Trace elemental distribution in soil and compost-derived humic acid molecular fractions and colloidal organic matter in municipal wastewater by flow field-flow fractionation-inductively coupled plasma mass spectrometry (flow FFF-ICP-MS)

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Abstract
Humic acids (HA) are heterogeneous, dark-colored, organic macromolecules found in soil and aquatic systems that play a crucial role in complexation, transportation and cycling of trace elements in ecosystems. Flow field-flow fractionation-inductively coupled plasma mass spectrometry (flow FFF-ICP-MS) is applied to study trace metals complexed to soil-, peat- and compost-derived HA and colloidal organic matter (COM) in municipal wastewater. Monomodal elemental fractograms with different polydispersities demonstrate that Cu, Fe, Mn, Pb and Zn are bound to a broad range of HA molecular size fractions. Molecular weights (Mp) and diffusion coefficients (Dp) at peak maximum for HA of various origins were determined by flow FFF. The average Mp of nine replicate measurements of investigated HA ranged from 3950 to 3790 Da while their diffusion coefficients varied from 1.72 × 10-6 cm2 s−1 for peat-derived HA to 1.66 × 10-6 cm2 s−1 for compost-derived HA, indicating very small variations among the HA under investigation. Elemental fractograms were also obtained for aqueous COM in municipal wastewater after on-line preconcentration. Fractograms clearly showed the gradual reduction of COM content for primary through secondary treatment. Elemental COM fractograms also demonstrated that the bulk of Al, Cu, Fe, Pb, Mn and Zn was complexed to organic macromolecules while a fraction of Al, Zn, Mn and halogens (Cl, Br and I) was present as dissolved ionic species. Copper, Fe and Zn signals decreased during wastewater treatment. This investigation demonstrates precipitation and subsequent removal of metal ions that are associated with COM as sludge.

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