Efficiency of Earthworms in Bioremediation of Soil Contaminated with Sewage Water and Heavy Metals

Abstract

This study evaluated the efficiency of two earthworm species (Eisenia fetida and Lumbricus terrestris) in the bioremediation of loam and clay soils contaminated with sewage water, Cadmium (Cd), and Lead (Pb), alongside assessing their biomass dynamics, antioxidant defense, and neurotoxic responses. A 60-day factorial experiment was conducted using a Randomized Complete Block Design (RCBD), with 10% fermented cow waste added as an organic amendment. The results revealed a clear ecological and physiological specialization; E. fetida exhibited the highest survival rate (72.50%), maximum biomass (34.75 g pot⁻¹), and peak cellular specific activity of superoxide dismutase (SOD) (54.90 units mg⁻¹ protein) under individual Pb exposure within the loam soil. In contrast, L. terrestris demonstrated exceptional efficiency in clay soil with a survival rate of 68.25%, higher biomass retention, and a superior adaptive antioxidant response to overcome the compact mechanical stress. The findings indicated that Cd possesses higher acute toxicity and cellular damage compared to Pb. Under dual contamination (W3), survival rates collapsed significantly (36.25%), and biomass plunged to 18.75 g pot⁻¹ due to severe synergistic stress. This severe toxicity caused a non-linear drop-in SOD activity, indicating a partial collapse of the defense mechanism. Furthermore, acetylcholinesterase (AChE) activity was heavily inhibited by heavy metals, dropping from a baseline of 86.45 nmol min⁻¹ mg⁻¹ protein to an absolute minimum of 21.65 nmol min⁻¹ mg⁻¹ protein under mixed treatments (W3), indicating acute neurotoxicity. Clay soil effectively acted as a protective shield, mitigating neurotoxic inhibition and preserving higher enzyme levels due to its high cation exchange capacity (30.84 cmol kg⁻¹). The study concludes that monitoring biomass, antioxidant profiles, and neurotoxic biomarkers provides a powerful framework for evaluating multi-metal contamination risks and earthworm-mediated soil restoration.