Synthesis and characterization of arsenate-intercalated layered double hydroxides (LDHs): prospects for arsenic mineralization.

The arsenate-intercalated layered double hydroxide (LDH) of Mg and Al is synthesized by coprecipitation. The higher thermodynamic stability and the consequent lower solubility of the unitary arsenates preclude the formation of arsenate-intercalated LDHs of other metals directly from solution. However other M/Al-AsO(4) (M=Co, Ni, Zn) LDHs could be prepared by anion exchange, showing that arsenate intercalation proceeds topotactically. The intercalation of various species of As(V) into the interlayer of LDHs and the subsequent arsenate carrying capacity are dependent upon the pH of the solution. Upon thermal decomposition, the intercalated arsenate ion undergoes reductive deintercalation to give a mixture of As(III) and As(V) oxides. The product oxides revert back to the LDH upon soaking in water on account of the compositional and morphological metastability of the former. This is in contrast with the phosphate-intercalated LDHs, in which the reversibility is suppressed, consequent to the formation of stable metal phosphates.