The state-of-the-art development of photocatalysts for the degradation of persistent herbicides in wastewater

Herbicides are one of the most recurring pollutants in the aquatic system due to their widespread usage in the agriculture sector for weed control. Semiconductor-based photocatalysts have gained recognition due to their ability to degrade and mineralize pollutants into harmless by-products completely. Lately, many studies have been done to design photocatalysts with efficient separation of photogenerated charge carriers and enhanced light absorption. Photocatalyst engineering through doping with metal and non-metal elements and the formation of heterojunction are proven effective for minimizing the recombination of electron-hole pairs and enlarging the absorption in the visible light region. This review focuses on discussing and evaluating the recent progress in the types of photocatalysts and their performance in the remediation of herbicides in wastewater. The development of innovative hybrid technologies is also highlighted. The limitations and challenges of photocatalysis technology in the present literature have been identified, and future studies are recommended.

Bismuth Oxyhalides for NOx Degradation under Visible Light: The Role of the Chloride Precursor

Photocatalysis is a green technology for tackling water and air contamination. A valid alternative to the most exploited photocatalytic material, TiO2, is bismuth oxyhalides, which feature a wider bandgap energy range and use visible radiation to attain photoexcitation. Moreover, their layered structure favors the separation of photogenerated electron–hole pairs, with an enhancement in photocatalytic activity. Controlled doping of bismuth oxyhalides with metallic bismuth nanoparticles allows for further boosting of the performance of the material. In the present work, we synthesized Y%Bi-doped BiO(Cl0.875Br0.125) (Y = 0.85, 1, 2, 10) photocatalysts, using cetyltrimethylammonium bromide as the bromide source and varying the chloride source to assess the impact that both length and branching of the hydrocarbon chain might have on the framing and layering of the material. A change in the amount of the reducing agent NaBH4 allowed tuning of the percentage of metallic bismuth. After a thorough characterization (XRPD, SEM, TEM, UV-DRS, XPS), the photocatalytic activity of the catalysts was tested in the degradation of NOx under visible light, reaching a remarkable 53% conversion after 3 h of illumination for the material prepared using cetylpyridinium chloride.

An ultrasound-assisted photocatalytic treatment to remove an herbicidal pollutant from wastewaters

Pollutants of emerging concern contaminate surface and ground water. Advanced oxidation processes treat these molecules and degrade them into smaller compounds or mineralization products. However, little information on coupled advanced oxidation techniques and on the degradation pathways of these pollutants is available to identify possible ecotoxic subproducts. In the present work, we investigate the ultrasound assisted photocatalytic degradation pathway of the herbicide Isoproturon. We worked in batch mode in a thermostatic glass reactor. We compared the activity of nanometric TiO₂ P25 with that of Kronos 1077, a micrometric TiO₂. We discuss the individual, additive and synergistic degradation action of photolysis, sonolysis, sonophotolysis, and sonophotocatalysis by varying catalyst loading and/or ultrasound power for the last three techniques. With 0.1 g L^-1 catalyst, photocatalysis and sonophotopcatalysis completely degrade Isoproturon within 240 min and 60 min, respectively (>99% conversion). Sonophotocatalysis breaks Isoproturon down into smaller molecules than photocatalysis alone.

Ultra-small and recyclable zero-valent iron nanoclusters for rapid and highly efficient catalytic reduction of p-nitrophenol in water

The synthesis of nanoscale zero-valent iron (NZVI) nanoclusters with dimensions ranging from 20 to 100 nm for the control of environmental pollutants has received substantial attention. However, due to the strong van der Waals and magnetic attraction forces of ZVI, synthesizing ZVI nanoclusters with a subnanometre size while retaining their surface activity and avoiding aggregation is challenging. Moreover, NZVI particles can be oxidized easily after the removal of contaminants even in anoxic environments, which makes the recovery and recycling of the particles very difficult. Here, for the first time, ultra-small zero-valent iron (ZVI) nanoclusters are successfully prepared in a micelle assisted method under mild conditions, and can be recycled simply. It is found that by encapsulating Fe3+ within the micelles, controlling the release of sulfur ions (S2-) from thiourea and forming the FeS nanoparticles as intermediates, the ZVI nanoclusters are produced with a precisely controlled size (<1 nm). A large number of zero-valent iron nanoclusters were assembled into quasi-spherical assemblages (with around 5 nm size), in which most of the nanoclusters exist discretely because of being coated by entangled hydrocarbon chains of the surfactant. The ZVI nanoclusters (with a diameter of <1 nm) exhibit excellent dispersibility and accessibility in solution, presenting significantly enhanced catalytic activity in the removal of p-nitrophenol from water. The as-prepared ZVI nanoclusters possess excellent stability and durability with the aid of NaBH4. Their catalytic activity/reusability can be comparable to those of the commonly used noble metal catalysts.