Chemical fixation of toxic metals in stainless steel pickling residue by Na2S∙xH2O, FeSO4∙6H2O and phosphoric acid for beneficial uses

Abstract

The leaching concentrations of different metals in stainless steel pickling residue (SSPR) were determined and the toxic metals were treated using Na₂S∙xH₂O, FeSO₄∙6H₂O, and phosphoric acid. A modified European Community Bureau of Reference (BCR) sequential extraction was used to identify the speciation of the concerned metals. Results showed that SSPR contains a large amount of Ca (58.41%), Fe (29.44%), Cr (3.83%), Ni (2.94%), Mn (2.82%) and some of Al, Cu, Mg, Zn. Among them, Cr and Ni were the most toxic metals in SSPR, thus the raw SSPR falls into hazardous waste category due to the leaching amount of Cr. In addition, the leached Cr was identified as Cr⁶⁺ (MgCrO₄) in the waste. BCR test revealed that risk assessment code (RAC) of Cr and Ni were 33.29% and 61.7%, indicating they posed "high" and "very high" risk to the environment, respectively. After fixing by Na₂S∙xH₂O and FeSO₄∙6H₂O, the leaching concentrations of Cr and Ni were less than 1.5 and 0.5 mg/L, respectively. After fixing by Na₂S∙xH₂O and FeSO₄∙6H₂O the treated SSPR can be safely reused as roadbed materials, concrete and cement aggregates. This study provides a useful implication in treatment and beneficial reuse of heavy metal-containing hazardous wastes.

Keywords: Beneficial reuses; Chemical fixation; Cr(VI); Leaching toxicity; Stainless steel pickling residue.