Removel of Uranium from Aqueous Solutions by ... - Science Direct

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ScienceDirect Procedia Environmental Sciences 31 (2016) 382 – 391

The Tenth International Conference on Waste Management and Technology (ICWMT)

Removel of uranium from aqueous solutions by spirodela punctata as the mechanism of biomineralization Xiaoqin Nie1, Faqin Dong2*, Mingxue Liu3, Shiyong Sun2, Gang Yang3, Wei Zhang4, Yilin Qin2, Jialin Ma1, Rong Huang1, Junyuan Gong1 1Fundamental Science on Nuclear Wastes and Environmental Safety Laboratory, Southwest University of Science and Technology, Mianyang 621010, P. R. China 2 Key Laboratory of Solid Waste Treatment and Resource Recycle, Ministry of EducationˈSouthwest University of Science and Technology, Mianyang 621010, P. R. China 3 School of Life Science and Engineering, Southwest University of Science and Technology, Mianyang 621010, P. R. China 4 Analytical and Testing Center of Southwest University of Science and Technology, Mianyang 621010, P. R. China

Abstract Click here and insert your abstract text. This study evaluates the feasibility of biosorption of uranium from aqueous solutions using living and dried S. punctata. The uranium maximum removal efficiency for the plant cultivar occurrs in the soluation at pH 4-5 and the uranium removal efficiency exceeds 90%. In the kinetics studies, the dried powder of duckweed can reach nearly 80% adsorption within 5 min, and the batch adsorption equilibrium can be reached within 24 h for the living and dried powder of duckweed. Both for the living and dried powder of duckweed, the experimental data were fitted by the pseudo-second-order rate model with the degree of fitting (R2) higher than 0.99. The uranium was deposited onto the root surface of living S. punctata as 100-500 nanometer size schistose structures that consist of primarily U (82.5%,wt%) and P(8.76 %,wt %), and no carbon; while no similar uranium phosphate minerals were found on the dried powder of S. punctata, as revealed by scanning electron microscope and energy dispersive spectrometer analysis. The results suggest that there is one particularly promising biosportion processes, namely, biomineralization. And the root surface of the living S. punctata plays an important role in the uranium phosphate precipitate processes, not only play as a carrier of the functional groups but also as a substrate inducing the nucleation of uranium-phosphate nanoparticles. The dried powder of S. punctata adsorption of U is mainly through the effect of electrostatic attraction, ion exchange, and complexation coordination. © 2016 The Authors. Published by Elsevier B.V This is an open access article under the CC BY-NC-ND license © 2015 The Authors. Published by Elsevier B.V. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of Tsinghua University/ Basel Convention Regional Centre for Asia and the Pacific. Peer-review under responsibility of Tsinghua University/ Basel Convention Regional Centre for Asia and the Pacific

* Corresponding author. Tel.: 086-816-6089013; fax: 086-816-6089013. E-mail address: [email protected]; [email protected]

1878-0296 © 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of Tsinghua University/ Basel Convention Regional Centre for Asia and the Pacific doi:10.1016/j.proenv.2016.02.060

Xiaoqin Nie et al. / Procedia Environmental Sciences 31 (2016) 382 – 391

Keywords: Spirodela punctata; Uranium; Biosorption; Mechanism

1.

Introduction

Studies have shown that plant can remove significant amount of uranium in the large areas that have low concentration of uranium contaminated water by accumulation, rhizosphere filtration, biosorption, complexation and other mechanisms[6, 11, 19, 22, 24]. Uranium has not yet been found to be the necessary nutrient for the plant, but the roots of some terrestrial and aquatic plants, such as the Helianthus annuus L., Phaseolus vulgaris, Zea mays and Callitrichaceae members could absorb a large number of U from uranium waste water[6, 14, 25]. There are experiments shown that uranium concentrations ultimately reached the water quality control standards of the U.S. Environmental Protection Agency by sunflowers. Compared with terrestrial plants, aquatic plants have more advantage for in situ reducing uranium concentration in the uranium contaminated water [4, 22]. The concentration factor of uranium of small S. punctata is 2.87 × 103 and 1.567 × 103 in the still water and flowing surface running water, respectively [15]. Four basic mechanisms by which organisms can immobilize uranium are (1) reductive precipitation of U (VI) to U (IV) by organisms, (2) uptake and accumulation of U (VI) by organisms, (3) adsorption onto cell surface, and (4) precipitation of U (VI) with inorganic phosphate released or produced by cells from the hydrolysis of phosphate containing compounds. Polyphosphate accumulation and release is generally related to cellular energetics for survival and growth. Many investigators have found the formation of U (VI)–phosphate minerals as needlelike fibrils on the cell surface of microorganisms[2, 16, 18, 21, 26]. The researchers have reported that biomineralization of U (VI)–phosphate minerals may be complementary to bioreduction, but also that U (VI)–phosphate biomineralization may be preferable to bioreduction in certain environments due to its utility in a wide range of chemical and redox conditions. It may be also serving as a detoxification mechanism, as proposed recently for the hydrolysis of exogenous organophosphate compounds with other microorganisms[7]. However, the mechanisms whereby the plants that could cause the uranium removal from water are poorly understood. In this study, the S. punctata has been chosen because its rapid proliferation, high tolerance to waste levels, and excellent uptake ability of heavy metal and uranium[1, 4, 8, 12, 20, 23]. The biosorption characteristics of uranium by both living and dried powder of S. punctata were investigated and the uranium products formed on the root of living and dried powder of S. punctata has been characterized by two methods: Scanning Electron Microscopy (SEM) and Energy X-ray Dispersion Spectroscopy (EDS).

2.

Methods

2.1 Regents All chemical reagents used were analytical grade unless otherwise stated. All solutions were prepared with de-ionized water. All laboratory plastic-ware and glassware were soaked in 10%( v/v) nitric acid (HNO3) and thoroughly rinsed with deionized water prior to use. UO2 (NO3)2·6H2O was used to prepare the uranium (VI) solution. The pH of the uranium solution was adjusted to required values by using 0.1 mol/L of sodium carbonate (Na2CO3) and sodium bicarbonate (NaHCO3) and hydrochloric acid (HCl) solution.

2.2 Spirodela punctata Spirodela punctata was obtained from the natural pond in the Southwest University of Science and Technology (China). The plants after collection were rinsed thoroughly with tap water. This was followed by washing three times with deionized water in order to get a clean biomass that is free from silt, sand, most of diatoms and other epiphytic organisms. And the healthy fronds were carefully selected and cultivated in 50 % Hoagland’s nutrient medium with aerated intermittently in the (GXH-250-ċ) illumination incubator with controlled temperatures˄20°C/night and 25°C/day), and relative humidity (65 %) and a photoperiod of 12 hours (Simulated natural light intensity). The S. punctata from the nutrient medium before experiment were rinsed for 30 min using 20 mM Na2-EDTA solution and then were washed thoroughly with deionized water, to exchange and remove the adhering nutrient medium on roots. The dried powder of S. punctata was air-dried at an ambient temperature of 25±2 ◦C and stored as whole biomass at room temperature.

383

384


2.3 Biosorption experiments Unless otherwise indicated, for all the biosorption experiments, 3 g (fresh weight) of living S. punctata or 250 mg (dried weight) of dried powder of S. punctata was introduced into a 200 mL of uranium solution having an initial uranium concentration of 5 mg/L, in a 250 mL conical flasks. After 2 h of shaking at 150 rpm and 25 ć, the supernatant was separated by filtration and used for estimating the dissolved uranium concentration.

2.4 Estimation of uranium Estimation of uranium was done by arsenazo (III) method. Briefly, 1 mL sample was mixed with 5 mL of CH2ClCOOHCH3COONa buffer and 1 mL of arsenazo (III) (0.06 %) and diluted with hydrochloric acid (4 M) to a total volume of 25 mL, then assayed for uranium concentration after 10 min using a spectrophotometer at a wavelength of 650 nm. For each of the experiments, solutions without S. punctata were used as controls. The data presented in the result represent the average of triplicate readings ±standard error. Statistical tests for analysis of variance (One-way ANOVA and Tukey’s significance test) were carried out on the experimental results using OriginPro8.0 software. The uranium removal efficiency (R) was calculated by the following formula:

R˄%˅

˄C0 C˅ u 100% C0 (1)

The biosorption quantity (Q) of uranium per unit S. punctata (mg of U/g fresh or dry weight of S. punctata) was calculated using following equation:

Q

˄C0 C˅V W

(2)

where ‘C0’ and ‘C’ are the concentrations of uranium (mg/L) in the initial and final solutions, respectively, ‘V’ is the volume of uranium solution used in liters and ‘W’ is the amount of S. punctata used in grams.

2.5 SEM and EDS analyzing SEM is one of the useful equipments to visualize the surface of roots and EDS is a useful instrument to evaluate the compositional characteristics of elements. The surface characters of the root surface of living S. punctata and dried powder were analyzed by SEM (Ultra 55) coupled with EDS (Oxford IE450X-Max80). The samples of living S. punctata were placed on cover slips and air-dried, then mixed in a 2.5% glutaraldehyde solution for 5 h. Next, samples were dehydrated by 20 min incubations in a graded ethanol series (30%, 50%, 70%, 90% and 100% ethanol). Finally, the samples were air-dried. The samples of dried powder of S. punctata were dried at 75°C in a forced-air convection oven for 48 hours. Before using, the samples were sputter-coated with 4 nm of gold particles.

3.

Results and discussion

3.1 Biosorption characteristics of uranium

3.1.1 Effect of pH on uranium biosorption Initial pH plays an important role in the biosorption of uranium from the aqueous solutions. It influences both the speciation of uranium in the aqueous solution and the binding sites present on the surface of biomass. The effect of pH on biosorption of uranium onto S. punctata was studied in order to find out the optimum pH for the biosorption process, and to find out whether the S. punctata was able to show a good uranium uptake at a wide range of pH values. The maximum of uranium removal efficiency of living and dried S. punctata were also observed at pH 5. The uranium removal efficiency of living S. punctata increased from 4% to 89% as the pH of the solution was increased from 3 to 5, and then decreased from 89% to 65% as the pH of the solution was increased from 5 to 7. For dried S. punctata, like living S. punctata, the greatest uranium accumulation occurred at pH 5. The uranium removal efficiency of dried S. punctata increased from 66% to 82% as the pH of the solution was increased from 3 to 5, and then decreased from 82% to 68% as the pH of the solution was

385


increased from 5 to 7. The removal efficiencies and biosorption quantity of uranium versus pH for both the living S. punctata and dried S. punctata were plotted in (Fig. 1), which both result in a bell shaped curve. This suggests that the ability of S. punctata to uptake uranium was influenced by the pH of water, due to the differences in uranium speciation in the solution. The reason for the significantly decrease of biosorption for living S. punctata at pH 3 could be because of strong acidic conditions lead to severe damages of the living S. punctata, and at this pH there is a high concentration of H+ and H3O+, which compete with uranyl ions for the binding sites on the surface of the biomass. At pH 4-5, uranium was present predominantly as oxidated free uranyl cations, the most amenable form for biosorption, and was readily adsorbed to the root surface of living S. punctata[14], resulting in a significant increase of uranium bioaccumulation on the root of living S. punctata. The reason for increased biosorption by dried S. punctata at pH 4 and 5 could be due to the presence of functional groups like carboxyl, hydroxyl, sulphate, amino, and phosphate present on the surface of biomass, which ionize at pH around their pK values. Maybe the functional groups which are in ionized form at pH 4 or 5 not be ionized at pH 3. Decrease in the uptake of uranium at higher pH could be due to the formation of uranyl carbonate complexes, as the initial pH of the solution was adjusted with Na2CO3 and NaHCO3, and also, atmospheric CO2 plays a role in the formation of hexavalent uranyl carbonate complexes above pH 6[27]. At higher pH values the aqueous carbonate competes with surface binding sites for uranyl ions and reduces the availability of uranium for biosorption by both living and dried S. punctata. As optimum pH for biosorption process by living and dried S. punctata were found to be 5, all the subsequent experiments were conducted at this pH. 0.40

100

0.24

0.16

R Q 20

0

3

4

5

6

7

R (%)

60

80

3.6

60

2.7

1.8

40

R Q

0.08

20

0.00

0

Q (mg/g dry wt.)

0.32

Q (mg/g fresh wt.)

R(%)

(b)

80

40

4.5

100

(a)

0.9

3

pH

4

5

6

7

0.0

pH

Fig. 1. Effect of pH on uranium (VI) biosorption by living (a) and dried (b) S. punctata: V =200 mL, ma= 3g (fresh weight, FW), mb= 0.25g (dried weight, DW), pH=3-7, t=2 h,T = 25 ◦C, C0[u] = 5mg/L.

3.1.2 Effect of initial uranium concentration on uranium biosorption The effect of initial uranium concentration on the biosorption by living and dried S. punctata are shown in Fig. 2(a) and (b). The plot shows two phases. In the first phase, a steep increase in removal efficiency and biosorption quantity are seen for both the living and the dried S.punctata., And in the second phase, the rates of increase in biosorption quantity were lower and the removal efficiency begins to decrease. The steep increase in the removal efficiency was observed for the initial uranium concentration of 1–50 mg/L. The highest value of biosorption quantity was 4.05±0.23 mg/g (FW) by living S. punctata, with uranium concentration of 189.30 mg/L, and 131.76±6.44 (DW) mg/g by dried S. punctata with a uranium concentration of 85.30 mg/L. Since the mass of S. punctata is constant, the number of binding sites is same. And the number of uranyl ions increases as the initial uranium concentration increases. At low initial uranium concentration solution, saturation of living and dried S. punctata by uranyl ions could not be achieved as the number of uranyl ions is smaller than the number of binding sites present on the S. punctata. By increasing the concentration of initial uranium in the solution, it results in the increase of biosorption quantity, untill the saturation of living and dried S. punctata is attained. Once the binding sites present on the dried S. punctata becomes saturated with the uranium, the availability of binding sites for the uranium decreases[3]. In this study, the saturation of living and dried S. punctata both were not achieved when the initial uranium concentration reached at 250 mg/L.

386


5

100

80

4

80

120

60

3

60

90

100

(b)

R Q 20

0

0

50

100

150

200

250

R (%)

60

40

1

20

0

0

150

R Q

0

50

C0[u](mg/L)

100

150

Q (mg/g dry wt.)

2

40

Q (mg/g fresh wt.)

R (%)

(a)

30

200

250

0

C0[u](mg/L)

Fig. 2. Effect of initial U (VI) concentration on uranium biosorption by living (a) and dried (b) S. punctata V =200 mL, ma= 3g (FW), mb= 0.25g (DW), pH=5, t=12 h, T = 25 ◦C, C0[u] = 1-250 mg/L.

3.1.3 Effect of biomass concentration on uranium biosorption The effect of biomass concentration on the biosorption of uranium by living and dried S. punctata are shown in Fig. 3(a) and (b). Biomass concentration appears to influence the biosorption process. The removal efficiency was found to be more than 75% and 90% for living and dried S. punctata, respectively, and no significant increase with biomass concentration. And the biosorption quantity was found to decrease concomitantly with the increase of in biomass concentration. For living S. punctata, highest value of biosorption quantity (1.57±0.13 mg/g) was observed at the biomass concentration of 2.5 g/L (FW). The observed biosorption quantity for the biomass concentration of 10 g/L (FW) was 0.42±0.03 mg/g. On further increasing the biomass concentration to 15 g/L (FW), the decrease in biosorption quantity was not too steep, as shown in Fig. 3(a). A similar trend is found in dried S. punctata. The highest value of biosorption quantity observed was 3.82±0.24 mg/g at 1 g/L (DW) of biomass concentration. At high uranium concentration of S. punctata, there would have been a fast superficial adsorption onto the surface of S. punctata, which would have resulted in a low uranyl ion concentration in the solution. At a low uranyl ion concentration, the uranyl ion sorbed is low, because of the lower driving force (by a lower concentration gradient pressure). Thus an efficient use of sorptive capacity of the living and dried S. punctata is not reflected. 100

(a)

0.8

20

0.4

0.5

1.0

1.5

2.0

2.5

3.0

0.0

ma(g)

3.6

R Q

60

R (%)

40

4.5

2.7

40

1.8

20

0.9

0 0.0

0.4

0.8

1.2

1.6

2.0

Q (mg/g dry wt.)

1.2

R Q

(b)

80

Q (mg/g fresh wt.)

60

0

100

1.6

80

R (%)

2.0

0.0

mb(g)

Fig. 3. Effect of biomass concentration on uranium sorption by living (a) and dried (b) S. punctata V =200 mL, ma= 0.5-3g (FW), mb= 0.25-2 g (DW), pH=5, t=12 h,T = 25 ◦C, C0[u] = 5 mg/L.

3.1.4 Effect of contact time on uranium biosorption The study of effect of contact time on the biosorption of uranium by S. punctata revealed that the equilibrium state could be achieved in 12h with the biosorption quantity of 0.31±0.13 mg/g (FW) by living S. punctata (Fig 4a), and in 2h with the biosorption quantity of 3.39±0.71 mg/g (DW) by dried S. punctata (Fig 4b). Fig 4a indicates that sorption took place in two stages, first one was rapid surface adsorption and the second one was a slow intracellular diffusion. After the equilibrium was achieved, biosorption quantity remained constant. For S. punctata, the initial stage of sorption was rapid, followed by a slow stage. Especially for the dried S. punctata, in first 10 min of contact, almost 80% removal efficiency and biosorption quantity of

387


3.20±0.14 mg/g (DW) could be achieved. The higher removal efficiency in initial stage of biosorption could be due to electrostatic interactions, between uranyl ions and surface ligands on the S. punctata. These binding sites present on surface of the biomass start binding to uranyl ions as soon as they come in contact with each other. As time progresses availability of binding sites reduces, thus reducing the rate of biosorption. Rapid sorption that we have observed for dried S. punctata is considered as a good characteristic of a biosorbent, as it allows short contact time between solution and sorbent. 100

(a)

4.2

100

0.35

(b)

80

0.28

60

0.21

40

0.14

3.5

80

20

0

0

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R (%)

2.1 40 1.4

0.07

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Q (mg/g dry wt.)

R Q

Q (mg/g fresh wt.)

R (%)

2.8 60

R Q 0.7

0

10

20

30

40

50

0.0

t (h)

t (h)

Fig. 4. Effect of contact time on uranium sorption by living (a) and dried (b) S. punctata V =200 mL, ma= 3 g (FW), mb= 0.25 g (DW), pH=5, t=10min-24 h, T = 25 ◦C, C 0[u] = 5 mg/L.

3.1.5 Kinetic modeling Kinetics of uranium uptake by S. punctata was modeled using the pseudo-first-order and pseudo-second-order Lagergren equation. The pseudo-first-order and pseudo-second-order reaction of Lagergren for sorption can be expressed as equation (3) and (4), respectively:

ln(qe qt ) ln qe k1t t q

1 1 ( )t 2 (k2 qe ) qe

(3)

(4)

Where qe and qt are the amount of uranium sorbed per unit weight (mg/g, fresh or dry weight) of living or dried S. punctata at equilibrium and at any time t (min), respectively, and the value of the rate constant k1 (min−1) and k2 (g/(mg·min)) is the rate constant of pseudo-first-order and pseudo-second-order sorption, respectively. The values obtained from the plots of pseudo-first-order, and pseudo-second-order kinetics are shown in Tab. 1. The kinetics data of sorption by living and dried S. punctata both could be well described by pseudo-second-order plot and the R2value of pseudofirst-order rate kinetics were 0.66 and 0.96 by living and dried S. punctata, respectively, and for pseudo-second-order rate kinetics were both greater than 0.99 by living and dried S. punctata. The observed experimental qe values (0.31 and 3.39 by living and dried S. punctata, respectively) were close to the values of qe (0.31 and 3.71 by living and dried S. punctata, respectively) obtained from the slope of the linear plot for the pseudo-second-order rate kinetics (t/qt vs.t), as shown in Tab. 1. The initial rate of uptake (h) could be calculated using the expression h=k2qe2. It was found to be 8.8×10-5 (mg/g) min (FW) of living S. punctata and 56.6 (mg/g) min (DW) of dried S. punctata, respectively. Tab. 1 Pseudo-first-order, pseudo-second-order, and experimental values for living (a) and dried (b) S. punctata. S.

Experimental,

Pseudo-first-order mode

Pseudo-second-order mode 2

Initial rate, 2

punctata

qe (mg/g)

k1[g/(mg·min)]

qe(mg/g)

R

k2[g/(mg·min)]

qe(mg/g)

R

h (mg/g)min

living

0.3141

0.0010

0.0802

0.6589

0.0009

0.3124

0.9978

0.878×10-4

dried

3.7340

0.0007

0.4927

0.9594

4.1014

3.7160

0.9998

56.6348

388


3.1.6 Equilibrium modeling Adsorption curve data were fitted to linearised Langmuir and Freundlich adsorption isotherms. The Langmuir isotherm is a means to interpret hyperbolic adsorption data. It is basically the same equation used in Michaelis–Menten enzyme kinetics, and in this paper it describes the adsorption of uranyl ions to a finite number of ligand sites in a single layer on the living or dried S. punctata surface. The Langmuir and Freundlich isotherm can be expressed as equations (5) and (6), respectively:

1 qe

1 1 1 qm ka qm Ce

ln qe

1 ln K f (ln Ce ) n

(5)

(6)

Where ‘qm’ and‘qe’ in this paper is the maximum and equilibrium uranium uptake (mg/g), and ‘Ce’is the residual uranium concentration in the solution (mg/L). The constant ‘ka’(the ratio of adsorption/desorption rates related to energy of adsorption) and ‘Kf’(a measure of uranium adsorption capacity) represents the adsorption equilibrium constant of Langmuir isotherm (L/mg) and Freundlich isotherm, respectively, and ‘1/n’ the intensity of adsorption. The fitting parameters of Langmuir and Freundlich isotherms for the biosorption of uranium onto the living and dried S. punctata are shown in Tab. 2. The uranium loading capacity (qm) was found to be 18.52 mg/g (FW) and 72.6 mg/g (DW) for the biosorption of uranium by living and dried S. punctata, respectively. The data did not fit well with both the Langmuir (R2 = 0.55) and Freundlich (R2= 0.63) isotherm for living S. punctata. Also for dried S. punctata, data did not fit well with both Langmuir (R2= 0.62) and Freundlich (R2 =0.88) isotherm. ‘n’ was 2.24 and 1.49, for living and dried S. punctata, respectively. The value obtained for ‘n’ in the range 1 < n < 10, being the suggestive of a beneficial adsorption [3], indicated a beneficial biosorption of uranium both by living and dried S. punctata. Tab. 2 Values obtained from Langmuir and Freundlich isotherms

S. punctata ka/(L·mg-1) living dried

Langmuir isotherm qm/(mg·g-1)

R2

Kf

Freundlich isotherm n

R2

0.0099

18.5185

0.5519

0.2255

2.24

0.6267

0.08932

72.4638

0.6195

5.7116

1.49

0.8831

3.2 Biosorption mechanisms of uranium

3.2.1 SEM and EDS analysis Fig.5 shows SEM micrographs, Fig.6 and Tab. 3 shows EDS spectra of the root surface of S. punctata before and after exposure to 10-200mg/L of uranium for 24 hours, respectively. The SEM images show that there has some of the hexahedral calcium oxalate crystal similar on the root surface of living S. punctata before exposure to uranium (Fig 5(a)), after 24 hours of exposure to 10 mg/L (b), 100 mg/L(c), 200 mg/L (d) of uranium. The SEM micrographs show that the root surface was covered with many clusters resembling small uranium lamellar crystal formations, the quantity of lamellar crystal formations increased with the increase in initial uranium concentration (Fig. 5(b) ǃ(c) ǃ (d)). And these observations were confirmed by EDS analytical results, comparing uranium spectrums before/after biosorption (Fig. 6). In Fig. 6(a), The EDS spectra of the small box labeled portions on root surfaces show the composition of C, O, Na, and P to S. punctata, as shown in the EDS spectrum. In Fig. 6(b), the distinct uranium peaks in the EDS spectra of the cluster on the root surface can be observed after biosorption and the uranium mass ratio of the cluster spot was measured to be 82.53% of the total component weight (Tab.3). Flaky crystal of the root surfaces to 10-200 mg/L uranium for 24h show the similar essential composition of O, P and U to S. punctata. The content of phosphorus increased greatly (from 0.24% to 8.76%, wt %) and peak of carbon disappeared after adsorption. The content of Si may be due to the thin S. punctata root thickness on the carrier of glass cover slip. EDS data show the systematic association of U with P in all the lamellar crystal formations after uranium treatment. The molar ratio of U to P was calculated to be near 1.0 from EDS semi quantitative analysis in these U-P complex or similar, that U-phosphate precipitates (Data shown in Tab.3). , And we have do not observed similar uranium crystal on the dried S. punctata (Data not shown). The result indicating that P groups are


key coordination sites for the root surface complexes, and strongly suggests that the root cell surface of the living S. punctata plays an important role in the U-phosphate precipitates process, not only as a carrier of the functional groups but also as a substrate inducing the nucleation of phosphate nanoparticles. The similar results have been found with U, Yb and Ce absorbed by yeast [9, 10, 17]. These findings suggest that biomineralization on the root surface might be the major mechanisms to remove uranium from waste water by S. punctata. Some previous studies have reported that the sorbed U(ě) on the cell surfaces reacts with P release form the yeast to form H-autunite by local saturation[21]. While there was not found similar uranium- phosphate crystal plate on the root surface of dried powder of S. punctata after uranium biosorption. a

b

d

c

Fig.5. SEM images of the living S. punctata root before (a) and after (b: 10 mg/L), (c: 100 mg/L ), (d: 200 mg/L ) U biosorption

a

389

390


b

Fig. 6. The EDS results of the living root of S. punctata root before (a) and after (b: 200mg/L U) biosorption. (□) the spot for EDS analysis. Tab. 3. The element distribution results of the root surface of the living S. punctata before (a) and after (b: 200mg/L U) biosorption. (Gold is present due to the treatment.) Element

C O Na Mg Si P Ca Au U Totals

4.

(a)

(b)

Weight%

Atomic%

Weight%

Atomic%

48.34 36.52 2.34 0.61 7.29 0.24 1.22 3.44 -100.00

59.63 33.82 1.51 0.37 3.84 0.11 0.45 0.26 -100.00

-6.54 0.87 -1.30 8.76 --82.53 100.00

-36.42 3.37 -4.13 25.19 --30.89 100.00

Conclusions

The results presented here are the living and dried S. punctata exhibited good potential for application in removal of uranium from aqueous solutions (1-250 mg/L), and the living S. punctata root surface capable of U immobilization by two different mechanisms: first adsorption of uranium to root surfaces, then precipitation of uranium with biogenic phosphate ligands. Overall, our results offer a first step toward understanding the phosphate release and uranium removal by S. punctata. The phosphate might be released from the hydrolysis of organophosphate, but in this study it was not independently verified.

Acknowledgements

This work was supported by the National Basic Research Program of China (973 Program: 2014CB846003), the China National Natural Science Foundation (Grant No. 41502316, 41272371, 41472310), the Prior Research Foundation of Fundamental Science on Nuclear Waste and Environmental Security Laboratory (Grant No. 15yyhk11), and the Doctor Foundation of Southwest University of Science and Technology (Grant No.15zx7109). We are very grateful to Professor Guohua Ma and Mengjun Chen for their helpful suggestions. Warmest thanks are also expressed to Mr. Qian Wang for his kind help.


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