Hexavalent chromium, or Cr(VI), is a toxic and carcinogenic pollutant. It can be removed from water, e.g., by precipitation, membrane separation, or adsorption. The reduction of Cr(VI) species to the significantly less toxic Cr(III) can also be helpful. Metal–organic frameworks (MOFs) can be promising candidates for adsorption. The titanium-based MOF MIL-125(Ti)-NH₂, for example, has a large specific surface area and is interesting for heavy-metal adsorption. However, the amino groups in this MOF do not have a high affinity for Cr(VI). Amidoxime groups, in contrast, can provide a fast adsorption rate and high selectivity for Cr(VI) removal.

Chen Xu, China Academy of Engineering Physics, Mianyang, and colleagues have functionalized MIL-125(Ti)-NH₂ with amidoxime groups for the first time and shown that the product (MIL-125(Ti)-AO) can be used for Cr(VI) adsorption. The team first synthesized MIL-125(Ti)-NH₂ via a hydrothermal method from 2-aminoterephthalic acid and tetrabutyl titanate. MIL-125(Ti)-AO was then prepared in two steps: First, MIL-125(Ti)-NH₂ was reacted with glutaraldehyde to introduce an aldehyde group, followed by the introduction of diaminomaleonitrile. The diaminomaleonitrile intermediate was then converted to the desired MIL-125(Ti)-AO.

The functionalized MOF showed good Cr(VI) adsorption performance with a maximum adsorption capacity of 271 mg·g^–1. In addition, Cr(VI) is reduced to less toxic Cr(III) during the adsorption. The team found that the chromium concentration in water (with an initial concentration of 60 mg·L^–1) after treatment with the functionalized MOF was below the limit for drinking water recommended by the World Health Organization (WHO).

• Functionalized Titanium-Based MOF for Cr(VI) Removal from Wastewater,
  Yefan Qing, Wei Gao, Yantong Long, Yijin Kang, Chen Xu,
  Inorg. Chem. 2023.
  https://doi.org/10.1021/acs.inorgchem.2c04501