Vermicomposting of coal fly ash using epigeic and epi-endogeic earthworm species: nutrient dynamics and metal remediation

Abstract

Huge amounts of coal fly ash (FA), produced during power generation, has led to several environmental problems associated with metal pollution and the burden of its disposal. Vermicomposting has emerged as a cost-effective technique in suitable management of FA. Two epigeic earthworm species: Eisenia fetida, and Eudrilus eugeniae and one epi-endogeic species: Lumbricus rubellus were selected for vermiremediation of coal FA. The investigation addresses the changes in the earthworm biomass, number, nutrient content and metal content in the treatments of FA amended with cow-dung (CD) at periodical intervals of vermicomposting. Metal (Cr, Cu, Cd, Zn, Ni and Pb) removal efficiency of the three earthworm species has been observed. Bioaccumulation factor (BAF) and changes in the microbiological fauna of the different mixtures were also witnessed. Earthworm and cocoon count showed an increase in trend with duration of vermicomposting. The concentration of total nitrogen, phosphorous and potassium increased in all the treatments. The maximum bacterial count (8.0 × 10⁵ CFU g⁻¹) and fungal count (3.8 × 10³ CFU g⁻¹) were observed in FA + CD (1 : 3) mixture, comprising E. eugeniae. Significant reduction (p < 0.05) in the metal concentration of the treatments along with subsequent increase in metal content of earthworm tissues using Flame Atomic Absorption Spectrophotometer was observed. The treatment FA + CD (1 : 3) gave the best results in terms of nutrient enhancement and metal removal. Maximum metal reduction in the treatment, FA + CD (1 : 3) was 58.82% for Cr by E. fetida, 71.94% for Ni by E. eugeniae, and 51.67% for Cu by L. rubellus. Highest BAF value was obtained for Ni (3.31) in E. eugeniae driven treatment FA + CD (1 : 3). The maximum metallothionein production was observed in E. eugeniae followed by E. fetida and L. rubellus.