Bioenhancement Mechanism Of Biological Sponge Iron System In The Treatment Of Refractory Organic Matter

The application of zero-valent iron coupled with microorganisms（bio-ZVI systems）,has gained considerable attention in recent years for eliminating recalcitrant organic matters from wastewater.However,most of the current research focuses on the degradation of pollutants and the optimization of operating parameters.There is insufficient understanding of the formation of enhanced microorganisms,the interaction between zero-valent iron and microorganisms,and the degradation mechanism.The large-scale application of this technology brings certain limitations.In this paper,sponge iron（SFe,a special zero-valent iron）with strong redox performance,high specific surface energy,and large specific surface area was used.SFe was coupled with microorganisms to form the special bio-ZVI systems,named the biological sponge iron system（SFe-M）.A variety of refractory organic wastewater was treated by the SFe-M system,and the treatment effect and microbial structure characteristics were preliminarily analyzed.Then the influence of SFe dosage on the treatment effect and Fenton-like effect during the long-term operation were investigated with ciprofloxacin（CIP）as a pollutant.On this basis,the structural characteristics of microorganisms in the SFe-M system at different operating stages were explored to reveal the evolution characteristics of enhanced microorganisms.Afterwards,the SFe in the SFe-M system was removed.The operation effect and microbial community changes of the domesticated strengthened microorganisms were investigated.In addition,the tafel curve was used to express the corrosion characteristics of SFe in the SFe-M system.At the same time,the relationship between microorganisms,Fe（Ⅱ）,and ROS during the degradation of pollutants were analyzed.To ascertain the relationship between microorganisms and Fenton-like in the system.Finally,the contribution of each degradation force in the process of degrading pollutants was determined through parallel comparative experiments.And the CIP degradation intermediate was measured.The strengthen degradation mechanism of the SFe-M system for the organic matter was further discussed.The research results enrich the research content of the degradation mechanism of the bio-ZVI systems,and provide data reference and technical support for the promotion of the bio-ZVI systems in the treatment of refractory organic wastewater.The main conclusions of the paper are as follows:（1）Compared with conventional SBR,the removal rate of SFe-M system for aniline wastewater,nitrobenzene wastewater,CIP wastewater,and actual acrylic fiber wastewater was significantly improved.The SFe-M system has wide adaptability to the treatment of refractory organic wastewater.The presence of SFe promoted the growth of some microorganisms in the treatment of nitrobenzene wastewater and actual acrylic fiber wastewater.The existence of SFe was conducive to the screening and enrichment of obligate degrading bacteria.（2）When the dosage of SFe in the SFe-M system was 90 g/L,the stability and degradation effect on COD and CIP was the best.The Fenton-like effect in the SFe-M system originated from the mutual promotion of microorganisms and SFe.And the iron dose significantly affected the Fenton-like effect.When the dosage of SFe was 45 g/L,the Fenton-like effect of the SFe-M system was far less than that of 90 g/L and 135 g/L.However,when the iron dose was increased to 135 g/L,the Fenton-like effect of the system could not be significantly improved.Therefore,90 g/L of SFe can promote Fenton-like more economically and effectively.（3）The richness and diversity of the microbial community structure in the SFe-M system both showed the maximum except at the stage of CIP concentration of 0.5 mg/L.At the phylum level,there was no obvious difference in each reactor at different stages.Proteobacteria and Bacteroidetes were inhibited at the stage of 0.5 mg/L,while Planctomycetes and Chloroflexi began to enrich when the concentration of CIP was 3 mg/L.At the genus level,the community structure of SFe-M system and SFe system were similar,and SFe promoted the enrichment of Zoogloea and Tessaracoccus at the stage of 0.5 mg/L.At the stage of 1 mg/L,SFe strengthened the tolerance of Saccharimonadales to CIP and promoted the enrichment of Thermomonas and Berkelbacteria.At the stage of 3 mg/L,the difference of microbial community structure between the SFe-M system and the SFe system increased.Nakamurella,Terrimonas,Silanimonas,Arenimonas were the main contributors to the enhanced degradation of organic matter in the SFe-M system.At the stage of 5 mg/L,the dominant bacteria in the SFe-M system were Nakamurella,Reyranella,Pe M15.Among them,the enrichment of Reyranella reflected the integrity of the microbial structure of the SFe-M system,while Pe M15 indirectly reflected the existence of the hypoxic zone.These microorganisms with different functions and characteristics made the community structure of the SFe-M system more complete and diversified.（4）The microbial structure of the SFe-M system changed significantly during the reactor operation.At the phylum level,the SFe-M system was mainly composed of Proteobacteria,Actinobacteria,Patescibacteria,Bacteroidetes,Chloroflexi.In the four stages of operation,Proteobacteria was gradually transformed into enrichment due to initial inhibition,Chloroflexi was gradually enriched under the strengthening of SFe,and Bacteroidetes was gradually eliminated.At the genus level,the SFe-M system formed a unique microbial structure at different stages with the continuous evolution of microorganisms.In the stable period,nearly 60%of the microorganisms in the initial seed sludge were enriched to form an enhanced microbial system,such as Tetrasphaera,Nitrospira,Azospira,Thermomonas.In different operating stages,the dominant bacteria showed differences,which reflected the microbial evolution process in the SFe-M system.LFf Se analysis showed that the number of significantly different species in the SFe-M system decreased in the initial stage（0.5 mg/L,1mg/L）,increased in the later period（3 mg/L,5 mg/L）.The most contributing species for pollutants degradation at the stage of 0.5 mg/L,1 mg/L,3 mg/L,5 mg/L were Saccharimonadia（c）,Propionibacteriaceae（f）,Gammaproteobacteria（c）,Alphaproteobacteria（c）,respectively.（5）The strengthened microorganisms in the SFe-M system had a strong dependence on SFe.The stability of the sludge structure deteriorated after removing SFe.After 35 d,the CIP degradation rate was close to 0,the Fenton-like effect disappeared,and the microbial metabolism ability decreased.There was no doubt that the richness and diversity of the microbial community structure were significantly reduced.The phylum was still composed of Proteobacteria,Actinobacteria,Patescibacteria,Bacteroidetes,Chloroflexi,and Acidobacteria.Although the phylum-level species did not change significantly,the relative abundance of the original dominant bacteria decreased gradually as the reaction progressed.At the genus level,new resistant bacteria（such as Acinetobacter,Microtrichaceae）appeared on the 10 th and35th days.However,they could not exist stably for a long time.Therefore,the effective operation of the enhanced microorganisms could only be maintained for about 35 d after leaving the SFe.（6）In the SFe-M system,the self-corrosion tendency of SFe gradually increased.From 0d to 35 d,SFe corrosion changed from cathodic diffusion to cathodic diffusion and anode activation.The corrosion rate of SFe increased from 0.066 mm/a to 12.633 mm/a,which ensured the continuous release of Fe（Ⅱ）in the system.Interestingly,·OH and Fe（Ⅱ）had almost the same change trend.The average yield of H₂O₂ in the SFe-M system with enhanced microbial structure is 3 times of the sum of the SFe-M system with ordinary activated sludge（2#ordinary activated sludge+SFe）and the simple SFe system（3#SFe）.The continuous release of Fe（Ⅱ）was affected by the microorganisms structural in the system.It can be seen that the synergy of microorganisms,Fe（Ⅱ）and Fenton-like effects contributed the pollutants degradation,and the three were always in dynamic equilibrium.The strengthening factors of2#ordinary activated sludge+SFe and 5#SFe-domesticated sludge+SFe were 1.04 and 2.98respectively.In other words,there was an obvious coupling effect between SFe and microorganisms,and the domesticated microorganisms have a stronger synergistic effect with SFe.The piperazine ring,C-F bond,cyclopropane ring,carboxyl group on the CIP and the intermediate products were attacked by·OH or metabolized by microorganisms,and were completely removed through four degradation pathways.Among them,the contribution of microbial metabolism accounted for 82.4%,microbial Fenton-like effect accounted for13.1%,SFe/O₂ Fenton-like effect accounted for 3.3%.