Ternary adsorption of Auramine-O, Rhodamine 6G, and Brilliant Green onto Arapaima gigas scales hydroxyapatite: Adsorption mechanism investigation using CCD and DFT studies

Abstract

The simultaneous-competitive adsorption of Auramine-O (AO), Rhodamine 6G (R6G), and Brilliant Green (BG) dyes onto hydroxyapatite (HAP) produced from pirarucu (Arapaima gigas) scales was investigated by Fourier Transform Infrared (FTIR), Central Composite Design (CCD), and Density Functional Theory (DFT). Pirarucu scales were chosen as the precursor of the bioadsorbent owing to the large volume of this unutilized waste produced by the Brazilian pisciculture, which has increased the concern about solid disposal in the Amazon region. The CCD matrix presented each dye initial concentration as a factor influencing the removal of the other dyes. DFT was utilized to indicate the preferential order of adsorption. FTIR analyses of the dye-adsorbed HAP confirmed that the cationic dyes were removed from aqueous solution owing to interactions with anionic groups present on the HAP surface. High values of BG initial concentration negatively influenced the adsorption of AO, according to CCD, DFT, and FTIR results. Desirability analysis based on CCD results revealed the simultaneous optimum conditions of the ternary adsorption to be: HAP dosage 1.09 g.L−1; AO, R6G, and BG initial concentration 77, 29, and 39 mg.L−1, respectively, yielding percentage removals of AO, R6G, and BG as 93.78%, 23.94%, and 99.94%, respectively. The adsorption isotherms at 28, 38, and 48 °C suggested that the dyes in the ternary mixture were adsorbed in monolayer on the HAP surface (modified Langmuir model). The CCD, DFT, and FTIR studies effectively aimed to propose the possible multicomponent adsorption mechanism. This study shows that pirarucu scales is a residue biomass that can be easily processed to obtain an efficient, low-cost, and selective adsorbent regarding the studied dyes.