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Boric acid is one of the most mobile inorganic contaminant species in nature due to its pK a of 9.23. Co-precipitation of borate with hydroxyapatite (HAp: Ca 5 (PO 4 ) 3 OH) facilitates the simultaneous removal of borate with co-existing oxoanions in natural waters. The cost of phosphate is an impediment to industrialize the co-precipitation of borate with HAp for treatment of geothermal waters. In the present work, an inexpensive industrial by-product of magnesium ammonium phosphate (MAP) derived from sewage sludge, was examined as a phosphate source. MAP includes 89% pure magnesium ammonium phosphate, resulting in better performance than the pure chemical form of NH 4 H 2 PO 4 , because Mg 2+ and Al 3+ (trace elements in MAP product) play roles in enhancing the removal rate of borate and lowering the equilibrium borate concentration. These ions have a good affinity with phosphate to nucleate crystal seeds independently of powdery Ca sources. To reduce the bulky volume of solid residues, hot isostatic pressing (HIP) was applied. There is structural water in HAp; therefore, the greatest volume reduction was achieved with 78.3 ± 2.0% (n = 3). Additionally, a synergic effect to suppress the released borate, greater than the sequential combination of calcination and cold isostatic pressing was accomplished in the toxicity contents leaching procedure (TCLP) test. This is not due to larger crystal sizes alone, but it is derived from boron stabilization in HAp at an atomic level by the synergic effect of heating and pressing simultaneously., (Copyright © 2020 Elsevier Ltd. All rights reserved.)

The treatment of the geothermal water discharged through mining activity is a critical issue because the rate of discharge is 12,000 m 3 per day and the discharge contains high concentrations of borate (>20 mg/L) and arsenate (ca. 0.4 mg/L) as well as silicate and carbonate. The simultaneous reduction of borate and arsenate concentrations to acceptable levels was successfully performed by co-precipitation with hydroxyapatite (HAp). Although the coexisting high concentrations of carbonate act as a disturbing element, the co-precipitation equilibrium of borate was shifted to lower values by adjusting the P/Ca molar ratio, and the removal rate of borate was accelerated by using Al 3+ additives, resulting in the efficient reduction of borate within 1 h. The initially immobilized boron in HAp is in the tetragonal form, which probably occupies the hydroxyl sites in HAp, gradually transforming into the trigonal form in the solid state, as interpreted by 1 H NMR and 11 B-NMR. The coexisting silicate was also immobilized in an ellestadite form, as confirmed by 29 Si-NMR measurements. Arsenate and silicate were immobilized before borate in geothermal water. A dissolution assay of borate in the solid residues after co-precipitation with HAp verified the acceptable stability of borate, which is independent of the amount of added Al 3+ ., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

Discharge from accidental nuclear power plants includes boric acid, which is used as a neutron absorbent in nuclear reactors. Co-precipitation of borate with hydroxyapatite (HAp), using Ca(OH) 2 , is known to be an effectively fast method for stabilization of borate as well as coexisting radioactive nuclides. To reduce bulky volume of solid residues after co-precipitation, calcination is necessary to investigate the chemical stability of targets. Calcination at 850 °C resulted in the high crystalization of HAp with formation of x CaO·B 2 O 3 as a by-phase in which x increased with a decrease in the borate contents. After calcination, the lattice parameter a of HAp showed a reentrant curve and c showed a convex curve with an increase in borate contents. A dissolution assay revealed that calcination sometimes increases the borate moiety and that the acceptable B contents in HAp are lower than 1.59 mmol/g-calcined HAp. These results imply that during calcination of HAp, some borate is excluded to form the by-phase x CaO·B 2 O 3 , which is relatively insoluble in water, but some other fractions might be additionally emitted from the amorphous phase to weakly bind the calcined products. [ABSTRACT FROM AUTHOR]