Biostimulation enhanced the biotic degradation of hexabromocyclododecane in sediments

Abstract

The aim of this study was to investigate biodegradation of γ-hexabromocyclododecane (γ-HBCDD) under conditions mimicking three bioremediation strategies: (i) biostimulation: addition of sodium formate and ethanol to stimulate biodegradation as the carbon source and electron donor, respectively; (ii) bioaugmentation: addition of an enrichment culture of Dehalobium chlorocoercia strain DF-1; and (iii) natural attenuation: no amendments. To differentiate between biotic and abiotic mechanisms affecting γ-HBCDD degradation, four control microcosms were set up as sterile, negative, abiotic, and contaminant control. Sediment microcosms were prepared in 20-mL bottles and operated as duplicate sacrificial reactors with a sediment-to-liquid ratio of 3 g wet solid:3.5 mL liquid. Total incubation time was 36 days with sampling every 4 days, except the last day. γ-HBCDD contents of sediments were extracted using ultrasonication and analyzed using GC-MS. Four control microcosms were used to observe the effect of (i) microbial activity (sterilization with mercuric chloride and autoclaving), i.e., sterile; (ii) microbial culture without DF-1 cells, i.e., negative control; (iii) sediments, where kaolinite is used instead of sediments, i.e., abiotic control; and (iv) γ-HBCDD, where no analyte is added, i.e., contaminant control. Biostimulation showed the highest γ-HBCDD biodegradation rate (k = 0.0542 day−1) and enhanced biodegradation compared to natural attenuation (k = 0.0155 day−1). Bioaugmentation (k = 0.0123 day−1) with DF-1 strain showed a sharp decrease at the beginning, but could not maintain this trend afterwards. Paired comparison of microcosms yielded no statistically significant difference between bioaugmentation and natural attenuation; hence, DF-1 strain did not improve degradation when compared to natural attenuation. This was also substantiated by observations from the negative control set. Sterile and abiotic control sets showed no significant concentration change in time. Consequently, adsorption was not considered as a significant mechanism acting on γ-HBCDD concentration change in our sediment microcosms. Thus, γ-HBCDD decrease observed in bioremediation microcosms was attributed to microbial activity. We reported effective analyte degradation with biostimulation. This was the first study to test bioaugmentation for HBCDD degradation, but we observed no enhancement of degradation with the DF-1 strain tested. Previous studies observed HBCDD reduction in their sterilized controls, hence reported total biotic and abiotic degradation rate. In this study, comparative evaluation of three test and four control microcosms enabled identification of only anaerobic biodegradation rates for γ-HBCDD, providing useful information for bioremediation of contaminated sites.