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 NO 3 – . In contrast, it has recently been reported that LDHs with certain compositions show high selectivity for NO 3 – . 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 SO 4 2– and F – decreased, whereas that for NO 3 – increased. In the case of F – , the diameter of which is smaller than the height of the flat orientation of NO 3 – , this result indicates that an ion sieve effect is not the only factor that determines selectivity. Other factors, such as the interactions of NO 3 – with the interlayer surface and with interlayer water, may also be involved.