Effect of soil organic matter content and pH on the toxicity of ZnO nanoparticles to Folsomia candida

• Toxicity of ZnO–NP and ZnCl2 to Folsomia candida determined in four soils.
• Test soils differed in organic matter content (2.37–14.7%) and pH (5.0–6.8).
• Higher Zn porewater concentrations and higher toxicity for ZnCl2 than for ZnO–NP.
• Zn porewater concentrations were affected by organic matter content and pH.
• ZnO–NP and ZnCl2 toxicity more affected by pH than by organic matter content.

Organic matter (OM) and pH may influence nanoparticle fate and effects in soil. This study investigated the influence of soil organic matter content and pH on the toxicity of ZnO–NP and ZnCl2 to Folsomia candida   in four natural soils, having between 2.37% and 14.7% OM and pHCaCl2pHCaCl2 levels between 5.0 and 6.8. Porewater Zn concentrations were much lower in ZnO–NP than in ZnCl2 spiked soils, resulting in higher Freundlich sorption constants for ZnO–NP. For ZnCl2 the porewater Zn concentrations were significantly higher in less organic soils, while for ZnO–NP the highest soluble Zn level (23 mg Zn/l) was measured in the most organic soil, which had the lowest pH. Free Zn2+ ion concentrations were higher for ZnCl2 than for ZnO–NP and were greatly dependent on pH (pHpw) and dissolved organic carbon content of the pore water. The 28-d EC50 values for the effect of ZnCl2 on the reproduction of F. candida increased with increasing OM content from 356 to 1592 mg Zn/kg d.w. For ZnO–NP no correlation between EC50 values and OM content was found and EC50 values ranged from 1695 in the most organic soil to 4446 mg Zn/kg d.w. in the higher pH soil. When based on porewater and free Zn2+ concentrations, EC50 values were higher for ZnCl2 than for ZnO–NP, and consistently decreased with increasing pHpw. This study shows that ZnO–NP toxicity is dependent on soil properties, but is mainly driven by soil pH.