Synthesis of modulator-driven highly stable zirconium-fumarate frameworks and mechanistic investigations of their arsenite and arsenate adsorption from aqueous solutions

Here, we synthesized a modulator (benzoic acid (BA))-driven zirconium-fumaric acid (Zr-fum) metal-organic framework (MOF) and investigated in detail the adsorption mechanism of arsenic oxyanions (AsO₄³⁻ and AsO₃³⁻) and their stability before and after adsorption from water. Without the support of the modulator, BA, Zr-fum formed an amorphous MOF due to the occurrence of quick precipitation of both zirconium and -fum. Various amounts of BA, 0-10 eq. to ZrCl₄, were used to control the surface charge on the MOF nanoparticles, which directly corresponds to the colloid stability of the MOF and helps to obtain a defect-free Zr-fum MOF for high uptakes of AsO₄³⁻ and AsO₃³⁻ from water. With the concentration of 3 eq. BA, the adsorption capacities of both arsenic oxyanions on the Zr-fum MOF were found to be higher than those on other MOFs. The CHN analysis indicated that the excess of -fum present does not alter the adsorption densities of AsO₄³⁻ and AsO₃³⁻ on MOFs in addition to the negligible amount of BA except for the Zr-fum-10 eq. BA MOF where it is present in higher amounts, as confirmed by NMR studies. The physicochemical properties of the synthesized MOFs with/without the modulator support adsorbents before and after the adsorption of AsO₄³⁻ and AsO₃³⁻ were extensively characterized using several advanced instrumental techniques. The maximum uptake performance of AsO₄³⁻ (1.159 mmol g⁻¹) and AsO₃³⁻ (1.121 mmol g⁻¹) was obtained using the Langmuir adsorption isotherm. The adsorption of both AsO₄³⁻ and AsO₃³⁻ takes place by electrostatic interaction/complexation and ligand exchange with Zr-fum-5 eq. BA. In addition, the Zr-fum-5 eq. BA was recycled up to four times at a sustained efficiency after washing with 0.1 M NaOH. Owing to their high uptake capacities of AsO₄³⁻ and AsO₃³⁻, the synthesized MOFs are expected to have potential applications as adsorbents for practical applications.