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Dissolution of β-MnOOH particles by ligands: pyrophosphate, ethylenediaminetetraacetate, and citrate

Klewicki, J. K. and Morgan, J. J. (1999) Dissolution of β-MnOOH particles by ligands: pyrophosphate, ethylenediaminetetraacetate, and citrate. Geochimica et Cosmochimica Acta, 63 (19-20). pp. 3017-3024. ISSN 0016-7037. doi:10.1016/s0016-7037(99)00229-x.

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Experiments describing the extent of dissolution of β-MnOOH(s) suspensions vs. time using measurements of both total dissolved Mn and Mn(III) complex species show that the ligands pyrophosphate (P₂O₇⁴⁻), ethyelenediaminetetraacetate (EDTA⁴⁻), and citrate (CIT³⁻), each in large excess with respect to solids concentration and estimated surface sites, promote dissolution by different mechanisms and at different rates. Pyrophosphate dissolution of β-MnOOH yields predominately the Mn(III)P₂O₇(aq) complex. EDTA⁴⁻ causes reductive dissolution, with no Mn(III) complexes of the ligand observed in solution. Citrate causes ligand-promoted dissolution in the initial stages of reaction, after which solution phase reactions bring about reduction of Mn(III)CIT(aq) to Mn(II) solution species. Rates of dissolution at circum-neutral pH show somewhat higher rates at lower pH. Initial rates of ligand-promoted dissolution, R₀, by pyrophosphate were in the range ∼ 8 × 10⁻⁸ to 6 × 10⁻⁷ mol g⁻¹ s⁻¹ between pH 8 and 6.5. For reductive dissolution by EDTA, initial rates, R0, ranged from ∼ 8 × 10⁻⁷ to 2 × 10⁻⁶ mol g⁻¹ s⁻¹ between pH 8 and 7. At pH 6.3, initial dissolution rate with citrate was ∼ 6 × 10⁻⁶ mol g⁻¹ s⁻¹. Results of this exploratory study of MnOOH dissolution by ligands at circum-neutral pH suggest that manganese cycling in natural waters may involve loss of MnOOH particles promoted by naturally-occurring or pollutant ligands with formation of Mn(III) aqueous complexes.

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Additional Information:© 1999 Elsevier. Received 15 October 1998, Revised 29 April 1999, Accepted 29 April 1999, Available online 22 November 1999. We thank Alan T. Stone for valuable discussions concerning the experiments with EDTA. Janet G. Hering offered helpful advice about appropriate kinetic expressions. Critical, constructive questions and suggestions by the anonymous reviewers proved very beneficial. Financial support from the Mellon Foundation and a General Motors Research grant is gratefully acknowledged.
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Andrew W. Mellon FoundationUNSPECIFIED
Issue or Number:19-20
Record Number:CaltechAUTHORS:20201209-122140846
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Official Citation:J.K. Klewicki, J.J. Morgan, Dissolution of β-MnOOH particles by ligands: pyrophosphate, ethylenediaminetetraacetate, and citrate, Geochimica et Cosmochimica Acta, Volume 63, Issues 19–20, 1999, Pages 3017-3024, ISSN 0016-7037,
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:106984
Deposited By: George Porter
Deposited On:09 Dec 2020 22:46
Last Modified:16 Nov 2021 18:58

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