LEACHABILITY OF SOLIDIFIED (Ba,Ra)S04 SEDIMENTS IN
SIMULATED SETTLING PONDS
T. W. Constable, Process Development Engineer
Wastewater Technology Centre
Environment Canada
Burlington, Ontario L7R 4A6
W. J. Snodgrass, Consulting Engineer
IEC Beak Consultants, Ltd.
Mississauga, Ontario L4V 1P1
INTRODUCTION
The decanted effluent from uranium tailings basins contains large amounts of dissolved Ra-226. To
remove this Ra-226, the effluent is treated with barium chloride (BaCl2) to precipitate barium-radium
sulphate [(Ba,Ra)S04] according to a reaction of the type:1
BaCl2 + Ra2+ + SO,,2" ** 2 CI" + (Ba,Ra)S04 (1)
The sediments are stored on the bottom of the treatment pond and the treated effluent is discharged
to a local water course. However, redissolution of Ra-226 from these sediments may occur after the
tailings area has been abandoned and thus cause contamination of receiving waters.
This study was conducted to investigate the rate at which this dissolution occurred and to determine
if this rate could be decreased by solidification of the sediments. Solidification (also referred to as
chemical fixation or encapsulation) involves mixing the sediments with chemicals to produce a solid
matrix analogous to concrete. The sediments used in the study were dredged in 1976 from the bottom
of the North Nordic Lake settling pond in Elliot Lake, Ontario, which receives the effluent from the
Nordic tailings area.
EXPERIMENTAL PROCEDURE
Four rectangular steel-supported polyethylene tubs, 61 cm wide, 91 cm long and 81 cm deep, were
used as simulated settling ponds to hold two replications of two (Ba,Ra)S04 sediment compositions
(Figure 1). Each tub contained 0.07 m3 of sediments over a base of silica sand. Two of the tubs
received untreated sediments and two received sediments which had been solidified using the proprietary Chemfix® process.2 This solidification process increased the volume of the sediments by 23%.
The solidified sediments were cured for two weeks, after which all loose material on top of the
solidified sediments was removed by vacuum and all ponds were filled with distilled water. The tubs
were located inside the Wastewater Technology Centre in Burlington, Ontario, and were kept at 18 to
20°C.
A constant head of 41 cm of water was maintained in each pond during the 1616-day monitoring
period. A peristaltic pump was used to continuously add distilled water to the settling ponds at a flow
rate of approximately 1 litre/day, giving a hydraulic loading of 0.18 cm/day and a detention time of
228 days. Effluent was collected continuously from an outflow port located opposite the inflow port
at the same hydraulic head, and composited for 1, 2 or 4 months (compositing periods increased as the
study progressed). The composited effluent samples were analyzed for pH, conductivity, dissolved
and suspended solids, selected trace metal concentrations and radionuclide levels (dissolved Ra-226,
Pb-210, Th-232, Th-230, Th-228, gross alpha and gross beta, and suspended gross alpha and beta).
The acute lethality of the effluents was examined using static 96-hour bioassay tests with daphnia
(Daphnia pulex) for the initial screening tests and rainbow trout (Salmo gairdneri) for the rest of the
tests. Initial tests with daphnia and rainbow trout were conducted in undiluted effluents. Since it was
expected that high pH might be causing the toxicity observed in these initial tests, subsequent tests
with rainbow trout were conducted in undiluted effluents which were adjusted prior to testing to pH 7
using either concentrated sulphuric acid or sodium hydroxide.
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