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Ni(II) Sorption on the Fracture Filling Mineral Chlorite
KTH, School of Chemical Science and Engineering (CHE), Chemistry.
KTH, School of Chemical Science and Engineering (CHE), Chemistry.
2009 (English)In: SCIENTIFIC BASIS FOR NUCLEAR WASTE MANAGEMENT XXXII, 2009, Vol. 1124, 531-536 p.Conference paper, Published paper (Refereed)
Abstract [en]

Abundance of the mineral chlorite as fracture filling material in granitic bedrock motivates sorption studies for quantification of the retention of radionuclides on this mineral. The activation product Ni-63 is an important component in spent nuclear fuel, accounting for a large contribution to the high activity level, and further motivates sorption studies of Ni on chlorite. Earlier sorption studies have been performed on larger mineral pieces; however, it is questionable if these data are representative for fracture filling material. Chlorite from a borehole core from Oskarshamn, Sweden, from the depth of 944 m has been characterized and the chemical composition determined prior to the experiments. The thickness of the fracture filling mineral is at maximum a couple of millimeters; therefore, careful removal of the thin chlorite layer from the core was performed with a carbide tool. The fraction was sieved to a size distribution of 63-118 mu m and ultrasonically washed. The major oxides were found to be 34.4 % SiO2, 21.6 % MgO, 15.4 % Al2O3 and 12.9 % Fe2O3 determined by Scanning Electron Microscopy-Energy Dispersive Spectroscopy (SEM-EDS) analysis. Sorption of Ni(II) to the sample was performed in 0.1 M NaClO4 with a batch technique inside a glove-box with Ar-atmosphere, using Ni-63 as a tracer with the Ni carrier concentration of 10(-6) and 10(-8) M. The percent sorption in these experiments was 57% at pH 5.5, 85 % at pH 6.5 and 92 % at pH 8.3, with the last value corresponding to maximum sorption No significant differences between the experiments performed in the two concentrations of 10(-6) and 10(-8) M were found. The degree of sorption on this fracture filling material is of the same magnitude as earlier studies of Ni sorption on chlorite provided from larger mineral pieces, using the same size fraction of the material. Sorption of Ni is, as expected, strongly pH dependent and Ni-63 escaping a breached canister will, reaching chlorite within the surrounding bedrock, be retarded due to strong sorption.

Place, publisher, year, edition, pages
2009. Vol. 1124, 531-536 p.
Series
Materials Research Society Symposium Proceedings, ISSN 0272-9172
Keyword [en]
dissolution rates; ph; 25-degrees-c
National Category
Chemical Sciences
Identifiers
URN: urn:nbn:se:kth:diva-24163ISI: 000270898900070Scopus ID: 2-s2.0-70449382071OAI: oai:DiVA.org:kth-24163DiVA: diva2:344402
Conference
32nd Symposium on Scientific Basis for Nuclear Waste Management held at the 2008 MRS Fall Meeting Boston, MA, DEC 01-05, 2008
Note
QC 20100819Available from: 2010-08-19 Created: 2010-08-19 Last updated: 2010-08-19Bibliographically approved
In thesis
1. Chlorite: Geochemical properties, Dissolution kinetcis and Ni(II) sorption
Open this publication in new window or tab >>Chlorite: Geochemical properties, Dissolution kinetcis and Ni(II) sorption
2009 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In Sweden, among other countries, a deep multi-barrier geological repository, KBS-3, is planned for the burial of nuclear waste. One of the barriers is identified as the grantic bedrock itself and in this environment chlorite is present at surfaces in fracture zones.

This thesis is focused on characterisation of chlorite samples and studies of their dissolution and sorption behaviour, in order to verify chlorites capacity to retard possible radionuclide migration in the case of leaking canisters.

Chlorite dissolution of has been studied in the pH interval 2-12, and as expected the dissolution is highest at acidic pH and at most alkaline pH, whereas dissolution is lowest at near neutral pH values. Chemical and physical properties of chlorites clearly influence the dissolution rates, and at steady-state dissolution rates in the interval 10-12 ‑ 10-13 mol g-1 s-1 was observed.

Sorption studies were performed since Ni(II) is one of the important activation products in spent nuclear fuel and sorption data on minerals like chlorite are lacking. Ni(II) sorption onto chlorite was studied using batch technique as a function of; pH, concentration of Ni(II), ionic strength and solid concentrations. As expected, the sorption of Ni(II) onto chlorite was pH dependent, but not ionic strength dependent, with a sorption maximum at pH ~ 8, and with a Kd of ~ 103 cm3/g. This confirms that the Ni(II) sorption onto chlorite is primarily acting through surface complexation. The acid-base properties were determined by titrations and described by a non-electrostatical surface complexation model in FITEQL. Further, the sorption results were fit with a 2-pK NEM model and three surface complexes, Chl_OHNi2+, Chl_OHNi(OH)+ and Chl_OHNi(OH)2, gave the best fit using FITEQL.

Abstract [sv]

Sverige är ett av de länder som planerar ett geologiskt slutförvar kallad KBS-3, bestående av ett antal barriärer, för placering utav det använda kärnbränslet. En av dessa barriärer är identifierad som själva berggrunden där det tilltänkta förvaret kommer att byggas och i denna miljö förekommer klorit på granitytor i sprickzoner.

Denna doktorsavhandling karakteriserar kloriter och studerar deras upplösnings- och sorptionsbeetende, för att kunna bestämma huruvida kloriter är utav betydelse som naturlig barriär för eventuell radionuklidtransport från det använda kärnbränslet.

Upplösning av klorit har undersökts i pH intervallet 2-12 och graden av upplösningen är som förväntat högst vid sura respektive mest basiska pH och lägst där pH är neutralt. Denna studie bekräftar att den kemiska sammansättning och de fysikaliska egenskaper hos kloriterna påverkar upplösningshastigheterna och vid steady‑state har upplösningshastighet bestämts till 10-12 ‑ 10-13 mol g-1 s-1.

Sorptionsstudier genomfördes då Ni(II) är en viktig aktiveringsprodukt och data rörande Ni(II) sorption till klorit saknas. Ni(II) sorption till klorit har studerats i; varierande pH, olika initiala Ni(II) koncentrationen, olika jonstyrka och olika fastfas förhållanden där individuella satser i serie har nyttjats. Som förväntat är sorptionen av Ni(II) till klorit pH beroende men inte jonstyrkeberoende och ett sorpions maximum observerades vid pH ~ 8, med ett Kd‑värde på ~ 103 cm3/g. Från detta dras slutsatsen att sorptionen av Ni(II) till klorit sker mestadels genom ytkomplexering. Syra-bas egenskaperna hos kloriterna bestämdes genom titreringar och bekrevs med en icke‑elektrostatisk modell i FITEQL. Vidare har passning av sorptionsresultaten utförts med en 2-pK NEM-modell och tre ytkomplex, Chl_OHNi2+, Chl_OHNi(OH)+ och Chl_OHNi(OH)2, vilket gav den bästa passningen av data med FITEQL.

Place, publisher, year, edition, pages
Stockholm: KTH, 2009. xii, 81 p.
Series
Trita-CHE-Report, ISSN 1654-1081 ; 2009:9
Keyword
chlorite, dissolution kinetics, Ni(II) sorption, surface complexation
National Category
Other Basic Medicine
Identifiers
urn:nbn:se:kth:diva-10139 (URN)978-91-7415-247-0 (ISBN)
Public defence
2009-04-24, D2, Lindstedsvägen 5, entreplan, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
QC 20100819Available from: 2009-05-05 Created: 2009-03-25 Last updated: 2010-08-19Bibliographically approved

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