Investigation on Effective Neutralization Process of Acid Mine Drainage Containing High Amount of Mn and Zn by Additions of δ-MnO2 Adsorbent and Oxidizing Agent

This study examined effective removal methods for high amounts of manganese (Mn) and zinc (Zn) in acid mine drainage (AMD) by addition of different neutralizing agents (NaOH and NaClO) and synthesized birnessite (δ-MnO2) using two-type AMD samples which Mn and Zn concentrations were 778 and 410 mg L−1 for A mine and 18.0 and 5.51 mg L−1 for B mines, respectively. The precipitation mechanism of these metal ions was investigated by geochemical modeling (PHREEQC) and X-ray absorption near edge structure (XANES) analysis. Mn concentrations were below the effluent standard (10 mg L−1) at pH 9–10 with the NaOH neutralization, whereas it was accomplished at lower pH (6–7) condition with the NaClO addition; it could act as an oxidizing agent, resulting that most of Mn precipitated as δ-MnO2. Zn concentrations decreased below the effluent standard (2 mg L−1) at pH 8–9 using both neutralizing agents. XANES analysis results indicated Zn was removed by the surface complexation formation on manganite and δ-MnO2 surface. More effective removal of Mn and Zn from AMD was found around pH 6 when a sufficient amount of δ-MnO2 was added to both AMD before the NaOH neutralization; a geochemical model coupling charge distribution multisite ion complexation revealed the triple-corner-sharing on δ-MnO2 was the most reasonable mechanism. Our result suggests that the presence of sufficient δ-MnO2 was the most effective for high Mn and Zn contents AMD treatment; however, ferrous ion (Fe2+) can inhibit the adsorption reaction and decompose δ-MnO2. Thus, pre-precipitation of Fe2+ is required to enhance the effect of δ-MnO2 on Mn and Zn removals from AMD

Investigation of Cerium Reduction Efficiency by Grinding with Microwave Irradiation in Mechanochemical Processing

This study evaluated the efficiency of cerium reduction by grinding with microwave irradiation in mechanochemical processing. Grinding experiments with microwave irradiation were conducted using an agitating mixer. Since the structure of the ground samples was amorphous and the cerium concentration was much lower than those of other elements, the valence change and structural change of cerium after grinding with microwave irradiation were investigated using X-ray absorption fine structure (XAFS) analysis in the cerium K-edge. The X-ray absorption near-edge structure (XANES) analysis revealed that a portion of tetravalent cerium was reduced to trivalent cerium by grinding with microwave irradiation. In addition, it was confirmed by extended X-ray absorption fine structure (EXAFS) analysis that oxygen vacancies were produced as a result of the cerium reduction reaction. To evaluate the efficiency of cerium reduction efficiency, the percentage reduction by grinding with microwave irradiation was compared to that by planetary ball milling and microwave irradiation. As a result, it was revealed that the efficiency of cerium reduction via grinding with microwave irradiation was higher than that via microwave irradiation and the same as that via planetary ball milling. Moreover, a larger amount of tetravalent cerium could be reduced to trivalent cerium by grinding with microwave irradiation than when using planetary ball milling and microwave irradiation

Prediction of Acid Mine Drainage Quality for the Next Decades by a Tank Model and Multiple Regression Analysis

Acid mine drainage (AMD) generation is a serious problem from the environmental and economic perspectives, because it contains a large amount of heavy metals and because there are high costs associated with maintaining the facilities, purchasing neutralizing agents, and disposing sludges; which are required for the treatment process. In this study, changes in AMD quantity and quality for the next decades were predicted by a three series tank model in three stages, by combining the first order kinetic calculation of sulfide minerals' dissolution for two metal mines (X and Y) in Japan. Results from the AMD quality model represented the decrease of heavy metal concentrations below the effluent standard values in 30–140 years, by considering dilution and/or additional dissolution by heavy rain and snow melting, although these predicted values diverged by our previous model. However, the low correlation coefficient values (0.23–0.63) observed between the measured values of heavy metal concentration and the values calculated by our new model, mean that other chemical reactions, such as sulfate and/or carbonate mineral dissolution could greatly affect the AMD quality. In fact, there was no correlation between the metal potential calculated by our model and the real distribution of sulfide minerals at X mine. Our results therefore indicate that specific geochemical reaction and geological information should be included in the AMD quality prediction model, to estimate more accurately the fluctuation of each heavy metal concentration during different seasons

Kinetic Modeling and Mechanisms of Manganese Removal from Alkaline Mine Water Using a Pilot Scale Column Reactor

Manganese (Mn) is a major element in various aqueous and soil environments that is sometimes highly concentrated in mine water and other mineral processing wastewater. In this study, we investigated Mn removal from alkaline mine water (pH > 9) with an Mn-coated silica sand packed into a pilot-scale column reactor and examined the specific reaction mechanism using X-ray absorption near-edge structure (XANES) analysis and geochemical kinetic modeling. The kinetic effect of dissolved Mn(II) removal by birnessite (δ-Mn(IV)O2) at pH 6 and 8 was evaluated at different Mn(II)/Mn(IV) molar ratios of 0.1–10. Our results confirmed the positive effect of the presence of δ-MnO2 on the short-term removal (60 min) of dissolved Mn. XANES analysis results revealed that δ-MnO2 was more abundant than Mn(III)OOH in the reactor, which may have accumulated during a long-term reaction (4 months) after the reactor was turned on. A gradual decrease in dissolved Mn(II) concentration with depth was observed in the reactor, and comparison with the kinetic modeling result confirmed that δ-MnO2 interaction was the dominant Mn removal mechanism. Our results show that δ-MnO2 contents could play a significant role in controlling Mn removability from mine water in the reactor

Role of Resource Circularity in Carbon Neutrality

With the help of circular strategies, products can be used longer (i.e., reuse, repair, and refurbish). Products that are difficult to use will be recycled efficiently. The present paper provides actionable guidelines for reducing environmental impact at all stages of a product’s life cycle, including the manufacture and assembly of the materials that make up the product, environmental impacts during use, and environmental impacts at final disposal, as well as specific actions and evaluation mechanisms. The circular economy is a concept that encompasses specific actions and their evaluations. To clarify the contribution of this circular economy to carbon neutrality, the present paper highlights how it is important to recognize the role of carbon as both an energy carrier and a material. CO2 is a waste product from burning and powering carbon. CO2 must be disposed of like any other waste product, but carbon itself is also an energy carrier. Thus, when promoting the carbon cycle, it is important to harmonize carbon’s function as a material with its role as an energy carrier. The further introduction of renewable energy and societal shift towards circular economy would contribute to carbon neutrality and more resource efficient use in a mutually complementary manner