Effects of Initial pH and Carbonate Rock Dosage on Bio-Oxidation and Secondary Iron Mineral Synthesis.

The effect of pH is a key factor in biomineralization mediated by Acidithiobacillus ferrooxidans to promote the transformation of Fe into secondary iron minerals. This study aimed to investigate the effects of initial pH and carbonate rock dosage on bio-oxidation and secondary iron mineral synthesis. Variations in pH and the concentrations of Ca2+, Fe2+, and total Fe (TFe) in the growth medium of A. ferrooxidans were examined in the laboratory to determine how they affect the bio-oxidation process and secondary iron mineral synthesis. The results showed that in systems with an initial pH of 1.8, 2.3, and 2.8, the optimum dosages of carbonate rock were 30, 10, and 10 g, respectively, which significantly improved the removal rate of TFe and the amount of sediments. At an initial pH of 1.8 and a carbonate rock dosage of 30 g, the final removal rate of TFe reached 67.37%, which was 28.03% higher than that of the system without the addition of carbonate rock, and 36.9 g·L-1 of sediments were generated, which was higher than that of the system without the addition of carbonate rock (6.6 g·L-1). Meanwhile, the number of sediments generated by adding carbonate rock were significantly higher than those without the addition of carbonate rock. The secondary minerals were characterized by a progressive transition from low crystalline assemblages composed of calcium sulfate and subordinated jarosite, to well crystal-line assemblages composed of jarosite, calcium sulfate, and goethite. These results have important implications for comprehensively understanding the dosage of carbonate rock in mineral formation under different pH conditions. The findings help reveal the growth of secondary minerals during the treatment of AMD using carbonate rocks under low-pH conditions, which offers valuable information for combining the carbonate rocks with secondary minerals to treat AMD.

Adsorption Characteristics of Heavy Metals Pb2+ and Zn2+ by Magnetic Biochar Obtained from Modified AMD Sludge.

Acid mine drainage (AMD) sludge can be used to prepare adsorbent materials for the removal of heavy metals in water, which is an effective means for its resource utilization. Magnetic modified biochar (MMB), which can be recovered by magnetic separation, was prepared from sludge generated from the carbonate rock neutralization treatment of AMD and rice straw agricultural waste. Unmodified biochar (UMB) was obtained from rice straw and chemically modified and treated by ultraviolet radiation to produce MMB. The Pb2+ and Zn2+ adsorption capacities of UMB and MMB were investigated. Simultaneously, the materials were characterized by SEM, FTIR, BET, and ZETA. The results showed that the specific surface area (130.89 m2·g-1) and pore volume (0.22 m2·g-1) of MMB were significantly increased compared to those of UMB (9.10 m2·g-1 and 0.05 m2·g-1, respectively). FTIR images showed that MMB was successfully loaded with Fe3O4. The adsorption process of Pb2+ and Zn2+ onto MMB was consistent with the Langmuir adsorption isotherm and second-order kinetic models, with maximum adsorption capacities of 329.65 mg·g-1 and 103.67 mg·g-1, respectively. In a binary system of Pb2+ and Zn2+, MMB preferentially binds Pb2+. The adsorption efficiencies of MMB reached >80% for Pb2+ and Zn2+.

Study on the precipitation of iron and the synchronous removal mechanisms of antimony and arsenic in the AMD under the induction of carbonate rocks.

The ecological environment can be severely polluted and destroyed by the acid mine drainage (AMD) generated during the exploration and utilization of minerals. However, neutralized by carbonate rocks (a natural material in Karst regions), the AMD secondary iron flocs containing a large number of iron oxides or hydroxide can be precipitated in AMD. The metal ions, such as antimony (Sb) and arsenic (As), can be effectively removed by these neutralizing products. In this paper, the neutralization reaction of different acid solutions in an iron-antimony-arsenic system was induced by carbonate rocks to explore the removal effect of metals during this neutralization process. Meanwhile, taking the release amounts of iron (Fe), Sb, and As as well as the phase transformation of minerals at different pH levels as stability indexes, we quantitatively analyzed the chemical stability of AMD neutralizing products (secondary iron flocs) containing Sb and As under the typical acid-base environment (pH = 3.0 ~ 9.0) of AMD and other waters. Results showed that the neutralization reaction with carbonate rocks induced the co-precipitation of Fe with Sb and As. When the concentration ratio of Fe, Sb, and As was 30:1:1, the pH of AMD raised from 3.0 to 7.28 within 72 h, and the three elements were removed by 99%, 85%, and 90%, respectively. After soaking the AMD secondary iron flocs in an acid environment (pH = 3.0) for 30 days, the release amount of Fe reached its peak of 0.070 mg/g. Then, when the pH value increased to 4.0, the As and Sb showed their maximum release amounts of 14.90 µg/g and 19.19 µg/g, respectively. In addition, under acidic conditions, these AMD secondary iron flocs were easily transformed into the goethite with better crystallinity and higher structural stability. This study could help reveal the development of the secondary mineral during the treatment of AMD by carbonate rocks and understand the release characteristics of metals from AMD secondary products containing Sb and As, which sheds light on and provides theoretical foundations for the passive treatment of AMD containing these two elements in the future.