Electrochemically Enhanced Adsorption of Aluminum from Sodium

Sep 13, 2013 - ABSTRACT: The purity of sodium carbonate solution is critical for membrane electrolysis to efficiently produce sodium hydroxide and ...
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Electrochemically Enhanced Adsorption of Aluminum from Sodium Carbonate Solution by Activated Carbon Fibers Yanling Zhang,† Yang Tang,‡ Qian Qiu,‡ Yongmei Chen,‡ Yanzhi Sun,‡ Pingyu Wan,‡ and X. Jin Yang*,† †

Beijing Key Laboratory of Membrane Separation Process and Technology, and ‡National Fundamental Research Laboratory of New Hazardous Chemicals Assessment and Accident Analysis, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China ABSTRACT: The purity of sodium carbonate solution is critical for membrane electrolysis to efficiently produce sodium hydroxide and bicarbonate. This study reports electrochemically enhanced adsorption of aluminum from sodium carbonate solution by activated carbon fibers. The parameters examined included: bias potential, pH, aluminum concentration, and sodium carbonate concentration. The electrosorption process followed the Lagergren first-order kinetics and the experimental results were fitted to the Langmuir and Freundlich isotherms. The results demonstrate that the adsorption can be well-described by the Langmuir isotherms and occurs mainly in the monolayer.

1. INTRODUCTION The Bayer process has dominated for alumina production from bauxite ores over 100 years but has a typically low production efficiency of ∼50%. Numerous efforts have been made for improving the efficiency of the Bayer process, but the efficiency has remained almost unchanged, because of the inherent phase equilibrium limitation of the Na2O−Al2O3−H2O system.1−3 Increasing the production efficiency of the Bayer process has been the first priority of the alumina industry and has posed a significant technical challenge.4,5 Recently, we proposed an efficient and sustainable process6 for production of alumina from bauxite ores by membrane electrolysis of sodium carbonate, which produces concentrated NaOH solution for ore leaching and digestion and NaHCO3 solution for precipitating Al(OH)3 from the digestion caustic liquor [sodium aluminate, NaAl(OH)4]. The basic chemistry of the process is summarized as follows: Ore digestion: Al 2O3 ·3H 2O + 2NaOH → 2NaAl(OH)4

observed that the electrolytic cell voltage gradually increased with electrolysis time due to precipitates of hydroxides and carbonates on the surface of the membrane. By carefully controlling the condition of Al(OH)3 precipitation step and overdosing NaHCO3 by 11%, we decreased the aluminum concentration in the solution of 1.6 M NaOH and 4.85 M NaAl(OH)4 to 0.5−1 mg L−1.6 However, it is difficult to reduce the aluminum concentration further, because of the existence of trace negatively charged aluminum species in a metastable state. In alkaline solution, aluminum exists mainly in the form of Al(OH)4− and possibly several other forms, such as Al(OH)52−, Al(OH)63−, and Al2O(OH)2−,7,8 leading to great difficulties in reducing the concentration of aluminum in alkaline solutions. In addition, the formation of dawsonite (Na2O·Al2O3·2CO2· nH2O) small particles is prone to occur in sodium carbonate solution.9 The adsorption of aluminate ions and dawsonite particles could be enhanced on the electrode surface due to polarization effects.10,11 Activated carbon has been widely employed for the removal of aluminum in wastewater treatment plants,12 and electrochemically enhanced removal of aluminum and other elements from wastewater has been reported.12−15 Choksi and Joshi12 reported the adsorption behavior of aluminum and nickel on four different adsorbents (including activated carbon) from wastewater, and they found that the aluminum adsorption capacity was clearly pH-dependent and the adsorption capacity of activated carbon showed 1.08-fold increments with the increase of pH from 6 to 7.5. Afkhami and Conway13 investigated the electrosorption of Cr(VI), Mo(VI), W(VI), V(IV), and V(V) oxy-ions from water at low concentrations (2.0 × 10−4 M) on high-area C-cloth electrodes and found that positive polarization of the C-cloth increased adsorption of Cr(VI), Mo(VI), and V(V) oxy-ions while the negative

(1)

Al(OH)3 precipitation: NaAl(OH)4 + NaHCO3 → Al(OH)3 ↓ + Na 2CO3 + H 2O

(2)

Membrane electrolysis: Na 2CO3 + 2H 2O → NaHCO3 (anode) + NaOH(cathode) + H 2 ↑(cathode) +

1 O2 ↑(anode) 2

(3)

This new process is not limited by the equilibrium of Na2O− Al2O3−H2O system and, therefore, the production efficiency was enhanced by 110%.6 To reduce electricity consumption and increase the durability of the cation ion-exchange membrane, metallic element impurities in the resultant sodium carbonate solution are required to be as low as possible. We © 2013 American Chemical Society

Received: Revised: Accepted: Published: 14449

April 23, 2013 September 2, 2013 September 13, 2013 September 13, 2013 dx.doi.org/10.1021/ie401290v | Ind. Eng. Chem. Res. 2013, 52, 14449−14455

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Figure 1. Schematic of the experimental setup for electrosorption of aluminum from sodium carbonate solutions.

using nylon thread, were used as the working electrode. The distance between the working electrode and reference electrode was 3−5 mm. The volume of solution was 120 mL. A DF-101S collector thermostat heating magnetic stirrer (Henan Yuhua Corporation, China) was used in the water tank to control the temperature and speed during the electrosorption experiments. 2.4. Procedure for Electrosorption Experiments and Data Analysis. Aluminum stock solution (35 g L−1) was prepared by dissolving 51 g of Al(OH)3 into 500 mL of a 4.5 M NaOH solution. Sodium carbonate-bicarbonate buffer solution (pH 10.2) containing 0.6 mg L−1 aluminum was employed for electrosorption experiments, unless otherwise specified. The solution was prepared by combining 250 mL of 1 M sodium carbonate solution with 100 mL 1 M sodium bicarbonate solution and adding Al stock solution. During the electrosorption, 2 mL of the solution was sampled at a certain time interval and transferred to a 25-mL colorimeter tube. The solution was adjusted to pH 1.96−2.0 by adding 1 M HCl (3.5 mL). Then, one drop of 1.0 g/L para-nitrophenol solution was added and the solution was adjusted to a light-yellow color by aqueous ammonia solution (1:6). The sample was acidified with 0.5 M HNO3 to colorless and the following reagent solutions were added: 8 mL of CH3COOH−CH3COONa buffer solution (pH 5.5), 1.0 mL of ascorbic acid (10 g/L), 1.0 mL of sodium thiosulfate solution (200 g/L), 2.0 mL of cetyltrimethylammonium bromide (CTAB) (0.2 g/L), 1.0 mL of polyethylene glycol octyl phenyl ether (0.5%), and 2.0 mL of chrome azurol S solution (0.5 g/L). Finally, the sample was diluted with water to the mark to develop the coloring process for 30 min. Al was analyzed by UV−vis spectrophotometer (Shimadzu Corporation, Japan, Model V-2550) at 615 nm.17 The experiments were carried out at 20 °C and the adsorption amount of aluminum was calculated using the expression

polarization enhanced desorption of Mo(VI) and V(V) oxyions. Huang et al.14 investigated the electrosorption of Cu(II) ions from wastewater with modified activated carbon fiber (ACF) cloth electrodes and found that the equilibrium adsorption capacity at 0.3 V was twice as high as that at open circuit. In summary, electrosorption by activated carbon materials is an effective approach for removal of trace amounts of metal elements in aqueous solutions and the major factors include the bias potential, pH of solution and concentrations of metal element and electrolyte.13−16 In this study, electrosorption of aluminum from sodium carbonate solutions was investigated on ACFs attached to dimensionally stable anode (DSA, titanium ruthenium plate). The electrosorption kinetics and isotherms of aluminum by ACF with different bias potentials and concentrations of aluminum and sodium carbonate were examined.

2. EXPERIMENTAL SECTION 2.1. Chemicals and Materials. The ACF cloth made from viscose was obtained from Beijing Evergrow Resources Co., Ltd., China. The ACF cloth’s specific area is 1178 m2 g−1, and the micropore area of the active carbon fiber is 905 m2/g. Sodium carbonate, aluminum hydroxide, sodium hydroxide, and sodium bicarbonate (analytical grade) were purchased from Beijing Chemicals and used as received. 2.2. Pretreatment of ACFs. The commercial viscose-based ACF cloth was cut in rectangles (25 mm × 40 mm, 0.1 g). The cut ACF cloth was washed with deionized water, then boiled in 1 M NaOH for 30 min, and washed with deionized water thoroughly to remove NaOH. After that, the cloth was boiled in 1 M HCl solution for 30 min and washed with deionized water to neutral. Finally, it was dried in a vacuum drying oven for 12 h at 105 °C and stored in a desiccator after cooling. 2.3. Instrumentation and Adsorption Cell. Adsorption experiments were carried out in a three-dimensional (3D) electrode system (Figure 1). The polarization of the ACFs was controlled by a CH Instruments Model 660C electrochemical workstation (Shanghai Chenhua Corporation, China). A saturated calomel electrode (SCE) served as the reference electrode, and a titanium ruthenium plate (25 mm × 40 mm) was employed as auxiliary electrode. A DSA electrode and an ACF cloth (25 mm × 40 mm), sandwiched by a plastic grille

qt =

(C0 − Ct ) × V mACFs

(4)

where qt (mg g−1) is the adsorption amount of aluminum at time t (min). C0 and Ct (mg L−1) are the concentration at the beginning and time t (min), respectively. V (L) is the volume of solution and mACFs (g) is the mass of ACFs. The adsorption rate constant of aluminum was determined from the Lagergren equation, a first-order adsorption kinetics18 (eq 5): 14450

dx.doi.org/10.1021/ie401290v | Ind. Eng. Chem. Res. 2013, 52, 14449−14455

Industrial & Engineering Chemistry Research log(qe − qt ) = log qe −

k1t 2.303

Article

(5)

−1

where qe (mg g ) is the adsorption amount of aluminum at equilibrium and k1 (min−1) is the adsorption rate constant. Isotherms were fitted by the classical empirical models of Langmuir19 (eq 6) and Freundlich20 (eq 7): ⎛ bCe ⎞ qe = qmax ⎜ ⎟ ⎝ 1 + bCe ⎠

(6)

qe = kFCe1/ n

(7) −1

where qmax (mg g ) is the maximum saturation adsorption capacity at the constant temperature, Ce (mg L−1) the equilibrium concentration of Al in solution, b (L mg−1) the Langmuir constant related to the energy of adsorption, kF the Freundlich constant related to the adsorption capacity of Al, and 1/n is the indication of the tendency of Al to be adsorbed. The essential characteristics of the Langmuir isotherm can be expressed by a dimensionless constant called the equilibrium parameter RL:21 1 RL = 1 + bC0 (8)

Figure 3. Electrosorption kinetics of aluminum at different bias potentials between 0 and 600 mV in sodium carbonate−bicarbonate solution (pH 10.2, initial [Al] = 0.6 mg L−1).

Table 1. Lagergren Parameters of Aluminum Electrosorption on ACF at Different Bias Potentials

3. RESULTS AND DISCUSSION 3.1. Determination of Electrosorption Potential. The cyclic voltammetry at a scan rate of 10 mV s−1 were measured

bias potential (mV)

qe (mg g−1)

k1 × 10−2 (min−1)

R2

0 200 300 400 600

0.133 0.185 0.271 0.377 0.454

2.629 3.081 3.238 3.436 4.406

0.9746 0.9858 0.9827 0.9907 0.9938

adsorption rate was investigated, and the results are shown in Figure 3. The electrosorption reached equilibrium at ∼120 min. The experimental results were fitted into the first-order adsorption kinetics using the Lagergren equation (eq 5) (see Table 1). The results clearly demonstrate that the adsorption capacity and rate constant increase with the bias potential and the equilibrium adsorption capacity at 600 mV was more than three times greater than that without applying a bias potential (0 mV). The good fitting indicates that the electrosorption of aluminum onto ACF-DSA electrodes from sodium carbonate solution followed the first-order kinetics. During the adsorption experiments, the pH of solution remained constant, suggesting that no Faradic reaction occurred and the increased adsorption with increasing the bias potential was caused by the polarization effect. 3.3. Adsorption Isotherms. Langmuir isotherm and Freundlich isotherm were used to fit the experimental data for aluminum electrosorption on ACFs from sodium carbonate solutions (see Table 2 and Figure 4). The Langmuir isotherm fits the experimental data better than the Freundlich isotherm, indicating that aluminum is mainly adsorbed in the form of monolayer adsorption. RL values between 0 and 1 indicated favorable adsorption in the Langmuir isotherm.21 Compared to the equilibrium adsorption capacity, the slope of maximum adsorption capacity against the bias potential is much greater (see Tables 1 and 2), i.e., the maximum adsorption capacity at 600 mV was 1.347 times that at 0 mV, whereas the equilibrium adsorption capacity was 3 times higher. 3.4. Effect of Initial Aluminum Concentration. Figure 5 shows the experimental and Lagergren-fitting results of different initial aluminum concentrations. The amount of aluminum adsorbed increased with increasing initial aluminum concentration. Increasing the aluminum concentration increased the concentration gradient from the surface of carbon fiber to the bulk of the solution. However, the adsorption

Figure 2. Cyclic voltammograms of sodium carbonate−bicarbonate solutions (pH 10.2) containing 0 and 0.6 mg L−1 aluminum.

in sodium carbonate−biocarbonate solution (Figure 2). The current increased rapidly beyond the potential range of −430 mV and 650 mV. This indicates that H2O is electrochemically decomposed beyond this range. No Faraday reaction was observed between 0 and 600 mV. Therefore, the electrosorption bias potential was controlled between 0 and 600 mV in the following experiments to enhance the adsorption of aluminum on ACFs from sodium carbonate−bicarbonate solutions. 3.2. Adsorption Kinetics. In the electrosorption of aluminum on ACFs from sodium carbonate solutions, factors influencing the adsorption kinetics include bias potential, pH, and the concentration of aluminum and sodium carbonate. The bias potential is a key factor driving ions and polarized species onto the ACF-DSA electrode surface.22 The effect of electrochemical polarization on the adsorption capacity and 14451

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Table 2. Isotherm Parameters of Aluminum Adsorption on ACFs at Different Bias Potentials Langmuir

Freundlich

bias potential (mV)

qmax

b

RL

R2

kF

1/n

R2

0 200 300 400 600

0.546 0.594 0.684 0.711 0.736

0.839 1.527 2.251 3.567 5.301

0.665 0.522 0.425 0.318 0.239

0.9607 0.9977 0.9835 0.9987 0.9701

0.240 0.342 0.458 0.532 0.593

0.498 0.399 0.352 0.232 0.199

0.9458 0.9861 0.9718 0.9903 0.9841

Figure 7. Effect of sodium carbonate concentration on electrosorption process ([Al] = 0.6 mg L−1, bias potential = 600 mV).

Figure 4. Adsorption isotherms of aluminum in sodium carbonate− bicarbonate solution under different bias potentials at 20 °C (pH 10.2, [Al] = 0.6 mg L−1).

Figure 8. Adsorption equilibrium of Al on activated carbon fibers in NaOH and Na2CO3−NaHCO3 solutions (pH 10.2−12.2; Al concentration = 0.6 mg/L).

Figure 5. Effect of initial aluminum concentration on adsorption capacity in sodium carbonate−bicarbonate solution (pH 10.2, bias potential = 600 mV).

percentage decreased from 50% at 0.61 mg L−1 to 22% at 2.16 mg L−1. Similar trends were also observed for vanadium(IV) from aqueous solutions by aluminum-pillared clay.23 3.5. Effect of pH. After ore digestion by NaOH, a typical digested liquor consists of 1.6 M NaOH and 4.85 M Al(OH)4. The addition of NaHCO3 neutralizes the hydroxide and, therefore, decomposes Al(OH)4 (see reaction 2). The pH of the solution decreased to 11.2 at a stoichiometric dose of 1.3 M NaHCO3 and further to 10.7 by overdosing 10%.6 Aluminum is an amphoteric substance and therefore, the pH of the solution is a key factor determining the charge (form) of aluminum species in solutions. The effect of pH on the adsorption kinetics is shown in Figure 6. It can be seen that the higher the pH, the higher the amount of aluminum adsorbed on ACFs. The amount of aluminum adsorbed increased from 0.34 mg g−1 to 0.74 mg g−1 with an increase of pH from 9.2 to 12.2. This may be mainly due to the electrostatic interaction. Increasing pH of the solution may enhance the formation of Al(OH)4− ions and

Figure 6. Effect of pH on electrosorption process in sodium carbonate−bicarbonate solution (Al = 0.6 mg L−1, bias potential = 600 mV).

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Figure 9. Electrodesorption of aluminum from the carbon fibers as a function of applied potential (electrosorption, 6 h; electrodesorption, 8 h).

Figure 11. X-ray diffraction (XRD) spectrum of carbon fibers before and after the sorption experiments. The diamond symbols represent the diffraction peaks of carbon.

therefore, promotes their attraction to the positively charged ACF cloth. It should be noted that increasing the pH of the solution would result in higher aluminum concentrations. The aluminum concentration was found to be 210 mg L−1 at pH 11.2 by a stoichiometric dose of NaHCO3.6 Such a high aluminum concentration would quickly saturate the ACFs.

Therefore, pH values of 10.2−10.8 are recommended at which both a sufficient adsorption rate by electrosorption and a minimum residual Al concentration from aluminate solution by NaHCO3 decomposition could be achieved. 3.6. Effect of Electrolyte Concentration. Figure 7 shows that the amount of aluminum absorbed decreases as the

Figure 10. Scanning electron microscopy (SEM) images of ACF electrodes: (a) before sorption, (b) after sorption at 0 mV, (c) after sorption at 600 mV, and (d) after desorption at −600 mV. 14453

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Notes

concentration of Na2CO3 increases. This is likely to be due to the competition between Al(OH)4− and CO32− and increased resistance of electrical double layer under higher carbonate concentrations. The ionic species are surrounded by an electrical diffused double layer during electrosorption processes, and higher electrolyte concentration results in a thicker double layer, leading to greater adsorption resistance.24 Figure 8 shows a comparison of Al adsorption between NaOH and carbonate− bicarbonate solutions. Under the identical pH, the adsorption capacity is greater in NaOH solution than in carbonate− bicarbonate solution. This decrease of Al adsorption capacity is most likely due to the competition of carbonate−bicarbonate ions. 3.7. Desorption of Al and Regeneration of Activated Carbon Fiber. The regeneration of the ACFs after sorption was carried out by reversing the charge of the electrodes through electrodesorption. The electrodesorption percentage increased with the applied voltage, but the maximum removal percentage was 80% under the voltage of −600 mV (Figure 9). This means that the electrodesorption is not sufficient to regenerate the carbon fibers. One of the possible explanations was that part of the Al(OH)4− adsorbed on the carbon fibers was converted to other forms (e.g., aluminum hydroxides and dawsonite), which were not able to be removed via electrodesorption. The scanning electron microscopy (SEM) images indicate some differences before and after electrodesorption (Figure 10), although the XRD analysis does not show aluminum hydroxide peaks (Figure 11). For regeneration of the carbon fibers, a chemical cleaning process (e.g., the use of HCl) is recommended after electrodesorption. In fact, HCl was used for cleaning the new fibers (see the Experimental Section). Note that the adsorption capacity of the activated carbon fiber cloth investigated here is relatively low. Enhancement in adsorption capacity is important for industrial applications and surface modification of the carbon fibers would be worthy of investigation.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the Natural Science Foundation of China (No. 21176024).

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4. CONCLUSIONS The production practice in alumina industry has revealed that the relatively low production efficiency of the Bayer process for extraction of alumina from bauxite ores is mainly caused by slow rate and poor efficiency of the Al(OH)3 seeded precipitation from caustic aluminate liquor. Our previous work showed that the NaHCO3 carbonation significantly accelerates the Al(OH)3 precipitation rate from aluiminate liquor and the removal of trace amounts of Al from the carbonation’s byproduct Na 2 CO 3 is important to the regeneration of NaHCO3 and NaOH by membrane electrolysis. This study demonstrates that the electrochemical adsorption onto activated carbon fiber cloth is effective and efficient for the removal of trace levels of aluminum from sodium carbonate solution. The adsorption follows the first-order kinetics and fits the Langmuir isotherm better than the Freundlich isotherm. Increasing aluminum concentration resulted in greater amount of aluminum adsorbed, but decreased adsorption percentage and kinetics constant. The amount of aluminum adsorbed increased with increasing pH of the solution but decreased with increasing sodium carbonate concentration.

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