1 Introduction

Nuclear energy is an green source for electricity generation, due to the characteristics of safety, efficiency, low-cost, and minimal emission of greenhouse gases [1, 2]. However, inappropriate handling or accidents due to leakage of nuclear material can cause the unintended release of radionuclides, which makes effective management of radioactive waste a challenging issue. It poses a high risk to human health and results in a long-term contamination for the environment [3, 4]. The disposal of volatile radionuclides is particularly challenging, since they can easily diffuse by the water or atmosphere. Radioiodine is the crucial isotope of concern, owing to its high radioactivity and chemical toxicity [5,6,7]. In general, 131I isotope has a short half-life period of about 8 days and relatively high radioactivity, whereas 129I is a fission product with a long half-life period of 1.57 × 107 years and it strongly interferes with human metabolic processes, which causes serious health problems [8, 9]. Therefore, prompt and effective treatment of radioiodine is urgently required.

Many technologies have been developed for the capturing of radioactive iodine from the exhaust gas stream, such as wet scrubbing and solid adsorption. As compared to the wet scrubbing method, solid adsorption is considered as an effective and convenient strategy for iodine removal, owing to its high capacity, simple operation, easy post-treatment, and so on [7, 10, 11]. However, the use of traditional activated carbon as an adsorbent for iodine is less favorable, due to its flammability and unfavorable conditions of regeneration experiments [12]. Silver-containing materials have a strong affinity for iodine and remarkable high-temperature resistance, but high manufacturing cost and toxicity are their main disadvantages [5, 7]. Alternative materials, such as metal-organic frameworks (MOFs) [13], covalent organic frameworks (COFs) [14, 15], and conjugated microporous polymers (CMPs), usually possess high iodine loading capacity, due to their high specific surface areas, adjustable porosity characteristics, and accessibility for surface functionalization. However, they possess low structural stability, high-cost, and complicated preparation process [9, 16]. Hence, the development of efficient and low-cost iodine capturing methods that are environment-friendly and easy-to-operate, is highly desirable.

Biomass-based components are natural renewable resources for functional materials design [17, 18], and have the potential for capturing of gases in the nuclear reactor. In addition, they have the advantages of low-cost, non-toxic, and possess abundant binding sites [10, 19, 20]. Studies have shown that the recognition and adsorption of iodine molecule are due to the numerous active sites, such as –OH, –COOH, –NH2, –NHR, etc. [9, 21,22,23]. Particularly, biomass-derived porous materials are more favorable for adsorption applications, because of their ultra-low density and high specific surface areas [24, 25]. In our previous work, a composite collagen aerogel was prepared with polyphenol modification by a simple cross-linking reaction, which showed an excellent capture capacity (222.67 wt%) for iodine vapor [9]. The advantages, such as high adsorption capacity, environmental friendliness, and low-cost raw materials make the biomass-based porous material an ideal adsorbent for capturing of radioactive iodine. Although these materials have the advantages of facile preparation and high capturing capacity, their practical applications have always been limited by inherent flammability, which could cause easy migration of iodine or security issues [26]. In fact, the adsorption is usually performed at high temperature to expedite the adsorption process and remove the competitive water on the matrix [16, 27]. Incorporation of flame retardant additives is a common strategy to improve the flame retardancy of biomass-derived porous material. Previous studies have shown hydroxyapatite (HAP) to be a candidate because of its good biocompatibility, non-toxicity, especially high thermal stability [28, 29]. Importantly, HAP showed excellent flame retardancy in cellulose-based aerogel and flexible inorganic nanocomposite paper, and the synergistic flame retardancy in nanomaterials of poly(ethylene-co-vinyl acetate) [28,29,30].

Herein, a composite gelatin cryogel (HAP@Ge) with high flame-retardancy, was prepared by immobilization of HAP nanorods on Ge cryogel. This strategy was expected to not only endow the Ge cryogel with excellent flame retardancy, but also improve the adsorption capacity for iodine vapor. Furthermore, the flame retardancy and iodine adsorption mechanism of the HAP@Ge cryogel were confirmed in detail with cone calorimetric experiments and X-ray assisted studies. Overall, this study aimed to determine whether the combination of Ge cryogel and HAP nanorods could provide a useful strategy for radioiodine treatment.

2 Experimental section

2.1 Materials

Gelatin powder (AR), iodine (AR) and other reagents, including calcium chloride (CaCl2, AR), sodium hydrogen phosphate (Na2HPO4, AR), sodium hydroxide (NaOH, AR), hydrochloric acid (HCl, 37%), and anhydrous ethanol (CH3CH2OH, AR) were all provided by Kelong Chemical Co. Ltd. (Chengdu, China).

2.2 Preparation of HAP nanorods

Firstly, 500 mL of 0.08 mol/L CaCl2 solution (pH 11) was prepared. 200 mL of Na2HPO4·12H2O solution (0.12 mol/L) was slowly added into CaCl2 solution while vigorously stirring and keeping the pH value constant with 1 mol/L NaOH solution. Then, the beaker containing the white suspension was placed in a 70 ºC water bath and stirred continuously for 12 h. Finally, the sample was washed more than three times using deionized water and freeze-dried for 72 h to obtain HAP nanorods.

2.3 Preparation of HAP@Ge cryogel

4 g of gelatin powder was dissolved in deionized water and stirred at 50 ºC for 30 min to ensure complete dissolution. Then, different amounts of HAP (the mass ratios of hydroxyapatite to gelatin were 0:1, 0.2:1, 0.5:1, and 1:1) were added into 50 mL of the gelatin solution (2 wt %) and continuously stirred for 20 min to obtain uniform mixtures. Next, 0.1 mL of glutaraldehyde (50 wt %) was also dripped into the above solutions and stirred for 30 min to complete the cross-linking. Thereafter, the mixtures were poured into plastic petri dishes (with 90 mm diameter) and pre-treated at 4 ºC for 1 h. Finally, the samples were freeze-dried for 72 h to obtain HAP@Ge composite cryogels with different HAP contents, which were labeled based on the HAP to Ge ratio as HAP@Ge(0/1), HAP@Ge(0.2/1), HAP@Ge(0.5/1), and HAP@Ge(1/1).

2.4 Characterization

The mapping of elements and morphological analysis of samples were performed by scanning electron microscopy (SEM, Ultra55, Carl Zeiss, Germany) and transmission electron microscopy (TEM, Libra200, Carl Zeiss, Germany). The FT-IR spectra were obtained by Fourier transform infrared spectroscopy (FT-IR, Spectrum Two N, PE, USA) within a spectral range of 400–4000 cm−1. Cone calorimetry experiments were conducted on cryogel samples with dimensions of 30 mm × 25 mm × 6 mm when exposed to an external heat flux of 30 kW/m2, which used an FTT standard cone calorimeter (FTT0007, British) based on ISO 5660 standard procedures. The temperature changes of cryogels with time were recorded by infrared thermography (FLIR T540, USA) to analyze the dynamic heat transfer and dissipation properties of samples. The binding energies of the elements in samples were measured by X-ray photoelectron spectroscopy (XPS, Escalab250, Thermo, Fisher Corporation, USA). Thermal analysis of adsorbents was conducted by thermogravimetric analysis, and the temperature was changed from room temperature to 800 ºC with a heating and cooling rate of 10 ºC/min under nitrogen atmosphere (TGA/SDTA851e, Mettler Toledo, Switzerland). UV–Vis absorption spectra were recorded on a UV–Vis spectrophotometer (UV-5500PC, Shanghai Metash Instruments Co. Ltd., Shanghai, China).

2.5 Adsorption of iodine vapor

The iodine vapor adsorption experiments were performed according to a previous literature reference, with some modifications [11]. Briefly, 1 g of iodine was put in the bottle and 0.1 g of HAP@Ge cryogel was placed on a filter paper above the iodine crystal. Then, the bottle was capped and heated at 75 ºC for a certain time. Next, the bottle was cooled to room temperature and taken out for analysis. Adsorption experiments without iodine were also conducted under the same conditions, which served as the control. The adsorption capacity for iodine by HAP@Ge cryogel was calculated according to the following equation:

$${q}_{{\rm t}}=\frac{{m}_{{\rm t}}-{m}_{0}}{{m}_{0}}\times 100\%$$
(1)

where qt (mg/g) is the iodine adsorption capacity and m0 (g) and mt (g) are the masses of the samples before and after iodine adsorption, respectively.

3 Results and discussion

3.1 Characterization of HAP@Ge cryogel

Figure 1a–c show the preparation process and applications of HAP@Ge cryogel. The composite cryogel was prepared by simple crosslinking reaction and it possessed ultra-high porosity and abundant functional groups, that are required for effective capturing of iodine. HAP nanorods could be fixed in the porous structure of Ge cryogel through cross-linking of gelatin chains. After cross-linking, the originally white color of the HAP@Ge cryogel changed to light yellow (Additional file 1: Fig. S1). The effect of HAP content on the microstructure of HAP@Ge cryogel was studied by preparing the composite cryogels with different HAP contents. All the HAP@Ge cryogels exhibited uniform surfaces and porous structures. Meanwhile, the addition of HAP increased the compressive strength of the Ge cryogel, which acted as a support under certain pressure (Additional file 1: Fig. S2).

Fig. 1
figure 1

a–c Schematic illustration of the preparation process and applications of HAP@Ge cryogel. d, e SEM images of pure Ge and HAP@Ge cryogels. f Nitrogen adsorption and desorption isotherms of HAP@Ge cryogels with different mass ratios of HAP and Ge

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The surface morphologies of Ge and HAP@Ge cryogels were studied by SEM. As depicted in Fig. 1d, Ge cryogels exhibited a porous structure consisting of many enclosed flakes, whereas HAP@Ge(0.5/1) cryogel contained complex and irregular flakes with high specific surface area (Fig. 1e). However, with higher content of HAP (1/1), the structure was compact, which was unfavorable for adsorption of iodine (Additional file 1: Fig. S3). TEM images in Additional file 1: Fig. S4 showed that HAP possessed a rod-like structure with diameters in the range of 10–20 nm and lengths between 20 and 60 nm. More importantly, EDX results demonstrated the successful combination of HAP with Ge cryogel. It also showed the homogeneous dispersion of HAP in this cryogel (Additional file 1: Fig. S5). The surface areas and porosities of HAP@Ge cryogels with different HAP contents were also analyzed by nitrogen adsorption-desorption experiments. In Fig. 1f, the N2 adsorption curve of Ge cryogel exhibited almost no hysteresis. Compared with Ge cryogel, the adsorption curve of HAP@Ge cryogel grew steeper, the hysteresis loop broadened, and the specific surface area and pore volume also increased (as shown in Table 1). This indicated the presence of highly mesoporous and macroporous structures after the fixation of HAP. This could be attributed to the accumulation of HAP on the structure of Ge cryogel.

Table 1 Porosity parameters of the HAP@Ge aerogels
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3.2 Flame retardancy of HAP@Ge cryogel

3.2.1 Reaction to Fire and cone calorimetry results

To investigate the flammability property of Ge and HAP@Ge cryogels, the samples was placed on an asbestos mesh, which was fixed above an alcohol burner and make it contact with the flame. The digital photographs of samples at different times are depicted in Additional file 1: Fig. S6. It can be seen that Ge cryogel burned very quickly after being ignited, resulting in some residual char of random shape. In the case of HAP@Ge cryogels, HAP could prevent the ignition of Ge in a highly efficient manner and also mechanically support the porous residues. Obviously, the residues of HAP@Ge cryogels retained their initial state.

Cone calorimetry is identified as a useful test technique to evaluate the flammability performance of many materials. The relevant parameters such as time to ignition (tign), time to PHRR (tp), heat release rate (HRR), especially peak of HRR (PHRR), and the rate of smoke release (RSR) for Ge and HAP@Ge cryogels are presented in Fig. 2a–c and Table 2. It can be seen that Ge cryogel burned very rapidly and showed a PHRR value of 190 kW/m2 at 14 s. Compared to Ge cryogel, PHRR values of HAP@Ge(0.2/1), HAP@Ge(0.5/1), and HAP@Ge(1/1) cryogel were reduced by 7.0%, 10.4%, and 43.5%, respectively. Moreover, they showed much prolonged tign and tp values. The tign increased from 3 s for Ge to 12 s for HAP@Ge cryogels, whereas tp increased from 14 to 32 s. Meanwhile, the peak of RSR significantly decreased from 1.548 to 0.867 with increasing HAP loadings. This indicated that HAP@Ge cryogel had significant flame retardancy. Fire performance index FPI and fire growth rate FIGRA, which were defined as tign divided by PHRR and PHRR divided by tp, respectively, are two significant parameters that could be obtained from the data of cone calorimetry to analyze the safety of materials. A better flame retardancy for materials are suggested by the lower FIGRA values and higher FPI values. As seen in Table 2, FPI of HAP@Ge cryogel increased from 0.0157 to 0.112 and FIGRA decreased from 13.597 to 3.358, as compared to pure Ge cryogel. This was consistent with the previous results.

Fig. 2
figure 2

The curves of a HRR, b RSR, and c CO2 of pure Ge and HAP@Ge cryogels under heat flux of 30 kW/m2. d Schematic diagram showing heat transfer and dissipation mechanism of HAP@Ge cryogel

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Table 2 Cone calorimeter data of pure Ge and HAP@Ge composite aerogels under a heat flux of 30 kW/m2
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Apart from the decrease in heat release rate, suppression of smoke production during burning is important to minimize the fire hazards. As shown in Fig. 2b and Table 2, the rate of smoke release (RSR) and total smoke release (TSR) of HAP@Ge cryogel decreased with an increase of HAP loading ratios though it had a low ratio of CO/CO2. This result indicated that HAP@Ge cryogel showed a improved smoke suppression performance compared with Ge cryogel. The TSR and RSR values decreased from 21.244 to 12.136 and 1.549 to 0.867, respectively, which suggested that the immobilization of HAP can not only enhance the flame retardant property but suppress the release of smoke, thus improve fire safety of Ge cryogel.

3.2.2 Dynamic heat transfer and dissipation properties

The heat transfer property for these cryogels was evaluated by recording the temperature changes as a function of time. All cryogels with dimensions of 3 cm × 2.5 cm × 0.6 cm were placed on the center of a plate equipped on thermostatic heater at the set temperature of 155 °C, as illustrated in Fig. 3a. The temperatures at the lateral-surfaces of Ge and selected HAP@Ge(0.5/1) cryogels could be monitored using an by an infrared imaging devices, then relevant digital images at different times were compared. It was evident that the temperature of Ge cryogel underwent a faster increase during the first 10 s than HAP@Ge(0.5/1). The temperature (P2 point) at the center increased from 45.7 °C to 67.2 °C (ΔT = 21.5 °C) in the initial 10 s for HAP@Ge cryogel, while the increase was 46.4 °C to 82.7 °C (ΔT = 36.3 °C) for Ge cryogel (Fig. 3b and c). This indicated that HAP@Ge cryogels exhibited poor thermal conductivity, suggesting an excellent heat insulation performance during the heating process. After 120 s, both the samples showed similar digital photographs, which indicated that the samples reached a state of thermal equilibrium, and formed a stable gradient distribution of temperature within them.

The heat dissipation property of Ge cryogel and HAP@Ge cryogel are shown in Fig. 3e. Prior to the recording of the optical photographs, all cryogels were placed on the plate of a heater to attain thermal equilibrium and transferred immediately onto a support at room temperature. The Ge and HAP@Ge cryogels showed the similar heat dissipation tendency. Both the cryogels exhibited high rate of heat dissipation during the initial 10 s and decreased to a low value to reach thermal equilibrium at 60 s. However, HAP@Ge cryogel presented a reduction in temperature with a lower rate compared with Ge cryogel. The temperatures at the centers of HAP@Ge and Ge cryogels decreased from 93.5 °C to 64.5 °C (ΔT = 29 °C) and 97.2 °C to 40.3 °C (ΔT = 56.9 °C), respectively. The results showed that HAP@Ge cryogel could effectively prevent rapid heat release to the environment and thereby address the safety concerns due to heat accumulation. Therefore, HAP present in the Ge cryogel acted as a thermal barrier and slowed down the heat transfer inside the material and also the dissipation of heat to the environment, as illustrated in Fig. 2d. This can effectively prevent the occurrence of safety incidents caused by further thermal degradation or combustion of this cryogel.

Fig. 3
figure 3

a Schematic illustration of the experimental setup and selected point (P2). Real-time variations in lateral-surface temperature with time during heating b, c and d, e cooling for pure Ge and HAP@Ge (0.5/1) cryogels

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3.3 Iodine vapor capture of HAP@Ge cryogel

The adsorption of iodine by HAP@Ge cryogel was investigated by conducting a series of adsorption experiments. The iodine adsorption capacity of HAP@Ge cryogels increased rapidly within 12 h, decreased after 12 h, and then reached an adsorption equilibrium as depicted in Fig. 4a. The maximum adsorption capacity at equilibrium was > 1837 mg/g for all HAP@Ge samples. The high iodine adsorption capacity and rapid rate could be ascribed to the multi-scale porous structure and immobilization of HAP on the cryogel surface. Further studies of the adsorption process were conducted by fitting the experimental data, and the detailed steps are shown in Fig. 4b, c and Table 3. It can be seen that the data of adsorption kinetics for iodine vapor by HAP@Ge cryogel was more consistent with the pseudo-second-order model, which showed a correlation coefficient R2 of 0.9986. This indicated that the adsorption process was chemical adsorption.

Table 3 The parameters of pseudo-first-order and pseudo-second-order kinetic models of HAP@Ge(0.5/1) aerogel for iodine vapor
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Moreover, the adsorptivity of HAP@Ge cryogel for iodine vapor increased with an increase in HAP content (Fig. 4d). The pure Ge cryogel had a low iodine vapor capture capacity of 1096 mg/g at equilibrium. On the other hand, HAP@Ge cryogel with different loadings (0.2/1, 0.5/1 and 1/1) exhibited the I2 adsorption capacities of 2240, 2693, and 1837 mg/g, respectively. Meanwhile, the maximum adsorption capacity was reached when the mass ratio of HAP to Ge was 0.5/1. Interestingly, pure HAP nanorods showed almost no adsorption of iodine (~ 3.5 mg/g) (Additional file 1: Fig. S7). This could be explained by the fact that the increased number of HAP nanorods could provide more active sites for iodine capture during the initial period, which resulted in the increase in iodine vapor capture capacity. However, further increase in HAP content led to a decrease in porosity of the Ge cryogel, as shown in Fig. S3, that led to a low capacity of iodine vapor capture. For pure HAP nanorods, the porous structure could no longer support the HAP nanorods, which led to poor dispersivity. This was unfavorable for the diffusion of iodine molecules in the material, thus worsening the adsorption of iodine. Nonetheless, the adsorption results were better than most of the reported biomass-based materials, such as collagen-based adsorbents [9,10,11], MOF-cellulose composite [24], lignin-based nanofibers [19], etc. In addition, the above mentioned adsorbent materials required complicated preparation process and were costly. Compared to them, HAP@Ge cryogel was developed by using simple synthesis process based on low-cost raw materials that derived mainly from industrial leather waste. Further, this cryogel had strong flame retardancy, which made it an attractive biomass-based adsorbent for iodine vapor capture.

Thermogravimetry was used to investigate the thermal stability of HAP@Ge cryogel as shown in Fig. 4e. It was evident that the Ge sample decomposed at 280 ºC and the mass loss reached 73.4% at about 500 ºC. However, for HAP@Ge cryogel, the values of mass loss decreased with an increase in HAP content and reached 47.3% when the mass ratio of HAP and Ge was 1/1. This indicated that HAP could improve the thermal stability of Ge cryogel, due to the thermal shielding effectiveness of the HAP immobilized in Ge cryogel, resulting in higher residual weight [31]. Notably, the mass loss for HAP@Ge cryogel after iodine adsorption reached 49.2%. The increase in mass loss from 80 to 280 ºC was due to iodine loss. It indicated that the absorbed iodine on HAP@Ge cryogel was lost as a result of physical sublimation.

The stability and recyclability of iodine capture by HAP@Ge cryogel were also investigated. Results showed that iodine could be released from HAP@Ge cryogel when placed in ethanol solution. Obviously, the color of ethanol became dark brown due to the rapid desorption of iodine. Moreover, this iodine-containing solution was diluted 10 times using ethanol and analyzed by UV–Vis spectrophotometry (Fig. 4f). It showed that the iodine desorption in ethanol solution reached a state of equilibrium within 12 min.

Fig. 4
figure 4

a Effect of time on the iodine vapor adsorption performance of HAP@Ge cryogels. Fitted curves of adsorption kinetics with b pseudo first order and c pseudo second order kinetics model. d Iodine adsorption capacities of HAP@Ge cryogels. e TGA curves for HAP@Ge cryogels. f Photographs of the desorption process of HAP@Gecryogel-I2 in ethanol solution and UV–Vis absorption spectra for desorption

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3.4 Iodine adsorption mechanism

The inset image in Fig. 5a showed that the color of HAP@Ge cryogel changed to black and there was a shrinkage in the volume of the absorbent after the capturing of iodine vapor. More importantly, the surface structure and morphology of the material showed obvious changes after iodine adsorption. The pores on the surface of the cryogel seemed to be filled with adsorbed iodine, resulting in a denser structure (Fig. 5a). Furthermore, EDX elemental mapping was used for determining the elemental composition of the HAP@Ge cryogel-I2. It was clear that the distribution of iodine was mainly uniform on surface of HAP@Ge cryogel, which proved the successful capturing of iodine by the HAP@Ge cryogel (Fig. 5b). Meanwhile, Additional file 1: Fig. S8 showed the emergence of strong peaks due to high content of iodine after capturing of iodine. This result showed the presence of large amount of iodine in the material and further proved the successful adsorption of iodine by HAP@Ge cryogel.

The functional groups present in different samples were identified by FT-IR spectrometry, as illustrated in Fig. 5c. FT-IR spectrum of Ge cryogel showed characteristic peaks ascribed to amide I, amide II, amide III, and hydroxyl groups at 1633 cm−1, 1547 cm−1, 1238 cm−1, and 3291 cm−1, respectively. In comparison to Ge cryogel, the spectrum of HAP@Ge cryogel showed the characteristic peaks of HAP, including the absorption peak of 1027 cm−1, which was attributed to the stretching modes of PO43−groups. Moreover, the peaks at 597 cm−1 and 563 cm−1 were assigned to the bending modes of the O-P-O bonds of PO43− groups [31, 32]. Further, the peak corresponding to –OH strengthened after immobilization of HAP, which indicated that HAP was successfully immobilized on the HAP@Ge cryogel. Obviously, after capturing of iodine, the peaks belonging to amide I, amide II, and amide III shifted from 1637 cm−1 to 1619 cm−1, 1545 cm−1 to 1523 cm−1, and 1239 cm−1 to 1228 cm−1, respectively. In addition, the characteristic absorption peaks of P-O bond were changed from 597 cm−1 to 601 cm−1 and 563 cm−1 to 559 cm−1. These results proved that capturing of iodine vapor using HAP@Ge cryogell was mainly dominated by the C=O and –NHR groups of gelatin and PO43− groups of HAP.

XPS analysis was further conducted to determine the chemical states of the elements in the samples. Figure 5d shows the XPS spectrum of HAP@Ge cryogel, before and after iodine adsorption. The two peaks at 618 eV and 631 eV appeared after iodine adsorption that corresponded to I3d5/2 and I3d3/2, respectively, which indicated a successful adsorption of iodine on HAP@Ge cryogel. The I3d peaks of HAP@Ge cryogel-I were deconvolved into three components (Fig. 5e), including I− (617.97 and 629.52 eV), I3− (618.56 and 630.12 eV) and I2 (620.31 and 631.78 eV) [33,34,35]. The O1s spectrum of HAP@Ge cryogel (Fig. 5f) was deconvoluted into two peaks of 530.9 eV and 531.58 eV assigned to –OH and C-O-R, respectively. However, the two peaks of –OH and C=O after capturing of iodine shifted from 530.9 eV to 531.19 eV and 532.99 eV to 531.88 eV, respectively,. This indicated that the –OH and C=O groups induced the adsorption of iodine. Meanwhile, it was evident that the peak of N1s barely underwent changes after loading of HAP, but it shifted from 399.2 to 339.8 after capturing of iodine (Fig. 5g). This indicated that –NHR groups were not involved in HAP loading, but participated in the adsorption process for iodine vapor [36]. Meanwhile, the two characteristic peaks of C1s, 287.45 eV (C=O) and 285.79 eV (C-N), shifted to 288.01 eV and 286.13 eV, respectively (Fig. 5h), which proved that C=O and C-N played a strong part in iodine adsorption [37, 38]. Furthermore, the peak of P 2p (Fig. 5i) shifted from 132.93 eV to 134.03 eV after adsorption of iodine due to the strong reaction between PO43− and iodine molecules [39]. Results analysis confirmed that the capturing of iodine by HAP@Ge cryogel was based on the inducing of adsorption by –OH, C=O, –NHR and PO43− groups.

Fig. 5
figure 5

a SEM image of HAP@Ge cryogel-I2 (Inset is the digital photograph of HAP@Ge cryogel after iodine adsorption). b EDX elemental mapping of HAP@Ge cryogel-I2. c FT-IR spectra of Ge cryogel, HAP@Ge cryogel and cryogel-I2. d Full survey XPS spectra of HAP@Ge cryogel and HAP@Ge cryogel-I2, respectively. High-resolution XPS spectrum of e I 3d, f O 1s, g N 1s, h C 1s and i P 2p before and after iodine adsorption

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The mechanism of iodine vapor capture on HAP@Ge cryogel was proposed according to the results analysis, as shown in (Fig. 6). Firstly, the porous structure in HAP@Ge cryogel offered effective channels for the diffusion of iodine, whereas the numerous complex and irregular flakes provided sufficient active sites for the capturing of iodine. In addition, iodine molecules acted as Lewis acid, due to which the PO43− groups on HAP@Ge cryogel enhanced the capturing sites that resulted in high adsorption capacity for iodine. The electrostatic attraction between PO43− groups and iodine caused the polarization of iodine molecules and formed a polarization state of I− [40, 41]. As the concentration of iodine increased, a complex state of I3− was obtained since the I− could further combine with another iodine molecule [22, 35], which was consistent with the analysis results from XPS and FT-IR data. Meanwhile, the –NHR groups of HAP@Ge sample could induce the adsorption of iodine by the transfer of lone pair of electrons [42]. In addition, some iodine molecules were captured through physical adsorption. Therefore, the presence of sufficient number of active sites and multi-scale porous structure was contributed to the effective adsorption of iodine on HAP@Ge cryogel, which enhanced the chemical binding and physical capturing for iodine.

Fig. 6
figure 6

The schematic diagram of iodine capture mechanism by HAP@Ge cryogel

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4 Conclusions

In this work, a composite HAP@Ge cryogel was prepared by immobilizing HAP nanorods on Ge cryogel for enhancing the capturing of iodine. This method was more economical and convenient, as compared with most adsorption materials. The as-prepared HAP@Ge cryogel showed slower dynamic heat transfer and dissipation rate. The immobilization of HAP in Ge cryogel caused a significant improvement in its flame retardancy and smoke suppression properties. Particularly, the material containing 50 wt% HAP nanorods reduced by 43.5% in PHRR value and retained its original shape after ignition. The results of iodine adsorption of the material showed that the immobilization of HAP nanorods could also greatly enhanced the effective capturing of iodine, which resulted in a high adsorption capacity. The maximum equilibrium adsorption capacity of HAP@Ge cryogel for iodine vapor reached 2693 mg/g. The high adsorption capacity for iodine was attributed to the multi-scale porous structure in HAP@Ge cryogel, which offered effective channels for the diffusion of iodine, whereas the numerous flake-like structures provided sufficient number of active sites for iodine adsorption. Importantly, the synergistic effect between HAP nanorods and Ge cryogel was involved in the porous structure of HAP@Gecryogel, wherein, HAP nanorods provided more active sites for Ge cryogel and the Ge cryogel endowed the HAP nanorods with good dispersivity. During the capturing of iodine by HAP@Ge cryogel, the chemical adsorption mechanism was dominant, due to the induction of adsorption by PO43−, –OH, C=O, and –NHR groups on HAP@Ge cryogel. Moreover, complex porous structure of HAP@Ge cryogel was beneficial for the efficient physical adsorption of iodine. Thus, HAP immobilized HAP@Ge cryogel has high potential in applications as a low-cost, safe, and efficient adsorbent for the effective capturing of iodine vapor in the exhaust gas stream of post-treatment plants.