(231f) Boron Removal from Aqueous Solution by Bipolar Membrane Electrodialysis
Boron compounds in water at high concentration is known to be toxic to humans and plants. The World Health Organization (WHO) has limited boron concentration in drinking water to less than 0.5 ppm. Because of the increased use of desalinated seawater and underground water for drinking water and large amount of boron emissions via wastewater from the manufacturing process, the removal of boron from aqueous solutions has been an important issue in recent years. There are several methods used for boron removing from aqueous solutions, such as precipitation, adsorption, solvent extraction and ion-exchange method. However, the precipitation and the adsorption methods are efficient only under high boron concentration. In the case of solvent extraction and ion-exchange, post regeneration process causes cost increases. In aqueous solutions, boron is mainly present in the form of boric acid. According to the dissociation reaction (pKa 9.1), boric acid exists in the form of borate ions in alkaline condition. Since borate ion is a monovalent anion, electrodialysis can be utilized to remove boric ions from aqueous solutions. Additionally, the dissociation reaction of boric acid into borate ions can be enhanced by combining water splitting reaction caused by bipolar membrane. In this study the feasibility of the boron removable process using electrodialysis method combined with bipolar membrane water splitting reaction was investigated. The basic unit of the boron removal system is composed of two compartments separated by anion exchange membrane, and the compartments are sandwiched by two sheets of bipolar membranes. The boron contained solution is introduced into one compartment (feed compartment) and an electrode solution is introduced into another compartment (concentrate compartment). When an electrical potential is applied across the system, water molecules in the bipolar membranes will be split into protons and hydroxyl ions. According to the electrical field applied across the system, hydroxyl ions will be transported to the feed compartment. Due to the increase of hydroxyl ions, pH of the boron contained solution will increase and the dissociation of boric acid into borate ions will be enhanced. Generated borate ions will be transported from feed compartment to concentrate compartment through the anion exchange membrane by function of the electrical field. Thus boron will be removed from boron contained solution and concentrated in the electrolyte solution. An experimental study was conduced in a laboratory-scale electrodialysis apparatus to examine the feasibility of the boron removal process. Sodium tetraborate solution (100 mg-B/L) was used as both the feed solution and the electrolyte solution. 10 basic units were placed between two electrodes at the both end. The membrane effective area was 117.5 cm2 and the distance between two membranes was 0.75 mm. The electrodialysis was operated with a constant electrical potential at 10 V. When the electrical potential was applied across the stack, the pH of the feed solution was increased with time, while that of the electrolyte solution was decreased with time. At the same time, decrease of the boron concentration in the feed solution and increase of the boron concentration in the electrolyte solution was observed. The boron concentration in the feed solution reached <10 mg-B/L in 60 minutes. The power requirement for boron removal calculated from the experimental result was 2.65 kWh/m3. The estimated cost of electricity required for electrodialysis process was equal to $0.13/m3.
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