Dissolution of beta-MnOOH particles by ligands: Pyrophosphate, ethylenediaminetetraacetate, and citrate

Experiments describing the extent of dissolution of beta-MnOOH(s) suspensio ns vs. time using measurements of both total dissolved Mn and Mn(III) compl ex species show that the ligands pyrophosphate (P2O74-), ethyelenediaminete traacetate (EDTA(4-)), and citrate (CIT3-), each in large excess with respe ct to solids concentration and estimated surface sites, promote dissolution by different mechanisms and at different rates. Pyrophosphate dissolution of beta-MnOOH yields predominately the Mn(III)HP2O7(aq) complex. EDTA4- cau ses reductive dissolution, with no Mn(III) complexes of the ligand observed in solution. Citrate causes ligand-promoted dissolution in the initial sta ges of reaction, after which solution phase reactions bring about reduction of Mn(III)CIT(aq) to Mn(II) solution species. Rates of dissolution at circ um-neutral pH show somewhat higher rates at lower pH. Initial rates of liga nd-promoted dissolution, R-0, by pyrophosphate were in the range similar to 8 x 10(-8) to 6 x 10(-7) mol g(-1) s(-1) between pH 8 and 6.5. For reducti ve dissolution by EDTA, initial rates, R-0, ranged from similar to 8 x 10(- 7) to 2 x 10(-6) mol g(-1) s(-1) between pH 8 and 7. At pH 6.3, initial dis solution rate with citrate was similar to 6 x 10(-6) mol g(-1) s(-1). Resul ts of this exploratory study of MnOOH dissolution by ligands at circum-neut ral 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. Copyright (C) 1999 Elsevier Science Ltd.