Uso da torta da linhaça (Linum usitatissimum L.) como biossorvente para remoção de compostos orgânicos em soluções aquosas

Abstract

This study aimed to analyze the potential of flaxseed cake in adsorbing the reactive red 4B dye. The adsorbent was obtained after oil extraction from ground flaxseeds; the resulting cake was washed with water, dried in an oven at 50 °C, and stored. The flaxseed cake was characterized through pH at the point of zero charge (pHpzc), scanning electron microscopy, and Fourier-transform infrared spectroscopy. Batch adsorption experiments were conducted under different operational conditions, varying initial solution pH, adsorbent dosage, and agitation speed. In all experiments, the biosorbent was mixed with 50 mL of pollutant solution (C₀ = 100 mg L⁻¹) and kept under constant agitation for a set period. After the experiments, the biosorbent was separated by centrifugation (3000 rpm, 10 min) and the supernatant analyzed using UV-Vis spectrophotometry (518 nm). A 2² central composite rotational design was used to optimize the initial pH and adsorbent dosage, with adsorption capacity as the response variable. Kinetic and equilibrium studies were also performed to determine equilibrium time and maximum adsorption capacity, respectively, as well as fixed-bed column adsorption experiments. The biosorbent characterization revealed a pHpzc of 6.0. Possible macropores were observed, along with changes in the 1374 and 1317 cm⁻¹ bands, which may be associated with carboxylate groups and deformation vibrations of C–H or –OH bonds. Adsorption tests showed the best dye removal at pH 2.0, with adsorbent dosage of 1.09 g·L⁻¹, 100 rpm, and room temperature (25 °C). The adsorption kinetics indicated an equilibrium time around 1440 minutes with a capacity of approximately 34.7 mg g⁻¹. The pseudo-second-order model best fit the experimental data (R² = 0.9968), suggesting possible chemisorption. In the equilibrium study, the Sips model showed the best fit (R² = 0.9955), indicating heterogeneous surface adsorption with a maximum capacity of about 153 mg g⁻¹. In a continuous system, with initial concentration of 100 mg L⁻¹ and flow rate of 100 mL h⁻¹, the breakthrough and saturation points for the fixed-bed column were 240 and 570 minutes, respectively. The adsorption capacity of flaxseed cake for other pollutants was also evaluated, with capacities of 10.3 mg·g⁻¹ for the herbicide 2,4-D (C₀ = 100 mg L⁻¹) and 3.1 mg g⁻¹ for the pharmaceutical metformin hydrochloride (C₀ = 20 mg L⁻¹). These results suggest the potential use of flaxseed cake as a low-cost biosorbent for the removal of textile dyes, agrochemicals, and pharmaceuticals from aqueous solutions, making it a sustainable and accessible alternative for use in wastewater treatment plants.