Removal of Pb(II) from Acid Mine Drainage with Bentonite-Steel Slag Composite Particles

: 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 e ﬀ ects 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 di ﬀ raction analyses. The e ﬀ ect 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 e ﬀ ect. 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 di ﬀ usion and in-particle di ﬀ usion, the liquid ﬁlm di ﬀ usion rate being higher than the in-particle di ﬀ usion 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).


Introduction
Acid mine drainage (AMD) is an inevitable by-product of mining production in the case of the occurrence of sulfide minerals [1,2]. AMD is highly acidic and contains high concentrations of toxic substances, including heavy metal ions [3,4]. Consequently, the discharge of untreated AMD into natural water receivers can cause significant damage to aquatic flora and fauna [5,6]. In addition, groundwater pollution due to AMD is a serious problem too. To mitigate the negative effects of AMD on ecosystems and human health, several treatment approaches have been developed, including neutralization precipitation [7], electrochemical [8], membrane separation [9,10], microbial treatment [11,12], constructed wetland [13], and adsorption [14] methods. Among these, the neutralization precipitation and adsorption methods [15,16] are widely used.
Traditionally, AMD is neutralized using lime. However, when lime neutralizes wastewater, the generated metal hydroxide has a low specific gravity and can be easily broken into small particles when strongly stirred or transported. Therefore, its sedimentation rate is very low and solid-liquid

Preparation of BSC
Bentonite and steel slag powder were mixed in a ratio of 1:1, and 5% of total mass Na 2 CO 3 was added. After the mixture was evenly mixed, an appropriate amount of DW was added, and the mixture was stirred; then, composite particles with a particle size of 2 mm were prepared using an extrusion granulator. The particles were aged for 12 h in a dark, ventilated location; then, they were heated in a crucible in a preheated muffle furnace at temperatures of 523 K to 773 K for 60 min. After roasting, the particles were allowed to cool naturally to obtain the adsorbent. The chemical compositions and physical properties of BSC are listed in Table 3.

Effect of pH on Pb(II) Adsorption
A batch adsorption test was performed in a thermostated shaker (298.0 ± 0.5 K, 100 rpm). The BSC dosage was fixed as 10 g L −1 , while the initial solute concentration of Pb(II) was 100 mg L −1 . pH 0 was changed from 1 to 6.5 with increments of 0.5; in particular, pH 0 was adjusted by adding 0.1 M HNO 3 or 0.1 M NaOH solution. After allowing the reaction to continue for 24 h, the resulting equilibrium solution was filtered. The equilibrium concentration of the solution was determined using an atomic absorption spectrophotometer (AAS) (Hitachi Z-2000, Tokyo, Japan), while the equilibrium pH (pH e ) was determined using a glass electrode potentiometer.

Adsorption Kinetics
Kinetics experiments were conducted in a thermostated shaker (298 ± 0.5 K, 100 rpm). The BSC dosage was fixed at 10 g L −1 , while the initial solute concentrations of Pb(II) were 200 and 300 mg L −1 . The equilibration times were set between 5 to 540 min. Samples were extracted in a controlled temperature environment at different intervals and filtered. The concentration of Pb(II) in the filtrate was determined using the AAS.

Effects of Temperature and Solute Concentration
Batch tests were conducted to study Pb(II) adsorption at different temperatures and solution concentrations. The temperature range was from 278 to 318 K with increments of 10 K. The BSC dosage was fixed at 10 g L −1 with initial solute concentrations of 100, 200, 400, 500, and 600 mg L −1 . All samples were equilibrated for 24 h in a thermostated shaker at 100 rpm; then, the equilibrium Pb(II) concentrations were measured using the AAS to calculate the amount of Pb(II) adsorbed on the sorbent.
In order to ensure the reliability of our test results, two groups of batch tests were conducted in parallel and mean values were recorded; in addition, blank sample tests were conducted to test the initial concentration of Pb(II).
The adsorption amount at equilibrium q e (mg g −1 ) and removal efficiency R (%) of Pb(II) by BSC was calculated as follows: where C 0 and C e are the initial and equilibrium solute concentration (mg L −1 ), respectively, V is the volume of solution (L), and m is the quantity of BSC (g).

Microstructure Characterization
The surface microstructures of BSC before and after the reaction, as well as the precipitate after the reaction, were analyzed using a FEI Quanta 200 scanning electron microscope. The XRD-6100 X-ray diffractometer was used to conduct micro analysis of the pre-reaction BSC, post-reaction BSC, and mineral phase of the post-reaction precipitate. The diffraction angle ranged from 5 • to 90 • , and the scanning rate was 6 • /min.

Effect of pH on Pb(II) Adsorption and Associated Mechanisms
From Figure 1, it can be observed that when the initial pH (i.e., pH 0 ) increased from 1 to 2, the Pb(II) removal rate increased from 39.44% to 99.98%, and the equilibrium pH (i.e., pH e ) increased from 1.55 to 8.36. In addition, when pH 0 was between 2 to 6.5, the Pb(II) removal rate approached 100%, while pH e remained between 9 and 10. Thus, it is clear that if pH 0 is controlled between 2 to 6.5, BSC could effectively remove Pb(II) from the solution.

Microstructure Characterization
The surface microstructures of BSC before and after the reaction, as well as the precipitate after the reaction, were analyzed using a FEI Quanta 200 scanning electron microscope. The XRD-6100 X-ray diffractometer was used to conduct micro analysis of the pre-reaction BSC, post-reaction BSC, and mineral phase of the post-reaction precipitate. The diffraction angle ranged from 5° to 90°, and the scanning rate was 6°/min.

Effect of pH on Pb(II) Adsorption and Associated Mechanisms
From Figure 1, it can be observed that when the initial pH (i.e., pH0) increased from 1 to 2, the Pb(II) removal rate increased from 39.44% to 99.98%, and the equilibrium pH (i.e., pHe) increased from 1.55 to 8.36. In addition, when pH0 was between 2 to 6.5, the Pb(II) removal rate approached 100%, while pHe remained between 9 and 10. Thus, it is clear that if pH0 is controlled between 2 to 6.5, BSC could effectively remove Pb(II) from the solution. The lower the pH0 of solution containing Pb(II), the more the H + ions in it. These H + ions would have competed with Pb(II) for adsorption; in particular, the adsorption sites on BSC would be occupied by a large number of H + ions, which would have impeded Pb(II) adsorption on BSC. Furthermore, because of the limited alkalinity release of BSC, the higher the number of H + ions in the solution, the more the OHions that will be consumed by them, leading to a reduction in the number of OHions that could form hydroxides with Pb(II), consequently weakening the precipitation removal effect of BSC on Pb(II). Therefore, when pH0 was low, the removal rate was low.
Because BSC released alkalinity to regulate the pH of the solution, the pH of the solution changed constantly. It was observed that the effect of pH on adsorption process was similar to that of the formation of soluble and insoluble lead hydrolysates on the pH value [38]. In particular, when pH < 7, Pb(II) in the solution existed in the form of Pb 2+ ; therefore, the primary adsorption mechanism was ion exchange, i.e., H + , Na + , and Ca 2+ ions on the exchangeable adsorption sites of BSC were replaced by Pb 2+ ions. In contrast, when 7 < pH < 10, Pb(II) occurred in the form of Pb(OH) + and Pb(OH)2; thus, the removal of Pb(II) by BSC included Pb(OH) + adsorption, complexation, and Pb(OH)2 precipitation. Lastly, when pH > 10, Pb(OH)2 and Pb(OH) 3-were the main forms in which Pb(II) existed, and thus, the removal of Pb(II) by BSC was primarily due to Pb(OH)2 precipitation.
The interaction of Pb(II) in solution with minerals [39,40] present in BSC can be expressed as follows: The lower the pH 0 of solution containing Pb(II), the more the H + ions in it. These H + ions would have competed with Pb(II) for adsorption; in particular, the adsorption sites on BSC would be occupied by a large number of H + ions, which would have impeded Pb(II) adsorption on BSC. Furthermore, because of the limited alkalinity release of BSC, the higher the number of H + ions in the solution, the more the OH − ions that will be consumed by them, leading to a reduction in the number of OH − ions that could form hydroxides with Pb(II), consequently weakening the precipitation removal effect of BSC on Pb(II). Therefore, when pH 0 was low, the removal rate was low.
Because BSC released alkalinity to regulate the pH of the solution, the pH of the solution changed constantly. It was observed that the effect of pH on adsorption process was similar to that of the formation of soluble and insoluble lead hydrolysates on the pH value [38]. In particular, when pH < 7, Pb(II) in the solution existed in the form of Pb 2+ ; therefore, the primary adsorption mechanism was ion exchange, i.e., H + , Na + , and Ca 2+ ions on the exchangeable adsorption sites of BSC were replaced by Pb 2+ ions. In contrast, when 7 < pH < 10, Pb(II) occurred in the form of Pb(OH) + and Pb(OH) 2 ; thus, the removal of Pb(II) by BSC included Pb(OH) + adsorption, complexation, and Pb(OH) 2 precipitation. Lastly, when pH > 10, Pb(OH) 2 and Pb(OH) 3− were the main forms in which Pb(II) existed, and thus, the removal of Pb(II) by BSC was primarily due to Pb(OH) 2 precipitation.
The interaction of Pb(II) in solution with minerals [39,40] present in BSC can be expressed as follows: where ≡S represents the adsorption site bonded with a hydroxyl group in BSC; ≡ S − OH + 2 , ≡ S − OH, and ≡ S − O − are protonated, neutral, and ionized hydroxyl groups, respectively; and ≡X represents the permanently charged site with a negative charge for cations.
Considering the nature of the steel slags in BSC, an exchange interaction of the slag glass [38] in solution can be expressed as follows: Furthermore, hydrolysis and precipitation could be expressed as follows: Figure 2 shows the Pb(II) adsorption curve over time. It can be observed that a higher initial concentration of Pb(II) leads to a higher adsorption amount. This can be explained as follows: the higher the initial concentration of Pb(II), the greater the concentration gradient between BSC and water, the stronger the driving force experienced by ions to diffuse to the surface of the BSC particles, and more the number of ions removed via precipitation, leading to a larger adsorption amount. For further analysis, the experimental data were fitted using pseudo-first-order kinetic, pseudo-second-order kinetic, and intra-particle diffusion models. The pseudo-first order kinetic equation [41] is expressed as follows:

Effect of Contact Time and Adsorption Kinetics
where e q and t q are the amounts of solute adsorbed per unit adsorbent at equilibrium and at any time, respectively (mg g −1 ), and 1 k is the pseudo-first order rate constant of the adsorption process The pseudo-first order kinetic equation [41] is expressed as follows: where q e and q t are the amounts of solute adsorbed per unit adsorbent at equilibrium and at any time, respectively (mg g −1 ), and k 1 is the pseudo-first order rate constant of the adsorption process (min −1 ). The pseudo-second order kinetic equation is expressed as follows: where k 2 is the pseudo-second order rate constant of the adsorption process (g mg −1 min −1 ). The equation for the intra-particle diffusion model is expressed as follows: where k i is the intra-particle diffusion constant (mg g −1 min −1/2 ) and C is the intercept. The kinetic model parameters of Pb(II) adsorption on BSC are listed in Table 4. The pseudo-second-order kinetic fitting appears to be the best, with correlation coefficients (R 2 ) of 0.99 and 0.98 for the initial Pb(II) concentrations of 200 mg L −1 and 300 mg L −1 , respectively. The equilibrium adsorption amounts of Pb(II) calculated using the pseudo-second-order kinetic equation were 20.60 mg g −1 and 31.54 mg g −1 , respectively, which were consistent with the corresponding experimental results of 19.87 mg g −1 and 29.86 mg g −1 . Furthermore, Figure 3c shows multi-line fitting, indicating that the total rate of adsorption is determined by two stages, namely liquid film diffusion and in-particle diffusion because BSC is a porous medium. In particular, Pb(II) is adsorbed from the liquid phase to BSC through three consecutive steps: First, in the membrane diffusion stage, Pb(II) diffuses to the outer surface of BSC through an imaginary fluid dielectric film. Second, Pb(II) diffuses from the outer surface of BSC to the inner pore of the particle, thus diffusing to the inner surface. Third, in the equilibrium stage, the adsorption reaction is balanced. As can be inferred from the data in Table 3, the diffusion boundary layer thickness in particles (C 2 ) was greater than that in the liquid film (C 1 ); therefore, the diffusion rate of the liquid film (k i1 ) was significantly higher than that in the particles (k i2 ).
through an imaginary fluid dielectric film. Second, Pb(II) diffuses from the outer surface of BSC to the inner pore of the particle, thus diffusing to the inner surface. Third, in the equilibrium stage, the adsorption reaction is balanced. As can be inferred from the data in Table 3, the diffusion boundary layer thickness in particles ( 2 C ) was greater than that in the liquid film ( 1 C ); therefore, the diffusion rate of the liquid film ( 1 i k ) was significantly higher than that in the particles ( 2 i k ).   Figure 4 shows the isothermal adsorption curves of Pb(II) onto BSC at different temperatures. According to the classification of isothermal adsorption curves by Giles et al. [42], the isothermal adsorption curves showed an "L" shape at reaction temperatures of 278, 288, and 298 K, and an "H" shape at reaction temperatures of 308 and 318 K. The adsorption amount of Pb(II) onto BSC increased with an increase in the concentration of Pb(II). Within the range of the Pb(II)  Figure 4 shows the isothermal adsorption curves of Pb(II) onto BSC at different temperatures. According to the classification of isothermal adsorption curves by Giles et al. [42], the isothermal adsorption curves showed an "L" shape at reaction temperatures of 278, 288, and 298 K, and an "H" shape at reaction temperatures of 308 and 318 K. The adsorption amount of Pb(II) onto BSC increased with an increase in the concentration of Pb(II). Within the range of the Pb(II) concentrations that we tested for (i.e., 100-600 mg L −1 ), the adsorption capacity of BSC for Pb(II) did not reach saturation. The Langmuir, Freundlich, D-R, and Brunauer Emmett Teller(BET) models were used to analyze the experimental data to explain the adsorption mechanism of Pb(II) on BSC. The linear plots are shown in Figure 5. concentrations that we tested for (i.e., 100-600 mg L −1 ), the adsorption capacity of BSC for Pb(II) did not reach saturation. The Langmuir, Freundlich, D-R, and Brunauer Emmett Teller(BET) models were used to analyze the experimental data to explain the adsorption mechanism of Pb(II) on BSC.

Adsorption Isotherms
The linear plots are shown in Figure 5.
where e C is the equilibrium solute concentration (mg L −1 ), e q is the adsorption amount at equilibrium (mg g −1 ), m q is the monolayer saturation capacity (mg g −1 ), and b is the Langmuir constant (L mg −1 ). The Freundlich isotherm model can be expressed as follows: where e C and e q are defined in the same manner as in Equation (19), F K is the Freundlich constant (L g −1 ) indicating the adsorption capacity, and n is the isotherm constant indicating the adsorption intensity.
The D-R model can be expressed as follows: where m q is the monolayer saturation capacity (mol g −1 ), k is the model constant of adsorption energy (mol 2 kJ −2 ), and ε is the Polanyi potential, which is given by: where the unit for e C should in mol L −1 .
The mean free energy of adsorption E is: In general, adsorption is attributed to surface adsorption by means of ion exchange when |E| is between 8.0 and 16.0 kJ mol −1 , while to physical adsorption when |E| is between 1.0 and 8.0 kJ mol −1 .
The BET model can be expressed as follows The Langmuir isotherm model [41] can be expressed as follows: where C e is the equilibrium solute concentration (mg L −1 ), q e is the adsorption amount at equilibrium (mg g −1 ), q m is the monolayer saturation capacity (mg g −1 ), and b is the Langmuir constant (L mg −1 ). The Freundlich isotherm model can be expressed as follows: where C e and q e are defined in the same manner as in Equation (19), K F is the Freundlich constant (L g −1 ) indicating the adsorption capacity, and n is the isotherm constant indicating the adsorption intensity.
The D-R model can be expressed as follows: ln q e = ln q m − kε 2 , where q m is the monolayer saturation capacity (mol g −1 ), k is the model constant of adsorption energy (mol 2 kJ −2 ), and ε is the Polanyi potential, which is given by: where the unit for E should in mol L −1 .
The mean free energy of adsorption E is: In general, adsorption is attributed to surface adsorption by means of ion exchange when |E| is between 8.0 and 16.0 kJ mol −1 , while to physical adsorption when |E| is between 1.0 and 8.0 kJ mol −1 .
The BET model can be expressed as follows where C e and q e are defined in the same manner as in Equation (19), q 0 is the monolayer saturation capacity (g g −1 ), C s is the saturated concentration of the adsorbate (g L −1 ), and B is the BET constant.
where e C and e q are defined in the same manner as in Equation (19), 0 q is the monolayer saturation capacity (g g −1 ), s C is the saturated concentration of the adsorbate (g L −1 ), and B is the BET constant.   It can be seen from Table 5 that the fitting effect of the four isothermal models is good, and the correlation coefficients of Langmuir and BET models are relatively high, indicating that Langmuir and BET models are more suitable to describe the isothermal adsorption process of Pb(II) onto BSC than the other models. It should be noted that the adsorption of Pb(II) by bentonite satisfied the Langmuir model. In addition, the alkalinity released by BSC reacts with Pb(II) to form a hydroxide, which accumulates layer by layer on BSC, which continues to adsorb Pb(II). A synergistic adsorption-coagulation effect occurs, leading to the appearance of multiple layers locally on the surface of BSC, which satisfies the BET model.  It can be seen from Table 5 that the fitting effect of the four isothermal models is good, and the correlation coefficients of Langmuir and BET models are relatively high, indicating that Langmuir and BET models are more suitable to describe the isothermal adsorption process of Pb(II) onto BSC than the other models. It should be noted that the adsorption of Pb(II) by bentonite satisfied the Langmuir model. In addition, the alkalinity released by BSC reacts with Pb(II) to form a hydroxide, which accumulates layer by layer on BSC, which continues to adsorb Pb(II). A synergistic adsorption-coagulation effect occurs, leading to the appearance of multiple layers locally on the surface of BSC, which satisfies the BET model.  When the temperature was increased from 278 to 318 K in increments of 10 K, the monolayer saturated adsorption capacities calculated via the Langmuir isothermal model were 43.63, 45.99, 53.53, 56.79 and 70.18 mg g −1 , respectively, which was, in essence, the same as that calculated using the BET model. Thus, it was verified that the adsorption of Pb(II) on BSC satisfied the Langmuir and BET models.
Using the Freundlich model, n was calculated to be greater than 1, indicating that the adsorption process was spontaneous. However, when the temperature was 308 K and 318 K, n was greater than 2, indicating that adsorption was spontaneous and easier. These results proved that the adsorption of Pb(II) by BSC was an endothermic process, and the increase of temperature was beneficial to the adsorption of Pb(II) by BSC. For the same set of temperatures as above, the saturated adsorption capacities calculated using the D-R model were 107.06, 146.99, 214.69, 238.96, and 310.42 mg g −1 , respectively. It can be observed that, at the same temperature, the saturated adsorption capacity obtained using the D-R model was larger than that obtained using the Langmuir model; this is because the D-R model assumed an ideal state in which all micropores were filled with Pb(II), which was difficult to achieve in practice. Furthermore, when the temperature was increased from 278 to 318 K in increments of 10 K, the average free energy of adsorption (E) was −7.64, −8.56, −9.19, −11.27, and −13.99, respectively, indicating that the adsorption process at the four temperatures other than 278 K was dominated by ion exchange, compared with physical adsorption at 278 K.

Adsorption Thermodynamics
The thermodynamic [43] behavior of Pb(II) adsorption on BSC was evaluated using the following equations: where K c is the distribution coefficient of the solute between adsorbent and solution in equilibrium (q e /C e ), R the air constant, T is the temperature (K), ∆H 0 is the change of enthalpy, ∆S 0 is the change of entropy, and ∆G 0 the standard Gibb's free energy. Equations (25) and (26) can be written in a linearized form between ln(K c ) and 1/T as follows: Figure 6 is the line fitted according to the linear equation between ln Kc and 1/T, the values of ∆H 0 and ∆S 0 can be calculated from the intercept and slope of the plots. Table 6 lists the thermodynamic parameters of Pb(II) adsorption on BSC. When the concentration of Pb(II) is unchanged, the Gibb's free energy decreased with an increase of temperature and is all negative, indicating that the adsorption reaction of BSC to Pb(II) is spontaneous; it should be noted that the higher the temperature, the stronger the spontaneity. In contrast, with an increase in the concentration of Pb(II), ∆H 0 decreases and is all positive, indicating that adsorption reaction is an endothermic process. Furthermore, when the concentration of Pb(II) was increased from 100 mg L −1 to 600 mg L −1 , all ∆S 0 values were positive, but decreased from 141.84 J mol −1 K −1 to 9.94 J mol −1 K −1 , indicating that the degree of freedom of the Pb(II)-BSC system increased with the adsorption reaction; in particular, the higher the initial concentration of Pb(II), the greater the disorder degree of the system. thermodynamic parameters of Pb(II) adsorption on BSC. When the concentration of Pb(II) is unchanged, the Gibb's free energy decreased with an increase of temperature and is all negative, indicating that the adsorption reaction of BSC to Pb(II) is spontaneous; it should be noted that the higher the temperature, the stronger the spontaneity. In contrast, with an increase in the concentration of Pb(II), 0 H Δ decreases and is all positive, indicating that adsorption reaction is an endothermic process. Furthermore, when the concentration of Pb(II) was increased from 100 mg L −1 to 600 mg L −1 , all 0 S Δ values were positive, but decreased from 141.84 J mol −1 K −1 to 9.94 J mol −1 K −1 , indicating that the degree of freedom of the Pb(II)-BSC system increased with the adsorption reaction; in particular, the higher the initial concentration of Pb(II), the greater the disorder degree of the system.       Figure 7 shows the SEM images of the BSC before the reaction, BSC after the reaction, and precipitate generated after the reaction. BSC before the reaction exhibits an uneven surface and large pores, which are conducive to ion absorption, alkalinity release, and sediment accumulation. On the surface of the BSC after the reaction, there are dense clusters of aggregates, and many small pores have generated, possibly due to the continuous accumulation of sediments. These small pores can continue to facilitate adsorption. The sediment formed after the reaction is larger than the particles of the BSC after the reaction, in the form of loose large flakes or needles.  Figure 8 shows the XRD patterns of the BSC before the reaction, BSC after the reaction, and the precipitate generated after the reaction. According to (a), the mineral phases of BSC before the reaction are mainly CaCO 3 , SiO 2 , Na 2 AlSi 3 O 8 OH, and Ca 2 SiO 4 , which are the main components of calcite, quartz, bentonite, and steel slag respectively. According to (b), the main mineral phases of BSC after the reaction are CaCO 3 Figure 8 shows the XRD patterns of the BSC before the reaction, BSC after the reaction, and the precipitate generated after the reaction. According to (a), the mineral phases of BSC before the reaction are mainly CaCO3, SiO2, Na2AlSi3O8OH, and Ca2SiO4, which are the main components of calcite, quartz, bentonite, and steel slag respectively. According to (b), the main mineral phases of BSC after the reaction are CaCO3, SiO2, PbO, PbO2, Pb2O3, Pb2SiO4, Pb4SiO6, Pb5Si4O8(OH)10, Ca2PbO4, and Na6PbO5. This indicates that CaCO3 and SiO2 do not change during the reaction, while the other phases are new minerals generated during Pb removal. BSC releases Ca 2+ , Na + , and OHions in solution. PbO, PbO2, and Pb2O3 are generated by the thermal decomposition of Pb(OH)2 generated by the reaction of Pb 2+ and OHions; Pb2SiO4 and Pb4SiO6 are generated by the electrostatic adsorption of Pb 2+ and silicate on BSC; Pb5Si4O8(OH)10 is generated by Pb 2+ , OH -, and silicate ions; and Ca2PbO4 and Na6PbO5 are generated by the thermal decomposition of compounds generated by Ca 2+ , Na + , Pb 2+ , and OHreactions. According to (c), the reaction precipitates are mainly PbCO3, Pb4(SO4)(CO3)2(OH)2, Pb3(CO3)2(OH)2, and Pb3(CO2)2(OH)2. When in contact with water, BSC will release CO3 2− , SO4 2− , and OH − ions into the solution, where PbCO3 is generated by the electrostatic adsorption of Pb 2+ and CO3 2− . Pb4(SO4)(CO3)2(OH)2, Pb3(CO3)2(OH)2, and Pb3(CO2)2(OH)2 are complexes formed by coordination between Pb 2+ , CO3 2− , SO4 2− , and OH -.

Conclusions
In this study, the adsorption behavior of Pb(II) onto BSC was studied, thus providing a theoretical basis for AMD treatment containing Pb(II). First, we observed that the dependence of the adsorption process on pH was similar to that of the formation of soluble and insoluble hydrolysates of Pb(II) on the pH value. The adsorption mechanism included ion exchange, complexation, precipitation, and a synergistic adsorption-coagulation effect. Hydrolysis and precipitation of Pb(II), as well as the interactions between Pb(II) and minerals and slag glass in BSC, were expressed by equations. Second, the pseudo-first-order kinetic, pseudo-second-order kinetic, and intra-particle diffusion models were used to fit the kinetic data. The pseudo-second-order kinetic model was the most suitable to describe the adsorption kinetics of Pb(II) onto BSC. In addition, it was observed that the total adsorption rate was controlled by liquid film diffusion and intra-particle diffusion. Third, the isothermal adsorption data were analyzed using the Langmuir, Freundlich, D-R, and BET models. The Langmuir and BET models led to the best isotherms. Fourth, thermodynamic analysis revealed that the adsorption process was spontaneous and endothermic, and that the degree of freedom of the Pb(II)-BSC system increased with time. Lastly, the BSC before and after the reaction, as well as the precipitation after the reaction, were characterized by SEM and XRD. In this study, the kinetic, isothermal adsorption, thermodynamic properties, mechanism, and microscopic characterization of Pb(II) adsorption on BSC were studied. The results offer further scope to study the desorption behaviors and to perform dynamic experiments. Therefore, to conclude, BSC is a good adsorbent for treating Pb(II)-containing AMD. It is a new multifunctional mineral material that allows neutralization, adsorption, coagulation, and filtration.