Influence of soil on outcome of nanoparticles

To understand soil influence, wheat, a widely grown crop over the world, was exposed to TiO2 nanoparticles (0, 100, 500 mg·kg−1) in four different types of soil. After several weeks of exposure, Ti quantification in soil, soil leachates and wheat leaves was assessed. Ti distribution in roots and leaves and speciation (is Ti complexed with other molecules, for example) in leaves  were also determined using techniques available on LUCIA ( X-ray fluorescence microspectrocopy and micro-XANES) and at LEEL (micro particle induced X-ray emission, µPIXE, and Rutherford Backscattering spectroscopy, RBS, especially suitable for the study of light elements, C, N, O). Finally nanoparticle phytotoxicity was evaluated through plant development parameters: height, fresh and dry weights and chlorophyll content.

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Very mobile nanoparticles

The results obtained in the study demonstrated that TiO2 nanoparticles can be significantly taken up with no speciation modification by wheat seedlings after a three week exposure in a sandy soil in which 500 mg·kg−1 of engineered nanoparticles have been added. This is also in this soil type that nanoparticles were able to reach in higher quantities the leachates. Thus if a contamination occurs in a sand, nanoparticles will be very mobile towards terrains storing water and bioavailable to crop plants representing a potential risk for trophic transfer.

No sign of phytotoxicity

However, sand is not a relevant medium for agriculture. The same trend has been highlighted in silty sand but in smaller proportions. In contrast, in loamy sand and clayey loam no difference has been evidenced after addition of nanoparticles to the system: no Ti concentration increase in the soil, partly due to the high natural Ti concentration in the soil, no increase in plant roots and leaves and little to no increase in the soil leachates. This difference in TiO2 nanoparticle behavior can be related to three characteristics of the soil used: organic carbon content, clay content and cationic exchange capacity. In all conditions, no acute sign of phytotoxicity was detected in the endpoints analyzed.

TiO2 nanoparticles remain in the soil

The overall conclusion for agro-ecosystems is that TiO2 nanoparticle contamination (after a short term exposure: three weeks) seems to have little impact on plant health and low chance of uptake as well as low risk of leaching to the aquifers. The soil displaying the highest risk for food safety would be the silty sand but this soil had little nutrients for plant growth and was not the best featured for agriculture. However, those results also imply that TiO2 nanoparticles will remain in the soil with possible consequences for soil micro and macro-organisms.

The study is published in: Science of The Total Environment., 630: 609–617. (2018).