Ultrathin MnO2-Coated FeOOH Catalyst for Indoor Formaldehyde Oxidation at Ambient Temperature: New Insight into Surface Reactive Oxygen Species and In-Field Testing in an Air Cleaner.

Herein, we tailored a series of ultrathin MnO2 nanolayers coated on the surface of commercial goethite (α-FeOOH) by a facile in situ chemical precipitation method. α-FeOOH inhibited the MnO2 crystal growth via the incorporation of K+ ions between MnO2 and α-FeOOH interfaces during the synthesis process. The hybrid design of MnO2 with an ultrathin nanolayer structure could reduce the electron transfer resistance and bring abundant oxygen vacancies, accelerating the activation of molecular O2 to generate more oxygen-free radical species and favoring the thermodynamic HCHO oxidation. The ROS quenching in gas/aqueous systems and DRIFTS results demonstrated that •O2- was responsible for HCHO oxidization, which assisted the preliminary intermediate dioxymethylene dehydrogenation into formate species. The 25%MnO2@FeOOH(25wt% of MnO2) catalyst was subsequently loaded into the filter substrates of a commercial air cleaner and tested in an indoor room with actual application conditions. As a result, the composite filter could eliminate different initial concentrations of HCHO (150-450 ppb) to the WHO guideline value (≈81 ppb) within 60 min. Furthermore, the 25%MnO2@FeOOH sample was also effective against the representative bacteria and mold in indoor air. This study provides new insight into the role of the chemisorbed ROS for HCHO oxidation at ambient temperature.

Understanding Competitive Phosphate and Silicate Adsorption on Goethite by Connecting Batch Experiments with Density Functional Theory Calculations.

Despite the biogeochemical importance of phosphate fate and transport in aquatic environments, little is known about how competition with other common aqueous oxyanions affects its retention by mineral surfaces. Here, we examined the competitive uptake of phosphate and silicate on goethite over a wide pH range, using batch measurements supported by DFT calculations. The results show selective adsorption of phosphate at pH < 4 and silicate at pH > 10 with little to no competitive effect. However, between 4 < pH < 10, the total phosphate and silicate loading was found to be almost equal to that of silicate loading from single-component solution, revealing a proportionate competition for surface site types and a competitive effect controlling their mutual retention. DFT-calculated adsorption energies and charge density redistributions for various surface complexes on different charged (101) and (210) facets are consistent with the trends observed in batch measurements, suggesting that the observed behavior reflects the primary controlling influence of goethite surface chemistry at the molecular scale. An important implication is that at the circumneutral pH in most environmental systems, where iron oxyhydroxides comprise much of the reactive interfacial area, unbound phosphate concentrations may be strongly controlled by dissolved silicate concentration, and vice versa.

Phosphate adsorption kinetics and equilibria on natural iron and manganese oxide composites.

Although it is well known that phosphate retention in soils and sediments is strongly influenced by binding to secondary iron oxides, there have been relatively few studies examining its adsorption/desorption behavior on multicomponent particles of realistic natural complexity. In this study, natural Mn-rich limonite (LM), was used to prepare naturally complex Fe- and Mn-oxide composite materials to examine phosphate adsorption/desorption. To clarify the role of the Mn-oxides, results for the LM sample were compared to those for an acid treated version (LAT), in which the acid-extractable Mn-oxide fraction has been selectively eliminated while leaving the Fe-oxide fraction intact. The saturated adsorption capacity on LAT was almost double that on LM, suggesting that phosphate adsorption to the iron oxides is strongly occluded by the Mn-oxide fraction. This result is reinforced by the comparing the pH dependence and fits to adsorption isotherms, and by desorption experiments and STEM-EDS mapping showing that phosphate loading on Mn-oxides was limited. Hence, although the collective results confirm that phosphate uptake and strong binding is selectively controlled by the Fe-oxide fraction, our study reveals that the Mn-oxide fraction strongly interferes with this process. Therefore, phosphate uptake behavior on metal oxides cannot be predicted solely on the basis of the Fe-oxide fraction present, but instead must take into account the deleterious impacts of other intimately associated phases. For co-diagenetic Fe/Mn-oxide composites in particular, Mn-oxides appear to severely limit phosphate uptake on the Fe-oxide fraction, either by hindering access to binding sites on the Fe-oxide or by lowering their affinity for P.

New Synthesis of nZVI/C Composites as an Efficient Adsorbent for the Uptake of U(VI) from Aqueous Solutions.

New nanoscale zerovalent iron/carbon (nZVI/C) composites were successfully prepared via heating natural hematite and pine sawdust at 800 °C under nitrogen conditions. Characterization by SEM, XRD, FTIR, and XPS analyses indicated that the as-prepared nZVI/C composites contained a large number of reactive sites. The lack of influence of the ionic strength revealed inner-sphere complexation dominated U(VI) uptake by the nZVI/C composites. Simultaneous adsorption and reduction were involved in the uptake process of U(VI) according to the results of XPS and XANES analyses. The presence of U-C/U-U shells demonstrated that innersphere complexation and surface coprecipitation dominated the U(VI) uptake at low and high pH conditions, respectively. The uptake behaviors of U(VI) by the nZVI/C composites were fitted well by surface complexation modeling with two weak and two strong sites. The maximum uptake capacity of U(VI) by the nZVI/C composites was 186.92 mg/g at pH 4.0 and 328 K. Additionally, the nZVI/C composites presented good recyclability and recoverability for U(VI) uptake in regeneration experiments. These observations indicated that the nZVI/C composites can be considered as potential adsorbents to remove radionuclides for environmental remediation.

Recyclable Naturally Derived Magnetic Pyrrhotite for Elemental Mercury Recovery from Flue Gas.

Magnetic pyrrhotite, derived from the thermal treatment of natural pyrite, was developed as a recyclable sorbent to recover elemental mercury (Hg0) from the flue gas as a cobenefit of wet electrostatic precipitators (WESP). The performance of naturally derived pyrrhotite for Hg0 capture from the flue gas was much better than those of other reported magnetic sorbents, for example Mn-Fe spinel and Mn-Fe-Ti spinel. The rate of pyrrhotite for gaseous Hg0 capture at 60 °C was 0.28 µg g min-1 and its capacity was 0.22 mg g-1 with the breakthrough threshold of 4%. After the magnetic separation from the mixture collected by the WESP, the spent pyrrhotite can be thermally regenerated for recycle. The experiment of 5 cycles of Hg0 capture and regeneration demonstrated that both the adsorption efficiency and the magnetization were not notably degraded. Meanwhile, the ultralow concentration of gaseous Hg0 in the flue gas was concentrated to high concentrations of gaseous Hg0 and Hg2+ during the regeneration process, which facilitated the centralized control of mercury pollution. Therefore, the control of Hg0 emission from coal-fired plants by the recyclable pyrrhotite was cost-effective and did not have secondary pollution.

Influence of Different Electron Acceptors on the Anaerobic Degradation of Curly-Leaf Pondweed.

Curly-leaf pondweed (Potamogeton crispus) was utilized as the representative to investigate the biodecomposition process of aquatic plants under different reducible conditions. Results showed that the methane production was inhibited when different electron acceptors (Fe(III), and ) were available. The methane production was decreased by 57% when Fe(III) and or were both available compared to the control. The degradation efficiency of hemicellulose and lignin with Fe(III) and were increased significantly. This provided a theoretical basis for slowing down the emissions of methane.

Manganese sulfide-sulfur and limestone autotrophic denitrification system for deep and efficient nitrate removal: Feasibility, performance and mechanism.

Despite the great potential of sulfur-based autotrophic denitrification, an improvement in nitrate removal rate is still needed. This study used the desulfurized products of Mn ore to develop the MnS-S0-limestone autotrophic denitrification system (MSLAD). The feasibility of MSLAD for denitrification was explored and the possible mechanism was proposed. The nitrate (100 mg/L) was almost removed within 24 h in batch experiment in MSLAD. Also, an average TN removal of 98 % (472.0 mg/L/d) at hydraulic retention time of 1.5 h in column experiment (30 mg/L) was achieved. MnS and S0 could act as coupled electron donors and show synergistic effects for nitrate removal. γ-MnS with smaller particle size and lower crystallinity was more readily utilized by the bacterium and had higher nitrate removal efficiency than that of α-MnS. Thiobacillus and Sulfurimonas were the core functional bacterium in denitrification. Therefore, MnS-S0-limestone bio-denitrification provides an efficient alternative method for nitrate removal in wastewater.

Autocatalytic degradation of Cu-EDTA in the Calcite/PMS system: Singlet oxygen and Cu(III).

The simultaneous removal of heavy metal complexes (HMCs) and heavy metal ions presents a significant challenge in treating wastewater. To address this, we propose a Calcite/Peroxymonosulfate (Calcite/PMS) system aimed at simultaneously decomplexing Cu-EDTA and removing Cu ions. Calcite/PMS system could achieve 99.5 % Cu-EDTA decomplexation and 61.9 % Cu ions removal within 60 min under initial conditions of Cu-EDTA (10 mg/L), Calcite (3 g/L), and PMS (2 mM). Singlet oxygen (1O2) emerged as the predominant reactive species responsible for Cu-EDTA decomplexation, which selectively targeted the N-C bonds in the Cu-EDTA structure to produce intermediates with lower biotoxicity than EDTA. Interestingly, solid phase Cu(III) (≡Cu(III)) promoted the generation of superoxide radicals (O2•-) with a contribution of up to 72.8 %. Subsequently, nascent ≡Cu(III) and O2•- accelerated the degradation of intermediates. Besides, coexisting organic substances inhibited Cu-EDTA decomplexation, whereas inorganic ions had a weak impact. After five cycles of use, the Calcite/PMS system retained 99.3 % efficiency in decomplexing Cu-EDTA. This investigation provides valuable insights into using calcite to remove HMCs and enhances our comprehension of the decomplexation intermediates accelerating HMCs degradation.

Assessment of nitrogen and phosphorus removal in an intermittently aerated sequencing batch reactor (IASBR) and a sequencing batch reactor (SBR).

Biological nitrogen and phosphorus removal was investigated in an intermittently aerated sequencing batch reactor (IASBR) and a sequencing batch reactor (SBR). The removal efficiencies of ammonium-nitrogen (NH4(+)-N) were 100% in both reactors in steady operation state. The total nitrogen (TN) removal efficiencies were 90.4% in the IASBR and 79.3% in the SBR, while the total phosphorus (TP) removal efficiencies were 88.8% in the IASBR and 82.3% in the SBR. The efficiencies of simultaneous nitrification and denitrification (SND) were 90.4% in the IASBR and 79.3% in the SBR, indicating that the IASBR was more efficient than the SBR in SND. The sludge in the IASBR had a P release capability of 16.6 mg P/g VSS (volatile suspended solids) but only 7.5 mg P/g VSS in the SBR.

The pH-sensitive transformation of birnessite and its effect on the fate of norfloxacin.

Birnessite plays a crucial role in regulating the fate of contaminants in soil, which is affected by the crystal structure of birnessite. In this study, the transformation of triclinic birnessite to hexagonal birnessite was examined at various pH values, and their reactivity towards norfloxacin was investigated. The findings indicate that the conversion from triclinic birnessite to hexagonal birnessite occurs under pH conditions lower than 7. The lower of the solution pH where the birnessite formed, the higher the surface reactivity. Throughout the transformation process, the migration of Mn3+ and the increased interlayer protons generated more reactive oxygen species, which enhanced the surface reactivity towards norfloxacin. Specifically, at a conversion pH of 1, the norfloxacin removal rate significantly increases from 14% to 97% compared to triclinic birnessite. The mechanism of norfloxacin removal by triclinic and hexagonal birnessite is illustrated. These findings provide valuable insights into the dynamic transformation of birnessites in aqueous environments with varying pH values and their impact on norfloxacin removal.

The synergistic effect of calcite and Cu2+ on the degradation of sulfadiazine via PDS activation: A role of Cu(â ¢).

A system of Cu2+/calcite/PDS was constructed to degrade sulfadiazine (SDZ). Different from the traditional Cu-mediated activation, a low concentration of Cu2+ that met drinking water standards (≤ 1 mg/L) transformed into Cu(Ⅱ) solid in the presence of calcite, and then enhanced the degradation of SDZ via PDS activation over a pH range from 3 to 9. According to scavenger and chemical probe experiments, Cu(Ⅲ), rather than radicals (hydroxyl radicals and sulfate radicals) and singlet oxygen, was the predominant reactive species, which was responsible for the degradation of SDZ. Based on the results of XRD, ATR-FTIR, and CV curves et al., CuCO3 was the main complex with high reactivity for PDS activation to form Cu(Ⅲ). Moreover, detailed degradation pathways of sulfadiazine were proposed according to the UPLC-ESI-MS/MS and their toxicity was predicted by ECOSAR. Besides, the real water matrix would not seriously affect the degradation of SDZ in the Cu2+/calcite/PDS system. In summary, this study reveals a new insight into the synergistic effect of Cu2+ and calcite on the SDZ degradation, and promotes an understanding of the environmental benefits of natural calcite.

Ce(SO4)2/α-Fe2O3 selective catalytic reduction of NOx with NH3: preparation, characterization, and performance.

To achieve a low-cost, high-activity denitrification catalyst with excellent water and sulfur resistance, goethite and Ce(SO4)2·4H2O were used to prepare Ce(SO4)2/α-Fe2O3 composite catalyst by the impregnation way and investigated the effect of Ce(SO4)2 on the properties of goethite. Ce(SO4)2/α-Fe2O3 with various preparation conditions for denitration was systematically discussed, and its structure and properties were characterized by XRD, BET, TEM, XPS, H2-TPR, and NH3-TPD methods. The results showed that Ce(SO4)2/α-Fe2O3 over the Ce/Fe molar ratio of 0.02 and calcination temperature of 350 ℃ had excellent catalytic activity, resistance to sulfur, and water properties and stability. When NOx initial concentration was 500 ppm, gas hourly space velocity was 36,000 h-1 and its reaction temperature was 300 ℃; the NOx conversion efficiency was maintained at over 95% along with 300 ppm SO2 and nearly 100% couple with 10% H2O. Its superior performance was mainly attributed to the enhancement of the surface adsorbed oxygen and acidity of α-Fe2O3 by cerium sulfate. The multiple advantages of Ce0.02/α-Fe2O3(350) made it feasible for practical engineering application.

Utilization of methylene blue-adsorbed halloysite after carbonization to activate peroxymonosulfate degrading phenol: Performance and mechanism.

In this study, a new low-cost carbon-based material was prepared via the carbonization of methylene blue adsorbed halloysite (CMH) at different temperatures in a nitrogen atmosphere, which was named CMH-T (T-Temperature). The performance of CMH-T was explored and the effects of initial pH values, catalyst dosage, phenol (PE) concentrations, peroxymonosulfate (PMS) concentrations, and water background compounds on PE degradation were investigated systematically. The results indicated that CMH800 exhibited the best performance to activate PMS for degrading PE. Specifically, 92% PE was degraded within 30 min with a constant rate (kobs) of 0.1186 min-1 in the CMH800/PMS system. Furthermore, CMH800 was efficient over a wide pH range (pH 3-9) and showed a slight inhibition to inorganic anions. Quenching experiments, electron spin resonance (ESR) analysis, and electrochemical analysis confirmed that PE was degraded through non-radical pathways dominated by single oxygen (1O2) and mediated electron transfer processes in the CMH800/PMS system. In addition, the predicted toxicity of intermediates through ECOSAR software based on QSAR (Quantitative Structure - Activity Relationship) model indicated that most of the intermediates had a low risk to water environment. Therefore, the CMH800 has a good potential for wastewater treatment applications.

The removal performance and mechanisms of tetracycline over Mn-rich limonite.

Naturally occurring Mn-rich limonite mainly composed of goethite and manganese oxides was used to remove tetracycline (TC) from the aqueous solution. The effects of dosage, initial solution pH, temperature, and coexisting anions on TC removal were investigated. Results showed that 95% of TC (30.0 mg·L-1) was removed in a wide pH range of 3.0-9.0 by limonite with high specific surface area (145.0 m2·g-1) and mesoporous structure. The presence of Cl-, NO3-, and SO42- in the studied concentration range did not influence TC removal efficiency significantly, while PO43- inhibited the adsorption of TC over limonite due to the competition with TC for active sites. Integrated with the FT-IR analysis, electrostatic interaction and complexation were proved to be the adsorption mechanisms of TC by limonite. The quenching experiments and ESR analysis revealed that singlet oxygen (1O2) also was involved in TC degradation. In addition, limonite displayed an efficient recycling performance and stability after four cycles. This study revealed that the Mn-rich limonite was a promising adsorbent for TC removal from aqueous solutions and promoted the application of natural mineral material in the environmental field.

H2O2 activation over Co substitution in Fe1-xS for tetracycline degradation: Effect of Co substitution.

In this work, the effect of Co substitution in the Fe1-xS (CSP) on the activation of H2O2 to degrade tetracycline (TC) is investigated. A series of CSP samples with different Co content are synthesized via a high-temperature sulfidation method and characterized by XRD, XPS, SEM, and electrochemical analysis. The result showed that low Co content (≤1%) promotes the catalytic activity of Fe1-xS, while excessive Co (1%﹤x ≤ 3%) inhibits its catalytic activity. The investigation of Behnajady-Modirshahla-Ghanbery kinetic model (BMG) showed that the maximum initial degradation rate of TC over 1.0% CSP/H2O2 was 1.6 times than that of in CSP/H2O2 system. The Box-Behnken with Response Surface Methodology was employed to verify optimum condition for TC degradation. The quenching experiments and ESR determined that ·OH, ·O2- and 1O2 were involved in TC degradation with the treatment of 1.0% CSP/H2O2 system. Electrochemical analysis, ·OH quantification, and metal ion concentrations measure reveal that Co substitution accelerates electron transfer efficiency and Fe2+ regeneration. Furthermore, nine intermediates are identified and the possible degradation pathway of TC is proposed. The unique effect of Co provides novel insight and efficient strategies for improving the reactivity of iron sulfide.

Nitrate removal from groundwater in columns packed with reed and rice stalks.

Nitrate leaching contaminates groundwater. The objective of this study was to determine if reed and rice stalks could enhance denitrification and reduce nitrate leaching into groundwater. Artificial groundwater spiked with nitrate and field groundwater samples were tested in the columns in sand reactors packed with either reed or rice stalks. The maximum nitrate removal rates were determined to be 1.93 and 1.97 mg nitrate-N l(-1) h(-1), respectively, in the reed and rice stalk-packed columns. The maximum nitrate-nitrogen removal rate in reactors packed with reed stalk was 1.33 mg nitrate-N l(-1) h(-1) when experimented with natural groundwater. Chemical oxygen demand consumption was higher when rice stalk (176.1 mg l(-1)) was used as the substrate, compared to reed stalk (35.2 mg l(-1)) at the same substrate dosage. No nitrite accumulation was detected during the test. The results demonstrate that agricultural byproducts, such as reed and rice stalks, may be used as substrate amendments for enhanced denitrification in natural settings, such as lakeside lagoons, ditches or wetlands.

Anaerobic co-digestion of landfill leachate and acid mine drainage using up-flow anaerobic sludge blanket reactor.

A laboratory-scale up-flow anaerobic sludge blanket (UASB) reactor was developed and constructed for the treatment of landfill leachate and acid mine drainage (AMD). The removal of chemical oxygen demand (COD), sulfate, and metal ions was studied. The maximum COD and sulfate removal efficiency reached 75% and 69%, respectively, during the start-up phase of the UASB. The hydraulic retention time (HRT) had a significant influence on the system. The maximum removal efficiency for COD and sulfate reached 83% and 78%, respectively, at an HRT of 20 h. The methane production process competed with the sulfate reduction process in the UASB. The fractionation of metals in the sludge was analyzed to facilitate metal recovery in a later processing stage. The most abundant sulfate-reducing bacteria was Desulfobulbus, and the methanogen archaeal community in the reactor was mainly composed of Methanobacterium.

Dual role of macrophytes in constructed wetland-microbial fuel cells using pyrrhotite as cathode material: A comparative assessment.

In the recent years many studies have shown that wetland plants play beneficial roles in bioelectricity enhancement in constructed wetland-microbial fuel cell (CW-MFC) because of the exudation of root oxygen and root exudates. In this study, the long-term roles of plants on the bioelectricity generation and contaminant removal were investigated in multi-anode (Anode1 and Anode2) and single cathode CW-MFCs. The electrode distances were 20 cm between Anode1-cathode and 10 cm between Anode2-cathode, respectively. Additionally, the employment of natural conductive pyrrhotite mineral as cathode material was firstly investigated in CW-MFC system. A cathode potential of -98 ± 52 mV to -175 ± 60 mV was achieved in the unplanted (CW-MFC 1), and planted CW-MFCs with Iris pseudacorus (CW-MFC 2), Lythrum salicaria (CW-MFC 3), and Phragmites australis (CW-MFC 4). The maximum power densities of Anode1-cathode and Anode2-cathode were 8.23 and 15.29 mW/m2 in CW-MFC 1, 8.51 and 1.67 mW/m2 in CW-MFC 2, 5.67 and 3.15 mW/m2 in CW-MFC 3, and 7.59 and 14.71 mW/m2 in CW-MFC 4, respectively. Interestingly, smaller power density was observed at Anode2-cathode, which has shorter electrode distance than Anode1-cathode in both CW-MFC 2 and CW-MFC 3, which indicates the negative role of oxygen released from the flourished plant roots at Anode2 micro-environment in power production. Therefore, recovering power from commercial CW-MFCs with flourished plants will be a challenge. The contradiction between keeping short electrode distance and avoiding the interference from plant roots to maintain anaerobic anode may be solved by the proposed modular CW-MFCs.

Performance of Nano Zero-Valent Iron Derived from the Decomposition of Siderite in the Removal of Phosphate.

Natural siderite was selected as a raw material for preparing nano zero-valent iron (nZVI). The efficiency of the as-synthesized nZVI for PO3-4-P removal was investigated, and the effects of the annealing temperature, pH, initial PO3-4-P concentration, adsorption temperature and oxygen were investigated. The results indicated that after annealing at 550 °C, nZVI exhibited an average crystal size of 56.3 nm and a surface area of 14.1 m²/g. A decrease in pH and an increase in oxygen availability enhanced the removal efficiency. The adsorption process, which was spontaneous and exothermic according to the thermodynamic analysis, agreed well with the pseudo-second-order kinetic model. Based on the Langmuir equilibrium isotherms, the capacity of nZVI to adsorb phosphorus was determined to be 33.18 mg/L. The optimized conditions for the experimental conditions were defined by an orthogonal experiment as follows: initial P concentration 2 mg/L, initial pH 4, iron dose 2 g/L, adsorption time 60 min. The experimental results suggested that the as-prepared nZVI was a promising adsorbent for the removal of phosphate.