| Agricultural habitat is closely connected and inseparable with the living environment of human being. The pollution and deterioration of natural environment, particularly of air, water and soil bring huge damage to agricultural habitat. Recently, the pollution problems of agricultural habitat and crops in China become more and more serious, and the soil environmental quality continues to decrease, especially the secondary salinization of greenhouse soil grows more and more obviously. The greenhouse soil keeps remains in the long-term closed environment with a high multiple crop index and heavy application of fertilizers. The excess nutrients remain in soil due to the ineffective absorption of crops, which leads to a large number of salt accumulation in the surface soil, and thus causes soil secondary salinization. In the composition of salt ions, nitrate is the main anion accumulated in soil, which accounts for67%～76%in the soil anions. Under the action of nitrite bacteria and nitrifying bacteria, the nitrogen fertilizers applied in soil can be converted into nitrite and nitrate, respectively. Nitrate can also be reduced to nitrite owing to microbial action. The increasing accumulation of nitrate and nitrite can not only influence soil ecological environment, and pollute the quality of groundwater the pollution, but also be absorbed and enriched by crops, which may threaten to agricultural food safety and human health. Therefore, nitrate and nitrite become important indices for soil pollution control. Based on the current situation of secondary salinization of greenhouse soil, it is of great significance to develop rapid and sensitive detection technique to control the agricultural environmental pollution, to remediate the contaminated habitat, to ensure agricultural product safety and protect human health.Currently, the determination methods of nitrate and nitrite mainly include the spectroscopy, chromatography, electrochemical sensor, capillary electrophoresis, and etc. Among them, the most commonly used method is spectrophotometry. The detection limit of nitrite is0.003mg/L by using hydrochloric naphthaline ethylenediamine spectrophotometry, while that of nitrate is0.02mg/L by using phenoldisulfonic acid spectrophotometry. Spectrophotometry may be affected by the interference of coexisting ions in complicated environment, so an appropriate treatment need be done to eliminate it. Moreover, its operation is cumbersome and needs a long time to analyze. Despite its high sensitivity, ion chromatography has many shortcomings, such as time-consuming, sample pretreatment and expensive instrument, and is inappropriate to be used online analysis. Capillary electrophoresis has high separation efficiency, and consumes small quantity of the sample. What’s more, it is poor in reproducibility and labor-intensive when applied to analyze batch samples, so it is unsuited for process analysis. The device is simple by the electrochemical method. It is easy to operate with a high speed, and easy to implement on-line monitoring, which can make up for the lack of other methods. Based on the actual needs of the detection of nitrate and nitrite in the soil, in this research, the carbon nano-tubes/toluidine blue composite modified electrode is built to detect nitrite, and a rapid and sensitive method is formed to detect nitrite and nitrate, which both are applied to the determination of nitrite and nitrate in the soil.In this thesis, we fabricated a novel nitrite sensor via simply in situ electropolymerization of toluidine blue (TB) on the multi-walled carbon nanotubes (MWCNTs) modified glassy carbon eletrode. The resulting PTB-MWCNTs composite film electrode exhibited good electrochemical response for oxidation of nitrite. The main results are summarized as follows.(1) The oxidation peak current with the square root of scan rate (v1/2) in the range of10-150mV/s has a linear regression equation was obtained as ip=2.583v1/2+9.380(R2=0.993, n=11), The anodic peak potential shifts with the scan rate, indicating the chemical irreversibility of the nitrite electrocatalytic oxidation process.(2) Acidic conditions are more favorable for a good response and the maximum current (36.74μA) is obtained at pH5.00; the experiment data indicated that the best electrochemical response of nitrite at the sensor was get when the volume of1mg/ml MWCNTs solution was8μL and the electropolymerization cycles was25with a san rate of50mV/s.(3) Under the optimized experiment conditions, the response current has a good linear relationship with successive injection of nitrite in the range of39nmol/L to1.1mmol/L and the detection limit is low to19nmol/L (signal to noise ratio is3); the detect of nitrate was followed by reduction of nitrate to nitrite and repeating the nitrite analysis, calculation of the nitrate concentration can then be obtained by diffrernce.(4) Stability and reproducibility are two key factors to evaluate PTB/MWCNTs composite film based nitrite sensor performance. In the amperometric experiment, for nine successive measurements of2.0μmol/L nitrite at the same sensor, the amperometric response was nearly the same with a relative standard deviation (RSD)3.8%. The RSD for measurement of2.0μmol/L nitrite with five different nitrite sensors was4.1%. The nitrite sensor remained93.7%of its initial response after it was kept in air for a week, and the RSD was2.6%; and84.4%of its initial response for2months.200folds of Na+, K+, Ca2+, Mg2+, Cu2+, A13+, Fe3+, Zn2+, Cd2+, Pb2+, Hg2+, SO42-, NO3, Cl-, and100folds of CO32-, and I, Br-have almost no influence on the current response of nitrite. The small variation of recovery in the range of80.00%and103.17%suggests good reliability of the PTB/MWCNTs composite film sensor, which is reliable to detect nitrite in soil samples.(5) The contents of nitrite and nitrate in greenhouse soil samples collected from the experimental farm of Southwest University were determined by the developed electrochemical sensing method. The obtained results were in good accordance with those by the standard analytical methods. |
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