Cleaning of olive mill wastewaters by visible light activated carbon doped titanium dioxide

Abstract

Carbon doped titanium dioxide (CDT) was tested as catalyst for photodegradation of phenolic compounds of olive mill wastewater (OMW). The activation of the catalyst was triggered by exposure to visible light radiation. The cleaning effectiveness of this catalyst towards the polluted wastewater from olive oil industry was demonstrated by means of HPLC and UV-visible spectroscopy combined with phenol compound determination. The photodegradation activity was tested on systems having different initial concentration of phenols and in the presence of different amounts of CDT. By introducing a suitable parameter, namely the ratio between the amount of catalyst and the amount of total phenols Ti/TPh, it was demonstrated that the proposed degradation method could be scaled up without losing its effectiveness. The OMW decolorization occurring in the presence of CDT particles under visible light radiation is marked enough to be directly appreciated with the naked eye. The decolorization is strongly associated with the removal of phenols. In fact, while bleaching the solutions, CDT successfully removed 70% of the phenols in 24 hours. HPLC analysis demonstrates that CDT was effective in degrading the higher part of the phenols of OMW. An exception is represented by hydroxytyrosol that seemed to have high resistance in the first 24 hours of treatment.

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1. Introduction

Olive cultivation and olive oil production are part of the local tradition and rural economy throughout Mediterranean regions. Currently, three processes are available for the extraction of olive oil, the traditional press, the two-phase method and the three-phase method. All these systems generate a large quantity of by-products. The two by-products formed by both the traditional press and the three-phase method are a solid residue known as olive press cake and large amounts of aqueous liquid known as olive-mill wastewater (OMW).

OMW comes along with different environmental problems like toxicity towards plants and microorganisms or degradation of soil and water quality. Thus, these effluents unless disposed of properly can result in serious environmental damage. OMW is, in fact, extremely rich in phenolic compounds, which are in part responsible for very high values of chemical oxygen demand (COD) and have very strong antimicrobial and phytotoxic properties. Phenolic compounds are a class of molecules consisting of a hydroxyl group directly bonded to an aromatic hydrocarbon group.¹ These compounds are ubiquitous in plants and for the human diet.^2,3 Several phenolic compounds, because of their antioxidant properties, possess several common biological and chemical properties.^4–7 Besides all the benefits related to human, environmental and food areas, the presence of a so huge amount of phenolic compound in the OMW represents a serious environmental treat. These substances are resistant to biological degradation and their disposal on agricultural soils will cause their accumulation, thus determining problems of soil fertility and groundwater contamination. In addition, the low limit values of polyphenols disposal and their widespread and threatening presence have accelerated the search for advanced and economically attractive treatment technologies for their removal.

In recent years, research efforts have been directed towards the development of efficient treatment technologies in the pollutants removal area highlighting the importance of synthesizing hybrid materials that are self-supporting, without losing the properties of adsorbent devices which are also important to such processes.^8–11

Several technologies, such as adsorption, co-precipitation, coagulation, filtration and evaporation in open ponds and reverse osmosis have been used to treat OMW with a particular interest towards combined physico-chemical processes.^12–15 Alternatively biological treatments have been reported, like the use of aerobic or anaerobic micro-organisms.¹⁶

Oxidation processes based on conductive diamond, supercritical water oxidation, electro-oxidation, Fenton oxidation and UV-light/TiO₂ oxidation show a suitable ability to mineralise an extensive range of organic compounds.^17–19

The application of titanium dioxide as heterogeneous photocatalyst is well established for the remediation of water and for air purification.^20,21 However, titanium dioxide has a band gap of 3.2 eV, which can be activated only under UV-light irradiation, at wavelengths lower than 387 nm. Phenolic compounds in aqueous solutions were successfully degraded by means of photocatalysis using TiO₂.^22,23 The practice is based on photo activation of titanium dioxide with UV light, which leads to a sequence of reactions resulting in the production of oxidants. The so formed compounds (hydroxyl radicals) can easily react with organic compound on the TiO₂ surface.^24,25

Efforts have been made to explore methods to provide the photoactivation of titanium dioxide under visible light. Doping of TiO₂ represents a widely used approach, for developing titanium dioxide based materials, for environmental applications.²⁶ In this direction, different methods for the synthesis of carbon doped TiO₂ particles have been proposed to improve the photocatalytic activity.^27–31 Ren and co-workers synthesized a visible-light-active TiO₂ photocatalyst prepared through carbon doping using glucose as carbon source, performed by a hydrothermal method at low temperature.³² They demonstrated that the synthesized carbon doped TiO₂ had a band gap energy of 3.01 eV. Recently we demonstrated the enhanced photocatalytic activity of this type of carbon-doped TiO₂ (CDT) towards caffeic acid and showed how the use of this type of particles is highly appropriate for pollutants removal.³³

Another important issue that should be taken into account is the large variety of compounds typically found in this kind of effluents that, in many cases, brings only to a partial treatment of OMW. In fact, the complexity of OMW calls the need to identify the mechanism of each oxidation process and eventually the presence of inhibition/cooperative effect. In this respect, a detailed study based on the experimental condition could give important information for the realization of a standard method that leads to the complete oxidation process.

The aim of this work was to study the efficiency of the visible light activated CDT photocatalyst towards the OMW compounds.

2. Experimental

2.1. Materials

Glucose, titanium isopropoxide (97%), ethanol absolute, potassium chloride, sodium carbonate, ethyl acetate, Folin–Ciocalteu's reagent 2 M and the standard phenols (gallic and caffeic acid, hydroxytyrosol, verbascoside, oleuropein) were from Sigma-Aldrich and used without further treatment. Pure water used in all the experiments was obtained by Humancorp purification system.

2.2. OMW

OMW samples were obtained in the year 2014 from a traditional olive oil mill-press located in the Molise region (Italy). No chemical additives were used during the olive oil production. The OMW were characterized and stored at −18 °C before use. The characterization of the OMW is reported in Table 1. Before performing photoxidation tests, the raw effluent was diluted with deionized water and filtered to remove the solid content. To better characterize the phenolic composition of the OMW, the water phase was splitted in two fractions according to the procedure previously described by De Leonardis et al.³⁴ Briefly, 10 mL of OMW were extracted four times with 5 mL of ethyl acetate. Solvent was removed from both the aqueous and acetate fractions and the residue was dissolved in distilled water for HPLC analysis.

Table 1 Main characteristics of the tested OMW

Total solids (g L^−1)	115
COD (g L^−1)	100
Total lipids (g L^−1)	3.0
Total nitrogen (g L^−1)	0.7
Total phenols OMW (g L^−1)	8.6
Conductivity (mS cm^−1)	11.88
Density (kg L^−1)	1.036
pH	5.04

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2.3. Catalyst

Glucose-doped titanium preparation. Carbon-doped TiO₂ (CDT) was synthesized following the method reported by Ren et al.³² Briefly, the amorphous TiO₂ particles were prepared by controlled hydrolysis of titanium isopropoxide in ethanol mixed with potassium chloride. After this step, the sample was stirred to produce a white precipitate, and then the suspension was aged for 24 hours. The suspension was collected by filtration and overdried to yield amorphous TiO₂ particles. Carbon-doped TiO₂ was synthesized by adding a glucose solution to amorphous TiO₂ powder (0.25 g of TiO₂ and 0.018 g of glucose). The suspension was heated at the temperature of 160 °C for 12 h and the resulting powder was washed several times with water and ethanol.³³

2.4. Photodegradation

Photocatalytic degradation of phenolic compounds was carried out by using CDT activated under visible irradiation. The irradiation experiments were performed by placing the samples in a homemade reactor where the photocatalytic activity was activated with 180 W biolux OSRAM fluorescent lamps (6500 K). The photoemission spectrum of the fluorescence lamps provides visible light in the range of 400–800 nm. The distance between the light source and the bottom of the solution was ∼15 cm. 10 mL of OMW with appropriate dilution factor and different amounts of CDT were placed in 25 mL glass flasks and mechanically stirred. The temperature was kept constant at 25 °C. Samples were placed in the reactor and illuminated for various time intervals. At predetermined time intervals, aliquots of the sample were withdrawn, centrifuged at 10 000 rpm for 10 minutes and analyzed. All experiments were carried out in triplicate.

2.5. Analytical methods

2.5.1 Total phenols. Total phenols content was analyzed colorimetrically by the Folin–Ciocalteu's method by using an independent caffeic acid calibration curve (0–0.25 mg mL⁻¹) as reference. Briefly, 500 μL Folin–Ciocalteu's reagent (0.2 M) were added to 100 μL of sample (diluted or not according to the initial concentration of phenols) and placed in the dark for 4 minutes. Successively, 400 μL of sodium carbonate 7.5% (w/v) were added and the cuvettes were allowed to stand in the dark, at room temperature for 1 h before the detection of the absorbance at 760 nm. The percentage of phenolic compounds removal was calculated as the percentage of TPh/TPh₀, where TPh₀ is the initial concentration of phenolic compound and TPh is the concentration of phenolic compound at time t.

2.5.2 Color detection. UV-visible spectra were collected using a double-beam thermostated spectrometer (Cary 100-Varian), in the 200–800 nm region with cells with a path length of 1 cm. Changes in sample color were monitored at the wavelength of 380 nm to evaluate the level of decolorization occurring during photocatalytic treatment.

2.5.3 High performance liquid chromatography (HPLC). A UV/vis detector Varian ProStar 230 instrument (Mulgrave, AUS) set at 280 nm and a Kinetex 5u C18 100A (150 × 4.6 mm) (Phenomenex, USA) column were utilized. Chromatographic separation was carried out according to the condition previously reported.³⁵ Hydroxytyrosol was quantified through a corresponding calibration curve derived from a plot of area counts versus concentration.

2.5.4 COD. COD concentrations were analysed using the vial test LCK 314 of Dr Lange (15–150 mg O₂ per L).

2.5.5 Total solid, total lipid, total nitrogen and density.
Total solids. 30 mL of OMW were dried overnight at 105 °C in a porcelain crucible, cooled in a desiccator and weighted using a 4-digit analytical balance.
Total lipid. 30 mL of OMW were extracted five times with hexane. Total nitrogen was determined by the Kjeldahl method. Density measurement was done with a Gibertini (Milano) hydrostatic balance.

3. Results and discussion

3.1. TPh analysis and Ti/TPh ratio

The photocatalytic activity of carbon-doped TiO₂ has been tested for the degradation of OMW phenols by lighting, with visible light radiations, an aqueous suspension containing OMW and CDT particles. A first series of trials was made, by measuring the changes of total phenol content, TPh, of OMW having fixed dilution factor and variable amount of CDT particles. Commonly, for such reactions, the extent of degradation strongly depends on the amount of catalyst and on the initial substrate content, on the ability of the substrate to be adsorbed on the surface of the catalyst, as well as on other parameters such as pH and O₂ concentration.³⁶ In this study, a useful parameter based on the ratio between the TiO₂ amount used (expressed in mg) and the initial phenol content (expressed in mg L⁻¹) is introduced. Hereafter this parameter will be indicated as Ti/TPh. Fig. 1 shows the removal ability of carbon doped titanium against OMW for samples with different values of Ti/TPh (50, 100 and 200). OMW used in these degradation experiments was diluted 200 times, in order to reduce the concentration of possible interfering compounds. The main characteristics of the OMW raw effluent, used in this study, are listed in Table 1. As shown in Fig. 1, CDT presents a high propensity to reduce the TPh content in the OMW. In fact, as can be seen from the experiments, after 24 h, CDT allowed the removal of 33, 67 and 71% of TPh from samples having Ti/TPh of 50, 100 and 200, respectively. From this representation, it is noticeable that the higher part of the whole photodegradation process occurs within the first hours (6 hours). In fact, for Ti/TPh 100 and 200 no substantial decrease of TPh was noticed from the sixth to the twenty-fourth hour. The data presented in Fig. 1, on the one hand demonstrate that the higher is the Ti/TPh ratio, the higher is the amount of phenols that can be removed from the water solution. On the other hand, the data indicate that, at high Ti/TPh ratios, no significant differences are longer detectable. These results, ultimately, allowed the identification of a value of the ratio Ti/TPh of about 100, as a good compromise between the amount of catalyst used and a high yield of TPh degradation.

Fig. 1 Total phenols degradation profiles as function of time. OMW (diluted 200 times) under light irradiation with different amounts of carbon doped TiO₂. Ti/TPh ratios 50, 100 and 200.

The role of the light as activator of the degradation process and the contribution of adsorption phenomena for this reaction is presented in Fig. 2. In order to compare the simple adsorption phenomenon with the photodegradation process, the TPh removal proofs carried out in the presence and in absence of light are reported together. From this experiment, it is evident that taking the sample in the absence of light, only the adsorption of phenolic compounds onto CDT is detectable. By lighting the sample, the photodegradation process allows the removal of TPh with a rate comparable with the one of the sample placed under light from the beginning. This behavior is in agreement with Baransi and coworkers³⁷ and with our previous work where it was demonstrated that, for the caffeic acid degradation induced by CDT, the whole process is governed by the synergic occurrence of both adsorption and photodegradation mechanisms.³³ Furthermore, in Fig. 2, the profile of TPh removal from a sample illuminated in absence of CDT indicates the stabilization of OMW under visible light irradiation and the absence a self-degradation process.

Fig. 2 Total phenols degradations profiles as function of time (Ti/TPh = 100). Black full dots: CDT-OMW under light condition; black empty squares: CDT-OMW under dark condition for 24 hours and successively treated with light; black full square: OMW without CDT under light condition.

3.2. UV-vis absorption spectroscopy

Once established that the phenols are the main compounds involved in the photocatalytic process, in order to correlate the TPh degradation with the whitening process, a decolorization analysis was performed on OMW treated with the catalyst and placed under visible light. It is well known that the decolourizing process is mainly attributed to the oxidized forms of phenols.³⁴ The disappearance of color from the suspension was followed by UV-vis absorption spectroscopy, following the absorbance change at 380 nm. Fig. 3 shows the absorbance spectra during the photocatalytic treatment of OMW solution (dilution factor 1 : 20) for a sample with a Ti/TPh ratio = 100. The decrease of absorbance at 380 nm upon irradiation reveals the effect of the photodegradation process. As exposed in Fig. 3, more than 60% of color was removed from OMW within 6 h, indicating a fast decolorization rate in the first steps of the reaction. As shown, the extent of whitening of OMW, ascribable to the use of CDT particles, is marked enough to be directly appreciated also with the naked eye. In the inset of Fig. 3, the OMW supernatants before and after 6 h of treatment are reported.

Fig. 3 Whitening of OMW in the presence of carbon doped TiO₂. The UV-visible spectra (diluted 200 times) show the decrease of the absorbance at 380 nm within 6 hours. The photograph in the inset shows the color of a sample (Ti/TPh = 200) before and after light irradiation in the presence of carbon doped TiO₂.

3.3. OMW phenol composition

A phenol composition analysis was made through HPLC. In Fig. 4A the phenol composition of OMW ethyl acetate extract is shown. Chromatogram A reports a variety of phenolic compounds with significant biochemical activity.³⁴ Chromatogram B refers to the untreated OMW sample (obtained by injecting OMW diluted 20 times). Chromatogram C comes from OMW treated with CDT and visible light for 23 hours. By analyzing chromatogram A, besides the presence of phenols like tyrosol, verbascoside, hydroxycinnamic family compounds and oleuropein, it was noticed that hydroxytirosol, is undoubtedly the most representative of phenol species of the OMW, in agreement with previous studies (37.3% as peak area counts versus sum of total peak area counts).^34,38 The second phenol for quantity is verbascoside (10.7%), while tyrosol was present at 2.5%. Hydroxycinnamic family compounds, eluted from 17.0 to 21.5 minutes (Fig. 4A), exhibit a maximum absorbance both at 280 and 320 nm and represented, on the whole, the 12.4%. Finally, the main peak eluted in the oleuropein zone was about 3%. Chromatogram of the fraction obtained from the solvent extraction appeared to be the fingerprint chromatogram of OMW diluted 20 times and directly injected in the HPLC (Fig. 4B) and above all, the hydroxytyrosol peak was well resolved and identifiable.

Fig. 4 (A) Ethyl acetate extract; (B) OMW-CDT time 0; (C) OMW-CDT after 23 hours. (1) Hydroxytyrosol; (2) tyrosol; (3) verbascoside; (4) hydroxycinnamic acid zone; (5) oleuropein zone.

HPLC analysis performed on the CDT treated water (Ti/TPh 100) showed that after 6 h a large amount of constituents was removed from the solution and after 23 h almost all the representative species were removed from the water solution (Fig. 4B and C). From these chromatograms the effectiveness of CDT, particularly on the phenol component, can be established. By analyzing the chromatogram after 23 h of treatment, important evidence can be inferred. While all the most representative phenols species were removed from water the hydroxytirosol peak is still present. In Fig. 5 the removed amounts of hydroxytirosol are illustrated together with TPh decrease during the treatment. As shown, while all the other phenols are rapidly degraded, the degradation of hydroxytyrosol appears slower.

Fig. 5 Total phenols and hydroxytyrosol removal profiles as function of time. OMW under light irradiation with Ti/TPh = 100.

Remarkably, while almost all the other phenols are breaking down with this treatment, hydroxytyrosol in the early stages of the reaction maintains a high resistance to the presence of the catalyst. Actually this behavior was previously shown during a degradation performed with an enzymatic catalyst, where it was found that in the presence of other phenols and the enzyme polyphenoloxidase, the hydroxytyrosol oxidation was significantly inhibited.³⁵ The above results suggest that OMW is a potential substrate for producing hydroxytyrosol through CDT particles.

The effects of the whole treatment have also been considered by measuring the COD variation of the OMW. By analyzing the COD values for samples having Ti/TPh ratio of about 100, a loss of 25% was found for the CDT treated OMW solution. The differences in terms of removal between TPh and COD should not surprise. In fact, the TPh fraction (8 g L⁻¹) represents only a small fraction compared with the total solid fraction (115 g L⁻¹), as reported in Table 1. The small COD decrease, for analogues systems, is widely reported in literature. Owing to the presence of different constituents, a higher COD removal calls for further pre-treatments like sonication.^23,39 Anyway, the loss of color was in agreement with removal of TPh, thus it could be inferred that the decolorization has to be assigned to the oxidations of phenolic compounds.

On the other hand, the higher phenols removal can be related to the CDT micropore size (8 nm) that fits the phenolic compounds better than the other organic compounds present in OMW, such as lignin and sugars. In fact, CDT is a mesoporous material and, its physical properties (126.5 m² g⁻¹)³² should provide a suitable surface to remove a great amount of substrate from the water solution.

As for the specificity towards phenols, it is established that photocatalytic reactions in the presence of TiO₂ are based on a free radical reaction initiated by light.⁴⁰ Titanium dioxide is considered an efficient photocatalyst that can form hydrogen and oxygen from water and active species such as hydroxyl radical (OH˙). Under our experimental conditions, the visible light source activated the production of hydroxyl radicals by direct excitation of CDT, thus triggering phenolic compounds photocatalysis.⁴¹

As reported above, the mechanism of adsorption is the only mechanism that drives the removal of OMW in the dark. The action of the light adds to the adsorption mechanism, the photodegradation of phenols. Besides this, parameters such light, micropore size of the catalyst and the titanium dioxide/TPh ratio play important roles in the OMW phenolic compounds photodegradation process.

4. Conclusions

Carbon doped titanium dioxide (CDT) activated by visible light was employed effectively for total phenol photooxidation, decolorization, and COD removal from olive mill wastewater (OMW). Kinetic studies demonstrate the importance of the initial relationship between the catalyst and phenol content namely, Ti/TPh ratio, to the extent of phenols removal. Under appropriate experimental conditions, more than 70% of total phenols can be removed from the water solution in 24 h. Furthermore the fast decolorization rate of the first steps of the reaction, ascribable to the use of CDT particles, was correlated with the phenols removal.

It was also evidenced that after 24 hours of treatment while all the most representative phenols species were removed from water, the hydroxytirosol showed a high resistance to the treatment. Further studies will provide valuable insights on the use of CDT treated OMW as a potential substrate for producing hydroxytyrosol rich waters.

Acknowledgements

The authors wish to thank Dr Giuseppe Moffa for the technical support.

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