Investigating the effects of biochar and activated carbon amendment on the microbial community response in a volatile petroleum hydrocarbon - contaminated gravelly sand

Abstract

Laboratory batch and long-term column experiments were conducted to investigate the effects of wood-chip biochar and coal-derived activated carbon amendment on the microbiology of a volatile petroleum hydrocarbon (VPH) - contaminated gravelly sand. First, a stable isotope-labelled mono aromatic compound – toluene was used as a model VPH to gain insights into the mineralization of VPHs by soil microorganisms in the presence and absence of biochar or activated carbon. The biodegradation of a mixture of 12 VPHs was subsequently monitored in batch microcosms over a duration of 6-19 days by measuring headspace CO2 concentration. Further analysis was carried out by characterizing changes in the soil microbial community composition using next generation sequencing techniques – 454 pyrosequencing and Ion torrent sequencing. Increases in the levels of headspace CO2 in contaminated soil batches as compared to live and abiotic controls to which no VPHs were added indicated a stimulation of microbial activity in the batches through VPH addition. By fitting a maximum specific growth rate of 0.6 h-1 (in line with published rates), it was possible to match model predictions of 45CO2 and 44CO2 concentrations with the experimentally determined data. Half-saturation constants of 4.06 x 103 mgL-1, 7.76 x 102 mgL-1 and 1.83 x 102 mgL-1 were predicted for soil, soil & BC and soil & AC respectively, much higher than values reported in the literature. Differences in the half-saturation constant suggests that sorbent amendment affects the microbial ecology, by making microorganisms which can utilize substrates at lower concentrations more competitive. Yield coefficients (g biomass-C relative to g (biomass-C + CO2-C)) compared more closely in the nutrient (N & P) amended soils ranging from 4.83±0.46 in soil and biochar to 7.86±0.72 in unamended soil, than in the batches without nutrients, 4.1±3.1 in soil & BC, 17.7±5.2 in soil and 13.7±4.6 in soil & AC. Sorbent amendment thus reduced yield coefficients, thereby slowing the growth of VPH degrading biomass. Microbial community structure analysis revealed an increase in the relative abundance ranking of members of the genera Pseudomonas, Pseudoxanthomonas, and Arenimonas by up to 32 folds and in the families Nocardioidaceae and Pseudomonadaceae by at least 32 folds in sorbent amended and unamended soil batches and columns compared to their initial soil conditions. Consequently, amending soils with 2% BC or AC changed the biokinetics of VPH degradation by rendering VPHs less bioavailable, but did not appear to have any detrimental effects on the VPH degrading bacteria both in the short- and long-term, and may serve as a sustainable, cost-effective approach for enhancing the natural attenuation of VPHs in soil, thus addressing the challenge of petroleum hydrocarbon contamination.