Strontium adsorption on clays at low concentration

低濃度におけるストロンチウムの粘土への吸着

Zigong Ning1, Munehide Ishiguro2, Luuk K. Koopal3, Tsutomu Sato4

1Graduate School of Agriculture, Hokkaido University, 2Research Faculty of Agriculture, Hokkaido

University, 3 Physical Chemistry and Soft Matter, Wageningen University and Research, Wageningen,

The Netherlands; 4Faculty of Engineering, Hokkaido University

Abstract: Sr2+ adsorption on kaolinite, illite and vermiculite at low Sr2+ concentration under the different concentrations of NaCl solution was measured by the batch method. The calculated distribution coefficients (KD) was constant at low Sr2+ concentration but decreased with increasing Sr2+ concentration. The Sr2+ isotherms could be fitted well to the Langmuir adsorption equation, but the adsorption constant, KL, should be interpreted as conditional constant depending on the Na+ concentration because of competition between Sr2+ and Na+ for the adsorption sites.

Key words：adsorption, clay, strontium, distribution coefficient, Langmuir

1. Introduction Strontium (Sr)-90 is one of the radioactive elements of nuclear fuel materials emitting beta particles. Its half-life is 29 years, and it is harmful to the human health. During the nuclear power plant accidents in Fukushima, Japan and Chernobyl, Ukraine, large amounts of radioactive species, including Sr-90, were released into the environment. As strontium has similar properties as calcium, it is by mistake readily taken up and deposited in human bone and leads to bone tumors and tumors of the blood-cell forming organs due to the beta particles emitted from Sr-90. As the radioactive damage is serious, the environmental fate of Sr2+ must be considered, especially at low Sr2+ concentrations. The objectives of the present study are to investigate both the characteristics of Sr2+ adsorption on four clays minerals at different NaCl concentrations with emphasis to low Sr2+

concentrations.

2. Materials and Methods Kaolinite (Iriki, Kagoshima, JP; 98.1% of kaolinite, 1.9% of alumiteillite), IMt-2 (Silver Hill, Montana, USA; 90% of illite, 10% of quartz), and vermiculite (Sukagawa, Fukushima, JP; 100% of vermiculite) were considered. Less than 2 µm

diameter of clays were obtained by method of centrifugation. The experiment of Sr2+ adsorption on clays were conducted by the batch method under the different NaCl concentrations and pH. The Sr and Na concentrations were measured by inductively coupled plasma mass spectrometry (ICP-MS). The the Sr2+ distribution coefficients were derived and used to describe the Sr2+ migration. The Langmuir model assumes ideal adsorption behavior in a monolayer and considers the adsorption capacity as independent parameter and neglects valence effects.

3. Results and Discussion The Sr adsorption isotherms at different conditions are depicted in Fig.1. The double logarithmic plots are about linear (slope ≈1) at low [Sr], but level off at higher concentrations. This implies that, at given conditions, the distribution coefficient, KD = {X2Sr}/[Sr], where {X2Sr} is the Sr adsorption (mmol kg-1), decreases with increasing [Sr]. The slope of the isotherm decreases at higher [Sr] because the adsorption sites on clays become progressively occupied by Sr. The Sr adsorption decreases with increasing [NaCl] because of adsorption competition with Na and screening of the electrostatic attraction. The latter has been

confirmed by the fact that the negative ζ potential becomes less negative with increasing [NaCl]. The adsorption isotherms agreed well with the Langmuir equation. The constant KD values at low [Sr] and the decrease of KD values with increasing [Sr] can be explained with the Langmuir equation. The fitting of the Sr adsorption data to the Langmuir equation is presented in Fig. 2 as double logarithmic plot. The values of the adsorption affinity or Langmuir constant, KL and the adsorption capacity, QSr,m are collected in Table 1. The values of KL strongly decrease with increasing NaCl concentration, and QSr,m decreases only slightly with increasing NaCl concentration, but

not very dependent on NaCl concentration. The fact that KL is strongly dependent on NaCl concentration indicates that KL should be interpreted as a conditional constant and that the competition between Sr and Na is important. The higher cation exchange capacity of the clay results in the larger values of QSr,m ( Vermiculite > Iliite> Kaolinte). The larger values of KL represent the larger adsorption energy, △G, according to the equation: △G=RT ln(KL). The result (Kaolinite > Illite ≈ Vermiculite) indicates that edge sites produce the higher affinity for Sr; Edge sites are predominant for Kaolinite while basal sites are predominant for Illite and Vermiculite.

Fig.1. Sr2+ adsorption isotherms on clays (symbols) at pH 6.5 and different salt concentrations. The dotted lines are lines of equal distribution coefficient, KD. (△0.1, □1, ○10 mmol L-1 NaCl)

Fig.2. Comparison of the experimental Sr2+ adsorption data at pH 6.5 and different salt concentrations (symbols) with the Langmuir equation (solid lines). (△0.1, □1, ○10 mmol L-1 NaCl)

Table 1 Langmuir isotherm parameter for Sr2+ adsorption on clays Kaolinite Illite Vermiculite

Na conc. (mmol L-1)

QSr,m (mmol kg-1)

KL (L mmol-1)

QSr,m (mmol kg-1)

KL (L mmol-1)

QSr,m (mmol kg-1)

KL (L mmol-1)

10 11.0 60 38.6 37 126.2 36

1 11.6 1100 40.5 202 127.5 264 0.1 12.0 3300 42.6 339 - -

KD=750KD=10000

KD=40000

0.01

0.1

1

10

100

0.00001 0.0001 0.001 0.01 0.1

Srad

sorp

tion

(mm

olkg

-1)

Sr equilibrium conc. (mmol L-1)

KaoliniteKD=14000

KD=1500

KD=8000

0.1

1

10

100

0.0001 0.001 0.01 0.1

Sr a

dsor

ptio

n (m

mol

kg-1

)

Sr equilibrium conc. (mmol L-1)

Illite

KD=4500

KD=33500

1

10

100

0.0001 0.001 0.01 0.1

Sr a

dsor

ptio

n (m

mol

kg-1

)

Sr eqilibrium conc. (mmol L-1)

Vermiculite

0.01

0.1

1

0.00001 0.001 0.1

Q/Q

Srm

Sr conc. (mmol L-1)

Kaolinite

0.01

0.1

1

0.0001 0.001 0.01 0.1 1

Q/Q

Srm

Sr conc. (mmol L-1)

Illite

0.01

0.1

1

0.0001 0.01 1

Q/Q

Srm

Sr conc. (mmol L-1)

Vermiculite