Overcoming Zn–Carboxylate Mismatch with a Zinc-Based Metal Hydroxide Organic Framework for Robust Arsenate Adsorption

Highly alkaline, arsenate (As(V))-rich wastewater from hydrometallurgy poses severe environmental and health risks. Although metal–organic framework (MOF)-based adsorbents offer high capacity, their hydrolytic instability under alkaline conditions restricts practical applications. Inspired by the layered stability of metal hydroxides and the coordination flexibility of MOFs, we reconfigure MOF-5(Zn) into Zn2(OH)2(BDC), a robust metal hydroxide organic framework (MHOF), by coordinating Zn–OH–Zn sheets with 1,4-benzenedicarboxylic acid (BDC). As we expected, coordinating Zn–OH–Zn sheets with BDC could overcome the Zn–carboxylate mismatch, creating a robust framework with locally hydrophobic environments and outstanding alkaline stability. Zn2(OH)2(BDC) reached adsorption equilibrium within 120 min and exhibited a maximum As(V) capacity of 149.32 mg/g, representing 14.2 and 145.3% improvements over MOF-5(Zn) and Zn(OH)2, respectively. Zn2(OH)2(BDC) also maintained high adsorption efficiency over four regeneration cycles and continuously treated 2500 mL of 10 mg/L As(V) solution (pH = 11) in fixed-bed column tests before breakthrough. This work demonstrates that merging MOF designability with hydroxide robustness yields a durable sorbent and charts a rational path toward efficient arsenate remediation in challenging alkaline metallurgical wastewaters.