Enhanced trichloroethylene biodegradation: Roles of biochar-microbial collaboration beyond adsorption

Trichloroethylene (TCE) is a pollutant widely found in groundwater, especially in the heavily contaminated industrial sites. Biological dechlorination method is environmentally friendly and low-cost. However, microorganisms grow slowly and their activity is susceptible to environmental fluctuations. This study used biochar as an additive to promote anaerobic biodegradation of TCE with mixed culture. Results showed that biochar with dose of 0.1-0.4% (w/v) brought a rapid initial decrease of TCE concentration by 39.4-88.8% in 24 h via adsorption mechanism. Biochar produced at 500 °C pyrolysis temperature (BC500) achieved the highest TCE adsorption in comparison to BC300 and BC700. Subsequently, a significantly shortened microbial stagnation phase (from 85 h to 37 h) was observed in the system with the presence of biochar. During the exponential growth phase, BC700 outperformed BC300 and BC500 in terms of TCE degradation efficiency. Electrochemical analysis demonstrated that BC700 possessed the greatest electron transfer capability. Finally, biochar shortened the time for achieving 100% removal of TCE by 54.5-69.7% (from approximate 330 h to 100-150 h). Even at high concentration of TCE (20-30 mg·L^-1) that could lead to serious microbial growth inhibition, the TCE degradation efficiency could be recovered in the presence of BC500. The high-throughput sequencing data revealed that biochar promoted the relative abundance of co-metabolizing dechlorinating microorganisms (Pseudomonas, Burkholderia) in the aqueous solution, and simultaneously led to the selective colonization of reductive dechlorinating microorganisms (Enterobacteriaceae, Clostridium) attached on biochar surface. On the other hand, biochar addition decreased the relative abundance of hydrogen-competing microorganisms, thereby forming an efficient co-metabolism-reductive dechlorination system. These findings allow a better understanding of the promotion mechanism of biochar for microbial dechlorination technology supporting the biochar-assisted bioremediation in practice.