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Research Article pubs.acs.org/journal/ascecg

Increasing the Greenness of Lignocellulosic Biomass Biorefining Processes by Means of Biocompatible Separation Strategies L. Xavier,*,†,‡,§ F. J. Deive,∥ M. A. Sanromán,∥ A. Rodríguez,∥ M. S. Freire,§ J. González-Á lvarez,*,§ P. Gortáres-Moroyoqui,† S. Ruíz-Cruz,† and R. G. Ulloa†

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†

Department of Biotechnology and Food Science, Technological Institute of Sonora, 5 de febrero 818 sur, Colonia Centro, 85130 Ciudad Obregón, Mexico ‡ Instituto de Ingeniería Química, Facultad de Ingeniería, Universidad de la República Oriental del Uruguay, Julio Herrera y Reissig 565, 11300 Montevideo, Uruguay § Department of Chemical Engineering, Universidade de Santiago de Compostela, Rúa Lope Gómez de Marzoa, s/n, 15782 Santiago de Compostela, Spain ∥ Department of Chemical Engineering, University of Vigo, Edificio Isaac Newton-Campus Lagoas Marcosende, 36310 Vigo, Spain S Supporting Information *

ABSTRACT: The extraction of phenolic acids from wheat straw (WS) using more environmentally friendly and competitive means is targeted in the present work. The ability of Tween 20 (polyoxyethylene (20) sorbitan monolaurate) as a phenolics extractant in aqueous solutions of a biocompatible ionic liquid (choline dihydrogencitrate) obtained after biomass hydrolysis has been demonstrated. To accomplish the extraction as required, the existence of biphasic areas in model aqueous ternary systems from 20 to 60 °C was ensured as a prior step to evaluate the extraction yields and partition coefficients of ferulic and p-coumaric acids at different Tween 20 concentrations. It was observed that the surfactant content turned out to be a key parameter to attain high extraction levels of phenolic acids (up to 97% of total phenols, 89% of ferulic acid, and 93% of pcoumaric acid at the greatest surfactant concentration). Furthermore, at this surfactant concentration, the total phenols content (TPY) after extraction was 4.51 mg of gallic acid equivalent (GAE)/g of WS dry basis (d.b.), antioxidant activity TEAC (Trolox equivalent antioxidant capacity) was 4.26 μmol equivalents of Trolox (TRE)/g WS d.b., and antioxidant activity for DPPH (2,2-diphenyl-1picrylhydrazyl) was 0.458 μmol TRE/g WS d.b. The proposed extraction method did not entail bioactivity impairment, as reflected the analysis of free radical scavenging capacity and trolox equivalents antioxidant capacity. KEYWORDS: Choline dihydrogencitrate, Wheat straw, Ferulic acid, p-Coumaric acid, Antioxidant activity

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INTRODUCTION

Different researchers have converged upon the idea that acid hydrolysis of WS could be a suitable tool to break lignin,8 and its alkaline delignification may yield a hydrolyzed rich in phenolic acids like ferulic acid and p-coumaric acid.9 These acids bear paramount scientific interest due to their antioxidant, preservative, antimicrobial, anti-inflamatory, and chemoprotective properties.10 The search for greener and more competitive biorefining processes has prompted the replacement of traditional sulfuric acid and sodium hydroxide by biocatalytic ones in the presence of neoteric solvents such as ionic liquids.11 They have been commonly defined as organic salts, whose melting temperature lies below 100 °C and possess properties like their tunability which allow designing countless taskspecific new compounds for a plethora of applications.12

The replacement of nonrenewable fossil resources by sustainable feedstocks has been in the limelight for the last years.1 In this sense, waste minimization strategies must go hand in hand with the valorization of residues.2 Among agricultural wastes, wheat straw (WS) is ranked as the second most favorable lignocellulosic material in terms of abundance and availability, beaten only by rice straw.3 This inexpensive and abundant resource is mainly composed of cellulose, hemicellulose, and lignin, and the fractionation has already been reported.4 Cellulose and hemicellulose have been demonstrated to be quite easily transformed to fermentable sugar for biofuels production or can be converted in chemicals such as furfural, xylitol, and lactic acid.5,6 However, lignin is a phenol-rich polymer, whose selective conversion into chemically useful compounds still constitutes a challenging issue to be addressed.7 © 2017 American Chemical Society

Received: December 28, 2016 Revised: February 14, 2017 Published: March 7, 2017 3339

DOI: 10.1021/acssuschemeng.6b03188 ACS Sustainable Chem. Eng. 2017, 5, 3339−3345

Research Article

ACS Sustainable Chemistry & Engineering

Alkaline hydrolysate samples were then neutralized (HBN) using 2 M HCl prior to aqueous two-phase extraction. Determination of Binodal Curves and Tie-Lines. The mapping of the immiscibility region started by the addition of distilled water to binary mixtures of ChDHC and Tween 20 until solid vanishing, thus delimiting the solid +2L region. The exact composition of the ternary mixture was determined by weighing in an analytical Sartorius Cubis MSA balance. Later, drops of water were added under constant stirring until a clear solution was detected, indicating that the monophasic region had been attained. The area between these two points is the biphasic region.22 These experiments were performed in a thermostatically controlled glass cell equipped with a magnetic stirrer at different temperatures ranging from 20 to 60 °C measured with a digital thermometer (F200 ASL). The tie-lines (TLs) determination was carried out by preparing a mixture with a composition falling in the biphasic region. The mixture was stirred for 30 min and left to settle for 12 h in order to reach the thermodynamic equilibrium. Then, the phases were carefully separated, and their density and refractive index were measured with a digital vibrating tube densimeter (Anton Paar DSA-48 digital) and a Dr. Kernchen ABBEMAT WR refractometer, respectively. Density and refractive index calibration curves of the coexisting phases were made with known composition samples of the ternary mixture to determine the mass fraction from 25 to 60 °C. Finally, the composition of each phase was ascertained by using the calibration curve of the binodal data.19 Extraction and Separation Procedure. The amount of HBN used in the extraction process was fixed at 3 g, and the concentrations of the components of the ATPS were selected on the basis of the binodal and tie-line data. A predetermined quantity of ionic liquid was dissolved in HBN, and the pH was measured (Table S1). Then, the corresponding quantity of Tween 20 according to the composition selected (Table S1) was added into the salt aqueous solution to form the ATPS. The amount of liquid used in the extraction was fixed at 15 g, so the remainder was completed with water if necessary. The stirring and settling period was the same as that described in the tie-line determination, and the phases were separated with a syringe. Total phenol content was determined for both phases at several temperatures (20, 40, 50, and 60 °C). Finally, the total phenol content, DPPH, TEAC, p-coumaric acid, and ferulic acid were determined for both phases as indicated further down. All the experiments were carried out in triplicate, and the results shown correspond to mean values and their corresponding deviations. The partition coefficient K was determined as the ratio between the corresponding biomolecule in the top phase and that in the bottom phase; that is,

Among the ionic liquids, cholinium-based ionic liquids have been pointed out as an attractive alternative due to the low toxicity of the cation and its biodegradability,13 and their suitability for biomass hydrolysis has already been highlighted.11 Therefore, the presence of cholinium-based ionic liquids after the hydrolysis in these previous research works can also be seen as an opportunity to implement competitive and biocompatible separation strategies for the extraction of phenolics from WS. In this sense, the application of aqueous two-phase systems (ATPS) is usually regarded as an environmentally friendly alternative, with advantages like the presence of water in both phases, rapid phase disengagement, and the possibility to operate under mild operating conditions.14 This interest has been translated into the application of ATPS for the extraction and purification of several biological products such as antioxidants, dyes, or antibodies.14−16 Notwithstanding the fact that the traditional ATPS include polymers and salts, the emergence of ionic liquids has opened up new opportunities for the development of novel systems where these molten salts may be salted out by inorganic or organic salts or can act as phase promoters in polymer or tensoactive aqueous solutions.17,18 In this regard, the nonionic surfactant Tween 20 can be considered as an excellent model tensoactive due to its biocompatibility and its potential interest in lignocellulosic biomass hydrolysis.19,20 In the present work, the ability of ATPS composed of a cholinium-based ionic liquid containing biodegradable and nontoxic anion dihydrogen citrate (ChDHC) and Tween 20 to extract phenolic acids from WS lignin has been evaluated. The partition of ferulic and p-coumaric acids was analyzed for different temperatures of operation and Tween 20 concentration in the system, and the existence of antioxidant activity was checked by well-known methods.

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MATERIALS AND METHODS

Chemicals. Gallic acid, sodium carbonate, and potassium persulfate were purchased from Panreac (Barcelona, Spain). Acetic acid, acetonitrile, methanol, and Folin−Ciocalteu reagent were supplied by Merck (Darmstadt, Germany). ChDHC (dihydrogen citrate), ABTS (2,2′-azino-bis(3-ethyl benzothiazoline-6-sulfonic acid)), DPPH (2,2-diphenyl-1-picrylhydrazyl), Trolox ((±)-6-hydroxy-2,5,7,8-tetramethylchromane-2-carboxylic acid), HPLC (high performance liquid chromatography) standards (p-coumaric and ferulic acid), and ethanol were acquired from Sigma-Aldrich (Steinheim, Germany). Tween 20 was delivered by Faga Lab (Sinaloa, México). Raw Material and Pretreatment. WS samples were supplied by a local producer in Sonora (Mexico) and were air-dried until equilibrium humidity (∼5.3%) and ground in a blade mill up to a particle size between 0.5 and 1.0 mm. WS was submitted to acid hydrolysis (4% H2SO4) for 30 min at 121 °C and 103 kPa using a solid−liquid ratio of 1:10 g/g in order to ensure maximum presence of phenolics.21 Once the reaction was complete, the solid (AWS) was recovered by filtration and air-dried in order to reduce its moisture. Finally, an alkaline delignification (2 M NaOH) was carried out at 121 °C and 103 kPa for 30 min using the same solid−liquid ratio (1:10 g/g).8 The choice of acid and alkaline treatment was performed under the concept of biorefinery where all streams are valorized. As already reported, the acid hydrolysis used in this work leads to the highest performance with monosaccharides (xylose, mannose, and glucose). Xylose may be used for the production of chemical products such as xylitol and phenolic compounds.9 Besides, if the hydrolysate sample is neutralized and the necessary nutrients are added, lactic acid can be produced.8,9 The solid rich in cellulose may be used as substrate for the production of sugar, ethanol, and acid lactic.9

K=

C TPi C BPi

(1)

where CTPi (mg/L) and CBPi (mg/L) are the concentrations of the corresponding biomolecule i in the top and the bottom phases, respectively. The extraction efficiency (EE) is defined as the percentage ratio between the corresponding biomolecule in the top phase and the sum in both phases EE (%) =

C TPi

C TPi × VTP × 100 × VTP + C BPi × VBP

(2)

where VTP and VBP are the volumes of the top phase and bottom phase, respectively. The sum of the corresponding biomolecule in both phases is the mass of the corresponding biomolecule at HBN used at the ATPS. Total Phenols Content (TPY). Total phenols content was determined by the Folin−Ciocalteu method. Briefly, a mixture containing 0.5 mL of the extract, 2.5 mL of a 1:10 v/v dilution of Folin−Ciocalteu reagent, and 2 mL of an aqueous solution of Na2CO3 (75 g/L) was heated at 50 °C for 5 min, and the absorbance was measured at 760 nm in a Shimadzu UV/vis−mini-1240 spectrophotometer after cooling.24 The phenols content was calculated as gallic 3340


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Figure 1. Aqueous immiscibility in a ternary system composed of Tween 20 (1) + ChDHC (2) + H2O (3) at 20 °C (black square), 40 °C (red trianglel), 50 °C (green circle) and 60 °C (blue triangle). volume was 20 μL.27 Prior to analysis, alkaline hydrolysate samples were neutralized using 2 M HCl and filtered through 0.22-μm filter. Quantification was done by the external standard method using the standards ferulic and p-coumaric acids. The calibration curve was made using the standards of ferulic and p-coumaric acids at various concentrations (25−250 mg/mL), and results were expressed as milligrams of phenolic acid/g WS d.b. Statistical Analysis. Data were reported as mean ± SD (standard deviation) of triplicate determinations. The existence of significant differences among the results was analyzed. One-way analysis of variance (ANOVA) was used followed by the Tukey’s HSD or Dunnett T3 test, depending on whether equal variances could be assumed or not. All statistical tests were performed at a 5% significance level using PASW Statistics 18 software.

acid equivalent (GAE) from the calibration curve of gallic acid standard solutions (2−40 mg/mL). The total phenols content was expressed as millgrams of gallic acid equivalent (GAE)/g of WS dry basis (d.b.). All analysis were carried out in triplicate. DPPH Radical-Scavenging Capacity. The procedure relies upon the spectrophotometric method proposed by Barreira et al.,25 which evaluates the decrease in absorbance at 517 nm when an antioxidant is added to a DPPH (2,2-diphenyl-1- picrylhydrazyl) fresh solution. A mixture containing 0.3 mL of diluted extract and 2.7 mL of fresh DPPH solution (60 μmol/L in an aqueous solution of methanol at 80%) was prepared. The samples were incubated for 20 min in the dark at room temperature, and the absorbance was measured at 517 nm. The calibration curve was previously prepared by using Trolox in an 80% methanol aqueous solution as the reference solution at various concentrations (0.04−0.30 mmol/L), and the results were expressed as micromole equivalents of Trolox (TRE)/g WS d.b. Trolox Equivalent Antioxidant Capacity (TEAC). The procedure relies upon the method proposed by Re et al.26 First, an induction of the ABTS·+ radical was produced by reaction of a disolution of ABTS (7 mmol/L) and potassium persulfate (2.45 mM). The former reactives were mixed and stored for 40 h in the dark at room temperature. The ABTS·+ solution was diluted with distilled water to an absorbance of 0.700 (±0.020) at 734 nm. A mixture of 25 μL of diluted extract and 2.5 mL of ABTS·+ reagent was made, and the absorbance was measured at 734 nm after 6 min in the dark at room temperature. The calibration curve was made using a Trolox solution in 50% (v/v) of ethanol as standard (0.2−2.0 mmol/L), and the results were expressed as micromoles of Trolox (TRE)/g WS d.b. Phenolic Acids Quantification. Ferulic and p-coumaric acids were analyzed by HPLC (Shimadzu) using a reverse phase C-18 column (5 μm, 4.6 mm × 250 mm Waters Spherisorb) and a diode array detector (DAD) at 320 nm. The equipment consists of two pumps (Shimadzu LC 10 AD VP), oven (Shimadzu CTO-10 AC VP), DAD sensor (Shimadzu DAD SPD-M10A VP), system controller (SCL-10 AVP), and autosampler (Shimadzu SIL 10 AD VP). The system was monitored and controlled by Class VP software (v5.04). The elution was carried out in isocratic mode at a flow rate of 1 mL/ min at 25 °C. The mobile phase was composed of 20% (v/v) acetonitrile and 80% (v/v) water (1% acetic acid), and the injection

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RESULTS AND DISCUSSION Characterization of the Immiscibility Region. The development of more environmentally benign biomass biorefining processes has encouraged us to bet on the role of both ionic liquids and surfactants as key compounds to promote lignocellulosic feedstock hydrolysis.11,20 Starting from this premise, the investigation of the phase segregation capacity of these compounds could be the cornerstone to advance toward more competitive and greener biomass valorization strategies. Hence, the first stage of this work consisted of demonstrating the existence of immiscibility regions in aqueous solutions of a biocompatible ionic liquid and surfactant (ChDHC and Tween 20). The experimental equilibrium data shown in Figure 1 and listed in Table S2 demonstrate the existence of three regions: one monophasic (L) lying over the binodal curve, one biphasic between the binodal curve and the solid−liquid equilibrium (L +L), and one triphasic where the solid ionic liquid coexists with two liquid phases (S+2L). Additionally, the biphasic region can be classified as an island-type L-L system (type 0 in Treybal classification),28 as the binary mixtures ChDHC−water, 3341


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ACS Sustainable Chemistry & Engineering ChDHC−Tween 20, and Tween 20−water are miscible, and the biphasic gap is just detected in the ternary area. From the data obtained at different temperatures (Figure 1), it becomes clear that the regions of immiscibility become greater at higher temperature values. This fact can be interpreted in the light of the different hydrogen bonding capacity of Tween 20 and ChDHC at different temperatures. Thus, it seems that the interaction of the nonionic surfactant with water molecules is weakened at higher temperatures, making water hydration easier by ChDHC molecules and the consequent phase splitting. Therefore, water molecules are more easily excluded from the surfactant-rich phase as the hydrophobic character of the top phase is increased at greater temperature. This trend matches the previous results obtained in research works tackling phase segregation in aqueous solutions of ionic liquids and nonionic surfactants.23,29 Additionally, the binodal data were characterized by fitting to the following empirical models:30−32 w1 = exp(a + bw2 0.5 + cw2 + dw2 2)

(3)

w1 = e + fw2 0.5 + gw2

(4)

w1 = hln(w2 + i) + j

(5)

The TLs data are also represented in Figure 2, and the concentrations of the phases in equilibrium are detailed in Table S3, together with the tie-line length (TLL) and the slope (S), which were calculated as follows: TLL = [(w1t − w1b)2 − (w2t − w2b)2 ]0.5

S=

(6)

where wexp and wadjust represent the experimental and theoretical values, respectively, and nDAT is the number of experimental points. The values of the fitting parameters are listed in Table 1 together with the corresponding standard deviations and allow concluding the suitability of eq 3 to more appropriately describe the binodal data, as can also be noticed in Figure 2 as solid lines. The same model turned out to be the most adequate one for the experimental data obtained for another choline-based ionic liquid in the presence of tween surfactants.19 Table 1. Correlation Parameters and Standard Deviation for Ternary System Tween 20 (1) + ChDHC (2) + H2O (3) from 20 to 60°C T (°C)

a

b

c

d

σ

20 40 50 60

−2.3348 −1.2431 0.4029 0.0057 e

−6.1113 −16.5272 −36.4216 −35.6312 f

19.75 47.53 113.95 118.68

−121.82 −191.92 −409.95 −457.51

0.0020 0.0012 0.0010 0.0015 σ

20 40 50 60

0.0812 0.1164 0.1374 0.1178 h

20 40 50 60

−2.5650 −2.4907 −2.4755 −2.0507

−0.1435 −0.3646 −0.5199 −0.4525 i 8.2638 8.2937 8.3220 8.3199

g −0.0726 0.2392 0.4757 0.4275 j

0.0033 0.0026 0.0034 0.0040 σ

5.4798 5.3281 5.3014 4.3922

0.0046 0.0051 0.0074 0.0077

(w1t − w1b) (w2t − w2b)

(8)

where t and b stand for the top and the bottom phases, respectively. The analysis of the data reveal that greater concentrations of ChDHC in the bottom phase correlate with higher amounts of Tween 20 in the upper phase, which means that the existence of more ionic liquid molecules involve a greater competition for the water molecules, thus segregating more nonionic surfactant to the top layer. In parallel, greater TLL values are obtained as the ionic liquid content is increased. Partitioning of Phenolic Acids. Once the existence of the immiscibility region in ATPS composed of ChDHC and Tween 20 was demonstrated and properly characterized at different temperatures, it can be concluded that the operation at 20 °C does not entail great deleterious effects in terms of immiscibility area and tie-line data. Additionally, different research works have pointed out that a decrease in the partition coefficient K may occur at greater temperature values.33 In the same line, the operation at high temperatures could also affect the stability of phenolic compounds due to chemical degradation reactions or thermal decomposition, which would in turn lead to a decreased antioxidant activity of the extracts.34 In this particular case, the data (not shown) confirmed that the total phenols yields in the top phase were not significantly different at different temperatures, so 20 °C was hereinafter selected for the extraction assays. The following goal was to identify and quantify the phenolic acids content and antioxidant activity in WS hydrolysates after the alkaline delignification (HBN). Therefore, HPLC-DAD and standard total phenol, DPPH, and TEAC antioxidant assays were carried out to characterize the obtained hydrolysates. The HPLC chromatogram of the HBN can be visually inspected in Figure 3 and clearly reveals the presence of two peaks at different retention times that correspond to p-coumaric and ferulic acid at concentrations of 0.176 and 0.091 mg/g WS d.b., respectively. Besides, TPY of 4.167 mg/g WS d.b. was obtained. Therefore, the amount of CA y FA represents about 5.78% of the TPY. While it is difficult to compare values with other works in the literature due to differences in pretreatments, cultives, etc., our results lie in the order of many others. For instance, Akpinar and Usal reported 9.8% using alkaline hydrolysis of WS from Turkey.8 Max et al. reported 2.77% from alkaline hydrolysis of trimming vine shoots.9 Moreira et al. reported 9.8% from brewer’s spent grain.35 In relation to antioxidant activity, the results shown in Table 2 indicate total phenol yields and DPPH radical-scavenging capacity similar to those reported recently by Akpinar and Usal,8 which confirms the potential of the feedstock employed in this work to be used in the extraction process. As the phase diagrams were characterized with model aqueous solutions of ChDHC and Tween 20, the absence of possible components in the HBN modifying the immiscibility region should be demonstrated. Therefore, several binodal points corresponding to the ATPS composed of ChDHC,

where w1 is the mass fraction of Tween 20, w2 is the mass fraction of ChDHC, and a, b, c, d, e, f, g, h, i, j are the fittings parameters obtained by minimization of the standard deviation (σ). ⎛ ∑nDAT (w − w )2 ⎞0.5 exp adjust i ⎟ σ = ⎜⎜ ⎟ n DAT ⎝ ⎠

(7)

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Figure 2. TLs for the system Tween 20 (1) + ChDHC (2)+ H2O (3) at 20 °C (black square), 40 °C (red triangle), 50 °C (green circle) and 60 °C (blue triangle). Symbols represent experimental binodal (void) and TL (full) data. Dashed lines are guides to the eye and dotted lines refer to the empirical model.

Figure 3. Identification of p-coumaric acid (peak 1) and ferulic acid (peak 2) in WS neutralized delignified hydrolysates by HPLC.

Table 2. Phenolic Acids, TPY, DPPH, and TEAC RadicalScavenging Capacity in HBN

Figure 4. Comparison of experimental binodal curves for the systems composed of Tween 20 (1) + ChDHC (2) + H2O (3) (black square) and Tween 20 (1) + ChDHC (2) + HBN (4) (red circle) at 20 °C.

bioactivity parametersa p-coumaric acid (mg/g WS d.b.) ferulic acid (mg/g WS d.b.) DAD TPY (mg GAE/g WS d.b.) DPPH (μmol TRE/g WS d.b.) TEAC (μmol TRE/g WS d.b.) a

0.176 0.091 4.617 0.757 7.551

± ± ± ± ±

0.006 0.002 0.020 0.014 0.840

concluding the absence of any interference modifying the position of the binodal curve, so the system can be effectively used for this kind of hydrolysates. In view of this information, the HBN was extracted at different ATPS compositions located in the longer TL of the system operating at 20 °C in order to attain greater purities in both phases. Additionally, the influence of feed composition in this TL was evaluated, as this factor may strongly affect the partition of phenolic acids due to the final phase volume ratio being varied. Three different feed compositions were studied, as

Values are presented as mean ± SD (n = 3).

Tween 20, and HBN were ascertained, and they are represented in Figure 4 in an orthogonal representation in order to ease the comparison with those obtained for the model systems employing pure water. The experimental data allow 3343


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Figure 5. Phenolic acids extraction of top phase in systems composed of Tween 20 (1) + ChDHC (2) + HBN (4) at 20 °C for different surfactant concentrations in feed stream (F1, F2, and F3). Black and white bars represent p-coumaric and ferulic acid, respectively. Stripped bars stand for the partition coefficients.

Table 3. DPPH, TEAC, TPY, and Extraction Efficiency of Phenols in Top Phase at Different Feed Compositions and 20°C

a

feed

DPPHa (μmol TRE/g WS d.b.)

TEACa (μmol TRE/g WS d.b.)

TPYa (mg GAE/g of WS d.b.)

EEa (%)

F1 F2 F3

0.485 ± 0.018 0.478a ± 0.016 0.458a ± 0.016

5.24 ± 1.19 4.23a ± 0.75 4.26a ± 0.71

1.29 ± 0.14 3.12b ± 0.04 4.51b ± 0.04

28.0 ± 2.9 67.5 ± 0.8 97.6 ± 0.9

a

a

a

Values are presented as mean ± SD (n = 3). In each column, values with different letters are significantly different (p < 0.05).

from lignin hydrolysates maintaining the high levels of antioxidant acitivity observed in the original extracts.

can be noticed in Figure 5. Phenolic acids concentrations, partition coefficient (K), and extraction efficiency (EE) have been monitored. The results are also shown in this figure, and they are listed in Table S4. In all cases, the pH remained constant (slightly greater than 4), so the effect of the feed concentration can be analyzed without the interference of this crucial variable. Generally speaking, it is clear that both pcumaric and ferulic acids preferentially migrate to Tween 20rich phase (K > 1). The reason behind this behavior may be that the pH of the operation is hindering the formation of hydrogen bonds with water molecules, thus promoting the molecule salting out to the top phase, in line with the conclusions reported for different phenolic acids.36 Furthermore, the partition coefficient is always higher for pcoumaric acid than for ferulic acid, even up to about 50%, which is also translated to greater extraction levels (up to 93%). This fact may be attributed to the different hydrophobicity of both molecules, as inferred from their different log P values.37 Thus, the most hydrophobic p-coumaric acid (log P = 1.29) presents a lower hydrogen bonding capacity with water molecules than ferulic acid (log P = 0.78). Finally, the antioxidant activity and total phenolic content were evaluated and compared to the HBN in order to discard possible deleterious effects of the selected system on the bioactivity of the extracts. The data compiled in Table 3 reveal that, although a slight decrease is recorded in DPPH radicalscavenging activity, the TEAC is almost maintained. Apart from that, the TPY and the extraction efficiency of phenols are drastically improved (up to 3.5 times, respectively) when Tween 20 concentration in the feed is increased from 28% to 60% (feed stream 1 to 3). The reason could lie in the existence of an additional volume of surfactant allowing to overcome saturation problems and promoting the preference partition of the product toward the top phase.38 In summary, the present work demonstrates the ability of this biocompatible ATPS to extract p-coumaric and ferulic acids

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CONCLUSIONS The present work has demonstrated the suitability of a completely biocompatible platform consisting of ChDHC and Tween 20 to generate immiscibility windows in aqueous solutions at temperatures from 20 to 60 °C, observing a slight increase in the total biphasic area at more elevated temperatures. After having mapped the tie-lines and binodal curves, all the data were suitably modeled by fitting to mathematical equations. The application of the ChDHC−Tween 20 ATPS to the extraction of p-coumaric and ferulic acid revealed high extraction percentages (>90%) and partition coefficients for high surfactant concentrations in the feed stream, without losing the total phenolic content of a real sample of neutralized delignified WS hydrolysate.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b03188. Experimental and theoretical data of aqueous two-phase systems can be found in Tables S1−S3. Phenolic acids concentrations, partition coefficient, and extraction efficiency (EE) are listed in Table S4. (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: luxavier@fing.edu.uy (L.X.). *E-mail: [email protected] (J.G.-A.). ORCID

J. González-Á lvarez: 0000-0001-9829-9589 3344


Research Article

ACS Sustainable Chemistry & Engineering Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS F.J. Deive thanks the Spanish Ministry of Economy and Competitiveness (MINECO) and Xunta de Galicia for funding through a Ramón y Cajal contract (RyC-2013-14225) and the research project ED431F 2016/007, respectively.

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