Enhanced removal of hexavalent chromium from contaminated soil using an amphoteric–anionic mixed surfactant (SDAO/SDS): Efficiency, mechanism, and environmental implications

• A binary SDAO/SDS surfactant system was proposed for efficient and environmentally safe washing of Cr(VI)-contaminated soil. • Synergistic amphoteric–anionic interactions enhanced electrostatic attraction, micelle formation, and Cr(VI) desorption, leading to >90% removal. • Structural and speciation analyses confirmed that the mixed system effectively mobilized and immobilized Cr(VI) without altering soil mineral integrity, offering a green remediation pathway. The remediation of Cr(VI)-contaminated soils requires mobilization of strongly bound species without damaging soil structure. A mixed surfactant system of N, N-dimethyl-N-dodecylamine N-oxide (SDAO) and sodium dodecyl sulfate (SDS) was evaluated and achieved 92.3% Cr(VI) removal, outperforming single SDAO (83.0%) and SDS (63.1%), with broad pH tolerance (pH 3-10) with stable performance and a local maximum near pH ∼8. Pseudo-second order kinetics provided a superior empirical description, consistent with surface-controlled, multi-site interactions. BCR fractionation showed a shift from labile to less mobile fractions, while FTIR indicated residual surfactant adsorption and XPS showed no significant Cr valence change, supporting a non-redox pathway. Overall, Cr(VI) removal is inferred to proceed via interfacial modification, mixed-micelle mobilization, and adsorption-site blocking that suppresses re-adsorption and promotes transfer to the aqueous phase. The pH-responsive adaptability of amphoteric SDAO, combined with the dispersive and solubilizing capacity of anionic SDS, underpins the observed synergy and highlights the mixed system as an efficient, environmentally compatible strategy for washing Cr(VI)-contaminated soils.