| Solid phase carbon source denitrification technology has been proved to be a promising method for nitrate removal from waterbody with low C/N ratio.Most of the current studies focus on the effect and screening of external reinforcement of carbon sources,but the specific process and mechanism of denitrification were not very clear.In this paper,cellulose,starch,chitin,polybutylene succinate(PBS),polycaprolactone(PCL)and polylactic acid(PLA)were used as carbon source substrates to investigate the denitrification performance and microbial community composition.Sequential batch experiments were used to investigate the carbon release process and the subsequent denitrification process of hydrolysate.In addition,the denitrification effects of the denitrification effects of hydrolysate DOM and the basic molecules of corresponding polymers were compared,so as to analyze the mechanism of different solid carbon sources producing different denitrification effects.The results showed that the content and composition of dissolved organic matter(DOM)released by solid carbon source had a high correlation with the denitrification effect.Among them,chitin could release enough DOM which was easy to be utilized by microorganisms and had a certain chemical reduction effect,thus the denitrification rate was the fastest.The denitrification rates of cellulose and starch were similar,but the total organic carbon(TOC)content released by cellulose was lower,and the fitting coefficient of the Monod kinetics equation was lower,thus the denitrification process was limited by the carbon source concentration.There was no significant difference in the carbon release of artificial materials compared with natural materials,but the overall denitrification effect of artificial materials was weaker than that of natural materials,which may be related to the relatively high proportion of components in DOM that were not easy to be utilized by microorganisms.The results of microbial community analysis showed that different types of solid carbon sources would affect the composition of microbial structure in the system,but the overall similarity was still high.Among them,the main denitrification species was gamma-Proteobacteria,and natural materials contained more major denitrifying microbial species than artificial materials at the generic level.Chitin because of a certain antibacterial and derived from the microbial category of other systems,but still has a rich denitrifying bacteria.The results showed that the total carbon release of cellulose,starch and PCL was relatively high,but the carbon release amount was not the only determining factor affecting the reaction process,which together with DOM components determined the bioavailability of carbon sources.The addition of hydrolytic enzymes could accelerate the initial carbon release process of each carbon source and slightly add to the total carbon release.At the same time,the addition of hydrolase could overcome the restriction of available DOM concentration on denitrification rate when cellulose,PBS and PLA were carbon sources.The hydrolysis of starch,chitin,PCL and PLA showed a high rate of nitrate degradation,while the accumulation of nitrite in the hydrolysate of cellulose,starch and PBS was obvious.The reason was that the hydrolysate of chitin,PCL and PLA contained more DOM available to microorganisms,while the hydrolysate of cellulose and PBS was relatively short of such DOM.DOM in starch hydrolysate would further decompose and reduce the p H value of solution,which would affect the removal of nitrite and lead to the accumulation of nitrite.The denitrification effect of the basic molecules corresponding to the solid carbon source was explored in this paper.When the monomer molecules of carbon source were carbon sources,the denitrification performance were not significantly better than hydrolysates,so complex DOM composition was more conducive to the denitrification process.When monomer molecules existed as a single carbon source,their properties would significantly affect the reaction process.When glucose and caprolactone were carbon sources,the denitrification was inhibited by a large decrease in p H value,while the corresponding hydrolysate contained more complex intermediates,which could be used by microorganisms and at the same time,the p H value could be buffed to a certain extent.Acetyl glucosamine may affect the reaction due to its bacteriostatic effect.By contrast,chitin hydrolysate was more conducive to microbial growth.The system of butylglycol succinate coexistence and lactic acid system showed excellent nitrate degradation performance as the corresponding hydrolysate after p H value was adjusted to the appropriate value.Both systems were rich in DOM of-COO-and-OH groups,so the multifunctional carbon source system was more conducive to microbial utilization.Further analysis of DOM components in hydrolysates of each carbon source showed that the DOM composition of hydrolysates would affect its bioavailability.DOM rich in-COO-group or C=O double bond was more conducive to microbial utilization than DOM rich in-OH group or C=C double bond.The hydrolysates of chitin,PCL and PLA showed excellent denitrification performance and no obvious accumulation of nitrite.Their DOM could be effectively utilized by microorganisms and may be rich in more-COO-groups.In contrast,cellulose and starch hydrolysates may have more C=C double bonds in DOM,and PBS hydrolysates may have DOM containing-OH groups.In conclusion,when cellulose,PBS and PLA were carbon sources,the hydrolysis process was the limiting process of denitrification.The bioavailability of DOM would affect the denitrification process,and a complex DOM system was more conducive to denitrification.In hydrolysate DOM,components rich in-COO-and C=O groups were more easily utilized to promote denitrification,which was the reason why DOM in hydrolysate of chitin,PCL and PLA had better denitrification effect.This study provides a theoretical basis for the selection of carbon sources for nitrate removal in water bodies with low C/N ratio. |
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