A review on the transformation of birnessite in the environment: Implication for the stabilization of heavy metals

Design and construction of magnetic nanomaterials and their remediation mechanisms for heavy metal contaminated soil

Soil heavy metal pollution poses huge threat to ecosystem and human health. In-situ chemical remediation aims to immobilize free heavy metals in soil through adding passivators, thereby greatly reducing the mobility and bioavailability of heavy metals. Magnetic nanomaterials (MaN) have strong adsorption and immobilization capabilities for heavy metals due to their significant surface effects, small size effects and interfacial effects. Compared with traditional remediation materials, MaN can be recovered and reused using external magnetic fields. These advantages give MaN broad application prospects in the field of soil remediation. This work provides a comprehensive review of the application of MaN in heavy metal contaminated soil, including the design and application effect of various types of MaN, the influence of MaN on soil properties, environmental toxicity, and microbial composition, the in-situ remediation mechanism of MaN on heavy metal contaminated soil. On the other hand, there are potential risks associated with the remediation of heavy metal contaminated soil using MaN, including their impact on the soil ecosystem and biosafety concerns, requiring further research. Finally, this review proposes the future prospects for the application of MaN in the remediation of heavy metal polluted soil.

Molecular response to the influences of Cu(II) and Fe(III) on forming biogenic manganese oxides by Pseudomonas putida MnB1

The biogeochemical cycle of biogenic manganese oxides (BioMnOx) is closely associated with the environmental behavior and fate of various pollutants. It is significantly interfered by many metals, such as Cu and Fe. However, the bacterial molecular responses are not clear. Here, the effects of Cu(II) and Fe(III) on oxidation of manganese by Pseudomonas putida MnB1 and the bacterial molecular response mechanisms have been studied. The bacterial oxidation of manganese were promoted by both Fe(III) and Cu(II) and the final manganese oxidation rate of the Cu(II) group exceeded 16 % that of the Fe(III) group. The results of transcriptome indicated that Cu(II) promoted manganese oxidation by up-regulating the expression levels of multicopper oxidase (MCO) and peroxidase(POD), and by stimulating electron transfer, while Fe(III) promoted this process by accelerating the electron transfer and nitrogen cycling, and activating POD. The protein-protein interaction (PPI) network indicated that the MCO genes (mnxG and mcoA) were directly linked to the copper homeostasis proteins (cusA, cusB, czcC and cusF). Cytochrome c was closely related to the genes related to nitrogen cycling (glnA, glnL, and putA) and electrons transfer (cycO, cycD, nuoA, nuoK, and nuoL), which also promoted manganese oxidation. This study provides a molecular level insight into the oxidation of Mn(II) by Pseudomonas putida MnB1 with Cu(II) and/or Fe(III) ions.

Simultaneous removal of nitrate, manganese, zinc, and bisphenol a by manganese redox cycling system: Performance and mechanism

The manganese(Mn) redox cycling system in this work was created by combining Mn(IV)-reducing bacteria MFG10 with Mn(II)-oxidizing bacteria HY129. The biomanganese oxides (BMO) generated by strain HY129 were transformed by strain MFG10 to Mn(II), finishing the Mn redox cycling, in which nitrate (NO3--N) was converted to nitrite, which was further reduced to nitrogen gas. The system could achieve 85.7 % and 98.8 % elimination efficiencies of Mn(ⅠⅠ) and NO3--N, respectively, at Mn(ⅠⅠ) = 20.0 mg/L, C/N = 2.0, pH = 6.5, and NO3--N = 16.0 mg/L. The removal of bisphenol A (BPA) and zinc (Zn(II)) at 36 h reached 91.7 % and 89.7 % under the optimal condition, respectively. Furthermore, the Mn redox cycling system can reinforce the metabolic activity and electron transfer activity of microorganisms. The findings showed that the adsorption by bioprecipitation throughout the Mn cycling was responsible for the elimination of Zn(II) and BPA.

Cadmium Isotope Fractionation during Adsorption onto Edge Sites and Vacancies in Phyllomanganate

Cadmium (Cd) geochemical behavior is strongly influenced by its adsorption onto natural phyllomanganates, which contain both layer edge sites and vacancies; however, Cd isotope fractionation mechanisms at these sites have not yet been addressed. In the present work, Cd isotope fractionation during adsorption onto hexagonal (containing both types of sites) and triclinic birnessite (almost only edge sites) was investigated using a combination of batch adsorption experiments, extended X-ray absorption fine structure (EXAFS) spectroscopy, surface complexation modeling, and density functional theory (DFT) calculations. Light Cd isotopes are preferentially enriched on solid surfaces, and the isotope fractionation induced by Cd2+ adsorption on edge sites (Δ114/110Cdedge-solution = -1.54 ± 0.11‰) is smaller than that on vacancies (Δ114/110Cdvacancy-solution = -0.71 ± 0.21‰), independent of surface coverage or pH. Both Cd K-edge EXAFS and DFT results indicate the formation of double corner-sharing complexes on layer edge sites and mainly triple cornering-sharing complexes on vacancies. The distortion of both complexes results in the negative isotope fractionation onto the solids, and the slightly longer first Cd-O distances and a smaller number of nearest Mn atoms around Cd at edge sites probably account for the larger fractionation magnitude compared to that of vacancies. These results provide deep insights into Cd isotope fractionation mechanisms during interactions with phyllomanganates.

The dehydration mechanism of Na and K birnessites: a comprehensive multitechnique study

The structural, spectroscopic and electronic properties of Na and K birnessites were investigated from ambient conditions (birA) to complete dehydration, and the involved mechanisms were scrutinized. Density Functional Theory (DFT) simulations were employed to derive structural models for lamellar A0.33MnO2·xH2O (A = Na+ or K+, x = 0 or 0.66), subsequently compared with the experimental results obtained for Na0.30MnO2·0.75H2O and K0.22MnO2·0.77H2O materials. Thermal analysis (TGA-DSC), X-ray diffraction (XRD), Fourier Transform Infrared (FTIR) spectroscopy, and Near Ambient Pressure X-ray Photoemission Spectroscopy (NAP-XPS) measurements were conducted for both birnessites. Dehydration under vacuum, annealing, or controlled relative humidity were considered. Results indicated that complete birnessite dehydration was a two-stage process. In the first stage, water removal from the interlayer of fully hydrated birnessite (birA) down to a molar H2O/A ratio of ∼2 (birB) led to the progressive shrinkage of the interlayer distance (3% for Na birnessite, 1% for K birnessite). In the second stage, water-free (birC) domains with a shorter interlayer distance (20% for Na birnessite, 10% for K birnessite) appeared and coexisted with birB domains. Then, birB was essentially transformed into birC when complete dehydration was achieved. The vibrational properties of birA were consistent with strong intermolecular interactions among water molecules, whereas partially dehydrated birnessite (birB) showed a distinct feature, with 3 (for Na-bir) and 2 (for K-bir) vibrations that were reproduced by DFT calculations for organized water into the interlayer (x = 0.66). The study also demonstrated that the electronic structure of Na birnessite depends on the interlayer water content. The external Na+ electronic level (Na 2p) was slightly destabilized (+0.3 eV binding energy) under near ambient conditions (birA) compared to drier conditions (birB and birC).

Polyethyleneimine-modified iron-doped birnessite as a highly stable adsorbent for efficient arsenic removal

Remediation of arsenic contamination is of great importance given the high toxicity and easy mobility of arsenic species in water and soil. This work reports a new and stable adsorbent for efficient elimination of arsenic by coating polyethyleneimine (PEI) molecules onto the surface of iron-doped birnessite (Fe-Bir). Characterization results of surface microstructure and crystalline feature (scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared spectrometer (FTIR) and X-ray photoelectron spectroscopy (XPS), etc.) suggest that Fe-Bir/PEI possesses a fine particle structure, inhibiting the agglomeration of birnessite-typed MnO2 and offering abundant active sites for arsenic adsorption. Fe-Bir/PEI is capable of working in a wide pH range from 3 to 11, with an efficient removal capacity of 53.86 mg/g at initial pH (pH0) of 7. Meanwhile, commonly coexisting anions (NO3-, SO42-, and Cl-) and cations (Na+, K+, Ca2+ and Mg2+) pose no effect on the arsenic removal performance of Bir/PEI. Fe-Bir/PEI exhibits a good reusability for arsenic removal with low Mn and Fe ions leaching after 5 cycles. Besides, Fe-Bir/PEI possesses efficient remediation capability in simulated As-contaminated soil. The modification of PEI in Fe-Bir/PEI can adsorb newly formed As(V), which is impossible for the adsorbent without PEI. Further, the arsenic removal mechanism of Fe-Bir/PEI is revealed with redox effect, electrostatic attraction and hydrogen bonding.

Understanding the role of manganese oxides in retaining harmful metals: Insights into oxidation and adsorption mechanisms at microstructure level

The increasing intensity of human activities has led to a critical environmental challenge: widespread metal pollution. Manganese (Mn) oxides have emerged as potentially natural scavengers that perform crucial functions in the biogeochemical cycling of metal elements. Prior reviews have focused on the synthesis, characterization, and adsorption kinetics of Mn oxides, along with the transformation pathways of specific layered Mn oxides. This review conducts a meticulous investigation of the molecular-level adsorption and oxidation mechanisms of Mn oxides on hazardous metals, including adsorption patterns, coordination, adsorption sites, and redox processes. We also provide a comprehensive discussion of both internal factors (surface area, crystallinity, octahedral vacancy content in Mn oxides, and reactant concentration) and external factors (pH, presence of doped or pre-adsorbed metal ions) affecting the adsorption/oxidation of metals by Mn oxides. Additionally, we identify existing gaps in understanding these mechanisms and suggest avenues for future research. Our goal is to enhance knowledge of Mn oxides' regulatory roles in metal element translocation and transformation at the microstructure level, offering a framework for developing effective metal adsorbents and pollution control strategies.