A feasibility study on clearance levels for real U(VI)-contaminated soil and the mechanisms involving metal ions after neutralization of effluents generated from the soil-washing process

Remediation of soil contaminated with U(VI) and treatment of the resulting soil-washing effluents are investigated in this study. Specifically, we examined the changes in metal concentrations in wastewater based on (i) solution pH, (ii) soil type, and (iii) number of soil-washing steps. We then evaluated the feasibility of these experiments to meet clearance levels (<81 U(VI) ppm or 1 Bq∙g⁻¹). Our findings indicate that three washing cycles with H₂SO₄ concentrations stronger than 0.05 M were sufficient to satisfy self-disposal clearance levels, regardless of soil type. Besides, we determined the relationship between the concentrations of elements and U as a function of the soil-washing steps and change of solution pH, respectively. Finally, we proposed precipitation mechanisms based on the variations in metal compositions in the precipitate depending on the soil-washing steps. Meanwhile, hydrazine was used as a neutralizing agent to avoid interference with monovalent cations dissolved in the soil (e.g., Na⁺). In conclusion, we elucidated the relationship between metal concentrations in the wastewater and the composition of the precipitate after neutralization. We believe the findings of this study can significantly advance the remediation of U(VI)-contaminated soil through soil-washing processes and the treatment of the associated wastewater by pH adjustment for element removal via neutralization for precipitation.