Surface decontamination of protective duplex oxide layers on stainless steel waste using deep eutectic solvents

Abstract

The decontamination capabilities of deep eutectic solvents (DESs) formed from choline chloride (ChCl) and ptoluenesulfonic acid monohydrate (PtsA), ChCl:PtsA, under different conditions (hydrated, heated, and agitated) were tested with simulant oxidized stainless steel 304 specimens. Although the leaching rates were satisfactory under all conditions, hydrated and stirred ChCl:PtsA at 60 degrees C showed the fastest leaching rate of 0.1647 mg/min. Oxidized specimens with an average mass gain of 1.2 +/- 1 mg were leached, and their masses were reduced by 558 +/- 22 mg after 26 h. These results were understood by improved physical properties of ChCl:PtsA upon hydration. Metal oxide solubility of CoO and NiO increased with water, and those of Cr2O3 and Fe3O4 decreased with hydration. Importantly, the use of choline chloride-based DESs in decontamination applications may significantly reduce the cost of decontamination because these DESs can be mass-produced and their components are both easily obtainable and economical. Also, DESs are biodegradable and eco-friendly. The different speciation of Co and Ni, which bond with Cl-, compared with Fe and Cr, which bond with H2O, illustrated the potential for a metal recovery for secondary liquid waste reduction.