Solubility of thorium in 0.1 M NaCl solution and in saline and fresh anoxic reference groundwater

The es­ti­ma­tions of so­lu­bi­li­ty va­lues of dif­fe­rent ra­dio­ac­ti­ve ele­ments for the Fin­nish sa­fe­ty as­sess­ment of spent fuel dis­po­sal in­dica­ted a need to de­ter­mi­ne the so­lu­bi­li­ty va­lues for some es­sen­tial ele­ments, e.g. tet­ra­va­lent ac­ti­ni­des. Tho­rium, which ex­hi­bits main­ly one oxi­da­tion sta­te, +IV, in na­tu­ral con­di­tions, was cho­sen as the nucli­de with which to start with.

The so­lu­bi­li­ty stu­dies were star­ted with 0.1 M NaCl so­lu­tion to test the dif­fe­rent met­hods nee­ded in de­ter­mi­ning the so­lu­bi­li­ty va­lues. The work was con­ti­nued with sa­li­ne and fresh synt­he­tic si­mu­lant groundwa­ter. Wit­hin the so­lu­bi­li­ty stu­dy, a stea­dy sta­te of so­lu­tion was ap­proac­hed from both over­sa­tu­ra­tion and un­der­sa­tu­ra­tion. All the ex­pe­ri­ments were con­duc­ted in anoxic, car­bo­na­te-free at­mosp­he­ric con­di­tions. The de­ve­lop­ment of crys­tal­li­ni­ty of the so­lu­bi­li­ty-li­mi­ting so­lid pha­se, mic­roc­rys­tal­li­ne Th oxi­de (ThO₂(act)), was sur­veyed by X-ray dif­frac­tion pat­terns. Scan­ning elect­ron mic­rosco­pe (SEM), trans­mis­sion elect­ron mic­rosco­pe (TEM) and a dy­na­mic light-scat­te­ring la­ser were used to find pos­sibly for­med tho­rium col­loids from the ult­ra­filt­ra­tion membra­nes and so­lu­tions. In­fra­red spect­rosco­py (FT-IR) was used in cha­rac­te­rizing pos­sible car­bo­na­te struc­tu­res exis­ting in a so­lid for­med in the over­sa­tu­ra- tion ex­pe­ri­ment in fresh re­fe­rence groundwa­ter.

At this sta­ge of the work, the so­lu­bi­li­ty va­lues of tho­rium are between 10^-10 and 10^-8 M. The to­tal dis­sol­ved car­bon concent­ra­tion in the so­lu­tion was 10^-6-10^-5 M. X-ray dif­fraction mea­su­rements showed the so­lids to be mic­roc­rys­tal­li­ne. Ins­pec­tion of the ult­ra­filtra­tion membra­nes with SEM, or fil­te­red so­lu­tion with TEM and dy­na­mic light-scat­tering la­ser gave no in­dica­tion of the pre­sence of tho­rium col­loids. IR spect­ro­met­ry gave an in­dica­tion of the trans­fer of car­bo­na­tes from so­lu­tion into so­lid pha­se in the over­sa­tura­tion ex­pe­ri­ment with fresh re­fe­rence groundwa­ter.

Hydro­geoc­he­mical mo­del­ling (EQ3/​6 code) was used to com­pu­te the so­lu­bi­li­ty va­lues of Th ba­sed on the so­lu­bi­li­ty of mic­roc­rys­tal­li­ne ThO₂. The com­pu­ted re­sults gave a Th concent­ra­tion of about 2·10^-9 M in the stu­died sys­tems. The pos­sible influence of CO₂ on the specia­tion and so­lu­bi­li­ty of Th was eva­lua­ted by com­pu­ting using the mea­su­red dis­sol­ved inor­ga­nic car­bon con­tent in the samples.