Ilija Plecas

The batch study of Sr2 + sorption by bone char

Considering the excellent sorption properties of synthetic calcium hydroxyapatite (HAP) towards many divalent cations, the potential application of bone char, the natural source of HAP, for sequestering Sr2 +ions from aqueous solutions has been studied in batch conditions. Contact time, initial solution pH and initial Sr2 + concentrations were varied to examine the effect of these process parameters on the amount of Sr2 + sorbed. The kinetics of Sr2+ sorption was found to be a 2-step process, with contact time of 24 h required for attaining equilibrium. The sorption isotherm was well fitted with Langmuir and DKR theoretical models. Sorption of Sr2 + on bone char was found to be a favorable, thermodynamically feasible and spontaneous process, with the maximum sorption capacity of 0.271 mmol/g and sorption energy of 11.09 kJ/mol. The sorption was pH-independent in the initial pH range 4-10, as a result of excellent buffering properties of bone char (constant final pH), while for pH > 10 sorbed amounts of Sr2 + increased due to attractive electrostatic forces between negatively charged sorbent surface and positively charged metal ions. On the basis of the amount of Ca2 + released and final pH decrease in respect to the point of zero charge of bone char (pHPZC), two possible mechanisms of Sr2 + sorption were identified: ion-exchange and the formation of complex compounds with HAP and carbon active surface sites. The amounts of Sr2 + leached from bone char increased with the increase of Ca2 + content and the decrease of solution pH. In comparison with synthetic HAP, bone char represents a cost-effective alternative for Sr2 + sequestering.

Kinetic Study of Sr2+ Sorption by Bone Char

Abstract The effect of particle size, bone char mass, initial pH, and metal concentration on the kinetics of Sr2+ sorption by bone char was studied and discussed. Considering the sorbed amounts of Sr2+, solution pH changes, changes of Ca2+ concentrations and Ca/Sr molar ratios, with time, it was concluded that surface complexation reactions are dominant in the first, more rapid stage of the sorption process, while the contribution of the ion-exchange mechanism increases with time and becomes more significant in the second, slower phase. Under all investigated experimental conditions, the pseudo-second-order model was found to provide high correlation coefficients and the equilibrium amounts of Sr2+ sorbed comparable to the values obtained experimentally.

The role of external and internal mass transfer in the process of Cu2+ removal by natural mineral sorbents.

The kinetics of Cu2+ sorption on to zeolite, clay and diatomite was investigated as a function of initial metal concentrations. For consideration of the mass transfer phenomena, single resistance models based on both film and intraparticle diffusion were tested and compared. The obtained results suggested that the rate‐limiting step in Cu2+ sorption strongly depended on the sorbent type, as well as on initial cation concentration. The decrease in external mass transfer coefficients with the increase in initial metal concentrations was in excellent agreement with expressions based on Sherwood and Schmidt dimensionless numbers. The internal diffusivities through zeolite particles were in the range 1.0 × 10−11 to 1.0 × 10−13 m2/min, depending on the Cu2+ concentration and the applied theoretical model.

The effect of process parameters on kinetics and mechanisms of Co2+ removal by bone char

Bone char powder, composed mainly of poorly crystalline hydroxyapatite (Ca10(PO4)6(OH)2), carbon and CaCO3, has potential applicability in the removal of Co2+ ions from contaminated effluents. In the present study, the influence of process parameters: particle size, agitation speed, initial pH and initial sorbate concentration, onto kinetics and mechanism of Co2+sorption was studied and discussed. In order to describe and compare time evolution of the process under different conditions, the experimental data were analyzed using pseudo–first, pseudo–second and Vermeulens kinetic models. Generally, experimental results were best fitted with the pseudo–second-order model, which accurately predicted the equilibrium sorbed amounts. The pseudo–second-order rate constant was the most influenced by variations in initial metal concentration and pH, in the investigated ranges. The conclusions about sorption mechanism were derived based on Co2+ amounts sorbed during time, as well as considering solution pH changes, changes of Ca2+ amounts released into liquid phase and Ca2+/Co2+ molar ratios. It was concluded that rapid sorption stage was governed by surface complexation reactions, whereas the contribution of the ion-exchange mechanism increased with time and became more significant in the second, slower phase. Experimentally determined maximum sorption capacity towards Co2+, under optimal conditions, was found to be 0.38 mmol/g. The results show that bone char represents cost-effective alternative to synthetic hydroxyapatite sorbent.

Leaching behavior of 137Cs in concrete-based nuclear waste forms

Abstract Determination of retardation factors and coefficients of distribution using a simplified mathematical model for analyzing the migration of leachate and radioactive material contained in radioactive waste burial concrete trench systems has been developed. Results show that concrete for engineered trench systems secures radionuclide preservation in solidified medium for longer than 300 years. These results will be used for constructing future radioactive waste storing centers.

Development of solidification techniques for radioactive sludge produced by a research reactor

Abstract In the last forty years in the “Vinca” Institute, as a result of the operation of two research reactors, named RA and RB, and as a result of the application of radionuclides in medicine, industry and agriculture, radioactive waste materials of different levels of specific activity were generated. As a temporary solution, it is proposed that radioactive waste materials be stored in two interim storages. Radwaste materials that were immobilized in the inactive matrices are to be placed into concrete containers, for further manipulation and disposal. The present paper reports the results on the preliminary removal of sludge from the bottom of the spent fuel storage pool in the RA reactor, mechanical filtration of the pool water and sludge conditioning and storage.

Modeling of physico–chemical characteristics of concrete for filling trenches in radioactive waste management

Abstract An optimization of concrete used for immobilization of radionuclides 60Co and 137Cs, is presented. A relatively simple mathematical model is given, which permits minimization of leach rate and permeability and maximization of compressive strength. An optimal solution, based on experimental data, is given. These results will be used for a future Serbian radioactive waste disposal center.

Curing Time Effect on the Fraction of 137Cs From Immobilized Radioactive Evaporator Sludge by Portland Cement

Available in abstract form only. Full text of publication follows: Traditional methods of processing evaporator concentrates from NPP are evaporation and cementation. These methods allow to transform a liquid radioactive waste into the rather inert form, suitable for a final disposal. To assess the safety for disposal of radioactive mortar-waste composition, the leaching of {sup 137}Cs from immobilized radioactive evaporator concentrate into a surrounding fluid has been studied. Leaching tests were carried out in accordance with a method recommended by IAEA. Curing conditions and curing time prior to commencing the leaching test are critically important in leach studies since the extent of hydration of the cement materials determines how much hydration product develops and whether it is available to block the pore network, thereby reducing leaching. Incremental leaching rates Rn(cm/d) of {sup 137}Cs from evaporator concentrates after 180 days were measured. The results presented in this paper are examples of results obtained in a 20-year concrete testing project which will influence the design of the engineer trenches system for future central Serbian radioactive waste storing center. (authors)

Mathematical Modelling of Transport Phenomena in Concrete Porous Media

Two fundamental concerns must be addressed when attempting to isolate low-level waste in a disposal facility on land. The first concern is isolating the waste from water, or hydrologic isolation. The second is preventing movement of the radionuclides out of the disposal facility, or radionuclide migration. Particularly, we have investigated here the latter modified scenario.To assess the safety for disposal of radioactive waste-concrete composition, the leakage of 137 Cs from a waste composite into a surrounding fluid has been studied. Leakage tests were carried out by original method, developed in Vinca Institute [1,2,3,4,5]. Transport phenomena involved in the leaching of a radioactive material from a cement composite matrix are investigated using three methods based on theoretical equations [6,10]. These are: the diffusion equation for a plane source an equation for diffusion coupled to a first-order equation, and an empirical method employing a polynomial equation‥ The results presented in this paper are from a 25-year mortar and concrete testing project that will influence the design choises for radioactive waste packaging for a future Serbian radioactive waste disposal center.

Curing Time Effect on the Fraction of 137Cs From Cement-Ion Exchange Resins-Bentonite Clay Composition

To assess the safety of disposal of radioactive waste material in cement, curing conditions and time of leaching radionuclides 137 Cs have been studied. Leaching tests in cement-ion exchange resins-bentonite matrix, were carried out in accordance with a method recommended by IAEA. Curing conditions and curing time prior to commencing the leaching test are critically important in leach studies since the extent of hydration of the cement materials determines how much hydration product develops and whether it is available to block the pore network, thereby reducing leaching. Incremental leaching rates Rn (cm/d) of 137 Cs from cement-ion exchange resins-bentonite matrix after 180 days were measured. The results presented in this paper are examples of results obtained in a 20-year concrete testing project which will influence the design of the engineer trenches system for future central Serbian radioactive waste storing center.Copyright