Določanje izotopske sestave molibdena v vodi in sedimentih kraškega jezera Brljan

Hauptman, Žan (author), Dolenec, Matej (mentor)

Večina raziskav izotopske frakcionacije Mo je bila narejena v morskih okoljih, medtem ko je malo podatkov za površinske vode, predvsem zaradi nizkih koncentracij Mo. Raziskave so pokazale pomembno izotopsko frakcionacijo Mo izotopov na meji med oksičnimi in anoksičnimi pogoji (na redoks meji). V okviru magistrske naloge je bil cilj optimizirali postopek priprave vzorcev za določevanje izotopske sestave Mo (98Mo/95Mo oz. 97Mo/95Mo) v jezerski vodi, jezerskih sedimentih in porni vodi iz Brljanskega jezera na reki Krki (Hrvaška). Za odstranitev interferenčnih elementov je bilo potrebno optimizirati kromatografski postopek, s katerim smo ločili večji del matrice in interferenčne elemente (Mn, Fe in Zr) od Mo. V ta namen se je najbolje izkazala anionsko izmenjalna smola AG1-X8. Nato je bil cilj določiti izotopsko sestavo Mo v vzorcih jezerske vode, sedimentov in porne vode z uporabo MC-ICP-MS. V sedimentu smo dodatno določili še redoks potencial (Eh), elementno (ICP-MS) in mineralno (XRD) sestavo. Ugotovili smo, da večina sedimenta v Brljanskem jezeru izvira iz dedritičnega materiala, s katerim v jezero prehaja tudi večji del Mo. V sedimentih se mešata siliciklastična in karbonatna faza. Dedritični Mo ni vezan na karbonatno fazo. Koncentracija raztopljenega Mo v vodi Brljanskega jezera so nizke (0,46 µg/L +/- 0,03 µg/L) z visoko ?97Mo (+1,52 ‰ +/- 0,100 ‰). Sedimenti iz K5 profila imajo koncentracije Mo med 1,24 mg/kg in 1,56 mg/kg ter ?97Mo od 0,22 ‰ do 0,79 ‰. V K13 imajo sedimenti Mo med 0,78 mg/kg in 1,46 mg/kg ter imajo večji razpon vrednosti ?97Mo (-0,07 ‰ in +1,03 ‰) kot pa K5. Razlika v ?97Mo med vodnim stolpcem in sedimenti odraža manjšo frakcionacijo, ki je najverjetneje posledica adsorpcije Mo na Fe-Mn okside na vzorčnem mestu K13. V K5 sedimentih na frakcionacijo najverjetneje vpliva redukcija sulfata z mikrobno aktivnostjo ter vezava izotopsko lažjega Mo na železov sulfid in njegovo nalaganje v sedimentu. Ključne besede: molibden, izotopi, frakcionacija, redoks pogoji, jezerski sedimenti, geokemija

• Language: Slovenian
• Keywords: molibden, izotopi, frakcionacija, redoks pogoji, jezerski sedimenti, geokemija
• Work type: Master's thesis/paper
• Organization: NTF - Faculty of Natural Sciences and Engineering
• Year: 2023
• PID: 20.500.12556/RUL-149769
• COBISS.SI-ID: 165811971
• Publication date in RUL: 09.09.2023

Secondary language

• Language: English
• Title: Determination of the isotopic composition of Molybdenum in waters and sediments of a karstic lake Brljan

Abstract

Most research on molybdenum (Mo) isotopic composition has been done in marine environments, while relatively few studies have been conducted in freshwater lacustrine environments due to the low concentrations of Mo. Most studies have revealed significant Mo isotope fractionation at the boundary between oxic and anoxic conditions. In this study, a sample preparation method for determining the isotopic composition of Mo (98Mo/95Mo or 97Mo/95Mo) in lake water, lake sediments, and porewater. To remove interfering elements, it was necessary to optimise a chromatographic procedure to separate a major part of the matrix and interfering elements (Mn, Fe, and Zr) from Mo. For this purpose, anion exchange resin AG1-X8 proved to be the most effective. These methods—in combination with MC-ICP-MS—were used to determine the isotopic composition of Mo in lake water, sediments, and porewater from Lake Brljan on the Krka River (Croatia). Redox potential (Eh) in sediments, and elemental (ICP-MS) and mineral (XRD) composition of the sediments were also determined. Results indicate that most of the sediment in Lake Brljan originates from detrital material, through which a significant portion of Mo is transported to the lake, unbound to the carbonate phase; siliciclastic and carbonate phases are mixed in the sediments. Mo concentrations in Lake Brljan are low (0.46 µg/L +/- 0.03 µg/L) with high concentrations of δ97Mo (+1.52 ‰ +/- 0.100 ‰). Sediments from the K5 profile displayed Mo concentrations between 1.24 mg/kg and 1.56 mg/kg, and δ97Mo from 0.22 ‰ to 0.79 ‰. K13 sediments cpntain levels of Mo between 0.78 mg/kg and 1.46 mg/kg. δ97Mo value in K13 sediments is greater than in K5, and has a larger offset (between -0.07 ‰ and +1.03 ‰). The offset between δ97Mo in the water column and sediments reflects minor fractionation, likely due to light Mo isotope adsorption onto Fe-Mn oxide particles at the K13 sampling site. We believe that fractionation in the K5 sediments can be related to microbial sulfate reduction processes in which the binding of isotopically lighter Mo onto iron sulfide likely influences fractionation and deposition in the sediment.

• Keywords: molybdenum, isotopes, fractionation, redox proxies, lake sediments, geochemistry