**1. Introduction**

Over the last decades, many ecosystems have been altered by human activities, causing the contamination of the environment. Heavy metals, as well as radioactive materials, have been extensively used in industrial applications, medicine, military activity, or various research fields. Despite strict loyalty to all laboratory safety procedures, it is still very possible to encounter heavy metal or radionuclide contamination. During the use of materials containing heavy metals or radioactive metals, various surfaces, such as concrete, steel, glass, rubber, plastic materials, or painted surfaces, from a laboratory, a shooting range [1,2], an industrial or a nuclear facility [3–5], can be contaminated with these hazardous materials.

"Toxic metals", including "heavy metals" or "radioactive metals", are compounds that pose severe environmental problems, negatively affecting the health and the safety of humans at the same time. In very low concentrations, some heavy metals are necessary

to support life, but at higher concentrations, they become poisonous due to their bioaccumulation. Occupational exposure to lead is one of the most widespread overexposures. High potential exposures sources include firing ranges, car batteries or pigment industries. Another prevalent heavy metal is mercury. Typical sources of mercury exposure include mining and refining of gold and silver ores. Another category of "toxic metals" is represented by radioactive metals. Radioactive metals are natural or synthetic isotopes of natural non-radioactive metals that can release alpha (α), beta (β), and gamma (γ) radiation [6] In certain circumstances, these metals can be useful for humans, being employed for cancer treatment, material engineering, or for power generation. Uranium is one of the most valuable radioactive material of the modern world. It is the main raw material for nuclear bombs and nuclear power plants. Cesium and strontium are high-yield fission products that are present in significant amounts in fuel pond waters and reprocessing stream liquors [7]. Radioactive contamination can occur as a result of working with radioisotopes, an accident, or even a terrorist attack [8]. A nuclear explosion is followed by the production of a considerable quantity of radioactive cesium isotope 137Cs (1.6 times greater than 90Sr) [3]. 137Cs has long-term consequences due to relatively long half-life [9]. 90Sr is utilized in medicine and industry, but it generates significant concerns regarding the fallout from nuclear weapons or nuclear accidents. The probability of 90Sr being released as a part of a nuclear reactor accident is lower than the one of 137Cs because it is much less volatile, but 90Sr is probably the most hazardous element of the radioactive effect from a nuclear weapon.

A major problem with exploiting radioactive metal is represented by wastes. Once released into the environment they will result in possibly catastrophic effects that may last long periods and cause malignant illnesses. While "high-level radioactive waste (HLW)" mainly comes from spent fuel from commercial or research reactors, reprocessing of spent fuel, nuclear weapons, or propulsions industry, "low-level radioactive waste (LLW)" comes from hospitals and industry, as well as the nuclear fuel cycle [6]. External contamination occurs when a heavy metal or a radioactive material, in the form of dust, powder, or liquid, encounters an object or a person. External contamination can become internal if the hazardous material enters their bodies through ingestion, inhalation, or skin. Unfortunately, surface contamination with heavy metals and radioactive metals can, very often, become airborne or can be easily transferred by contact [10].

Considering the significant number of environmental incidents caused by heavy metals or radioactive materials, an overabundance of formulations for decontamination have been developed, and described in the literature, to successfully address different types of contamination scenarios. Efficient decontamination techniques are essential for minimizing occupational exposures, facilitating waste management, restricting the potential accidental release of hazardous materials, and allowing the reuse of some of the components from nuclear reactors, industrial installations, laboratory equipment, shooting ranges, etc. Well-weighted decisions must be taken when choosing between passing the entire system through the decontamination process or replacing just the contaminated equipment/component [11]. Several in situ chemical decontamination technologies, which can be applied for the removal of these hazardous materials, have been developed: wiping with textiles wetted with a decontamination solution [12]; wet vacuum treatment (when the vacuum cleaner is charged with a decontamination solution) [13]; electrochemical decontamination with use of external electrode [14]; foam decontamination [15]; decontamination by etching pastes and gels [16]; decontamination by removable polymer coatings [5,17]; and decontamination by sorbents [16]. A major problem in chemical decontamination is the production of a high volume of secondary waste that needs additional treatment for radionuclides removal. Moreover, to increase the decontamination factor and accelerate the decontamination process, chemical methods involve the use of concentrated acid solutions and temperatures up to 70–90 ◦C, which could endanger the health and safety of the workers that are manipulating these corrosive and toxic materials. Electrochemical decontamination is limited by the size of the bath in which the contaminated object must

be immersed and could not be used on an industrial scale [12]. Using strippable coatings has the advantage of higher efficiency with simpler equipment, fewer chemical reagents, and less waste volume. Among the above-mentioned techniques, decontamination by removable polymer coatings is described in the literature as being the most rapid and cost-efficient technique [16]. Koryakovskiy et al. explained that the application of pastes and gels to the surface takes about 20 to 30 s/m2, while their removal takes additional 45 to 90 s/m2 or even more to wash the residues; still less productive is an electric brush having a treatment rate of about 8 to 10 min/m2 or electrochemical methods, which are expensive and difficult to manage [16].

It is very important to reduce the work time in the contaminated areas because prolonged exposure to these hazardous metals can lead to serious health and safety problems. Moreover, finding a method to remove and fix the contaminants (in a polymeric matrix for example), efficiently with the shortest possible personnel exposure time, is imperative. In this context, removable polymer coatings fulfill most of the requirements listed above.

This method of decontamination was used for the first time on radioactively hazardous facilities of the naval forces, on nuclear submarines. Since then, polyvinyl alcohol-based decontamination formulations were used on a large scale. These polymeric compositions were employed for Chernobyl accident response activities and displayed the highest decontamination degrees for most of the contaminated materials [16].

However, despite the positive findings, certain problems remain regarding the removable polymeric coatings. One of them is related to their chemical composition because some of the film-forming decontamination solutions contain volatile solvents or corrosive components. In addition, they contain "old-generation" chelating agents which are not biodegradable, thus leading to supplementary disposal issues. Another problem is related to their viscosity control, which also needs improvement, because the available commercial solutions have low viscosities and poor adhesion to smooth surfaces and cause serious problems for the decontamination of vertical surfaces, flowing down by gravity. As already mentioned, decontamination of surfaces with strippable coatings is a technique that has been extensively studied in the last decade, due to its multiple advantages, but especially because it is a decontamination method that generates a considerably smaller amount of post-treatment waste [10]. Even though polyvinyl alcohol (PVA), employed in nearly all these types of strippable coatings, is a biodegradable polymer (in specific circumstances), it is still a synthetic polymer and requires special conditions for biodegradation [18]. Moreover, PVA has gained attention lately, being among the major pollutants of industrial wastewater in the textile industry [19]. Therefore, the environmental issues include not only the well-known contaminating agents but also the polymeric materials due to the problems arising from their subsequent disposal. Thus, finding solutions to improve the biodegradability of these decontamination solutions should become a main concern for an environmentally responsible collective attitude. Sodium alginate (SA) could represent an adequate candidate for decontamination applications due to its unique set of properties. SA is a biodegradable hydrophilic linear polysaccharide obtained from marine brown algae [20]. The major advantage of alginates is represented by their liquid-gel behavior in aqueous solutions, due to the ion exchange phenomenon that occurs between the sodium ions in alginate and other divalent ions (calcium ions especially), leading to a gel structure with higher viscosity. In the presence of divalent ions, the G-blocks of alginate participate at the formation of the intermolecular crosslinking, thus gaining enhanced mechanical properties [21]. Depending on the concentration of divalent ions in the system, the crosslinking process can be temporary or permanent. At lower concentrations of Ca ions, the temporary association of the chains can occur, leading to viscous, thixotropic solutions, while at higher Ca ions concentrations precipitation or gelation will occur due to permanent crosslinking phenomena [21]. In the literature, numerous papers have shown that the gelforming kinetics have a significant influence on its functional properties involving porosity, swelling behavior, stability, biodegradability, gel strength, or biocompatibility [21,22]. Polysaccharides, including cellulose, chitosan, pectin, or alginate, possess the ability to

produce films/coatings [23]. Numerous studies have revealed that some films obtained by employing alginate displayed improved barrier and mechanical properties [23,24].

In the actual context, considering all the shortcomings of the existing decontamination methods detailed above, this work describes a novel approach towards obtaining novel water-based film-forming decontamination solutions, containing "green" chelates, for an ecological tactic of efficiently removing heavy metals and radionuclides. These innovative biodegradable solutions can generate resistant and easy-peelable polymeric nanocomposite coatings, due to their specially designed composition, comprising a polymeric blend (polyvinyl alcohol and sodium alginate), with excellent film-forming abilities conjoined with the reinforcing effect brought by bentonite nanoclay. Once these aqueous solutions are applied on the contaminated surface, the complexation of the contaminants occurs, together with their entrapment in the polymer-clay system. After solvent evaporation, they form resistant continuous films that can be easily removed from the surface by simply being peeled off, thus ensuring fast and efficient decontamination. These modern decontamination solutions generate a considerably lower volume of post-decontamination wastes than the traditional methods. The material resulting after decontamination can be easily compacted and temporarily stored in a special small container, until it can be further disposed of as hazardous waste. Another purpose of this study was to evaluate the effect obtained by reducing the PVA amount employed in the strippable coatings through the introduction of SA, an eco-friendlier alternative. Thus, using different amounts of SA, we were able to reduce the PVA concentration in the nanocomposite films while ensuring the same performance: homogenous film-forming ability, ease of film removal, thermal, and mechanical resistance. Another advantage brought by these alginate-based solutions consists of the ability of SA to bind divalent heavy metal ions or radioactive metals, which ensures higher decontamination degrees in this case. Moreover, these novel alginate–based decontamination solutions have higher viscosities that increase in the presence of divalent ions, due to the crosslinking phenomenon, thus allowing the use of these decontamination solutions also on vertical surfaces. Decontamination can possibly occur through two distinct paths: chemical interaction (complexation by "green" chelating agents [25–27] or SA), and physical interaction (adsorption by bentonite nanoclay). These two mechanisms, in conjunction with alginate crosslinking [28], enhance the overall efficiency of the decontamination solutions.

The novelty of this work consists of an innovative way of combining the ability of polyvinyl alcohol to produce films, with the remarkable adsorption capacity of bentonite nanoclay, together with the chelating ability of alginate and "new-generation" "green" complexing agents: iminodisuccinic acid (IDS) and 2-phosphonobutane-1,2,4-tricarboxylic acid (PBTC), to obtain powerful, versatile, and environmentally-friendly water-based solutions for surface decontamination of heavy metal or radioactive metals. Both "green" chelating agents, PBTC and IDS, are well-known in literature for being efficient in chelating metals in aquatic environments [29]. In comparison with the classical complexing agent EDTA, these new generation "green" complexation agents are considered eco-friendly materials, due to their biodegradability. From our knowledge, we report here for the first time, a decontamination solution, containing two "green" chelates: PBTC and IDS, utilized together with sodium alginate as a complexing agent and, at the same time, as an integrant part of eco-friendly peelable polymeric coatings, for the efficient removal heavy metals or radioactive metals from contaminated surfaces. This paper comprises structural, thermal, and mechanical characterization of the newly synthesized nanocomposites through various analytical techniques and decontamination tests on various types of surfaces (glass, metal, painted metal, plastic, and textile sample from the CBRN individual protection equipment) contaminated with heavy metals and three types of radioactive solutions: alpha (241Am), beta (90(Sr−Y)) and gamma (137Cs).

#### **2. Experimental**

#### *2.1. Materials*

Reagent: iminodisuccinic acid (**IDS**—BAYPURE® CX 100 solid G (>78% Iminodisuccinic acid Na4 salt, <15% Aspartic acid, Na2 salt, <5% Fumaric acid Na2 salt, <0.7% Hydroxysuccinic acid, Na2 salt, <0.5% Maleic acid Na2 salt and <4% Water)—Lanxess, Cologne, Germany), 2-phosphonobutane-1,2,4-tricarboxylic acid (**PBTC**—BAYHIBIT® AM (40.0–42.5 % PBTC-Na4 content in water)—Lanxess, Cologne, Germany), poly(vinyl alcohol) (**PVA** with 98–99% hydrolysis degree, DP ≈ 1700–1800, Mw ≈ 115000 Da—Loba Chemie, Mumbai, India), nano-clay hydrophilic bentonite (**BT**, Sigma–Aldrich, St. Louis, MO, USA), Glycerol ≥ 99% (**GLY**—Sigma–Aldrich, St. Louis, MO, USA); Sodium alginate (Special Ingredients®, **SA**, Garlenda, Savona, Italy). Metal solutions: caesium sulphate (Cs2SO4— 99.99% trace metal basis, Sigma–Aldrich—0.005 M, 0.05 M and 0.5 M Cs2SO4 aqueous solutions); lead standard for AAS (1000 mg/L ± 4 mg/L Pb in nitric acid, Sigma–Aldrich); strontium standard for AAS (1000 mg/L ± 4 mg/L Sr in nitric acid, Sigma–Aldrich); cobalt standard for AAS (1000 mg/L ± 4 mg/L Co in nitric acid, Sigma–Aldrich). Radioactive solutions: 137Cs; 241Am; 90(Sr-Y). Tested surfaces: stainless steel sheets (10 mm × 10 mm × 0.4 mm classical 18/8 stainless steel), galvanized metal sheets (10 mm × 10 mm × 0.4 mm), brass sheets (10 mm × 10 mm × 0.5 mm) and cooper sheets (10 mm × 10 mm × 0.2 mm), metal (100 mm × 100 mm × 0.5 mm, MIL-46100 military grade steel), painted metal (100 mm × 100 mm × 0.5 mm, paint based on urethane modified resin), plastic (100 mm × 100 mm × 1 mm, polycarbonate), glass (100 mm × 100 mm × 5 mm) and BC/SP2 (Romanian Army Individual Protective Equipment (IPE) material sample (100 mm × 100 mm × 0.5 mm)).

#### *2.2. Methods*

#### 2.2.1. Synthesis of the Decontamination Solutions

The water-based decontamination solutions were obtained according to the procedure described below. The correlation between the composition of the decontamination solutions and the sample IDs is summarized in Table 1. Every decontamination solution contains water, 5% PVA, 1% BT nano-clay, 2.5% glycerol, and different sodium alginate concentration (0%, 0.25%, 0.5%, 0.75%, or 1%), indicated in Table 1. Each of the last two samples (GD-3-PBTC and GD-3-IDS), contained 1% chelating agent (CA). The aqueous decontamination solutions were obtained through the following steps: the first one consisted in the dissolution of the chelating agent in water (for GD-3-PBTC and GD-3-IDS samples), followed by the dispersion of bentonite by ultrasonication. The next step consisted in the addition of the alginate water solution, followed by the dissolution of PVA. The last step consisted in the addition of glycerol. The solutions were stored at 2–5 ◦C until they were employed for decontamination tests. All the decontamination solutions containing SA exhibited a liquid-gel behavior in the presence of divalent ions, changing from a relatively low viscosity solution to a gel structure. They were allowed to dry on the target surface, forming thin films, which were subsequently removed by being peeled off. The nanocomposite films were subjected to different analytical investigations, described in Characterization Section 2.3.

#### 2.2.2. Nanocomposite Films Preparation

To evaluate the properties of the nanocomposite films obtained from the decontamination solutions, various analytic techniques were employed (described in the Characterization section). The films subjected to analysis were obtained through casting method. Decontamination solutions were placed in rectangular glass molds (12 cm × 12 cm × 2 cm), and they were allowed to dry on a plane horizontal surface, at room temperature and 50–55 relative humidity. Usually, the drying time for these film-forming materials is below 24 h, depending on the thickness of the film and the environmental conditions.


**Table 1.** Composition of the gel-forming decontamination solutions.

\* % calculated from the total mass of the decontamination solution.

#### 2.2.3. Controlled Contamination for SEM-EDS Analysis

Non-radioactive cesium was employed for evaluating in-depth and surface contamination on three different types of metallic surfaces. For this purpose, Cs2SO4 solutions were employed for the contamination of stainless-steel surfaces (square-shaped metallic coupons: 1 cm × 1 cm × 0.05 cm) with 3 types of finishing: mirror-finish (SSMF), grindedfinish (SSGF), and etched-finish (SSEF)—immersed in royal water). To reproduce in-depth contamination [30], stainless steel coupons were immersed for 30 min of in 0.005 M Cs2SO4 aqueous solution followed by a thermal treatment at 700 ◦C, in a furnace, for 2 h. To simulate superficial contamination, three different concentrations of Cs2SO4 aqueous solutions were employed: 0.005 M, 0.05 M, and 0.5 M. The metallic coupons were placed in Petri dishes and the contamination solution was added until they were completely covered by liquid. Then, the coupons were maintained at 40 ◦C until the complete evaporation of water. For the decontamination tests, metallic coupon was covered with decontamination solution. Once the drying process (complete evaporation of water, at 25 ◦C, approximately 8 h) ended, the strippable coatings were easily peeled off from the metallic coupons. Both, polymeric films containing the contaminant and decontaminated surfaces were subsequently subjected to SEM/EDS (Billerica, MA, USA) analysis. This investigation offered preliminary qualitative information about the decontamination process. To quantitatively evaluate the decontamination efficiency, decontamination tests described below were performed.

#### 2.2.4. Decontamination Tests

Decontamination tests aimed to evaluate the removal efficacy of herein reported decontamination solutions. For this purpose, controlled contamination with heavy metals and radionuclides was performed prior to the decontamination step. For the evaluation of the decontamination degree, two distinct analytic investigations were performed: (1) atomic absorption spectrometry and (2) alpha, beta, and gamma radiation measurements.
