Suppression processes of anionic pollutants released from fly ash by various Ca additives

High-efficient stabilization and solidification of municipal solid waste incineration fly ash by synergy of alkali treatment and supersulfated cement

Municipal solid waste incineration fly ash (IFA) designated as hazardous waste poses risks to environment and human health. This study introduces a novel approach for the stabilization and solidification (S/S) of IFA: a combined approach involving alkali treatment and immobilization in low-carbon supersulfated cement (SSC). The impact of varying temperatures of alkali solution on the chemical and mineralogical compositions, as well as the pozzolanic reactivity of IFA, and the removal efficiency of heavy metals and metallic aluminum (Al) were examined. The physical characteristics, hydration kinetics and effectiveness of SSC in immobilizing IFA were also analyzed. Results showed that alkali treatment at 25 °C effectively eliminated heavy metals like manganese (Mn), barium (Ba), nickel (Ni), and chromium (Cr) to safe levels and totally removed the metallic Al, while enhancing the pozzolanic reactivity of IFA. By incorporating the alkali-treated IFA and filtrate, the density, compressive strength and hydration reaction of SSC were improved, resulting in higher hydration degree, finer pore structure, and denser microstructure compared to untreated IFA. The rich presence of calcium-aluminosilicate-hydrate (C-(A)-S-H) and ettringite (AFt) in SSC facilitated the efficient stabilization and solidification of heavy metals, leading to a significant decrease in their leaching potential. The use of SSC for treating Ca(OH)2- and 25°C-treated IFA could achieve high strength and high-efficient immobilization.

Alkaline industrial wastes - Characteristics, environmental risks, and potential for mine waste management

The large quantities of alkaline industrial wastes that are generated globally have the potential to be valorized in various applications instead of being landfilled. This study evaluated the potential reuse of green liquor dregs (GLD), wood ashes, coal ash, red mud, mussel, scallop, and oyster shells to control acid and metalliferous drainage (AMD). Low hydraulic conductivities (10^-7 to 10^-9 m/min) suggest that covers constructed from fine-grained GLD, red mud, coal ash and wood fly ash can limit the formation of AMD. Static and kinetic test leachates of pH 5.8 to 10.6 indicate that the tested materials can neutralize acidic drainage and immobilize metal(loid)s by precipitation. The alkalinity is proportional to the amount and reactivity of carbonate and hydroxide fractions with red mud followed by coal ash being the most alkaline over 100 weeks and wood ashes the least. The tested industrial wastes generate leachates with a low metal(loid) risk when screened against the Australian freshwater guidelines. However, oxyanions including Al, Cr, Cu, Se, and V were leached in deleterious concentrations ≤100 times more than the guidelines because of their mobility in alkaline conditions. The outcomes of this study highlighted that alkaline industrial wastes can be potentially used in the long-term remediation of AMD as part of an environmentally sustainable and cost-effective integrated mine waste management strategy.

Immobilization of Zn(Ⅱ) and Cu(Ⅱ) in basic magnesium-sulfate-cementitious material system: Properties and mechanism

To solve the environmental problems caused by heavy metal pollution, a new cementitious material (basic magnesium sulfate cement, BMSC) was developed for the solidification of Cu²⁺/Zn²⁺. First, the effects of different amounts of Cu²⁺/Zn²⁺ on the properties (compressive strength, setting time, pH, and leaching toxicity) of the BMSC were investigated. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS) were used to investigate the effects of different amounts of Cu²⁺/Zn²⁺ on the phase and microstructure of BMSC. The results showed that Cu²⁺/Zn²⁺ inhibited the hydration of BMSC, reduced compressive strength, and prolonged the setting time. The results of the leaching tests showed that the BMSC system exhibited high immobilization efficiency (up to 99%) for Cu²⁺/Zn²⁺. Further, the BMSC solidification matrix exhibited excellent acid resistance (compressive strength >40 MPa after 28 days of immersion). The physical phase analysis showed that the main phases of BMSC were the 5Mg(OH)₂-MgSO₄-7 H₂O (5-1-7) phase and Mg(OH)₂, and the crystal structure refinement analysis suggested that Cu²⁺/Zn²⁺ ions were substituted with Mg²⁺ in the 5-1-7 phase. It was confirmed that the solidification mechanism of BMSC on Cu²⁺/Zn²⁺ is mainly performed by chemical complexation and ionic substitution.

Highly-efficient fluoride retention in on-site solidification/stabilization of phosphogypsum: Cemented paste backfill synergizes with poly-aluminum chloride activation

Phosphogypsum (PG) is a massively generated hazardous by-product in the phosphorus industry. Large-scale, efficient, profitable on-site recycling is an emerging topic for promoting sustainable phosphorus circularity and mitigating potential human exposure. In this work, we integrated a green and low-cost additive polymeric aluminum chloride (PAC) into the binder design of PG immobilization. The overall experimental results illustrate that the incorporation of PAC can efficiently promote the cement hydration reaction, with amorphous phases increased from 25.9 wt% (control group) to 27.5 wt% (with 2 g/L PAC). The macro-investigations indicate that the PAC optimized the porosity and mechanical properties of specimens, facilitating a mechanically stable solidified matrix for extrapolating its field engineering application. The detailed micrographs and elemental mapping demonstrate that apart from co-existing with the hydration products, the PAC agent plays a role in the immobilization of fluoride. Herein, the combined optimization enhanced the fluoride retention capacity due to the precipitated additional hydration products, comparable encapsulation, and high adsorption ability of PAC agents. Therefore our design of PAC-augmented binders can open up a new field of PG on-site solidification/stabilization application that ensures efficient fluoride retention in a technically feasible and financially profitable methodology.

Efficient remediation of heavily As(III)-contaminated soil using a pre-oxidation and stabilization/solidification technique

The high mobility of As(III) makes it difficult to remediate heavily As(III)-contaminated soil. A novel remediation technique that combines pre-oxidation and stabilization/solidification (PO + S/S) is proposed in this study to remediate heavily As(III)-contaminated soil. After oxidizing As(III) in the contaminated soil using Fenton's reagent, FeCl₃·6H₂O was used as a chemical stabilizing agent to reduce the toxicity and mobility of As. Finally, Portland cement (PC) was used for solidification. The effects and mechanisms of the proposed technique were studied using unconfined compressive strength tests, leaching tests, sequential extraction procedure (SEP), and a series of spectroscopic/microscopic investigations. The experimental results showed that the addition of FeCl₃·6H₂O increased the strength of the curing body because the hydration degree of PC and pore structure were improved. Portland cement can increase the pH of the curing body. At a 1:1 Fe to As molar ratio and a 15 wt% PC dosage, the leached As concentration decreased to 3.25 mg L^-1, and the remediation efficiency reached 99.54%. The SEP results showed that the PO + S/S treatment converted As into more stable phases and effectively reduced the potential mobile phase risk. The majority of As was bound to hydrated iron oxides; however, the increased pH affected the Fe-As interactions and prompted the release of As from the surface of the hydrated iron oxides. Spectroscopic/microscopic investigations indicated that the PO + S/S treatment converted As(III) to less toxic and less mobile As(V) and then immobilized by the encapsulation of calcium silicate hydrate and ion exchange of ettringite. This study provides a scientific basis and theoretical support for the effective remediation of heavily As(III)-contaminated soil.

Chemical Speciation and Leaching Behavior of Hazardous Trace Elements in Coal Combustion Products from Coal-Fired Power Stations in China

This paper reports on the chemical speciation and leaching behavior of a selected group of hazardous trace pollutants in lignite and lignite-petcoke blend co-combustion products from three power stations in China. The evaluation of speciation results showed that, during combustion, oxidizable elements, mainly As and Mo, bound to organic matter and sulfides in coals were mostly transferred to easily water-soluble forms or to slightly acidic states in the ashes. This manner was the most readily bioavailable condition for such an environment. The evaluation of the leaching results shows that the use of petroleum coke as co-fuel has an impact on the ash composition and on the leaching behavior of some inorganic trace pollutants such as Mo and V. The leaching results compared to the European waste acceptance criteria for landfills reveal that the Mo and As' leaching yield brand the coal combustion products as materials that necessitate preventative measures to reduce their potential leaching. Future work will be focused on the application of our novel chemical stabilization method to these coal ashes to reduce the mobility of elements such as Mo and As, and other potentially leachable elements, and on the use of the resulting ash with aggregate products as a substitute for concrete production.

Treatment of municipal solid waste incineration fly ash: State-of-the-art technologies and future perspectives

Municipal solid waste incineration (MSWI) fly ash is considered as a hazardous waste that requires specific treatment before disposal. The principal treatments encompass thermal treatment, stabilization/solidification, and resource recovery. To maximize environmental, social, and economic benefits, the development of low-carbon and sustainable treatment technologies for MSWI fly ash has attracted extensive interests in recent years. This paper critically reviewed the state-of-the-art treatment technologies and novel resource utilization approaches for the MSWI fly ash. Innovative technologies and future perspectives of MSWI fly ash management were highlighted. Moreover, the latest understanding of immobilization mechanisms and the use of advanced characterization technologies were elaborated to foster future design of treatment technologies and the actualization of sustainable management for MSWI fly ash.

A novel flameless oxidation and in-chamber melting system coupled with advanced scrubbers for a laboratory waste plant

This is the first study integrate the flameless oxidation (FO) and in-chamber melting (ICM) processes in a primary chamber of a laboratory waste incinerator to improve energy and emission performances. Two liquid burners created a twin-cyclonic fluid field that achieved the FO and ICM in the same chamber. The first cyclone provided a well-mixed and lower temperature FO to reduce auxiliary diesel consumption, NOx and PM emissions by 25.8%, 30.9%, and 79.2%, respectively, from the original system. The hot gases produced by FO enhance the ICM process and transformed the bottom ashes to stabler slags, in turn meeting the regulations for nonhazardous wastes. The other cyclone enhanced the drying and water-gas shift reaction in the drying zone by recirculating the CO and enthalpy from FO and ICM. Eventually, the residual CO, hydrocarbons, and H₂ were sent to the secondary chamber for further oxidation. A computational fluid dynamic simulation supported the fluid field assumption posed in this study. Moreover, advanced scrubbers were employed after thermal treatments to reduce HCl and SO₂ by 81.8% and 38.8% and further retarded the corrosion rate in the baghouse supporting cage by 87.7%. The precursors of condensable particulate matter were reduced by condensation and finally removed in the baghouse. Nevertheless, the emissions of the high- and mid-molecular-weight polycyclic aromatic hydrocarbons were greatly reduced by 60.8-93.1% and 80.2-99.9%, respectively. Consequently, the new system reduced annual emissions by 40.7-87.6% and operating costs by 41.5%, allowing recovery of the remodification investment in 20.5 months.

High-efficiency and low-carbon remediation of zinc contaminated sludge by magnesium oxysulfate cement

Electroplating sludge is classified as a hazardous waste due to its extremely high leachability of potentially toxic elements. This study concerns the use of magnesium oxysulfate cement (MOSC) for the stabilisation/solidification (S/S) of Zn-rich electroplating sludge. According to X-ray diffraction and thermogravimetric analyses, Zn was mainly immobilised through both chemical interaction and physical encapsulation in the MOSC hydrates of 5Mg(OH)₂·MgSO₄.7H₂O (5-1-7) phase. The crystal size analysis, elemental mapping, and extended X-ray absorption fine structure (EXAFS) analysis proved that the Zn²⁺ was also incorporated in the structure of 5-1-7 phase. Unlike Portland cement system, hydration kinetics, setting time, and compressive strength of the MOSC system were only negligibly modified by the presence of Zn, indicating its superior compatibility. Subsequent S/S experiments demonstrated that the MOSC binder exhibited an excellent performance on immobilisation efficiency of Zn (up to 99.9%), as well as satisfying the requirements of setting time and mechanical strength of sludge S/S products. Therefore, MOSC could be an effective and sustainable binder for the treatment of the Zn-rich industrial wastes.

Stabilisation/solidification of municipal solid waste incineration fly ash by phosphate-enhanced calcium aluminate cement

Landfill disposal of municipal solid waste incineration fly ash (MIFA) presents significant environmental and economic burden. This study proposed a novel and high-efficiency approach for stabilisation/solidification (S/S) of MIFA by phosphate-modified calcium aluminate cement (CAC). Experimental results showed that the presence of Pb (the most leachable metal contaminant in the MIFA) retarded the early-stage reaction of CAC, resulting in an extension of setting time and a significant decline of compressive strength of CAC pastes. The incorporation of phosphate additives (10 wt% of binder), especially for trisodium phosphate, in CAC system effectively mitigated the negative impact of Pb on the CAC reaction and reduced the Pb leachability. Elemental mapping results illustrated that Pb²⁺ coordinated with phosphate to generate insoluble precipitates (e.g., Pb₃(PO₄)₂). The S/S treated MIFA samples fulfilled the compressive strength and leachability requirements for on-site reuse. Overall, this study demonstrated that phosphate-modified CAC is a promising binder for S/S of hazardous MIFA.

Leaching of As and Se from coal fly ash: fundamental study for coal fly ash recycling

Coal fly ash (CFA) is a useful recycled resource for uses such as cement raw material. To manage and evaluate safety for effective utilization of CFA, the leaching concentration and amounts of toxic elements in CFA need to be determined. In this study, 38 types of CFA and aged CFA generated in Japan were used to measure the occurrence and leaching concentration range of As and Se. In addition, the leaching characteristics over the long term were examined using statistical analysis. Leaching concentrations of As and Se from CFAs were in the range of 0.001-0.163 mg/L (average: 0.025 mg/L, median: 0.014 mg/L) and 0.001-0.189 mg/L (average: 0.071 mg/L, median: 0.055 mg/L), respectively. In general, the concentrations of aged CFAs were less than those of the CFAs with a few exceptions. Leaching concentrations of As and Se in the tank leaching test changed with time, and As and Se concentrations in the dispersions increased with stirring time. In contrast, pH of the dispersion decreased with time. The relation between As or Se and CFA factors showed that As or Se and pH or Ca were highly correlated. However, in aged CFAs for long-term use, the correlation coefficient for the relation between As and other factors was low while that for Se-S was high. Considering the effective utilization of CFA as a long-term recyclable resource, the leaching processes of As and Se in CFA would change with time depending on the environmental conditions.

Structural characterizations of fly ash-based geopolymer after adsorption of various metal ions

Geopolymer, an amorphous substance, has been viewed as good adsorbent or catalyst and attracted much attentions from all over the world. In order to achieve the better applications of geopolymer in these fields, a deep understanding of the microstructure of geopolymer would be strongly required. In the present study, geopolymer was synthesized from coal fly ash, and the structural analysis of geopolymer after adsorption of various metal ions (Li⁺, Cs⁺, Sr²⁺ and Co²⁺) was studied using XRD, SEM-EDX, FTIR, UV-VIS DRS, TG-DTA as well as surface area and pore distribution analysis. Pair distribution function preferably illustrated that geopolymer was successfully prepared from calcined fly ash. Geopolymer possesses different affinities towards various metal ions. After exchanging with other metal ions, the main structure of geopolymer was maintained. Metal ions with a large radius would have greater effect on the existing state and amount of water molecules in geopolymer. Moreover, the specific surface area of geopolymer after exchanging with metal ions decreased as a function of the radius of them. The spectra corresponding to d-d transitions indicated that the Co²⁺ could be incorporated into the deformed six-member rings or eight-membered rings. It could be deduced that the sites for ion exchange could be different rings or even cavities distributed on the surface layer of geopolymer. Furthermore, the rings distributed in the geopolymer structure were predominant in the 6-, 8-, 10- or even 12-member rings to maintain the structure stability and charge balance with the cations.

Evaluation of chemical stabilisation methods of coal-petcoke fly ash to reduce the mobility of Mo and Ni against environmental concerns

Reducing the potential leaching of Mo and Ni from the fly ash (FA) of petroleum coke is an increasingly important issue as Asia and Europe's demand is expected to drastically intensify as continuing urbanisation and technological innovation demands ever more electricity. In the present study, we investigated coal combustion products (CCP) from a large coal-fired power station fed with a 56:44 coal/petroleum coke blend. Results revealed that leachable concentrations of Mo and Ni from FA were in the upper non-hazardous limit and in the inert limit, respectively (2003/33/EC). Whilst common prevention measures for Mo and Ni based on the adsorption capacity of boiler slag (BS), a mixture of BS: goethite, and jarosite, were considered insufficient to reduce the potential leaching of Mo into FA leachates, a novel chemical stabilisation method based on an aggregate product of portlandite and FA immobilised both Mo and Ni such that the resulting concentrations were below the limits established in the abovementioned 2003 EC Decision. Precipitation may be responsible for the fixation of Mo and Ni in the FA: portlandite aggregates as Ca(MoO₄) and NiMoO₄, respectively. The findings of this novel study support the use of this aggregate to reduce FA pollutants, which will be of particular interest to nations that remain largely coal/petroleum coke-dependant.

Red mud-enhanced magnesium phosphate cement for remediation of Pb and As contaminated soil

Lead (Pb) and arsenic (As) contaminated soil poses severe threats to human health. This study proposes a novel approach for synchronous stabilisation/solidification (S/S) of Pb and As contaminated soil and explains the immobilisation mechanisms in red mud-modified magnesium phosphate cement (MPC). Experimental results show that incorporation of red mud in MPC binder retarded over-rapid reaction and enhanced compressive strength via the formation of (Al,Fe,K)PO₄·nH₂O compounds as indicated by X-ray diffractometer (XRD) and elemental mapping. The presence of Pb had a marginal effect on the MPC reaction; however, the presence of As suppressed the generation of MgKPO₄·6H₂O, leading to a significant delay of setting time and a reduction of compressive strength. Extended X-ray absorption fine structure (EXAFS) analysis proved that Pb²⁺ strongly coordinated with the PO₄^3-, whereas AsO₂- gently coordinated with K⁺. The MPC binder displayed an excellent immobilisation efficiency for Pb (99.9%), but was less effective for As. The use of red mud enhanced the As immobilisation efficacy to 80.5% due to strong complexation between AsO₂- and Fe³⁺. The treated soils fulfilled requirements of metal(loid) leachability and mechanical strength for on-site reuse. Therefore, red mud-modified MPC can be an effective binder for sustainable remediation of Pb and As contaminated soil.

Immobilizing Pertechnetate in Ettringite via Sulfate Substitution

Technetium-99 immobilization in low-temperature nuclear waste forms often relies on additives that reduce environmentally mobile pertechnetate (TcO₄^-) to insoluble Tc(IV) species. However, this is a short-lived solution unless reducing conditions are maintained over the hazardous life cycle of radioactive wastes (some ∼10,000 years). Considering recent experimental observations, this work explores how rapid formation of ettringite [Ca₆Al₂(SO₄)₃(OH)₁₂·26(H₂O)], a common mineral formed in cementitious waste forms, may be used to directly immobilize TcO₄^-. Results from ab initio molecular dynamics (AIMD) simulations and solid-phase characterization techniques, including synchrotron X-ray absorption, fluorescence, and diffraction methods, support successful incorporation of TcO₄^- into the ettringite crystal structure via sulfate substitution when synthesized by aqueous precipitation methods. One sulfate and one water are replaced with one TcO₄^- and one OH^- during substitution, where Ca²⁺-coordinated water near the substitution site is deprotonated to form OH^- for charge compensation upon TcO₄^- substitution. Furthermore, AIMD calculations support favorable TcO₄^- substitution at the SO₄^2- site in ettringite rather than gypsum (CaSO₄·2H₂O, formed as a secondary mineral phase) by at least 0.76 eV at 298 K. These results are the first of their kind to suggest that ettringite may contribute to TcO₄^- immobilization and the overall lifetime performance of cementitious waste forms.

A review of soil carbon dynamics resulting from agricultural practices

Literature related to the carbon cycle and climate contains contradictory results with regard to whether agricultural practices increase or mitigate emission of greenhouse gases (GHGs). One opinion is that anthropogenic activities have distinct carbon footprints - measured as total emissions of GHGs resulting from an activity, in this case, "agricultural operations". In contrast, it is argued that agriculture potentially serves to mitigate GHGs emissions when the best management practices are implemented. We review the literature on agricultural carbon footprints in the context of agricultural practices including soil, water and nutrient management. It has been reported that the management practices that enhance soil organic carbon (SOC) in arid and semi-arid areas include conversion of conventional tillage practices to conservation tillage approaches. We found that agricultural management in arid and semi-arid regions, which have specific characteristics related to high temperatures and low rainfall conditions, requires different practices for maintenance and restoration of SOC and for control of soil erosion compared to those used in Mediterranean, tropical regions. We recommend that in order to meet the global climate targets, quantification of net global warming potential of agricultural practices requires precise estimates of local, regional and global carbon budgets. We have conducted and present a case study for observing the development of deep soil carbon profile resulting from a 10-year wheat-cotton and wheat-maize rotation on semi-arid lands. Results showed that no tillage with mulch application had 14% (37.2 vs 43.3 Mg ha^-1) higher SOC stocks in comparison to conventional tillage with mulch application. By implementing no tillage in conjunction with mulch application, lower carbon losses from soil can mitigate the risks associated with global warming. Therefore, it is necessary to reconsider agricultural practices and soil erosion after a land-use change when calculating global carbon footprints.

Co-sorption of Sr2+ and SeO42- as the surrogate of radionuclide by alginate-encapsulated graphene oxide-layered double hydroxide beads

Graphene oxides (GO) and layered double hydroxides (LDHs) were applied to produce alginate beads for the remove of ⁹⁰Sr²⁺ and ⁷⁹SeO₄^2-. The Freundlich isotherm indicated that the Sr²⁺ sorptions were based on the energetically heterogeneous multilayer surfaces. In contrast, the sorption behavior of SeO₄^2- fitted to the Langmuir adsorption isotherm models, indicating that the removal of SeO₄^2- was caused by the ion-exchange of LDHs. The synthesized LDH/GO alginates beads were also applied for setting up small-bore adsorption columns with loading synthetic SeO₄^2- and Sr²⁺ contaminated wastewater. Based on the water chemistry, the adsorbed amount of Sr²⁺ significantly increased after using alginates beads, which was attributed to the functional groups of either GO or alginic acid. The incorporated SeO₄^2- was highly depended on the contents of fabricated LDHs in alginate beads. Specifically, the adsorption capacity of Sr²⁺ (0.85-0.91 mmol/g) on GO slightly increased after alginates fabrication. Therefore, it was deduced that this layered material was partially exfoliated during the manufacture and thus increased the sorption sites. Applications of LDH/GO alginates beads in the removal of both Sr²⁺ and SeO₄^2- in water and soil treatment have a significant impact on the environmental remediation.

The leaching mechanism of heavy metals (Ni, Cd, As) in a gasification slag during acidification

The gasification slag by acidification can leach abundant heavy metals. In this paper, the fate of heavy metals (Ni, Cd, and As) in the raw slag and the acidified slag that treated by HAc and HCl was systematically investigated combined with Density Functional Theory (DFT) calculations. The results show that the content of Ni and Cd is reduced with an increasing acid concentration and meets the regulatory standards by 7 M HAc and 3 M HCl, respectively. Most of Ni combined with gehlenite is released as gehlenite dissolves during acid treatment, whereas Cd in combination with gehlenite and iron compounds is hard to release at lower HAc concentrations. Unexpectedly, the content of As tends to elevate at a higher concentration of HAc, which is due to the increase in the content of Ca by new Ca-compound formation and the higher binding capacity of Ca to As according to DFT results. Additionally, if the acid-base ratio reaches about 2.0 by acid treatment, there would be a maximum leaching rate. It is recommended that acid concentration should be controlled to avoid a secondary risk of heavy metals.

Characterization and environmental implications of selenate co-precipitation with barite

This study investigated the sequestration of dissolved selenate (SeO₄^2-) via co-precipitation in barite for a range of SeO₄^2- concentrations (0-~8650 mg/L), as well as its release at near neutral pH conditions (pH = ~5.5-6.5). Solid precipitates were characterized via X-ray diffraction and subsequent Rietveld refinements, Raman spectroscopy, Brunauer-Emmett-Teller surface area analyses, scanning electron microscopy, electron probe microanalyses (EPMA), inductively coupled plasma optical emission spectroscopy (ICP-OES), and X-ray absorption spectroscopy (XAS). ICP-OES results suggested barite efficiently removed >99% of SeO₄^2- from the test solutions during all co-precipitation experiments. EPMA results showed the SeO₄^2- was sequestered from the aqueous phase via co-precipitation with barite. XAS analyses indicated the SeO₄^2- tetrahedron is incorporated into the barite structure by substituting for sulfate (SO₄^2-) and bonding to Ba²⁺ atoms through bidentate mononuclear and bidentate binuclear complexes. Dissolution data showed the release of SeO₄^2- sequestered in barite to the aqueous phase is unlikely due to the low solubility and stability of the barite phase. As such, co-precipitation of SeO₄^2- with barite could be effective for removing SeO₄^2- from waters affected by mining and metallurgical operations.

Mechanism analysis of selenium (VI) immobilization using alkaline-earth metal oxides and ferrous salt

The immobilization of selenate (SeO₄^2-) 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 SeO₄^2- was reduced to SeO₃^2- in the CaO-based reaction within 7 days. Then, the generated SeO₃^2- was mainly sorbed onto the iron-based minerals (Fe₂O₃ 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 SeO₄^2- (approximately 45.50%) was reduced to SeO₃^2- 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 (SeO₃^2- and SeO₄^2-) 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.

Spectroscopic and first-principles investigations of iodine species incorporation into ettringite: Implications for iodine migration in cement waste forms

Low-level radioactive wastes are commonly immobilized in cementitious materials, where cement-based material can incorporate radionuclides into their crystal structure. Specifically, ettringite (Ca₆Al₂(OH)₁₂(SO₄)₃∙26H₂O) is known to stabilize anionic species, which is appealing for waste streams with radioactive iodine (¹²⁹I) that persists as iodide (I-) and iodate (IO₃-) 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 IO₃- exhibited a high affinity for ettringite via anion substitution for SO₄^2- (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 IO₃- was stabilized in ettringite by hydrogen bonding and electrostatic forces. Substituting IO₃- for SO₄^2- 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 IO₃- 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.

Immobilization mechanism of Se oxyanions in geopolymer: Effects of alkaline activators and calcined hydrotalcite additive

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 SeO₃^2- and SeO₄^2- 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 SeO₃^2- and SeO₄^2- (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 SeO₃^2- and SeO₄^2- 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 SeO₃^2- and SeO₄^2- 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.

Leaching of elements from cement activated fly ash and slag amended soils

Very few studies have investigated the leaching characteristics of cement activated fly ash and slag treated soils, although the inclusion of cement significantly enhances the material pH and may alter the leachability of elements. In this study the leaching behavior and mechanisms of chromium (Cr), copper (Cu), iron (Fe) and sulfur (S) from cement activated fly ash and slag stabilized soils were evaluated. An array of synthetic precipitation leaching procedure (SPLP), batch water leach test (WLT), toxicity characteristic leaching procedure (TCLP) and pH-Static leach tests were conducted. A geochemical equilibrium model Visual MINTEQ was implemented to identify the leaching controlling mechanisms of the metals. Results indicated that, the leached concentrations of Cr, Cu, Fe and S in SPLP, WLT and TCLP effluents were in the range of 0.016-0.74 mg/L, 0.013-0.17 mg/L, 0.019-0.27 mg/L and 1.78-234 mg/L, respectively. Quantitative comparisons between the standard test procedures suggested the necessity of multiple test methods for a comprehensive leaching assessment. Cr and Cu showed amphoteric leaching behaviors, whereas Fe and S followed cationic leaching patterns. According to the geochemical analyses, amorphous Cr(OH)₃; tenorite and Cu(OH)₂; ferrihydrite and goethite; gypsum and anhydrite; could control the leaching of Cr, Cu, Fe and S, respectively. The effluent Cr concentrations frequently exceeding the U.S. EPA specified maximum contaminant level of 0.1 mg/L. Yet, the use of cement activated fly ash and slag mixed soils could be beneficial, since less toxic trivalent Cr (III) was identified through geochemical modeling.

Immobilization of cesium in fly ash-silica fume based geopolymers with different Si/Al molar ratios

Geopolymers are considered as promising matrixes for waste solidification. However, the effects of the Si/Al molar ratio of geopolymer on the immobilization efficiencies for metal ions have not been fully studied and understood. In the present study, geopolymers with different Si/Al ratios were synthesized from coal fly ash and silica fume. Adsorption tests were conducted to evaluate their immobilization efficiencies for Cs⁺. The results indicated that geopolymer with low Si/Al ratio could have a better immobilization performance for Cs⁺ than that with high Si/Al ratio. High Si/Al ratio could contribute to a more compact structure of geopolymer. Each sorption process fitted better with the pseudo-second-order model, and all of them were governed by film diffusion. However, the diffusion mode was gradually closed to particle diffusion with the increase in the Si/Al ratio. Both Langmuir and Freundlich models could well fit the sorption data, and the free energy of each sorption process decreased with the increase in the Si/Al ratio according to D-R equation. The distribution of AlO₄ tetrahedron in the geopolymer structure plays a significant role in the immobilization of Cs⁺. Low Si/Al ratio could result in that more AlO₄ tetrahedrons distribute in the small rings (<eight-member), which has stronger locking effects on Cs⁺. However, high Si/Al ratio leads to the distribution of AlO₄ tetrahedrons mainly in larger rings (≥eight-member), and this could contribute to the high leaching amount of Cs⁺. In addition, high-temperature treatment could contribute to the formation of nepheline or pollucite in geopolymer matrix. These minerals locked Cs⁺ in their structures, and the leaching amount of Cs⁺ was reduced correspondingly from high levels (26.36%, 27.26%, and 66.92%) to very low levels (0.67%, 0.53%, and 0.95%).