Immobilization mechanism of cesium in geopolymer: Effects of alkaline activators and calcination temperature.

Geopolymer is always regarded as a promising material for the immobilization of radioactive waste. In the present study, the stabilization of Cs in geopolymers activated by NaOH and Na2SiO3 solutions and calcined at various temperatures was studied via toxicity characteristic leaching procedure (TCLP), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscope and energy dispersive spectroscopy (SEM-EDS), solid-state nuclear magnetic resonance (SSNMR), and N2 adsorption-desorption isotherm. For both NaOH-activated and Na2SiO3-activated geopolymers, the leaching concentrations of Cs decreased with the increase of calcination temperature. Specifically, most of the amorphous substance was crystallized to nepheline at 1000 °C for NaOH-activated geopolymer, and Cs+ can be incorporated into the structure of nepheline, contributing to the reduction of Cs leaching concentration. However, the amorphous structure was still maintained for Na2SiO3-activated geopolymer even after calcination at 1000 °C. It has been deduced that the main structure of Na2SiO3-activated geopolymer after calcination at 1000 °C should be in short-range order and Cs+ can be locked in a micro "crystal" structure. In addition, the change of specific surface area was not fully consistent with the decreasing trend of Cs leaching concentration. Therefore, the inner structure and the specific surface area of geopolymer should have a combined effect on the leaching behavior of Cs. This study can provide new insights into the application of geopolymer to immobilize radionuclides.

The Carboxyl Functionalized UiO-66-(COOH)2 for Selective Adsorption of Sr2.

Efficient and selective removal of 90Sr is an important process for the safe use of nuclear energy. Herein, we investigate and assess the Sr2+ adsorption properties of a metal-organic framework UiO-66-(COOH)2 functionalized by non-bonded carboxylic groups. This MOF is an exciting class of free carboxylic functionalized MOFs that combine chemical stability with gas sorption, dye elimination, and conductivity. Specifically, we show that uniformly distributed carboxyl and water stability make it accessible for loading Sr2+ without structural changes. The FTIR spectroscopy, PXRD analysis, XPS, and SEM-EDS studies show excellent stability as well as the strong affinity between -COOH active site and Sr2+. This strong coordination interaction guarantees a high adsorption capacity of 114 mg g-1 within 5 h (pH 5 and 298 K). Combined kinetic and thermodynamic studies show that the surface complexation is strong chemisorption and cost-effective spontaneous process (ΔG = -5.49 kJ mol-1~-2.16 kJ mol-1). The fact that UiO-66-(COOH)2 not only possesses a high adsorption capacity, but also enables selectivity to Sr2+ in the presence of similar radius ions Na+ and K+, prefigures its great potential for the practical treatment of radioactive Sr2+ in polluted water.

Application of Geopolymer in Stabilization/Solidification of Hazardous Pollutants: A Review.

Geopolymers, as a kind of inorganic polymer, possess excellent properties and have been broadly studied for the stabilization/solidification (S/S) of hazardous pollutants. Even though many reviews about geopolymers have been published, the summary of geopolymer-based S/S for various contaminants has not been well conducted. Therefore, the S/S of hazardous pollutants using geopolymers are comprehensively summarized in this review. Geopolymer-based S/S of typical cations, including Pb, Zn, Cd, Cs, Cu, Sr, Ni, etc., were involved and elucidated. The S/S mechanisms for cationic heavy metals were concluded, mainly including physical encapsulation, sorption, precipitation, and bonding with a silicate structure. In addition, compared to cationic ions, geopolymers have a poor immobilization ability on anions due to the repulsive effect between them, presenting a high leaching percentage. However, some anions, such as Se or As oxyanions, have been proved to exist in geopolymers through electrostatic interaction, which provides a direction to enhance the geopolymer-based S/S for anions. Besides, few reports about geopolymer-based S/S of organic pollutants have been published. Furthermore, the adsorbents of geopolymer-based composites designed and studied for the removal of hazardous pollutants from aqueous conditions are also briefly discussed. On the whole, this review will offer insights into geopolymer-based S/S technology. Furthermore, the challenges to geopolymer-based S/S technology outlined in this work are expected to be of direct relevance to the focus of future research.

Elemental distributions of solid waste collected from the germanium extraction process.

The solid waste produced from the germanium extraction process has attached much attention to its potential germanium sources. However, the elemental distribution of solid waste is still unclear. Therefore, the solid waste was studied using a sequential extraction procedure and characterizations including XRD, FTIR, XPS, SEM-EDS, and XAFS. It has been found that Ca, S, Fe, and Si could present crystal occurrence forms such as calcium sulfate, iron oxide hydroxide, or quartz. Furthermore, Si and Al can form a certain amount of amorphous substance. Accordingly, the sequential leaching results tell that Ca and S can be mostly leached out in pure water or weak acid solution, and more than 50% of Fe, Al, and Si were leached out in the reducible or oxidizable environment. Additionally, a part of S could be associated with Pb, generating a mostly Pb-bearing sulfate structure. Most of Zn was leached out from the reducible step, and only a very small part of Zn presented in the residual state, indicating that the majority of Zn might exist in an oxidation state and a small amount of Zn is associated in the amorphous phase. In terms of Ge, As, and Cr, almost all of them existed in the residual state. Ge should be in the occurrence of Si/Al amorphous structure. Similarly, Cr should be most likely to associate with silicates. Furthermore, As is mainly associated with iron mineral through the formation of the binuclear bidentate corner-sharing complex.

Associations of Gangue Minerals in Coal Flotation Tailing and Their Transportation Behaviors in the Flotation Process.

The mass production of flotation tailings has become a serious risk to the environment. Re-concentration of tailings is one of the best ways to solve this problem, which requires a better understanding of flotation tailings. In the present work, flotation kinetics, timed-release flotation, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS), and solid-state nuclear magnetic resonance (SSNMR) were used to analyze the properties of flotation tailings with different particle sizes and densities, the occurrence and binding state of gangue minerals in tailing, and the transportation behaviors in the re-flotation process. Flotation results showed that the flotation yield exhibited little change with the extension of flotation time, while the ash content of the froth concentrates increased. An increase of the flotation time could reduce the ash content of the obtained product. The characterization results confirmed that the main gangue minerals in the tailings were kaolinite and quartz. With the decrease of particle size or the increase of floating and sinking density, the contents of kaolinite and quartz increased. However, due to the different dissemination characteristics of kaolinite and quartz in the tailings, the distributions of kaolinite and quartz in the different particle sizes and densities of tailing had differences. Although both kaolinite and quartz could exist as monomers, kaolinite was more easily associated with coal. Based on the above cognition, a new flotation method is proposed for coal flotation tailing. A part of the concentrates in the early stage of flotation should be scraped out quickly. Then, the concentrates obtained in the later stage of flotation are collected and merged into the concentrates obtained during the early stage of flotation, while the secondary tailing is directly pumped into the raw feed system.

Immobilization of strontium in geopolymers activated by different concentrations of sodium silicate solutions.

Sodium silicate is always used as an activator for the synthesis of geopolymer. However, the effect of sodium silicate concentration on the geopolymer used as adsorbent was still unclear. Therefore, the immobilization of Sr2+ in geopolymers activated by different concentrations of sodium silicate was studied through kinetic and isotherm modeling and solid characterizations including XRD, FTIR, TG, SEM-EDS, and N2 adsorption-desorption isotherm. The adsorption amount of Sr2+ decreased with the sequence of S1, S2, and S3. According to the kinetic and isotherm modeling results, these sorption processes fitted better with pseudo-second-order, mainly governed by film diffusion. However, the diffusion mode was gradually closed to particle diffusion as for the sequence of S3, S2, and S1. Besides, the Langmuir model can be more befitting to sorption data than the Freundlich model, and the free energies decreased with the order of S1, S2, and S3. In addition, the specific surface areas did not change regularly with S1, S2, and S3. Thus, the distribution of Al tetrahedrons has a decisive role in the sorption process of Sr2+, even though the specific surface area is also a critical factor. More Al tetrahedrons can be formed under the activation of sodium silicate with higher concentration, leading to the low Si/Al molar ratio of the as-synthesized geopolymer.

A Bibliometric Analysis of Research Progress and Trends on Fly Ash-Based Geopolymer.

The objective of this work is to present the research progress and applications of fly ash-based geopolymer, and summarize the future research hotpots. Since 1998, scholars have made important contributions to the study of fly ash-based geopolymer, and a large number of research studies have been published. Therefore, a bibliometric analysis for the determination of the research status, trend, and history of fly ash-based geopolymer was conducted in the present study. A total of 4352 publications on fly ash-based geopolymer were collected between 1998 and 2022, with an increasing trend year by year. China and Australia are the largest contributors to the field, and the research institutions in each country cooperate closely. In addition, the most contributing research areas are MATERIALS SCIENCE, ENGINEERING, and CONSTRUCTION & BUILDING TECHNOLOGY. The keywords including fly ash, compressive strength, and mechanical property are the most frequently appearing words. On the whole, the development of fly ash-based geopolymer could be divided into three stages including the replacement of ordinary Portland cement, the development of multifunctional materials, and the reduction of environmental impact by the conversion of solid waste. This overview could provide an important guidance for the development of fly ash-based geopolymer.

Influence of aluminate and silicate on selenate immobilization using alkaline-earth metal oxides and ferrous salt.

Effects of aluminate and silicate species on the SeO42- immobilization using alkali-earth metal oxides and ferrous species have not been clearly elucidated. In the present study, Al and Si species were separately added into MgO/Fe(II) and CaO/Fe(II) reactions containing SeO42-, studied by toxicity characteristic leaching procedure (TCLP), X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), X-ray absorption fine structure (XAFS), and PHREEQC simulation. Approximately 42 % of SeO42- was reduced to SeO32- for MgO/Fe(II) reaction in the presence of Al species, being consistent with the case without Al species. The Al species only showed slight inhibition of Se leaching for the MgO/Fe(II) reaction. Most of Se oxyanions were adsorbed onto Mg(OH)2 through outer-sphere complexation. For CaO/Fe(II) reaction, all of SeO42- was reduced to SeO32- with or without Al species. However, the Se leaching amount (3 %) of sample added with Al species (CE3) is much lower than that (12 %) of sample without Al species (CE2). This is mainly because SeO32- can be sorbed onto the iron-based minerals through binuclear bidentate corner-sharing (2C) complexation instead of monodentate mononuclear corner-sharing (1V) complexation of the case without Al species. On the other hand, SeO42- was not reduced to SeO32- in the presence of silicate, and almost all of Se was leached out for silicate-contained samples except CaO/Fe(II) reaction with the addition of Al species. This is due to the polymerization of Al and Si species under a high-alkalinity environment, thereby stabilizing SeO42- in the amorphous silicon-aluminum structure and contributing to the decrease of Se leaching.