Keywords

12.1 Introduction

Assorted anthropogenic activities are major sources of radionuclides in the environment. Establishments like nuclear fuel cycle enterprises, nuclear reactor facilities, and nuclear weapon tests explosions result in pollution of radionuclides especially in soil and adjoining water bodies (Gupta et al. 2016). Once contaminated, the radionuclide can migrate far away distance from the release source and expanding the contaminated zone radioactively (Gupta and Walther 2014). Further, radionuclide can enter into the animal food chain as a result of consumption of contaminated food grown in the area irrigated with water having radionuclide(s) (Walther and Gupta 2015). Effective, appropriate and eco-friendly technologies are required to mitigate the problem of radionuclide contamination in natural water. Natural materials can be used which may help in rehabilitation and reduction in migration of radionuclides execution green technological concept. Among green sorbents for radioactive pollutants removal various materials are present like activated carbons , cellulose, lignin, humic acids and phytoplankton. This chapter encompasses the up-to-date information on the development of green sorbents and their use for removal of radioactive wastes. It gives the main sorption characteristics of a number of sorbents based on chemically modified natural and technical cellulose with respect to yttrium, strontium, uranium, cesium and some other radionuclides .

12.2 Radioactive Contamination of Natural Waters

Radioactive contamination of the environment is an important part of contamination affecting the quality of the environment and public health. In general, natural and artificial (anthropogenic) radionuclides are two types of radionuclides in the environment; however, some radionuclides such as tritium (3H) and radiocarbon (14C) have both natural and anthropogenic sources of release to the environment. According to the recommendations of the International Commission on Radiological Protection (ICRP 2007), approaches to control of radioactivity in the environment should be the same for both natural and artificial radionuclides .

Natural radioactivity in the environment is presented as long-lived primordial radionuclides (40K, 87Rb, 238U, 235U, 232Th and some others) (Gupta et al. 2014, 2016), and cosmogenous radionuclides (3H, 14C, 7Be). However, uranium, thorium and their daughter nuclides (mainly, 226Ra, 228Ra, 210Po and 210Pb) contribute more than half of annual internal dose of humans (Gupta et al. 2014). Typical natural waters containing elevated activities of these nuclides are ground waters, especially near uranium/thorium deposits (Lauria and Godoy 2002; Kozłowska et al. 2007; Onishchenko et al. 2010) and mining sites (Chałupnik et al. 2017; Lourenço et al. 2017).

Among the main anthropogenic radionuclides occurring in the environments, there are fission products (129, 131I, 90Sr – 90Y, 134, 137Cs, 106Ru, 99Tc), transuranium elements (long-lived isotopes of Np, Pu, Am and in a less degree heavier elements) and activation products (3H, 14C, 60Co, 55Fe, 56Мn, etc.). Nuclear tests in 1950s–1960s resulted in release of 2∙1020 Bq to the environment (Choppin 2006); global fallout still remains to be the dominant source of anthropogenic radionuclides in the environment (Povinec et al. 2004). This type of radioactive contamination is very intensive at the former test sites (Vintró et al. 2009). Another one important source of anthropogenic radionuclide in the environment is daily work of plants for plutonium production and irradiated nuclear fuel reprocessing such as La Hague in France, Sellafield in the Great Britain, Savannah River Site in the USA (Beals and Hayes 1995) and Mayak PA in Russia (Shutov et al. 2002; Shishkina et al. 2016) and some others. Finally, the third main source is nuclear accidents at nuclear power plants and enterprises of nuclear fuel cycle. Two most significant accidents are Chernobyl disaster in 1986 resulted in a world-wide contamination of rivers (Vukovic et al. 2006), seas (Polikarpov et al. 1991) and groundwater (Roux et al. 2014) as well as Fukishima Dai-ichi accident in 2011 (Ueda et al. 2013; Fuma et al. 2017). Release of radionuclides after Chernobyl disaster was estimated to be 7∙1017 Bq; approximately 90% of this activity was presented as the short-lived 131I, whereas 137Cs activity was near to 7∙1016 Bq (Myasoedov 1997). Release at Fukushima Dai-ichi was near to 10% of Chernobyl release.

Besides radioactive decay, radionuclides may be eliminated from natural water bodies due to natural processes, such as (co)precipitation, sorption, and evaporation (for radon in surface waters). For example, Simpson et al. (1982) reported that fallout strontium-90 has been largely removed from the lake water in California, probably as a result of co-precipitation with calcium carbonate. Also, it is well-known that cesium is intensively sorbed by natural clayey mineral such as zeolites (Chesnokov et al. 1999, 2000); whereas plutonium interacts with oxide minerals (Romanchuk et al. 2016). It is reported that 60Co is sorbed by bottom sediments (Murray and Mayer 1986) presenting predominantly in hydrophilic fraction of sediments (Caron and Mankarios 2004). The rest part of radionuclides may be intercepted by plants (Hirono and Nonaka 2016) and animals (Kennamer et al. 2017) and thus be involved into migration through food chains. Finally, the radionuclides occurring in water and living creatures may be consumed by humans causing the exposure of internal irradiation doses. Thus, elimination of radionuclides from drinking water and prevention of their migration through food chains is a very important task from the point of view of radiation safety and public health.

12.3 Role of Green Technologies in Rehabilitation of Radioactively Contaminated Landscapes

Solving the problem of radiation safety to the population and the environment may be possible as a result of use of effective and eco-friendly methods for rehabilitation of radioactively contaminated natural environment and water bodies (Shao et al. 2014; Olszewski et al. 2016). Green technologies based on natural materials became a significant part of these methods. Green sorbents can be obtained from organic components of natural ecosystems; therefore, these sorbents meet all ecological requirements. They possess a number of properties being important for decontamination of aqueous media, such as hydrophilic surface, suitable pores structure, functional groups on the surface, mechanical and chemical stability .

Natural organic raw materials and phytogenic farm wastes are the most attractive materials for green sorbents production (Bhatnagar and Sillanpää 2017). Sorption materials based on vegetable substance and wood waste such as sawdust, wood chippings, etc. may possess successful mechanical, chemical and radiation stability, very low cost and the possibility of further volume decrease by pyrolysis. Wood and such wood components as cellulose, lignin and their derivatives are also relatively low cost sorbents (Hokkanen et al. 2016). Production of these sorbents requires low capital investment and it can be organized at any region that would allow for decrease of transport charges.

Activated carbons , waste of food industry, natural and technical cellulose, lignin, phytoplankton and humic acids are used as green sorbents for various radionuclides separation from various aqueous media. Basic properties and examples of use of these green sorbents are described below.

12.4 Green Sorbents in Radioactive Pollutants Removal

12.4.1 Activated Carbons Based on Vegetable Materials

Activated carbons are the most widely used sorption materials among all green sorbents. These porous carbonic sorbents may be obtained using various raw materials such as turf, coal (Liu et al. 2014; Zhong et al. 2016), wood (Vanderheyden et al. 2016), wood waste (Caccin et al. 2013), etc. Classical methods of activated carbons production include primary heat of the raw material without air access resulting in elimination of volatile compounds and formation of macroporous structure of the material. Classification of pore is as following: micropores have the size up to 2 nm, mesopores are 2–20 nm and macropores have the size more than 20 nm. After this, the carbon is activated through oxidation by oxygen , carbon dioxide or water vapor at the temperature of 800–900 °С (Voronina et al. 2010):

$$ \mathrm{C}+{\mathrm{O}}_2\to {\mathrm{CO}}_2 $$
$$ 2\mathrm{C}+{\mathrm{O}}_2\to 2\mathrm{C}\mathrm{O} $$
$$ \mathrm{C}+{\mathrm{H}}_2\mathrm{O}\to \mathrm{C}\mathrm{O}+{\mathrm{H}}_2 $$
$$ \mathrm{C}+{\mathrm{CO}}_2\to 2\mathrm{CO} $$

The activation process results in formation of micropores. Thus, as a result of this two-stage treatment, porous substances with sponge-like structure are obtained. The nature of raw material and the synthesis method strongly affect pore size of the activated carbon. Furthermore, activation of a carbon results in formation of oxide compounds closely associated with the carbon surface. After contact with water, these compounds are transformed into carboxylic or hydroxyl functional groups conditioning cation and anion exchange properties of activated carbons .

Activated carbons possess relatively low ion exchange capacity and almost do not sorb ionic forms of radionuclides. However, they can be used for separation of radionuclides being associated with organic matter and colloidal particles. Activated carbons are used for elimination of 131I, 90Sr, 90Y, 137Cs, and 106Ru from natural waters. Easy peptization and low chemical stability are the main disadvantages of activated carbons. Cation exchange activated carbons based on coal, wood and turf after treatment by concentrated sulfuric , phosphoric and some other acids possess elevated chemical stability in alkaline solutions (Voronina et al. 2010, 2015a). Acidic treatment allows for formation of additional ion exchange functional groups on the surface of the activated carbon. For example, sulfuric acid treatment results in appear of sulfate ion exchange groups. Thus, change of ion exchange properties, chemical and mechanical stability of the carbon occurs as a result of sulfuric acid treatment .

Use of prospective raw materials and new methods of chemical activation for production of activated carbons allows for increase of porosity, selectivity and exchange capacity of these products. Moloukhia et al. (2016) described the method of synthesis and physicochemical properties of activated carbons based on coconut shells. The method allows for introduction of new ion exchange functional groups, increase of surface area and pores volume of the activated carbon. Coconut shells are preliminary treated by 20% H3PO4 in order to improve texture of the surface and increase its porosity . After this, carbonizing is performed by heating this paste at the temperature of 500 °С for 3 h with the heating rate of 10 °С per minute (Moloukhia et al. 2016). Chemical modification of the activated carbon based on coconut shells are performed via sequential oxidizing by 4М H2O2 and 6М HNO3 solutions. The activated carbon shows a high affinity for hydrolyzable ions Eu3+ and Се3+, but a significantly lower affinity for Sr2+ and Cs+. In addition, the results have shown a low selectivity of the activated carbon in presence of Na+ ions.

Ershov et al. (1993a) suggested a method for synthesis of a sorption material from wood sawdust via soft oxidation of carbonized sawdust . The first stage of synthesis includes sawdust carbonizing in a rotating pipe furnace at the temperature of 300–350 °С for 2 h. The second stage includes oxidation by air at the temperature of 200–250 °С for 2 h. After this treatment, a new ion exchange material, oxidized wood carbon was obtained. Its total ion exchange capacity determined by KОН solution is ~5.0 mg-eqv/g; content of strong acid carboxylic groups, weak acid groups and phenolic groups is 2.0, 2.7 and 0.3 mg-eqv/g respectively (Bykov 2011). Ershov et al. (1993b, c) and Bykov (2011) studied physicochemical properties of sorption-active materials based on modified phytomaterials using modern methods of analysis; the authors determined content of ion exchange groups and studied the kinetics of sorption of cesium, strontium , uranium, americium, thorium, plutonium and technetium from various aqueous solutions .

It is shown that oxidized wood carbon possess good sorption properties; distribution coefficients and total exchange capacities of oxidized wood carbon for cesium, thorium, americium and plutonium are higher than these values obtained for untreated sawdust and birch activated carbon. Bykov (2011) reported the following distribution coefficients obtained for oxidized wood carbon (L/kg): Cs+ − 5.4·103, UO22+ − 3.5·102, Am3+ − 5.8·103, Pu4+ − 1.8·103, Th4+ − 2.1·103. The oxidized wood carbon was tested on both simulated solutions and real groundwater samples from influence zone of Chernobyl nuclear power plant. As it is reported, Kd values of radionuclides 90Sr, 137Cs, 243Am and 239Pu obtained on the aqueous extract from a soil sample from radioactive waste storage near Buryakovka settlement are 7, 5.2·103, 1.1·103 and 1.2·104 L/kg respectively. The data obtained on simulated solutions coincided with those obtained on the real samples of radioactively contaminated natural water .

The last but not the least one important example of activated carbons use for radionuclide separation is connected with radon elimination form radon-rich underground waters (Turtiainen et al. 2000; Alabdula’aly and Maghrawy 2011; Karunakara et al. 2015; Guo et al. 2017). Due to an extended surface area, activated carbons can successfully separate isotopes of radon due to absorption mechanism . This is mainly for the most long-lived isotope 222Rn with the half-life of 3.83 days. Relatively low exchange capacity of activated carbons is not a problem in this case because of a very low concentration of radon in water. For example, even 10,000 Bq/L of 222Rn corresponds to its concentration of as low as 8∙10−15 mol/L. The sorption method is very effective; it provides 95–100% elimination of radon . As compared with physical methods of radon elimination from drinking water such as air bubbling, boiling, etc., the main advantages of absorption by activated carbons are additional elimination of radon short-lived daughter radionuclides (214Pb, 214Bi) and prevention of radon transfer to air (Semenishchev et al. 2017). As an example, in our experiment, we have studied 222Rn sorption from water by four types of activated carbons. In this experiment, 5 L of natural water from a spring near Ekaterinburg city (Russia) containing nearly 100 Bq/L of 222Rn was passed through an 18-mm column containing 20 g of an activated carbon at the flowrate of 100 mL/min. The results are given in Table 12.1. The Carbotech (Germany) activated carbon showed the best results; however, in this case other types of activated carbon were also enough to decrease the radon level down to values less than the maximal permissible level (60 Bq/L).

Table 12.1 The degrees of radon sorption by certain types of activated carbon . It can be noted that Carbotech (Germany) activated carbon showed the best selectivity for radon
Full size table

12.4.2 Chemically Modified Waste of Food Industry

Annually, renewable, large-tonnage and cheap wastes of food industry can be used as sorbents for radionuclides separation from various aqueous solutions . Velichko and his research group obtained a number of phytosorbents based on malt sprouts (PS-710, PS-744, PS-745 and PS-761) and on sawdust (PS-768). The method of synthesis included high-temperature treatment of raw materials by a solution containing carbamide, dimethyl formamide and phosphoric acid. Use of the phytosorbents based on sawdust, malt sprouts and barley husk for heavy metals and radionuclides separation from various types of natural water is described in a number of publications (Velichko et al. 2002; Medvedev et al. 2003; Likhacheva 2005). The authors have found radionuclides distribution coefficients from water of Irtyash Lake and industrial water bodies of the Mayak PA. It was shown that the chemical composition of water significantly affect sorption properties of the sorbents. As reported, values of distribution coefficients of 137Сs, 90Sr and uranium under various conditions are n·102–103 and n·103 L/kg respectively. The PS-728 sorbent (modified sawdust) showed the best sorption properties for strontium and uranium; whereas, the PS-761 (based on malt sprouts) was the most effective for cesium sorption.

12.4.3 Sorbents Based on Natural and Technical Cellulose

Sawdust, having huge functional surface, which can sorb ionic and colloidal forms of radionuclides as well as radioisotopes being sorbed on fine suspensions (Voronina et al. 2010). The main advantage of sawdust use for radioactive water treatment is the possibility of its further pyrolysis resulting in a multifold decrease of volume of radioactive waste. The weight of sawdust after pyrolysis decreases by 40 times or more. Radioactive isotopes are strongly fixed on the surface of the ash that makes further radioactive deposition easier. However, some radionuclides may evaporate during the pyrolysis process resulting in air radioactive contamination. Prevention of this secondary contamination requires the use of special complex equipment. Ion exchange capacity of the cellulose is usually low; therefore, use of cellulose is reasonable in case of diluted solutions with low concentrations of the main components. It is possible to improve ion exchange properties of the cellulose due to either chemical oxidation or introduction of various functional groups (Wang et al. 2017). Sorbents based on cellulose may appear as fibrous granulated materials as well as textile (acetyl cellulose). Use of textile allows for more rapid sorption kinetics as compared with fibrous granulated materials .

Thin-layer inorganic sorbents (TLIS) were synthesiszed by chemical deposition of thin films of sparingly soluble compounds onto the surface of natural cellulose (sawdust) (Betenekov et al. 1976). TLIS retain improved sorption and kinetic properties with respect to radionuclides due to increase of internal diffusion rate. Sorption properties of selected TLIS are given in Table 12.2.

Table 12.2 Distribution coefficients (Kd) and rate constants (βc1 and βc2 for the first and the second sections of kinetic curves ) for various radionuclides from seawater by TLIS based on wood cellulose
Full size table

Bykov and Ershov (1996) obtained a sorbent with a high phosphor content (16.5% wt.) and high affinity to uranium as a result of wood phosphorylation. The maximal value of Kd for uranium (1.9·104 L/kg) was reached at pH 2–3; for Tc(VII), the maximal Kd value at pH 9 is reported to be 75.9 L/kg. The phosphorylated wood is not selective for cesium and strontium radionuclides ; therefore, sorption of these radionuclides is possible only in absence of Na+, Н+ and Сa2+ ions.

Besides wood cellulose and products of its modification, technical cellulose being obtained from waste of food industry can be used as a sorbent. Minakova (2008) described a method for synthesis of technical cellulose from waste of corn production. Sorption properties and exchange capacity of this cellulose may be improved by chemical modification. The method for synthesis of surface-modified sorbents based on various sorption-active supports (hydrated titanium and zirconium dioxides, aluminosilicates and, technical cellulose, etc.) was developed by Voronina et al. (2012, 2015b, 2017; Voronina and Nogovitsyna 2015). Voronina et al. (2013) studied sorption properties of untreated and chemically modified samples of the technical cellulose with respect to 137Cs, 90Sr and 90Y radionuclides on spiked tap water at static and kinetic conditions . Table 12.3 presents distribution coefficients of various radionuclides sorption from tap water by untreated and chemically modified technical cellulose obtained waste of corn production.

Table 12.3 Distribution coefficients of various radionuclides sorption from tap water by technical cellulose and surface-modified sorbents based on cellulose
Full size table

The data from Table 12.2 show that sorption properties of the technical cellulose depend on the type of vegetable raw material : the cellulose obtained from straw shows higher distribution coefficients as compared with the cellulose obtained from husk. However, selectivity of untreated cellulose for cesium does not exceed 102–103 L/kg. Thermal treatment of the technical cellulose in presence of ozone or H3PO4 results in increase of Kd values by the order of a magnitude due to incorporation of additional function groups into the sorbent. Furthermore, the results showed that the technical cellulose cannot sorb 90Sr; however, it can sorb its daughter radionuclide 90Y with distribution coefficients as high as 103–104 L/kg.

Carboxylic groups without hydrogen bonds among them are the main sorption-active functional groups of cellulose materials. The value sorption capacity of the cellulose (0.011–0.1 mg-eqv/g) is conditioned by the quantity of the carboxylic groups in the polymer matrix; this quantity is affected by the quality of decontamination from organic and inorganic impurities as well as by the method of thermal treatment (Nikiforova et al. 2009).

Furthermore, the technical cellulose has a developed fibrous surface that gives a good possibility for sorption of radionuclides presenting as colloidal particles . This is a quite important possibility because Voronina et al. (2015b) reported that the part of colloidal fraction of 137Cs in spiked samples of tap water varied from 10% in spring to 50% in autumn. This difference is conditioned by various chemical composition of tap water at various seasons .

Carboxylic groups are weak acid cation exchange functional groups; therefore, they can dissociate at neutral and alkaline pH ranges. Figure 12.1 shows the pH dependences of cesium sorption by the technical cellulose obtained from waste of corn production. It is obvious that degree of cesium sorption by the samples of the technical cellulose is maximal at the pH range of 5–9.

Fig. 12.1
figure 1

pH dependences of cesium sorption by cellulose from oat husk (a) and cellulose from rice husk (b). It can be seen that unmodified cellulose shows a relatively low selectivity for cesium and works mostly at neutral and slight alkaline media

Full size image

The results showed that carboxylic groups provide very low selectivity for radionuclides . Sorption of radionuclides by the technical cellulose occurs mainly due to retention of their colloidal forms that is confirmed by the experiments on yttrium and cesium sorption. Yttrium shows more affinity to colloids formation than cesium; therefore, cellulose sorbs yttrium better than cesium. By contrast, the technical cellulose does not sorb strontium presenting in a solution only as ionic forms.

Chemical modification of the cellulose with obtaining nickel-potassium ferrocyanide allows for introduction of new sorption sites resulting in a significant increase of cellulose selectivity for cesium. Mixed nickel-potassium ferrocyanide based on cellulose from rice husk showed the best sorption properties; for this sorbent , cesium distribution coefficient was found to be as high as 3.9·105 L/kg (Voronina et al. 2013).

The relationships described above, agree well with the data obtained by Minakova (2008). The technical cellulose from rice husk possess a low degree of crystallinity (25%), extended absorbency (150 g/m2) and a high water retention (280%); due to these properties, this cellulose is easily saturated by modifying solutions and therefore, its modification occurs more effectively. As compared with other types of cellulose, the cellulose from rice husk contains more amorphous clusters that are probably conditioned by special vegetation process and morphological structure of rice. High content of amorphous clusters affects easy penetration and retention of aqueous solutions.

Thus, the cellulose from waste of food industry modified by nickel-potassium ferrocyanide possesses excellent sorption and kinetic properties ; it can be successfully used for rehabilitation of natural water bodies and decontamination of natural waters and technogenic solutions containing 137Cs.

12.4.4 Hydrolytic Lignin

Hydrolytic lignin is a solid by-product of hydrolysis of wood and some other vegetable materials. It consists of various chemical organic compounds. Hydrolytic lignin contains approximately 10–11% of methoxylic, 5–6% of carboxylic and 3% of phenolic functional groups in terms of the absolutely dry product (Chudakov 1983).

Nikiforov and Yurchenko (2010) showed that distribution coefficient of 90Sr by hydrolytic lignin at distilled water is ~ 12 ± 1 L/kg; whereas, for lignin treated by concentrated sodium carbonate, ammonium hydroxide and sodium hydroxide solutions, Kd values increased up to 417 ± 21, 353 ± 19 and 4532 ± 124 L/kg respectively. In addition, these authors studied the effect of Na+ and Ca2+ concentrations on 90Sr sorption. It was shown that distribution coefficient of 90Sr in 0.01 M NaCl and CaCl2 solutions decreased by factors of 3 and 40 respectively as compared with normal Kd values obtained in distilled water .

Sorption of Ra, Th and U by hydrolytic lignin from solutions with a composite chemical structure was studied by Rachkova et al. (2006). Based on the achieved results, the method of rehabilitation of radioactively contaminated soils using hydrolytic lignin was suggested (Rachkova and Shuktomova 2008).

As other green sorbents, hydrolytic lignin can be chemically modified in order to increase its selectivity for radionuclides. For example, the sorbent based on phosphorylated lignin with high phosphorus content (16.6% wt.) was obtained for actinides sorption. For this sorbent, uranium and thorium distribution coefficients are 9.7∙103 and 3.0∙103 L/kg respectively (Bykov and Ershov 2009, 2010).

12.4.5 Living and Dead Phytoplankton

In natural water bodies, phytoplankton participates in radionuclides sorption and natural decontamination of water due to radionuclides transfer from water to bottom sediments; thus, phytoplankton may be considered as a prospective biosorbent . The effectiveness of biological mechanism of radionuclides interception and transfer from the surface to lower layers was mentioned by Warner and Harrison (1993). Vertical distribution of plutonium in seawater is conditioned by on the one hand weight and density of plutonium-containing particles and on the other hand interception of plutonium by phytoplankton living at a certain depth (Romanchuk et al. 2016). Maximal plutonium concentrations were found at the depth of 100–177 m.

Both accumulation factor and distribution coefficient may be used for quantitative characterization of a biosorbent. The phytoplankton shows higher accumulation factors in fresh water as compared with seawater due to rather lower concentration of mineral salts. For example, accumulation factor of 90Sr by algae from seawater is 20, whereas for fresh water this value is 5⋅105; accumulation factors of iron by fish is 105 and 103 in river and ocean water respectively (Voronina et al. 2010). Phytoplankton accumulates activation radionuclides (65Zn, 60Co, 55Fe, 56Мn) better than fission products .

Polyakov et al. (2016) studied the difference of sorption behavior of living and dead freshwater plankton. Sorption of Sr, Ba, Mn, Fe, rare earth elements, Th and U is studied in this work. It is shown that for almost all elements (excluding Mg) sorption by living plankton is more intensive and with higher values of distribution coefficients than by dead plankton; the reverse dependence is observed for magnesium. For these radionuclides, distribution coefficients vary from 103 to 105 L/kg. It is also shown than living plankton has a higher sorption capacity as compared with dead plankton .

12.4.6 Humic Acids and Their Derivatives

Humic acids are natural organic compounds with unstable structure and composition. They contain many types of ion exchange functional groups such as –CОOH, –OH, –OCH3, etc. that can exchange hydrogen by alkaline and alkaline earth cations. In soils, humic acids usually present as calcium, magnesium, ammonium and sodium humates rather than free acids. These organic compounds affect radionuclides behavior in underground waters (Geckeis et al. 2011). Humic compounds can affect the fate of radionuclides in the environment as well as possibility of their separation from waters and soils. The presence of humic acids in groundwater may cause either increase of decrease of radionuclide mobility due to formation of complex compounds (Koopal et al. 2001). The type of this influence depends on chemical properties of a radionuclide .

Covering the surface of mineral particles, humic acids may block their sorption sites. In this case, 137Cs may increase its mobility because it is strongly sorbed by clayey minerals, whereas stability constants of cesium humates are very low (Lofts et al. 2002; Voronina et al. 2010). Helal et al. (2007) reported that Cs+ forms weak complex compounds with humic acids and does not bind with fulvic acids. No similar dependence is observed for strontium. By contrast to cesium, mobility of plutonium significantly decreases in presence of humic acids because plutonium interaction with humates is more preferable than sorption on clayey minerals.

Use of humic acids as sorbents is very complicated because of their high solubility in aqueous solutions at pH > 3.5 (Guthrie et al. 2003; Baker and Khalili 2003). It is experimentally shown that humic acids are nonselective sorbents. Precipitates of humic acids can sorb thorium (IV) and uranium (VI) ions with distribution coefficients Kd ~ 103–104 L/kg; for strontium, aluminum and iron (II) ions distribution coefficients are found to be ~103 L/kg (Polyakov 2007; Volkov 2015). Distribution coefficients of chromium (III), manganese (II) and alkaline earth metals significantly decrease down to 100−102 L/kg with the increase of concentration of humic acid in the solution; this can be explained by formation of complex compounds of these cations with free humic acid (Volkov 2015).

Presence of humic acids in soil solutions results in an increase of Pu and Am leaching from soils by a factor of 4–8, depending on the type of a soil (Goryachenkova et al. 2009). Pu and Am distribution in various groups of humic and fulvic acids with different molecular weight and size were studied. Understanding the regularities of the actinides distribution in fractions of humic acid makes it possible to affect actinides mobility: decrease of actinides mobility may be achieved by controlling the dispersed state of humic acids in aquifers .

Humic matter possesses reducing properties; therefore, they are able to reduce some actinides from the highest oxidation states to the tetravalent or pentavalent state. For example, Np(VI) and Pu(VI) may be reduced to Np(V) or Np(IV) and Pu(V) or Pu(IV) respectively (Sapozhnikov et al. 2006; Shcherbina et al. 2007). Plutonium forms various complex compounds with humic and fulvic acids. These reactions can be described as either interaction with a functional group or interaction with the whole macromolecule. Humic acids can form organomineral aggregates with the size of 60–500 nm; behavior of these particles is an average between behavior of soluble complex compounds and colloidal particles. As it is shown by Romanchuk et al. (2016), plutonium complexes with organic compounds may exist as dissolved forms or colloidal particles. Thus, actinides binding into soluble humate complex compounds should affect actinides behavior in the environment. Furthermore, humate complex compounds are available for interception by plants; thus, plutonium can be involved into migration in food chains.

Humic acids can affect the sorption properties of natural and synthetic inorganic sorbents with respect to radionuclides. This affect is intensively reported by examples of Pb(II) sorption by goethite (Orsetti et al. 2006), Co(II), Sr(II) and Se(IV) sorption by goethite (Masset et al. 2000), Np sorption by goethite and hematite (Khasanova et al. 2007), U(VI) and Np(V) sorption by kaolinite (Niitsu et al. 1997; Křepelova et al. 2006), Th (IV) sorption by bentonite (Xu et al. 2006). Polyakov et al. (2015) studied sorption of Cs (I), Мg (II), Са (II), Al (III), Co, Ce (III), Th (IV) and U (VI) by iron (III) hexcyanoferrate (II). Malakhov and Voronina (2016) reported the effect of presence of humic acids on sorption of Сs (I) and Sr (II) by clinoptilolite, glauconite and nickel-potassium ferrocyanide based on these minerals .

Perminova et al. (1996) showed the possibility of synthesis of humic acids derivatives with required properties via chemical modification of natural humic acids. These derivatives can be used for radionuclides elimination from contaminated underground waters. Pumping the soluble humic acids derivatives into a water bearing stratum through a system of wells allows for creation of a geochemical barrier without soil extraction; this should reduce the cost of activities for decontamination of radioactive underground water (Kalmykov et al. 2005; Shcherbina 2009).

12.5 Conclusion and Future Prospective

The problem of radionuclides elimination from large volumes of natural media (soils, natural waters with various salt contents including drinking water) cannot be solved using synthetic inorganic sorbents because of their high cost and low affinity to the environment. In spite of all advantages of synthetic inorganic sorbents (high Kd values, selectivity and exchange capacity), these sorbents find a limited applications. Synthetic sorbents may become a potential secondary source of the environmental pollution by chemical compounds due to destruction of the sorbents after a long-term contact with various aqueous solutions . Using these sorbents is possible only after the study of their chemical stability at the conditions similar to natural; the study of their toxicity and carcinogenicity should be also performed.

Sorption methods based on use of natural or chemically modified natural materials may become an eco-friendly way for decontamination of natural ecosystems from radionuclides and decrease of radionuclides migration in the environment. For example, natural inorganic sorbents (zeolites, etc.) have been already widely used for decontamination of radioactively contaminated natural waters and soils . Green sorbents attract some practical interest and have certain perspectives. Among the green sorbents, there are old and reputed types widely used for water decontamination, such as activated carbons . In addition, some new materials were developed and studied for last decades. New methods of chemical modification of vegetable raw materials allow for improvement of physicochemical and sorption properties of these materials. These green sorbents show successful chemical stability, and selectivity comparable with that of natural inorganic sorbents. In contrast to inorganic sorbents, the natural organic materials allow obtaining sorbents with various textures such as granules, fibres and textile. Due to these various textures, green sorbents may be successfully used in both static and kinetic regimes of sorption . A complex approach combining use of natural organic and inorganic sorbents will allow improving the quality of the environment due to decrease of radioactive contamination .