Steel pickling rinse water sludge: Concealed formation of Cr(VI) driven by the enhanced oxidation of nitrite

Abstract Steel processing hydroxide sludge (SPHS) is a waste from the treatment of rinse water generated during stainless steel processing. Cr(VI) concentrations of up to 20 mg/L were observed in the leachate of a landfill mono-section receiving SPHS. This was surprising since the rinse water is free of Cr(VI) and compliance tests of the fresh SPHS gave no indication of Cr(VI) mobilization. The aim of this study was to identify the processes conducive to the formation of Cr(VI) with emphasis on the effect of H 2 O 2 used in the elimination of NO 2 − . Rinse water, SPHS, and the landfill leachate were characterized in samples collected over a period of several months. The water treatment process was downscaled to laboratory conditions and the generated SPHS was investigated for Cr(VI) desorption. Results showed that overdosing H 2 O 2 with respect to NO 2 − caused the formation of Cr(VI) which adsorbed to the SPHS and was only desorbed in presence of a competing anion. This hampers the recognition of the true leaching potential in compliance testing. Therefore, Cr(VI) leaching from the landfill is attributed to patchy zones of deposited SPHS that was generated from process water treatment at overdosed H 2 O 2 . In conclusion, Cr(VI) formation may circumvented by an automated process control based on the monitoring of either NO 2 − or H 2 O 2 .