Novel Z-scheme MgFe2O4/Bi2WO6 heterojunction for efficient photocatalytic degradation of tetracycline hydrochloride: Mechanistic insight, degradation pathways and density functional theory calculations

In this study, a new Z-scheme MgFe2O4/Bi2WO6 heterojunction was successfully prepared by hydrothermal and wet ball milling process. The results of the study showed that after 90 min of visible light exposure, the photocatalytic degradation of tetracycline hydrochloride (TCH) by 25%-MgFe2O4/Bi2WO6 heterojunction was as high as 95.82%, and the highest photocatalytic rate (0.0281 min-1) was 4.61 and 3.43 times higher than that of pure Bi2WO6 (0.0061 min-1) and MgFe2O4 (0.0082 min-1), respectively. Furthermore, spin-polarized density functional theory (DFT) calculations were performed to provide additional evidence of the presence of a Z-scheme charge transfer mechanism between MgFe2O4 and Bi2WO6. We investigated the effects of initial TCH concentration, pH, coexisting ions and different water sources on the efficiency of photocatalytic degradation of TCH in composite samples. The recovery experiments demonstrated that the MgFe2O4/Bi2WO6 composites had good stability and repeatability. A series of experimental results showed that 25%-MgFe2O4/Bi2WO6 had a larger specific surface area, better ultraviolet and visible absorbance, superior charge transfer and higher efficiency of photogenerated electron-hole pair separation. This paper provides new ideas for the design and preparation of new Z-scheme heterojunctions and has great prospects for practical applications in the field of wastewater treatment.

Modulating Cu valence state in Cu and graphene oxide composites for electrocatalytic tetracycline hydrochloride degradation

Cu and graphene oxide composites (Cu-GO) were designed by anchoring Cu+ via oxygen groups in GO based on the heavy co-relationships of copper (Cu) anode electrocatalytic activity with Cu valence state. With the consumption of oxygen groups under various pyrolysis temperatures, the Cu valence state changed from Cu ions (as CuCl2 and CuCl) to Cu oxide (CuO and Cu2O) and the final metallic Cu. In which the Cu+ in CuCl was more favorable for electrocatalytic oxidation than other Cu valence states. Due to the dramatic contribution of 1O2 and active chlorine, 100% degradation efficiency was achieved using tetracycline hydrochloride (TCH) as the target pollutant. Cu+ showed a selective preference for 1O2 and active chlorine triggering, rather than metallic Cu. Under the attack of 1O2 and active chlorine, the degradation intermediates of TCH were then provided by LC-MS results. The final results not only prove the feasibility of the Cu-GO/electrocatalysis system for pollution control but also shed light on the anode design via Cu valence state modulation.

In situ synthesis of Ag3PO4/C3N5Z-scheme heterojunctions with enhanced visible-light-responsive photocatalytic performance for antibiotics removal

Environmental pollution caused by antibiotics-containing wastewater has attracted increasing attention. Considering the superior photocatalytic performance and the issue of photo-corrosion over silver-based photocatalysts, it is desirable to construct silver-based photocatalysts with high photostability. Herein, a serious of Ag₃PO₄/C₃N₅ nanocomposites with Z-scheme band alignment were rationally designed and fabricated for tetracycline hydrochloride (TCH) removal. A variety of characterizations were employed to systematically study the phase structure, morphology and microstructure, optical properties, surface chemical states, and photocatalytic performance of the as-fabricated photocatalysts. The as-prepared Ag₃PO₄/C₃N₅ nanocomposites exhibited superior photocatalytic activity and photochemical stability than a single component toward TCH removal, meanwhile, the photocatalytic performance was not increased with the increasing amount of Ag₃PO₄. The possible photocatalytic mechanism (Z-scheme mechanism) was investigated and verified. The Z-scheme heterojunctions formed between Ag₃PO₄ and C₃N₅ is the main reason for the enhanced photocatalytic activities.

Peroxymonosulfate-enhanced photocatalysis by carbonyl-modified g-C3N4 for effective degradation of the tetracycline hydrochloride

In this work, carbonyl-modified g-C₃N₄ (CO-C₃N₄) is prepared through one-step calcination of the melamine-oxalic acid aggregates. The visible light-assisted photocatalytic degradation efficiency of the tetracycline hydrochloride (TCH) for CO-C₃N₄ is significantly enhanced by introducing the peroxymonosulfate (PMS), and the apparent rate constant is greatly increased from 0.01966 min^-1 in CO-C₃N₄/vis system to 0.07688 min^-1 in CO-C₃N₄/PMS/vis system. It is found that carbonyl for CO-C₃N₄ might offer possible reactive sites for PMS activation and collection sites of photo-generated electrons, greatly accelerating carrier's separation for PMS activation. The favorable conditions, such as the higher catalyst dosage, higher PMS amount and alkaline pH, contribute to TCH degradation. The deleterious effects of co-existing anions on the TCH degradation efficiency are ranked in a decline: H₂PO₄^- > SO₄^2- > HCO₃^- > NO₃^- > Cl^-, and it may be affected by the type and amounts of anions and active radicals generated. The radical trapping tests and electron spin resonance (ESR) detection display that the O₂^-, h⁺, ¹O₂, OH and SO₄^- all contribute to TCH degradation. Meanwhile, possible degradation mechanism, intermediates and degradation pathway of TCH are revealed in CO-C₃N₄/PMS/vis system. This study will offer a new insight for constructing PMS activation with carbonyl modified g-C₃N₄ photocatalysis system to achieve effective treatment of organic wastewater.