Environment

[PY1-13] Immobilization of borate from the discharged water in mining activity by co-precipitation with hydroxyapatite

○Yoshikazu Hayashi¹, Binglin Guo¹, Tsuyoshi Hirajima¹, Keiko Sasaki¹ (1. Kyushu university)

Keywords:Hydroxyapatite, Borate, Geothermal water, Co-precipitation

Geothermal water often contains high concentrations of boron owing to hydrothermal activity. More than 250 types of boron-bearing minerals have been known in nature. While boron is regarded as one of essential elements for the human body, excessive intake of boron causes health hazards. In Japan the maximum contamination limit (MCL) of boron and its compounds is regulated as 10 mg/L. There is a mine site in Japan where the geothermal water is discharged daily 12,000 m³ including 22 mg/L of boron in average. In addition, it contains arsenate over the MCL, as well as carbonate and silicate. This water chemistry makes the treatment of borate difficult. In the present work, co-precipitation of borate with hydroxyapatite (HAp; Ca₁₀(PO₄)₆(OH)₂) was applied, since HAp is one of the stable compounds with very low solubility product (K_sp = 6.62×10^-126 at 25 ºC) and known as an environmental material as cationic/anionic exchanger. Although coexisting carbonate and silicate are expected to inhibit the removal of borate by co-precipitation with HAp, accompanied with Ca(OH)₂ as a mineralizer in the presence of phosphate, borate concentration was reduced to under the MCL within 2 hrs and arsenate and silicate were successfully removed under the detection limits. Combining ¹¹B-NMR with ¹H-NMR results for the solid residues, tetragonal boron (B(OH)₄^-) occupied the OH sites of HAp at the initial stage, however, gradually hydroxyl ions were arranged the OH sites and some tetragonal boron seemed to transform into trigonal boron at the equilibrium. ²⁹Si-NMR results revealed that silicate was stably immobilized in an ellestadite-like form. Further the removal rate of borate was significantly enhanced by Al³⁺ additives. It was also confirmed the solid residues can be effectively reused as adsorbents to remove Sr²⁺.