Study on the heavy metal immobilization mechanism in the alkali-activated red mud-ground granulated blast furnace slag-based geopolymer

ABSTRACT

This study presents an investigation into the challenge of alleviating heavy metal pollution while using red mud (RM) as an industrial byproduct, focusing on its application in the preparation of geopolymers. Synthesis RM-ground granulated blast furnace slag (GGBS)-based geopolymer (RMG) and studied with particular attention to optimizing compressive strength through modifying key parameters: RM content, Na₂SiO₃ modulus, and water-to-binder ratio. The immobilization of heavy metals, particularly lead (Pb) and copper (Cu), within geopolymer was thoroughly examined. Results indicate that optimal compressive strength was achieved at a 40 wt.% RM content, a Na₂SiO₃ modulus of 1.8, and a water-to-binder ratio of 0.65, with 28-day compressive strengths reaching 36.9 MPa. A 1% mass of heavy metals was observed to improve the mechanical characteristics of the geopolymer; however, beyond this threshold resulted in detrimental effects. The immobilization capabilities of RMG under various environmental conditions were robust, with immobilization efficiencies exceeding 97% for Pb and 94% for Cu. The immobilization mechanism was found to involve physical encapsulation, with Cu uniquely forming covalent bonds with non-bridging oxygens within the polymeric structure, creating stable Si-O-Cu bonds. This study highlights the potential of geopolymer as a viable technology for mitigating environmental impacts associated with RM disposal by effectively immobilizing heavy metals, thus facilitating safe and sustainable resource utilization. This work contributes to the field by demonstrating a novel approach to the valorization of industrial waste, offering a promising solution for the management of RM while addressing the critical issue of heavy metal pollution.

Keywords:
Red mud; Ground granulated blast furnace slag; Compressive strength; Solidification/stabilization technology; Immobilization mechanism

1. INTRODUTION

The generation of industrial waste has experienced a significant increase in recent years, primarily driven by rapid urbanization and industrial growth. However, much of this waste is still handled unsafely or insufficiently, and it is frequently disposed of using crude techniques like stockpiling or landfilling [¹]. This practice results in excessive disposal costs in addition to the waste of precious land resources. Moreover, the waste’s heavy metal content may seep into the soil and water, endangering human health and causing ecological contamination [¹,²,³,⁴]. Stabilization/solidification (S/S) technology prevents or reduces the migration of pollutants, allowing harmful waste to be transformed into harmless forms. A traditional approach to dealing with heavy metal waste is cement stabilization [⁵, ⁶]. It is known for its ability to produce huge quantities of material on a wide scale and its adaptability to different raw materials [¹,⁷]. However, there are certain drawbacks to this method, including high CO₂ emissions [⁸,⁹,¹⁰,¹¹], and insufficient effectiveness in solidifying heavy metals [¹², ¹³]. Consequently, a critical area of research within the domain of heavy metal stabilization technology is the exploration of durable alternative cementitious materials [¹⁴, ¹⁵].

Geopolymer is a category of inorganic binders that use silicon dioxide and aluminum oxide-rich compounds as raw materials under alkaline activation [¹⁶]. It can form a three-dimensional network structure, effectively encapsulating heavy metal [¹⁷, ¹⁸]. In contrast to conventional binders, especially ordinary Portland cement (OPC), geopolymers demonstrate benefits including reduced energy consumption [¹⁸], improved durability [¹⁹, ²⁰], and superior immobilization of heavy metals [²¹, ²²]. Materials commonly used to synthesize geopolymers include fly ash [²³, ²⁴], metakaolin [²⁵, ²⁶], silica fume [²²], and sludge [²⁷]. However, the imbalance in waste utilization can increase costs for sourcing these solid wastes. As a result, industrial solid waste that is low-cost and easily accessible is more appropriate for the synthesis of geopolymers.

RM is an industrial byproduct produced during alumina mining from bauxite, causing serious environmental concerns when disposed of improperly [²⁸]. RM is produced 1.25 tons for every ton of alumina [²⁹]. RM is currently mostly stored via a process of dry storage, which entails first removing the majority of the moisture through mechanical pressure filtering and then allowing the residual material to dry naturally. Unfortunately, this approach can pollute nearby air and water resources and necessitate large land resources. Many academics are researching the wide range of uses of RM [³⁰], with particular attention to its ability to recover important metals like iron, titanium, gallium, and scandium. RM has also been used in the construction sector, where it is used as geopolymers [³¹], roadbed materials [³²], ceramic tiles [³³], cement [³⁴], and other applications. The elevated alkalinity of RM renders it an optimal adsorbent for heavy metals and other contaminants in soil and wastewater [³⁵]. It has also shown promise as an effective reagent for remediating acidic environments [²⁹]. RM is also critical for achieving environmental cleanup in bauxite mining areas. The elevated alumina and silica content of RM renders it a suitable precursor for geopolymers [¹⁶]. Utilizing RM to synthesize geopolymers can successfully address the issue of waste of land resources and environmental pollution. RM serves as an appropriate precursor for geopolymer synthesis owing to its elevated silica and alumina content [¹⁶]. Using mechanical activation, chemical activation, and thermal activation, several researchers have tried to increase the reactivity of RM [³⁶,³⁷,³⁸]. YE et al. [³⁹] treated RM with an alkaline thermal activation process to create a single-component geopolymer, which, after 28 days had a compressive strength of 31.5 MPa. Scholars have studied the effect of activation temperature on RM, and the research shows that under the thermal activation condition of 700 °C, the Si-O and Al-O bonds in aluminosilicates of RM will break, leading to a decrease in polymerization degree and an increase in activity index from 0.69 to 0.85 [⁴⁰]. Synthesized geopolymers using municipal solid waste incineration fly ash and RM, which can effectively solidification heavy metals in the raw materials [¹⁷].

This study evaluates the feasibility of employing RM and GGBS for geopolymer synthesis. The study examines how the RM content, Na₂SiO₃ modulus, and water-to-binder ratio affect the geopolymers’ compressive strength. The effectiveness of immobilizing Pb²⁺ and Cu²⁺ in various environmental settings is assessed, emphasizing the immobilization capacities of each compound. Furthermore, using multiple characterization methods, the immobilization mechanisms of the Pb²⁺ and Cu²⁺ within geopolymers are discussed.

2. MATERIAIS AND METHODS

2.1. Materials

The raw materials for synthesizing geopolymers were commercially available GGBS, S95 grade gray-white powder. RM from Guangxi Xinfa Aluminum Group (Baise, Guangxi province of China). Dry and grind the RM, then sieve it through a 200 mesh (74 μm diameter) screen to achieve uniformity and fineness. The geopolymer precursor particle size distribution was detecte by laser particle sizer (Mastersizer 3000, Malvern, UK), and the results show in Figure 1. RM was sieved through 200 mesh, and it is evident from the Figure 1 that its particle sizes were all less than 74 μm, with the largest proportion of 2.76 μm particles and a D50 of 4.09 μm. The particle sizes of S95 grade GGBS were all less than 40 μm, with the largest proportion of 7.64 μm particles and a D50 of 5.52 μm. The diminutive particle size of the raw materials indicates that RM and GGBS possess a larger quantity of active sites to facilitate the synthesis of geopolymers in alkaline environments.

Figure 1
Raw material particle size distribution and cumulative particle size distribution: (a) RM; (b) GGBS.

RM and GGBS were analyzed for mineralogical composition and chemical fractions by X-ray diffraction (XRD) and X-ray fluorescence (XRF) spectrometers. The results are displayed in Figure 2 and Table 1. The XRD of the RM showed that its mineral phase was mainly hematite. This work utilizes thermal activation to augment the reactivity of RM, given that haematite is an inert mineral. The RM is subjected to calcination in a muffle furnace, with the temperature incrementally raised from 0°C to 800°C, followed by a calcination duration of 2 hours. The broad peaks between 22° and 40° indicate the presence of an amorphous phase. The XRF results show that the main compounds of RM are 42.10% Fe₂O₃, 17.80% CaO, 11.80% SiO₂, and 13.60% Al₂O₃, and the GGBS main compounds are 43.10% CaO, 29.20% SiO₂, and 24.30% Al₂O₃. The electron microscopy of RM and GGBS is shown in Figure 3. The particles of GGBS are more irregular than those of RM, and the edges of the particles have many edges, which may be favorable for geopolymer formation.

Figure 2
Raw material XRD patterns.

Table 1
Chemical fractions of RM and GGBS (%).

COMPOSITION	Fe2O3	Al2O3	SiO2	CaO	MgO2	NaO2	TiO2	P2O5	SiO3
RM	42.10	13.60	11.80	17.80	0.90	5.19	7.05	0.37	0.22
GGBS	0.42	14.30	29.20	43.10	8.15	0.51	0.94	–	2.4

Figure 3
Raw material electron micrograph: (a)RM; (b)GGBS.

2.2. Mixing ratios of geopolymers and preparation of specimens

Different RM-based geopolymers were prepared by adjusting the RM content, Na₂SiO₃ modulus, and water-to-binder ratio. The specific ratio design of the geopolymers is shown in Table 2. The process of preparation is as follows: first, the modulus of the original Na₂SiO₃ solution was 3.3 (SiO₂/Na₂O molar ratio = 3.3), and the concentration was 34%. The modulus of Na₂SiO₃ was adjusted to 1.3, 1.5, 1.8 and 2.0 by the addition of NaOH, while the concentration was maintained at 34% by the addition of deionised water, the details shown in Table 3. A magnetic stirrer was then used for 20 minutes to ensure that the NaOH particles were fully dissolved into the Na₂SiO₃ solution. The Na₂SiO₃ solution mixture generates much heat during the stirring process, so it was necessary to seal the mixture to prevent excessive water evaporation. A certain mass of RM and GGBS was first weighed for thorogh mixing, then Na₂SiO₃ solution was added for stirring. The slurry was mixed slowly for 3 minutes using a slurry mixer and then quickly for 5 minutes. To eliminate air bubbles, the blended slurry was put into a silicone mold measuring 40 × 40 × 40 mm and vibrated for two minutes. In the curing phase, the samples were initially cured at 60°C for 24 hours. This curing process is crucial for enhancing the mechanical properties of the material, significantly improving the initial strength of the geopolymer. The samples are relocated to the environment and maintained at ambient temperature until the designated period for subsequent mechanical tests. Figure 4 displays the preparation process flow chart.

Table 2
The specific ratio design of the RM-based geopolymer.

NO.	RM (WT%)	GGBS (WT%)	MODULUS	W/B
R20	20	80	1.5	0.6
R40	40	60	1.5	0.6
R60	60	40	1.5	0.6
R80	80	20	1.5	0.6
M1.3	40	60	1.3	0.6
M1.5	40	60	1.5	0.6
M1.8	40	60	1.8	0.6
M2.0	40	60	2.0	0.6
W0.50	40	60	1.8	0.50
W0.55	40	60	1.8	0.55
W0.60	40	60	1.8	0.60
W0.65	40	60	1.8	0.65

Table 3
The details of modulus adjustment of the Na₂SiO₃ solution.

MODULUS	Na2SiO3 (g)	NaOH (g)	DEIONISED WATER (g)
1.3	100	16.08	31.22
1.5	100	15.54	24.35
1.8	100	8.71	16.91
2.0	100	6.80	13.19

Figure 4
Preparation process of geopolymer.

This work also seeks to examine the impact of heavy metals Pb and Cu on the mechanical characteristics of RM-based geopolymers. Furthermore, it aims to clarify the processes via which these heavy metals were immobilized in the matrix of geopolymers. To achieve this, different quantities of Pb²⁺ and Cu²⁺ were incorporated into the geopolymers, employing their corresponding nitrates, Pb(NO₃)₂ and Cu(NO₃)₂, at ratios of 0.6%, 0.8%, 1.0%, and 1.2% of the total mass of RM and GGBS. The nitrates were initially dissolved in deionized water before incorporation into the powdered mixture, then together with a Na₂SiO₃ solution and powdered mixture in a stirring vessel, ensuring comprehensive mixing to produce a homogeneous RM-based geopolymer including the heavy metals. Table 4 presents the design of the mixing ratios for the geopolymers, encompassing those with significant metal additions and blank controls.

Table 4
The design mixing ratios of the geopolymers introducing heavy metals and the blank controls.

NO.	RM (WT.%)	GGBS (WT.%)	MODULUS	W/B	PB2+ (WT.%)	CU2+ (WT.%)
W0.65	40	60	1.8	0.65	0	0
0.6 Pb	40	60	18	0.65	0.6	0
0.8 Pb	40	60	1.8	0.65	0.8	0
1.0 Pb	40	60	1.8	0.65	1.0	0
1.2 Pb	40	60	1.8	0.65	1.2	0
0.6 Cu	40	60	18	0.65	0	0.6
0.8 Cu	40	60	1.8	0.65	0	0.8
1.0 Cu	40	60	1.8	0.65	0	1.0
1.2 Cu	40	60	1.8	0.65	0	1.2
0.6 PbCu	40	60	18	0.65	0.3	0.3
0.8 PbCu	40	60	1.8	0.65	0.4	0.4
1.0 PbCu	40	60	1.8	0.65	0.5	0.5
1.2 PbCu	40	60	1.8	0.65	0.6	0.6

2.3. Geopolymer performance testing

2.3.1. Mechanical performance test

After 3d, 7d, and 28d of curing, the samples were subjected to compressive testing. The compression rate was 2 mm/min until failure occurred. The sample’s compressive strength is determined by averaging three tests conducted in parallel.

2.3.2. Heavy metal leaching test

After curing of 28d, the geopolymers containing heavy metals were subjected to grinding and subsequently passed through a 2 mm sieve. Following the Chinese national standards HJ/T300-2007 and HJ/T299-2007, acetic acid and nitric acid sulfate were employed as leaching agents to facilitate the extraction of heavy metals from the geopolymer matrix. The leaching conditions of heavy metals are detailed in Table 5. The heavy metal concentration of the bleaching agent was detected using inductively coupled plasma optical emission spectroscopy (ICP-OES). The efficiency of the geopolymer in immobilizing heavy metals was evaluated by calculating the heavy metal immobilization efficiency (IE) using the formula outlined in Equation (1). This calculation provides insights into the performance of the geopolymer as a potential material for environmental remediation and waste management applications.

Table 5
The leaching conditions of heavy metals.

STANDARD	LEACHING AGENTS	PH	LIQUID/SOLID RATIO (ML/G)	SHAKING TIME (H)	TEMPERATURE (°C)
HJ/T300-2007	acetic acid	2.64 ± 0.05	20:1	8 h	25 ± 2
HJ/T299-2007	sulfuric acid nitrate	3.20 ± 0.05	10:1	8 h	25 ± 2

(1)$$IE=\frac{m_0-C\times V}{m_0}\times 100\%$$

In this context, m₀ denotes the initial heavy metal content within the geopolymer matrix before leaching. The symbol C indicates the concentration of Pb or Cu present in the leachate, which is the liquid that has come into contact with the geopolymer and has the potential to dissolve and transport contaminants. Finally, V denotes the volume of the leachate collected during the leaching process.

2.3.3. Characterization method

Analyze the mineral composition of geopolymer through XRD. Employing fourier transform infrared spectroscopy (FTIR) to examine the properties of chemical bonds. Scanning electron microscopy with energy dispersive spectrometry (SEM-EDS) was used to study the micromorphology and elemental distribution. Moreover, X-ray photoelectron spectroscopy (XPS) was utilized to examine the valence states and binding energies in geopolymers.

3. RESULTS AND DISCUSSIONS

3.1. Mechanical performance analysis of RM-based geopolymer

3.1.1. The compressive strength of RM-based geopolymer

Figure 5 displays the results of compressive strength. Figure 5(a) shows that as the proportion of RM content increases, the strength of the geopolymer decreases after an initial increase. The highest compressive strength emerges when the RM content is 40 wt.%. At 3d, 7d, and 28d, the strength reached 25.2 MPa, 27.4 MPa, and 30.3 MPa, respectively. When the RM content exceeds 40 wt.%, the strength of RMG sharply decreases. The compressive strength of R60 and R80 at 28d was 16.9 MPa and 8.9 MPa, respectively, reflecting reductions of 44.2% and 70.7% in comparison to R40. The aluminosilicate components in RM have lower cementitious activity than GGBS. Inert minerals like hematite can reduce the proportion of reactive substances in the raw materials, negatively impacting the polymerization reaction [⁴¹].

Figure 5
Compressive strength of geopolymer sample: (a) RM content, (b) Na₂SiO₃ modulus, and (c) water-to-binder rati

In Figure 5(b), with the modulus of Na₂SiO₃ rises, the compressive strength increases slightly before rapidly decreasing. At the modulus of Na₂SiO₃ was 1.8, the compressive strength at 3d, 7d, and 28d reached 30.7 MPa, 32.1 MPa, and 35.5 MPa, respectively. However, when the Na₂SiO₃ modulus was lower than 1.8, the excess alkali content led to the weathering and brittleness of the RMG. In contrast, when the modulus exceeds 1.8, the alkali content decreases, which hinders the activation of the active ingredients and inhibits the polymerization reaction [⁴²]. Therefore, it is crucial to accurately determine the appropriate modulus of Na₂SiO₃ to obtain optimal compressive strength results.

Figure 5(c) shows a positive correlation between compression strength and the water-to-binder ratio. Due to the fine particles of RM and GGBS, improving the water-to-binder ratio impacts the migration rate of geopolymers and promotes the depolymerization of the raw material. As a result, geopolymer has the maximum compressive strength at a water-to-binder ratio of 0.65, and the compressive strength at 3d, 7d, and 28d attained 32.2 MPa, 35.1 MPa, and 36.9 MPa, respectively.

3.1.2. The impact of heavy metals on geopolymers’ compressive strength

Figure 6 shows the compressive strength of geopolymers introduced with different levels of heavy metals and its blank control group at 3d, 7d, and 28d. In the control group W0.65, the compressive strength was 36.9 MPa at 28d. While added heavy metals in geopolymer, the compressive strength decreased slightly. The compressive strength showed a tendency to rise after the addition of heavy metals but then to decline as the addition of Pb and Cu increased. When the heavy metal content is 1%, the geopolymer attains its peak compressive strength. The compressive strengths of 1% Pb, 1% Cu, and 1% PbCu at 28d are 29.8 MPa, 28.1 MPa, and 26.1 MPa, respectively. When the heavy metal content exceeds 1%, the strength decreases, mainly due to the reaction between heavy metal Pb²⁺ and Cu²⁺ with -OH, which reduces the alkalinity of the solution. Both the polymerization of [SiO₄] and [AlO₄] and the release of active silicon and aluminum from geopolymer precursor materials are hampered due to the reduction of alkaline conditions. An increased trend occurs when the heavy metal level is less than 1%, suggesting that the addition of Pb and Cu alters the geopolymer’s composition. This alteration is primarily caused by the hydroxide precipitation and silicate formation in the geopolymers, which increases the mechanical strength of the geopolymers.

Figure 6
Effect of heavy metals on compressive strength.

3.2. RM-based geopolymer heavy metal immobilization

Figure 7 illustrates the efficiency of heavy metal immobilization in RM-based geopolymers cured for 28 days across different leaching conditions. The results indicate that as the additional heavy metals rise, Pb and Cu’s immobilization efficiency in the RM-based geopolymer diminishes progressively. The immobilization efficiency of these two heavy metals consistently exceeds 94%, indicating that geopolymers are effective at immobilization heavy metals. In acetic acid solution, the curing efficiency of Pb is as high as 99.14%, the curing efficiency of Cu is 97.12%, and the curing efficiency of PbCu is as high as 98.78%. There is a certain difference in the immobilization effect of different heavy metals, and the Pb is better than that of Cu. This is explained by the fact that the Cu element has a smaller molar mass than the Pb element. Under the same mass, Cu requires more solidification sites, resulting in more leaching of Cu, so its solidification efficiency will be lower than that of Pb. Simultaneously adding heavy metals Pb and Cu. It is evident that Pb’s immobilization efficiency is still superior to Cu’s. An increase in the pH value of the leachate in a solution of sulfuric acid and nitric acid makes it easier for heavy metals to stick to geopolymers. This is primarily attributable to the tendency of certain Pb and Cu to create hydroxide precipitates in alkaline conditions, which are easily dissolved under acidic conditions, making geopolymers more effective in fixing heavy metals in alkaline environments.

Figure 7
Immobilization efficiency of Pb and Cu in geopolymers: (a) acetate buffer solution; (b) sulfuric acid nitrate.

3.3. XRD analysis

Figure 8 displays the XRD patterns of geopolymers with different RM contents and added heavy metals. In Figure 8 (a), hematite is evident in all geopolymers, indicating that it is a persistent phase that is not easily soluble in alkaline environments [⁴³, ⁴⁴]. In all RMGs, a wide peak between 22° and 38° can be observed, which is due to the geopolymer gel and C-S-H gel during hydration [³⁷, ⁴⁵]. In alkaline environments, reactive silica, aluminum, and calcium phases of the raw materials dissolve to form new phases. It is worth noting that C-S-H gel is the main reaction product of geopolymer. Early studies showed that the adhesive synthesized from calcium-rich materials (such as GGBS) can produce C-S-H gel when activated with an alkaline solution [⁴⁶].

Figure 8
XRD patterns of RM-based geopolymers, (a) different RM contents, (b) addition of different heavy metal contents.

Figure 8(b) shows the variations in geopolymer’s mineral composition with and without the incorporation of heavy metals. Because of the high chemical stability of hematite in RM, it maintains its integrity even after polymerization. Wide peaks can also be observed between 22° and 38° (2θ), indicating the formation of amorphous polymers. The image also shows that adding heavy metals will not result in the geopolymer producing additional diffraction peaks, indicating that silicate-based gel products remain the primary source of the geopolymer’s strength. Additionally, typical peaks associated with precipitation Pb and Cu hydroxides are absent from the XRD spectra of geopolymers due to the low crystallinity of some phases and the low detection limit of XRD. Next, we will further analyze the immobilization forms of Pb and Cu in geopolymers from the perspective of chemical bonding.

3.4. FTIR analysis

The chemical bonding of the raw material and geopolymer samples was examined by FTIR spectroscopy, and the results are presented in Figure 9. The comparative study facilitates the examination of alterations in the chemical bonding of the geopolymer with varying quantities of RM and differing heavy metal contents. The raw material RM has a peak at 432 cm⁻¹, however, in the geopolymer, it transitions to a higher vibrational frequency of 447 cm⁻¹. This peak is indicative of the bending vibration of the Si-O bond, and this transition results from the alteration in structural order during the geopolymerization process [⁴⁷]. The stretching vibration of -OH was detected at 3446 cm⁻¹ in RM, moving to 3446–3458 cm⁻¹ in geopolymer with varying RM concentrations, and increasing to 3450 cm⁻¹ following the incorporation of heavy metals.The bending vibration of the H-O-H bond at 1593 cm⁻¹ shifts to 1597–1652 cm⁻¹ in geopolymer with varying RM concentrations and increases to 3456 cm⁻¹ with the incorporation of heavy metals. This finding suggests that Ca(OH)₂ participates in the hydration reaction. The stretching vibration of O-C-O at 1421–1440 cm⁻¹ confirms the presence of carbonization in the geopolymer. The interval of 979 to 1001 cm⁻¹ pertains to the stretching vibrations of the Si-O-T structure (T denotes Si or Al), and the peak values in this range also indicate the extent of polymerization of the geopolymer.

Figure 9
FTIR profiles:(a) raw material; (b) geopolymer samples with different RM content and heavy metals at 28d.

3.5. SEM-EDS analysis

Figure 10 is the microstructure morphology and elemental characteristics of RM-based geopolymer, and the EDS results are shown in Table 6. The SEM shows that the geopolymers have a dense microstructure. The dense microstructure of these geopolymers, primarily composed of C-S-H and C-A-S-H gels, serves as the foundation for geopolymers’ strength. The emergence of inevitable cracks can be ascribed to the shrinkage phenomenon observed during the compression test. The EDS results show that the geopolymer matrix mainly comprises O, Al, and Si. These three elements connect to form a three-dimensional O-Si-O-Al-O-Al network structure characteristic. Notably, both Pb and Cu were detected in the geopolymer samples, suggesting the successful immobilization of heavy metal ions within geopolymers. Adding Cu to the sample with 1.0 Cu changed the elements of sodium, aluminum, and silicon in a big way, but this change was less intense in the sample with 1.0Pb. This disparity indicates that heavy metal Pb solidification through geopolymers predominantly depends on physical encapsulation and chemical bonding [¹²].

Figure 10
SEM-EDS investigation of heavy metals with and without addition: (a) W0.65; (b) 1.0 Pb; (c) 1.0 Cu; (d) 1.0 PbCu.

Table 6
Percentage composition of each element in EDS analysis (atomic ratio, %).

SCAN POINT	O	Na	Al	Si	Pb	Cu
#1	55.62	10.45	13.09	20.83	/	/
#2	67.96	26.84	4.15	11.05	/	/
#3	46.33	10.89	15.82	25.81	1.15	/
#4	59.11	4.94	14.73	20.16	1.06	/
#5	55.99	28.61	6.03	7.90	/	1.47
#6	55.02	27.54	4.12	12.66	/	0.66
#7	63.25	12.07	11.46	11.50	0.29	0.87
#8	53.54	17.31	6.59	21.84	0.38	0.35

Figures 11, 12, and 13 show the elemental mapping results of geopolymers with different heavy metals added. From these figures, it can be seen that in the plate-like structure, both Pb and Cu elements are uniformly distributed in the geopolymer. However, in Figure 11, there are microstructural features similar to flakes and particles in 1.0Pb, and the distribution of Pb elements avoids these areas. This indicates that the structural features resembling flakes and particles are not conducive to the fixation of Pb in geopolymers.

Figure 11
SEM-EDS elemental mapping of the 1.0 Pb geopolymer sample.

Figure 12
SEM-EDS elemental mapping of RMG samples with 1.0% Cu contained.

Figure 13
SEM-EDS mapping of RMG samples with 1.0% PbCu contained.

3.6. XPS analysis

Figure 14 displays the XPS wide-scan results of W 0.65, 1.0 Pb, 1.0 Cu, and 1.0 PbCu. The elements detected in the geopolymer include C, O, Si, Al, Na, Pb, and Cu. The appearance of element C further suggests the carbonization of geopolymer samples in the air.

Figure 14
The XPS wide-sweep spectra of RMG.

Figures 15 and 16 show the narrow-scan results of XPS for Si2p and Al2p, respectively. The introduction of heavy metals Pb²⁺ and Cu²⁺ causes a gradual increase in the binding energy of Si2p and Al2p. In the W0.65 sample, the binding energy for Si2p is measured at 102.18 eV, while Al2p is recorded at 74.08 eV. Upon the addition of heavy metals, these values shift; in the 1.0Pb sample, Si2p and Al2p binding energy increase to 102.38 eV and 74.18 eV, respectively; in the 1.0Cu sample, the values are similarly 102.38 eV and 74.28 eV; and in the 1.0 PbCu sample, they shift to 102.28 eV and 74.28 eV. These changes indicate a modification in the electronic environment surrounding these elements owing to the presence of heavy metals.

Figure 15
The spectra of binding energy for Si2p in RMG: (a) W0.65; (b) 1.0 Pb; (c) 1.0 Cu; (d) 1.0 PbCu.

Figure 16
The spectra of binding energy for Al2p in RMG: (a) W0.65; (b) 1.0 Pb; (c) 1.0 Cu; (d) 1.0 PbCu.

Figure 17 shows the XPS narrow scan results of O1s. In geopolymers, O1s spectra can be roughly divided into three categories: Si-O-Si/Al (siloxane bond), Si-O-Na (nonbridging oxygen), and Si-OH (silanol bond) [⁴⁷, ⁴⁸]. O1s (peak1) represents Si-OH (silanol bond), and O1s (peak2) represents Si-O-Si/Al (siloxane bond). Due to the introduction of Pb²⁺ and Cu²⁺, the peak of O1s has subtle changes. In the 1.0Pb sample, the O1s (peak1) improved from 532.28 eV to 532.85 eV and O1s (peak2) improved from 531.18 eV to 531.30 eV. In the 1.0 Cu sample, the O1s (peak1) improved from 532.28 eV to 533.08 eV and O1s (peak2) improved from 531.18 eV to 531.28 eV. In the 1.0 PbCu sample, the O1s (peak1) improved from 532.28 eV to 533.18 eV and O1s (peak2) improved from 531.18 eV to 533.18 eV. Due to the Pb²⁺ and Cu²⁺ having a higher ionic potential, after introducing the heavy metals, it applies attractive force to electrons around Si, Al, and O elements in geopolymers. This electronic attraction reduces the electron cloud density around Si, Al, and O elements, thereby weakening the shielding effect of electrons on atomic nuclei. This increases the interaction between atomic nuclei and electrons in Si, Al, and O, increasing binding energy [⁴⁸, ⁴⁹].

Figure 17
The spectra of binding energy for O1s in RMG: (a) W0.65; (b) 1.0 Pb; (c) 1.0 Cu; (d) 1.0 PbCu.

Figure 18 shows the narrow scan results of the binding energies of Pb4f and Cu2p. Figure 18 (a) shows the Pb4f peaks of 1.0 Pb and 1.0 PbCu at 143.18 eV and 138.28 eV, which are associated with Pb4f5/2 and Pb 4f7/2, respectively. These peaks reflect the binding energy characteristics of Pb-O bonds in geopolymer structures [¹³, ¹⁸, ²³]. Figure 18 (b) illustrates that the Cu2p peaks observed at 953.38 eV and 933.48 eV, peaks are associated with Cu-O, suggesting that Cu is present as Cu-O-T and Cu(OH)₂. The interaction of Cu and the [SiO₄] structure results in the formation of Cu-O structures, at the same time, some Cu-O interacts with the Si-O framework in the geopolymer, resulting in the creation of a Si-O-Cu structure that is embedded within the geopolymer matrix. Furthermore, a minor fraction of Cu exists as a free Cu-O structure within the geopolymers. Additionally, some of the heavy metal Cu precipitates as insoluble hydroxides [⁴⁸, ⁵⁰].

Figure 18
The XPS narrow-sweep spectra of RMG: (a) Pb 4f; (b) Cu 2p.

3.7. The immobilization mechanism of Pb and Cu in RMG

The immobilization of Pb and Cu in geopolymer is a complex process involving physical and chemical interactions. The ways of immobilization heavy metals in geopolymers include physical encapsulation, adsorption effect, chemical bonding, chelation, precipitation reaction, and complexation reaction. The immobilization effect of geopolymers on heavy metals is mainly attributed to physical encapsulation and chemical bonding. Figure 19 shows two immobilization forms: physical encapsulation and chemical bonding.

Figure 19
Schematic diagram of geopolymer immobilization heavy metals.

During the polymerization process of geopolymers, aluminosilicates dissolve in alkaline solutions. Then, Si and Al undergo a condensation reaction to form an amorphous three-dimensional network structure. These newly formed gels effectively encapsulate Pb and Cu in geopolymer gel. The dense microstructure of the gel product acts as an effective barrier, significantly inhibiting the migration and release of encapsulated heavy metal species [⁵¹]. Even under different environmental conditions, this barrier can maintain its integrity. Therefore, the physical encapsulation mechanism plays a crucial role in significantly reducing the leaching of heavy metal elements from geopolymer matrices. In addition, the immobilization of heavy metal ions in the RM-based geopolymer matrix is mainly achieved through the formation of covalent Si-O-M or Al-O-M bonds, where M represents heavy metal cations. These chemical interactions significantly enhance structural stability and reduce the mobility of the added heavy metal species.

4. CONCLUSIONS

This study aims to synthesize a novel geopolymer using RM and GGBS to achieve safe disposal of solid waste. Firstly, the effects of different RM contents, Na₂SiO₃ modulus, and water-to binder ratio on the compressive strength of geopolymers were studied. Secondly, evaluate the immobilization effect of geopolymer on heavy metals through leaching tests and analyze the heavy metal immobilization mechanism. The findings and conclusions of this study are summarized as follows.

(1)

As the RM content gradually increases, the strength of the geopolymer first increases and then decreases. When the RM content is 40 wt.%, the Na₂SiO₃ modulus and water-cement ratio are 1.8 and 0.65, the 28d compressive strength of the geopolymer attains its maximum value of 36.9 MPa. The introduction of Pb²⁺ and Cu²⁺ will have adverse effects on the polymerization process, manifested as a decrease in compressive strength.

(2)

The immobilization efficiency of geopolymer for Pb²⁺ and Cu²⁺ in acetic acid environment is as high as 99.88% and 97.78%, respectively. The immobilization efficiency of Pb²⁺ and Cu²⁺ will decrease with the introduction of heavy metals, but the immobilization efficiency is above 93%. In the environment of sulfuric acid nitric acid, the curing efficiency of Pb²⁺ and Cu²⁺ is above 97.97% and 94.76%, respectively. The curing efficiency of Pb²⁺ is higher than that of Cu²⁺, which can be attributed to the difference in atomic weight and ionic radius.

(3)

The XRD results indicate that introducing Pb²⁺ and Cu²⁺ does not result in new crystalline phases, and Pb²⁺ and Cu²⁺ are physically encapsulated and immobilization within the geopolymer. The FTIR results also indicate that the chemical bond properties of the geopolymer have not undergone significant changes. The results of SEM-EDS indicate that the structure of the geopolymer is compact, and Pb²⁺ and Cu²⁺ are successfully immobilization in the geopolymer. The results of XPS indicate that oxygen-containing groups play a crucial role in the immobilization of heavy metals. Cu²⁺ is immobilization in the form of Cu-O-Si in the geopolymer.

In summary, this study not only optimized the mechanical properties of RM-based geopolymer, but also revealed their potential in heavy metal immobilization mechanism, providing a new perspective and solution for solid waste resource utilization and environmental protection.

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Publication Dates

• Publication in this collection
  17 Feb 2025
• Date of issue
  2025

History

• Received
  08 Nov 2024
• Accepted
  27 Dec 2024