Evidence of Multiple Sorption Modes in Layered Double Hydroxides Using Mo As Structural Probe

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.

Super-stable mineralization of Cu, Cd, Zn and Pb by CaAl-layered double hydroxide: Performance, mechanism, and large-scale application in agriculture soil remediation

The synthesized CaAl-layered double hydroxide (CaAl-LDH) shows excellent performance in potentially toxic metals (PTMs) removal, and the removal capacity of CaAl-LDH toward Cu²⁺, Zn²⁺ and Pb²⁺ in aqueous solution is 502.4, 315.2 and 600.0 mg/g respectively. Cu²⁺ and Zn²⁺ are removed through isomorphic substitution of laminate Ca and dissolution-reprecipitation, leading to the formation of CuAl-LDH and ZnAl-LDH mineralization products. Pb²⁺ is removed by the complexation and precipitation to form Pb₃(CO₃)₂(OH)₂. The application of CaAl-LDH in laboratory-scale soil remediation shows that target PTMs are gradually mineralized into relatively stable oxidizable and residual state, and the immobilization efficiency of available Cu, Zn, Cd and Pb reaches 84.62 %, 98.66 %, 96.81 % and 70.27 % respectively. In addition, practical application in farmland results in the significant reduction of available Cu, Zn, Cd and Pb with the immobilization efficiency of 30.15 %, 67.30 % and 57.80 % and 38.71 % respectively. Owing to the super-stable mineralization effect of CaAl-LDH, the content of PTMs in the roots, stems and grains of cultivated buckwheat also decreases obviously, and the growth and yield of buckwheat are not adversely affected but improved. The above prove that the super-stable mineralization based on CaAl-LDH is a promising scheme for the remediation of PTMs contaminated agriculture soil.

Regulation of Structure and Anion-Exchange Performance of Layered Double Hydroxide: Function of the Metal Cation Composition of a Brucite-like Layer

As anion-exchange materials, layered double hydroxides (LDHs) have attracted increasing attention in the fields of selective adsorption and separation, controlled drug release, and environmental remediation. The metal cation composition of the laminate is the essential factor that determines the anion-exchange performance of LDHs. Herein, we review the regulating effects of the metal cation composition on the anion-exchange properties and LDH structure. Specifically, the internal factors affecting the anion-exchange performance of LDHs were analyzed and summarized. These include the intercalation driving force, interlayer domain environment, and LDH morphology, which significantly affect the anion selectivity, anion-exchange capacity, and anion arrangement. By changing the species, valence state, size, and mole ratio of the metal cations, the structural characteristics, charge density, and interlayer spacing of LDHs can be adjusted, which affect the anion-exchange performance of LDHs. The present challenges and future prospects of LDHs are also discussed. To the best of our knowledge, this is the first review to summarize the essential relationship between the metal ion composition and anion-exchange performance of laminates, providing important insights for regulating the anion-exchange performance of LDHs.

Solidification/stabilization of highly toxic arsenic-alkali residue by MSWI fly ash-based cementitious material containing Friedel's salt: Efficiency and mechanism

Arsenic-alkali residue (AAR) and MSWI fly ash (MFA) are hazardous wastes, which still lack effective treatment methods. In this study, a novel solidification/stabilization (S/S) method for AAR with MFA-based cementitious material (MFA-CM) containing Friedel's salt was proposed. The efficiency and mechanism of S/S was mainly focused. Abundant Friedel's salt as well as a few C-S-H gel and ettringite (AFt) were found as hydration products of MFA-CM. 12% of AAR was well solidified/stabilized by MFA-CM, accompanied by As leaching concentration reducing from 10,687 mg/L to less than 5 mg/L. In order to investigate S/S mechanism of As, removal mechanism of As during co-precipitation synthesis of Friedel's salt was studied. During co-precipitation process, As was successively removed by formation of calcium arsenate precipitates, formation of As-Friedel's salt (replacement of Cl^- by AsO₄^3-), and adsorption of Friedel's salt. The S/S mechanism of As by MFA-CM was found to be similar to the removal mechanism of As during co-precipitation. With the prolonging of curing time, As was mainly solidified/stabilized by formation of calcium arsenate precipitates and As-Friedel's salt, and adsorption of Friedel's salt. Thus, this study provides a novel harmless treatment method for highly toxic arsenic-containing wastes by "treating the wastes with wastes".

Mechanisms and thermodynamic modelling of iodide sorption on AFm phases

Both, experimental and modelling evidence is presented in this study showing that interlayer anion exchange is the dominant sorption mechanism for iodide (I^-) on AFm phases. AFm phases are Ca-Al(Fe) based layered double hydroxides (LDH) known for their large potential for the immobilization of anionic radionuclides, such as dose-relevant iodine-129, emanating from low- and intermediate-level radioactive waste (L/ILW) repositories. Monosulfate, sulfide-AFm, hemicarbonate and monocarbonate are safety-relevant AFm phases, expected to be present in the cementitious near-field of such repositories. Their ability to bind I^- was investigated in a series of sorption and co-precipitation experiments. The sorption of I^- on different AFm phases was found to depend on the type of the interlayer anion. Sorption R_d values are very similar for monosulfate, sulfide-AFm and hemicarbonate. A slightly higher uptake occurs by AFm phases with a singly charged anion in the interlayer (HS-AFm) as compared to AFm with divalent ions (monosulfate), whereas uptake by hemicarbonate is intermediate. No significant sorption occurs onto monocarbonate. Our derived thermodynamic solid solution models reproduce the experimentally obtained sorption isotherms on HS-AFm, hemicarbonate and monosulfate, indicating that anion exchange in the interlayer is the dominant mechanism and that the contribution of I^- electrostatic surface sorption to the overall uptake is negligible.

Ultrastrong Anion Affinity of Anionic Clay Induced by Its Inherent Nanoconfinement

Layered double hydroxide (LDH), the only anionic clay in the environment, plays a key role in natural ion transportation. The ion retention effect of LDHs was traditionally attributed to ion exchange with low affinity. Here, we demonstrated an ultrastrong interaction between anions and LDHs induced by their inherent nanoconfinement using chromium ore processing residue (COPR) that contained several Cr(VI)-bonded LDHs as a probe. Hydrocalumite (Ca/Al-Cl LDH) was verified as the primary phase for Cr(VI) retention through two types of interactions such as ion exchange and Cr-Ca coordination. More significantly, the confined spacing between two layers of hydrocalumite provided spatial restriction and shielding effects to the intercalated Cr(VI), which enhanced Cr-Ca coordination by shortening the bonding distance and modulating the binding angle to achieve the lowest bonding energy. Such enhancement boosted Cr(VI) affinity up to 3.2 × 10⁵ mL/g, which was 1-3 orders of magnitudes higher than ion exchange. The universality of this mechanism was verified using another Mg/Al-Cl LDH and various anions. This study broke the traditional awareness of low ion affinities of LDHs limited by single ion exchange and disclosed an essential mechanism for unexpected ion retention effects of anionic clays in nature.

Selenite Sorption on Hydrated CEM-V/A Cement in the Presence of Steel Corrosion Products: Redox vs Nonredox Sorption

Reinforced cementitious structures in nuclear waste repositories will act as barriers that limit the mobility of radionuclides (RNs) in case of eventual leakage. CEM-V/A cement, a ternary blended cement with blast furnace slag (BFS) and fly ash (FA), could be qualified and used in nuclear waste disposal. Chemical interactions between the cement and RNs are critical but not completely understood. Here, we combined wet chemistry methods, synchrotron-based X-ray techniques, and thermodynamic modeling to explore redox interactions and nonredox sorption processes in simulated steel-reinforced CEM-V/A hydration systems using selenite as a molecular probe. Among all of the steel corrosion products analyzed, only the addition of Fe⁰ can obviously enhance the reducing ability of cement toward selenite. In comparison, steel corrosion products showed stronger reducing power in the absence of cement hydrates. Selenium K-edge X-ray absorption spectroscopy (XAS) revealed that selenite immobilization mechanisms included nonredox inner-/outer-sphere complexations and reductive precipitations of FeSe and/or Se(0). Importantly, the hydrated pristine cement showed a good reducing ability, driven by ferrous phases and (bi)sulfides (as shown by sulfur K-edge XAS) originated from BFS and FA. The overall redox potential imposed by hydrated CEM-V/A was determined, hinting to a redox shift in underground cementitious structures.

Achieving arsenic concentrations of <1 μg/L by Fe(0) electrolysis: The exceptional performance of magnetite

Consumption of drinking water containing arsenic at concentrations even below the World Health Organization provisional limit of 10 μg/L can still lead to unacceptable health risks. Consequently, the drinking water sector in the Netherlands has recently agreed to target 1 μg/L of arsenic in treated water. Unfortunately, in many poor, arsenic-affected countries, the costs and complexity of current methods that can achieve <1 μg/L are prohibitive, which highlights the need for innovative methods that can remove arsenic to <1 μg/L without costly support infrastructure and complicated supply chains. In this work, we used Fe(0) electrolysis, a low cost and scalable technology that is also known as Fe(0) electrocoagulation (EC), to achieve <1 μg/L residual dissolved arsenic. We compared the arsenic removal performance of green rust (GR), ferric (oxyhydr)oxides (Fe(III) oxides) and magnetite (Mag) generated by EC at different pH (7.5 and 9) in the presence of As(III) or As(V) (initial concentrations of 200-11,000 μg/L). Although GR and Fe(III) oxides removed up to 99% of initial arsenic, neither Fe phase could reliably meet the 1 μg/L target at both pH values. In contrast, EC-generated Mag consistently achieved <1 μg/L, regardless of the initial As(V) concentration and pH. Only solutions with initial As(III) concentrations ≥2200 μg/L resulted in residual arsenic >1 μg/L. As K-edge X-ray absorption spectroscopy showed that Mag also sorbed arsenic in a unique mode, consistent with partial arsenic incorporation near the particle surface. This sorption mode contrasts with the binuclear, corner sharing surface complex for GR and Fe(III) oxides, which could explain the difference in arsenic removal efficiency among the three Fe phases. Our results suggest that EC-generated Mag is an attractive method for achieving <1 μg/L particularly in decentralized water treatment.

Adsorption of low-concentration mercury in water by 3D cyclodextrin/graphene composites: Synergistic effect and enhancement mechanism

The efficient removal of mercury from aqueous media remains a severe challenge in ensuring environmental safety, especially for low-concentration mercury, which requires adsorbents with high mercury affinity. In this work, we reported a nanocomposite of β-cyclodextrin and three-dimensional graphene (3D CD@RGO) to enhance the adsorption affinity and capacity for mercury with low concentrations. Characterization of the nanocomposite revealed that cyclodextrin was well dispersed on the 3D graphene support structure to provide highly exposed hydroxyl groups. Adsorption experiments showed that CD@RGO exhibited different adsorption behaviors for mercury within different concentration ranges of 0.2-4.0 mg/L and 4.0-10.0 mg/L, and the adsorption affinity for the former range (K_L = 10.05 L/mg) was 1.5 times higher than that for the latter range (K_L = 6.69 L/mg). Moreover, CD@RGO had a high adsorption efficiency of 96.6% with a superb adsorption affinity (172.09 L/g) at C_e = 0.01 mg/L, which is 6.70 and 41.25 times higher than that of RGO and RCD (physical mixture of RGO and cyclodextrin), respectively, indicating a synergistic effect of CD@RGO for mercury adsorption. This enhancement can be attributed to the transformation of the adsorption mechanism from the outer-sphere force of electrostatic interaction in RGO to the inner-sphere surface complexation in CD@RGO.

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

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.

Double-Edged Effect of Humic Acid on Multiple Sorption Modes of Calcined Layered Double Hydroxides: Inhibition and Promotion

Layered double hydroxides (LDHs) are a typical class of anionic clay minerals whose structural memory effect has been widely used in pollutant adsorption. However, the influencing mechanism of humic acid (HA) on the structural memory effect in adsorption is not clear. In this study, HA was extracted from black soil and sediments, and its effect on the structural memory effect of LDHs with different divalent metals was evaluated in adsorption. Borate complexed with HAs and HAs promoted the dissolution of magnesium-calcined LDHs (Mg-CLDH), which enhanced their adsorption rate by Mg-CLDH. However, the adsorbed HA caused a decline in the crystallinity of the regenerated Mg-LDH and an incomplete structural transformation, thereby resulting in decreased adsorption capacity. After the complexation of HAs with borate, the resulting compound was adsorbed on the surface of Zn-CLDH. The adsorption rate of borate was effectively improved in the initial stage, but at the same time slowed down the hydration and structural regeneration of Zn-CLDH. Meanwhile, the surface-adsorbed HAs also prevented borate from entering the newly formed layer inside the particles and led to a significant decrease in adsorption performance. When Ca-CLDH was used to adsorb borate, the process mainly occurred through the formation of ettringite. However, the presence of HAs enhanced the stability of the restructured LDHs and hindered the dissolution of Ca-CLDH and the reaction with B(OH)₄^- to form ettringite during the regeneration process, which severely inhibited the sorption of borate.

XANES-Based Determination of Redox Potentials Imposed by Steel Corrosion Products in Cement-Based Media

The redox potential (Eh) in a cementitious nuclear waste repository is critical to the retardation behavior of redox-sensitive radionuclides (RNs), and largely controlled by embedded steel corrosion but hard to be determined experimentally. Here, we propose an innovative Eh determination method based on chemical/spectroscopic measurements. Oxidized nuclides (U^VI, Se^IV, Mo^VI, and Sb^V) were employed as species probes to detect the Eh values imposed by steel (Fe⁰) and steel corrosion products (magnetite/hematite, and magnetite/goethite couples) in cement pore water. Nuclides showed good sorption affinity, especially toward Fe⁰, in decreasing K_d order for U > Sb > Se > Mo under both N₂ and H₂ atmospheres. The reduced nuclide species were identified as UO₂, U₄O₉, FeSe, FeSe₂, Se⁰, Sb⁰, and Sb₂O₃, but no redox transformation occurred for Mo. Eh values were obtained by using the Nernst equation. Remarkably, their values fell in a small range centered around -456 mV at pH ∼ 13.5 for both Fe⁰ and Fe-oxyhydroxides couples. This Eh value appears to be controlled by the nanocrystalline Fe(OH)₂/Fe(OH)₃ or (Fe_{1- x},Ca _x)(OH)₂/Fe(OH)₃ couple, whose presence was confirmed by pair distribution function analyses. This approach could pave the way for describing the Eh gradient in reinforced concrete where traditional Eh measurements are not feasible.

Thermodynamic and crystallographic model for anion uptake by hydrated calcium aluminate (AFm): an example of molybdenum

Amongst all cement phases, hydrated calcium aluminate (AFm) plays a major role in the retention of anionic species. Molybdenum (Mo), whose ⁹³Mo isotope is considered a major steel activation product, will be released mainly under the form of MoO₄²⁻ in a radioactive waste repository. Understanding its fate is of primary importance in a safety analysis of such disposal. This necessitates models that can both predict quantitatively the sorption of Mo by AFm and determine the nature of the sorption process (i.e., reversible adsorption or incorporation). This study investigated the Cl⁻/MoO₄²⁻ exchange processes occurring in an AFm initially containing interlayer Cl in alkaline conditions using flow-through experiments. The evolution of the solid phase was characterized using an electron probe microanalyzer and synchrotron high-energy X-ray scattering. All data, together with their quantitative modeling, coherently indicated that Mo replaced Cl in the AFm interlayer. The structure of the interlayer is described with unprecedented atomic-scale detail based on a combination of real- and reciprocal-space analyses of total X-ray scattering data. In addition, modeling of several independent chemical experiments elucidated that Cl⁻/OH⁻ exchange processes occur together with Cl⁻/MoO₄²⁻ exchange. This competitive effect must be considered when determining the Cl⁻/MoO₄²⁻ selectivity constant.

Selenite Uptake by Ca-Al LDH: A Description of Intercalated Anion Coordination Geometries

Layered double hydroxides (LDHs) are anion exchangers with a strong potential to scavenge anionic contaminants in aquatic environments. Here, the uptake of selenite (SeO₃^2-) by Ca-Al LDHs was investigated as a function of Se concentration. Thermodynamic modeling of batch sorption isotherms shows that the formation of SeO₃^2--intercalated AFm (hydrated calcium aluminate monosubstituent) phase, AFm-SeO₃, is the dominant mechanism controlling the retention of Se at medium loadings. AFm-Cl₂ shows much stronger affinity and larger distribution ratio (R_d ∼ 17800 L kg^-1) toward SeO₃^2- than AFm-SO₄ (R_d ∼ 705 L kg^-1). At stoichiometric SeO₃^2- loading for anion exchange, the newly formed AFm-SeO₃ phase results in two basal spacing, i.e., 9.93 ± 0.06 Å and ∼11.03 ± 0.03 Å. Extended X-ray absorption fine structure (EXAFS) spectra indicate that the intercalated SeO₃^2- forms inner-sphere complexes with the Ca-Al-O layers. In situ X-ray diffraction (XRD) shows that basal spacing of Ca-Al LDHs have a remarkable linear relationship with the size of hydrated intercalated anions (i.e., Cl^-, SO₄^2-, MoO₄^2-, and SeO₃^2-). Contrary to AFm-SeO₃ with inner-sphere SeO₃^2- complexes in the interlayer, the phase with hydrogen-bonded inner-sphere complexed SeO₃^2- is kinetically favored but thermodynamically unstable. This work offers new insights about the determination of intercalated anion coordination geometries via XRD analyses.

Fulvic acid anchored layered double hydroxides: A multifunctional composite adsorbent for the removal of anionic dye and toxic metal

A novel multifunctional composite adsorbent which possesses the ability for anion exchange and toxic metal complexation has been synthesized by the hybridization of layered double hydroxides (LDH) and fulvic acid (FA) in this study. The results show that FA with lots of functional groups can be effectively and stably anchored on the surface of LDH through coagulation process without occupying the interlayer of LDH. Therefore, the anion exchange ability remains and the adsorption capacity of Orange II can reach 1.9mmol/g, which is almost as much as stoichiometric anion exchange capacity of pure LDH. Moreover, the composite adsorbent's adsorption capacity of Cu²⁺, Pb²⁺, Ni²⁺ and Cd²⁺ can also get to 2.25mmol/g, 0.98mmol/g, 0.99mmol/g and 0.16mmol/g respectively with an adsorption preference order of Cu²⁺>Pb²⁺>Ni²⁺>Cd²⁺. In addition, Orange II and toxic metals are able to be simultaneously removed by this composite adsorbent, and the adsorption of toxic metals can be enhanced by the synergetic adsorption of Orange II. Anion exchange with Cl^- in LDH matrix accounts for the adsorption of Orange II, while the adsorption of toxic metal is mainly attributed to the complexation of carboxyl functional group derived from FA.