Radioactive contamination of the environment has occurred not only through use of radiological dispersal devices (dirty bomb) but also through other means like destruction of nuclear reactors or by virtue of an industrial or military nuclear accident. The radioactive contaminants released during any accident or incident includes cesium-137, strontium-90, iodine-131, cobalt-60, americium-241 etc. (FDA, 2006). Barium-141, Cesium-137, and strontium-90 are produced from the nuclear fission of uranium and plutonium. Cesium-137 is decays into barium-137. With such an extensive use of radionuclides there is an increased possibility of humans getting exposed to them leading to external and/or internal contamination. A nuclear disaster leads to both external and internal contamination of the radioisotopes. External contamination is associated with the short-term effects of radiations. It includes the contamination of skin surface, foodstuff, water and gamma radiations from the spread of radioisotopes in the atmosphere. About 95% of decontamination can be achieved by removal of clothing, and washing with soap or detergent and water (FDA, 2006).
Internal contamination occurs through ingestion, inhalation or absorption through skin contact of radioactive materials. Exposure to radioactive or nonradioactive materials via ingestion is a major exposure pathway to human and animals. Many factors, such as chemical nature, and physical and biological half life of radionuclides or metal ions affect its absorption, distribution, metabolism and elimination in human body. Once inside the body, radionuclides pose major health problems due to their emission characteristics in the form of alpha, beta particles or gamma rays. Although they are present in micro-quantity, which is far below the metal toxicity level but the radio-toxicity (toxicity due to emitted radiation) is very high. Although some metal ions, such as cobalt and iron are essential for maintaining normal physiological functions, but at higher concentration or their radioactive isotopes leads to poisoning. The metal ions like mercury, cesium, thallium and strontium have the greatest potential to cause harm on account of their extensive use. Following entry into the blood, most of the ions are excreted through kidney and some of them get accumulated into their target organs or tissues. Once inside the body they affect liver, kidney, hematopoietic and nervous system. Further, it causes various disorders like cardiac irregularities, anxiety, tremor and paralysis. Its presence in bones can cause bone cancer, cancer of nearby tissues, and leukemia. After entering the body, biological behavior of cesium and thallium is similar to that of potassium and is excreted by the bile in enterohepatic recirculation (Avery 1995) whereas strontium behaves like calcium (EPA 2017). After entering the body, most of the cesium-137 (biological half life, 70–110 days) and thallium (biological half life, 8–10 days) gets deposited into soft tissues. After Sr-90 is absorbed, it acts like Ca+ 2 and is readily absorbed into bones and teeth, where it can cause cancer of bone, bone marrow and soft tissue surrounding bone (ATSDR, 2014). The removal of toxic metals and radionuclides from body is required to prevent its accumulation in organs or tissues.
Potential agents avert the adverse effects of toxic metal ions by removing them from the body. Chelating and diluting agent helps to accelerate the removal of toxic metal ions through the kidneys (Queiroz et al., 2003; Klaassen, 1996; Gurer and Ercal, 2000). Adsorption over conventional methods such as chelation, reverse osmosis and hemoperfusion is an easy and advantageous process for the removal of radionuclides and toxic metal ions. The finding of new technologies has drawn attention to algae based adsorbents or agents that can remove radionuclides/toxic metals from the body. However, systematic studies are needed to establish a safe and efficacious alga based adsorbent/agents over conventional agents. Algae like Spirulina and Chlorella long been associated with detoxification, specifically the detoxification of toxic/heavy metals. They are highly effective in binding and elimination of toxic metals like lead, cadmium, chromium, mercury, strontium, thallium etc. (Sandau et al., 1996; Rangsayatorn et al., 2004; Chojnacka et al., 2004, 2005; Yadav et al. 2020, 2021a, 2021b).
Spirulina is blue-green algae, consumed as dietary food supplement in pulverized form. In addition to its high nutritional value, it is reported as an excellent detoxifying agent (Chojnacka et al., 2004). Therefore, the present study was undertaken to evaluate the potential of Spirulina for adsorption and removal of non-radioactive isotopes of radionuclides. Spirulina is a genus of blue-green algae belonging to the family of Oscillatoriaceae. Spirulina is a filamentous cyanobacterium composed of individual cells (about 8 µm diameter), which grows in subtropical, alkaline lakes with a temperature optimum of about 35°C (Masojídek and Torzillo, 2014). Spirulina is the commercial name refers to the dried biomass of Spirulina platensis (also known as Arthrospira platensis) (Gershwin and Belay, 2007). The two species which are most commonly utilized are Spirulina platensis and Spirulina maxima. Dried Spirulina contains about 5% water, 55–60% protein, 10–20% carbohydrates, 9–14% lipids, 0.8–1.5% chlorophyll, minerals and vitamins (De Smet, 1997; Khan et al., 2005).
In addition to its ability to bind to toxic metals or radionuclides, Spirulina also contains a lot of important vitamins and minerals that could help during detoxification. Spirulina contains a variety of metal-binding functional groups such as carboxyl, amino, phosphoryl, hydroxyl and carbonyl groups, which has high affinity towards various metal ions (Chojnacka et al., 2005; Fang et al., 2011). Therefore, pulverized (broken cell wall) Spirulina platensis were used in this study to bind/adsorb and remove various metal ions viz., cobalt, strontium, barium, cesium and thallium. Binding efficiency was determined in terms of adsorbent weight, contact time, pH and metal ions concentration. The maximum binding capacity of Spirulina for cobalt, strontium, barium, cesium and thallium were determined by using Langmuir and Freundlich adsorption isotherm models (Ayawei et al., 2017; Yadav et al., 2020). In addition to this, the in vivo removal efficacy of Spirulina was evaluated for strontium and thallium in Swiss albino mice.