17:15 〜 19:15
[HRE12-P10] 酸性坑廃水処理施設におけるマンガン沈殿物の鉱物学的および地球化学的特徴
キーワード：酸性坑廃水、マンガン、マンガン酸化物
Acid mine drainage (AMD) generated at active and abandoned mines has been conventionally treated through neutralization and solid-liquid separation (thickener). Recently, some facilities have implemented sand filtration tanks to remove suspended solids after the thickener. Dissolved divalent manganese ions (Mn2+) are one of the difficult-to-treat ions due to their slow oxidation kinetics. As a result, after neutralization, manganese remains in the treated water at a concentration of about one-tenth of Japan effluent standard of 10 ppm. Subsequently, it precipitates as manganese oxides with sand or manganese precipitated sand (MPS) within sand filtration tanks. The accumulation of manganese oxide may result in operational challenges such as filter clogging and reduced filtration efficiency. Conversely, MPS completely removed the Mn2+ that entered the sand filtration tank. This study aimed to elucidate the formation processes and characteristics of manganese oxides formed in sand filters at an AMD treatment facility in an abandoned mine. The initial sand media and manganese precipitated sand (MPS) aged over varying periods (0.75, 4.75, and 7.15 years) were examined through mineralogical and geochemical analyses.
Both particle size and weight increase through the growth of MPS with aging. X-ray Fluorescence (XRF) analysis demonstrated that the SiO2 content of MPS decreased from 85% (initial sand) to 5% over 7.15 years of aging, whereas the Mn2O3 content increased from 0% to 55%. Considering the weight change, approximately 45% of SiO2 was lost, while other components (e.g., Mn, Al, Fe, and Ca) were added from the initial sand with an increased growth in MPS. Furthermore, Scanning Electron Microscope (SEM) analysis revealed that sand-like structures were present in the core of MPS after 0.75 years but became indistinct after 4.75 and 7.15 years. The SEM image also exhibits distinct inner and outer structures. Chemical mapping by energy-dispersive spectroscopy (EDS) indicated that Si and Fe, initially concentrated in the core of the young MPS, were dispersed more uniformly with aging. However, Al was strongly concentrated on the outer shell and may play a significant role in the formation of the outer shell. Transmission electron microscopy (TEM) analysis revealed that the outer structure was primarily composed of layered minerals, which were identified as birnessite by electron diffraction. BET analysis demonstrated that the surface area increased with age, which was attributed to the gradual increase in the outer layer structure during MPS formation. It can be inferred that the layered structure of the MPS may facilitate the incorporation of dissolved elements from external sources.
Efficiency of MPS to remove Mn2+ and Zn2+ ions was evaluated through neutralization experiments of a solution containing 10 ppm of these ions with and without MPS. The results demonstrated that in the acidic region (pH 3), MPS dissolved into the solution, while over a wide pH range (4–8), removal efficiencies of Zn2+ and Mn2+ ions by MPS exceeded 50% after one hour of agitation. This suggests that the MPS may be used to remove these metal ions near the neutral conditions.
Both particle size and weight increase through the growth of MPS with aging. X-ray Fluorescence (XRF) analysis demonstrated that the SiO2 content of MPS decreased from 85% (initial sand) to 5% over 7.15 years of aging, whereas the Mn2O3 content increased from 0% to 55%. Considering the weight change, approximately 45% of SiO2 was lost, while other components (e.g., Mn, Al, Fe, and Ca) were added from the initial sand with an increased growth in MPS. Furthermore, Scanning Electron Microscope (SEM) analysis revealed that sand-like structures were present in the core of MPS after 0.75 years but became indistinct after 4.75 and 7.15 years. The SEM image also exhibits distinct inner and outer structures. Chemical mapping by energy-dispersive spectroscopy (EDS) indicated that Si and Fe, initially concentrated in the core of the young MPS, were dispersed more uniformly with aging. However, Al was strongly concentrated on the outer shell and may play a significant role in the formation of the outer shell. Transmission electron microscopy (TEM) analysis revealed that the outer structure was primarily composed of layered minerals, which were identified as birnessite by electron diffraction. BET analysis demonstrated that the surface area increased with age, which was attributed to the gradual increase in the outer layer structure during MPS formation. It can be inferred that the layered structure of the MPS may facilitate the incorporation of dissolved elements from external sources.
Efficiency of MPS to remove Mn2+ and Zn2+ ions was evaluated through neutralization experiments of a solution containing 10 ppm of these ions with and without MPS. The results demonstrated that in the acidic region (pH 3), MPS dissolved into the solution, while over a wide pH range (4–8), removal efficiencies of Zn2+ and Mn2+ ions by MPS exceeded 50% after one hour of agitation. This suggests that the MPS may be used to remove these metal ions near the neutral conditions.