Coordination geometry of Zn^(2+) on hexagonal turbostratic birnessites with different Mn average oxidation states and its stability under acid dissolution

Hexagonal turbostratic birnessite,with the characteristics of high contents of vacancies,varying amounts of structural and adsorbed Mn^（3＋）,and small particle size,undergoes strong adsorption reactions with trace metal（TM）contaminants.While the interactions of TM,i.e.,Zn^（2＋）,with birnessite are well understood,the effect of birnessite structural characteristics on the coordination and stability of Zn^（2＋）on the mineral surfaces under proton attack is as yet unclear.In the present study,the effects of a series of synthesized hexagonal turbostratic birnessites with different Mn average oxide states（AOSs）on the coordination geometry of adsorbed Zn^（2＋）and its stability under acidic conditions were investigated.With decreasing Mn AOS,birnessite exhibits smaller particle sizes and thus larger specific surface area,higher amounts of layer Mn^（3＋）and thus longer distances for the first Mn／O and Mn／Mn shells,but a low quantity of available vacancies and thus low adsorption capacity for Zn^（2＋）.Zn K-edge EXAFS spectroscopy demonstrates that birnessite with low Mn AOS has smaller adsorption capacity but more tetrahedral Zn（^（IV）Zn）complexes on vacancies than octahedral（^（VI）Zn）complexes,and Zn^（2＋）is more unstable under acidic conditions than that adsorbed on birnessite with high Mn AOS.High Zn^（2＋）loading favors the formation of^（VI）Zn complexes over^（IV）Zn complexes,and the release of Zn^（2＋）is faster than at low loading.These results will deepen our understanding of the interaction mechanisms of various TMs with natural birnessites,and the stability and thus the potential toxicity of heavy metal pollutants sequestered by engineered nano-sized metal oxide materials.

Constructing crystalline homophase carbon nitride S-scheme heterojunctions for efficient photocatalytic hydrogen evolution

The number of photogenerated carriers involved in the photocatalytic reaction is one of the main factors influencing the photocatalytic activity,and constructing S-scheme heterojunctions can significantly en-hance the migration of photogenerated carriers,which is regarded as an effective method.In this study,Au nanoparticles(NPs)-supported crystallized heptazine/triazine-based carbon nitride(AHTCN)S-scheme heterojunction photocatalysts are successfully prepared by photoreduction methods.Except for the Au NPs function as an electron mediator,the experiment results and DFT calculations demonstrate that the Fermi energy level of crystallized heptazine/triazine-based heterojunction(HTCN)is pulled down after anchoring Au NPs,and thus the electron transfer path of HTCN changed from Type II-scheme to S-scheme.Owing to S-scheme heterojunction,the optimal AHTCN-2(2 wt.%Au loaded)exhibits the best photocat-alytic hydrogen evolution with a production rate of 715.2μmol h^(−1)g^(−1),which significantly outperforms that of the HTCN.This work delivers a new strategy for the construction of S-scheme heterojunctions.