Efficient adsorption of methyl red, methyl orange and bromothymol blue by polyamine resin: Role of linearity and size of carboxylate in enhanced encapsulation and molecular matching

Dyes constitute the major contribution to the environmental organic pollutants. Besides being hazardous to aquatic life, dyes are highly toxic and injurious to human health. Herein, we disclose the synthesis of a novel hyper cross-linked, dubbed as BPA-PEA, for the adsorptive removal of methyl red (MR), methyl orange (MO) and bromothymol blue (BTB). BPA-PEA displayed maximum adsorption capacities (q_max) for the adsorptive removal of these ionic dyes as 230.6 mg/g, 117.7 mg/g, and 80.42 mg/g for MR, MO and BTB, respectively. Density Functional Theory (DFT) simulations indicate a spontaneous binding process, with binding order closely aligns with the experimentally observed order (MR > MO > BTB). Moreover, the higher interaction in the resin-MR complex was believed due to the linearity and smaller size of the carboxylate group in the MR. Response Surface Methodology (RSM) optimization shows excellent suitability from their statistical parameter as well as low SSE value with the experimental values. The kinetic and isothermal data suggest that the adsorption process followed a chemisorption pathway and adsorption equilibrium was achieved within 2, 6, and 18 h for BTB, MR, and MO, respectively. A significant influence of pH was also observed in the adsorption of MO (5.5), MR (6.1), and BTB (5.3). The BPA-PEA was found to be thermally stable (T_d = 348 °C) and mesoporous in nature, having a high surface area (78.3 m² g⁻¹). The adsorption mechanism is mainly governed by electrostatic interaction between the dyes and adsorbent, however, other adsorptive mechanism such as H-bonding interactions, hydrophobic interactions or π-π interactions were also involved. In addition, the adsorption efficiency of BPA-PEA was maintained for consecutive 5 cycles, with slightly decline (less than 5 mg/g for MR, MO, and BTB) in the adsorption capacity. Given the high adsorption capacity, short adsorption time, and reusability, BPA-PEA can serve as a promising adsorbent for the removal of anionic dyes from wastewater.