Enhanced Extraction of Heavy Metals from Gypsum-based Hazardous Waste by Nanoscale Sulfuric Acid Film at Ambient Conditions

Low-cost, low-energy extraction of heavy metal(loid)s (HMs) from hazardous gypsum cake is the goal of the metallurgical industry to mitigate environmental risks and carbon emissions. However, current extracting routes of hydrometallurgy often suffer from great energy inputs and substantial chemical inputs. Here, we report a novel solid-like approach with low energy consumption and chemical input to extract HMs by thin films under ambient conditions. Through constructing a nanoscale sulfuric acid film (NSF) of ~50nm thickness on the surface of arsenic-bearing gypsum (ABG), 99.6% of arsenic can be removed, surpassing the 50.3% removal in bulk solution. In-situ X-ray diffraction, infrared spectral, and ab initio molecular dynamics (AIMD) simulations demonstrate that NSF plays a dual role in promoting the phase transformation from gypsum to anhydrite and in changing the ionic species to prevent re-doping in anhydrite, which is not occurred in bulk solutions. The potential of the NSF is further validated in extracting other heavy metal(loid)s (e.g., Cu, Zn, and Cr) from synthetic and actual gypsum cake. With energy consumption and costs at 1/200 and 1/10 of traditional hydrometallurgy separately, this method offers an efficient and economical pathway for extracting HMs from heavy metal-bearing waste and recycling industrial solid waste. Environmental Implication Toxic heavy metal gypsum-based wastes, targeted for disposal due to their danger, encounter obstacles in the recrystallization treatment, mainly due to high energy demands and high chemical input, hindering efforts to achieve carbon neutrality goals. Thus, heavy metal-containing gypsum was selected as a target hazardous waste in this study to investigate the potential mechanisms and pathways of heavy metal removal triggered by nanoscale liquid films constructed on solid surfaces at room temperature. This study introduces an eco-friendly approach to remove heavy metals and reclaim calcium sulfate from gypsum-based waste, contributing valuable insights to the effective management of gypsum-based waste.