Sorption behaviour of microelements in aqueous solutions in the presence of humic acid

Migration of stable and radioactive forms of microelements in soil as a result of interaction of atmosphere fallouts with monazite sand is one of the factors of a local radiological situation in Sverdlovsk Region of Urals, Russia. Though no experimental data exist on the physicochemical state in soil of such water soluble products as Th(IV), U(VI), lanthanides (REE), thermodynamic analysis predicts crucial role of humic acids (HA) in the complexation of these elements in natural heterogeneous aqueous media. The object of our research consisted in the experimental evaluation of the impact of Th(IV), U(VI), REE(III) physicochemical state on their partitioning between a cation exchanger (CE) and aqueous solution of HA (800-900 mg HA/L). Determination of equilibrium abundance of the elements in the CE, insoluble fraction of HA and water allowed us to trace both general (εb) and partial (εKY2, εHA) behaviour of more than 20 major and minor elements in the system 'HA-CE-HCl-NaOH-H2O' as a function of pH. Sorption system 'CE-HCl-NaOH-H2O-microelement' was selected as a reference one with regard to the chemical state of microelements. It was shown that sorption behaviour of Th(IV), U(VI), lanthanides(III) and a number of other microelements toward CE in the reference system is fully determined by the formation of aqua-hydroxo- and mixed hydroxo-carbonate complexes of these elements. For example, in the case of U(VI), the partitioning of positively charged hydroxo complexes leads to cupola-shaped isotherm of sorption due to the formation of non-adsorbable uranium carbonate complexes at pH>6. Presence of HA in sorption system changes substantially the sorption behaviour of microelements due to the formation of strong HA-complexes at pH>4 and insoluble HA solid at pH<4. The peculiarities of sorption behaviour of Th(IV), U(VI) and lanthanides(III) in the investigated systems are discussed in detail. (author)