Determination of dissolved oxygen in water by the Winkler method: Performance modelling and optimisation for environmental analysis

Evaluating the impacts of drilling and extraction activities on the marine carbonate system in the natural gas fields of Beibu Gulf, Northern South China Sea

Natural gas fields are typically located in shallow gulfs. Previous studies have predominantly focused on gas leakage and its subsequent toxic effects on marine organisms; however, the impacts of accidental CO2 leaks on carbonate dynamics during drilling and extraction remain poorly understood. In this study, we investigate carbonate parameters in two gas fields to elucidate the influences of extraction activities and natural processes on carbonate dynamics in Beibu Gulf, situated in the northern South China Sea (nSCS). Our findings indicate that Beibu Gulf acts as a CO2 source during the late spring season, with an air-sea CO2 flux ranging from 1.1 to 4.4 mmol m-2 d-1. Spatially, higher sea surface pCO2 and temperatures were recorded at 479 ± 17 μatm and 29.6 ± 0.3 °C respectively in the Dongfang gas field within the inner gulf, compared to values of 462 ± 20 μatm and 27.6 ± 0.6 °C observed in the Ledong gas field located in the outer gulf. In the Ledong gas field, carbonate dynamics are primarily influenced by mixing between offshore subsurface water and river plumes, with no significant contributions from extraction activities noted. Conversely, dissolved inorganic carbon (DIC) levels within the Dongfang gas field exhibited two extremes: a consumption of 13 μmol kg-1 in surface waters alongside an addition of 20 μmol kg-1 in subsurface waters. Although enhanced biological production may lead to decreased surface pCO2 levels, elevated surface pCO2 values observed at Dongfang can likely be attributed to higher sea surface temperatures. In subsurface layers, we quantify and determine the origin of excess DIC based on δ13CDIC analysis, attributing these carbon inputs to organic matter respiration following surface biological utilization of atmospheric CO2. This study provides the first evidence of high biological DIC consumption in surface waters and DIC generation in subsurface waters within a shallow semi-enclosed bay, a phenomenon previously seen only in river estuaries. We demonstrate that natural processes predominantly govern carbonate dynamics within these gas fields, while any potential influences from extraction activities appear negligible.

Photosensitized Reduction and Polymerization for One-Step Preparation of Leucomethylene Blue Hydrogel as a Visual Indicator for Both Gaseous and Dissolved Oxygen

Oxygen is crucial for many chemical and biological processes, and its facile detection is of great significance in our daily lives. In this study, we report a leucomethylene blue (LMB)-encapsulated hydrogel visual indicator for the detection of both gaseous and dissolved Oxygen (DO). The photosensitization of methylene blue (MB) not only lead to its reduction to colorless LMB but also resulted in hydrogel polymerization, thus allowing the one-step preparation of the LMB-hydrogel. Meanwhile, the photosensitized reduction of MB was quite fast (5000-fold faster than the classical glucose reduction). In this manner, the blue color of MB could be completely decayed within only 1 min. Also, the efficient polymerization triggered by MB photosensitization ensured the rapid preparation of LMB hydrogels within 10 min. By placing the oxygen indicator in air or water, oxygen can specifically oxidize the colorless LMB-hydrogel to the blue MB-hydrogel. When coupled with a smartphone, the oxygen indicator exhibited a linear response to DO in the range 0.23-10 mg/L with a detection limit of 0.077 mg/L. The LMB-hydrogel indicator was successfully explored for visual evaluation of vacuum degree during food packaging. The LMB-hydrogel, with the advantages of low cost, ease of preparation, as well as facile use, is a promising visual indicator for both gaseous and dissolved oxygen, especially for in-house usage.

Ozonation using a stainless-steel membrane contactor: Gas-liquid mass transfer and pharmaceuticals removal from secondary-treated municipal wastewater

A tubular porous stainless steel membrane contactor was characterized in terms of ozone-water mass transport, as well as its application in removing 23 pharmaceuticals (PhACs) detected in the secondary-treated municipal wastewater, under continuous mode operation. The volumetric mass transfer coefficient (KLa) was evaluated based on liquid flow rate, gas flow rate, and ozone gas concentration. The KLa values were substantially improved with an increment in liquid flow rate (1.6 times from 30 to 70 dm3 h-1) and gas flow rate (3.6 times from 0.30 to 0.85 Ndm3 min-1) due to the improved mixing in the gas-liquid interface. For the lowest liquid flow rate (30 dm3 h-1), the water phase boundary layer (82%) exhibited the major ozone transfer resistance, but it became almost comparable with membrane resistance for the highest liquid flow rate (70 dm3 h-1). Additionally, the influence of the specific ozone dose (0.39, 0.53, and 0.69 g O3 g DOC-1) and ozone inlet gas concentration ( [Formula: see text]  = 27, 80, and 134 g Nm-3) were investigated in the elimination of 23 PhACs found in secondary-treated municipal wastewater. An ozone dose of 0.69 g O3 g DOC-1 and residence time of 60 s resulted in the removal of 12 out of the 23 compounds over 80%, while 17 compounds were abated above 60%. The elimination of PhACs was strongly correlated with kinetic reaction constants values with ozone and hydroxyl radicals (kO3 and kHO•), leading to a characteristic elimination pattern for each group of contaminants. This study demonstrates the high potential of membrane contactors as an appealing alternative for ozone-driven wastewater treatment.

Enhancement of oxygen sensing performance of metalloporphyrin film modified with nano Al2O3 powder

In this work, the metalloporphyrin film of PtOEP/Poly (St-co-TFEMA) modified by nano Al₂O₃ powder was prepared, and the enhancement performance of oxygen sensing was studied in detail. It was verified that the modified film extended the residence time of oxygen through the characterization of SEM. The phosphorescence intensity changes with the content of Al₂O₃ and PtOEP were studied. The Stern-Volmer equations before and after modification of Al₂O₃ powder was compared. It was found that the linearity of the calibration curve was still high, but the oxygen sensitivity value of K_SV was significantly improved due to the increase of quenching probability between indicator and oxygen. Finally, the stability test shows that the Al₂O₃@PtOEP/Poly (St-co-TFEMA) oxygen sensing film presents a strong anti-photo-bleaching ability, and the response time is shortened.

Dissolved oxygen sensor based on the fluorescence quenching method with optimal modulation frequency

Measurement of dissolved oxygen (DO) in liquid samples is of vital importance in both industrial and biomedical fields. In this paper, a DO sensor based on the fluorescence quenching method has been built. The measurement principle is based on fluorescence lifetime detection, which is indicated by the phase difference between an excitation light signal and a fluorescence signal. The nonlinear effect of the fluorescent material has been taken into consideration to obtain a more accurate fitting model. The performance of the system varying with the modulation frequency of excitation light signals is also reported. Modulation frequency mainly affects the sensitivity and phase resolution ratio of the system. The system at the optimized modulation frequency has a good degree of fitting with R² value of 0.9981 and a small relative error of 0.79%. The study shows that this kind of sensor with optimal modulation frequency has good performance, which can be used in many important fields.