Improvement of bivalent metal dispersion in ternary layered double oxide (LDO) for efficient anion removal

Abstract

In this study, the removal of tripolyphosphate (TPP) was performed over Mg2-xCaxFe layered double oxide (Fe-LDO, x = 0.0-2.0) and Mg2-xCaxAl layered double oxide (Al-LDO, x = 0.0-2.0). For the preparation of LDO, the thermal process of the pristine LDHs was carried out by thermal gravity analysis (TGA). It is indicated that the binary metal in Mg2-xCaxAl layered double hydroxide (Al-LDH, x = 0.5-1.5) was well dispersed while Mg2-xCaxFe-LDH showed two separated Ca-Fe-LDH and Mg-Fe-LDH phases in the sample. In TPP removal, the removal amount of TPP over Mg2Fe-LDO and Mg2Al-LDO was 11.8 and 21.4 mg P/g, respectively. In contrast, Ca2Fe-LDO and Ca2Al-LDO provided 121.0 and 185.0 mg P/g of TPP removal capacity, respectively. Moreover, on ternary Mg2-xCaxFe-LDO and Mg2-xCaxAl-LDO (x = 0.5-1.5), the sorption amount of TPP in Al-LDO was higher than that in Fe-LDO. Especially, 117 mg P/g of TPP removal amount was recorded in the case of Mg0.5Ca1.5Al-LDO. This was attributed to the fact that Al-LDO could be reverted to Mg-Al-LDH framework, which provided OH-metal on the solid surface to improve the TPP adsorption on the adsorbent. The other reason for TPP removal was Ca doping into the Mg-Al-LDH framework in the reformed LDH from LDO, which improved the affinity of TPP to the adsorbent by TPP precipitation. Therefore, our result proposed that it was advisable and feasible to use LDO for the removal of TPP.