Solid-Phase Extraction of Pesticides in Water Using Corncob Biochar

Corncob biochar was produced by pyrolisis (400 °C, N₂ flow of 3 L/min, yield of 42%) and applied as an alternative adsorbent in solid-phase extraction (SPE) for pesticide control in water. Part of the biochar was activated with KOH (3 mol/L) and HCl (3 mol/L). The non-activated biochar (NB) and activated biochar (AB) were characterized by Fourier-transform infrared spectroscopy (FTIR) and point of zero charge (PZC) analyses, with PZC pH values of 7.37 and 6.07 obtained for NB and AB, respectivey. For AB, Boehm titration, energy-dispersive X-ray spectroscopy (EDX), scanning electron microscopy (SEM), and Brunauer-Emmett-Teller (BET) analyses indicated the presence of acidic groups, high silicon content, surface area of 99.59 m²/g, and pore volume of 0.265 cm³/g, respectively. The adsorption of pesticides in water (1 μg/mL) was performed by SPE using C18 and AB. The eluents that provided the best recoveries were acetonitrile and dichloromethane/methanol (50:50). Pesticide analysis was performed by LC/MS, with a C18 column and a mobile phase of water (5 mmol/L NH₄HCO₂) and methanol (5 mmol/L NH₄HCO₂). The effects of the variables adsorbent amount, sample volume, and eluent volume were evaluated using 2³ factorial design for SPE with AB. Among 13 pesticides, 10 presented recoveries between 75.9 and 117%. Statistical analysis (Pareto chart, response surface, and ANOVA) showed that the best condition was obtained with 100 mg of AB, 50 mL of sample, and 15 mL of eluent, while the use of 5 mL of eluent also provided satisfactory results. Under the optimal conditions, the SPE method exhibited good linear relation (r² ˃ 0.99) in the linear range of 1–1000 μg/L. Detection and quantification limits ranged from 0.01 to 0.04 μg/L and from 0.1 to 0.4 μg/L_, respectively.