Reductive debromination of bromo-substituted C2 aliphatics using a biochar-iron(II) composite

BACKGROUND Tribromoethylene (TrBE), dibromoethylenes (DBEs) and 1,2-dibromoethane (DBA) are widespread, toxic and persistent contaminants found in anaerobic soil and groundwater. The abiotic debromination of TrBE, DBEs and DBA using a layered iron(II)-iron(III) hydroxide (green rust, GR) with biochars acting as electron mediators was studied. A screening of biochar substrates was conducted. Reactive biochars were chosen for debromination of TrBE, DBEs and DBA in deionized water and groundwater. Post-pyrolysis activation with CO2 was tested for reactivity enhancement. RESULTS Bone meal (BM) and shrimp shell (SS) biochars resulted in 60% and 90% debromination of TrBE, respectively. The SS biochar catalyzed debromination by elimination reactions of TrBE and DBEs resulting in formation of acetylene, while DBA was reduced to ethylene. All debrominations in deionized water followed pseudo-first-order kinetics with half-lives t(1/2) of 5.1, 1.3, 2.6 and 9.5 h for TrBE, cDBE, tDBE and DBA, respectively. For similar experiments in moderately hard groundwater, removal t(1/2) values were on average 2.6 times slower, but still considered fast. Compared to removal t(1/2) in deionized water, post-pyrolysis activation with CO2 of an acid-treated SS biochar produced a t(1/2) 11 times shorter for removal of TrBE and two times shorter for DBA. CONCLUSIONS Biochars are efficient mediators of reductive debromination of TrBE, DBEs and DBA. TrBE, DBEs and DBA can be effectively debrominated in deionized water and groundwater samples using a GR-SS biochar composite. The mediating ability of the SS biochar can be further improved by post-pyrolysis CO2 activation. (c) 2022 The Authors. Journal of Chemical Technology and Biotechnology published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry (SCI).