Using Ca-Fe layered double hydroxide transformation to optimise phosphate removal from waste waters

Single phase Ca-Fe layered double hydroxide (LDH) minerals containing Cl- species in the interlayer was synthesized by coprecipitation with a Ca-II: Fe-III ratio of 2: 1. In both phosphate (PO4) free water and at low aqueous PO4 concentration, the LDH was fully transformed into a mixture of a ferrihydrite-like material, calcite and soluble calcium species. Mossbauer spectroscopy and transmission electron microscopy showed that phosphate was removed by the ferrihydrite like phase that contained a significant quantity of Ca. At high phosphate concentration the Ca species released from the LDH precipitated to form hydroxyapatite leading to a maximal removal capacity of similar to 130 mg P-PO4 g(-1). The Ca-Fe LDH was deposited onto a pozzolana volcanic rock in order to perform a column experiment under hydrodynamic conditions for 70 days. A high removal capacity was observed, a q(B) of similar to 4 mg P-PO4 g(-1) was measured at the breakthrough of the column, however the pH in the outflow was measured to be higher than 11. Such an increase was due to the very high solubility of the Ca-Fe LDH.