Role of Particle Size and Soil Type in Toxicity of Silver Nanoparticles to Earthworms

Abstract

Silver nanoparticles (NPs) are an emerging contaminant of concern due to their increased use. The earthworm Eisenia fetida was exposed to a range of concentrations of AgNO3 and two polyvinylpyrolidone coated Ag NPs with different particle size distributions. They were exposed in two different soils: a naturally occurring sandy loam and a standardized artificial soil. The AgNO3 significantly reduced E. fetida growth and reproduction at 7.41 ± 0.01 mg kg−1 Ag in the sandy loam but only reproduction was affected at concentrations of 94.1 ± 3.2 mg kg−1 in the artificial soil. In the artificial soil, significant (α = 0.05) reproductive toxicity was only observed in organisms exposed to the Ag NPs at concentrations approximately eight times higher than those at which the effects from ionic Ag were observed. Eisenia fetida exposed to either AgNO3 or Ag NPs in the sandy loam accumulated significantly (α = 0.05) higher concentrations of Ag than those exposed in the artificial soil and had higher bioaccumulation factors. Earthworms exposed to AgNO3 also accumulated significantly higher concentrations of Ag than those exposed to Ag NPs. No differences in toxicity were observed between the two size distributions. Extended x‐ray absorption fine structure spectroscopy analysis of the soils indicated that the Ag was approximately 10 to 17% Ag(I), suggesting that Ag ions may be responsible for effects on growth and development caused by exposure to Ag NPs. Our results also suggest that soil type is a more important determinant of Ag accumulation from Ag NPs than particle size.

Metal‐based nanoparticles in soil: Fate, behavior, and effects on soil invertebrates

“…Although size influences the toxicity, soil type was found to be more important than particle size [55]. In a sandy loam, the earthworm E. fetida accumulated significantly more Ag from Ag NPs exposure than in artificial soil.…”

“…The accumulation of Ag seems to be greater in the earthworm E. fetida exposed to the corresponding AgNO 3 salt than from NPs [55,83]. However, extended X-ray absorption fine structure spectroscopy analysis in soil showed that Ag in earthworms exposed to Ag NPs suggests that accumulation was not caused only by the ionic form (i.e., ions dissolved from NPs), but that nondissolved particles contributed to Ag accumulation in the organisms [85].…”

“…The avoidance rate for Ag NPs and AgNO 3 was the same only at the end of the 48-h avoidance test; although AgNO 3 -contaminated soil was avoided immediately by the animals [84]. Based on another study [55], we concluded that the release of Ag ions from Ag NPs was not the major factor responsible for the avoidance effects because the exposure time (48 h) would not be long enough to dissolve a similar amount of ions as presented in the AgNO 3 -based exposure [84].…”

“…In soils, Au NPs of different sizes (20 and 55 nm) showed 8.0 and 3.4% dissolution of Au ions after 28 d, respectively [54]. Approximately 10 to 17% of Ag NPs (30-50 nm) were found to be transformed to Ag(I) after 28 d, suggesting oxidative dissolution to ions [55]. The dissolution of CeO 2 NPs has been shown to be very low in different types of soil [56].…”

“…In a sandy loam, the earthworm E. fetida accumulated significantly more Ag from Ag NPs exposure than in artificial soil. Sandy loam characteristics, such as lower pH and organic content, resulted in greater availability of Ag ions [55]. Also, a natural sandy loam resulted in greater avoidance of Ag NPs by earthworms in comparison with artificial soil [84].…”