Efficiency and limitations of deionized water washing for 137Cs decontamination from crude Zinc Oxide ash: Mechanistic insight and implications for Zinc recovery.

Crude zinc oxide (ZnO) ash of Waelz process contamination by Cesium-137 (137Cs) presents distinct decontamination challenges compared with steel electric arc furnace dust (EAFD). In this work, multi-stage washing with deionized water were systematically evaluated to address the decontamination of 137Cs from the ZnO ash. The washing was conducted at an ambient temperature and unadjusted pH (neutral condition) as a baseline treatment. The limiting factors of radionuclide removal were also investigated. The results showed more than half of the 137Cs was removed during the first washing stage followed by extraction efficiencies plateaued after the third stage, with only 71-78% cumulative removal after six stages. 137Cs residual activities in the ZnO remained substantially above regulatory exemption limits, demonstrating an insufficiency of water washing alone for complete decontamination. Elemental and mineralogical analyses indicated the matrix-specific retention mechanism within the ZnO ash that limits the extractability of 137Cs, including lattice incorporation and occlusion within the zincite-rich ZnO matrix. The washing process selectively removed soluble species while preserving the zinc-bearing phase. Due to the neutral pH that suppresses zincite dissolution while still mobilizing the exchangeable fraction, this method is highly effective as a reagent-free pre-treatment step. These findings establish baseline performance limits for deionized water decontamination of ZnO ash and provide mechanistic insight for the development of hybrid treatment strategies aimed at improving 137Cs removal while maintaining zinc recovery potential.