Strontium Contamination in the Environment

This chapter gives an overview of the properties of strontium as a chemical element, the methods of its determination, data on its abundance in the environment, and its impact on the human organism in daily life. Strontium is one of the alkaline earth group elements, two of which, calcium and magnesium, are among the ten most abundant elements in the Earth’s crust. Chemical properties of strontium are analyzed in comparison with properties of other group II elements. The chemical properties of strontium are quite similar to those of calcium and barium, resulting in similar behavior in the environment and biological objects, and also making determination of strontium by chemical methods difficult, especially in environmental samples. Therefore, traditional chemical methods of strontium determination are replaced by modern physical and physicochemical analytical methods that lower the detection limit and simplify sample treatment techniques. This chapter is not focused on radioactive isotopes of strontium, for example, ⁹⁰Sr, a long-lived beta-emitting fission product causing long-term environmental contamination. Because the stable and radioactive isotopes of an element act identically in most physical, chemical, and biological processes, the methods for pre-concentration and separation of strontium in the analytical techniques described here can be used successfully for determination of the radioisotopes of strontium.

The chapter gives an overview of properties and application of stable and radioactive isotopes of strontium. Natural strontium consists of the following four stable isotopes: ⁸⁴Sr (0.56%), ⁸⁶Sr (9.86%), ⁸⁷Sr (7.00%), and ⁸⁸Sr (82.58%). Among them, ⁸⁷Sr is a radiogenic isotope being the decay product of the long-lived natural beta-emitting isotope ⁸⁷Rb; it is widely used in geology for rocks and minerals dating as well as for systematization of origin of various rock formations. A unique ⁸⁷Sr/⁸⁶Sr ratio in each region became a useful tool for tracing geographical origin of water, archaeological artifacts, and foods. Besides stable isotopes, a number of radioactive isotopes of strontium from ⁷³Sr to ¹⁰⁷Sr are also known; among them, relatively long-lived isotopes are ⁹⁰Sr, ⁸⁹Sr, ⁸²Sr, and ⁸⁵Sr. The most long-lived radioactive isotope ⁹⁰Sr with the half-life of 28.9 years is one of the most contaminants of the environment because of radiation accidents. Being a significant component of irradiated nuclear fuel and radioactive waste after spent fuel reprocessing, ⁹⁰Sr is used in production of radioisotope thermoelectric generators, as beta radiation sources for radiometric and dosimetry applications, as well as in nuclear medicine as a mother nuclide for isotopic generators of ⁹⁰Y being used for therapy in oncology. Applications of ⁸²Sr and ⁸⁵Sr in nuclear imaging and ⁸⁹Sr in radiotherapy are described in the chapter.

Strontium, a highly reactive alkaline earth metal, is very stable in natural occurrence minerals; however, the mobility of its isotope ⁹⁰Sr produced from the nuclear fallout is one of the deadly fission products. Celestite, SrSO_4, is the most important primary source of it following the exploitation via either the black ash or direct leach process. For which, the illustration of the thermal and aqueous chemistry of strontium is very crucial. In the case of ⁹⁰Sr, its separation from the other radionuclide, most specifically over the ¹³⁷Cs from the high-level waste (HLW) of fission products, is vital, converting the HLW to low-level waste (LLW). Liquid-liquid (solvent) extraction technology has been widely accepted for the efficient separation and recovery of strontium from the fission products, as the radionuclide already remains in its soluble form therein the waste solution. Interaction strategy between the metal ion and dipole from the donor atom of crown ether is prominently being used, whereas the poorly hydrated anions of dicarbollide, a boron cluster with a π-bonded trivalent cobalt ion, form ion-pair neutral compounds in the extraction process. In this chapter, the extraction processes of strontium from both natural mineral and synthetic source of the waste fission solution are being discussed, which includes the process technology, adopted techniques behind the technology, crucial points, and key parameters.

The migration of strontium to the geosphere is one of the major concerns in a proper handling of radionuclide waste solutions. The costly chemical reagents of toxic nature, generating secondary pollution with low efficiency for a minute concentration of metal ions, make their physicochemical separation rather limited in the application. Adsorption onto solid substances like activated carbon, kaolinite, montmorillonite, and colloidal silica is simpler in operation and the most robust; however, their ineptitude at high sodium concentration and very acidic pH are the principal disadvantages of it. The microbial techniques using various biomaterials have been identified as the potential alternative because of being effective to uptake lower content of radionuclides, inexpensive, free of secondary pollution, and exhibition of electronegative characteristics through the different organic functional group present therein. The passive removal of metals, termed biosorption, requires the selectivity and high metal uptake capacity of substrates for the applied remediation scenarios. A detailed description of cell wall constituents of fungi, bacteria, and algae is provided in order to explain the selective sorption and/or accumulation of strontium from the radiotoxic waste solutions. The influential role of cellular structure, cell wall, storage, and extracellular polysaccharides is evaluated for the strontium sequestration. Furthermore, the binding mechanisms are discussed, including the involvement of key functional groups and the exhibited ion-exchange process.

Strontium (Sr) is one of the plentiful elements of earth’s crust. However, human activities related to nuclear reactor, nuclear weapons explosions and accidental fallouts release substantial amount of radiostrontium. Among radioisotopes, ⁹⁰Sr is the most important, which affects organisms, including plants, and enters into the food chain. ⁹⁰Sr is a β-emitter and produces ⁹⁰Y and ⁹⁰Zr through succeeding radioactive decay. Further, ⁹⁰Sr bear a resemblance to calcium affecting plants uptake of Ca and is extremely injurious inducing metabolic imbalances within the tissues. Sr uptake by plants is therefore a crucial matter to understand the scope of phytoremediation. Several external factors including soil properties and availability of the element at plant root zone determine the mobility and uptake of ⁹⁰Sr by plants. However, selected plants should qualitatively have equal efficiency of high uptake and high biomass production. Plants’ native anti-stress physiological mechanisms play an important role in this issue. This chapter deals with the probable issues for overall phytoremediation strategies of radiostrontium to ameliorate the contaminated soil in an eco-friendly manner.

Strontium abundantly occurs in nature in the rocks and soils in the form of sulfate and carbonate minerals, i.e., celestite (SrSO₄) and strontianite (SrCO₃), in nonradioactive and nontoxic forms. However, a small amount of strontium as radioactive ⁹⁰Sr has been contributed to the geo-environment by the anthropogenic activities of nuclear testing and fission reactions. The contamination caused by the radioactive by-products of such activities poses major concerns due to its ease of entry into the ecosystem. The ⁹⁰Sr has a prolonged half-life of 28.9 years and can progressively enter into the geo-environment and also the life cycle of organisms living in close proximity to such contaminated sites. The mobility of ⁹⁰Sr migration in the geo-environment is often favored due to its similarity to calcium ions but can be retarded through the strong interactions with soil organic materials, clay minerals, and other oxides present in the environment. Owing to the severity of radioactive Sr contamination, this chapter thus deals with the various pathways of strontium dispersal into the geo-environment. Additionally, for the contaminated sites, studies on sorption-desorption behavior for selection of suitable remediation technology are deliberated herein. This chapter also explores the extent to which phytoremediation, an in situ modification, can be used to reclaim soils contaminated with ⁹⁰Sr.

In this chapter, activity concentration of ⁹⁰Sr in the soil and plants from different types of ecosystems in the Belarussian sector of the exclusion zone of Chernobyl NPP after three decades of accident is shown. Lateral migration of ⁹⁰Sr and variability of its activity in the topsoil in different forest types are discussed here. Influence of soil properties and peculiarities of species in plant communities on the accumulation of the radioisotope in the main compartments of ecosystems is presented. The obtained results may be actuals for designing of mathematical landscape models intended for the forecast of space-time redistribution of radioisotopes after emergency emissions. The authors proved that soil litter is an integral indicator of affinity the living parts of an ecosystem regarding the radioisotope.

The presence of some enterprises of the nuclear fuel cycle (NFC) in the Ural region has caused the formation of a radioactive contamination on large territories that exceeds the level of global fallouts typical for the middle latitudes of the Northern Hemisphere. Currently, additional sources of man-made radionuclides in the Urals are (1) the operating NFC enterprises, the largest of which are “Mayak” Production Association and Beloyarsk Nuclear Power Plant (permissible emissions), and (2) contaminated zones that are sources of secondary pollution of the environment. The activity of PA “Mayak” created the most problematic areas: the Techa River, which was polluted with liquid radioactive wastes, and the East Ural Radioactive Trace (EURT), which was the result of the Kyshtym accident of 1957. These local zones possessed very high levels of radionuclides, including ⁹⁰Sr. The inventory of ⁹⁰Sr reached 70,000 kBq/m² in the EURT head part. Prolonged input of global fallout and gas-aerosol emissions from the NFC enterprises and transport of radionuclides from contaminated areas supported during a long time stable global (1.3 Bq/m²) and regional (1.6–3.0 Bq/m²) levels of ⁹⁰Sr in the Urals. Migration of ⁹⁰Sr within geochemical conjugations depends on the sources of intake (gas-aerosol emission or liquid discharges) of the radionuclide into the environment. The depth of vertical migration and the character of the distribution of the radionuclide depend on the regime of moistening and the type of soils.

The significant part of the territory (43.3%) of State nature conservation research institution “Polessye State Radiation Ecological Reserve” is occupied by the pine stands. The forest litter in case of mossy, heath, bilberry types of pine forest remained to be the accumulator of large part of ⁹⁰Sr retaining from 26 to 38% of its inventory. The common peculiarity of distribution of ⁹⁰Sr through mineral part of soil profile in pine stands was its accumulation by upper 0–5 cm under litter layer with following decreasing in dipper layers. The part of ⁹⁰Sr absorbed by mineral soil layer varied from 21 to 36% of its inventory in pine forests. The ability of tree organs and tissues of pine plantations to accumulate ⁹⁰Sr was decreasing in the following order: bark > roots > wood on the automorphic (well drained) soils and roots > bark > wood on the semihydromorphic soils. The transfer of ⁹⁰Sr to the wood of understory vegetation was decreased in the order: oak > Persian berry > mountain ash. For the leaves, the same row was mountain ash > Persian berry > oak. Each species of understory vegetation was characterized by its own specificity of ⁹⁰Sr absorption by the organs and tissues. The content of ⁹⁰Sr in species of alive soil cover in pine forests was decreasing in the following order: adderspit > dicranum moss > Schreber’s big red stem moss. Their aboveground phytomass accumulated more ⁹⁰Sr than roots. The experimental data described in the paper were received in the period 2014–2017.

Strontium, a relatively abundant alkaline element in the earth’s crust, occurs in four stable isotopes, ⁸⁴Sr, ⁸⁶Sr, ⁸⁷Sr and ⁸⁸Sr. The separation of soluble Sr²⁺ ion from water, mainly seawater, can be achieved through one or a combination of methods such as adsorption, chemical precipitation, ion exchange, membrane technology and solvent extraction, amongst which adsorption and membrane processes are popular solutions. The regeneration of spent adsorbents along with Sr recovery is the inherent advantage of the adsorption process. Natural adsorbents such as alginate microspheres, attapulgite, bentonite, dolomite, goethite, hematite and natural zeolites and inorganic ion-exchange materials, viz. activated carbon, antimony oxide, artificial zeolites, carbon and titanate nanotubes, gel and macroporous resins, titanium oxide and synthetic birnessite, have been used for immobilization of Sr. Industrial wastes (coal fly ash and industrial sludges) and agricultural byproducts (almond green hull, eggplant hull, moss and waste rice straw) are also potential Sr adsorbents. The adsorption process is greatly influenced by pH, initial concentration of contaminant, temperature and textural characteristics of the adsorbents. Membranes from polymeric and ceramic materials have also been used for Sr attenuation, and hybrid membrane technologies using multiple membranes have been found to be effective.

Stable isotopes of strontium are common components of natural waters, soils, and soil solutions, whereas radioactive isotopes such as Sr-90 are components of liquid radioactive waste and pollutants of the environment. Sorption method is used for decreasing toxicity of drinking waters when elevated concentrations of stable isotopes are present as well as for deactivation of aqueous media containing radioactive isotopes of strontium. The chapter discusses natural and artificial sorbents for strontium sorption from various aqueous solutions. Mechanisms of strontium sorption by various types of sorbents and factors affecting strontium sorption are described. The effect of concentrations of cations and anions in a solution on sorption and redistribution of strontium between liquid and solid phase is shown.

This chapter provides an outline of alkaline earth metals’ contamination caused by the industrial and nuclear pollution. The assessment has been done for quantifying sources (viz. natural and anthropogenic) of the alkaline earth metals along with describing the physicochemical characteristics of calcium, strontium, and barium. The isotopes of strontium, calcium, and barium are found to have significant impact not only onto the human health but also the geoenvironment. In this view, estimation of risk assessment caused by these metals is mandatory and also described in this chapter. Notably, calcium is one of the most significant elements in human body in the form of bone skeleton. In contrast, strontium and barium are non-essential elements for human beings; however, due to their similar characteristics with calcium, they get managed to enter within the human body via accumulation onto the tissues and bones. Therefore, the quantification of strontium and barium is imperative that can be done by measuring ratios with respect to calcium (as, Ba:Ca and Sr:Ca).