Sorption of terephthalate anions by calcined and uncalcined hydrotalcite-like compounds

Abstract

The uptake of terephthalate anions (TA) from aqueous solutions by Mg–Al–CO3 layered double hydroxides (LDHs) has been investigated. The sorption process, that takes place by the reconstruction of the calcined LDH and involves the intercalation and adsorption of TA, is much more efficient in the uptake of such anion than the pure adsorption process occurring in the parent LDH. The extraction ratio (defined as the ratio between the sorbed quantity of TA and the total amount of TA in the system) observed in the concentration range usually found in industrial wastes of purified terephthalic acid (PTA) production plants varied from 83 to 57% for the calcined LDH. The investigation of the sorption kinetics showed that this process is relatively slow, and occurs by a two step mechanism, i.e. a fast rehydration of the mixed oxide (calcined LDH) with intercalation of hydroxyl anions, and a slow anion exchange of the hydroxyl anions by TA. The displacement of OH− by TA increased the pH. Therefore, the pH is a parameter of paramount importance in the process, since it affects the ratio between TA and OH− in solution and, consequently, in the LDH interlayer. Recycling of the sorbent, in cycles of calcination and reconstruction in TA solution, showed a small reduction (around 10%) in the uptake capacity up to the 5th cycle. This small reduction can be related to the easy elimination of the TA anions by calcination and, unlike previously reported results, to the absence of Al(III)-rich phase segregation. Therefore, Mg–Al-LDHs can be considered as very promising sorbents for TA.