Chromate anion was effectively concentrated on an anion exchanger, layered double hydroxide (LDH), from aqueous solution. The LDHs used in this study were synthesized by hydrothermal reactions using urea hydrolysis as reported previously (Ogawa and Kaiho, 2002). The ion exchange was conducted at pH=3 to concentrate larger amount of dichromate (Cr2O7 2-). The adsorption isotherms indicated the strong interactions of chromate anions with LDHs. Among the tested LDHs (MgAl, NiAl, CoAl, and LiAl-LDHs), LiAl-LDH gave the best result in terms of the adsorbed Cr amount. Since the reaction was started from carbonate forms of layered double hydroxides, only the carbonate anions are released to the environment after the anion exchange. All these facts indicated that the present layered double hydroxides are promising materials for the environmental remediation from Cr contaminated water. If compared with commercially available LDHs and those synthesized by the conventional co-precipitation methods, the LDHs synthesized by hydrothermal reaction using urea hydrolysis were shown to be superior because of the stability and the ease of solid-liquid separation after the ion exchange.
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