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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.

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.

Fly ash always contains many toxic elements which can be released into environment, thereby easily leading to environmental contaminations. In order to dispose fly ash safely, related strategies are needed. In this investigation, two kinds of hydroxylated calcined dolomites (HCD60 and HCD100) were used as the additives and compared with lime on the leachabilities of anionic species from fly ash. Both additives were found effective in reducing the leaching concentrations of these elements, which was better than that of only lime addition. Mg(OH)2 and MgO were believed to play important roles in the hydration reaction of fly ash. In the presence of Mg(OH)2 and MgO, there were more hydration products including calcium silicate hydrate, ettringite, hydrocalumite and other Layered double hydroxides (LDHs) generated which were effective candidates for anion removal. Thus, the final leaching results were controlled by these newly formed phases through adsorption, incorporation or encapsulation. On the other hand, compared with Mg(OH)2, MgO can promote the formation of hydration products in a larger extent because of the hydration process of MgO into Mg(OH)2. There was no systematic trend in the promotion of fly ash hydration by Mg(OH)2 or MgO because it had a close relationship with the properties of original fly ash. Objectively, hydroxylated calcined dolomites can be promising candidate additives for reduction of toxic elements leaching from fly ash.

The elevated level of selenium (Se) in the aqueous system presents long-term ecological risks. Hydroxylated calcined dolomites (HCDs) are considered as potentially sustainable lime sources due to their lower production temperatures and resourceful than lime. In this study, HCDs were developed for the removal of Se contaminated water. The HCDs exhibited a better performance to co-precipitate with Se oxyanions than pure Ca(OH)2. Considering that HCDs consisted of Ca(OH)2 with MgO and Mg(OH)2, the function of Mg compounds was also elaborated by various characterization techniques. Mg compounds were proved to enhance the precipitation process and Se removal. STEM-EDX observation revealed that SeO32– was incorporated into the ettringite structure with Mg compounds, whereas SeO42– exhibited a affinity to Mg compounds. Extended X-ray absorption fine structure (EXAFS) results for the MgO/Mg(OH)2 after the reaction proved that both SeO32– interacted with Mg(OH)2 and MgO via inner-sphere complexes whereas SeO42– formed outer-sphere complexes with these Mg compounds. Besides, Mg compounds also influenced the surface charge of solid residues and thus enhanced SeO32– removal. This study provides a fundamental understanding of the roles of Mg compounds in the removal of Se oxyanions during environmental remediation. Besides, the HCDs were proved to be sustainable Ca sources for waste/wastewater remediation.

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 N2 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 (Na3.25(OH)0.25(H2O)12)(AsO4) 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.

In this investigation, the possibility of carboxylic acid used as collector to float low-rank coal was explored and alkane was also used to be compared. The effect of different lengths of carbon ch...

In this investigation, the floc-flotation performance of ultrafine coal slimes in the large-scale 5500 × 7000 mm cyclonic-static micro-bubble flotation column (FCSMC) was explored. A laser particle size analyzer was used to indicate the changes in particle size distribution with or without adding a flocculant. The results show that adding a flocculant can contribute to a high content of particles with a larger size in feed, compared with raw feed without adding a flocculant. The flotation results show that the yield of concentrate is 66.69%, with the ash of 7.38% when using 5500 × 7000 mm FCSMC without a flocculant. However, the yield reaches 77.39% with an ash content of 8.70% using a combination of FCSMC and flocculant. The analyses of size distribution of flotation products show that size fractions with the highest and lowest recoveries are +0.125−0.074 mm and −0.045 mm, respectively. The recovery of −0.045 mm fraction sees a 12.29% rise from 62.14 to 74.43% with the ash increasing by only 1%, ...

The influence of silicate on the structural memory effect of layered double hydroxides (LDHs) has been rarely reported. In this study, five kinds of calcined LDHs (CLDHs) were synthesized and used as adsorbents for the sorption of selenium with or without silicate, under the initial pH 10 and 13, respectively, characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), infrared spectroscopy (IR), nuclear magnetic resonance (NMR), X-ray photoelectron spectroscopy (XPS), and N2 adsorption-desorption isotherm. The results indicated that silicate can significantly affect the phase transformation of CLDHs, and the sorption amounts of selenite and selenate dramatically decreased in the presence of silicate. Specifically, silicate can react with MgO and Al2O3 in CLDHs to generate magnesium silicate hydrate and geopolymer-like substance which were covered on the surface of particles, blocking the hydroxylation of metal oxides. However, higher pH suppressed the interaction between MgO and silicate and enhanced the formation of geopolymer-like substance, which promoted the regeneration of LDHs. Al in CLDHs plays a critical role in the regeneration of LDHs. Besides, the ternary oxides (CLDH-2, Mg2Al0.75Fe0.25-oxide; CLDH-3, Mg2Al0.5Fe0.5-oxide) possessed larger specific surface areas (127.7 and 158.2 m2/g) and consequently presented more resistance to the effect of silicate.

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 SeO32– and SeO42– was explored for the first time using geopolymer activated by different alkaline solutions (NaOH and Na2SiO3) with and without calcined hydrotalcite (CHT), characterized by TCLP, XRD, FTIR, TG, NMR, XAFS, and N2 adsorption-desorption isotherm. Na2SiO3-activated geopolymers without CHT additive showed lower leaching percentages of SeO32– and SeO42– (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 SeO32– and SeO42– in both NaOH- and Na2SiO3-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 Na2SiO3-geopolymers. Due to the formation of hydrotalcite, the CHT additive contributed to immobilize SeO32– and SeO42– in NaOH-activated geopolymers. However, this phenomenon was not observed in Na2SiO3-activated geopolymers. Thus, the leaching amount of Se greatly increased from Na2SiO3-activated geopolymers with CHT additive. This study provides new insights on the application of geopolymer to immobilize anionic pollutants.

Boric acid is one of the most mobile inorganic contaminant species in nature due to its pKa of 9.23. Co-precipitation of borate with hydroxyapatite (HAp: Ca_5(PO_4)_3OH) facilitates the simultaneous removal of borate with co-existing oxoanions in natural waters. The cost of phosphate is an impediment to industrialize the co-precipitation of borate with HAp for treatment of geothermal waters. In the present work, an inexpensive industrial by-product of magnesium ammonium phosphate (MAP) derived from sewage sludge, was examined as a phosphate source. MAP includes 89% pure magnesium ammonium phosphate, resulting in better performance than the pure chemical form of NH_4H_2PO_4, because Mg^+ and Al^ (trace elements in MAP product) play roles in enhancing the removal rate of borate and lowering the equilibrium borate concentration. These ions have a good affinity with phosphate to nucleate crystal seeds independently of powdery Ca sources. To reduce the bulky volume of solid residues, hot isostatic pressing (HIP) was applied. There is structural water in HAp; therefore, the greatest volume reduction was achieved with 78.3 ± 2.0% (n = 3). Additionally, a synergic effect to suppress the released borate, greater than the sequential combination of calcination and cold isostatic pressing was accomplished in the toxicity contents leaching procedure (TCLP) test. This is not due to larger crystal sizes alone, but it is derived from boron stabilization in HAp at an atomic level by the synergic effect of heating and pressing simultaneously.

The immobilization of selenate (SeO_4^) 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^ was reduced to SeO_3^ in the CaO-based reaction within 7 days. Then, the generated SeO_3^ was mainly sorbed onto the iron-based minerals (Fe_2O_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^ (approximately 45.50%) was reduced to SeO_3^ 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^ and SeO_4^) 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.

There are few reports to explore immobilization of undesirable anionic species released from fly ash (FA) blended cement. This study clarifies the stabilities of several undesirable anionic species, through dissolution tests under a different matrix. The immobilization mechanism mainly involves co-precipitation with ettringite and hydrocalumite, the marginal formation of insoluble Ca salts, and occlusion in calcium silicate hydrates and amorphous phases produced during the pozzolanic reaction. The products released some amounts of chromate, arsenate, and selenite but not borate in dissolution tests. Elution of borate was the most effectively immobilized by Ca additives, owing to selective incorporation in ettringite. Moreover, selenite showed a trend of dissolving in acidic conditions, presumably owing to the dissolution of CaSeO3 with relatively high Ksp in addition to coprecipitation with hydrocalumite and ettringite. Under the present mixing condition of cementation, only selenite is a cautionary species which excessively eluted over the environmental standard.

Fly ash usually contains a considerable amount of toxic elements that can be leached into the environment, thereby easily leading to serious contaminations. In this work, the leaching behaviors of poisonous elements including boron (B), phosphorus (P), vanadium (V), chromium (Cr), arsenic (As), selenium (Se), molybdenum (Mo), antimony (Sb), and tungsten (W) from fly ash were explored by sequential extraction. Importantly, the associations of these elements in fly ash were discussed based on their leaching and X-ray absorption near-edge structure (XANES) results. From the XANES results, it was observed that V(IV), Cr(III), As(V), Se(IV), and W(IV) were their main states of existence in fly ash. In terms of leaching results, large amounts of Mo and W were leached into pure water, which indicated their high mobilities. Furthermore, the occurrence of Mo in fly ash was mainly in the form of oxides, and W had complex associations including WX4 (X can be monovalent anions), its reduction state or association with the elements that can be oxidized, and existence in silicates. B was as easily released into the environment as Mo and W. It can have several associations with the other cations, such as Ca2+, Na+, and Mg2+, and occurs in silicates. In contrast, most of the Cr and Sb were locked in silicates, indicating that they were very stable in fly ash. In addition, P, V, and As can exist within the structure of silicates as well. However, a considerable amount of them leached in the reduction step with a low pH. Hence, they can be associated with Ca2+, Na+, Mg2+, or Fe3+. In terms of Se, oxidation processes played an important role in controlling its leaching because of the oxidation of Se(IV) to Se(VI). Calcium selenite should be the predominant form of Se in fly ash.

Coking coal, the raw material of a coke plant, was applied to the adsorption of coking wastewater. In this study, coking coal was directly treated with sulfuric acid to improve its surface properties and adsorption ability. Acid treatment was carried out at various concentrations, by varying from 0.001 to 1 mol/L. The samples were characterized by ash content analysis, scanning electron microscope (SEM), N2 adsorption-desorption analysis, Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), wettability analysis, and zeta potential analysis. These results demonstrated that H+ could react with inorganic minerals, which resulted in a significant variation of the chemical composition and the structure of coal surface. Furthermore, both the ash content and the surface content of O = C-O, C = O and C-O groups declined gradually as the concentration of sulfuric acid increased, while the surface area and pore volume of micropore, the lipophilic and hydrophobic properties, and zeta potential magnitude increased, resulting in enhanced hydrophobic and Van der Waals' forces between the fine coal and organic pollutants. Characterization modification showed a better performance in adsorption, the removal rate enhanced from 23% to 42% after treated by 1 mol/L sulfuric acid. It was concluded that the acid activation modified the lipophilic and hydrophobic properties, the surface charge properties, surface area and pore volume, the content of oxygen functional groups, all of which could be potentially useful in wastewater adsorption.

Geopolymers are always considered as promising materials for the treatment of radioactive wastes. In order to extend the application of geopolymer to the immobilization of anionic species, a novel composite of layered double hydroxide/geopolymer (LDH/GEO) was synthesized and applied for cosorption of Cs+ and SeO42−. The ability of LDH/GEO to sorb Cs+ was maintained as that of pure GEO, even though the surface of geopolymer was homogeneously covered by the LDH platelets. The sorption of Cs+ onto LDH/GEO composite occurred via ion exchange, which was controlled by particle diffusion. It is different with Cs+ sorption onto pure GEO governed by film diffusion. Therefore, “Pocket diffusion” was proposed for the particle diffusion as the case of LDH/GEO because this kind of diffusion would be restricted in a certain distance around the ring entrance gate due to the amorphous essence of GEO. For SeO42− sorption by LDH/GEO, ion-exchange with the interlayer NO3− and surface sorption could be the main mechanisms. Importantly, the sorption speed of SeO42− achieved by LDH/GEO composite was much faster than that by pure LDH. In the binary system (Cs++ SeO42−), the sorption of Cs+ was slightly suppressed compared to the single system, which might be due to the formation of ion-pair complex of [CsSeO4]−. However, it did not have negative effect on the SeO42− sorption. In the presence of other cations or anions, the cosorption performances of Cs+ and SeO42− were satisfactorily obtained. Furthermore, the Cs+ and SeO 4 2 - sorption densities were superior to the previously reported values. The combined MgAl-LDH/geopolymer composite could be a promising material for the immobilization of Cs+ and SeO42−, and this work would provide guidance for the development of geopolymer-based materials for environmental applications.