L.B.Prozorov, V.V.Martianov, M.Yu.Chtcheglov,
V.B.Nikolayevsky, V.L.Tarasov
Mos SIA "Radon"

Main problem of ground decontamination technologies is the most full extraction and multiple reduction of radioactive materials volume which are the subject of RAW placement. The majority of developed methods of grounds cleaning is based on the leaching of radioactive elements (using various chemical reagents) and chemical solutions processing (the result of leaching). Among them an electrokinetic method of ground cleaning including processes of leaching, electrodialisis and electroosmosis takes special place. This method can have certain advantages as compared with other metods, especially in the case of low water permability grounds and soils.

The complex researches on the cleaning of grounds from ²²⁶Ra and ²³⁸U were conducted in the following sequence:

• conditions of radionuclides leaching;
• possibility of electrokinetic ground cleaning;
• choice and check of the treatment method secondary received liquid radioactive wastes with the purpose of a diminution of volumes which are being a subject to RAW placement.

The researches were conducted in laboratory conditions. Two given samples of the ground ( 20 kgs each) contaminated with radium and uranium were selected. Scale - spectral analysis data of the ground samples are submitted in Tables I and II.

The contents of uranium and thorium (ground sample No1) are dozens of times lower than significant amounts. Therefore researches were directed only for radium decontamination. The ground sample No2 is characterized by uranium mono-contamination.

The contents of other (unradioactive) metals was determined in the samples of the grounds (Table III.).

The laboratory experiments on determination of various chemical reagents efficiency for ground cleaning from radium and uranium were conducted in static conditions by the way of air - dry ground contact with liquid phase, solutions of salts and acids. Experiment conducted under room temperature conditions in closed glasses using periodic intermixing and mechanical mixer for 24 hours. The ratio of solid and liquid phases (S/L) was 1:5 and 1:2 respectively.

Liquid and solid phases were divided by filtering after the test, and ²²⁶Ra and ²³⁸U contents in water solution were determined by the scale-spectral method, using the peak multichannel analyzer with Ge-Li detector.

NH₄Cl, CaCl₂, BaCl₂, AlCl₃ and FeCl₃ solutions, and also hydrochloric and nitric acid were tested as desorption reagents for ²²⁶Ra. Nitric acid, potessium and sodium salts were used for ²³⁸U desorption. The results of the experiments for ²²⁶Ra desorption are indicated in Table IV.

The highest degree of ²²⁶Ra extraction (70 %) within one stage of processing was reached using 1N solutions of acids and cloride iron.

The large efficiency of three-charge cations of aluminium (52 % extraction) and iron (70 % extraction) and low efficiency of one and two charge cations (NH₄⁺, Ca²⁺, Ba²⁺) indirectly specify the ion-changing character of radium sorption by the ground.

The adding of ammonium and calcium salts into hydrochloric solution acid results in insignificant increase of radium extraction with the comparison of mono-acid solution (from 55,3 % up to 58,8 % and 61,4 %). The diminution of the ground cleaning efficiency up to 42,8 and 27,4 % is observed in a case when the barium and iron solutions with the hydrochloric acid are applied.

The obtained outcomes show, that most effective reagents for ground cleaning from radium are the solutions of chloric iron, and hydrochloric acids. Therefore these reagents are chosen for further researches.

The results of uranium leaching (S:L=5:1) showed, that the most effective reagents were as follows:

• HNO₃ (68% during the first stage of leching);
• HCl ( 60% during the first stage of leaching);
• KHCO₃ ( 48% during the first stage of leaching).

Application of NaHCO₃ has not given a positive effect. The reagents containing SO₄^2--ions, were not applied because the large content of Ca,MgCO₃ was in the ground, and CaSO₄ changing ground structure was being formed.

The kinetics of radium leaching (Fig.1.) was investigated. It was found out, that an equilibrium in the ground - solution system is formed enough quickly for 1,5 hours.

Researches on uranium desorption have shown, that the balance in the ground - solution system is formed for 1.5-2 hours after beginning the experiment.

The dependence on radium leaching from various concentration of HNO₃ and FeCl₃ was determined.

The growth of HNO₃ concentration up to 0,5 M/l results in sharp growth of radium extraction in the solution (up to 60 % for one stage of leaching, the ratio S:L = 1:5). Then this dependence becomes smooth. The further acid concentration magnification results in maximum radium extraction up to 90 % conteining 2M/l of nitric acid.

The influence of initial concentration of iron salt on radium extraction is observed only up to 0,1M/l (Table V). The maximum ²²⁶Ra extraction degree of from the ground comes 74%. The further magnification of concentration does not result in growth of radium transition in the solution.

The amount of reagents (E = 70 %) comes to about 81g/kg for 0,1M FeCl₃ and 315g/kg for 1M HNO₃.

It is necessary to note, that other chemical elements also pass in the solution from the ground (TableVI).

The experiments on radium leaching at the ratio S:L = 1:2 also were conducted. It was made for reduction of radioactive solutions volumes formed during the ground cleaning.

The fact of diminution of radium extraction degree at the L:S reduction for the majority of reagent concentration was determinated ( except 2M HNO₃ solution, where the degree of extraction was ~ 88 %).

It is also necessary to note insignificant decrease of radium extraction from 74 up to 65 % using 0,33M (53,6 g/l) solution of FeCl_3. Specific amount of reagents comes to 107g/kg for FeCl₃ and 252 g/kg for HNO₃.

The repeated processing of the ground was conducted by the fresh reagent solutions for reaching deeper cleaning. 21 % of radium (0,33M FeCl₃) and 10 % of radium (2M HNO₃) were extracted in addition. The general extraction comes to 86% and 98 % for salt and acid respectively.

Thus, radium leaching at low significances of S:L = 1:2 ensures effective ground cleaning and high radionuclide concentration in liquid phase. The important result is also volume reduction of solutions which are the subject to further treatment.

The data of static researches were used for dynamic column experience (reagent filtration through the ground). The nitric acid (concentration - 31 gms/l) solution was used, as a desorption reagent for ²²⁶Ra leaching.

The solutions of nitric (1M/l) and hydrochloric (1M/l) acids and KHCO₃ (1M/l) were selected for ²³⁸U desorption.

The average velocity of filtration was 0,32 cm/hours.

The experimental data of radium extraction are submitted in Fig 2.

Dependences of ²²⁶Ra extraction degree (E %) and its specific radio-activity in the solution (C, Bq/ml) on the amount of filtered reagent (V) were obtained. The dependence of kind C = f (t) has a diffusion maxima (E ~ 8 %) at a volume of filtrate ~ 210 ml, that corresponds to N = 2.4.

The pH significance during the leaching changed from 7 up to 0,3, coming closer to the initial solution pH.

The general radium extraction grows constantly with the N magnification (the cycle number), reaching E=89 %, V = 840ml, i.e. N~10.

Dependences of ²³⁸U extraction degree (E %) on the filtered reagent number of cycle (N) were obtained (Fig.3).

The peaks of uranium concentration correspond to values N=1.5 (HHO₃), N=1.3 (HCl) and N=2.2 (KHCO₃). The maximum values of uranium extraction (E %) were achieved at values N=6 (HNO₃ -92%, HCl - 82%, KHCO₃ - 80%).

The electrokinetic cleaning of the grounds has the stages as follows:

• creation of direct current electrical field with the help of electrode devices;
• transferring of radioactive element in mobile form (as ions) by means of leaching or desorption with the help of chemical reagents;
• moving of ions-contaminants to the zone of electrode device;
• radionuclides removal from the electrode devices, and radioactive solutions treatment.

We used process of water electrolysis for shaping ions of hydrogen, which moves from the anode to the cathode and forms an acid medium. It gives a possibility of conducting electrokinetic ground cleaning without adding chemical reagents.

The electrokinetic cell was used for making experiments. The cell consists of three chambers: the working chamber, the cathode chamber and the anode chamber. The special diaphragms separated electrode chambers from the working chamber.

The cation-changing diaphragm (membrane) was used both for prevention of inflow OH-ions into the ground and decrease of the return diffusion stream of cations-radionuclides.

The ultrofiltering diaphragm (membrane) is a neutral filter throagh ions-contaminants from the anode chamber pass freely. Titanium-and-platinum plate was used as anode, and corrosion-proof steel was used as cathode.

The force of the current was constant and was supported automatically: in the first period - 50mA, and after 30 day period - 25mA.

90 % of ²²⁶Ra from the ground was extracted for 82 day. 70 % of ²²⁶Ra was extracted from the cathode zone and 20 % of ²²⁶Ra was extracted from the anode zone. The further electroeffect during consequent 10 day has not reduced in additional ²²⁶Ra extraction.

94% of ²³⁸U from the ground was extracted for 32 days. It is necessary to note, that significant part of uranium (up to 60 %) migrated as anion form (application of KHCO₃). Application of acids resulted in mainly cathion form of uranium migration.

The experiment conducted in laboratory conditions showed a possibility of ground cleaning from ²²⁶Ra and ²³⁸U up to 90%.

Secondary liquid radioactive wastes form after application of reagent or electrokinetic method of cleaning. The effective ²²⁶Ra removal from contaminated solutions can be reached using radium sedimentation with BaSO₄. Uranium sorption was not considered.

The check of method efficiency was conducted using two chemical solutions:

• solution N1 - obtained after electrokinetic method implementation (specific radioactivity 5150 Bq/l);
• solution N2 - obtained after ²²⁶Ra leaching with FeCl₃ (specific radioactivity 2190 Bq/l.)

These solutions contained way elements as follows: Fe²⁺³⁺, Ca²⁺, Zn²⁺.

Therefore a smole amount of concentrated sulfuric acid (1 % from volume of the solution) was added in the solutions. Then 2ml of BaCl₂ (C = 10g/l.) was gradually added.

More than the 100-multiple SO₄^2- surplus (because of Ba²⁺ ) was supported during the sedimentation. The time of solution intermixing made 2 hours. After upholding the solution was filtered and than it was analyzed for ²²⁶Ra determination. In filtrate ²²⁶Ra was not found out within the limits of analysis sensitivity.

The conducted experiments showed a possibility of complicated structure concentrated solutions cleaning from ²²⁶Ra using sedimentation method. It has allowed to reduce the volume of solid radioactive wastes dozens of times.

The conducted laboratory researches have shown a possibility of ground cleaning from radium and uranium, using both the reagent and electrokinetic methods.

The solutions of nitric and hydrochloric acids, and also solutions containing Fe³⁺, have the greatest ²²⁶Ra extraction efficiency.

The solutions of nitric and hydrochloric acids have the greatest ²³⁸U extraction efficiency.

The static (²²⁶Ra) researches showed, that the magnification of extraction degree happens to growth nitric acid concentration up to 2M/l (126g/l) and iron sult up to 0,1M/l (16,2gms/l). The maximum significances of ²²⁶Ra extraction (88 % and 74 %) are reached during the first stage of leaching. The maximum significances of ²³⁸U extraction (68 % and 62 %) are also reached for the first stage of leaching.

The dynamic tests showed, that the initial contamination can be reduced up to 90% (²²⁶Ra and ²³⁸U). The specific radioactivity of the ground after the experiment has made 1,1.10³ Bq/kgs, that is lower than an allowable radioactivity level. The specific amount of HNO₃ has made 275 g/kg.

The electrokinetic researches have showed, that the initial contamination of ²²⁶Ra and ²³⁸U can be reduced up to 90% also. However duration of this process makes dozens of days (especially for ²²⁶Ra). Therefore its application is expedient when the collection and transportation of the contaminated ground is impossible (it is IN SITU method). The amount of the electric power have made 3 kW.h/kg.

Sedimentation tests showed, that the ²²⁶Ra sedimentation with BaSO₄ happens quantitatively. It was experimentally noted, that decontamination of the ground with consequent ²²⁶Ra accumulation in the sulphate form allows to reduce the mass of solid radioactive wastes more than 100 times.

1. Sanitary rules and reversion with radioactive wastes (SPORO-85).,1986, M.Atomizdat, :4-54.
2. Bakhurov V.G., Lutsenko I.K., Shashkina N.N., 1965. Radioactive wastes of uranium factories. M.Atomizdat., : 15-45.
3. Reiniers R.S.& Krenkel P.A. 1974. Kinetics of mercury adsorption in sediments, J. Water Polut. Control Fed., 46(2): 215
4. Hamed,J., Y.B. Acar & R.J. Gale 1991. Pb (II) removal from kaolinite by electrokinetics, J. Geotech. Eng., ASCE, 117(2): 241-271
5. I.A.Sobolev, L.B.Prozorov, A.S.Barinov & V.V. Martyanov. 1996. Experiments of applying electromigration to soil contaminated with ¹³⁷Cs, J.Enviroment. Geotechn., 6(2):1089-1094

BACK