Enhanced selective separation of mixed anionic and cationic dyes by whey soybean protein foam fractionation via a two-stage pH adjustment strategy

The discharge of colored effluents into the environment causes significant ecological disruption. Selectively separating individual dyes from multi-component systems remains a critical challenge in foam fractionation. Herein, we investigated a novel foam fractionation method for the selective removal of organic dyes, employing whey soybean protein (WSP) for environmentally friendly treatment. WSP, a natural protein extracted from abundantly available soybean meal (SBM) agro-industrial waste, was utilized to remove the anionic dye Congo red (CR) and the cationic dye Malachite green (MG) from aqueous solutions. WSP exhibited remarkable surface activity and formed robust interfacial films, enhancing both foaming capacity and stability. Furthermore, we elucidated the interaction mechanisms between the dyes (CR and MG) and WSP through spectroscopic methods and molecular docking simulations, including electrostatic interactions, hydrogen bonding, hydrophobic interactions, salt bridges, and π-cation interactions. Building upon single-dye removal experiments, we developed a novel two-stage pH adjustment strategy. This strategy enabled the selective removal of 97.1 % of CR from the mixed dye solution at pH 3. Subsequently, subjecting the pre-treated solution to foam fractionation at pH 9 achieved remarkable removals of 97.9 % of MG. This work provides an effective approach for developing foam fractionation agents, addressing the urgent need for mixed-dyes wastewater treatment.