Spectroscopic and microscopic characteristics of natural aquatic nanoscale particles from riverine waters

• UF coupled with EEM-PARAFAC is used for characterizing fluorescent property of COM.
• Fluorescent components exist in colloidal form with heterogeneous distribution.
• Visual property of nanoparticle measured by AFM is governed by diverse waterbody.
• Colloidal Fe derived from UF is predominant across the rivers.
• Majority of total dissolved Mn is ultrafilterable in riverine water.

The physicochemical characterization of colloids in waters is vital for the understanding of their crucial role in transportation of pollutants and biogeochemical processes of trace elements in aquatic systems. However, there are still lack of knowledge about quantitative properties of natural aquatic nanoscale particles owing to their small size and matrix complexity. In the present work, the complementary spectroscopic and microscopic methods were applied to characterize the fluorescent properties of organic colloids, morphology of complex nanoparticles and colloidal speciation of Fe and Mn in rivers, namely Pu River (PR), Xi River (XR) and Haicheng River (HCR), located in northeast of China. Parallel factor analysis (PARAFAC) of excitation-emission matrix (EEM) spectra reveals ten fluorescent components occurring in colloidal organic matter fractionated by ultrafiltration process with cut off 1 kDa or 100 kDa membrane: five humic-like fluorophores (C1, C2, C3, C6 and C8) and five protein-like components (C4, C5, C7, C9 and C10). The majority of humic-like substances is present in small size colloids (1–100 kDa) in these waters and the protein-like materials are, not truly dissolved or “free”, but associated with colloidal materials. Atomic force microscopy (AFM) measurement indicates that small near-spherical particles with height dimensions up to 6 nm are main form in the three rivers and occasionally fibrillary material. The significant shift in the size distribution of natural colloids occurs across three rivers with average particle size of 1.43 ± 1.01 nm in the XR, 1.53 ± 1.22 nm in the PR and 2.22 ± 1.31 nm in the HCR. Analysis by Inductively Coupled Plasma-Optical Emission Spectrometer (ICP-OES) combined with ultrafiltration shows that over 73% of total dissolved Fe and < 45% of total dissolved Mn are present in the colloidal phase despite diverse distribution of colloidal Fe and Mn in small size (1–100 kDa) or large size (> 100 kDa) fractions across selected rivers.