Microstructure and Pb 2 + Adsorption Properties of Blast Furnace Slag and Fly Ash based Geopolymers

: In this study, a blast furnace slag (BFS) and ﬂy ash (FA) based adsorbent geopolymer to be used for removing Pb 2 + from aqueous solutions were synthesized using the hydrothermal method at 60 ◦ C for 24 h, and then cured at 25 ◦ C for another six days. The alkali activator applied in this work was a combination of sodium hydroxide and sodium silicate solutions at a mass ratio of 2. The geopolymer slurry was adjusted to a Si / Al molar ratio of 3. A BFS-based geopolymer (GS) having a speciﬁc area of 23.56 m 2 / g and pore size and volume of 7.8 nm and 73 cm 3 / kg, respectively, surpassed the raw material surface by approximately 13-fold. An FA-based geopolymer (GA) having a speciﬁc area of 35.97 m 2 / g and a size and porous volume of 9 nm and 124 nm, respectively, surpassed the raw material surface by approximately 23-fold. In addition, GS and GA showed a cation exchange capacity (CEC) of 241.30 and 286.96 Meq / 100 g, respectively. X-ray di ﬀ raction (XRD) determined sample crystallinity and it was proven by scanning electron microscopy (SEM), showing that both geopolymers were constituted of unreacted particles surrounded by amorphous and semi-amorphous products. Through Fourier transform infrared spectroscopy (FTIR), a band that was assigned to the asymmetric stretching vibration of Si-O-M (M = Na + and / or Ca 2 + ) non-bridging oxygen type was observed, which suggested that Na and Ca could serve as exchangeable ions in the ionic exchange process. Adsorption test data indicated that good adsorption was obtained when a neutral pH was used at room temperature, and the adsorption isotherm showed that GA had more adsorption sites than GS, which meant greater maximum adsorption capacity.


Introduction
Urbanization and industrialization have led to serious problems for the environment due to the heavy metal pollution in wastewater. It is estimated that industrial activity in México will generate a wastewater volume that is close to 2.1 millions of square meters by 2030 [1]. Heavy metals are highly hazardous pollutants, because of their high toxicity, non-degradability, and tendency to accumulate in living organisms [1][2][3][4]. Several studies have identified industrial wastewater sources containing lead, which include metal smelting processes, the manufacture of lead-based paints, metallurgy, mining, and alloys [5,6]. Waste emissions from these industries are the main cause of lead contamination in food, as they accumulate in animal tissues and organs and, subsequently, go into the production The raw BFS was supplied with agglomerated particles with average size of 3.7 cm, and they were subjected to a size reduction process by grinding; this process was carried out in a rotating balls mill with nineteen steel ball of 540 g each for a period of 2 h; with this grinding time, finer and non-agglomerated particles are obtained that facilitate the synthesis of geopolymers [39]. The BFS resulted in milling, and the FA were sectioned with sieving (45 µm ASTM E11-95 [40]) to obtain a surface area of 1.79 m 2 /g and 1.37 m 2 /g for the BFS and FA, respectively.
For the alkali activator solution (AAS), a mix of sodium hydroxide (NaOH) pellets with ±97% purity, containing sodium carbonate (Na 2 CO 3 ) in 1% at 10 M and sodium silicate solution (Na 2 O 3 Si) with a modulus (SiO 2 /Na 2 O ratio) of 2.8 was used. The mix was obtained according to a mass ratio (Na 2 OSiO 3 /NaOH) of 2.
Aluminum hydroxide (Al(OH) 3 ) containing aluminum oxide (Al 2 O 3 ) between 50% and 57.5% was used as the source of aluminum (AS).
Distilled water with a pH between 6 and 7.5 and total dissolved solids between 2 and 5 mg/L was used as the solvent for NaOH.
The aqueous Pb 2+ solution was prepared by dissolving Pb(NO 3 ) 2 in deionized water (standard solution). The concentration was diluted before the adsorption experiment. The pH value of the Pb 2+ solution was adjusted while using 0.1 M HNO 3 and 0.1 M NaOH.

Geopolymer Synthesis
BFS and FA were individually combined with AS, and then mechanically mixed with the AAS to create a geopolymer slurry that was adjusted to a mass molar ratio Si/Al of 3. BFS-based geopolymer slurry have molar ratios SiO 2 /Al 2 O 3 = 5.26, Na 2 O/SiO 2 = 0. 48  After it was mixed until homogeneous (about 1 min.), the geopolymer slurry was cast into a plastic mould (2 × 2 × 2 cm), and then subjected to hydrothermal curing at 60 • C for 24 h until it achieved geopolymerization, and was then subjected to water-submerged curing at 25 • C for another six days. The geopolymer samples were then dried in oven at 100 • C for 24 h, cooled, ground, sieved (150 µm ASTM E11-95 [40]), and labelled as GS (BFS-based geopolymer) and GA (FA-based geopolymer).

Microstructure Analysis
Materials crystallinity was determined by X-Ray diffraction (XRD) while using a Bruker D8 Advanced diffractometer (Centro de Investigación en Materiales Avanzados S.C.(CIMAV), Chihuahua, Chih., México). XRD analysis was performed in the 2θ range of 15 • to 70 • in order to verify the change from the crystalline phases of the raw materials to geopolymers. Fourier transform infrared spectroscopy (FTIR) was carried out using tablets mixed with KBr in a Bruker spectrophotometer Minerals 2020, 10, 808 4 of 17 model ATR Tensor 27 and the analysis was carried out in the frequency range of 4000-400 cm −1 to identify the functional groups of BFS, GS, FA, and GA. Peak deconvolution using Gaussean peak fitting was performed.
For SEM observation, hardened samples fragments were coated with gold to obtain the necessary conductivity to acquire the secondary electron images, and to observe porosity, microcracks, and crystals that are present in geopolymer matrix. The geopolymer morphology was obtained by SEM while using a Hitachi SU3500 scanning electron microscope (Advanced Materials Research Center (CIMAV), Chihuahua, Chih. México), at 3000×, 7000×, 13,000×, and 30,000×.
The texture properties were measured by N 2 adsorption via a Brunauer-Emmett-Teller (BET) method using a Quantachrome Autosorb automated gas sorption system.

Cation Exchange Capacity (CEC)
The materials cation exchange capacity was determined by the Na + ion saturation with 1.0 N sodium acetate (CH 3 COONa), and subsequently a Na + ion exchange by NH 4+ with 1.0 N ammonia acetate (CH 3 COONH 4 ). The CEC was estimated by an equation following [41]: where:

Batch Adsorption Operation
Lead adsorption was investigated in batch tests at a constant temperature of 25 • C. The tests were carried out, as follows: 0.5 g of geopolymer sample was added to 40 mL of Pb 2+ solution with a different initial concentration (5,10,20,50,100,200, and 500 mg/L) at pH values that ranged from 2 to 8 for a period of time of six days.
For each test, the lead concentration was determined by atomic absorption spectroscopy (AAS), while using the atomic absorption spectrophotometer GBC Avantar 4324 (Universidad de Guanajuato, Gto., México), and calculating the uptake percentage using the following equation [4]: where: C 0 = Initial concentration of heavy metal solution (mg/L) C e = Remaining equilibrium or final concentration (mg/L) The adsorption amount was calculated using the following equation [42]: where: Thus, once the value of q was obtained for each C e value, those values were correlated to the Langmuir-Freundlich [43] model and their adsorption isotherms were obtained.  Figure 1 presents the XRD pattern of BFS and GS. In the patterns, the main peak is located around 30 • 2θ, which corresponds to Calcite (CaCO 3 ) and it is associated with a large amount of CaO in raw BFS and its interaction with CO 2 in the environment. This phase constitutes most of the BFS and GS. The other carbonate found is magnesite (MgCO 3 ) around 33 • 2θ due to a small but significant amount of MgO in the raw material. Calcium is linked to Si and Al, forming calcium aluminosilicate, also called gehelenite (Ca 2 Al(SiAl)O 7 ). Finally, a small peak around 27 • 2θ is attributed to Quartz (SiO 2 ). Because all of these phases appear in both BFS and GS, it is possible to note that the crystalline structure of the raw material did not completely dissolve during the geopolymerization process. However, it is possible to observe wide dispersion peaks and a hump between 25-40 • 2θ identifying the amorphous structure of several typical metallurgical slags [44] and synthetized geopolymers, like GS [39,45] the amorphous structure of several typical metallurgical slags [44] and synthetized geopolymers, like GS [39,45]. In GS, it can observe new phases calcium aluminosilicate hydrated gel (CASH), such as chabasite-Ca (Al3.6Ca1.76H19.74O33.87Si8.4) and chantalite (Al2CaH4O8Si), with triclinic and tetragonal structures, respectively, as well as sodium aluminosilicate hydrated (NASH) gel in a phase, called Analcime (Al2H4Na1.862O13.667Si4), with an isometric crystal structure [46]. On the other hand, Figure 2 presents the patterns of FA and GA. It is possible to observe that the FA is composed of quartz (SiO2) corresponding to a peak around 27° 2θ and mullite (Al6Si2O13). After geopolymerization, the ions of these phases were redirected in an activated process and formed NASH-type gels, such as analcime (Al2H4Na1.862O13.667Si4) and ussingite (AlHNa2O9Si3), with orthorhombic and triclinic structure, respectively.

Mineralogical Analyses
GS and GA both present phases that contain Al in their structure. This could indicate a greater number of exchange sites in the geopolymer, as the Al in the structure represents the sites where metal ions are exchanged for compensating ions in the solid matrix (Na and Ca in this case). Table 2 summarizes the crystalline phases identified using XRD [47][48][49][50][51].   On the other hand, Figure 2 presents the patterns of FA and GA. It is possible to observe that the FA is composed of quartz (SiO 2 ) corresponding to a peak around 27 • 2θ and mullite (Al 6 Si 2 O 13 ). After geopolymerization, the ions of these phases were redirected in an activated process and formed NASH-type gels, such as analcime (Al 2 H 4 Na 1.862 O 13.667 Si 4 ) and ussingite (AlHNa 2 O 9 Si 3 ), with orthorhombic and triclinic structure, respectively. GS and GA both present phases that contain Al in their structure. This could indicate a greater number of exchange sites in the geopolymer, as the Al in the structure represents the sites where metal ions are exchanged for compensating ions in the solid matrix (Na and Ca in this case). Table 2 summarizes the crystalline phases identified using XRD [47][48][49][50][51]. Figure 3 shows the spectra that were obtained for BFS and GS. In both spectrums, it can be observed the bands between 500 and 700 cm −1 associated with the vibrations of Si-O-Si or O-Si-O bonds, and both are attributed to Quartz. In raw material, there is a band at 541 cm −1 , which was Minerals 2020, 10, 808 6 of 17 assigned to the symmetric stretches of the Si-O bonds, and the 710 cm −1 band is generated by in-plane bending vibrations of the same kind of bond, as well as the 690 cm −1 band in GS. Both types of vibration correspond to quartz [52].       The aluminosilicate characteristic band is located around 1000 cm −1 . This band indicates asymmetric stretches on the Si-O-Si or Si-O-Al bonds of unreacted raw material. In BFS, it is situated at 1018 cm −1 and, in GS, it is displaced to 998 cm −1 . This behavior is because a three-dimensional network of Si-O bonds has not been well formed, as the glassy structure of the BFS is enriched with Ca and a large amount of this element is present at early ages in the solution. It consumes the Si and Al units, as they react preferentially with Ca to form a CASH gel instead of accumulating continuously. Figure 4 represents the deconvolution analysis of BFS and GS and shows the changes of molecular vibration that were present in the aluminosilicate characteristic band. The amorphous structure and the BFS intermolecular bonds produce more bands that together form a broad band. GS forms a tight band instead. In addition, because the position of this band depends on the Si/Al ratio of the product formed, the band moves at lower frequencies due to the rise in the tetrahedral aluminum content [51,[53][54][55][56][57][58][59]. The replacement of Si 4+ by Al 3+ results in the T-O-T (T = Si or Al) angle becoming more acute, and the band changing to lower frequencies because the Al-O bond is longer than the Si-O bond. The degree of substitution of the Si atoms by Al atoms has an important effect in the direction of the displacement of this band. On the other hand, bands located at wavenumbers 876 cm −1 in the raw material and at 879 cm −1 in GS may be attributed to the vibration in non-bridging oxygen bonds with Si-O-M (M = Na + , Ca 2+ ). Na + ions are produced by the alkali activator (NaOH + Na 2 O 3 Si), and the Ca 2+ ions are produced by the main component in BFS (CaCO 3 ). These two ions are the interchangeable ions in the geopolymer. They may be released from the structure and leave a space for the metal ion (Pb 2+ ), whereby its presence is essential to the efficiency of the ion exchange mechanism [60][61][62].      1076 cm −1 and around 1007 cm −1 in GA. In addition to the BFS-based geopolymers, we can observe a small but important displacement to a lower signal in the deconvolution study in Figure 6. FA presents different bands that, just like BFS, could indicate the Si-O-Si or Si-O-Al bonds in aluminosilicate phases; once the geopolymerization is carried out, the number of bans decreases and GS and GA show a broad band instead, which may indicate that the Si-O-Si or Si-O-Al bonds correspond to amorphous NASH and CASH gel phases. The case of GA indicates that the Si and Al units were consumed by a high amount of Na, contributed by alkali activator, in order to form NASH type gel.

Fourier Transform Infrared Spectroscopy (FT-IR)
Minerals 2020, 10, x FOR PEER REVIEW 8 of 17 occurs in GA. After geopolymerization, this band becomes broader, which is possibly due to an increase in water molecules by the hydrolysis process effect. In general, it is possible to observe in all materials that the most intense band corresponds to the asymmetric stresses of Si-O-Si or Al-O-Al bonds. This is because BFS and FA mainly conform to aluminosilicates. In the BFS case, the main component is CaO in CaCO3 form. For this reason, the carbonates band (around 1400 cm −1 ) is more intense in BFS than in FA.   Figure 7 shows SEM images for GS. In images "a" and "b," we can observe a predominantly spherical shape with a smooth surface. This is the typical morphology of BFS after grinding with a rotating ball mill. The slag is composed of crystalline phases and, during the grinding, the bonds between the molecules and atoms are broken, thus leaving the shape of the surface broken [64]. The BFS grains are covered with amorphous and irregular structures. In image "c", it is possible to distinguish unreacted calcite grains, covered by amorphous CASH type gels. At last, in image d, irregular phases can be found, but with defined orthorhombic type forms, which can be attributed to the Analcime phase that is found in XRD studies. Analcime has relatively structured unit cells that are based on non-intersecting channels with four, six, and eight members of oxygen rings, and 16 sites occupied by sodium in the smallest 24 cavities, and by water molecules located in the 16 largest channels [46]. occurs in GA. After geopolymerization, this band becomes broader, which is possibly due to an increase in water molecules by the hydrolysis process effect. In general, it is possible to observe in all materials that the most intense band corresponds to the asymmetric stresses of Si-O-Si or Al-O-Al bonds. This is because BFS and FA mainly conform to aluminosilicates. In the BFS case, the main component is CaO in CaCO3 form. For this reason, the carbonates band (around 1400 cm −1 ) is more intense in BFS than in FA.   Figure 7 shows SEM images for GS. In images "a" and "b," we can observe a predominantly spherical shape with a smooth surface. This is the typical morphology of BFS after grinding with a rotating ball mill. The slag is composed of crystalline phases and, during the grinding, the bonds between the molecules and atoms are broken, thus leaving the shape of the surface broken [64]. The BFS grains are covered with amorphous and irregular structures. In image "c", it is possible to distinguish unreacted calcite grains, covered by amorphous CASH type gels. At last, in image d, irregular phases can be found, but with defined orthorhombic type forms, which can be attributed to the Analcime phase that is found in XRD studies. Analcime has relatively structured unit cells that are based on non-intersecting channels with four, six, and eight members of oxygen rings, and 16 sites occupied by sodium in the smallest 24 cavities, and by water molecules located in the 16 largest channels [46]. However, the atmospheric carbonation band is located in GA 1428 cm −1 . This band is caused by vibrations in the O-C-O bonds and it indicates the presence of sodium bicarbonate in the samples. Finally, the bands caused by vibration in H-O-H and -OH bonds were also present in the geopolymers in the raw material. This bands corresponds to physically and chemically bonded water the same as occurs in GA. After geopolymerization, this band becomes broader, which is possibly due to an increase in water molecules by the hydrolysis process effect.

Scanning Electron Microscopy (SEM)
In general, it is possible to observe in all materials that the most intense band corresponds to the asymmetric stresses of Si-O-Si or Al-O-Al bonds. This is because BFS and FA mainly conform to aluminosilicates. In the BFS case, the main component is CaO in CaCO 3 form. For this reason, the carbonates band (around 1400 cm −1 ) is more intense in BFS than in FA. Figure 7 shows SEM images for GS. In images "a" and "b," we can observe a predominantly spherical shape with a smooth surface. This is the typical morphology of BFS after grinding with a rotating ball mill. The slag is composed of crystalline phases and, during the grinding, the bonds between the molecules and atoms are broken, thus leaving the shape of the surface broken [64]. The BFS grains are covered with amorphous and irregular structures. In image "c", it is possible to distinguish unreacted calcite grains, covered by amorphous CASH type gels. At last, in image d, irregular phases can be found, but with defined orthorhombic type forms, which can be attributed to the Analcime phase that is found in XRD studies. Analcime has relatively structured unit cells that are based on non-intersecting channels with four, six, and eight members of oxygen rings, and 16 sites occupied by sodium in the smallest 24 cavities, and by water molecules located in the 16 largest channels [46]. Thus, we can prove that the geopolymer is formed by crystalline, semi-crystalline, and amorphous phases; generally, the percentages of unreacted BFS covered by geopolymerization products, which will be in charge of giving the material the ion exchange property [65].  Figure 8 shows the SEM images for GA, by imaging that it is possible to observe irregular and amorphous structures, which are attributed to the reaction products obtained after geopolymerization (NASH type gels). In this image, it is possible to appreciate a hollow sphere shape, which is characteristic of FA structure. However, not many of these shapes are observed that can indicate a high reactivity of raw material. In image "b", the semi-spherical particles completely surrounded by the reaction product and NASH type gels. In image "c" and "d", we can observe irregular particles with triclinic structure. This structure is attributed to the ussignite phase [66]. Thus, we can prove that the geopolymer is formed by crystalline, semi-crystalline, and amorphous phases; generally, the percentages of unreacted BFS covered by geopolymerization products, which will be in charge of giving the material the ion exchange property [65]. Figure 8 shows the SEM images for GA, by imaging that it is possible to observe irregular and amorphous structures, which are attributed to the reaction products obtained after geopolymerization (NASH type gels). In this image, it is possible to appreciate a hollow sphere shape, which is characteristic of FA structure. However, not many of these shapes are observed that can indicate a high reactivity of raw material. In image "b", the semi-spherical particles completely surrounded by the reaction product and NASH type gels. In image "c" and "d", we can observe irregular particles with triclinic structure. This structure is attributed to the ussignite phase [66]. Minerals 2020, 10, x FOR PEER REVIEW 10 of 17

Physical Adsorption of Gases by the BET Method
The specific surface of GS is 23.56 m 2 /g and the pore volume and size were 73 cm 3 /kg and 7.8 nm, respectively, surpassing by about 26-fold the surface of the raw FA. By about 13-fold the surface of the raw BFS. In the case of GA, the specific surface was 35.97 m 2 /g and the pore volume and size were 124 cm 3 /Kg and 9 nm, respectively, surpassing by about 26-fold the surface of the raw FA.
The increase in specific surface with respect to raw material corresponds to an addition of aluminum for adjusting the Si/Al ratio of 3, and also corresponds to the grinding process after geopolymerization. It favors the adsorption mechanism, as there is an increase in exchange sites in the geopolymer.

CEC
CEC depends on the number of available exchange sites in the geopolymer. According to the experimental results, GS and GA have an exchange capacity of 241.30 and 286.96 Meq/100 g, respectively. In the DRX and MEB results, it was observed that the generation of phases, such as Analcime, Chantalite, Chabasite-Ca, and Ussignite, which had a high aluminium content, promoted the generation of a large number of exchange sites, and, hence, favored the adsorption capacity of the geopolymer.

Adsorption Potential for Pb 2+ Ions
FA and BFS, according to Nguyen et al., can adsorb for itself positive metal ions by Coulombic forces on negatively charged particles at pH above 3 in FA and above pH 6 in BFS; it is important to mention that the pHPZC of each of these materials is 3 and 6, respectively; however, adsorption by other forces can occur at all pHs independent of the PZC [67], it is possible that this mechanism occurs in geopolymers due to the presence of unreacted phases, like quartz, in the particles. The attractive forces of positively charged metal ions into GA and GS are due to relatively weak Van Der Waals forces and at active sites due to ion exchange of Ca + and Na + ions that are present in the geopolymer

Physical Adsorption of Gases by the BET Method
The specific surface of GS is 23.56 m 2 /g and the pore volume and size were 73 cm 3 /kg and 7.8 nm, respectively, surpassing by about 26-fold the surface of the raw FA. By about 13-fold the surface of the raw BFS. In the case of GA, the specific surface was 35.97 m 2 /g and the pore volume and size were 124 cm 3 /Kg and 9 nm, respectively, surpassing by about 26-fold the surface of the raw FA.
The increase in specific surface with respect to raw material corresponds to an addition of aluminum for adjusting the Si/Al ratio of 3, and also corresponds to the grinding process after geopolymerization. It favors the adsorption mechanism, as there is an increase in exchange sites in the geopolymer.

CEC
CEC depends on the number of available exchange sites in the geopolymer. According to the experimental results, GS and GA have an exchange capacity of 241.30 and 286.96 M eq /100 g, respectively.
In the DRX and MEB results, it was observed that the generation of phases, such as Analcime, Chantalite, Chabasite-Ca, and Ussignite, which had a high aluminium content, promoted the generation of a large number of exchange sites, and, hence, favored the adsorption capacity of the geopolymer.

Adsorption Potential for Pb 2+ Ions
FA and BFS, according to Nguyen et al., can adsorb for itself positive metal ions by Coulombic forces on negatively charged particles at pH above 3 in FA and above pH 6 in BFS; it is important to mention that the pH PZC of each of these materials is 3 and 6, respectively; however, adsorption by other forces can occur at all pHs independent of the PZC [67], it is possible that this mechanism occurs in geopolymers due to the presence of unreacted phases, like quartz, in the particles. The attractive forces of positively charged metal ions into GA and GS are due to relatively weak Van Der Waals forces and at active sites due to ion exchange of Ca + and Na + ions that are present in the geopolymer structure. The high specific surface area due to the porosity of GA and GS and volume pore are factors influencing the adsorption capacity [68]. Figure 9 shows the adsorption test data at 2, 4, 6, and 8 pH. The maximum removal of lead (approximately 95% in GS and 99% in GA) was reached at a pH of 6, and it decreased at a pH of 8. According to the lead speciation graph, from about 6 pH, hydroxide species are formed, which cause precipitation on the geopolymer surface and block access to geopolymer pores for lead ions.
(approximately 95% in GS and 99% in GA) was reached at a pH of 6, and it decreased at a pH of 8. According to the lead speciation graph, from about 6 pH, hydroxide species are formed, which cause precipitation on the geopolymer surface and block access to geopolymer pores for lead ions.
At a pH of 2 and 4, lead is in the ionized form in solution, and this allows access through the pores and, therefore, access to exchange sites by Van Der Waals forces, which indicates that geopolymers enhance their adsorption capacity in acidic media [69]. Figure 10 shows the effect of the initial Pb 2+ concentration (C0) on geopolymer adsorption capacity. In GS, adsorption capacities that are greater than 90% are obtained up to 100 mg/L concentration, due to the high CEC and porosity caused by aluminium-rich phases (analcime, chantalite, and chabasite-Ca). At 5 mg/L concentration, a removal of 95% is reached, which increases to 99.9% at 20 mg/L. Subsequently, as the concentration increases, the removal percentage decreases by 20%. In GA, the uptake increases to 10 mg/L, reaching approximately 98%. At 20 and 50 mg/L, there is a decrease, and at 100, 200, and 500, an increase. The uptake obtained at 20 and 50 are inconsistent, probably due to an error in the pH adjustment [42].   At a pH of 2 and 4, lead is in the ionized form in solution, and this allows access through the pores and, therefore, access to exchange sites by Van Der Waals forces, which indicates that geopolymers enhance their adsorption capacity in acidic media [69]. Figure 10 shows the effect of the initial Pb 2+ concentration (C 0 ) on geopolymer adsorption capacity. In GS, adsorption capacities that are greater than 90% are obtained up to 100 mg/L concentration, due to the high CEC and porosity caused by aluminium-rich phases (analcime, chantalite, and chabasite-Ca). At 5 mg/L concentration, a removal of 95% is reached, which increases to 99.9% at 20 mg/L. Subsequently, as the concentration increases, the removal percentage decreases by 20%. In GA, the uptake increases to 10 mg/L, reaching approximately 98%. At 20 and 50 mg/L, there is a decrease, and at 100, 200, and 500, an increase. The uptake obtained at 20 and 50 are inconsistent, probably due to an error in the pH adjustment [42].

Langmuir-Freundlich Adsorption Isotherm
The equilibrium isotherm shows the quantity of adsorbate (Pb 2+ ) that can be adsorbed by the adsorbent (GS and GA) in proportion to the equilibrium concentration in liquid phase. Furthermore, the equilibrium curve helps to explain certain phenomena that are associated with the adsorbate-adsorbent interaction [42,70]. The initial equilibrium curve shape of GS ( Figure 11) and GA ( Figure 12) have been identified as the L2 and S1 types, respectively, according to the Giles classification [71], and the best mathematical model that represents GS is the Langmuir model, and the Freundlich model for GA. In this study, the equilibrium isotherm used was the Langmuir-Freundlich (R 2 = 0.992, GS and R 2 = 9544, GA). The L-type shape indicates that the higher the concentration, the greater the adsorption amount until the available exchangeable sites reach their limit. Thereby, there is a competition between the metal ions for the available sites, and this kind of isotherm indicates that adsorption occurs because relatively weak forces as Van Der Waals. In the subclass 2 curves, there is no intermolecular interaction between the solute, which forms a long plateau that is represented by the maximum adsorption capacity (q m ), indicating a saturation of the adsorbent monolayer before the additional adsorption at new sites, and the process becomes highly favorable, this Pb 2+ adsorption in geopolymers based FA was reported by Al-Zboon et.al [72]. A high energy barrier is necessary, and this indicates that q m depicts the maximum number of exchangeable sites, in this case, 38.20 mg/g. The S-type shape shows the opposite behavior to the L-type, because the orientation tends to be vertical for solute molecules at higher concentrations, and more sites are available for adsorption, which agreed with the CEC results. Subclass 1 of an S-type isotherm indicates completely vertical behavior of adsorption capacity, which is possibly caused by the precipitation of the solute on the adsorbent surface causing the curve slope to increase [46]. GA has a q m of 83.20 mg/g, according to the Langmuir-Freundlich model.

Langmuir-Freundlich Adsorption Isotherm
The equilibrium isotherm shows the quantity of adsorbate (Pb 2+ ) that can be adsorbed by the adsorbent (GS and GA) in proportion to the equilibrium concentration in liquid phase. Furthermore, the equilibrium curve helps to explain certain phenomena that are associated with the adsorbateadsorbent interaction [42,70]. The initial equilibrium curve shape of GS ( Figure 11) and GA ( Figure  12) have been identified as the L2 and S1 types, respectively, according to the Giles classification [71], and the best mathematical model that represents GS is the Langmuir model, and the Freundlich model for GA. In this study, the equilibrium isotherm used was the Langmuir-Freundlich (R 2 = 0.992, GS and R 2 = 9544, GA). The L-type shape indicates that the higher the concentration, the greater the adsorption amount until the available exchangeable sites reach their limit. Thereby, there is a competition between the metal ions for the available sites, and this kind of isotherm indicates that adsorption occurs because relatively weak forces as Van Der Waals. In the subclass 2 curves, there is no intermolecular interaction between the solute, which forms a long plateau that is represented by the maximum adsorption capacity (qm), indicating a saturation of the adsorbent monolayer before the additional adsorption at new sites, and the process becomes highly favorable, this Pb 2+ adsorption in geopolymers based FA was reported by Al-Zboon et.al [72]. A high energy barrier is necessary, and this indicates that qm depicts the maximum number of exchangeable sites, in this case, 38.20 mg/g. The S-type shape shows the opposite behavior to the L-type, because the orientation tends to be vertical for solute molecules at higher concentrations, and more sites are available for adsorption, which agreed with the CEC results. Subclass 1 of an S-type isotherm indicates completely vertical behavior of adsorption capacity, which is possibly caused by the precipitation of the solute on the adsorbent surface causing the curve slope to increase [46]. GA has a qm of 83.20 mg/g, according to the Langmuir-Freundlich model.

Conclusions
GS and GA are comprised of crystalline, semi-crystalline, and amorphous phases. The geopolymerization process generates CASH-and NASH-type gels, which are driven by high Ca and Na content, thus generating active sites for the ion exchange process.
GA generates NASH type gels only, and it has higher amorphicity and, hence, a more specific surface, porosity, and CEC.
Adsorption isotherms accorded well with the Langmuir-Freundlich adsorption isotherm model, and the maximum adsorption capacities for GS and GA were 38.20 and 83.20 mg/g, respectively.
Additionally, adsorption occurs because of the relatively weak Van Der Waals forces in the reaction, allegedly by the ion exchange of Ca + and Na + present in the geopolymer structure.
The Pb 2+ adsorption is strongly affected by pH; better adsorption is obtained when a solution with neutral pH is used at room temperature. Funding: This research was funded by Fondo Sectorial de Investigación para la Educación SEP-CONACYT, grant number A1-S-15401 and the APC was funded by grant number A1-S-15401.