CHARACTERIZING ANAEROBIC BIODEGRADATION OF GASOLINE WITH LOW MOLECULAR WEIGHT ORGANIC ACID ANIONS AND PHOSPHATE AMENDMENT

Abstract

Bioremediation is a feasible method to clean up petroleum hydrocarbons (PHC) contaminated soil. But bioremediation is often limited by nutrient bioavailability, PHC bioaccessibility, microbial activity and environmental factors. It was hypothesized low molecular weight organic acid anions (LMOAA) can enhance PHC biodegradation by increasing PHC bioavailability, increasing phosphorus (P) bioavailability, or stimulating hydrocarbon degrader community. A microcosm study and a field study were conducted to evaluate this hypothesis. In the microcosm study, 10-100 mM citrate increased petroleum hydrocarbon bioavailability to enhance in situ anaerobic gasoline degradation. Lower citrate addition, 1.0 mM and 1.75 mM, accelerated ex situ biodegradation for benzene and gasoline respectively, probably through changing P bioavailability. In the field, two large bore injectors were constructed in a gasoline contaminated cold region calcareous site, for in situ biostimulation solution delivery. Two biostimulation solutions were applied. The first solution containing 11 mM MgSO4, 1 mM H3PO4, and 0.08 mM HNO3 (phosphate amendment) stimulated the site for about 4 months. Then, 10 mM citric acid was incorporated into the existing biostimulation solution (phosphate and citrate amendment) for another 8 months. Dissolved P in groundwater and bioavailable organic P in soil were increased after citrate addition, which corresponded to the decrease for benzene, toluene, ethylbenzene, and xylene (BTEX) in groundwater and F1-BTEX (C6-C10 with BTEX subtracted) decrease in soil. Citrate addition also increased benzoate degradation N (bzdN, encoding ATP-dependent benzoyl-CoA reductase subunit N) gene prevalence and culturable anaerobic PHC degraders population in soil. After applying phosphate amendment, the bacterial community structure changed in both soil and groundwater. However, the influence of citrate on microbial community differed between soil and groundwater. Citrate selectively stimulated anaerobic hydrocarbon degraders in groundwater, and reversed soil bacterial community structure, which corresponded to the rebounded adsorbed phosphate. In addition, a method trapping 13CO2 produced from 13C-labelled contaminates was developed and successfully assessed PHC mineralization rate using the cavity ring-down spectrometry. This research highlights the positive effect of LMOAA on anaerobic PHC biodegradation through increasing PHC/P bioavailability and stimulating PHC degraders.