Mechanistic Insights on the Roles of Function Speciation in Zirconium-Based Metal–Organic Frameworks for High-Capacity As(III) Removal

Here, a series of functionalized zirconium-based metal–organic frameworks (Zr-MOFs) were synthesized by using ZrCl4 and l-aspartic acid (ASP), 3-mercapto succinic acid (MSA), and succinic acid (SUC) as organic linkers for direct As(III) removal from contaminated water. The Zr-MOFs are nonfunctional Zr-SUC, amine-decorated Zr-ASP, and thiol-decorated Zr-MSA. Mechanistic studies reveal that the introduction of thiol and especially amine functional groups has significantly increased the adsorption capacity and affinity toward As(III) species. The As(III) adsorption capacities are 126 mg/g for Zr-SUC at pH 7, 314 mg/g for Zr-ASP at pH 7, and 250 mg/g for Zr-MSA at pH 2. The origin of maximum capacity at different pH for Zr-ASP and Zr-MSA is related to the different chemical speciation of functional groups at varying pH levels due to secondary mechanisms such as protonation/deprotonation and redox transformation. The investigations based on X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FT-IR) spectroscopy reveal that not only the functionalization of the framework and donor–acceptor interactions between functional groups and As(III), but also the chemical speciation of the functional groups must be considered to develop efficient adsorbents for real-life applications.