Precipitation-filtering technology for uranium waste solution generated on washing-electrokinetic decontamination

Graphical abstract: A recycling process diagram for the volume reduction of waste solution generated from washing-electrokinetic decontamination. - Highlights: • A process for recycling a waste solution generated was developed. • The total metal precipitation rate by NaOH in a supernatant after precipitation was the highest at pH 9. • The uranium radioactivity in the treated solution upon injection of 0.2 g of alum was lower. • After drying, the volume of sludge was reduced to 35% of the initial sludge volume. - Abstract: Large volumes of uranium waste solution are generated during the operation of washing-electrokinetic decontamination equipment used to remove uranium from radioactive soil. A treatment technology for uranium waste solution generated upon washing-electrokinetic decontamination for soil contaminated with uranium has been developed. The results of laboratory-size precipitation experiments were as follows. The total amount of metal precipitation by NaOH for waste solution was highest at pH 11. Ca(II), K(I), and Al(III) ions in the supernatant partially remained after precipitation, whereas the concentration of uranium in the supernatant was below 0.2 ppm. Also, when NaOH was used as a precipitant, the majority of the K(I) ions in the treated solution remained. The problem of CaO is to need a long dissolution time in the precipitation tank, while Ca(OH)₂ can save a dissolution time. However, the volume of the waste solution generated when using Ca(OH)₂ increased by 8 mL/100 mL (waste solution) compared to that generated when using CaO. NaOH precipitant required lower an injection volume lower than that required for Ca(OH)₂ or CaO. When CaO was used as a precipitant, the uranium radioactivity in the treated solution at pH 11 reached its lowest value, compared to values of uranium radioactivity at pH 9 and pH 5. Also, the uranium radioactivity in the treated solution upon injection of 0.2 g of alum with CaO or Ca(OH)₂ was lower than that upon injection of 0.4 g of alum. The results of industrial scale precipitation experiments were as follows. When NaOH was injected as a precipitant, the uranium radioactivity in the treated solution was the lowest of the three precipitants, and the dried sludge volume was the smallest. However, when the treated solution was recycled as a washing solution or electrolyte, K(I) ions can be accumulated in the treated solution. When Ca(OH)₂ was injected as a precipitant, the volume of the generated waste solution was found to increase. After drying for 24 h, the volume of sludge was reduced to 35% of the initial sludge volume. Meanwhile, a recycling process diagram for the volume reduction of waste solution generated from soil washing and electrokinetic decontamination was developed through experiments