Adsorption kinetics of Sr ions on clay

ADSORPTION KINETICS OF Sr IONS ON CLAY

B inay B IL G IN an d G onul K E C E L I

Department ofPhysical Chemistry, Faculty o f Engineering, Istanbul University Avcilar 34850 Istanbul, TURKEY

The adsorption o f metal ions in aqueous solution onto clay minerals is important in many areas o f science and technology. These adsorption studies are needed in order to estimate rates o f transport o f the nuclides in the event o f water penetration into and through the repository. The adsorption behaviour o f Sr2+, one o f the more important nuclides present in nuclear waste, has been extensively studied for some mineral-solution systems. In this study adsorption o f Sr ions on halloysite type clay has been investigated using 90Sr as tracer. The adsorption experiments were carreid out using the batch method and initial concentrations o f Sr ions ranged from 10"1 to 10'6 M. It was found that % adsorption values and distribution coefficient decreases with increasing initial Sr ion concentration. Adsorption experiments were performed as a function o f shaking time at 293 K. Initially, the % adsorption o f Sr ion increases rapidly, but then the process slow down. The slow adsorption may be due to the diffusion o f the ions into the pores o f the clays. Kinetic investigations were performed to elucidate the mechanism o f adsorption o f Sr ions. The first order rate constants were calculated for different initial concentrations o f Sr ions. It is seen that distribution coefficient decrease with increase the temperature. In order to predict thermodynamic behaviour o f the adsorption process on halloysite type clay. Thermodynamic constants AH°, AS0 and AG° were calculated at 293 K, 313 K, 333 K and 343 K. IN T R O D U C T IO N

The removal o f radionuclides from high level liquid wastes is importance in the partitioning process. Sr-90 is an important source o f radioactivity in the waste from the nuclear power industry. The adsorption o f radionuclides in aqueous solution onto clay minerals is important in many areas o f science and technology. These adsorption studies are needed in order to estimate rates o f transport o f the nuclides in the event o f water penetration into and through the repository. Clay minerals have been employed for the processing o f radioactive liquid wastes, owing to their relatively large adsorption capacity and high selectivity. The adsorption behaviour o f Sr2+ has been extensively studied for some mineral solution systems[13].

In this study, adsorption o f Sr ions on halloysite type clay has been investigated using 90Sr as tracer.

M E T H O D

The sorption experiments were carried out using the batch method and initial concentration o f Sr ions ranged from 10"6 to 10_1M. Experiments were performed as a function o f concentration

o f adsorbate, shaking time and temperature. The sorption percentage (Ads.,%) and distribution coefficient, KD, was calculated as:

A — A Ads.,% = —i----f- x100 Ai KD _ Ai - Af V —--- L x — Af m

W here; A i and Af are initial and final activities (Bq/cm3) o f the solution phase, respectively, V (cm3) is the volume o f the solution and m (g) is the amount o f clay.

R E SU LT S

Fig. 1 shows the variation o f %Ads., o f Sr ions adsorbed as a function o f time. As it is seen from the figure that the amount o f Sr adsorbed increases with time and concentration o f sr ions and then attains a constant value after 30 (min.).

Sr2+ removal by the clay tends to follow Lagergren’s first order rate expression,

log(q. q)= logq.

-W here qe and q are the equilibrium and initial Sr ion concentrations in the clay respectively. t is

the time (min) and kads. is the first order rate constant.

As can be observed from Fig.2, the linearity o f the plots indicated the appliabillity o f the first order kinetics for the system. The adsorption rate constants were determined from the slopes o f the plots and were found as follows: k 1= 0.051, k2 = 0.064, k3 = 0.066, k4 = 0.074, k5 = 0.069,

k6 = 0.070 respectively.

In batch adsorption processes the adsorbate molecules diffuse into the interior o f the porous adsorbent and rate process usually depends upon t 12 rather than t. Fig.3 shows the extent o f adsorption o f Sr ions as a function o f t 12 an different concentrations. As can be observed the plot is linear for a wide range o f contact times but does not go through the origin. This indicates that the mechanism o f Sr adsorption on the clay is complex and both the surface adsorption as well as the interparticle diffusion contribute to the rate determining step[4].

The thermodynamic parameters for the adsorption o f Sr ions on the clay were calculated from linear variation o f ln KD vs. 1/T using relation, (Fig.4.);

ln Kd = AS° R

AH0 RT

The values o f AG0 calculated are shown in Table 1 which is an indication o f a spontaneous process, that is to say that the adsorptive forces are strong enough to break the potential barrier. The positive AH0 value is a further confirmation that the adsorption process is endothermic. The positive value o f AS0 shows the existence o f some structural changes at solid-liquid interface.

%

A

d

s

T able 1: The Enthalpy, Entropy and Gibbs Free Energy Changes Obtained from The Sorption Data. C (M ) AH°(kJ/m ol) x1 0-3 AS0(J/m olK ) x10-3 AG0(kJ/m ol) at T = 293K AG0(kJ/m ol) at T = 313K AG0(kJ/m ol) at T = 333K AG0(kJ/m ol) at T = 313K R 10-1 7.64 5.62 -1.64 -1.75 -1.86 -1.92 0.92 10-2 8.56 12.72 -3.72 -3.97 -4.23 -4.35 0.98 10-3 7.98 13.21 -3.86 -4.13 -4.39 -4.52 0.97 10-4 5.40 10.80 -3.16 -3.37 -3.59 -3.69 0.91 10-5 8.31 22.19 -6.49 -6.94 -7.38 -7.60 0.96 10-6 12.72 39.99 -11.70 -12.5 -13.3 -13.7 0.96 Time (min)

Fig.1. Adsorption o f Sr2+ as a function o f time at different concentrations.

Time (min)

Fig.2. Lagergren equation for Sr2+ adsorption at different concentrations.

% A d s . 80.0 1.0 10 'm 10' 2m

Fig.3. Plots for Sr2+ adsorptionagainst time (min)12 at different concentrations.

Fig.4.Relationship between distribution coefficient and 1/T at different concentrations

R E F E R E N C E S

1) P. Rafferty, S.Y. Shiao, C.M. Binz, R.E. Meyer, Inorg. Nucl. Chem., 43 (1981) 797 2) V.I. Spitssyn, V.V. Gromev, At. Energy (Engl. Transl.), 5 (1958) 1341

3) J.R. Eliason, Amer. Mineralogist, 51(1966)

4) Faraday F.O. Orumwense, J. Chem. Tech. Biotechnol., 65 (1996) 363-369