Perchlorate (ClO4-) and pertechnetate (TcO4-) exhibit similar adsorption characteristics on alkyl quaternary ammonium-modified montmorillonite (Mt), and (TcO4-)-Tc-99m normally coexists with Sr-90(2+) in radionuclide-contaminated water. In this study, hexadecyl pyridinium (HDPy)-modified Mt (OMt) was encapsulated in alginate beads to inhibit HDPy release and simultaneously immobilize ClO4- and Sr2+ ions. The release of HDPy was remarkably reduced (78 times) from OMt after alginate encapsulation. Adsorption of ClO4- and Sr2+ on the obtained composite demonstrated synergistic effects, with adsorption capacities reaching 0.542 and 0.484 mmol/g, respectively. Compared to the single-adsorbate system, adsorption capacities of ClO4- and Sr2+ increased significantly. The characterization of solids using X-ray diffraction, Fourier transform infrared spectroscopy, C-13 nuclear magnetic resonance, and X-ray photoelectron spectroscopy, as well as the chemical analysis of the aqueous solution, demonstrated that HDPy+-COO- disintegration accounted for the adsorption synergy. HDPy was extracted from the Mt interlayer space during the synthesis of OMt/alginate and then partially re-intercalated back after interacting with ClO4- during the adsorption of ClO4- and/or Sr2+. In the binary-adsorbate system, the synergy-induced adsorption capacity was superior to many previously reported adsorbents, implying that OMt/alginate beads can be a promising adsorbent for the remediation of aqueous solutions contaminated with multiple radionuclides. (C) 2019 Elsevier Inc. All rights reserved.