Biological conversion of low-grade coal discard to a humic substance-enriched soil-like material

Abstract

A mutualistic relationship between grasses, coal-degrading fungi, and arbuscular mycorrhizal fungi was proposed to account for the phyto-biodegradation of coal discard. In this study pot trial experiments were carried out to confirm transformation of the carbonaceous substrate, in the presence of a suite of coal degrading fungi and arbuscular mycorrhizal fungi, into a humic-enriched soil-like material in the Cynodon dactylon/coal rhizosphere. The results show that after 47 weeks of C. dactylon growth on coal discard the concentration of humics increased from (62.9 ± 1.5) to (112.1 ± 5.4) mg/kg. Substrate humic acid-like substance concentration positively correlated (r2 = 0.95) with accumulation of above ground C. dactylon biomass. FTIR spectroscopy of the extracted humic-like substances confirmed both product identity and increased oxidation of the coal discard substrate. Substrate ash content and electrical conductivity declined coincident with an increase in humic acid-like substance concentration, which together reduced the intensity of acidity in the C. dactylon/coal discard rhizosphere. These observations support the proposal that biological oxidative degradation of coal discard leads to increased humic-like substance concentration and formation of a soil-like material. Results have profound implications for use of coal discard as an organic substrate to replace topsoil in phyto-bioremediation strategies for sustainable large-scale rehabilitation of coal discard dumps.