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• Chemical states of As, Cr, and Se in fly ash are determined. • HCD enhanced the immobilization of released pollutants. • Ettringite formation is the crucial factor to immobilize pollutants. • MgO and Mg(OH) 2 enhance the formation of ettringite and hydrocalumite. Harmful trace elements, which are initially included in the coal fly ash, have the potential to be leached when coal fly ash comes in contact with water. This causes a risk of pollutant species being released, considering the long lifetime of building structures where coal fly ash was applied. Some Ca additives effectively function to suppress the release of anionic pollutants; however, the detailed suppression processes remains unclear. In this work, the influences of various Ca additives on the released anionic pollutants (B, F, S, As, and Cr) was systematically investigated. According to the comprehensive results of solution data with the solid characterization, the 60% hydroxylated calcined dolomite (HCD 60) was the best Ca additive for the suppression of different anionic pollutants since this Ca source not only simply provides an alkaline reagent but also supplies MgO and Mg(OH) 2 , which affect the phase transformation that accompanies with hydration. The phase transformation occurs from Ca(OH) 2 to ettringite via hydrocalumite, which is the most important suppression processes of released pollutants. The precipitation of Ca salts is another pathway to immobilize these pollutants. In this scheme, MgO and Mg(OH) 2 were proven to enhance the formation of ettringite and hydrocalumite, respectively. [ABSTRACT FROM AUTHOR]

In this study, the feasibility of using zero-valent iron (ZVI) and Fe 3 O 4 -loaded biochar for Pb immobilization in contaminated sandy soil was investigated. A 180-day incubation study, combined with dry magnetic separation, chemical extraction, mineralogical characterization, and model plant (ryegrass, namely the Lilium perenne L.) growth experiment was conducted to verify the performance of these two materials. The results showed that both amendments significantly transferred the available Pb (the exchangeable and carbonates fraction) into more stable fractions (mainly Fe/Mn oxides-bound Pb), and ZVI alone showed a better performance than the magnetic biochar alone. The magnetic separation and extended X-ray absorption fine structure (EXAFS) analysis proved that Fe (oxyhydr)oxides on aged ZVI particles were the major scavengers of Pb in ZVI-amended soils. In comparison, the reduced Pb availability in magnetic biochar-amended soil could be explained by the association of Pb with Fe/Mn (oxyhydr)oxides in aged magnetic biochar, also the possible precipitation of soil Pb with soluble anions (e.g. OH−, PO 4 3−, and SO 4 2−) released from magnetic biochar. ZVI increased ryegrass production while Fe 3 O 4 -loaded biochar had a negative effect on the ryegrass growth. Moreover, both markedly decreased the Pb accumulation in aboveground and root tissues. The simple dry magnetic separation presents opportunities for the removal of Pb from soils, even though the efficiencies were not high (17.5% and 12.9% of total Pb from ZVI and biochar-treated soils, respectively). However, it should be noted that the ageing process easily result in the loss of magnetism of ZVI while the magnetic biochar tends to be more stable and has high retrievability during the dry magnetic separation application. [Display omitted] • Both ZVI and biochar decreased available Pb in soil and reduced Pb uptake by ryegrass. • EXAFS demonstrated the major contribution of ferrihydrite and/or birnessite for Pb immobilization. • Dry magnetic separation presents opportunity for the permanent removal of Pb from soils. [ABSTRACT FROM AUTHOR]

The effect of Si/Al molar ratio of geopolymer on the immobilization of Se and As oxyanions was studied through leaching test and solid characterizations including XRD, FTIR, TG, NMR, XAFS, and N 2 adsorption-desorption isotherm. As a whole, the leaching percentages of Se and As oxyanions increased with the increase of the Si/Al molar ratio of geopolymer. Linear combination fitting confirmed that most of selenite, selenate and arsenate ions existed in geopolymers through electrostatic interaction. Thus, Al tetrahedrons in geopolymer structure control the charge stability for these oxyanions to a large extent. Differently, as for arsenate ions, they were recrystallized into an arsenate compound (Na 3.25 (OH) 0.25 (H 2 O) 12)(AsO 4) in geopolymers. The additive of these pollutants has an adverse effect on the compactness of geopolymer, then influencing the leaching performance in turn. However, the changes in leaching results did not follow the variation trend of specific surface areas and pore volumes of geopolymers with different Si/Al ratios. The number and distribution of Al tetrahedron and compactness of geopolymer have a synergistic effect on the immobilization of these oxyanions. Besides, the compressive strengths of geopolymer samples are always higher than 20 MPa, which meets the requirement of safe disposal of hazardous waste. Image 1 • The percentages of Se and As leaching increased with the increase of the Si/Al ratio. • Selenite, selenate and arsenite existed in geopolymer by electrostatic interaction. • Arsenate ions were crystallized into an arsenate compound in geopolymer. • Al distribution and compactness have a synergistic effect on the S/S process. The number and distribution of Al tetrahedron largely determine the stabilities of Se and As oxyanions in geopolymers. [ABSTRACT FROM AUTHOR]

The immobilization of selenate (SeO 4 2–) using metal oxides (CaO and MgO) and ferrous salt as the immobilization reagents were examined by the leaching test and solid-phase analysis via XRD, XAFS, TGA, and XPS. The results indicated that nearly all of SeO 4 2– was reduced to SeO 3 2– in the CaO-based reaction within 7 days. Then, the generated SeO 3 2– was mainly sorbed onto the iron-based minerals (Fe 2 O 3 and FeOOH) through the formation of both bidentate mononuclear edge-sharing (1E) and monodentate mononuclear corner-sharing (1V) inner-sphere surface complexes, suggested by PHREEQC simulation and EXAFS analysis. Differently, less amount of SeO 4 2– (approximately 45.50%) was reduced to SeO 3 2– for the MgO-based reaction. However, if the curing time increases to a longer time (more than 7 days), the further reduction could occur because there are still Fe(II) species in the matrix. As for the associations of Se in the solid residue, most of the selenium (SeO 3 2– and SeO 4 2–) was preferentially distributed onto the Mg(OH) 2 through outer-sphere adsorption. Definitely, this research can provide a deep understanding of the immobilization of selenium using alkaline-earth metal oxide related materials and ferrous substances. Image 1 • The immobilization of SeO 4 2– using metal oxides and ferrous salt was studied. • Different immobilization mechanisms were shown in CaO- and MgO-based reactions. • SeO 4 2– was immobilized via reduction and inner-sphere sorption in CaO-based reaction. • SeO 4 2– was immobilized via reduction and outer-sphere sorption in MgO-based reaction. [ABSTRACT FROM AUTHOR]

• The immobilization mechanisms of I– and IO 3 – in ettringite are different. • IO 3 – is energetically favorable to be accommodated in ettringite. • The energy barrier is observed for I– accommodation in ettringite. • The structural water determines the ions immobilization in ettringite. Low-level radioactive wastes are commonly immobilized in cementitious materials, where cement-based material can incorporate radionuclides into their crystal structure. Specifically, ettringite (Ca 6 Al 2 (OH) 12 (SO 4) 3 ∙26H 2 O) is known to stabilize anionic species, which is appealing for waste streams with radioactive iodine (129I) that persists as iodide (I–) and iodate (IO 3 –) in the cementitious nuclear waste repository. However, the structural information and immobilization mechanisms of iodine species in ettringite remain unclear. The present results suggested minimal I– incorporation into ettringite (0.05 %), whereas IO 3 – exhibited a high affinity for ettringite via anion substitution for SO 4 2– (96 %). The combined iodine K-edge extended X-ray absorption fine structure (EXAFS) spectra and first-principles calculations using density functional theory (DFT) suggested that IO 3 – was stabilized in ettringite by hydrogen bonding and electrostatic forces. Substituting IO 3 – for SO 4 2– was energetically favorable by –0.41 eV, whereas unfavorable substitution energy of 4.21 eV was observed for I– substitution. Moreover, the bonding charge density analysis of the substituted IO 3 – and I– anions into the ettringite structure revealed the interaction between intercalated ions with the structural water molecules. These results provided valuable insight into the long-term stabilization of anionic iodine species and their migration in cementitious nuclear waste repository or alkaline environments. [ABSTRACT FROM AUTHOR]

• The immobilization of selenite and selenate in geopolymer was explored. • Two kinds of activators were used for geopolymer synthesis. • The effect of calcined hydrotalcite on the Se leaching was discussed. • Electrostatic interaction is the main association mode of selenium in geopolymer. • Compactness of geopolymer plays a critical role in the immobilization of selenium. Geopolymers have been widely adopted to stabilize the cationic pollutants. However, few studies have focused on the immobilization of anionic species. In this study, the immobilization of SeO 3 2– and SeO 4 2– was explored for the first time using geopolymer activated by different alkaline solutions (NaOH and Na 2 SiO 3) with and without calcined hydrotalcite (CHT), characterized by TCLP, XRD, FTIR, TG, NMR, XAFS, and N 2 adsorption-desorption isotherm. Na 2 SiO 3 -activated geopolymers without CHT additive showed lower leaching percentages of SeO 3 2– and SeO 4 2– (approximately 10 % and 18 %) than NaOH-activated geopolymers (approximately 58 % and 74 %). It has been proven that electrostatic interaction is the main association mode of SeO 3 2– and SeO 4 2– in both NaOH- and Na 2 SiO 3 -activated geopolymers. Hence, compactness plays a vital role in the Se leaching from geopolymer. The addition of CHT reduced the compactnesses of both NaOH- and Na 2 SiO 3 -geopolymers. Due to the formation of hydrotalcite, the CHT additive contributed to immobilize SeO 3 2– and SeO 4 2– in NaOH-activated geopolymers. However, this phenomenon was not observed in Na 2 SiO 3 -activated geopolymers. Thus, the leaching amount of Se greatly increased from Na 2 SiO 3 -activated geopolymers with CHT additive. This study provides new insights on the application of geopolymer to immobilize anionic pollutants. [ABSTRACT FROM AUTHOR]

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 Na 2 SiO 3 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 N 2 adsorption-desorption isotherm. For both NaOH-activated and Na 2 SiO 3 -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 Na 2 SiO 3 -activated geopolymer even after calcination at 1000 °C. It has been deduced that the main structure of Na 2 SiO 3 -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. [Display omitted] • Leaching concentration of Cs decreased with the increase of calcination temperature. • Lower leaching amounts of Cs can be always achieved for Na 2 SiO 3 -activated geopolymer. • NaOH-activated geopolymer can be mostly crystallized to nepheline after calcination. • Less crystallized nepheline was found in Na 2 SiO 3 -activated geopolymer after calcination. [ABSTRACT FROM AUTHOR]