NoteHeavy metal immobilization in aqueous solution using calcium phosphate and calcium hydrogen phosphates
Introduction
Immobilization of aqueous heavy metal cations, particularly lead cation, with calcium hydroxyapatite (Ca10(PO4)6(OH)2, CaHAp) has received attention as an effective means of removing those harmful metals from aqueous solution [1], [2], [3], [4], [5], [6], [7], [8]. As a mechanism for lead immobilization with CaHAp, absorption of lead on the surface of CaHAp followed by cation exchange with calcium to afford insoluble Pb10(PO4)6(OH)2 (PbHAp) was suggested [1], [2], [3]. In contrast, several groups suggested that the dissolution of CaHAp provided phosphate anion for the subsequent precipitation of PbHAp from aqueous solution containing lead cation as [4], [5], [6], [7] This dissolution–precipitation mechanism suggests that phosphates and hydrogen phosphates, which can dissolve more easily than CaHAp (solubility product constant (SPC) =6.62×10−126 [9]) into aqueous solution followed by the formation of H2PO−4, may be employed for CaHAp. However, there are rather few reports on the immobilization of heavy metals in aqueous solution with phosphates and hydrogen phosphates other than CaHAp except our communication [10]. In the present paper, we report immobilization of aqueous heavy metal cations such as Pb2+, Cu2+, Co2+, and Cd2+ with β-Ca3(PO4)2 (SPC =2.07×10−33 [9]), CaHPO4·2H2O (SPC =2.59×10−7 [9], and solubility = 0.017 at 298 K [11]), and Ca(H2PO4)2·H2O (solubility = 1.3 at 303 K [11]) at 293 K. In the present study, the solubility of these phosphates and solution pH have been focused on to reveal the contribution of the dissolution–precipitation mechanism to the immobilization.
Section snippets
Experimental
All chemicals, which were purchased from Wako Pure Chemicals, Osaka, Japan, were of high purity and used as supplied. Standard solutions for ICP measurement of Ca, Pb, Co, Cu, Cd, and P were obtained from Kanto Kagaku, Tokyo, Japan. An aqueous solution (100 ml), to which a known quantity of heavy metal cations as nitrate or acetate had been added, was stirred at 120 rpm in a constant temperature bath at 293 K for 20 min. The initial solution pH was adjusted by the addition of HNO3 when needed.
Results and discussion
The employment of β-Ca3(PO4)2 (3.9 mmol Ca2+/0.4 g β-Ca3(PO4)2) for the removal of Pb2+ (4.2 mmol/100 ml, supplied as nitrate) resulted in 97% immobilization of Pb2+ at 6 h, while complete removal of Pb2+ was observed with CaHPO4·2H2O (2.3 mmol Ca2+/0.4 g CaHPO4·2H2O) and Ca(H2PO4)2·H2O (1.6 mmol Ca2+/0.4 g Ca(H2PO4)2·H2O) from the aqueous solution of Pb2+ (2.6 and 1.8 mmol/100 ml, respectively) as shown in Fig. 1. It should be noted that the quantities of Pb2+ lost from the solution are
Acknowledgements
This work has been funded by Grants-in-Aid for Scientific Research (C) to S.S. and (B) to T.T. (KAKENHI 14550801 and 13480178, respectively) from the Japan Society for the Promotion of Science (JSPS) and by the Kurita Water and Environment Foundation (No. 14058 to S.S.), to which our thanks are due.
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