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Novel phase diagrams of aqueous two-phase systems based on tetrahydrofuran + carbohydrates + water: Equilibrium data and partitioning experiments
Accepted Manuscript Novel phase diagrams of aqueous two-phase systems based on tetrahydrofuran + carbohydrates + water: Equilibrium data and partitioning experiments Kênia M. Sousa, Gustavo E.L.O. Maciel, Filipe S. Buarque, Adriana J. Santos, Maria N. Marques, Eliane B. Cavalcanti, Cleide Mara F. Soares, Álvaro S. Lima PII:
S0378-3812(16)30548-9
DOI:
10.1016/j.fluid.2016.11.001
Reference:
FLUID 11311
To appear in:
Fluid Phase Equilibria
Received Date: 8 August 2016 Revised Date:
28 October 2016
Accepted Date: 1 November 2016
Please cite this article as: K.M. Sousa, G.E.L.O. Maciel, F.S. Buarque, A.J. Santos, M.N. Marques, E.B. Cavalcanti, C.M.F. Soares, E.S. Lima, Novel phase diagrams of aqueous two-phase systems based on tetrahydrofuran + carbohydrates + water: Equilibrium data and partitioning experiments, Fluid Phase Equilibria (2016), doi: 10.1016/j.fluid.2016.11.001. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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TOC – Graphical Abstract
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Water Tetrahydrofuran
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Carbohydrate
Diuron
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Novel phase diagrams of aqueous two-phase systems based on tetrahydrofuran +
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carbohydrates + water: equilibrium data and partitioning experiments
3 4 Kênia M. Sousaa, Gustavo E.L.O. Maciela, Filipe S. Buarquea, Adriana J. Santosb, Maria N.
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Marquesa,b, Eliane B. Cavalcantia,b, Cleide Mara F. Soaresa,b, Álvaro S. Limaa,b,*
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a
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SE, Brazil
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b
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SE, Brazil
Instituto de Tecnologia e Pesquisa. Av. Murilo Dantas, 300. CEP: 49032-490, Aracaju –
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Universidade Tiradentes, Av. Murilo Dantas 300, Farolândia. CEP: 49032-490, Aracaju-
*To whom correspondence should be addressed: E-mail [email protected]. Phone: +55
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7932182115. Fax: +55 7932182190.
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ABSTRACT This work addresses the partitioning of diuron, an herbicide heavily employed in
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agriculture, using the aqueous two-phase system formed by tetrahydrofuran and
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carbohydrates. Phase diagrams using tetrahydrofuran (THF) and six monosaccharides, two
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disaccharides and three commercial sugars at 298.15 K and 0.1 MPa were constructed. The
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equilibrium phases were further characterized by determining the density and viscosity at
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283.15–333.15 K and 0.1 MPa. Finally, the diuron extraction was assessed by the partition
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coefficient (KDIU) and recovery in the top-phase (RT). The number of equatorial hydroxyl
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groups present in the carbohydrates’ structures is the driving force for the phase separation;
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disaccharides are stronger inducers of the ATPS formation than monosaccharides. The
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differences in the density and viscosity of the phases allowed an easy phase separation. In
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all cases, the diuron partitions preferentially to the THF-rich phase with KDIU ranging from
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1.23 (commercial glucose) to 16.19 (pure glucose) and recovery between 49.45 %
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(commercial glucose) and 92.70 % (pure glucose), demonstrating the applicability in the
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partition of diuron.
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Keywords: partitioning; tetrahydrofuran; carbohydrates; herbicide; extraction.
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1. Introduction Aqueous two-phase systems (ATPS) were first introduced by Beijernick in 1896 [1].
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In the last few decades, they have become an option in liquid-liquid extraction systems due
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to their characteristics such as low interfacial tension and good mass transfer, which allow
5
high yields; their environmentally friendly features (e.g., high water content); and the ease
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of scaling up [2,3]. A wide range of molecules have been successfully separated, recovered
7
and purified using ATPS, such as enzymes (pectinase [4], lipase [5]), antibiotics [6],
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antibodies [7,8], alkaloids (gallic acid [9] and quinine [10]), metals [11], and dyes [12].
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ATPS can be formed when two structurally different compounds are dissolved in
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water above a certain concentration [13]. The success of an ATPS depends on the choice of
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system constituents, which are usually two polymers (dextran + dextran, dextran +
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polyethylene glycol (PEG), dextran + Ficoll and PEG/maltodextrin [14,15]); polymer +
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salt (PEG + potassium phosphate [16]), and recently, ionic liquids + salts [17–21], ionic
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liquids + sugar [22], ionic liquids + PEG [23,24], organic solvents + salts (alcohol +
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potassium phosphate [25], organic solvents + carbohydrates/polyols (acetonitrile +
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carbohydrates/polyols [26,27] and organic solvent + dextran [28].
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Recently, tetrahydrofuran (THF) has also been proposed as a constituent of
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ATPS [29,30]. THF (C4H8O) is a cyclic ether that is miscible with water in all
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proportions at room temperature [31,32] and is highly flammable (flashpoint -21.2
20
°C), with a vapor pressure of 21.6 kPa at 25 °C [33]. This organic solvent is employed
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as a solvent of dyes and lacquers, a precursor in the synthesis of succinic acid, a
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reaction medium, and a polymerization solvent for fat oils, among other applications
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[34].
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The goal of this study is to associate carbohydrate and THF to develop a novel
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aqueous two-phase system for use in the partition of water-soluble herbicides such as
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ACCEPTED MANUSCRIPT diuron. It is well known that carbohydrates do not present any environmental or health
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concerns, and according to Banton and co-workers [33], THF does not have an
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adverse impact on human health and the environment. In animals, the acute toxicity
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potential is low to moderate by different routes. Although absorption through the skin
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is rapid, skin irritation is slight. Genetic toxicity has been negative, as well as
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reproductive toxicity, but it produces benign tumors in rats, which is not relevant to
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human health. Therefore, it can be said that the components of the system do not have
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any risk to human health and the environment. Thus, these ATPS based on
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carbohydrates and THF may be used for the separation, extraction/recovery or
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detection of diuron from aqueous media.
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Diuron (N-(3,4-dichlorophenyl)-N,N-dimethyl-urea) is a crystalline compound
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belonging to the phenylamide group [35]. According to Giacomazzi and Cochet [36],
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diuron is a non-ionic and non-volatile substance whose melting point is 158–159 °C
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and water solubility is 42 mg L-1. Its octanol-water partition coefficient (log Kow = 2.6)
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characterizes it as a hydrophobic compound, which has been widely used in pre- or
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post-emergent treatment in agricultural crops [37]. The presence of diuron in drinking
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water and food is extremely harmful to human health due to its carcinogenicity [38],
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which makes this herbicide a dangerous aquatic pollutant.
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2. Material and Methods
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2.1 Materials
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Tetrahydrofuran - THF (HPLC grade, 99.9 wt%) was purchased from Sigma-
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Aldrich®. The carbohydrates used here were D-(+)-xylose (99 wt%), D-(-)-fructose (> 98
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wt%, L-(+)-arabinose (> 99 wt%), D-(-)-arabinose (> 99 wt%), D-(+)-mannose (> 99
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wt%), D-(+)-glucose (> 95 wt%) and sucrose (> 95.55 wt%). D-(+)-maltose (98 wt%) was
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purchased from Vetec®. The commercial sugars (viz. sucrose - Pinheiro, glucose - Yoki®
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and fructose – Doce Menor®) were acquired from the local market in Aracaju-Sergipe,
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Brazil (Supporting Information Table A.1). Figure 1 depicts the chemical structure of the
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compounds. Deionized water was used in all experiments.
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5 2.2 Phase Diagrams
This study concerns the measurement of novel phase diagrams for ATPS based on
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tetrahydrofuran and monosaccharides (D-(+)-xylose, D-(+)-fructose, L-(+)-arabinose, D-(-
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)-arabinose, D-(+)-mannose, D-(+)-glucose), disaccharides (D-(+)-sucrose and D-(+)maltose), as well as commercial sugars (sucrose, glucose and fructose).
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The experimental data were determined gravimetrically, within an uncertainty of ±
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10-4 g, by the cloud point method at 298.15 ± 1.00 K and 0.10 ± 0.01 MPa. This procedure
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follows a protocol already established in our previous works [25,26]. Aqueous solutions of
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carbohydrates (≈ 40–70 wt%) and tetrahydrofuran (≈ 80 wt%) were prepared. Drop-wise
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addition of the tetrahydrofuran was carried out to each solution of carbohydrate until the
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visual detection of a cloud point (biphasic region). Subsequently, drop-wise addition of
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water was performed until the solution became clear (monophasic region). This protocol
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was carried out under constant stirring and was repeated several times in order to obtain
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sufficient data for the construction of a liquid-liquid equilibrium binodal curve. The
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experimental binodal curves were adjusted to Merchuk’s equation [39]:
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[CAR ] = A × exp( B[THF ]0.5 − C[THF ]3 )
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where [CAR] and [THF] are, respectively, the carbohydrate and tetrahydrofuran mass
24
fraction percentages, and A, B and C are the adjustable parameters.
(1)
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ACCEPTED MANUSCRIPT Two mixture points in the biphasic region of the diagram phases were chosen.
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Initially, stock solutions of each carbohydrate were prepared, and subsequently, pure THF
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was added. The mixtures were vigorously stirred, and after reaching equilibrium (at least 12
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h at 298.15 ± 1.00 K), the phases were carefully separated and weighed to within ± 10-4 g.
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To obtain the components’ concentration in the top and bottom phases, Equations (2)–(5)
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were solved (mass balance), and the tie-lines were determined individually.
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[CAR]T = A × exp(B[THF]T0.5 − C[THF]T3 )
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[CAR]B = A × exp(B[THF]0B.5 − C[THF]3B )
(2) (3)
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[THF ] M 1 − α [ CAR ]T = − [THF ] B α α
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[THF ]M 1 − α [THF ]T = − [THF ] B α α
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where the subscripts “T” and “B” designate the top (tetrahydrofuran rich-phase) and bottom
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(carbohydrate rich-phase) phases, respectively, and “M” represents the mixture composition.
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The parameter α is the ratio between the weight of the top phase and that of the overall
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mixture.
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The tie-line lengths (TLLs) were calculated using Equation (6), which uses the
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(5)
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(4)
concentrations of [CAR] and [THF] in each phase.
TLL =
([THF ]T
− [THF ] B ) + ([ CAR ]T − [CAR ] B ) 2
2
(6)
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2.3 Partitioning and Recovery of Diuron Two mixture points (20–45 wt% and 25–45 wt% of carbohydrate-THF,
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ACCEPTED MANUSCRIPT respectively) were selected in the biphasic region. The ATPS were prepared in graduated
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centrifuge tubes (15 mL) and contained the appropriate amounts of carbohydrates,
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tetrahydrofuran and an aqueous solution containing diuron in order to obtain the maximum
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possible diuron concentration in each system (i.e., 0.12 mg L-1). Then, the system
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constituents were vigorously stirred, centrifuged at 3000 rpm for 10 min and finally placed
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at room temperature (12 h), using a thermostatic bath Marconi MA-127, to reach
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equilibrium. The vials were sealed to avoid THF vaporization. The two phases were
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carefully withdrawn using a pipette for the top phase and a syringe with a long needle for
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the bottom phase. The weights were measured in a balance, and the volumes were
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determined in graduated test tubes [27].
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The diuron concentrations in the top and bottom phases were determined by
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spectroscopy using a UV-Vis spectrophotometer (Varin Cary 50 Bio UV/Vis) at a
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wavelength of 250 nm. A calibration curve previously established using diuron as a
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standard in different concentrations (0.02, 0.2, 1, 2, 4, 6 and 8 mg L-1), and as a blank,
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either water (calibration curve) or the corresponding phase in the analysis (partitioning
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process) was used. Before the analyses, the samples were diluted at a ratio of 1:5; the
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diuron quantification was performed in triplicate.
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The partition coefficient of diuron (KDIU) was defined as the ratio between the
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diuron concentrations in the top phase (CT) and the bottom phase (CB). The volume ratio
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(Rv) and recovery percentages of diuron for the top (RT) were measured to evaluate the
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diuron partitioning using to Equations (7)–(9).
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K DIU =
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Rv =
CT CB
(7)
VT VB
(8)
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RT =
100 1 1+ R ×K v
DIU
(9)
where V is phase volume.
3 The pH of each phase was assayed using a pHmeter HANNA – HI9321.
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2.4 Density and Viscosity
The density and viscosity of the acetonitrile-rich and carbohydrate-rich phases
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were determined at different temperatures (298.15–328.15 K), with an uncertainty of ±
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0.02 K, using an automated SVM 3000 Anton Paar rotational Stabinge viscosimeter-
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densimeter. The density had an absolute uncertainty of 5 x 10-4 g.cm-3, while the
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relative uncertainty of dynamic viscosities is 0.35 %. The viscosimeter-densimeter was
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calibrated using a standard solution composed of mineral oils with different viscosities
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and densities.
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3. Results and Discussion
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3.1 Phase Diagrams
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To avoid the negative influence of the molecular weight of the system components in
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the evaluation of the phase separation, the binodal curves are represented in molality units;
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the data are shown in Figures 2–4, and the weight fractions are presented in Tables A.2–A.4
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(Supporting Information). It should be highlighted that for all studied ATPS, the THF-rich
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phase corresponded to the top phase, while the carbohydrate-rich phase corresponded to the
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bottom phase.
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ATPS are formed due to the competition between the constituents of the system in
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the creation of hydration complexes, which largely depends on its hydrogen-bond-accepting
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strength [19]. In the studied systems, tetrahydrofuran was the common constituent; for this
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reason, the capability of phase forming was ascribed to the different carbohydrates used. The binodal curves for the systems with several monosaccharides and THF are
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depicted in Figure 2. Indeed, the hydration ability of monosaccharides and the phase
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separation facility are proportional and can be ranked in the following increasing order: D-
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(+)-xylose < D-(-)-arabinose ≈ L-(+)-arabinose ≈ D-(-)-fructose < D-(+)-mannose < D-(+)-
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glucose. This sequence agreed with that found in the literature for ATPS formed by
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carbohydrates and acetonitrile [26].
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Carbohydrates have a structure with several hydroxyl groups that allow a high
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affinity with water, and consequently promotes the sugaring-out effect [40]. The hydroxyl
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group has a dual electron donor/acceptor character, which is involved in hydrogen bonding.
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Therefore, disaccharide (8 hydroxyl
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monosaccharide (5 or 4 hydroxyl groups) phases. Moreover, aldo-hexoses (5 hydroxyl
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groups) are more adept at forming hydrogen bonds and trigger easier phase separation than
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aldo-pentoses (4 hydroxyl groups). Lima and co-workers [27] also observed the effect of the
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hydroxyl group on the phase-forming ability.
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groups) phases formed more easily than
For carbohydrates with the same number of hydroxyl groups, the sequence of phase-
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formation ease is ordered by the correlation with the number of equatorial hydroxyl groups
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(Ne-OH): maltose (7.2), sucrose (6.3), glucose (4.6), xylose (3.5), mannose (3.3), fructose
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(3.0), arabinose (2.6) [41,42]. Uedaira and co-workers [43] reported the strong correlation
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between Ne-OH (number of equatorial hydroxyl group) and the hydration capability of
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carbohydrates because the equatorial hydroxyl group stabilizes the water structure; in
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addition, Engberts and co-workers [44] described the ratio of equatorial and axial hydroxyl
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groups in hydration phenomena. This sequence of Ne-OH agrees with the phase-formation
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ability found here, except for xylose.
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ACCEPTED MANUSCRIPT Comparing the phase-forming capability of isomeric monosaccharide (aldose –
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glucose and ketoses – fructose) conformation, it is noted that six-membered aldoses have a
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more favorable structure for hydrogen bonding with water because the equatorial hydroxyl
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groups orientated within the plane of the six-membered ring accommodate more water
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molecules in their vicinity [45].
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To evaluate the effect of the enantiomers, the binodal curves of D-(-)-arabinose and
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L-(+)-arabinose were compared: it was observed that they overlapped. In other words, the
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compounds had the same phase-forming capability. Additionally, the orientation of the
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hydroxyl group at carbon 2 in epimers of the carbohydrate altered the phase forming ability,
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either to aldoses with five carbon atoms (D-(-)-arabinose and D-(+)-xylose) or aldoses with
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six carbon atoms (D-(+)-glucose and D-(+)-mannose).
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For disaccharides, the binodal curve of D-(+)-maltose is closer to the origin of the
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axes, meaning that the phases are separated more easily than those in the binodal curve
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using D-(+)-sucrose (Figure 3), which agrees with Ne-OH 7.2 (maltose) and 6.3 (sucrose).
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Figure 4 depicts the binodal curves of systems based on THF and pure and
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commercial carbohydrates (fructose, glucose and sucrose). The increasing order to induce
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the ATPS formation is D-(-)-fructose ≈ fructose commercial < sucrose commercial ≈ D-(+)-
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sucrose ≈ glucose commercial < D-(+)-glucose; therefore, the forming-phase using pure and
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commercial carbohydrates was similar for fructose and sucrose. Maulyn and co-workers
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[46] found by HPLC no inverted sugars in different commercial sucroses but very low
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concentrations of minerals. Commercial crystallized fructose has on average 0.5 %
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impurities [47]. The low concentration of impurities in commercial sugar did not affect the
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phase-formation. Conversely, the discrepancies between the binodal curves for commercial
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and pure glucose at low THF concentrations were probably derived from impurities (e.g.,
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maltose, isomaltose and maltotriose) present in commercial glucose syrup, as reported by
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Pontoh and Low [41]. At high THF concentrations, these impurities were diluted, with the
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binodal curves becoming similar. The advantage of using commercial carbohydrates was
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their price. The price of pure carbohydrates was greater than that of commercial carbohydrates
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(US$ 47.16 vs. US$ 2.30 – glucose; US$ 147.38 vs. US$ 13.54 – fructose and US$ 91.53 vs.
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US$ 0.60 – sucrose). The prices of commercial carbohydrates were obtained directly from
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the local supermarket in July 2016. Therefore, the use of commercial carbohydrates reduces
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process costs while not significantly affecting the phase-forming ability (except for glucose).
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Parameters A, B and C were adjusted by least-squares regression; the standard
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deviations (σ) and correlation coefficient are displayed in Table 1. In general, good
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correlation coefficients (0.9975 < R2< 0.9997) were obtained for all systems, indicating that
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Equation (1) adequately correlated the experimental binodal curves.
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The experimental TLs, respective tie-line length (TLL), and the composition for the
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studied systems, are all shown in Table 2. The graphical representation of the TLs can be
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found in the Supporting Information (Figures A.1–A.11).
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Measurements of density and viscosity between 283.15 and 333.15 K were used to
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characterize the top and bottom phases. These parameters were important for the design
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and scale-up of the extraction processes and were determined for both phases of ATPS
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with compositions 20–45 wt% and 25–45 wt% for carbohydrate and THF, respectively
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(Figures 5–6 and Tables A.5–A.8). Differences in the density between the two phases
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allowed faster and easier phase separation. For the systems studied, the THF-rich phases
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(top phases) were less dense than the corresponding carbohydrate-rich phases. Compared
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to the polymer-salt ATPS at the temperature reported in the literature [49], the densities
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ranged from 0.881 g cm-3 (D-arabinose at 318 K) to 1.230 g cm-3 (pure glucose at 298.15
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K) for the carbohydrate-rich phase, and from 0.875 g cm-3 (commercial sucrose at 318.15
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in density between the carbohydrate-rich phase and the THF-rich phase at 298.15 K ranged
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from 0.235 g cm-3 (pure glucose) to 0.322 g cm-3 (commercial glucose) for the system
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consisting of 20–45 wt% (carbohydrate – THF) and from 0.261 g.cm-3 (commercial
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glucose) to 0.352 g cm-3 (glucose) for the system consisting of 25–45 wt% (carbohydrate –
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THF), while for ATPS composed of PEG-6000 and triammonium citrate, the difference
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was only 0.08 g cm3 [49]. Hence, these systems separated easier and faster than that
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formed by PEG-6000.
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The phase densities decreased linearly with temperature for all carbohydrates. A
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comparison between pure carbohydrates and their commercial version for the studied
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systems shows little difference for sucrose and fructose in contrast to glucose, probably
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due to the composition of the glucose syrup used.
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The curves of viscosity versus temperature showed a typical exponential decrease.
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This decrease of viscosity was assigned to intra- and intermolecular interactions [50], of
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which H-bonding is the most important for the systems under study. The carbohydrate-rich
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phase was shown to be more viscous than the corresponding THF-rich phase for all
17
systems investigated. Our viscosity data for sucrose, glucose and fructose agreed with
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those reported in the literature [51,52]. Commercial sucrose and commercial fructose had
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the same viscosity for system 1 (20–45 wt% carbohydrate – THF) but were slightly
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different for system 2 (25–45 wt% for carbohydrate – THF).
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ATPS based on PEG and different salts presented viscosities in the PEG-rich phase
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between 9.7 mPa.s (PEG-1000 - 20 wt% + K3PO4 – 11.4 wt%) and 202.0 mPa.s (PEG-
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8000 – 21.9 % + K3PO4 – 12.3 %) [53]. At this temperature, the viscosity of the
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carbohydrate rich-phase ranged from 8.54 (D- and L-arabinose) to 106.83 mPa.s (maltose)
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for the system consisting of 20–45 wt% (carbohydrate – THF) and from 13.44 mPa.s
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ACCEPTED MANUSCRIPT (commercial glucose) to 209.58 mPa.s (maltose) for the system consisting of 25–45 wt%
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(carbohydrate – THF). The aforementioned data indicated that ATPS based on
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carbohydrate and THF and ATPS based on PEG + K3PO4 present similar viscosities for
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25–45 wt% (carbohydrate – THF); however, systems formed by 20–45 wt% (carbohydrate
5
– THF) are easier and faster to separate than ATPS based on PEG + K3PO4.
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3.2 Partitioning and Recovery of Diuron in ATPS
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The ATPS presented here have been studied as an alternative technique for the
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extraction of diuron, an herbicide widely used in sugarcane cultivation. Figure 7 (Supporting
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Information Table A.9) shows the partition coefficient of diuron in an aqueous two-phase
11
system based on carbohydrates and THF. Two different tie lines were investigated: 20–45
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wt% and 25–45 wt% in carbohydrates and THF, respectively. The choice of the mixture
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points was based on the phase-forming ability of all systems studied, while trying to
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minimize the use of THF.
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The experimental data reflected the affinity of diuron for the tetrahydrofuran rich-
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phase (1.23 < KDIU < 16.19), except for the system 25–45 wt% of L-(+)-arabinose–THF. The
17
aromatic ring of diuron influenced the hydrophobic characteristic and the migration of
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diuron to the acetonitrile-rich phase. Lima and co-workers [9] reported a similar observation
19
in the partition of gallic acid in ATPS based on polyethylene glycol and potassium
20
phosphate. Their results had a good correlation with the octanol-water coefficient of diuron
21
(log Kow = 2.60) [35], indicating the affinity of diuron to the more hydrophobic phase.
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Indeed, tetrahydrofuran (log Kow = 0.53) was more hydrophobic than carbohydrates (-2.30 <
23
log Kow< -4.70) [54].
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The pH values ranged from 4.91–7.07 for the top phase and 3.72–5.76 for the bottom
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phase (Table 3). The speciation curve (Supporting Information Figure A.12) shows that the
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ACCEPTED MANUSCRIPT pH does not change the chemical structure of diuron, which presents a positive charge on the
2
nitrogen of the aromatic ring and a negative charge on the carbonyl, i.e., a net charge of 0,
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corresponding to a neutral form. In this way, diuron tended to migrate to the top phase. This
4
migration suggested that the main interactions controlling this partition were hydrophobic
5
interactions.
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The recovery in the top phase was higher than 51.19 % except for that of L-
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arabinose, reaching 92.70 % for glucose (Figure 8 and Table A.9 of Supporting
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Information). Commercial carbohydrates exhibited contradictory behavior: for glucose and
9
fructose, the recoveries were lower than in systems with commercial sugar, although these
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values were similar for fructose and commercial fructose. Martinez and co-workers [55]
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reported a diuron recovery of 98.8 % using a solid phase extraction.
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4. Conclusions
A novel aqueous two-phase system based on carbohydrate (six monosaccharides,
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two disaccharides and three commercial sugars) and tetrahydrofuran dextran has been
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proposed. The phase diagrams with two tie-lines each were determined at 298.15 K and 0.1
17
MPa. In general, the number of equatorial hydroxyl groups of carbohydrates is the driving
18
force behind the phase-forming. Disaccharides form ATPS with tetrahydrofuran more
19
easily than monosaccharides, the same trend as aldoses compared with ketoses. Moreover,
20
the system based on carbohydrates (20 wt%) and THF (45 wt%) has differences in
21
densities and lower viscosities compared to ATPS based on PEG and K3PO4, allowing an
22
easier phase separation.
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23
The application of carbohydrates and tetrahydrofuran aqueous two-phase systems
24
for the partitioning of diuron was demonstrated to be a good strategy. The diuron migrates
25
preferentially to the top phase (i.e., the THF-rich phase). The highest values of the partition
14
ACCEPTED MANUSCRIPT coefficient KDIU and the recovery of diuron of the top phase (RT) were 16.19 and 92.70 %
2
for systems consisting of D-(+)-glucose (25 wt%) and tetrahydrofuran (45 wt%),
3
respectively. Commercial sugars presented differences in terms of density, viscosity, KDIU
4
and RT compared to pure carbohydrates; however, the results demonstrated that they may
5
be successfully used to partition diuron in aqueous two-phase systems based on
6
carbohydrates and tetrahydrofuran.
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1
7 Acknowledgments
9
We thank the funding agencies FAPITEC, CAPES and FINEP for financial support and CNPq for the scholarship of G.E.L.O. Maciel.
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ACCEPTED MANUSCRIPT Table 1. The data of the respective regression parameters (A, B and C) obtained by Merchuk’s equation, standard deviations (σ) and correlation coefficients (R2) are for the ternary systems composed of tetrahydrofuran + carbohydrates + water, at 298.15 ± 1 K and 0.1 ± 0.01 MPa. B±σ
C±σ
R2
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A±σ
Carbohydrate
156.31 ± 4.12 -0.314 ± 0.0061 1.10 x 10-6 ± 1.24 x 10-7
0.9998
D-(-)-fructose
131.04 ± 5.64 -0.323 ± 0.010 1.39 x 10-6 ± 2.59x10-7
0.9982
L-(+)-arabinose
221.40 ± 4.62 -0.453 ± 0.005 4.25 x 10-7 ± 1.19 x 10-7
0.9997
D-(-)-arabinose
96.57 ± 1.46
-0.296 ± 0.004 2.03 x 10-6 ± 1.45 x 10-7
0.9994
D-(+)-mannose
160.99 ± 2.85 -0.358 ± 0.006 4.72 x 10-7 ± 7.07 x 10-7
0.9995
D-(+)-glucose.
128.37 ± 2.16 -0.320 ± 0.006 6.25 x 10-6 ± 7.46 x 10-7
0.9993
D-(+)-sucrose
141.97 ± 3.30 -0.308 ± 0.005 8.81 x 10-7 ± 1.09 x 10-7
0.9998
D-(+)-maltose
101.68 ± 9.61 -0.246 ± 0.019 1.38 x 10-6 ± 3.45 x 10-7
0.9975
Commercial sucrose 183.35 ± 9.34 -0.369 ± 0.011 2.45 x 10-9 ± 2.12 x 10-7
0.9993
Commercial glucose 114.73 ± 3.44 -0.244 ± 0.010 4.41 x 10-6 ± 6.50 x 10-7
0.9989
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D-(+)-xylose
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Commercial fructose 154.35 ± 3.69 -0.346 ± 0.007 1.92 x 10 -6 ± 2.55 x 10-7 0.9994
ACCEPTED MANUSCRIPT Table 2. Experimental (liquid + liquid) equilibrium data of tie-lines (TLs) and tie-line lengths (TLLs) values of the coexisting phases for the tetrahydrofuran (THF) + carbohydrate (CAR)+ water in ATPS at 298.15 K and 0.1 MPa.a Weight fraction composition (wt%) [CAR]M
[THF]M
[CAR]T
[CAR]B
[THF]B
TLL
19.99 44.94 11.88 54.83 44.55 14.96 51.55 24.95 44.93 8.82 63.85 54.27 10.51 70.07 D-(-)-fructose 19.99 45.01 6.61 63.79 44.35 10.82 65.04 24.95 44.92 5.24 69.79 55.95 6.95 80.17 L-(+)-arabinose 19.95 44.96 5.09 65.18 43.53 12.89 64.89 24.96 44.99 4.71 67.48 57.35 8.90 78.76 D-(-)-arabinose 19.99 45.00 4.55 67.54 46.69 6.03 74.56 24.96 44.98 3.74 71.67 58.44 2.88 87.88 D-(+)-mannose 19.96 45.08 9.38 59.01 44.36 12.98 54.82 24.90 44.91 7.77 65.66 54.37 9.21 73.19 D-(+)-glucose. 20.00 45.00 1.40 67.14 50.82 8.29 76.85 24.94 44.97 0.79 72.37 59.55 5.72 88.85 D-(+)-sucrose 19.94 45.92 11.72 57.63 37.63 18.443 46.63 24.98 45.00 9.03 66.01 50.69 11.16 68.87 D-(+)-maltose 19.95 44.95 10.73 61.36 34.70 18.72 48.91 24.98 45.04 9.53 65.05 54.93 6.24 74.29 Commercial sucrose 19.95 44.92 11.15 57.58 38.89 17.67 48.60 24.93 44.83 9.78 63.08 52.64 11.44 67.11 Commercial glucose 20.00 44.86 6.46 60.54 47.94 12.54 63.44 25.03 45.00 3.97 67.52 60.77 7.75 83.18 Commercial fructose 20.03 44.97 6.63 61.26 47.54 12.00 64.39 24.86 44.83 4.92 67.81 56.44 9.00 78.61 a Standard uncertainties u are u([CAR] or [THF]) = 0.01, u(T) = 1 K, and u(p) = 10 kPa.
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D-(+)-xylose
[THF]T
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ACCEPTED MANUSCRIPT Table 3. pH values of the tetrahydrofuran (top)- and carbohydrate (bottom)-rich phases at 298.15 K and 0.1 MPa.a 25 wt% carbohydrate +
45 wt% tetrahydrofuran
45 wt% tetrahydrofuran +
Bottom phase
Top phase
Bottom phase
D-(+)-xylose
4.9
5.7
5.9
5.6
D-(-)-fructose
7.0
5.4
6.9
4.9
L-(+)-arabinose
4.9
4.8
5.0
4.9
D-(-)-arabinose
6.7
3.8
6.5
3.7
D-(+)-mannose
5.5
5.5
6.8
5.7
D-(+)-glucose
6.4
5.6
D-(+)-sucrose
7.0
4.9
D-(+)-maltose
6.5
4.6
Commercial sucrose
7.0
Commercial glucose
6.8
Commercial fructose
5.9
SC 6.5
4.7
7.0
5.7
7.0
5.5
5.6
6.5
5.4
3.7
6.4
3.9
4.9
4.9
4.9
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Standard uncertainties u are u(pH) = 0.1, u(T) = 1 K, and u(p) = 10 kPa.
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Top phase
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20 wt% carbohydrate +
ACCEPTED MANUSCRIPT Figure Capitation
Figure 1. Molecular structure of carbohydrates and tetrahydrofuran used to form aqueous two-phase system.
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Figure 2. Phase diagrams for the ternary system composed by monosacharides + THF + water at 298.15 ± 1 K and 0.10 ± 0.01 MPa: () D-(+)-xylose; () D-(-)-fructose; () L(+)arabinose; () D-(-)-arabinose; () D-(+)-mannose and () D-(+)-glucose.
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Figure 3. Phase diagrams for the ternary system composed by THF + disaccharides + water at 298.15 ± 1 K and 0.10 ± 0.01 MPa: () D-(+)-sucrose; () D-(+)-maltose.
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Figure 4. Phase diagrams for the ternary system composed by carbohydrates + THF + water at 298.15 ± 1 K and 0.10 ± 0.01 MPa: () D-(-)-fructose; () fructose commercial; () D-(+)glucose; () glucose commercial; () D-(+)-sucrose and () sucrose commercial. Figure 5. Experimental density (ρ / g.cm-3) for different aqueous two-phase system
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composed of 20–45 wt% (System 1), and 25–45 wt% for carbohydrate – tetrahydrofuran (System 2), respectively, at 298.15 ± 1 K and 0.10 ± 0.01 MPa:-D(+)-xylose, - D-(-)-fructose, - L-(+)-arabinose, - D-(-)-arabinose, - D-(+)-
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mannose, - D-(+)-glucose, -D-(+)-sucrose, - D-(+)-maltose, - Commercial
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sucrose, - Commercial fructose, - Commercial glucose. Figure 6. Experimental viscosity (η / mPa.s) for different aqueous two-phase system composed of 20–45 wt% (System 1), and 25–45 wt% for carbohydrate – tetrahydrofuran (System 2), respectively, at 298.15 ± 1 K and 0.10 ± 0.01 MPa:-D-(+)-xylose, - D-(-)fructose, - L-(+)-arabinose, - D-(-)-arabinose, - D-(+)-mannose, - D-(+)-glucose, -D-(+)-sucrose, - D-(+)-maltose, - Commercial sucrose, - Commercial fructose, - Commercial glucose. Figure 7. Partition coefficient of diuron (KDIU) in different aqueous two-phase systems at 1
ACCEPTED MANUSCRIPT 298.15± 1 K and 0.10 ± 0.01 MPa: 20–45 wt% and 25–45 wt% for carbohydrates tetrahydrofuran, respectively. Figure 8. Recovery of diuron in the top phase (RT) in different aqueous two-phase systems at 298.15± 1 K and 0.10 ± 0.01 MPa: 20–45 wt% and 25–45 wt% for carbohydrates -
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tetrahydrofuran, respectively.
2
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Figure 1. Molecular structure of carbohydrates and tetrahydrofuran used to form aqueous
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two-phase system.
3
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Figure 2. Phase diagrams for the ternary system composed by monosacharides + THF + water at 298.15 ± 1 K and 0.10 ± 0.01 MPa: () D-(+)-xylose; () D-(-)-fructose; () L-
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(+)arabinose; () D-(-)-arabinose; () D-(+)-mannose and () D-(+)-glucose.
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Figure 3. Phase diagrams for the ternary system composed by THF + disaccharides + water
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at 298.15 ± 1 K and 0.10 ± 0.01 MPa: () D-(+)-sucrose; () D-(+)-maltose.
5
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Figure 4. Phase diagrams for the ternary system composed by carbohydrates + THF + water at 298.15 ± 1 K and 0.10 ± 0.01 MPa: () D-(-)-fructose; () fructose commercial; () D-(+)-
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glucose; () glucose commercial; () D-(+)-sucrose and () sucrose commercial.
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Figure 5. Experimental density (ρ / g.cm-3) for different aqueous two-phase system
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composed of 20–45 wt% (System 1), and 25–45 wt% for carbohydrate – tetrahydrofuran (System 2), respectively, at 298.15 ± 1 K and 0.10 ± 0.01 MPa:-D(+)-xylose, - D-(-)-fructose, - L-(+)-arabinose, - D-(-)-arabinose, - D-(+)mannose, - D-(+)-glucose, -D-(+)-sucrose, - D-(+)-maltose, - Commercial
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sucrose, - Commercial fructose, - Commercial glucose.
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Figure 6. Experimental viscosity (η / mPa.s) for different aqueous two-phase system composed of 20–45 wt% (System 1), and 25–45 wt% for carbohydrate – tetrahydrofuran (System 2), respectively, at 298.15 ± 1 K and 0.10 ± 0.01 MPa:-D-(+)-xylose, - D-(-)fructose, - L-(+)-arabinose, - D-(-)-arabinose, - D-(+)-mannose, - D-(+)-glucose,
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-D-(+)-sucrose, - D-(+)-maltose, - Commercial sucrose, - Commercial fructose,
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- Commercial glucose.
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Figure 7. Partition coefficient of diuron (KDIU) in different aqueous two-phase systems at
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298.15± 1 K and 0.10 ± 0.01 MPa: 20–45 wt% and 25–45 wt% for carbohydrates -
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tetrahydrofuran, respectively.
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Figure 8. Recovery of diuron in the top phase (RT) in different aqueous two-phase systems at 298.15± 1 K and 0.10 ± 0.01 MPa: 20–45 wt% and 25–45 wt% for carbohydrates -
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tetrahydrofuran, respectively.
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ACCEPTED MANUSCRIPT Highlights
Tetrahydrofuran and carbohydrates form aqueous two-phase systems at 298.15 K.
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Number of equatorial OH is driving force to form ATPS. Commercial sugars form also two-phase system similar a pure compounds.
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Diuron migrates preferentially to the THF rich-phase.
S0378-3812(16)30548-9
DOI:
10.1016/j.fluid.2016.11.001
Reference:
FLUID 11311
To appear in:
Fluid Phase Equilibria
Received Date: 8 August 2016 Revised Date:
28 October 2016
Accepted Date: 1 November 2016
Please cite this article as: K.M. Sousa, G.E.L.O. Maciel, F.S. Buarque, A.J. Santos, M.N. Marques, E.B. Cavalcanti, C.M.F. Soares, E.S. Lima, Novel phase diagrams of aqueous two-phase systems based on tetrahydrofuran + carbohydrates + water: Equilibrium data and partitioning experiments, Fluid Phase Equilibria (2016), doi: 10.1016/j.fluid.2016.11.001. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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TOC – Graphical Abstract
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Water Tetrahydrofuran
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Carbohydrate
Diuron
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Novel phase diagrams of aqueous two-phase systems based on tetrahydrofuran +
2
carbohydrates + water: equilibrium data and partitioning experiments
3 4 Kênia M. Sousaa, Gustavo E.L.O. Maciela, Filipe S. Buarquea, Adriana J. Santosb, Maria N.
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Marquesa,b, Eliane B. Cavalcantia,b, Cleide Mara F. Soaresa,b, Álvaro S. Limaa,b,*
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10 11 12
a
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SE, Brazil
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b
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SE, Brazil
Instituto de Tecnologia e Pesquisa. Av. Murilo Dantas, 300. CEP: 49032-490, Aracaju –
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Universidade Tiradentes, Av. Murilo Dantas 300, Farolândia. CEP: 49032-490, Aracaju-
*To whom correspondence should be addressed: E-mail [email protected]. Phone: +55
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7932182115. Fax: +55 7932182190.
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ABSTRACT This work addresses the partitioning of diuron, an herbicide heavily employed in
3
agriculture, using the aqueous two-phase system formed by tetrahydrofuran and
4
carbohydrates. Phase diagrams using tetrahydrofuran (THF) and six monosaccharides, two
5
disaccharides and three commercial sugars at 298.15 K and 0.1 MPa were constructed. The
6
equilibrium phases were further characterized by determining the density and viscosity at
7
283.15–333.15 K and 0.1 MPa. Finally, the diuron extraction was assessed by the partition
8
coefficient (KDIU) and recovery in the top-phase (RT). The number of equatorial hydroxyl
9
groups present in the carbohydrates’ structures is the driving force for the phase separation;
10
disaccharides are stronger inducers of the ATPS formation than monosaccharides. The
11
differences in the density and viscosity of the phases allowed an easy phase separation. In
12
all cases, the diuron partitions preferentially to the THF-rich phase with KDIU ranging from
13
1.23 (commercial glucose) to 16.19 (pure glucose) and recovery between 49.45 %
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(commercial glucose) and 92.70 % (pure glucose), demonstrating the applicability in the
15
partition of diuron.
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Keywords: partitioning; tetrahydrofuran; carbohydrates; herbicide; extraction.
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1. Introduction Aqueous two-phase systems (ATPS) were first introduced by Beijernick in 1896 [1].
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In the last few decades, they have become an option in liquid-liquid extraction systems due
4
to their characteristics such as low interfacial tension and good mass transfer, which allow
5
high yields; their environmentally friendly features (e.g., high water content); and the ease
6
of scaling up [2,3]. A wide range of molecules have been successfully separated, recovered
7
and purified using ATPS, such as enzymes (pectinase [4], lipase [5]), antibiotics [6],
8
antibodies [7,8], alkaloids (gallic acid [9] and quinine [10]), metals [11], and dyes [12].
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ATPS can be formed when two structurally different compounds are dissolved in
10
water above a certain concentration [13]. The success of an ATPS depends on the choice of
11
system constituents, which are usually two polymers (dextran + dextran, dextran +
12
polyethylene glycol (PEG), dextran + Ficoll and PEG/maltodextrin [14,15]); polymer +
13
salt (PEG + potassium phosphate [16]), and recently, ionic liquids + salts [17–21], ionic
14
liquids + sugar [22], ionic liquids + PEG [23,24], organic solvents + salts (alcohol +
15
potassium phosphate [25], organic solvents + carbohydrates/polyols (acetonitrile +
16
carbohydrates/polyols [26,27] and organic solvent + dextran [28].
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Recently, tetrahydrofuran (THF) has also been proposed as a constituent of
18
ATPS [29,30]. THF (C4H8O) is a cyclic ether that is miscible with water in all
19
proportions at room temperature [31,32] and is highly flammable (flashpoint -21.2
20
°C), with a vapor pressure of 21.6 kPa at 25 °C [33]. This organic solvent is employed
21
as a solvent of dyes and lacquers, a precursor in the synthesis of succinic acid, a
22
reaction medium, and a polymerization solvent for fat oils, among other applications
23
[34].
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The goal of this study is to associate carbohydrate and THF to develop a novel
25
aqueous two-phase system for use in the partition of water-soluble herbicides such as
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ACCEPTED MANUSCRIPT diuron. It is well known that carbohydrates do not present any environmental or health
2
concerns, and according to Banton and co-workers [33], THF does not have an
3
adverse impact on human health and the environment. In animals, the acute toxicity
4
potential is low to moderate by different routes. Although absorption through the skin
5
is rapid, skin irritation is slight. Genetic toxicity has been negative, as well as
6
reproductive toxicity, but it produces benign tumors in rats, which is not relevant to
7
human health. Therefore, it can be said that the components of the system do not have
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any risk to human health and the environment. Thus, these ATPS based on
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carbohydrates and THF may be used for the separation, extraction/recovery or
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detection of diuron from aqueous media.
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Diuron (N-(3,4-dichlorophenyl)-N,N-dimethyl-urea) is a crystalline compound
12
belonging to the phenylamide group [35]. According to Giacomazzi and Cochet [36],
13
diuron is a non-ionic and non-volatile substance whose melting point is 158–159 °C
14
and water solubility is 42 mg L-1. Its octanol-water partition coefficient (log Kow = 2.6)
15
characterizes it as a hydrophobic compound, which has been widely used in pre- or
16
post-emergent treatment in agricultural crops [37]. The presence of diuron in drinking
17
water and food is extremely harmful to human health due to its carcinogenicity [38],
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which makes this herbicide a dangerous aquatic pollutant.
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2. Material and Methods
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2.1 Materials
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Tetrahydrofuran - THF (HPLC grade, 99.9 wt%) was purchased from Sigma-
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Aldrich®. The carbohydrates used here were D-(+)-xylose (99 wt%), D-(-)-fructose (> 98
24
wt%, L-(+)-arabinose (> 99 wt%), D-(-)-arabinose (> 99 wt%), D-(+)-mannose (> 99
25
wt%), D-(+)-glucose (> 95 wt%) and sucrose (> 95.55 wt%). D-(+)-maltose (98 wt%) was
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purchased from Vetec®. The commercial sugars (viz. sucrose - Pinheiro, glucose - Yoki®
2
and fructose – Doce Menor®) were acquired from the local market in Aracaju-Sergipe,
3
Brazil (Supporting Information Table A.1). Figure 1 depicts the chemical structure of the
4
compounds. Deionized water was used in all experiments.
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5 2.2 Phase Diagrams
This study concerns the measurement of novel phase diagrams for ATPS based on
8
tetrahydrofuran and monosaccharides (D-(+)-xylose, D-(+)-fructose, L-(+)-arabinose, D-(-
9
)-arabinose, D-(+)-mannose, D-(+)-glucose), disaccharides (D-(+)-sucrose and D-(+)maltose), as well as commercial sugars (sucrose, glucose and fructose).
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The experimental data were determined gravimetrically, within an uncertainty of ±
12
10-4 g, by the cloud point method at 298.15 ± 1.00 K and 0.10 ± 0.01 MPa. This procedure
13
follows a protocol already established in our previous works [25,26]. Aqueous solutions of
14
carbohydrates (≈ 40–70 wt%) and tetrahydrofuran (≈ 80 wt%) were prepared. Drop-wise
15
addition of the tetrahydrofuran was carried out to each solution of carbohydrate until the
16
visual detection of a cloud point (biphasic region). Subsequently, drop-wise addition of
17
water was performed until the solution became clear (monophasic region). This protocol
18
was carried out under constant stirring and was repeated several times in order to obtain
19
sufficient data for the construction of a liquid-liquid equilibrium binodal curve. The
20
experimental binodal curves were adjusted to Merchuk’s equation [39]:
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[CAR ] = A × exp( B[THF ]0.5 − C[THF ]3 )
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where [CAR] and [THF] are, respectively, the carbohydrate and tetrahydrofuran mass
24
fraction percentages, and A, B and C are the adjustable parameters.
(1)
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ACCEPTED MANUSCRIPT Two mixture points in the biphasic region of the diagram phases were chosen.
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Initially, stock solutions of each carbohydrate were prepared, and subsequently, pure THF
3
was added. The mixtures were vigorously stirred, and after reaching equilibrium (at least 12
4
h at 298.15 ± 1.00 K), the phases were carefully separated and weighed to within ± 10-4 g.
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To obtain the components’ concentration in the top and bottom phases, Equations (2)–(5)
6
were solved (mass balance), and the tie-lines were determined individually.
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[CAR]T = A × exp(B[THF]T0.5 − C[THF]T3 )
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[CAR]B = A × exp(B[THF]0B.5 − C[THF]3B )
(2) (3)
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[THF ] M 1 − α [ CAR ]T = − [THF ] B α α
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[THF ]M 1 − α [THF ]T = − [THF ] B α α
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where the subscripts “T” and “B” designate the top (tetrahydrofuran rich-phase) and bottom
13
(carbohydrate rich-phase) phases, respectively, and “M” represents the mixture composition.
14
The parameter α is the ratio between the weight of the top phase and that of the overall
15
mixture.
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The tie-line lengths (TLLs) were calculated using Equation (6), which uses the
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(5)
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(4)
concentrations of [CAR] and [THF] in each phase.
TLL =
([THF ]T
− [THF ] B ) + ([ CAR ]T − [CAR ] B ) 2
2
(6)
21 22 23
2.3 Partitioning and Recovery of Diuron Two mixture points (20–45 wt% and 25–45 wt% of carbohydrate-THF,
6
ACCEPTED MANUSCRIPT respectively) were selected in the biphasic region. The ATPS were prepared in graduated
2
centrifuge tubes (15 mL) and contained the appropriate amounts of carbohydrates,
3
tetrahydrofuran and an aqueous solution containing diuron in order to obtain the maximum
4
possible diuron concentration in each system (i.e., 0.12 mg L-1). Then, the system
5
constituents were vigorously stirred, centrifuged at 3000 rpm for 10 min and finally placed
6
at room temperature (12 h), using a thermostatic bath Marconi MA-127, to reach
7
equilibrium. The vials were sealed to avoid THF vaporization. The two phases were
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carefully withdrawn using a pipette for the top phase and a syringe with a long needle for
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the bottom phase. The weights were measured in a balance, and the volumes were
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determined in graduated test tubes [27].
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The diuron concentrations in the top and bottom phases were determined by
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spectroscopy using a UV-Vis spectrophotometer (Varin Cary 50 Bio UV/Vis) at a
13
wavelength of 250 nm. A calibration curve previously established using diuron as a
14
standard in different concentrations (0.02, 0.2, 1, 2, 4, 6 and 8 mg L-1), and as a blank,
15
either water (calibration curve) or the corresponding phase in the analysis (partitioning
16
process) was used. Before the analyses, the samples were diluted at a ratio of 1:5; the
17
diuron quantification was performed in triplicate.
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The partition coefficient of diuron (KDIU) was defined as the ratio between the
19
diuron concentrations in the top phase (CT) and the bottom phase (CB). The volume ratio
20
(Rv) and recovery percentages of diuron for the top (RT) were measured to evaluate the
21
diuron partitioning using to Equations (7)–(9).
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K DIU =
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Rv =
CT CB
(7)
VT VB
(8)
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RT =
100 1 1+ R ×K v
DIU
(9)
where V is phase volume.
3 The pH of each phase was assayed using a pHmeter HANNA – HI9321.
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2.4 Density and Viscosity
The density and viscosity of the acetonitrile-rich and carbohydrate-rich phases
8
were determined at different temperatures (298.15–328.15 K), with an uncertainty of ±
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0.02 K, using an automated SVM 3000 Anton Paar rotational Stabinge viscosimeter-
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densimeter. The density had an absolute uncertainty of 5 x 10-4 g.cm-3, while the
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relative uncertainty of dynamic viscosities is 0.35 %. The viscosimeter-densimeter was
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calibrated using a standard solution composed of mineral oils with different viscosities
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and densities.
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3. Results and Discussion
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3.1 Phase Diagrams
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To avoid the negative influence of the molecular weight of the system components in
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the evaluation of the phase separation, the binodal curves are represented in molality units;
19
the data are shown in Figures 2–4, and the weight fractions are presented in Tables A.2–A.4
20
(Supporting Information). It should be highlighted that for all studied ATPS, the THF-rich
21
phase corresponded to the top phase, while the carbohydrate-rich phase corresponded to the
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bottom phase.
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ATPS are formed due to the competition between the constituents of the system in
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the creation of hydration complexes, which largely depends on its hydrogen-bond-accepting
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strength [19]. In the studied systems, tetrahydrofuran was the common constituent; for this
2
reason, the capability of phase forming was ascribed to the different carbohydrates used. The binodal curves for the systems with several monosaccharides and THF are
4
depicted in Figure 2. Indeed, the hydration ability of monosaccharides and the phase
5
separation facility are proportional and can be ranked in the following increasing order: D-
6
(+)-xylose < D-(-)-arabinose ≈ L-(+)-arabinose ≈ D-(-)-fructose < D-(+)-mannose < D-(+)-
7
glucose. This sequence agreed with that found in the literature for ATPS formed by
8
carbohydrates and acetonitrile [26].
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Carbohydrates have a structure with several hydroxyl groups that allow a high
10
affinity with water, and consequently promotes the sugaring-out effect [40]. The hydroxyl
11
group has a dual electron donor/acceptor character, which is involved in hydrogen bonding.
12
Therefore, disaccharide (8 hydroxyl
13
monosaccharide (5 or 4 hydroxyl groups) phases. Moreover, aldo-hexoses (5 hydroxyl
14
groups) are more adept at forming hydrogen bonds and trigger easier phase separation than
15
aldo-pentoses (4 hydroxyl groups). Lima and co-workers [27] also observed the effect of the
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hydroxyl group on the phase-forming ability.
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groups) phases formed more easily than
For carbohydrates with the same number of hydroxyl groups, the sequence of phase-
18
formation ease is ordered by the correlation with the number of equatorial hydroxyl groups
19
(Ne-OH): maltose (7.2), sucrose (6.3), glucose (4.6), xylose (3.5), mannose (3.3), fructose
20
(3.0), arabinose (2.6) [41,42]. Uedaira and co-workers [43] reported the strong correlation
21
between Ne-OH (number of equatorial hydroxyl group) and the hydration capability of
22
carbohydrates because the equatorial hydroxyl group stabilizes the water structure; in
23
addition, Engberts and co-workers [44] described the ratio of equatorial and axial hydroxyl
24
groups in hydration phenomena. This sequence of Ne-OH agrees with the phase-formation
25
ability found here, except for xylose.
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ACCEPTED MANUSCRIPT Comparing the phase-forming capability of isomeric monosaccharide (aldose –
2
glucose and ketoses – fructose) conformation, it is noted that six-membered aldoses have a
3
more favorable structure for hydrogen bonding with water because the equatorial hydroxyl
4
groups orientated within the plane of the six-membered ring accommodate more water
5
molecules in their vicinity [45].
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To evaluate the effect of the enantiomers, the binodal curves of D-(-)-arabinose and
7
L-(+)-arabinose were compared: it was observed that they overlapped. In other words, the
8
compounds had the same phase-forming capability. Additionally, the orientation of the
9
hydroxyl group at carbon 2 in epimers of the carbohydrate altered the phase forming ability,
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either to aldoses with five carbon atoms (D-(-)-arabinose and D-(+)-xylose) or aldoses with
11
six carbon atoms (D-(+)-glucose and D-(+)-mannose).
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For disaccharides, the binodal curve of D-(+)-maltose is closer to the origin of the
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axes, meaning that the phases are separated more easily than those in the binodal curve
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using D-(+)-sucrose (Figure 3), which agrees with Ne-OH 7.2 (maltose) and 6.3 (sucrose).
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Figure 4 depicts the binodal curves of systems based on THF and pure and
16
commercial carbohydrates (fructose, glucose and sucrose). The increasing order to induce
17
the ATPS formation is D-(-)-fructose ≈ fructose commercial < sucrose commercial ≈ D-(+)-
18
sucrose ≈ glucose commercial < D-(+)-glucose; therefore, the forming-phase using pure and
19
commercial carbohydrates was similar for fructose and sucrose. Maulyn and co-workers
20
[46] found by HPLC no inverted sugars in different commercial sucroses but very low
21
concentrations of minerals. Commercial crystallized fructose has on average 0.5 %
22
impurities [47]. The low concentration of impurities in commercial sugar did not affect the
23
phase-formation. Conversely, the discrepancies between the binodal curves for commercial
24
and pure glucose at low THF concentrations were probably derived from impurities (e.g.,
25
maltose, isomaltose and maltotriose) present in commercial glucose syrup, as reported by
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Pontoh and Low [41]. At high THF concentrations, these impurities were diluted, with the
2
binodal curves becoming similar. The advantage of using commercial carbohydrates was
3
their price. The price of pure carbohydrates was greater than that of commercial carbohydrates
5
(US$ 47.16 vs. US$ 2.30 – glucose; US$ 147.38 vs. US$ 13.54 – fructose and US$ 91.53 vs.
6
US$ 0.60 – sucrose). The prices of commercial carbohydrates were obtained directly from
7
the local supermarket in July 2016. Therefore, the use of commercial carbohydrates reduces
8
process costs while not significantly affecting the phase-forming ability (except for glucose).
9
Parameters A, B and C were adjusted by least-squares regression; the standard
10
deviations (σ) and correlation coefficient are displayed in Table 1. In general, good
11
correlation coefficients (0.9975 < R2< 0.9997) were obtained for all systems, indicating that
12
Equation (1) adequately correlated the experimental binodal curves.
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The experimental TLs, respective tie-line length (TLL), and the composition for the
14
studied systems, are all shown in Table 2. The graphical representation of the TLs can be
15
found in the Supporting Information (Figures A.1–A.11).
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Measurements of density and viscosity between 283.15 and 333.15 K were used to
17
characterize the top and bottom phases. These parameters were important for the design
18
and scale-up of the extraction processes and were determined for both phases of ATPS
19
with compositions 20–45 wt% and 25–45 wt% for carbohydrate and THF, respectively
20
(Figures 5–6 and Tables A.5–A.8). Differences in the density between the two phases
21
allowed faster and easier phase separation. For the systems studied, the THF-rich phases
22
(top phases) were less dense than the corresponding carbohydrate-rich phases. Compared
23
to the polymer-salt ATPS at the temperature reported in the literature [49], the densities
24
ranged from 0.881 g cm-3 (D-arabinose at 318 K) to 1.230 g cm-3 (pure glucose at 298.15
25
K) for the carbohydrate-rich phase, and from 0.875 g cm-3 (commercial sucrose at 318.15
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ACCEPTED MANUSCRIPT K) to 0.907 g cm-3 (xylose at 298.15 K) for the tetrahydrofuran-rich phase. The differences
2
in density between the carbohydrate-rich phase and the THF-rich phase at 298.15 K ranged
3
from 0.235 g cm-3 (pure glucose) to 0.322 g cm-3 (commercial glucose) for the system
4
consisting of 20–45 wt% (carbohydrate – THF) and from 0.261 g.cm-3 (commercial
5
glucose) to 0.352 g cm-3 (glucose) for the system consisting of 25–45 wt% (carbohydrate –
6
THF), while for ATPS composed of PEG-6000 and triammonium citrate, the difference
7
was only 0.08 g cm3 [49]. Hence, these systems separated easier and faster than that
8
formed by PEG-6000.
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The phase densities decreased linearly with temperature for all carbohydrates. A
10
comparison between pure carbohydrates and their commercial version for the studied
11
systems shows little difference for sucrose and fructose in contrast to glucose, probably
12
due to the composition of the glucose syrup used.
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The curves of viscosity versus temperature showed a typical exponential decrease.
14
This decrease of viscosity was assigned to intra- and intermolecular interactions [50], of
15
which H-bonding is the most important for the systems under study. The carbohydrate-rich
16
phase was shown to be more viscous than the corresponding THF-rich phase for all
17
systems investigated. Our viscosity data for sucrose, glucose and fructose agreed with
18
those reported in the literature [51,52]. Commercial sucrose and commercial fructose had
19
the same viscosity for system 1 (20–45 wt% carbohydrate – THF) but were slightly
20
different for system 2 (25–45 wt% for carbohydrate – THF).
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ATPS based on PEG and different salts presented viscosities in the PEG-rich phase
22
between 9.7 mPa.s (PEG-1000 - 20 wt% + K3PO4 – 11.4 wt%) and 202.0 mPa.s (PEG-
23
8000 – 21.9 % + K3PO4 – 12.3 %) [53]. At this temperature, the viscosity of the
24
carbohydrate rich-phase ranged from 8.54 (D- and L-arabinose) to 106.83 mPa.s (maltose)
25
for the system consisting of 20–45 wt% (carbohydrate – THF) and from 13.44 mPa.s
12
ACCEPTED MANUSCRIPT (commercial glucose) to 209.58 mPa.s (maltose) for the system consisting of 25–45 wt%
2
(carbohydrate – THF). The aforementioned data indicated that ATPS based on
3
carbohydrate and THF and ATPS based on PEG + K3PO4 present similar viscosities for
4
25–45 wt% (carbohydrate – THF); however, systems formed by 20–45 wt% (carbohydrate
5
– THF) are easier and faster to separate than ATPS based on PEG + K3PO4.
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3.2 Partitioning and Recovery of Diuron in ATPS
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The ATPS presented here have been studied as an alternative technique for the
9
extraction of diuron, an herbicide widely used in sugarcane cultivation. Figure 7 (Supporting
10
Information Table A.9) shows the partition coefficient of diuron in an aqueous two-phase
11
system based on carbohydrates and THF. Two different tie lines were investigated: 20–45
12
wt% and 25–45 wt% in carbohydrates and THF, respectively. The choice of the mixture
13
points was based on the phase-forming ability of all systems studied, while trying to
14
minimize the use of THF.
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The experimental data reflected the affinity of diuron for the tetrahydrofuran rich-
16
phase (1.23 < KDIU < 16.19), except for the system 25–45 wt% of L-(+)-arabinose–THF. The
17
aromatic ring of diuron influenced the hydrophobic characteristic and the migration of
18
diuron to the acetonitrile-rich phase. Lima and co-workers [9] reported a similar observation
19
in the partition of gallic acid in ATPS based on polyethylene glycol and potassium
20
phosphate. Their results had a good correlation with the octanol-water coefficient of diuron
21
(log Kow = 2.60) [35], indicating the affinity of diuron to the more hydrophobic phase.
22
Indeed, tetrahydrofuran (log Kow = 0.53) was more hydrophobic than carbohydrates (-2.30 <
23
log Kow< -4.70) [54].
AC C
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24
The pH values ranged from 4.91–7.07 for the top phase and 3.72–5.76 for the bottom
25
phase (Table 3). The speciation curve (Supporting Information Figure A.12) shows that the
13
ACCEPTED MANUSCRIPT pH does not change the chemical structure of diuron, which presents a positive charge on the
2
nitrogen of the aromatic ring and a negative charge on the carbonyl, i.e., a net charge of 0,
3
corresponding to a neutral form. In this way, diuron tended to migrate to the top phase. This
4
migration suggested that the main interactions controlling this partition were hydrophobic
5
interactions.
RI PT
1
The recovery in the top phase was higher than 51.19 % except for that of L-
7
arabinose, reaching 92.70 % for glucose (Figure 8 and Table A.9 of Supporting
8
Information). Commercial carbohydrates exhibited contradictory behavior: for glucose and
9
fructose, the recoveries were lower than in systems with commercial sugar, although these
10
values were similar for fructose and commercial fructose. Martinez and co-workers [55]
11
reported a diuron recovery of 98.8 % using a solid phase extraction.
M AN U
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6
12 13
4. Conclusions
A novel aqueous two-phase system based on carbohydrate (six monosaccharides,
15
two disaccharides and three commercial sugars) and tetrahydrofuran dextran has been
16
proposed. The phase diagrams with two tie-lines each were determined at 298.15 K and 0.1
17
MPa. In general, the number of equatorial hydroxyl groups of carbohydrates is the driving
18
force behind the phase-forming. Disaccharides form ATPS with tetrahydrofuran more
19
easily than monosaccharides, the same trend as aldoses compared with ketoses. Moreover,
20
the system based on carbohydrates (20 wt%) and THF (45 wt%) has differences in
21
densities and lower viscosities compared to ATPS based on PEG and K3PO4, allowing an
22
easier phase separation.
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23
The application of carbohydrates and tetrahydrofuran aqueous two-phase systems
24
for the partitioning of diuron was demonstrated to be a good strategy. The diuron migrates
25
preferentially to the top phase (i.e., the THF-rich phase). The highest values of the partition
14
ACCEPTED MANUSCRIPT coefficient KDIU and the recovery of diuron of the top phase (RT) were 16.19 and 92.70 %
2
for systems consisting of D-(+)-glucose (25 wt%) and tetrahydrofuran (45 wt%),
3
respectively. Commercial sugars presented differences in terms of density, viscosity, KDIU
4
and RT compared to pure carbohydrates; however, the results demonstrated that they may
5
be successfully used to partition diuron in aqueous two-phase systems based on
6
carbohydrates and tetrahydrofuran.
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1
7 Acknowledgments
9
We thank the funding agencies FAPITEC, CAPES and FINEP for financial support and CNPq for the scholarship of G.E.L.O. Maciel.
11
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ACCEPTED MANUSCRIPT Table 1. The data of the respective regression parameters (A, B and C) obtained by Merchuk’s equation, standard deviations (σ) and correlation coefficients (R2) are for the ternary systems composed of tetrahydrofuran + carbohydrates + water, at 298.15 ± 1 K and 0.1 ± 0.01 MPa. B±σ
C±σ
R2
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A±σ
Carbohydrate
156.31 ± 4.12 -0.314 ± 0.0061 1.10 x 10-6 ± 1.24 x 10-7
0.9998
D-(-)-fructose
131.04 ± 5.64 -0.323 ± 0.010 1.39 x 10-6 ± 2.59x10-7
0.9982
L-(+)-arabinose
221.40 ± 4.62 -0.453 ± 0.005 4.25 x 10-7 ± 1.19 x 10-7
0.9997
D-(-)-arabinose
96.57 ± 1.46
-0.296 ± 0.004 2.03 x 10-6 ± 1.45 x 10-7
0.9994
D-(+)-mannose
160.99 ± 2.85 -0.358 ± 0.006 4.72 x 10-7 ± 7.07 x 10-7
0.9995
D-(+)-glucose.
128.37 ± 2.16 -0.320 ± 0.006 6.25 x 10-6 ± 7.46 x 10-7
0.9993
D-(+)-sucrose
141.97 ± 3.30 -0.308 ± 0.005 8.81 x 10-7 ± 1.09 x 10-7
0.9998
D-(+)-maltose
101.68 ± 9.61 -0.246 ± 0.019 1.38 x 10-6 ± 3.45 x 10-7
0.9975
Commercial sucrose 183.35 ± 9.34 -0.369 ± 0.011 2.45 x 10-9 ± 2.12 x 10-7
0.9993
Commercial glucose 114.73 ± 3.44 -0.244 ± 0.010 4.41 x 10-6 ± 6.50 x 10-7
0.9989
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D-(+)-xylose
AC C
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Commercial fructose 154.35 ± 3.69 -0.346 ± 0.007 1.92 x 10 -6 ± 2.55 x 10-7 0.9994
ACCEPTED MANUSCRIPT Table 2. Experimental (liquid + liquid) equilibrium data of tie-lines (TLs) and tie-line lengths (TLLs) values of the coexisting phases for the tetrahydrofuran (THF) + carbohydrate (CAR)+ water in ATPS at 298.15 K and 0.1 MPa.a Weight fraction composition (wt%) [CAR]M
[THF]M
[CAR]T
[CAR]B
[THF]B
TLL
19.99 44.94 11.88 54.83 44.55 14.96 51.55 24.95 44.93 8.82 63.85 54.27 10.51 70.07 D-(-)-fructose 19.99 45.01 6.61 63.79 44.35 10.82 65.04 24.95 44.92 5.24 69.79 55.95 6.95 80.17 L-(+)-arabinose 19.95 44.96 5.09 65.18 43.53 12.89 64.89 24.96 44.99 4.71 67.48 57.35 8.90 78.76 D-(-)-arabinose 19.99 45.00 4.55 67.54 46.69 6.03 74.56 24.96 44.98 3.74 71.67 58.44 2.88 87.88 D-(+)-mannose 19.96 45.08 9.38 59.01 44.36 12.98 54.82 24.90 44.91 7.77 65.66 54.37 9.21 73.19 D-(+)-glucose. 20.00 45.00 1.40 67.14 50.82 8.29 76.85 24.94 44.97 0.79 72.37 59.55 5.72 88.85 D-(+)-sucrose 19.94 45.92 11.72 57.63 37.63 18.443 46.63 24.98 45.00 9.03 66.01 50.69 11.16 68.87 D-(+)-maltose 19.95 44.95 10.73 61.36 34.70 18.72 48.91 24.98 45.04 9.53 65.05 54.93 6.24 74.29 Commercial sucrose 19.95 44.92 11.15 57.58 38.89 17.67 48.60 24.93 44.83 9.78 63.08 52.64 11.44 67.11 Commercial glucose 20.00 44.86 6.46 60.54 47.94 12.54 63.44 25.03 45.00 3.97 67.52 60.77 7.75 83.18 Commercial fructose 20.03 44.97 6.63 61.26 47.54 12.00 64.39 24.86 44.83 4.92 67.81 56.44 9.00 78.61 a Standard uncertainties u are u([CAR] or [THF]) = 0.01, u(T) = 1 K, and u(p) = 10 kPa.
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D-(+)-xylose
[THF]T
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Carbohydrate
ACCEPTED MANUSCRIPT Table 3. pH values of the tetrahydrofuran (top)- and carbohydrate (bottom)-rich phases at 298.15 K and 0.1 MPa.a 25 wt% carbohydrate +
45 wt% tetrahydrofuran
45 wt% tetrahydrofuran +
Bottom phase
Top phase
Bottom phase
D-(+)-xylose
4.9
5.7
5.9
5.6
D-(-)-fructose
7.0
5.4
6.9
4.9
L-(+)-arabinose
4.9
4.8
5.0
4.9
D-(-)-arabinose
6.7
3.8
6.5
3.7
D-(+)-mannose
5.5
5.5
6.8
5.7
D-(+)-glucose
6.4
5.6
D-(+)-sucrose
7.0
4.9
D-(+)-maltose
6.5
4.6
Commercial sucrose
7.0
Commercial glucose
6.8
Commercial fructose
5.9
SC 6.5
4.7
7.0
5.7
7.0
5.5
5.6
6.5
5.4
3.7
6.4
3.9
4.9
4.9
4.9
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Standard uncertainties u are u(pH) = 0.1, u(T) = 1 K, and u(p) = 10 kPa.
AC C
a
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Top phase
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Carbohydrate
20 wt% carbohydrate +
ACCEPTED MANUSCRIPT Figure Capitation
Figure 1. Molecular structure of carbohydrates and tetrahydrofuran used to form aqueous two-phase system.
RI PT
Figure 2. Phase diagrams for the ternary system composed by monosacharides + THF + water at 298.15 ± 1 K and 0.10 ± 0.01 MPa: () D-(+)-xylose; () D-(-)-fructose; () L(+)arabinose; () D-(-)-arabinose; () D-(+)-mannose and () D-(+)-glucose.
SC
Figure 3. Phase diagrams for the ternary system composed by THF + disaccharides + water at 298.15 ± 1 K and 0.10 ± 0.01 MPa: () D-(+)-sucrose; () D-(+)-maltose.
M AN U
Figure 4. Phase diagrams for the ternary system composed by carbohydrates + THF + water at 298.15 ± 1 K and 0.10 ± 0.01 MPa: () D-(-)-fructose; () fructose commercial; () D-(+)glucose; () glucose commercial; () D-(+)-sucrose and () sucrose commercial. Figure 5. Experimental density (ρ / g.cm-3) for different aqueous two-phase system
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composed of 20–45 wt% (System 1), and 25–45 wt% for carbohydrate – tetrahydrofuran (System 2), respectively, at 298.15 ± 1 K and 0.10 ± 0.01 MPa:-D(+)-xylose, - D-(-)-fructose, - L-(+)-arabinose, - D-(-)-arabinose, - D-(+)-
EP
mannose, - D-(+)-glucose, -D-(+)-sucrose, - D-(+)-maltose, - Commercial
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sucrose, - Commercial fructose, - Commercial glucose. Figure 6. Experimental viscosity (η / mPa.s) for different aqueous two-phase system composed of 20–45 wt% (System 1), and 25–45 wt% for carbohydrate – tetrahydrofuran (System 2), respectively, at 298.15 ± 1 K and 0.10 ± 0.01 MPa:-D-(+)-xylose, - D-(-)fructose, - L-(+)-arabinose, - D-(-)-arabinose, - D-(+)-mannose, - D-(+)-glucose, -D-(+)-sucrose, - D-(+)-maltose, - Commercial sucrose, - Commercial fructose, - Commercial glucose. Figure 7. Partition coefficient of diuron (KDIU) in different aqueous two-phase systems at 1
ACCEPTED MANUSCRIPT 298.15± 1 K and 0.10 ± 0.01 MPa: 20–45 wt% and 25–45 wt% for carbohydrates tetrahydrofuran, respectively. Figure 8. Recovery of diuron in the top phase (RT) in different aqueous two-phase systems at 298.15± 1 K and 0.10 ± 0.01 MPa: 20–45 wt% and 25–45 wt% for carbohydrates -
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tetrahydrofuran, respectively.
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Figure 1. Molecular structure of carbohydrates and tetrahydrofuran used to form aqueous
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two-phase system.
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Figure 2. Phase diagrams for the ternary system composed by monosacharides + THF + water at 298.15 ± 1 K and 0.10 ± 0.01 MPa: () D-(+)-xylose; () D-(-)-fructose; () L-
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(+)arabinose; () D-(-)-arabinose; () D-(+)-mannose and () D-(+)-glucose.
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Figure 3. Phase diagrams for the ternary system composed by THF + disaccharides + water
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at 298.15 ± 1 K and 0.10 ± 0.01 MPa: () D-(+)-sucrose; () D-(+)-maltose.
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Figure 4. Phase diagrams for the ternary system composed by carbohydrates + THF + water at 298.15 ± 1 K and 0.10 ± 0.01 MPa: () D-(-)-fructose; () fructose commercial; () D-(+)-
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glucose; () glucose commercial; () D-(+)-sucrose and () sucrose commercial.
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Figure 5. Experimental density (ρ / g.cm-3) for different aqueous two-phase system
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composed of 20–45 wt% (System 1), and 25–45 wt% for carbohydrate – tetrahydrofuran (System 2), respectively, at 298.15 ± 1 K and 0.10 ± 0.01 MPa:-D(+)-xylose, - D-(-)-fructose, - L-(+)-arabinose, - D-(-)-arabinose, - D-(+)mannose, - D-(+)-glucose, -D-(+)-sucrose, - D-(+)-maltose, - Commercial
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sucrose, - Commercial fructose, - Commercial glucose.
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Figure 6. Experimental viscosity (η / mPa.s) for different aqueous two-phase system composed of 20–45 wt% (System 1), and 25–45 wt% for carbohydrate – tetrahydrofuran (System 2), respectively, at 298.15 ± 1 K and 0.10 ± 0.01 MPa:-D-(+)-xylose, - D-(-)fructose, - L-(+)-arabinose, - D-(-)-arabinose, - D-(+)-mannose, - D-(+)-glucose,
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-D-(+)-sucrose, - D-(+)-maltose, - Commercial sucrose, - Commercial fructose,
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- Commercial glucose.
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Figure 7. Partition coefficient of diuron (KDIU) in different aqueous two-phase systems at
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298.15± 1 K and 0.10 ± 0.01 MPa: 20–45 wt% and 25–45 wt% for carbohydrates -
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tetrahydrofuran, respectively.
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Figure 8. Recovery of diuron in the top phase (RT) in different aqueous two-phase systems at 298.15± 1 K and 0.10 ± 0.01 MPa: 20–45 wt% and 25–45 wt% for carbohydrates -
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tetrahydrofuran, respectively.
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ACCEPTED MANUSCRIPT Highlights
Tetrahydrofuran and carbohydrates form aqueous two-phase systems at 298.15 K.
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Number of equatorial OH is driving force to form ATPS. Commercial sugars form also two-phase system similar a pure compounds.
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Diuron migrates preferentially to the THF rich-phase.