Determination of Nanoparticle Uptake, Distribution, and Characterization in Plant Root Tissue after Realistic Long-Term Exposure to Sewage Sludge Using Information from Mass Spectrometry

The use of nanoparticles (NPs) in numerous products and their potential accumulation causes major concern for humans and the environment. Until now, the uptake of NPs in plant tissue was mostly shown under greenhouse conditions at high doses and short exposure periods. Here, we present results on the uptake of particulate silver (Ag) and cerium dioxide (CeO2) in the tissues of Triticum aestivum, Brassica napus, and Hordeum vulgare, after exposure to sewage sludge treated with nano-Ag (NM300 K at 1.8 and 7.0 mg/kg sludge per dm soil) and nano-CeO2 (NM212 at 10 and 50 mg/kg sludge per dm soil). All plants were cultivated in a rural area near the German town Schmallenberg according to the common regional crop rotation on outdoor lysimeters. The highest concentrations measured were 86.4 mg/kg for Ag (Hordeum vulgare) and 94 mg/kg for Ce (Triticum sativum). Analysis of plant samples revealed the presence of Ag mainly in its ionic form. However, the occurrence of nano- and larger sized particles of Ag and CeO2 was observed as well. Quantitative shares of the particulate fraction of the total element concentration were estimated up to 22.4% for Ag and up to 85.1% for CeO2. A high abundance of particle agglomerates in the phloem suggests upward transport of the nanoparticles to other plant parts. A small number of agglomerates in the xylem suggests a downward transport and subsequent accumulation in the root phloem. Exemplary investigations of Brassica napus root exposed to nano-CeO2 revealed no accumulation of the pristine material in the cell nucleus; however, CePO4 was found. The presence of this substance points to a dissolution of the low soluble CeO2 in planta and subsequent precipitation. Furthermore, for the first time, mixed NP-salt agglomerates, composed of Ca3PO4+ and K3SO4+ NPs, could be observed within Brassica napus root tissue.