Amine-modified mesoporous silica for quantitative adsorption and release of hydroxytyrosol and other phenolic compounds from olive mill wastewater

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Amine-modified mesoporous silica for quantitative adsorption and release of hydroxytyrosol and other phenolic compounds from olive mill wastewater Asma Yangui a,b,∗, Manef Abderrabba a, Abdelhamid Sayari c a

Institut Préparatoire aux Etudes Scientifiques et Techniques (IPEST), Laboratoire Matériaux, Molécules et Applications (LMMA), BP 51 La Marsa 2070, Carthage, Tunisia Université de Tunis El Manar, Faculté des Sciences de Tunis, Campus Universitaire 2092, Tunis, Tunisia c Department of Chemistry, Centre for Catalysis Research and Innovation (CCRI), University of Ottawa, Ottawa, Ontario K1N 6N5, Canada b

a r t i c l e

i n f o

Article history: Received 17 May 2016 Revised 20 October 2016 Accepted 27 October 2016 Available online xxx Keywords: Mesoporous materials Phenolic compounds adsorption Selective hydroxytyrosol adsorption Olive mill wastewater

a b s t r a c t A series of synthesized and commercial adsorbents were used for the treatment of olive mill wastewater (OMW). SBA-15 and P-10 silicas functionalized by post-synthesis grafting with 3-trimethoxysilylpropyl diethylenetriamine (TRI) and commercially available activated carbon (AC) were used for the removal and recovery of phenolic compounds from actual olive mill wastewater. Using UV spectrophotometric and HPLC analyses, the concentrations of total phenols, hydroxytyrosol and tyrosol in OMW were found to be 8.21 g L–1 , 2.14 g L–1 and 0.50 g L–1 , respectively. The recovery of phenolic compounds after adsorption was performed by solvent extraction using 0.1 M HCl in ethanol. Under otherwise the same conditions, all three adsorbents removed hydroxytyrosol quantitatively, but activated carbon showed the highest uptake of total phenols, i.e. 87% versus 75% and 67% for TRI-SBA-15 and TRI-P-10. The overall efficiencies for adsorption and recovery of hydroxytyrosol were 100%, 91% and 37% for TRI-SBA-15, TRI-P-10 and AC, respectively. Recycling experiments revealed that no loss of efficiency occurred after three consecutive adsorption–regeneration cycles. Furthermore, triamine-functionalized SBA-15 gave the highest discoloration percentage of 61% compared to the other materials. © 2016 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

1. Introduction The olive oil production in the Mediterranean countries is of great economic importance as it represents 98% of the world’s production [1]. In Tunisia, about 1.7 million ha representing more than 30% of the cultivable land is used for olive agriculture [2], making this country one of the most important producers of olive oil. Unfortunately, oil extraction processes generate huge amounts of polluted liquid by-product, referred to as olive mill wastewater (OMW). In fact, the annual discharge of OMW exceeds 30 million m3 , creating the same annual amount of pollution as 22 million people [3]. OMW is considerably destructive to the environment, especially if thrown without previous treatments since it is characterized by high acidity and significant concentrations of organic compounds comprised mostly of phenolic acids, which are generally resistant to biodegradation and responsible for the toxicity of the agro-

∗

Corresponding author. E-mail address: [email protected] (A. Yangui).

industrial wastewater [4]. As required by legislation in Tunisia, OMW has to be collected in evaporation ponds to mitigate its effect on the environment [5]. Nonetheless, owing to the large volume of OMW generated on a yearly basis, its negative impact on the environment cannot be ignored. Up to date, no viable solution is available for the proper treatment of OMW. Although numerous pretreatment methods were investigated [6], this liquid waste is often discharged directly into the environment. In some cases, faced with huge quantities of OMW, producers chose to spread it on agricultural land as fertilizer [7], but the uncontrolled discharge of this wastewater may not only adversely affect the soil properties, but contaminate the aquifer [8]. Therefore, olive-producing countries have been searching for viable technologies to minimize the amount of generating wastewater in the first place, and to mitigate its adverse effect on the environment. For example, a twophase process for olive oil extraction was developed in the early nineties to reduce the amount of liquid waste, compared to the more commonly employed three-phase oil extraction process or the traditional press [9,10]. Therefore, OMW effluents must be treated by removing most of their organic content, in particular phenol and its

http://dx.doi.org/10.1016/j.jtice.2016.10.053 1876-1070/© 2016 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

Please cite this article as: A. Yangui et al., Amine-modified mesoporous silica for quantitative adsorption and release of hydroxytyrosol and other phenolic compounds from olive mill wastewater, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.10.053

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derivatives, prior to their release in the environment. In addition, some of the phenolic compounds found in significant amounts in OMW, exhibit many health benefits. Among the wide variety of phenolic compounds detected in OMW, hydroxytyrosol proved to be a potent radical scavenger with excellent antioxidant, anti-inflammatory, antiatherogenic and antiplatelet properties [11]. Hence, combining OMW treatment with the recovery of valuable phenols appears to be of great interest, both economically and environmentally. Due to its highly beneficial properties, several procedures have been established to yield hydroxytyrosol by different means, including chemical synthesis [12], enzymatic catalysis [13] and decomposition of natural oleuropein occurring in OMW in the presence of acetic acid [14]. Earlier studies proved that hydroxytyrosol is the most abundant phenolic compound in OMW [15]. Nonetheless, the nature of phenolic compounds in OMW and their concentrations vary significantly, depending on the origin of the wastewater and its storage conditions. The recovery of phenolic compounds by adsorption is a very promising technique because of its simplicity and reversibility. Several studies dealt with the removal and recovery of phenolic compounds from OMW [16–22] and other liquid sources [23] using adsorption, also known as solid phase extraction. Extensive research effort has been focused on the discovery and development of new adsorbents for the recovery of phenolic compounds from OMW. For example, activated carbon, natural zeolites, dried Azolla plant and Amberlite XAD16 resin were tested for their adsorption capacity [24]. Other adsorbents included layered double hydroxide, hydroxyaluminum–iron-coprecipitate and hydroxyaluminum–ironmontmorillonite [25]. Since their discovery [26], periodic mesoporous silicas and their derivatives have been gaining increasing importance because of their high surface area, narrow pore size distribution with controllable average pore diameter, and their chemical and structural versatility. SBA-15 material is among the most popular mesoporous silicas because of its high mechanical stability, and large and adjustable pores, leading to fast mass transfer kinetics [27]. Surface-grafting of mesoporous silicas with organosilanes is another convenient tool to modify their surface properties and improve their adsorption capacity and selectivity toward organic or inorganic compounds [28,29]. In particular, surfacemodified mesoporous silicas were used as efficient adsorbents for organic and inorganic pollutants for environmental remediation [29]. For example, although pristine mesoporous SBA-15 silica was used for adsorption of phenolic compounds from red wine [23], amine and thiol-functionalized SBA-15 showed better performance for phenol adsorption from aqueous solutions [30]. The objective of this work was to investigate the adsorption and desorption properties of commercial materials (activated carbon, P-10 silica), laboratory-synthesized mesoporous silica (SBA15) and triamine (TRI)-modified mesoporous silicas (TRI-SBA-15 and TRI-P-10) for the treatment of OMW, in order to develop a reversible solid-phase extraction process for the selective removal and recovery of the most valuable low-molecular weight phenols. Commercial P-10 silica was compared to laboratorysynthesized SBA-15, for the purpose of investigating the effect of structural properties (e.g. surface area, pore size, pore volume) on phenol adsorption, if any. To increase the adsorption capacity of the materials toward phenolic compounds, post-synthesis grafting of 3-trimethoxysilylpropyl diethylenetriamine was carried out on both SBA-15 and P-10 silicas. Furthermore, since AC is known to be an excellent adsorbent for organic compounds, its adsorption capacity was also investigated for comparative purpose.

2. Experimental 2.1. Materials OMW was collected from a modern unit of olive oil extraction located in Tunisia, and stored in a plastic container in a refrigerator at 4 °C. AC was purchased from Strem Chemicals (USA). Silica P-10 was a gift from Fuji Silysia Chemicals (Japan). Hydroxytyrosol and tyrosol used for product identification and HPLC calibration were acquired from Sigma Aldrich (Canada). Tetraethylorthosilicate (TEOS) used as a silica source for SBA-15 synthesis, and triblock poly (ethylene oxide)-b-poly (propylene oxide)-b-poly (ethylene oxide) copolymer Pluronic P123 (MW = 5800) used as structure directing surfactant and 3-trimethoxysilylpropyldiethylenetriamine, herein referred to as triamine (TRI), used for surface functionalization were all obtained from Aldrich (Canada). Ultra-high purity gases (N2 , air) were supplied by Linda, Canada. All solvents and reagents were used without additional purification. 2.2. Adsorbent synthesis 2.2.1. SBA-15 synthesis SBA-15 silica was prepared according to the procedure proposed elsewhere [31,32]. In a Teflon container, 4 g of Pluronic P123 was stirred in 30 mL of H2 O and 120 mL of a 2 M HCl solution at 35 °C for 16 h until complete dissolution. Then, 8.5 g (40.8 mmol) of TEOS was added at the same temperature and stirred for 20 h. The Teflon vessel was then placed in an autoclave and heated at 100 °C for 24 h in static conditions. Subsequently, the solid was recovered by filtration, washed and dried. The resulting product was calcined at 550 °C in flowing air. 2.2.2. Post-synthesis functionalization of SBA-15 and P-10 1 g of SBA-15 or P-10 was dried under vacuum at 100 °C for 1 h; then dispersed in 30 mL of toluene and stirred for 15 min. The flask was then placed in an oil bath maintained at 85 °C using a temperature-controlled hotplate. Subsequently, 3 mL of triamine was added to the mixture and left stirring under reflux for 20 h. The mixture was filtered and washed with toluene. The recovered solid was dried under vacuum at 50 °C for 2 h. The final materials were referred to as TRI-SBA-15 and TRI-P-10. 2.3. Characterization All adsorbents used in this study were characterized by nitrogen adsorption–desorption measurements at 77 K using a Micromeritics ASAP 2020 volumetric apparatus. Samples were first degassed under vacuum for 3–5 h. The specific surface areas of the materials were determined by the BET (Brunauer, Emmett and Teller) method using nitrogen adsorption data within the relative pressure range of 0–0.3. The pore size distribution was determined by the KJS (Kruk–Jaroniec–Sayari) method [33]. The pore volume was calculated as the amount of liquid nitrogen adsorbed at P/P0 = 0.99. The amine content was determined by thermogravimetric analysis using a Q500 (TA Instruments) apparatus. The weight loss during the samples calcination at 10 0 0 °C was used to calculate the amine loading, according to the method reported by Harlick and Sayari [34]. 13 C Cross-polarization, magic angle spinning nuclear magnetic resonance (CP MAS NMR) spectra were collected for TRI-SBA-15 and TRI-P-10 at room temperature on a Bruker ASX200 instrument in a magnetic field of 4.7 T. The resonance frequency was 50.32 MHz, respectively. The spinning frequency was 4.5 kHz. The contact time was 2 ms and the recycle delay was 2 s. Analysis

Please cite this article as: A. Yangui et al., Amine-modified mesoporous silica for quantitative adsorption and release of hydroxytyrosol and other phenolic compounds from olive mill wastewater, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.10.053

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by ATR-FTIR spectroscopy was carried out using a Thermo Scientific Nicolet 6700 instrument in the region between 400 and 50 0 0 cm–1 . The instrument was equipped with a zinc selenide ATR crystal. 2.4. Analytical methods 2.4.1. UV–visible measurement Total phenolic compounds content was determined using a UV–visible spectrophotometer (Cary 300, Varian) according to the method reported by Singleton et al. [35]. A series of gallic acid solutions (60, 120, 180, 240 and 300 mg L–1 ) were prepared and used to establish the calibration curve. UV–vis measurements were carried out as follows. Aliquots of gallic acid solutions (100 μL) were mixed with 6 mL of distilled water, 500 μL of Folin & Ciocalteu’s reagent and 1.5 mL of Na2 CO3 (20% in water). The solutions were then adjusted to 10 mL with distilled water and stirred vigorously, after 2 h of incubation, the absorbance was measured at 760 nm. The same procedure was used to determine total phenolic compounds in OMW. 2.4.2. HPLC measurement Qualitative and quantitative measurements of different phenolic compounds were achieved using a high performance liquid chromatography (HPLC) apparatus (Agilent 1200) in reverse phase equipped with a diode array detector (DAD) set at 280 nm. The analysis time was 50 min and the separation was performed on a C18 column (1.8 μm; 50 × 2.1 mm). The column temperature was maintained at 30 °C, the flow rate was 0.1 mL min–1 and the injection volume was 20 μL. The separation was made in gradient mode using two mobile phases, namely an aqueous solution of 0.01% acetic acid and acetonitrile according to the gradient schedule shown in Table S1 (Supplementary Information). 2.5. Adsorption–desorption–regeneration experiments Adsorption and desorption experiments were carried out in glass vials as follows: 0.6 g of the adsorbent was added to 10 mL of OMW, and the bottle was hermetically sealed. The mixture was stirred at 150 rpm for 24 h at room temperature (22 °C) using a magnetic stirrer. After filtration, the resulting aqueous phase (treated OMW) was analyzed by UV–visible spectrophotometry between 250 and 500 nm. The concentrations of total phenolic compounds, hydroxytyrosol and tyrosol in the filtrate were determined as described in Section 2.4. The adsorption efficiency (%A) was calculated using Eq. (1), where C0 and C are the initial and equilibrium concentrations of phenolic compounds (mg L–1 ).

% A=

C0 − C × 100 C0

(1)

The desorption was carried out on the adsorbent recovered by filtration, immediately after the adsorption experiment. The solid was extracted with 2 mL 0.1 M HCl in ethanol. The solvent separated by filtration was placed in a rotary evaporator operating at 180 rpm at 40 °C to recover the desorbed solid fraction.

3

Table 1 Structural properties of the materials. Samples

Surface area (m2 g– 1 )

Pore size (nm)

Pore volume (cm3 g– 1 )

Amine content (mmol N g– 1 )

SBA-15 TRI-SBA-15 P-10 TRI-P-10 AC

925 43 353 14 945

6.4 4.2 17.3 5.5 3.8

1.12 0.03 1.65 0.01 0.52

– 6.60 – 5.26 –

The desorption efficiency (%D) was calculated using Eq. (2), where Cad and Cd are the concentrations (mg L–1 ) of adsorbed and desorbed phenols, relative to the starting OMW.

% D=

Cad − Cd × 100 Cad

(2)

The regeneration procedure of the adsorbents depended on the material being tested. For TRI-SBA-15 and TRI-P-10, since the amine groups were protonated during the desorption step in the presence of 0.1 M HCl, these groups were regenerated via neutralization for 1 h in 0.1 M NaHCO3 aqueous solution, then washed with distilled water and dried under vacuum at 50 °C for 3 h. As for AC, it was also washed with 0.1 M NaHCO3 aqueous solution and distilled water, then dried under vacuum at 105 °C for 1 h. 3. Results and discussion 3.1. Material characterization The main structural characteristics for the adsorbents as determined by nitrogen adsorption–desorption measurements are listed in Table 1. The surface functionalization of the silica by postsynthesis grafting with triamine is shown in Scheme 1. After triamine grafting, the surface areas decreased from 925 to 43 m2 g–1 and from 353 to 14 m2 g–1 for SBA-15 and P-10, respectively. The pore volume and pore diameter also decreased upon amine-grafting on SBA-15 (0.03 versus 1.12 cm3 g–1 and 4.2 versus 6.4 nm, respectively) and P-10 (5.5 versus 17.3 cm3 g–1 and 0.01 versus 1.65 nm, respectively). These findings indicate that the organosilane groups were grafted on the internal surface of the materials. The amine content as measured by TGA for TRI-SBA-15 and TRI-P10 is also listed in Table 1. 13 C MAS NMR data for TRI-SBA-15 and TRI-P-10 data provided direct evidence for the removal of P123 block copolymer and the occurrence of grafted triamine on mesoporous SBA-15 and P-10 silicas. As shown on Fig. 1, NMR signals at 11 and 23.4 ppm are attributable to C1 and C2, whereas, the signal at ca. 49 ppm is a composite associated with the remaining C3 to C7 carbon atoms. The signal at 166.6 ppm is attributable to carbamate obtained by reaction of amine groups with atmospheric CO2 [36,37]. Attenuated total reflectance Fourier transform infrared (ATRFTIR) spectra were recorded for both TRI-SBA-15 and TRI-P-10. Fig. 2 shows comparative spectra of SBA-15 before and after grafting with triamine. Two bands at 3440 cm–1 and 1639 cm–1

Scheme 1. Post-synthesis grafting of SBA-15 and P-10 silicas.

Please cite this article as: A. Yangui et al., Amine-modified mesoporous silica for quantitative adsorption and release of hydroxytyrosol and other phenolic compounds from olive mill wastewater, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.10.053

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41.3 23.4 49.2 11.0

C3,C4,C5,C6

O C 5 C6 C2 NH2 NH O Si O NH C7 C1 C3 C4

C1 C2 C7

-50

0

166.66

50

100

150

200

Chemical shift (ppm) Fig. 1.

100

13

C CP MAS NMR spectrum of TRI-SBA-15.

(a)

Transmittance (%)

(b)

90 80

1650 3440

1639

3280

1580 2930 2816

70

787 800

1470

960

3.3. Adsorption experiments and thermodynamic study

1380 1150

60 50

as tyrosol and its derivatives are present in the olive pulp and olive pits, respectively [14], but very abundant in OMW because they are generally polar and water-soluble compounds. The nature and composition of phenolic compounds in the aqueous phase depend on the history of OMW, and evolve with time, depending on the storage conditions. In particular, the hydroxytyrosol content depends on the degree of hydrolysis of its ester form, oleuropein [42]. Phenolic compounds in OMW were characterized by HPLC using a DAD set at 280 nm. They were identified based on their order of elution and retention times compared to pure standard compounds (Fig. S1). The obtained chromatographic profiles of OMW are in good agreement with literature data [43]. The concentrations of total phenols, hydroxytyrosol and tyrosol in our OMW sample were found to be 8.21 g L–1 , 2.14 g L–1 and 0.50 g L–1 , respectively. However, no oleuropein was detected in the current OMW sample, presumably because it was completely hydrolyzed. Many factors could be behind the absence of this compound in our sample, such as climate conditions, the period of harvest, the maturation of olives, olive oil extraction process, quality of soil and pesticides, etc. Earlier studies [43] indicated that after 6–12 months of OMW incubation at pH of 2–4, total conversion of oleuropein to hydroxytyrosol takes place. This is consistent with the lack of oleuropein in our OMW sample and the occurrence of important quantities of hydroxytyrosol (2.14 g L–1 ) and tyrosol (0.5 g L–1 ).

1080

4000 3500 3000 2500 2000 1500 1000 500 -1 Wavelength (cm ) Fig. 2. FTIR spectra of non-functionalized (a) SBA-15 and (b) TRI-SBA-15.

attributed to Si–OH groups and to adsorbed H2 O molecules, respectively were observed [38,39]. Typical silica network bands at 800 and 1080 cm–1 attributed to the symmetric and antisymmetric vibrations of Si–O–Si respectively, were also observed. The band at 960 cm–1 was assigned to the deformation Si–OH groups [38]. The band at 3280 cm–1 was associated with N–H elongation for the triamine-grafted material, which was further confirmed by the band at 787 cm–1 , typical of N–H “wag”, usually occurring between 700 and 900 cm–1 . Moreover, primary amine vibration occurring between 1580 and 1650 cm–1 was observed. The absorption band at 1380 cm–1 and the shoulder at 1150 cm−1 for TRI-SBA-15 were assigned to C–N stretching vibrations [40]. Bands at 1470, 2930 cm–1 and 2816 cm–1 were assigned to CH2 deformation, aliphatic CH stretch and to the symmetric CH stretching vibrations, respectively [41]. Similar data (not shown) were obtained for TRI-P-10. These results provide evidence for the effective functionalization of SBA-15 and P-10 with triamine groups. 3.2. OMW composition Olive oil production is conventionally achieved by crushing the olives with their pits to produce a thick paste. Large quantities of water are added continuously during this step, a process known as malaxation, then the paste is pressed to generate two phases, namely an oil-rich phase immediately collected, and a wastewater phase referred to as vegetation water or olive mill wastewater. Oleuropein and its derivatives and monophenolic compounds such

3.3.1. Adsorption experiments The adsorption efficiency and recovery of phenolic compounds in the OMW sample were determined by adsorption and solvent extraction measurements. The obtained data for the adsorption percentages of phenolic compounds and the color removal are listed in Table 2. Hydroxytyrosol, which is the most valuable component of the vegetation water, was found to be quantitatively adsorbed on both amine-functionalized materials as well as on AC (Fig. 3). Therefore, the novel adsorbents seem to be more efficient than amberlite resins such as XAD16, IRA96 and XAD7, which showed adsorption efficiencies in the range of 75–79% of the original hydroxytyrosol, even though the solid to liquid ratio was higher (70 versus 60 g L–1 ) and the initial concentration of hydroxytyrosol was much lower (0.6 g L–1 versus 2.14 g L–1 ) than in the current study [17]. TRI-SBA-15 exhibited slightly higher adsorption efficiency of tyrosol compared to TRI-P-10; however, AC showed the best uptake for the same compound, as shown in Table 2. Regarding the total uptake of phenolic compounds, AC showed an adsorption efficiency of 87%, whereas TRI-SBA-15 and TRI-P-10 had adsorption efficiencies of 75% and 67%, respectively (Table 2; Fig. 3). These results were qualitatively consistent with the spectrophotometric analysis of filtrates as illustrated in Fig. 4. Moreover, the tested materials exhibited significant color removal, with TRI- SBA-15 being the most efficient, i.e., 61% versus 55% and 41% for AC and TRI-P-10, respectively (Fig. 4). The foregoing data indicate that although AC exhibited a higher overall adsorption capacity for phenols, amine-modified silicas were more selective toward hydroxytyrosol, the most sought after component. Electron donor–acceptor interactions could occur between phenolic compounds and AC. This would involve the interaction between the aromatic ring of the phenol which is an electron acceptor and the carbonyl groups on AC which are strong electron donors due to their high dipole moment [44]. The high uptake and selectivity of TRI-SBA-15 and TRI-P-10 toward hydroxytyrosol are associated with the occurrence of surface amine groups. Notice that in the presence of a solid to liquid ratio of 60 g L–1 , the removal of hydroxytyrosol was quantitative, indicating that there is room for further optimization

Please cite this article as: A. Yangui et al., Amine-modified mesoporous silica for quantitative adsorption and release of hydroxytyrosol and other phenolic compounds from olive mill wastewater, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.10.053

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Table 2 Adsorption efficiencies (%A) of total phenols, hydroxytyrosol, tyrosol and dark color using different adsorbentsa . Total phenols removedb –1

Hydroxytyrosolb e

Materials

AQ (g L )

%A

SSE

None P-10 SBA-15 TRI-P-10 TRI-SBA-15 AC

8.21d 2.46 3.12 5.53 6.20 7.07

0 30 38 67 75 87

0.0353 0.0030 0.0361 0.0590 0.0521

(a) (b) (c) (d)

Tyrosolb

Dark color (395 nm)c

–1

e

AQ (g L )

%A

SSE

AQ (g L )

%A

SSE

2.14d 0.56 0.62 2.14 2.14 2.12

0 26 29 100 100 99

0.0106 0.0124 0.0 0 01 0.0 0 01 0.0 0 02

0.50d 0.12 0.17 0.19 0.24 0.48

0 24 34 30 48 97

0.0074 0.0023 0.0070 0.0062 0.0017

Abs (nm)

%A

SSEe

2.60 0.21 0.36 1.53 1.02 1.17

0 8 14 41 61 55

0.0133 0.0070 0.0183 0.0012 0.0277

Results were obtained by performing three replicates for each experiment. AQ: adsorbed quantities. Abs: absorbance at 395 nm of filtered-OMW before (None) and after treatment diluted 10 times. content of total polyphenols, hydroxytyrosol and tyrosol originally present in OMW. SSE: sum of squared errors.

%A

(e)

–1

110 100 90 80 70 60 50 40 30 20 10 0

Total phenols Hydroxytyrosol Tyrosol Color

HO

pH=4.2

H

OH

O

Hydrogen bond O O O P-10

H

Si

H

H

H

N+

N+

N+

SBA-15 TRI-P-10 TRI-SBA-15 AC

H

Hydrogen bond

Fig. 3. Adsorption efficiencies (%A) of total phenols, hydroxytyrosol, tyrosol and dark color using P-10, SBA-15, TRI-P-10, TRI-SBA-15 and AC.

O

H H

bo nd ogen r d O y H

H

H HO

A b s o r b a n c e ( a u.) .

5 4

None TRI-P-10 TRI-SBA-15 AC

280 nm

3

HO

OH

Scheme 2. The interaction mechanism between TRI-SBA-15 and hydroxytyrosol.

2

395 nm

1 0

250

HO

300

350 400 Wavelength (nm)

450

500

Fig. 4. UV–vis spectra for non-treated OMW (None) and treated OMW in the presence of TRI-P-10, TRI-SBA-15 and AC (diluted 50 times).

of the adsorption conditions. Compared to TRI-SBA-15 adsorbent, TRI-P-10 may be more advantageous because of the lower cost of P-10 silica as opposed to SBA-15. Since the adsorption experiments were carried out in slightly acidic medium (pH of OMW 4.2), it is expected that the amine functional groups on the surface of TRI-SBA-15 and TRI-P-10 be protonated, and hence the occurrence of ammonium groups as shown in Scheme 2. In addition, phenolic compounds are present in the liquid medium in undissociated molecular form (pH < pka of phenol) [45]. Therefore, two paths could explain the adsorption mechanism. On the one hand, the hydrogen atoms of the ammonium groups may interact with the oxygen of the phenol hydroxyl group through hydrogen bonding [46] (Scheme 2). On the other hand, particularly under slightly less acidic conditions, adsorption may occur as a result of acid–base interactions. Adsorption takes place via the electron donor–acceptor process between the basic

sites of amine groups in TRI-SBA-15 and TRI-P10 and the weakly acidic hydroxyl groups of phenol. Table 2 shows that the relatively higher phenols adsorption capacity of SBA-15 silica compared to P-10, does not reflect the large difference in their surface areas. Moreover, both functionalized silicas adsorbed about as much total phenol, and three to four times more hydroxytyrosol compared to their non-functionalized counterparts, despite the drastic decrease in surface and pore volume. These findings indicate that the structural properties, such as the BET surface area, pore size and volume may not play a critical role in the adsorption of phenols. In contrast, it is inferred that the amine functional groups are the actual phenol adsorption sites. This contention is consistent with data reported by Khalid et al. [47], which indicated that phenolic compounds are efficiently adsorbed on basic sites, with no correlation between the adsorption percentage and the structural properties of the adsorbent. Likewise, comparing the adsorption capacity of phenols on three polymeric materials modified with amide or urea functional groups, Huang et al. [46] found that differences in surface area, pore volume and pore size did not affect the adsorption performance of the materials. They also concluded that the adsorption of phenol is driven by hydrogen bonding between phenol and the oxygen of the surface functional groups.

Please cite this article as: A. Yangui et al., Amine-modified mesoporous silica for quantitative adsorption and release of hydroxytyrosol and other phenolic compounds from olive mill wastewater, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.10.053

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Table 3 Thermodynamic values adsorption of total phenols on TRI-SBA-15, TRI-P-10 and AC.

where R is the ideal gas constant (8.314 J K−1 mol−1 ), T is the temperature in Kelvin (K), r H° (K J mol–1 ) and r S° (K J mol–1 ) were deduced from the slope and intercept of the linearized Van’t Hoff plot of ln (KC ) versus 1/T. The calculated thermodynamic properties are listed in Table 3. The adsorption enthalpies for TRI-SBA-15 and TRI-P-10 were found to be negative, indicating that the adsorption is exothermic, whereas a positive value was obtained for AC which indicates that the adsorption on AC is endothermic. Adsorption Gibbs free energies, were found to be negative for all adsorbents, which indicates that the adsorption process is spontaneous. As seen in Table 3, the adsorption enthalpy on TRI-SBA-15 was slightly higher than TRI-P-10, indicating the occurrence of stronger interactions. This is also consistent with the fact that r G° was more negative for TRI-SBA-15 than TRI-P-10. These differences are also partly due to the difference in amine loading, since TRI-SBA-15 exhibits higher amine content than TRI-P-10 (6.6 versus 5.26 mmol N g–1 ).

ethanol as a non-toxic solvent for phenolic compounds recovery was demonstrated to be an effective approach. However, employment of acidified ethanol gave rise to improved desorption efficiency and higher recovery of both hydroxytyrosol and tyrosol (Table 4). This is consistent with a previous study dealing with the desorption of phenolic compounds from OMW on Amberlite resins [17]. The results of desorption experiments revealed significant differences between triamine-modified silicas and AC. Table 4 shows that hydroxytyrosol, the most valuable compound in OMW, was recovered quantitatively in the presence of TRI-SBA-15. This important finding indicates that TRI-SBA-15 is highly efficient for both adsorption and recovery of hydroxytyrosol. A total amount of 345 mg of hydroxytyrosol per g of phenolic extract was obtained when phenol-loaded TRI-SBA-15 was extracted using acidified ethanol. The overall efficiency achieved for the recovery of hydroxytyrosol in the presence of TRI-P-10 was slightly lower than that obtained with the TRI-SBA-15 (90.7% versus 100%), leading to the conclusion that triamine-functionalized materials are highly efficient for hydroxytyrosol concentration. Although all three adsorbents showed quantitative adsorption of hydroxytyrosol (Table 2), AC exhibited significantly lower recovery of only 36.9%. Nonetheless, AC showed the highest overall performance for adsorption and recovery of tyrosol, corresponding to 96% versus only 38% and 48 % for TRI-P-10 and TRI-SBA-15, respectively. For both post-synthesis grafted materials (TRI-SBA-15 and TRIP-10), regeneration was performed in the presence of a 0.1 M NaHCO3 solution in order to neutralize the amine functional groups after exposure to an acid solution during the desorption process. Then, they were placed in a vacuum oven at 50 °C for 3 h. The drying temperature was low enough to avoid damaging the amine groups by oxidative degradation [48]. Activated carbon was also treated with 0.1 M NaHCO3 solution, washed with water and placed for 1 h in a vacuum oven at 105 °C. For recycling experiments, the same adsorption conditions were used after the regeneration of the materials as described earlier. After three successive adsorption–desorption–regeneration cycles, the experimental data confirmed the effectiveness of the procedure developed since the working adsorption capacity remained constant, indicating that all materials exhibit excellent stability under the employed experimental conditions (Fig. 5). These findings are very significant for further development of solid phase extraction as a promising technology for the recovery of hydroxytyrosol from vegetation water.

3.4. Desorption–regeneration studies

4. Conclusion

In addition to exceptional adsorption performance, it is required that an adsorbent be treated to recover valuable compounds, then regenerated and reused repetitively for economic viability. The recovery of phenolic compounds adsorbed on AC, TRI-SBA-15 and TRI-P-10 was carried out using solvent extraction. The use of

Triamine-modified SBA-15 and P-10 silicas were synthesized and thoroughly characterized using N2 adsorption–desorption measurements, FTIR, 13 C MAS NMR and TGA. Along with activated carbon, they were used for the treatment of olive mill wastewater with the specific purpose of recovering hydroxytyrosol, because of

r H° (KJ mol– 1 )

Kc (L mg– 1 )

Temperature (K)

293 313 333

51.409 39.876 33.090

293 313 333

32.028 27.616 21.878

293 313 333

124.035 142.574 162.444

r S ° (KJ mol– 1 )

r G ° (KJ mol– 1 )

TRI-SBA-15 –8.951

0.002

–9.597 –9.591 –9.688

TRI-P-10 –7.682

0.003

–8.445 –8.635 –8.542

AC 5.467

0.059

–11.743 –12.907 –14.093

3.3.2. Thermodynamic study The effect of temperature on the adsorption process was investigated and the thermodynamic properties of total phenols adsorption on TRI-SBA-15, TRI-P-10 and AC were determined. The adsorption enthalpy change (r H°), the entropy change (r S°) and Gibbs free energy change (r G°) were calculated using the linearized Van’t Hoff equation as follows:

r G◦ = −RT ln(Kc ) ln(Kc ) =

r S ◦ R

−

(3)

r H ◦

(4)

RT

Table 4 Desorbed phenols and overall efficiency of removal and recovery of phenols in the presence of TRI-SBA-15, TRI-P-10 and ACa . Adsorbent

Desorbed phenols (g L– 1 )

Adsorbed –1

P-10 SBA-15 TRI-P-10 TRI-SBA-15 AC (a) (b) (c)

phenols (g l )

Hydro

2.46 3.12 5.53 6.20 7.07

0.51 0.60 1.94 2.14 0.79

b

SSE

c

0.0147 0.0081 0.0358 0.0030 0.0554

Overall efficiency (%) c

Tyrosol

SSE

0.10 0.15 0.19 0.24 0.48

0.0030 0.0016 0.0 0 02 0.0014 0.0027

Hydrob

Tyrosol

23.8 28.0 90.7 100 36.9

20.0 30.0 38.0 48.0 96.0

The recovery was carried out in acidified ethanol. Hydro: hydroxytyrosol. SSE: sum of squared errors.

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P-10 SBA-15 TRI-P-10 TRI-SBA-15 AC

100 90 80 70

%A

60 50 40 30 20 10 0

Cycle 1

Cycle 2

Cycle 3

Fig. 5. Adsorption cycles of hydroxytyrosol on regenerated adsorbents.

its exceptional health benefits. TRI-SBA-15 outperformed the two other materials in terms of adsorption and recovery of hydroxytyrosol as well as wastewater decolorization. Activated carbon was found to be the most efficient material based on the overall removal of phenols, but the release of adsorbed hydroxytyrosol was very limited. Regarding the adsorption and recovery of tyrosol, activated carbon also showed the most promising behavior. The current findings are clearly a promising step toward the development of a viable process for the treatment of OMW. Further investigations are underway to achieve (i) quantitative removal and recovery of hydroxytyrosol under optimum conditions, (ii) total removal of organic material from OMW, and (iii) total decolorization. This study indicates that TRI-SBA-15 is to be used to achieve the first goal, whereas AC can be used in a second stage to achieve the other two objectives. Therefore, a dual bed solid extraction process could be contemplated. Acknowledgments This work was carried out at the Centre for Catalysis Research and Innovation (CCI) at the University of Ottawa, Canada. The financial support of the Ministry of Higher Education and Scientific Research of Tunisia is acknowledged. Thanks to Fuji Silysia Chemicals (Japan) for providing a P-10 silica sample. Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jtice.2016.10.053. References [1] Magdich S, Abid W, Boukhris M, Ben Rouina B, Ammar E. Effects of long-term olive mill wastewater spreading on thephysiological and biochemical responses of adult Chemlali olive trees (Olea europaea L.). Ecol Eng 2016;97:122–9. [2] Baccari M, Bonemazzi F, Majone M, Riccardi C. Interaction between acidogenesis and methanogenesis in the anaerobic treatment of olive oil mill effluents. Water Res 1996;30:183–99. [3] Aktas ES, Imre S, Ersoy L. Characterization and lime treatment of olive mill wastewater. Water Res 2001;35:2336–40. [4] Erses Yay AS, Volkan Oral H, Onay TT, Yenigüna O. A study on olive oil mill wastewater management in Turkey: a questionnaire and experimental approach. Resour Conserv Recycl 2012;60:64–71. [5] Jarboui R, Sellami F, Azri C, Gharsallah N, Ammar E. Olive mill wastewater evaporation management using PCA method: case study of natural degradation in stabilization ponds (Sfax, Tunisia). J Hazard Mater 2010;176:992–1005. [6] Rizzo L, Lofrano G, Grassi M, Belgiorno V. Pretreatment of olive mill wastewater by chitosan coagulation and advanced oxidation processes. Sep Purif Technol 2008;63:648–53. [7] Altieri R, Esposito A. Olive orchard amended with two experimental olive mill wastes mixtures: effects on soil organic carbon, plant growth and yield. Bioresour Technol 2008;99:8390–3.

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Please cite this article as: A. Yangui et al., Amine-modified mesoporous silica for quantitative adsorption and release of hydroxytyrosol and other phenolic compounds from olive mill wastewater, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.10.053