U(VI) Adsorption onto Low Dose Radiation Acclimated Tradescantia Fluminensis

Jun Liu (  edwardnuclear_liu@163.com ) Chengdu University of Technology Junxiang Shu Chengdu University of Technology Shilong Shi Chengdu University of Technology Chao Li Chengdu University of Technology Bing Li Chengdu University of Technology Dabiao Liang Chengdu University of Technology Fei Li Chengdu University of Technology Tu Lan Sichuan University Jiali Liao Sichuan University Ning Liu Sichuan University Kunming Zhang Xinjiang Uygur Autonomous Region Research Institute of Measurement and Testing


Introduction
Hexavalent uranium can easily migrate in the environment, resulting in many cases of uranium contamination to the pedosphere and hydrosphere. Several tens of mg/L uranium could be measured around the waste disposal sites, especially in some acid mine water (Li et al. 2015b). Besides, the migration and enrichment of U(VI) in human body through the food chain could cause severe toxic injury and radiation damage to the kidneys, livers and other organs. For human health and ecological security protection reasons, uranium removal from contaminated nuclear industrial e uent makes a lot of sense.
Adsorption takes advantages of low cost, operation simplicity, eco-friendly technology and high wastewater treatment e ciency (de Freitas et al. 2019). Many biomass adsorbents, such as coir pith (Parab et al. 2005), Trichoderma harzianum (Akhtar et al. 2007), Rhizopus arrihizus (Wang et al. 2010), Arthrobacter (Carvajal et al. 2012), tea wastes (Li et al. 2015a), Saccharomyces cerevisiae (Zheng et al. 2018), marine fungus (Han et al. 2020) etc. have been tested. For example, Aytas et al. (Aytas et al. 2011) found that algae and yeast immobilized on silica gel could improve interaction properties between the bi-functionalized biocomposite and U(VI) ions. Bayramoglu et al. (Bayramoglu et al. 2015) improved U(VI) adsorption capacity via the synthesis and application of polyethyleneimine and amidoxime modi ed Spirulina platensis. The binding functional groups such as carboxylate, hydroxyl, amidoxime and others on live, dead or modi ed biomass surface play a key role in coordinating U(VI) in solutions (Wang et al. 2015). However, the biomass modi cation methods mainly focused on chemical modi cation, researches on radiation effects on biomass mainly aimed at the assessment of enzymatic activity or production of microorganisms. Potential radiation effects, especially for plants are rarely studied.
Highly energetic particles produced by radiation could destroy the cell structure or induce the chemical reactions to attack cell. The major difference between radiation damages of animals and plants' cells is that the former is fatal, whereas plants display excellent cell turnover and regeneration property. Plants growing in nuclear radiation exclusion zone may also have the repair mechanism of DNA protection. Early reports (Amiro &Sheppard 1994) found that even the dose rates reached up to 65 mGy/ h, the herbaceous plant community still thrived. Low dose radiation (LDR) has a certain stimulation on plants include the dormancy breaking, germination promotion, acceleration of growth and development, rooting induction, increase of yield and disease resistance improvement, etc (Zaka et al. 2004). Besides, the meristem in plants is believed to have sensitivity to radiation, and the mechanism may be involved in the repairing enzymes activity caused by the biological organism injury. But as far as we know, no studies reported on U(VI) adsorption onto plants that are growing in radiation area. Moreover, studies on U(VI) adsorption onto these unique biological samples will be conducive to deducing the potential effects of LDR acclimation on U(VI) adsorption e ciency by biomass adsorbents.
Tradescantia uminensis is a perennial evergreen herb that grown in warm and humid climatic conditions, native to central Brazil, Uruguay and Paraguay and belong to commelinaceae. Herein, U(VI) adsorption e ciency was rst determined for both wild and LDR acclimated Tradescantia uminensis samples that collected from radiation area in Southwest China. The potential effects of LDR acclimation were deduced, and the key factors such as contact time, pH, ionic strength and U(VI) concentraion affected on U(VI) adsorption had been conducted. In addition, U(VI) loaded samples were also characterized by FTIR, SEM-EDS, TG-DSC, and XPS for purpose of the possible reactive mechanisms discussion.

Characterization
Thermo Nicolet 6700 spectrophotometer (USA) was used for recording the FTIR to identify the surface functional groups. Surface morphology and EDS analyses were conducted using a ZEISS SUPRA 40 (Germany) and X-Max (UK). A TGA/DSC2 apparatus (Mettler-Toledo, Switzerland) carried out the TG-DSC data acquisition. The electronic structure information was obtained by an Axis-Ultra, Kratos (UK).

Collection of Tradescantia uminensis biomass
Tradescantia uminensis biomass was collected from the radiation area (Located in the grassland of Applied Nuclear Techniques in Geosciences Key Laboratory of Sichuan, Chengdu University of Technology, Sichuan, China) that contains the Th-232 isotope radioactive source over a period of 10 years. Two kinds of Tradescantia uminensis biomass were tested, i.e. the wild Tradescantia uminensis growing in the natural background radiation environment (< 214 ± 37.4 nGy/h) and the LDR acclimated Tradescantia uminensis growing in the radiation environment that exceeding the maximum background limits (> 214 ± 37.4 nGy/h) (Fig. 1). Two kinds of Tradescantia uminensis biomass samples are randomly and equally collected from the range of the above two area, respectively.

Pretreatment of Tradescantia uminensis biomass adsorbents
Silt and sand on the fresh and sliced biomass (with roots) were dried at 90°C for 24 h after rinsing with running water. Then transferred it to a mortar and grind to powder with a 200-mesh sieve. The treated biomass was sealed and stored in a desiccator as use.
2.5 U(VI)determination U(VI) concentration was measured on a UV-2450 UV-VIS spectrophotometer (SHIMADZU, Japan) following the Arsenazo-III spectrophotometric method (Liu et al. 2016a). And the only improvement is that the percent content of Arsenazo-III decreased from 0.1-0.06% for economic considerations.

Adsorption experiments
Adsorption of U(VI) by Tradescantia uminensis was conducted using static equilibrium method. 0.01 g Tradescantia uminensis was added into 25 mL U(VI) working solutions with various concentrations after pH adjustment. The mixture was continuously shaken at 25℃ until the adsorption equilibrium was achieved. Uranium concentrations in the supernatant liquid were determined via the mentioned above method at 651.8 nm. The adsorption capacity q e (mg/g) was calculated as follows: where C 0 and C e refer to the initial and equilibrium U(VI) concentrations (mg/L), respectively, V is the solution volume (L), and m is the biomass adsorbent mass (g).

Effect of impact factors
Kinetics of the adsorption process could be determined by the study of time effect. Clearly, during the rst 15 min, U(VI) adsorption amount increased rapidly then reached almost saturation at 180 min ( Fig. 2A). The ultimate uranium adsorption capacity of the wild and LDR acclimated Tradescantia uminensis were ~15.2 mg/g and ~19.0 mg/g, respectively.
U(VI) adsorption on the wild and LDR acclimated Tradescantia uminensis as a function of initial pH value was shown in Fig. 2B. The adsorption amount increased linearly form 0.5 and 2.0 mg/g to 15 2015) indicated that cellulose, treated by high absorbed dose irradiation in oxygen, formed -C=O and -COOH groups because of the oxidative degradation. Accordingly, uranium adsorption capacity of LDR acclimated Tradescantia uminensis enhanced because of the higher amounts of the carboxyl groups existed on adsorbent's surface compared to the wild one.
The competition between cations and UO 2 2+ for the active sites may also control the adsorption process.
Herein, NaNO 3 , a key electrolyte in nuclear wastewater, was selected as the affecting factor, and the results are displayed in Fig. 2C. One can observe a good tolerance of Tradescantia uminensis even the Na +

Adsorption kinetics and isotherms
Adsorption kinetics study aimed at evaluation of adsorption e ciency and mechanism controlling. Herein, different kinetic models, i.e. the pseudo-rst-order model and the pseudo-second-order model were tted.
Eqs. (2) and (3) expressed the the linear form: ln(q e -q t )=lnq e -k 1 t 2 t q t = 1 where q t is the U(VI) adsorbed amount (mg/g) at any time t, k 1 (min −1 ) and k 2 (mg/g min −1 ) are the pseudorst-order and pseudo-second-order rate constants. The tting results and the relevant kinetics parameters are plotted and listed in Fig. 3 and Table 1, respectively.
A higher correlation coe cient (R 2 ) and a value of calculated q e2 that is close to q e,exp based on pseudosecond-order model indicates that adsorption process may be involved in sharing or exchanging of electrons between the adsorbent and adsorbate, in other words, chemisorption dominates the rate-limiting step of the reaction. Gül et al. (Gül et al. 2019) suggested that UO 2 2+ adsorption on lichen mainly through the electrostatic interactions and the interaction of the carboxyl groups. Khani (Khani 2011) indicated that uranium removal by Padina sp. algae biomass could be associated with ion exchange mechanism. Considering the abundant functional groups, such as -COOH and -OH existed on Tradescantia uminensis surface, they could provide surface active sites for uranium adsorption. Furthermore, the q e values of the LDR acclimated Tradescantia uminensis was always higher than that of the wild Tradescantia uminensis, indicating the potential improvement U(VI) adsorption capacity of Tradescantia uminensis affected by LDR acclimation.
Bhat et al. (Bhat et al. 2008) found that uranium adsorption curve data of Catenella repens (a red alga) could be well described by both the linearized Langmuir and Freundlich adsorption isotherms at pH 4.5. Therefore, adsorption isotherms had also been evaluated ( Fig. 7 and Table 2).
The equilibrium data was evaluated by the Langmuir and Freundlich models, which could be given by linear Eq. (4) and (5):   After U(VI) adsorption, the band at 1626 and 1384 cm −1 that related to -COOH and -OH gradually weakened ( Fig. 4c and d). Besides, a new IR absorption bands correlated with the O-U-O stretching for U(VI) appeared at 925 and 917 cm −1 (Jain et al. 2018). These may be acceptable evidence for the surface complexation between carboxyl and hydroxyl groups and U(VI) occurring on Tradescantia uminensis surface.

SEM-EDS
The surface morphologies and element types could be observed and detected via the SEM and EDS analyzes. Obviously, C and O are the mainly elements on the plant surface, and the morphologies of leaves and stems with plant bers displayed clearly on the Tradescantia uminensis surface (Fig. 5). Actually, any

TG-DSC
The TG-DSC curves of the wild and LDR acclimated Tradescantia uminensis before and after U load were shown in Fig. 6. As shown, two stages, associated with approximate 15.0~20% (room temperature tõ 100°C) and 46~48% (100°C to 500°C) weight loss in Tradescantia uminensis, could be divided. The loss of the moisture and low organic content of the samples and the pyrolysis of organic matter such as cellulose, hemicellulose and lignin could be the main reasons (Müsellim et al. 2018). For all heating rates, an exothermic behavior could be found in the reactions on the basis of DSC plots.

XPS
The bonding environment and surface chemical composition of the wild and LDR acclimated Tradescantia uminensis before and after U load were analyzed through XPS. U4f peak, presented in wide scan data (Fig.  7a) on U load sample, was the clear evidence for the U adsorption ability of Tradescantia uminensis. Further analysis of valence state from the tting curves of U4f spectrum high-resolution XPS spectra (Fig. 7b) 2014) also found that the O1s spectra upon γ-irradiation for few-layered graphene materials displayed few variations, and they seem to occur quite randomly. However, the normalized intensity of the hydroxyl increased for both the U(VI)-loaded Tradescantia uminensis.
Three peaks can be observed in C1s XPS spectra for Tradescantia uminensis samples, as depicted in Fig.  7d. The peaks at binding energy of ~284.8, 286.3±0.2 and 288.2±0.2 eV can be related to the intensity of C=C, C-O (hydroxyl) and C=O. Increasing irradiation dose weakened the relative intensity of the C-O (hydroxyl) and C=O bands. Wan et al. (Wan et al. 2005) also noted that the amount of -OH in C1s region of carbon bers decreased after gamma irradiation. Besides, the hydroxyl normalized intensity also increased for both U(VI)-loaded Tradescantia uminensis, which is in line with the results of O1s.

Probable adsorption mechanisms
On the basis of FTIR and XPS spectra, the amounts of carboxyls and hydroxyls on the surface of the wild Tradescantia uminensis increased after LDR acclimation, accompanying by the -OH and -COOH groups coordination for U(VI). Moreover, multilayer chemical adsorption on a heterogeneous surface was described well for the U(VI) adsorption by both the wild and LDR acclimated Tradescantia uminensis according to the isotherm and kinetics analyses. In general, the complexation of carboxyls and hydroxyls on the surface of the LDR acclimated Tradescantia uminensis played a key role in UO 2 2+ adsorption. Besides, the results of batch adsorption experiments correlated well with the characterization data. The probable mechanisms of U(VI) adsorption on the wild and LDR acclimated Tradescantia uminensis were schematically shown in Fig.  8.

Conclusions
In summary, we rst report a plant, i.e. Tradescantia uminensis colleted from the radiation area for adsorption of uranium. Due to the potential LDR acclimation effect, the uranium adsorption capacity of Tradescantia uminensis improved, and the q e values the the wild and LDR acclimated Tradescantia uminensis were about 16 mg/g and 20 mg/g when pH initial was 4.5 and C 0 was 50 mg/L. Adsorption kinetics demonstrated a multilayer adsorption on Tradescantia uminensis surface. Higher pH and initial uranium concentration bene ted the U(VI) adsorption process, whereas ionic strength has no obvious effect on the q e values. Characterization results suggested that the carboxyl and hydroxyl groups on Tradescantia uminensis surface may complex U(VI), and the LDR acclimated Tradescantia uminensis contained a larger number of the two groups than the wild ones. Besides, good structural stability of the LDR acclimated plant remained during the U(VI) adsorption process. In summary, the LDR acclimated Tradescantia uminensis may be a novel candidate for radioactive wastewater uranium removal.

Declarations
Availability of data and materials Not applicable Funding Financial support from the National Natural Science Foundation of China (Grant No. 22006004) and the Fundamental Research Funds for the Central Universities is gratefully acknowledged.