Flocculation of trace metals from boreal acid sulphate soils over an estuarine gradient

Among the most important processes affecting the global biogeochemical cycles of carbon, nutrients and trace metals is the physicochemical transformation of terrestrial dissolved and colloidal material into suspended particles in estuaries. The gradient of increasing salinity and pH towards seawater induces the flocculation of riverine dissolved organic matter to the particulate form, providing a mechanism for the capture of dissolved metals to the newly formed particles. The role of pH is particularly relevant in the context of acid sulphate (AS) soils that can generate extremely low pH conditions and high soluble trace metal concentrations in river waters.

This study investigated organic matter flocculation, and trace metal (Al, Fe, Co, Mn and Cu) association with the newly formed organic particles, by the mixing of natural acidic river water and synthetic seawater in the laboratory. River water was collected from the Laihianjoki and Sulvanjoki rivers that are among the most AS-soil-impacted rivers in Finland and Europe. Continuous bucket experiments with an in situ LISST-100X particle size distribution analyser and YSI EXO2 multiparameter water quality sonde were run to simulate the development of suspended particle pool over the local salinity gradient from 0 to 6. Jar experiments with discrete salinity treatments were carried out to investigate metal behaviour with the collection of flocculated material on glassfibre filters. The filter samples were subjected to persulfate digestion of organic particles and metal oxyhydroxides (pH <2.3) and the digestion supernatants were analysed for metal concentrations by ICP-MS.

It was found that Al and Fe were strongly transferred to the particle pool and captured in large (>80 µm) organic flocs already in the lowest salinity range from 0 to 2, with a slightly lower transfer rate at higher salinities. The increasing concentration of small (median 11 µm) flocculi particles above pH ca. 5.5 (salinity 1 in Laihianjoki, salinity 2 in Sulvanjoki) indicate that the precipitation of Al and Fe oxyhydroxides contributed to the particle pool as a minor component. Co and Mn were weakly associated with organic complexes and relatively persistent in solution; however, the transfer of Co to the particle pool was roughly twice of that of Mn, indicating at least partial decoupling of Co from Mn oxyhydroxide precipitation. Cu to a significant proportion (<60 %) was bound to organic particles in the acidic and humic-rich rivers, and this proportion did not change significantly during mixing with seawater. The findings of this study demonstrate that salinity and pH in the estuarine mixing are important controls for the seaward transport and environmental impacts of metal loading from boreal AS-soil-impacted rivers.