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In high-fluorine geological background areas, the supergene geochemical process of fluorine has an impact on regional environmental health and safety, which has long been the focus of attention. However, in karst areas characterized by a high incidence of endemic fluorosis, the migration and transformation of fluorine in surface water have not received sufficient attention. This study investigated the joint influence of the weathering and erosion of carbonate rocks and coal-bearing strata on a typical small watershed. Accordingly, 239 samples representing 13 periods of hydrochemical samples were systematically collected to clarify the source, migration, and transformation characteristics of fluorine in surface water. The results revealed that the pH of the Huatan River was low in the rainy season and high in the dry season. The annual variation range of fluorine concentration was 0.11–0.40 mg/L. Although mining development produced acid mine drainage with high fluorine concentration, its impact at the watershed scale appeared to be limited. In terms of spatial scale, the concentration of fluorine in the Huatan River increased gradually from upstream to downstream. The dissolution of fluorite and other fluorine-bearing minerals had not reached the saturation state. The fluorine in the watershed primarily came from the dissolution of fluorine-bearing minerals, followed by the contribution of atmospheric precipitation. The release of fluorine adsorbed on the surface of clay minerals was not the main source of this element. The leaching of the watershed was shown to represent a critical transport process concerning fluorine in the Huatan River, and evaporation had a notable impact on the enrichment of fluorine in water.

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+.

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.