Study On Enhanced Anaerobic Removal Of Nitro Compounds By Biochar Stabilized Sulfide-modified Nanoscale Zero-valent Iron

Due to the highly recalcitrant and toxicological nature of nitro compounds(NACs),conventional anaerobic bioprocess is usually limited by low removal rate,poor stability and weak anti-shock loading capacity.Thus,it is of great theoretical and practical significance to develop the treatment for NACs wastewater with low cost,high efficiency and environmental friendliness.In this study,nitrobenzene(NB)was selected as a model pollutant because it is the most widely used NACs.A sulfide-modified nanoscale zero-valent iron(S-n ZVI)coupled anaerobic system was developed for efficient NB reduction.Through immobilization technology,biochar(BC)stabilized sulfide-modified nanoscale zero-valent iron(S-n ZVI@BC)were designed and manufactured in the aim of optimizing the reduction performance of S-n ZVI.Through the investigation of stability and economy,S-n ZVI@BC coupled anaerobic biological system for efficient NB removal was developed to provide technical guidance for NACs pollution control.A S-nZVI coupled anaerobic system was developed for the enhanced reduction of NB through the optimization of S/Fe molar ratio.The results indicated that the performance and stability of the S-n ZVI coupled anaerobic system for NB removal were remarkably improved as S/Fe molar ratio of 0.3(0.3S-n ZVI).The secretion of extracellular polymeric substances(EPS),transformation of volatile fatty acids(VFAs),yield of methane and activity of key enzymes could be efficiently improved by 0.3S-n ZVI.Furthermore,high-throughput sequencing analysis suggested that the enhanced reduction of NB in the 0.3S-n ZVI coupled anaerobic system could be attributed to the improved biodiversity and the enrichment of species related to reduction(Desulfovibrio,Caldisericum),fermentation(Magnetospirillum,Longilinea),electroactivity(Phaselicystis)and methanogenesis(Methanosphaerula,Methanomassiliicoccus,Methanosarcina,Methanomethylovorans).S-nZVI@BC was fabricated via a one-step liquid phase reducing method.S-nZVI@BC had a clearly uniform core/shell structure with ?-Fe as the core and amorphous Fe S as the shell.In addition,S-n ZVI particles were well-dispersed on the BC surface as well as in the channels.After the surface modification,Fe S shell with good electrical conductivity could protect the n ZVI core for the prevention of rapid passivation,and BC with stable structure could alleviate n ZVI agglomeration for the enhancement of reducibility.As compared to S-n ZVI and blank BC,S-n ZVI@BC processed high reducibility with NB removal efficiency of 100% and aniline(AN)formation efficiency of 84.6±1.7%.Furthermore,a possible reaction mechanism for enhanced NB removal by S-n ZVI@BC was proposed,including chemical adsorption of NB onto S-n ZVI@BC,direct reduction by S-n ZVI and enhanced electron transfer by BC.S-n ZVI@BC were adopted for coupled anaerobic system with continuously operation for 96 d to systematically investigate the feasibility of the enhanced reduction of NB.The results demonstrated that both NB reduction and AN formation could be substantially facilitated in the S-n ZVI@BC coupled anaerobic system compared to other anaerobic ones coupled with n ZVI or S-n ZVI.The presence of S-n ZVI@BC resulted in the formation of densely packed aggregates,evidently increased the EPS content,promoted the VFAs transformation and stimulated the methane yield.Furthermore,species related to reduction(Denitrovibrio),fermentation(Bacteroides,Longilinea),methanogenesis(Methanosarcina,Methanomethylovorans),as well as electroactivity(Phaselicystis,Pelobacter,Thiobactillus)were considerably enriched in the S-n ZVI@BC coupled anaerobic system.In addition,Sn ZVI@BC could stimulate enzymatic activities closely related to microbial activity and NB transformation,resulting in the improvement of NB removal efficiency and the reduction of operational cost.The observed efficient and stable performance highlights the potential for long-term operation and full-scale application of the S-n ZVI@BC coupled anaerobic system,particularly for highly recalcitrant NACs removal.In order to further improve the reduction performance of S-nZVI@BC,BC was modified before the loading of S-n ZVI through three methods,namely oxidant(H2O2)pretreatment,alkali(Na OH)pretreatment and acid(HCl)pretreatment.Modified biochars supported S-n ZVI(S-n ZVI@m BC),i.e.,S-n ZVI@H2O2-BC,S-n ZVI@Na OH-BC and S-n ZVI@HCl-BC,were successfully synthesized and their reducibility for NB removal were compared.The results indicated that NB reduction and AN formation performance followed the order: S-n ZVI@HClBC > S-n ZVI@Na OH-BC > S-n ZVI@BC > S-n ZVI@H2O2-BC.S-n ZVI could be evenly distributed onto HCl-BC,due to the increased surface area,negative surface charge and increased acidic functional groups on HCl-BC.The elevated electron transfer efficiency between S-n ZVI and NB was achieved with more C=O and-COOH groups on HCl-BC,which was rather crucial for the enhancement of NB reduction by S-n ZVI@HCl-BC.