Investigating bisulfide sorption onto bentonite through laboratory batch experiments

Bentonite clay is a key part of the engineered barrier system of the deep geological repositories (DGRs), designed by many nuclear nations worldwide to safely house the used nuclear fuel. However, the copper coated used fuel containers (UFCs) may undergo corrosion due to bisulfide (HS-) transport through the bentonite towards the containers. Understanding the sorption behaviour of HS- is therefore critical in understanding the HS- transport dynamics and in assessing the long-term safety of the DGR. As such, this study investigated HS- sorption onto bentonite, through laboratory batch experiments and microscopic/spectroscopic analyses, under the influence of various experimental conditions, such as contact time (1-120 h), temperature (10-40 degrees C), liquid to solid mass ratios (L:S) (100-1000), initial HS- concentration (1-6 mg L-1). The study results indicated that HS- sorption onto bentonite occurred faster and the equilibrium sorption capacity increased (by 3%) with increasing temperature (from 10 degrees C to 40 degrees C). Several established kinetic and isotherm models were applied to the experimental data to provide insight into the key processes driving HS- sorption onto bentonite. The desorption test results indicated that HS- was irreversibly sorbed on bentonite. The surface analyses of the bentonite test samples were conducted using scanning electron microscopy along with energy dispersive spectroscopy. These results suggested that the sorption might have occurred due to chemical reactions of HS- with the iron present in bentonite and subsequent formation of iron monosulfide (FeS). The findings of this study provided critical information to better understand the underlying sorption mechanism of HS- on bentonite, which can reduce HS- transport in the DGR.