B. Kim, Q.T. Nguyen, M.E. Sherman, L. Nguyen, D.M. Aruguete, S.W. Eisenman
Keywords: doped aluminum oxide nanoparticles, lemma minor, azolla filiculoides, urban stream, biomass, and uptake
Summary:Platinum group elements (PGEs), such as platinum (Pt) and palladium (Pd), have long been favored as active components in catalysts for their high activity and reaction selectivity, as well as their intrinsic resistance to corrosion. In particular, PGEs are the active components of three-way catalytic converters in automobiles, as coated on a ceramic monolith, such as cordierite, with a PGE-containing alumina washcoat. Typically, gamma-aluminum oxide (-Al2O3) is the most widely used washcoat for automotive exhaust catalysts. While Pd and Pt are rare elements in the earth’s crust, a significant contribution from automobile emissions has resulted in their concentration exceeding the natural background levels by up to two orders of magnitude. Wide distribution and accumulation of PGEs have been extensively noted in various terrestrial environments including (roadside) soils, sediments, as well as waterways. In this study, we therefore examined the effect of pristine 10 nm -Al2O3 nanoparticles (NPs) and Pd or Pt-doped -Al2O3 (Pd/Al2O3, Pt/Al2O3, respectively) NPs on two representative freshwater aquatic plants that are commonly found in urban streams, duckweed, Lemna minor (L. minor), and waterfern, Azolla filiculoides (A. filiculoides), by measuring their biomass and PGEs uptake as a function of NPs concentrations using the OECD guidelines. A series of Pd/Al2O3, Pt/Al2O3, or Al2O3 NPs suspension solutions were added to the growth medium to reach the target concentrations of 50, 100, 500, and 1,000 ppm, respectively. With increasing Pd/Al2O3 or Al2O3 NPs concentration, L. minor showed a decrease in their fresh biomass, approximately 11.4% and 7.4% reduction compared to the control by the highest treatment (1000 ppm), respectively, but did not affect A. filiculoides. In contrast, Pt/Al2O3 caused 7.3% reduction in the A. filiculoides fresh biomass compared to the control, but not L. minor. The plant growth inhibition results seem to be closely related to the plants’ ability to uptake and accumulate elements of interest. For instance, a higher Pd accumulation by L. minor (30.1 ppm) than A. filiculoides (22.9 ppm) was noted. Further, Pt accumulation by A. filiculoides (43.6 ppm) occurred approximately three-fold higher than L. minor (15.5 ppm) at the highest treatment. For all the treatments, Al concentration measured in A. filiculoides was much higher than that in L. minor. Increased concentration of Pd or Pt and Al in both plants resulted in lower nutrient uptake, including Ca, Mg, Fe, Mn, S, and P. Thus, PGEs and Al released from the NPs appear to be responsible for the reduced fresh biomass and enhanced PGEs bioaccumulation in both species, while presenting species-dependence in elemental uptake. These findings demonstrate that dopants greatly modify surface properties of the pristine NPs, and thus, present differences in their bioavailability and uptake when interacting with aquatic plants in urban stream environment.