The phosphorous removal by adding chemicals has problems such as high capital cost, chemical sludge handling, etc.. In addition, such chemicals as alum, iron salt, and lime can be used only once and can not be recovered for further use, which makes chemical treatment costly. So it is required to develop recyclable chemicals which keep the original treatability. Layered Double Hydroxide (LDH), which can keep the original capacity of ion exchange, could be as an alternative chemical for phosphorous removal.
LDH is a family of hydroxylated hydrate compounds of general formula: $[M(Ⅱ)-{1-x}M(Ⅲ)_x(OH)_2][X^{m-}{x/m}.nH_2O]$. These compounds consist of a positively charged brucite $(M(OH)_2)-$ like octahedral layer and a negatively charged interlayer containing anions and water molecules. The layer can be stacked and the balancing interlayer anions can be exchanged for other anions.
LDH ($Mg-Al-CO_3$ system) and LDH-Cl (Mg-Al-Cl system) were synthesized by a coprecipitation method, and were characterized by X-ray powder diffraction, FT-IR, $^13C-CP-MAS NMR$, $^27Al-MAS NMR$, TGA/DSC, ICP-AES and other methods.
The ion exchange reactions of LDH-Cl and calcined LDH in aqueous phosphate solution were investigated with various aspects such as phosphate uptake according to the kind of LDH, phosphate concentration's variation, the effect of competitive anion, isotherm for phosphate uptake, etc.. From the result, the phosphate uptake by LDH-Cl was found to be 2.35-2.83 meq of P/g and the isotherm of that followed a typical langmuir type adsorption. The most serious obstacle anion against phosphate uptake was carbonate because the selectivity of carbonate on LDH is the highest.
To investigate the possibility of LDH's recycle employing "memory effect", calcined LDH (CLDH) was repeatedly used for 6 times. From the fifth time, the final phosphate uptake capacity decreased. The consecutive reconstruction mechanism of CLDH is not yet known but it is certain that the possibility of LDH's recycle is promising.
