Kinetics of Mn(II) adsorption and catalytic oxidation on the surface of ferrihydrite

Effect of Mn(II) photochemical oxidation on Cd immobilization in hematite

Hematite, a commonly stable iron oxide in the environment, which can not only adsorb Cd in the environment, but also catalyze the photochemical oxidation of Mn(II) in the environment. However, the impact of Mn(II) on the structure of hematite and the adsorption of Cd during the surface oxidation of hematite remains unknown. In this study, we investigated the surface and structural changes of hematite after the photochemical oxidation of Mn(II), as well as the geochemical behavior of Cd during this process. The results demonstrate that Mn(II) was oxidized to Mn(III/IV) on the hematite surface, with some Mn(III) being incorporated into the hematite structure. Simulations using XRD data showed that higher Mn(II) concentrations resulted in increased levels of Mn doping, leading to significant variations in the hematite unit cell. This was further confirmed through FTIR and Raman spectroscopy characterization. The oxidation of Mn(II) on the hematite surface resulted in a shift in surface charge from positive to negative, enhancing the adsorption capacity of Cd. However, when Mn(II) exceeded 0.4 mM, the immobilization of Cd within the system decreased. This was attributed to the competitive adsorption of Mn(II) and a reduction in the relative abundance of Mn(IV) oxides.

Induced transformation of antimony trioxide by Mn(II) oxidation and their co-transformed mechanism

Antimony (Sb) is a toxic and carcinogenic element that often enters soil in the form of antimony trioxide (Sb₂O₃) and coexists with manganese (Mn) in weakly alkaline conditions. Mn oxides such as birnessite have been found to promote the oxidative dissolution of Sb₂O₃, but few researches concerned the co-transformations of Sb₂O₃ and Mn(II) in environment. This study investigated the mutual effect of abiotic oxidation of Mn(II) and the coupled oxidative dissolution of Sb₂O₃. The influencing factors, such as Mn(II) concentrations, pH and oxygen were also discussed. Furthermore, their co-transformed mechanism was also explored based on the analysis of Mn(II) oxidation products with or without Sb₂O₃ using XRD, SEM and XPS. The results showed that the oxidative dissolution of Sb₂O₃ was enhanced under higher pH and higher Mn(II) loadings. With a lower Mn(II) concentration such as 0.01 mmol/L Mn(II) at pH 9.0, the improved dissolution of Sb₂O₃ was attributed to the generation of dissolved intermediate Mn(III) species with strong oxidation capacity. However, under higher Mn(II) concentrations, both amorphous Mn(III) oxides and intermediate Mn(III) species were responsible for promoting the oxidative dissolution of Sb₂O₃. Most released Sb (∼72%) was immobilized by Mn oxides and Sb(V) was dominant in the adsorbed and dissolved total Sb. Meanwhile, the presence of Sb₂O₃ not only inhibited the removal of Mn(II) by reducing Mn(III) to Mn(II) but also affected the final products of Mn oxides. For example, amorphous Mn oxides were formed instead of crystalline Mn(III) oxides, such as MnOOH. Furthermore, rhodochrosite (MnCO₃) was formed with the high Mn(II)/Sb₂O₃ ratio_, but without being observed in the low Mn(II)/Sb₂O₃ ratio. The results of study could help provide more understanding about the fate of Sb in the environment and the redox transformation of Mn.

Effects and mechanisms of Al substitution on the catalytic ability of ferrihydrite for Mn(II) oxidation and the subsequent oxidation and immobilization of coexisting Cr(III)

Al(III)-substituted ferrihydrite existing in natural soils is more common than pure ferrihydrite; however, the effects of Al(III) incorporation on the interaction between ferrihydrite, Mn(II) catalytic oxidation, and coexisting transition metal (e.g., Cr(III)) oxidation remain elusive. To address this knowledge gap, Mn(II) oxidation on synthetic Al(III)-incorporated ferrihydrite and Cr(III) oxidation on the previously formed Fe-Mn binaries were investigated in this study via batch kinetic studies combined with various spectroscopic analyses. The results indicate that Al substitution in ferrihydrite barely changes its morphology, specific surface area, or the types of surface functional groups, but increases the total amount of hydroxyl on the ferrihydrite surface and enhances its adsorption capacity toward Mn(II). Conversely, Al substitution inhibits electron transfer in ferrihydrite, thereby weakening its electrochemical catalysis on Mn(II) oxidation. Thus, the contents of Mn(III/IV) oxides with higher Mn valence states decrease, whereas those of lower Mn valence states increase. Furthermore, the number of hydroxyl radicals formed during Mn(II) oxidation on ferrihydrite decreases. These inhibitions of Al substitution on Mn(II) catalytic oxidation subsequently cause decreased Cr(III) oxidation and poor Cr(VI) immobilization. Additionally, Mn(III) in Fe-Mn binaries is confirmed to play a dominant role in Cr(III) oxidation. This research facilitates sound decision-making regarding the management of Cr-contaminated soil environments enriched with Fe and Mn.

Simultaneous alleviation of Cd availability in contaminated soil and accumulation in rice (Oryza sativa L.) by Fe-Mn oxide-modified biochar

Fe-Mn oxide-modified biochar (BC-FM) was used to remediate Cd-contaminated soil and mitigate Cd accumulation in rice. The roles of Fe and Mn in soil Cd immobilization and in controlling Cd uptake by rice were investigated via X-ray photoelectron spectroscopy (XPS) characterization and chemical analysis. Fe and Mn loaded on BC-FM increased the removal efficiencies of CaCl₂ extractable Cd in soil and Cd in pore water compared to those in only biochar (BC)-treated soil, with maximum removal rates at 67.9 % and 77.8 %, respectively. The XPS results indicated that the redox reactions of the Fe-Mn oxides on BC-FM surface affected Cd immobilization in the soil. The Fe (II/III) components on BC-FM were primarily converted to Fe₃O₄ in the soil system, which may form stable complexes with Cd²⁺ (Fe-O-Cd) during the entire rice growth period, and Cd may be bound to MnO or Mn₂O₃ in the form of CdMn₂O₄. The excellent adsorption performance of BC-FM enhanced by Fe-Mn oxides reduced the available Cd in the soil and stimulated Fe and Mn transport in rice, thereby inhibiting Cd accumulation in the aerial parts of rice. Cd concentrations in brown rice under BC-FM treatments reached the national safety standard (0.2 mg/kg, GB2762-2017). And BC-FM significantly increased the biomass of brown rice with a maximum rate of 26.8 %. These findings suggest that BC-FM could be used as an efficient material for Cd-contaminated soil remediation, and Fe-Mn plays important role in immobilizing Cd in soil and reducing Cd transport in rice.

Mechanism of mercapto-modified palygorskite in reducing soil Cd activity

Mercapto-modified palygorskite (MP) is an efficient novel amendment with superior ability to decrease soil Cd bioavailability, but the unclear immobilization mechanism has become the bottleneck of its performance improvement and precise application. In order to clarify the Cd reducing mechanism of MP, long-term and short-term soil incubation with three types of soils (paddy soil, alluvial soil and yellow mountain soil) and sorption verification experiments were conducted to investigate the dynamic process of soil labile Cd impacted by MP and the synergetic effects on labile Fe, Mn, S and dissolved organic carbon via in-situ diffusive gradients in thin-films and soil solution sampling techniques. MP with four dosages rapidly and continuously decreased soil labile Cd contents by 14.50 % ∼ 89.16 % in long-term incubation, meanwhile low-dosage MP reduced soil labile Fe and Mn contents, but high-dosage MP increased their contents. With MP dosages increased, the effects of Fe-Mn oxides on soil labile Cd content gradually weakened. MP effectively promoted the reduction of Fe adsorbed by clay minerals and enhanced their ability to adsorb Cd. Short-term incubation showed that MP could decline soil labile Cd by 7.17 % ∼ 44.74 %, especially at the dosage 0.4 %. MP was a reduction catalyst to facilitate Fe reduction, which profited for clay minerals adsorbing Cd. The sorption experiments indicated that 0.30 % MP could adsorb 73.34 % Cd²⁺, promote the release of Fe²⁺ from the soil, and stimulate the ability of clay minerals to adsorb Cd. The results revealed that MP decreased soil labile Cd content within 2 d, and MP made soil Cd activity change out of the influence of soil Fe/Mn redox system. The mechanism will be beneficial for the large-scale application of MP in safe utilization of Cd contaminated soil.

Passivation of lead and cerium in soil facilitated by biochar-supported phosphate-doped ferrihydrite: Mechanisms and microbial community evolution

The massive exploitation and application of heavy metals and rare earth elements (REEs) lead to their exceeding the standard in soil. Herein, a new type of biochar supported phosphorus doped ferrihydrite (P-FH@BC) has been designed and enhance passivation of Pb and Ce in soil. SEM images of P-FH@BC showed P-FH nanoparticles adhered to the natural cavity and large pore diameter on the surface of biochar, which greatly avoided the agglomeration of nanoparticles. The residual state of lead or cerium increased 161.4% or 43.9% by adding 3% P-FH@BC after 90 days of incubation in 500 mg/kg lead or cerium simulated contaminated soil. The passivation of cerium by P-FH@BC is obviously inhibited with the coexistence of lead. The results of P-FH@BC magnetically separated from the soil characterization indicate that complexation, co-precipitation and the formation of secondary minerals mainly contribute to the high efficiency passivation ability of P-FH@BC for lead and cerium. By changing the addition of P-FH@BC, the soil pH can be adjusted and the soil organic matter and P contents can be improved. Moreover, P-FH@BC is an environmentally friendly material without ecotoxicity. And bacterial richness and diversity in soil were improved after passivation of Pb and Ce by adding P-FH@BC.

Photocatalytic oxidation of Mn(II) on the surface of Bi2.15WO6via the ligand-to-metal charge transfer (LMCT) pathway

Biotic and abiotic oxidation of Mn(II) in aqueous environments is an important process for the cycling of many elements. However, the mechanism involved in photocatalytic oxidation of Mn(II) has not been clearly elucidated yet. In this study, the photocatalytic oxidation of Mn(II) on the surface of self-doped Bi_2+xWO₆ (Bi_2.15WO₆) under visible light was conducted. Kinetics results show that visible light apparently accelerates the oxidation of Mn(II) to Mn(III, IV) oxides on Bi_2.15WO₆. The average oxidation states (AOS) of manganese reach 2.18 after 80 min of reaction under visible light at pH 8.50. Characterizations indicate the formation of Bi(III)-O-Mn(II) surface complexes between Mn(II) and surface Bi(III) on Bi_2.15WO₆, which then decreases the bandgap of [Bi_2.15WO₆ + Mn(II)]_light (2.53 eV) compared with those of [Bi_2.15WO₆ + Mn(II)]_dark (2.72 eV) and pure Bi_2.15WO₆ (2.86 eV), suggesting the contribution of the ligand-to-metal charge transfer (LMCT) pathway to the photocatalytic oxidation of Mn(II). Moreover, the addition of inorganic oxidants with strong oxidizing capacities (such as Cr₂O₇^2-, NO₃^- or NO₂^-) significantly increases the oxidation rate of Mn(II), further verifying the contribution of the LMCT pathway to Mn(II) oxidation. We therefore suggest that the LMCT pathway is one of the important oxidation routes for Mn(II) oxidation on Bi_2.15WO₆ under visible light.

Denitrification performance of nitrate-dependent ferrous (Fe2+) oxidizing Aquabacterium sp. XL4: Adsorption mechanisms of bio-precipitation of phenol and estradiol

In this study, a nitrate-dependent ferrous (Fe²⁺) oxidizing strain under anaerobic conditions was selected and identified as XL4, which belongs to Aquabacterium. The Box-Behnken design (BBD) was used to optimize the growth conditions of strain XL4, and the nitrate removal efficiency of strain XL4 (with 10% inoculation dosage, v/v) could reach 91.41% under the conditions of 30.34 ℃, pH of 6.91, and Fe²⁺ concentration of 19.69 mg L^-1. The results of Fluorescence excitation-emission matrix spectra (EEM) revealed that the intensity of soluble microbial products (SMP), aromatic proteins and the fulvic-like materials were obvious difference under different Fe²⁺ concentration, pH, and temperature. X-ray diffraction (XRD) data confirmed that the main components of bio-precipitation were Fe₃O₄ and FeO(OH), which were believed to be responsible for the adsorption of phenol and estradiol. Furthermore, the maximum adsorption capacity of bio-precipitation for phenol and estradiol under the optimal conditions were 192.6 and 65.4 mg g^-1, respectively. On the other hand, the adsorption process of phenol and estradiol by bio-precipitation confirmed to the pseudo-second-order and Langmuir model, which shows that the adsorption process is chemical adsorption and occurs on the uniform surface.