Fe–Al–organic Colloids Control of Trace Elements in Peat Soil Solutions: Results of Ultrafiltration and Dialysis

Abstract

Size fractionation of $40 major and trace elements (TE) in peat soil solutions from the Tverskaya region (Russia) has been studied using frontal filtration and ultrafiltration through a progressively decreasing pore size (5, 2.5, 0.22 lm, 100, 10, 5, and 1 kD) and in situ dialysis through 6-8 and 1 kD membranes with subsequent analysis by ICP-MS. In (ultra) filter-passed permeates and dialysates of soil solutions, Fe, Al, and organic carbon (OC) are well correlated, indicating the presence of mixed organo-mineral colloids. All major anions and silica are present in ''dissolved'' forms passed through 1 kD membrane. According to their behavior during filtration and dialysis and association with mineral or organic components, three groups of elements can be distinguished: (i) species that are weakly affected by size separation operations and largely (>50-80%) present in the form of dissolved inorganic species (Ca, Mg, Li, Na, K, Sr, Ba, Rb, Cs, As, Mn) with some proportion of small (1-10 kD) organic complexes (Ca, Mg, Sr, Ba), (ii) biologically essential elements (Co, Ni, Zn, Cu, Cd) mainly present in the fraction smaller than 1 kD and known to form strong organic complexes with fulvic acids, and, (iii) elements strongly associated with aluminum, iron and OC in all ultrafiltrates and dialysates with 30-50% being concentrated in large (>10 kD) colloids (Ga, Y, REEs, Pb, Cd, V, Nb, Sn, Ti, Zr, Hf, Th, U). For most trace metals, the proportion in the colloidal fraction correlates with their first hydrolysis constant. This implies a strong control of negatively charged oxygen donors present in inorganic/organic colloids on TE distribution between aqueous solution and colloid particles. It is suggested that these colloids are formed during plant uptake of Al, Fe, and TE from mineral matrix of deep soil horizons and their subsequent release in surface horizons after litter degradation and oxygenation on redox or acid/base fronts. Dissolved organic matter stabilizes Al/Fe colloids and thus enhances trace elements transport in soil solutions.

Diurnal variations of trace metals and heterotrophic bacterioplankton concentration in a small boreal lake of the White Sea basin

“…Similar technique has been used to account for metal speciation both in the water column and in the pore waters (Carignan et al, 1985;Alfaro-De la Torre et al, 2000;Gimpel et al, 2003). The duration of this dialysis procedure was between 24 and 48 h. Pokrovsky et al (2005) showed in laboratory experiments that, for both 1 and 6-8 kDa membranes, an equilibrium distribution is attained within 6-12 h, in agreement with the manufacturer's specifications. At the same time, Carignan et al (1985) argued that much longer equilibration time is necessary to achieve equilibrium dialysis distribution in porewaters.…”

“…Another important environmental issue is the fate of organic carbon in lakes that is likely to significantly control the CO 2 balance in high latitudes (Smith et al, 2004;Shirokova et al, 2009). High concentrations of dissolved organic matter (DOM) and thus, the colloidal status of most trace elements (TE) is the most important characteristic feature of TE geochemistry in European Russian Arctic zone (Pokrovsky and Schott, 2002;Pokrovsky et al, 2005). This change in metal speciation (i.e., the proportion of organo-mineral colloids and humic/fulvic complexes of trace metals, size distribution in colloidal fraction) between different water reservoirs or in the water column is expected to be controlled by: (1) biological processes of primary production/mineralisation and the release of organic exudates, both on diurnal (photosynthesis) and monthly (summer bloom, spring and autumn overturn) scales occurring in the water column; and (2) hydrological conditions of the watershed that controls the delivery of allochtonous organic matter from the soil reservoirs via riverine inflow, which in turn, depends on intra-soil processes, watershed lithology, and, if present, anthropogenic and atmospheric input.…”

From soil water to surface water – how the riparian zone controls element transport from a boreal forest to a stream

“…As discussed above, the Fe colloids appear to be missing in the riparian zone, leaving only the organic colloids as possible carriers (Neubauer et al, 2013). There is also strong evidence that organic matter is responsible for carrying a wide range of elements in boreal waters (e.g Gustafsson et al, 2000;Pokrovsky et al, 2005;Andersson et al, 2006;Dahlqvist et al, 2007;Pokrovsky et al, 2010;Vasyukova et al, 2010;Aiken et al, 2011). Furthermore, it has been demonstrated that the affinity for organic matter of various metals is essential for understanding their transport and spatial variability in the boreal landscape .…”

Environmental Impacts—Terrestrial Ecosystems

“…5), enhanced mobilisation of soil organic matter and transport of terrestrial DOC to the limnic ecosystems and the North Sea are likely. Dissolved organic matter affects ecosystem nutrient availability (Carpenter et al 2005), acidification of limnic systems (Oliver et al 1983) and solubility, transport and toxicity of heavy metals and organic pollutants (Carter and Suffet 1982;Pokrovsky et al 2005). It also regulates the photochemistry of natural waters (Zafiriou et al 1984) and influences aquatic production of algae and bacteria (Wetzel 1992;Carpenter and Pace 1997).…”