High adsorption performance of synthesized hexametaphosphate green rust towards Cr(VI) removal and its mechanism explorations

In-situ groundwater remediation of contaminant mixture of As(III), Cr(VI), and sulfanilamide via electrochemical degradation/transformation using pyrite

Electrochemical advanced oxidation processes (EAOPs) are effective in removing persistent contaminants from groundwater. However, their practical applicability depends significantly on various site-specific characteristics. Therefore, the primary objective of this investigation was to study the feasibility of EAOPs and pyrite, which is a sulfide mineral, to effectively remove the mixture of arsenic (As (III)), chromium (Cr (VI)), and sulfanilamide in groundwater. We conducted a comparison of three systems: (1) EAOP alone, (2) pyrite alone, and (3) a combined EAOP and pyrite system. In EAOP alone, sulfanilamide was effectively oxidized (80%), while the electrochemical transformation of As(III)/Cr(VI) into As(V)/Cr(III) was limited. In just the pyrite system, As(III), Cr(VI), and sulfanilamide were adsorbed onto the surface of pyrite (60%, 20%, and 18%). Neither the EAOP nor the pyrite system alone could effectively treat the contaminants mixture. Nonetheless, the combined system completely removed As(III), Cr(VI), and sulfanilamide by the synergistic reaction. This could be attributed to the formation of green rust, a natural adsorbent mineral produced as a reaction of dissolved iron, generated via electrochemical pyrite oxidation, with the groundwater electrolyte (e.g., CO3 or SO4). This system harmonized the combined approach of EAOP and pyrite to effectively eliminate both organic and inorganic contaminants. ENVIRONMENTAL IMPLICATION: A paper proposed electrochemical oxidation (EO) with pyrite to remove both organic and inorganic contaminants from groundwater. The removal performance of the combined system was evaluated, and the synergistic mechanism was revealed. The combination of EO and pyrite with synergistic removal effectively removed the mixture of both contaminants. This could be attributed by the formation of green-rust by electrochemical activation for pyrite. Compared to the single system of EO and pyrite alone, the combined system with EO and pyrite improved removal performance. Results suggested that the combined system could be used for green groundwater remediation.

Synthesis and characterization of green rust-deposited MoS2 composites for adsorptive removal of EDTA-chelated Ni(II) in wastewater

Ethylenediamminetetraacetatonickel(II) (EDTA-Ni(II)) has emerged as a significant soil and groundwater contaminant due to the increasing agricultural and industrial activities, posing environmental challenges. This study focuses on addressing the reactivity of green rust (GR), which can be hindered by oxidation with oxygen, limiting its effectiveness in remediation processes. To overcome this limitation and enhance the adsorptive capacities, the combination of sulfate green rust (SO4-GR) with various Fe(II)/Fe(III) ratios with a high-surface-area adsorbent, MoS2, resulting in the formation of binary composites of green rust-deposited MoS2 (MSGs) were explored. The aim was to improve the removal efficiency of EDTA-Ni(II) from contaminated wastewater. To characterize the MSGs, a comprehensive analysis using XRD, SEM, TEM, FTIR, and X-ray absorption spectroscopy was performed. The surface areas of the MSGs were smaller than that of MoS2 but larger than that of the SO4-GRs, indicating a promising composite material. XANES spectra analysis revealed that both MSGs and SO4-GRs exhibited a mixture of ferrous and ferric ions, as evident from their spectral positioning between FeO and Fe2O3. The optimal pH for efficient removal of EDTA-Ni(II) was 3, which resulted in removal efficiencies of 45.6%, 47.3%, 46.0%, and 46.2% for MSG 1, MSG 2, MSG 3, and MSG 4 after 24 h, respectively. Reducing the initial concentration of EDTA-Ni(II) to 50 mg Ni(II)/L effectively doubled the removal efficiency. Notably, as EDTA-Ni(II) was removed, an increased leaching of iron was observed, leading to a total iron concentration exceeding 40 mg/L for the composites with higher Fe(II)/Fe(III) ratios. These findings underscore the potential of MSG as a promising material for degrading EDTA-Ni(II) in contaminated wastewater, offering a viable solution to mitigate the environmental impact of this emerging contaminant. This study contributes to the understanding of green rust reactivity and provides valuable insights for developing effective strategies to address the challenges associated with EDTA-Ni(II) contamination.

A comprehensive investigation of zeolite-rich tuff functionalized with 3-mercaptopropionic acid intercalated green rust for the efficient removal of HgII and CrVI in a binary system

In this study, the 3-mercaptopropionic acid (MA) was chosen to achieve the anionic intercalation into the green rust (GR) materials (MA-GR). The zeolite-rich tuff functionalized with the MA-intercalated GR (MA-GR-tuff) was subsequently synthesized and used to remove both Hg^II cations and Cr^VI anions in a binary system. MA-GR-tuff showed the best adsorption capacities to both Hg^II and Cr^VI among the adsorbent materials. The optimal combination of parameters was determined as the molar ratio of Fe^II to Fe^III of 3.5, the molar ratio of OH^- to the total iron of 3.75, the molar ratio of MA to the total iron of 2.5, and the mass ratio of the total iron to the tuff of 1.25. The pseudo-first-order kinetic model was appropriate in describing the kinetic sorption of Cr^VI by MA-GR-tuff. Both the pseudo-first-order kinetic model and Elovich were suitable for explaining Hg^II sorption. The maximum monolayer adsorption capacities of MA-GR-tuff towards Cr^VI and Hg^II were 185.19 mg/g and 72.99 mg/g, respectively. More flocs and plumes were formed in the MA-GR while the intercalation and more pores and crevices of different sizes were found in the MA-GR-tuff. Sulfhydryl complexation and the molecular sieve of tuff obviously both played a role in influencing the adsorption process. This study directly overcomes the drawback brought by the natural tuff to the treatment of a cationic-and-anionic binary system and supplies a new kind of tuff-based adsorbent for the potential use for the remediation of HM-contaminated wastewater.

Preparation and permeation recognition mechanism of Cr(vi) ion-imprinted composite membranes

The Cr(vi) ion-imprinted composite membranes (Cr(vi)-IICMs) were prepared by using the surface imprinting method. The template ion was Cr(vi), the functional monomer was 4-vinylpyridine (4-VP), and the nylon filter membrane (nylon-6) was the support membrane. Non-imprinted composite membranes (NICMs) were prepared under the same conditions as the corresponding Cr(vi)-IICM. The adsorption effect of the imprinted membrane can reach 2.4 times that of the corresponding non-imprinted membrane. Meanwhile, the adsorption quantity of Cr(vi)-IICM was 34.60 μmol·g−1. The physical characteristics of membranes were confirmed by Brunauer–Emmett–Teller and scanning electron microscopy. Inductively coupled plasma emission spectrometry was used to analyze their adsorption properties and permeation selectivity. Cr(vi)-IICM and NICM were both mesoporous materials from the structural characterization and performance test results. Their adsorption behavior conformed to the Langmuir isotherm adsorption model. The permeation recognition mechanism of Cr(vi)-IICM was the Piletsky’s gate model. The IICM still has excellent permeability selectivity to Cr(vi) in the presence of competitive ions. The results provided a reference for the isolation and enrichment of Cr(vi).

Enhancement of the heterogeneous adsorption and incorporation of uraniumVI caused by the intercalation of β-cyclodextrin into the green rust

The influence of intercalated anions on the structure and composition of green rusts supplies a theoretical possibility for the investigation of the structural modification of Fe^II/Fe^III (oxyhydr)oxide materials. β-Cyclodextrin was intercalated into the mixed-valent iron-based hydroxide layers to synthesize new green rust materials (β-CD GRs), pursuing high-capacity uranium^VI (U^VI) sorption. The molar ratios of Fe^II to Fe^III and the molar ratios of β-CD GR to Fe^II + Fe^III had a significant effect on the synthesis of β-CD GRs. The synthesis process was further optimized by the quadric predictor and desirability function in a central composite design in combination. Both strong acidity and alkalinity were harmful to the adsorption of β-CD GRs towards U^VI. The pseudo-first-order kinetic model and Langmuir isotherm model were appropriate in fitting the whole adsorption process. The maximum monolayer adsorption capacity of β-CD GRs was 2548.61 mg/g. The presence of mimic groundwater constituents explicitly deteriorated the interaction between β-CD GR and U^VI species. Nanoscale nodules and particles were formed on the β-CD GR after the adsorption experiments. The peaks at 1159 and 609 cm^-1 vanished with the band at 1103 cm^-1 being left-shifted to 1117 cm^-1 in the FTIR spectra of β-CD GR during the heterogeneous process. The intercalation of β-CD brought obvious enhancement of U^VI species sorption to the GR material, which was combinedly driven by several reaction pathways and different from the unmodified GRs.

Effective adsorption of quadrivalent cerium by synthesized laurylsulfonate green rust in a central composite design

The layered laurylsulfonate intercalated green rust (lauryl-S GR) was synthesized to evaluate the influence of synthesis parameters and aqueous conditions on the adsorption of Ce^IV. The maximum adsorption capacity of 305.58 mg/g by lauryl-S GR was predictably obtained. The pseudo-first-order kinetic model was appropriate in fitting the whole uptake process in a weak acid environment. Three isotherm models including Langmuir, Freundlich, and Tempkin were all reliable in depicting the isotherm adsorption process. The maximum monolayer adsorption capacity of lauryl-S GR towards Ce^IV was 315.46 mg/g. Ce species including CeO and Ce₂O₃ besides CeO₂ were matched in the XPS distribution, directly indicating the reduction reaction brought by Fe^II in the GR occurred to hydrated Ce^IV ions during the adsorption. Nano-sized Ce particles attached to the lauryl-S GRs after the adsorption experiments were observed in the morphological characterization. Flocculated materials were formed on the surface of the lauryl-S GR at a pH of 7, which further reduced the active sites and disrupted the continuous uptake of Ce^IV to the lauryl-S GR. This study expands the application of GRs and supplies an ideal iron-based material for the construction of the affiliated recovery pathway to the traditional separation of Ce.

A microwave irradiation-persulfate-formate system for achieving the detoxification and alkali-activated composite geopolymerization of the chromate-contaminated soil

A microwave (MA) irradiation-persulfate-formate system was constructed to detoxify Cr contamination and solidify the geopolymerization of the alkali-activated composite material. Three series of experiments were correspondingly conducted to evaluate the treatment for the chromate-contaminated soil. The changes in the molar ratios of formate to persulfate and the mass rates of fortifier to soil led to a significantly greater reduction of Cr^VI in the detoxification experiments. The increase of blast furnace slag from 50% to 80% in the composite cementitious materials (CCM) intensified the immobilization efficiencies of chromate and the compressive strengths of geopolymer blocks. MA irradiation potentially enhanced the binding of Ca cations to the aluminosilicate compounds. The degree of reaction in the phenomenological kinetics model mathematically verified the geopolymerization process. Ettringite was formed within the structure of the geopolymer in the coupling system. Sulfate radicals released from persulfate not only contributed to the detoxification process but also strengthened the immobilization process.

A green rust-coated expanded perlite particle electrode-based adsorption coupling with the three-dimensional electrokinetics that enhances hexavalent chromium removal

A green rust-coated expanded perlite (GR-coated Exp-p) microelectrode was synthesized and incorporated into a column-mode three-dimensional electrokinetic (3D-EK) platform to effectively pursue a continuous Cr(VI) removal from the aqueous solution. Brucite-like layers of GR were decorated onto the Exp-p material. The molar ratio of Fe(II) to Fe(III) played a most vital role among the three synthesis factors in influencing the performance of the particle electrode. For the equilibrium adsorption experiments, the target maximum adsorption capacity of 122 mg/g was predicted by a target optimizer and desirability function at the conditions following the pH of 4.7, the initial concentration of 172.4 mg/L, the dosage of 0.28 g/L, and the temperature of 28.96 °C, respectively. SO₄^2-, Cl^-, and NO₃^- fiercely competed with Cr(VI) anions in the acidic conditions for the locally positive sites. A low concentration and a slow flow were favored in the column-mode 3D-EK platform. The pseudo-first-order and Langmuir models were suitable for describing the kinetics and isotherms of the adsorption process, respectively. Cr(VI) anions were electrostatically attracted to the silanol groups and GR surface of the adsorbent, subsequently reduced in both heterogeneity and homogeneity, and finally immobilized by coordinating with silanediol groups and silanetriol groups.

Wheat Straws and Corn Straws as Adsorbents for the Removal of Cr(VI) and Cr(III) from Aqueous Solution: Kinetics, Isotherm, and Mechanism

In this paper, the adsorption properties of wheat straw (WS) and corn straw (CS) for Cr(VI) and Cr(III) in solution were studied. The effects of adsorption time, pH of the solution, temperature, and initial concentration of metal ions on adsorption capacity were investigated. The adsorption mechanism was discussed. The results showed that the adsorption isotherms of WS and CS for Cr(VI) and Cr(III) satisfied the Langmuir equation. By fitting the Langmuir equation, the saturated adsorption capacity of WS for Cr(VI) and Cr(III) can reach 125.6 and 68.9 mg g^-1, and that of CS for Cr(VI) and Cr(III) can reach 87.4 and 62.3 mg g^-1 , respectively. The adsorption kinetics conformed to the pseudo-second-order kinetic equation. The effect of temperature on the adsorption capacity was not significant. Physical diffusion and chemical adsorption coexist in the process of adsorption of metal ions by straws, and chemical adsorption is dominant, and the effect of physical diffusion on the chemical adsorption rate can be neglected. It can be seen from the experimental results that the treatment of chromium-containing wastewater by using cheap and easily available wheat straw and corn straw had a remarkable effect. The adsorbed straw could be completely desorbed and had excellent recyclability, indicating that the straws are ideal adsorbents.