Slavko Dimovic

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 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.

Evaluation study of cobalt(II) and strontium(II) sorption–desorption behavior for selection of soil remediation technology

Sorption–desorption properties of cobalt(II) and strontium(II) ions were studied using a soil sample from the vicinity of the Serbian radioactive waste processing and interim storage facilities. The mobility of the cations in the soil was evaluated and compared with the intention to facilitate the selection of optimal remediation strategy in case of accidental soil contamination with radioactive cobalt-60 and strontium-90 isotopes. A systematic sorption study was performed through a series of batch experiments at different aging times, cation concentrations and pH. Kinetics experiments revealed that sorbed amounts of cobalt(II) continuously increased with contact time until quasi-equilibrium was reached, while initial fast strontium(II) sorption was followed by a desorption step. Based on the shapes of the sorption isotherms and calculated sorption parameters, it was concluded that cobalt(II) sorbed more selectively and strongly than strontium(II). Sequential extraction showed that, regardless of the initial content of contaminants in the soil and the aging time, high amounts of both cations were bonded to relatively mobile fractions: strontium(II) in the exchangeable, while cobalt(II) in the carbonate and ferromanganese oxide fraction. Strontium(II) was readily desorbed in acidic, calcium(II) and ethylenediaminetetraacetic acid-containing media, whereas complexing agents such as citric and tartaric acids at low pH were more effective reagents for cobalt(II) desorption. The results from the present study indicate that chemical extraction can be considered as remediation option for strontium(II)- and cobalt(II)-contaminated soil.

Amendment Type and Dose Effects onto Coexisting Copper, Lead, and Nickel Ions Distribution in Soil

The use of soil additives for toxic metals chemical stabilization aims to decrease in situ the pollutants’ mobility and availability. In this study, the effectiveness of rinsed red mud (RBRM) and annealed animal bones (B400) was compared in terms of Cu, Pb and Ni stabilization in two contaminated soils with contrasting properties Dystric Cambisol (CM dy) and Rendzic Leptosol (LP rz). The mobility of metals in unamended soil samples (control) and samples amended with 1% and 5% of selected additives were compared using sequential extraction protocol. The relative content of metals in readily and potentially available fractions was higher in CM dy (62% Pb, 13% Cu, and 31% Ni in exchangeable fraction) than in LP rz (< 5% of Pb, Cu, Ni in exchangeable fraction). In CM dy, both additives have caused a decrease in metal mobility with an increase of their doses. The effect of 5% sorbent addition was most pronounced related to Pb immobilization, provoking decrease of exchangeable Pb content to < 10%. Furthermore, B400 addition has redistributed investigated metals from the exchangeable to the residual phase more effectively than RBRM, and its effect on metal mobility decreased in the order Pb > Cu > Ni. Amending of LP rz soil had limited effects with no apparent decrease in exchangeable metal content. The effects of soil type variation, the type of additive and the additive dose onto metal mobility were compared according to ANOVA results. The content of readily and potentially available forms of metals was found to be (i) significantly correlated with all investigated variables for Pb, (ii) significantly correlated with soil type for Cu, and (iii) not in significant correlation with selected variables for Ni. Complex impacts of soil properties and treatment conditions on the mobility of co-contaminants emphasize the need for an individual approach to each case of contamination.

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