Determination of Electroactive Organic Acids in Sugarcane Vinasse by

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Determination of Electroactive Organic Acids in Sugarcane Vinasse by High Performance Anion-Exchange Chromatography with Pulsed Amperometric Detection Using a Nickel Nanoparticle Modified Boron-Doped Diamond Graziela C. Sedenho,† José Luiz da Silva, Maísa A. Beluomini, Acelino C. de Sá, and Nelson R. Stradiotto* Department of Analytical Chemistry, Institute of Chemistry of Araraquara, Universidade Estadual Paulista (UNESP), Rua Prof. Francisco Degni, 55, 14800-060, Araraquara, São Paulo, Brazil ABSTRACT: Ethanol production process generates a huge quantity of vinasse. A suitable destination for this byproduct may be its utilization as source of chemical substances, by recovery within the biorefinery process. Vinasse is rich in organic acids, which present value-added due to their many industrial applications. In this context, the present work aimed the development of an anion-exchange chromatographic method with pulsed amperometric detection, using oxidized nickel nanoparticle modified boron-doped diamond electrode, to determine industrially interesting electroactive organic acids, such as lactic acid, malic acid, and tartaric acid, in sugarcane vinasse. The chromatographic separation was carried out in a CarboPac PA 1 column under gradient elution employing different proportions of 0.10 mol L−1 NaOH in 0.25 mol L−1 CH3COONa and deionized water. Under these conditions, lactic acid, malic acid, and tartaric acid were separated in 27 min. The limits of detection were 1.2 × 10−4 mol L−1 for lactic acid, 6.1 × 10−5 mol L−1 for malic acid, and 2.8 × 10−5 mol L−1 for tartaric acid. The concentration of each organic acid in sugarcane vinasse was determined to be (1.2 ± 0.3) × 10−1 mol L−1 lactic acid, (2.7 ± 0.6) × 10−3 mol L−1 malic acid, and (9.9 ± 1.0) × 10−4 mol L−1 tartaric acid. The values of recovery between 97.4 and 107.6% indicated the method has excellent accuracy. Our results showed the present method is attractive for routine analysis during the ethanol production process because of the not costly and not time-consuming sample preparation, no need for organic solvent, rapid run time, and satisfactory separation. Thus, it can contribute to the process of utilization of sugarcane vinasse as a source of value-added chemical substances.

1. INTRODUCTION

fertilization cannot always dispose of the total volume of vinasse produced in each season. In this context, in recent years, research has focused on finding alternative and adequate uses or treatments for vinasse in order to alleviate the vinasse problem and/or to improve the economic viability and sustainability of bioethanol production by production of other products besides ethanol and sugar.7−9 Some alternative destinations for vinasse have emerged, such as anaerobic or aerobic digestion,4,8 production of livestock feed,5 energy prodution,10,11 reuse in fermentation processes,12 and yeast production.5 Another alternative destination for vinasse is its utilization as a source of chemical substances, by recovery within the biorefinery process. Although the sugarcane vinasse chemical composition may vary,3 the organic fraction of vinasse is basically composed of alcohols, sugars, sugar derivatives, phenols, and organic acids.6,10,13−16 In particular, organic acids are present in sugarcane vinasse in large amounts because they are the principal products of sugar fermentation. Lactic acid and acetic acid are the major organic acids in sugarcane vinasse, which also contains citric acid, malic acid, oxalic acid, formic acid, propionic acid, and tartaric acid.6,10,13−16 Some of

In the past few decades, worldwide production and consumption of biofuels has increased significantly and it is expected that biofuel demand will increase further in the coming years. This is result of the search for renewable energy sources, mainly stimulated by environmental impact concerns and the reduction of fossil energy sources. Commercial production of biofuels, especially bioethanol, is well-established in the United States and Brazil.1 According to UNICA (Brazilian Sugarcane Industry Association), ethanol production in Brazil during the 2014/2015 season was 28.4 billion L.2 In the ethanol production process, industries generate large quantities of a waste, generally known as vinasse or stillage. On average, 10−15 L of vinasse is generated for each liter of ethanol, during the distillation step of ethanol.3,4 According to this proportion, the amount of sugarcane vinasse generated in Brazil in 2014/2015 season was estimated to be 300 billion L. In general, the surgacane vinasse presents a dark color and consists basically of water (93%) and organic compounds and minerals (7%).3 Because of the high content of water, nutrients, and organic matter, this effluent has been largely employed for irrigation and fertilization of soil. However, indiscriminate and inadequate disposal of sugarcane vinasse in crops can pollute the soil and the groundwater and cause negative effects on plants.3,5,6 As result, the utilization in soil irrigation and © XXXX American Chemical Society

Received: October 24, 2016 Revised: February 8, 2017 Published: February 13, 2017 A

DOI: 10.1021/acs.energyfuels.6b02783 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

pulsed amperometric detection (PAD). In our previous work, nickel nanoparticles have proved to be a good catalyst for organic acid oxidation in alkaline condition;42 for this, oxidized nickel nanoparticle modified boron-doped-diamond electrode (NiNPs-BDD) was employed, which is greatly compatible with HPAEC separation.

these compounds have interesting characteristics, such as acidulant, antioxidant, and biodegradable properties, which allow several applications in food,17,18 and the cosmetic,19,20 pharmaceutical,21 and chemical21,22 industries. Thus, the valueadded organic acids and the large content of these compounds in sugarcane vinasse can make this byproduct a source of chemicals. The commonly employed methods for identifying and quantifying organic acids in different matrixes are high performance liquid chromatography (HPLC),23−27 gas chromatography (GC),28,29 and electrophoresis (EC).26,30 However, for an efficient GC analysis, appropriate sample preparation is required, such as a cleanup step and/or sample derivatization. This because, in general, carboxylic acids show low volatility, strong polarity, and high solubility in water.29 Regarding carboxylic acid analysis by EC, in many cases, indirect photometric detection or sample derivatization is required, which makes the method complicated.26,30 These additional steps may restrict the practical application of the GC and EC methods for organic acid determination. Because of this, HPLC methods have been used more often for organic acid determination. Among the liquid chromatographic modes, high performance anion-exchange chromatography (HPAEC) has been used with success for separation of organic acids, based on interaction of these ionizable molecules with a anionexchange resin depending on their dissociation constants and hydrophobicities.31−35 However, the major problem of organic acids analysis by liquid chromatography is that aliphatic organic acids do not exhibit chromophores or fluorophores, generally. This fact limits the use of traditional spectrophotometric detectors, because sample derivatization is necessary. In this context, electrochemical detectors, mainly amperometric detectors, have become popular for detection of organic acids. In addition, amperometric detectors offer good sensitivity, selectivity, and wide linear range.31−35 In order to improve the amperometric performance, chemically modified electrodes with metal or metal oxide nanoparticles have been used for detection of a variety of organic analytes,33,36,37 including organic acids.31−33 Chemically modified electrodes with metal nanoparticles use much less material than bulk electrodes and have advantages in analytical approaches, because they provide enhancement of mass transport, improvement of the signal/noise ratio, and show higher effective surface area and sensitivity.38 Nickel nanoparticles are especially attractive because nickel is low cost, has low toxicity, and shows excellent catalytic activity toward the oxidation of a wide range of organic compounds, which also enables its application in sensors and detectors.39,40 Several materials can be used as substrates for metal nanoparticles, such as other metals, glassy carbon, graphite, and boron-doped diamond (BDD). Among them, BDD shows highly desirable electrode properties, which make it an attractive substrate for sensitive dynamic electroanalytical experiments. These properties include low capacitance, wide aqueous potential window, very low background capacitance relative to other carbonaceous and metal electrodes, and a mechanical and chemical robustness allowing application to extreme experimental conditions.41 In this context, the goal of the present work was to devise an anion-exchange chromatographic method for determination of industrially interesting electroactive organic acids, namely lactic acid, malic acid, and tartaric acid, in sugarcane vinasse, using

2. EXPERIMENTAL SECTION 2.1. Reagents and Solutions. Nickel(II) chloride hexahydrate (≥98%, NiCl2·6H2O) and anhydrous sodium acetate (≥98%, C 2H3NaO2) were purchased from Synth; glacial acetic acid (≥99.8%, C2H4O2), sulfuric acid (≥95%, H2SO4), sodium hydroxide (≥98%, NaOH), lactic acid (≥98%, C3H6O3), malic acid (≥99%, C4H6O5), and tartaric acid (≥99.5%, C4H6O6) were purchased from Sigma-Aldrich. All the solutions were prepared with deionized water from the Milli-Q system from Millipore at a resistivity of no less than 18.2 MΩ cm−1 at 25 °C. Acetate buffer (pH 5) was prepared using glacial acetic acid and anhydrous sodium acetate. NiCl2 solution was prepared in acetate buffer, and lactic acid, malic acid, and tartaric acid stock solutions were prepared in 1.0 mol L−1 NaOH, which was employed as supporting electrolyte. 2.2. Apparatus. Electrochemical experiments were carried out using a potentiostat/galvanostat (AUTOLAB Model PGSTAT 30) controlled by GPES 4.9 software. All experiments were carried out using a conventional single-compartment cell with three electrodes. The working electrode was a plate of BDD from Element Six (Didcot, U.K.) with an area of 1.0 cm2 and resistivity of 0.02−0.18 Ω cm. The exposed area of BDD was 0.20 cm2. A platinum wire and Ag/AgCl (KCl 3.0 mol L−1) were used as counter and reference electrodes, respectively. The experiments were performed at room temperature. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) were performed on a scanning electron microscope from Jeol, Model JSM 7500F. ImageJ software (National Institutes of Health, USA) was used to measure the NiNPs size. The chromatographic measurements were carried out with an 850Professional IC (Metrohm, Swiss), equipped with extender module 872 (postcolumn) and an autosampler 863-Compact Autosampler (Metrohm, Swiss). The anion-exchange chromatographic column was the DIONEX CarboPac PA 1 (4 × 250 mm) with guard column CarboPac PA 1 column (4 × 50 mm). An electrochemical cell with wall-jet configuration was used in the amperometric detector. The counter and reference electrodes were platinum and palladium, respectively. 2.3. Nickel Nanoparticle Modified BDD Electrode. Prior to the modification with NiNPs, the surface of the BDD electrode was cleaned by holding at 3.00 V (vs Ag/AgCl) in 0.50 mol L−1 H2SO4 for 200 s, followed by −3.00 V (vs Ag/AgCl) during the same interval of time. NiNPs were electrodeposited onto the BDD surface by reduction in acetate buffer (pH 5) containing 1.0 × 10−3 mol L−1 NiCl2. For this, BDD was held at −1.20 V (vs Ag/AgCl) until an electrodeposition charge of 90 mC was achieved.42 Thereafter, the electrode was rinsed, transferred to a 1.0 mol L−1 NaOH solution, and then subjected to 50 potential cycles between 0.00 and 0.60 V at 50 mV s−1 to form βNi(OH)2 species.43 2.4. Chromatographic Method. For chromatographic separation, two eluents were used: a mixture of 0.10 mol L−1 NaOH in 0.25 mol L−1 CH3COONa (eluent A) and deionized water (eluent B). The gradient program was the following: 0−1 min, 5% A; 1−12 min, 5− 17% A; 12−14 min, 17−100% A; 14−27 min, 100% A. The flow of mobile phase was 1.0 mol L−1, the column temperature was 20 °C, and the injection volume was 20 μL. In order to keep the ionic strength constant in the electrochemical detector and to ensure an adequately alkaline medium for electroactivity of NiNPs, a post-column pump with flow of 0.3 mL min−1 of 0.40 mol L−1 NaOH was used. The quadruple waveform for the PAD was as follows: E1, 0.48 V for 300 ms; E2, 0.10 V for 50 ms; E3, 0.50 V for 100 ms; and E4, 0.48 V for 150 ms. 2.5. Preparation of the Sample for Chromatographic Separation. Vinasse sugarcane was obtained from a sugar and B

DOI: 10.1021/acs.energyfuels.6b02783 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels alcohol plant in the region of Araraquara-SP/Brazil. The sample was centrifuged for 20 min at 4000 rpm and 4000g in an Excelsa Baby I Model 206 centrifuge. The supernatant was collected and filtered through filters with pores of 5, 0.47, and 0.22 μm, in sequence. Before the injections, the filtered sample was diluted in deionized water 80 times for analysis of lactic acid and five times for analysis of malic acid and tartaric acid.

3. RESULTS AND DISCUSSION 3.1. Characterization of the Nickel-Modified BDD Electrode. The surface of BDD modified with oxidized NiNPs was morphologically characterized by SEM. In Figure 1, representative SEM images at low and high magnifications Figure 2. Histogram showing diameter distribution of NiNPs.

Figure 3. Cyclic voltammograms, at 50 mV s−1, in 1.0 mol L−1 NaOH (curve I, solid line) and in the presence of 4.0 × 10−2 mol L−1 lactic acid (curve II, dashed line), 4.0 × 10−2 mol L−1 malic acid (curve III, dotted line), and 2.0 × 10−2 mol L−1 tartaric acid (curve IV, dashed− dotted line), using NiNPs-BDD.

(curve II), malic acid (curve III), and tartaric acid (curve IV), using NiNPs-BDD. In curve I, the redox couple observed at ca. 0.34 V (vs Ag/AgCl) is attributed to the oxidation of Ni(II) to Ni(III), and reduction to Ni(III) to Ni(II); see eq 1. Ni(OH)2 + OH− ⇌ NiOOH + H 2O + e−

(1)

With organic acid addition, a pronounced increase of anodic peak current (Ipa) and an anodic shift in the peak potential are observed (see curves II, III, and IV); this behavior evidences the catalytic oxidation of the organic acids by Ni(III) species.42 The decrease of cathodic peak current (Ipc) can be attributed to the catalytic cycling of NiOOH species during the chemical reaction of organic acid oxidation, according to eq 2.44 The most pronounced electrochemical response observed for tartaric acid is due to the two hydroxyl groups able to be oxidized, which gives the transfer of four electrons, while lactic acid and malic acid have just one oxidizable group, transferring two electrons (see chemical structures in Figure 4). The lactic acid, malic acid, and tartaric acid did not exhibit significant electrooxidation response on the unmodified BDD electrode in alkaline medium (not shown).

Figure 1. SEM images of NiNPs-BDD at an acceleration voltage of 10 kV and magnifications of 10 000 (A) and 50 000 times (B).

show the BDD surface was homogeneously covered by monodispersed spherical NiNPs. The histogram in Figure 2 shows the diameter of most NiNPs varied from 80 to 100 nm, with a mean diameter of 89 nm ± 15 nm. The EDX analysis (not shown) showed the presence of carbon, from the substrate, oxygen, and nickel, due to the modification with oxidized NiNPs, as expected. 3.2. Organic Acid Electrooxidation. When placed in the alkaline solution, the electrodeposited metallic nickel is electrochemically converted to nickel hydroxide at cathodic potential, which is electrooxidized to nickel oxyhydroxide at approximately 0.40 V (vs Ag/AgCl), as described in the eq 1.40−42 Figure 3 shows typical cyclic voltammograms in 1.0 mol L−1 NaOH solution (curve I) and in the presence of lactic acid

3.3. Detection and Separation of Organic acids. According to voltammetric experiments in the presence of organic acids (see Figure 3), these compounds can be detected by measurement of the electrical current generated through their oxidations on the surface of the NiNPs when potentials C

DOI: 10.1021/acs.energyfuels.6b02783 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 4. Chemical structures of lactic acid (A), malic acid (B), and tartaric acid (C). Figure 5. Chromatogram of a standard mixture containing 1.0 × 10−3 mol L−1 lactic acid (1), 7.5 × 10−4 mol L−1 malic acid (2), and 3.0 × 10−4 mol L−1 tartaric acid (3).

higher than ca. 0.40 V (vs Ag/AgCl) are applied. However, the products of these oxidation reactions can adsorb strongly on the surface of the electrode, which can cause its poisoning. Because of this, PAD mode was chosen, since it allows cleaning of the electrode surface between the measurements and obtaining stable amperometric responses.31,33,35,36,45 For this, a sequence of four pulses of potential was employed: The first pulse (E1) was the detection potential, where the organic acids are oxidized. During this interval of time, the data were collected. In the second pulse, E2, a less anodic potential was applied to reduce the surface of the electrode and to remove the possible adsorbed oxidation products. A higher potential was applied in the third pulse (E3) in order to guarantee the formation of NiOOH electroactive species. Last, in the fourth pulse (E4) was applied the same potential as in the E1 in order to stabilize the surface of the electrode before starting the next cycle of potential pulses. The values of the potential and duration of each pulse that provide the highest detectability to the organic acids were the following: E1, 0.48 V for 300 ms; E2, 0.10 V for 50 ms; E3, 0.50 V for 100 ms; and E4, 0.48 V for 150 ms. In alkaline conditions, nickel-based electrodes show a high electrochemical activity for electrooxidation of hydroxylcontaining organic compounds.42 Because carboxylic acids become anionic in alkaline conditions, an anion-exchange column can be employed for separation of these compounds. However, deprotonated organic acids and additional −OH groups can show high affinity toward the anion-exchange stationary phase. Because of this, the chromatographic separation, in suitable retention time, requires a mobile phase with a stronger elution ability than −OH. In this way, mixtures of sodium acetate and sodium hydroxide have been used as eluent in order to solve this problem.31,32,35,45 The separation conditions, such as the composition of the mobile phase, gradient program, flow, and temperature of the column, were studied in order to achieve the best-possible chromatographic separation of the analytes in a suitable run time. The gradient elution mode was employed because of the difference of affinity between mono- and dicarboxylic acids toward the anionexchange column. The optimal separation was achieved employing as eluent the mixture of 0.10 mol L−1 NaOH with 0.25 mol L−1 CH3COONa (eluent A) and deionized water (eluent B), at 1.0 mol L−1 under the following gradient program: 0−1 min, 5% A; 1−12 min, 5−17% A; 12−14 min, 17−100% A; 14−27 min, 100% A; and column temperature of 20 °C. The Figure 5 shows a typical chromatogram of a standard mixture containing lactic acid, malic acid, and tartaric acid. Lactic acid is the first compound to elute, at 11.31 min, followed by malic acid and tartaric acid, at 24.10 and 26.06 min,

respectively. Malic acid and tartaric acid showed higher retention times than lactic acid and required increase in mobile-phase strength because they are dicarboxylic compounds and, thus, they have stronger affinity to the anionexchange stationary phase. The rise in the baseline between ca. 15 min and ca. 17 min occurs because of the increase of the proportion of eluent “A” up to 100%. 3.4. Calibration, Limits of Detection, and Stability. After the optimization of the chromatographic conditions, analytical curves were constructed to evaluate the analytical parameters, which can be found in Table 1. For the three organic acids, the peak areas varied linearly with the concentration over 1 or 2 orders of magnitude range, with determination coefficients (r2) of at least 0.995. The limits of detection (LOD) were deduced from 3 × (standard deviation for a calibration curve/sensitivity).46 The NiNPs-BDD electrode showed a good temporal stability under the chromatographic conditions. A freshly prepared electrode, after 6 h of continuous injections of a standard mixture containing the three analytes, exhibited a signal decrease of less than 15%. The retention time reproducibility expressed as the relative standard deviation (RSD) was lower than 1% (n = 5). 3.5. Accuracy. To determine the analytical accuracy, the recovery was measured in the sugarcane vinasse sample by spiking with three different concentrations of each organic acid (see Table 2). The values of recovery were from 97.4 to 107.6%, with RSD lower than 7%. The found concentrations were statistically compared to nominal concentrations using Student’s t test. t calculated values did not exceed the theoretical ones at 95% (n = 3) of confidence level. This indicates that there were no significant differences between the found concentrations and the nominal ones. Thus, the recovery test showed that the developed analytical method can be considered suitable and accurate for the determination of lactic acid, malic acid, and tartaric acid in sugarcane vinasse. 3.6. Determination of Organic Acids in Sugarcane Vinasse. The developed HPAEC-PAD method using NiNPsBDD electrode was used for determining the contents of the three organic acids in sugarcane vinasse. A typical chromatogram of a sugarcane vinasse sample is shown in Figure 6. The organic acids were identified in the sample chromatogram according to the retention times observed in the standard chromatogram shown in Figure 5. The retention time for lactic acid in the sample was the same as in the mixture standard, 11.31 min; on the other hand, malic acid and tartaric acid had D

DOI: 10.1021/acs.energyfuels.6b02783 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Table 1. Quantitative Parameters in HPAEC-PAD of Organic Acids at NiNPs-BDD Electrode (n = 3) linear range (mol L−1) −4

LOD (mol L−1)

−3

3.0 × 10 −4.0 × 10 8.0 × 10−5−1.5 × 10−3 4.0 × 10−5−1.0 × 10−3

lactic acid malic acid tartaric acid

lactic acid

malic acid

tartaric acid

5.00 1.00 1.50 5.00 7.50 1.00 1.00 2.50 5.00

× × × × × × × × ×

−4

10 10−3 10−3 10−4 10−4 10−3 10−4 10−4 10−4

found concn (mol L−1) 5.12 1.02 1.52 5.15 7.31 1.01 1.08 2.64 4.87

× × × × × × × × ×

−4

10 10−3 10−3 10−4 10−4 10−3 10−4 10−4 10−3

r2

7.5 × 10 3.5 × 104 5.4 × 105

0.995 0.997 0.996

1.2 × 10 6.1 × 10−5 2.8 × 10−5

recovery (%)

RSD (%)

102.3 ± 1.9 102.2 ± 4.6 101.0 ± 4.6 103.1 ± 6.8 97.5 ± 2.3 100.1 ± 3.3 107.6 ± 4.0 105.5 ± 2.9 97.4 ± 1.0

1.8 4.5 4.6 6.6 2.4 3.3 3.7 2.7 1.0

3

± 0.08, and 0.15 ± 0.01 g L−1, respectively. The content of each organic acid is according to the literature.6,13,15

Table 2. Recoveries of Organic Acids in HPAEC-PAD Using NiNPs-BDD Electrode (n = 3) nominal concn (mol L−1)

sensitivity (nA L mol−1)

−4

4. CONCLUSIONS In this study, an analytical method was developed to determine three industrially interesting organic acids in sugarcane vinasse, by HPAEC with PAD employing NiNPs-BDD. The present method is advantageous because of the very simple sample preparation, because there is no need for organic solvent, because of the rapid run time, and because of satisfactory separation of the complex sample. Furthermore, great accuracy, high sensitivity, and good stability were observed. Thus, the results show a practical and efficient chromatographic method for routine analysis of organic acids in sugarcane vinasse, which can contribute to utilization of this byproduct as a source of value-added chemical substances.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +55 16 3301 9621. ORCID

Graziela C. Sedenho: 0000-0001-8696-5978 José Luiz da Silva: 0000-0002-7130-9684 Maísa A. Beluomini: 0000-0003-3441-1572 Acelino C. de Sá: 0000-0002-9811-4879 Present Address †

Figure 6. Chromatogram of sugarcane vinasse. (1) Lactic acid, (2) malic acid, and (3) tartaric acid.

G.C.S.: Institute of Chemistry of São Carlos, Universidade de São Paulo (USP), Avenida Trabalhador São-carlense, 400, 13560-970, São Carlos, SP, Brazil. Author Contributions

shifts of 2.1 and 1.4%, respectively, in their retention times in comparison to the standard mixture (see Figure 5). These changes can be attributed to the matrix effect and may be originated from the competition between the analytes and the matrix components. Lactic acid, malic acid, and tartaric acid were separated from the other compounds of the matrix with enough resolution, since the values of resolution between the two consecutive peaks were higher than 1.47 Organic acid concentrations were determined by standard addition method because a sample blank cannot be obtained, and this method is able to minimize to the maximum the matrix interferences.46 In this way, the samples were spiked with known concentrations of the standards from 5.0 × 10−4 to 2.0 × 10−3 mol L−1 lactic acid, from 2.5 × 10−4 to 1.0 × 10−3 mol L−1 malic acid, and from 5.0 × 10−5 to 5.0 × 10−4 mol L−1 tartaric acid. A linear relationship was observed when peak areas were plotted against the added concentration of each organic acid. The regression equation for lactic acid was y = 1.25 × 104x + 19.66, with r2 of 0.999; for malic acid was y = 4.89 × 104x + 25.03, with r2 of 0.995; and for tartaric acid was y = 3.53 × 105x + 71.70, with r2 of 0.993. By extrapolation of the line and considering the sample dilution, the concentrations found in sugarcane vinasse were (1.2 ± 0.3) × 10−1 mol L−1 lactic acid, (2.7 ± 0.6) × 10−3 mol L−1 malic acid, and (9.9 ± 1.0) × 10−4 mol L−1 tartaric acid, which correspond to 10.8 ± 2.7, 0.36

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank FAPESP (2013/09833-5 and 2014/238465) for financial support.

E

ABBREVIATIONS HPLC = high performance liquid chromatography GC = gas chromatography EC = electrophoresis HPAEC = high performance anion-exchange chromatography PAD = pulsed amperometric detection NiNPs = nickel nanoparticles BDD = boron-doped-diamond electrode NiNPs-BDD = nickel nanoparticle modified boron-dopeddiamond electrode SEM = scanning electron microscopy EDX = energy-dispersive X-ray spectroscopy Ipa = anodic peak current DOI: 10.1021/acs.energyfuels.6b02783 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

(30) Golubenko, A. M.; Nikonorov, V. V.; Nikitina, T. G. J. Anal. Chem. 2012, 67, 778−782. (31) Beluomini, M. A.; da Silva, J. L.; Stradiotto, N. R. Anal. Methods 2015, 7 (6), 2347−2353. (32) Casella, I. G.; Gatta, M. J. Chromatogr. A 2001, 912 (2), 223− 233. (33) Casella, I. G.; Gatta, M.; Desimoni, E. J. Chromatogr. A 1998, 814 (1−2), 63−70. (34) Casella, I. G.; Gatta, M. J. Agric. Food Chem. 2002, 50 (1), 23− 28. (35) Hu, Q.; Tan, L.; Heng, Z.; Su, X.; Zhang, T.; Jiang, Z.; Xiong, X. Energy Fuels 2012, 26 (5), 2942−2947. (36) da Silva, J. L.; Beluomini, M. A.; Stradiotto, N. R. J. Sep. Sci. 2015, 38, 3176−3182. (37) Prabhu, S. V.; Baldwin, R. P. Anal. Chem. 1989, 61 (8), 852− 856. (38) Baldwin, R. P.; Thomsen, K. N. Talanta 1991, 38 (1), 1−16. (39) Campbell, F. W.; Compton, R. G. Anal. Bioanal. Chem. 2010, 396, 241−259. (40) Miao, Y.; Ouyang, L.; Zhou, S.; Xu, L.; Yang, Z.; Xiao, M.; Ouyang, R. Biosens. Bioelectron. 2014, 53, 428−439. (41) Toghill, K. E.; Compton, R. G. Electroanalysis 2010, 22, 1947− 1956. (42) Sedenho, G. C.; Lee, P. T.; Toh, H. S.; Salter, C.; Johnston, C.; Stradiotto, N. R.; Compton, R. G. J. Phys. Chem. C 2015, 119, 6896− 6905. (43) Guzmán, R. S. S.; Vilche, J. R.; Arvía, A. J. J. Electrochem. Soc. 1978, 125 (10), 1578−1587. (44) Fleischmann, M.; Korinek, K.; Pletcher, D. J. Electroanal. Chem. Interfacial Electrochem. 1971, 31, 39−49. (45) Rothenhöfer, M.; Grundmann, M.; Bernhardt, G.; Matysik, F. M.; Buschauer, A. J. Chromatogr. B: Anal. Technol. Biomed. Life Sci. 2015, 988, 106−115. (46) Snyder, L. R.; Kirkland, J. J.; Dolan, J. Introduction to Modern Liquid Chromatography, 3rd ed.; Wiley: Hoboken, NJ, 2010. (47) Snyder, L. R.; Kirkland, J. J.; Glajch, J. L. Practical HPLC Method Development, 2nd ed.; John Wiley & Sons: New York, 1997.

Ipc = cathodic peak current LOD = limit of detection RSD = relative standard deviation

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DOI: 10.1021/acs.energyfuels.6b02783 Energy Fuels XXXX, XXX, XXX−XXX