Solid-state NaOH treatment of montmorillonite clay was used to enhance the removal of Cs+ and Sr2+. Through this facile and low-cost modification, montmorillonite with a large BET surface area (117.1 m(2) g(-1)) and many surface functional groups (SiAOANa), demonstrated enhanced sorption kinetics (89% removal for 40 mg L-1 Cs+ and 23 mg L-1 Sr2+ in 1 h) with a sorption capacity of 290.7 mg g(-1) for Cs+ and 184.8 mg g(-1) for Sr2+, greatly exceeding the low sorption capacity (137.0 mg g(-1) for Cs+ and 15.6 mg g(-1) for Sr2+) of pristine montmorillonite. SEM-EDS and XPS analyses revealed that Cs+ and Sr2+ were ion-exchanged with Na+ on the surface functional groups formed following NaOH treatment. The performance of NaOH-treated montmorillonite was stable following gamma-ray irradiation (at 6 Gy h(-1) for 30 min) and across a broad range of pHs (3 to 11), exhibiting a high distribution coefficient (Kd) of 1.5 x 10(3) mL g(-1) for Cs+ (1.58 mg L-1) and 3.7 x 10(3) mL g(-1) for Sr2+ (1.64 mg L-1) under groundwater conditions where various cations including Na+, K+, and Ca2+ (V/m = 1 L/g) were present. The proposed method demonstrated great improvement of the sorption capacity of an abundant and inexpensive montmorillonite. (C) 2018 Elsevier B.V. All rights reserved.