S.V.MIKHEIKIN, A.N.ALEKSEEV, L.V.PRONINA, A.YU.SMIRNOV
A.A.Bochvar Institute of Inorganic Materials
(SRC RF VNIINM)
123060 P.O.Box 369 Moscow, Russia
Fax: ++7- 095-196-4168
E-mail: serge@bochvar.ru
8910.g23@g23.relcom.ru

1. To reduce the radioactive dust carryover, VNIINM in collaboration with the MSU has developed new polymeric compounds (trade mark MM-1, MT-1, MN-1) based on interpolyelectrolyte complexes (IPEC). The technique and demonstration sowing-irrigator were constructed and tested in Chernobyl.
2. Some 1.0-1.5 l/m² of the compound containing 2-3% of polymers can provide a protective layer strength as high as 5.0 kg/m² as a function of the soil type. Radioactive substance dust carry-over has also been evaluated on a laboratory scale at air flow rates of 7.5, 10.0, 15.0 and 20.0 m/s. IPEC and related coating resistance to gamma-irradiation is using a Co-60 source at gamma- dozes up to 10⁶ Gy. It is found that the IPEC retentivity shows no decrease at these doses. Moreover, its moisture- swelling ability typical of IPEC ( mild hydrogel ) improves the growth of vegetation to form a stable sod which gives a contaminated area a final localisation. Polymer structureformers were experimentally tested using the confinement technique developed. MM-1 was used at the Chernobyl NPP site in 1986-1990 on a commercial scale. Also, polymers developed can be helpful in reducing highly-saline sand transfer from dried-up bottom of the area of the Aral See ecological calamity. Tests carried out under the Polyarny Ural since 1995 suggest that the polymers are useful for remediation of forest-tundra soils.

Large area (over 2 mln. ha.) of contaminated soil as a result of the Chernobyl accident is the major contributor of aerosol air-borne pollution since May 1986 /1/.

Essentially all accidental radioactive releases whatever their nature are well known to be tightly found to the organic fraction (by chemical interaction with humic and fulvic acids) and the mineral fraction by adhesion (hot particles) or sorption of soluble species (especially, Sr and Cs). Thus, secondary spread of radioactive precipitates can be prevented through polymeric mixtures capable of sticking separate particles together into large aggromerates. There is a wide range of such mixtures often used for preventing soil erosion in agriculture. But the Chernobyl experience shoved their low efficiency. Widespread lignosulfonate-based polymers are not rainfall resistant. Latex films are rapidly disributed on topsoil with a consequent radioactive migration.

To diminish radioactive dust carryover, VNIINM in collaboration with the Moscow State University (MSU) have developed new polymeric compounds (trade name MM-1, MT-1, MN-1) based on interpolyelectrolyte complexes (IPEC).

IPECs are the products of interactions between oppositely charged macromolecules /2/. These ionic interactions are of a distinct cooperative nature that is responsible for the relatively high stability of IPECs with respect to their dissociation into constituent polyelectrolytes. IPECs are amphiphilic macromolecular compounds, i.e., contain both hydrophobic (b) and hydrophilic (a) sites (Fig.4). Sites formed by coupling polyionic counterparts are sufficiently hydrophobic because of mutual screening by polyions of their charges. Hydrophilic sites include polyelectrolyte units that are not involved in the ionic interactions between oppositely charged macromolecules. Because of the reversibility of the IPEC formation, hydrophobic and hydrophilic sites are able to spontaneously exchange their location within IPECs. These peculiarities of the IPEC structure provide a unique opportunity for interactions of IPECs with colloidal particles and surfaces of different natures. Introduced into disperse systems IPECs can adjust themselves for the hydrophobic - hydrophilic balance of microenvironment by the «trial and error» method. Thus, IPECs can be considered as intelligent (smart) materials due to their ability to adapt themselves to complex structure of disperse systems via rapid exchange processes and to realize the optimal set of bonds with different colloidal particles and surfaces.

The technological procedure of the preparation and utilization of IPEC binders is quite simple. It includes (i) the preparation of dilute aqueous polyelectrolyte mixtures at high concentrations of low-molecular weight salts (for instance, mineral fertilizers) when ionic interactions between oppositely charged polyions are completely suppressed (Fig1), (ii) the introduction of these mixtures into dispersed systems usually by splashing polyelectrolyte mixtures on soil or ground surfaces (Fig2,3) and (iii) washing of disperse systems by water in order to remove salts. The decrease in salt concentrations leads to the IPEC formation.

With mineral matters predominant on topsoil, account should be taken of separate slightly charged particles resulted from, firstly, the replacement of Al cations in montmorillonite by lower-charged cations to give electrostatic uncompensation of the crystal lattice surface layer. Secondly, the charge can result from particle dissociation of silanol Si-OH groups on surface irregularities.

The latter IPEC interact with negative silanol groups on the silica surface through ionic linkage (Fig.4) /2-6/.

Polymeric soil-stabilizers were tested both experimentally and in field conditions using the confinement technique developed. Specifically, MM-1 was demonstrated at the Chernobyl NPP site in 1986-1993 and over the Aral Sea area in distress. Quartz sand supplied from Chernobyl external areas in 1986 was used for laboratory experiments. Its properties are given in Table I.

The size distribution of beta activity within a sample clearly demonstrates a buildup of the major portion of radioactivity in small-sized fractions which are particularly susceptible to deflation. This agrees closely with the data of other researchers /7-8/.

Comparison between bar charts for gamma-activity distribution in soil depth measured as of 1986 and 1993 suggests that the dust carryover is of vital importance in the early years after contamination since later on the radioactivity transfers from the surface to depth.

An aqueous solution (1 l/m²) of polymers at a concentration of 2-4% sprayed on a surface produces a soil-polymer crust 3-5 mm thick. Polymerization time depends on the moisture content of soil and the ambient temperature. The polymerization comes to a close when the crust runs dry.

The following types of connections arising in stabilised polymer soil layer that depend upon the nature of polymer and its amount, have been found by optical microscopy:

1. continuous sands gluing together with impenetrable soil-polymer crust layer. Here every sand independently on its size, is filmed with thick (5-10 microns) polymer layer, moreover, the polymer occupies all the space among sands, forming thick impenetrable film.
2. sand particles gluing together with polymer only at their contact places. Water filtration and aeration of bottom sand layers take place through the sand particles free interspace.
3. connection of sand particles of different size, that are on considerable distance (from a few microns up to 1-2 mm) from each other with long and thin (about 20 microns) filaments-bridges.
4. adhering of small-size particles (from less than 5 microns up to 250 microns) to the surface of larger sands (more than 700 microns). Such adhering-attracting of fine disperse particles takes place during the polymer drying out due to the sand particle enveloping with drying solution. And fine disperse particles are picked up by such a solution from the space, formed by loosely located large particles, attracted and glued to the surface of large particles.

Polycation and polyanion interaction on the soil particle surface leads to the formation of soil-polymer crust layer insoluble in water but permeable and gas-penetrable. The presence of hydrophilic as well as hydrophobic bridges in the IPEC structure leads to the most optimum bond of the soil particles.

Protective properties, the amount to be sprayed as well as the choice of spraying equipment are dependent upon the working solution concentration (Table II) (See annex).

It was exposed to air at a rate ranging from 7.5 to 20.0 m/s in a wind tunnel. Laboratory tests show that the dust suppression is independent on the air rate and within the experimental error. Tests suggest the efficiency of the techniques developed for protecting against radioactive aerosol carryover at a wind rate no less than 20.0 m/s. As far as unprotected sand is concerned, its mass transfer averages 3.05 g/cm²min. The efficiency of a protective coat at similar wind rates is presented in Table III. (See annex)

Actual limits of gamma-irradiation at which protective coats developed are still kept essentially constant are a major objective of an investigation into the IPEC radiolytic decomposition. Gamma-dose rates of 5x10³ and 1.03 Gy/s at doses ranging from 10² to 1.5x10⁵ Gy were used for experiments. The irradiation in the range under study is shown to have little effect on protective properties. Only with 3.5x10⁶ Gy, MM-1 fails to form a protective coat. The behavior of g -irradiated MT-1 -based coating is similar to that of MM-1 -based one.

Initially, a mixture based on polyvinil alcohol (PVA) was used for eliminating the consequences of the Chernobyl accident /9/ . An area of 350 ha (0.5-1.0 l/m²) was coated in May 1986. Meant for inside applications, it proved to be low-resistant to exposure to the atmosphere. In June, the mixture was replaced by a 50% aqueous lignosulfonate-based polymeric solution (SSB) /9-11/.

Also, petroleum residues were partially applied for suppressing dust along Chernobul road shoulders. Later in 1987 the use of latex the disrtucted film of which was readily carried away along with radioactive matters adhered to it by wind and petroleum residues which negatively affected vegetation was minimized.

Even them it was evident that SSB and latex were not adequate to suppress dust over a long period of time. New IPEC-based MM-1 and MT-1 mixtures were tested in June-July 1986 and 1989, respectively. They were shown to be of high efficiency (Table IV) and atmospheric exposure ¾ resistance for a long time (more than 13 months). These polymers can be deposited manually, by army cars, hydromonitors and helicopters as mach as 1-1.5 l/m². Of aqueous polymeric solution (2-4%) delivered to the soil is adequate.

The complexes developed have passed laboratory and field tests in the site of the Chernobyl NPP. The behaviour of complexes and protective coatings has been studied under laboratory conditions at negative temperatures down to minus 40^OC. The protective coatings are shown to retain their properties at negative temperature as well as after thawing. The protective coatings microstructure investigations by optical microscopy have shown that no changes occur when the freezing-thawing cycle is repeated many times.

The MM-1 composition was tested in eliminating consequences of the Chernobyl accident in 1986-1990 (over 5000 tons) for contaminated soils stabilizing. MT-1 was also put to Chernobyl tests and recommended for further application.

Performance of IPECs is specified in Table V.

The Chernobyl experience with dust suppression revealed that the equipment available was not adequate to get a uniform protective coat /12-14/. Besides, for effective and long-term stabilization a sod should be used. In this context an integrated stabilization technique has been developed to apply MM-1 or MT-1 and perennial herb seeds on topsoil at a time. The technique was successfully employed for confinement of contaminated soils at Chernobyl. In collaboration with VNIIvodpolymer a sower-sprinkler prototype PPME-8 has been developed for applying more uniform coats concurrent with herb seeds to reduce aerosol airborne contamination and tested to advantage. The prototype made around a tractor T-150K is equipped with a tank, a pneumatic seed drill and a sprayer ( Fig 2).

The both polymeric compounds were successfully demonstrated over the Aral Sea area in the ecological distress. They were helpful in reducing highly-saline sand transfer from dried-up topsoils of the Aral Sea. With saline soils, MT-1 showed its superiority over other IPEC compositions in stability.

• as effective dust suppressing compounds;
• to localize small-dispersed high-toxic substances, including radioactive ones;
• to immobilize powder materials;
• as effective means against the wind and water soil erosion;
• as agents for better vegetation, especially in arid districts;
• dust setting in open-out mines and waste piles
• pyrifying industrial wastewater.

We are thankful to the Moscow State University team under the supervision of professor A.B.Zezin for their help in synthesizing IPECs.

The study is now financially supported by the International Science and Technology Centre (ISTC Project # 589).

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