Anion Selectivity of Layered Double Hydroxides: Effects of Crystallinity and Charge Density

Abstract

Layered double hydroxides (LDHs) have long been thought to have low selectivity for NO3–. In contrast, it has recently been reported that LDHs with certain compositions show high selectivity for NO3–. The reason for the high selectivity has been unclear, although the ion sieve effect has been proposed as a possible mechanism. Moreover, small LDHs have been reported to show good anion exchanging ability. Given this background, a systematic study was conducted using Mg-Al LDHs with two different crystallinities. High-crystallinity LDHs with lager particle sizes clearly showed selectivity changes that depended on the Mg/Al molar ratio, compared with low-crystallinity LDHs with smaller particle sizes. When the Mg/Al ratio increased, selectivity of high-crystallinity LDHs for SO42– and F– decreased, whereas that for NO3– increased. In the case of F–, the diameter of which is smaller than the height of the flat orientation of NO3–, this result indicates that an ion sieve effect is not the only factor that determines selectivity. Other factors, such as the interactions of NO3– with the interlayer surface and with interlayer water, may also be involved.