Precipitation-filtering technology for uranium waste solution generated on 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.

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.