Örjan Gustafsson

Temporal variations in the fractionation of the rare earth elements in a boreal river; the role of colloidal particles.

Rare earth element (REE) data from weekly sampling of the filtered (<0.45 μm) and suspended particulate phase during 18 months in the Kalix River, Northern Sweden, are presented together with data on colloidal particles and the solution fraction (<3 kDa). The filtered REE concentration show large seasonal and temporal variations in the river. Lanthanum varied between approximately 300 and 2100 pM. High REE concentration in the filter-passing fraction is related to increased water discharge and there is a strong correlation between the REE concentration, organic carbon, Al and Fe. Physical erosion of detrital particles plays a minor role for the yearly transport of particulate REE in this boreal river system. The suspended particulate fraction, which is dominated by non-detrital fractions, accounted for only 35% of the yearly total transport of La in the river. Approximately 10% of the REE were transported in detrital particles during winter. At spring-flood in May, about 30% of the LREE and up to 60% of the HREE where hosted in detrital particles. Ultrafiltration of river water during spring-flood shows that colloidal particles dominate the transport of filter-passing REE. Less than 5% of the filtered REE are found in the fraction smaller than 3 kDa. The colloidal fraction shows a flat to slightly LREE enriched pattern whereas the solution fraction (<3 kDa) show an HREE enriched pattern, compared with till in the catchment. Suspended particles show a LREE enriched pattern. Data indicate that the REE are associated with two phases in the colloidal (and particulate) fraction, an organic-rich phase (with associated Al–Fe) and an Fe-rich (Fe–oxyhydroxide) inorganic phase. The Ce-anomaly in the suspended particulate fraction in the river shows systematic variations, and can be used to interpret fractionation processes of the REE during weathering and transport. There was no anomaly at maximum spring-flood but during the ice-covered period the anomaly became more and more negative. The temporal and seasonal variations of the Ce-anomaly in the suspended particulate phase reflect transport of REE–C–Al–Fe-enriched colloids from the upper section of the till (and/or from mires) to the river at storm events.

The importance of colloids for the behavior of uranium isotopes in the low-salinity zone of a stable estuary

Particle-mediated removal processes of U isotopes were investigated during spring flood discharge in the low-salinity zone (LSZ, up to 3 practical salinity units [psu]) of a stable estuary. A shipboard ultrafiltration cross-flow filtration (CFF) technique was used to separate particles (>0.2 μm) and colloids (between 3000 daltons (3 kD) and 0.2 μm) from ultrafiltered water (<3 kD) containing “dissolved” species. Sediment traps were used to collect sinking material. Concentration of Fe and organic C, which are indicators of the major U carrier phases, were used to interpret the behavior of ^(234)U-^(238)U during estuarine mixing. nColloids dominated the river water transport of U, carrying ≈90% of the U. On entering the estuary, colloids accounted for the dominant fraction of U to about a salinity of 1 psu, but only a minor fraction ( 1 psu, there is a general correlation between U and salinity in all filtered fractions. The ^(234)U/^(238)U ratios in different filtered fractions and sinking particles were generally indistinguishable at each station and showed enrichment in ^(234)U, compared with secular equilibrium (δ^(234)U = 266–567). This clearly shows that all size fractions are dominated by nondetrital U. Consideration of U isotope systematics across the estuary reveals that substantial U exchange must occur involving larger particles at least to 1 psu and involving colloids at least to ≈1.5 psu. Further exchange at higher salinities may also occur, as the proportion of U on colloids decreases with increasing salinity. This may be due to decreasing colloid concentration and increasing stabilization of uranyl carbonate complexes during mixing in the estuary. nThe results show that although U is a soluble element that shows generally conservative mixing in estuaries, removal occurs in the very low salinity zone, and this zone represents a significant sink of U. Variation in composition and concentration of colloidal particles between different estuaries might thus be an important factor for determining the varying behavior of U between estuaries.

Evaluation of two cross-flow ultrafiltration membranes for isolating marine organic colloids

Abstract Laboratory and field studies were performed to evaluate two 1 kilo-Dalton (kD) cross-flow ultrafiltration (CFF) membranes (a Millipore Prep-scale CFF membrane constructed primarily from regenerated cellulose and an Amicon CFF polysulfone membrane) to isolate colloids (operationally defined as particles or macromolecules between 1 kD and 0.2–1 μ m) from sea water. We focused on three crucial aspects when applying the CFF technique: retention characteristics, sorptive potential and ultrafilter breakthrough. Lab results showed that both CFF systems retained ≥91% of a 3000 nominal molecular weight (NMW) dextran standard, consistent with the manufacturers rated cutoff. The Millipore membrane showed essentially no loss of a dextran standard, while 33% was lost for the same molecule onto the Amicon CFF membrane. Both membranes showed higher losses of a protein standard (Lactalbumin) added to sea water. For bulk organic carbon (OC), both membranes usually had reasonable recovery (100±10%) as long as the membranes were preconditioned. This was true for both lab experiments and field investigations in open ocean water off Bermuda. However, data from 234 Th and 230 Th analysis of samples from a station off Bermuda showed very large losses and hence low recovery from CFF. Results of these fractionated OC and 234 Th distributions are also discussed in the context of prior studies. Ultrafilter breakthrough of both high molecular weight (HMW) and low molecular weight (LMW) compounds may occur throughout the CFF process, especially when processing coastal sea water where COC is relatively enriched. A permeation coefficient model provides an overall reasonable fit to the data characterizing the permeation behaviour of CFF; the retentate prediction based on the model indicates that breakthrough becomes more significant after the concentration factor (cf) is higher than 5, which implies that fractionation of organic components increases at higher cf.

Phase Distributions of Hydrophobic Chemicals in the Aquatic Environment

At present, most predictors of the truly dissolved - directly bioavailable - aqueous fraction of xenobiotic chemicals are based on predicting the attenuation from total to dissolved concentrations using models of equilibrium partitioning with bulk organic matter. While certainly a wonderfully practical and, at least in laboratory systems, often successful approach. such organic-carbon normalizing partitioning models (e.g., ref. 1, 2) are frequently not able to explain field-observed in situ phase-distributions. In this overview, we will discuss situations where the bioavailable aqueous fractions may deviate from predictions of existing speciation models: (a) strong interactions with soot result in lower dissolved exposure, (b) poor interactions with bulk organic colloids result in higher dissolved exposure, and (c) nonequilibrium interactions in systems where the solid phases are dominated by living cells may result in either lower or higher dissolved fractions. Another apparently aberrant partitioning case to be discussed is the elevated fog-drop associations of hydrophobic chemicals, in excess of their Henry’s Law expectations, which may be explained by partitioning to the interface between air and water.