In-Depth Investigation of the Mechanism of Dehydration-Induced Phase Transformation from Nb3O7(OH) to H-Nb2O5: A Theoretical and Experimental Approach.

H-Nb2O5 is a promising energy material, which can be typically obtained from any other polymorph after conducting high temperature calcination (∼1273 K). Recently, a low-temperature dehydration from Nb3O7(OH) was employed to prepare H-Nb2O5 at 723 K for 2 h, and yet the transformation mechanism has remained unclear in the literature. Here, the dehydration kinetic and phase transformation mechanism of the Nb3O7(OH) were investigated for the first time by experiments, density functional theory, and molecular dynamics calculations. After dehydration, the orthorhombic Nb3O7(OH) initially transformed into an intermediate Nb-O compound with dislocations, preserving parent structure, and subsequently transformed into monoclinic H-Nb2O5. The activation energy for the transformation from Nb3O7(OH) to H-Nb2O5 was as low as 1.35 eV, compared to that of T-Nb2O5 to H-Nb2O5 (3.60 eV). Furthermore, the defect-rich H-Nb2O5 obtained from Nb3O7(OH), does not exhibit pristine bound exciton state due to severe recombination of photogenerated carriers, resulting in poor photocatalytic activity.

The effects of Fe2+ on sulfur-oxidizing bacteria (SOB) driven autotrophic denitrification.

With the short-term exposure to Fe2+, the mechanism of autotrophic denitrification and sulfide oxidation and the correlation between microbial community changes and environmental factors have been explored in the ADSOB process. RSM was used to optimize conditions for the maximum nitrate reduction and sulfide oxidation. About 88% of nitrate could be autotrophically denitrified to nitrogen by utilizing sulfide as the electron donor with the molar ratio C/N of 1.14 and S/N of 0.99 at pH 7.1. Lower Fe2+ additions can reduce TDS inhibition with dissolved sulfide to form FeS precipitates, while high amount of Fe2+ limited the mass transfer of NO3- and intermediate products such as S0 may be generated. High-throughput sequencing and RDA analysis revealed the correlation between ferrous iron, environmental factors and microorganisms. Sulfurospirillum, Rhodanobacter, Thauera and Thiobacillus were all slightly promoted at NFL level and inhibited at NFH level. And the narrow angles of the arrows indicated that Thauera, Sulfurospirillum and Thiobacillus were positively correlated with SO42- concentrations, while large angles indicated these bacteria were inversely related with TDS and NO3- arrows, which further confirmed that these bacteria played a dominant role in the ADSOB process, and can reduce NO3- by the oxidation of TDS. The correlation further indicated that lower Fe2+ additions have a promoting effect, while high concentrations have an inhibiting effect.

In-situ synthesis of 3D GA on titanium wire as a binder- free electrode for electro-Fenton removing of EDTA-Ni.

Ethylenediaminetetraacetic acid (EDTA) could form stable complexes with toxic metals such as nickel due to its strong chelation. The three-dimensional (3D) macroporous graphene aerogels (GA), which was in-situ assembled by reduced graphene oxide (rGO) sheets on titanium wire as binder-free electrode, was presented as cathode for the degradation of EDTA-Ni in Electro-Fenton process. The X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), Transmission Electron Microscope (TEM) and Brunauer-Emmett-Teller (BET) results indicated 3D GA formed three dimensional architecture with large and homogenous macropore structure and surface area. Cyclic Voltammetry (CV), Linear Sweep Voltammetry (LSV) and Rotating Ring-disk Electrode (RRDE) results showed that the 3D GA cathode at pH 3 displayed the highest current density and electrochemical active surface area (ECSA), and better two-electron selectivity for ORR than other pH value, confirming the 3D-GA cathode at pH 3 has the highest electrocatalytic activity and generates more H2O2. The factors such as pH, applied current density, concentration of Fe2+, Na2SO4, and aeration rates of air were also investigated. Under the optimum conditions, 73.5% of EDTA-Ni was degraded after reaction for 2h. Mechanism analysis indicated that the production of OH on the 3D GA cathode played an important role in the removal of EDTA-Ni in the 3D GA-EF process, where the direct regeneration of Fe2+ on the cathode would greatly reduce the consumption of H2O2. Therefore, it is of great promise for 3D-GA catalyst to be developed as highly efficient, cost-effective and durable cathode for the removal of EDTA-Ni.

Treatment of Ni-EDTA containing wastewater by electrocoagulation using iron scraps packed-bed anode.

The unique electrocoagulator proposed in this study is highly efficient at removing Ni-EDTA, providing a potential remediation option for wastewater containing lower concentrations of Ni-EDTA (Ni ≤ 10 mg L-1). In the electrocoagulation (EC) system, cylindrical graphite was used as a cathode, and a packed-bed formed from iron scraps was used as an anode. The results showed that the removal of Ni-EDTA increased with the application of current and favoured acidic conditions. We also found that the iron scrap packed-bed anode was superior in its treatment ability and specific energy consumption (SECS) compared with the iron rod anode. In addition, the packed density and temperature had a large influence on the energy consumption (ECS). Over 94.3% of Ni and 95.8% of TOC were removed when conducting the EC treatment at an applied current of 0.5 A, initial pH of 3, air-purged rate 0.2 L min-1, anode packed density of 400 kg m-3 temperature of 313 K and time of 30 min. SEM analysis of the iron scraps indicated that the specific area of the anode increased after the EC. The XRD analysis of flocs produced during EC revealed that hematite (α-Fe2O3) and magnetite (Fe3O4) were the main by-products under aerobic and anoxic conditions, respectively. A kinetic study demonstrated that the removal of Ni-EDTA followed a first-order model with the current parameters. Moreover, the removal efficiency of real wastewater was essentially consistent with that of synthetic wastewater.