Avaliação da remoção do corante índigo blue de efluente aquoso por adsorvente de baixo custo derivado de resíduo siderúrgico: estudo dos mecanismos de remoção

Abstract

Indigo blue (IB) is one of the most widely used dyes in denim dyeing and one of the most challenging from an environmental perspective, mainly due to its resistance to degradation. This characteristic necessitates the application of multiple processes to effectively remove this dye, making its treatment more complex and costly. This study investigated the removal process of IB dye present in synthetic effluent using a steelmaking by-product, blast furnace sludge (BFS), as a low-cost adsorbent. For the experiments, the dye was dispersed in an aqueous solution with sodium naphthalene sulfonate (SNS) as a dispersing agent, simulating the industrial dyeing process. The removal efficiency was evaluated through batch tests using pre-dried BFS. Characterization techniques, including FTIR, XRD, XRF, SEM, TGA, and N₂ adsorption/desorption, revealed that BFS primarily consists of iron oxide with a mesoporous and macroporous aggregate structure. The characterization analyses, along with kinetic and isotherm modeling, highlighted the adsorption processes of the IB dye and the NSS dispersant by LAF. The results indicated that the high affinity of the NSS dispersant for LAF, demonstrated by a high removal rate of 70% within the first 2 minutes, was attributed to the presence of oxides on the adsorbent surface, which promoted dispersant removal and facilitated the flocculation and sedimentation of IB. The kinetic data were best described by the pseudo-second-order (PSO) and Elovich models, based on the determination coefficient (R² > 0.99) and the sum of squared errors (SSE). The adsorption isotherm was best fitted to the Langmuir model (R² > 0.98, low SSE), with a maximum adsorption capacity (qmax) of 25.89 mg g⁻¹ and a separation factor (RL) of 0.378, indicating that the adsorption of NSS by LAF is favorable. Variations in pH, conductivity, and dispersant concentration indicated that the increase in IB particle size is a limiting factor in the adsorption process of this dye. The results demonstrated that the removal of IB dye by LAF occurs through adsorption and flocculation/sedimentation processes, resulting in a total removal of approximately 80% after these processes. Furthermore, it was found that the colloidal system formed by IB plays a crucial role in the removal mechanism and, consequently, in the treatment efficiency, opening new perspectives for the study and development of processes aimed at treating effluents containing this dye.