Research on the Adsorption Behavior of Heavy Metal Ions by Porous Material Prepared with Silicate Tailings

: Tailings generated from mineral processing have attracted worldwide concerns due to creating serious environmental pollution. In this work, porous adsorbents were prepared as a porous block by using silicate tailings, which can adsorb heavy metal ions from the solution and are easy to separate. The synthesized silicate porous material (SPM) was characterized by X-ray di ﬀ raction (XRD), Brunner–Emmet–Teller (BET), and scanning electron microscope (SEM). The material presented a surface area of 3.40 m 2 · g − 1 , a porosity of 54%, and the compressive strength of 0.6 MPa. The maximum adsorption capacities of Pb 2 + , Cd 2 + , and Cu 2 + by SPM were 44.83 mg · g − 1 , 35.36 mg · g − 1 , and 32.26 mg · g − 1 , respectively. The experimental data were ﬁtted well by the Freundlich and Langmuir adsorption models. The kinetics of the adsorption process were ﬁtted well by the pseudo-ﬁrst order kinetic equation. These results show that the porous materials prepared with silicate tailings could act as an e ﬀ ective and low-cost adsorbent for the removal of heavy metal ions from wastewater. This study may provide a new thought on the high-value utilization of tailing for alleviating environmental pressure.


Introduction
According to the report of the United Nations Development Program, only one-third of the world's population can get clean water [1], and heavy metal ions are one of the main pollutants that pollute water. Sources of heavy metal pollution are extensive, including paint, electroplating, metal finishing, fertilizer, electrical, pigment industries, and wood manufacturing, etc., which impact on the environment and human health [2]. Various wastewater treatment techniques have been used for the removal of heavy metal ions [3], such as chemical precipitation [4,5], ion exchange [6], adsorption [2,7], membrane filtration [8], and electrochemical treatment [9]. Among these methods, adsorption treatment is considered as a low-cost, high-efficient, eco-friendly, and easily operated method [10,11]. It is of great significance for exploring the proper adsorbents, since the severity of heavy metal pollution and high processing costs are increasing.
Solid waste is another serious environmental issue. Global solid waste production was about 11 billion tons in 2011, and about 1.74 tons solid wastes for per capita were generated in one year [12]. Since rapid urbanization and industrialization need a lot of mineral resources, and these processes will produce vast solid wastes (e.g., tailings), serious environmental problems are then raised. The untreated solid wastes have not only occupied a large area of land resource, but also pollute water and

SPMs Preparation
The SPM was prepared by a sintering method. Silicate tailings and bauxite were milled and sieved through a 200-mesh screen. The raw materials of 64.35 wt % silicate tailings, 27.15 wt % bauxite, 1.35 wt % Fe 2 (SO 4 ) 3 , 0.92 wt % MgSO 4 , 1.7 wt % NaHCO 3 , and 4.53 wt % bentonite were finely mixed, and water (w:v = 10:3.5) was added after mixing the raw materials. A certain amount of the mixture was uniaxially pressed into the mold (length 70 mm, width 50 mm, and height 30 mm) to obtain the green body, which was then placed into the drying oven for 4 h at 80 • C. In the drying process, the green body was expanded with the releasing gas, and the interconnected pores were then formed. Subsequently, the desiccated green body was placed into a muffle furnace with a ramp rate of 10 • C·min −1 from the room temperature to 1140 • C and kept for 20 min. After the sintering period, the enamel on the material surface was removed by polishing ( Figure S1). All the heat treatments of the green body were conducted with normal pressure and temperature.

Characterization
Mineralogical analyses of the raw materials and SPM were performed by X-ray diffraction (XRD, SeimensO8DISCOVER, Seimens, Berlin, Germany); the prepared samples were exposed to an X-ray with the 2θ angle varying between 10-80 • with Cu Kα radiation. The applied voltage and current were 40 kV and 30 mA, respectively. The major chemical compositions of SPM were analyzed by an X-ray fluorescence (XRF) spectrometer (AxiosmAX Panalytical. B.V, Milford, CT, USA). The microstructure, textural, and morphological characteristics of the SPM sample were observed by the JSM-6460LV scanning electron microscope (Japan electronics corporation, Tokyo, Japan). The specific surface area of the SPM was determined by an automatic specific surface analyzer (Quantachrome Instruments Monosorb, Quantachrome, Düsseldorf, Germany). Fourier transform infrared (FTIR) spectra were recorded on a Nicolet 6700 spectrometer (Nicolet, Waltham, MA, USA). The compressive strengths of the samples, which were cut from the cubes with a normal size of about 20 mm × 20 mm × 20 mm, were measured using an Inst LTD Hydraulic servo dynamic test system 8802 (Instron, London, UK). The porosity of the SPM was measured by a pycnometer and calculated by the formula (Equation (1)): where ρ g is the bulk density, ρ is the true density, and n is the porosity.

Determination Point of Zero Charge
To determine the point of zero charge [30], a series of 50-mL centrifugal tubes loaded 45 mL of 0.1 mol·L −1 of NaNO 3 solution were transferred. The solution pH values were adjusted from 2.0 to 10.0 by adding either 0.1 mol·L −1 of HNO 3 or 0.1 mol·L −1 of NaOH solution. The solution total volume in each flask was adjusted to 50 mL exactly by adding the NaNO 3 solution. Then, the pH 0 of the solutions were subsequently recorded. SPM (1 g) was immediately added to each centrifugal tube and securely capped. Then, the suspension was manually agitated, and the pH values of the supernatant liquid were recorded. The differences between the initial and final pH values (pH 0 −pH f ) were plotted through the pH 0 .

Adsorption Experiments
Batch mode experiments were conducted on the adsorption of heavy metals by SPM from solutions containing single heavy metal ions at an initial concentration of 125 mg·L −1 and an ionic strength of 0.01 mol·L −1 of NaNO 3 at a temperature of 30 • C. SPM adsorbents that were cut into small cubes (0.5 g) were added into 50-mL capped polyethylene bottles that contained 40 mL of single metal ion solutions; the competitive experiment used 40 mL of multiple metal ion solutions. The bottles were sealed and agitated at 150 revolutions per minute (rpm) in an orbital shaker. The energy-dispersive Minerals 2019, 9, 291 4 of 16 spectrum (EDS) was used to confirm that heavy metal ions have been adsorbed on the SPM. After the adsorption process, the mixed solution was centrifuged at 5000 rpm for 10 min, and the supernatant was reserved for Pb 2+ , Cd 2+ , and Cu 2+ analyses by inductively coupled plasma mass spectrometry (ICP-MS, PerkinElmer Elan 6000, PerkinElmer, Waltham, MA, USA). The experimental conditions are shown in Table 3. All the experiments were performed in triplicate, and the procured data were analyzed by Origin 8.0 (OriginLab, Northampton, MA, USA). The adsorption amounts of heavy metal ions at equilibrium state were calculated using Equation (2), and the removal efficiency was calculated using Equation (3).
Removal efficiency where q e is the adsorption capacity (mg·g −1 ), C 0 is the initial concentration of a heavy metal (mg·L −1 ); C e is the equilibrium concentration of a heavy metal (mg·L −1 ); V is the volume of solution (L), and M is the mass of adsorbent (g). The average value was obtained, and the error ranges were controlled within ±5%. The amount of each ion adsorbed on SPM was calculated and expressed as a percentage by using the difference between the amount of ion in the initial solution and the final solution.

Desorption Experiments
The desorption experiments were carried out with 0.1 mol·L −1 of HNO 3 solution. First, 0.5 g of SPM loaded with single heavy metals after adsorption was dried in the oven. After drying, the SPM was added into 40 mL of HNO 3 solution and shaken in a shaker at 30 • C for 24 h. Next, the mixture was centrifuged at 4000 rpm for 10 min, and the SPM precipitant was collected. The amount of desorbed heavy metal ions was determined by the amount of heavy metal ions in the solution.

Adsorption Isotherm of SPM
Langmuir and Freundlich adsorption isotherm models were used to depict the equilibrium between adsorbed Pb 2+ , Cd 2+ , and Cu 2+ on SPM (q e ) and ions concentration in solution (C e ) at constant temperature (30 • C). The ratio of adsorbent to solution volume was 12.5 g·L −1 , and the adsorption amount kept rising during heavy metal ions concentration ranging from 20 mg·L −1 to 1000 mg·L −1 .

Kinetic Study of SPM
The pseudo-first order and pseudo-second order kinetic models were used to describe the adsorption process at a heavy metal ions concentration of 125 mg·L −1 and temperature of 30 • C. The ratio of adsorbent to solution volume was 12.5 g·L −1 . The removal rate of heavy metals in the solution was increased with time from 0 to 28 h.

Column Experiments
The columns were constructed from a polymethyl methacrylate cylinder (inner diameter 25 mm, length 100 mm) which were packed with 16 g of SPM to yield a bed height of 90 mm. Single heavy metal ions solution with a concentration of 100 mg·L −1 was pumped upward through the column at a filtration velocity of 5 mL·min −1 and controlled by a peristaltic pump.

Characterization of the SPM
The zero-charge point was determined by the solid addition method, and the zero-charge point (pH 0 ) of SPM was 6.7 ( Figure 1a). The XRD patterns of SPM, bauxite, and tailings are shown in Figure 1b. The crystalline phases of raw materials were mainly composed of quartz, orthoclase, and diaspore. After the sintering process, the main crystalline phases of SPM were quartz, corundum, and hematite, which have been previously shown to be capable of adsorbing heavy metal ions [31][32][33].
Based on the BET analysis, the SPM has a specific surface area of 3.40 m ·g , and a porosity of 54%. The compressive strength of SPM is 0.6 MPa. The above characteristics of SPM basically accorded with the design requirements. SEM was used to observe the morphology of SPM, which was shown in Figure 2. Some favorable features of SPM were found; the material surface had geometrical properties with irregular shape and large agglomerates and the rugged surface, which would provide more sites for adsorbing heavy metal ions.
The FTIR spectra of SPM, silicate tailings, bauxite, and bentonite were shown in Figure 3. The peaks of these substances were mainly distributed in three regions: 3399-3695 cm −1 , 1608-1873 cm −1 , and 542-792 cm −1 . The characteristic absorption peaks at 3399 cm −1 and 3695 cm −1 were assigned to the Si-OH stretching vibration [30]. The peaks at 1629 cm −1 and 1873 cm −1 were attributed to C-O lattice vibrations [34]. The adsorption bond at 542 cm −1 represented the Si-O stretching vibration, and the adsorption bonds near 534-542 cm −1 were -H deformation vibration [35]. The Si-O symmetric stretching vibration occurred at 792 cm −1 [33,36]. The broad bonds near 1034-1047 cm −1 were assigned to stretching vibrations of the Si-O tetrahedral [36]. The peaks located at about 3695 cm −1 and 1822 cm −1 were approximately the Mg3O-H and Fe-O stretching vibrations in the spectra of SPM. Mg3O-H and Fe-O are presented in SPM, because MgSO4 and Fe2(SO4)3 were added during the preparation [35,37]. In the spectra of raw materials, included silicates, tailings, bentonite, and bauxite, peaks of C-O stretching vibration located at 1608-1629 cm −1 , 1871-1873 cm −1 , 1989 cm −1 and 2123 cm −1 were observed [37], which might be due to the existed impurities or a small amount of adsorbing carbon dioxide. The adsorption bonds of C-O at 1989 cm −1 and 2128 cm −1 disappeared in SPM, which could be due to the bicarbonate decomposition at high temperatures. Therefore, it could be concluded that the hydroxyl groups on the surface of SPM played an important role in the removal of heavy metals [38].  Based on the BET analysis, the SPM has a specific surface area of 3.40 m 2 ·g −1 , and a porosity of 54%. The compressive strength of SPM is 0.6 MPa. The above characteristics of SPM basically accorded with the design requirements. SEM was used to observe the morphology of SPM, which was shown in Figure 2. Some favorable features of SPM were found; the material surface had geometrical properties with irregular shape and large agglomerates and the rugged surface, which would provide more sites for adsorbing heavy metal ions.
The FTIR spectra of SPM, silicate tailings, bauxite, and bentonite were shown in Figure 3. The peaks of these substances were mainly distributed in three regions: 3399-3695 cm −1 , 1608-1873 cm −1 , and 542-792 cm −1 . The characteristic absorption peaks at 3399 cm −1 and 3695 cm −1 were assigned to the Si-OH stretching vibration [30]. The peaks at 1629 cm −1 and 1873 cm −1 were attributed to C-O lattice vibrations [34]. The adsorption bond at 542 cm −1 represented the Si-O stretching vibration, and the adsorption bonds near 534-542 cm −1 were -H deformation vibration [35]. The Si-O symmetric stretching vibration occurred at 792 cm −1 [33,36]. The broad bonds near 1034-1047 cm −1 were assigned to stretching vibrations of the Si-O tetrahedral [36].  3 were added during the preparation [35,37]. In the spectra of raw materials, included silicates, tailings, bentonite, and bauxite, peaks of C-O stretching vibration located at 1608-1629 cm −1 , 1871-1873 cm −1 , 1989 cm −1 and 2123 cm −1 were observed [37], which might be due to the existed impurities or a small amount of adsorbing carbon dioxide. The adsorption bonds of C-O at 1989 cm −1 and 2128 cm −1 disappeared in SPM, which could be due to the bicarbonate decomposition at high temperatures. Therefore, it could be concluded that the hydroxyl groups on the surface of SPM played an important role in the removal of heavy metals [38].

Effect of Contacting Time
The uptake capacities of single heavy metal ions were investigated as a function of time to determine an optimum contact time for the adsorption of heavy metal ions on SPM. The effects of contact time on adsorption amount of Pb 2+ , Cd 2+ , and Cu 2+ by SPM are shown in Figure 4. The equilibrium time was found to be 20 h. At equilibrium conditions, 88.4% of Pb 2+ , 51.2% of Cd 2+ , and 41.3% of Cu 2+ were removed from the solution. When interacting with heavy metal ions, the adsorption site on SPM gradually decreased, but the percentage of adsorption depended on the amount of heavy metal ions transported from the bulk liquid phase to the actual adsorption site, so the percentage of adsorption

Effect of Contacting Time
The uptake capacities of single heavy metal ions were investigated as a function of time to determine an optimum contact time for the adsorption of heavy metal ions on SPM. The effects of contact time on adsorption amount of Pb 2+ , Cd 2+ , and Cu 2+ by SPM are shown in Figure 4. The equilibrium time was found to be 20 h. At equilibrium conditions, 88.4% of Pb 2+ , 51.2% of Cd 2+ , and 41.3% of Cu 2+ were removed from the solution. When interacting with heavy metal ions, the adsorption site on SPM gradually decreased, but the percentage of adsorption depended on the amount of heavy metal ions

Effect of Contacting Time
The uptake capacities of single heavy metal ions were investigated as a function of time to determine an optimum contact time for the adsorption of heavy metal ions on SPM. The effects of contact time on adsorption amount of Pb 2+ , Cd 2+ , and Cu 2+ by SPM are shown in Figure 4. The equilibrium time was found to be 20 h. At equilibrium conditions, 88.4% of Pb 2+ , 51.2% of Cd 2+ , and 41.3% of Cu 2+ were removed from the solution. When interacting with heavy metal ions, the adsorption site on SPM gradually decreased, but the percentage of adsorption depended on the amount of heavy metal ions transported from the bulk liquid phase to the actual adsorption site, so the percentage of adsorption increased with time until saturation [39]. SPM could remove these heavy metal ions at different extents, and the removal abilities of different metal ions were in the order of Pb 2+ > Cd 2+ > Cu 2+ , which could be due to the difference in the ion-exchange capacity of the heavy metal ions adsorbed on SPM [40]. EDS results confirmed that Pb 2+ , Cd 2+ , and Cu 2+ were adsorbed on the SPM surface. As shown in Figure S2, the characteristic peaks of Pb, Cd, and Cu were observed in the EDS spectra, and the corresponding peak intensities on the energy spectra were increased compared to blanks. The removal percentage remains stagnant at 0.5 to 4 h, which might be contributed by the tiny and rich micropores filled in SPM. After the addition of SPM, the heavy metal ions were rapidly adsorbed onto the external surface of the SPM, since the external surface was in direct contact with the solution. Owing to the narrow diameter of micropores, the heavy metal ions needed more time to permeate into the pores, so it took a long time for SPM to get the adsorption saturation. Therefore, the first time to reach equilibrium could indicate the adsorption saturation of the outer surface, and the second equilibrium was probably the adsorption saturation of the inner surface of the pores. The amount of adsorption when reaching the second equilibrium is significantly higher than that of the first time. It could be inferred that the amount of heavy metal ions adsorbed on the internal surface of SPM was more than that adsorbed on the external surface.
Minerals 2019, 9, x FOR PEER REVIEW 7 of 16 and the removal abilities of different metal ions were in the order of Pb 2+ > Cd 2+ > Cu 2+ , which could be due to the difference in the ion-exchange capacity of the heavy metal ions adsorbed on SPM [40]. EDS results confirmed that Pb 2+ , Cd 2+ , and Cu 2+ were adsorbed on the SPM surface. As shown in Figure S2, the characteristic peaks of Pb, Cd, and Cu were observed in the EDS spectra, and the corresponding peak intensities on the energy spectra were increased compared to blanks. The removal percentage remains stagnant at 0.5 to 4 h, which might be contributed by the tiny and rich micropores filled in SPM. After the addition of SPM, the heavy metal ions were rapidly adsorbed onto the external surface of the SPM, since the external surface was in direct contact with the solution. Owing to the narrow diameter of micropores, the heavy metal ions needed more time to permeate into the pores, so it took a long time for SPM to get the adsorption saturation. Therefore, the first time to reach equilibrium could indicate the adsorption saturation of the outer surface, and the second equilibrium was probably the adsorption saturation of the inner surface of the pores. The amount of adsorption when reaching the second equilibrium is significantly higher than that of the first time. It could be inferred that the amount of heavy metal ions adsorbed on the internal surface of SPM was more than that adsorbed on the external surface.

Effect of Initial Concentration of Heavy Metal Ions
The influence of initial concentrations of heavy metal ions on the adsorption of Pb 2+ , Cd 2+ , and Cu 2+ by SPM was investigated, and the results are shown in Figure 5. The adsorption capacity sharply increased with the increase of the initial concentration of heavy metals ions ranging from 20 mg·L −1 to 1000 mg·L −1 . The maximum adsorption capacities of Pb 2+ , Cd 2+ , and Cu 2+ by SPM were 44.83 mg·g −1 , 35.36 mg·g −1 , and 32.26 mg·g −1 , respectively. The adsorption capacity of heavy metal ions increased, and the removal efficiency decreased with the increasing initial concentrations of heavy metal ions [39]. This indicated that the increase of initial concentrations of heavy metal ions contributed to enhance the driving force at the solid-liquid interface to increase the adsorption capacity until the adsorption sites were saturated [41,42]. In order to describe the adsorption behaviors well (i.e., the extent of adsorption and relevant mechanism), the experimental data were then fitted with the Freundlich and Langmuir isotherm models in Section 3.5.

Effect of Initial Concentration of Heavy Metal Ions
The influence of initial concentrations of heavy metal ions on the adsorption of Pb 2+ , Cd 2+ , and Cu 2+ by SPM was investigated, and the results are shown in Figure 5. The adsorption capacity sharply increased with the increase of the initial concentration of heavy metals ions ranging from 20 mg·L −1 to 1000 mg·L −1 . The maximum adsorption capacities of Pb 2+ , Cd 2+ , and Cu 2+ by SPM were 44.83 mg·g −1 , 35.36 mg·g −1 , and 32.26 mg·g −1 , respectively. The adsorption capacity of heavy metal ions increased, and the removal efficiency decreased with the increasing initial concentrations of heavy metal ions [39]. This indicated that the increase of initial concentrations of heavy metal ions contributed to enhance the driving force at the solid-liquid interface to increase the adsorption capacity until the adsorption sites were saturated [41,42]. In order to describe the adsorption behaviors well (i.e., the extent of adsorption

Effect of pH Value on the Adsorption of Heavy Metal Ions
During the adsorption process, the pH value is one of the important impact factors that can influence the heavy metal ions binding to the surface of the adsorbent, since the pH value of the solution would affect the binding sites on the surface of the adsorbent and the chemical nature of the adsorbent, and the hydrogen ions could strongly compete with heavy metal ions at low pH values [43,44]. In this work, the effect of pH value on the adsorption of heavy metal ions was investigated, and the results are shown in Figure 6. The adsorption curve results indicated that the effect of pH value on Cd 2+ adsorption was not distinct, but the effect was obvious for Pb 2+ and Cu 2+ , and the removal efficiency increased as the pH value increased from 2 to 6. The removal efficiencies of Pb 2+ and Cd 2+ reached their maximum of 82% and 71% at pH 6, respectively, while the removal efficiencies dropped down to 57% and 63% when the pH value was higher than 6. The removal efficiency of Cu 2+ reached a maximum of 98% at pH 7. The surface of SPM contained many active sites, and might become positively charged at a low pH value, thus increasing the competition between the heavy metal ions and H + for available adsorption sites. However, this competition decreased while these surface active sites became more negatively charged with the increasing pH value, which enhanced the adsorption of the positively charged metal ions through electrostatic force [45,46]. To rule out the effect of SPM on the pH of the solution, the results are shown in Table S1 and Figure S3. The adsorption results of SPM in this study are consistent with most of the previously reported silicate minerals. The Cu 2+ removal efficiency of 98% from the solution at pH 7 was attributed to Cu 2+ being favored to precipitate as Cu(OH)2 when the pH value was above 6 [39]. This phenomenon further proved that electrostatic interaction played an important role in the adsorption of Pb 2+ , Cd 2+ , and Cu 2+ . Thus, the optimal pH value for the removal of Pb 2+ , Cd 2+ , and Cu 2+ was determined to be 6 in this study.

Effect of pH Value on the Adsorption of Heavy Metal Ions
During the adsorption process, the pH value is one of the important impact factors that can influence the heavy metal ions binding to the surface of the adsorbent, since the pH value of the solution would affect the binding sites on the surface of the adsorbent and the chemical nature of the adsorbent, and the hydrogen ions could strongly compete with heavy metal ions at low pH values [43,44]. In this work, the effect of pH value on the adsorption of heavy metal ions was investigated, and the results are shown in Figure 6. The adsorption curve results indicated that the effect of pH value on Cd 2+ adsorption was not distinct, but the effect was obvious for Pb 2+ and Cu 2+ , and the removal efficiency increased as the pH value increased from 2 to 6. The removal efficiencies of Pb 2+ and Cd 2+ reached their maximum of 82% and 71% at pH 6, respectively, while the removal efficiencies dropped down to 57% and 63% when the pH value was higher than 6. The removal efficiency of Cu 2+ reached a maximum of 98% at pH 7. The surface of SPM contained many active sites, and might become positively charged at a low pH value, thus increasing the competition between the heavy metal ions and H + for available adsorption sites. However, this competition decreased while these surface active sites became more negatively charged with the increasing pH value, which enhanced the adsorption of the positively charged metal ions through electrostatic force [45,46]. To rule out the effect of SPM on the pH of the solution, the results are shown in Table S1 and Figure S3. The adsorption results of SPM in this study are consistent with most of the previously reported silicate minerals. The Cu 2+ removal efficiency of 98% from the solution at pH 7 was attributed to Cu 2+ being favored to precipitate as Cu(OH) 2 when the pH value was above 6 [39]. This phenomenon further proved that electrostatic interaction played an important role in the adsorption of Pb 2+ , Cd 2+ , and Cu 2+ . Thus, the optimal pH value for the removal of Pb 2+ , Cd 2+ , and Cu 2+ was determined to be 6 in this study. Minerals 2019, 9, x FOR PEER REVIEW 9 of 16

Competitive Adsorption among Three Heavy Metal Ions
It is of great significance to investigate the interaction between heavy metal ions and SPM, since various heavy metal ions usually coexist in the wastewater. The effects of interaction among heavy metal ions on the adsorption of Pb 2+ , Cd 2+ , and Cu 2+ were tested, and the results are shown in Figure 7. When the initial concentration of metal ions increased from 20 mg·L −1 to 100 mg·L −1 , the removal efficiency of heavy metal ions decreased significantly. The result indicated that the adsorption amount presented a significant difference between single metal and multi-metal ions adsorbed on the SPM. The adsorption capacity of Pb 2+ was larger than those of Cd 2+ and Cu 2+ . For example, the 42.8% of Pb 2+ was adsorbed on the SPM when the multi-metal ions concentration was 100 mg·L −1 , but the removal efficiencies of Cd 2+ and Cu 2+ were just 13.8% and 4.9% at the same concentration, respectively. In single metal experiments, the removal efficiencies of Pb 2+ , Cd 2+ , and Cu 2+ were higher than those of multi-heavy metal ions experiments at the same concentration. Since different heavy metal ions competed for adsorption sites, the removal efficiency in a single-metal system was higher than that of a multi-metal ions system. The higher concentration of mixed heavy metal ions resulted in the stronger competition among heavy metal ions. Pb 2+ could replace Cd 2+ and Cu 2+ to be adsorbed on the surface of the SPM; therefore, Pb 2+ was more competitive than Cd 2+ and Cu 2+ [47]. These different properties of heavy metal ions, such as ionic potential, electronegativity, softness capacity for hydroxylation, and position in the Irving-Williams series [48] also made the difference in removal efficiency among heavy metal ions adsorption. The preference of the sorbent for the Pb 2+ may be because the metal ions have the largest atomic weight and paramagnetic, and the most electronegative ion has the highest standard reduction potential as compared to Cu 2+ and Cd 2+ [49]. Another reason may be related to the hydration energy and hydrated ionic radius of heavy metals, and the lower hydration energy was easier for the adsorption. The hydration energy ranked as follows: Pb 2+ < Cd 2+ < Cu 2+ [36].

Competitive Adsorption among Three Heavy Metal Ions
It is of great significance to investigate the interaction between heavy metal ions and SPM, since various heavy metal ions usually coexist in the wastewater. The effects of interaction among heavy metal ions on the adsorption of Pb 2+ , Cd 2+ , and Cu 2+ were tested, and the results are shown in Figure 7. When the initial concentration of metal ions increased from 20 mg·L −1 to 100 mg·L −1 , the removal efficiency of heavy metal ions decreased significantly. The result indicated that the adsorption amount presented a significant difference between single metal and multi-metal ions adsorbed on the SPM. The adsorption capacity of Pb 2+ was larger than those of Cd 2+ and Cu 2+ . For example, the 42.8% of Pb 2+ was adsorbed on the SPM when the multi-metal ions concentration was 100 mg·L −1 , but the removal efficiencies of Cd 2+ and Cu 2+ were just 13.8% and 4.9% at the same concentration, respectively. In single metal experiments, the removal efficiencies of Pb 2+ , Cd 2+ , and Cu 2+ were higher than those of multi-heavy metal ions experiments at the same concentration. Since different heavy metal ions competed for adsorption sites, the removal efficiency in a single-metal system was higher than that of a multi-metal ions system. The higher concentration of mixed heavy metal ions resulted in the stronger competition among heavy metal ions. Pb 2+ could replace Cd 2+ and Cu 2+ to be adsorbed on the surface of the SPM; therefore, Pb 2+ was more competitive than Cd 2+ and Cu 2+ [47]. These different properties of heavy metal ions, such as ionic potential, electronegativity, softness capacity for hydroxylation, and position in the Irving-Williams series [48] also made the difference in removal efficiency among heavy metal ions adsorption. The preference of the sorbent for the Pb 2+ may be because the metal ions have the largest atomic weight and paramagnetic, and the most electronegative ion has the highest standard reduction potential as compared to Cu 2+ and Cd 2+ [49]. Another reason may be related to the hydration energy and hydrated ionic radius of heavy metals, and the lower hydration energy was easier for the adsorption. The hydration energy ranked as follows: Pb 2+ < Cd 2+ < Cu 2+ [36].

Desorption of Heavy Metal Ions
The result of single heavy metal ions desorption from SPM is shown in Figure 8. During the desorption process, Cd 2+ and Cu 2+ presented greater desorption efficiency than Pb 2+ . Heavy metal ions were released from the surfaces of SPM over the first nine hours of the desorption process, and 15.8% of Cd 2+ , 16.7% of Cu 2+ , and 35.8% of Pb 2+ were desorbed from SPM, respectively. The loosely bound metal from the surface of the adsorbent could not be washed away before the desorption experiment, so a very strong desorption occurred at the beginning of the desorption experiment. The desorption isotherm indicated that Cd 2+ and Cu 2+ could be strongly bonded on the surface of the SPM, which should be due to the existence of sites for specifically adsorbing Cd 2+ and Cu 2+ on the SPM surface. This result indicated that more Cd 2+ and Cu 2+ could be adsorbed on specific sites than nonspecific sites, while a higher proportion of Pb 2+ was adsorbed on a non-specific site [40]. This also indicated that more non-specific sites were on the SPM surface, leading to the better adsorption capacity for Pb 2+ by SPM, but the binding stability for Pb 2+ was lower as compared with those of Cd 2+ and Cu 2+ [48].
The raw material for the preparation of SPM was silicate tailings, and the main components were the silicate with great adsorption capacity to heavy metals ions. Table 4 shows the adsorption results of reports on various silicate solid wastes. Compared with silicate solid wastes, SPM was sintered into block, which retained the excellent adsorption capacity of silicate solid wastes and was easy to separate from solution, avoiding secondary pollution. Thus, the preparation of SPM provides a way for reusing solid waste such as silicate tailings.

Desorption of Heavy Metal Ions
The result of single heavy metal ions desorption from SPM is shown in Figure 8. During the desorption process, Cd 2+ and Cu 2+ presented greater desorption efficiency than Pb 2+ . Heavy metal ions were released from the surfaces of SPM over the first nine hours of the desorption process, and 15.8% of Cd 2+ , 16.7% of Cu 2+ , and 35.8% of Pb 2+ were desorbed from SPM, respectively. The loosely bound metal from the surface of the adsorbent could not be washed away before the desorption experiment, so a very strong desorption occurred at the beginning of the desorption experiment. The desorption isotherm indicated that Cd 2+ and Cu 2+ could be strongly bonded on the surface of the SPM, which should be due to the existence of sites for specifically adsorbing Cd 2+ and Cu 2+ on the SPM surface. This result indicated that more Cd 2+ and Cu 2+ could be adsorbed on specific sites than nonspecific sites, while a higher proportion of Pb 2+ was adsorbed on a non-specific site [40]. This also indicated that more non-specific sites were on the SPM surface, leading to the better adsorption capacity for Pb 2+ by SPM, but the binding stability for Pb 2+ was lower as compared with those of Cd 2+ and Cu 2+ [48].

Adsorption Isotherm of SPM
The adsorption equilibriums are usually described by isotherm equations whose parameters represent the surface properties and affinity of the adsorbent. The adsorption isotherms can be generated on the basis of theoretical models, the most common of which are the Langmuir model and the Freundlich model [45]. The Langmuir (Equation (4)) and Freundlich (Equation (5)) adsorption isotherm models can be written as follows: The raw material for the preparation of SPM was silicate tailings, and the main components were the silicate with great adsorption capacity to heavy metals ions. Table 4 shows the adsorption results of reports on various silicate solid wastes. Compared with silicate solid wastes, SPM was sintered into block, which retained the excellent adsorption capacity of silicate solid wastes and was easy to separate from solution, avoiding secondary pollution. Thus, the preparation of SPM provides a way for reusing solid waste such as silicate tailings.

Adsorption Isotherm of SPM
The adsorption equilibriums are usually described by isotherm equations whose parameters represent the surface properties and affinity of the adsorbent. The adsorption isotherms can be generated on the basis of theoretical models, the most common of which are the Langmuir model and the Freundlich model [45]. The Langmuir (Equation (4)) and Freundlich (Equation (5)) adsorption isotherm models can be written as follows: ln q e = lnk f + 1 n lnC e The Langmuir and Freundlich models can describe the adsorption mechanism on the surface well, and they can account for these experimental results in a wider range concentration [57,58]. According to the Langmuir adsorption model and Freundlich adsorption model, the values of q e followed the sequence: Pb 2+ > Cd 2+ > Cu 2+ , which matched to the batch experiment. In fact, the data obtained from the experiment of adsorption fitted the two models well ( Table 5), indicating that the adsorption sites were uneven and non-specific. The adsorption coefficient is greatly consistent with the conditions that support favorable adsorption. This result suggested that more than one type of active site took part in the heavy metal ions adsorption, such as some specific adsorption sites that were consistent with the speculation of Section 3.3 [42].

Kinetic Study of SPM
The pseudo-first order kinetic model and pseudo-second order kinetic model have been diffusely used to describe the removal of pollutants from solution in different fields [28]. Here, the tested pseudo-first order (Equation (6)) and pseudo-second order (Equation (7)) kinetics models can be written as follows: ln(q e − q t ) = lnq e − k 1 t (6) where q e is the mass of heavy metal ions adsorbed at equilibrium (mg·g −1 ), q t is the mass of metal adsorbed at time t (h), and k 1 and k 2 are the pseudo-first order model and the pseudo-second order model rate constants of adsorption. The kinetic adsorption data were satisfactorily fitted to the pseudo-first order model and pseudo-second order model ( Table 6). The R 2 values of the pseudo-first order model and pseudo-second order model for Pb 2+ , Cd 2+ , and Cu 2+ were~0.915-0.984 and 0.908-0.982, respectively, and the equilibrium adsorption capacities predicted from the models were approximately equal to the corresponding experimental values. The diffusion process of heavy metal ions will affect the interaction between heavy metal ions and the SPM surface; thus, the adsorption rate will be affected. If the R 2 values were considered, the pseudo-first order model appeared to fit the data better than the pseudo-second order model. The better-fitted results suggested that the diffusion process might be the prior rate-limiting step in the adsorption.

Column Experiments
Column experiments were conducted for determining the single ion adsorption characteristics of Pb 2+ , Cd 2+ , and Cu 2+ in the dynamic adsorption process, respectively (Figure 9), and the adsorption parameters are listed in Table 7. The Thomas model satisfactorily described the adsorption behavior of the different heavy metal ions from SPM (R 2 = 0.913~0.934). SPM had better adsorption ability for Pb 2+ as compared to Cd 2+ and Cu 2+ in the column experiments, and the time to reach the plateau of Ct/C0 value was significantly longer for Pb 2+ than other metal ions. The results can be obtained from Figure 8 that the breakthrough speed was the slowest and the adsorption curve was the least steep for Pb 2+ . The breakthrough point of Cu 2+ appeared at about 30 min, followed by Cd 2+ at 60 min and Pb 2+ at 110 min. The adsorption capacity order of the column from the breakthrough curves was Pb 2+ > Cd 2+ > Cu 2+ , which suggested that Pb 2+ was the most easily to bond to the adsorption sites of SPM in the column experiment. This is consistent with the results of the batch experiment, because Pb 2+ presented stronger adsorption properties due to its greater ion size, while Cu 2+ and Cd 2+ were lower affinity to bind to the adsorption groups [59]. The maximal capacity of the column experiment was significantly lower than that of the batch experiment, because the heavy metal ions flowing through the columns probably had less contact time, and the adsorption equilibrium was not reached, unlike in the batch equilibrium experiment [60]. Therefore, SPM is an appropriate adsorbent to adsorb heavy metal ions from the contaminated aqueous media in column adsorption application.

Conclusions
Preparing the silicate tailings into foamed silicate porous materials for heavy metal adsorption from solution is an effective way to alleviate environmental pressures from tailings, and can attain sustainability. Porous materials were successfully fabricated by silicate tailings in this work, and it could be used as an adsorbent to remove Pb 2+ , Cd 2 , and Cu 2+ from aqueous solution with high adsorption capacities. The adsorption performance of SPM was greatly affected by the pH value and the other co-existence of heavy metal ions in the solution. Referring to the heavy metal ions on the SPM, the selective adsorption order of these metals was Pb 2+ > Cd 2+ > Cu 2+ . In addition, experimental data were suitable for the Langmuir model and the Freundlich model, indicating that the adsorption sites were uneven and less non-specific. The kinetic model fitting results indicated that the adsorption process was limited by heavy metal ions diffusion and chemisorption processes. The experimental results indicate that the SPM prepared in this study is a potential adsorbent for the removal of heavy metal ions from solution.
Supplementary Materials: The following are available online at www.mdpi.com/xxx/s1, Figure S1: Photographs of SPM, Figure S2: EDS spectra of SPM after adsorb heavy metal ions (a-Pb, b-Cd, c-Cu), Figure S3: Effect of pH value on the ion concentration, Table S1: The effect of SPM on the initial concentration of the solution.

Conclusions
Preparing the silicate tailings into foamed silicate porous materials for heavy metal adsorption from solution is an effective way to alleviate environmental pressures from tailings, and can attain sustainability. Porous materials were successfully fabricated by silicate tailings in this work, and it could be used as an adsorbent to remove Pb 2+ , Cd 2 , and Cu 2+ from aqueous solution with high adsorption capacities. The adsorption performance of SPM was greatly affected by the pH value and the other co-existence of heavy metal ions in the solution. Referring to the heavy metal ions on the SPM, the selective adsorption order of these metals was Pb 2+ > Cd 2+ > Cu 2+ . In addition, experimental data were suitable for the Langmuir model and the Freundlich model, indicating that the adsorption sites were uneven and less non-specific. The kinetic model fitting results indicated that the adsorption process was limited by heavy metal ions diffusion and chemisorption processes. The experimental results indicate that the SPM prepared in this study is a potential adsorbent for the removal of heavy metal ions from solution.
Supplementary Materials: The following are available online at http://www.mdpi.com/2075-163X/9/5/291/s1, Figure S1: Photographs of SPM, Figure S2: EDS spectra of SPM after adsorb heavy metal ions (a-Pb, b-Cd, c-Cu, d-blank), Figure S3: Effect of pH value on the ion concentration, Table S1: The effect of SPM on the initial concentration of the solution.