In viewpoint of development of photocatalyst for water purification based on advanced oxidation process, hydroxyapatite (HAP-400) and fluoraptite (FAP) were treated with Bi(NO₃)₃ in DMSO/H₂O mixed solvent (DMSO = dimethylsufoxide) to give the corresponding Bi-treated apatites, HAP-400-Bi and FAP-Bi, respectively. The treated apatites were investigated as catalysts in photodegradation of dyes, alizarin red S (ARS) and methyl orange (MO), in water under H₂O₂-absent/present condition. Photodegradation rates of ARS under H₂O₂-absent condition reached 73% for HAP-400-Bi and 83% for FAP-Bi, accompanied with the corresponding ARS adsorption rates at 62% for HAP-400-Bi and 69% for FAP-Bi, respectively. On the other hand, MO was photodegraded at less than 4% under the same condition, possibly attributed to little adsorbabilities of MO on the prepared Bi catalysts. Eventually, photodegradation of ARS by using HAP-400-Bi or FAP-Bi under H₂O₂-present condition, namely photo-Fenton like condition, resulted in the complete ARS consumption rate of 100% for both of the catalysts, and considerably high mineralization rates of 40% for HAP-400-Bi and 62% for FAP-Bi. Generation of hydroxyl radical •OH as an activated oxygen species was confirmed by the following two methods: One employs 2-propanol as a •OH scavenger; The other employs N, N-dimethylamino-4-nitrosoaniline (DMNA) as an indirect •OH detector. It was found that •OH radical species is predominantly formed via the prepared Bi catalysts under photo-Fenton like condition to offer an oxidative reaction field, and that self-photodegradation of dye, especially ARS, partly proceeds by electron transferring interaction between dye and Bi catalyst.

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Starting powders of silicon-substituted beta-tricalcium phosphate (β-TCP/Si) were prepared by the polymerized complex method. Silicon ions were blended up to 3.0 mol% of the total P ions content. The raw product powder was calcined at 1000 °C, formed into pellets, and sintered at 1120 °C for 24 h. The resultant sintered bodies showed that a single phase of β-type tricalcium phosphate (β-TCP) can be obtained with silicon ions addition of 0-2.0 mol% of the total P ions content. The substitutional solid solution of silicon ions in β-TCP affected the sinterability of the obtained sintered bodies by inhibiting the grain growth and densification of the sintered bodies. On the other hand, the samples prepared with silicon ions additions of more than 2.5 mol% of the total P ions content were mixed phases with β-TCP, α-TCP and silicocarnotite (Ca₅(PO₄)2SiO₄).

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In the present study, we developed a technology for concentrating and recovering phosphorus from slag-like phosphorus-containing unused resources and applied it to slag discharged during the latest steelmaking process, that is, double-slag refining process (DRP). The technology we developed consists of the following four processes: Process (1) is the initial acid elution; Process (2) involves alkali precipitation; Process (3) is the second acid elution; and, Process (4) utilizes ion-exchange. In Process (1), the addition of DPR slag to 0.5 M of a nitric acid solution for 24 min resulted in sufficient phosphorus dissolution. In Process (2), ammonia was added to the dissolved solution, and phosphorus was precipitated with high efficiency. The timing of the addition of ammonia significantly influenced the removal of silicon and iron, which would have been inconvenient to accomplish in subsequent processes. In Process (3), the precipitation obtained in Process (2) was re-dissolved in a nitric acid solution. The dissolution of phosphorus together with other elements progressed sufficiently, and we confirmed that silicon could be completely separated as silica by using high-concentration nitric acid at this stage. The fact that silicon could be removed during Process (3) was an important finding, since silicon could not have been separated in the Process (4). In Process (4), by passing the phosphorus-containing solution obtained in Process (3) through an ion exchange resin, elements other than phosphorus and silicon could be removed, which confirms that the range of applications for this technology could be expanded.

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