Mechanistic insights into collaborative removal of NOx and chromium from flue gas in denitrifying biotrickling filter

Nitrogen oxides (NOx) and chromium can cause harm to human health and ecological environment. An aerobic denitrifying biotrickling filter (DBTF) for simultaneous removal of NOx and chromium from flue gas was investigated. NOx and chromium removal efficiency could reach up to 98 % and 88 %, respectively. Halomonas, Stappia, Pseudomonas and UG_Peptococcaceae were the core chromium-resistance- denitrifying genus. Antioxidative stress proteins (RpoS, SOD, OxyR and catalase) and anti-heavy metal proteins (MntA, FbpA, TorB and ChrE etc.) and the denitrifying enzymes (NarH, NarI, NapA, NapB, NirK, NirS and NorB) involved in the regulation of chromium stress and the reduction of NOx, as shown by 16 S rDNA, metagenomic and metaproteomic sequencing. The biofilm was characterized by FTIR, XPS, EEM, and SEM-EDS. XPS spectra indicated the formation of a chromium carbonate by microbial induced carbonate precipitation. Chromium speciation in biofilm mainly consisted of 52.8 % organic matter bound state, 35.1 % carbonate bound state and 10.7 % exchangeable state. Flue gas chromium could be removed in the form of chromium carbonate bioprecipitate and HA-Cr via complexation of humic acids in extracellular polymeric substances (EPS), biosorption and biodeposition. DBTF was capable of NOx reduction and chromium removal by denitrification. The results provide a new potential for the collaborative control of NOx and heavy metals from flue gas by the DBTF.