Soil-Powered Device For In-Situ Monitoring Of Heavy Metal:Device Construction And Performance

At present,the problem of water pollution in paddy fields is serious.In particular,heavy metals such as lead and cadmium in the water of paddy fields seriously threaten the safety of rice production and human life.In order to reduce and control heavy metal pollution in water bodies of paddy fields,detection technologies applicable to heavy metal pollution in water bodies of paddy fields should be developed as soon as possible,so as to control pollution discharge from the source and avoid serious pollution in paddy fields.Although traditional heavy metal detection methods such as Inductively Coupled Plasma–Mass Spectrometry and Atomic Absorption Spectrometry can detect accurately and quantitatively,they are difficult to be used for in-situ long-term monitoring of heavy metal pollution in water bodies of paddy fields.Based on soil microbial fuel cell（SMFC）,it is expected to establish a self-powered biosensor device to realize in situ monitoring of heavy metal pollution in paddy water.SMFC can use the abundant electroactive microbial flora and organic matter in the soil to achieve"self-powered"operation.At the same time,the addition of heavy metals will affect the output of SMFC,thus generating a response signal.Therefore,combining SMFC with bioelectrochemical sensors,it is possible to develop in situ long-term monitoring devices for heavy metal pollution in water bodies of paddy fields.Based on SMFC,this paper constructs an in-situ monitoring device for heavy metals in water bodies of paddy fields,and initially constructs a self-powered intelligent monitoring system for soil,which initially solves the problem of inconvenient signal transmission when SMFC biosensors are used in practice.The main research contents and results are summarized as follows:（1）Construct an efficient and stable SMFC,and analyze the electricity generation performance of SMFC.In this study,a variety of SMFC devices were designed for the selection of SMFC devices in the actual paddy water environment.An efficient and stable SMFC was constructed using paddy soil as anode substrate.The anode uses carbon felt with high specific surface area as the electrode material,and the cathode uses carbon felt and Pt/C carbon cloth which are widely used as the cathode electrode material.The results show that after 50 days of acclimation,the output voltage of SMFC for carbon felt cathode and Pt/C cloth cathode is stable at240 m V and 310 m V respectively.The polarimetric power density test results show that the maximum power density of carbon felt cathode SMFC and Pt/C carbon cloth cathode SMFC are 35 m W/m²and 42 m W/m²,respectively.The electrical performance of both SMFC and PT/C carbon cloth cathode is about 2～5 times higher than that of the same type of SMFC in the references.The number of live bacteria on the anode was 10000 times that in the paddy field soil.Meanwhile,the results of microbial community analysis showed that desulphurized bulb was the main electrogenerator on the anode.These results indicate that the enrichment of highly active electrogenic bacteria in anode is the key factor for the construction of efficient and stable SMFC.（2）A carbon felt cathode SMFC biosensor was constructed to monitor four typical heavy metal ions:Cd²⁺,Zn²⁺,Pb²⁺and Hg²⁺.The SMFC biosensor uses carbon felt with high specific surface area as the electrode material.The anode is placed in the paddy field soil and the cathode is placed in the water above the paddy field soil.The external resistance starts to operate.The electrochemical test and heavy metal monitoring of the SMFC biosensor were carried out.The results showed that the carbon felt cathode SMFC biosensor operated continuously and efficiently for 4 months in the paddy field soil environment,and the output voltage was stable at 400 m V.At the same time,SMFC biosensor has a signal response to Cd²⁺,Zn²⁺,Pb²⁺,Hg²⁺,and has a good linear response to heavy metals in the range of 1～20mg/L,R²>0.98.The response mechanism of heavy metals was analyzed,and the electron acceptor was provided by adding Cd²⁺,Zn²⁺,Pb²⁺or Hg²⁺to promote the cathode reaction and increase the output voltage.The SMFC biosensor was tested with real samples,and the detection error was less than 10%.（3）The soil self-powered intelligent monitoring system is preliminarily designed.The intelligent monitoring system includes SMFC power supply,boost converter,data acquisition and signal wireless transmission module,signal receiving module.The SMFC is connected in series to increase the power supply voltage to3.5 V to supply power to loads.The power input voltage of 3.5 V is increased to the output voltage of 5 V through the boost converter for data acquisition and wireless signal transmission module.The heavy metal response signal of SMFC biosensor is received through the mobile phone APP.The results show that under the condition of3.5 V power input voltage,the data acquisition and signal wireless transmission module can work for 250 s,and can receive the signal of SMFC biosensor response to heavy metals on the mobile phone.The response signals of 0.5～20 mg/L Pb²⁺were monitored,and the linear correlation coefficient R²was 0.961,which proved the feasibility of the soil self-powered intelligent monitoring system.At present,the design of the system is still in the laboratory research stage.The SMFC system used for power supply is small,and cannot power the monitoring device for a long time.However,the design of the system provides a way to further solve the problem of SMFC sensor signal transmission.In summary,in this paper,carbon felt,a carbon-based material with high specific surface area and electrical conductivity,is used as electrode material to construct an efficient and stable SMFC.Using carbon felt cathode as sensing element,a carbon felt cathode SMFC biosensor which can be used to monitor heavy metal pollution in paddy water was constructed.The preliminary design of soil self-powered intelligent monitoring for monitoring SMFC biosensor response signals.SMFC biosensor and intelligent monitoring system provide a new idea and method for in-situ on-line monitoring and response signal transmission of heavy metal pollution in paddy water.