Transformation of oil scum as magnetic char for effective recovery of Cu and Ni from EDTA-bearing wastewater

Abstract

Blackish ropy scum was collected from the scum vessel of the floatation tank from the petrochemical wastewater treatment plant. Such oil scum is conventionally burned as an additive in a rotary cement kiln. Herein, the scum was separated as magnetic char and combustible gas/tar via a one-step pyrolysis route with the addition of Fe-rich sludge, and then, the magnetic char was modified by grafting functional amido/thiol groups for enriching heavy metals Cu and Ni from EDTA-bearing wastewater. The results showed that the oil scum had 31.1 wt% water content and can be separated as 27.1 wt% char and 41.8 wt% combustible gas/tar after pyrolysis in nitrogen gas at 600 °C for 20 min with the addition of 5% waste cold-rolling sludge. The separated char showed a good magnetic response of 1.91 emu g⁻¹, the raw char had BET surface area of 96.1 m² g⁻¹, nearly twice that of bifunctional char and total surface site concentration (H_s) of 3.12 mmol g⁻¹ but showed low removal efficiencies of 15.2% for Cu and 8.6% for Ni from EDTA-bearing wastewater. After the graft of two amido/thiol groups, the formed bifunctional char can also be easily separated from water by a magnet after use, and showed superior performance for the removal of 90.2% Cu and 70.2% Ni, even though its BET surface area and H_s apparently decreased to 49 m² g⁻¹ and 1.86 mmol g⁻¹. On the bifunctional char, the amido/thiol groups spontaneously reacted with free Cu and Ni in competition with EDTA, to form stable mesh-like chelation, resulting in the high removal of Cu and Ni from the wastewater. The bifunctional char showed desirable adsorption performance after regeneration five times by acid dilution and was used as filter material in the filter column to enrich Cu and Ni from wastewater. The enriched Cu and Ni were collected as a Cu and Ni halite with the CuCl₂/NiCl₂ contents of 47.6 wt% and 39.8 wt%. Thus, the oil sludge was completely recycled as value-added products of combustible gas/tar and magnetic char, especially, the magnetic char showed practical performance in the enrichment of heavy metals from EDTA-bearing wastewater.