence of some macropores. A slightly higher specific surface area for the Z–Fe sample (17.3 m2 g−1 in comparison to 15.3 m2 g−1 for Z) could be due to formation of small iron oxide micro crystallites on the modified natural zeolite sample. XPS was finally applied to characterize the materials, indicating that elements present on the surface in a quantity high enough were indeed detected (O, C, Si, Al, K, Na, Ca, Fe, see Fig. 3) and that the Z–Fe sample was characterized by some increase in the iron component and the presence of zinc was also confirmed (Table 3). In both Z and Z–Fe samples, iron was present as Fe(III) species, as pointed out by the Fe 2p3/2 signal located at 712 eV [53]. 3.2. Electrochemical behavior, electrocatalytic reduction of H2 O2 and amperometric sensors for H2 O2 Fig. 4 compares the voltammetric response of Z- and Z–FeMCPEs in phosphate buffer at pH 7. As shown, only Z–Fe gave rise

Fig. 2. (A) FTIR spectra and (B) N2 adsorption–desorption isotherms of the crude (Z) and modified (Z–Fe) zeolitic volcanic tuff samples. In part (B) of the figure, pore size distribution has been added as inset.

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Table 3 Mass concentration of the elements identified by XPS on the surface of the crude (Z) and modified (Z–Fe) zeolite samples. Element

Na 1s Zn 2p3/2 Fe 2p3/2 O 1s Ca 2p C 1s K 2p3/2 Si 2p Al 2p

Mass concentration (%) Z

Z–Fe

0.9 – 1.8 42.6 1.9 18.3 1.2 28.2 5.1

0.7 0.2 2.1 40.2 1.8 24.1 0.8 25.3 4.8

to a well-defined response, which can be attributed to the reduction of Fe(III) species that have been accumulated in the course of electroplating wastewaters treatment. In contrast, no noticeable signal can be seen on the voltammetric curve obtained with the non-modified Z sample, suggesting that Fe(III) species in the pristine solid are electrochemically silent, contrarily to what was reported for intraframework iron in clays [54,55]. The value of the mid-wave potential for Z–Fe-MCPE was −0.38 V vs. Ag/AgCl/KClsat , at 50 mV s−1 , and current values were directly proportional to the square root of the potential scan rate, indicating diffusioncontrolled electron transfer processes. Such diffusion-controlled electron transfer process was often observed at zeolite modified electrodes, independently of the charge transfer mechanism (intraor extra-zeolite electron transfer [56]), and can arise from two contributions: (i) the physical diffusion of electroactive species from their sites in the zeolite to the surface of carbon grains constituting the ZMCPE and (ii) counterions transport for maintaining charge balance after electron transfer (as also reported for other systems

Fig. 3. XPS spectra for (Z) zeolitic volcanic tuff sample highlighting the Fe 2p3/2 signal (inset).

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(by ca. 100 mV) in comparison to Z–Fe-MCPE. These results indicate that iron-enriched zeolites are likely to provide an attractive catalytic effect towards H2 O2 reduction, which agrees well with previous works reporting similar electrocatalytic effects for ironrich clays [55,58] or other Prussian Blue modified surfaces [59]. In the present case, however, iron species responsible for the catalytic behavior seem to be those brought from the electroplating wastewaters treatment as the non-modified Z sample did not give rise to any electrocatalytic effect in spite of iron content not dramatically lower than that in Z–Fe (Table 1). This suggests that structural iron sites present in the pristine zeolite sample could not mediate the reduction of hydrogen peroxide, contrarily to the mechanism reported for intraframework iron centers in clays [55,58], and that the electrocatalytic effect should be mainly due to mobile Fe(III) species. After the initial reduction of Fe(III) into Fe(II), the proposed mechanism to explain the electrocatalytic behavior can be one of Fenton type: Fe(II) + H2 O2 → Fe(III) + HO− + HO• Fe(III) + H2 O2 → H+ + [Fe(OOH)]2+ Fig. 4. Cyclic voltammograms for (a) Z-MCPE (- - -) and (b) Z–Fe-MCPE (—) modified carbon paste electrodes (20% zeolite). Experimental conditions: scan rate, 50 mV s−1 ; supporting electrolyte, 0.1 M phosphate buffer (pH 7).

[57]). As good stability of voltammetric responses was observed upon multiple potential scanning (i.e., no noticeable loss of iron in solution), it can be stated that the second contribution would be dominant. In the presence of H2 O2 , this cathodic signal was found to increase proportionally to the analyte concentration but this signal enhancement was significantly dependent on the modified electrode composition and configuration (Fig. 5). For instance, enhancement factors of 1.78, 1.52, and 1.20 (calculated using the following equation ((Ipeak )[H O ]=2 mM − 2 2

(Ipeak )[H

2 O2 ]=0

)/(Ipeak )[H

2 O2 ]=0

, for currents measured at −0.4 V)

have been respectively obtained with Z–Fe-MCPE (30% zeolite), Z–Fe-MCPE (20% zeolite), Z–Fe-MCPE with zeolite particles physically immobilized onto the electrode surface. Control experiments made on unmodified CPE (see part “B” in Fig. 5) confirm the role played by the Z–Fe modifier as the direct reduction of H2 O2 was of smaller intensity and shifted towards more cathodic values

[Fe(OOH)]2+ → Fe(II) + HO2 · One should finally mention that all experiments have been performed in deaerated solutions (absence of oxygen) as molecular oxygen is reduced at −0.4 V on CPE so that it would superimpose to that of iron species and affect thereby the voltammetric response. In an attempt to increase such mobility and therefore the sensitivity of the electrode to H2 O2 , similar experiments have been repeated at pH 3 (acidic media are known to enhance the response of Fe(III)-exchanged ZMEs [32]). As shown in Fig. 6, reduction of Fe(III) from the zeolite was always present at the same potential of −0.38 V (pH did not affect characteristic reduction potentials in the 3–7 range (Fig. 7); lower pH values have been avoided to prevent from zeolite framework dissolution and higher values to avoid precipitation of Fe(II) hydroxides) and the sensitivity of H2 O2 detection was noticeable but additional signals also appeared on the voltammetric curves (notably around −0.1 V). Elucidation of this behavior is beyond the scope of this paper but one can suggest that the additional signals are probably due to the copper (present at 0.017 mg g−1 in Z–Fe) system in view of their shape and position on the potential axis. Note that peak currents for Fe(III) reduction at pH 3 were lower than at pH 7 (Fig. 7), suggesting some loss of

Fig. 5. Cyclic voltammograms for (A) Z–Fe-MCPE (20% zeolite), (B) Z–Fe-MCPE (30% zeolite) and CPE, (C) Z–Fe-MCPE (zeolite deposited on surface), recorded in absence and in presence of H2 O2 . Experimental conditions: scan rate, 10 mV s−1 ; supporting electrolyte, 0.1 M phosphate buffer (pH 7).

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Fig. 6. Cyclic voltammograms for Z–Fe-MCPE (20% zeolite), recorded in absence and in presence of 2 mM H2 O2 at pH 3 (scan rate, 10 mV s−1 ). Inset is enlargement of the upper part of the figure.

these species in solution (and thus no more available for electrochemical detection), which is consistent with the fact that Fe(III) species become soluble in such acidic medium. Constant-potential amperometry in hydrodynamic conditions was then applied to evaluate the potential interest of the modified electrodes for practical H2 O2 sensing. This was performed from batch amperometric measurements at various applied potentials in the −0.4 to 0 V (vs. Ag/AgCl/KClsat ), in stirred solutions at two pH values (7 and 3). As expected sensitivity was found to increase upon rising potentials towards the negative direction (e.g., values of 0.03, 0.57, and 2.2 mA M−1 cm−2 have been observed for Z–FeMCPE (20% zeolite) at pH 7 when applying potentials of respectively −0.1, −0.2, and −0.3 V). More surprising was the sensitivity drop (by ca. 50%) observed when passing from the Z–Fe-MCPE (20% zeolite) to the Z–Fe-MCPE (30% zeolite), which could be explained by the initially lower background currents observed in the absence of H2 O2 (compare parts A and B in Fig. 5). Also, the sensitivity val-

Fig. 8. Calibration curve for H2 O2 at Z–Fe-MCPE (20% zeolite) at pH 7 (the insets shows an enlargement of lower concentrations range and the log I vs. log[H2 O2 ] representation). Experimental conditions: applied potential, −0.2 V vs. Ag/AgCl/KClsat ; stirring speed, 500 rpm.

ues observed at pH 3 were higher than those obtained at pH 7, for all applied potentials, especially at low H2 O2 concentration. For example, current values recorded by applying a cathodic potential of −0.2 V to Z–Fe-MCPE (20% zeolite) for H2 O2 analysis at concentrations ranging between 10 and 100 ␮M at pH 7 were one order of magnitude higher than those sampled at pH 3 in the same conditions, in agreement with the aforementioned possibility of Fe(III) dissolution in acidic medium. A typical calibration curve obtained at pH 7 is depicted in Fig. 8, showing the existence of two linear concentration ranges. In the best case, the following analytical parameters describe the sensor performance: linear ranges, from 10 to 150 ␮M and from 0.3 to 100 mM; detection limit, ∼ =60 ␮M (S/N 3); response time, ∼ =1 min. The slope of the “log I − log[H2 O2 ]” dependence (see inset of Fig. 8), examined in the concentration range corresponding to the linear amperometric response, was always close to unity, confirming that the H2 O2 sensor works under kinetic control (Koutechy–Levich criteria), corresponding to reaction of 1st order between H2 O2 and iron from the zeolite. 4. Conclusions

Fig. 7. The dependence of signal potential and current response on pH for Z–FeMCPE (20% zeolite), scan rate, 10 mV s−1 .

Solid carbon paste electrodes incorporating iron-enriched natural zeolites (mainly of clinoptilolite type) are proposed and successfully used to develop new sensors for hydrogen peroxide. The electrode response to hydrogen peroxide is based on electrocatalysis by Fe(III) centers in the zeolite. Only the zeolitic volcanic tuff samples that have been used beforehand to treat wastewaters from electrochemical plating industry gave rise to significant electrocatalytic behavior with respect to H2 O2 reduction, suggesting that mobile iron species (in the form of Fe(III) as pointed out by XPS) should be responsible for the sensor response. This work demonstrates that used adsorbents (i.e., intrinsically waste materials) can be recycled as electrode modifiers, showing better performance

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than the non-used pristine solid. The analytical characteristics (e.g. lower detection limit in comparison to other modified electrodes based on mediators [19,59]) of obtained solid carbon paste electrodes demonstrate the possibility to use them as amperometric sensors for H2 O2 detection but practical use would need to consider the selectivity aspects (i.e., influence of interference) which are beyond the scope of the present report.

[27] [28] [29] [30] [31] [32] [33] [34] [35]

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