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.
Introduction
The electrokinetic decontamination process holds great promise for the remediation of contaminated soil because it has a high removal efficiency and is time-effective for low permeability of soil (Cho et al., 2011, Yang and Chang, 2011, Kaneta et al., 1992). When the washing-electrokinetic decontamination process was applied to the decontamination of the soil contaminated with uranium, a lot of waste solution volume was generated. That is, using washing-electrokinetic equipment as shown in Fig. 1, about three drums of waste solution were generated for the decontamination of a drum of contaminated soil by washing-electrokinetic equipment as shown in Fig. 1. Therefore, the development of a treatment process for the generated waste solution is needed for the reuse of the treated waste solution.
The waste solution generated in the washing- electrokinetic process contains UO22+, Al(III), Fe(III), Ca(II), and Mg(II) ions. Sorption from solutions equilibrated with CaCO3 showed maximum UO22+ adsorption at pH 8.4. At pH > 8.4, UO22+ adsorption was identical for calcium-free and calcium containing solutions (Dong et al., 2005, Niu et al., 2009). Uranyl ions were precipitated completely by ammonium hydroxide solution at pH 11 (Ikeda et al., 2002). Although Fe(III), Al(III), and Cr(III) ions in the solution precipitated almost completely, the amounts of precipitation of Ni(II) and Cu(II) ions were very low. All metal ions except Al(III) ions were precipitated effectively by aqueous NaOH at pH 13; Al(III) ion was precipitated by aqueous NaOH at pH 6. Uranium was rapidly removed as the pH increased. However, the removal of uranium was found to occur at a lower pH (∼4) in the presence of sediments than in the groundwater alone (at pH ∼ 5.2) (Luo et al., 2009). The dominant forms of U and Tc in groundwater and in sediments were U(VI) and Tc(VII) (Gu et al., 2003, Istok et al., 2004). However, uranyl can be associated with nitrate (as UO2NO3+) or with sulfate [as UO2SO4, or UO2(SO4)22−] at such a low pH owing to the high nitrate and sulfate concentration (Couston et al., 1995, Langmuir, 1978, Moulin et al., 1998, Zhou and Gu, 2005, Gu et al., 2004, Bhattacharya et al., 1982). Pertechnetate was poorly retained by the sediment, and thus has often been observed along with nitrate in groundwater. On the other hand, reduced forms of U(IV) and Tc(IV) were known to be particle reactive or readily immobilized in the sediment under strong reducing conditions (Gu et al., 2003, Istok et al., 2004). Because of stringent product specifications and environmental problems, the use of hydrogen peroxide as a precipitant to precipitate uranium has received considerable attention (McFarlane and Rollwagen, 1982). The pH of the solution, and the temperature and duration of the precipitation of peroxide are of vital importance to the production of uranium peroxide (Kunin and Preuss, 1956).
In the nuclear fuel cycle, an anion exchange resin has been used for uranium recovery from the leaching solution of ores: uranium ores are dissolved in a sulfuric acid solution and uranium oxide in the leaching solution is recovered from other metal elements by anion exchange resin (Chia and Cooper, 1986, Badawy, 2003). In addition, Chelating polymer adsorbents containing amidoxime groups have received considerable attention in the separation of uranium owing to their selectivity and ability to form chelates (Badawy and Dessouki, 2003, NizamEl-Din et al., 2000). In recent years, various authors have carried out the separation of U and Th in different matrices using solid phase extraction, applying an octadecyl silica membrane disk (Shamsipur et al., 2000), Empore chelating resin disk (Miura et al., 2000), TEVA resin (Carter et al., 1999, Yokoyama et al., 1999), Dowex IX8 and Dowex 50WX (Alhassanieh et al., 1999, Karivam et al., 1998). The selective binding of uranyl ion has also been reported by several researchers using 2,2-dihydroxyazobenzene attached to crosslinked polystyrene, covered with highly populated quaternary ammonium cations (Lee et al., 1999), molecularly impregnated Chelex-100 polymer (Bae et al., 1999), chelating resin containing a 4-(2-thiazolyazo) resorcinol functional group and polypyrrole resin (Mann and Todd, 2000).
In this study, a treatment technology for uranium waste solution generated upon washing-electrokinetic decontamination for soil contaminated with uranium was developed. First, a treatment process suitable to the contamination characteristics of washing-electrokinetic waste solution was proposed. Second, the experimental precipitation conditions for precipitation and filtration were compared through many experiments. Third, a process diagram for recycling the waste solution generated from soil washing-electrokinetic decontamination was developed through experiments.
Section snippets
Drawing up a treatment method for uranium waste solution
It was considered that the washing-electrokinetic decontamination process was the best method for removal of uranium from contaminated soil near South Korean nuclear facilities (Bae et al., 1999). The concentration of uranium in the waste solution generated from washing-electrokinetic decontamination was 230 ppm and the concentrations of Mg(II), K(I), Fe(II), and Al(III) ions were comparatively high. A treatment method was proposed for the recycling of the generated uranium waste solution as
Laboratory-size precipitation experiments with various precipitants
Table 2 shows the metal concentrations in the supernatant after precipitation with NaOH at different pH levels for the treatment of the waste solution. The total amount of metal precipitation induced by NaOH 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. However, at pH 13, the concentration of uranium in the supernatant increased.
Table 3 shows the metal
Conclusions
The total amount of metal precipitation by NaOH for waste solutions 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 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
Acknowledgement
This work was supported by the Nuclear Research & Development Program of the Korea Science and Engineering Foundation (KOSEF), in a grant funded by the South Korean Government (MEST).
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