Tetracycline Degradation in the UV-heterogeneous Fenton-like System With BiFeO3 Catalyst

Surplus tetracycline (TC) in the water body causes damage to the ecology balance and human health. Therefore, an efficient strategy was proposed, namely, the UV-heterogeneous Fenton-like system with BiFeO3 (BFO) catalyst, to eliminate TC pollution. This work successfully integrated the photocatalytic oxidation system with the heterogeneous Fenton-like system, cooperating with the photolysis of H2O2. These coupled effects could boost the reduction of Fe (Ⅲ) to Fe (Ⅱ) and depress the recombination of photogenerated charges, further promoting the generation of reactive species, and ultimately facilitating the TC degradation and mineralization. The catalytic of the prepared BFO was stable with no secondary pollution, and BFO could be recovered by an extra magnet to reuse. Compared with other advanced oxidation processes, this system showed an outstanding performance in TC degradation and mineralization, and TC and TOC removal efficiencies could reach 100% and 74.92%, respectively. Moreover, the possible mechanisms for TC degradation involved that TC was degraded by oxidation species generated by the synergistic effect in this system, such as superoxide radicals (·O- 2), hydroxyl radicals (·OH), and positive holes (h+). Intermediate products in the TC degradation process mainly were products at m/z=459, m/z=445, and m/z=134.

Heterojunctions for Photocatalytic Wastewater Treatment: Positive Holes, Hydroxyl Radicals and Activation Mechanism under UV and Visible Light

Forming heterojunctions by coupling two or more semiconductors is an important strategy to develop stable and efficient photocatalysts able to operate both under near-UV and visible light. Five novel heterojunction systems were synthesized in the present study, using a modified sol-gel method: Bi2Mo3O12/TiO2, ZnFe2O4/TiO2, FeTiO3/TiO2, WO3(US)/TiO2 and WO3/TiO2. These heterojunction semiconductors were characterized by using XRD, SEM and EDX, UV–Vis diffuse reflectance spectroscopy and BET. Their photocatalytic activities were evaluated using methyl orange (MO) degradation under both near-UV and visible light. From the various heterojunctions developed, the WO3(US)/TiO2 photocatalyst was the one that showed the highest photocatalytic efficiency with this being assigned to the formation of a double heterojunction involving anatase, rutile and monoclinic WO3 phases. On this basis, a photocatalyst activation mechanism applicable to near-UV and visible light irradiation was proposed. This mechanism explains how the photogenerated electrons (e–) and positive holes (h+) can be transferred to the various phases. As a result, and given the reduced holes and electron recombination surface, hydroxyl radicals found were more abundant. To confirm this assumption, hole formation in the valence band was studied, using hole-scavenging reactions involving ion iodine (I–), while hydroxyl radical production used fluorescence spectroscopy.

Real Time Monitoring of Groundwater Fluorescence: Principle and Applicability in Nepal

As Nepal sits on the Indo-Eurasian plate boundary, it is highly susceptible to catastrophic earthquakes which have been posing a grave threat to the people of the country. Even though earthquake is one of the most destructive natural phenomena, its occurrence is still unpredictable. Advance warning of disastrous earthquakes is crucial so that the damage due to it is minimal. Different changes in the properties of groundwater prior to earthquake have been detected which can give important hints. Among them, the increase in fluorescent intensity of groundwater comprises a significant earthquake precursor. The positive holes, formed due to tectonic stresses in rocks with peroxy defects, interact with groundwater modifying the fluorescence intensity of water. In this study, we investigate the effect of seismic activity on the groundwater fluorescence intensity. The necessity of this examination for the country is also demonstrated. Taking into account the large variation in the groundwater quality and types of rock in Nepal, the investigation of fluorescent intensity using fluorometer may unravel different uncertainties and limitations. The simplicity of fluorometer in installation, methodology, maintenance, and its sensitivity up to large catchment area make it suitable for such investigation. Till now, variations of the groundwater fluorescent intensity have not been deeply studied in Nepal. This is of high importance in terms of earthquake forecast considering the high seismic activity in Nepal which lies on a very seismically active zone.

Titanium Dioxide/Graphene and Titanium Dioxide/Graphene Oxide Nanocomposites: Synthesis, Characterization and Photocatalytic Applications for Water Decontamination

The use of titanium dioxide, TiO2 as a photocatalyst in water decontamination has witnessed continuous interest due to its efficiency, stability, low toxicity and cost-effectiveness. TiO2 use is limited by its large band gap energy leading to light absorbance in the UV region of the spectrum, and by the relatively fast rate of recombination of photogenerated electrons and positive holes. Both limitations can be mitigated by using carbon-TiO2 nanocomposites, such as those based on graphene (G) and graphene oxide (GO). Relative to bare TiO2, these nanocomposites have improved photocatalytic activity and stability under the UV–visible light, constituting a promising way forward for improved TiO2 photocatalytic performance. This review focuses on the recent developments in the chemistry of TiO2/G and TiO2/GO nanocomposites. It addresses the mechanistic fundamentals, briefly, of TiO2 and TiO2/G and TiO2/GO photocatalysts, the various synthesis strategies for preparing TiO2/G and TiO2/GO nanocomposites, and the different characterization techniques used to study TiO2/G and TiO2/GO nanocomposites. Some applications of the use of TiO2/G and TiO2/GO nanocomposites in water decontamination are included.

Structure, Chemical Composition, and Catalytic Behaviour of Stoichiometric and Non-Stoichiometric LaMnO3 Toward Deep Oxidation of Ethanol

A series of structurally modified LarMnO3±δ (r = 0.80, 1.00, 1.25) perovskites have been investigated for ethanol deep oxidation in the temperature range 100–300°C. The catalysts were characterized by X-ray diffraction (XRD), Brunauer–Emmet–Teller (BET) surface area analysis, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), redox titration and CO2 adsorption. All samples are single perovskite phases without segregated phases. For La- and Mn-under-stoichiometric samples, the defects generated to maintain structure electroneutrality are positive holes and anion vacancies whereas for stoichiometric solids, the defects are positive holes and cation vacancies. The surface oxidation state and composition are critically controlled by the phase composition of the bulk. XPS results showed that the decrease in bulk La/Mn ratio induced a decrease in surface La enrichment. A concomitant decrease of basic site concentration was shown by CO2 adsorption. With lower surface La content, excess Mn sites and increasing concentrations of surface Mn4+ were observed. Catalytic properties in the oxidation reaction of ethanol are attributed to the variability of the manganese oxidation state, basic character of the material surface, which is related to the La/Mn atomic ratio, and to the oxygen storage capacity in the crystalline lattice of the catalysts. It was shown that La0.8MnO3–δ presents the best catalytic activity, due to low surface basicity (lowest La/Mn ratio) and high redox properties (highest Mn4+/Mn3+ ratio).

Multiple Effect of pH on Photocatalytic Degradation of Orange II Using TiO2

The performance of UV/TiO2 (100 mg/L TiO2) process for Orange II degrdation was emphatically compared at pH 2.3, 6.9 and 11.5 by means of UV-Visible spectra, ionic chromatography, TOC, and HPLC analyses. The results revealed that the reaction intermediates were produced in different contents with the increasing reaction time due to the different pH conditions. The dye was oxidized more completely at pH 2.3 while the highest color removal percentage was achieved at pH 11.5. It was proved that active electrons, which is photo-generated along with positive holes on TiO2, not only contribute to the discoloration and mineralization of the dye due to the producing of oxidative species such as O2•- and •OOH, but also contribute to the discoloration of Orange II as reductive species. The most possible mechanism was put forward to explain the key roles of active electrons in dye degradation by UV/TiO2 process at pH 2.3 and 11.5.