Abandoned lead and zinc (Pb-Zn) mines around the world produce large amounts of acid mine drainage (AMD) containing Pb(II), which is toxic and accumulates in the environment and in living organisms. Bentonite-steel slag composite particles (BSC) are a new type of acid mine drainage (AMD) treatment material that can remove heavy metal ions and reduce acidity. To date, there have been no reports on the treatment of Pb(II)-containing AMD using BSC. Therefore, the effects of pH, reaction time, temperature, and Pb(II) concentration on the adsorption of Pb(II) onto BSC were studied. Moreover, the BSC before and after the reaction, as well as the precipitation after the reaction, were characterized by scanning electron microscopy and X-ray diffraction analyses. The effect of pH on the adsorption process is similar to that of the formation of soluble and insoluble hydrolysates of Pb(II) on pH. The adsorption mechanism includes ion exchange, complexation, precipitation, and synergistic adsorption–coagulation effect. Adsorption kinetics are best-fit with the pseudo-second order kinetics model ( R 2 > 0.98). Furthermore, the total adsorption rate is controlled by liquid film diffusion and in-particle diffusion, the liquid film diffusion rate being higher than the in-particle diffusion rate. The isothermal adsorption of Pb(II) onto BSC fit well with Langmuir and Brunauer Emmett Teller (BET) isotherms ( R 2 > 0.995), and both single layer adsorption and local multilayer adsorption were observed. Thermodynamic analysis revealed that the adsorption process is spontaneous and endothermic, and that the degree of freedom increases with time. In summary, this study provides a theoretical basis for the use of BSC in treating AMD containing Pb(II).

Novel multifunctional adsorbent bentonite-steel slag composite particles (BSC) were developed for highly efficient and synergistic treatment of heavy metal ions in acid mine drainage (AMD). Single-factor experiments were performed to examine the influence of different parameters on the adsorption effect, alkalinity release quantity, and loss rate of the composite particles. Based on these results, an L9(4 3 ) orthogonal experiment was carried out, and the optimum levels and order of the factors were determined by range analysis. Finally, the optimum preparation process of the composite particles was determined: a bentonite-steel slag proportion of 5:5, Na 2 CO 3 content of 5%, aging time of 12 h, calcination particle size of 2 mm, calcination temperature of 500 • C, and calcination time of 60 min. The isothermal adsorption of optimum BSC fit well with Langmuir and Brunauer-Emmett-Teller (BET) isotherms (R 2 R 2 > 0.997). A synergistic adsorption-coagulation effect occurs, leading to the appearance of multiple layers locally on the surface of BSC, which satisfies the BET model. To understand the preparation mechanism of the BSC, bentonite, steel slag, uncalcined BSC, and the optimum BSC were characterized using scanning electron microscopy (SEM), Brunauer-Emmett-Teller (BET) surface area analysis, X-ray powder diffraction (XRD), and Fourier-transform infrared spectroscopy (FTIR). The results indicate that calcination led to an increase in the average pore radius, total pore volume, and specific surface area (S BET ) in the optimum BSC; numerous pores were present on its layered surface. Although the layer spacing increased after calcination, the structure of the dioctahedra remained unchanged. Exchangeable Na + , montmorillonite, and alkaline components were present between the optimum BSC layers. Water and impurities were removed after calcination. The BSC not only released an alkalinity-neutralising acid but also induced a synergistic adsorption-coagulation effect that removed heavy metal ions. It is an excellent multifunctional protective material for the mining environment, that can treat AMD-containing heavy metal ions. Sustainability 2020, 12, 18 2 of 27 Current techniques for treating AMD containing heavy metals include the neutralization precipitation [11], electrochemical [12], membrane separation [13-15], microbial [16,17], constructed wetland [18], and adsorption [19,20] methods. Neutralization precipitation and adsorption are widely used [21,22]. However, neutralization precipitation requires a large amount of chemical agents, such as neutralizer and flocculent [23,24], which leads to difficulties in solid-liquid separation and high processing costs [25,26]. Besides various adsorbent materials, activated carbon is frequently used for adsorption, but has a low adsorption capacity for heavy metal ions, has high costs, and cannot neutralize acid [27][28][29]. At present, most treatments use a single method, and the treatment effect is poor. If two methods are connected in series, the process flow is long,...