Кинетика сорбции цезия-137 и стронция-90 глинами

Abstract

Проведены экспериментальные модельные исследования обменного и необменного поглощения ¹³⁷Cs и ⁹⁰Sr природными глинами (бентонитом, палыгорскитовой глиной, каолином и глиной смешанного состава). Выявлен эффект ремобилизации радионуклидов, находившихся в фиксированной форме. Определены величины констант скорости процессов ионообменной сорбции и десорбции ¹³⁷Cs и ⁹⁰Sr, а также констант скорости необменной сорбции (фиксации) и ремобилизации этих радионуклидов на реакционных центрах разных типов для глин различного минерального состава.

Проведено експериментальні модельні дослідження обмінного та необмінного поглинання ¹³⁷Cs і ⁹⁰Sr природними глинами (бентонітом, палигорськітовою глиною, каоліном та глиною змішаного складу). Виявлено ефект ремобілізації радіонуклідів, що перебували у фіксованій формі. Визначено величини констант швидкості іонообмінної сорбції та десорбції ¹³⁷Cs і ⁹⁰Sr, а також константи швидкості необмінної сорбції (фіксації) та ремобілізації ¹³⁷Cs і ⁹⁰Sr на реакційних центрах різних типів для глин різного мінерального складу.

Experimental model studies of ion-exchange and unexchange sorption of ¹³⁷Cs and ⁹⁰Sr by natural clays of different mineral composition were carried out. Study of dynamics of radionuclide sorption by clays showed, that 99.6—99.8 % of ¹³⁷Cs is absorbed by clays during the first day, the relative parts of ion-exchange and unexchange sorption vary depending on mineral composition of clays. In accordance with a ¹³⁷Cs unexchange sorption power the clays may be arranged in a row: multimineral clay < palygorskite < kaolin < bentonite. The effect of radionuclide remobilization was found, in other words a reverse conversion of radionuclide fixed species to ion-exchange one, potentially able to water migration. This effect is especially evident in the case of ⁹⁰Sr sorption by clays. A remobilization of the ¹³⁷Cs, absorbed by multimineral and palygorskite clays, is rather noticeable too. ¹³⁷Cs absorption by these clays is characterized by the significant contribution of ion-exchange sorption over a long time. Although bentonite has a great ¹³⁷Cs absorption power, the process of temporal remobilization takes place even in dynamics of ¹³⁷Cs sorption by that kind of clay. The data obtained from our model experiments were processed by mathematical modeling of kinetics of radionuclide ion-exchange and unexchange sorption with the use of multi-position absorption model. It is established that both ionexchange and unexchange ¹³⁷Cs sorption by clays takes place on the reaction centers of three types. The processes of two opposite directions — sorption (fixing) of ¹³⁷Cs and its desorption (remobilization) — proceed on the centers of each type. The resulting distribution of ¹³⁷Cs between fixed and ion-exchangeable species on the centers of different types is determined by competition of sorption processes. Ion-exchange sorption of ⁹⁰Sr by clays takes place on the reaction centers of four types, unexchange one — on the reactionary centers of three types. The values of rate constants of the processes of ¹³⁷Cs and ⁹⁰Sr ion-exchange sorption and desorption of ¹³⁷Cs and ⁹⁰Sr, as well as rate constants of their unexchange sorption (fixing) and remobilization on the reaction centers of different types were estimated for investigated clays. Study of radionuclide sorption power of clays in dynamics, taking into account the time factor, combined with traditional static researches, allows more reliable estimation of clay efficiency as a protective layer for radioactive waste repository.