Competition between three organic anions during regeneration process of calcined LDH

Abstract

The calcination of Layered Double Hydroxides (LDH) produces mixed oxides which can undergo rehydration when in contact with aqueous solution containing anions, regenerating the layered structure of LDH, with these anions in the interlayer space. This process is known as ‘ memory effect’. In this work we studied the regeneration of LDH, by sorption of organic anions as terephthalate (TA), benzoate (BA) and 4-methyl-benzoate (m-BA), from the same aqueous solution, starting from the product of calcination of carbonate-containing LDH. The selectivity of the sorbent to each organic anion during reconstruction of the layer structure was also investigated. Results showed a higher extraction rate for TA species and a lower one for BA species. From the results, an order was proposed for the removal of anions as TA≫m-BA>BA, which is related to the capacity of each anion to stabilize the LDH layer. PXRD patterns showed peaks with basal spacing characteristic of intercalated TA anions. The recycling capacity of the sorbent during five cycles of calcination/sorption was also investigated. This capacity decreased after each calcination/sorption cycle, probably due to the formation of magnesium oxide and spinel phases in the material.