Aggregation and Dispersion Behaviours of Riverine Trace Metals (Fe, Al, V, Mn, Ni, and Zn) and Organic Matter in Freshwater and Estuarine Conditions: A case study in Shira and Midori Rivers, Kumamoto, Japan

The aggregation and dispersion of metals and organic matter are an important morphological alteration process for their transportation and bioavailability in coastal areas. However, variable mixing behaviours can be observed for some substances (e.g. Mn and Cu) due to the variable interaction systems in natural systems. In this study, riverine freshwater in the Shira and Midori rivers, Kumamoto, Japan, was mixed with artificial seawater to investigate the aggregation and dispersion behaviours of trace metals (i.e. Fe, Al, Mn, Zn, Cu, V, and Ni) and organic matter. In particular, their interactions were examined with differentiating the fast and slow transformations and considering the effects of suspended substances. Comparisons of sequential processes, including seawater mixing, decantation, centrifugation, and multiple filtrations, illustrated the aggregation and dispersion characteristics of the metals and organic matter as follows. A strong aggregating nature was evident for Fe in estuarine systems. In addition, the slow aggregation of Fe was accelerated by river-borne suspended substances. Small Fe (oxy)hydroxide particles were the major forms of dissolved Fe in both freshwater and estuarine systems and were partly associated with the other metals. In contrast, Zn and Ni were characterised by strong dispersion properties, although it can resorb onto suspended substances in estuarine systems. Synchronous behaviours with Zn and Ni were observed for V, Cu, and organic matter in the Midori River. The adsorption of Mn onto suspended substances was evident in the freshwater systems instead of the estuarine systems. Meanwhile, the behaviour of Mn is known to be dependent on its abundance in suspended forms, its redox state, and the influence of Fe. V and Cu, which are non-conservative in nature, were affected by other metals such as Fe, Zn, and Ni. Al, whose behaviour is largely dependent on the target estuary, which was also affected by other metals. Slow aggregation of organic matter was induced by suspended substances which were produced by fast transformation. In addition, evident interactions between suspended and dissolved substances were observed with the behaviours of Fe, Zn, Ni, and organic matter, indicating that the deposition and dispersion at the early stage of estuarine mixing can influence the subsequent slow transformation in real environments.