How does acid treatment to remove carbonates affect the isotopic and elemental composition of soils and sediments?

Milena Fernandes; Evelyn Krull

doi:10.1071/EN07070

RESEARCH ARTICLE

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How does acid treatment to remove carbonates affect the isotopic and elemental composition of soils and sediments?

Milena Fernandes ^A ^C and Evelyn Krull ^B

+ Author Affiliations

- Author Affiliations

^A South Australian Research and Development Institute, Aquatic Sciences Centre, PO Box 120, Henley Beach, SA 5022, Australia.

^B CSIRO Land and Water, PMB 2, Glen Osmond, SA 5064, Australia.

^C Corresponding author. Email: fernandes.milena@saugov.sa.gov.au

Environmental Chemistry 5(1) 33-39 https://doi.org/10.1071/EN07070
Submitted: 27 September 2007 Accepted: 9 January 2008 Published: 22 February 2008

Environmental context. The ability to accurately determine the elemental and isotopic composition of soils and sediments has important implications to our quantitative understanding of global biogeochemical cycles. However, the analysis of organic carbon in solid matrices is a time-consuming task that requires the selective removal of carbonates, a treatment that has the potential to significantly alter the composition of the original sample. In the present work, we compare three of the most common acid treatments used for carbonate removal, and critically evaluate their effect on the content and isotopic signature of organic carbon and nitrogen in both soils and sediments.

Abstract. In the present work, we compared the efficacy of three acid treatments in removing carbonates from soils and sediments for elemental and isotopic analysis. The methods tested were (1) refluxing with H₂SO₃; (2) in situ treatment with H₂SO₃ in silver capsules; and (3) treatment with HCl followed by rinsing with water. Refluxing with H₂SO₃ led to substantial losses of organic carbon and nitrogen, but comparatively small nitrogen isotopic shifts. The in situ treatment was inadequate for carbonate-rich samples (contents ≥30%) as a consequence of the formation of a mineral precipitate. Treatment with HCl led to substantial losses of nitrogen from carbonate-rich samples, and deviations in nitrogen isotopic signatures (δ¹⁵N) of up to 3.7‰. δ¹⁵N values showed no significant difference between acid-treated and untreated samples or between treatments, although variability was high and influenced by sample composition. Carbonate-poor samples showed no statistical difference in δ¹³C values between treatments, whereas carbonate-rich samples tended to be more ¹³C-depleted when treated with HCl, potentially suggesting the preferential preservation of ¹³C-depleted compounds (e.g. lipids or lignin).

Additional keywords: hydrochloric acid, nitrogen, organic carbon, stable isotopes, sulfurous acid.

Acknowledgements

We wish to thank Sonja Venema and Genevieve Mount (SARDI Aquatic Sciences), Janine McGowan and Athina Massis (CSIRO Land and Water) for sample preparation and analyses, and Stuart McClure (CSIRO Land and Water) for carbon and nitrogen IRMS analyses and advice.

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How does acid treatment to remove carbonates affect the isotopic and elemental composition of soils and sediments?

Acknowledgements

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