Heterogeneous photocatalytic oxidation for the removal of organophosphorus pollutants from aqueous solutions: A review

Organophosphorus pollutants (OPs), which are compounds containing carbon‑phosphorus bonds or phosphate derivatives containing organic groups, have received much attention from researchers because of their persistence in the aqueous environment for long periods of time and the threat they pose to human health. Heterogeneous photocatalysis has been widely applied to the removal of OPs from aqueous solutions due to its better removal effect and environmental friendliness. In this review, the removal of OPs from aqueous matrices by heterogeneous photocatalysis was presented. Herein, the application and the heterogeneous photocatalysis mechanism of OPs were described in detail, and the effects of catalyst types on degradation effect are discussed categorically. In particular, the heterojunction type photocatalyst has the most excellent effect. After that, the photocatalytic degradation pathways of several OPs were summarized, focusing on the organophosphorus pesticides and organophosphorus flame retardants, such as methyl parathion, dichlorvos, dimethoate and chlorpyrifos. The toxicity changes during degradation were evaluated, indicating that the photocatalytic process could effectively reduce the toxicity of OPs. Additionally, the effects of common water matrices on heterogeneous photocatalytic degradation of OPs were also presented. Finally, the challenges and perspectives of heterogeneous photocatalysis removal of OPs are summarized and presented.

Remediation techniques for uranium removal from polluted environment - Review on methods, mechanism and toxicology

Uranium, a radionuclide, is a predominant element utilized for speciality requirements in industrial applications, as fuels and catalyst. The radioactive properties and chemical toxicity of uranium causes a major threat to the ecosystem. The hazards associated with Uranium pollution includes the cancer in bones, liver, and lungs. The toxicological properties of Uranium are discussed in detail. Although there are many methods to eliminate those hazards, this research work is aimed to describe the application of bioremediation methods. Bioremediation methods involve elimination of the hazards of uranium, by transforming into low oxidation form using natural microbes and plants. This study deeply elucidates the methods as bioleaching, biosorption, bioreduction and phytoremediation. Bioleaching process involves bio-oxidation of tetravalent uranium when it gets in contact with acidophilic metal bacterial complex to obtain leach liquor. In biosorption, chitin/chitosan derived sorbents act as chelators and binds with uranium by electrostatic attraction. Bio reduction employs a bacterial transformation into enzymes which immobilize and reduce uranium. Phytoremediation includes phytoextraction and phytotranslocation of uranium through xylems from soil to roots and shoots of plants. The highest uranium removal and uptake reported using the different methods are listed as follows: bioleaching (100% uranium recovery), biosorption (167 g kg^-1 uranium uptake), bioreduction (98.9% uranium recovery), and phytoremediation (49,639 mg kg^-1 uranium uptake). Among all the techniques mentioned above, bioleaching has been proved to be the most efficient for uranium remediation.

Synthesis, Characterization and Visible-Light Photocatalytic Activity of Solid and TiO2-Supported Uranium Oxycompounds

In this study, various solid uranium oxycompounds and TiO₂-supported materials based on nanocrystalline anatase TiO₂ are synthesized using uranyl nitrate hexahydrate as a precursor. All uranium-contained samples are characterized using N₂ adsorption, XRD, UV–vis, Raman, TEM, XPS and tested in the oxidation of a volatile organic compound under visible light of the blue region to find correlations between their physicochemical characteristics and photocatalytic activity. Both uranium oxycompounds and TiO₂-supported materials are photocatalytically active and are able to completely oxidize gaseous organic compounds under visible light. If compared to the commercial visible-light TiO₂ KRONOS^® vlp 7000 photocatalyst used as a benchmark, solid uranium oxycompounds exhibit lower or comparable photocatalytic activity under blue light. At the same time, uranium compounds contained uranyl ion with a uranium charge state of 6+, exhibiting much higher activity than other compounds with a lower charge state of uranium. Immobilization of uranyl ions on the surface of nanocrystalline anatase TiO₂ allows for substantial increase in visible-light activity. The photonic efficiency of reaction over uranyl-grafted TiO₂, 12.2%, is 17 times higher than the efficiency for commercial vlp 7000 photocatalyst. Uranyl-grafted TiO₂ has the potential as a visible-light photocatalyst for special areas of application where there is no strict control for use of uranium compounds (e.g., in spaceships or submarines).

Stabilization of Photocatalytically Active Uranyl Species in a Uranyl-Organic Framework for Heterogeneous Alkane Fluorination Driven by Visible Light

When photoactivated, the uranyl ion is a powerful oxidant capable of abstracting hydrogen atoms from nonactivated C-H bonds. However, the highly reactive singly reduced [U^VO₂]⁺ intermediate is unstable with respect to disproportionation to the uranyl dication and insoluble tetravalent uranium phases, which limits the usage of uranyl ions as robust photocatalysts. Herein, we demonstrate that photoactivated uranyl ions can be stabilized by immobilizing and separating them spatially in a uranyl-organic framework heterogeneous catalyst, NU-1301. The visible-light-photoactivated uranyl ions in NU-1301 exhibited longer-lived U(V) and radicals than those in homogeneous counterparts, as evidenced by X-ray photoelectron spectroscopy and time-dependent electron paramagnetic resonance, leading to higher turnovers and enhanced stability for the fluorination of nonactivated alkanes.

Mesoporous Titanium Dioxide: Synthesis and Applications in Photocatalysis, Energy and Biology

Mesoporous materials are materials with high surface area and intrinsic porosity, and therefore have attracted great research interest due to these unique structures. Mesoporous titanium dioxide (TiO₂) is one of the most widely studied mesoporous materials given its special characters and enormous applications. In this article, we highlight the significant work on mesoporous TiO₂ including syntheses and applications, particularly in the field of photocatalysis, energy and biology. Different synthesis methods of mesoporous TiO₂-including sol⁻gel, hydrothermal, solvothermal method, and other template methods-are covered and compared. The applications in photocatalysis, new energy batteries and in biological fields are demonstrated. New research directions and significant challenges of mesoporous TiO₂ are also discussed.

Assembly of thiacalix[4]arene-supported high-nuclearity Cd24 cluster with enhanced photocatalytic activity

A high-nuclearity nanoscale Cd24 cluster has been hydrothermally synthesized by assembly of Cd4-TC4A (H4TC4A = p-tert-butylthiacalix[4]arene) second building units (SBUs) and in situ generated peroxy(mono)phosphate PO53- groups and peroxyphenoxide groups of TC4A. The cluster was structurally characterized by single crystal X-ray diffractions. Photocatalytic studies revealed that the highest nuclearity Cd,S-co-rich Cd24 cluster exhibits enhanced photocatalytic water splitting activities compared to the sandwich Cd4(TC4A)2 (Cd4) cluster under the same conditions in the absence of a co-catalyst. The nanostructure of Cd24 incorporated both peroxyphosphate and peroxyphenoxide groups, which increased the metal coordination numbers to give more labile Cd-O/S bonds and is believed to be the key feature that enables the significant photocatalytic water splitting activities.