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)HP2O7(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 x 10-8 to 6 x 10-7 mol g-1 s-1 between pH 8 and 6.5. For reductive dissolution by EDTA, initial rates, R0, ranged from ~ 8 x 10-7 to 2 x 10-6 mol g-1 s-1 between pH 8 and 7. At pH 6.3, initial dissolution rate with citrate was ~ 6 x 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.