Enhanced bioremediation of TCE-contaminated groundwater using gamma poly-glutamic acid as the primary substrate

In situ bioremediation has been applied to remediate trichloroethylene (TCE)-contaminated groundwater, and primary substrates need to be supplied to enhance the reductive dechlorination. Gamma poly-glutamic acid (γ-PGA) is a natural occurring and biodegradable biopolymer, which consists of glutamic acid monomers crosslinked between γ-carboxyl and α-amino functional groups. Microcosm experiments were performed to assess the feasibility and effectiveness of TCE dechlorination using γ-PGA as the primary substrate. The dominant dechlorinating bacteria and bacterial communities were determined using nucleotide sequence analyses and 16S rRNA-based denatured-gradient-gel electrophoresis profiling. The variations in Dehalococcoides spp. (DHC) were evaluated using the real-time polymerase chain reaction technique. Results show that γ-PGA supplement created anaerobic conditions and resulted in significant increase of total organic carbon [1930 mg/L (1 mL γ-PGA in 30 mL groundwater)]. About 99% of TCE (initial concentration = 4.3 mg/L) was degraded after 85 days and acidification was not observed because the released amine from γ-PGA could react with water and form ammonia, which resulted in alkaline conditions due to its basic characteristic. Solution pH was maintained neutral and benefited the growth of DHC. The supplement of γ-PGA enhanced the growth of DHC from 1.03 × 104 to 2.6 × 106 gene copies/g soil, and the TCE dechlorination rate was enhanced. The increase in ethene concentrations indicated the complete TCE dechlorination with γ-PGA addition. γ-PGA globule (with a zeta potential of −19.8 mv) had three major diameters in micro to nano-scale ranges. This allowed γ-PGA to migrate into different soil formations and distribute within soil pores evenly. Results suggest that γ-PGA can be used as the carbon substrate to enhance the efficiency of reductive dechlorination of chlorinated compounds.