Cheap and biodegradable amino acid-based deep eutectic solvents for radioactive iodine capture via halogen bonds

Journal Pre-proof Cheap and biodegradable amino acid-based deep eutectic solvents for radioactive iodine capture via halogen bonds

Yu Chen, Honglian Liang, Xinbo Qin, Yi Liu, Shuang Tian, Yuanyuan Yang, Shujun Wang PII:

S0167-7322(19)36967-3

DOI:

https://doi.org/10.1016/j.molliq.2020.112615

Reference:

MOLLIQ 112615

To appear in:

Journal of Molecular Liquids

Received date:

20 December 2019

Revised date:

25 January 2020

Accepted date:

29 January 2020

Please cite this article as: Y. Chen, H. Liang, X. Qin, et al., Cheap and biodegradable amino acid-based deep eutectic solvents for radioactive iodine capture via halogen bonds, Journal of Molecular Liquids(2020), https://doi.org/10.1016/j.molliq.2020.112615

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© 2020 Published by Elsevier.

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Cheap and biodegradable amino acid-based deep eutectic solvents for radioactive iodine capture via halogen bonds Yu Chen*, Honglian Liang, Xinbo Qin, Yi Liu, Shuang Tian, Yuanyuan Yang, Shujun Wang*

Department of Chemistry and Material Science, Langfang Normal University, Langfang 065000, Hebei, China

Yu Chen, E-mail: [email protected]; Phone: +86-316-2188211; Fax: +86-316-2112462.

*

Shujun Wang, E-mail: [email protected]; Phone: +86-316-2188211; Fax: +86-316-2112462.

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*

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Abstract: Entrapment of hazardous radioactive iodine has attracted much attention due to the

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release of radioactive iodine during the application of nuclear energy, such as nuclear waste disposal and nuclear disaster. Deep eutectic solvents (DESs) are deemed as green solvents,

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designable solvents and solvents of the 21st century. Amino acids are cheap, highly

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biocompatible and highly biodegradable. Here, we for the first time develop amino acid-based DESs for iodine capture with high efficiency. Effect of structural factors (e.g., mole ratio,

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composition and composition) and external factors (e.g., mass, concentration and volume of iodine solution) on iodine capture is comprehensively studied. Cysteine:lactic acid (1:8) owns

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the highest rate and capacity for iodine capture, which shows similar efficiency with previous reports but owns lower cost and higher biodegradability. The high iodine-capturing efficiency of cysteine:lactic acid (1:8) is ascribed to the dominating halogen-bonding interaction (ca. 90%). This work opens a new route for the sustainable capture of radioactive iodine with high efficiency.

Keywords: Deep eutectic solvents; Dynamic process; Halogen-bonding interaction; Ionic liquids; Kinetics; Radioactive iodine capture.

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1. Introduction Nuclear energy is the efficient and clear energy consuming little energy and occupying little space. In accompany with the nuclear energy, there is much nuclear waste released. Some of the nuclear waste (e.g., fission of radioactive metal) is produced unavoidable and controllable; however, the nuclear waste from leak and disaster (e.g., Fukushima nuclear explosion and Chernobyl nuclear accident) is enormous, hazardous and catastrophic. Human health and ecological system would be severely damaged for decades even over hundreds of years due to

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the super-long half-life of radioactive elements. Radioactive iodine (e.g.,

125/129/131

I2) is one of

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the radioactive elements.[1] Long-term contact with radioactive iodine is deemed as one of

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the carcinogenic factors of thyroid cancer.[2] The hard decay of some radioactive iodine isotope could be exemplified by the ca. 15.7 million years half-life of

129

I2.[3] The threat of

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radioactive iodine would need the efficient entrapment of radioactive iodine in a green,

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sustainable and cheap way.

Metal-containing materials are reported to show favorable iodine-removing efficiency.[4] the

metal-containing

materials

are

expensive,

scare

and

even

non

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However,

environmental-friendly. Thus, the iodine removal by metal-containing materials is not suitable

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for widespread industrial application. Metal-free ionic liquids (ILs) are one of the materials to capture radioactive iodine with high efficiency without the usage of metal elements.[4-9] Mu et al. conducted the pioneering work of iodine uptake by ILs in 2014.[4] Halogen-bonding interaction was found to the main form of interaction between iodine and ILs.[4] Owing to the halogen-bonds, the reported highest efficiency by Mu’s group could reach 96% within 5 h by [BMIM][Br].[4] Liu et al. found that imidazolate-based ILs could absorb 8.7 g and 17.5 g iodine per g IL at 30 oC and 100 oC, respectively.[7] Wang et al. further investigated the halogen-bonding interaction between iodine and ILs systematically and found that anion of ILs was the key factor affecting strength of halogen-bonds (i.e., Cl ≈ Br ≈ I > Ac ≈ NO3 > BF4).[5] Xue et al. developed polyethylene glycol-based ILs (e.g., PEG/NaI system) for the first time to achieve the highest iodine-removing efficiency of 86% within 3 h.[6] Addition of dimethylsulfoxide and dimethyl formamide increased the removal 2

Journal Pre-proof efficiency, while the presence of water, methanol and acetonitrile in PEG-based ILs depended on the nature of ILs.[6] However, ILs also own several drawbacks. For example, most of the ILs are expensive, hygroscopic[10, 11] and complicated to be synthesized up to now. ILs tends to decompose or evaporate at a not very high temperature after long-term exposure to heat, electricity or light.[12, 13] Moreover, many kinds of ILs were not readily biodegradable.[14] ILs might also contaminate the soil, air and water. Some researchers concluded that the common idea of ILs with low toxicity was not correct.[15] Therefore, it is still necessary and challenging to

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develop new metal-free, cheap, efficient and biodegradable absorbents for iodine capture.

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Deep eutectic solvents (DESs) are thus proposed for the efficient iodine entrapment.

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DESs include two components (i.e., hydrogen-bond donor and hydrogen-bond acceptor) or more connected by hydrogen bonds (H-bonds).[16-18] Apart from H-bonds, other kinds of

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interactions (e.g., pi-pi, halogen bonds and host-guest interaction) would also contribute to the

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formation of DESs.[19-21] DESs are proposed as the alternative to ILs due to the low toxicity, high biocompatibility, cheap raw material and easy synthesis procedure.[22, 23]

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Numerous components and colorful interacting types would render DESs as the highly designable solvents.[22] Therefore, DESs have been widely applied in many areas, such as

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CO2/SO2 uptake[24-26], biomass utilization[27], nanomaterial synthesis[28] and so on. Mu et al. for the first time utilized choline-based and ammonium-based DESs to capture radioactive iodine. ChNO3:methylurea (1:2), ChPF6:methylurea (1:2), ChClO4:methylurea (1:2) and ChBF4:methylurea (1:2) showed negligible iodine removal efficiency; however, ChCl:methylurea (1:2), ChBr:methylurea (1:2) and ChI:methylurea (1:2) could absorb iodine with

high

efficiency.[29]

Particularly,

ChI:methylurea

(1:2)

showed the

highest

iodine-removing efficiency via halogen-bonding interaction between iodine anion in DES with iodine molecule.[29] Chen et al. also has developed PEG-based DESs for iodine entrapment and investigated the corresponding factors.[30] It was found that the impact of mass and mole ratio was negligible; however, the presence of organic solvent, the change of molecular weight, and different components of DES would obviously affect the iodine removal efficiency.[30] PEG200:thiourea+ethanol mixture was the best iodine-capturing absorbent among all the pure and mixed PEG-based DESs investigated.[30] There are only 3

Journal Pre-proof two reports (i.e., above) related to the radioactive iodine capture by DESs up to now, to the best of our knowledge. Although there are many improvements for choline-based[29] and PEG-based[30] DESs to capture iodine, it is still necessary to develop new types of DESs in a cheap and green way. For example, the best DES ChI:methylurea[29] owns the expensive component ChI, which is not favorable for widespread application of DESs in radioactive iodine removal. Thiourea in the best PEG200:thiourea is also deemed as the cancerogen although PEG200 is green and sustainable. The simultaneous achievement of cheap, biodegradable and efficient iodine

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capture by DESs is still challenging.

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The success synthesis of amino acid-based DESs could be corroborated by previous

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reports.[31-33] Here, we for the first time use amino acid-based DESs for iodine entrapment. All the components of amino acid-based DESs are highly green and highly biocompatible.

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Moreover, the raw materials of amino acid-based DESs are very cheap. Results show that

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amino acid-based DESs could absorb radioactive iodine with high efficiency. Effects of mass, mole ratio, component, composition, volume and concentration of iodine solution are

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systematically investigated. In situ two-dimensional UV-Vis spectra show that the best cysteine:LA (1:8) uptakes iodine mainly via halogen bonds before 120 min (90%) and via

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induced force after 120 min (10%). The dominating halogen bonds is the main reason for high iodine removal efficiency of cysteine:LA (1:8). The iodine-capturing rate and efficiency by amino acid-based DESs is comparable to that of the reported references; however, amino acid-based DESs are cheaper and more biodegradable.

2. Material and methods 2.1. Materials LA (AR, 85-90%%) and L-lysine (98%) were purchased from Aladdin Biochemical Technology Co. Ltd.. L-leucine (99%), L-tryptophan (99%), L-cysteine (99%), L-methionine (98%), glycerol (99.5%) and glycine (99%) were purchased from Beijing Innochem Technology Co. Ltd.. Iodine (> 99.8%) was supplied by Sinopharm Chemical Reagent Co., Ltd.. Cyclohexane (99.5%) was bought from Fuchen Chemical Reagents Co., Ltd.. The above chemicals (Scheme 1) were used without further purification. 4

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2.2. Synthesis of DESs

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Scheme 1. Components of amino acid-based DESs and other chemicals investigated.

Amino acid-based DESs were synthesized with the methods similar to previous reports.[29,

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30, 34, 35] Simply, the components of DESs were mixed with a certain mole ratio and then heated with magnetic stirring at ca. 80 oC until a clear solution appeared. The success

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synthesis of amino acid-based DESs could be corroborated by previous reports [31-33] and infrared (IR) spectra (Figures S1 and S2). Furthermore, theoretic calculation also corroborated the successful synthesis of amino acid-based DESs by the change of bond length, interaction energy (Scheme S1) and charge distribution (Scheme S2). The water content was less than 800 ppm in the synthesized amino acid-based by Karl-Fisher Titrator. 2.3. Iodine capture measurement Iodine-removing process was similar to our previous report.[30] Typically, 0.2 g DESs (in quartz cuvette) was used to capture 2 mL iodine cyclohexane solution (0.4774 g L-1). Investigation on the effect of DESs mass, concentration and volume of absorbate on the iodine capture would change the mass, concentration and volume correspondingly. The iodine-absorbing process was detected by spectrophotometer (UV1200, Macylab Instruments Inc.) at 523 nm (Figure S3).[4, 29, 30] Absorbance was recorded at 1 min, 5 min, 10 min, 0.5 5

Journal Pre-proof h, 1 h, 1.5 h, 2 h, 3 h, 4 h, 5 h, 6 h, and 7 h, respectively. The three-time iodine-capturing experiments by selecting representative amino acid DES cysteine:LA (1:8) implied that data of iodine capture were reliable (Figure S4). 2.4. UV-Vis spectra and photograph DESs leucine:LA and cysteine:LA (0.2 g in quartz cuvette) were selected as the representative amino acid-based DESs to conduct in-situ UV-Vis spectra of iodine (2 mL cyclohexane solution) capture (UV2550, Shimadzu, Japan). The UV-Vis spectra lasted for 500 min with the interval of 5 min. Difference spectra were derived on the basis of the

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spectrum at the begin. Perturbation–correlation moving-window two-dimensional correlation

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spectroscopy (PCMW2D-COS) UV-Vis spectra were also deduced, which were similar to the

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previous reports.[30] Photograph was taken at a specific time point by loading 0.2 g DESs and 2 mL iodine cyclohexane solution in the glass bottle with the cap sealed.

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2.5. IR spectra

Representative DESs (i.e., leucine:LA (1:8), cysteine:LA (1:8)), their components and their

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mixture with iodine were selected to record IR spectra (Bruker Tensor 27, 4 cm-1, 60 scans

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and 400~4000 cm-1). IR spectra of liquid samples were recorded by coating on the surface of potassium bromide (KBr) pellets. IR spectra of solid samples were recorded by mixing with

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KBr powder to press homogenous pellets. During the IR-measuring process, allochroic silicagel was put nearby the sample to reduce the effect of moisture. 2.6. Theoretical calculation

The basis set DFT B3LYP/6-311++G(d, p) was used for theoretic calculation, which was the same as that of Mu’s report.[29] For the iodine atom, effective core potential basis set aug-cc-Pvdz-PP was applied.[5] Counterpoise for the Gaussian basis set superposition error was also applied. The optimized structures were determined after calculating several possible structures.

3. Results and discussion 3.1. Effect of structural factors on iodine capture Structural factors include mass of amino acid-based DESs, the presence of water, concentration and volume of iodine cyclohexane solution. Composition is the most important 6

Journal Pre-proof factor affecting the iodine capture as shown in Figure 1. The order of iodine-removing efficiency could be listed as cysteine:LA > methionine:LA > tryptophan:LA > glycine:LA > valine:LA ≈ leucine:LA independent of the mole ratio (Figure 1a). When increasing the mole ratio of amino acid to La, the difference in iodine capture between valine:LA and leucine:LA becomes smaller. When the mole ratio reaches 1:16, the iodine removal efficiency for valine:LA (1:16) and leucine:LA (1:16) is almost the same. Moreover, cysteine:LA and leucine:LA own the highest and lowest iodine-uptaking efficiency for DESs varying in composition, respectively.

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Effect of mole ratio of DESs on iodine capture is also studied. Cysteine:LA (1:8) and

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leucine:LA (1:8) are selected as the representative amino acid-based DESs for analysis

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(Figure 1b). Unexpectedly, we find that the impact of mole ratio could be ignored. For other kinds of amino acid-based DESs varying in mole ratio (Figures S5-S8), the effect of mole

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ration on the iodine entrapment is negligible or very little. Transforming the components from

iodine capture (Figure 1c).

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two (i.e., cysteine:LA) to three (i.e., cysteine:LA:glycerol) also has ignored influence on

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It thus could be concluded that the importance of factors is: composition > mole ratio ≈ components based on the data investigated. Tuning the composition would be the most

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efficient way for the improvement of the iodine removal efficiency. For all the amino acid-based DESs to capture iodine, cysteine:LA and leucine:LA own the highest and lowest iodine-capturing efficiency for DESs, respectively. Therefore, cysteine:LA is selected to investigated external factors on iodine capture. Moreover, cysteine:LA and leucine:LA are selected as two representative amino acid-based DESs for mechanism investigation, as discussed below.

7

(a) 1:8

1:16

1:12

100

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leucine:LA (1:12) leucine:LA (1:16) leucine:LA (1:8)

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cysteine:LA (1:8) cysteine:LA (1:12) cysteine:LA (1:16)

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Time / min

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cysteine:LA methionine:LA tryptophanine:LA glycine:LA valine:LA leucine:LA

cysteine:LA (1:8) cysteine:LA:glycerol (1:8:2) cysteine:LA:glycerol (1:8:4) cysteine:LA:glycerol (1:8:8)

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Removal efficiency / %

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Figure 1. Effect of structural factors on iodine capture by amino acid-based DESs: components with different mole ratio (a), mole ratio in cysteine:LA (b), mole ratio in

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leucine:LA (c) and components (d).

3.2. Effect of external factors on iodine capture

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Cysteine:LA (1:8) is selected as the representative amino acid-based DES to study the external factors due to the highest iodine-capturing efficiency. The investigated external

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factors are mass of cysteine:LA (1:8), concentration of water in water+cysteine:LA (1:8) mixture, concentration of iodine cyclohexane solution and volume of iodine cyclohexane solution.

Four cysteine:LA (1:8) samples with different mass (0.1 g, 0.15 g, 0.2 g, 0.25 g) are selected to study the effect of mass on the iodine capture. Results imply that the mass own negligible influence on iodine capture (Figure 2a). We had expected that increasing the weight of cysteine:LA (1:8) would increase the rate and the steady-state iodine capture capacity due to the higher content of absorbent. However, it is not true. Figure 2a shows that the rate and capacity of iodine removal is almost the same when the mass of cysteine:LA (1:8) is changed. It is highly possible that only the surface of cysteine:LA (1:8) dominates the iodine capture, while the interior part of cysteine:LA (1:8) is difficult for iodine molecule to penetrate. It might be due to the fact that the iodine-absorbing process is static without stir. We did not use magnetically stir during the iodine-capturing process for the purpose of 8

Journal Pre-proof keeping negligible interference on absorbance from stirred solution. We have found that the absorbance would change significant if we applied the stir. Thus, it is highly possible than iodine in cyclohexane is absorbed on the surface of cysteine:LA (1:8). The kinetic curves (Figures 1 and 2) of resembling Langmuir absorption could also corroborate the iodine absorption on the surface. Water is the ubiquitous contaminate, therefore, the effect of water in cysteine:LA (1:8) on iodine capture should also be investigated. Addition 20%, 40%, 60% (mass fraction) of water in cysteine:LA (1:8) while keeping the total mass the same would unexpectedly have

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negligible effect on iodine capture (Figure 2b). This result is consistent with the report by

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Chen.[30] It means that cysteine:LA (1:8) could be potentially used to capture iodine

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efficiently in the presence of humid environment. When the iodine cyclohexane is more diluted, the rate of iodine removal by cysteine:LA (1:8) is highly improved (Figure 2c).

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Similarly, the volume of iodine cyclohexane would also be a factor influencing iodine capture.

(a)

100

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80 60 40

0 0

0.1 g 0.15 g 0.2 g 0.25 g

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cysteine;LA (1:8) cysteine;LA (1:8) cysteine;LA (1:8) cysteine;LA (1:8)

20

Removal efficiency / %

100

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Removal efficiency / %

Namely, a higher rate of iodine removal is observed when the volume is less (Figure 2d).

100

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(b)

80 60 cysteine:LA (1:8)

40

0% water 20% water 40% water 60% water

20 0 0

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(c)

80 60

cysteine:LA (1:8)

40

iodine cyclohexane 20% 1.5 mL iodine cyclohexane 40% 1.5 mL iodine cyclohexane 80% 1.5 mL iodine cyclohexane 100% 1.5 mL

20 0 0

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(d)

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cysteine:LA (1:8)

40

iodine cyclohexane 1 mL iodine cyclohexane 1.5 mL iodine cyclohexane 2 mL iodine cyclohexane 2.5 mL

20 0 0

100

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300

400

500

Time / min

Time / min

Figure 2. Effect of external factors on iodine capture by representative amino acid-based DES cysteine:LA (1:8): mass of cysteine:LA (1:8) (a), concentration of water in water+cysteine:LA (1:8) mixture (b), concentration of iodine cyclohexane solution (c) and volume of iodine cyclohexane solution (d). 9

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3.3. Comparison with iodine capture by reported DESs Comparison with iodine capture by reported DESs is necessary. There are only two previous reports on the iodine capture by DESs. Mu et al. reported that ChI:methylurea owned the highest iodine-capturing efficiency.[29] Chen et al. found that PEG200:thiourea was the best PEG-based DES for iodine capture among all the PEG-based DESs investigated.[30] Here, we find that cysteine:LA (1:8) is the best amino acid-based DES for iodine capture. Comparison for the three DESs would conclude that cysteine:LA (1:8) in our work own the

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similar iodine removal efficiency with the previous reports (Figure 3). Although Chen et al.

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stated that the presence of ethanol in PEG200:thiourea would enhance the iodine-capturing

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rate, the high volatility of ethanol would be not very favorable for the widespread application of DESs-related system for practical iodine entrapment.

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More importantly, amino acid-based DESs including the best cysteine:LA (1:8) for

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iodine capture in this work is highly cheap and biodegradable. Both components (e.g., cysteine and La) of amino acid-based DESs are easily obtained and even edible. It means that

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amino acid-based DESs would not be harmful to the human beings and the ecosystems. However, ChI in ChI:methylurea is expensive,[29] which would hinder the industrial

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application for iodine capture. Thiourea in PEG200:thiourea[30] is redeemed as the type III cancerogen by International Agency for Research on Cancer of World Health Organization. Therefore, iodine capture by amino acid-based DESs is very promising for widespread application due to the low cost, high accessibility, high biodegradability and high degradability. Furthermore, compared to iodine capture by ILs,[4, 6, 7] the similar conclusion could also be drawn.

10

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80 60 ChI:methylurea, Mu et al. [26] PEG200:thiourea (1:2), Chen et al. [27] cysteine:LA (1:8), This work

40 20

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Removal efficiency / %

100

0

100

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0

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500

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Time / min

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Figure 3. Comparison of iodine-capturing efficiency by DESs: the best DESs from Mu et

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al.;[29] the best DES from Chen et al.;[30] the best DES from this work.

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3.4. Kinetics and dynamic mechanism

Cysteine:LA (1:8) and leucine:LA (1:8) are chosen as the representative amino acid-based

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DESs to investigate the kinetics and evolution mechanism between DESs and iodine. Kinetic equation W=W∞(1-e-kt)[30] is utilized as the representative model to fit the relationship between iodine capture capacity (W, %) and time (t, min), where k and W∞ are fitted parameters implying the iodine capture rate and steady-state iodine-removing efficiency, respectively (Table 1). R2 of the fitted curves for cysteine:LA (1:8) and leucine:LA (1:8) are 0.982 and 0.999, respectively (Table 1), suggesting a favorable regression with the model W=W∞(1-e-kt). The iodine-capturing capacity within 7 h (W7h) of cysteine:LA (1:8, 100%) is much higher than that of leucine:LA (1:8, 12.5%). It means that the iodine removal ability of cysteine:LA (1:8) is better than that of leucine:LA (1:8). This tendency could be corroborated by the value of W∞ for cysteine:LA (1:8, 98.2%) and leucine:LA (1:8, 15.1%). Iodine removal rate of cysteine:LA (1:8, k=0.024 % min-1) is ca. 6 times that of leucine:LA (1:8, k=0.004 % min-1). 11

Journal Pre-proof Table 1: Fitted and derived parameters of iodine capture by amino acid-based DESs with the equation W=W∞(1-e-kt). DESs

R2

k

W∞

W7h

/ % min-1

/%

/%

cysteine:LA (1:8)

0.024

98.2

100%

0.982

leucine:LA (1:8)

0.004

15.1

12.5

0.999

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The higher iodine removal efficiency of cysteine:LA (1:8) than leucine:LA (1:8) could also be demonstrated by photograph and IR spectra. The color of iodine solution captured by

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cysteine:LA (1:8) becomes violet into nearly white within 1.5 h; however, it is still highly

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violet in leucine:LA (1:8) at 7 h for iodine capture (Figures 4a and 4b). Due to the high removal efficiency of iodine by cysteine:LA (1:8), the cap of glass bottle is nearly white

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(Figure 4c). The low removal efficiency of iodine by leucine:LA (1:8) would make iodine

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more easily volatile inner the bottle, thus leading to the bottle cap yellow (Figure 4d). The higher iodine absorption by cysteine:LA (1:8) than leucine:LA (1:8) could be ascribed to a

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higher halogen-bonding interaction between iodine and cysteine:LA (1:8) than between iodine and leucine:LA (1:8), as demonstrated by IR spectra as below. The IR band shift is obvious

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for cysteine:LA (1:8) after absorbing iodine (Figure 4e). Particularly, two new IR peaks appears and one IR peak disappears for cysteine:LA (1:8) before and after iodine capture (Figures S9 and S10). However, there is no new IR peak occurring or no IR peak disappearing for leucine:LA (1:8) before and after iodine capture (Figure S11).

12

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(a) 0 min 1 min 5 min 10 min 0.5 h

1h

1.5 h

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6h

7h

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(b)

(e)

cysteine:LA (1:8)+iodine

1631.8 1625.9

cysteine:LA (1:8) 1458.1 1456.2

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7h

Intensity

(d) 0 min

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-p

1638 1624 1610

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Intensity

1.5 h

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(c)

1h

Intensity

0 min 1 min 5 min 10 min 0.5 h

1464 1456 1448 1410 1400 1390

Wavenumber / cm-1

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Figure 4. Visualized color change of solution in the glass bottle for the iodine capture by

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cysteine:LA (1:8) (a) and leucine:LA (1:8) (b). Visualized color change of the cap of the glass bottle before and after the iodine capture by cysteine:LA (1:8) (c) and leucine:LA (1:8) (d).

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IR band shift of cysteine:LA (1:8) before and after iodine capture (e).

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Then, we apply UV-Vis spectra to demonstrate difference in iodine capture between cysteine:LA (1:8) than leucine:LA (1:8). Conventional UV-Vis spectra show that absorbance reduces significantly for iodine capture by cysteine:LA (1:8, Figure 5a), while absorbance reduces only a little for iodine capture by leucine:LA (1:8, Figure 5b). It corroborates again a higher iodine removal efficiency of cysteine:LA (1:8) than leucine:LA (1:8). Difference spectra display more evident evidence, i.e., obvious difference for cysteine:LA (1:8, Figure 5c) while little difference for leucine:LA (1:8, Figure 5d). The evolutional mechanism of amino acid-based DESs to capture iodine is also studied by PCMW2D-COS UV-Vis spectra. Synchronous mode (s-PCMW2D-COS) of iodine capture by cysteine:LA (1:8, Figure 5e) and leucine:LA (1:8, Figure 5f) suggests that the iodine-capturing rate by cysteine:LA (1:8, -5.5) is much higher than that by leucine:LA (1:8, -0.41). This again supports that iodine-entrapping rate of cysteine:LA (1:8) is much higher than that of leucine:LA (1:8). Furthermore, asynchronous mode (as-PCMW2D-COS) shows 13

Journal Pre-proof that cysteine:LA (1:8, Figure 5g) and leucine:LA (1:8, Figure 5h) own dividing points at ca. 120 min and ca. 60 min, respectively. Halogen bonds dominate the iodine capture before the dividing point and induced force dominates after the dividing point.[30] It thus could be concluded that ca. 90% of iodine is captured via halogen-bonding interaction, while ca. 10% of iodine is entrapped via induced force by cysteine:LA (1:8, Figure 6a). However, the ratio of halogen of halogen bonds and induced force is 3% and 97% for the entrapment of iodine by leucine:LA (1:8), respectively (Figure 6b). It is consistent with the previous finding that halogen-bonding interaction is the main interaction for absorbents to capture iodine.[4, 29, 30]

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Higher ratio of halogen-bonding interaction between amino-acid DESs (e.g., cysteine:LA (1:8)

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vs leucine:LA (1:8)) and iodine would contribute to a higher iodine removal efficiency (e.g.,

-1.5

500 min

0.0 500

550

-2.0

600

Wavelength / nm 0.0 0 min

550

Intensity

5 min interval

1.0 500 min

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100% vs 12.5%).

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Figure 5. UV-Vis spectra of iodine capture by cysteine:LA (1:8) (a) and leucine:LA (1:8) (b) from 0 min to 500 min with the interval of 5 min. Difference UV-Vis spectra of iodine capture by cysteine:LA (1:8) (c) and leucine:LA (1:8) (d). S-PCMW2D-COS of iodine capture by cysteine:LA (1:8) (e) and leucine:LA (1:8) (f). As-PCMW2D-COS of iodine capture by cysteine:LA (1:8) (g) and leucine:LA (1:8) (h).

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(a)

(b) 3%

90%

10% 97%

Halogen bonds

Induced force

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Figure 6. Proportion of halogen bonds and induced force of iodine capture by cysteine:LA

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(1:8) (a) and leucine:LA (1:8) (b).

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To verify the halogen bonds between representative amino acid-based DESs (i.e., cysteine:LA and leucine:LA) and iodine, theoretic calculation is used to corroborate

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the halogen-bonding interaction. Interaction energy (ΔH) between DESs and iodine

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shows that ΔH of cysteine:LA+iodine (-20.5 kJ mol-1) is more negative than leucine:LA+iodine (-16.6 kJ mol-1, Scheme 2). It indicates a higher halogen-bonding

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interaction between cysteine:LA and iodine than between leucine:LA and iodine, which corroborating the higher iodine removal efficiency by cysteine:LA than by

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leucine:LA. The stronger intermolecular H-bonding interaction (ΔH=-56.0 kJ mol-1) between cysteine and LA in cysteine:LA does not prevent cysteine:LA binds iodine with a higher halogen-bonding interaction. It is because iodine mainly interacts with cysteine:LA and leucine:LA via I-S and I-O halogen bonds, respectively. A higher halogen-bonding interaction between cysteine:LA and iodine than between leucine:LA and iodine could also be verified by the more elongated I-I bond length in cysteine:LA+iodine (2.797 Å) than in leucine:LA+iodine (2.755 Å). Intermolecular H-bonding interaction for both cysteine:LA and leucine:LA is also changed after interacting with iodine. Apart from bond length and ΔH, the higher absolute change of iodine

atom

charge

in

cysteine:LA+iodine

(-0.382

and

0.511)

than

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leucine:LA+iodine (-0.179 and 0.332) provides additional evidence for explaining the higher iodine removal efficiency by cysteine:LA than by leucine:LA (Scheme S3). 15

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Scheme 2. Bond length (Å) and interaction energy of cysteine:LA, leucine:LA,

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cysteine:LA+iodine and leucine:LA+iodine. O (red), N (blue), S (yellow), C

4. Conclusion

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(grey), I (violet) and H (white).

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In summary, highly cheap, biocompatible and biodegradable amino acid-based DESs are for first time used to capture radioactive iodine efficiently. Mass of DESs, mole ratio, the presence of water, components have negligible effect on iodine entrapment; nevertheless, effect of other factors (e.g., composition, concentration and volume of iodine solution) on iodine capture is obvious. Cysteine:LA (1:8) and leucine:LA (1:8) are found to be the best and worst amino acid-based DESs to capture iodine, respectively. Compared to previous reports, the best cysteine:LA (1:8) in this work shows similar iodine-capturing rate and capacity but owns lower cost and higher biodegradability. The higher rate and capacity of cysteine:LA (1:8) than leucine:LA (1:8) could be ascribed to the higher ratio of halogen-bonds between cysteine:LA (1:8) and iodine (90%) than between leucine:LA (1:8) and iodine (3%). This work for the first time provides a new way for the sustainable treatment of radioactive iodine with efficient rate and capacity by amino acid-based DESs. Other 16

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Author information Corresponding author * *

Yu Chen, E-mail: [email protected]; Phone: +86-316-2188211; Fax: +86-316-2112462. Shujun Wang, E-mail: [email protected]; Phone: +86-316-2188211; Fax:

Notes

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The authors declare no competing financial interests.

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+86-316-2112462.

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Acknowledgment

This work was supported by the Natural Science Foundation of Hebei Province

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(B2019408018), the Fundamental Research Funds for the Universities in Hebei Province

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(JYQ201902) and the Self-financing Project of Science and Technology Research in Colleges

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Graphic Abstract

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Cheaper and more biodegradable amino acid-based DESs are for the first time found to

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capture radioactive iodine efficiently and sustainably.

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Journal Pre-proof Author Statement

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Chen Yu: Conceptualization, Methodology, Software, Writing-Reviewing and Editing, Project administration; Liang Honglian: Investigation, Supervision; Qin Xinbo, Liu Yi, Tian Shuang, Yuanyuan Yang: Data curation; Qin Xinbo, Liu Yi: Software, Formal analysis; Wang Shujun: Resources, Supervision.

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Journal Pre-proof Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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Highlights

Amino acid-based DESs is highly cheap and green

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Amino acid-based DESs captures iodine for the first time

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High efficiency and rate

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Halogen-bonding interaction dominates

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