Enhanced Biodegradation Performance And Mechanism Of Cyclic Compounds In Coal Pyrolysis Wastewater By Lignite Activated Coke

Abundant inferior coal resource can be transformed into coal-based solid,liquid and gas products with high value-added by low-rank coal pyrolysis technology.However,the coal pyrolysis wastewater(CPW)generated during the process is a kind of highly toxic wastewater which is very difficult to degrade,and the near-zero liquid discharge of CPW becomes the bottleneck of sustainable development of coal pyrolysis industry.The cyclic compounds in CPW is the key factor lead to poor degradation performance,high biological toxicity,which restrict the near-zero discharge of wastewater.In order to reduce the biological toxicity,and considering in-situ utilization of industrial products,coal-based carbon like lignite activated coke(LAC)and nano-Fe₃O₄ loaded activated coke(Nano-Fe₃O₄@LAC)were studied to enhance the performance and mechanism of biodegradation of cyclic compounds in CPW under micro-aerobic environment.For typical cyclic compounds in CPW,the single component toxicity and joint toxicity were analyzed to reveal the toxicity mode of action.Phenolic compounds was the main source of acute toxicity,which could be easily activated and cause irreversible damage by rapidly enhancing cell oxidative stress.Nitrogen heterocyclic compounds(NHCs)and polycyclic aromatic hydrocarbons(PAHs)presented reversible baseline toxicity.Most of the cyclic compounds in CPW were narcotic compounds,while polyhydroxyphenols were the main reactive toxicants.The joint toxicity of phenols,NHCs and PAHs was first dominated by the baseline toxicity of hydrophobic substances,and then the irreversible acute toxicity of phenolic compounds,increasing with time.The adsorption performance of activated coke materials for cyclic compounds in CPW,as well as the detoxification mechanism during adsorption was discussed.LAC exhibted the best adsorption capacity for cyclic compounds,and the removal rates of chemical oxygen demand(COD)and total phenols(Tph)in 48 h were39.07%and 44.06%,respectively.Compared with activated carbon(AC),LAC with more mesoporous and oxygen-containing functional groups could selectively adsorbes methyl phenols,NHCs and other toxic and refractory organic compounds.Because of the advantages of mesoporous and positive charge,nano-Fe₃O₄@LAC exerted stronger selective adsorption for macromolecular PAHs and electronegative NHCs.By adsorbing and removing phenols and hydroxyl activated organic compounds,activated coke materials reduced reactive toxici ty of CPW.Meanwhile,activated coke materials also reduced the initial joint toxicity by selectively adsorbing fused ring NHCs and PAHs.In addition,the adsorption sites of activated coke materials were easy to regenerate in situ and could couple with mi croorganisms to achieve resilient adsorption.The performance and mechanism of LAC enhanced biodegradation of cyclic compounds under micro-aerobic environment were studied.Because of the acute toxicity of phenols,the concentration of phenols showed a significant effect on the biodegradation of cyclic compounds.The tolerance threshold of micro-aerobic(DO=1.0-2.0 mg/L)activated sludge system is the lowest(200 mg/L).NHCs and PAHs synergistically inhibited the degradation of phenols.Under low DO(0.3-0.5mg/L)environment,LAC could significantly reduce the toxic pressure of phenols,and the increase of hydraulic retention time(HRT)can also improve the degradation performance under high phenolic loading.When HRT=48 h and Tph=500 mg/L,the enhanced system exerted the best degradation performance,the toxicity was reduced by 87.65%,and the biodegradability was kept at 0.24.LAC promoted the biomass growth,floc aggregation and stability of activated sludge via adsorption detoxification.Under high DO environment,phenols and other toxic substances destroyed the microbial community of activated sludge,seriously reduced the species diversity,and made microbes highly concentrated in Thauera(49.90%),Zoogloea(6.40%),Pseudoxanthomonas(5.67%)and other bacteria with strong phenolic tolerance.Under low DO condition,by selective adsorption to remove some toxic and refractory organics,LAC promoted the microbial diversity and caused distribution of microbes more evenly.Moreover,facultative anaerobic bacteria such as Alicycliphilus(19.05%)and Acidovorax(7.13%)were the core microbes,and the unique electroactive bacteria such as Ignavibacterium(3.07%)were enriched,which strengthened the interspecific cooperation of microbial community.Adsorption contributed only 20-30%to cyclic compounds removal,which was mainly determined by biodegradation,and meta-cleavage was the main way of phenolic degradation.Based on the enhanced dissimilatory microbial iron reduction(DIR),nano-Fe₃O₄@LAC played an important role for electron transfer,and thus the performance and mechanism of enhanced biodegradation of cyclic compounds by nano-Fe₃O₄@LAC were also explored.The effects of phenolic concentration,coke dosage and HRT on the removal rate of COD and Tph of enhanced system(FEBS)was little,but it could significantly reduce the toxicity and improve the biodegradability of CPW,especially the reactor with 2 g/L nano-Fe₃O₄@LAC,whose effluent dispalyed nontoxicity and high biodegradability(BOD ₅/COD=0.55),relating to its advantage of degrading NHCs.The result was very critical for refractory wastewater treatment.It revealed that nano-Fe₃O₄@LAC enhanced activated sludge characteristics by improving its activity,strength and stability.Meanwhile,nano-Fe₃O₄@LAC also promoted the distribution of Alicycliphilus in suspended sludge which specifically degraded cyclic compounds.As for biofilm on the activated coke materials,Syntrophohabdus,Geobacter and Ignavibacterium were enriched.The syntrophic metabolism between these bacteria promoted the ring-cleavage of refractory cyclic compounds adsorbed on the nano-Fe₃O₄@LAC and released small molecular acids into the suspended sludge to increase biodegradability of wastewater.Electroactive bacteria which enriched by Fe₃O₄relied on iron redox to accelerate the DIR process and drove the ring-cleavage reactions of cyclic compounds.Metagenomics analysis showed that facultative anaerobes were the dominant species in activated sludge,and Alicycliphilus was the core genus of microbial community.Catechol 1,2-dioxygenase and catechol 2,3-dioxygenase dominated the benzene ring-cleavage reaction.Comamonas testosteroni involved into most degradation processes of cyclic compounds.It participated in the anaerobic and aerobic ring-cleavage reaction of benzene,and dominated the microbial community for nitrogen heterocycle ring-cleavage,mainly by anaerobic way.Azotobacter closely cooperated with Alicycliphilus and Acidovorax in organic and inorganic nitrogen metabolism,and they could metabolize ammonia nitrogen that produced during the degradation process of NHCs.Through DIR process,Ignavibacterium genus eriched on biofilm and suspended sludge promoted the electron transfer for ring-cleavage reaction.Therefore,nano-Fe₃O₄@LAC enhanced the biodegradation of cyclic compounds indirectly by promoting the DIR process.