Improved reduction efficiency, cycling performance, and removal rate of hexavalent chromium by adding water-soluble salts.

Recently, the reaction speed and cycle performance of hexavalent chromium reduction over microsized zero-valent iron (ZVI) with an Fe0 core and iron oxide (FeOx) shell structure have been improved by activating the Fe0-core electrons through electromagnetic coupling between Fe0-core electrons and charges (hexavalent chromium in solution, double-charge layers of the ZVI/solution interface). Herein, the abovementioned electromagnetic coupling was greatly increased by adding salt (CH3COONa, NaCl, NaNO3, and Na2SO4) in the hexavalent chromium solution to increase the charge response. Adding salt greatly improved the reaction speed and cycle performance of hexavalent chromium reduction. It took 8 min to reduce hexavalent chromium with CH3COONa to below the discharge standard of wastewater in the first cycle and 20 min after reducing for 20 cycles. The best apparent rate of constant value (0.416 (min)-1) is nearly four times larger than those without salts. X-ray diffraction and X-ray photoelectron spectroscopy revealed the production of amorphous iron oxide shell with salt. The salt improves the hexavalent chromium reduction speed and cycle performance and impedes the Fe0-core-electron transfer via the produced Fe2O3, resulting in existence of an optimized salt dosage. This work aims to provide an effective route for enhancing the removal efficiency and cycle performance of heavy-metal-ion reduction via Fe0. And this work also proposes a novel viewpoint that adding salt in waste water would increase the electromagnetic coupling between the charges in solution and Fe0-core electrons which could finally activate the redox reaction.

Photoreduction properties of novel Z-scheme structured Sr0.8La0.2(Ti1-δ 4+Ti δ 3+)O3/Bi2MoO6 composites for the removal of Cr(vi).

Novel Z-scheme structured Sr0.8La0.2(Ti1-δ 4+Ti δ 3+)O3/Bi2MoO6 (LSTBM) composites were prepared via a facile two-step solvothermal method. Several characterization techniques were employed to investigate the phases, microstructures, compositions, valence states, oxygen vacancies, surface oxygen absorption, energy band structures and lifetime of photoproduced carriers. It was found that the lifetime and transfer of the photoproduced carriers of LSTBM were better than those of Bi2MoO6 (BMO) and Sr0.8La0.2(Ti1-δ 4+Ti δ 3+)O3 (LSTO). The LSTBM with a molar ratio of BMO/(LSTO + BMO) = 0.07 (denoted as LSTBM7) showed 1.9 and 3.1 times removal rates than those for BMO and LSTO, respectively. Importantly, the built-in electric field in the heterojunction of LSTBM and Ov-s, especially in Ov-s on the higher-Fermi-level side of the heterojunction, had co-played roles in prolonging the lifetime and improving the transfer of photogenerated carriers. The photoproduced e- played a dominant role in reducing Cr(vi) to Cr(iii) and the produced Cr(iii) tends to form Cr(OH)3 and adsorb onto the surface of the photocatalyst to decrease the nucleation energy. The possible reduction route for Cr(vi) to Cr(iii) over LSTBM7 was figured out. This study implies that inducing Ov-s on the higher-Fermi-level side of the Z-scheme heterojunction is a more effective route for separating the photogenerated electrons and holes and improving the transfer of photogenerated carriers.