Insight in transformations of nano-metallic and ionic platinum forms in different soil types in the context of Pt immobilization

Platinum is emitted by road traffic mainly in the form of metallic particles. Interaction of Pt-NPs with soil causes their chemical transformation that may result in dissolution. Investigation of soil - Pt-NPs interactions presented in this study focuses on assessing the influence of soil type on Pt mobility in soil enriched in its metallic and ionic forms. Studied soil types included peat soil (high content of organic matter), sandy soil, chalk loam soil and transformed soil collected next to a road with high traffic (Zabrze, Poland), to which citrates were added to mimic the rhizosphere activity. Solid-liquid extractions based on modified BCR protocols were applied to establish mobile and organic fractions, and Pt was determined with both voltammetry and ICP-MS. Cross-comparison of the results of these two techniques allows to conclude about Pt-NPs transformation into Pt(II). The mobility of Pt in transformed soil and sandy soil (about 10% extractability with CH₃COOH) is significantly higher than in clay (4-5%) and peat soil (0.4-0.8%). Metallic Pt-NPs with small diameters can be effectively transformed into ionic forms. Their content in mobile fraction reaches 30-50%, and in oxidizable fraction - even 75-80%. Higher mobility of Pt was observed after incubation in the presence of citrates, however it is not due to a transition of Pt-NPs into ionic forms but results from limited interaction of small NPs with the soil matrix.