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Chronoamperometric and chronopotentiometric investigation of Kraft black liquor
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Chronoamperometric and chronopotentiometric investigation of Kraft black liquor R.C.P. Oliveira a, M. Mateus b, D.M.F. Santos a,* Center of Physics and Engineering of Advanced Materials (CeFEMA), Instituto Superior Tecnico, Universidade de Lisboa, 1049-001 Lisbon, Portugal b Center for Natural Resources and the Environment (CERENA), Instituto Superior Tecnico, Universidade de Lisboa, 1049-001 Lisbon, Portugal a
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abstract
Article history:
Black liquor is an effluent of pulp and paper mills. The electrolysis of Kraft black liquor is
Received 19 September 2017
drawing high attention, as it leads to lignin recovery by anodic electrodeposition and
Received in revised form
cathodic production of hydrogen. However, the lignin oxidation process on black liquor is
5 January 2018
still poorly understood, requiring further fundamental studies to allow its comprehension.
Accepted 8 January 2018
Therefore, detailed electrochemical studies are being carried out on the topic, which will
Available online xxx
allow a deeper insight in the involved electrode processes. The Kraft black liquor is physicochemically characterised, with the determined high values of pH, conductivity and
Keywords:
lignin content (12.4, 460 mS cm1 and 48.8 g dm3, respectively) demonstrating the suit-
Kraft black liquor electrolysis
ability of black liquor as an electrolyte medium. Voltammetry, chronoamperometry (CA)
Lignin oxidation
and chronopotentiometry (CP) techniques are done in black liquor solutions using Pt, Ni
Lignin recovery
and AISI 304 SS electrodes to investigate the lignin oxidation process. The number of
Waste valorisation
exchanged electrons is determined for each electrode material from CA and CP measurements using classic Cottrell and Sand analyses, respectively, with the faradic efficiency of Ni being close to that of Pt. The low-cost and high efficiency suggest Ni as a promising electrode material for application in black liquor electrolysers. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The Kraft process is the most used pulping process in pulp and paper industries, holding 90% of the total production capacity. It involves the removal of the lignin, an organic polymer that acts as the glue between cellulose and hemicellulose in the wood, to isolate the cellulose fibres in the pulp [1e3]. For that, the wood chips are placed in a pressurised reactor (digester) with cooking chemicals (NaOH and Na2S) to promote the lignin's separation. Subsequently, the digester content is washed, originating pulp (mainly composed of cellulose fibres)
and black liquor, the major by-product produced by pulp mills, containing water, cooking chemicals, hemicellulose and lignin. For each ton of produced pulp are generated ca. 7 tons of black liquor [3e7]. Currently, the black liquor is concentrated in a multipleeffect evaporator and burned in a recovery furnace to recover the inorganic chemicals used during the cooking process and produce energy and heat, a well-known 80-yearold process named Tomlinson recovery [4,6e8]. Although being widely used, the process presents several disadvantages, such as the generation of air pollutants (NOx, SOx) harmful for the human health and environment and low
* Corresponding author. E-mail address: [email protected] (D.M.F. Santos). https://doi.org/10.1016/j.ijhydene.2018.01.046 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Oliveira RCP, et al., Chronoamperometric and chronopotentiometric investigation of Kraft black liquor, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.01.046
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efficiency (as the boiler requires excessive energy to concentrate the black liquor before incinerating it) [8]. In addition, pulp mills face capacity issues due to the increasing demands; during production peaks the amount of black liquor produced may exceed the capacity of the recovery furnace, being a pulp production bottleneck that forces a temporary stoppage of production [9]. Thus, it is essential to develop alternative processes for the treatment of the black liquor [9]. Black liquor contains lignin, the second most abundant organic polymer in nature [10,11]. Lignin has high economic value and a large range of applications, namely in phenolic resins, epoxies, adhesives, polyolefins, binders, polyurethane copolymers and admixtures for concretes, just to name a few [3,4,10]. If the lignin recovery from black liquor is done inside the pulp mill, it could even be used to feed the lime kiln, reducing the fossil fuel needs [12]. Having these in mind, lignin recovery processes have been studied as an economic, practical and ecologic way to use the excess of black liquor [4,12e26]. Some of these processes include the use of membranes [12,14,18,22,27,28], although it has high costs and drawbacks associated with the membranes sealing [14,27]. Acidification techniques [15e17,21,23] are also particularly noteworthy, as the black liquor's lignin precipitates when the pH is decreased (normally for pH < 9). In fact, a sequential acidification is used in the LignoBoost™ [15] and LignoForce™ [16,23] commercial processes. The lignin produced using LignoBoost™ contains sulphur, making its use difficult for certain applications [9,15]. In the case of Lignoforce™ system, O2 is used as oxidant before the black liquor's acidification, to increase the lignin purity (namely by decreasing the sulphur content in the final solid) and decrease the chemicals needs [16,23]. However, these methods have high investment costs and require the use of chemicals [14]. Moreover, both use the so-called “strong black liquor” generated after the evaporation process, thus being necessary to spend additional energy for the water removal before applying these methods [15,16,23]. On the other hand, the electrolysis of black liquor has been recently envisaged [4,19,20,24,28]. This process leads to lignin deposition on the electrolyser anode with simultaneous generation of hydrogen on the cathode side [4,19,20,24]. Thus, the proposed electrochemical treatment of the Kraft black liquor allows the generation of a clean fuel with high calorific value on the cathode and the recovery of a compound with economic value [4]. Despite the importance of the process, there are still very few detailed studies in the topic, meaning that the underlying mechanisms that govern the lignin oxidation (and consequent deposition) at the anode of the black liquor electrolyser cell are yet to be fully understood. For that reason, we have carried out a series of electrochemical studies on black liquor, which allow a deeper insight in the involved electrode processes. Specifically, the work presents cyclic voltammetry (CV), chronoamperometry (CA) and chronopotentiometry (CP) studies of black liquor oxidation on platinum (Pt), nickel (Ni) and AISI 304 stainless steel (SS) electrodes. These techniques are used to obtain kinetic parameters that allow assessing the efficiency of the anode process in black liquor electrolysis. Several physicochemical properties of the black liquor solution are also evaluated.
Experimental Characterisation of the black liquor samples All black liquor samples used in this study were provided by a Kraft pulp mill that uses exclusively Eucalyptus Globulus (a hardwood tree) wood. The physical characterisation of those samples was done at room temperature. The pH was determined using a Hanna Instruments PH20-01 pH meter with HI1110B pH electrode, the conductivity was determined using a Hanna Instruments HI8733 conductivity meter and the density was determined using a 50 mL glass pycnometer following ASTM D1217-15 standard test method [29]. The dry solids content in the black liquor was determined by evaporating most of the water in a Heidolph VV200 rotary evaporator followed by drying the samples in a Nabertherm LE 2/11/R6 muffle at 120 C, until constant weight is achieved. To determine the organic/inorganic ratio of the black liquor dry solids, the solid ash content was obtained according to typical determination of biomass ash procedure [30]. The lignin content in the black liquor dried solids was determined using thermogravimetric analysis in N2-inert atmosphere. For that, 10.9 mg of the solid sample was inserted in a NETZSCH STA 409 PC thermogravimetric balance using heating rate of 10 C min1 (from 25 to 1200 C). The cooling process was performed by a JULABO 5 water bath. A NETZSCH Proteus Thermal Analysis software was used for controlling the experiments.
Electrochemical studies Electrochemical measurements were performed in an electrochemical cell containing 125 mL of black liquor at room temperature. The cell comprised a typical three electrode arrangement, with platinum (Pt, Metrohm 60305100, A ¼ 1 cm2), nickel (Ni, A ¼ 0.79 cm2) or AISI 304 stainless steel (SS, A ¼ 8 cm2) working electrode, a saturated calomel electrode (SCE, Metrohm 60701100) as a reference electrode, and a Pt mesh counter electrode (Johnson Matthey, A ¼ 50 cm2). The distance between working and reference electrodes was the lowest possible in order to minimise the ohmic drop. A PAR 273A computer-controlled potentiostat/galvanostat (Princeton Applied Research, Inc.) and the associated PowerSUITE package were employed for total control of the experiments and data acquisition. The cell was magnetically stirred in between experiments to keep the black liquor sample homogeneous, as it contains solids in suspension that can sediment during the measurements, modifying the experimental conditions. Voltammograms were run at 50 mV s1 for each working electrode to make an initial assessment of the electrochemical behaviour of Kraft black liquor. In CA, potentials between 0.5 V and 0.6 V were applied, with the transient currents following a stepwise change of the electrode potential from the open circuit potential (OCP, no oxidation) to a value where the lignin oxidation was sufficiently fast so that its concentration at the electrode surface was essentially zero, i.e. diffusion-controlled. Currents ranging from 2.5 to 24 mA cm2 were applied in CP measurements, with the electrode
Please cite this article in press as: Oliveira RCP, et al., Chronoamperometric and chronopotentiometric investigation of Kraft black liquor, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.01.046
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potential shifting from the OCP to a potential value where lignin oxidation could occur.
Results and discussion Chemical characterisation of the samples The measured values of conductivity, density, pH, dry solids content and organic/inorganic ratio of the Kraft black liquor samples is presented in Table 1. These values were found to be in good agreement with those previously reported [3,31]. The high ionic conductivity and alkaline pH of the black liquor are important features for a good electrolyte. The high conductivity reflects the electrolyte good ionic conduction, minimising the ohmic resistance in the system and enabling a fast charge transfer in solution. The alkaline pH is also extremely advantageous, as it allows the use of cheaper electrode materials, like transition metals, instead of the expensive noble metal-based electrodes frequently required for acid media. Additionally, the high lignin concentration leads to a more efficient electrode process, by decreasing the mass-transfer resistance within the electrolyte [32e35]. The total lignin content in the black liquor dry solids was determined by thermogravimetric analysis in inert atmosphere [4]. Fig. 1 shows the thermal decomposition of the Kraft black liquor dry solids, which proceeds through four main stages. The first mass loss occurring between the room temperature and 200 C corresponds to the residual humidity loss, i.e., the water associated with hemicellulose and organic compounds whose boiling points, bp, are in this temperature range, as dimethyl sulphide (bp ¼ 38 C) and dimethyl disulphide (bp ¼ 112 C) [36]. The second stage occurs in the 200e550 C temperature range and corresponds to the pyrolysis of organic compounds, as hemicellulose (200e260 C), low molecular weight organic acids (200e300 C) and lignin [36e38]. Different lignin degradation temperature ranges can be found in the literature [36e41]; those differences can be attributed to the complex structure of lignin, which is dependent on its source (type of wood), heating rate and type of degradation atmosphere during the cooking process and extraction process (acidification, oxidation, electrooxidation, membrane processing, etc.) [41]. Herein, it is assumed that lignin degradation occurs in the 240e600 C temperature ~ al et al. with range, as reported in the studies by Sebio-Pun Eucalyptus Globulus lignin [39]. In this range there is the breaking of the bonds between the monomeric units of lignin, generating the release of phenols [38]. The decomposition of
Fig. 1 e Thermogravimetric analysis of Kraft black liquor dry solids (adapted from Ref. [4]).
aromatic rings occurs above 400 C, specifically demethoxylation and/or demethylation of the aromatic ring [38]. Taking this into account and assuming that the degradation of other components of black liquor is negligible in the 240e600 C range, a value of 26.5 wt.% was determined for the lignin composition in the black liquor dry solids. This value is in agreement with those reported by Gebremeskel et al. for the total lignin content in hardwoods using acidification (30.2 wt.%) and capillary zone electrophoresis (31.2 wt.%) [42]. From the above data, the lignin mass concentration, Cm (mol dm3), in the black liquor samples can be assessed using Eq. (1), Cm ¼ Cds Clds rbl
(1)
where Cds is the dry solids ratio in the black liquor (0.167 or 16.7 wt.%), Clds is the lignin ratio in the black liquor's dry solids (0.265 or 26.5 wt.%), rbl is the density of the black liquor (1103 g dm3). Thus, a value of 48.8 g dm3 was determined for Cm. Considering the molar mass of lignin, Mlig, from Eucalyptus Globulus Kraft black liquor (1000 g mol1 [43]), it is possible to determine the value of 4.88 102 mol dm3 for the molar concentration, C, of lignin in black liquor from the expression C ¼ Cm/Mlig. As stated above, together with the high pH and high ionic conductivity, the determined high lignin concentration is an extremely important parameter for the black liquor use as electrolyte in industrial electrochemical processes, as the high lignin content decreases the masstransfer resistance, thus enhancing the efficiency of the electrode process [32].
Electrochemical studies Voltammetric characterisation
Table 1 e Physicochemical properties of the Kraft black liquor samples. Conductivity/mS cm1 Density/g dm3 pH Dry solids content/wt.% Organic/inorganic ratioa a
Value considered in a dry basis.
460 1103 12.4 16.7 1.0
The voltammograms obtained in black liquor solution at 50 mV s1 using platinum (Pt), nickel (Ni) and AISI 304 stainless steel (SS) electrodes in natural diffusion conditions are presented in Fig. 2. Fig. 2 shows that all three tested electrodes present welldefined oxidation peaks in the forward scan (Fig. 2A) and the typical cathodic currents associated with hydrogen evolution reaction (HER) (Fig. 2B). Two sharp oxidation peaks are observed at Pt at potentials of 0.22 V and 0.10 V, with peak
Please cite this article in press as: Oliveira RCP, et al., Chronoamperometric and chronopotentiometric investigation of Kraft black liquor, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.01.046
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Fig. 2 e Positive scan CVs run in black liquor at 50 mV s¡1 using Pt, Ni and SS electrodes (A) and comparison of the corresponding cathodic polarisation curves (B).
current densities, jp, of 4.7 mA cm2 and 7.9 mA cm2, respectively. The lignin oxidation at Ni electrode is recorded as a broader peak at potential of 0.07 V and a jp of 2.9 mA cm2. AISI 304 SS presented an anodic peak with intensity of 3.9 mA cm2 at 0.57 V, a potential significantly shifted to the right, demonstrating the considerably lower activity for lignin oxidation of SS compared to the other two metals. The absence of cathodic peaks in the backscan (Fig. 2A) suggests the irreversibility of the lignin oxidation process at the tested electrodes, in agreement with previous studies [20]. Regarding the cathodic direction scans in black liquor (Fig. 2B), the onset potential for HER at Pt electrode was ca. 1.10 V, whereas for Ni and SS it was 1.20 V and 1.30 V, respectively. The current densities recorded for Pt, Ni and SS electrodes at 1.50 V were 49.8, 17.7 and 2.2 mA cm2, respectively. Despite the superior activity of Pt, these results also attest the high activity of Ni both for lignin oxidation and HER.
Chronoamperometric study Considering the anodic behaviour observed during the initial voltammetric assessment, namely the potential range of the oxidation peaks, a CA study was performed for each working electrode. CA measurements involve following the current response with time after applying a specific electrode potential [44]. In this case, potentials ranging between 0.5 and 0.6 V were applied. Fig. 3D shows typical current densitytime (j vs. t plot) transients in Pt, Ni and SS electrodes for an applied potential of 0.1 V. CA measurements in black liquor are based in an instantaneous potential change from the OCP, where no oxidation occurs, to a more positive applied potential. Thus, all compounds susceptible to oxidation in the vicinity of the electrode are oxidised almost instantly, generating high current. That is followed by a current decrease due to a concentration drop of the species next to the electrode and finally by the current stabilisation, a region of the CA known as the diffusion-controlled region. The stable current in this latter region is controlled by the diffusion of new species to the working electrode surface [35]. This typical behaviour was observed for all electrodes (Fig. 3D). Ni and Pt electrodes present very similar current transients, confirming that Ni has a catalytic behaviour close to Pt, with the lignin oxidation occurring at similar potentials. As for SS, the measured low current densities in the CA
Fig. 3 e CA response on black liquor at ¡0.4 V and 0.1 V using Pt (A), Ni (B) and SS (C) electrodes and comparison of the CAs obtained at 0.1 V with the three tested materials (D).
are in good agreement with the voltammetric scans (Fig. 2A), as the oxidation currents recorded in the CV at 0.1 V are negligible. In order to compare the CA response at different potentials, Fig. 3AeC presents CAs obtained for each electrode at 0.4 V and at 0.1 V. The application of more positive potentials leads to higher current values, a typical behaviour in CA. The higher currents generated in the beginning of a CA measurement are directly dependent on the applied potential. When the applied potential is more positive, the consumption of the species is faster due to an accelerated oxidation kinetics, resulting in higher initial current [35]. The values of initial, ji, and stabilised, js, current densities, as well the rate of initial current decay in the CAs obtained for each electrode material at applied potentials of 0.4 V and 0.1 V (Fig. 3AeC) are shown in Table 2. As expected, all electrodes presented higher currents when the applied potential was increased from 0.4 V to 0.1 V. It is interesting to notice that the stabilised currents for Pt and Ni electrodes do not present significant differences, with a considerably higher current decay at 0.1 V. This suggests that the lignin oxidation at Pt and Ni proceeds with high kinetics, being the similar values of the stabilised currents determined by the ligninate ions diffusion to the electrode surface. On the other hand, currents at SS do not decay at the same rate as those of Pt and Ni. This suggests that the anodic process at SS is controlled by the electron transfer, at both applied potentials, which reflects the much lower electrocatalytic activity of AISI 304 SS for lignin oxidation. In order to analyse the lignin oxidation kinetics at the three tested electrode materials, j-t1/2 plots were drawn for each potential applied in the CAs. Fig. 4A shows the CA for Ni electrode at 0 V and the corresponding j vs. t1/2 plot, whose linearity shows that for the selected experimental conditions, the applied potential is in the diffusion-controlled region and
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Table 2 e Analysis of the current transients obtained at ¡0.4 V and 0.1 V using Pt, Ni and SS electrodes. E vs. SCE ¼ 0.4 V
Electrode material
Pt Ni SS
E vs. SCE ¼ 0.1 V
ji/mA cm2
js/mA cm2
Current decay/%
ji/mA cm2
js/mA cm2
Current decay/%
2.5 1.8 0.4
1 0.8 0.2
60 56 50
18.5 15.4 1.9
2.7 1.9 0.8
85 88 58
the current transients exhibit a Cottrellian behaviour, allowing the use of Cottrell's equation [35,44] (Eq. (2)), j ¼ nFD1=2 Cp1=2 t1=2
(2)
where n is the number of exchanged electrons, F is Faraday's constant (96485 C mol1), D is the lignin diffusion coefficient in the black liquor (3.65 107 cm2 s1 at 25 C), C is the lignin concentration in black liquor (4.89 105 mol cm3), j is the transient current density and t is the time. Thus, the number of exchanged electrons were calculated for each electrode material from the slope of the j vs. t1/2 regression for each applied electrode potential in the 0.5 V to 0.6 V range. This enables plotting a potential distribution of the calculated n values for Pt, Ni, and SS (Fig. 4B). It is very interesting to notice that the plot for Pt in Fig. 4B presents two maximum values, at 0.2 V (n ¼ 3.3) and at 0.1 V (n ¼ 3.5), in agreement with the Pt CV shown in Fig. 2A, having two well-defined peaks at the same potentials. Regarding Ni and SS, those electrodes presented maximum values of n at 0.1 V (n ¼ 2.9) and 0.2 V (n ¼ 2.7), respectively. As in the voltammetric analysis, the n maximum value for Ni appears at identical potentials to that of Pt, with the potential for the SS maximum n being shifted to the right. It should be noted that electrochemical reactions usually occur at restricted potentials. Thus, a different response of the system is expected for each applied potential, depending on the involved oxidation process. As lignin is a large molecule with a complex and lessknown oxidation mechanism, it is likely that several intermediate species are formed during the ligninate ion oxidation, which can vary for each tested electrode material [45e47]. Still, the analysis of Fig. 4B clearly demonstrates that there are specific potentials for each material at which the lignin oxidation process occurs with higher faradic efficiencies.
Fig. 4 e Current density transient and corresponding j-t¡1/2 plot for Ni at 0 V (A) and variation of the n values as a function of the applied potential for each electrode material (B).
Chronopotentiometric study To further understand the kinetics of lignin oxidation in Kraft black liquor solutions, additional CP studies were also run. In CP, the current flowing on the working electrode is instantaneously stepped from zero to some finite value [35], being recorded the potential vs. time response [48]. Herein, current densities ranging from 4 to 24 mA cm2 were applied in the CP studies. Fig. 5AeC shows typical CP curves obtained for Pt, Ni and SS electrodes at four different applied currents and Fig. 5D contains a direct comparison of CP curves taken for the three electrodes at 15 mA cm2. The lignin oxidation step at Pt electrode starts at low potentials (ca. 0.2 V), whereas that at Ni occurs at somewhat higher potentials, and a considerably high overpotential is required for the lignin oxidation at SS electrode (Fig. 5D). As illustrated in Fig. 5AeC, in CP measurements, following the application of a current pulse, the potential increases from the OCP until a potential value where the oxidation of the species near the electrode is possible. Specifically, ligninate ion is oxidised at the electrode surface at this potential and, during this process, the diffusion layer grows, which explains the small slope in the oxidation step. Eventually, diffusion can no longer supply enough species to provide the required current and the potential goes up until the next electrochemical process can occur, in this case, the
Fig. 5 e CPs run in black liquor at Pt (A), Ni (B) and SS (C) electrodes and comparison of CPs obtained at 15 mA cm¡2 with the three tested materials (D). Inset shows the t1/2 vs. j¡1 plots for application in Sand equation.
Please cite this article in press as: Oliveira RCP, et al., Chronoamperometric and chronopotentiometric investigation of Kraft black liquor, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.01.046
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OH oxidation to O2 [49]. The time required for the complete lignin depletion from the electrode surface (before reaching the next potential swing) is called the transition time, t. As expected, higher currents lead to shorter transition times. So, the t values obtained in the CP studies were registered for each applied current, with the inset of Fig. 5D showing the t1/2 vs. j1 plots for the three electrodes. The good linearity of the plots allows the application of Sand equation (Eq. (3)) for the determination of kinetic parameters for the black liquor oxidation [35,49]. 1
t1=2 ¼ nFCðpDÞ1=2 ð2jÞ
(3) 1/2
1
From the slope of the t vs. j plots (inset of Fig. 5D), Eq. (3) leads to n values of 7.3, 3.7 and 2.7 for Pt, Ni and SS electrodes, respectively. These n values obtained by CP analysis can be closely related with the n value potential distribution obtained by CA (Fig. 4B). In fact, the n value of 7.3 found for Pt by CP is close to the sum of the number of exchanged electrons (6.8) in the two maximum n values determined by CA. As for Ni, an n value of 3.7 electrons was achieved, which is 27% higher than the observed in Fig. 4B (2.9). Lastly, SS presented the same value of n (2.7) both for CP and CA n-distribution peak, which is related to the slow-electron transfer at SS. Considering all the above results, it is possible to conclude that the material with better catalytic activity for lignin oxidation is certainly Pt, closely followed by Ni, and with SS showing poor activity. This order has been verified both by CA and CP analysis and agrees well with the initially obtained anodic scan (Fig. 2A). The high performance of Ni, despite being outperformed by Pt, suggests the application of the former as a low-cost electrode material for industrial black liquor electrolysers.
Conclusions In this work, Kraft black liquor samples were physicochemically characterised and their electrochemical behaviour during electrooxidation of lignin was evaluated by chronoamperometry (CA) and chronopotentiometry (CP) techniques using Pt, Ni and AISI 304 stainless steel working electrodes. A preliminary voltammetric characterisation was also done to have an overview of the electrochemical response of the black liquor (both for lignin oxidation and for hydrogen evolution) and compare it with the CA and CP results. Black liquor samples presented an alkaline pH, good conductivity and considerable lignin content, demonstrating to be an adequate medium for application of electrolysis. The voltammetric characterisation showed that the lignin oxidation occurs at relatively low potentials at Ni and Pt electrodes (less than 0.1 V) while at SS the process is slightly hindered, occurring at potentials higher than 0.5 V. CA measurements (at 0.1 V) showed that the current densities in the diffusioncontrolled region were ca. 85%, 88% and 58% (for Pt, Ni and SS, respectively) lower than those obtained in the beginning of the CA experiments, suggesting that the lignin oxidation at Pt and Ni electrodes proceeds with fast kinetics, while SS presents the lowest electrocatalytic activity for lignin oxidation. The application of Cottrell analysis to the CAs allowed determining the number of exchanged electrons, n, as a function of
the applied potential, with the results agreeing well with the voltammetric characterisation. CP measurements in the black liquor revealed for all three materials the expected decrease of the transition times, t, for lignin oxidation with the increase of the applied current density. The analysis of the recorded t values in light of Sand equation allowed obtaining n values of 7.3, 3.7 and 2.7 for Pt, Ni and SS electrodes, respectively, which closely matched those determined by CA. CP results also confirmed that lignin oxidation starts at low potentials at Pt (ca. 0.2 V), followed by Ni at a potential slightly higher, while SS requires high overpotentials for lignin oxidation, confirming the inferior activity of SS electrode compared to the other two materials. In summary, it is demonstrated that Pt shows the best performance for lignin oxidation in Kraft black liquor, with the highest catalytic activity, lowest overpotential and highest faradic efficiency. However, taking into account the high price of Pt and the good results presented by Ni, it is forecasted that Ni electrodes would be the best option for an industrial black liquor electrolyser when both price and catalytic activity are considered.
Acknowledgements ~ o para a Cie ^ncia e a The authors would like to thank Fundac¸a Tecnologia (FCT, Portugal) for funding contract no. IF/01084/ 2014/CP1214/CT0003 (D.M.F. Santos) and for grant BL166/2016IST-ID within the associated IF/01084/2014 project (R.C.P. Oliveira).
references
[1] Chakar FS, Ragauskas AJ. Review of current and future softwood Kraft lignin process chemistry. Ind Crops Prod 2004;20:131e41. [2] Azadi P, Inderwildi OR, Farnood R, King DA. Liquid fuels, hydrogen and chemicals from lignin: a critical review. Renew Sustain Energy Rev 2013;21:506e23. Passos ML. Chemical composition [3] Cardoso M, de Oliveira ED, and physical properties of black liquors and their effects on liquor recovery operation in Brazilian pulp mills. Fuel 2009;88:756e63. [4] Oliveira RCP, Mateus M, Santos DMF. Black liquor electrolysis for hydrogen and lignin extraction. ECS Trans 2016;72:43e53. [5] Biermann CJ. Handbook of pulping and papermaking. 2nd ed. California, United States: Academic Press; 1996. [6] Cao C, Guo L, Chen Y, Guo S, Lu Y. Hydrogen production from supercritical water gasification of alkaline wheat straw pulping black liquor in continuous flow system. Int J Hydrogen Energy 2011;36:13528e35. [7] Ahmed RA, Kader AHA. Effect of additives on electrical resistivity of pulp black liquor- sawdust blends. Int J Mod Eng Res 2013;3:183e8. [8] Cao C, Xu L, He Y, Guo L, Jin H, Huo Z. High-efficiency gasification of wheat straw black liquor in supercritical water at high temperatures for hydrogen production. Energy Fuels 2017;31:3970e8. [9] Pinto P. Conversion of lignin to high added-value chemicals and materials: an opportunity to pulp and paper industry and biorefineries. In: XXIII TECNICELPA e International forest, pulp and paper conference; 2016.
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[10] Ghatak HR, Kundu PP, Kumar S. Thermochemical comparison of lignin separated by electrolysis and acid precipitation from soda black liquor of agricultural residues. Thermochim Acta 2010;502:85e9. [11] Watkins D, Nuruddin M, Hosur M, Tcherbi-Narteh A, Jeelani S. Extraction and characterization of lignin from different biomass resources. J Mater Res Technol 2015;4:26e32. [12] Arkell A, Olsson J, Wallberg O. Process performance in lignin separation from softwood black liquor by membrane filtration. Chem Eng Res Des 2014;92:1792e800. [13] De S, Bhattacharya PK. Flux prediction of black liquor in cross flow ultrafiltration using low and high rejecting membranes. J Membr Sci 1996;109:109e23. € nsson AS, Nordin AK, Wallberg O. Concentration and [14] Jo purification of lignin in hardwood Kraft pulping liquor by ultrafiltration and nanofiltration. Chem Eng Res Des 2008;86:1271e80. [15] Tomani P. The lignoboost process. Cellul Chem Technol 2010;44:53e8. [16] Kouisni L, Gagne A, Maki K, Holt-hindle P, Paleologou M. The LignoForce System™ for the recovery of lignin from black liquor: feedstock options, odour profile and product characterization. ACS Sustain Chem Eng 2016;4:5152e9. [17] Masoud MS, Farag HA, Zaatout AA, Kamel HM, ElGreatly MH, Ahmed HM. Recovery of some valuable components from industrial wastes. Sep Sci Technol 2017;57:512e9. [18] Haddad M, Mikhaylin S, Bazinet L, Savadogo O, Paris J. Electrochemical acidification of Kraft black liquor: effect of fouling and chemical cleaning on ion exchange membrane integrity. ACS Sustain Chem Eng 2017;5:168e78. [19] Ghatak HR. Electrolysis of black liquor for hydrogen production: some initial findings. Int J Hydrogen Energy 2006;31:934e8. [20] Ghatak HR, Kumar S, Kundu PP. Electrode processes in black liquor electrolysis and their significance for hydrogen production. Int J Hydrogen Energy 2008;33:2904e11. [21] Paula SCS. Precipitation of lignin from Kraft black liquor. Universidade do Porto; 2010. [22] Uloth VC, Wearing JT. Kraft lignin recovery: acid precipitation versus ultrafiltration. I. Laboratory test results. Pulp Pap Can 1989;90:T310e60. [23] Kouisni L, Holt-Hindle P, Maki K, Paleologou M. The LignoForce System™: a new process for the production of high-quality lignin from black liquor. J Sci Technol Prod Process 2012;2:6e10. [24] Cloutier JN, Azarniouch MK, Callender D. Electrolysis of weak black liquor Part II: effect of process parameters on the energy efficiency of the electrolytic cell. J Appl Electrochem 1995;25:472e8. [25] Jin W, Tolba R, Wen J, Li K, Chen A. Efficient extraction of lignin from black liquor via a novel membrane-assisted electrochemical approach. Electrochim Acta 2013;107: 611e8. € [26] Ohman F, Wallmo H, Theliander H. Precipitation and filtration of lignin from black liquor of different origin. Nord Pulp Pap Res J 2007;22:188e93. [27] Haddad M, Mikhaylin S, Bazinet L, Savadogo O, Paris J. Electrochemical acidification of Kraft black liquor by electrodialysis with bipolar membrane: ion exchange membrane fouling identification and mechanisms. J Colloid Interface Sci 2017;488:39e47. [28] Nong G, Zhou Z, Wang S. Generation of hydrogen, lignin and sodium hydroxide from pulping black liquor by electrolysis. Energies 2016;9:1e11.
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[29] ASTM D1217-15. Standard test method for density and relative density (specific gravity) of liquids by Bingham pycnometer. West Conshohocken, PA: ASTM International; 2015. [30] ASTM E1755-01(2015). Standard test method for ash in biomass. West Conshohocken, PA: ASTM International; 2015. ~ o do licor negro de Eucalipto [31] Andreuccetti MT. Caracterizac¸a ~ o e correlac¸a ~ o das suas propriedades. na etapa de evaporac¸a Universidade Estadual de Campinas; 2010. [32] Zoski CG. Handbook of electrochemistry. 1st ed. Amsterdam, The Netherlands: Elsevier; 2007. [33] Brownson DAC, Banks CE. Interpreting electrochemistry in: the handbook of graphene electrochemistry. 1st ed. London, United Kingdom: Springer; 2014. [34] Pletcher D, Greef R, Peat R, Peter LM, Robinson J. Instrumental methods in electrochemistry. 1st ed. Cambridge, United Kingdom: Woodhead Publishing Limited; 2011. [35] Bard AJ, Faulkner LR. Electrochemical methods: fundamentals and applications. 2nd ed. Texas, United States: John Wiley & Sons, Inc; 2001. [36] Nassar MM. Thermal studies on Kraft black liquor. J Chem Technol Biotechnol 2007;34:21e4. [37] Song X, Bie R, Ji X, Chen P, Zhang Y, Fan J. Kinetics of reed black liquor (RBL) pyrolysis from thermogravimetric data. BioResources 2015;10:137e44. [38] Vallejos ME, Felissia FE, Curvelo AAS, Zambon MD, Ramos L, Area MC. Chemical and physico-chemical characterization of lignins obtained from ethanol-water fractionation of bagasse. BioResources 2011;6:1158e71. ~ al T, Naya S, Lo pez-Beceiro J, Tarrı´o-Saavedra J, [39] Sebio-Pun Artiaga R. Thermogravimetric analysis of wood, holocellulose, and lignin from five wood species. J Therm Anal Calorim 2012;109:1163e7. [40] Domı´nguez JC, Oliet M, Alonso MV, Gilarranz MA, Rodrı´guez F. Thermal stability and pyrolysis kinetics of organosolv lignins obtained from Eucalyptus globulus. Ind Crops Prod 2008;27:150e6. [41] Brebu M, Vasile C. Thermal degradation of lignin e a review. Cellul Chem Technol 2010;44:353e63. € rngren P, Jacobs A. Simplified [42] Aldaeus F, Schweinebarth H, To determination of total lignin content in Kraft lignin samples and black liquors. Holzforschung 2011;65:601e4. [43] Pinto PC, Evtuguin DV, Neto CP, Silvestre AJD, Amado FML. Behavior of Eucalyptus globulus lignin during Kraft pulping. II. Analysis by NMR, ESI/MS, and GPC. J Wood Chem Technol 2002;22:109e25. [44] Santos DMF, Sequeira CAC. Determination of kinetic and diffusional parameters for sodium borohydride oxidation on gold electrodes. J Electrochem Soc 2009;156:F67e74. [45] Tolba R, Tian M, Wen J, Jiang ZH, Chen A. Electrochemical oxidation of lignin at IrO2-based oxide electrodes. J Electroanal Chem 2010;649:9e15. [46] Pan K, Tian M, Jiang Z-H, Kjartanson B, Chen A. Electrochemical oxidation of lignin at lead dioxide nanoparticles photoelectrodeposited on TiO2 nanotube arrays. Electrochim Acta 2012;60:147e53. [47] Shao D, Liang J, Cui X, Xu H, Yan W. Electrochemical oxidation of lignin by two typical electrodes: Ti/SbSnO2 and Ti/PbO2. Chem Eng J 2014;244:288e95. [48] Andreuccetti MT, Leite BS, D'Angelo JVH. Eucalyptus black liquor e density, viscosity, solids and sodium sulfate contents revisited. O Papel 2011;72:52e7. [49] Santos DMF, Sequeira CAC. Chronopotentiometric investigation of borohydride oxidation at a gold electrode. J Electrochem Soc 2010;157:F16e21.
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Chronoamperometric and chronopotentiometric investigation of Kraft black liquor R.C.P. Oliveira a, M. Mateus b, D.M.F. Santos a,* Center of Physics and Engineering of Advanced Materials (CeFEMA), Instituto Superior Tecnico, Universidade de Lisboa, 1049-001 Lisbon, Portugal b Center for Natural Resources and the Environment (CERENA), Instituto Superior Tecnico, Universidade de Lisboa, 1049-001 Lisbon, Portugal a
article info
abstract
Article history:
Black liquor is an effluent of pulp and paper mills. The electrolysis of Kraft black liquor is
Received 19 September 2017
drawing high attention, as it leads to lignin recovery by anodic electrodeposition and
Received in revised form
cathodic production of hydrogen. However, the lignin oxidation process on black liquor is
5 January 2018
still poorly understood, requiring further fundamental studies to allow its comprehension.
Accepted 8 January 2018
Therefore, detailed electrochemical studies are being carried out on the topic, which will
Available online xxx
allow a deeper insight in the involved electrode processes. The Kraft black liquor is physicochemically characterised, with the determined high values of pH, conductivity and
Keywords:
lignin content (12.4, 460 mS cm1 and 48.8 g dm3, respectively) demonstrating the suit-
Kraft black liquor electrolysis
ability of black liquor as an electrolyte medium. Voltammetry, chronoamperometry (CA)
Lignin oxidation
and chronopotentiometry (CP) techniques are done in black liquor solutions using Pt, Ni
Lignin recovery
and AISI 304 SS electrodes to investigate the lignin oxidation process. The number of
Waste valorisation
exchanged electrons is determined for each electrode material from CA and CP measurements using classic Cottrell and Sand analyses, respectively, with the faradic efficiency of Ni being close to that of Pt. The low-cost and high efficiency suggest Ni as a promising electrode material for application in black liquor electrolysers. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The Kraft process is the most used pulping process in pulp and paper industries, holding 90% of the total production capacity. It involves the removal of the lignin, an organic polymer that acts as the glue between cellulose and hemicellulose in the wood, to isolate the cellulose fibres in the pulp [1e3]. For that, the wood chips are placed in a pressurised reactor (digester) with cooking chemicals (NaOH and Na2S) to promote the lignin's separation. Subsequently, the digester content is washed, originating pulp (mainly composed of cellulose fibres)
and black liquor, the major by-product produced by pulp mills, containing water, cooking chemicals, hemicellulose and lignin. For each ton of produced pulp are generated ca. 7 tons of black liquor [3e7]. Currently, the black liquor is concentrated in a multipleeffect evaporator and burned in a recovery furnace to recover the inorganic chemicals used during the cooking process and produce energy and heat, a well-known 80-yearold process named Tomlinson recovery [4,6e8]. Although being widely used, the process presents several disadvantages, such as the generation of air pollutants (NOx, SOx) harmful for the human health and environment and low
* Corresponding author. E-mail address: [email protected] (D.M.F. Santos). https://doi.org/10.1016/j.ijhydene.2018.01.046 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Oliveira RCP, et al., Chronoamperometric and chronopotentiometric investigation of Kraft black liquor, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.01.046
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efficiency (as the boiler requires excessive energy to concentrate the black liquor before incinerating it) [8]. In addition, pulp mills face capacity issues due to the increasing demands; during production peaks the amount of black liquor produced may exceed the capacity of the recovery furnace, being a pulp production bottleneck that forces a temporary stoppage of production [9]. Thus, it is essential to develop alternative processes for the treatment of the black liquor [9]. Black liquor contains lignin, the second most abundant organic polymer in nature [10,11]. Lignin has high economic value and a large range of applications, namely in phenolic resins, epoxies, adhesives, polyolefins, binders, polyurethane copolymers and admixtures for concretes, just to name a few [3,4,10]. If the lignin recovery from black liquor is done inside the pulp mill, it could even be used to feed the lime kiln, reducing the fossil fuel needs [12]. Having these in mind, lignin recovery processes have been studied as an economic, practical and ecologic way to use the excess of black liquor [4,12e26]. Some of these processes include the use of membranes [12,14,18,22,27,28], although it has high costs and drawbacks associated with the membranes sealing [14,27]. Acidification techniques [15e17,21,23] are also particularly noteworthy, as the black liquor's lignin precipitates when the pH is decreased (normally for pH < 9). In fact, a sequential acidification is used in the LignoBoost™ [15] and LignoForce™ [16,23] commercial processes. The lignin produced using LignoBoost™ contains sulphur, making its use difficult for certain applications [9,15]. In the case of Lignoforce™ system, O2 is used as oxidant before the black liquor's acidification, to increase the lignin purity (namely by decreasing the sulphur content in the final solid) and decrease the chemicals needs [16,23]. However, these methods have high investment costs and require the use of chemicals [14]. Moreover, both use the so-called “strong black liquor” generated after the evaporation process, thus being necessary to spend additional energy for the water removal before applying these methods [15,16,23]. On the other hand, the electrolysis of black liquor has been recently envisaged [4,19,20,24,28]. This process leads to lignin deposition on the electrolyser anode with simultaneous generation of hydrogen on the cathode side [4,19,20,24]. Thus, the proposed electrochemical treatment of the Kraft black liquor allows the generation of a clean fuel with high calorific value on the cathode and the recovery of a compound with economic value [4]. Despite the importance of the process, there are still very few detailed studies in the topic, meaning that the underlying mechanisms that govern the lignin oxidation (and consequent deposition) at the anode of the black liquor electrolyser cell are yet to be fully understood. For that reason, we have carried out a series of electrochemical studies on black liquor, which allow a deeper insight in the involved electrode processes. Specifically, the work presents cyclic voltammetry (CV), chronoamperometry (CA) and chronopotentiometry (CP) studies of black liquor oxidation on platinum (Pt), nickel (Ni) and AISI 304 stainless steel (SS) electrodes. These techniques are used to obtain kinetic parameters that allow assessing the efficiency of the anode process in black liquor electrolysis. Several physicochemical properties of the black liquor solution are also evaluated.
Experimental Characterisation of the black liquor samples All black liquor samples used in this study were provided by a Kraft pulp mill that uses exclusively Eucalyptus Globulus (a hardwood tree) wood. The physical characterisation of those samples was done at room temperature. The pH was determined using a Hanna Instruments PH20-01 pH meter with HI1110B pH electrode, the conductivity was determined using a Hanna Instruments HI8733 conductivity meter and the density was determined using a 50 mL glass pycnometer following ASTM D1217-15 standard test method [29]. The dry solids content in the black liquor was determined by evaporating most of the water in a Heidolph VV200 rotary evaporator followed by drying the samples in a Nabertherm LE 2/11/R6 muffle at 120 C, until constant weight is achieved. To determine the organic/inorganic ratio of the black liquor dry solids, the solid ash content was obtained according to typical determination of biomass ash procedure [30]. The lignin content in the black liquor dried solids was determined using thermogravimetric analysis in N2-inert atmosphere. For that, 10.9 mg of the solid sample was inserted in a NETZSCH STA 409 PC thermogravimetric balance using heating rate of 10 C min1 (from 25 to 1200 C). The cooling process was performed by a JULABO 5 water bath. A NETZSCH Proteus Thermal Analysis software was used for controlling the experiments.
Electrochemical studies Electrochemical measurements were performed in an electrochemical cell containing 125 mL of black liquor at room temperature. The cell comprised a typical three electrode arrangement, with platinum (Pt, Metrohm 60305100, A ¼ 1 cm2), nickel (Ni, A ¼ 0.79 cm2) or AISI 304 stainless steel (SS, A ¼ 8 cm2) working electrode, a saturated calomel electrode (SCE, Metrohm 60701100) as a reference electrode, and a Pt mesh counter electrode (Johnson Matthey, A ¼ 50 cm2). The distance between working and reference electrodes was the lowest possible in order to minimise the ohmic drop. A PAR 273A computer-controlled potentiostat/galvanostat (Princeton Applied Research, Inc.) and the associated PowerSUITE package were employed for total control of the experiments and data acquisition. The cell was magnetically stirred in between experiments to keep the black liquor sample homogeneous, as it contains solids in suspension that can sediment during the measurements, modifying the experimental conditions. Voltammograms were run at 50 mV s1 for each working electrode to make an initial assessment of the electrochemical behaviour of Kraft black liquor. In CA, potentials between 0.5 V and 0.6 V were applied, with the transient currents following a stepwise change of the electrode potential from the open circuit potential (OCP, no oxidation) to a value where the lignin oxidation was sufficiently fast so that its concentration at the electrode surface was essentially zero, i.e. diffusion-controlled. Currents ranging from 2.5 to 24 mA cm2 were applied in CP measurements, with the electrode
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e7
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potential shifting from the OCP to a potential value where lignin oxidation could occur.
Results and discussion Chemical characterisation of the samples The measured values of conductivity, density, pH, dry solids content and organic/inorganic ratio of the Kraft black liquor samples is presented in Table 1. These values were found to be in good agreement with those previously reported [3,31]. The high ionic conductivity and alkaline pH of the black liquor are important features for a good electrolyte. The high conductivity reflects the electrolyte good ionic conduction, minimising the ohmic resistance in the system and enabling a fast charge transfer in solution. The alkaline pH is also extremely advantageous, as it allows the use of cheaper electrode materials, like transition metals, instead of the expensive noble metal-based electrodes frequently required for acid media. Additionally, the high lignin concentration leads to a more efficient electrode process, by decreasing the mass-transfer resistance within the electrolyte [32e35]. The total lignin content in the black liquor dry solids was determined by thermogravimetric analysis in inert atmosphere [4]. Fig. 1 shows the thermal decomposition of the Kraft black liquor dry solids, which proceeds through four main stages. The first mass loss occurring between the room temperature and 200 C corresponds to the residual humidity loss, i.e., the water associated with hemicellulose and organic compounds whose boiling points, bp, are in this temperature range, as dimethyl sulphide (bp ¼ 38 C) and dimethyl disulphide (bp ¼ 112 C) [36]. The second stage occurs in the 200e550 C temperature range and corresponds to the pyrolysis of organic compounds, as hemicellulose (200e260 C), low molecular weight organic acids (200e300 C) and lignin [36e38]. Different lignin degradation temperature ranges can be found in the literature [36e41]; those differences can be attributed to the complex structure of lignin, which is dependent on its source (type of wood), heating rate and type of degradation atmosphere during the cooking process and extraction process (acidification, oxidation, electrooxidation, membrane processing, etc.) [41]. Herein, it is assumed that lignin degradation occurs in the 240e600 C temperature ~ al et al. with range, as reported in the studies by Sebio-Pun Eucalyptus Globulus lignin [39]. In this range there is the breaking of the bonds between the monomeric units of lignin, generating the release of phenols [38]. The decomposition of
Fig. 1 e Thermogravimetric analysis of Kraft black liquor dry solids (adapted from Ref. [4]).
aromatic rings occurs above 400 C, specifically demethoxylation and/or demethylation of the aromatic ring [38]. Taking this into account and assuming that the degradation of other components of black liquor is negligible in the 240e600 C range, a value of 26.5 wt.% was determined for the lignin composition in the black liquor dry solids. This value is in agreement with those reported by Gebremeskel et al. for the total lignin content in hardwoods using acidification (30.2 wt.%) and capillary zone electrophoresis (31.2 wt.%) [42]. From the above data, the lignin mass concentration, Cm (mol dm3), in the black liquor samples can be assessed using Eq. (1), Cm ¼ Cds Clds rbl
(1)
where Cds is the dry solids ratio in the black liquor (0.167 or 16.7 wt.%), Clds is the lignin ratio in the black liquor's dry solids (0.265 or 26.5 wt.%), rbl is the density of the black liquor (1103 g dm3). Thus, a value of 48.8 g dm3 was determined for Cm. Considering the molar mass of lignin, Mlig, from Eucalyptus Globulus Kraft black liquor (1000 g mol1 [43]), it is possible to determine the value of 4.88 102 mol dm3 for the molar concentration, C, of lignin in black liquor from the expression C ¼ Cm/Mlig. As stated above, together with the high pH and high ionic conductivity, the determined high lignin concentration is an extremely important parameter for the black liquor use as electrolyte in industrial electrochemical processes, as the high lignin content decreases the masstransfer resistance, thus enhancing the efficiency of the electrode process [32].
Electrochemical studies Voltammetric characterisation
Table 1 e Physicochemical properties of the Kraft black liquor samples. Conductivity/mS cm1 Density/g dm3 pH Dry solids content/wt.% Organic/inorganic ratioa a
Value considered in a dry basis.
460 1103 12.4 16.7 1.0
The voltammograms obtained in black liquor solution at 50 mV s1 using platinum (Pt), nickel (Ni) and AISI 304 stainless steel (SS) electrodes in natural diffusion conditions are presented in Fig. 2. Fig. 2 shows that all three tested electrodes present welldefined oxidation peaks in the forward scan (Fig. 2A) and the typical cathodic currents associated with hydrogen evolution reaction (HER) (Fig. 2B). Two sharp oxidation peaks are observed at Pt at potentials of 0.22 V and 0.10 V, with peak
Please cite this article in press as: Oliveira RCP, et al., Chronoamperometric and chronopotentiometric investigation of Kraft black liquor, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.01.046
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Fig. 2 e Positive scan CVs run in black liquor at 50 mV s¡1 using Pt, Ni and SS electrodes (A) and comparison of the corresponding cathodic polarisation curves (B).
current densities, jp, of 4.7 mA cm2 and 7.9 mA cm2, respectively. The lignin oxidation at Ni electrode is recorded as a broader peak at potential of 0.07 V and a jp of 2.9 mA cm2. AISI 304 SS presented an anodic peak with intensity of 3.9 mA cm2 at 0.57 V, a potential significantly shifted to the right, demonstrating the considerably lower activity for lignin oxidation of SS compared to the other two metals. The absence of cathodic peaks in the backscan (Fig. 2A) suggests the irreversibility of the lignin oxidation process at the tested electrodes, in agreement with previous studies [20]. Regarding the cathodic direction scans in black liquor (Fig. 2B), the onset potential for HER at Pt electrode was ca. 1.10 V, whereas for Ni and SS it was 1.20 V and 1.30 V, respectively. The current densities recorded for Pt, Ni and SS electrodes at 1.50 V were 49.8, 17.7 and 2.2 mA cm2, respectively. Despite the superior activity of Pt, these results also attest the high activity of Ni both for lignin oxidation and HER.
Chronoamperometric study Considering the anodic behaviour observed during the initial voltammetric assessment, namely the potential range of the oxidation peaks, a CA study was performed for each working electrode. CA measurements involve following the current response with time after applying a specific electrode potential [44]. In this case, potentials ranging between 0.5 and 0.6 V were applied. Fig. 3D shows typical current densitytime (j vs. t plot) transients in Pt, Ni and SS electrodes for an applied potential of 0.1 V. CA measurements in black liquor are based in an instantaneous potential change from the OCP, where no oxidation occurs, to a more positive applied potential. Thus, all compounds susceptible to oxidation in the vicinity of the electrode are oxidised almost instantly, generating high current. That is followed by a current decrease due to a concentration drop of the species next to the electrode and finally by the current stabilisation, a region of the CA known as the diffusion-controlled region. The stable current in this latter region is controlled by the diffusion of new species to the working electrode surface [35]. This typical behaviour was observed for all electrodes (Fig. 3D). Ni and Pt electrodes present very similar current transients, confirming that Ni has a catalytic behaviour close to Pt, with the lignin oxidation occurring at similar potentials. As for SS, the measured low current densities in the CA
Fig. 3 e CA response on black liquor at ¡0.4 V and 0.1 V using Pt (A), Ni (B) and SS (C) electrodes and comparison of the CAs obtained at 0.1 V with the three tested materials (D).
are in good agreement with the voltammetric scans (Fig. 2A), as the oxidation currents recorded in the CV at 0.1 V are negligible. In order to compare the CA response at different potentials, Fig. 3AeC presents CAs obtained for each electrode at 0.4 V and at 0.1 V. The application of more positive potentials leads to higher current values, a typical behaviour in CA. The higher currents generated in the beginning of a CA measurement are directly dependent on the applied potential. When the applied potential is more positive, the consumption of the species is faster due to an accelerated oxidation kinetics, resulting in higher initial current [35]. The values of initial, ji, and stabilised, js, current densities, as well the rate of initial current decay in the CAs obtained for each electrode material at applied potentials of 0.4 V and 0.1 V (Fig. 3AeC) are shown in Table 2. As expected, all electrodes presented higher currents when the applied potential was increased from 0.4 V to 0.1 V. It is interesting to notice that the stabilised currents for Pt and Ni electrodes do not present significant differences, with a considerably higher current decay at 0.1 V. This suggests that the lignin oxidation at Pt and Ni proceeds with high kinetics, being the similar values of the stabilised currents determined by the ligninate ions diffusion to the electrode surface. On the other hand, currents at SS do not decay at the same rate as those of Pt and Ni. This suggests that the anodic process at SS is controlled by the electron transfer, at both applied potentials, which reflects the much lower electrocatalytic activity of AISI 304 SS for lignin oxidation. In order to analyse the lignin oxidation kinetics at the three tested electrode materials, j-t1/2 plots were drawn for each potential applied in the CAs. Fig. 4A shows the CA for Ni electrode at 0 V and the corresponding j vs. t1/2 plot, whose linearity shows that for the selected experimental conditions, the applied potential is in the diffusion-controlled region and
Please cite this article in press as: Oliveira RCP, et al., Chronoamperometric and chronopotentiometric investigation of Kraft black liquor, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.01.046
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Table 2 e Analysis of the current transients obtained at ¡0.4 V and 0.1 V using Pt, Ni and SS electrodes. E vs. SCE ¼ 0.4 V
Electrode material
Pt Ni SS
E vs. SCE ¼ 0.1 V
ji/mA cm2
js/mA cm2
Current decay/%
ji/mA cm2
js/mA cm2
Current decay/%
2.5 1.8 0.4
1 0.8 0.2
60 56 50
18.5 15.4 1.9
2.7 1.9 0.8
85 88 58
the current transients exhibit a Cottrellian behaviour, allowing the use of Cottrell's equation [35,44] (Eq. (2)), j ¼ nFD1=2 Cp1=2 t1=2
(2)
where n is the number of exchanged electrons, F is Faraday's constant (96485 C mol1), D is the lignin diffusion coefficient in the black liquor (3.65 107 cm2 s1 at 25 C), C is the lignin concentration in black liquor (4.89 105 mol cm3), j is the transient current density and t is the time. Thus, the number of exchanged electrons were calculated for each electrode material from the slope of the j vs. t1/2 regression for each applied electrode potential in the 0.5 V to 0.6 V range. This enables plotting a potential distribution of the calculated n values for Pt, Ni, and SS (Fig. 4B). It is very interesting to notice that the plot for Pt in Fig. 4B presents two maximum values, at 0.2 V (n ¼ 3.3) and at 0.1 V (n ¼ 3.5), in agreement with the Pt CV shown in Fig. 2A, having two well-defined peaks at the same potentials. Regarding Ni and SS, those electrodes presented maximum values of n at 0.1 V (n ¼ 2.9) and 0.2 V (n ¼ 2.7), respectively. As in the voltammetric analysis, the n maximum value for Ni appears at identical potentials to that of Pt, with the potential for the SS maximum n being shifted to the right. It should be noted that electrochemical reactions usually occur at restricted potentials. Thus, a different response of the system is expected for each applied potential, depending on the involved oxidation process. As lignin is a large molecule with a complex and lessknown oxidation mechanism, it is likely that several intermediate species are formed during the ligninate ion oxidation, which can vary for each tested electrode material [45e47]. Still, the analysis of Fig. 4B clearly demonstrates that there are specific potentials for each material at which the lignin oxidation process occurs with higher faradic efficiencies.
Fig. 4 e Current density transient and corresponding j-t¡1/2 plot for Ni at 0 V (A) and variation of the n values as a function of the applied potential for each electrode material (B).
Chronopotentiometric study To further understand the kinetics of lignin oxidation in Kraft black liquor solutions, additional CP studies were also run. In CP, the current flowing on the working electrode is instantaneously stepped from zero to some finite value [35], being recorded the potential vs. time response [48]. Herein, current densities ranging from 4 to 24 mA cm2 were applied in the CP studies. Fig. 5AeC shows typical CP curves obtained for Pt, Ni and SS electrodes at four different applied currents and Fig. 5D contains a direct comparison of CP curves taken for the three electrodes at 15 mA cm2. The lignin oxidation step at Pt electrode starts at low potentials (ca. 0.2 V), whereas that at Ni occurs at somewhat higher potentials, and a considerably high overpotential is required for the lignin oxidation at SS electrode (Fig. 5D). As illustrated in Fig. 5AeC, in CP measurements, following the application of a current pulse, the potential increases from the OCP until a potential value where the oxidation of the species near the electrode is possible. Specifically, ligninate ion is oxidised at the electrode surface at this potential and, during this process, the diffusion layer grows, which explains the small slope in the oxidation step. Eventually, diffusion can no longer supply enough species to provide the required current and the potential goes up until the next electrochemical process can occur, in this case, the
Fig. 5 e CPs run in black liquor at Pt (A), Ni (B) and SS (C) electrodes and comparison of CPs obtained at 15 mA cm¡2 with the three tested materials (D). Inset shows the t1/2 vs. j¡1 plots for application in Sand equation.
Please cite this article in press as: Oliveira RCP, et al., Chronoamperometric and chronopotentiometric investigation of Kraft black liquor, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.01.046
6
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e7
OH oxidation to O2 [49]. The time required for the complete lignin depletion from the electrode surface (before reaching the next potential swing) is called the transition time, t. As expected, higher currents lead to shorter transition times. So, the t values obtained in the CP studies were registered for each applied current, with the inset of Fig. 5D showing the t1/2 vs. j1 plots for the three electrodes. The good linearity of the plots allows the application of Sand equation (Eq. (3)) for the determination of kinetic parameters for the black liquor oxidation [35,49]. 1
t1=2 ¼ nFCðpDÞ1=2 ð2jÞ
(3) 1/2
1
From the slope of the t vs. j plots (inset of Fig. 5D), Eq. (3) leads to n values of 7.3, 3.7 and 2.7 for Pt, Ni and SS electrodes, respectively. These n values obtained by CP analysis can be closely related with the n value potential distribution obtained by CA (Fig. 4B). In fact, the n value of 7.3 found for Pt by CP is close to the sum of the number of exchanged electrons (6.8) in the two maximum n values determined by CA. As for Ni, an n value of 3.7 electrons was achieved, which is 27% higher than the observed in Fig. 4B (2.9). Lastly, SS presented the same value of n (2.7) both for CP and CA n-distribution peak, which is related to the slow-electron transfer at SS. Considering all the above results, it is possible to conclude that the material with better catalytic activity for lignin oxidation is certainly Pt, closely followed by Ni, and with SS showing poor activity. This order has been verified both by CA and CP analysis and agrees well with the initially obtained anodic scan (Fig. 2A). The high performance of Ni, despite being outperformed by Pt, suggests the application of the former as a low-cost electrode material for industrial black liquor electrolysers.
Conclusions In this work, Kraft black liquor samples were physicochemically characterised and their electrochemical behaviour during electrooxidation of lignin was evaluated by chronoamperometry (CA) and chronopotentiometry (CP) techniques using Pt, Ni and AISI 304 stainless steel working electrodes. A preliminary voltammetric characterisation was also done to have an overview of the electrochemical response of the black liquor (both for lignin oxidation and for hydrogen evolution) and compare it with the CA and CP results. Black liquor samples presented an alkaline pH, good conductivity and considerable lignin content, demonstrating to be an adequate medium for application of electrolysis. The voltammetric characterisation showed that the lignin oxidation occurs at relatively low potentials at Ni and Pt electrodes (less than 0.1 V) while at SS the process is slightly hindered, occurring at potentials higher than 0.5 V. CA measurements (at 0.1 V) showed that the current densities in the diffusioncontrolled region were ca. 85%, 88% and 58% (for Pt, Ni and SS, respectively) lower than those obtained in the beginning of the CA experiments, suggesting that the lignin oxidation at Pt and Ni electrodes proceeds with fast kinetics, while SS presents the lowest electrocatalytic activity for lignin oxidation. The application of Cottrell analysis to the CAs allowed determining the number of exchanged electrons, n, as a function of
the applied potential, with the results agreeing well with the voltammetric characterisation. CP measurements in the black liquor revealed for all three materials the expected decrease of the transition times, t, for lignin oxidation with the increase of the applied current density. The analysis of the recorded t values in light of Sand equation allowed obtaining n values of 7.3, 3.7 and 2.7 for Pt, Ni and SS electrodes, respectively, which closely matched those determined by CA. CP results also confirmed that lignin oxidation starts at low potentials at Pt (ca. 0.2 V), followed by Ni at a potential slightly higher, while SS requires high overpotentials for lignin oxidation, confirming the inferior activity of SS electrode compared to the other two materials. In summary, it is demonstrated that Pt shows the best performance for lignin oxidation in Kraft black liquor, with the highest catalytic activity, lowest overpotential and highest faradic efficiency. However, taking into account the high price of Pt and the good results presented by Ni, it is forecasted that Ni electrodes would be the best option for an industrial black liquor electrolyser when both price and catalytic activity are considered.
Acknowledgements ~ o para a Cie ^ncia e a The authors would like to thank Fundac¸a Tecnologia (FCT, Portugal) for funding contract no. IF/01084/ 2014/CP1214/CT0003 (D.M.F. Santos) and for grant BL166/2016IST-ID within the associated IF/01084/2014 project (R.C.P. Oliveira).
references
[1] Chakar FS, Ragauskas AJ. Review of current and future softwood Kraft lignin process chemistry. Ind Crops Prod 2004;20:131e41. [2] Azadi P, Inderwildi OR, Farnood R, King DA. Liquid fuels, hydrogen and chemicals from lignin: a critical review. Renew Sustain Energy Rev 2013;21:506e23. Passos ML. Chemical composition [3] Cardoso M, de Oliveira ED, and physical properties of black liquors and their effects on liquor recovery operation in Brazilian pulp mills. Fuel 2009;88:756e63. [4] Oliveira RCP, Mateus M, Santos DMF. Black liquor electrolysis for hydrogen and lignin extraction. ECS Trans 2016;72:43e53. [5] Biermann CJ. Handbook of pulping and papermaking. 2nd ed. California, United States: Academic Press; 1996. [6] Cao C, Guo L, Chen Y, Guo S, Lu Y. Hydrogen production from supercritical water gasification of alkaline wheat straw pulping black liquor in continuous flow system. Int J Hydrogen Energy 2011;36:13528e35. [7] Ahmed RA, Kader AHA. Effect of additives on electrical resistivity of pulp black liquor- sawdust blends. Int J Mod Eng Res 2013;3:183e8. [8] Cao C, Xu L, He Y, Guo L, Jin H, Huo Z. High-efficiency gasification of wheat straw black liquor in supercritical water at high temperatures for hydrogen production. Energy Fuels 2017;31:3970e8. [9] Pinto P. Conversion of lignin to high added-value chemicals and materials: an opportunity to pulp and paper industry and biorefineries. In: XXIII TECNICELPA e International forest, pulp and paper conference; 2016.
Please cite this article in press as: Oliveira RCP, et al., Chronoamperometric and chronopotentiometric investigation of Kraft black liquor, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.01.046
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e7
[10] Ghatak HR, Kundu PP, Kumar S. Thermochemical comparison of lignin separated by electrolysis and acid precipitation from soda black liquor of agricultural residues. Thermochim Acta 2010;502:85e9. [11] Watkins D, Nuruddin M, Hosur M, Tcherbi-Narteh A, Jeelani S. Extraction and characterization of lignin from different biomass resources. J Mater Res Technol 2015;4:26e32. [12] Arkell A, Olsson J, Wallberg O. Process performance in lignin separation from softwood black liquor by membrane filtration. Chem Eng Res Des 2014;92:1792e800. [13] De S, Bhattacharya PK. Flux prediction of black liquor in cross flow ultrafiltration using low and high rejecting membranes. J Membr Sci 1996;109:109e23. € nsson AS, Nordin AK, Wallberg O. Concentration and [14] Jo purification of lignin in hardwood Kraft pulping liquor by ultrafiltration and nanofiltration. Chem Eng Res Des 2008;86:1271e80. [15] Tomani P. The lignoboost process. Cellul Chem Technol 2010;44:53e8. [16] Kouisni L, Gagne A, Maki K, Holt-hindle P, Paleologou M. The LignoForce System™ for the recovery of lignin from black liquor: feedstock options, odour profile and product characterization. ACS Sustain Chem Eng 2016;4:5152e9. [17] Masoud MS, Farag HA, Zaatout AA, Kamel HM, ElGreatly MH, Ahmed HM. Recovery of some valuable components from industrial wastes. Sep Sci Technol 2017;57:512e9. [18] Haddad M, Mikhaylin S, Bazinet L, Savadogo O, Paris J. Electrochemical acidification of Kraft black liquor: effect of fouling and chemical cleaning on ion exchange membrane integrity. ACS Sustain Chem Eng 2017;5:168e78. [19] Ghatak HR. Electrolysis of black liquor for hydrogen production: some initial findings. Int J Hydrogen Energy 2006;31:934e8. [20] Ghatak HR, Kumar S, Kundu PP. Electrode processes in black liquor electrolysis and their significance for hydrogen production. Int J Hydrogen Energy 2008;33:2904e11. [21] Paula SCS. Precipitation of lignin from Kraft black liquor. Universidade do Porto; 2010. [22] Uloth VC, Wearing JT. Kraft lignin recovery: acid precipitation versus ultrafiltration. I. Laboratory test results. Pulp Pap Can 1989;90:T310e60. [23] Kouisni L, Holt-Hindle P, Maki K, Paleologou M. The LignoForce System™: a new process for the production of high-quality lignin from black liquor. J Sci Technol Prod Process 2012;2:6e10. [24] Cloutier JN, Azarniouch MK, Callender D. Electrolysis of weak black liquor Part II: effect of process parameters on the energy efficiency of the electrolytic cell. J Appl Electrochem 1995;25:472e8. [25] Jin W, Tolba R, Wen J, Li K, Chen A. Efficient extraction of lignin from black liquor via a novel membrane-assisted electrochemical approach. Electrochim Acta 2013;107: 611e8. € [26] Ohman F, Wallmo H, Theliander H. Precipitation and filtration of lignin from black liquor of different origin. Nord Pulp Pap Res J 2007;22:188e93. [27] Haddad M, Mikhaylin S, Bazinet L, Savadogo O, Paris J. Electrochemical acidification of Kraft black liquor by electrodialysis with bipolar membrane: ion exchange membrane fouling identification and mechanisms. J Colloid Interface Sci 2017;488:39e47. [28] Nong G, Zhou Z, Wang S. Generation of hydrogen, lignin and sodium hydroxide from pulping black liquor by electrolysis. Energies 2016;9:1e11.
7
[29] ASTM D1217-15. Standard test method for density and relative density (specific gravity) of liquids by Bingham pycnometer. West Conshohocken, PA: ASTM International; 2015. [30] ASTM E1755-01(2015). Standard test method for ash in biomass. West Conshohocken, PA: ASTM International; 2015. ~ o do licor negro de Eucalipto [31] Andreuccetti MT. Caracterizac¸a ~ o e correlac¸a ~ o das suas propriedades. na etapa de evaporac¸a Universidade Estadual de Campinas; 2010. [32] Zoski CG. Handbook of electrochemistry. 1st ed. Amsterdam, The Netherlands: Elsevier; 2007. [33] Brownson DAC, Banks CE. Interpreting electrochemistry in: the handbook of graphene electrochemistry. 1st ed. London, United Kingdom: Springer; 2014. [34] Pletcher D, Greef R, Peat R, Peter LM, Robinson J. Instrumental methods in electrochemistry. 1st ed. Cambridge, United Kingdom: Woodhead Publishing Limited; 2011. [35] Bard AJ, Faulkner LR. Electrochemical methods: fundamentals and applications. 2nd ed. Texas, United States: John Wiley & Sons, Inc; 2001. [36] Nassar MM. Thermal studies on Kraft black liquor. J Chem Technol Biotechnol 2007;34:21e4. [37] Song X, Bie R, Ji X, Chen P, Zhang Y, Fan J. Kinetics of reed black liquor (RBL) pyrolysis from thermogravimetric data. BioResources 2015;10:137e44. [38] Vallejos ME, Felissia FE, Curvelo AAS, Zambon MD, Ramos L, Area MC. Chemical and physico-chemical characterization of lignins obtained from ethanol-water fractionation of bagasse. BioResources 2011;6:1158e71. ~ al T, Naya S, Lo pez-Beceiro J, Tarrı´o-Saavedra J, [39] Sebio-Pun Artiaga R. Thermogravimetric analysis of wood, holocellulose, and lignin from five wood species. J Therm Anal Calorim 2012;109:1163e7. [40] Domı´nguez JC, Oliet M, Alonso MV, Gilarranz MA, Rodrı´guez F. Thermal stability and pyrolysis kinetics of organosolv lignins obtained from Eucalyptus globulus. Ind Crops Prod 2008;27:150e6. [41] Brebu M, Vasile C. Thermal degradation of lignin e a review. Cellul Chem Technol 2010;44:353e63. € rngren P, Jacobs A. Simplified [42] Aldaeus F, Schweinebarth H, To determination of total lignin content in Kraft lignin samples and black liquors. Holzforschung 2011;65:601e4. [43] Pinto PC, Evtuguin DV, Neto CP, Silvestre AJD, Amado FML. Behavior of Eucalyptus globulus lignin during Kraft pulping. II. Analysis by NMR, ESI/MS, and GPC. J Wood Chem Technol 2002;22:109e25. [44] Santos DMF, Sequeira CAC. Determination of kinetic and diffusional parameters for sodium borohydride oxidation on gold electrodes. J Electrochem Soc 2009;156:F67e74. [45] Tolba R, Tian M, Wen J, Jiang ZH, Chen A. Electrochemical oxidation of lignin at IrO2-based oxide electrodes. J Electroanal Chem 2010;649:9e15. [46] Pan K, Tian M, Jiang Z-H, Kjartanson B, Chen A. Electrochemical oxidation of lignin at lead dioxide nanoparticles photoelectrodeposited on TiO2 nanotube arrays. Electrochim Acta 2012;60:147e53. [47] Shao D, Liang J, Cui X, Xu H, Yan W. Electrochemical oxidation of lignin by two typical electrodes: Ti/SbSnO2 and Ti/PbO2. Chem Eng J 2014;244:288e95. [48] Andreuccetti MT, Leite BS, D'Angelo JVH. Eucalyptus black liquor e density, viscosity, solids and sodium sulfate contents revisited. O Papel 2011;72:52e7. [49] Santos DMF, Sequeira CAC. Chronopotentiometric investigation of borohydride oxidation at a gold electrode. J Electrochem Soc 2010;157:F16e21.
Please cite this article in press as: Oliveira RCP, et al., Chronoamperometric and chronopotentiometric investigation of Kraft black liquor, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.01.046