**4. Increase/decrease of radionuclide mobility as essential soil remediation strategy**

**3.3. Factors influencing radionuclide mobility in the soil**

262 Soil Contamination - Current Consequences and Further Solutions

of carbonates and have a high saturation of base cations (K+

utes to the soil CEC and to the water‐holding capacity.

**Chemical form Cs+ Sr2+ PuO2**

pH decrease Increase Increase Increase Clay content decrease Increase Increase Increase

Humus content low Not clear Decrease Decrease CEC decrease Increase Increase Increase Aging Decrease Weak effect Decrease

of some important pollutants is given in **Table 2**.

acidity in soils comes from H+

A capacity of the soil itself to immobilize radionuclide is the main factor controlling activity concentrations available to biota, and it operates in conjunction with the numerous external factors. Soil texture and structure, mineral composition, organic components, redox potential (Eh) and pH, as well as rainfall, climate changes, and soil management, are recognized as important for radionuclide mobility [54]. The pH of the soil, cation exchange capacity (CEC), and total organic carbon (TOC) are the physicochemical characteristic most often correlated with the distribution of the radionuclides [40]. Alkaline soils are characterized by the presence

surface charge of minerals is a major contributor to soils CEC and influences the soil's ability to retain important nutrients and the pollutants. The texture of a soil is based on the relative content of sand (0.05–2.00 mm), silt (0.002–0.05 mm), and clay (<0.002 mm) fraction. Due to the finest granulation, clays minerals exhibit the largest surface area, important for soil chemistry and CEC, but also for water‐holding capacity important for transporting nutrients and pollutants to soil organisms and plants. In addition, soil organic matter significantly contrib‐

Based on the literature data, the influence of soil properties and other condition on the mobility

**Radionuclide**

Sand content decrease Decrease Decrease Decrease Increase

**Table 2.** The effect of soil physicochemical properties and aging on the mobility of radionuclides [55, 56].

Apart from soil type, different sources of variability may influence the fractionation patterns and cause the shift from less available to more available fractions, or vice versa. Generally, the increase of contaminant concentration not only increases the overall activity in the soil but also leads to redistribution from the less to the more available fractions [57]. Radioactive contam‐ ination introduces new elements into the ecosystem and, in distinction from the transport of stable elements and NORM, transfer of contaminants through the trophic chains occurs under non‐equilibrium conditions. Consequently, ageing affects a decrease in the chemical mobility

**Mobility**

**Cs Sr, Ra U, Pu I**

, Ca2+, Mg2+, and Na+

**2+, Pu(NO3) 3+** **I2, I− , IO3 −**

**CH3I**

and Al3+ ions in the soil solution and sorbed to soil surfaces. The

), whereas

As the environmental conditions change, the distribution of pollutant also changes, causing the increase or the decrease in mobility. Knowledge of such dependencies represents the theoretical background for the development of mobilization/immobilization remediation methods. Furthermore, exploration and development of suitable solid and liquid media are fundamental in support of these technologies. Mobilization techniques imply weakening of bonds with the soil constituents provoking desorption, dissolution, and chalation of the pollutant [61, 62]. On the other hand, the general idea of the radionuclide immobilization (stabilization) is to induce chemical reactions, precipitation, and other processes which cause redistribution of the contaminants from more labile to more stable forms [61, 63]. Both principles exhibit certain benefits and drawbacks. Stabilization techniques are usually less expensive and easier to perform in comparison with the alternative processes; however, the total activity concentrations remain in the soil, posing a constraint for the future uses. Other‐ wise, the techniques based on the pollutant exclusion from the soil matrix represent a perma‐ nent solution for the contaminated site. However, transportation, consumption of the chemicals and the energy, and further management of the resulting liquid phase with the extracted pollutants, make these techniques complicated and costly. Remediation activities may also result in some negative effects on the soil properties, including fertility; thus, evaluation of suitable strategies and decision‐making process require detailed knowledge of all these aspects.

#### **4.1. Extraction of radioactive contaminants from the soil matrix**

Chemical extraction is the technique that stimulates the redistribution of contaminants from the solid phase to the solution, in order to selectively remove the contamination, or to enhance its physical separation [61, 64]. The contaminated soil is excavated and treated off‐site. After

the treatment, the soil is returned to its original location, while the activity remains concen‐ trated in the extraction medium. The extract is subsequently treated to precipitate the activity and return the leaching reagents to the process. Otherwise, the extracting solutions can be implemented *in situ*, to increase the radionuclide mobility in the soil and enhance their subsequent uptake by plants (combination with phytoextraction) [65].

Radionuclides in the soil can be re‐mobilized by four principal means [66]: (1) changes in the acidity, (2) changes in the ionic strength of the solution, (3) changes in the soil redox potential, and (4) formation of soluble complexes. To extract the pollutants, acids operate on the ion‐ exchange principle, and by dissolution of soluble soil components. Highly concentrated solutions of inorganic salts displace the radionuclides from ion‐exchangeable sites by mass action, and if implemented at low pH this effect is combined with the effects of acid leaching. Chelating agents solubilize metals through complexation, while redox manipulation aims to enhance solubilization by the change of valence and thus chemical properties. The most common chemical agents are inorganic salts (CaCl2, NaCl), mineral acids (HCl, H2SO4, HNO3), and complexing agents (EDTA, DTPA, oxalate, citrate, etc.) [61, 65, 67].

Selection of the proper chemical extracting reagent is influenced primarily by the radionuclide type, its speciation pattern and the characteristics of the soil. Pollutants that are majorly accumulated in ion‐exchangeable, carbonate, and Fe, Mn oxide fractions are the most suited for the removal by chemical leaching [68]. The soils characterized by low pH, low content of clay, and humic substances are the promising candidates for such treatments [61].

In order to extract the target metal from the soil environment, the strength of the radionuclide‐ chelating agent complex must overcome the strength of the bonds keeping raionuclide attached to the soil surface. The efficiency of EDTA is superior, and it is usually applied at pH 4–8, as the EDTA‐complexes can be re‐adsorbed on soil surface sites at lower pH [69]. In addition to the high price, selectivity of EDTA towards target radionuclides, its recovery and reuse are the major drawbacks. Furthermore, its low degradability can be a persistent problem after the soil treatment. Thus provided that they enable efficient removals of pollutants, and acidic and salt‐containing solutions are more acceptable due to lower environmental impact and the ease of regeneration.

In the comprehensive investigation of appropriate chelating agent for the extraction of vari‐ ous radionuclides, the regressive empirical predictive model was developed as a selection tool [62]. Using as the input variables, the properties of the chelators, various stability con‐ stants, radionuclide distribution, and the soil properties (mineralogical composition, pH, clay content, CEC, etc.), the following adequate chelator for target radionuclide were pro‐ posed: EDTA, DTPA, and nitrilotris(methylene)triphosphonic acid (NTTA) for Ba and Ra; 2‐ aminoethanethiol, EDTA, DTPA, thiobis(ethylenenitrilo)tetraaceticacid (TEDTA), and N‐2‐ acetamidoiminodiacedicacid (ADA) for Pb and Th; whereas iminodiaceticacid (IDA), nitrilo‐triaceticacid (NTA), and ethylenediiminodiacetic acid (EDDA) were suggested for the extra‐ction of Pu and U.

Selective removal of 137Cs and 90Sr from soil poses a problem, due to the lack of suitable complexing agents [61]. Although certain crown ethers form complexes with these cations, due to the toxicity and high cost of such agents, large‐scale agricultural applications are impractical. Solutions of HCl, CaCl2, EDTA, tartaric, and citric acid, with different concentra‐ tions of reagents, were applied to soil artificially contaminated with Sr2+ and Co2+ ions [39]. Due to its predominant association with ion‐exchangeable fraction, Sr2+ ions were efficiently desorbed using Ca2+ or acidic solutions. On the other hand, Co2+, which was largely distributed between carbonate and Fe, Mn‐oxide fractions, was leached most efficiently by complexing agents.

Chemical extraction processes have a large potential in the rehabilitation of the soil that have undergone radioactive contamination and their effectiveness can be additionally improved by optimizing reagent type and concentration, soil/solution ratio, pH, contact time, mixing, and other factors.

#### **4.2. Radionuclide immobilization (stabilization) by soil amendments**

the treatment, the soil is returned to its original location, while the activity remains concen‐ trated in the extraction medium. The extract is subsequently treated to precipitate the activity and return the leaching reagents to the process. Otherwise, the extracting solutions can be implemented *in situ*, to increase the radionuclide mobility in the soil and enhance their

Radionuclides in the soil can be re‐mobilized by four principal means [66]: (1) changes in the acidity, (2) changes in the ionic strength of the solution, (3) changes in the soil redox potential, and (4) formation of soluble complexes. To extract the pollutants, acids operate on the ion‐ exchange principle, and by dissolution of soluble soil components. Highly concentrated solutions of inorganic salts displace the radionuclides from ion‐exchangeable sites by mass action, and if implemented at low pH this effect is combined with the effects of acid leaching. Chelating agents solubilize metals through complexation, while redox manipulation aims to enhance solubilization by the change of valence and thus chemical properties. The most common chemical agents are inorganic salts (CaCl2, NaCl), mineral acids (HCl, H2SO4, HNO3),

Selection of the proper chemical extracting reagent is influenced primarily by the radionuclide type, its speciation pattern and the characteristics of the soil. Pollutants that are majorly accumulated in ion‐exchangeable, carbonate, and Fe, Mn oxide fractions are the most suited for the removal by chemical leaching [68]. The soils characterized by low pH, low content of

In order to extract the target metal from the soil environment, the strength of the radionuclide‐ chelating agent complex must overcome the strength of the bonds keeping raionuclide attached to the soil surface. The efficiency of EDTA is superior, and it is usually applied at pH 4–8, as the EDTA‐complexes can be re‐adsorbed on soil surface sites at lower pH [69]. In addition to the high price, selectivity of EDTA towards target radionuclides, its recovery and reuse are the major drawbacks. Furthermore, its low degradability can be a persistent problem after the soil treatment. Thus provided that they enable efficient removals of pollutants, and acidic and salt‐containing solutions are more acceptable due to lower environmental impact

In the comprehensive investigation of appropriate chelating agent for the extraction of vari‐ ous radionuclides, the regressive empirical predictive model was developed as a selection tool [62]. Using as the input variables, the properties of the chelators, various stability con‐ stants, radionuclide distribution, and the soil properties (mineralogical composition, pH, clay content, CEC, etc.), the following adequate chelator for target radionuclide were pro‐ posed: EDTA, DTPA, and nitrilotris(methylene)triphosphonic acid (NTTA) for Ba and Ra; 2‐ aminoethanethiol, EDTA, DTPA, thiobis(ethylenenitrilo)tetraaceticacid (TEDTA), and N‐2‐ acetamidoiminodiacedicacid (ADA) for Pb and Th; whereas iminodiaceticacid (IDA), nitrilo‐triaceticacid (NTA), and ethylenediiminodiacetic acid (EDDA) were suggested for the

Selective removal of 137Cs and 90Sr from soil poses a problem, due to the lack of suitable complexing agents [61]. Although certain crown ethers form complexes with these cations,

clay, and humic substances are the promising candidates for such treatments [61].

subsequent uptake by plants (combination with phytoextraction) [65].

264 Soil Contamination - Current Consequences and Further Solutions

and complexing agents (EDTA, DTPA, oxalate, citrate, etc.) [61, 65, 67].

and the ease of regeneration.

extra‐ction of Pu and U.

Despite the fact that the main objective of the soil remediation was the removal of the maximum amount of pollution, the major obstacles for the routine application of such an approach are the processing and the disposal of the radioactive waste resulting from the soil clean‐up [70]. The insufficient storage capacities, especially for waste classified as low level, long‐lived, are significant and global problem. As a consequence, immobilization treatments are being rapidly developed, with main goals to reduce the risk of exposure and uptake by biota, and the risk of the spread of contamination.

The application of soil amendments is performed on site (*in situ)* which makes such technol‐ ogies fast, simple, and effective. Alternatively, soil amendments can be applied in *ex situ* process, where soil is firstly physically removed from the site, pretreated, mixed with a stabilizing amendment, and then returned to its original location [71].

As the most of the radionuclides in soil exist in the cationic form, increase in pH, clay content, and CEC lead to an increase in pollutant stability (**Table 2**). Consequently, water‐soluble and water‐insoluble amendments are applied, with a role to modify the environmental conditions in favor of radionuclide stabilization or to directly interact with the contaminants (or both).

In order to raise pH and lower pollutants accessibility to plants, the materials traditionally applied to soil are carbonates, lime, and phosphates [72]. Other soil amendments that are currently in use or are under consideration and verification have been modeled after stabili‐ zation or encapsulation agents (such as cement) used for safe disposal of radioactive and hazardous wastes. Various forms of aluminosilicates, phosphates, carbonates, silicates, oxides, and hydroxides were largely investigated [65, 72]. In general, solid matrices that have shown superior immobilization potential towards radioactive ions in aqueous solutions are suitable for testing in the contaminated soil. Based on the numerous investigations of the sorption affinities and capacities toward variety of radioactive pollutants, the most prominent groups of materials are aluminosilicates [73–80] and phosphates [81–89]. The main operating mecha‐ nisms are quite different for these two groups: while aluminosilicate addition to soil increases the number of sorption sites, phosphate materials, mainly from the apatite group, act through several removal mechanisms (ion‐exchange, formation of specific surface complexes, and structural incorporation of pollutants by co‐precipitation and dissolution/precipitation processes).

Aluminosilicates, primarily clay minerals, and zeolites are inorganic ion‐exchangers with high surface area, which have been conventionally used for water treatment processes, for the treatment of liquid nuclear waste, and for the protection against nuclear waste leaking [79– 90]. Natural zeolites are the framework aluminosilicates, with variable porosity due to which they can selectively capture the ions having an appropriate radius. Zeolites are excellent sorbents of fission products that otherwise exhibit very low affinity for sorption on solid surfaces (such as Cs and Sr isotopes [78, 80]. Clay minerals (montmorillonite, vermiculite) are layered aluminosilicates, in which ion‐exchange is typically associated with cations situated in clay mineral interlayers [72]. The stabilization of Cs+ and Sr2+ contamination in the sandy soils was tested using different synthetic and natural zeolites [91]. With the addition rate of 1%, the maximum reduction of soil‐to‐plant transfer factor of 12.5 for Cs+ and 24.5 for Sr2+ions, was observed, as well as the significant changes in cationic composition and pH of the soil. By comparing the effect of various materials onto Sr2+ immobilization in the soil, zeolite has been identified as the most efficient, followed by bone char, synthetic hydroxyapatite, and phos‐ phate rock [92]. The most of the results have been obtained on the laboratory level or out of small‐scale field applications, while in solving the actual problems of soil contamination, applications are generally connected with Chernobyl and Fukushima disasters.

The other promising group of materials is the phosphate group. Among different soluble and sparingly soluble phosphate bearing materials, hydroxyapatite (Ca10(PO4)6(OH)2, HAP) exhibited superior physicochemical and sorption properties, that is, low solubility in water, high specific surface area, high buffering capacity, and the high sorption capacities towards variety of cationic and anionic pollutants [93]. HAP is by far the most selective to U and Pb, due to the removal mechanism which involve dissolution of HAP and precipitation of thermodynamically more stable Pb and U containing phases [87, 94]. In soil, apatite matrices were highly effective for U uptake; however, the increase of organic matter content influenced the decrease of amendments efficiency [95]. Furthermore, the selectivity and capacity of HAP towards Pu, Co, Ni is very high, moderate for Sr, while low considering Cs and Tc [78, 81–84, 86–88].

Comparing different apatite forms (synthetic, mineral, and biogenic), the product extracted from fish bones exhibited the best sorption properties, due to CO3 2− substitutions, low trace metal concentrations, poor crystallinity, and high microporosity necessary for optimal performance in the field [87]. Giving that this sorbent is produced from the commercial fish industry waste, it is both environmental friendly and cost‐effective for large‐scale operations. However, the bioavailability of essential trace elements was found to decrease at high HAP addition rates (5%), while uptake of As by plants was found to increase after HA treatment [96]. These results demonstrate that HAP application for the remediation of contaminated soil must be optimized and controlled.

In addition to animal bones as the source material for apatite production, many other industrial by‐products, wastes, and recycled materials are being tested as potential soil additives [65, 72]. In order to preserve natural mineral resources and reduce the costs of the immobilization treatments, application of such materials may represent a sustainable alternative. Another benefit comes from the reduction of the amount of accumulated wastes and their impact on the environment. Coal fly ash and bauxite residue (red mud) are mineral, oxide‐based, residues, which exhibit high sorption potential for a range of radioactive pollutants [97–101].

Fly ash has a silt loam texture (<90% of the particles having a diameter of <0.010 mm), and it is composed mainly of aluminosilicate structures, quartz, mullite, hematite, magnetite, and calcite [102]. The pH values of fly ash vary in the wide range 4.5–12.0, depending on the content of sulfur in the parent coal. Fly ash was considered as an additive in agriculture, for improving soil properties [102], and also as an additive for stabilization of heavy metals in polluted soil, with the promising results [103–105].

Red mud is by product obtained after bauxite processing, which primarily consists of Fe, Al, Si, and Ti oxides and zeolite‐like minerals [106]. Due to the nature of Al extraction process, this material exhibits extremely high pH (10–12), and it is high capacity sorbent especially for pollutants in cation form. Numerous laboratory, pot, and field studies were conducted in the past years regarding red mud utilization in remediation of heavy metal polluted soils, and its potentials (both as a liming additive, and as a sorbent) have been demonstrated [107]. However, radionuclides, as pollutants, have gained much less attention and the further research in this field is encouraged.

In general, there is a lack of the long‐term studies on the overall effects of waste material addi‐ tions on the soil properties. The variation in the composition of waste material and by‐prod‐ ucts adds uncertainty to their performance, and moreover, leaching of potentially hazardous substances from the waste material itself must be carefully evaluated. The activity levels of natural radinuclides can be elevated in fly ash and red mud with respect to parent coal and bauxite ore, therefore, a special attention should be paid to this aspect in order to keep activity levels in the permitted limits for soil.
