Preparation of a metal-phosphate/chromium oxide nanocomposite from Cr(III)-containing electroplating sludge and its optical properties as a nanopigment

A nanocomposite composed of metal-phosphates and chromium oxide was prepared from a Cr(III)-containing electroplating sludge (CES) by a facile three-step (extraction-precipitation-calcination) process. Optimal process parameters were determined, and the structure of the metal-phosphate/chromium oxide nanocomposite (MPCON) was investigated by field-emission scanning electron microscopy, Fourier transform infrared spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. The results show that the optimal extraction pH is 2.0. The MPCON presents a polyhedral morphology with average particle size of around 100 nm. The components of MPCON vary from AlPO4/Cr2O3 to Mg3(PO4)2/AlPO4/Cr2O3 at different solution pH during precipitation. Meanwhile, the optical performance of the nanocomposite as a pigment is discussed. The reflectance of MPCON-6.5 in the near-infrared range is around 56%, making it a strong prospect to be used as a functional pigment in energy-efficient buildings. This study proposes a novel recycling process for the conversion of CES into high-value products, which is beneficial for the treatment of waste.