Stable and recyclable polyporous polyurethane foam highly loaded with UIO-66-NH2 nanoparticles for removal of Cr(Ⅵ) in wastewater

Efficient and recoverable adsorption technology plays a key role in removing the heavy metal Cr(Ⅵ) from wastewater. The metal-organic frameworks (MOFs) are very unstable in aqueous solution and their structure is easy to be destroyed. Of all MOFs, UIO-66-NH2 (Zr-MOF) is widely used to adsorb heavy metal ions in wastewater. But UIO-66-NH2 has disadvantages of small size and easy aggregation. When it is employed as the absorbent of industrial wastewater, it is impossible to be recycled after absorption. Therefore, in this study taking advantage of the excellent properties of polyurethane foam (PUF) such as porous network, light weight and abundant binding sites, UIO-66-NH2 were loaded on PUF by in-situ growth method to obtain a composite UIO-66-NH2@PUF. BET and MIR results confirmed the co-existence of macropore, micropore and mesopore. The absorption kinetics and isotherms indicated that the adsorption capacity of UIO-66-NH2@PUF to Cr(Ⅵ) was 92 mg/g and Langmuir model could better describe adsorption process, as well as the process belongs to monolayer adsorption. After six cycles, the removal rate of UIO-66-NH2@PUF still reached to 79%. XRD, EDS and determination result of Zr(Ⅳ) concentration showed that the exfoliation of Zr(Ⅳ) was negligible, which proved UIO-66-NH2@PUF had excellent chemical stability after repeated use. The adsorption mechanism was the comprehensive effect of ligand exchange and Lewis acid-base interaction between Zr(Ⅳ) and Cr(Ⅵ) which was concluded by XPS. Owing to these attributes, loading MOFs powder on porous carrier makes it possible to reuse MOFs and avoid secondary pollution.