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Photocatalytic Activity of Combustion Synthesized ZrO2 and ZrO2TiO2 Mixed Oxides Sneha Polisetti,† Parag A. Deshpande,† and Giridhar Madras*,† †
Department of Chemical Engineering, Indian Institute of Science, Bangalore 560012, India ABSTRACT: Tetragonal ZrO2, synthesized by solution combustion technique, was found to be photocatalytically active for the degradation of anionic dyes. The compound was characterized by FT-Raman spectroscopy, X-ray photoelectron spectroscopy, FT-infrared spectroscopy, UVvis spectroscopy, BET surface area analysis, and zero point charge pH measurement. A high concentration of surface hydroxyl groups was observed over the catalyst, as confirmed by XPS and FTIR. The photocatalytic degradation of orange G, amido black, remazol brilliant blue R, and alizarin cyanine green (ACG) was carried out with this material. The effect of pH, inorganic salts, and H2O2 on the activity of the catalyst was also studied, and it was found that the catalyst maintained its activity at a wide range of pH and in the presence of inorganic salts. Having established that ZrO2 was photocatalytically active, mixed oxide catalysts of TiO2ZrO2 were also tested for the photocatalytic degradation of ACG, and the 50% ZrO2TiO2 mixed oxides showed activity that was comparable to the activity of TiO2.
’ INTRODUCTION TiO2 has been used as a photocatalytic material extensively. Newer techniques are constantly being adopted for obtaining TiO2 with enhanced photocatalytic activity. However, a large portion of materials research is devoted to harness the photocatalytic activity of TiO2 in the form of hybrid materials. The purposes of introducing a second component are obtaining high surface area, higher thermal and mechanical stability, and improved surface characteristics.1 Several oxides have been reported for synthesizing hybrid composite photocatalytic materials including TiO2ZrO2,1 TiO2Al2O3,2 TiO2V2O5,3 TiO2SiO24 and TiO2-carbon nanotubes.5 However, it is interesting to explore the photocatalytic activity of the second component in pure form. If the second component in its pure form is also photocatalytically active, then enhanced activity of the material may be expected owing to the possible synergistic effects. Synergistic effects in photoactivity in case of TiO2/carbon nanotube systems have indeed been observed.6 There are a few studies which report the activity of pure and metal doped ZrO2 as a photocatalyst for pollutant degradation and other applications.79 However, applications of ZrO2 for semiconductor photocatalysis have not been studied extensively. Therefore, photocatalytic activity of pure ZrO2, synthesized by solution combustion technique, was investigated in this study. The method of synthesis plays an important role in governing the properties of the final products. In the case of photocatalytic materials, the method of synthesis can result in differences in the band gap owing to which compounds of different photocatalytic activities can be obtained. Solution combustion technique1012 offers a unique way of size and phase control of the final product. We have previously reported the synthesis of TiO2 using the solution combustion technique, which resulted in nanocrystalline materials with a lower band gap and higher photocatalytic activity compared to commercial TiO2.10 A small amount of substitution of carbon in the structure of TiO2 also resulted in the enhancement of the photocatalytic properties. r 2011 American Chemical Society
The band gap of ZrO2 is generally accepted to be 5 eV, although values as low as 2.3 eV have been reported, depending on the method of synthesis employed.13 We have previously reported11 solution combustion synthesized metal ion substituted-ZrO2 and nanocrystallinity was observed in the samples. Therefore, the optical properties of solution combustion synthesized ZrO2 were determined in this study. The compound showed a band gap of 3.5 eV, which made the compound a potential photocatalytic material. As mentioned earlier, investigation of the photocatalytic activity of mixed oxides of TiO2 is of interest. The synthesized ZrO2 was also used for making ZrO2TiO2 mixed oxides, and the photocatalytic activity of the mixed oxides was also tested. Photocatalytic activity of solution combustion synthesized ZrO2 was tested for the degradation of four anionic dyes. Industrial wastewater contains several other materials including inorganic salts, which are used especially in the textile industry for fixing color onto the fabric. The process of photocatalytic degradation of organics involves charge transfer and electron exchanges. Such processes may be affected by the presence of charged species in the solution. Therefore, it is important to ensure that the compound retains activity even in the presence of such species. Therefore, the effect of the presence of salts in the solution on the photocatalytic activity of the compound was studied. Similarly, the effect of pH was also studied because of the widely varying pH of effluents from different sources. The effect of H2O2 on the enhancement of photocatalytic rates has also been determined. Special Issue: Ananth Issue Received: February 20, 2011 Accepted: June 27, 2011 Revised: June 25, 2011 Published: June 27, 2011 12915
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Industrial & Engineering Chemistry Research Thus the objectives of the study were 2-fold namely (a) to synthesize ZrO2 by the solution combustion technique and demonstrate its photocatalytic activity for the degradation of dyes under a wide variety of conditions and (b) to show the advantage of using ZrO2TiO2 mixed oxides and demonstrate the synergy of these mixed oxides for photocatalysis.
’ EXPERIMENTAL SECTION Synthesis and Characterization. ZrO2 was synthesized by the solution combustion technique. The precursor solution was made by dissolving stoichiometric amounts of zirconium nitrate (Zr(NO3)4•5H2O, Rolex, India) and glycine (C2H5NO2, S.D. Fine Chem, India) in deionized water. The molar ratio of glycine to zirconium nitrate used was 2.2:1, as determined by making a balance over the oxidizing and reducing valences. The clear solution was taken in a crystallizing dish and heated in a preheated muffle furnace at 350 °C. The catalyst formed after the combustion was finely ground and calcined at 500 °C for 24 h. The bright white powder obtained after calcination was used for testing the photocatalytic activity without further treatment. The compound was characterized by FT-Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), UVvis spectrophotometry, FT-infrared spectroscopy (FTIR), and BET surface area analysis. FT-Raman spectrum was recorded using the NXR-FT Raman module (Thermo Scientific, USA) which used Ge detector and Nd:YVO4 laser. XPS were recorded using Thermo Fisher Scientific Multilab 2000 (England) instrument. Thin pellets of the sample were made without dilution with graphite. The sample was degassed in an ultrahigh vacuum chamber, and the spectra were recorded using AlKR radiation (1486.8 eV). UVvis spectra and FTIR spectra were recorded on PerkinElmer machines. Thin pellets of 5 wt % of the samples were made in KBr for recording FTIR spectra. The surface area of the compound was determined using Micromeritics ASAP 2020 apparatus with N2 adsorptiondesorption cycles. The pH at the zero point charge of the compound was measured by the pH drift method.14 Photocatalytic Activity Tests. Photocatalytic degradation experiments were carried out in quartz tube reactors with provision of water circulation for cooling. Strong adsorption of cationic dyes was observed over the catalyst. Therefore only the degradation of anionic dyes was studied. Four anionic dyes, viz., Orange G (OG), Amido Black (AB), Remazol Brilliant Blue R (RBBR), and Alizarin Cyanine Green (ACG), were used for the photocatalytic degradation experiments. Whereas OG and AB belong to azo and diazo classes of the dyes, respectively, RBBR and ACG belong to anthraquinone class of the dyes. Three different concentrations of the dyes viz. 25 ppm, 50 ppm, and 100 ppm were made and tested for the photocatalytic degradation. The dye solution was kept in the reactor, and the lamp was positioned over the dye solution. Catalyst was added to the dye solution, and the solution was stirred without irradiation for 2 h. The adsorption of the dyes over the catalyst in all the cases was found to be less than 5% of the initial concentration. UV source was then irradiated, and the samples were taken at regular intervals. A high pressure mercury vapor lamp of 125 W was placed inside the quartz tube. The lamp radiated predominantly at 365 nm with an incident intensity of 4.24 107 Einstein L1 s1 and a photon flux of 7.2 W m2. The dye solution with catalyst suspension was centrifuged to remove finely suspended catalyst particles. The concentration of the dye in the solution was
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determined using a UVvis spectrophotometer (Shimadzu 1700, Japan), calibrated against standard dye solutions. Photocatalytic degradation of ACG was also carried out with ZrO2TiO2 mixed oxides. The combustion-synthesized ZrO2 obtained after calcination was finely ground in a mortar pestle to obtain a very fine powder. A known amount of commercial Degussa Aeroxide TiO2 P-25 catalyst was added to a known amount of ZrO2 so as to obtain a mixture of ZrO2TiO2 in the required wt%. The mixture was ground again in a mortar pestle for 30 min so as to obtain a uniform mixture. Both ZrO2 as well as TiO2 were white in color, and therefore analysis of homogeneity on the basis of uniformity of the color of the product was not possible. This necessitated long grinding times to obtain a homogeneous mixture. The finely ground mixture was then used directly for the experiments. It is to be noted that no heat treatment of the mixture was carried out. Therefore, the mixed oxides were actually physical mixtures and not solid solutions. Therefore, the interactions during the reactions can be assumed to be those taking place at the grain boundaries. Three mixed oxides with 25 wt %, 50 wt %, and 75 wt % of ZrO2 were used with commercial TiO2. An initial dye concentration of 50 ppm and total catalyst concentration of 1 g/L was maintained for all the reactions. The above-mentioned experiments were carried out in natural pH of the dye solutions containing the catalyst. The effect of pH of the solution was studied by adjusting the pH of the dye solution containing the catalyst, using dilute HCl and NaOH (both from Merck, India). The actual pH of the solution was determined using a pH meter (Waterproof pH Testr 30, Eutech instruments, Singapore). Control experiments were carried out without the catalyst and UV irradiation to observe the effect of HCl and NaOH. No appreciable decrease in the concentration of the dyes was observed for 2 h of the reaction. To study the effect of ions on the photocatalytic degradation of dyes LiCl, NaCl, KCl, CaCl2, and Na2SO4 (all from S.D. Fine Chem, India) were added to the reaction mixture such that the concentration of the salt in the mixture was 0.02 M. Dye concentration of 50 ppm and catalyst concentration of 1 g/L was maintained. Solutions containing different concentrations of H2O2 (Merck, India), from 0 to 150 mM, were prepared to study the effect of hydrogen peroxide. In these sets of experiments also, control experiments were carried out in the presence of only salts and H2O2, and no appreciable decrease in the concentration of the dye was observed for 2 h of reaction.
’ RESULTS AND DISCUSSION Structural Analysis. FT-Raman Spectroscopy. The XRD pattern of ZrO2 is shown in Figure 1a. This could be used for determining the crystal structure of the compound. However, the broadening of diffraction lines in XRD patterns made the discrimination between the cubic and tetragonal ZrO2 difficult. FT-Raman is an established technique for phase confirmation, and this has been reported for successfully determining the crystal structure of ZrO2.1517 We have previously used XRD as well as FT-Raman spectroscopy for solution combustion synthesized ZrO2 using oxalyldihydrazide as fuel.11 The nanocrystallinity of the material was apparent from the XRD data; the phase could be confirmed only with the help of FT-Raman spectroscopy. Therefore, we used FT-Raman spectroscopy in this study for determining the crystal structure. The FT-Raman spectrum for ZrO2 is shown in Figure 1b. For cubic ZrO2, a single 12916
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Figure 1. (a) XRD and (b) FT-Raman spectra of ZrO2.
Figure 2. SEM image of ZrO2.
peak at 490 cm1 is expected, whereas for ZrO2 with tetragonal structure, multiple peaks are observed. These peaks typically occur at 145, 270, 315, 460, 600, and 640 cm1. This makes the distinction between the cubic and tetragonal phase clear. From the figure, it is clear that ZrO2 crystallized in tetragonal phase.
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Figure 3. XPS of (a) Zr3d and (b) O1s in combustion synthesized ZrO2.
The peaks in the spectrum correspond to the tetragonal structure, and the cubic phase was absent. Scanning Electron Microscopy. SEM image of ZrO2 powder is shown in Figure 2. It can be seen from the image that the compound was porous with flake-like morphology. Large agglomerates were not observed, and the particles were separate with an average particle size of 25 μm. A porous structure of the particles is also advantageous for the photocatalytic activity of the compound. X-ray Photoelectron Spectroscopy. The XPS of Zr3d and O1s are provided in Figure 3(a,b) and have been reported earlier.11,18 The spectrum in Figure 3(a) is the characteristic spectrum of Zr3d with Zr in +4 oxidation state, thus confirming the formation of ZrO2. Well separated 3d(5/23/2) peaks can be observed at 181 and 184 eV. O1s spectra was found to be broad (Figure 3(b)) with the main peak at 530 eV and a high energy shoulder. The peak at 530 eV is due to the elemental O1s. The shoulder corresponds to the presence of a peak at 532 eV. The peak at 532 eV in the XPS of O1s is attributed to the presence of oxygen, bonded in the compound as the -O-H group. Therefore, the peak in the XPS at 532 eV shows the presence of surface hydroxyl groups in the compound. This was also confirmed by FTIR spectroscopy. A detailed discussion using FTIR spectroscopy is provided later. The spectra of Ti2p and O1s in TiO2 can be seen in Figure 4 (a,b). Ti was found to be in the +4 state. Similar to the spectrum of O1s in ZrO2, a peak at 530 eV with a shoulder at 532 eV was observed in the spectrum of O1s in TiO2. It can be inferred from 12917
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Figure 6. KubelkaMunk plot for ZrO2. The inset shows the DRS spectra of ZrO2.
Figure 4. XPS of (a) Ti2p and (b) O1s in TiO2.
Figure 7. FTIR spectra of TiO2 and ZrO2.
Figure 5. Valence band XPS of TiO2 and ZrO2.
this observation that surface hydroxyl groups were also present in TiO2. The relative magnitude of the peaks can be used to determine the relative amounts of surface hydroxyl groups in the compounds. The ratio of intensities of peaks at 532 and 530 eV in the two compounds are compared, and it is clear from Figure 3b and Figure 4b that the amount of surface hydroxyl groups in ZrO2 was higher compared to the amount of surface groups in TiO2. The valence band spectra of ZrO2 and commercial TiO2 are shown in Figure 5. It can be seen that valence band in ZrO2 was
populated, and the valence band maxima was found to be nearly 3.3 eV below the Fermi level, as determined from the linear interpolation method.19 This clearly showed that the synthesized compound could potentially be used as a photocatalyst, and excitation of electrons for valence band to conduction band using UV radiation was possible. Nearly the same valence band maxima was observed for TiO2 also. However, it can be seen from Figure 5 that the population of the valence band in case of TiO2 was relatively smaller compared to that of ZrO2. UVVis Spectroscopy. The diffuse reflectance spectrum (DRS) of ZrO2 was recorded, and the KubelkaMunk method was used to determine the band gap. The KubelkaMunk plot for the compound is shown in Figure 6, and the DRS is shown in the inset of Figure 6. The band gap, as determined from this plot, was found to be nearly 3.5 eV. This was in a good agreement with the valence band spectrum from which the valence band maxima was found to be 3.3 eV below the Fermi level. An increase in the absorbance can be observed in the range of 200 to 300 nm. The charge transfers in this energy range correspond to the charge transfers between Zr and O. Combustion synthesis12 results in the introduction of carbon in the structure of the compound. Enhanced absorption of the compound in high energy range may be attributed to the presence of carbon in the structure of the compound thus enhancing the charge transfers. 12918
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Figure 8. Variation of the concentration of various dyes with time during photocatalytic degradation over ZrO2.
Ciuparu et al.20 have studied in detail the effect of phase and synthesis conditions on the band gap exhibited by ZrO2. They could synthesize high surface area ZrO2 of different phases, and the DRS were recorded in order to determine the band gap of the material. Band gaps from 3.6 to 5.1 eV were observed for different phases. For tetragonal ZrO2, a band gap as low as 3 eV has been reported by Navio et al.21 Cubic ZrO2 also showed a band gap in the range of 3.33.6 eV in the studies by Nocoloso et al.22 Clearly the band gap for ZrO2 is a strong function of synthesis conditions, crystal structure, and crystallite size. Fourier Transform Infrared Spectroscopy. FTIR spectra of TiO2 and ZrO2 are shown in Figure 7. A wide band at 3440 cm1 showed the presence of surface hydroxyl groups. The hydroxyl groups were detected in the XPS of O1s in both TiO2 as well as ZrO2. This was further confirmed by FTIR spectroscopy. It can be seen from Figure 7 that the amount of hydroxyl groups in ZrO2 was much higher compared to the amount of hydroxyl groups in TiO2. The spectra shown in Figure 7 were taken with equal weights of ZrO2 and TiO2. The background was subtracted from both the spectra by first converting the transmittance to absorbance, and the absolute % transmittance was obtained for both the spectra after background subtraction. Therefore, the intensity of the peaks in Figure 7 directly gave an idea about the amount of the groups present in the sample. A higher amount of surface hydroxyl groups over ZrO2 again showed the potential of the compound for the photocatalytic activity. We have previously carried out a similar analysis of surface hydroxyl groups using FTIR for solution combustion synthesized TiO2.10 For the case
of TiO2 also, the amount of surface hydroxyl groups were found to be higher resulting in higher activity of the catalyst. BET Surface Area Analysis. BET surface area of ZrO2 was determined using N2 adsorptiondesorption. The surface area of ZrO2 was found to be 12 m2/g, while the surface area of TiO2 was earlier reported to be 50 m2/g.10 ZrO2 had a mean pore diameter of 36 nm, and TiO2 had a mean pore diameter of 30 nm. Therefore, the surface area of ZrO2 was much less compared to the surface area of TiO2. Zero Point Charge pH Measurement. The pH at the zero point charge of ZrO2 was determined using the pH drift method described in the literature.14 Solutions of pH between 2 and 12 were prepared using dilute HCl and NaOH solutions, and 20 mg of the compound was dispersed in 20 mL of each of these solutions for 24 h. The final solution pH was measured and was plotted against the initial pH. The point where the curve crossed the initial pH = final pH line was taken as the pH at the zero point charge. It was found to be 3.9 for the combustion synthesized zirconia, which was similar to the values reported in other studies.23,24 Catalytic Activity Tests. Photocatalytic degradation of the dyes is a complex reaction involving the conversion of the dye to CO2 and H2O via a series of intermediates. Complete conversion of the dye to CO2 and H2O is referred to mineralization. We refer to the degradation of the dye as the decrease in the concentration of the dye with time. Analysis of mineralization by following the reduction in total organic carbon has to be carried out for ensuring the complete removal of the hazardous material. 12919
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Figure 9. Variation of the concentration of ACG with time during photocatalytic degradation over ZrO2TiO2 mixed oxide catalysts. C0 = 50 ppm.
We have used degradation rather than mineralization for the assessment of the photocatalytic activity. The variation of concentration of the various dyes with time during the photocatalytic degradation in the presence of ZrO2 is shown in Figure 8. All the dyes showed photocatalytic degradation with time. Control experiments were carried out in the absence of the catalyst by irradiating the solution with UV radiation. The degradation of the dyes due to photolysis was found to be lesser than 10% in all the cases. Nearly 80% degradation of OG was observed in 2 h with 25 ppm initial concentration. The overall percentage degradation decreased with an increase in the initial concentration of the dye. 50% and 22% degradation was obtained with initial concentrations of 50 ppm and 100 ppm, respectively. In case of AB, almost complete degradation was observed within 90 min for 25 ppm initial concentration, and 50% degradation was observed with 100 ppm initial concentration. The activity of the catalyst was found to be higher for the degradation of AB. Similarly, almost complete degradation of RBBR in 2 h and ACG in 1 h was observed. The degradation was found to be the highest for ACG and AB with 90% and 92.5% degradation respectively with initial 50 ppm concentration and 60% and 52% degradation respectively with 100 ppm initial concentration. The degradation of ACG was carried out with mixed ZrO2 TiO2 compounds with 50 ppm as the initial ACG concentration. The variation of normalized ACG concentration with time is shown in Figure 9. It can be seen from Figure 9 that the activity of mixed oxides was high, and almost complete degradation of ACG was observed within 2 h even when the amount of ZrO2 in the mixed oxide was as high as 50%. We carry out the analysis of the relative rates further with the help of rate parameters discussed later. The kinetics of photocatalytic degradation of dyes has been reported to usually follow pseudofirst-order kinetics2527 dC ¼ k0 C dt C ln ¼ k0 t C0
r ¼
ð1Þ ð2Þ
Equation 1 can be solved to obtain eq 2, where C0 and C denote the initial concentration and concentration at time t, respectively.
Figure 10. Variation of natural log of the normalized concentration with time for the degradation of anionic dyes over ZrO2.
Thus the plot of variation of natural log of the normalized concentration with time will be linear, and the slope of the regressed line will yield the rate coefficient of degradation. The initial rates for the photocatalytic degradation of dyes over ZrO2 were determined with eq 2. The rate constant in the expression signifying the rate of reaction was determined by plotting the natural logarithm of the normalized concentration with time. The logarithm of the normalized concentration with time for the degradation of different dyes over ZrO2 is shown in Figure 10, and the plots were found to be linear for all four dyes. The first-order rate constants (k0, min1 103), obtained from the regressed lines, are 5.7, 4.1, 16.5, and 14.4 for OG, RBBR, AB, and ACG, respectively. The degradation rates of ACG and AB were found to be comparable, while the degradation of the diazo dye (AB) was found to be higher than the degradation rate of the monoazo dye (OG). The reason for the differences in the rates of degradation can be attributed to the ease of the formation of intermediates during the primary and secondary hydroxylation.25 The formation of the intermediates is dependent on the interaction of the dye with the active species (OH radicals), and, therefore, the structure of the dyes has to be considered for the activity. The different bonds present in the dye have variable strengths and thus have different ease of breakage. The formation of inorganic species can take place during the degradation for the dyes that contain atoms like N and S. For the case of the S atom, the ease of degradation of the dye depends upon the identity of the oxygenated S species and their position of attachment in the dye. SO3 attached to benzene ring, for example, shows higher reactivity than that attached to a naphthalene ring. Since such structural differences are present in the case of the dyes studied in this study, differences in the degradation rates are observed. An azo bond is present in OG which provides scission center for the degradation of the dye. Dyes having diazo bonds provide more such scission centers, and thus the rate of the degradation of these dyes are higher than that of the monoazo dye. A thorough analysis of the effects of different groups on bond scission and thus the degradation of the dye can be found in ref 25 and the references therein. In this study, it was observed that the diazo dye, AB, degraded at a faster rate than the mono azo dye, AB. The degradation rate of the diazo dye is similar to the degradation rate of the anthraquinonic dye, ACG. This is consistent with our 12920
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Figure 12. Variation of the concentration of ACG with time during photocatalytic degradation at different pH over ZrO2 in the presence of different salts.
Figure 11. Variation of (a) natural log of the normalized concentration with time and (b) rate constant with %ZrO2 for the degradation of ACG over ZrO2TiO2 mixed oxide catalysts.
previous study that discusses the effect of functional groups on the photocatalytic degradation of dyes with TiO2.25 The variation of natural logarithm of normalized ACG concentration with time in the presence of various catalysts is shown in Figure 11a. The rate constants were determined for the degradation of ACG in the presence of mixed oxides from eq 2. The rate constants (k0, min1 103) were found to be 37.5, 37.3, 34.0, 28.3, and 14.4 for the degradation of ACG over 0 wt %, 25 wt %, 50 wt %, 75 wt %, and 100 wt % ZrO2. In this case, 0 wt % ZrO2 corresponds to TiO2. Figure 11b shows the variation of first-order rate constants with the composition of the catalyst. It is clear from the values of the rate constant that the effect of ZrO2 increases with an increase in the amount of ZrO2 in the mixed oxide and nearly the same activity as commercial TiO2 is observed in an oxide mixture having 50% ZrO2. We have previously studied27 the effect of catalyst concentration for the photocatalytic degradation of dyes over TiO2 and linear increase in the rate of degradation was observed with an increase in catalyst concentration up to 1 g/L. In this study, the catalyst concentration was maintained at 1 g/L, and the activity of mixed oxide with 50 wt % TiO2 was found to be similar to the activity of TiO2. This clearly showed the role of ZrO2 in the mixed oxide and enhancement in the activity to make the activity of the mixed oxide the same as that of TiO2 was due to ZrO2. From FTIR analysis, the amount of surface hydroxyl groups in ZrO2 was higher compared to that over TiO2. However, the band gap of the
compound was also higher compared to TiO2. Therefore, ZrO2 alone showed lesser activity compared to TiO2 but the ability of ZrO2 to retain surface hydroxyl groups was found to be beneficial for mixed oxide, and it was possible to obtain enhanced photocatalytic activity due to mutual interaction of the two compounds. Apart from the initial concentration of the dye in the solution and the effect of % ZrO2 in mixed oxide, the effect of pH of the solution, ions, and H2O2 were also studied for the photocatalytic degradation of ACG. These are described below. Effect of pH. In the above experiments, the pH of the solution was not altered, and the natural pH of the dye solutions containing the catalyst was found to be close to 10.5. To study the effect of pH of the solution on the degradation of the dye ACG, experiments were carried out at different values of acidic and basic pH from 3 to 12. There was no appreciable degradation (
