| 1. |
Keishi Oyama, Kazuhiko Shimada, Jun-ichiro Ishibashi, Hajime Miki, Naoko Okib, Silver-catalyzed bioleaching of enargite concentrate using moderately thermophilic microorganisms, HYDROMETALLURGY, 10.1016/j.hydromet.2018.03.014, 177, 197-204, 2018.05, Effect of silver (Ag) catalyst in bioleaching of enargite (Cu3AsS4) concentrate was studied using mixed cultures of moderately thermophilic acidophilic microorganisms at 45 degrees C. Addition of Ag2S enabled selective Cu dissolution from enargite while suppressing pyrite oxidation: At the highest Ag2S concentration of 0.04%, Cu recovery reached 96% while Fe dissolution was suppressed to reach only 29% by day 72. Overall results from thermodynamic calculation, liquid/solid analyses and kinetic study suggested that Ag-catalyzed bioleaching of enargite concentrate proceeds via formation of at least two types of secondary products (chalcocite, Cu2S; trisilver arsenic sulfide, Ag3AsS4): Addition of Ag2S as Ag catalyst thermodynamically and microbiologically contributed to lowering solution redox potentials during bioleaching, consequently satisfying E-ox (Cu2S) |
| 2. |
Keishi Oyama, Kazuhiko Shimada, Jun-ichiro Ishibashi, Keiko Sasaki, Hajime Miki, Naoko Okibe, Catalytic mechanism of activated carbon-assisted bioleaching of enargite concentrate, HYDROMETALLURGY, 10.1016/j.hydromet.2020.105417, 196, 2020.09, The catalytic mechanism of activated carbon-assisted bioleaching of enargite concentrate (enargite 37.4%; pyrite 47.3%) was investigated by employing microbiological, electrochemical and kinetic studies. By using moderately thermophilic microorganisms at 45 degrees C, the final Cu dissolution was improved from 36% to 53% at 0.2% (w/v) activated carbon. An excess activated carbon addition showed an adverse effect. The enargite mineral itself favored higher solution redox potential (E-h) for solubilization. However, the dissolution of co-existing pyrite, which also favors high E-h, immediately hindered enargite dissolution through the passivation effect. The surface of activated carbon functioned as an electron mediator to couple RISCs oxidation and Fe3+ reduction, so that elevation of the E-h level was controlled by offsetting microbial Fe3+ regeneration. As long as the E-h level was suppressed at |
| 3. |
Keishi Oyama, Kyohei Takamatsu, Kaito Hayashi, Yuji Aoki, Shigeto Kuroiwa, Tsuyoshi Hirajima, Naoko Okibe, Carbon-Assisted Bioleaching of Chalcopyrite and Three Chalcopyrite/Enargite-Bearing Complex Concentrates, MINERALS, 10.3390/min11040432, 11, 4, 2021.04, Overcoming the slow-leaching kinetics of refractory primary copper sulfides is crucial to secure future copper sources. Here, the effect of carbon was investigated as a catalyst for a bioleaching reaction. First, the mechanism of carbon-assisted bioleaching was elucidated using the model chalcopyrite mineral, under specified low-redox potentials, by considering the concept of E-normal. The carbon catalyst effectively controlled the E-h level in bioleaching liquors, which would otherwise exceed its optimal range (0 |
| 4. |
Naoko Okibe, Kaito Hayashi, Keishi Oyama, Kazuhiko Shimada, Yuji Aoki, Takahiro Suwa, Tsuyoshi Hirajima, Bioleaching of Enargite/Pyrite-rich "Dirty" Concentrate and Arsenic Immobilization, MINERALS, 10.3390/min12040449, 12, 4, 2022.04, Bioleaching of arsenic (As)-rich, so-called "dirty" concentrates can produce additional Cu value from the flotation waste while simultaneously releasing toxic As. This study bioleached three such concentrates of varying pyrite/enargite ratios ([Py]/[Ena] = 0.7, 1.3 and 2.4) at a pulp density of 20%. The dissolution behavior of Cu and As in relation to the solution redox potential (Eh) was studied with and without activated carbon (AC) as a potential Eh-controlling catalyst. At this high pulp density, Eh was naturally suppressed, without a need for AC dosing, to |
| 5. |
A sulfidation process for recovering Ni from an acidic spent catalyst, and the chemical characterization of the sulfide precipitates. |
| 6. |
Investigation of liberation mechanism of electronic scrap by impact energy measurements and DEM simulations. |
| 7. |
Juan Qin, Yeting Fang, Jian Shi, Chiharu Tokoro, Mauricio Córdova-Udaeta, Keishi Oyama, Juncheng Zhang, Waste-Based Ceramsite for the Efficient Removal of Ciprofloxacin in Aqueous Solutions, International Journal of Environmental Research and Public Health, 10.3390/ijerph20065042, 20, 6, 5042-5042, 2023.03, Ciprofloxacin (CIP), a compound with bioaccumulation toxicity and antibiotic resistance, is frequently detected in water at alarming concentrations, which is becoming an increasing concern. In this study, a low-cost ceramsite was developed from industrial solid wastes through sintering to remove CIP from wastewater. The effects of adsorbent dosage, initial pH, contact time, initial CIP concentration, and temperature were explored. More than 99% of CIP (20–60 mg/L) was removed at around pH 2–4 by the ceramsite. The kinetic data fitted well with the pseudo-second-order model, revealing that chemisorption was the main rate-determining step. The isotherm data was better described by the Freundlich model, suggesting that CIP was removed by the formation of multiple layers on the heterogeneous surface. Moreover, the removal efficiency was practically higher than 95% during five regeneration cycles, when different regeneration methods were used, including calcination, HCl, and NaOH washing, indicating that the ceramsite exhibited outstanding reusability in removing CIP. The primary mechanism of CIP removal by the ceramsite was found to be the synergism of adsorption and flocculation, both of which depended on the release of Ca2+ from the ceramsite. In addition, strong Ca-CIP complexes could be formed through surface complexation and metal cation bridging between Ca2+ and different functional groups in CIP.. |
| 8. |
Keishi Oyama, Kentaro Hayashi, Yusei Masaki, Takaya Hamai, Shigeshi Fuchida, Yutaro Takaya, Chiharu Tokoro, Geochemical Modeling of Heavy Metal Removal from Acid Mine Drainage in an Ethanol-Supplemented Sulfate-Reducing Column Test, Materials, 10.3390/ma16030928, 16, 3, 928-928, 2023.01, A passive treatment process using sulfate-reducing bacteria (SRB) is known to be effective in removing heavy metals from acid mine drainage (AMD), though there has been little discussion of the mechanism involved to date. In this work, a sulfate-reducing column test was carried out using supplementary ethanol as an electron donor for microorganisms, and the reaction mechanism was examined using geochemical modeling and X-ray absorption fine structure (XAFS) analysis. The results showed that Cu was readily removed from the AMD on the top surface of the column (0–0.2 m), while Zn and Cd depletion was initiated in the middle of the column (0.2–0.4 m), where sulfide formation by SRB became noticeable. Calculations by a developed geochemical model suggested that ethanol decomposition by aerobic microbes contributed to the reduction of Cu, while sulfide produced by SRB was the major cause of Zn and Cd removal. XAFS analysis of column residue detected ZnS, ZnSO4 (ZnS oxidized by atmospheric exposure during the drying process), and CuCO3, thus confirming the validity of the developed geochemical model. Based on these results, the application of the constructed geochemical model to AMD treatment with SRB could be a useful approach in predicting the behavior of heavy metal removal.. |
