Abstract
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(43) 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, Na2CO3 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 (SBET) 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.
Highlights
Acid mine drainage (AMD) is a major pollution source in mining areas [1,2]
To optimize the bentonite–steel slag proportion, the Na2CO3 content was set to 5% of the total mass of the composite material, the aging time was set to 24 h, the calcination particle size was set to 1–2 mm, the calcination temperature was set to 500 ◦C, and the calcination time was set to 60 min
The experimental results showed that steel slag plays an active role in the removal of heavy metal ions by releasing alkalinity, increasing ion removal, and reducing the loss rate
Summary
Acid mine drainage (AMD) is a major pollution source in mining areas [1,2]. AMD has a pH < 3, and contains high concentrations of toxic elements, such as heavy metal ions, which cannot be reused [3,4]. If AMD is discharged into the environment without treatment, it can directly harm humans, animals, and plants [5,6], especially when it contains heavy metals like Pb(II), Cd(II), and Cr(III) [7,8]. Current techniques for treating AMD containing heavy metals include the neutralization precipitation [11], electrochemical [12], membrane separation [13,14,15], microbial [16,17], constructed wetland [18], and adsorption [19,20] methods. 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]. If two methods are connected in series, the process flow is long, and the processing cost is high [30]
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