A modified constrained sediment probe [4] packed with agarose based resin anddiffusive gels were exposed to uranium spiked sediment core to obtain high resolutionprofiles of manganese, iron and uranium to show its performance
The diffusive equilibrium in thin films technique (DET) was used to obtain complementary high resolution pore water profiles of total concentrations of dissolved metals species
Sediment sample was taken at a sampling site of Krencak pond, Netin, Czech Massif,Czech Republic, GPS-coordinates: 49°24'41.87"N, 15°56'19.129"E) during spring 2011remediation period, homogenized, incubated before and after spiking of sediment coreby depleted uranium (DU) standard solution
Segmented sediment DGT and DET probes were deployed in sediment core for one week, and pH and redox potential depth profiles were also measured
Mobile metals species were taken up by the Spheron-Oxin® based resin gel:
Pseudo steady-state concentration gradient in the diffusive gel was established, andthus metal flux to the resin gel was governed by interfacial metal concentration Ca
Metals were eluted from the strips of resin and diffusive gels by 1 M nitric acid, andsubsequently determined by ICP MS (Agilent 7700 Series)
NEW MODIFICATION OF DIFFUSIVE GRADIENT IN THIN FILM TECHNIQUE (DGT) FOR DETERMINATION OF METALS IN SEDIMENTS
M. Gregušová, B. DočekalInstitute of Analytical Chemistry of the ASCR, v. v. i., Veveří 97, 602 00 Brno, Czech Republic
INTRODUCTION EXPERIMENTALThe diffusive gradient in thin film (DGT) technique hasbeen used for measurement of concentrations and fluxesof labile metal species in environmental systems, incharacterization of waters, soils and sediments. When DGTprobes are exposed to aquatic systems over a certain timeperiod, metal species diffuse through the well-defineddiffusion layer into the binding resin gel and provide in situtime averaged concentrations of mobile metal species [1].In sediments, the probes disturb local sediment/pore waterequilibrium under well-defined conditions, so that thesediment response, metal re-supply fluxes can bemonitored in situ [2-3].The scope of the work was to investigate performance of amodified DGT probe for sediment characterization.
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DGT technique is widely used to asses fluxes of metals in sediment porewaters. High resolution of a modified probe and the preconcentrationcapability of DGT technique provides direct information on remobilizationprocesses of metals in sediments. The interference free determination ofmanganese, iron and uranium by the Spheron-Oxin based resin gel can giveadditional data on uranium reactions that are driven by redox processes.Combination of DGT technique with ICP MS analysis can also potentiallyprovide information on sources of uranium in sediments and relatedenvironmental systems.
ACKNOWLEDGEMENTCONCLUSIONThis work was performed within theInstitutional research plan AVO Z40310501and the project No. P503/10/2002 of the CzechScience Foundation.
RESULTS & DISCUSSION
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The DGT technique can provide information on geochemical cycling and fluxes ofmetals. As documented in graphs, the modified segmented probe enables to obtainsediment depth profiles of metals with the resolution down to the millimeter level.The depth profile of redox potential exhibits significant changes to negative valuesclose to the water-sediment interface, and it remains fairly constant in deeper horizonsof the sediment core. This indicates that the pond sediment was anoxic underlaboratory model conditions. Manganese and iron depth profiles show changes inmetal speciation, i.e. the reductive mobilization in accordance with the redox profile ofthe model sediment. Uranium data show a good correlation between the uptake ofuranium by the DGT probe and the amount of uranium spikes in the sediment core.Transport of uranium from spiked zones into upper sediment horizons, even thoughinto overlaying water was significantly hindered by the redox conditions, whichindicates reductive immobilization of uranium in deeper horizons of sediment core.
Determination of 238U and 235U isotopes by DET and DGT technique showed that the isotopiccomposition of uranium varies within both depth profiles. Uranium DET profile reflects finalequilibrium concentration of uranium isotopes in sediment pore water at the end of one weekdeployment period. In contrary, DGT profile shows averaged record of uranium isotopes takenup over the whole deployment period. Consequently, comparison of both types of profiles cangive an information on kinetics of transport and changes in metal speciation. The sedimentsample exhibited natural uranium isotope abundance with a 238U/235U ratio of 140. The spikeswere based on a depleted uranium standard solution with the isotopic ratio of 281.Consequently, the DET measured isotope ratio decreases stepwise in the depth profile due toisotopic dilution by natural abundant uranium isotopes (sediment contained 0.49 ± 0.01 mg/kgU), i.e. upwards the profile according to spike locations from the value of 280 to a value ofapproximately 260 at the water/sediment interface. Evidently, this effect of isotopic dilution ismore pronounced in the DGT profile, in which natural isotopic ratio of 145 was found in theoverlaying aqueous phase.
[1] H.Zhang, W.Davison, Anal.Chem. 67 (1995) 3391.[2] H.Zhang, W.Davison, B. Knight, S. McGrath, Environ.Sci.Technol. 32 (1998) 704.[3] M.P.Harper, W.Davison, H.Zhang, W.Tych, Geochim.Cosmochim.Acta 62 (1998)
2757. [4] B.Dočekal, M.Gregušová, Analyst, 137 (2012), 502 – 507.
REFERENCES
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