Microwave Discharge Electrodeless Lamps. Part VIII: Continuous On-Site Solar Energy Remediation of Contaminated Water

Solar Photooxygenations for the Manufacturing of Fine Chemicals-Technologies and Applications

Photooxygenation reactions involving singlet oxygen (1O2) are utilized industrially as a mild and sustainable access to oxygenated products. Due to the usage of organic dyes as photosensitizers, these transformations can be successfully conducted using natural sunlight. Modern solar chemical reactors enable outdoor operations on the demonstration (multigram) to technical (multikilogram) scales and have subsequently been employed for the manufacturing of fine chemicals such as fragrances or biologically active compounds. This review will highlight examples of solar photooxygenations for the manufacturing of industrially relevant target compounds and will discuss current challenges and opportunities of this sustainable methodology.

Development of a Hg-free UV light source incorporating a Kr/Br2 gas, and its application for wastewater treatments

Mercury lamps are typically the major light sources in water treatments. However, the use of mercury has raised some concerns with regard to the Minamata Convention on Mercury. As such, Hg-free microwave discharged electrodeless lamps (MDELs) that incorporate a rare gas and a halogen gas (R/H-MDEL) have been investigated with such Hg-free mixture filler gases as Kr/Cl₂, Xe/Cl₂, and Kr/Br₂ (R/H). Of these, only the Kr/Br₂-MDEL lamp is self-ignited at an inner pressure of 15 Torr when irradiated with microwave radiation. Accordingly, a novel Kr/Br₂ three-layer MDEL (Kr/Br₂-MDEL) photoreactor was fabricated to assess the optimal gas composition and gas pressure toward its performance vis-à-vis the treatment of model wastewaters contaminated with the tartrazine dye in aqueous media and with Escherichia coli (E. coli) bacteria. The extent of degradation of the tartrazine dye and sterilization of E. coli increased with irradiation time, with microwave radiation power (100, 200, and 300 W), and with increased sample flow rate 0.4 L min^‒1 to 0.8 L min^‒1. The tartrazine-contaminated wastewater was treated at a flow rate of 0.4 L min^‒1 for 60 min of microwave irradiation by three different protocols that resulted in UV (62%) >> UV/ROS (24%) > ROS (0%); ROS denotes reactive oxygen species. After 5 min irradiation of the E.coli wastewater, also at 0.4 L min^‒1, the order was UV (99.5%) ≈ UV/ROS (99.3%) >> ROS (14.5%). For comparison, the photosterilization of E. coli with an equivalent Hg/Ar-MDEL light source was also nearly complete (99.7%). Thus, the suitability of the environmentally friendlier Kr/Br₂ gas fill to replace Hg/Ar filler gas in MDELs for the photoelimination of organic pollutants and microbial disinfection in aqueous media has been demonstrated.

Microwave-Enhanced Photolysis of Norfloxacin: Kinetics, Matrix Effects, and Degradation Pathways

Degradation of norfloxacin (NOR) was studied using a combination of microwave and UV irradiation methods (MW/UV process). Remarkable synergistic effect was found between MW and UV light. The removal rate with the MW/UV process was much faster than that with UV light irradiation only. Degradation of NOR followed second-order kinetics and ~72% of NOR could be removed in the first 5 min of MW/UV reaction. Influence of inorganic ions (cations (K⁺, Mg2+, Ca2+, Cu2+) and anions (Cl-, SO₄2-, NO₃-, CO₃2-)), humic acid (HA) and surfactants (cation, anion, and non-ionic) on the degradation of NOR by the MW/UV process was investigated. Among the ions, Cu2+ and NO₃- ions inhibited the degradation of NOR. The presence of HA and surfactants in water showed a slight inhibition on the NOR removal. Furthermore, the NOR degradation in the MW/UV process was primarily caused by the ·OH-photosensitization steps. Seven intermediates formed by the oxidation of NOR were identified and three reaction pathways were proposed. Removals of NOR in tap water (TW), synthetic wastewater (WW), river water (RW), and seawater (SW) were also studied, which demonstrated that the MW/UV process was an effective oxidation technology for degrading fluoroquinolone antibiotics in different water matrices.

Solar Photochemical Synthesis: From the Beginnings of Organic Photochemistry to the Solar Manufacturing of Commodity Chemicals

Natural sunlight offers a cost-efficient and sustainable energy source for photochemical reactions. In contrast to the lengthy and small-scale "flask in the sun" procedures of the past, modern solar concentrator systems nowadays significantly shorten reaction times and enable technical-scale operations. After a brief historical introduction, this review presents the most important solar reactor types and their successful application in preparative solar syntheses. The examples demonstrate that solar manufacturing of fine chemicals is technically feasible and environmentally sustainable. After over 100 years, Ciamician's prophetic vision of "the photochemistry of the future" as a clean and green manufacturing methodology has yet to be realized. At the same time, his warning "for nature is not in a hurry but mankind is" is still valid today. It is hoped that this review will lead to a renewed interest in this truly enlightening technology, that it will stimulate photochemists and photochemical engineers to "go back to the roots onto the roofs" and that it will ultimately result in industrial applications in the foreseeable future.