Celso Hissao Maeda

The discharge of harmful dyes in water bodies is a serious pollution problem, dangerous for the ecosystem’s equilibrium and human health. In this sense, the aim of this work was to determine the influence of electrolytes (NaCl, KCl, CaCl₂ and MgCl₂) in the adsorption of Reactive Blue BF-5G dye, the most common dye used in industrial process for fabric colouring, using bovine bone char as the adsorbent. The bovine bone char was characterized by pH of point of zero charge (pH_PZC), N₂ adsorption–desorption isotherms, Fourier transform-infrared spectroscopy (FT-IR) and X-ray diffractometry (XDR). The characterization revealed a mesoporous structure (pore mean diameter of 94 Å and S_BET ∼107 m² g^-1) with negative charge distribution at the surface (pH_PZC= 3.8). The adsorption experiments revealed that the presence of KCl enhanced the material adsorption capacity (q_max = 195 mg g⁻¹), that the Sips isotherm best fitted the experimental data (R² > 0.9 except for KCl solution) and the adsorption process was mono- and multilayered. The kinetic adsorption experiments indicated that the inorganic electrolytes increased the initial adsorption velocity and the data was best modelled by the surface diffusional model (SDM), implying a resistance (aqueous > CaCl₂> NaCl > MgCl₂> KCl) to mass transfer at the surface of the pores which, in turn, prevented the dye diffusion to the interior of the adsorbent (q_e = 71 mg g⁻¹). Therefore, small quantities of KCl can be used to lower the mass transfer resistance and provide higher adsorption capacity with reduced time of operation, thus increasing the overall process efficiency. GRAPHICAL ABSTRACT

In the textile industry, the amount of dye unfixed in fabrics is discarded with wastewaters. Effluents of this nature can be treated efficiently by adsorption on activated bone char, but the reuse of adsorbent is necessary for the technique to be economically feasible. Therefore, the objective of this work was to study the process of desorption of BF-5G blue dye from a bone char fixed-bed column. Solutions of sodium chloride, acetic acid and ethyl alcohol were tested as regenerating agents. Due to the hydrophobicity effect of organic solvent molecules, the highest desorption capacity was observed for ethyl alcohol solution, and the fixed bed was reused after six cycles of adsorption. The other solutions did not promote significant desorption. The results showed that adsorption of the dye involved irreversible interactions between adsorbate molecules and bone char. However, the use of acetic acid solution resulted in the neutralisation of some of the adsorbent surface charges, allowing the fixed bed to operate for a longer time in the second cycle than in the first.