THE GEOCHEMICAL EFFECTS OF BENZENE, TOLUENE, AND XYLENE (BTX) BIODEGRADATION

The geochemical effects of microbially mediated degradation of aromati c hydrocarbons were observed as changes in solution composition of an artificial groundwater in packed-sand laboratory columns. Benzene, tol uene, and xylene, both individually and in a combined fashion, were us ed as substrates in biodegradation experiments conducted under oxygena ted and anoxic conditions in columns filled with quartz, calcite, or F e3+-coated quartz sand. Typically, column effluent had increased conce ntrations of dissolved inorganic C, decreased pH, and decreased concen trations of NO3 and dissolved O-2 relative to column influent. Efficie ncy of CO2 generation was similar for the three different substrates, ranging from 22.5 to 26.6% organic C converted to CO2. When all three substrates were combined, the percentage of CO2 produced fell within t he range observed in the single substrate experiments. Nitrate disappe arance was more varied as a function of substrate identity, with great est amounts lost when toluene was the substrate. Calcite dissolved as a result of CO2 generated during the biodegradation reactions, and emp irically calculated dissolution rates varied between 1.9 and 4.0 x 10( -9) mmol cm(-2) s(-1). The calcite dissolution rate was slower than th e biodegradation rate, as evidenced by excess generation of CO2 relati ve to Ca2+ production. The decrease in pH was less in experiments with calcite present than in those with quartz sand present due to bufferi ng by calcite dissolution. Dissolution of Fe oxyhydroxides was not obs erved under any experimental conditions. (C) 1997 Elsevier Science Ltd .