Effective separation of metal impurities from gypsum nanosludge: synergism of mechanical force and metal species regulation

Abstract

The effective separation of metal impurities from gypsum sludges is crucial for both environmental protection and resource recovery. However, it is seriously limited by their entrapment within calcium sulfate crystal lattices. This study presents a universal strategy for metal extraction through combined control of mechanical force and metal species regulation, which effectively separated P, Cr, As, Sr, Cd, and Hg from gypsum sludges with separation efficiencies all above 94.0%, especially for As (99.8%) and Hg (99.2%). Such an exciting effect was due to the precise control of a two-step dehydration–rehydration transformation of gypsum. The process initiated by the mechanical force reduced the gypsum particle size from the microscale (∼10 μm) to the nanoscale (<50 nm), which facilitated the dehydration process of gypsum–bassanite to exclude the doped metals. In the subsequent rehydration process, the nanoparticles were also beneficial for disrupting the calcium sulfate framework of bassanite, leading to the full release of entrapped metals. Additionally, the application of species-regulating agents changed the species of released metals, preventing their re-incorporation into the calcium sulfate. This approach offered a promising method for the separation and recovery of heavy metals from gypsum sludges, providing valuable insights into the treatment of heavy metal-containing solid wastes.