Spectrophotometric determination of manganese in acidified matrices from (pore)waters and from sequential leaching of sediments.

Manganese (Mn) is a major redox reactive element in marine sediments and it plays an important role in the biogeochemical cycles of carbon, nitrogen, phosphorus, or trace metals. Mn cycle in marine sediments is characterized by an alternation of oxidation and reduction processes depending on physicochemical and biological conditions: assessing the quantification and the speciation of Mn is thus an essential issue to understand redox reaction-transport processes in sedimentary deposits. Solid Mn phases can be determined through chemical extractions techniques that permits selective leaching of operationally defined Mn fractions. Mn oxides and oxyhydroxides are extracted with an ascorbate leaching solution, while the whole Mn (oxyhydr-)oxides and Mn associated to carbonates is extracted with HCl. An existing spectrophotometric method allows to quantify Mn dissolved in (sea) water. We present here a modified version, which permits to measure Mn in acidified matrices, including ascorbate and HCl solutions. A metallic substitution occurs between a Cd-TCPP complex and Mn at pH 7.5-8.0, with imidazole as catalyst. We propose here to use a NaHCO solution to dilute the samples in order to be in the necessary pH range to perform the metal substitution. Using this method, Mn(II,III) concentrations were determined in standard solutions with a precision of 3% within a concentration range of 0.5-80 μM. The procedure was successfully applied to determine Mn in acidified pore waters and in ascorbate and HCl sequential extractions from muddy sediments of the Bay of Biscay. Spectrophotometric results agreed closely with results from atomic absorption spectrometry, validating the proposed method.