Recovery of Syringic Acid from Industrial Food Waste with Aqueous

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Recovery of syringic acid from industrial food waste with aqueous solutions of ionic liquids Emanuelle L. P. de Faria, Ana M. Ferreira, Ana Filipa M. Cláudio, Joao A.P. Coutinho, Armando J.D. Silvestre, and Mara G. Freire ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.9b02808 • Publication Date (Web): 18 Jul 2019 Downloaded from pubs.acs.org on July 19, 2019

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ACS Sustainable Chemistry & Engineering

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Recovery of syringic acid from industrial food

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waste with aqueous solutions of ionic liquids

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Emanuelle L. P. de Faria†, Ana M. Ferreira†, Ana F. M. Cláudio†, João A. P.

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Coutinho†, Armando J. D. Silvestre†, Mara G. Freire†*

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†

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Campus Universitário de Santiago, 3810-193 Aveiro, Portugal.

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*E-mail address: [email protected]

CICECO - Aveiro Institute of Materials, Department of Chemistry, University of Aveiro,

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KEYWORDS. food industrial wastes; syringic acid; extraction; recovery; ionic liquids.

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ABSTRACT. Phenolic acids present in industrial food waste display a broad range of

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biological activities and related health benefits, among which their strong antioxidant and

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free radical scavenger activities are the most investigated. However, food waste is still

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scarcely considered as an alternative source for these compounds, and volatile organic

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solvents for their extraction are still the preferred choice. In this work, aqueous solutions

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of ionic liquids (ILs) with hydrotropic or surfactant character were investigated to

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improve the solubility and effectively extract syringic acid from Rocha pear peels, a

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relevant waste of the food industry. The solubility of syringic acid in aqueous solutions

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of a wide variety of ILs at different concentrations at 30ºC was first ascertained. The

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results obtained show that ILs that behave as cationic hydrotropes are the best option to

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enhance the solubility of syringic acid in aqueous media, with increases in solubility of

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up to 84-fold when compared with water. After identifying the most promising IL

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aqueous solutions, a response surface methodology was used to optimize operational

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extraction conditions (extraction time, solid-liquid (biomass-solvent) ratio and

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temperature), leading to a maximum extraction yield of syringic acid of 1.05 wt.% from

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pear peels. Both the solvent and biomass reuse were additionally investigated,

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demonstrating to overcome the biomass-solvent ratio constraints and mass transfer effects

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and leading to extraction yields of 2.04 and 2.22 wt.%. Although other methods for the

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recovery of syringic acid can be applied, taking advantage of the hydrotropy phenomenon

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and on the solubility of syringic acid dependency with the IL concentration, water was

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used as an anti-solvent allowing to obtain 77% of the extracted phenolic acid. A

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continuous countercurrent process conceptualized for large-scale applications, and that

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further allows the solvent recycling after the recovery of syringic acid, is finally proposed.

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INTRODUCTION

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Biomass is a source of high-value compounds with relevant biological activities such as

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phenolic compounds, one of the most abundant families of secondary metabolites in

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plants.1 The interest in phenolic compounds results from their broad range of biological

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activities and related health benefits, among which the strong antioxidant and free radical

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scavenger activities are well-established.2 These properties are responsible for their

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application in the food, nutraceutical, cosmetic and pharmaceutical industries.3 Examples

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of phenolic compounds present in biomass are, among others, vanillic, gallic,

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protocatechuic, ellagic, and syringic acids, and quercetin, vanillin and resveratrol.1

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Most of these compounds can be obtained from waste generated by agroforest and

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agrofood industries, thus contributing to the full valorization of feedstocks in an

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integrated biorefinery perspective, and, ultimately, contributing towards a circular

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economy.4 Accordingly, the development of sustainable extraction, purification and

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recovery processes of high-value compounds from food industry waste, suitable of

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industrial implementation, is nowadays a crucial need and in line with the United Nations

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Sustainable Development Goals.5-6

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Most of these compounds, and particularly those with recognized health benefits, are of

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high cost due to the complex and multistep methods required for their extraction,

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purification and recovery. The extraction of these compounds from biomass is commonly

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carried out using volatile organic solvents,7 which recently have been combined with

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microwave- and ultrasound-assisted methods to improve the extraction efficiency.8

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Supercritical CO2 extraction and alternative solvents, such as ionic liquids and deep

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eutectic solvents, have also been investigated.8-10 In addition to the extraction step, further

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purification steps are required, typically carried out by solid-phase extraction, back3 ACS Paragon Plus Environment


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extraction with organic solvents, evaporation of the solvents or compounds (when

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applicable), and induced precipitation.9

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Amongst the several solvents that can be used for the extraction of bioactive compounds

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from food industry waste, and taking into account their envisioned consumption by

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humans, water certainly corresponds to the most benign solvent and preferred choice.

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Nevertheless, many bioactive compounds display a limited solubility in water,11

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restricting its use for their effective extraction from biomass. To increase the solubility of

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bioactive compounds in aqueous solutions, additives such as surfactants or hydrotropes

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can be used.12,13 Surfactants have long alkyl side chains, have well-defined critical micelle

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concentration (CMC) values and are able to form micelles. Therefore, surfactants may be

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used in micelle-mediated extraction processes, which are particularly relevant to enhance

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the solubility and to extract highly hydrophobic compounds, such as triterpenic acids.14,15

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On the other hand, hydrotropes do not have a CMC nor form micelles, but can increase

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the solubility of given solutes in aqueous media by the formation of hydrotrope-solute

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aggregates.14 The aggregation of a target solute with a given hydrotrope occurs above a

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minimum hydrotrope concentration and is due to the establishment of favorable

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interactions between the two species, including dispersion forces between less polar

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moieties and specific interactions such as hydrogen-bonding and π–π interactions.

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Hydrotropes are usually anionic or cationic aromatic ring functionalized compounds with

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a sulphate, sulfonate or carboxylate group.16,17 Besides enhancing the solubility of target

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compounds in aqueous media, hydrotropes play an important role in the stabilization of

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aqueous solutions and on tailoring their viscosity.18,19 Furthermore, since the solubility of

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a given solute in aqueous media depends on the hydrotrope concentration, it is possible

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to design strategies for the solute recovery using water as anti-solvent. In addition to the

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well-known ability of designed ILs to act as surfactants, it was recently demonstrated that 4 ACS Paragon Plus Environment

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some ILs in aqueous media behave as hydrotropes.14,20,21 This was shown by determining

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the solubility of vanillin and gallic acid in aqueous solutions of a wide range of ILs

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aqueous solutions, where an increase in the solubility of up to 40-fold was observed when

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using ILs with hydrotropic character.13,14 Dynamic light scattering, nuclear magnetic

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resonance and molecular dynamics simulations studies were additionally employed,

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allowing to confirm the presence of IL-biomolecule aggregates.13,14, 22-25

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Based on the need of developing cost-effective and sustainable processes to extract and

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recover bioactive compounds from industry waste, in this work we investigated a series

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of ILs to improve the solubility of syringic acid in aqueous media and for its extraction

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from Rocha pear peels. The interest in syringic acid is related with its bioactive properties,

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namely antioxidant, antiproliferative, antiendotoxic, antimicrobial, anti-inflammatory,

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and anticancer activities.26 Pears in general are rich in phenolic compounds, such as

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syringic, chlorogenic, ferulic and coumaric acids.27-30 Among these, syringic acid is one

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of the phenolic compounds present at higher concentrations in pears (9.5–21.26 mg/100

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g fresh pears).27-30 The Rocha pear was chosen because of its economic importance for

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the agrofood industry in Portugal, with an average annual production of 173,000 tons.31

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Rocha pear is used to produce juices, jams and other food products, generating vast

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amounts of peel residues.

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We first determined the solubility of syringic acid in aqueous solutions of a wide range

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of ILs and concentrations to identify the most promising ILs, and then applied the best IL

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aqueous solutions to extract the target phenolic acid from pear peels. In order to optimize

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the extraction operational conditions, namely temperature, solid-liquid (biomass-solvent)

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ratio and time of extraction, a factorial planning was applied. Both the solvent and

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biomass reuse have been investigated, allowing to propose a continuous countercurrent

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process conceptualized for large-scale applications, which further allows the solvent 5 ACS Paragon Plus Environment


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recycling after the recovery of syringic acid by the addition of water that acts as anti-

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solvent. For the first time, combined solubility data and extractions from biomass are

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applied to demonstrate the relevance of the hydrotropy phenomenon displayed by ILs.

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EXPERIMENTAL SECTION

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Materials. Syringic acid (> 99 % pure), whose chemical structure is shown in Figure 1,

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was purchased from Sigma-Aldrich and used as received. The water employed was

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double distilled, passed across a reverse osmosis system, and further treated with a Milli-

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Q plus 185 water purification apparatus. A large variety of ionic liquids (ILs) with

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hydrotropic or surfactant character were investigated in aqueous solutions aiming at

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improving the solubility and extraction of syringic acid from pear peels. The chemical

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structures of the investigated ILs are depicted in Figure 1. The ILs investigated were 1-

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butyl-3-methylimidazolium

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methylimidazolium

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methylimidazolium hydrogensulfate ([C₄C1im][HSO4], >98% pure),

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methylimidazolium chloride ([C₄C₁im]Cl, 99% pure), 1-butyl-3-methylimidazolium

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dicyanamide ([C₄C₁im][N(CN)2], >98% pure), 1-butyl-3-methylimidazolium acetate

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([C₄C₁im][Ac], >98% pure), 1-methyl-3-octylimidazolium chloride ([C₈C₁im]Cl, 99%

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pure), 1-butyl-3-methylpyridinium chloride ([C₄C₁py]Cl, 99% pure), 1-butyl-1-

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methylpiperidinium chloride ([C₄C₁pip]Cl, 99% pure), 1-butyl-1-methylpyrrolidinium

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chloride ([C₄C₁pyrr]Cl, 99% pure), tetrabutylammonium chloride ([N4444]Cl, ≥97%

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pure),

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triisobutyl(methyl)phosphonium tosylate ([Pi(₄₄₄)₁][TOS], 98% pure), cholinium chloride

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[Ch]Cl, ≥99% pure), cholinium acetate ([Ch][Ac], 98% pure), cholinium butanoate

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(Ch][But], >97 wt.% pure), cholinium hexanoate ([Ch][Hex], >97 wt.% pure), cholinium

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octanoate ([Ch][Oct], >97 wt.% pure), and cholinium decanoate ([Ch][Dec], >97 wt.%

tosylate

thiocyanate

tetrabutylphosphonium

([C₄C₁im][TOS],

([C₄C₁im][SCN],

chloride

98%

>98%

([P₄₄₄₄]Cl,

pure), pure),

96%

1-butyl-31-butyl-31-butyl-3-

pure),


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pure). The imidazolium-, pyridinium-, piperidinium- and pyrrolidinium-based were

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purchased

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triisobutyl(methyl)phosphonium tosylate were kindly supplied by Cytec Industries Inc.

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Tetrabutylammonium chloride and cholinium chloride were acquired from Sigma-

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Aldrich. With the exception of cholinium acetate that was purchased from Iolitec, the

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remaining cholinium carboxylate ILs were synthesized by us, using the previously

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published protocol.32 Before use, all ILs were dried under vacuum (10-2 Pa) at 30ºC for a

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minimum of 48 h.

from

Iolitec.

The

tetrabutylphosphonium

chloride

and

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Figure 1. Chemical structures of syringic acid and cations/anions of the ionic liquids

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investigated.

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Solubility of syringic acid in ILs aqueous solutions. IL aqueous solutions were

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prepared with concentrations ranging from 0.1 to 4.0 mol‧L-1. Syringic acid was added in 7 ACS Paragon Plus Environment


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excess to each IL aqueous solution and equilibrated under constant stirring and fixed

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temperature using an Eppendorf Thermomixer Comfort equipment. Solubility data were

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determined at 30°C, using optimized equilibration conditions, namely a stirring velocity

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of 750 rpm and an equilibration time of at least 72 h. Samples were then centrifuged in a

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Hettich Mikro 120 centrifuge, during 20 min at 4500 rpm, to separate the macroscopic

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solid (syringic acid) and liquid (IL aqueous solution saturated with syringic acid) phases.

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After centrifugation, samples were placed in an air bath equipped with a Pt 100 probe and

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PID controller at the temperature used in equilibrium assays for more 2 h. Samples of the

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liquid phase were collected and diluted in ultra-pure water, and the amount of syringic

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acid was quantified through UV-spectroscopy, using a SHIMADZU UV-1700, Pharma-

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Spec Spectrometer, at a wavelength of 271 nm, using an established calibration curve.

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Controls containing each IL at the same concentration were used in all experiments as

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control samples. At least three individual samples were prepared and quantified. The

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same procedure was applied to determine the solubility of syringic acid in pure water.

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Extraction of syringic acid from pear peels using IL aqueous solutions. Peels from

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fresh Portuguese Rocha pears, purchased in a local supermarket, were manually removed,

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dried at 25 °C for 2 days, and ground with a commercial coffee grinder. Weighted

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amounts (±10−4 g) of ground pear peels were added to [C₄C₁im]Cl aqueous solutions. This

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IL was used on this set of experiments due to its significant hydrotropic effect and ability

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to improve the solubility of syringic acid in aqueous media, as identified in the previous

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set of experiments. Operational conditions, namely temperature, solid-liquid (biomass-

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solvent) ratio, and time of extraction were optimized by a 23 factorial planning to

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simultaneously analyze various operational conditions and to identify the most significant

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parameters that enhance the syringic acid extraction yield.33 Student’s t-test was used to

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evaluate the statistical significance of the adjusted data. The suitability of the model was 8 ACS Paragon Plus Environment

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determined by evaluating the lack of fit, the regression coefficient (R2) and the F-value

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obtained from the analysis of variance (ANOVA). In the factorial planning the central

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point was experimentally considered at least in triplicate. Additional 12-20 experiments

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per factorial planning were carried out, for which several operational conditions were

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repeated to guarantee the accuracy of the data. The StatSoft Statistica 10.0© software was

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used for all statistical analyses. Further details related with the 23 factorial planning used

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are provided in the Supporting Information.

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After the extraction step, the IL aqueous solutions were separated from the biomass by

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centrifugation (at 5000 rpm for 10 min using an Eppendorf centrifuge 5804), and the

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supernatant was filtered using a 0.20 μm syringe filter. A 200 μL aliquot was taken, mixed

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with 800 μL of mobile phase (ultra-pure H2O + 0.2% acetic acid) used in the HPLC-DAD

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analysis, and filtered over a 0.2 μm syringe filter. The quantification of syringic acid in

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each solution was carried out using a HPLC-DAD (Shimadzu, model PROMINENCE).

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HPLC analyses were performed with an analytical C18 reversed-phase column (250 ×

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4.60 mm), Kinetex 5 μm C18 100 Å, from Phenomenex. The mobile phase consisted of

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77.5% of ultra-pure H2O + 0.2% acetic acid and 22.5% of acetonitrile. The separation

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was conducted in isocratic mode, at a flow rate of 1.0 mL‧min−1 and using an injection

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volume of 10 μL. DAD was set at 271 nm. Each sample was analyzed at least in duplicate.

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The column oven and the autosampler operated at 30°C. Calibration curves were prepared

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using pure and commercial syringic acid aqueous solutions. For comparison purposes,

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methanol and dichloromethane were also used as solvents to carry out the extraction of

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syringic acid from pear at the optimized conditions. The reported syringic acid extraction

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yield corresponds to the percentage ratio between the weight of pure syringic acid

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extracted and the total weight of dried biomass.



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Reusability of biomass and solvent, and syringic acid recovery. In order to develop a

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sustainable extraction and recovery process, as well as to infer the solvent saturation

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effects and maximum amount of the target compound present in the biomass, two

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strategies were investigated: (i) reuse of the biomass employing 5 new aqueous solutions

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of ILs to the same biomass sample; and (ii) reuse of the solvent by applying 5 successive

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cycles of extraction using the same IL aqueous solution. Both approaches were applied at

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the optimum operational conditions. These assays also allowed us to propose the use of

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IL aqueous solutions in a continuous countercurrent mode extraction process.

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After 5 extraction cycles with the reuse of the IL aqueous solution, which allowed the

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solvent saturation, water was finally added as an anti-solvent, inducing the precipitation

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and recovery of syringic acid. Testes were carried out by the addition of 1, 5, 10, 15 and

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25 mL of distilled water to 0.5 mL of the IL aqueous solutions containing syringic acid

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after the extraction step. After, the solution was centrifuged at 5000 rpm for 15 min, and

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vacuum filtered with a 0.45 μm microporous membrane. HPLC-DAD analysis of the

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aqueous solutions was carried out to determine the recovery yield of syringic acid by

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precipitation. The recovery yield corresponds to the percentage ratio between the total

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weight of precipitated syringic acid and the weight of syringic acid initially present in the

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IL aqueous solution. The obtained precipitate/solid residue was dried in an air oven at

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room temperature for 2 days, and further analyzed by HPLC-DAD to guarantee its

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stability and purity.

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RESULTS AND DISCUSSION

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Solubility of syringic acid in ILs aqueous solutions. In a first set of experiments we

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determined the solubility of syringic acid in aqueous solutions of ILs at concentrations

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ranging from 0.1 to 4.0 mol‧L-1, at 30°C, aiming at better understanding the role of ILs to 10 ACS Paragon Plus Environment

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improve the solubility of natural phenolic acids in aqueous media and to identify the most

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promising IL aqueous solutions to proceed with the syringic acid extraction from food

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industry waste, namely pear peels. The solubility results are given in Table S2 in the

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Supporting Information. The solubility of syringic acid at 30ºC in pure water determined

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by us is 1.43 ± 0.08 g‧L-1 (0.0072 mol‧L-1). The effect of the IL chemical structure and IL

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concentration on the solubility of syringic acid at 30ºC is depicted in Figure 2. These

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results are shown as solubility enhancement, namely S/S0, where S and S0 represent the

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solubility of the syringic acid in each IL aqueous solution and in pure water, respectively.

Figure 2. Solubility enhancement of syringic acid in aqueous solutions of ILs (a): ( ) [C₄C₁im]Cl, ( ) [C8C1im]Cl, ( ) [P4444]Cl, ( ) [N4444]Cl, ( ) [C4C1pyrr]Cl, ( ) [C4C1pip]Cl, ( ) [C4C1py]Cl, ( ) [Ch]Cl); (b) ( ) [C₄C₁im]Cl, ( ) [C₄C₁im][TOS], ( ) [C₄C₁im][HSO4], ( ) [C₄C₁im][N(CN)2], ( ) [C₄C₁im][SCN], ( ) [C₄C₁im][Ac], ( ) [Pi(444)1][TOS], ( ) [P4444]Cl, ( ) [Ch][Cl], ( ) [Ch][Ac], ( ) [Ch][But], ( ) [Ch][Hex], ( ) [Ch][Oct] and ( ) [Ch][Dec]). Lines have no physical meaning and are only guides for the eye. 237

The results obtained reveal a synergetic effect of the two solvents (water and IL) on the

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solubility of syringic acid, where a maximum in solubility occurs along the IL



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concentration. The IL concentration where this maximum occurs depends however on the

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IL chemical structure and hydrotrope or surfactant behavior, with surfactants usually

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performing better at lower concentrations. A similar behavior was reported by Cláudio et

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al.14 on the solubility of vanillin and gallic acid in aqueous solutions of ILs. Amongst all

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the ILs studied, [C₄C₁im]Cl is the best IL identified with an increase in the solubility of

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syringic acid in aqueous media up to 84-fold (120.3 ± 0.3 g‧L-1 in the IL aqueous solution

245

versus 1.43 ± 0.08 g‧L-1 in pure water).

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Figure 2a shows results for the following ILs: [C₄C₁im]Cl, [C8C1im]Cl, [P4444]Cl,

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[N4444]Cl, [C4C1pyrr]Cl, [C4C1pip]Cl and [C4C1py]Cl, which share a common anion

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(chloride), thus allowing to evaluate the IL cation effect on enhancing the syringic acid

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solubility. Among these only [C8C1im]Cl presents a surfactant behavior, with a

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previously reported critical micellar concentration (CMC) of 238 mM,34 whereas the

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remaining ILs of this series are expected to act as hydrotropes. The overall capacity of

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these ILs to improve the solubility of syringic acid (appraised at the maximum solubility

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of syringic acid) follows the order: [C₄C₁im]Cl > [C4C1py]Cl > [P4444]Cl > [C4C1pip]Cl

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> [N4444]Cl > [C8C1im]Cl > [C4C1pyrr]Cl > [Ch]Cl. The obtained trend reflects the ability

255

of the IL cation to act as hydrotrope in the solubility of syringic acid. With the exception

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of cholinium- and pyrrolidinium-based ILs that perform worst than the surfactant

257

[C8C1im]Cl, all remaining ILs display a significant ability to increase the solubility of

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syringic acid while behaving as hydrotropes. In particular, with [Ch]Cl the solubility of

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syringic acid only slightly increases, reaching a maximum of solubility enhancement of

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1.64, suggesting that neither [Ch]+ nor Cl− have a significant effect on improving the

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solubility of the target phenolic compound. When comparing the solubility of syringic

262

acid in aqueous solutions containing [C₄C₁im]Cl and [C8C1im]Cl, it is clear that ILs with


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263

shorter alkyl chains and with hydrotrope characteristics perform better at enhancing the

264

solubility.

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When using ILs with surfactant characteristics, the solubility enhancement occurs due to

266

the IL self-aggregation and possibility of incorporating hydrophobic solutes in the micelle

267

core. With this type of ILs, the enhancement of solubility is more pronounced at low

268

concentrations of IL, close to the critical micellar concentration, but seems to quickly

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saturate at low concentrations above which a much weaker effect on the solubility of the

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syringic acid is observed. This behavior was observed by Cláudio et al.14 while evaluating

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the variation of the solubility of vanillin in aqueous solutions of [CnC1im]Cl ILs, with 2

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≤ n ≤ 14, concluding that ILs with shorter alkyl chain lengths ([CnC1mim]Cl, n = 2–6)

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behave as hydrotropes, whereas ILs with longer alkyl side chains ([CnC1im]Cl, n = 8–

274

14)34,35 behave as surfactants. However, the data corresponding to [C8C1im]Cl and

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[Ch][Oct] (cf. Figure 2a and discussion below on the IL anion effect) seem to indicate the

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presence of combined effects and mechanisms, particularly when dealing with

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moderately hydrophobic species such as syringic acid. These combined effects seem to

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appear with ILs with alkyl side chains that are in the length threshold to form micelles,

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being this the case of [C8C1im]Cl and [Ch][Oct]. Hydrotropes cannot form micelles, and

280

no competitive or combined effects occur with these. However, for ILs with surfactant

281

behavior and depending on the solute hydrophobicity, the solubility may be enhanced by

282

the formation of micelles and by the formation of solute-IL aggregates. For instance, with

283

[Ch][Dec] that has a lower CMC, the maximum solubility value of syringic acid occurs

284

at a concentration of IL < 0.5 M, whereas for ILs acting as hydrotropes the maximum

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solubility values occur for concentrations of IL > 2 M. With [C8C1im]Cl and [Ch][Oct],

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the maximum in the solubility of syringic acid occurs in between, thus indicating that

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combined effects of both phenomena may exist. 13 ACS Paragon Plus Environment


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Although the formation of solute–hydrotrope complexes based on π…π interactions

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(between the aromatic moieties of the substrate and aromatic hydrotropes) has been used

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to explain the hydrotropy concept,36,37 the results obtained with non-aromatic ILs, such

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as [N4444]Cl, [P4444]Cl, [C4C1pip]Cl and [C4C1pyrr]Cl, reveal that the hydrotropic effect

292

cannot be explained as resulting only from this type of interactions. This observation is

293

in agreement with the findings of Cláudio et al.,14 as well as with other works in which

294

conventional hydrotropes have been studied.38-41 Overall, hydrotropes enhance the

295

solubility of the target solute by the formation of solute–hydrotrope aggregates, which

296

are formed due to favorable interactions between the two species, including dispersion

297

forces between less polar moieties and specific interactions such as hydrogen-bonding

298

and π–π interactions.

299

Figure 2b shows the results in solubility enhancement for the two following series of ILs:

300

(i) [C₄C₁im]Cl, [C₄C₁im][TOS], [C₄C₁im][HSO4], [C₄C₁im][N(CN)2], [C₄C₁im][SCN],

301

and [C₄C₁im][Ac], and (ii) [Pi(444)1][TOS] and [P4444]Cl. These two sets of ILs share a

302

common cation, allowing to address the IL anion effect on the syringic acid solubility. In

303

the same figure additional results for the ILs [Ch][Ac], [Ch][But], [Ch][Hex], [Ch][Oct]

304

and [Ch][Dec] are given to address the effect of carboxylate-based anions combined with

305

the cholinium cation to improve the syringic acid solubility in water. Among these,

306

[Ch][Oct] and [Ch][Dec] display surface-active properties.42,43

307

All ILs investigated with the [C₄C₁im]+ cation display an important hydrotropic effect,

308

thus improving the solubility of syringic acid in aqueous media according to the following

309

anion trend (taken at the maximum solubility enhancement): Cl- > [Ac]- > [TOS]- >

310

[N(CN)2]- > [SCN]- > [HSO4]-. With the [P4444]-based ILs, the same trend was observed

311

for the ILs comprising the Cl- and [TOS]- anions. If compared with previous trends

312

obtained for ILs used to improve the solubility of vanillin and gallic acid,14 these results 14 ACS Paragon Plus Environment

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313

again confirm that the hydrotropy dissolution is solute dependent. According to the

314

literature, most of the industrially used hydrotropes are anionic, and often contain a

315

phenyl group, such as tosylate.44 However, according to the results shown in Figure 2b,

316

[C₄C₁im]Cl and [C₄C₁im][Ac] perform better than [C₄C₁im][TOS], and [P4444]Cl performs

317

better than [Pi(444)1][TOS], demonstrating that tosylate does not act as an hydrotrope able

318

to improve the solubility of syringic acid in aqueous solutions. As observed with the

319

cationic hydrotropes previously discussed, this result further supports the idea that π⋯π

320

interactions do not contribute to the hydrotropic effect.38-40,45 Although chloride is not an

321

hydrotrope, and has a marginal effect upon the solubility of syringic acid as shown by the

322

results of [Ch]Cl, aqueous solutions of [C₄C₁im]Cl are the best solvent identified. All

323

these results therefore indicate that for syringic acid there is not a synergetic effect of both

324

IL ions to improve its solubility, as it would be expected by using ILs where both ions

325

have hydrotrope characteristics, e.g. [C₄C₁im][TOS] and [C₄C₁im][N(CN)2].

326

It has been previously reported that the solubility enhancement by hydrotropes is more

327

effective for more hydrophobic solutes,14 as ascertained when comparing the solubility

328

of vanillin and gallic acid (with values of the logarithm of the octanol-water partition

329

coefficient, log(Kow), as follows: 1.23 and 0.72, respectively46). Syringic acid has an

330

intermediate log(Kow) value of 1.04,46 but more significant improvements in solubility

331

have been observed applying ILs as hydrotropes if compared to vanillin and gallic acid.

332

Furthermore, in the current work, the IL cation plays a major role on the design of

333

effective hydrotropes. These results suggest that the strength of interactions between ILs

334

and phenolic compounds are different, being solute-specific, which could be however

335

useful to design selective solvents and effective recovery strategies.



Page 16 of 33

336

In addition to [C₄C₁im]-based ILs, cholinium-based ILs have also been investigated. Both

337

short alkyl side chain cholinium carboxylates (acetate, butanoate and hexanoate) that

338

could act as hydrotropes, and long alkyl side chain cholinium carboxylate (octanoate and

339

decanoate), able to form micelles in aqueous solutions and with reported CMCs,42,43 were

340

studied. Contrarily to the [C8C1im]Cl previously discussed that is a cationic surfactant,

341

[Ch][Oct] and [Ch][Dec] are anionic surfactants. The results obtained show that the

342

maximum in the syringic acid solubility occurs at lower IL concentrations as the alkyl

343

side chain at the carboxylate anion is increased. Furthermore, the maximum in solubility

344

enhancement occurs with [Ch][Hex] and [Ch][Oct], but strongly decreases with the better

345

surfactant – [Ch][Dec]. The surfactant-based cholinium carboxylate ILs perform better

346

than [C8C1im]Cl, meaning that anionic surfactants are more appropriate to increase the

347

solubility of syringic acid in aqueous media. However, favorable and specific interactions

348

occurring between the carboxylate anions and syringic acid cannot be discarded since

349

[C₄C₁im][Ac] is one of the best ILs (after [C₄C₁im]Cl) identified in the imidazolium-

350

based series of ILs. Still, [C₄C₁im][Ac] is a better hydrotrope to enhance the solubility of

351

syringic acid than [Ch][Ac], reinforcing the relevance of cationic IL hydrotropes. These

352

results disclose that the IL cation and anion may have different roles, being the hydrotropy

353

effect towards syringic acid dominated by the IL cation. Overall, according to the results

354

discussed above and considering that aqueous solutions of [C₄C₁im]Cl at 3.5 mol‧L-1 were

355

identified as the best solvent, able to increase 84-fold the solubility of syringic acid when

356

compared to pure water, aqueous solutions of this IL were used to optimize the

357

operational conditions of extraction of syringic acid from Rocha pear peels by a response

358

surface methodology, as described below.

359

Extraction of syringic acid from Rocha pear peels using aqueous solution of ILs.

360

Although pure ILs have been described as solvents for the extraction of value-added 16 ACS Paragon Plus Environment

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361

compounds from biomass, e.g. ellagic acid,47 artemisinin,48 limonene,49 betulin,50 among

362

others, aqueous solutions of ILs display a high potential and additional advantages due to

363

the use of lower amounts of IL, with intrinsic environmental and economic benefits.

364

Aqueous solutions of ILs are solvents of lower toxicity and cost when compared to the

365

respective pure ILs, while decreasing the overall viscosity of the solvent and enhancing

366

mass transfer. Furthemore, aqueous solutions of ILs are more selective, not allowing the

367

dissolution of all biomass. Accordingly, whenever possible, IL aqueous solutions should

368

be used instead of pure ILs.

369

After evaluating the best IL aqueous solutions as solvents for syringic acid, a factorial

370

planning of 23 (3 factors and 2 levels) was used to optimize the operational conditions for

371

the syringic acid extraction from Rocha pear peels. The results obtained were analyzed

372

statistically with a confidence level of 95% (data shown in the Supporting Information –

373

Table S5 and Figure S1). This methodology allows the study of the relationship between

374

the response (extraction yield of syringic acid) and the independent variables/operational

375

conditions that influence the extraction yield, namely extraction time (t, min), biomass-

376

solvent weight ratio (S/L ratio, weight of dried biomass per weight of solvent) and

377

temperature (T, ºC). The extraction yield corresponds to the percentage ratio between the

378

weight of extracted syringic acid and weigth of dried biomass (pear peels). Based on the

379

solubility results shown in Figure 2 and maximum solubility enhancement of syringic

380

acid of 84-fold, aqueous solutions of [C₄C₁im]Cl at 3.5 mol‧L-1 were used. Additional

381

studies by increasing the IL concentration were not carried out since the maximum in

382

solubility occurs at 3.5 M and because an increase in the IL concentration increases the

383

solution viscosity, thus not favorably contributing to the mass transfer phenomenon.

384

Furthermore, an increase in the IL concentration increases the solvent cost and toxicity.46



Page 18 of 33

385

Variance analysis (ANOVA) was used to estimate the statistical significance of the

386

variables and their interactions. The experimental points used in the second factorial

387

planning, the model equation, the extraction yield of syringic acid obtained

388

experimentally and the respective calculated values using the correlation coefficients

389

obtained in the statistical treatment, as well as all the statistical analysis, are shown in the

390

Supporting Information. Based on the statistical model and results (Tables S4 and S5 in

391

the Supporting Information) given in detail in the Supporting Information, the average

392

relative deviation between the experimental and the predicted values is 0.45%, supporting

393

the good description of the experimental results by the statistical model.

394

The influence of the three variables on the extraction yield of syringic acid is illustrated

395

in Figure 3, with the respective detailed data given in Table S4 in the Supporting

396

Information. It is evident that extraction time is a significant parameter leading to a region

397

of maximum yield of extraction at 60 min. The solid-liquid ratio also has a relevant impact

398

in the syringic acid extraction yields, increasing by increasing the solvent volume.

399

Additionally, higher temperatures are more efficient for the extraction of syringic acid

400

from pear peels, although this is the variable with the weakest influence on the extraction

401

yield of syringic acid (between 45 and 60ºC). Overall, the parameters that have a higher

402

impact on the extraction yield of syringic acid (as can be seen in the pareto chart, Figure

403

S1 in the Supporting Information), are the extraction time and the solid-liquid ratio. The

404

optimized operational conditions found for the extraction of syringic acid occur at a

405

temperature of 50ºC, an extraction time of 60 min, and a solid-liquid ratio of 0.10,

406

providing a syringic acid extraction yield of 1.05 wt.%.


Page 19 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


407 408

Figure 3. Response surface plots (left) and contour plots (right) on the extraction yield of

409

syringic acid with the combined effects of (a) T (ºC) and t (min), (b) T and S/L ratio and

410

(c) S/L ratio and t, using aqueous solutions of [C₄C₁im]Cl at 3.5 M.

411

Some organic solvents were also evaluated at the optimized conditions for comparison

412

purposes, with dichloromethane and methanol leading to extraction yields of 1.51 wt.%

413

and 1.68 wt.%, respectively. Although a lower extraction yield is obtained with IL 19 ACS Paragon Plus Environment


Page 20 of 33

414

aqueous solutions, these are still competitive solvents since the use of volatile organic

415

solvents is avoided. Furthermore, by reusing the IL aqueous solutions similar yields are

416

obtained, as shown below, supporting their high potential as alternative solvents to extract

417

syringic acid from biomass.

418

The two last sets of results on the solubility and extraction of syringic acid from biomass

419

suggest that the high performance demonstrated by ILs aqueous solutions may be a major

420

outcome of the enhanced solubility afforded by ILs with hydrotrope characteristics, and

421

not only from the biomass disruption as usually reported.

422

Reusability of biomass and solvent, and syringic acid recovery. Aiming to infer the

423

maximum amount of syringic acid present in biomass, the same sample of Rocha pear

424

peels was sequentially extracted with “fresh” IL aqueous solutions using the optimum

425

conditions previously identified in five successive extraction cycles. The results obtained

426

are shown in Figure 4a and given in detail in the Supporting Information – Table S6. It is

427

shown that the syringic acid present in the biomass sample is not fully extracted in the

428

first cycle (1.05 wt.%) of extraction and it is possible to achieve a maximum yield of 2.22

429

wt.% after 5 extraction cycles with “fresh” IL aqueous solutions. Thus, in a single

430

extraction approach half of syringic acid still remains in biomass; however, if several

431

extraction cycles are implemented the total extraction yield can surpass the ones obtained

432

with conventional organic solvents.

433

The reusability of the extraction solvent using new Rocha pear peels was also investigated

434

for 5 cycles to maximize the cost-efficiency and sustainability character of the developed

435

process (Figure 4b; detailed results given in the Supporting Information - Table S7). After

436

each extraction, the solid-liquid mixture was filtered, and the IL aqueous solution was

437

reused with new pear peel samples. As summarized in Figure 4b, the extraction yield 20 ACS Paragon Plus Environment

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438

obtained in the first extraction cycle (1.07 wt.%) could be improved by reusing the solvent

439

and applying it to new biomass samples, achieving a yield of 2.04 wt.% after the fifth

440

cycle of reuse of the IL aqueous solution.

441 442

Figure 4. Syringic acid extraction yields from Rocha pear peels with the biomass (a) and

443

solvent (b) reuse at the optimized operational conditions.

444

These improvements in the extraction yield were not identified before in the factorial

445

planning due to constraints of the solid-liquid ratio and mass transfer phenomenon, but if

446

both the solvent and biomass are reused a significant increment on the extraction yields

447

is obtained. Accordingly, an extraction continuous process operating in countercurrent,

448

in which the solvent and biomass are reused in a continuous mode seems to be the most

449

adequate option to be applied in large-scale applications. A schematic representation of

450

the envisioned process is given in Figure 5.



Page 22 of 33

SOLVENT RECYCLING AND SYRINGIC ACID RECOVERY H2O IL aqueous solution (1.5 M)

Distillation Biomass

IL aqueous solution recovery

N+1

N

1

Biomass

SYRINGIC ACID

SOLID-LIQUID EXTRACTION

451 452

Figure 5. Scheme of the continuous extraction process operating in countercurrent.

453

Although the use of aqueous solutions of aprotic ILs has shown to be an alternative over

454

pure ILs to extract added-value compounds from biomass, the non-volatile nature of

455

aprotic ILs represents a major drawback when envisaging the target compound recovery

456

since a simple evaporation step cannot be applied. Based on this limitation and knowing

457

that the solubility of the target compound largely depends on the IL (hydrotrope)

458

concentration as shown in Figure 2, the recovery of syringic acid from the IL aqueous

459

solution was carried out by dilution with water, which acts as anti-solvent. At the end of

460

the fifth extraction cycle of the previous experiments (reuse/saturation of the solvent), the

461

saturated IL aqueous solution (0.5 mL) was diluted with increased volumes of water (1,

462

5, 10, 15 and 25 mL) (cf. the Supporting Information, Table S8 and Figure S2). By

463

applying this process, 77% of the extracted syringic acid can be recovered by precipitation

464

by the addition of the largest volume of water, and with high purity (≈ 94%) as shown in

465

the chromatogram given in the Supporting Information – Figure S3. Impurities addressed

466

mainly comprise other phenolic acids and aromatic compounds absorbing in the same


Page 23 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


467

wavelength and present in pear peels, such as arbutin, gallic acid, (+)-catechin,

468

chlorogenic acid, caffeic acid, (-)-epicatechin, coumaric acid and ferulic acid.30 Finally,

469

the recovery of syringic acid by this process allows the elimination of the IL from syringic

470

acid, thus overcoming the toxicity concerns that could be associated to ILs, and the IL

471

recycling after an evaporation step to decrease the water content, as shown in the process

472

given in Figure 5. Although the addition of a large amount of water to recover syringic

473

acid may not be the most sustainable approach, syringic acid can be recovered by other

474

approaches, such as solid-phase extraction, as carried out by Quental et al.51 to recover

475

phenolic acids from IL aqueous solutions.

476

Even though syringic acid obtained by synthetic routes is not an expensive compound,

477

the same is not true for syringic acid obtained from natural sources. Nowadays, there is a

478

high demand of natural products with relevant bioactive properties by the food and

479

cosmetic industries52, justifying the interest behind the developed and proposed process.

480

Furthermore, it is usually claimed that ILs are expensive compounds, thus compromising

481

their application at a large-scale. It should be remarked that if obtained at a large scale, a

482

low-cost ILs was applied - a cost analysis for the application of several ILs at a large scale

483

can be found in the literature.9

484

Overall, the results obtained demonstrate that aqueous solutions of ILs with hydrotropic

485

characteristics are improved solvents to extract syringic acid from biomass. Furthermore,

486

it is possibility to recover syringic acid by the addition of water inducing its precipitation,

487

while allowing the IL recycling.

488

CONCLUSIONS

489

It was shown that aqueous solutions of ILs that behave as cationic hydrotropes are

490

remarkable solvents for syringic acid, leading to enhancements up to 84-fold in solubility 23 ACS Paragon Plus Environment


Page 24 of 33

491

when compared to pure water. Due to these solubility enhancements, the same ILs

492

aqueous solutions lead to good extraction yields of syringic acid from pear peels. The

493

operational extraction conditions were optimized by factorial planning resulting in a

494

maximum extraction yield of 1.05 wt.%. With the goal of developing sustainable

495

extraction strategies, both the biomass and the IL aqueous solutions were reused, allowing

496

to reach extraction yields of syringic acid of 2.04 wt.% and 2.22 wt.%, respectively. These

497

values are significantly higher than those obtained with dichloromethane and methanol at

498

the same operational conditions (1.51 wt.% and 1.68 wt.%, respectively). Based on these

499

results, and possibility of reusing both the solvent and biomass, a continuous process was

500

conceptualized and proposed for large-scale applications, acting in continuous and

501

countercurrent mode, in which the solvent is recycled after the recovery of syringic acid

502

by precipitation with water as anti-solvent. This strategy allows to recover 77% of the

503

extracted phenolic acid. The results reported here have a significant impact on the

504

understanding of the role of ILs aqueous solutions in the extraction of added-value

505

compounds from biomass and on the design of processes for their recovery based on the

506

hydrotropy concept.

507

ASSOCIATED CONTENT

508

Supporting Information. Additional information on the factorial planning, solubility of

509

syringic acid in ILs aqueous solutions, experimental points used in the factorial planning,

510

model equations, yields of syringic acid experimentally obtained, statistical analysis

511

connected to the response surface methodology, yield of syringic acid by the reuse of

512

biomass and IL aqueous solution, percentage recovery of syringic acid with the addition

513

of water and HPLC-DAD chromatogram of the recovered syringic acid.

514

AUTHOR INFORMATION


Page 25 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


515

Corresponding Author

516

*E-mail address: [email protected]; Tel: +351-234-401422; Fax: +351-234-370084;

517

Notes

518

The authors declare no competing financial interest.

519

ACKNOWLEDGMENT

520

This work was developed within the scope of the project CICECO-Aveiro Institute of

521

Materials, FCT Ref. UID/CTM/50011/2019, financed by national funds through the

522

FCT/MCTES, and projects Multibiorefinery (POCI-01-0145-FEDER-016403) and Deep

523

Biorefinery (PTDC/AGR-TEC/1191/2014), financed by national funds through the

524

FCT/MEC and when appropriate co-financed by FEDER under the PT2020 Partnership

525

Agreement. E.L.P. Faria acknowledges financial support from CNPq (Conselho Nacional

526

de Desenvolvimento Científico e Tecnológico) for the PhD grant (200908/2014-6). This

527

work was presented at the 13th International Chemical and Biological Engineering

528

Conference (CHEMPOR 2018). The authors acknowledge the Scientific and Organizing

529

Committees of the Conference for the opportunity to present this work.

530

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531

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532

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For Table of Contents Use Only

SOLVENT RECYCLING Ionic liquid aqueous solution

SOLID-LIQUID EXTRACTION

1

FOOD WASTE

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


689

Pear Peels

H2O

N+1 N Table S29 in the Supporting Information.

Pear Peals

SYRINGIC ACID RECOVERY

HYDROTROPY: Solubility and Extraction Enhancement

690

Synopsis: Aqueous solutions of ionic liquids with hydrotrope characteristics increase

691

both the solubility and extraction yield of syringic acid from food industry waste.


Recovery of Syringic Acid from Industrial Food Waste with Aqueous

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