Produção de hidrocarvões a partir de resíduos lignocelulósicos amazônicos e sua aplicação para a remoção de contaminantes em efluentes sintéticos

Abstract

Increasing industrialization and urbanization have increased pollutant levels in aquatic ecosystems, especially toxic metals and synthetic dyes. Adsorption, using low-cost materials, has emerged as an efficient method for remediating these contaminants. In this context, this study proposed the production of hydrochars from two lignocellulosic residues abundant in the Amazon: açaí seeds and curauá mucilage. It evaluated their application for the removal of methylene blue (AM) and hexavalent chromium (Cr(VI)). The hydrochars, modified with NaOH and H3PO4, and unmodified hydrochars, were synthesized at 126 and 220 °C. Increasing the temperature to 220 °C, combined with acid modification, resulted in materials with higher carbon content, high aromaticity, and surface area (~34.36 m² g-1 ). In contrast, the synthesis at 126 °C preserved a higher density of oxygenated functional groups and obtained the highest yields (~60%). Regarding AM adsorption, it was most efficient at alkaline pH (8-10), with removals greater than 95%. The adsorption kinetics best fit the pseudo-second-order model and the isotherms the Langmuir model, indicating a thermodynamically spontaneous and favorable monolayer chemisorption process. For chromium (VI) adsorption, the process was favored in an acidic medium (pH 2), with removals greater than 90%. The factorial design confirmed pH as the variable of greatest statistical significance. The best kinetic fit was to the pseudo-second-order model, and the thermodynamics indicated a spontaneous process. The maximum Cr(VI) adsorption capacity reached 110.81 mg g⁻¹ for the curauá hydrochar treated with acid at 45°C. The results validate the açaí and curauá-derived hydrochars as promising adsorbents for contaminated water remediation and corroborate the potential of a circular bioeconomy in the Amazon region.