INTRODUCTION:

The radioactive wastes arising from various points in the nuclear fuel cycle vary in volume, physical form, chemical composition and concentration. For radioactive waste, this means isolating or diluting it such that the rate or concentration of any radiotoxic ions returned to the biosphere is harmless. To fulfill this, practically all radioactive waste is contained and managed, with some clearly needing deep and permanent burial.

Nuclear power is known by the large amount of energy produced from a very small amount of fuel, and the amount of waste produced during this process is also relatively small. However, much of the waste produced is radioactive and therefore must be carefully managed as hazardous material. Radioactive waste is not unique to the nuclear fuel cycle. Radioactive materials are used extensively in medicine, agriculture, research, manufacturing, non-destructive testing, and minerals exploration. Unlike other hazardous industrial materials, however, the level of hazard of all radioactive waste – its radioactivity – diminishes with time

Gaseous Wastes and Radioactive Aerosols:

The radioactive aerosols are generated mainly by three sources: emission of activated corrosion and fission products, radioactive decay of gases to volatile elements and the adsorption of volatile radionuclides on existing suspended materials. The principal species involved are halogens, noble gases, tritium and Carbon-14. All the gaseous effluents at nuclear plants are treated before discharge to the atmosphere to remove most of the radioactive components from the effluents.

Liquid Wastes:

Nuclear reactors cooled and moderated by water generate more liquid waste than those cooled by gas. The volume of liquid wastes generated at boiling water reactors (BWR) are significantly higher than at pressurized water reactors (PWR). Active liquid wastes are created by the clean up of primary coolant (PWR, BWR), cleanup of the spent fuel storage pond, drains, wash water and leakage waters. Decontamination operations at reactor sites generate liquid wastes containing corrosion products and a wide variety of organic such as oxalic and citric acids. The high level liquid waste (HLLW) contains 99% of the nonvolatile fission products. Wet solids are another category of wastes generated at nuclear power industry. They include different kinds of spent ion exchange resins, filter media and sludges.

Lester [1], Naren et al [2], Bhattacharya and Venkobachar [3], Som Shankar Dubey et al [4], Grasso et al [5] and Thakuria et al [6] have presented the excellent views on the technologies available for the removal of heavy metals from wastewater and several physicochemical methods have been recommended for the removal of heavy metal ions from wastewater. Among those precipitation, cementation, coagulation, foam flotation, reverse osmosis membrane process, solvent extraction, ion exchange, electrodialysis and adsorption are commonly employed and therefore being discussed for the purpose. Biosorption also provides an unconventional method for removal of heavy metals through uptake by algae, fungi and other aquatic plants growing abundantly in ponds and streams. The specific processes selected to remove contaminants from aqueous phase, can be used, singularly or together in various combinations. Before selecting a suitable technique it is essential to characterize the wastewater properly. Beyond this point, a detailed screening should be performed that takes into consideration technical, economic, regulatory, and operability factors. This screening should help to identify a smaller group of processes with a better likelihood of success for a particular waste stream. To do so, a basic understanding of the different types of available physicochemical treatment processes is needed.

Evaporation:

This method provides good decontamination as well as volume reduction. Although its application in the radioactive waste management can give rise to some technical problems as in corrosion. However, corrosion can be reduced by pH adjustment, volume reduction by evaporation of low level radioactive effluents is so effective that the clean condensate could be discharged to the environment without further treatment. Evaporation method has been successfully utilized, to decontaminate the a-particle radioactivity at Kyoto research reactor.

Precipitation

In chemical precipitation, soluble contaminants are converted into their insoluble forms by chemical reactions or by change in composition of the solvent that diminish the contaminant solubility [7,8]. Heavy metals are generally removed from waste by precipitating them as insoluble hydroxides, carbonates or sulphides [9,10]. Precipitation of Hg²⁺, Ce³⁺ as hydrated oxides is quite often used to reduce their concentration in wastewater [11-13]. Difficulty appears in hydroxide precipitation due to appreciable solubility of some metal hydroxides and post treatment of effluent to bring pH within permissible discharge limits. On the other hand, carbonate precipitates are found denser than hydroxides, consequently resulting in decrease sludge volumes. Sulphide precipitation gives higher removal efficiency over wide range of pH along with effective and complete precipitation with extremely low solubility [14,15].

Cementation:

Cementation is an electrochemical process of precipitation in which the metal of interest is displaced from solution by a metal higher in the electromotive series [8]. This process is used to remove and recover reducible metallic ions, such as precipitation of silver from photographic processing solutions and copper from printed circuit solutions, wherein copper ions are reduced to their elemental state (Cu⁰) by reaction with (Fe⁰).

Reverse Osmosis/Membrane Process:

In reverse osmosis, pressure is applied to the more concentrated side to drive water into less-concentrated side across a semipermeable membrane [7, 16]. The membranes employed generally have very high water and very low salt permeability. Cellulose acetate and aromatic polyamides are two most commonly used materials. The key advantage of reverse osmosis is that it gives purified and high quality effluent that can be reused, while the waste stream that requires disposal can be concentrated and reduced in volume. Chemical conditioning to produce particles of near zero zeta potential minimizes the membrane fouling caused by suspended materials in wastewater [17]. However, fouling of membrane and high cost of membrane are the weaknesses of this process.

Coagulation/Floculation:

Coagulation is the reaction that takes place upon the addition of coagulant to wastewater and results in the formation of insoluble products of reaction between coagulant and impurity to be removed. Floculation is the process of building the coagulated particles into floc that is large enough and dense enough to settle [18]. Aluminium sulphate, ferric sulphate and ferric chloride are most commonly used coagulants. Inorganic polysilicate polymers are considered to be safer from human health point than conventional organic polyelectrolytes [19, 20].

Floatation:

Floatation is the process of converting suspended and some colloidal, emulsified and dissolved substances into the floating matter. Floated agglomerated sludges can be readily and continuously removed from the surface of liquid by skimming [21-23]. Foam floatation has distinct advantage when dealing with large volumes of highly contaminated wastes that are quite dilute in ions to be removed. In a study, Thackston et el [24] reported that ferric chloride forms ferric hydroxide floc to carry lead ions present in a simulated waste. Jorne and Rubin [25] and Chatman [26] proposed mechanism and model for flotation. Michelsen et al [27] used a dilute surfactant solution to produce micro bubble foam using a packed column with glass beads.

Solvent Extraction/Liquid-Liquid Extraction:

Solvent extraction is the technique for separation of constituents from a liquid solution by contact with another liquid in which the constituents are more soluble [8]. The constituents are transferred from one liquid to other but remain, in most cases, chemically unchanged obeying Nernst distribution law. The separation of solvent and contaminant can be accomplished by air or steam stripping, distillation, evaporation or second solvent extraction. Typical extraction solvent include crude oil, light oil, benzene, toluene, isopropyl ether, butyl acetate, methylisobutyl ketone and methyl chloride.

Ion-exchange:

The ion-exchange process involves the exchange of an ion (anion or cation) held on a solid surface with another ion of similar charge that is present in the bulk waste stream [7,16]. The contaminants can be recovered by regeneration of the ion-exchange material using alkaline or acid washings. Ion-exchange process is well suited for the detoxification of large flows of wastewaters containing relatively low trace levels of heavy metal contaminants [28-31].

Adsorption:

To achieve the stringent standards for disposal of effluent into natural water bodies, adsorption offers one of the most expedient approaches to meet the problem as a final step in a series of techniques to be used. Adsorption is a general term, in case of solid-liquid interfaces, which describes the attachment of molecular/ionic species from solutions to solid surfaces. In principle, the process is very simple and as effective as it can provide an ultimate answer to water treatment technology. The process involves the use of solid materials having high surface area and capable of undergoing regeneration. A wide variety of adsorbents used for wastewater treatment include activated carbon, clay minerals, fly ash, coal, peat moss, agricultural byproducts, biopolymers, starch xanthate, hydrous metal oxides, metal titanates and acidic salts of polyvalent metal ions.

(a) Activated Carbon:

Adsorption by activated carbon is usually performed using powdered activated carbon (PAC) in complex-mix reactors or granular activated carbon (GAC) in column or fluidized-bed reactors. However, selection of an appropriate activated carbon type is also an important factor in achieving the end. For some adsorbate Chang and Wu [32] found chemical regeneration of activated carbon to be economically viable alternative to thermal regeneration. Activated carbon obtained from low cost agricultural byproducts like rice husks, almond shell, olivestone, coconut shell [33], Hazelnut shell [34] and jute have also been used for adsorption of metal ions. However, the problem associated with activated carbon, is the fragility of its particles which therefore makes difficult its regeneration.

(b) Clay Minerals and Sand:

Clay minerals are crystalline hydrous silicates endowed with structure of layered lattice type. As a consequence of their structure, clay minerals are found to be excellent for adsorption of heavy metals in treatment of wastewater. Kaolinite, illite, and montmorillonite are widely used for adsorption of metal ions [35-38].

(c) Fly Ash and Peat:

Fly ash, which is the waste product of electric power plants and also obtainable in large quantities during burning of coal at high temperature, has been tried by few investigators as an adsorbent. Utilization of coal in treatment of wastewater has been well reviewed [39]. Modified peat was also used by some researchers for adsorption of Cs¹³⁷, Yb¹⁶⁹, and UO₂²⁺ [40].

Surface properties of sludge particles from an extended aeration plant have been investigated by Tien and Huang [41]. The results indicate that adsorption of heavy metals follows second order rate expression and surface reaction determines the rate of adsorption rather than mass transport process [42].

(d) Metal Oxides:

Several types of adsorbents have been developed in recent years, of which hydrous oxide of polyvalent metals have attracted particular attention for removal of anionic/cationic species from aqueous solutions [43-45]. These are found to be mechanically and chemically more stable than commercially available organic exchangers. Hydrous oxides have excellent exchange capacity, stability against high temperature and radiation doses. Hydrous metal oxides do not involve any biological contamination which might arise in case of some organic ion exchangers and in addition they can be converted into dense ceramic materials by pressure sintering. Furthermore, they are highly leach resistant and capable for long term storage of municipal and industrial wastes in solid forms.

CONCLUSION:

It was observed from our discussion that for the removal of radiotoxic ions various techniques could be used as per the need. No technique is fully reliable in complete removal, each have their own merits and demerits in the applications. As per the requirements one can use the best available technique for the efficient removal of the ions.

ACKNOWLEDGEMENT:

I like to thank Heads of Department of Chemistry Deen Dayal Upadhayay Gorakhpur University, Gorakhpur and GITAM Institute of Technology, GITAM (Deemed to be University), Visakhapatnam for providing necessary facilities in authoring this manuscript.

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Received on 26.11.2018 Modified on 22.12.2018

Asian J. Research Chem. 2019; 12(1): 37-40.

DOI: 10.5958/0974-4150.2019.00009.9