Enhanced humification by carbonated basic oxygen furnace steel slag - II. Process characterization and the role of inorganic components in the formation of humic-like substances

► Carbonated steel slag promotes the polycondensation of model humic precursors. ► Fe- and Mn-oxides in steel slag can function as Lewis acids to oxidize precursors. ► FLAs are the inevitable state of transition of organic matter and HLA precursors. ► Steel slag is a milder material for promoting humification and stabilizing more OC. ► Humification can suppress release of metals from steel slag under acidic condition.