Environ. Sci. Technol. 2006, 40, 5233-5243
Electrodialysis with Bipolar Membranes for Sustainable Development CHUANHUI HUANG AND TONGWEN XU* Laboratory of Functional Membranes, School of Chemistry and Material Science, University of Science and Technology of China, Hefei 230026, P. R. China
Electrodialysis with bipolar membranes (EDBM) is a kind of technology that integrates solvent and salt dissociation. It can realize salt conversion without second salt pollution or provide H+ and OH-/alkoxide ions in situ without salt introduction. Thus, it inherently possesses economical and environmental benefits. Moreover, its technological compatibility gives rise to new functions when it couples with other technologies, such as complexion, ion exchange, extraction, and adsorption. In view of the above peculiarities, EDBM has found many interesting applications in chemistry, food processing, biochemical industries, and environmental protection. However, its development has been restricted by such factors as lack of recognition of its contribution to industrial ecology, high membrane cost, insufficient research investment, and scarce operation experience. This paper compiles an introduction to this technology from the perspective of industrial ecology and conducts an extensive examination into EDBM applications. Its purpose is to gather synergic strength from academia, industry, and government to perfect EDBM for sustainable development.
Introduction In the aspiration to meet the needs of the present and posterity, sustainability has been widely endorsed as the overarching goal of environmental policy (1). As the science of sustainability with emphasis on the careful use and reuse of resources, industrial ecology is centered around “design for the environment (DFE)” and “green chemistry” (2). The former is to elucidate and incorporate environmental impacts into the development of technology during the design stage of a new product or process; the latter is to use chemistry techniques and methodologies to reduce or eliminate the use or generation of materials which are hazardous to human health and the environment (2-4). They work to complement an integral part of industrial ecology and sustainable technology. Electrodialysis with bipolar membranes (EDBM) is a type of technology which is based on DFE and green chemistry. It can realize some new synthesis processes to achieve the maximal utilization of resources and pollution prevention (5). It can be flexibly coupled with many other technologies and obtain a better function by means of technological symbiosis. It can realize closing loops (6) when inputting waste materials as feedstock and carrying out production, resource regeneration, and effluent treatment at the same time. In this sense, EDBM plays the same role as “photo* Corresponding author phone: 86 551 3601587; fax: 86 551 3601592; e-mail:
[email protected]. 10.1021/es060039p CCC: $33.50 Published on Web 07/22/2006
2006 American Chemical Society
FIGURE 1. Chronology of EDBM documents. Source: www. scopus.com. [Search settings: TITLE-ABS-KEY (bipolar membrane electrodialysis). Search date: Apr 3, 2006.] synthesizers” in industrial ecosystems and inherently possesses economical and environmental benefits. EDBM has gained more attention in the last two decades (Figure 1). It has been applied to or studied for chemistry, food processing, biochemical industries, and environmental protection. From 1986 to 2004, the bipolar membrane (Aqualytics and Neosepta) installed worldwide totaled 3010 m2 (U.S., 1660 m2; Asia, 650 m2; Europe, 700 m2) (7). However, since its preparation a half century ago (8), the bipolar membrane technology has not developed at a desirable pace. On one hand, there is a lack in recognition of the role EDBM plays in industrial ecology and sustainable development. On the other, there exist some hurdles to cross when bringing this technology to practice. This paper makes an introduction to this technology from the perspective of industrial ecology. Its purpose is to draw the attention from academia, industry, and government. Thus, more breakthroughs will be motivated to bring this technology to perfection and more practice will be contributed to sustainable development.
Overview: Background of Bipolar Membrane Processes Bipolar Membrane and Its Function. A bipolar membrane is a composition of a cation-selective layer and an anionselective layer (9). Its typical function is manifested when being applied under reverse potential bias (Figure 2 a). Under this condition, electrolyte ions will be depleted in the transition region and the current carriers will stem from solvent dissociation. At the present, there are only two kinds of solvents reported to dissociate in bipolar membranes: water (H2O) and methanol (CH3OH) (10). They split into H+ and OH- and H+ and CH3O-, respectively. As for the water splitting, it accounts for most of the functions of bipolar membrane processes applied so far. There is no agreement on the mechanism of water splitting in the transition region VOL. 40, NO. 17, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Bipolar membrane and EDBM: BP, bipolar membrane; A, anion-selective membrane; C, cation-selective membrane; M+, cation; X-, anion; H+, hydrogen ion; R, OH or CH3O. Key: (a) bipolar membrane and its function; (b) acid and base/alkoxide production; (c) acid production; (d) base/alkoxide production. of bipolar membranes, but two possible contributing factors have been used for explanation: (a) the catalytic effect of fixed ionogenic groups or other added chemicals (11); (b) the second Wien effect (12-14). The former has been the effective guideline for the interface modification of bipolar membranes, with the purpose of reducing the energy consumption for water splitting. The studied materials used for interface modification include metal hydroxides (15, 16), PEG, PAMAM, bovine serum albumin, PVA (17), silica sol (18), etc. As for the methanol splitting, there is much less research. However, it should not be ignored because it is of significance to realize some new synthesis paths, which embody the tenets of green chemistry. Furthermore, the methanol splitting initiates the dissociation of organic solvents in bipolar membranes, and thus, many other protic solvents are expected to be applied in EDBM to meet specific needs. When it comes to membrane preparation, a bipolar membrane can be formed by (a) laminating (heat-pressing or gluing) an anion exchange membrane and a cation one, back to back, (b) introducing positively charged fix groups and negatively charged ones to different sides of a neutral film, or (c) casting a cation (or anion) exchange polyelectrolyte solution on a commercial anion (or cation) exchange membrane. The latter seems to be the most attractive due to its low cost, easy operation, and flexibility to achieve the desired membrane properties. Table 1 provides information on some available bipolar membrane properties and suppliers (9, 19-23). The ideal bipolar membrane should have high permselectivity, low electrical resistance and water splitting voltage drop, high current efficiency, good chemical and mechanical stability, long lifetime, and no “ballooning”. Notably, high permselectivity is very crucial to alleviate the salt leakage resulted from co-ion migration. EDBM and Its Configurations. EDBM is a successful functional integration of the solvent dissociation of bipolar membranes and the salt dissociation of conventional electrodialysis. In practice, three cell configurations (Figure 2b5234
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d) are chosen to produce acid and/or base/alkoxide from the corresponding salts according to practical requirements. Certainly, these solvents can generate ions via electrolysis or electro-electrodialysis; however, EDBM has some outstanding preponderancies. Primarily, there is no gas or byproduct generated, i.e., no H2 or O2 in the case of H2O dissociation (24) and no H2, CO2, or other byproducts of alcohol oxidation in the case of alcohol dissociation (10). This leads to a lower voltage drop and maximal energy utilization. Additionally, only one pair of electrodes is needed to make many bipolar membranes function (split the solvent) at the same time, which results in some other advantages, i.e., space-saving, easier installation and operation, and less initial investment. Ultimately, inorganic and organic salts can be converted to the corresponding acids and bases/alkoxides at the same time and in the same EDBM stack.
EDBM for Acid and/or Base Production A large quantity of neutralization reactions exists in industrial chemistry, i.e., acids react with bases, forming salts and water. Unfortunately, a lot of these salts are not able to find a market and thus discharged into the environment, resulting in a resource waste and environmental pollution. The ideal way to supply the acid and base and utilize the salt and water is to find a path conjugating with neutralization. EDBM is such a conjugate path: salts dissociate in the same way as in conventional electrodialysis and the corresponding ions form acids and bases with the H+ and OH- ions supplied by the water splitting in bipolar membranes. In such a way, EDBM forms closing loops with neutralization reactions and carries out acid/base production, acid/base regeneration, and saltcontaining effluent treatment at the same time. This brings EDBM the analogy of a “photosynthesizer” in industrial ecosystems. Inorganic Acids and Bases. There have been some inorganic salts reported to generate acids and/or bases: NaCl (25); Na2CO3 (26); NaNO3 (27); Na2SO4 (28, 29); Na3PO4 (30). The corresponding base and acids are NaOH, HCl, H2CO3,
TABLE 1. Bipolar Membrane Properties and Suppliers (9, 19-23) membrane
thickness/mm
voltage drop/V
efficiency/%
dimens/m
FuMA-Tech GmbH, Germany >99b 0.50 × 2.00 >92b 0.50 × 1.00
FuMA-Tech FT-FBI FuMA-Tech FTBM
0.180 0.450
