Biodegradation of Sorbed 2,4-Dinitrotoluene in a Clay-Rich, Aggregated Porous Medium

The availability of clay-sorbed 2,4-dinitrotoluene (2,4-DNT) for degradation by Burkholderia sp. strain DNT was investigated in column experiments. Artificial aggregates of montmorillonite glued to glass spheres served as the sorbent. Sorption isotherms and bacterial kinetic parameters were determined in batches. Sorption of 2,4DNT to clay aggregates gave reasonable fit to the Langmuir equation. The degradative activity of Burkholderia sp. strain DNT followed Michaelis-Menten kinetics. This allowed inferring bioavailable concentrations in the presence of clay from degradation rates. It appeared that montmorillonitesorbed 2,4-DNT was readily available to Burkholderia sp. strain DNT. However, despite the accumulation of biomass in the columns due to filtration, absolute degradation rates remained constant, and specific rates continuously decreased toward the end of the experiments. Removal of suspended cells by miscible displacement led to a drastically reduced degradation rate that was not due to decreasing desorption, as 2,4-DNT concentrations in column effluents increased simultaneously. Decreasing degradation could be explained fairly well assuming that the specific activity of suspended cells remained at the initial value of 0.93 nmol mg dw-1 min-1, whereas the specific activity of adhered bacteria steadily dropped to 0.12 nmol mg dw-1 min-1. A likely explanation is the prolonged exposure (up to 6 h) to 2,4-DNT and nitrite for adhered cells, compared with a maximum exposure for suspended cells of 19.5 min, i.e., their residence time in the column. According to the Michaelis-Menten equation, the initial activity corresponded to a bioavailable concentration that exceeded the aqueous equilibrium concentration in the absence of bacteria by a factor of roughly two. The most probable explanation is a shift of the sorption equilibrium in the presence of cells, as direct accessibility of sorbed 2,4-DNT for suspended cells can be excluded.