Methane oxidation and microbial exopolymer production in landfill cover soil

Abstract

In laboratory simulations of methane oxidation in landfill cover soil, methane consumption consistently increased to a peak value and then declined to a lower steady-state value. It was hypothesized that a gradual accumulation of exopolymeric substances (EPS) contributed to decreased methane uptake by clogging soil pores or limiting gas diffusion. This study was conducted to detect and quantify EPS in soil from columns sparged with synthetic landfill gas and from fresh landfill cover cores. Polysaccharide accumulations were detected with alcian blue stain. EPS was observed adhering to soil particles and as strands associated with, but separate from soil grains. Glucose concentrations in laboratory soil columns averaged 426 mg kg −1 dry soil, while in a column sparged with air the average glucose concentration in a horizon was 3.2 mg glucose kg −1 dry soil. Average glucose concentrations in two of four cores sampled from a closed landfill ranged from 600–1100 mg kg −1 dry soil, while control cores averaged 38 mg glucose kg −1 dry soil. Viscosity due to EPS was measured by comparing filtration rates of soil suspensions. Soil extracts from the upper horizons of laboratory columns sparged with landfill gas filtered at about one-third the rate of extracts from the lower horizons, and the landfill core with the highest glucose content also produced highly viscous extracts. Breakthrough curves measured in columns before and after methane exposure were similar, so that short-circuiting due to clogging was not occurring. The data support the hypothesis that EPS impeded oxygen diffusion to an active biofilm and limited the extent of methane oxidation.