RESUMEN
Magnesium hydrogen phosphate, calcium hydroxyapatite, and strontium hydroxyapatite were successfully prepared from sol consisting of sodium alginate and Na4P2O7 with Mg2+, Ca2+, and Sr2+ in the corresponding nitrates, respectively. It is revealed that the order of the addition of those substrates and the role of sodium alginate are important factors for the preparation of desired phosphate compounds. According to the previous paper on the preparation of calcium hydroxyapatite, sodium alginate was mixed with aqueous Na4P2O7, followed by the addition of the aqueous divalent cations, resulting in the poor formation of the target phosphates. However, as a revised sol-gel technique, sodium alginate was added to the mixture of Na4P2O7 and aqueous Mg2+ and Sr2+, resulting in a rather favorable formation of MgHPO4 and strontium hydroxyapatite, respectively, while the sol thus obtained was stable within a few days. However for aqueous Ca2+, calcium hydroxyapatite could not be obtained through the revised sol-gel technique. In the preparation of magnesium hydrogen phosphate, sodium alginate contributes mainly to the sol formation of the precursor. The ion exchange between Na+ in sodium alginate and aqueous Ca2+ was important for the preparation of calcium hydroxyapatite. In contrast, the reaction of sodium alginate with the mixture of Na4P2O7 and aqueous Sr2+ afforded strontium hydroxyapatite at the specific ratio of those three substrates. The structure of calcium and strontium phosphates prepared from the revised sol-gel process evidently depended on the amount of sodium alginate introduced into the mixture of Na4P2O7 and the corresponding divalent cations.
RESUMEN
In order to recycle magnesium ammonium phosphate (MgNH4PO4.6H2O: MAP) obtained from MAP process, which is one of the attractive processes for removal of aqueous ammonium and phosphate from wastewater, ammonium elimination from MAP to magnesium phosphates and ammonium incorporation into the magnesium phosphates have been investigated in the present study. It is confirmed that magnesium hydrogen phosphate (MgHPO4) is favorably obtained from the ammonium elimination from MAP at temperatures greater than 353 K, although magnesium phosphate (Mg3(PO4)2) and magnesium pyrophosphate (Mg2P2O7) have been suggested as possible candidates. Based on the dissolution-precipitation mechanism for the removal of aqueous ammonium with magnesium phosphates, three magnesium phosphates were employed for the removal of aqueous ammonium. The order of the removal rate of the aqueous ammonium was MgHPO4>Mg3(PO4)2>Mg2P2O7, as expected from the solubility of those magnesium phosphates. The removability of the solid obtained from ammonium elimination of MAP is also confirmed. The present results show that MAP can be employed as an advanced material for the removal/recovery of ammonium, although it is generally accepted that an excess of MAP obtained from the wastewater treatment can be only used as a slow-acting fertilizer.
RESUMEN
This study examines the possibilities for removing heavy metal cations from water with calcium phosphate, calcium hydrogen phosphate, and calcium dihydrogen phosphate at 293 K. It was reported that immobilization of aqueous heavy metal cations, which is known to be one of the characteristic properties of calcium hydroxyapatite, proceeded favorably with these phosphates. Calcium phosphate, calcium hydrogen phosphate, and calcium dihydrogen phosphate could favorably remove Pb2+ from aqueous solution. Calcium hydrogen phosphate also removed aqueous Cu2+, Co2+, and Cd2+, whereas these cations were not immobilized by calcium phosphate and calcium dihydrogen phosphate. A contribution of the dissolution-precipitation mechanism to immobilization with these phosphates is suggested.