Application And Mechanism Study Of Microbial Electrochemical Sensor For Oil Pollution And CO₂ Emission Monitoring

Currently,fossil fuels are still the main source of energy worldwide,among which the oil and its products may leak into environmental water bodies during their extraction,transportation and use,causing non-negligible ecological harm.For example,during inland waterway transportation,sudden water traffic accidents,routine vessel operations,and cargo handling can lead to the invasion of oil into water bodies;For another example,oil pipelines due to weld interface rupture or failure of ancillary equipment will result in hidden,small oil leakages and drippings,which enters sensitive water bodies through migration and enrichment,endangering water quality and threatening the safety of water supply.The scale of these oil pollution discharges is small,but involves a wide range and a long period,and the discharge time and space points are uncertain,and thus most of them are difficult to supervise and cannot establish a special collection and treatment system.The use of oil pollution detection technology to set up real-time monitoring around the water can detect oil pollution sources in advance for emergency measures to be taken.However,the existing oil pollution detection technologies（spectroscopy,chemistry,etc.）have disadvantages of high operating costs,complex operations,which are unsuitable for large-scale applications.Besides oil pollution,from the perspective of the entire life cycle of fossil fuels,its terminal product CO₂ is also one of the main causes of global greenhouse effect.To achieve large-scale CO₂ emissions reduction,carbon capture and storage technology has received widespread attention,including the inevitable leakage problem during the implementation process.Where and when these leakage occure is unpredictable,so a multi-point CO₂ long-term online monitoring system needs to be established.While the existing CO₂ detection instruments（spectroscopy,chemistry,etc.）are costly and complicated in operation,and thus not suitable here.According to literature analysis,among the existing MES sensors,the microbial fuel cell（MFC）type sensors are simple,economical,and self-sustainable,which is suitable for long-term sensing monitoring systems with large-scale and multi-point structures.However,there has been no relevant research on detecting oil pollution and CO₂ for MFC sensors.Based on this,this paper innovatively designs a Vertical Floating air-Cathode MFC（VFC-MFC）sensor,the main feature of which is the three-phase junction of"atmosphere-cathode-electrolyte"at the VFC cathode.Oil pollution can restraint oxygen by covering the three-phase junction,and meanwhile CO₂ gas can alter the cathodic reaction environment by contacting the three-phase junction,thereby both affecting the MFC signal.The main contents of this paper include the configuration selection,response performance,characterization and modelling of oil sensing,as well as the response characteristics and mechanism of CO₂ sensing.（1）In order to compare different MFC configurations to oil pollution,this study designed and started-up three common MFC configurations and the VFC-MFC configuration,and conducted comparative tests of oil pollution and electrolyte shocks.The results showed that VFC-MFC had advantages of fast start-up,high baseline voltage,and the best sensitivity and responsiveness to oil pollution shocks,which was suitable as an oil pollution sensor with both alarming and detection functions.（2）This study evaluated the oil sensing performance of VFC-MFC comprehensively by using standard,continuous,long-term and different factors influencing oil response tests,and receiver operating characteristics（ROC）analysis.The results showed that VFC-MFC had a relatively stable baseline signal（441.3 mV）,and after contacting with oil pollution,the signal dropped to a steady-state response value within a certain response time（single detection mode:1.83-11.5 h,continuous detection mode:about 1.4 h）.The steady-state voltage difference between them was positively correlated with the oil pollution/accumulated amount,with an average change rate of about 27.2 mV·mL^-1,which was the calibration curve used for oil detection.The characteristics of calibration curve were relatively stable within a certain temperature range,but were greatly affected by configuration conditions（cathode receiving area,anode surface area,and external resistance）and operating conditions（electrolyte organic concentration）.For example,when the organic concentration was lower than the critical range(62.8-42.1 mg-COD·L^-1),the signal of VFC-MFC collapsed and cannot detect oil normally.After long-term stable operation（86 d）,the signal drift rate of VFC-MFC was 0.25 mV·d^-1.On the other hand,with accuracy as the target,the ROC analysis obtained the best strategy for oil alarming was 20 min of signal acquisition frequency and 1.47 mV of signal abnormality threshold.The pilot-scale application results of VFC-MFC in actual water bodies showed that although its baseline signal was relatively low（141.2 mV）,it still had decent oil alarming function and preliminary oil detection function.（3）This study characterized the changes of VFC-MFC before and after oil detection using electrochemical and microscopic imaging techniques.The polarization curve results showed that with the increase of oil pollution,the output voltage and power of VFC-MFC were suppressed with significant concentration polarization phenomenon.The measured internal resistance increased from 1207.9Ω（zero oil）to6803.6Ω（5.0 mL oil）.The electrochemical impedance spectroscopy（EIS）results showed that the main reasons for the increase in internal resistance were the cathodic charge transfer impedance and material（oxygen）transfer impedance caused by the oil pollution,accounting for 53.9%and 44.9%,respectively.By using a power management system（PMS）to recover VFC-MFC energy for self-operation,the results showed that the working frequency of 1.0-5.9 h^-1 could be ensured under specific detection conditions.According to the imaging analysis of the cathode,the oil adhered to the carbon fiber surface forming oil film,with bubble structures for partial oxygen transmission.Further analysis showed that the total thickness of the oil film is the sum of the average thickness and the edge increment formed by the contact angle（1.67 mm）.（4）Based on oil impeding oxygen transport and altering electrode surface characteristics,this study established static and dynamic mechanistic models,with the former highly fitting steady-state signals（R²>0.97）,and the latter further highly fitting transient signals（variance of 3.87 mV）.Besides,the dynamic model integrated with a two-population competition theory,successfully fitted the signal collapse and slow-down under critical and deficient organic concentration conditions.（5）Based on the low resistance of gas transfer of the VFC-MFC configuration,this study investigated its response performance to CO₂ gas emissions.The results showed that the signal response characteristics（peak value,peak position,peak area,etc.）of VFC-MFC were closely related to the CO₂ gas injection rate,injection duration,and injection volume.The theoretical concentration changes of CO₂ at the three-phase junction obtained by flow field simulation and multilayer diffusion simulation were highly consistent with the real-time signal of VFC-MFC（correlation of 99.8%）.In conclusion,VFC-MFC can effectively monitor oil pollution autonomously,and serves as an early oil pollution warning means for sensitive water,ensuring water supply safety.Meanwhile,it can achieve real-time monitoring of CO₂ emissions and serves as an effective evaluation means for carbon management.