Electroanalytical determination of the herbicide glyphosate in an environmental sample from the city of Gonçalves Dias, Maranhão

Abstract

Glyphosate (GLI) is a non-selective contact and post-emergence herbicide, being one of the main pesticides most sold in Brazil. Due to the increased concentration of this herbicide in soils, rivers and sediments, and the consequent environmental and human contamination, the present work proposes the development of a carbon paste electrode (EPC) associated with a low-cost carbon nanotube (NTC) easy production for electroanalytical determination of GLI in surface water samples. For that, studies were carried out in the techniques of cyclic voltammetry (VC), differential pulse voltammetry (DPV) and square wave voltammetry (VOQ), where the specific parameters of each electroanalytical technique were investigated. Thus, the electrochemical measurements were performed using an Ag/AgCl (KCl) electrode as a reference electrode, an auxiliary Pt electrode and the working electrode of EPC and EPC-NTC in a cell with a capacity of 10 mL, being used a mini-potentiostat/galvanostat (Ω Metrohm) connected to a managed Notebook with a PSTART software 1.1 computer program. The best responses regarding GLI oxidation in VC were verified in 0.1 mol L-1 phosphate buffer, pH 7, at 50 mV s-1, in a potential range of 1.0 V to 1.2 V And in the composition of the working electrode, the electrode with the proportion of 90% carbon paste and 10% carbon nanotube was the one that could perform the best scan against GLI oxidation. The parameters selected in VPD were: α = 100 mV, t = 10 ms and v = 10 mV s-1. As for the VOQ technique, α = 50 mV, F = 50 Hz and ∆E = 10 mV. In the repeatability study between the VPD and VOQ techniques, values of 1.03% in VPD and 0.18% in VOQ were obtained. As for the reproducibility study, the values were 1.08% in VPD and 1.50% in VOQ. The analytical curve performed using the VPD technique showed a correlation coefficient of 0.99949, with a LD = 5.14 x 10-6 mol L-1 and LQ = 1.71 x 10-5 mol L-1. And the analytical curve in VOQ demonstrated a correlation coefficient of 0.99971, with a LD = 1.45 x 10-5 mol L-1 and LQ = 4.83 x 10-5 mol L-1. Finally, the electroanalytical determination of glyphosate in surface water was carried out by the curve extrapolation method, which allowed obtaining a concentration of 4.27 x 10-5 mol L-1, a value almost 3 times higher than the expected value of 1.48 x 10-5 mol L-1. However, it can be seen that the artisanal electrode developed from carbon paste associated with carbon nanotube, proved to be considered acceptable in the presence of interfering agents. It is also concluded that the method met the demands of quantification of the herbicide glyphosate.