TRATAMENTO DA LAMA VERMELHA E APLICAÇÃO COMO ADSORVENTE PARA REMOÇÃO DE CORANTES TÊXTEIS E BISFENOL A

Abstract

Red mud (RM), a residue of the Bayer process, was investigated before and after acid treatment with HCl (treated red mud, TRM) to characterize it and evaluate its potential as an adsorbent for methylene blue (MB). The point of zero charge (pHₚzc) of TRM was 4.3, indicating a positively charged surface in an acidic medium and a negatively charged surface at higher pH values. Characterization techniques (XRF, XRD, SEM, Raman, and TG/DTG) showed that acid treatment selectively removed Na and Ca promoting the dissolution of labile phases and relative enrichment in hematite and goethite. Textural analyses indicated a shift from a type III isotherm (RM) to a type IV isotherm with hysteresis (TRM), an increase in BET surface area (86.22 → 138.01 m²·g⁻¹), and a higher mesoporous fraction. In the adsorption tests, the highest AM removal occurred at pH 3, suggesting the involvement of electrostatic interactions, hydrogen bonding, and short-range interactions. The adsorption kinetics fit best with the Elovich and pseudo-second-order models, indicating a heterogeneous surface. Adsorption equilibrium was evaluated using different isotherms, with the best statistical fit to the Dubinin–Radushkevich model, while the Freundlich model adequately described the heterogeneity of the system. Additionally, exploratory tests with bisphenol A (BPA) indicated removal dependent on the adsorbent/solution ratio, with a maximum efficiency of approximately 13.5% at a 1:10 (m/v) ratio. The lower efficiency compared to methylene blue was attributed to the neutral nature and hydrophobic character of BPA, which favor weaker interactions. Taken together, the results demonstrate that acid treatment increases the surface reactivity, porosity, and adsorption efficiency of red mud, making LVT a promising material for applications in environmental remediation processes targeting cationic organic contaminants and, to a lesser extent, emerging neutral organic compounds.