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Ind. Eng. Chem. Res. 2008, 47, 16-24
Technologies for Boron Removal Yonglan Xu and Jia-Qian Jiang* CEHE, Faculty of Engineering & Physics Science, C5, UniVersity of Surrey, Guildford, Surrey GU2 7XH, United Kingdom
Boron has been widely used in various manufacturing industries, and the level of boron in the industrial effluent can range from as high as several grams per liter to as low as a few micrograms per liter. Due to the potential adverse health effect of boron on human beings, the World Health Organization (WHO) has provided a guideline for the boron concentration in drinking water to be less than 0.5 mg/L. Therefore, the boroncontaining effluents or solutions should be treated before they could be discharged into natural water bodies. This paper critically reviews various technologies used for boron removal, including those recently developed, electrocoagulation (EC) and double-layered hydroxide (DLH) compounds, as well as conventional and advanced treatment technologies such as chemical precipitation, ion exchange, reverse osmosis, adsorption, liquidliquid extraction, and electrodialysis. Particular focus is given to the comparative performance and advantages and disadvantages of these technologies for boron removal. EC and DLH have been proven to be highly effective for boron removal from solutions with a high boron concentration, while a DLH compound alone can effectively treat solutions with low boron concentrations ( Fe(OH)3 > Al(OH)3. However, the problem for these reagents is their low mechanical strength, which makes them unsuitable for large-scale use in cyclic sorption-desorption processes. Using a sol-gel method could produce sorbents in the form of spherical granules of superior mechanical strength. Also, mixed zirconium-iron hydroxides are said to be both mechanically strong and chemically stable, with a high boron sorption capacity. The nature of the selective boron removal by hydrated zirconium dioxide has not yet been explained.79 The high affinity of zirconium hydroxide to boron compounds in aqueous solution has been explained by the formation of low solubility chemical compounds and variable composition: Na[ZrO(OH)x[B4O7]n]. The ratio varies from 1.7 to 2.8 B atoms per one Zr atom, as a result of OH- group substitutions in the tetraborate ions. Aluminum hydroxides have been studied for the boron removal.80 In the presence of atrazine, the amorphous aluminum hydroxides complex with boric acid or borate ions firmly; 42.1% more boron was removed than that without atrazine at an equilibrium status. The experimental data show that complexes (Al-OH-atrazine-B) might be formed on the activated sites (Al-OH) of amorphous aluminum hydroxides. However, in the presence of catechol, relatively low amounts of boron were adsorbed by aluminum hydroxides because the activated sites (Al-OH) were occupied by the phenolic rings of catechol. As the crystalline state of Al(OH)3 increases, the ability of boron to be adsorbed by amorphous Al(OH)3 is decreased. The existence of Al-catechol complexes has
22 Ind. Eng. Chem. Res., Vol. 47, No. 1, 2008
been reported,81 and possible complexes’ formulas were proposed as follows:82 Al(OH)2(O)B(C6H4O2) or [(C6H4O2)Al(O)2B(OH)2]2-, [(C6H4O2)2Al(O)B(OH)3]3-. Discussions and Concluding Remarks As discussed above, boron can be removed from water by various technologies. However, most of them have limitations and can only be applied conditionally. Chemical precipitation could effectively remove boron but requires adjusting water pH to a high level (pH >9). Therefore, this method causes high salinity and produces a voluminous amount of sludge for disposal. High costs of chemical demands and sludge disposal sometimes prohibit the use of chemical precipitation for boron removal. Activated carbon, impregnated activated carbon, and clay have been tested as a means of removing boron. The boron removal capacity of these materials is relatively low; they are unlikely to be economically applied to full-scale boroncontaining wastewater treatment. Liquid-liquid extraction offers the possibilities of the commercial recovery of high concentrated boron from industrial effluents, but the technique is only costeffective when the original boron concentration is to be at least 3 g/L as B2O3. A boron-specific ion-exchange resin offers very good selected ion exchange efficiency for boron. However, high costs of the system setting up and treatment of the regeneration effluents would limit its use. Reverse osmosis and electrodialysis can be effective in removing boron but require increasing the influent pH (>9); this results in high potentials of colloidal scaling and fouling which limit the use of these technmologies for the boron removal. In order to reach to the WHO recommended drinking water standard (0.5 mg/L), the multistage RO systems are required. The various difficulties and economical disadvantages of these methods led the researchers to perform new studies to develop robust and cost-effective methods. Electrocoagulation and Mg-Al double-layered hydroxide compounds are two recently tested technologies for boron removal. Electrocoagulation is a distinct economical and environmental choice for industrial, commercial, and municipal wastewater treatment. Except for the precipitation, the process also provides the freshly formed metal hydroxides which are a useful adsorbent for adsorbing boron. The capital and operating costs are usually significantly less than chemical coagulation. MgAl double-layered hydroxide compounds have high boron removal capacity. These two technologies have been proven to be highly effective for the boron removal from solutions with a high boron concentration, while a Mg-Al double-layered hydroxide compound alone can effectively treat solutions with low boron concentrations (
