Biocarvões de resíduos de café modificados com cloreto de magnésio para remoção do herbicida ácido 4-diclorofenoxiacético de meios aquosos

Abstract

Contamination of water bodies by persistent herbicides such as 4-chlorophenoxyacetic acid (4-CPA) demands the development of effective and sustainable remediation technologies. In this study, modified biochars were produced from two residues of the coffee agroindustry—coffee pulp and coffee husk—aiming at the removal of 4-CPA from aqueous media. The lignocellulosic precursors were subjected to pyrolysis at different temperatures and evaluated in their in natura form and after modification with 0.3% magnesium chloride (MgCl₂). The thermal stability and crystalline structure of the materials were confirmed, respectively, by thermogravimetry and X-ray diffraction, indicating carbon fixation with amorphous or dispersed mineral phases, intensified by the ash concentration effect. Morphological analyses (SEM) revealed a decisive structural distinction: while the biochar derived from the pulp (BC900) developed an irregular and collapsed alveolar structure, the biochar derived from the husk (BP900) preserved the tubular and fibrous organization of the original biomass, with the presence of channels that may favor adsorbate diffusion. Additionally, EDS analyses corroborated the chemical modification, evidencing the incorporation of magnesium (Mg) into the modified biochars. Adsorption tests indicated that chemical modification improved the removal capacity of 4-CPA, with emphasis on the modified husk biochar (BP900-0.3%), which achieved removals greater than 89% in aqueous media. The study revealed better adsorption performance at pH 2.0, a condition in which 4-CPA is in its molecular form, favoring dispersive interactions (π–π) and hydrogen bonding. Kinetic studies showed that the process followed the pseudo-second-order model, while equilibrium data exhibited a strong fit to the Freundlich model (R² > 0.96), suggesting adsorption on a heterogeneous surface. Reuse tests confirmed the material’s functionality for up to four regeneration cycles using ethanol. Finally, fixed-bed column experiments indicated operational feasibility under continuous flow, with a breakthrough time of 40 minutes. It is concluded that husk biochar modified at 900 °C constitutes a high-performance adsorbent, transforming an environmental liability into a technological solution for the treatment of herbicide-contaminated water.