In studying the transport and cycling of elements in ecosystems, filtration through a small pore-size filter (e.g., 450 nm) is used to discriminate between suspended particles and “dissolved” materials. Inherent in this dichotomous view is the assumption that the dissolved fraction consists largely of dissolved solutes; this has been challenged by over 10 years of work on fate and impacts of manufactured nanoparticles (1-100 nm particles). This work has both demonstrated the ubiquity of natural and incidentally formed nanoparticles, as well as nanoparticle uptake by organisms. However, it is not known to what degree natural and incidental nanoparticles and small colloids (1-450 nm) contribute to element cycles. As a first step towards addressing this knowledge gap, we examined the distribution of metals/metalloids between truly dissolved (< 1 nm) and small colloidal (1-450 nm) fractions using filtration, ultrafiltration, and ICP-MS. To further unpack colloids, we examined the composition of individual colloids using single particle ICP-MS using time of flight mass spectrometry to measure multiple elements per colloid. Our study systems were eight sites sampled along the metal-contaminated Clark Fork River, Montana, and six wastewater lagoon systems in the Clark Fork Watershed with elevated metal concentrations and which discharge directly or indirectly into the river.
Results/Conclusions
In wastewater, we found that cadmium, copper, iron, and lead were all > 50% colloidal, while chromium, manganese, and zinc were less abundant in the colloidal fraction, but still had median values from 27-48% colloidal. The analysis of individual particles in the Clark Fork River revealed that 78.3% of particles analyzed consisted of individual detectable elements, while particles with two, three, and four or greater elements represented 20, 1.4, and 0.3%, respectively. The dominant particles observed were iron particles with a median diameter of 74 nm. Manganese bearing particles were the second most abundant, and particles consisting of iron and manganese were the most common dual element particles. Several of the metals of concern in the Clark Fork were also found as single element particles (e.g., copper, arsenic, cadmium) while others like lead were found at low concentrations associated with iron or manganese particles. Given that colloids accounted for >50 % of a range of elements, and that organisms can take up elements in this size range, it seems highly likely that these small particles may play a big role in interactions between the biota and these elements, and thereby drive their cycling in these systems.