Design And Synthesis Of Bismuth-based Sensitive Materials For Electrochemical Detection Of Heavy Metal Ions In Groundwater

The current situation of heavy metal pollution in the groundwater of sites in China,such as mines and smelters,is becoming increasingly severe.Highly toxic heavy metals such as lead and cadmium can easily enter the human body through the water cycle,causing various diseases and serious damage to the body.Therefore,in the"Implementation Plan for the Prevention and Control of Groundwater Pollution"issued by the Ministry of Ecology and Environment in March 2019,the establishment of a groundwater environmental monitoring system is one of the urgent tasks at present.Currently,there are many detection methods for heavy metals,among which the anodic stripping voltammetry method is highly sensitive,has a short response time,is easy to operate,and has a small instrument volume,making it widely used in the in-situ detection of heavy metals in site groundwater.The sensitive material electrode is the key to the high sensitivity detection of heavy metals by the anodic stripping voltammetry method.Bismuth film electrodes have been widely studied as a substitute for traditional toxic mercury electrodes,but there are still application bottlenecks such as peeling and poor stability in practical applications.Therefore,developing a type of bismuth-based sensitive material with high stability and sensitivity is still a huge challenge and research hotspot in the field of heavy metal electroanalysis.This paper proposes strategies to design and synthesize new bismuth-based sensitive materials,including protective layer modification,carbon material loading,metal bismuth nanosizing,and in-situ formation of encapsulating structures,to address the application bottleneck issues of bismuth film electrodes,such as easy detachment,poor stability,and inferior detection performance.Three new bismuth-based electrochemical sensors were constructed based on these strategies,and their electrochemical methods for detecting Pb²⁺and/or Cd²⁺ions were established to evaluate their detection performance.The actual content of Pb²⁺and/or Cd²⁺ions in groundwater samples was detected using a standard addition method and compared with the ICP-OES detection results to evaluate their practical application ability.This paper mainly carried out the following specific work:（1）Three different Nafion modification strategies（Bi-Nafion,Nafion-Bi,and Nafion-Bi-Nafion）were designed using a Nafion protective layer modification strategy to enhance the mechanical strength of the bismuth film electrode,inhibit detachment during testing,and improve electrode stability and detection performance.The experimental results determined that the optimal Nafion modification strategy was the Bi-Nafion（B-N）,and the B-N electrode was optimized for Nafion loading amount,electrolyte p H,deposition potential,deposition time,and DPV parameters,and established a method for detecting Pb²⁺using this electrochemical sensor.Its linear range for detecting Pb²⁺was 30-300 n M,with a detection limit of 1.8 ppb（S/N=3）.Compared with a single bismuth film electrode,Nafion protective layer modification improved electrode stability,with a relative standard deviation of 6.2%for four consecutive tests.Actual groundwater samples from Dongguan City,Guangdong Province,China were tested,and the detected concentration of Pb²⁺in the real groundwater sample was 1.6 ppb,which is similar to the result obtained via ICP-OES（1.4 ppb）.（2）Based on the research work mentioned above（1）,a new strategy of carbon material loading and bismuth nanosizing was adopted,and a novel two-dimensional carbon material,graphdiyne（GDY）,was introduced as the substrate for loading bismuth nanodots（Bi NDs）.The high conjugated structure and rich alkyne bonds of GDY make its surface highly electronegative,which is beneficial for anchoring and enriching heavy metal ions.At the same time,the nanostructure of bismuth is more likely to expose its active sites,which can interact stably with GDY and heavy metal ions,reducing the impact of internal stress generated during the enrichment-stripping process on the stability of the bismuth electrode,and effectively improving the stability and detection performance of the electrode.A complete detection method for Pb²⁺ions using a GDY@Bi NDs electrochemical sensor was established,with a detection linear range of 20-1000 n M and a detection limit of 2.5 ppb（S/N=3）.The electrode was used to detect Pb²⁺in five real groundwater samples from four cities in China,with concentrations of 1.2,6.9,6.1,1.4,and 4.9 ppb,and the test results were similar to those obtained by ICP-OES.（3）Based on the research work mentioned above（2）,a strategy of in-situ formation of a wrapping structure was further adopted to design and synthesize a novel nanosized bismuth oxide@nitrogen-doped carbon nanosheets composite sensitive material derived from Bi-MOF heat treatment,in order to further improve the stability and detection performance of the bismuth electrode.In this composite sensitive material,nanosized bismuth oxide is tightly embedded in the carbon framework,which can effectively avoid its detachment during the detection process and improve the electrode stability.Nitrogen doping is not only conducive to the uniform distribution of nanosized bismuth oxide,but also provides abundant active sites for the enrichment of heavy metal ions such as Pb²⁺and Cd²⁺due to the negative nitrogen element.In addition,by using two heat treatment strategies to regulate the carbon,nitrogen,and bismuth composition of the material,simultaneous detection of Pb²⁺and Cd²⁺ions can be achieved,thereby improving its detection performance.The electrochemical sensor based on this composite material exhibited excellent performance in the detection of trace amounts of Pb²⁺and Cd²⁺ions,simultaneously detecting Pb²⁺in the concentration range of 10-1000 n M and Cd²⁺in the concentration range of 40-1500 n M,with detection limits of2.3 and 4.2 ppb（S/N=3）,respectively.It also showed good performance in the testing of real groundwater samples,simultaneously detecting Pb²⁺and Cd²⁺ions in real groundwater samples from a sampling point in Baise City,Guangxi Province,with a detected concentration of 4.3 ppb for Pb²⁺and 8.6 ppb for Cd²⁺.The test results were similar to those obtained by ICP-OES.