Sample pre-treatment

S e d i m e n t : The d r y M u r r u m b i d g e e R i v e r b ank s e d i m e n t was soak e d in w a t e r f o r 7 days, and t h e s e d i m e n t s l u r r y t h o r o u g h l y mixed. A s u b - s a m p l e w a s taken, and t h e w e t : d r y w e i g h t d e t e r m i n e d by d r y i n g o v e r n i g h t at 50°C. The w e t Bega R i v e r s e d i m e n t w a s w e t - s i e v e d in fresh water, and the v a r i o u s s i z e f r a c t i o n s r e t a i n e d . The w e t i d r y w e i g h t ratio was d e t e r m i n e d on the <63 /zm and 1 2 5 - 500/zm siz e f r a c t i o n s . Th e s e s e d i m e n t s l u r r i e s w e r e r e t a i n e d f o r the d e s o r p t i o n e x p e r i m e n t s .

A p o r t i o n of t h e <63 /zm f r a c t i o n of the Beg a s e d i m e n t was s u s p e n d e d in r i v e r water, and p l a c e d in an u l t r a - s o n i c bath for 3 m i n u t e s . S e d i m e n t a t i o n a n a l y s i s ( s e c t i o n 3.5) was then u sed to o b t a i n a s u s p e n s i o n

of p a r t i c l e s <20 /zm in size. An a l i q u o t of t h i s suspension was removed to determine the p a r t i c u l a t e mass concentration on a dry weight basis, and the

remainder of the suspension retained f o r the adsorption experiments. A l l

wet sediments were stored in sealed containers in a r e f r i g e r a t o r at 4*C

u n t i l required.

Water: The Murrumbidgee and Bega River water samples were co ll ec te d from

the same s i t e as the sediment. A seawater sample col lected from Tathra

wharf (Figure 5.2) was used to vary the s a l i n i t y of each experiment. A l l

water samples were f i l t e r e d through a 0.45 fm membrane f i l t e r and stored in

a dark room at 15‘ C u n t i l required.

6 .2 .2 Adsorption experiments Radium

A 226Ra stock s o lu ti on was prepared by adding a small qu a n t it y o f a

standard 226Ra sol u tio n (supplied by Amersham) to f i l t e r e d Bega River water

in a polypropylene b o t t l e . F i l t e r e d seawater (5% by volume) was added to

l i m i t adsorption of radium onto the walls of the b o t t l e , and the pFI

adjusted to 7.0 by ad di ti on o f a NaOH so lu ti on . The a c t i v i t y of the 226Ra

sol ut ion was -140 Bq/mL.

A l l experiments were c ar rie d out in duplicate in capped 25 mL polypropylene

b o t t l e s . Appropriate volumes of sediment suspension, r i v e r water and

seawater were added ( t o t a l volume -20 mL). S a l i n i t i e s were varied from

0.2 ppt ( r i v e r water only) to 35 ppt (98% seawater), and each series of

experiments were performed at sediment concentrations of 10, 100 and

1000 mg/L. The b o t t l e containing the sediment suspension was secured to a

revolving wheel f o r 24 h. A weighed qu antity (-200 mg) of 226Ra stock

solu tion was then added, and the suspension e q u i l i b r a te d f o r a f u r t h e r

24 h. At the end of the e q u i l i b r a t i o n period, the suspension was drawn

into a syringe and f i l t e r e d through a 0.45 fim membrane f i l t e r d i r e c t l y i n to

a 25 mL glass s c i n t i l l a t i o n v i a l . The dissolved 226Ra concentration was

determined by l i q u i d s c i n t i l l a t i o n counting (section 3.3).

P r o c e d u r a l b l a n k s c o n t a i n i n g only f i l t e r e d w a t e r and 2 2 6 Ra t r a c e r w e r e d e t e r m i n e d at a r a n g e of sali n i t i e s in o r d e r to asse s s the e x t e n t of s o r p t i o n losses of r a d i u m onto the w a l l s of the p o l y p r o p y l e n e b o t t l e and the m e m b r a n e f i l t e r . Losses w e r e g r e a t e s t f r o m the b l a n k of lowest salinity, with - 7 % of the added 2 2 6 Ra lost f r o m the 0.24 ppt solution. Losses d e c r e a s e d w i t h i n c r e a s i n g sali n i t y , and w e r e fo u n d to be n e g l i g i b l e at s a l i n i t i e s g r e a t e r t h a n 2 ppt.

The a c t i v i t y of 2 2 6 Ra a d s o r b e d by the s e d i m e n t (Rased), was c a l c u l a t e d at each s a l i n i t y u s i n g the e q u a t i o n

R ä s e d = R a t o t ~ ( R ä s o l / ( 1 _ f a d ) ) ( 1 )

w h e r e R a t o t is the 2 2 6 Ra t r a c e r a c t i v i t y ad d e d to the vial, R a s o i is the m e a s u r e d 2 2 6 Ra a c t i v i t y in solution, and f ad is t h e f r a c t i o n of d i s s o l v e d 2 2 6 Ra lost t h r o u g h s o r p t i o n by the a p p a r a t u s at t h a t salinity, d e r i v e d f r o m the p r o c e d u r a l blanks. The 2 2 6 Ra a c t i v i t y n a t u r a l l y p r e s e n t in t h e s e d i m e n t s u s p e n s i o n ( - 0 . 0 2 - 2 0 mBq) was m a n y o r d e r s of m a g n i t u d e lower tha n the a c t i v i t y of the a d d e d t r a c e r (-30 Bq), and w a s ignored.

B e c a u s e of the p o s s i b l e s h i e l d i n g of the vial wa l l s by the s u s p e n d e d s ediment, the v a l u e of f ad d e r i v e d f r o m the p r o c e d u r a l b l a n k s m a y have b e e n o v e r - e s t i m a t e d , p a r t i c u l a r l y for the 1000 mg/L e x p e r i m e n t s . C o n s e q u e n t l y , at low s a l i n i t i e s s e d i m e n t s h i e l d i n g m a y have r e s u l t e d in an o v e r - e s t i m a t e of the values of R a s e d , a n d hence K d . H o w e v e r t h e s e errors will not e x c e e d 0 . 5 % for R a sed, and 7 % f o r K d .

T h o r i u m

T h e 2 2 8 Th t r a c e r used in the a d s o r p t i o n e x p e r i m e n t s was o b t a i n e d fro m an 2 3 2 U solution, and was is o l a t e d us i n g an anion e x c h a n g e column ( Martin a n d H a n c o c k 1992). The p r e p a r a t i o n of t h e 2 2 8 Th s t o c k s o l ution a n d s e d i m e n t s u s p e n s i o n s w e r e identical to the 2 2 6 Ra a d s o r p t i o n e x p e r i m e n t s d e s c r i b e d above, except that e x p e r i m e n t s w e r e c o n d u c t e d at o n l y one s e d i m e n t s u s p e n s i o n c o n c e n t r a t i o n (10 mg/L). A p p r o x i m a t e l y 80 mBq of 2 2 8 Th t r a c e r w a s a d d e d to e a c h vial. The 2 2 8 Th f i l t r a t e c o n c e n t r a t i o n s w e r e a n a l y s e d u s i n g r a d i o c h e m i c a l and a l p h a - p a r t i c l e s p e c t r o s c o p y t e c h n i q u e s d e s c r i b e d in

Chapter 4. Because of the time-consuming nature of these analyses, only single experiments were performed at each s a lin it y .

Adsorption losses of 228Th onto the walls of the apparatus, as determined **■

by the procedural blanks, were much greater than 226Ra losses, and varied from 46% of the f i l t r a t e a c t iv it y at 0.2 ppt s a lin it y , to 54% at 28 ppt s a lin it y . The 228Th a c t iv ity adsorbed by the sediment was calculated in the same way as 226Ra (equation (1) above).

6 .2 .4 Desorption experiments

For both the kin e tic and s a lin it y experiments, the wet sediment (~10 g dry wt.) was weighed into a polypropylene b o ttle , and -800 g of a pre-mixed solution of riv e r water and seawater of the appropriate s a lin ity was added. Where diffe re n t S/L ratios were used, the weights of solution and sediment were varied accordingly. The b o ttle containing the sediment suspension was placed in an ultra-sonic bath fo r 3 min, shaken vigorously fo r 30 min by mechanical shaker, and then attached to a revolving wheel for the duration of the experiment. After a suitable period of time had elapsed, the sediment suspension was centrifuged, and the supernatant f ilt e r e d through a 0.45 fim membrane f i l t e r . The f i l t r a t e was weighed, and analysed fo r radium and thorium isotopes as described in Chapter 3.

The kinetics of radium and thorium desorption was studied by varying the equilibration time of freshwater sediments from 0.5 h to ~70 d. The s a lin it y of the solution used was 17.8 ppt (50% seawater) which is close to the s a lin ity of the NaCl solution used by Dickson (1985) in his kin etic experiments.

The s a lin ity experiments involved the e q uilib ratio n of riv e r bed sediments at d iffe re n t s a lin itie s , ranging from 1.8 ppt (5% seawater) up to 35.6 ppt (100% seawater). Preliminary experiments indicated that radium desorption reached a maximum in less than 4 h of e q u ilib ra tio n , and remained stable fo r at least 2-3 d. An eq uilib ratio n time of 20 h was subsequently selected for a ll s a lin ity experiments.

Both the kinetic and salinity experiments were run at high S/L ratios (12.5 g/L) so that the quantity of radium desorbed was sufficient to obtain a measurement of radium desorption with an acceptable level of uncertainty. This S/L ratio is the same as that used by Elsinger and Moore (1984) in

•v

radium desorption experiments on Pee Dee River sediment, allowing a direct comparison of the results of this work with those of sediments from another river.

The effect of S/L ratio on radium desorption was studied over a sediment suspension concentration range of 0.25-50 g/L, representing a 200 fold variation in sediment concentration.

6.3 Adso r p t i on Experiments - Results and Discussion