Carbon stable isotope analysis in groundwater remediation – The role of microbial biofilm communities and biochar in PCE and NAP degradation

Biochar as an alternative filter to activated charcoal was tested for the removal of tetrachloroethene (PCE) and Naphthalene (NAP) from contaminated groundwater by means of sorption and biodegradation in batch and column experiments. Microbial communities were extracted from the aqueous and solid phases and analysed using 16S rRNA gene amplicon sequencing. Microbial biomass and its carbon isotope composition were determined using microbial phospholipid fatty acids (¹³C-PLFA) analysis. This approach enabled a quantitative and functional observation of the microbial community besides identifying the relevant bacteria.

Molecular biological analyses of the PCE experiments confirmed that organo-halide respiration bacteria (OHRB) established after inoculation both in the batch and throughout the columns. PLFA analysis revealed that microorganisms and also those groups that can be assigned to the PCE-degrading organisms preferentially colonize biochar, while activated charcoal is avoided possibly due to the higher PCE sorption capacity of the activated charcoal and hence lower bioavailability of PCE. The carbon isotope value of the microorganisms (¹³C PLFA) indicates the use of biochar as a carbon source and/or the presence of strongly isotope-fractionating biochemical processes such as methanogenesis / methane oxidation. The microbial communities were influenced by the factor char and its physical/chemical properties. It is therefore advisable to choose the filter material not only on the basis of the sorption capacity, but above all on the synergy effects that leads to a permanently active microbial community and an extension of the filter life due to the continuous and complete degradation.

The living microbial biomass in the aerobic naphthalene columns was a factor of 4 higher than in the anaerobic PCE columns. The distribution of microorganisms was similar to that in the PCE degradation experiments; i.e. greater colonization of the biochar filters compared to the activated charcoal filters. Furthermore, the microorganisms responded to naphthalene supply with increased microbial biomass and naphthalene incorporation. Once the naphthalene addition was stopped, the microorganisms were able to convert sorbed naphthalene (in the presence of ethanol). This ability is a strong indicator of the sustainability and self-cleaning potential of the colonized biochars. However, high levels of colonization and biofilm development may increase the risk of clogging negatively impacting filter system sustainability. The insights gained from this study are crucial for advancing global efforts in groundwater remediation and sustainable environmental management.