Ivana Smičiklas

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.

Effect of experimental variables onto Co(2+) and Sr(2+) sorption behavior in red mud-water suspensions.

ABSTRACT The prospects of rinsed red mud (alumina production residue) utilization for liquid radioactive waste treatment have been investigated, with Co2+ and Sr2+ as model cations of radioactive elements. To evaluate the sorption effectiveness and corresponding binding mechanisms, the process was analyzed in batch conditions, by varying experimental conditions (pH, Co2+ and Sr2+ concentrations in single solutions and binary mixtures, contact time, and the concentration of competing cations and ligands common in liquid radioactive waste). Comparison of the Co2+ and Sr2+ sorption pH edges with the red mud isoelectric point has revealed that Co2+ removal took place at both positive and negative red mud surface, while Sr2+ sorption abruptly increased when the surface became negatively charged. The increase of initial cation content and pH resulted in increased equilibrium times and sorption capacity and decreased rate constants. From single metal solutions and various binary mixtures, Co2+ was sorbed more efficiently and selectively than Sr2+. While Sr2+ sorption was reduced by coexisting cations in the order Al3+ ≥ Ca2+ >Na+ ≥Cs+, removal of Co2+ was affected by Al3+ species and complexing agents (EDTA and citrate). Desorption of Co2+ was negligible in Ca2+ and Sr2+ containing media and in solutions with initial pH 4–7. Sr2+ desorption was generally more pronounced, especially at low pH and in the presence of Co2+. Collected macroscopic data signify that Co2+ sorption by red mud minerals occurred via strong chemical bonds, while Sr2+ was retained mainly by weaker ion-exchange or electrostatic interactions. Results indicate that the rinsed red mud represent an efficient, low-cost sorbent for Co2+ and Sr2+ immobilization.

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.

Concurrent Co2+ and Sr2+ sorption from binary mixtures using aluminum industry waste: Kinetic study

Multi-component sorption studies are essential to identify the applicability of red mud as a lowcost sorbent for the simultaneous removal of metal ions from wastewaters. Sorption kinetics of Co2+ and Sr2+ ions was investigated, at different total concentrations of mixtures and different molar ratios of two cations. Kinetics of metal sorption from binary systems was found to be well described by pseudo-second order rate model. Equilibrium sorbed amounts and equilibrium times for Co2+ sorption increased with the increase of its total concentration in the mixture, whereas pseudo-second order rate constants exhibited the opposite trend. Sr2+ sorption was strongly suppressed in the presence of Co2+ ions, and the removal efficiency decreased with increasing concentration and mole fraction of Co2+. Red mud can be used for simultaneous Co2+ and Sr2+ removal from mixtures of lower initial concentration, otherwise Co2+ sorption is dominant.

Radioactive Contamination of the Soil: Assessments of Pollutants Mobility with Implication to Remediation Strategies

Accidental releases, nuclear weapons testing, and inadequate practices of radioactive waste disposal are the principal human activities responsible for radioactive contamina‐ tion as a new and global form of soil degradation. Understanding the radionuclide distribution, mobility and bioavailability, as well as the changes caused by the variation of environmental conditions, is essential for soil rehabilitation. This chapter aims to highlight the importance of evaluating radionuclide distribution, for the selection of proper in situ or ex situ remediation strategy. Attention was focused onto remediation methods based on radioactive pollutants redistribution, for enhanced separation (chemical extraction) or containment (in situ immobilization). When the excavation and off‐site leaching treatments are uneconomic, impractical, or unnecessary, in situ stabilization by the addition of appropriate reactive materials is an alternative approach. The optimization of factors in control of chemical leaching methods, selection of cost‐ effective immobilization agents, especially among suitable wastes and by‐products, and verification of long‐term effects of remediating actions are the major challenges for future investigation in this field. Furthermore, the improvement and standardization of the methods for radionuclide speciation are necessary to enable comparison between studies and monitoring of the effects achieved by the soil treatments.

Evaluation of factors influencing Co2+ removal by calcinated bone sorbent using experimental design methodology

Experimental design methodology was applied for evaluation of factors influencing Co2+ sorption by thermally treated bovine bones. The major aim of this study was to determine factors which affect process the most, as well as their mutual interactions, in order to select conditions that provide maximum sorbent loading. Five process variables (sorbent mass, sorbate concentration, contact time, initial pH and agitation speed) were examined by full factorial design at two levels. The initial sorbate concentration and sorbent mass were found to be a principle factors influencing cation sorption. Furthermore, a considerable interaction effect between these two factors was detected. Optimal conditions for the maximum sorbent loading include the use of small sorbent doses and concentrated Co2+ solution, without any previous pH adjustment, at least if the pH of actual waste water is within tested range (3 < pH < 6). The contact time and agitation speed, which within investigated ranges had no significant effect on sorption, may be set at their minimum levels (1 h; 10 rpm) to shorten the reaction time and reduce energy consumption. The influence of process factors on other system responses (amounts of Ca2+ released from apatite phase of bones, and final pH values) were also determined and analyzed. Empirical mathematical models illustrated the dependences of responses on the process variables, whereas residual and statistical analysis confirmed model adequacy.

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.

Estimation of Cadmium uptake by tobacco plants from laboratory leaching tests

ABSTRACT The objective of the present study was to determine the impact of cadmium (Cd) concentration in the soil on its uptake by tobacco plants, and to compare the ability of diverse extraction procedures for determining Cd bioavailability and predicting soil-to-plant transfer and Cd plant concentrations. The pseudo-total digestion procedure, modified Tessier sequential extraction and six standard single-extraction tests for estimation of metal mobility and bioavailability were used for the leaching of Cd from a native soil, as well as samples artificially contaminated over a wide range of Cd concentrations. The results of various leaching tests were compared between each other, as well as with the amounts of Cd taken up by tobacco plants in pot experiments. In the native soil sample, most of the Cd was found in fractions not readily available under natural conditions, but with increasing pollution level, Cd amounts in readily available forms increased. With increasing concentrations of Cd in the soil, the quantity of pollutant taken up in tobacco also increased, while the transfer factor (TF) decreased. Linear and non-linear empirical models were developed for predicting the uptake of Cd by tobacco plants based on the results of selected leaching tests. The non-linear equations for ISO 14870 (diethylenetriaminepentaacetic acid extraction – DTPA), ISO/TS 21268-2 (CaCl2 leaching procedure), US EPA 1311 (toxicity characteristic leaching procedure – TCLP) single step extractions, and the sum of the first two fractions of the sequential extraction, exhibited the best correlation with the experimentally determined concentrations of Cd in plants over the entire range of pollutant concentrations. This approach can improve and facilitate the assessment of human exposure to Cd by tobacco smoking, but may also have wider applicability in predicting soil-to-plant transfer.