Abstract
The biodegradation of coal discard is being intensively studied in South Africa in an effort to develop passive methods for the successful revegetation and rehabilitation of waste dumps, to mitigate pollution, and facilitate mine closure. Bacteria were isolated from slurries of coal tailings and diesel-contaminated soil, screened for coal biodegradation competence, characterized, and the colonization and degradation of coal discard and geologically weathered coal investigated using individual isolates and consortia. Ten novel coal-degrading bacterial strains were isolated and characterized, the gene sequences deposited with GenBank, and the (wild-type) strains deposited at Microbial Culture Collection, India. The results from the present work show that bituminous coal discard and geologically weathered coal is used by these isolates as carbon and energy source. Isolated strains and consortia colonized and degraded both coal substrates.Growth rate of the isolates is faster and stationery phase achieved sooner in minimal medium containing geologically weathered coal. This observation suggests that the oxygen-rich weathered coal is a more friable substrate and thus readily colonised and biodegraded. A reduction in mass of substrate is demonstrated for both individual isolates and consortia. The changes in pH and associated media colouration occurred concomitant with formation of humic acid-like (HS) and fulvic acid-like substances (FS) which is confirmed following analysis of these products by FT-IR spectroscopy. It is concluded that preferential metabolism of alkanes from the coal substrates provided the carbon and energy for bacterial growth and transformation of the substrates to HS and FS.
Highlights
Discard coal is the low-energy-value residue after excavation of bituminous hard coal and may comprise a mix of waste coal with some soil and rock which in South Africa, is typically stockpiled in dumps
Results show that Citrobacter strain ECCN 19b, isolated from coal slurry and, Exiguobacterium strain ECCN 21b and Serratia strain ECCN 24b both isolated from diesel contaminated soil, were able to grow and proliferate when either coal discard or geologically weathered coal was used as substrate (Fig. 1a, b)
Following the successful demonstration of fungal biodegradation of hard coal in our laboratories (Igbinigie et al 2008; Sekhohola et al 2014), it seemed pertinent to bio-prospect for bacteria with similar coal degrading attributes
Summary
Discard coal is the low-energy-value residue after excavation of bituminous hard coal and may comprise a mix of waste coal with some soil and rock (i.e. spoil) which in South Africa, is typically stockpiled in dumps. Biological rehabilitation has become the strategy of choice and is considered to be economic, safe, more energy efficient and environmentally compatible, and potentially capable of providing a means to convert coal discard to ash and sulphurfree products of value (Reich-Walber et al 1997; Klein et al 2001; Hofrichter and Fakoussa 2001; Machnikowska et al 2002; Sekhohola et al 2013; Cowan et al 2016). At about the same time it was reported that wood-rot fungi quantitatively solubilised the low-rank coal, Leonardite (Cohen and Gabriele 1982). This was followed by confirmation of the microbial degradation of coal by a wide variety of biocatalysts including: Streptomyces viridosporous (Strandberg and Lewis 1987), Streptomyces flavovirens (Moolick et al 1989), Penicillium sp. While the detail of mechanisms involved in biodegradation of coal are still to be elucidated, oxidation of sub-bituminous coal by Pseudomonas fluorescens (Hazrin-Chong et al 2014), solubilisation of brown coal by Gordonia alkanivorans and Bacillus mycoides (Romanowska et al 2015), the production of humic substances from low-rank coal by strains of Bacillus mycoides, Microbacterium sp., Acinetobacter baumannii, and Enterobacter aerogenes (Valero et al 2014) together with release from liquefied coal of humic and fulvic acids by a novel recombinant Fusarium oxysporum laccase (Kwiatos et al 2018) and other coal biosolubilization mediators (David et al 2017), illustrate the potential to use microbes in developing sustainable rehabilitation strategies
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