ARTICLE pubs.acs.org/IECR
Utilization of Bipolar Membrane Electrodialysis for the Removal of Boron from Aqueous Solution Hiroki Nagasawa,*,† Atsushi Iizuka,†,‡ Akihiro Yamasaki,§ and Yukio Yanagisawa† †
Department of Environment Systems, Graduate School of Frontier Sciences, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8563, Japan ‡ Research Center for Sustainable Science & Engineering, Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Sendai, Miyagi 980-8577, Japan § Department of Materials and Life Science, Faculty of Science and Technology, Seikei University, 3-3-1 Kichijoji-kitamachi, Musashino, Tokyo 180-8633, Japan ABSTRACT: Bipolar membrane electrodialysis (BMED) was used to remove boron from aqueous solution. BMED was compared with conventional electrodialysis (ED) using aqueous solutions of sodium tetraborate containing 100 mg/L of boron. With use of BMED, more than 90% of the boron was removed under both acidic (pH 2.3) and basic (pH 9.1) conditions, whereas only 3545% of the boron was removed using conventional ED. Over 90% of the boron was removed from the diluate solution over a wide range of initial pH from 2.3 to 12.0. A high initial pH reduces the current efficiency of boron removal because of the high mobility and high concentration of hydroxide ions compared with those of borate ions. The power requirement for boron removal increased as the initial pH and concentration of sodium chloride increased, but decreased as the applied voltage was increased. BMED is a promising option for removal of boron from aqueous solution.
’ INTRODUCTION Boron is widely distributed throughout the environment, mainly in the form of boric acid or borates. Boron is an essential element in various industries, such as in the manufacture of glass and ceramics, in the semiconductor industry, and the detergent industry. Boron normally exists at low concentrations in the environment; however, higher concentrations are frequently found in water resources impacted by anthropogenic discharges, which can contain boron at concentrations of 10100 mg/L and higher, including sewage, industrial wastewater, and agricultural effluent.13 Although boron is an important micronutrient for plants, a high concentration of boron in irrigation water sources has a toxic effect on most plants.4 It has been reported that long-term intake of excess boron through drinking water is potentially toxic to humans and animals.5 The World Health Organization recommends that the maximum boron level in drinking water is 0.5 mg/L.6 To prevent health and environmental effects caused by high concentrations of boron in drinking and irrigation waters, the anthropogenic discharge of boron into the environment must be controlled. For these reasons, boron removal from industrial wastewaters has received a lot of attention in recent years. In Japan, the permitted boron concentration in industrial wastewater discharged into the environment is set at 10 and 230 mg/L for non-sea and sea environments, respectively. Although a number of physicochemical methods have been proposed for boron removal from industrial wastewaters, such as coagulationsedimentation,7 adsorption on metal hydroxide,8 solvent extraction,9 ion exchange,10 and the use of boronselective adsorbents,11,12 most of these methods are difficult to apply practically because of specific disadvantages. To remove boron to an acceptable level, precipitation and adsorption will r 2011 American Chemical Society
generate a large amount of sludge, which is difficult to dispose of. Solvent extraction, ion exchange, and the use of boron-selective resins are uneconomical because of their large consumption of acids and bases in the regeneration of extractants and resins. Membrane processes such as reverse osmosis,13,14 nanofiltration,15 and polymer-enhanced ultrafiltration16 have also been considered as alternatives for boron removal. However, in the cases of reverse osmosis and nanofiltration, boron rejection is usually insufficient near neutral pH. For polymer-enhanced ultrafiltration, specially synthesized functional polymers are employed for removal of boron to a satisfactory level. Electrodialysis (ED) has also been considered as an alternative method for boron removal.1719 In conventional ED treatment, the level of boron removal depends largely on the pH of the solution being treated. In general, conventional ED is available only for boron removal under alkaline conditions. This is because boric acid, which is the major chemical species of boron in aqueous solution, is a very weak acid with a pKa of 9.14. Thus, under alkaline conditions boron exists predominantly as borate ions (B(OH)4), which can be removed by ED, whereas under neutral and acidic conditions the main species is undissociated boric acid (B(OH)3), which is not removed by ED. In this study, to overcome the above shortcoming of conventional ED, bipolar membrane electrodialysis (BMED) was adopted for boron removal. BMED is the combined technology of electric field-enhanced water dissociation in bipolar membranes and the ion separation in conventional ED.20,21 Received: July 9, 2010 Revised: March 22, 2011 Accepted: March 25, 2011 Published: March 25, 2011 6325
dx.doi.org/10.1021/ie1014684 | Ind. Eng. Chem. Res. 2011, 50, 6325–6330
Industrial & Engineering Chemistry Research
ARTICLE
(NaCl), and the applied voltage on the boron removal performance of BMED were evaluated in terms of current efficiency and power requirement.
’ EXPERIMENTAL SECTION
Figure 1. Schematic diagrams of the membrane configurations for (a) BMED and (b) conventional ED. BPM, bipolar membrane; AEM, anion-exchange membrane; CEM, cation-exchange membrane.
The BMED system developed for removing boron from industrial wastewater is shown in Figure 1a. The removal of boron from wastewater using BMED proceeds in three main steps: dissociation of boric acid into borate ions upon electrobasification, electrodialytic migration of borate ions, and production of a concentrated boric acid solution upon electroacidification. Primarily, in the dissociation step, boric acid dissociates to form borate ions by reacting with hydroxide ions (OH) formed by water dissociation in the bipolar membrane. After the water dissociation step, borate ions in the wastewater migrate through the anion-exchange membrane toward concentrate compartments under the influence of an applied electric field. Finally, the migrated borate ions react with protons (Hþ) generated by the water-splitting reaction in bipolar membranes to form boric acid. Consequently, boron is removed from the wastewater, and a concentrated boric acid solution is obtained in the concentrate compartments. To confirm the feasibility of boron removal using BMED, experimental studies comparing the boron removal performance of BMED versus conventional ED were conducted. Furthermore, the effects of the initial pH, the concentration of sodium chloride
Experimental Set-up. Boron removal experiments were conducted using laboratory-scale ED equipment (AGC, Japan). The ED stack consisted of three pairs of diluate and concentrate compartments located between anode and cathode compartments. In this study, the commercially available membranes Selemion AMV (AGC, Japan), Selemion CMV (AGC, Japan), and Neosepta BP-1E (Astom, Japan) were used as the anionexchange, cation-exchange, and bipolar membranes, respectively. The effective area of each membrane was 117.5 cm2 and the width of each compartment was set at 1.5 mm. For each batch operation of the experiment, an aqueous solution of sodium tetraborate (Na2B4O7, 1.0 L) containing 100 mg/L of boron (9.25 mmol/L of boron) was supplied to the diluate and concentrate compartments, respectively. The initial pH of the aqueous solutions was controlled using aqueous solutions of hydrochloric acid and/or sodium hydroxide. An aqueous solution containing sodium sulfate (0.1 mol/ L, 1.0 L) was introduced to the electrode compartments as an electrode rinsing solution. Each solution was circulated through the ED stack using magnetic pumps and the flows for each compartment were introduced in parallel. The flow rate of each solution was set at 1.0 L/min, which is a linear flow rate of 3.7 cm/s. During the experiment, electric current was supplied to the ED stack at a certain constant voltage using a direct current power unit (Matsusada, Japan). Each experiment was continued for 90 min or until the current decreased to 0.01 A. Each experimental condition was performed at least three times. The pH of the solutions was measured with a pH meter (Horiba, Japan). The boron concentration was analyzed by spectrocolorimetry (V530, JASCO, Japan) using the azomethine-H method.22 Calculation of Current Efficiency and Power Requirement. The current efficiency and power requirement for boron removal were calculated to evaluate the boron removal performance of BMED. The current efficiency η is defined as the ratio of stoichiometric number of electrical charges needed for boron removal to the total electrical charge introduced into the ED stack, as shown in eq 1,
η¼
ðC0 Ct ÞVF Q
ð1Þ
where C0 and Ct are the concentration of boron in the diluate solution at time 0 and t, respectively, V is the volume of the diluate solution, and F is the Faraday constant. Q is the total electrical charge that passed through the ion-exchange membrane and is given by eq 2, Z Q ¼ nS
t
i dt
ð2Þ
0
where n is the number of pairs of diluate and concentrate compartments, S is the effective membrane area, and i is the current density. 6326
dx.doi.org/10.1021/ie1014684 |Ind. Eng. Chem. Res. 2011, 50, 6325–6330
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The power requirement for boron removal P is given by eq 3, Z
t
UiS dt P ¼
0
V
ð3Þ
where U is the applied voltage across the ED stack.
’ RESULTS AND DISCUSSION Comparison of BMED and Conventional ED. A comparative study between BMED and conventional ED was conducted using two membrane configurations as shown in Figure 1. The experiments were performed using an aqueous solution of Na2B4O7 with initial pH of 2.3 or 9.1 under a constant voltage of 15 V. To eliminate the effect of coexisting anionic species, the initial concentration of chloride ions (Cl) was adjusted to 10 mmol/L. Figure 2a illustrates the change in current over time. In the ED experiments, the current decreased to 0.01 A within 2025 min because of the increase in the electric resistance across the diluate compartment caused by the electrodialytic exclusion of ionic species such as sodium ions (Naþ) and Cl. However, in the BMED experiments, the current increased gradually after an initial decrease. The initial decrease in current is caused by the electrodialytic exclusion of Cl from the diluate solution. The subsequent increase in current is caused by the pH of the diluate solution increasing, which leads to more dissociation of B(OH)3 into B(OH)4. Figure 2b demonstrates the change in the boron concentration in the diluate solutions in the BMED and ED experiments. Boron is not removed from diluate solution to a satisfactory level in the ED experiments, with approximately 30 and 45% of boron removed from solutions with initial pH values of 2.3 and 9.1, respectively. In contrast, boron is removed to an acceptable level in the BMED experiments, with more than 90% of boron being removed from the diluate solutions with initial pH values of both 2.3 and 9.1 within 90 min. This significant difference in boron removal between BMED and ED is because of the difference in the variation of the pH of the diluate solutions, as shown in Figure 2c. In the BMED experiments, the pH of the diluate solution increased rapidly when a direct current was applied across the stack. This is caused by the migration of OH generated at the bipolar membranes. When the pH of the diluate solution becomes relatively high compared with the pKa of B(OH)3, its dissociation is enhanced and B(OH)4 becomes the predominant boron species. As a result, higher boron removal was achieved using BMED compared with using ED. Effect of the pH of the Feed Solution. The effect of the initial pH of the feed solution was examined using different pH of 2.3, 7.0, 9.1, and 12.0. To eliminate the effect of coexisting anionic species, the initial concentration of Cl was adjusted to 10 mmol/L. The experiments were performed under a constant voltage of 15 V. Figure 3 illustrates the effect of the initial pH on the change in boron concentration in the diluate solution (a) as a function of time and (b) as a function of total electric charge. Under all of the conditions studied, the boron concentration in the diluate solution decreased to
