Enhanced reactivity and mechanisms of copper nanoparticles modified green rust for p-nitrophenol reduction

Mineral-modulated Co catalyst with enhanced adsorption and dissociation of BH4- for hydrogenation of p-nitrophenol to p-aminophenol

Slow adsorption and dissociation kinetics of NaBH₄ onto the catalyst surface limit the hydrogenation reduction of hazardous p-nitrophenol to worthy p-aminophenol. Herein, we design a mineral-modulated catalyst to facilitate the rate-limiting step. Carbon-coated etched attapulgite (EAtp@C) is obtained by HF treatment. Co/EAtp@C is fabricated via anchoring cobalt nanoparticles (CoNPs) on the carrier EAtp@C. Compared to pure Co, the anchored CoNPs are more electronegative and stable, which provides abundant and stable active sites and accelerates the BH₄^- adsorption and dissociation. Therefore, Co/EAtp@C leads to nearly 100% reduction of p-nitrophenol to p-aminophenol within 8 min and its apparent rate constant K_app (0.69 min^-1) is 4 times higher than that of pure Co. Thermodynamic calculations show a lower activation energy (37.92 kJ mol-1) of Co/EAtp@C catalyst than that of pure Co. Co/EAtp@C also shows magnetic separability and good stability by remaining 98.6% of catalytic conversion rate after six cycles. Significantly, we detect the active species Co-H, and reveal the electron transfer mechanism between catalysts, BH₄^-, and p-nitrophenol by electrochemical method. These results offer a fundamental insight into the catalytic mechanism of p-nitrophenol hydrogenation for rational design of efficient catalysts.

Generation of atomic hydrogen by Ni-Fe hydroxides: Mechanism and activity for hydrodechlorination of trichloroethylene

Atomic hydrogen (H•) is highly reactive for the hydrodechlorination of trichloroethylene (TCE). In this work, we found that the coprecipitation of Ni²⁺ and Fe²⁺ at neutral pH led to an unprecedented catalytic generation of H•. The generated H• effectively dechlorinate TCE to nontoxic ethylene and ethane, and Fe²⁺ is the only electron donor. The abundant adsorbed H• produced with a Ni/Fe ratio of 0.4 enhances hydrogen evolution reaction causing a low efficiency for hydrodechlorination. In contrast, the active absorbed H• is generated in the crystal lattice of Ni-Fe hydroxides with a Ni/Fe ratio of 3.0 causing highly efficient hydrodechlorination of TCE. This work not only reveals the mechanism of catalytic hydrodechlorination by Ni-Fe hydroxides at neutral pH, but also provides a novel approach to detoxify TCE in contaminated water using facile precipitated Ni-Fe hydroxides.

New insights into stoichiometric efficiency and synergistic mechanism of persulfate activation by zero-valent bimetal (Iron/Copper) for organic pollutant degradation

Extensive studies have been devoting to investigating the catalytic efficiency of zero-valent iron (Fe⁰)-based bimetals with persulfate (PS), while little is known in the stoichiometric efficiency, underlying mechanisms and reaction center of zero-valent bimetallic catalysts in activating PS. Herein, nanoscale zero-valent Fe/Cu catalysts in decomposing 2,4-dichlorophenol (DCP) have been investigated. The results show that the increase of Cu ratio from 0 to 0.75 significantly enhances the DCP degradation with a rate constant of 0.025 min^-1 for Fe⁰ to 0.097 min^-1 for Fe/Cu(0.75) at pH ∼3.3, indicating Cu is likely the predominate reaction centers over Fe. The PS decomposition is reduced with the increase of Cu ratios, suggesting the stoichiometric efficiency of Fe/Cu in activating PS is notably enhanced from 0.024 for Fe⁰ to 0.11 for Fe/Cu(0.75). Analyses indicate Cu atoms are likely the predominant reaction site for DCP decomposition, and Fe atoms synergistically enhance the activity of Cu as indicated by DFT calculations. Both SO₄^⦁- and ⦁OH radicals are responsible for reactions, and the contribution of SO₄^⦁- is decreased at higher pH conditions. The findings of this work provide insight into the stoichiometric efficiency and the reaction center of Fe/Cu catalysts to activate PS for pollutant removals.

Preparation of highly dispersive and antioxidative nano zero-valent iron for the removal of hexavalent chromium

In this study, carboxymethyl cellulose (CMC) was employed to stabilize zero-valent iron nanoparticles (CMC-nFe⁰) to improve their dispersity and antioxidation for enhanced hexavalent chromium (Cr(VI)) removal. Scanning electron microscope (SEM) observation revealed that the nFe⁰ agglomerated in clusters, while the CMC-nFe⁰ connected as chains and presented higher dispersity. Therefore, compared with 54% of the nFe⁰, the Cr(VI) removal rate of the CMC-nFe⁰ increased by 0.8 time, reaching 97%. Besides, the nFe⁰ precipitated in 1 d and was obviously oxidized within 7 d under anoxic condition, leading to a rapid decease of Cr(VI) removal efficiency from 54% to 3% in 56 d. In contrast, the CMC-nFe⁰ showed no obvious subsidence and oxidized phenomenon within 14 d, which retained a relatively high Cr(VI) removal efficiency of 63% in 56 d, contributing to effective blockage of dissolved oxygen infiltrating from solution to nFe⁰ particles in presence of CMC. After reaction, the valence state distribution of Cr between solution and material surface indicated that Cr(VI) reduction was dominant comparing to physical adsorption to particles in the remediation process conducted by CMC-nFe⁰. In addition, lower initial pH and higher iron dosage facilitated Cr(VI) removal. Those results indicated that the dispersive and antioxidative characteristics of CMC-nFe⁰ were significantly superior to those of nFe⁰, and CMC stabilization thereafter can be a promising method to promote Cr(VI) elimination by nFe⁰.

Nitrate mediated biotic zero valent iron corrosion for enhanced Cd(II) removal

In this study, nitrate mediated biotic zero-valent iron (Fe⁰) corrosion was employed to enhance cadmium (Cd) removal from groundwater. In comparison with a 17.5% Cd(II) removal treated with abiotic Fe⁰, a 3.9 times higher Cd(II) removal of 86.2% was recorded in the nitrate-mediated biotic Fe⁰ system. Solids phase characterization confirmed that biogenic minerals such as green rust and iron sulfide could be formed in the nitrate-amended biotic Fe⁰ system, offering large amount of adsorption sites for Cd(II) removal. The decrease of nitrate concentration and the competition with cathodic hydrogen for biological nitrate reduction by extra organic substance such as sodium acetate both showed significant inhibition on Cd(II) removal, further proving that hydrogenotrophic denitrification was the main mechanism for enhanced Cd(II) removal. Besides, a relatively high Cd(II) removal efficiency was observed over a pH range of 5-8, and it increased with declining pH values. These results demonstrated that the bio-amended iron corrosion technology coupled Fe⁰-assisted H₂ production with hydrogenotrophic denitrification exhibited excellent Cd(II) removal capacity, which enabled this technology a promising potential for Cd(II)-contaminated groundwater treatment and an alternative strategy for Cd(II) and nitrate co-contaminated groundwater remediation.

Abiotic reduction of p-chloronitrobenzene by sulfate green rust: influence factors, products and mechanism

The reduction of p-chloronitrobenzene (p-CNB) by sulfate green rust (GR_SO4) was systematically studied. The results revealed that GR_SO4 has a good removal effect on p-CNB. The removal efficiencies of p-CNB by GR_SO4 improved with the increase of the pH value. The removal efficiencies in the presence of ions were better than that of GR_SO4 alone, while natural organic matter (NOM) could adsorb p-CNB, which competed with GR_SO4. The reductions of p-CNB by GR_SO4 under different conditions followed pseudo-first-order reaction kinetics except for the reactions in the presence of NOM. p-CNB was converted into p-chloroaniline (p-CAN), which produced p-nitrosochlorobenzene and p-chlorophenylhydroxylamine as the intermediate products. The results of the X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) showed GR_SO4 was gradually transformed into goethite. Fe(ii) in the GR_SO4 structure was the main electron donor involved in the reaction.

Sulfate green rust reduces p-chloronitrobenzene through the electron transfer from the structural Fe(ii) and transforms into goethite.