Photocatalytic Reduction and <i>In Situ</i> Superstable Mineralization of Cr ^VI to PW ₁₂ @ZnCr-LDH and Its Application for Efficient PET Glycolysis

The removal of toxic, highly soluble, and nonbiodegradable hexavalent chromium (CrVI) has been a major challenge for wastewater purification technologies. Herein, the in situ superstable mineralization strategy was applied for the complete removal of Cr (including CrVI and CrIII) in aquatic systems. Detailed quasi in situ X-ray diffraction (XRD) and transmission electron microscopy (TEM) revealed that under visible-light irradiation, CrVI can be in situ superstably mineralized in the form of PW12@ZnCr-LDH (Ksp ≈ 10–60) by using PW12@ZIF-8 (PW12 = H3PW12O40·nH2O). The removal efficiencies of total Cr can reach > 99.9% in 24 h, and the maximum mineralization capacities of PW12@ZIF-8 toward CrVI can achieve 224.6 mg g–1, surpassing most Cr mineralizers reported so far. On the basis of the results of X-ray absorption fine structure (XAFS), electron spin resonance (ESR), etc., the removal mechanism was proposed as follows: the CrVI can be reduced to CrIII by the intermediate ·O2– and ·OH generated by PW12@ZIF-8 under visible-light irradiation, and the CrIII in the presence of slowly released Zn2+ from PW12@ZIF-8, leading to in situ formation of PW12@ZnCr-LDH. Furthermore, the mineralized product PW12@ZnCr-LDH was used for polyethylene terephthalate (PET) glycolysis, which demonstrated a high BHET yield of 83.12% and excellent applicability to various postconsumer PET. This work provides a new pathway for the CrVI-containing wastewater treatment and utilization of the recycled Cr resource.