In situ self-assembly with heterostructures CAU-17@ZIF-8 adsorption material enhances the capture of methylene blue and Cd(II) in water.

When used individually, ZIF-8 suffers from hydrolysis in aqueous media, while CAU-17 is limited by its low specific surface area (64.71 m²/g). Therefore, we employed an in situ self-assembly strategy to construct a MOF-on-MOF heterostructure (CAU-17@ZIF-8) for the synergistic removal of Cd(II) and methylene blue (MB). The composite achieved maximum adsorption capacities of 607.65 mg/g for Cd(II) and 1025.38 mg/g for MB, with a 99% removal efficiency for low-concentration pollutants in real wastewater. Even after five cycles, 75% efficiency was maintained. Mechanistic studies combining batch experiments, characterization, and DFT calculations revealed that the heterojunction interface promotes substantial charge transfer from CAU-17 to ZIF-8, significantly enhancing electrostatic attraction toward cations. Simultaneously, new mesopores formed at the interface increase the specific surface area (239.62 m2/g) and provide rapid mass-transfer channels. MB adsorption is governed by electrostatic attraction, hydrogen bonding, and π-π interactions, while Cd(II) adsorption relies on ion exchange, coordination, and electrostatic attraction. This work demonstrates that constructing MOF-on-MOF heterojunctions is a promising strategy to overcome the intrinsic limitations of single MOFs in water remediation, thereby greatly enhancing the adsorption capacity toward methylene blue (MB) and Cd(II).