Desenvolvimento de eletrodos modificados com biocarvão da casca de café: potenciais aplicações para a quantificação do fungicida carbendazim e metais pesados cádmio e chumbo

Abstract

With the increase in coffee production, the husks resulting from the processing process of this bean are a widely generated agro-industrial residue with potential for environmental pollution. The pyrolysis of this biomass, in addition to being an alternative for recycling this waste, produces biochar, a material whose application in the development of sustainable pollutant quantification technologies has been studied. Rich in carbon and low cost, biochar can be used as an electrode modifier due to its ability to spontaneously adsorb organic and inorganic substances. This work proposes the use of coffee husk biochar as an electrode modifier for the determination of inorganic and organic pollutants. Firstly, a biochar-modified carbon paste electrode (BC-CPE) was manufactured and used for the simultaneous determination of Cd2+ and Pb2+ ions employing Differential Pulse Adsorptive Redissolution Voltammetry (DPAdsV). Several biochars (n = 8) were prepared under different conditions optimized by 2³ factorial design (final temperature, heating rate and heating time) and characterized by Electrochemical Impedance Spectroscopy (EIS), Fourier Transform Infrared Spectroscopy (FTIR) and Scanning Electron Microscopy and Energy Dispersive Spectroscopy (SEM-EDS). Using the optimized BC-CPE (25% m/m biochar pyrolyzed at 300 °C for 50 min at a heating rate of 10ºC/min) under ideal preconcentration conditions (10 min under stirring in a solution at pH 7 ,0), detection limits for Cd2+ and Pb2+ of 1.7 and 0.2 μg L−1, respectively, were obtained. The precision of the method was proven by repeatability (RSD < 4.5%; n = 5) and inter-electrode reproducibility (RSD < 4.4%; n = 5). The optimized method was applied for the simultaneous determination of Pb2+ and Cd2+ in water samples, obtaining recovery values between 81 and 122%. Subsequently, a biochar-modified composite based on 3D-printed insulating substrate coated with a conductive composite of graphite, biochar and acrylonitrile butadienestyrene (BC300-G-ABS) was manufactured and used for the determination of the fungicide carbendazim by Square Wave Voltammetry (SWV). The electrochemical measurement parameters pH and supporting electrolyte were optimized for the BC300-G-ABS (pH=2.0 and 0.2 mol L-1 phosphate solution). The optimized biochar percentage was 10% (m/m). The SWV parameters Estep, frequency and pulse amplitude were optimized as 2 mV, 15Hz and 120 mV, respectively. Under optimized conditions the BC300-G-ABS showed a LD of 0.05 µmol L-1 and LQ of 0.18 µmol L-1, values lower than the maximum value allowed for drinking water, which is approximately 0.63 µmol L-1 according to Ordinance 888/2021 of the Ministry of Health and the guidelines of the World Health Organization (WHO). The BC300-G-ABS exhibited precision for through interelectrode (RSD = 6.21%, n=6) and interday (RSD = 6.96%, n=10) reproducibility and repeatability (4.32; 2.93 and 2.66%, n=10 for 3, 6 and 9 µmol L-1 of carbendazim respectively). The BC300-G-ABS presented a linear range from 0.25 to 15 µmol L-1 (R = 0.998) and was used to quantify carbendazim in samples of drinking water and river water, the optimized method showed accuracy with recoveries varying between 92.7 and 96.43%. Biochar produced from coffee waste, in addition to being a green, sustainable and a high added value material, has also proven to be an excellent electrode modifier, which contributes to the circular economy, as agro-industrial waste is reused in the analysis of agricultural pesticides and heavy metals.