Effect of PFDS on the immobilization of Cs+ by metakaolin-based geopolymers in complex environments.

Metakaolin-based geopolymers are very promising materials for improving the safety of low and intermediate level radioactive waste disposal, with respect to ordinary Portland cement, due to their excellent immobilization performance for Cs+ and superior chemical stability. However, their application is limited by the fact that the leaching behavior of Cs+ is susceptible to the presence of other ions in the environment. Here, we propose a way to modify a geopolymer using perfluorodecyltriethoxysilane (PDFS), successfully reducing the leaching rate of Cs+ in the presence of multiple competitive cations due to blocking the diffusion of water. The leachability index of the modified samples in deionized water and highly concentrated saline water reached 11.0 and 8.0, respectively. The reaction mechanism between PDFS and geopolymers was systematically investigated by characterizing the microstructure and chemical bonding of the material. This work provides a facile and successful approach to improve the immobilization of Cs ions by geopolymers in real complex environments, and it could be extended to further improve the reliability of geopolymers used in a range of applications.

Additive manufacturing of geopolymers with hierarchical porosity for highly efficient removal of Cs.

Geopolymers (GPs) have emerged as promising adsorbents for wastewater treatment due to their superior adsorption stability, tunable porosity, high adsorption capacity, and low-energy production. Despite their great promise, developing GPs with well-controlled hierarchical structures and high porosity remains challenging, and the mechanism underlying the ion adsorption process remains elusive. Here we report a cost-effective and universal approach to fabricate Na or K GPs with sophisticated architectures, high porosity, and arbitrary cation species exchange by means of additive manufacturing and a surfactant. The introduction of sodium lauryl sulfate (SLS) enhanced the porosity of the GP adsorbents, yielding NaGP-lattice-10%SLS adsorbent with a high total porosity of 80.8 vol%. Combining static and dynamic adsorption tests, the effects of morphology, surfactant content, and cation species on Cs+ adsorption performance were systemically investigated. With an initial Cs+ concentration of 900 mg/L, the printed NaGP exhibited a maximum Cs+ adsorption capacity of 80.1 mg/g, outperforming other adsorbents reported so far. The quasi-second-order fit of the NaGP adsorbent showed overall higher R2 values than the quasi-first-order fit, indicating that the adsorption process was dominated by ion exchange. Combined with first-principles calculations, we verified that the content of water in the GP sod cages also affected the ion-exchange process between Na+ and Cs+.

Hydrothermal transformation of geopolymers to bulk zeolite structures for efficient hazardous elements adsorption.

This paper reports a facile route to prepare bulk zeolites with tunable phase compositions and microstructures by combining hydrothermal treatment and geopolymer precursor technique. Amorphous Na-based geopolymer (NaGP) is transformed into crystalline analcime following hydrothermal treatments. By systematically investigating the effects of hydrothermal conditions on the phase compositions and microstructures of the products, the optimal hydrothermal procedure is screened as treating NaGP in 1 M NaOH solution at 160 °C for 6 h. Furthermore, we achieve control over phase compositions of the resulting bulk zeolites by tailoring the initial Na/K ratio of geopolymer precursors. For instance, treating the geopolymer precursor with a Na/K ratio of 9: 1 under the optimal hydrothermal procedure leads to the formation of zeolite consisting of analcime and zeolite-P. The as-prepared adsorbents exhibit outstanding adsorption performance for the hazardous elements, among which analcime-zeolite-P shows an adsorption efficiency of 93.3% for Cs+, and NaGP exhibits an adsorption efficiency of 99.6% for Sr2+. Moreover, we reveal the mechanisms underlying the adsorption of Cs+ and Sr2+ in the adsorbents to be chemisorption. Meanwhile, ion exchanges also occur in NaGP and analcime-zeolite-P during Cs+ adsorption. These results render geopolymers and their derived bulk zeolites promising for hazardous elements adsorption.

Interplay between storage temperature, medium and leaching kinetics of hazardous wastes in Metakaolin-based geopolymer.

Notwithstanding the success of carbon-free nuclear power in the new energy industry, an effective way to address nuclear contamination is still lacking. As a generally accepted method to date, immobilization of radioactive nuclear wastes with inexpensive and environment-friendly matrix such as geopolymer has attracted extensive attention. In this contribution, Na-based and K-based metakaolin geopolymer were prepared to encapsulate simulated radioactive Cs+ and Sr2+ under different temperatures and environments during long-term leaching tests. The temperature-dependent and environmental-dependent leaching kinetics as well as their dominant leaching mechanisms have been revealed. The results showed that Na-based and K-based geopolymer exhibited better immobilization performance than that of Portland cement, ceteris paribus. For the immobilization of Cs+, Na-based geopolymer showed lower leaching rate than K-based geopolymer under the same leaching conditions. Both higher temperature and salt solution accelerated the leaching behaviors of Sr+ and Cs2+ from the encapsulation matrix. This contribution sheds light on understanding the dominant leaching mechanisms of hazardous elements under different storage environments and highlights the significance of salt-tolerant matrix for the immobilization of nuclear wastes.

A green and low-cost hollow gangue microsphere/geopolymer adsorbent for the effective removal of heavy metals from wastewaters.

In this paper, hollow gangue microspheres (GM) were introduced into a geopolymer matrix through a geopolymeric method; our aim was to synthesize a green and low-cost adsorbent (GM/KGP) for the removal of heavy metal ions (Cu2+, Cd2+, Zn2+, and Pb2+) from aqueous solutions. We investigated the microstructure of the GM/KGP adsorbent, as well as the effects of adsorbent dose, time, and temperature on adsorption behavior; moreover, an adsorption mechanism was proposed. The GM/KGP adsorbent possessed a typical broad amorphous structure and abundant O-containing functional groups on its surface. The adsorption of Cu2+, Cd2+, Zn2+, and Pb2+ onto the GM/KGP adsorbent fitted well to the pseudo-second-order kinetic model, while the equilibrium isotherm adsorption data were fitted well to the Langmuir equation. The adsorption mechanism GM/KGP was attributed to physical, chemical, and electrostatic attractions, as well as to ion exchange. We conclude that this novel adsorbent has great potential in removing heavy metal ions from contaminated wastewater.

Safe trapping of cesium into doping-enhanced pollucite structure by geopolymer precursor technique.

Safe trapping of radioactive nuclear waste such as 137Cs has become a grim issue with the popularity of nuclear power and the growing yield of nuclear waste. To address this intractable problem, pollucite has been screened out as an ideal material for the long-term storage of 137Cs due to its low leaching rate and good chemical and thermal stability. With an effort to further enhance its immobilization capability and practical values, we herein report a robust route to prepare pollucite from ion-doped geopolymer at low temperature (≤1000 °C), achieving high Cs content (33.37 wt.%) and the lowest leaching rate of Cs (2.51 × 10-4 g m-2 d-1) to date. Meanwhile, the leaching mechanism of Cs in pollucite is revealed via leaching tests coupled with SEM-EDS analysis. Therefore, this contribution may provide an alternative route to preparing pollucite and open up new possibilities in the immobilization of 137Cs for real-world applications.

Seed-mediated growth of ultra-thin triangular magnetite nanoplates.

We report a simple strategy for the growth of ultra-thin magnetite nanoplates. The injection of a large portion of precursor after stunted nucleation is favorable for both the survival of metastable structures in seeds and their subsequent development into anisotropic nanoparticles. The as-synthesized ultrathin triangular magnetite nanoplates are expected to have important applications as T1 contrast agents for magnetic resonance imaging.