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

Abstract

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.