Ecotoxicity assessment for environmental risk and consideration for assessing the impact of silver nanoparticles on soil earthworms

Abstract

Silver nanoparticles (AgNPs) are found in a range of commercial products due to their proven antibacterial properties. The unused silver nanoparticles (AgNPs) may make its way into the soil via biosolids that come from wastewater treatment or the effluent that comes from industrialisation processes, where it could be harmful to the organism that live in terrestrial ecosystems. In addition, silver ions are one of the most toxic forms of heavy metal released from dissolved silver nitrate (AgNO₃) and AgNPs through dissolution or oxidation. The study examined the effect of engineered AgNPs, and AgNO₃ on earthworms which are one of the most important bioindicator for determining toxicity in soil environment. Epigeic earthworm, Eudrilus eugeniae was exposed to soils spiked with equivalent concentrations of AgNPs or AgNO₃ at 0, 10, 100, and 200 mg kg^-1 in soil for 56 days of experiments. The survival and growth rate was recorded at 7^th, 14^th, 21^st, 28^th days and accumulation of Ag in earthworm tissue at 14^th and 28^th days, antioxidant enzymes at 28^th days and reproduction at 56^th days of experiment. Further, a short-term exposure of AgNPs and AgNO₃ was conducted to observe avoidance behaviour after 48 h of exposure. The result indicated that survivability was relatively low on exposure of AgNO₃ (83.3%) than AgNPs (86.7%) in 200 mg kg^-1 spiked soils, besides the growth was inhibited in both AgNPs (3.68%) and AgNO₃ (3.25%) at 28^th days. The uptake of Ag from AgNO₃ in the earthworm tissue was slightly higher than uptake of Ag from AgNPs and it showed concentration-dependent inhibitory effects on reproduction. In AgNO₃ spiked soil, a high level of the Malondialdehyde (MDA) based lipid peroxidation and increased activity of antioxidant enzyme catalase (CAT) was observed than AgNPs spiked soil. Similarly, glutathione (GSH), a cofactor for GPx and GST enzymes, was lower in AgNO₃-spiked soil than in AgNPs-spiked soil. In terms of avoidance behaviour, there was no discernible difference between the distribution of earthworms in AgNPs and AgNO₃ after 48 h. The study found E. eugeniae exhibits concentration-dependent alterations in its competence to survive, antioxidant enzymes, and reproduction. AgNO₃ was found to be more sensitive than AgNPs in the study. The research investigates the effect of AgNPs on earthworms in the soil ecosystem since this understanding is crucial for a comprehensive evaluation of AgNPs' environmental consequences.

Keywords: Antioxidant enzyme; Avoidance behaviour; Characterization; Engineered nanoparticle; Eudrilus eugeniae; Reproduction; Silver nanoparticle.

Figure 1

(a) SEM picture of synthesized silver nanoparticles showing well-distributed nanoparticles with a few regions of an agglomerated particles (b) FESEM image showing elemental mapping of synthesized AgNPs (c) Elemental analysis of AgNPs by EDX indicating 47% of silver.

Figure 2

(a) TEM images of synthesized nanoparticles showing particle size ranging from 33 nm and 44 nm approx. (b) HR TEM images showing the interplanar distances (d = 0.28 nm) (c) size distribution and (d) Area distribution of synthesized silver nanoparticles.

Figure 3

(a) SAED image reveals that the silver nanoparticles have polycrystalline structure (polycrystalline ring) (b) XRD pattern of silver nanoparticles specifying diffraction peaks at 2θ, crystallographic planes of fcc (111, 200, and 220) Ag crystals.

Figure 4

FTIR spectrum of silver nanoparticles showing presenting peaks.

Figure 5

SOD enzyme activity (mean ± standard deviation) in E. eugeniae exposed to AgNP and AgNo₃ at similiar concentration (0, 10, 100 and 200 mg kg⁻¹), with respect to control at 28 days of exposure. Note. Same letters in graph denote not significantly difference (p < 0.05) from control group, and between Ag treated group.

Figure 6

CAT enzyme activity (mean ± standard deviation) in E. eugeniae exposed to AgNP and AgNo₃ experiment of 28 days, with respect to control (0 exposures). Note. Means with different letters on bars, graph indicate significant differences (p < 0.05) from control and between treatments then within treatments (n = 10 worms per treatment) for both Ag (AgNPs and AgNo₃).

Figure 7

Total GSH content in E. eugeniae exposed to AgNP and AgNo₃ experiment of 28 days, with respect to control (0 exposures). Note. Means with different letters on bars, graph indicate significant differences (p < 0.05) from control and between treatments then within treatments (n = 10 worms per treatment) for both Ag (AgNPs and AgNo₃).

Figure 8

LPO (mean ± standard deviation) measured by MDA adducts formation in E. eugeniae to AgNP and AgNo₃ with respect to control (0 exposures) at 28 days of exeriment. Note. Means with different letters indicate significant differences (p < 0.05) from the respective controls (no exposure) and between treatments as well as within treatments (n = 10 worms per treatment) at each point for both Ag (AgNPs and AgNo₃).

Figure 9

Interrelationships between AgNPs and AgNo₃ in response to oxidative stress biomarkers (CAT, SOD, GSH, and LPO) of E. eugeniae exposed to Ag (AgNPs and AgNo₃: 0, 10, 100 and 200 mg kg⁻¹ similarly) during 28 days (a) Bipilot PCA analysis (b) Heat map analysis.