This review provides a comprehensive analysis of the role of chemical agents in enhancing the performance of geopolymers for the stabilization and adsorption of heavy metals. Geopolymers, synthesized from aluminosilicate sources activated under alkaline conditions, are recognized for their versatile structural and environmental benefits, including low carbon emissions and high chemical resistance. Their unique Si-O-Al framework supports both stabilization/solidification (S/S) and adsorption processes, making them an ideal polymeric matrix for the immobilization of hazardous heavy metals in contaminated environments. The review categorizes the heavy metal immobilization mechanisms into physical encapsulation, ion exchange, hydroxide precipitation, and chemical complexation, depending on the specific metal species and geopolymer formulation. The introduction of chemical stabilizing agents, such as dithiocarbamate, sodium sulfide, and trimercaptotriazine, significantly improves the encapsulation efficacy of geopolymers by promoting targeted reactions and stable metal complexes. These agents enable the effective S/S of metals, such as lead, cadmium, and chromium, reducing their leachability and environmental impact. In addition to solid waste management applications, geopolymers have shown promising adsorption capabilities for aqueous contaminants, with chemical modifications further increasing their affinity for specific heavy metals. This review evaluates the impact of different agents and synthesis conditions on the overall performance of geopolymers in heavy metal immobilization, highlighting advances in environmental applications and future research directions for sustainable hazardous waste treatment.

This study explores the potential of metakaolin-based geopolymers, activated using sodium hydroxide and sodium silicate, for the solidification and stabilization of heavy metals present in galvanic sludge—a hazardous industrial waste rich in chromium (Cr), nickel (Ni), and iron (Fe). The research investigates factors affecting the cold consolidation of the pastes, such as NaOH molarity (8 or 10 M) and time of preparation of activating solutions (24 h in advance or soon before the fresh paste preparation), the sequence of experimental steps (the sludge added to the fresh paste or to the powder of metakaolin) and amount of waste (10 or 20 per cent by weight over metakaolin). The final products were characterized by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), and environmental scanning electron microscopy (ESEM). Mechanical performance and durability assessments, including compressive strength and water stability tests, were conducted to evaluate the suitability of the geopolymer for construction applications. Leaching tests according to EU regulation demonstrated promising heavy metal immobilization, highlighting the effectiveness of the geopolymerization process in reducing metal leachability. It was found that the factors affecting immobilization are more evident for Cr than for Ni, whose immobilization percentages are very high. In particular, it was observed that preparing the mixture by adding sludge after metakaolin activation increased Cr immobilization from 83% to 89%. Similarly, preparing the activating solution 24 h before mixing the sludge and geopolymer increased the percentage from 89 to 95.

Alkali-activated aluminosilicate matrices are increasingly studied for their ability to stabilize hazardous metal contaminants via alkali activation at room temperature. In this study, metakaolin-based geopolymers were used to immobilize chromium and nickel salts, with systematic variation of key synthesis parameters, Na/Al molar ratio, metal concentration, anion type, and alkaline solution aging time, which have not been previously studied. A Design of Experiments approach was employed to study the effect of factors on metal leaching behavior and to better understand the underlying immobilization mechanisms. The analysis revealed that higher Na/Al ratios significantly enhance geopolymerization and reduce metal release, as supported by FTIR spectral shifts and decreased shoulder intensity. Notably, aging time had an influence on chromium behavior due to its effect on early silicate network formation, which can hinder the incorporation of chromium species. All tested formulations achieved metal immobilization rates of 98.8% or higher for both chromium and nickel. Overall, this study advances our understanding of geopolymer-based heavy metal immobilization.

Hazardous galvanic sludge waste (GSW) from the electroplating industry, produced at 100,000–150,000 tonnes/year in the EU and containing high concentrations of Cr and Ni was successfully treated using metakaolin-based geopolymers via Stabilization/Solidification (S/S). The experimental design incorporated chelating (sodium diethyl dithio carbamate, C₅H₁₀NS₂Na, DTC), reducing (sodium sulfide, Na₂S), and adsorbing (hydroxyapatite, Ca₅(PO₄)₃(OH), Hap) agents separately to improve heavy metal immobilization. The results demonstrated that Na₂S drastically decreased Cr release by −98.7% by reducing mobile Cr(VI) to insoluble Cr(III). DTC reduced Ni leaching by −93.4%, forming sparingly soluble Ni(II)(DTC)₂ complexes that precipitated within the matrix. Hap enhanced Ni retention by 55.5% via cation exchange but was ineffective for Cr due to electrostatic repulsion with the anion Cr(VI)O₄²⁻ at the geopolymer’s high pH. This work is the first to apply geopolymerization coupled with these chemical agents for S/S of as-received galvanic waste, offering a highly efficient, low-carbon strategy to manage this hazardous industrial residue.

Today, several conventional wastes (fly ash, ground granulated blast furnace slags, etc.) are used as valid precursors for geopolymer synthesis. However, there are several new wastes that can be studied to replace geopolymer precursors. This study investigates the behavior of four industrial wastes—suction dust (SW1), red mud (SW2), electro-filter dust (SW3), and extraction sludge (SW4)—as 20 wt.% substitutes for metakaolin in geopolymer synthesis. The objective is to assess how their incorporation before alkali activation affects the structural, thermal, mechanical, chemical, and antimicrobial properties of the resulting geopolymers, namely GPSW1–4. FT-IR analysis confirmed successful geopolymerization in all samples (the main Si-O-T band underwent redshift, confirming Al incorporation in geopolymer structures after alkaline activation), and stability tests revealed that none of the GPSW1–4 samples disintegrated under thermal or water stress. However, GPSW3 showed an increase in efflorescence phenomena after these tests. Moreover, compressive strength was reduced across all waste-containing geopolymers (from 22.0 MPa for GP to 12.6 MPa for GPSW4 and values lower than 8.1 MPa for GPSW1–3), while leaching tests showed that GPSW1 and GPSW4 released antimony (127.5 and 0.128 ppm, respectively) above the legal limits for landfill disposal (0.07 ppm). Thermal analysis indicated that waste composition influenced dehydration and decomposition behavior. The antimicrobial activity of waste-based geopolymers was observed against E. coli, while E. faecalis showed stronger resistance. Overall, considering leaching properties, SW2 and SW3 were properly entrapped in the GP structure, but showed lower mechanical properties. However, their antimicrobial activity could be useful for surface coating applications. Regarding GPSW1 and GPSW4, the former needs some treatment before incorporation, since Sb is not stable, while the latter, showing a good compressive strength, higher thermal stability, and leaching Sb value not far from the legal limit, could be used for the inner reinforcement of building materials.

This study investigates the potential of six illite-rich clay samples from the Lower Cretaceous outcrops of Zemlet El Beidha (southern Tunisia) as precursors for geopolymer synthesis. Τhe raw materials are predominantly illitic, although kaolinite remains a main reactive phase. Samples BHG ₂ and BHG ₅ were specifically selected for further investigation due to their relatively high kaolinite content compared to the remaining samples and due to their distinct secondary mineralogy, with BHG ₂ being rich in hematite and BHG ₅ being rich in calcite. This selection allowed for a comparative analysis of how different impurities affect geopolymerization. The samples were calcined at 550°C, 750°C and 950°C to determine the optimal dehydroxylation temperature. Alkali activation was systematically performed using 10, 12 and 14 M NaOH solutions to assess the influence of alkalinity on the polycondensation process. The 12 M NaOH was most effective for the dissolution of aluminosilicate phases without the detrimental effects of excess sodium. Structural characterization (X-ray diffraction, Fourier-transform infrared spectroscopy and scanning electron microscopy) confirmed that the greatest compressive strength (∼8 MPa) was achieved with BHG ₅ calcined at 750°C. Although this strength is lower than that of kaolinite-based geopolymers, it meets the requirements for non-structural applications, such as thermal insulation or lightweight masonry units. This research demonstrates a viable pathway for valorizing abundant illitic-kaolinitic Tunisian clays into sustainable construction materials.