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Distribution of TiO2 Nanoparticles in Acidic and Alkaline Soil and Their Accumulation by Aspergillus niger

The nanoparticles of TiO2 (TiO2 NP) have been used as a plant-growth stimulant or catalyst in pesticide formulas. However, due to high resistance of TiO2 NP to abiotic weathering, dissolved Ti is unlikely to act as an active compound in these preparations. Even if soil is acidic, TiO2 NP do not dissolve easily and preferably remain as undissolved particles. The low dissolution rates of inorganic nanoparticles in the soil environment make Ti in TiO2 NP largely unavailable for plants and soil microorganisms. To characterize the behavior of TiO2 NP in soil under different pH conditions, we analyzed TiO2 NP-size distribution in two soil materials, an alkaline and acidic one. We also cultivated Aspergillus niger, a fungus ubiquitously found in soils, in the growth medium spiked with TiO2 NP to assess accumulation of the nanoparticles in fungus. In soil suspensions, the dissolved Ti was present in low concentrations (up to 0.010 mg L−1). Most of the TiO2 NP remained in particulate form or appeared as aggregates sized 100–450 nm. In experiment on Ti accumulation by A. niger, TiO2 NP either settled down to the bottom of the flask with growth medium or were actually accumulated by the fungus; about 7.5% of TiO2 NP were accumulated in fungal mycelia. Most of the TiO2 NP remain in particulate form in soil solutions, regardless of soil pH. Filamentous fungus A. niger has the ability to accumulate bioavailable TiO2 NP, which hints at the possibility that some soil fungi can affect spatial distribution of this type of nanoparticles in soils.