Determination Of Hexavalent Chromium In Water By Differential Pulse Dissolution Voltammetry With Metal Coated/Nanocarbon Material Modified Electrodes

Objective The widespread use of chromium in industry has led to the contamination of hexavalent chromium（Cr（Ⅵ））,which can cause lesions and even cancerations in human skin,respiratory system,liver and kidney internal organs.Therefore,the development of analytical methods for the determination of Cr（Ⅵ）at trace levels have practical value.Electrochemical detection methods are receiving more and more attention because of their low cost,simple operation steps,short time consumption,high sensitivity,and easy automation.The working electrodes that can be selected for the determination of Cr（Ⅵ）by electrochemical analysis methods include Hg film electrode,Bismuth film electrode,etc.With the continuous research on nanomaterials,reduced graphene oxide（r GO）and carboxylated multi-walled carbon nanotubes（CMWCNTs）with large specific surface area and special structure and excellent electrical conductivity have been used for the modification of working electrodes.In this study,Bismuth film/carboxylated multi-walled carbon nanotubes/glass carbon electrode（Bi/CMWCNTs/GCE）and Mercury film/reduced graphene oxide/glass carbon electrode（Hg/r GO/GCE）were constructed for the detection of Cr（Ⅵ）in water using differential pulse solvation voltammetry（DPSV）in the trivalent chromium-diethylenetriaminepentaacetic acid-nitrate（Cr（III）-DTPA-NO₃-）adsorption catalytic system.Methods（1）The multi-walled carbon nanotubes（MWCNTs）were modified by the mixed acid method to obtain CMWCNTs,and the CMWCNTs were modified on the surface of the glassy carbon electrode by drop coating to obtain CMWCNTs/GCE,and then dried and immersed in the Bismuth plating solution to obtain Bi/CMWCNTs/GCE by electrochemical deposition,and the constructed modified electrodes were characterized by using scanning electron microscopy（SEM）,linear cyclic voltammetry（CV）,and DPSV.The amount of drop coating of CMWCNTs,the time of Bismuth deposition,the potential and time of chromium enrichment,the concentrations of diethylenetriaminepentaacetic acid（DTPA）and potassium nitrate in the substrate,and the p H conditions of the substrate were optimized for the detection of Cr（Ⅵ）under optimal conditions,and methodological validation experiments were performed to apply the established method to the detection of hexavalent chromium in water samples.（2）The electrochemical reduction method was applied to electrochemically reduce graphene oxide（GO）by linear cyclic voltammetry（CV）to obtain r GO,and then r GO was deposited on the surface of the glassy carbon electrode by electrochemical deposition to obtain r GO/GCE,which was then immersed in a mercury plating solution to obtain Hg/r GO/GCE by secondary electrochemical deposition.The constructed modified electrodes were characterized using SEM,CV,and DPSV.The time of Mercury deposition,the potential and time of chromium enrichment,the concentrations of diethylenetriaminepentaacetic acid（DTPA）and potassium nitrate in the substrate,and the p H conditions of the substrate were optimized,and the Cr（Ⅵ）was detected under the optimal conditions,and methodological validation experiments were performed to apply the established method to the detection of Cr（Ⅵ）in water samples.（3）The above two methods were compared with the national standard method for the determination of hexavalent chromium in water-diphenylcarbonyldihydrazide spectrophotometric method to verify the detection results.Results（1）SEM,CV,and DPSV data showed that the constructed Bi/CMWCNTs/GCE had improved sensitivity,enhanced electrical signal,and symmetrical and distinct peak shape.The detection was performed under the optimal experimental conditions,the drop coating volume of CMWCNTs was 4μL,the Bismuth deposition potential was-0.3 V,the Bismuth deposition time was 240 s,the Cr（Ⅵ）enrichment potential was-0.8 V,the Cr（Ⅵ）enrichment time was 120 s,the p H of the substrate was 6.0,the DTPA concentration was selected as 15 mmol/L,the KNO₃ concentration was selected as 1.25 mol/L,Cr（Ⅵ）showed a clear dissolution peak at-1.26V.In the range of 0.50-40.0μg/L,the linear regression equation was y=1.037x+31.41 with the correlation coefficient r=0.9995,the detection limit was0.18μg/L,and the spiked recoveries ranged from 95.07%to 105.29%with the RSDs of 1.81%-3.98%.The precision and reproducibility were good,and the interference experiments showed strong anti-interference ability of this method.（2）The constructed Hg/r GO/GCE was characterized by SEM,CV and DPSV,and the results showed that the electrode had the advantages of high sensitivity,strong electrical signal,symmetrical and obvious peak shape.The results of experimental condition optimization were that the Mercury deposition potential was-0.2 V,the Mercury deposition time was 120 s,the Cr（Ⅵ）enrichment potential was-1.0 V,the Cr（Ⅵ）enrichment time was 180 s,the p H value of the substrate was 6.0,the DTPA concentration was selected as 15 mmol/L,and the KNO₃ concentration was selected as 0.75 mol/L.The optimal experimental conditions were selected for the detection of Cr（Ⅵ）A clear dissolution peak was observed at-1.36V.In the range of 1.0-40.0μg/L the linear regression equation was:y=0.2272x+12.415,with the linear correlation coefficient r=0.9994,detection limit 0.68μg/L.The spiked recoveries ranged from99.03%to 103.22%with the RSDs of 2.67%to 4.15%.The precision,anti-interference ability,and reproducibility were good.（3）The collected lake water samples were tested by diphenylcarbodihydrazide spectrophotometry,and the results were compared with those of the two new methods established in this paper by paired t-test,which showed that the differences were not statistically significant（P>0.05）,indicating that the results of the two new methods based on Bi/CMWCNTs/GCE electrochemical sensor and Hg/r GO/GCE electrochemical sensor were accurate and reliable.Conclusions（1）A new method for the determination of Cr（Ⅵ）in water based on Bi/CMWCNTs/GCE electrochemical sensor was constructed based on a two-step method of drop coating followed by deposition of carboxylated multi-walled carbon nanotubes and Bismuth composite modified glassy carbon electrode,which can be stably and effectively combined with the modified materials,resulting in a significant increase in the surface activity of the modified electrode.The method is not only easy to operate and environmentally friendly but also has the advantages of low cost,high sensitivity,low detection limit,and environmental protection,which is suitable for the detection of Cr（Ⅵ）in water.（2）In this study,reduced graphene oxide and Mercury composite modified glassy carbon electrode was prepared by first electrochemical reduction of GO on the electrode surface,followed by constant potential deposition of Mercury film,and the electrochemical performance of the glassy carbon electrode was significantly improved after these two composite modifications,thus a new method based on Hg/r GO/GCE electrochemical sensor for the detection of Cr（Ⅵ）in water was established.The method is relatively simple,has the advantages of high sensitivity and low cost,and is suitable for the detection of Cr（Ⅵ）in water.（3）The peak position of Cr（Ⅵ）corresponding to the method based on Hg/r GO/GCE electrochemical sensor significantly lags behind that of the method based on Bi/CMWCNTs/GCE electrochemical sensor;Bi/CMWCNTs/GCE has higher stability,wider linear range,lower detection limit,higher sensitivity,and is environmental friendly.The established new method is consistent with the results obtained by the national standard method,indicating that the constructed two electrochemical sensor platforms are accurate and reliable for the detection of hexavalent chromium in water.