Ti-containing volcanic ash as photocatalyst for degradation of phenol

Abstract

Photocatalysis application in effluent decontamination is becoming one of the most promising green chemistry technologies. In this paper, natural volcanic ash particles have been characterized and used either as photocatalysts or as a support for a titania-containing photocatalyst. The natural material alone displayed good photocatalyst behaviour as a consequence of the presence of some titanium compounds in its composition. The original ashes were further loaded with TiO₂ through hydrolysis of TiOSO₄ under hydrothermal conditions. Activity tests of these Ti-loaded materials in the photocatalytic decomposition and elimination of phenol-type organic pollutants in water displayed good activity which makes them candidates to be used as photocatalysts for waste water treatment.

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Introduction

Photocatalysis is a subject of current interest related to its application in effluent decontamination. Photocatalytic degradation of organic pollutants is becoming one of the most promising green chemistry technologies.^1–3Phenol and phenolic compounds are common pollutants in industrial waste waters such as waste effluents of several industries (paper, herbicides, fungicides, etc.).⁴ These compounds are quite toxic and tend to degrade slowly in the environment, giving different aromatic intermediates, most of them are also of environmental concern.⁵ Heterogeneous photocatalysis using UV-radiation and TiO₂, either in the anatase form or as rutile–anatase mixtures is an interesting method for the treatment of water polluted with organic substances.^6,7 The improvement and optimization of TiO₂ as a photocatalyst is an important task for technical applications of heterogeneous photocatalysis.

Different forms of configuration of a photocatalytic reactor can be presented. The finely divided catalyst can be either dispersed in the irradiated aqueous solution as a slurry, or anchored on a suitable support such as a as fixed bed, fluidized bed or agitated-suspended particles system. According to various reports,⁸ mainly from laboratory scale investigations, slurry type reactors seem to be more efficient than those based on inmobilized catalyst. However, for engineering applications, there is an intrinsic drawback to the first option: the need for post-radiation particle–fluid separation treatment. The supported catalytic agents system exhibits advantages of catalyst recycling and for the ultimate goal of obtaining clean powder-free water.

TiO₂nanoparticles can be supported on some inorganic substrates. Indeed, several materials such as activated carbon, silica, polymers and natural porous materials have been used as support materials for TiO₂ and exhibited good activity in photocatalytic degradation.^9–15 For instance, titania coated carbon was found to display high photocatalytic activity under UV irradiation,¹⁶ but activated carbon is expensive.

In this paper, Ti-containing natural volcanic ash particles from volcanic lava occurring in the Canary Islands were used as a photocatalyst and as a carrier to deposit additional amounts of TiO₂ species. The photocatalytic activity of the original Ti-containing natural volcanic ash and of the TiO₂-loaded samples using this natural substrate was investigated by using the photodegradation of phenol as a test reaction for standard process and product. This natural and very cheap material appears to be a promising support/catalyst candidate because of its relatively good photoactivity.

Natural and supported material samples have been characterized by different techniques (N₂ adsorption–desorption at 77 K, mercury porosimetry, SEM-EDAX, X-ray diffraction, XPS, UV-vis spectrometry) and results related to their activity for photocatalytic degradation of phenol have been obtained. To the best of our knowledge, this is the first attempt to apply volcanic ashes as catalysts, and also as supports for catalysts for catalytic decomposition and elimination of various pollutants both in air and water with potential use in industrial waste treatments to protect the environment.

Experimental section

Photocatalyst preparation

Particles studied as photocatalyst or catalytic support were obtained by grinding and sieving (200–400 μm) natural volcanic ashes from volcanic zone soil in Tenerife, Canary Islands (support). The impregnation of these support particles with anatase-type TiO₂ (titanated support), was carried out using a hydrothermal method.^10,17,18 Reagent grade TiOSO₄ 1 M solution in deionized water was prepared. A suspension constituted by 100 mL of the above solution with volcanic ashes particles (20 g) was heated at 200 °C for 15 h in a Teflon container with a volume of 100 mL which was backed up by a stainless steel pressurized vessel. After hydrothermal treatment impregnated volcanic ash particles were washed with deionized water until a pH =7 value of the rinsed water was reached. The catalyst particles were then separated from the suspension by centrifugation and dried in air at 65 °C. Thus, by this hydrothermal treatment, fine particles of anatase were supported on the natural volcanic ashes (titanated support). Also, in this process, anatase-type fine particles were obtained separately by precipitation (residue) and were isolated by sieving below 200 μm.

Characterization methods

Natural volcanic ashes, titanated support and residue were examined by X-ray diffraction (XRD) using Cu Kα radiation. The surface morphology of the particles was monitored using a scanning electron microscope (SEM) (Jeol LTD, JSM-6300) coupled with an Oxford Link (ISIS L200Cs) detector to perform energy-dispersive analysis (EDAX). The samples were covered with a sputtered thin film of carbon for image acquisition and for EDAX analysis to avoid charging effects.

The specific surface area and pore structure of the samples was evaluated from the nitrogen adsorption–desorption isotherms recorded on a surface pore size analyzer (Gemini 5, Micromeritics) and mercury porosimetry (Autopore 4 mercury porosimeter, Micromeritics).

The X-ray photoelectron spectra of solids were recorded on a VG Escalab 200R spectrometer equipped with a hemispherical electron analyzer and an Mg Kα (hν = 1253.6 eV) X-ray source was used. The solids were placed in a copper holder mounted on a sample-rod in the pre-treatment chamber of the spectrometer. The samples were degassed at 10⁻⁵ mbar and then transferred to the ion-pumped analysis chamber, where residual pressure was kept below 7 × 10⁻⁹ mbar during data acquisition. The binding energies (BE) were referenced to the Al 2p peak (74.5 eV) to account for charging effects. The areas of the peaks were computed after fitting of the experimental spectra to Gaussian/Lorentzian curves and removal of the background (Shirley function). Surface atomic ratios were calculated from the peak area ratios normalized by the corresponding atomic sensitivity factors.¹⁹

The UV-vis spectra of the solid powder materials were recorded on a UV-vis spectrophotometer, equipped with a Varian Cary 3 UV-vis spectrometer equipped with an integration sphere. The powder was not diluted in any matrix to avoid a decrease in the absorbance. The spectra were recorded in diffuse reflectance mode and transformed by the instrument software to equivalent absorption Kubelka–Munk units.

Measurement of photocatalytic activity

The photocatalytic efficiency of the natural material (support), TiO₂ supported particles (titanated support) and commercial TiO₂ (Merck p.a.: 100% Anatase, BET 30 m².g⁻¹) were compared using the photocatalytic degradation of phenol as a standard test system.

Photocatalytic oxidation of phenol was carried out in a thermostated cylindrical quartz Heraeus UV reactor system (0.8 L). The reaction mixture inside the reactor was maintained in suspension by magnetic stirring. As light source, a 150 W medium pressure mercury vapour lamp (UV immersion lamp TQ-150) was used. The IR fraction of the incident beam was removed by the water placed in the double jacket of the photoreactor. The photon flux entering the reactor was 8.8 × 10⁻⁵ einstein s⁻¹. All the experiments were carried out at 20 °C and air was bubbled continuously through the solution, the concentration of catalyst in suspension was 3 g L⁻¹, sulfuric acid was added in order to adjust the pH of the solution²⁰ (pH = 3). To remove catalyst particulates before analysis, the water samples were filtered through 0.45 μm pore size cellulose acetate filters. The evolution of the initial phenol concentration (100 ppm) was followed through the evolution of its characteristic 270 nm absorption band using a filtered aliquot of the suspension taken at different irradiation times. Conversion, X, during each run was defined as X = ((C_initial − C(t))/C_initial), where C(t) is the phenol concentration at any time, t, and C_initial is the concentration at time t = 0 (just before the lamp was switched on). Before irradiation, suspensions were stirred for 60 min in the dark to ensure equilibrium of the solution with photocatalyst. Adsorption of phenol into materials used as catalyst was also evaluated in the dark, without UV irradiation. Only a 3% decrease of phenol concentration was seen by adsorption during the first 30 min, remaining constant with time.

Results and discussion

Materials characterization

The morphological and structural development of materials was investigated by SEM-EDAX. In Fig. 1, a series of SEM images belonging to the original natural volcanic material without anatase loading (support) are shown (Fig. 1 a–c) Similar SEM-EDAX images for anatase supported volcanic ashes (titanated support) are displayed in Fig. 1 d–f. The images of the natural volcanic ash surface confirm the relatively high porous structure of the material. Volcanic ashes appeared as an assembly of particles lower than 2 μm in size, which exhibit prismatic shapes when examined under high magnifications. The images of the titanated samples display the microstructure of the natural material whose holes appeared refilled by anatase crystallites. The titanium distribution was simultaneously evidenced by EDAX. As shown in Fig. 1d, titania particles with homogeneous morphology and large aggregates appear deposited along the substrate surface. Images recorded at much higher magnification (Fig. 1e–f) show prismatic particle shapes, which are covered by a snowfall of smaller anatase particles. Additionally, EDAX analysis of both materials confirms the presence of Ca, Si, Mg, Al, Fe and Ti elements in the volcanic material. For anatase mounted volcanic ashes, major elements found include Si, Al and Ti.

Fig. 1 SEM images. (a–c) Natural volcanic ash (support), (d–f) anatase supported volcanic ash (titanated support).

Fig. 2 displays the pore size distribution for both materials obtained from N₂ adsorption–desorption isotherms and mercury porosimetry. The wide pore size distribution displayed by the natural material is noticeable. Table 1 compiles the textural properties of natural material and titania-loaded samples. The textural parameters included in this table are: values of BET specific area (S_BET), micropore size (D), total pore area, porosity (ε) and density (ρ). Titania loading in the natural volcanic ash matrix increases only slightly the mesopore area, as determined by mercury porosimetry. This suggests that alteration of the arrangement of the support material was negligible. On the other hand, titania incorporation increases the specific BET area due to the microporosity increase.

Fig. 2 (a) N₂ adsorption and (b) mercury porosimetry pore size distributions for natural and titania loaded material.

Table 1 Textural properties of samples

Sample	N2 adsorption		Mercury porosimetry
	S BET/m^2 g^−1	D/Å	Area/m^2 g^−1	E (%)	ρ bulk/g mL^−1	ρ apparent/g mL^−1	D/μm
Support	5.47	3.5–5	60.78	69.9	1.090	3.632	128.7
Titanated support	10.45	3.5–5	60.83	67.8	1.016	3.158	118.8

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Fig. 3 shows X-ray diffraction patterns for the support, titanated support and residue in order to identify their principal constituents. In Table 2, the main constituents of the samples are shown, volcanic ashes (support) are mainly constituted by plagioclase (labradorite type), augite, ferrous magnesium hornblende and mixed metallic oxide. It can be noticed that Ti element is present in both augite and mixed metallic oxide. Anatase incorporation into volcanic ashes during the hydrothermal process can be detected in the titanated supportXRD by comparing with residue and support patterns. As is observed in Fig. 3, the X-ray diffraction peak corresponding to titania is also observed in titanated support, indicating that supported material has crystallized in a TiO₂ anatase phase. The average crystallite size calculated from Scherrer's equation from anatase (101) for both residue and titanated sample is 11.5 nm.

Fig. 3 X-Ray diffraction patterns of samples (support, titanated support and residue).

Table 2 Main component samples detected by X-ray diffraction

	Support	Titanated support	Residue
a ✗ = Not present in the sample. b ✓ = Present in the sample.
Anatase
TiO2	✗^a	✓^b	✓^b
Labradorite
(Na0.4Ca0.6)Al1.6Si2.4O8	✓^b	✓^b	✗^a
Mixed metallic oxide
(Mn0.298Zn0.3Fe0.405)(Mg0.019Fe1.861Al0.056Ti0.025Mn0.04O4)	✓^b	✓^b	✗^a
Magnesium hornblende, ferrous
(Na0.4K0.1)(Ca1.8Fe0.2)(Mg3.1Fe1.5Al0.4)(Si7AlO22)(OH)2	✓^b	✓^b	✗^a
Augite
(Mg,Fe,Ti,Al)(Ca,Fe,Na,Mn)(Si, Al)2O6	✓^b	✓^b	✗^a

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The chemical state of the elements and their relative abundance was determined by photoelectron spectroscopy. Binding energies (eV) for Ti 2p_3/2, O 1s core-levels of Ti-containing samples are collected in Table 3. Also for comparison, the Ti 2p doublets (2p_3/2 and 2p_1/2 peaks) and the O 1s profiles of the three samples are shown in Fig. 4 and Fig. 5, respectively. The binding energy of the main Ti 2p_3/2 peak appears at about 458.5 eV, indicating that titanium atoms are in the oxidation state Ti(IV) and surrounded by oxide ions in an octahedral environment. As can be seen in Fig. 5 and also in Table 3, the O 1s peak of impregnated and residue samples is constituted by three components that correspond to three kinds of oxygen species, whose binding energies fall within the range 529.9–533.6 eV. The peak located at 529.9 eV is attributed to the contribution of Ti–O bonds in the octahedrally coordinated TiO₂ crystal lattice. The second one located at 532.1 eV is related to hydroxyl groups anchored on the topmost layer of the TiO₂ surface and that found at 533.6 eV is typical of adsorbed molecular water.²¹ From peak intensities, normalized by atomic sensitivity factors, the Ti/Si surface ratios were computed (Table 4). The value of this ratio is moderate in the support ash material (0.059) indicating that not so much titanium is exposed in the original substrate. However, the value of the Ti/Si ratio increases by a factor close to 5 (0.311) due to the deposit of titania species on the outer surface. Other elements such as Si, Al and Fe coming from the ash substrate were also detected on the surface of the support and the impregnated sample (Table 3). These elements are the main constituent components of the minerals present in the ash. The decrease of Al/Si and Fe/Si ratios for the impregnated sample in relation to the support (Table 4) is due to the growth of a titania layer on the external (and also internal) surface of the ash grain thus limiting the probability of Al 2p and Fe 2p photoelectrons coming from sub-surface layers, to reach detector.

Table 3 Binding energies (eV) of samples

	Ti(2p3/2)	O(1s)	Si(2p)	Al(2p)	Fe(2p3/2)
Support	458.6	531.8	102.5	74.4	711.2
Titanated support	458.6	529.9 (24)	103.3	74.6	711.0
		532.1 (47)
		533.6 (29)
Residue	458.7	529.9 (37)	—	—	—
		532.1 (35)
		533.6 (28)

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Fig. 4 Ti 2p XPS spectra of samples (support, titanated support and residue).

Fig. 5 O 1s XPS spectra of samples (support, titanated support and residue).

Table 4 Surface atomic ratios

	Ti/Si	Al/Si	Fe/Si
Support	0.059	0.452	0.086
Titanated support	0.311	0.251	0.058

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The UV-vis spectra of the different solids expressed in terms of Kulbelka–Munk equivalent absorption units are presented in Fig. 6. As expected, titania residue has no absorption above its fundamental absorption sharp edge rising at 400 nm. The support and the titanated support can absorb at higher wavelengths than the titania residue, the absorption values corresponding to the impregnated sample being between those shown by titania residue and support. The shift observed for the support and for the titanated support corresponds to a decrease in the band gap energy, which should be related to a loss of crystallinity when titania is deposited over the support²² or, more probably, to the enhancement of the absorption at longer wavelengths by the presence of Ti compounds (mixed metallic oxide and augite) over the surface of the support and the titanated support, since the support is the sample that absorbs more in the red. These results suggest that these Ti compounds might produce an increment of surface electric charge of the oxides, which may lead to modifications of the fundamental process of electron/hole pair formation when visible radiation is applied. This should have a role in generating active sites for phenol photodegradation. On the other hand, as observed in Fig. 6, the residue, mostly composed by titania, is the sample that absorbs more at the shorter wavelengths, which can be attributed to the size quantification effect probably due to the presence of Ti-oxide particles smaller than 10 nm²³ and could account for a high photocatalytic activity in the UV region.

Fig. 6 Diffuse reflectance UV-vis spectra of support, titanated support and residue.

Activity measurements

The photocatalytic removal of phenol from aqueous solutions was investigated using both the original and TiO₂-loaded volcanic ash as photocatalyst. An increase in conversion with time was observed for all the samples, a steady state being reached at 4–5 h of reaction (Fig. 7). It is seen in Fig. 7 that the original ash (support) displays a high photodecomposition activity, giving a conversion of phenol removal of 85% at a reaction time of 5 h. Fig. 7 also shows that anatase-loaded on the volcanic ash (titanated support) is more active than the volcanic ash material (support), with which nearly complete removal of phenol (>95%) is achieved after 4 hours of reaction. A similar behavior is displayed by a commercial TiO₂ (Merck p.a.) sample under the same reaction conditions.

Fig. 7 Changes in phenol conversion with irradiation time (100 ppm phenol, 3 g L⁻¹catalyst).

In Table 5phenol conversions obtained after reaction for 1 hour are compared for different catalysts including support and titanated support particles. Good values of conversion are observed at 1 hour for the volcanic ashes particles relative to the commercial TiO₂ powders like TiO₂ (Merk p.a.), TiO₂ Degussa P-25 and TiO₂ Hombikat UV-100 reported in the bibliography.^24,25 Even though conversions observed at 1 h for volcanic ashes based photocatalysts are lower than TiO₂ Degussa P-25, both support and titanated support particles exhibit similar photoactivity to TiO₂ Hombikat UV-100 and TiO₂ (Merk p.a.) commercial powders photocatalyst respectively.

Table 5 Phenol photodegradation conversion (first hour) for different catalysts

Sample	Phenol conversion (%)
a This paper. b From ref. 23. c From ref. 24.
Support	43^a
Titanated Support	65^a
TiO2 (Merck p.a.)	64^a, 65^b
Hombikat UV-100 TiO2	40^c
Degussa P-25 TiO2	90^c

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These results on the performance of the Ti-containing volcanic ash material (support) for phenol removal from the aqueous phase appear quite useful because this material is very cheap. Indeed, the performance in the target reaction of the ash material can be substantially improved by incorporating a layer of titania on its surface (titanated support). Thus, this material could be a potential substrate displaying high efficiency for photodecomposition of organic pollutants in water. Volcanic ash material can be considered as a potential substrate for TiO₂ photocatalyst with good efficiency for phenol-type organic pollutant photo-decomposition in water.

The use of the catalyst as particles allows post-radiation treatment for particle–fluid separation, for catalyst recycling and for the ultimate goal of obtaining clean powder-free water.

In summary, this work describes the application of a Ti-containing volcanic ash as a suitable substrate for catalysts that might be beneficial for the decomposition and removal of various organic pollutants present in both air and water, with potential application in industrial waste stream treatments.

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