Synergistic effect of carboxyl and sulfate groups for effective removal of radioactive strontium ion in a Zr-metal-organic framework

Abstract

Rapid removal of radioactive strontium from nuclear wastewater is of great significance for environmental safety and human health. This work reports the effective adsorption of strontium ion in a stable dual-group metal-organic framework, Zr₆(OH)₁₄(BDC-(COOH)₂)₄(SO₄)_0.75 (Zr-BDC-COOH-SO₄), which contains strontium-chelating groups (-COOH and SO₄) and a strongly ionizable group (-COOH). Zr-BDC-COOH-SO₄ exhibits very rapid adsorption kinetics (<5 min) and a maximum adsorption capacity of 67.5 mg g^-1. The adsorption behaviors can be well fitted to the pseudo-second-order model and the Langmuir isotherm model. Further investigations indicate that the adsorption of Sr²⁺ onto Zr-BDC-COOH-SO₄ would not be obviously affected by solution pH and adsorption temperature. The feasible regeneration of the adsorbent was also demonstrated using a simple elution method. Mechanism investigation suggests that free -COOH contributes to the rapid adsorption based on electrostatic interaction, while the introduction of -SO₄ significantly enhanced the adsorption capacity. Thus, these results suggest that Zr-BDC-COOH-SO₄ is a potential candidate for Sr²⁺ removal. They also introduce dual groups as an effective strategy for designing high-efficiency adsorbents.