2.2.6. Evaluation of the Decontamination Efficacy through Radiation Measurement

Three standard radioactive solutions, 241Am for alpha radiation, 90(Sr-Y) for beta radiation, and 137Cs for gamma radiation, were employed for radioactive controlled contamination of 5 types of surfaces: metal, painted metal, plastic, glass, and BC/SP2. The radioactive controlled contamination was performed according to NATO standard AEP-58 [4]. Every tested surface measured a 10 cm<sup>2</sup> area and a specific quantity of each solution was uniformly dispersed on the surfaces to reach a value between 30 ÷ 80 Bq/cm<sup>2</sup> for medium contamination level and 300 ÷ 800 Bq/cm2 for high contamination level. The activity of the contaminated surface was measured at 10 min after the deposition of the radioactive solution. After measuring the initial activity (Ai) of the surfaces, 10 mL of decontamination solution was poured over the targeted area. The decontaminating solution was allowed to cure and dry. After the completion of this step (after 24 h), the coatings were peeled off and the final activity (Af) of the surface was measured again immediately after the removal of the nanocomposite film. The decontamination factor (DF) was calculated considering the initial activity of the contaminated surfaces and the final activity of the decontaminated surface, with the aid of the following formula (equivalent to the one utilized for heavy metals): DF = 100 (Ai − Af)/Ai, where Ai represents the initial contamination (Bq/cm2) and Af represents the residual contamination (Bq/cm2), measured after the removal of the nanocomposite film. DF was determined in accordance with NATO standard AEP-58 [4]. The radioactive activity investigations were carried out with Berthold dose/dose rate monitor. The measurements for each type of sample were repeated three times and the average value was reported.

#### *2.3. Characterization*

FT-IR spectra were obtained with the aid of a Perkin Elmer Spectrum Two with a Pike Miracle™ ATR modulus, at 4 cm−<sup>1</sup> resolution, 550 to 4000 cm<sup>−</sup>1. Thermogravimetric analysis (TGA) was performed on a Netzsch TG 209 F3 Tarsus instrument (Selb, Germany) under nitrogen atmosphere, a flow rate of 20 mL/min, on samples weighting approximately 4 mg, on a temperature ramp starting from ambient temperature up to 700 ◦C with 10 ◦C/min heating rate. A 710 Titan 2 universal strength testing machine, equipped with a 3000 N force cell was employed for tensile tests, performed according to ISO 37: 2011(E). The dumbbell-shaped specimens were obtained from the strippable nanocomposite films using

a cutting mold device that had 75 mm overall length. The test area implied a length of the narrow section of about 25 ± 1 mm. The variation of the length and force was continuously monitored with an accuracy of ±0.2% at a speed of 8.33 mm/s. Five specimens from each sample were analyzed. The mean values obtained for each material were plotted in a comparative stress–strain graph. To evaluate thermo-mechanical and viscoelastic properties of the nanocomposite coatings, measurements were conducted on a Discovery 850 DMA-TA Instruments, in single cantilever-bending mode. Experiments were run on samples (10 × <sup>30</sup> × 0.5 mm3 size), at a frequency of 1 Hz, and a temperature ramp starting from −80 ◦C to 200 ◦C with a heating rate of 5 ◦C/min. SEM-EDS analysis was performed with the aid of a Zeiss Gemini 500 microscope coupled with an XFlash 6 EDS detector from Bruker (Billerica, MA, USA). All data from the EDS were analyzed using the ESPRIT Software (version ESPRIT 2, Billerica, MA, USA). For the evaluation of the efficiency of heavy metal removal we employed an atomic absorption spectrometer, PerkinElmer AAnalyst™ 800 (Waltham, MA, USA) high-performance with WinLab32 software (Perkin Elmer, Waltham, MA, USA) for AA. The determinations were performed by subjecting samples to electrothermal atomization in a graphite furnace. The radionuclides decontamination efficiency was determined by measuring the activity of the targeted surfaces (contaminated with 241Am, <sup>90</sup> (Sr-Y), and 137Cs), before and after decontamination, using Berthold L123 dose/dose rate monitor with specific detectors for alpha, beta, and gamma radiation.

#### **3. Results and Discussion**

The decontamination process with strippable coatings involves the following three steps, presented in Scheme 1: (1) The decontamination solution is applied (poured or sprayed) on the surface contaminated with the "toxic metal" (heavy metal or radioactive metal). For exemplification, Scheme 1 describes contamination with cesium, because 137Cs is one of the radionuclides investigated in this study, but it could be replaced with any other metallic contaminant. The decontamination reaction occurs at the interface between the aqueous decontamination solution and the contaminated metallic surface. If this solution maintains its liquid state, contaminant ions are continuously extracted from the moistened metallic surface into the decontamination solution through complexation and adsorption mechanisms. As the solvent (water) is lost or the polymers undergo crosslinking, the metallic contaminants are entrapped inside the matrix of the polymeric nanocomposite. (2) After stage (1) is complete (less than 24 h), the dry nanocomposite film containing the contaminant can be easily peeled off from the metallic surface. The strong interactions between the components of these materials ensure the formation of a compact polymeric nanocomposite film which maintains its integrity when it is detached from the surface. (3) When the peeling process ends, the metallic surface is considered successfully decontaminated. The strippable coating containing the "toxic metals" should be compacted and sealed in a small container towards final disposal. According to legislation requirements, these materials should be disposed or incinerated as low-level waste [31], but their greatest advantage is represented by the small volume of material generated post-decontamination.

The schematical illustration (Scheme 1) presents only the main mechanisms of action of the active components from our decontamination solution: complexation (by chelating agents) and physical interactions (specific interactions between nano-clay and contaminant cations). Bentonite clay possesses five types of adsorption sites [32]: basal surface site (planar site), interlayer site, hydrated interlayer site, edge site, and the frayed edge site (FES) and their existence allows metal contaminants adsorption through distinct routes [32–34], part of them drawn in Scheme 1. The chelating effect of SA is also a factor which influences decontamination performances, but since it did not bring any outstanding improvements, it was intentionally omitted from this scheme, for the ease of visualization.

The nanocomposite strippable coatings were obtained through casting method, as described in Methods Section 2.2. Figure 1 illustrates the casting process. As can be seen in Figure 1a, the decontamination solution can be easily transferred on the contaminated surface. Depending on the surface type and position, these solutions can be simply poured

onto the surface, applied with a roller or a brush, or they can be applied by spraying technique. After being deposited on the targeted area, the decontamination process begins. The decontamination solution is allowed to dry and after the complete evaporation of the solvent (water), the nanocomposite film can be peeled off (Figure 1b), and thus the surface is decontaminated. These decontamination formulations are customizable, because the active ingredients of these decontamination solutions can be selected depending on the targeted contaminating agent (chemical, biological, radiological, or nuclear contaminants). In this study, "green" chelating agents and sodium alginate were employed, and the aim was "toxic metals" (heavy metals and radioactive metals) removal.

**Scheme 1.** Schematic illustration of decontamination process: (**1**) application of the decontamination solution on the contaminated surface; (**2**) decontamination process (exemplified by complexation or adsorption mechanisms); (**3**) removal of the contaminants entrapped into the nanocomposite film.

**Figure 1.** Casting process: (**a**) decontamination solution is transferred on the contaminated surface; (**b**) the surface is decontaminated after film removal.

The composition of the decontamination solutions and the interactions that occur between the components have a significant influence on the performances of the strippable coatings obtained. Thus, various decontamination formulations comprising different SA concentrations (Table 1) were developed to establish which composition of the polymeric matrix is adequate for this type of application.

The FT-IR spectra, illustrated in Figure 2, display the variation of the characteristic peaks of the decontamination strippable coatings with the increase in SA concentration.

**Figure 2.** FT-IR spectra of decontamination strippable coatings.

The O–H stretching vibrations at 3293 cm−<sup>1</sup> were slightly shifted to 3288 cm<sup>−</sup>1, probably due to the supplementary hydrogen bonds brought by increasing the amount of SA added in the decontamination solutions [35]. By increasing the concentration of SA, the two peaks associated with the asymmetric O–C–O stretching vibrations at 1630 cm−<sup>1</sup> and 1575 cm−<sup>1</sup> merged into a singular peak visible at 1610 cm<sup>−</sup>1, confirming that the interaction between SA and the other components of the decontamination films occurs also trough the carboxylic groups. The intensity of the characteristic peak for bentonite (attributed to Si–O stretching vibrations visible at 1029 cm−1) decreased with the addition of SA. A clearer indication of the interaction between the Si-OH group from the nanoclay and SA can be highlighted through a more visible peak, at 994 cm−1, also given by the Si–O stretching vibrations. The specific peak for mannuronic acid sequence of SA was visible at 820 cm−1, being slightly shifted in comparison with pure SA, due to the interactions established between SA and the other components, mainly hydrogen bonds formation. The peak at 850 is due to Si–O–Si vibrations, also slightly shifted in comparison with pure bentonite spectra.

The thermal and mechanical properties of the decontamination strippable coatings obtained by employing different concentrations of SA were investigated to establish the optimal composition for these strippable coatings designed for surface decontamination. Figure 3 illustrates the results of the TGA analysis performed on the synthesized materials for the assessment of their thermal stability.

As can it be observed in Figure 3a, all the strippable films exhibited a first weightloss of approximately 8% (up to approximately 140 ◦C), probably due to the loss of the residual water in their composition. Additionally, the results confirm the thermal stability of the polymeric nanocomposite films up to 140–150 °C, after which a second weight-loss stage is registered varying from 15% (GD-0, GD-1, and GD-2) to 7% (GD-3 and GD-4). Therefore, the addition of SA led to a smaller weight-loss for the material in this temperature range. As it can be noticed in Figure 3b, the peak at 220 °C, assigned to the third weight loss step, is slightly shifted to higher temperatures in the case of GD-2, GD-3, and GD-4 samples; thus, the increase in SA concentration leads to a slightly increased thermal stability for the samples, due to the higher number of hydrogen bonds established between SA and PVA [36].

**Figure 3.** TGA (**a**) and DTG (**b**) plots of decontamination strippable coatings.

Table 2 summarizes the results obtained from tensile tests. According to these results, the strippable films containing SA possess a higher elasticity. Figure 4 describes the correlation between the SA concentration and the mechanical properties of the nanocomposite strippable films. The value of the elastic modulus (λ = σ/ε) increases with the addition of SA, up to a concentration of approximately 0.8% SA, and then starts to decrease. Considering these results, an optimum amount of SA (approximatively 0.7–0.8%) permits the reduction in PVA concentration, thus affording decontamination strippable coatings with a lower environmental and superior mechanical and thermal properties.


**Table 2.** Mechanical properties nanocomposite films.

Fm is the maximum force recorded. Tensile stress, σ, was calculated, according to ISO37:2011(E), taking into account **W** (the average width of the narrow part of the specimen, mm) and **t** (the average thickness, mm): σ = Fm/(W·t). Tensile strain, ε, was calculated, according to ISO37:2011(E), taking into account **Lb** (length at breaking, mm) and **L0** (the initial length, mm): ε = 100·(Lb − L0)/L0. TSb is the tensile strength at break (tensile stress recorded at the moment of rupture). Eb stands for elongation at break. The average values for each sample were reported.

Thus, based on the above-mentioned analyses, it can be observed that GD-3 possess remarkable thermal properties and adequate mechanical properties for this type of application. Therefore, the solution containing 0.75% Na-Alginate (GD-3) was chosen as the basic polymeric matrix for the decontamination solutions containing also complexing agents. Thus, starting from this point, the following tests were performed only on the blank solution (GD-0), the solution with optimal level of alginate (GD-3), and the solutions comprising both alginate and chelating agents (GD-3-PBTC and GD-3-IDS).

The investigation of viscoelastic behavior is important for a proper design of the strippable coatings. Considering the results obtained from the tensile tests, which concluded that GD-3 displayed the best mechanical characteristics, four types of polymeric composites were selected to be subjected to DMA analysis: GD-0, GD-3, GD-3-PBTC, and GD-3-IDS. Figure 5 illustrates the comparative graph between these four types of decontamination polymeric composite films.

**Figure 4.** Correlation between the SA concentration and the mechanical properties of the films.

**Figure 5.** Loss and storage modulus of strippable coatings.

DMA results revealed distinct viscoelastic behavior of the cured strippable coatings depending on the chelating agents employed in the decontamination solutions. An adequate viscoelasticity is important for the peeling process of these coatings because their components should facilitate the final removal of the polymeric film from the decontaminated surface, while ensuring a high decontamination performance. When a stress is applied to a viscoelastic material [37] such as our decontamination coatings, during the peeling process, some parts of the long polymer chain change their position and when the stress is taken away, namely when the coating is completely peeled off, the accumulated back stresses will bring the polymeric nanocomposite film to its original shape, because, theoretically, the polymer chains will return to their initial position. When a stress is applied, the strippable coatings possessing higher viscoelasticity (higher storage and loss modulus values) will undergo a more significant molecular rearrangement. Whereas elasticity (described by the variation of storage modulus) is related to bond stretching, viscosity (described by the variation of loss modulus) is related to the diffusion of molecules inside an amorphous material [38]. From Figure 5, we can conclude that storage and loss modulus decrease in the following order: GD-0 > GD-3-PBTC > GD-3-IDS > GD-3, probably due to the different number of hydrogen bonds established inside the polymeric nanocomposite and depending on different strengths of these hydrogen bonds. The formed hydrogen-bonds network is responsible for the mechanical and structural properties of a material. Therefore, according to the values obtained for storage and loss modulus, it can be noticed that the strippable coatings without alginate and chelating agent exhibited the highest viscoelasticity, the one with alginate displayed the lowest viscoelasticity, while the ones containing IDS and PBTC display improved qualities compared to GD-3 [39].

The thermal properties of the polymeric nanocomposite films employed for decontamination were determined using the DMA technique because it can detect molecular relaxations, such as glass transition temperatures (Tg), with a higher accuracy than DSC or DTA [40,41]. According to the tan delta (δ) plots (Figure 6), the strippable coatings displayed an apparent double glass transition temperature (Table 3). This property was predictable because neat PVA (the polymer matrix employed in these decontamination nanocomposite films) shows typical behavior of a semi-crystalline polymer with two transition regions as well: Tg1 ≈ 30–60 ◦C and Tg2 ≈ 80–150 ◦C [39,42]. Our PVA nanocomposite peelable films possess significantly lower glass transition values due to their higher flexibility given by glycerol and the other additives. Therefore, the lower temperature amorphous glass transition, Tg1, varies from −61.8 ◦C to −51.6 ◦C and could be associated with the glass-rubber transition of the amorphous phase of the polyvinyl alcohol composites while the higher amorphous glass transitions, Tg2, varies from 5.7 ◦C to 14.6 ◦C and could be assigned to the relaxation of the PVA crystalline domains, representing the temperature range where amorphous, rubbery, and crystalline domains coexist [39,42,43]. Alginate and the chelating agents employed in these decontamination coatings also influence glass transition due to the interactions with the polymeric chain. The hydrogen bonds established between the hydroxyl groups of PVA and the other components of the strippable coating, influence the polymer chains mobility; therefore, both glass transition temperatures will be different for each strippable coating, depending on their composition, as can be observed in Table 3.

**Figure 6.** Tan Delta of strippable coatings.

**Table 3.** Characteristic temperatures \* for the polymeric films.


\* Tg1 and Tg2 correspond to the maximum value of each of the two tan delta peaks.

After characterizing the peelable coatings employed for decontamination, the next objectives consisted of the evaluation of the decontamination efficiency. This goal was accomplished through various analytical techniques: SEM-EDS, AAS, and radiation measurements.

The morphological evaluation of the targeted surfaces, prior- and post-decontamination, offered a preview on the decontamination efficiency through the qualitative information offered by this technique. Since decontamination performances also depend on the type of surface that requires decontamination, SEM/EDS analyses were carried out to observe the contaminant behavior on three types of stainless-steel surfaces: mirror-finish (SSMF), grinded-finish (SSGF), and etched-finish (SSEF). SEM analyses are made in a controlled environment, which cannot be contaminated with radioactive materials; thus, for these analyses, we employed 133Cs as simulant for 137Cs. For the controlled contamination of the above-described surfaces, a 0.005 M Cs2SO4 aqueous solution was utilized. The dispersion-pattern of the contaminant on each surface depends on the roughness of the employed material. As shown in Figure 7a, when applied to a mirror-finish metallic surface, contaminant droplets tend to clump in one place. The molecules of these aqueous droplets are held together by strong cohesive forces that are intermolecular attractive forces, which leads to the formation of large crystals that exhibit low-adhesion forces towards the mirror-finish metallic surface; therefore, they were easily removed using strippable coatings method. When surface roughness increases, the contaminant droplets tend to spread much more along the axes of the cracks: linear direction on grinded-finish stainless steel (Figure 7b), and dendritic direction on etched-finished stainless steel (Figure 7c), thus penetrating the interstices of the material.

**Figure 7.** SEM images of cesium surface contamination patterns on different types of surfaces (before decontamination): (**a**) mirror-finish stainless steel, (**b**) grinded-finish stainless steel, and (**c**) etched-finish stainless steel.

> For the controlled contamination process, we tried to reproduce cesium contamination at levels representative of those found in the nuclear industry, namely we started with 5 mM (C1) of Cs2SO4 aqueous solution, a similar concentration with those mentioned in the literature [7,44]. After being subjected to the controlled contamination process, stainlesssteel samples were further analyzed by the EDS technique, the mean values, and standard deviations for the determined and calculated values are reported in Table 4. The results obtained through SEM-EDS technique for the metallic samples subjected to controlled "artificial in-depth contamination", with C1 at 700 ◦C, did not render major differences in terms of decontamination factors obtained, since cesium could not be detected on the tested surface after decontamination. The only difference, when compared to superficial contamination with C1, consisted of the aspect of the contaminated surface after being subjected to the thermal treatment and the visible solid residues entrapped in the polymeric films (Figure S2 Supplementary Material). Since this attempt led to quite similar results with the samples subjected to "superficial" contamination, only the results obtained for superficial contamination are worth to be further detailed.

> As expected, the relative atomic concentration (at.%) [45] was higher on mirrorfinish surfaces where the droplets tended to agglomerate, and was lower in the cases of grinded-finish and etched-finish surfaces, according to the increasing number of surface imperfections, as, in these cases, the contaminant spread on the higher surface while

also entering the pores and cracks of the metallic sheets, leading, at the same time, to in-depth contamination.


**Table 4.** EDS analysis of the control sample.

\* Detailed in Supplementary Material in Figures S3–S5 and Tables S2–S4, respectively. The values reported for wt% and at% were calculated using the Bruker ESPRIT QUANTAX software of the scanning electron microscope. \*\* Abs. Errors (%) were calculate by Bruker ESPRIT QUANTAX software at 3 sigma for a single determination.

After the controlled contamination step, the decontamination solutions were poured over the contaminated surfaces and allowed to dry. All four types of decontamination solutions (GD-0, GD-3, GD-3-PBTC, and GD-3-IDS) were tested on mirror-finish, grindedfinish, and etched-finish stainless steel samples. Figure 8 illustrates the stainless-steel surfaces after the peeling process. Even though some polymer traces, probably resulting from the peeling process, are slightly observable, cesium crystals were not visible anymore. Thus, this information can serve as a screening method for the preliminary performance evaluation of the strippable films, thus confirming that all the four strippable coatings can be considered efficient for this type of surfaces, because the amount of residual cesium was undetectable after decontamination. Even if supplementary investigations were performed to prove and quantify the decontamination efficiency, SEM-EDS analysis offered, however, a reasonable preview of decontaminated surfaces.

**Figure 8.** SEM images of surfaces (after decontamination): (**a**) mirror-finish stainless steel, (**b**) grinded-finish stainless steel and, (**c**) etched-finish stainless steel. (These surfaces were contaminated with 0.005 M Cs2SO4 (C1) solution and decontaminated with GD-3-IDS.)

SEM-EDS analysis was also performed at higher contamination levels compared to those found in the literature, for a rough estimation of the upper limit of Cs retention in the polymeric coatings. For this purpose, two supplementary concentrated Cs2SO4 aqueous solutions (0.05 M (**C2**) and 0.5 M (**C3**)) were employed to find a correlation between the amount of Cs on the metallic surface and the upper limit of Cs retention in the polymeric coatings. The behavior of these strippable coatings on other types of metallic surfaces was also evaluated using galvanized metal (GM), brass (BS), and cooper substrates (CS). These three types of metallic surfaces (GM, BS, and CS) were contaminated with 0.005 M (**C1**), 0.05 M (**C2**), and 0.5 M (**C3**) Cs2SO4 aqueous solutions. Contamination and decontamination were performed according to the same procedure (described for **C1**) for all samples, but different amounts of residual cesium were still detected on some of the metallic surfaces, as can be observed in Table 5. Successful decontamination results were obtained for the metallic coupons contaminated with the lowest concentration of Cs (**C1**). For **C1**, after peeling the strippable decontamination coatings, undetectable amounts of residual cesium or a maximum of 0.66 Cs (at. %) for SC2-PBTC were measured for GMS. When the level of

contamination was increased, using 0.05 M (**C2**) and 0.5 M (**C3**) Cs2SO4, decontamination efficiency seemed to decrease. From Table 5, it can be noticed that after decontamination, residual Cs levels increased, according to the contamination level (C3 > C2 > C1), but the residual levels are also influenced by the type of the surface and the decontamination solution employed. Even so, for the usual contamination levels (**C1**) found in the literature for the nuclear industry, GD-3-IDS displayed the best decontamination performances, for all the tested surfaces; because in this case, the residual cesium was undetectable after decontamination. Figure 9 showed that employing different types of metallic surfaces led to Cs2SO4 crystals with distinct shapes, depending on the interaction with these metals. Therefore, on brass surfaces, the crystals have an elongated hexagonal shape, and on galvanized metal and copper surfaces, the crystals have a dendritic shape. When comparing the SEM images from Figure 9 with the ones illustrated in Figure 8, corelated with the data from Table 5, it can be noticed that as expected, the polymeric nanocomposite films become less efficient at higher levels of contamination (concentrations higher than 0.005 M). Thus, if this extreme situation were to be encountered in real contamination circumstances, several repeated decontamination procedures would be required for the complete removal of the contaminant. Another solution could be to increase the concentration of the chelating agent, but this might induce modifications in the properties of the strippable nanocomposite film, which is not desirable because it might reduce its performance.



It is useful to reiterate that the SEM-EDS method was employed only as screening method, the data obtained being useful only as preliminary results and complementary information for further investigations. Supplementary analyses with higher accuracy, which offer the possibility of better quantifying the contamination/decontamination levels, calculated according to AEP-58 NATO standard, were further employed (AAS technique and measurement of the activity of the radioactive materials) and are described in the following section of the study.

#### *Decontamination Tests*

Heavy metal uptake capacity of the strippable coating was evaluated by applying the decontamination solutions on glass surfaces contaminated with lead (Pb), strontium (Sr), and cobalt (Co), employed in this experiment as simulants for their radioactive analogues, were also subjected to the same procedure of analysis. Atomic adsorption spectrometry (AAS) was utilized to assess the concentration of "toxic metals" before and after decontamination. The data obtained from AAS analysis were utilized to calculate the decontamination efficacy of each solution, for every metal. The decontamination factor, DF = 100 (C0 − Cf)/C0 (DF is the decontamination factor, C0 is the initial metal concentration, and Cf is the final concentration) was calculated. Figure 10 illustrates a

comparative plot comprising the results obtained for glass surfaces contaminated with Pb, Sr, and Co.

**Figure 9.** Exemplification of SEM images after decontamination and EDS elemental mapping of Cs, S, and O (the metallic surfaces: (**a**) brass surface (BS), (**b**) galvanized metal surface (GM), and (**c**) copper surface (CS) contaminated with 0.05 M Cs2SO4 (C2) solution and decontaminated with GD-3-IDS.

**Figure 10.** Efficacy of the strippable coatings for toxic metals removal illustrated (DF calculus based on the concentration of metal before and after decontamination).

For each metal tested, different decontamination degrees were reached, depending on the intensity of the interaction between the targeted metal and the surface on which it was deposited and the interaction with the components of the decontamination solutions. As it can be observed in Figure 10, the reference sample (Bk0), exhibited lower decontamination efficiency for all the three metals tested, because, in this case, metal uptake involved only the physical interactions established between the nanoclay adsorbent and the contaminant. The metallic contaminant was entrapped and fixed in the matrix of the nanocomposite, and it was removed along with the exfoliation of the dry nanocomposite film. The influence of SA on the decontamination efficiency can be deduced from the values obtained and presented in Figure 10 with Alg label attributed to GD-3 decontamination solution. The chelating ability of sodium alginate significantly improved the decontamination performances, obtaining higher DF for all the metals tested. The "green chelates", IDS and PBTC had a significant positive influence on the decontamination efficacy of the solutions, confirmed by the results presented in Figure 10 (Alg-PBTC refers to GD-3-PBTC and Alg-IDS refers to GD-3-IDS). These results showed that IDS led to higher decontamination factors (as anticipated from the preliminary data obtained from SEM-EDS analysis). Therefore, these experiments proved the influence of each of the components on the performances of the decontamination solutions, showing that GD-3-IDS, especially, can be successfully employed for the removal of heavy metals from the contaminated surfaces.

Figure 11 offers information about the presence and abundance of the metals in the nanocomposite coatings after decontamination. However, because the process of obtaining clear AAS solutions required intermediary steps in this case (described in Methods Section 2.2), we cannot retrieve the expected concentration of metal in the values obtained, probably because a part of the metallic contaminant was fixed by bentonite. Even so, most of the contamination can be retrieved in Figure 11, while the results summarized in Figure 10 showed that on the decontaminated surface the toxic metals were only found as traces.

**Figure 11.** (**a**) Concentration of toxic metals found in the peeled nanocomposite film after decontamination; (**b**) SEM image of the nanocomposite film after decontamination of lead; (**c**) EDS image displaying C, O, and Pb elements present in the film obtained after decontamination of lead.

The most relevant tests in this study are represented by the decontamination investigations performed on radioactive materials. For this purpose, we used five different types of surfaces (metal-M, painted metal-PM, plastic-P, glass-G, and CBRN protective material—BC/SP2) and three radioactive solutions to generate alpha (241Am), beta (90(Sr-Y)), and gamma (137Cs) contamination. The decontamination solution employed for these experiments was GD-3-IDS, due to its remarkable efficiency, sustained by the previous results obtained in this study. The decontamination method herein described is in accordance with AEP-58 NATO standard. This military standard also establishes the minimal requirements for a new decontamination method to be considered efficient: the decontamination factor must have a value higher than 90% [4]. The results depicted in Figure 12 demonstrate that this criterion was successfully reached by GD-3-IDS decontamination solution. As can be seen, all the decontamination factors obtained indicated values above 90%, thus demonstrating that GD-3-IDS polymeric nanocomposite coatings efficiently removed radioactive contamination.

**Figure 12.** Decontamination factors obtained for GD-3-IDS decontamination solution in two distinct scenarios: (**a**) medium contamination and (**b**) high contamination (PM—painted metal; M—metal; P—plastic; G—glass; and BC/SP2—CBRN protective material).

In the first scenario described in Figure 12 implied a medium contamination level of 30 ÷ 80 Bq/cm2 while the second scenario implied a high contamination level of <sup>300</sup> ÷ 800 Bq/cm2. 241Am was tested only in medium contamination scenario, due to safety reasons. From Figure 12, it can be noted that decontamination efficiency of GD-3- IDS varies with the type of targeted surface, probably due to the different adherence of the polymeric matrix to each substrate and with the type of radionuclide employed for contamination. This can be explained by the different interactions established between the contaminant and the contaminated surface, and between the contaminant and the components of the decontamination solution (GD-3-IDS).

The advanced performances of the decontamination solutions developed in this study are sustained especially by the high contamination scenario, because even at this high contamination level, GD-3-IDS still managed to reach the 90%DF imposed by NATO standards. Therefore, even if the values obtained for DF were lower in high contamination than in the case of medium contamination, the strippable nanocomposite films can still be considered efficient in removing 241Am, 90(Sr-Y), and 137Cs contaminants.

#### **4. Conclusions**

Novel environmentally friendly surface decontamination solutions were successfully prepared based on PVA/SA/GLY/BT/IDS or PBTC aqueous mixtures. After deposition and during drying the solutions were capable to entrap heavy metals and radionuclide contaminants using physical and chemical processes by generating a composite polymeric film with good wetting capability on a large variety of solid surfaces displaying peeling-off capacity.

Chemical, mechanical, and thermal characterization of the polymeric films were performed using FTIR, tensile tests, TGA, and DMA techniques showing the influence of each component and allowing further optimization and selection of the formula for decontamination performance tests.

Controlled contaminations were performed on various surfaces metals (with different finishes), painted metal, plastic, glass, and CBRN protective material, according to NATO standard AEP-58 and using typical concentrations (or above) for contaminated sites found in the literature.

The decontamination effectiveness was first evaluated in a qualitative manner using SEM-EDS techniques followed by a quantitative approach employing AAS and surface activity measurements on live radioactive agents. The influence of SA, IDS, PBTC concentrations, and surface type over the DF was also emphasized. The presence of sodium alginate, and especially of chelating agents IDS/PBTC, decisively improves the decontamination factor.

Solution GD-3-IDS containing 5% PVA, 1% BT nano-clay, 2.5% glycerol, 0.75% sodium alginate, and 1% IDS chelating agent showed the best results as having decontamination factors that overpassed the DF imposed by NATO standards [4], DF being ≥90% for all surfaces tested and for the highest contaminations.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/article/10 .3390/polym13234194/s1. Figure S1—Different types of "polishing" for the stainless-steel coupons employed for decontamination tests prior to contamination step; Figure S2—Contamination (a) immersion in Cs2SO4 aqueous solution (C1 = 0.005 M); (b) "in-depth" contamination at 700 ◦C; (c) metallic coupons after in-depth decontamination; and Decontamination processes (d) applying one of the decontamination solutions; (e) peeled film; (f) different types of metallic samples prepared for SEM-EDS analysis after decontamination; Table S1—AAS instrumental parameters; Figure S3—EDS spectrum for SSMF contaminated with Cs; Table S2—Values obtained from EDS analysis for SSMF contaminated with Cs; Figure S4—EDS spectrum for SSGF contaminated with Cs; Table S3—Values obtained from EDS analysis for SSGF contaminated with Cs; Figure S5—EDS spectrum for SSEF contaminated with Cs; Table S4—Values obtained from EDS analysis for SSEF contaminated with Cs.

**Author Contributions:** Conceptualization, G.T., D.P., A.D., T.R., E.R. and R.G.; methodology, G.T., D.P., L.L. and T.R.; software, A.M., D.P., B.P., F.D., O.I. and M.U.; validation, G.T., D.P., A.D., T.R. and E.R.; formal analysis, G.T., D.P., A.D., A.M., R.G., A.P., A.M., B.P., L.L., F.A., S.A.B. and O.I.; investigation, G.T., D.P., A.D., A.M., R.G., F.A., S.A.B., L.L., F.D., A.P., A.M. and B.P.; resources, G.T., T.R., A.M., A.D., L.L. and R.G.; data curation, G.T., D.P., F.D., M.U., O.I., A.P., A.M., B.P. and F.A.; writing—original draft preparation, G.T., D.P., T.R., A.D., E.R. and R.G.; writing—review and editing, G.T., D.P., T.R., A.M., A.D., E.R., A.P., R.G., F.A., O.I., S.A.B., B.P., M.U., F.D. and L.L.; visualization, G.T., D.P., R.G., A.M. and F.D.; supervision, G.T., A.D., T.R., L.L. and E.R.; project administration, G.T., A.M., R.G., A.D., L.L. and T.R.; funding acquisition, G.T., A.M., R.G., L.L., A.D. and T.R. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by a grant of the Ministry of Research, Innovation and Digitization, CNCS/CCCDI—UEFISCDI through the National Project PN-III-P2–2.1-PTE-2019–0400, ctr. No. 49PTE/2020. L.L. would like to acknowledge the financial support of the Romanian Ministry of Research and Innovation and UEFISCDI through the Project PN-III-P2-2.1-PED-2019-1411, and Core Program PN19-03 (contract no. 21 N/08.02.2019).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

**Acknowledgments:** A.D. gratefully acknowledges financial support from the Competitiveness Operational Program 2014–2020, Action 1.1.3: Creating synergies with RDI actions of the EU's HORIZON 2020 framework program and other international RDI programs, MySMIS Code 108792, Acronym project "UPB4H", financed by contract: 250/11.05.2020. Authors are also grateful to Lanxess for donating the complexing agent BAYPURE® CX 100 and BAYHIBIT®. The article processing charges were supported by the Military Technical Academy "Ferdinand I".

**Conflicts of Interest:** The authors declare no conflict of interest.
