Removal of Phosphorous Using a Hydrotalcite-like Compound : Application to the Field of Medicine/Environmental Technology

Abstract

A Cl^- intercalated hydrotalcite-like compound (Cl/LDH) was prepared by neutralizing magnesium and aluminum chlorides with sodium hydroxide. A Cl^- ions in the interlayer of Cl/LDH can be exchanged with phosphate equivalently. The isotherm for phosphate uptake followed the Langmuir equation at pH 7.53 and 310K: it gave an ion-exchange capacity of 2.37mmol of P/g for phosphate. The process of phosphate/Cl^- ion exchange could be divided into three stages depending on the amount (P_<load>) of phosphate loading. In stage I (0%<P_<load><6%), the Cl^--intercalated phase (interlayer distance =0.30nm) alone was observed. In stage II (6%<P_<load><80%), the phase transition from the Cl^--intercalated phase to the phosphate-exchanged phase (interlayer distance =0.36nm) took place. In stage III (80%<P_<load><100%), the phosphate-exchanged phase alone was observed; the interlayer distance increased slightly (from 0.36 to 0.37nm) with an increase in the phosphate loading, indicating a formation of solid solution. The phosphate/Cl^- ion-exchange reaction with Cl/LDH was not affected by the presence of trypsin. On the other hand, the trypsin adsorption was significantly depressed by the loading of phosphate. In the removal of phosphate from brackish river water using Cl/LDH, Kawamoto et al. indicated that phosphorous concentrations in the river water went up from 0.1mg P dm^<-3> to 0.02mg P dm^<-3> under the condition of a column method with a space velocity of 5.0hr^<-1>. The value of 0.02mg P dm^<-3> was considered to be enough for the repression of algae growth. As a zero-emission type phosphorous removal system, Oshima et al. suggested that two-step regeneration system, which uses a NaOH-NaCl mixed aqueous solution as the desorbing solution and a MgCl_2 aqueous solution as the regenerating solution. Moreover, phosphate ions can be recovered as magnesium ammonium phosphate (MAP) from the desorbing solution in the system.