Research On Rapid Measurement Method And Instrument Development Of Heavy Metals In Water Based On Laser-induced Breakdown Spectroscopy

Heavy metal is one of the important micro-pollutants in water environment,Heavy metals in wastewater from industrial production and agricultural pesticides and fertilizers can pollute surface water and groundwater through surface runoff and leaching.It is characterized by non-degradation,bioaccumulation and high risk of trace amount.It can seriously endanger human health through food chain and is the most dangerous source of water pollution.The characteristics of low content,wide spread and rapid development of heavy metals in water pollution incidents are in sharp contradiction with the long sampling and detection cycle,slow response speed,strong professionalism and secondary pollution risk of traditional laboratory analysis methods,so it is of great significance to realize rapid detection of heavy metals in water bodies.During the growth of crops,water cycles in the form of surface runoff,soil absorption,crop absorption and transpiration,atmospheric sedimentation,etc.Heavy metals introduced in the above links will pose a potential threat to the normal growth of crops and the quality and safety of agricultural products.Therefore,the detection of heavy metals in water,soil,atmosphere and crops itself is of great value for studying the migration law of heavy metals and ensuring agricultural safety.Laser-induced Breakdown Spectroscopy（LIBS）,as a new rapid detection technology,has been widely used in industry,environment,geology,biopharmaceuticals,deep-sea and interstellar exploration and other fields because of its rapid and multi-element simultaneous detection without micro-loss,which also provides the possibility for the realization of the above goals.However,for liquid sample detection,due to the existence of laser focusing,liquid splash,energy jitter attenuation,low detection limit and other problems,the detection sensitivity and quantitative level of LIBS for liquid samples need to be improved.Based on the problems above,this study takes surface water and atmospheric condensate water in the process of agricultural water cycle as the research objects,and carries out the research on detection methods,signal enhancement methods,quantitative measurement methods and the development of miniaturized equipment.The main contents include the following parts:（1）In order to enhance the detection sensitivity of heavy metals in water,the LIBS method based on ion exchange resin was studied.The chelating resin was used for rapid enrichment and solid-liquid conversion of heavy metal element Cd in water.Combined with LIBS technology,the signal enhancement effect of this method was verified.The spectral variation law under different experimental parameters was analyzed,and a quantitative metrological model was constructed.The results showed that under the conditions of 500 mL sample water,0.05 g resin dosage and 40 min oscillation time,the correlation coefficient of calibration curve was 0.98,the RMSE_C was 4.14μg/L,the RMSEp of prediction set was 7.15μg/L,and the detection limit was 3.6μg/L,which was lower than the national detection standard limit of drinking water by 5μg/L.This method does not need chemical reagents and the chelating resin can be regenerated easily,so it is expected to be an environment-friendly detection method.（2）The toxicity of heavy metals in different valence states is quite different.Taking different valence states of chromium（Cr）in water as an example,the LIBS-LIF resonance enhancement measurement method based on selective adsorption was studied.Using highly selective ion exchange resin combined with LIBS technology,specific enrichment and detection of Cr（Ⅵ）in water was achieved,and combining with laser induced fluorescence resonance enhancement method,highly sensitive detection of total chromium content in water was accomplished.The results showed that the specific adsorption ability of Dusheng A12 resin for hexavalent chromium and the detection ability of total Cr in water samples after solid-liquid conversion based on LIBS-LIF method were 6-8 times higher than traditional LIBS method;The correlation coefficient of total Cr calibration curve was 0.97,the detection limit was 6μg/L,and the correlation coefficient of Cr（Ⅵ）calibration curve was 0.96.The relative error range of measuring Cr（Ⅵ）by actual samples was between 4.0%-10.3%.The method in this study showed good detection ability both for total Cr and highly toxic hexavalent chromium Cr（Ⅵ）.It has certain application value in screening and rapid detection of water pollution incidents.（3）Suspended droplets in the air are important sources of heavy metals in surface water.To solve this problem,methods using LIBS technology coupled with bionic concept was studied for heavy metals in atmospheric condensate water.A fast collection and capture method of atmospheric condensed water based on bionic condensed water collection on the back of desert beetle and bionic super-slippery surface of pitcher plant is proposed.A sensor device based on bidirectional control of semiconductor refrigeration combined with PID feedback control of dew point temperature was designed.Combining with super-hydrophobic surface coating and TEC reverse heating function.it could realize efficient convergence,morphology conversion and LIBS high sensitivity detection of condensed water.The results showed that the bionic sensor had the ability of continuously collecting atmospheric condensed water.For Cd and Pb simulated water samples,the correlation coefficients of calibration curves were 0.98(R²_Cd)and0.97(R²_Pb),respectively,and the detection limits were 11.1μg/L(LOD_Cd)and 56.6μg/L(LOD_Pb).This method could effectively identify heavy metal elements lithium（Li）,lead（Pb）,copper（Cu）,phosphorus（P）and radioactive element praseodymium（Pr）in condensed water samples in smoking scenes and industrial and mining scenes.Furthermore,based on PCA clustering analysis,the potential discrimination ability of this method for different pollution sources was also verified.（4）Design and development of miniaturized LIBS measurement system for heavy metals in water.In order to meet the requirements of rapid detection on-site,small integrated laser and spectrometer were chosen,and four parts work including optical path system design,sample bin structure design,electronic synchronous control system design,software design and algorithm integration were accomplished to construct a complete handheld LIBS system.According to the characteristics of samples and measurement methods in Chapter 3 and Chapter4,a standardized sample table component was designed,which could be vacuumized for sample bin or filled with protective gas,it was convenient for rapid detection of various samples.The detection methods based on LIBS and the development of miniaturized equipment in this study can lay a methodological foundation for rapid screening and detection of heavy metal pollution in water bodies and provide new ideas for subsequent expansion research.