Desorption behavior of phosphate species from layered double hydroxides

In phosphate species recovery, layered double hydroxides (LDHs) are good adsorbent, but show difficulties in its desorption. In this study, the desorption behavior was systematically investigated by changing several factors, which were drying condition, change with time, crystallinity and solution for desorption. Interlayer space of PO4exchanged LDHs could swell by adding water one day after synthesis when the LDHs were vacuum dried at 25 degrees C. However, the swelling ability was deteriorating with time, and was also lost by oven-drying at 110 degrees C. In the case of high-crystallinity LDH, clear results were observed: desorption ratio of phosphate species from the vacuumdried LDH decreased with time even by 1 M NaOH which was the most effective, and desorption ratios of phosphate species from the oven-dried LDH dropped sharply. In contrast, SO4-exchanged LDH, which were used for comparison, did not lose their swelling ability, and desorption ratios of sulphate were almost unchanged over time or by oven-drying at 110 degrees C. PO4-exchanged LDH stored in aqueous suspensions did not lose their swelling ability with time, and their desorption ratios of phosphate species remained unchanged over time. Therefore, above results suggest that interaction between phosphate species and host layers of LDHs in dried state causes the changes. In conclusion, the result that phosphate species desorption from dried high-crystallinity LDH deteriorated with time has not been reported previously, which will give a great impact on applications such as recovery of phosphate and slow release of phosphate.