Review of resource utilization of Fe-rich sludges: purification, upcycling, and application in wastewater treatment

This paper discusses the resource utilization of Fe-rich sludges generated as waste products from water treatment, hydromet-allurgy, surface finishing, and dye chemical industries. Apart from the conventional landfill disposal of such sludges, the work focuses on sludge purification for new commercial products, including iron red/black dyes, an iron concentrate powder, a polyferric flocculant, a catalyst, and a magnetic adsorbent. Among such purifications, a new strategy was developed to recycle Fe-rich sludges for a new Fe/S-bearing flocculant. Given that Fe-rich sludges may contain rare and/or heavy metals, the purification of sludges as high-purity hematite nanoparticles and other valuable products is detailed as a new insight. Accordingly, the mechanisms for the phase transformation of Fe-bearing minerals and the purification of valuable Fe oxides are deeply considered. The work summarizes the pilot-and/or field-scale application for recycling of Fe-rich sludge and proposes the development of a new Fe/S flocculant and a high-purity hematite product