Mg-Al layered double hydroxide (Mg-Al LDH) was modified with organic acid anions using a coprecipitation technique, and the uptake of heavy metal ions from aqueous solution by the Mg-Al LDH was studied. Citrate·Mg-Al LDH, malate·Mg-Al LDH, or tartrate·Mg-Al LDH, which had citrate3- (C6H5O73-), malate2- (C4H4O52-), or tartrate2- (C4H4O62-) anions intercalated in the interlayer, was prepared by dropwise addition of a mixed aqueous solution of Mg(NO3)2 and Al(NO3)3 to a citrate, malate, or tartrate solution at a constant pH of 10.5. These Mg-Al LDHs were found to take up Cu2+ and Cd2+ rapidly from an aqueous solution at a constant pH of 5.0. This capacity was mainly attributable to the formation of the citrate-metal, malate-metal, and tartrate-metal complexes in the interlayers of the Mg-Al LDHs. The uptake of Cu2+ increased in the order malate·Mg-Al LDH < tartrate·Mg-Al LDH < citrate·Mg-Al LDH. The uptake of Cd2+ increased in the order malate·Mg-Al LDH < tartrate·Mg-Al LDH = citrate·Mg-Al LDH. These differences in Cu2+ and Cd2+ uptake were attributable to differences in the stabilities of the citrate-metal, malate-metal, and tartrate-metal complexes. These results indicate that citrate3-, malate2-, and tartrate2- were adequately active as chelating agents in the interlayers of Mg-Al LDHs.
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