Ecotoxicological Assessment of the Single and Combined Effects of Nano-Zero-Valent Iron and Cadmium on Eisenia Fetida

Abstract

Nano-zero-valent iron (nZVI) is a soil conditioner used widely in soil remediation. However, studies have suggested that nZVI above threshold doses may adversely affect soil organisms and even exacerbate the toxicity of contaminants such as cadmium (Cd) in earthworms. This study investigated whether nZVI induces stress and enhances Cd toxicity in earthworms, by studying the responses of Eisenia fetida in a controlled laboratory experiment. Four treatments were established: control (no nZVI or Cd), nZVI (10 g∙kg⁻¹), Cd (30 mg∙kg⁻¹), and nZVI-Cd (nZVI: 10 g∙kg⁻¹; Cd: 30 mg∙kg⁻¹). The survival rate, biomass, Cd bioaccumulation, enzyme activity, and other indicators were measured. On exposure day 7, the levels of reactive oxygen species in the nZVI, Cd, and nZVI-Cd groups were 1.41, 1.48, and 1.62 times higher than those in the control group, respectively (P < 0.05). Over time, earthworm survival and biomass decreased, with the lowest survival rate and greatest biomass loss observed in the nZVI-Cd group on day 28. Tissue staining revealed that, under Cd or nZVI stress, the circular muscle layer of the earthworms had loosened, the intestinal tissue had partially detached, and the number of villi was reduced. Metagenomic sequencing indicated significant changes in the community structure of the earthworm intestinal flora under the different treatments. Both 30 mg∙kg⁻¹ Cd and 10 g∙kg⁻¹ nZVI induced an imbalance in the gut microbiota that was further exacerbated by combined exposure.

Keywords: bioaccumulation, Cadmium, Eisenia fetida, metagenomics, nano-zero-valent iron, oxidative damage