Mengkai Li

Impact of reflection on the fluence rate distribution in a UV reactor with various inner walls as measured using a micro-fluorescent silica detector

An assessment of the impact of ultraviolet (UV) reflection from inner walls is important for the accuracy of model predictions of fluence rate (FR) distribution and for the improvement of reactor efficiency. In this study, the FR distribution in an annular UV reactor with inner walls of various reflectances was measured in-situ by using a 360° response micro-fluorescent silica detector. The tests were performed in water with various transmittances ranging from 65% to 99% and with inner reactor walls composed of quartz/aluminum foil, quartz/stainless steel, or quartz/black cloth, whose reflection coefficients were determined to be 80.5%, 26.1% and 11.1%, respectively. The results demonstrate that an inner wall with a high reflection coefficient can lead to a marked increase in the weighted average FRs, thus greatly improving the reactor efficiency. Furthermore, the presently used FR distribution models could have an error of up to 35% for commonly used stainless steel walls as a result of the influence of inner-wall reflection. Finally, it was found that the uniformity of the FR distribution is strongly dependent on the diffuse reflection property of the inner wall, which could lead to a better fluence delivery distribution in the UV reactor. This work has potential application to increase the accuracy of model predictions as well as optimize the design of high-efficiency UV reactors.

In Situ Measurement of UV Fluence Rate Distribution by Use of a Micro Fluorescent Silica Detector

The fluence rate (FR) distribution in an ultraviolet (UV) reactor was determined experimentally in situ by use of a novel 360° micro fluorescent silica detector (MFSD). The UV response of the MFSD was systematically characterized, and the results indicated that this detector responded only to UV in the range from 210 to 280 nm. The nonlinearity was found to be less than 1% as the FR varied from 0.083 to 2110 μW/cm2. The luminescent signal increased by 0.11% for every degree increase in temperature in the studied range of 0-55 °C. FR distribution tests were performed in different media (air or water) with the water transmittance at either 95% or 85% determined in a 1-cm path length. The FR distribution of the near-lamp region (e.g., radius<50 mm) was well determined with the nearest distance to the sleeve being less than 3 mm. Comparisons were made between the experimental data and the calculations by use of the UVCalc model. This work demonstrates that the MFSD is a novel technique that can provide in situ and real-time measurements of the FR distribution in a UV reactor.

VUV/UV/Chlorine as an Enhanced Advanced Oxidation Process for Organic Pollutant Removal from Water: Assessment with a Novel Mini-Fluidic VUV/UV Photoreaction System (MVPS).

Vacuum ultraviolet (VUV) and ultraviolet (UV)/chlorine processes are regarded as two of many advanced oxidation processes (AOPs). Because of the similar cost of VUV/UV and UV lamps, a combination of VUV and UV/chlorine (i.e., VUV/UV/chlorine) may enhance the removal of organic pollutants in water but without any additional power input. In this paper, a mini-fluidic VUV/UV photoreaction system (MVPS) was developed for bench-scale experiments, which could emit both VUV (185 nm) and UV (254 nm) or solely UV beams with a nearly identical UV photon fluence. The photon fluence rates of UV and VUV output by the MVPS were determined to be 8.88 × 10(-4) and 4.93 × 10(-5) einstein m(-2) s(-1), respectively. The VUV/UV/chlorine process exhibited a strong enhancement concerning the degradation of methylene blue (MB, a model organic pollutant) as compared to the total performance of the VUV/UV and UV/chlorine processes, although the photon fluence of the VUV only accounted for 5.6% of that of the UV. An acidic pH favored MB degradation by the VUV/UV/chlorine process. The synergistic mechanism of the VUV/UV/chlorine process was mainly ascribed to the effective use of (•)OH for pollutant removal through formation of longer-lived secondary radicals (e.g., (•)OCl). This study demonstrates that the new VUV/UV/chlorine process, as an enhanced AOP, can be applied as a highly effective and energy-saving technology for small-scale water and wastewater treatment.

Effects of the oxide layer on cavity formation and He desorption in He implanted silicon

Samples of n-type Cz Si (1 0 0) and the same Si with a 220 nm oxide layer were implanted at room temperature with 40 or 160 keV He ions at the same dose of 5 × 1016 cm2. Cross-sectional transmission electron microscopy (XTEM) and thermal desorption spectroscopy (TDS) have been performed to study the effects of the oxide layer on cavity evolution and He desorption upon annealing. XTEM observations show that the presence of the oxide layer can inhibit the thermal growth of cavities, which show strong dependence on the energy of He ions. For 40 keV He implantation, shrinkage of the cavities was observed mainly in the Si region close to the original SiO2/Si interface. However, under 160 keV He implantation the presence of the oxide layer leads to a slight broadening of the cavity band, while the mean size of cavities in the band decreases. TDS measurements reveal that the top oxide layer on Si surface could induce the disappearance of the desorption peak at low annealing temperature and occurrence of a new peak at the intermediate annealing temperature. The results were tentatively discussed in combination with the role of the oxide layer in thermal evolution of both defects and He atoms.

Inspection of Feasible Calibration Conditions for UV Radiometer Detectors with the KI/KIO3 Actinometer

UV radiometers are widely employed for irradiance measurements, but their periodical calibrations not only induce an extra cost but also are time‐consuming. In this study, the KI/KIO3 actinometer was applied to calibrate UV radiometer detectors at 254 nm with a quasi‐collimated beam apparatus equipped with a low‐pressure UV lamp, and feasible calibration conditions were identified. Results indicate that a washer constraining the UV light was indispensable, while the size (10 or 50 mL) of a beaker containing the actinometer solution had little influence when a proper washer was used. The absorption or reflection of UV light by the internal beaker wall led to an underestimation or overestimation of the irradiance determined by the KI/KIO3 actinometer, respectively. The proper range of the washer internal diameter could be obtained via mathematical analysis. A radiometer with a longer service time showed a greater calibration factor. To minimize the interference from the inner wall reflection of the collimating tube, calibrations should be conducted at positions far enough away from the tube bottom. This study demonstrates that after the feasible calibration conditions are identified, the KI/KIO3 actinometer can be applied readily to calibrate UV radiometer detectors at 254 nm.

Accelerated degradation of sulfamethazine in water by VUV/UV photo-Fenton process: Impact of sulfamethazine concentration on reaction mechanism

The degradation of sulfamethazine (SMN) by VUV/UV photo-Fenton (VPF) process was investigated with a mini-fluidic VUV/UV photoreaction system. Compared with the conventional UV photo-Fenton process, the VPF process significantly enhanced the degradation and mineralization of SMN, because the VUV irradiation photolyzed H2O and accelerated the redox cycle of Fe3+/Fe2+ to generate more reactive oxygen species (ROS). Initial pH and concentrations of SMN, H2O2, Fe3+, inorganic anions (NO3-, HCO3-, and Cl-), and humic acid all considerably impacted SMN degradation in the VPF process. In particular, the initial SMN concentration significantly affected the absorption distributions of UV and VUV photons in the reaction solution, thus inducing a different reaction mechanism. At a lower SMN concentration (1.8μM), most of UV and VUV photons were absorbed by Fe3+ and H2O, respectively, so indirect oxidation by ROS mainly accounted for SMN degradation. However, at a higher SMN concentration (90μM), 89.2% of UV photons and 59.0% of VUV photons were absorbed by SMN, so direct photolysis also played an important role. In addition, HO and HO2 were identified as the main ROS in the VPF process. This study demonstrates that the VPF process can effectively remove organic micropollutants from water.

Experimental Assessment of Photon Fluence Rate Distributions in a Medium-Pressure UV Photoreactor

The performance of a medium-pressure (MP) mercury lamp photoreactor is strongly influenced by the spatial photon fluence rate (PFR) distributions which are wavelength-dependent. To address this issue, PFR distributions in an MP lamp photoreactor were measured using a 360-degree response microfluorescent silica detector (MFSD). To accurately express the optical behavior in an MP photoreactor, PFR, MFSD response PFR (PFRMFSD), and effective germicidal PFR (PFRGER) were defined and compared. The measured axial and radial PFRMFSD values agreed well with the corresponding results from a simulation model (UVCalc). The PFR and PFRGER were obtained from the measured PFRMFSD by using correction factors calculated by the UVCalc. Under identical UV transmittance (254 nm) conditions (75% and 85%), the weighted average PFRGER values were 13.3-18.7% lower than the corresponding PFR values, indicating that PFRGER, rather than PFR should be used in MP photoreactor design to meet disinfection standards. Based on measured lamp output, medium absorption spectrum, MFSD response, and microbial DNA response spectrum, the detailed relationships between the PFR, PFRMFSD, and PFRGER were elucidated. This work proposes a new method for the accurate description of wavelength-dependent PFR distributions in MP photoreactors, thus providing an important tool for the optimal design of these systems.

Quinone group enhances the degradation of levofloxacin by aqueous permanganate: Kinetics and mechanism

Quinone group is an important fraction of humic acid. The pseudo-first-order rate constant (kobs) of levofloxacin (LF) degradation by permanganate (MnO4-) significantly increased from 0.010 (without benzoquinone, BQ) to 0.042-0.443 min-1 at [BQ]o:[MnO4-]o (molar ratio) = 0.03-0.25 at pH 7.5, and an acidic pH facilitated LF degradation. In the presence of BQ, MnO4- was first reduced to Mn(II). Then, Mn(II) reacted with BQ to produce Mn(III) and semiquinone radical, which was promoted under acidic conditions. With dissolved oxygen available, Mn(III) further oxidized semiquinone radical to produce singlet oxygen (1O2) and superoxide radical (O2-) as well as regenerate BQ. In addition, MnO4- could also react with Mn(II) to produce Mn(III), whose complexation with semiquinone radical in turn promoted this reaction. Due to the predominant scavenging of O2- by BQ, 1O2 and Mn(III) mainly contributed to the accelerated LF degradation, with a notable formation of hydroxyl, ketone and endoperoxide groups in the degradation byproducts. This study helps better understand the role of natural organic matter in the degradation of organic micropollutants by MnO4- in water treatment.

A Green Method to Determine VUV (185 nm) Fluence Rate Based on Hydrogen Peroxide Production in Aqueous Solution

A mini‐fluidic vacuum ultraviolet/ultraviolet (VUV/UV) photoreaction system (MVPS) was developed in our previous study. Based on the MVPS, a green method to determine VUV fluence rate has been developed using the production rate of H2O2 when water is exposed to 185 nm VUV. The H2O2 production followed pseudo‐zero‐order reaction kinetics well over the first 10 min of VUV/UV exposure. This new method was well calibrated with a standard cis‐cyclooctene cis–trans photoisomerization actinometer as recommended by the International Union of Pure and Applied Chemistry. The apparent quantum yield for H2O2 production by 185 nm VUV irradiation of water was determined to be 0.024 ± 0.002. As the solution pH increased from 5.0 to 8.0, the H2O2 production rate decreased from 0.83 to 0.40 μm min–1. Dissolved oxygen had a negligible influence on the H2O2 production. This study proposes a novel VUV fluence rate determination method with advantages of nontoxicity, low detection limits, low costs and convenience, and it can be used as a good alternative to traditional actinometers.

Experimental Evaluation of Turbidity Impact on the Fluence Rate Distribution in a UV Reactor Using a Microfluorescent Silica Detector

Turbidity is a common parameter used to assess particle concentration in water using visible light. However, the fact that particles play multiple roles (e.g., scattering, refraction, and reflection) in influencing the optical properties of aqueous suspensions complicates examinations of their effects on ultraviolet (UV) photoreactor performance. To address this issue, UV fluence rate (FR) distributions in a photoreactor containing various particle suspensions (SiO2, MgO, and TiO2) were measured using a microfluorescent silica detector (MFSD). Reflectance of solid particles, as well as transmittance and scattering properties of the suspensions were characterized at UV, visible, and infrared (IR) wavelengths. The results of these measurements indicated that the optical properties of all three particle types were similar at visible and IR wavelengths, but obvious differences were evident in the UV range. The FR results indicated that for turbidity associated with SiO2 and MgO suspensions, the weighted average FR (WAFR) increased relative to deionized water. These increases were attributed to low particle photon absorption and strong scattering. In contrast, the WAFR values decreased with increasing turbidity for TiO2 suspensions because of their high particle photon absorption and low scattering potential. The findings also indicate that measurements of scattering and transmittance at UV wavelengths can be used to quantify the effects of turbidity on UV FR distributions.