Uptake of heavy metal ions from aqueous solution using Mg-Al layered double hydroxides intercalated with citrate, malate, and tartrate

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 citrate^3- (C₆H₅O₇^3-), malate^2- (C₄H₄O₅^2-), or tartrate^2- (C₄H₄O₆^2-) 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 citrate^3-, malate^2-, and tartrate^2- were adequately active as chelating agents in the interlayers of Mg-Al LDHs.