Adsorption of Cs onto biogenic birnessite: effects of layer structure

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Adsorption of Cs onto biogenic birnessite: effects of layer structure, ionic strength, and competition cations Qianqian Yu, Kazuya Tanaka, Naofumi Kozai, Fuminori Sakamoto, Yukinori Tani, and Toshihiko Ohnuki ACS Earth Space Chem., Just Accepted Manuscript • DOI: 10.1021/ acsearthspacechem.8b00042 • Publication Date (Web): 21 Jun 2018 Downloaded from http://pubs.acs.org on June 25, 2018

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ACS Earth and Space Chemistry

Adsorption of Cs onto biogenic birnessite: effects of layer structure, ionic strength, and competition cations Qianqian Yu 1,2*, Kazuya Tanaka 2, Naofumi Kozai 2, Fuminori Sakamoto 2, Yukinori Tani 3, Toshihiko Ohnuki 2,4

1. School of Earth Science, China University of Geosciences, Wuhan, 430074, China 2. Advanced Science Research Center, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan 3. Department of Environmental and Life Sciences, School of Food and Nutritional Sciences, University of Shizuoka, 52-1 Yada, Shizuoka 422-8526, Japan 4. Laboratory for Advanced Nuclear Energy, Institute of Innovative Research, Tokyo Institute of Technology, 2-12-1-N1-16 Ookayama, Meguro-ku, Tokyo, 152-8550, Japan

* Corresponding author: Qianqian Yu: [email protected]

1

ACS Paragon Plus Environment

ACS Earth and Space Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

Abstract

2

Although the adsorption of cesium (Cs) onto phyllosilicate minerals has been

3

widely studied, the effect of Cs on redox-sensitive biogenic Mn oxide is not well

4

understood. In this study, the structural transformation of biogenic birnessite

5

stimulated by commonly occurring natural heavy metals, and the influence of those

6

metals on the adsorption behavior of Cs over a wide range of concentrations (1×10-10

7

mol/L to 0.1 mol/L) was carefully examined via solution chemistry and XAFS

8

techniques. The Cs was reversibly adsorbed onto the mineral surface to form an

9

outer-sphere coordination for all biogenic birnessite samples. The presence of heavy

10

metals (e.g., Zn and Ni) during bio-oxidation of Mn(II), followed by acid treatment,

11

increased the number of available layer vacancies, which consequently increased the

12

adsorption capacity of Cs in the final product. The analysis of the dependence of

13

sorption values on the ionic strength showed distinct results on biogenic birnessite and

14

chemically synthesized birnessite. Trace ions (e.g. Mn2+) that were loosely bound to

15

biogenic birnessite were released into the solution when they came in contact with the

16

NaCl solution. The competition between these trace ions significantly influence Cs

17

adsorption on biogenic birnessite. The results in this study indicated that it is

18

necessary to account for the competition effect of trace ions, particularly when

19

considering poorly crystalline adsorbents, such as bio-minerals.

20 21

Keywords: Biogenic birnessite, Radioactive Cs, Adsorption model, Mn oxide, XAFS

2


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1

1. Introduction

2

The weapons testing, and nuclear accidents, such as those that occurred at

3

Chernobyl and Fukushima, may result in the release of radioactive cesium into the

4

environment, raising the need for remediation 1-6. The mobility and bioavailability of

5

cesium (Cs) in the environment is dependent on its adsorption and desorption

6

behaviors in soils and sediment. Cs has a strong and selective interaction with

7

phyllosilicate minerals, such as illite and vermiculite, either through weak

8

electrostatic attraction or through the formation of strong bonds via the partial sharing

9

of electrons between Cs and the oxygen-donor ligand sites of clay minerals

7-9

.

10

However, the interactions between Cs and non-phyllosilicate minerals remain

11

relatively unstudied.

12

Mn oxides are present in both aquatic and terrestrial environments

10, 11

. Despite

13

their minor abundances, Mn oxides play as scavengers of trace elements in soils,

14

sediments, and aquatic environments

15

oxides commonly occur at the interface between pore solutions and the primary soil

16

minerals. Moreover, their large specific surface area and high cation adsorption

17

capacity are well documented. In comparison with phyllosilicate minerals, Mn oxides

18

are more sensitive to seasonal changes and soil properties like pH, organic matter

19

content and type, redox conditions, and root exudates acting as chelates

20

Mn oxides with layer and tunnel structures are most frequently observed in soils, and

21

birnessite is typically the most reactive 17.

12-15

. As coatings on clay, silt, and sand, Mn

16

. Hydrous

22

The octahedral sheets of birnessite exhibit negative structural charges that arise

23

from vacancies, the isomorphous substitution of Mn(IV) by lower valence cations (e.g.

24

Mn(III) or Co(III)), and the unsaturated oxygen anions that are exposed at the particle

25

edge. A variety of cations can be adsorbed onto the sheets of birnessite to balance the

26

charge deficit. Mn octahedral layer symmetry and composition are sensitive to

27

coexisting cations during formation. The direct incorporation of Co into the birnessite

28

layer by coprecipitation results in reduce thermal stability and crystallinity 18. Doping

29

of Ni into birnessite led to a decreased in layer stacking 19. Coprecipitation of Ni with

30

biogenic birnessite enhanced vacant site formation

31

bio-oxidation of Mn modifies the layer stacking of biogenic birnessite

32

structural changes would further influence their effects on the geochemical behaviors

33

of other elements. For instance, partial substitution of Co into Mn octahedral layer 3


20

. Zn sorption during 21

. These


Page 4 of 45

22

1

caused improved catalytic activity with respect to olefin oxidation

2

hexagonal birnessite exhibited better performance to remove Pb(II) and As(III) from

3

solution phase

4

formation of vacancies in the Mn octahedral layer. However, adsorption capacities for

5

Pb2+ and Zn2+ by these Ni-doped birnessites are reduced, since most of vacancies

6

were occupied by a large amount of Ni 19. Birnessite could form in a wide variety of

7

geochemical conditions. The crystal structure of birnessite is affected by coexisting

8

heavy metals such as Zn2+ and Ni2+. Despite structural factors of birnessite affecting

9

adsorption of transition metals have been widely studied, their influence on Cs

10

. Co-doped

23

. The doping of Ni during birnessite crystallization enhances the

adsorption is still unknown.

11

Alkaline metals, including Li+, Na+, K+, and Cs+ are primarily located in the

12

interlayer of birnessite, where they bind to the interlayer water, and have a significant

13

effect on the stability of birnessite layer structures

14

energy among all alkaline metals, it does not bind the water molecules as tightly as

15

would a more highly charged cation

16

demonstrated a Cs adsorption capacity of 1×10-3 – 4.6×10-3 mol/g

17

interlayer cation exchange with Cs+ in birnessite is completed within hours

18

is accompanied by the release of the interlayer cations (e.g. K+, Na+) into the aqueous

19

phase. The rate of Cs cation exchange into hexagonal birnessite exhibited only a weak

20

dependence on pH, but the total amount of Cs loading in the interlayer region

21

increased dramatically above pH 6.5 +

24

. Cs+ has the lowest hydration

25

. Chemically synthesized birnessite 26, 27

. The

28, 29

, and

28

. Time-resolved XRD has revealed that the

+

22

ion-exchange between Na and Cs occurred via a sequential delamination and

23

reassembly of the birnessite sheets. Exfoliation of a given interlayer region allows for

24

the wholesale replacement of Na+ by Cs+

25

outer-sphere complex can form, allowing for Cs+ to reversibly adsorb onto the surface

26

of chemically synthesized birnessite 30, 31, which concur with previous reports that Cs

27

is located at the midpoint of the interlayer and is surrounded by water molecules 25, 28.

25

. Our recent studies have shown that the

28

Most studies of the interaction of Cs with birnessite are based on chemically

29

synthesized birnessite. However, most birnessite in terrestrial environments is

30

believed to be of biologic origin. This is because the oxidation rate of Mn(II) is up to

31

five orders of magnitude greater when formed biologically than in abiotically

32

Compared with chemically synthesized birnessite, biogenic birnessite generally

33

exhibits a poorer crystallinity with many more structural defects. Still, there are many 4


14, 32

.

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1

unknowns regarding the structural factors of biogenic birnessite affecting adsorption

2

of Cs+. Sasaki et al., (2014) investigated Cs+ adsorption on biogenic birnessite at high

3

levels. They found that the adsorption density of Cs+ on biogenic birnessite was

4

smaller than that on chemically synthesized birnessite, despite the former showed

5

larger adsorption densities of Co2+ and Ni2+ 30. Cs adsorption at trace levels has been

6

studied only on several hydrated Mn oxides

7

available to compare with biogenic birnessite.

29, 33, 34

. However, no structural data are

8

Multisite cation exchange models is an effective method to explain Cs adsorption

9

on phyllosilicate minerals 9, 35-38. Benedicto et al., (2014) modeled Cs adsorption onto

10

illite in the presence of different exchangeable cations. They found that large hydrated

11

cations such as Ca2+ induce expansion of illite layer, and finally increased Cs

12

adsorption capacity from near 200 meq/kg to 900 meq/kg

13

investigated the dependence of Cs sorption values on the ionic strength by using three

14

homoionic smectites. They found that competition effects of trace ions in solution

15

significantly influence sorption data interpretation

16

sorption site density for radiocesium on chemically synthesized birnessite and

17

todorokite by exploring a cation exchange model. Two types of adsorption sites were

18

observed for todorokite, and a small fraction of sorption site on todorokite possess

19

much higher selectivity coefficient than birnessite

20

the Cs uptake by biogenic birnessite, sorption data under a wide range of Cs

21

concentrations and ionic strengths are required. Unfortunately, no experimental data

22

are available for Cs adsorption on biogenic birnessite at trace loading (< 1×10-4

23

mol/L).

39

. Missana et al., (2014)

38

. We recently estimate the

31

. To comprehensively interpret

24

In the current study, a large set of experimental adsorption data was collected

25

under a wide variety of Cs+ concentrations (1×10-10 mol/L to 0.1 mol/L) and ionic

26

strengths (10-3 mol/L to 1 mol/L). The structural factors of biogenic birnessite

27

affecting adsorption of Cs+ were investigated via an ion-exchange model approach

28

combined with XAFS analysis. This study aims to give clarification of how the

29

structural transformation of biogenic birnessite, induced by the presence of common

30

naturally occurring heavy metals, affects the adsorption/desorption behavior of Cs.

31

2. Materials and methods

32

2.1 Preparation of biogenic and chemically synthesized birnessite 5



1

The Mn(II)-oxidizing fungus A. strictum KR21-2 was used to produce biogenic

2

birnessite 40, 41. The cultivation medium (HAY) was prepared following the procedure

3

of Miyata et al., (2004) and Tani et al., (2004)

4

medium is described in the Supporting Information. The aqueous stock solutions of

5

MnSO4, ZnSO4, and NiCl2 were filtered in advance with a sterile 0.2-µm membrane

6

filter (Advantec, Toyo Kaisha Ltd.) and added to the autoclaved HAY medium before

7

incubation. 1 mmol/L Mn(II) and a 100 µL spore suspension was added into the HAY

8

medium (50 mL) before incubation. For some treatment, either 0.5 mmol/L Zn(II), or

9

0.5 mmol/L Ni(II) was added in addition. Details of all incubation conditions are

10

summarized in Table 1. For all experiments, the pH of solution was kept at 7.0 using

11

20 mmol/L HEPES buffer solution. The medium was then incubated at 25 °C on a

12

reciprocal shaker (~100 strokes/min). After one week incubation, the solid products

13

were collected by filtration. These solid products were subsequently resuspended in

14

HCl solution at a pH value of 2.0 for 12 h, in order to remove adsorbed cation upon

15

the vacant site. After the reaction, the suspension was filtered through a 0.2-µm

16

membrane, washed by distilled water three times, and the solid products were

17

lyophilized using a FD-5N freeze-drier (EYELA, Tokyo, Japan) for 48 h. The final

18

product, which was derived from biogenic binressite, Zn-doped birnessite, and

19

Ni-doped birnessite, was named as BB, ZB, and NB, respectively.

42, 43

, and the composition of the HAY

20

Elemental compositions of solid products before and after acid wash were

21

measured using an inductively coupled plasma-atomic emission spectrometer

22

(ICP-AES, Vista-Mpx, Seiko, Japan) after decomposition in 0.25 mol/L hydrochloric

23

acid hydroxylamine. The cationic exchange capacity (CEC) is 7.28×10-3 mol/g Mn

24

for BB, 8.20×10-3 mol/g Mn for ZB, and 8.95×10-3 mol/g Mn for NB. These values

25

were obtained from the maximum Cs adsorption capacity that we observed and are

26

consistent with previous reports 31, 44.

27

As reference substances, chemically synthesized birnessite were prepared

28

according to Feng et al., (2004) 49. Briefly, one-hundred and twenty-five mL of 44 g

29

of NaOH were added to an aqueous solution of 125 mL of 14.96 g of MnCl2﹒4H2O

30

while stirring to form pink gel precipitates of Mn(OH)2. Then, a solution containing

31

3.982 g of KMnO4 in 250 mL of deionized water was slowly added to the above

32

mixture under vigorous stirring to form a dark gray precipitate. The reaction mixture

33

was stirred for another 30 min and aged at 60 °C for 12 h. The precipitate was 6


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1

collected on a filter with a pore size of 0.2 µm and then repeatedly washed with

2

purified water until the pH of the filtrate reached around 7. δ-MnO2 and triclinic

3

birnessite were prepared according to Villalobos et al., (2003) 46. MnCl2 and Mn2O3

4

were of special grade and purchased from Wako, Osaka, Japan.

5

2.2 Characterization

6

The powder X-ray diffractometer (XRD) patterns of birnessite samples were

7

obtained using a Multi Flex X-ray diffractometer (Rigaku, Akishima, Japan) at room

8

temperature with CuKα radiation (40 mA, 40 kV, at 0.02° 2θ intervals and 2s per

9

step). The relative humidity was measured during daily XRD measurement, which is

10

lower than 55%. The freeze-dried samples were ground with an agate mortar prior to

11

the XRD measurement.

12

The X-ray absorption fine structure (XAFS) spectra of the Mn K-edge for

13

birnessite samples before and after Cs adsorption were collected in transmission mode

14

at room temperature on BL 12C at the Photon Factory, KEK (Tsukuba, Japan). A

15

silicon (1 1 1) double-crystal monochromator was used to obtain the incident X-ray

16

beam. The intensities of the incident and transmitted X-rays were monitored with an

17

ionization chamber. The energy was calibrated by adjusting the second pre-edge peak

18

of δ-MnO2 (synthesized) to 6539 eV. All powder samples were lyophilized, diluted

19

with BN (Wako, special grade, Osaka, Japan) to adjust to 2 wt% of Mn, and were

20

pressed into a tablet with a diameter of 1 cm. To avoid the possibility of radiation

21

damage, only one spectrum was collected from each sample. The sample

22

homogeneity and the lack of sample damage by the X-ray radiation was confirmed by

23

the reproducibility of the spectra.

24

The local coordination structures of Cs in the birnessite were examined by Cs

25

LIII-edge EXAFS spectra. In order to obtain sufficient intensity, Cs-saturated samples

26

were prepared according to our previous work

27

equilibrated once with a 0.5-mol/L CsCl solution for 1 h, twice with a 0.1-mol/L CsCl

28

solution for 1 h, and once with a 0.05-mol/L CsCl solution for 6 h. The final

29

suspensions were filtered, rinsed carefully with distilled water to remove the CsCl

30

solution remaining in the sample and then sealed into polyethylene bags. The sample

31

was placed at an angle of 45° from the incident X-ray to measure the EXAFS

32

spectrum in fluorescence mode using a 19-element Ge solid-state detector. All spectra

33

were analyzed by using REX2000 software (Rigaku, Akishima, Japan). The analysis

31

. Briefly, the samples were

7



1

procedure was similar to that in our previous work 44, 47.

2

2.3 Cs adsorption experiments

Page 8 of 45

3

The adsorption kinetics were investigated first to determine the time required to

4

attain an adsorption equilibrium. Kinetic tests were carried out in 50-mL polyethylene

5

tubes containing 1 mol/L NaCl and 0.1 g/L the prepared birnessite sample. Two

6

different Cs concentrations (6.0×10-6 mol/L and 1.2×10-7 mol/L), traced with 137Cs,

7

were introduced into the suspension. Two mL of liquid samples were taken at time

8

intervals ranging from 1 h to 120 h to measure the Cs concentration. The liquid phases

9

were separated using a 0.2-µm pore size membrane for the analysis of the Cs

10

radioactivity. It has been confirmed in advance that the membrane does not adsorb Cs

11

during filtration.

12

The adsorption isotherm experiment was conducted at two background

13

electrolyte concentrations (10-3 mol/L and 1 mol/L) by varying the Cs concentration

14

from approximately 10-10 mol/L to 10-1 mol/L. For the experiment with a high Cs

15

concentration (higher than 10-6 mol/L), a non-radioactive chemical of high-purity

16

CsCl (Wako, special grade, Osaka, Japan) was used in addition to the radiotracer. For

17

each sample, 0.1 g/L Mn oxide was added. The suspensions were shaken for 2 days.

18

After the adsorption experiments, the solid and liquid phases were separated using a

19

centrifuge (10,000 rpm, 30 min) and the supernatant was filtered through a 0.2-µm

20

pore size membrane for measurements of the Cs concentration. The adsorption

21

amount (Q, mol/g) of Cs was calculated using the following equations:

22 23 24

Q =

( ⁄ )

(1)

and the distribution coefficient Kd (mL/g) was calculated with the following equation:

=

×( ⁄ )

× 1000

(2)

25

where C0 and Ce are the initial Cs concentration (mol/L) and the equilibrium Cs

26

concentration in the filtrate (mol/L), respectively and S/L is the solid-to-liquid ratio

27

(g/L).

28

2.4 137Cs measurement

29

The concentrations of

137

Cs in liquid samples were determined with a liquid

30

scintillation analyzer (Tri-Carb 2550TR/AB, Packard Instruments Company, Meriden,

31

CT, USA) by counting beta rays of

137

Cs

48-50

. The sample solution was added to the

8


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1

liquid scintillation cocktail before measurement (Ultima-Gold AB and FG, Packard

2

Instruments Company, Meriden, CT, USA).

3

2.5 Modeling background

4

The adsorption data were fitted using a multisite adsorption model based on the

5

premise that Cs is adsorbed by the Mn oxides by a cation exchange reaction: the ionic

6

exchange between Cs and Na at the surface of the birnessite (≡ bir) can be defined by

7

Equation 3:

8

Na≡ bir+Cs+ ↔ Cs≡ bir + Na+

9

The cation exchange reaction can be expressed with selectivity coefficients

10

(

) 51:

11

=

(3)

×

(4)

×

12

where αNa and αCs are the activities of Na and Cs respectively and NNa and NCs are the

13

equivalent fractional occupancies. The equivalent fractional occupancy is defined as

14

the sorbed quantity of the cations per mass divided by CEC. According to Bradbury

15

and Baeyens (1994)

16

determined by adsorption test. If Cs is present at trace levels, then NNa is

17

approximately 1, and the selectivity coefficient can be determined with Equation 5:

18

!

" = #$# ×

35

, selectivity coefficients of a cation at trace levels can be

%

%

× (&')

(5)

19

where γNa and γCs are the solution activity coefficients and can be calculated with the

20

Davies’ approximation:

21

Log+, = −0.51 × 0

√2 34√2

− 0.3 × 67

(6)

22

where I represents the ionic strength of the solution. These analytical expressions

23

allow the determination of apparent selectivity coefficients directly from the

24

adsorption data.

25

The verification of the experimentally determined parameters were performed 52

with the Gaines-Thomas equivalent ratio convention 51 for

26

using PHREEQC v2.17

27

the calculation of the exchanged species activity. The Davies equation was used to

28

account for the ionic strength correction for the solutes 53.

29

2.6 Sequential extraction

30

Sequential extraction was used to clarify the Cs partitions in the birnessite with 9



1

different treatments. Birnessite samples after adsorption of 10-9 mol/L Cs were

2

collected for the desorption experiment. The desorption experiments were carried out

3

in conditions that were identical to those used in the adsorption experiment (Section

4

2.3). All samples were rinsed with deionized water after removing the Cs solutions

5

before the sequential extraction test. The sequential extraction procedures were

6

modified based on the methods of Rauret et al. (1999) and Rigol et al. (1998; 1999)

7

54-56

8

solution for 24 h. The solution was separated from the minerals by centrifugation for

9

1 h at 10,000 rpm. The radioactivity of the supernatant was measured after filtration

10

through a 0.2-µm pore size membrane filter. This fraction of Cs was defined as the

11

weak exchangeable fraction (fNaCl). The minerals treated with NaCl solution were then

12

soaked with 40 mL of a 1 mol/L NH4Cl solution for 24 h after being washed with

13

deionized water. The NH4Cl solution was separated from the minerals by

14

centrifugation for 1 h at 10,000 rpm. This fraction was defined as the strong

15

exchangeable fraction (fNH4Cl). After being washed with deionized water, the solid

16

residues were completely decomposed in 5 mL of a 0.5 mol/L NH2OH·HCl solution

17

to determine the fraction of Cs in the solid residue (fresidue).

18

3. Results

19

3.1 Physicochemical Data

. Firstly, the Cs-adsorbed samples were contacted with 40 mL of a 1 mol/L NaCl

20

The elemental compositions of all birnessite samples before and after the acid

21

treatment were initially compared. After the acid treatment, the weight percentage of

22

Mn in biogenic birnessite decreased from 33.6% to 28.1% (Table 1). Note, the

23

biogenic birnessite contained significant concentrations of layer vacancies, which

24

resulted in a high negative charge on the octahedral sheet that was balanced by

25

interlayer Mn(II) or Mn(III) 15, 46, 47, 57. While under proton attack, these exchangeable

26

Mn ions were released into the solution. The Zn-doped birnessite contained 23.2%

27

Mn and 12.9% Zn (Table 1). Previous research base on EXAFS analysis shows that

28

Zn occurs in the interlayer as tetrahedral or octahedral triple-corner-sharing (TCS)

29

complexes on vacancies

30

Mn content decreased from 23.2% to 21.8% after the acid treatment. This result

31

indicated that most of surface adsorbed Zn could be replaced by protons. Similar

32

results were observed with Ni-doped birnessite. The Ni content decreased from 6.0%

21, 58

. The Zn content decreased from 12.9% to 0.2% and the

10


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1

to 0.8%, and the Mn content decreased from 26.1% to 13.4% after the acid treatment.

2

Decrease in Mn content might be attributed to release of exchangeable Mn in the

3

interlayer as well as dissolution of the Mn at the particle edge under proton attack.

4

3.2 XRD patterns

5

The structures of all acid treated samples before and after the Cs adsorption were

6

determined by X-ray diffraction. The XRD patterns of BB, ZB, and NB showed broad

7

peaks at 2.4 Å, and 1.4 Å (Fig. S1). The characteristics of the XRD patterns were

8

typical of poorly crystalline hexagonal turbostratic birnessite

9

basal diffraction peaks at 7.2 Å and 3.6 Å indicated a small number of randomly

10

stacked layers per diffraction particle. The peak intensities of BB were slightly higher

11

than ZB and NB. It has been reported that the presence of Zn and Ni modifies the

12

layer and interlayer structure of Mn oxide, which results in smaller crystal sizes and

13

more layer vacancies

14

adsorption, which is caused by the interference from Cs as a heavy scatterer (Fig. S1).

15

3.3 XAFS spectroscopy

46, 59

. The absence of

19-21, 58

. The peak intensities of all samples decreased after Cs

16

The Mn K-edge X-ray absorption near-edge structure (XANES) spectra for all

17

the birnessite samples displayed a distinct peak around 6558 eV (Fig. 1a). The

18

negative shifts in the shoulders of the birnessite samples were in the following order:

19

BB > ZB > NB

20

It has been reported that the more significant peak broadening near the 60

21

absorbance maximum may be reflecting a larger contribution from Mn(III)

.

22

Therefore, a linear combination fitting (LCF) was performed for all samples on a

23

fixed energy scale (6520 eV – 6590 eV) using the standards of MnCl2, Mn2O3, and

24

δ-MnO2; the fitting results are plotted in Fig. S4 and the obtained fractions of Mn(II)

25

(fMn(II)), Mn(III) (fMn(III)), and Mn(IV) (fMn(IV)) in BB, ZB, and NB were given in Fig.

26

1b and Table S1. The fMn(IV) increased in the order of BB < ZB < NB, and fMn(III)

27

decreased in the order of BB > ZB > NB. The data indicate Mn(III) make a larger

28

contribution in BB than in ZB and NB.

29

The experimental k3-weighted Mn K-edge EXAFS spectra and the corresponding

30

radical structure functions (RSF) of BB, ZB, and NB are shown in Fig. 2a and 2b. The

31

spectra of the δ-MnO2 and the triclinic birnessite were also plotted for comparison.

32

The δ-MnO2 had sharp peaks at 8.10 Å-1 and 9.25 Å-1 in k space, while the triclinic 11



Page 12 of 45

1

birnessite shows peak split and broadened at these region. According to Marcus et al.

2

(2004), the position and shape of the two wave resonances in region between 7.5Å-1

3

and 9.5 Å-1 is sensitive to the presence and distribution of Mn(III) in the manganese

4

layer

5

resonances are intense and symmetrical. When the amount of layer Mn(III) is higher,

6

the amplitudes are lower and the peaks are broadened on their left side. The spectrum

7

for triclinic birnessite shows a split peak in this region because of Jahn-Teller

8

distortion. The peaks at 8.10 Å-1, and 9.25 Å-1 tended to broaden with decreased

9

intensity in the order of NB > ZB > BB (Fig. 2a), which indicated that the amount of

10

61

. In the absence of layer Mn(III), as in stoichiometric δ-MnO2

46

, the two

layered Mn(III) increased in the order of NB < ZB < BB.

11

The radial structure functions (RSF) of all samples show two distinct peaks at R+

12

△R = 1.4 Å and 2.5 Å, indicating a single scattering photoelectron interaction in the

13

first Mn-O shell and Mn-Mn shell, respectively 15. Based on this point, shell-by-shell

14

fitting was performed for those two shells, and the fitting results are listed in Table 2.

15

For all biogenic samples, the peak intensity ratio of the Mn-Mn shell to the Mn-O

16

shell (Mn/O ratio) increased and the peak intensities at 5.2 (R+△R) increased in the

17

order of BB < ZB < NB (Fig. 2b). The peaks at 5.2 Å (R+△R) are attributed to

18

Mn-Mn-Mn multiple scattering (MS). Their intensities were significantly attenuated

19

by an ordered Mn(III) distribution 62. According to Zhu et al. (2010), Mn oxides with

20

more octahedral vacancies and less octahedral Mn(III) usually denotes increased

21

Mn/O ratios with higher peak intensities at 5.2 Å (R+△R)

22

results suggested that the amount of layer vacancies increased in the order of BB
ZB >

26

NB.

20

. Thus, the obtained

27

Figs. 3a and 3b show the experimental Cs LIII-edge EXAFS spectra and the

28

corresponding RSFs of Cs adsorbed on BB, ZB, and NB, as well as CsCl solution.

29

The oscillatory frequencies and the amplitudes were similar for all measured samples.

30

One intense peak at R+△R = 2.4 Å was observed in the RSFs for all biogenic

31

samples, which is similar to the spectra of CsCl solution. The Cs-O distance obtained

32

from shell-by-shell fitting was 2.87 Å, 2.98 Å, and 2.92 for BB, ZB, and NB,

33

respectively (Table 3). This result is in agreement with our previous results and 12


Page 13 of 45 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

suggests that Cs primarily forms outer-sphere coordination on birnessite 30, 31.

2

3.4 Adsorption of Cs on Mn oxides

3

Kinetic tests on BB, ZB, and NB were performed using two different Cs

4

concentrations: 6.0×10-6 mol/L and 1.2×10-7 mol/L. The adsorption equilibrium was

5

obtained within 12 hours (Fig. S2). Accordingly, the adsorption experiments were

6

carried out with a contact time of 2 days.

7

A comparison of the three isotherms for BB, ZB, and NB with the same ionic

8

strength of 0.001 mol/L at pH 7.0±0.2 is shown in Fig. 4a. Cs sorption amount

9

increase in the order of BB < ZB < NB. Adsorption isotherms for all samples showed

10

a non-linear character, which seems to be consistent with the existence of two

11

different adsorption sites. The first linear region with a slope of 1 was observed at low

12

Cs concentration (< 1×10-4 mol/g). A decrease in the dependence of Cs uptake on

13

dissolved Cs concentration appeared as the Cs adsorption amount was observed as Cs

14

concentrations exceeded 8.0×10-5. Changes of slope suggest a saturation of adsorption

15

sites (T1 site). After the saturation of T1 site, Cs uptake followed a linear relationship

16

until the amount of Cs adsorbed approached the CEC value. Therefore, the density of

17

the T1 sites was defined as the amount of sorption sites per unit weight of Mn, and

18

could be estimated directly from the point where the slope began to decrease (pointed

19

out by arrow in Fig. 4a), and the density of T2 sites could be estimated by subtracting

20

the T1 sites from the CEC value. The densities of the T1 and the T2 sites on two types

21

of Mn oxides are listed in Table 4.

22

The adsorption behavior as a function of the ionic strength was analyzed, and the

23

results were plotted in Fig. 5. The Cs adsorption decreased as the ionic strength of the

24

electrolyte increased (Fig. 5a). The non-linear feature of the adsorption curve became

25

obscure as the ionic strength increased to 1 mol/L (Fig. 5a, 5b, 5c). This feature was

26

obviously observed when the data were presented as the logarithm of the Kd values

27

(LogKd, mL/g) as a function of the logarithm of Cs in solution (LogC, mol/L) (Fig. 5d,

28

5e, 5f). The Kd values remained constant until the adsorbed Cs approached the CEC

29

value (Fig. 5d).

30

3.5 Modeling test

31

Firstly, the apparent selectivity coefficients (Log

) for BB, ZB, and NB

32

were calculated by equation 5. The adsorption curve was non-linear at the ionic 13



Page 14 of 45

1

strength of 0.001 mol/L, which indicated that more than one type of sorption site was

2

present. Therefore, the apparent Log

value was calculated based on the

3

assumptions that the Cs was predominately adsorbed onto the T1 sites at low

4

concentrations, and that the T2 site dominated Cs adsorption for high concentrations,

5

since the density of T1 sites was ignorable compared to T2 sites. The calculated results

6

were listed in Table 5. The mean value of the apparent Log

was 1.8±0.1 (T1

7

site), -1.4±0.0 (T2 site) for BB; 1.9±0.1 (T1 site), -1.3±0.0 (T2 site) for ZB; and

8

2.0±0.1 (T1 site), -1.4 0.2 (T2 site) for NB. The calculated results showed that

9

apparent Log

for BB, ZB, and NB was similar within the experimental error.

10

The non-linearity feature was not evident at an ionic strength of 1 mol/L, which

11

suggested that only one-type of sorption site was present. The same phenomenon has

12

been recently observed on chemically synthesized birnessite, which may attribute to

13

increased crystallinity and long range ordering of the MnO6 sheets when high

14

concentrations of electrolytes were added 31. Therefore, the apparent Log

value

15

was calculated directly by equation 5 through using CEC value as the site density. The

16

apparent Log

showed similar values among all samples, which was 2.2±0.1 for

17

BB; 2.3±0.1 for ZB; and 2.5±0.3 for NB.

18

Secondly, the verification of Log

values were performed by the 31

19

ion-exchange model following the procedure similar to that in our recent work

20

order to obtain an adsorption curve with the ionic strength of 0.001 mol/L to be

21

comparable with the experimental data, the site densities for T1 and T2 were used as a

22

fixed parameter (Table 4). The mean value of

23

the initial modeling, since the Log

value for three samples were similar (Table

24

5). The model was tuned by varying

25

calculated and used to determine which value best fit the experimental data. The best

26

fit was obtained via the following parameters: for all samples with an ionic strength of

27

0.001 mol/L, Log

(T1) = 1.2, Log

(T2) = -2.0 (Table 4).

.

. In

was used as an input value for

The Sum of Square Error (SSE) was

28

Note that for an ion-exchange reaction, selectivity coefficients of the sorption

29

sites were independent of the ionic strength. Theoretically, the same parameters could

30

be explored to predict the sorption of Cs with an ionic strength of 1 mol/L.

31

Interestingly, both LogQ and LogKd values were significantly underestimated when

32

compared with the adsorption data (blue dashed curve in Fig. 5).

33

Considering that the variation of the slope for the adsorption curve at ionic 14


Page 15 of 45 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

strength 1 mol/L was small, a one-site ion-exchange model was applied to predict the

2

adsorption curve for BB, ZB, and NB. The procedure was similar to the two-site

3

model described before. The CEC value was used as a fixed parameter, and the mean

4

value of

5

modeling. The best fit Log

= 2.3 was obtained. The calculated results were

6

plotted together with the experimental value in Fig. 5 (blue solid curve).

7

3.6 Sequential extraction of Cs from biogenic birnessites

(as shown in Table 4) was used as an input value for the initial

8

A sequential extraction scheme was developed to distinguish the fraction of Cs

9

associated with sites that had different affinities. Fig. 6 shows the Cs partition on BB,

10

ZB, and NB at the initial Cs concentrations of 1×10-9 mol/L. The extraction yields in

11

each fraction were nearly the same within all systems. The desorption fractions of Cs

12

from biogenic birnessite were 73% - 86% for the 1 mol/L NaCl solution, 12% - 22%

13

for the 1 mol/L NH4Cl solution, and less than 5% remained in residue. Results from

14

sequential extraction suggested that despite an increase in octahedral vacancy

15

concentrations, Cs was mainly reversibly adsorbed onto the surface of biogenic

16

birnessites.

17

4. Discussion

18

4.1 Chemical composition and layer structures in BB, ZB, and NB

19

The chemical composition and the Mn octahedral layer structure are sensitive to 18, 19, 20, 21, 23, 58

20

the coexisting cations, as well as solution pH

. An XAFS analysis

21

revealed that NB and ZB contain more layer vacancies than BB, which was verified

22

by the fact that the bond length between Mn-O and Mn-Mn for NB and ZB is shorter

23

than BB (Table 2). A XANES analysis further demonstrated that the Mn(III) content

24

in NB and ZB was relatively lower than in BB (Fig. 1). The analytical results clearly

25

showed that the doping of heavy metals such as Zn and Ni, followed by acid treatment,

26

could affect the Mn(III) concentrations, as well as the octahedral vacancy

27

concentrations in biogenic birnessite.

28

The effect of transition metals on the layer structures of Mn oxide has been

29

studied before. MnO2 nanoparticles that formed at the early stages of Mn

30

bio-oxidation contain significant concentrations of vacancies

31

such as Zn2+ and Ni2+ compete with the residual dissolved Mn(II) for the vacancies 15


62, 63

. Transition metals,


Page 16 of 45

1

and interfere with Mn(II) adsorption, subsequent oxidation, and incorporation into

2

vacancies 20, 58, 64. Accordingly, the doping of Ni or Zn during birnessite crystallization

3

generally enhances the formation of vacancies in the Mn octahedral layer.

4

Nevertheless, most of these vacant sites were bonded to divalent cations, such as

5

Zn(II), Ni(II), and Mn(II), and were not available for cation adsorption 19, 20, 58.

6

Acid treatment changed the chemical composition of birnessite in two ways.

7

First, Ni(II), Zn(II), or Mn(II) that were located on vacant sites were exchanged with

8

protons and released into solution. Results from elemental compositions analysis

9

showed that after the acid treatment, the Zn content in the ZB decreased from 12.89%

10

to 0.21%, and the Ni content in NB decreased from 6.03% to 0.83% (Table 1).

11

Secondly, the acidic condition might favor steady Mn(III) migration from layer to

12

interlayer sites

13

reaction kinetics on the surface of the three samples with the same pH value resemble

14

each other, this effect influenced three biogenic samples equally. Consequently, the

15

amount of layer vacancies in the final products followed the order of NB > ZB > BB.

65

. This consequently produced a new layer vacancies. Since the

16

The released fraction of Zn (98.4 %) from ZB was higher than the released

17

fraction of Ni (86.2 %) from NB when under the same treatment conditions (Table 1).

18

Previous XAFS results indicated that Zn primarily formed tetrahedral and/or

19

octahedral coordination upon layer vacant site

21, 66

, while a fraction of Ni was

20, 64, 67

20

incorporated into the Mn layers

21

ion that was located at the interlayer was more disposed to be released into solution

22

rather than the one located in the layer.

23

4.2 Effect of layer vacant site on Cs+ adsorption

24

. The present results suggested that the metal

The metal cations, such as Zn2+, Ni2+, Cd2+, and Cu2+ were directly adsorbed 21, 68-73

25

onto the vacant sites by establishing inner-sphere coordination

. Accordingly,

26

the adsorption capacities of birnessite for heavy metals were highly correlated with

27

the number of layer vacancies

28

interlayer sites to balance the layer charge. Cations, such as Na+, Li+, Mg2+, Ca2+, Sr2+,

29

and Ba2+ were regularly distributed within the interlayer because of their electrostatic

30

interaction

31

molecules as tightly as Na+. Therefore, Cs+ showed additional disordered distribution

32

in the a-direction in the interlayer

33

determined by Cs LIII-edge EXAFS, which indicated that Cs primarily formed

74

. Alkali and alkaline earth cations occupied the

12, 75

. Cs+ had lower hydration energy, as it did not bind to the water 25, 75

. The coordination environment of Cs was 16


Page 17 of 45 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

outer-sphere coordination on the birnessite layer (Fig. 3). The adsorption/desorption

2

test proved that most of Cs ions were reversibly adsorbed onto the birnessite (Fig. 6).

3

This suggest that like interlayer Na and K, Cs is located at the midpoint of the

4

interlayer and is surrounded by water molecules. A comparison of Cs adsorption on

5

BB, ZB, and NB showed that the adsorption capacity increased with the amount of

6

layer vacancies (Fig. 4). We thus postulate that the amount of layer vacancies affect

7

Cs uptake by the charge on the octahedral sheets. Although Cs could not attach to the

8

octahedral sheet vacancies to form an inner-sphere coordination in the manner of

9

Mn2+ or Zn2+, the octahedral MnO6 sheets become more negatively charged as the

10

amount of layer vacancies increases. To maintain neutrality of the surface charge,

11

more Cs+ must adsorb to surface sites. Thus, the change in surface charge as a

12

function of layer vacancies may explain the increased uptake of Cs in NB and ZB.

13

4.3 Effect of ionic strength on Cs+ adsorption

14

Background knowledge of the effect that ionic strength has on Cs adsorption was

15

essential when interpreting Cs desorption behaviors. For an ion-exchange reaction,

16

selectivity coefficients of the sorption sites should be independent of the ionic

17

strength. However, sorption data shows that the same selectivity coefficients could not

18

correctly predict sorption of Cs on biogenic birnessite with different ionic strengths.

19

As shown in Fig. 5, the adsorption of Cs with an ionic strength of 1 mol/L was

20

underestimated when the selectivity coefficients for adsorption data at an ionic

21

strength of 0.001 mol/L was used (blue dash curve in Fig. 5). Similarly, when the

22

adsorption of Cs with an ionic strength of 0.001 mol/L was predicted with the

23

parameter used for ionic strength of 1 mol/L, the predicted Cs adsorption curve would

24

be overestimated (red dash curve in Fig. 5). This result was markedly different from

25

Cs adsorption behavior on the chemically synthesized birnessite and todorokite

26

observed in our recent work 31. To confirm this point, the sorption data with initial Cs

27

concentration of 10-10 mol/L and a wide range of Na concentrations for biogenic and

28

chemically synthesized birnessite were collected, and the experimental dependence of

29

Log(Kd) on Log(Na) is plotted in Fig. 7.

30

For a typical ion-exchange reaction between Cs and Na, the dependence of

31

Log(Kd) on Log(Na) could be represented by a line with a slope of -1 38. For example,

32

the experimental adsorption dependence on the ionic strength compared well with the

33

theoretical behavior for chemically synthesized birnessite (slope = -1.01) as well as 17



Page 18 of 45

1

todorokite (slope = -0.86) (Fig. 7b). There were large variations observed within the

2

theoretical behavior for the biogenic samples: the slope of the curve was -0.18 for BB,

3

-0.14 for NB, and -0.18 for ZB (Fig. 7a), which was distinctly different from

4

chemically synthesized birnessite. The variation between the theoretical data and the

5

experimental results have been previously reported on clay minerals

6

possible explanation would be the non-electrostatic character of the interaction

7

between Cs and the clay surface. For example, Xu and Harsh (1990) attribute this

8

anomalous behavior to the formation of covalent bonds between Cs and the clay

9

surface

38, 76-78

. One

78

. However, the results from the sequential extraction did not support this

10

claim. The majority of the adsorbed Cs on the biogenic birnessite could be desorbed

11

by ion-exchange (Fig. 6), which indicated that a strong chemical bond was not formed

12

between the Cs and the birnessite layer. Our recent analytical results shows that Cs is

13

adsorbed on the chemically synthesized birnessite by ionic exchange 31. Considering

14

the structural similarity between biogenic and chemically synthesized birnessite, the

15

possibility that strong chemical bond formed between Cs and biogenic birnessite

16

surface was ruled out.

17

Another effect of ionic strength that could explain the change in selectivity 79-81

18

resulted from the changes in tactoid size and crystallinity

19

the interlayer sites could have differing affinities for Cs. When the ionic strength was

20

low, the mineral particles were probably highly dispersed, causing both the external

21

planar sites and the interlayer site to contribute to Cs adsorption. With increased

22

electrolyte concentration, the particle size increased and the proportion of interlayer

23

exchange

24

concentrations of electrolyte enhanced the crystallinity and the long range ordering of

25

the MnO6 sheets

26

sorption sites. Our recent research has shown that the non-linear character of the

27

adsorption curve for chemically synthesized birnessite attenuated as the ionic strength

28

increased

29

biogenic birnessite samples (Fig. 5). The increase in tactoid size could account for

30

these differences. However, the above considerations are unlikely to explain the

31

distinct behavior that was observed between the biogenic birnessite and the

32

chemically synthesized birnessite (Fig. 7).

sites

became

predominant.

Moreover,

. Both the external and

ion-exchange

with

high

25, 82

, which homogenized the difference between the two types of

31

. In the present study, a similar phenomenon was also observed for

33

Compared with the chemically synthesized birnessite, the biogenic birnessite

34

contains a significant amount of Mn (II, III) that is bound to microbial cells or 18


Page 19 of 45 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

adsorbed in the interlayers of biogenic birnessite 20, 66. Even following acid treatment,

2

trace amounts of cations were still present and could be released into solutions

3

through ion-exchange, so their effect could not be excluded. The concentrations of the

4

main trace cations found in the supernatant upon contact with BB, ZB, NB, and

5

chemically synthesized birnessite were determined. The concentrations of Mn in the

6

biogenic samples were 2-3 orders of magnitude higher than in the chemically

7

synthesized sample (Table 6). Competition effects of trace ions in solution has been

8

shown to significantly influence the Cs adsorption on smectites 38. This suggests that

9

the deviation from the theoretical behavior for biogenic samples was caused by the

10

competition for sorption sites of the existing trace ions (especially Mn2+) in the

11

aqueous solution. Considering that the selectivity of Mn2+ is much higher than Cs+,

12

small differences in the quantity of Mn2+ in solution could lead to significant

13

differences in measured Kd, especially when the Cs+ concentration is low. Accordingly,

14

all the experimental data set was fit with a finer tuning of the initial parameters to

15

consider the competitive effects of Mn in solution, and all the parameters used for the

16

fit of the experimental data are summarized in Table 7. As shown in Fig. 8, a much

17

improved fitting result was obtained after considering the competitive effect of Mn2+.

18

This result further prove that the different sorption behavior between biogenic and

19

chemically synthesized birnessite would be mainly attributed to the competition effect

20

from trace ions (such as Mn2+) that were loosely bound to biogenic birnessite. It

21

should be noted that a careful monitor of trace ions at the mineral-water interface is

22

necessary when surface properties of poorly crystalline adsorbents, such as

23

bio-minerals were considered.

24

5. Conclusion

25

The adsorption behavior of radioactive Cs over a wide variety of concentrations

26

that were placed on three biogenic birnessites that formed under different conditions

27

was studied. The adsorption parameters, assisted by ion-exchange modeling, were

28

determined. This is essential when quantitatively describing Cs adsorption under

29

environmental conditions.

30

The sorption occurred via a 1:1 ion-exchange process within all biogenic

31

birnessite samples. The Cs was reversibly adsorbed onto the mineral surface to form

32

an outer-sphere coordination. The doping of transition metals (e.g. Zn and Ni) during

33

bio-oxidation of Mn(II), followed by acid treatment, was effective in increasing the 19



1

number of available layer vacant sites, which consequently increased Cs adsorption

2

capacity in the final product.

3

Changes in electrolyte concentration give a clearly different result for Cs

4

adsorption on biogenic and chemically synthesized birnessite. Trace ions (e.g. Mn2+)

5

that were loosely bound to biogenic birnessite were found to be released into the

6

solution when they came into contact with NaCl solution. The competition between

7

these trace ions caused significant deviation of the experimental data from the

8

theoretical data. The present results indicate that it is necessary to account for the

9

competition effect of trace ions, especially when poorly crystalline adsorbents, such

10

as bio-minerals, were considered.

11

Acknowledgments

12

The project was partly supported by National Natural Science Foundation of

13

China (NO. 41703119); a Grant-in-Aid for Young Scientists (B) (26820410) from the

14

Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan;

15

Nature Science Foundation of Hubei Province of China (2017CFB139); the

16

Fundamental Research Funds for the Central Universities, China University of

17

Geosciences (Wuhan) (No. CUG170607, No. CUG170104). The XANES analysis

18

was performed with the approval of the Photon Factory, KEK (Proposal No.

19

2013G092).

20

Supporting Information

21

Composition of HAY medium; XRD patterns for biogenic birnessites before and

22

after exchange with Cs; Cs adsorption kinetics for biogenic birnessites; Contribution

23

of T1 and T2 sites to the Cs adsorption on biogenic birnessites with different ionic

24

strength; fitting results of Mn K-edge XANES spectra; Average oxidation states (AOS)

25

of Mn in biogenic birnessites before and after Cs adsorption obtained from XANES

26

analysis. These materials supplied as Supporting information is available free of

27

charge on the ACS publications website

28

Author information

29

ORCID 20


Page 20 of 45

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1

Qianqian Yu: 0000-0002-4003-0744

2

3

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Table 1 Incubation conditions of biogenic birnessite, Zn-doped birnessite, and Ni-doped birnessite and the elemental composition of the final products Element composition Mn addition Zn addition Ni addition before acid treatment (wt %) Sample (mmol/L) (mmol/L) (mmol/L) Mn Zn Ni Others biogenic birnessite 1.00 33.63 n.d. n.d. 66.37 Zn-doped birnessite 1.00 0.50 23.21 12.89 n.d. 63.90 Ni-doped birnessite 1.00 0.50 26.14 n.d. 6.03 67.83 n.d., not detected. Detection limits of not detected elements are Zn 0.002 mg/L, Ni 0.008 mg/L.

29


Element composition after acid treatment (wt %) Mn Zn Ni Others 28.05 n.d. n.d. 71.95 21.75 0.21 n.d. 78.04 13.39 n.d. 0.83 85.78


Page 30 of 45

Table 2 Optimized fitting parameters of EXAFS Mn K-edge for BB, ZB, and NB

Sample BB ZB NB

Mn-O1 R1 (Å)

N1

1.91(2) 3.86 1.89(0) 4.21 1.89(2) 4.12

Mn-Mn1 σ2 (Å2)

R2 (Å)

0.005 0.006 0.005

2.87(0) 4.66 2.85(4) 3.34 2.85(5) 4.03

N2

σ2 (Å2) 0.010 0.006 0.008

R (%)

∆E

4.08 2.40 2.13

7.622 4.586 5.208

N: effective coordination numbers; R(Å): interatomic distance; σ2: disorder contribution; The estimated standard deviation (e.s.d.) for R1 and R2 is less than 0.03 and for N1 is less than 0.8, and for N2 is less than 0.2. R(%) indicates the quality of fitting result and is expressed by the following formula: R = ∑@A B CDE (A) − A B CF (A)GH /∑@A B CDE (A)GH

30


Page 31 of 45 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 3 Local structure of Cs adsorbed on BB, ZB, and NB determined using Cs LIII-edge EXAFS Cs-O Sample R (%) ∆E R1 (Å) N1 σ2 (Å2) BB ZB NB

2.87(1) 2.98(4) 2.92(4)

9.85 14.65 14.11

0.01 0.03 0.01

1.068 0.886 1.994

2.17 2 0.12

N: effective coordination numbers; R(Å): interatomic distance; σ2: disorder contribution; R(%) indicates the quality of fitting result and is expressed by the following formula: R = ∑@A B CDE (A) − A B CF (A)GH /∑@A B CDE (A)GH

31



Page 32 of 45

Table 4 Parameters used for the best fit of the experimental adsorption curves presented in Fig. 5

Adsorbents NaCl (mol/L) BB

0.001

Site density* (mol/g) T1

T2

0.80×10-4 7.20×10-3 -4

T2

SSE

1.20

-2.00

0.22

1.20

-2.00

0.14

1.20

-2.00

0.33

0.001

1.00×10

NB

0.001

1.50×10-4 8.80×10-3

1

8.10×10

T1 -3

ZB BB

Selectivity coefficients Log Ksel

7.28×10

-3

-

2.30

-

0.59

-3

-

2.30

-

0.08

-

2.30

-

0.37

ZB

1

8.20×10

NB

1

8.95×10-3

*site density is the amount of sorption sites per unit weight of Mn

32


Page 33 of 45 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 5 Summary of the main results obtained in experimental test and apparent selectivity constants directly calculated with Equation 5

Adsorbents BB BB BB

NaCl

LogC

LogQ

(mol/L)

(mol/L)

(mol/g)

1×10

-3

1×10

-3

1×10

-3

BB

1×10-3

BB

1×10

-3

1×10

-3

1×10

-3

BB BB

-10.11 -9.01 -8.05 -7.08 -6.07 -3.01 -2.04

-9.53 -8.42 -7.44 -6.25 -5.31 -3.52 -2.56

Site density* (mol/g)

LogKd (mL/g)

Apparent LogKSEL

0.8×10

-4

3.71

1.79

0.8×10

-4

3.72

1.8

0.8×10

-4

3.74

1.82

0.8×10

-4

3.66

1.74

0.8×10

-4

3.6

1.68

7.2×10

-3

2.48

-1.38

7.2×10

-3

2.48

-1.35

-3

3.23

2.27

BB

1

-8.98

-8.75

7.28×10

BB

1

-8.05

-7.88

7.28×10-3

3.17

2.21

7.28×10

-3

3.12

2.16

7.28×10

-3

3.16

2.2

BB BB

1

-7.01

1

-6.02

1×10 1

ZB

1×10-3

ZB

1×10

-3

1×10

-3

1×10

-3

1×10

-3

1×10

-3

1×10

-3

ZB ZB ZB ZB

-5.86

-3

BBaverage BBaverage

ZB

-6.9

1.8±0.1 (T1); -1.4±0.0 (T2) 2.2±0.1 -10.14 -9.03 -8.06 -7.1 -6.08 -3.55 -2.99

-9.18

1×10-4

4.01

1.99

-8.18

1×10

-4

3.95

1.93

1×10

-4

3.96

1.94

1×10

-4

3.92

1.9

1×10

-4

-7.2 -6.05 -5.15 -3.89 -3.35

3.82

1.8

8.1×10

-3

2.66

-1.26

8.1×10

-3

2.64

-1.27

-3

3.36

2.34

ZB

1

-10.06

-9.7

8.2×10

ZB

1

-8.97

-8.5

8.2×10-3

3.48

2.46

8.2×10

-3

3.44

2.42

8.2×10

-3

3.3

2.28

8.2×10

-3

3.39

2.37

-3

3.15

2.14

3.13

2.12

ZB ZB ZB

1

-8

1

-7.01

1

-6

-7.56 -6.7 -5.62

ZB

1

-5.01

-4.86

8.2×10

ZB

1

-4.02

-3.89

8.2×10-3

-3

ZBaverage ZBaverage

1×10 1

NB

1×10-3

NB

1×10

-3

1×10

-3

NB

1.9±0.1 (T1); -1.3±0.0 (T2) 2.3±0.1 -10.14 -9.04 -8.08

-8.97

1.5×10-4

4.17

1.98

-7.92

1.5×10

-4

4.12

1.93

1.5×10

-4

4.17

1.98

-6.91 33



NB

1×10-3

NB

1×10

-3

NB

1×10

-3

-2.99

NB

1×10-3

-2.03

NB NB NB

1

-7.12 -6.09

-10.05

1

-8.97

1

-8.05

Page 34 of 45

-5.77

1.5×10-4

4.35

2.16

-4.88

1.5×10

-4

4.21

2.02

-3.33

8.8×10

-3

2.66

-1.28

-2.62

8.8×10-3

-9.47 -8.24 -7.18

2.41

-1.51

8.95×10

-3

3.57

2.52

8.95×10

-3

3.72

2.67

8.95×10

-3

3.88

2.83

-3

3.68

2.63

NB

1

-7.09

-6.4

8.95×10

NB

1

-6

-5.39

8.95×10-3

3.62

2.57

-3.83

8.95×10

-3

3.18

2.13

8.95×10

-3

3.17

2.13

NB NB NBaverage NBaverage

1

-4.01

1 1×10 1

-3

-2.83

-3

2.0±0.1 (T1); -1.4±0.2 (T2) 2.5±0.3

*site density is the amount of sorption sites per unit weight of Mn.

34


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Table 6 Concentration of main trace cations found in the supernatant upon the contact with biogenic birnessites. Maximum and minimum values were obtained after analysing different batches of the suspensions at different ionic strengths.

BB

(mol/L) +

K Mn2+ Ni2+

min 6.8×10-6 3.8×10-5 n.d.

ZB max 3.3×10-5 3.9×10-4 n.d.

min 5.0×10-6 2.4×10-5 n.d.

NB max 5.2×10-5 4.0×10-4 n.d.

min 3.6×10-6 2.4×10-5 1.7×10-6

n.d., not detected. Detection limits of not detected elements are Mn 0.0005 mg/L, Ni 0.008 mg/L.

35


max 4.6×10-5 4.5×10-4 1.7×10-6

Chemically synthesized birnessite min max -5 4.7×10 2.3×10-4 n.d. 4.6×10-7 n.d. n.d.


Page 36 of 45

Table 7 Parameters used for the best fit of the experimental adsorption curves presented in Fig. 8

Adsorbents

NaCl (mol/L)

Site densitya (mol/g)

BB

0.001

7.28×10-3

3

5.5

1.98

0.001

8.20×10

-3

3

5.5

2.36

8.95×10

-3

3

5.5

1.47

7.28×10

-3

3

5.5

1.49

-3

3

5.5

0.20

3

5.5

1.38

ZB NB BB

0.001 1

ZB

1

8.20×10

NB

1

8.95×10-3

Selectivity Selectivity coefficients coefficients SSE Log Ksel1b Log Ksel2c

a

Site density is the amount of sorption sites per unit weight of Mn Log Ksel1 is the selectivity coefficients between Cs and Na c Log Ksel2 is the selectivity coefficients between Mn and Na

b

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Fig. 1 (a) Mn K-edge XANES spectra of BB, ZB, and NB; (b) fractions of Mn(II), Mn(III), and Mn(IV) in BB, ZB, and NB, determined from XANES linear combination fittings.

37



Fig. 2 (a) Mn K-edge EXAFS spectra and (b) the corresponding Radial Structure Functions (RSF) of BB, ZB, and NB. The experimental data (solid line) are overlapped with fitting result (dashed line) and the parameters were summarized in Table 2.

38


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Fig. 3 (a) Cs LIII-edge EXAFS spectra and (b) the corresponding Radial Structure Functions (RSF) for Cs adsorbed on BB, ZB, and NB. The experimental data (solid line) are overlapped with fitting result (dashed line) and the parameters were summarized in Table 3.

39



Fig. 4 Cs sorption isotherms for BB, ZB, and NB at pH7.0±0.2 with an ionic strength of I = 1 mmol/L. (a) Data expressed as Log C vs. Log Q and (b) data expressed as Log C vs. Log Kd. Q value was normalized by Mn content for all biogenic samples. The solid lines represent the fitting result with the parameters of Table 4. The CEC value for two samples was marked with dotted line.

40


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Page 41 of 45 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fig. 5 Cs adsorption isotherms for (a, d) BB, (b, e) ZB, and (c, f) NB at pH 7.0±0.2 and different ionic strengths. Data in (a-c) was expressed as Log C vs. Log Q and data in (d-f) was expressed as Log C vs. Log Kd. Q value was normalized by Mn content for all biogenic samples. The solid lines represent the fitting result with the parameters of Table 4. The dash lines are examples of (false) simulations: the blue dash line represent simulation result with an ionic strength of 1 mol/L, when Log Ksel value for adsorption data at an ionic strength of 0.001 mol/L was used; the red dash line represent simulation result with an ionic strength of 0.001 mol/L, when Log Ksel value for adsorption data at an ionic strength of 1 mol/L was used.

41



Fig. 6 The partition of Cs on BB, ZB, and NB. The initial concentration of Cs were 1 ×10-9 mol/L for all samples.

42


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Fig. 7 Analysis of the dependence of Log(Kd) on the concentration of the main ions in solution. (a) BB, ZB, and NB; (b) Chemically synthesized birnessite and Todorokite.

43



Fig. 8 Examples of one site fitting of Cs adsorption on (a) BB, (b) ZB, and (c) NB at pH 7.0±0.2 and different ionic strengths. The solid lines represent the model calculation with the parameters of Table 7, when competition effect of Mn was considered.

44


Page 44 of 45

Page 45 of 45 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Graphic abstract

45


Adsorption of Cs onto biogenic birnessite: effects of layer structure

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