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Toxicity to soil microorganisms

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Chronic data are available for the toxicity of two upgraded ilmenite slags (>80% TiO2) to the nitrification activity of soil microorganisms in a natural soil, and the lowest NOEC amounts to 10,000 mg/kg dw. Regarding the toxicity of TiO2-NP to soil microbes, no differences between treatments with 100 mg n-TiO2/kg (highest test concentration) and the control were observed in nitrogen and carbon transformation tests according to OECD 216 and OECD 217, respectively (Hund-Rinke and Klawonn, 2013; Hund-Rinke et al., 2016).In sum, toxicity data from standard toxicity tests seems to indicate that micro-and nanosized TiO2 has a low potential for toxicity to soil microbes.

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Additional information

Microsized TiO2:

Chronic data are available for the toxicity of two upgraded ilmenite slags (>80% TiO2) to the nitrification activity of soil microorganisms in a natural soil, and the lowest NOEC amounts to 10,000 mg/kg dw.

Upgraded ilmenites (UGI) consist primarily (> 80%) of titanate, i.e., TiO2 with Fe, Al, Si, Mg and other metals, and are poorly soluble and not biodegradable. The transformation/dissolution of 3 UGI products at loadings of up to 100 mg/L over 28 d did not result in any significant release of titanium. Bulk and nano-TiO2 forms are also poorly soluble in environmental media (Brouwers, 2009; Klawonn, 2017 a-f). Based on similarity in composition and the same poor solubility of UGI and TiO2, read-across of ecotoxicological data from UGI to TiO2 is justified.

Nanosized TiO2:

Regarding the toxicity of TiO2-NP (P25 & NM-104) to soil microbes, no differences between treatments with 100 mg n-TiO2/kg (highest test concentration) and the control were observed in nitrogen and carbon transformation tests according to OECD 216 and OECD 217, respectively (Hund-Rinke and Klawonn, 2013; Hund-Rinke et al., 2016).

Data on soil microbial toxicity are also available from a supporting study, in which a non-standard test was performed. After 84 days of cultivating wheat (Triticum ssp.) in a sand-soil mixture spiked with TiO2-NP up to 1000 mg/kg, roots were colonised by AMF, abnormalities in mycorrhizal colonisation could not be observed, and N2O emissions and the amount of extracted DNA were similar to the control (Klingenfuss, 2014).

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