Photocatalytic Reduction and In Situ Superstable Mineralization of CrVI to PW12@ZnCr-LDH and Its Application for Efficient PET Glycolysis

Abstract

The removal of toxic, highly soluble, and nonbiodegradable hexavalent chromium (Cr^VI) 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 Cr^VI and Cr^III) in aquatic systems. Detailed quasi in situ X-ray diffraction (XRD) and transmission electron microscopy (TEM) revealed that under visible-light irradiation, Cr^VI can be in situ superstably mineralized in the form of PW₁₂@ZnCr-LDH (K_sp ≈ 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 Cr^VI 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 Cr^VI can be reduced to Cr^III by the intermediate ·O₂^– and ·OH generated by PW₁₂@ZIF-8 under visible-light irradiation, and the Cr^III 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 Cr^VI-containing wastewater treatment and utilization of the recycled Cr resource.