Characterization of Iron and Organic Carbon Colloids in Boreal Rivers and Their Fate at High Salinity

Abstract

Riverine colloids are important carriers of macronutrients, trace metals, and pollutants into marine waters. The aim of the current study was to extend the understanding of iron (Fe) and organic carbon (OC) colloids in boreal rivers and their fate at higher salinities. X-ray absorbance spectroscopy (XAS) and dynamic light scattering (DLS) were combined to explore Fe speciation and colloidal characteristics such as size and surface charge and how these are affected at increasing salinity. XAS confirmed the presence of two Fe phases in the river waters-Fe-organic matter (OM) complexes and Fe(oxy)hydroxides. From DLS measurements on filtered and unfiltered samples, three particle size distributions were identified. The smallest particles (10-40 nm) were positively charged and suggested to consist of essentially bare Fe(oxy)hydroxide nanoparticles. The largest particles (300-900 nm) were dominated by Fe(oxy)hydroxides associated with chromophoric molecular matter. An intermediate size distribution (100-200 nm) with a negative surface charge was presumably dominated by OM and containing Fe-OM complexes. Increasing the salinity resulted in a removal of the smallest distribution. Unexpectedly, both the intermediate and largest size distributions were still detected at high salinity. The collective results suggest that Fe(oxy) hydroxides and Fe-OM complexes are both found across the wide size range studied and that colloidal size does not necessarily reflect either Fe speciation or stability toward salinity-induced aggregation. The findings further demonstrate that also particles beyond the typically studied <0.45-μm size range should be considered to fully understand the riverine transport and fate of macronutrients, trace metals, and pollutants.When rivers discharge into estuaries, Fe displays a distinctly nonconservative behavior, with a major loss of Fe from the surface water following salinity-induced aggregation and sedimentation (Boyle et al., 1977;Kritzberg et al., 2014;Pokrovsky et al., 2014;Sholkovitz et al., 1978). Assessment of Fe speciation by X-ray absorption spectroscopy (XAS) has shown that Fe(oxy)hydroxides are highly susceptible to salinity-induced aggregation and are selectively lost across salinity gradients (Herzog et al., 2017). The Fe

How does calcium drive the structural organization of iron–organic matter aggregates? A multiscale investigation

“…The size of Fe-OM aggregates increases with the increasing Fe/OC ratio. These species were also observed in organic soil as Fe monomers and Fe oxyhydroxides bound to OM 22 and in boreal rivers as nano-Fh and Fe-OM complexes 23 . ThomasArrigo et al 6 reported the presence of lepidocrocite (Lp) and Fh in a peatland soil.…”

Northern High-Latitude Organic Soils As a Vital Source of River-Borne Dissolved Iron to the Ocean

“…For example, recent results have revealed that the Arctic Great Rivers act as significant sources of organically bound dissolved iron to the Arctic Ocean . Moreover, the dissolved iron input by Arctic–boreal rivers to coastal waters shows an increasing tendency due to profound changes in northern high-latitude catchment areas driven by permafrost thaw and the impact of warming on soil microbial communities . We present the current evidence for a significant contribution of humic-rich rivers, most notably Arctic–boreal rivers, to the ocean’s inventory of both, iron and iron-carrying SAHLs, with potentially wide-ranging effects on marine ecosystems and the oceanic sink for anthropogenic CO 2 .…”

Mercury Export From Freshwater to Estuary: Carbocentric Science Elucidates the Fate of a Toxic Compound in Aquatic Boreal Environments

“…Complexation of trace elements by OM in colloidal form would decrease their lability (Aiken et al, 2011;Stockdale et al, 2014;Cuss et al, 2020). Recently, technical advances have allowed the two iron-colloid phases to be distinguished (Herzog et al, 2020): Fe-OM complexes and Fe(oxy)hydroxides (as nanoparticles or associated with chromophoric molecular matter) and especially ferrihydrite, which is a common weathering product of Febearing minerals in boreal regions and an efficient trace metal sorbent in the aquatic environment (Wetzel, 2001;Jokinen et al, 2020). Despite our improved knowledge of the iron and carbon colloidal carrier phases, their specific association with mercury along the freshwater continuum is still unknown.…”

“…Subsequently, the conversion of dissolved OM into terrestrialorigin particulate organic matter (POM) through colloid-phase flocculation was demonstrated in subtropical (Benner and Opsahl, 2001), temperate (St. Pierre et al, 2020), boreal (Asmala et al, 2014;Kritzberg et al, 2014;Herzog et al, 2020) and arctic (Gonçalves-Araujo et al, 2015;Soerensen et al, 2017;Kipp et al, 2020) coastal waters. Flocculation is one of the marginal filter (or coastal filter) processes described by Lisitsyn (1995), who estimated that, in the world's rivers, 93-95% of the suspended matter and 20-40% of the dissolved matter in river discharge (pollution included) is deposited in mixing areas on average.…”