Identification of soil-specific aging processes of EINP

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Main processes to be addressed in the second phase of the whole research group are reaction, masking and catching of nanoparticles as well as the consequences of these processes on their mobility and ecotoxicology. Our project SOILMOBILE II will have a focus on those soil conditions which are identified as crucial for controlling these processes (concentration and valency of cations in solution, quality and concentration of DOM, concentration of EINP, specific surface of soil particles). In detail we will test the following hypotheses:Masking and dissolution. The dissolution of Ag EINP is controlled by mineral-organic coatings, by multivalent cations and by aggregation. Organic coatings decrease the solubility of the Ag0 EINP by decreasing the accessibility of EINP surfaces for oxidizing reagents. This effect of organic coatings increases with increasing molecular size of the organic matter and/or with increasing cation bridges between organic matter and Ag (cooperation with MASK and PORESURFACE). We will study the adsorption of (i) DOM compounds (<100 kDa, >100 kDa) extracted from various soil samples and (ii) two model organic molecules (polygalacturonic acid, citrate) to Ag0 EINPs at varying cation concentration and valency using isothermal nano-calorimetry. DOM coatings will be analyzed in cooperation with MASK. Furthermore, we will determine the effects of coatings and electrolytes on the dissolution kinetics of Ag0 NPs (using a micro-dialysis technique which is recently developed in our lab).Catching: The retention of EINP (Ag, Au, TiO2) by soil material is controlled by (a) transport into porous soil minerals rather than by (b) electrostatic interactions with soil particles. Under close-tonatural soil conditions homo-aggregation and filtration of stabilized EINP is of minor relevance compared to adsorption and penetration into soil aggregates.(a) EINP added to dry soil material may be transported via mass flow into the micro-aggregates of the soil. We will analyse the retention by soil micro-aggregates characterised for size and pore volume. The reversibility of EINP retention by soil aggregates will be analysed. In addition, we will map the spatial distribution of EINP using thin sections and SEM/EDX analyses (cooperation with PORENET).(b) We will quantify the adsorption of EINP as a function of their size and surface charge. In addition the heat of adsorption will be analyzed using ITC to identify the mechanism of EINP retention by soil particles (cooperation with PORENET, PORESURFACE).