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

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.