Degradation Mechanism Of P-Nitrophenol By Biochar And Influencing Factors

P-nitrophenol(PNP)is a refractory organic pollutant produced in production and life.It has strong carcinogenic,teratogenic,and mutagenic effects,and is widely distributed in soil and water environments.Because it is difficult to be biodegraded,PNP seriously endangers human health and the safety of the ecological environment.As a new type of functional material,biochar has attracted much attention.It is a kind of highly aromatic porous material with high carbon content produced by the cracking of biomass under complete or partial hypoxia.In recent years,the application in water and soil remediation has gradually increased.Biochar also has application potential in the treatment of PNP pollution.It can not only adsorb PNP,but also degrade PNP.Existing studies have shown that persistent free radicals(PFRs)on biochar can play a certain role in the degradation of PNP.However,the mechanism of non-radical degradation of PNP mediated by biochar has not yet been clearly understood.Therefore,we designed relevant experiments in a targeted manner,selecting biochars(B500 and B700)with pyrolysis temperatures of 500 ℃ and 700 ℃,and analyzing their degradation mechanisms and influencing factors.The research content is mainly expanded from four aspects:(1)the determination of non-radical active moieties;(2)the influence of the distribution of active moieties on the degradation of PNP;(3)the influence of the masking of active moieties on the degradation of PNP;(4)The effect of dissolved organic matter(DOM)in the active moieties on the degradation of PNP.The main conclusions of this study are as follows:(1)The determination of non-radical active moieties in the process of biochar degradation of PNP.This study found that B500 showed a stronger free radical signal than B700,but the degradation ratio of B700 to PNP was higher than that of B500,and the results of the two were opposite.It can be shown that persistent free radicals are not the only factor that controls the degradation of PNP by biochar,and there are other factors.Through Fourier infrared spectroscopy analysis,it is found that B700 has more redox active functional groups than B500,and the electron exchange capacity(redox ability)of B700 is also higher than that of B500.This result is consistent with the degradation rate result.Therefore,we believe that the non-radical moieties(redox active functional groups)on biochar also play a certain role in the degradation of PNP.The redox active moieties on the biochar will be covered by some impurities such as ash,which will weaken its role.Water washing treatment can expose these active groups.After water washing,the free radical signal and electron exchange capacity of biochar have been significantly increased,and the degradation ratio of PNP by biochar has also increased significantly.This shows that the water washing treatment removes the impurities covering the redox active moieties,exposes more redox active moieties,and enhances the redox ability of biochar and the strength of persistent free radicals,thereby increasing the resistance to PNP degradation.This result also verified that in the system of biochar degradation of PNP,the redox ability of biochar and persistent free radicals play an important role in the degradation of PNP.(2)The effect of DOM in the active group on the degradation of PNP.The water washing treatment can not only remove the ash in the biochar,but also separate the DOM from the biochar.DOM also has a certain degree of redox,but it is unclear whether it is involved in the degradation of PNP.This study found that DOM can also degrade PNP,and the degradation of PNP per unit mass of DOM is much higher than that of biochar particles.At the same time,DOM has no free radical signal,so the degradation that occurs in the DOM/PNP system does not participate in free radicals,and its degradation ability is mainly controlled by the redox ability of DOM.By analyzing the degradation ratio,it is concluded that the sum of the degradation ratio of DOM and the washed biochar particles to PNP is higher than the degradation ratio of the unwashed biochar to PNP.This indicates that DOM may block the pore structure of biochar while simultaneously inhibiting the degradation of PNP by biochar.(3)The influence of the distribution of redox active moieties on the degradation of PNP.Redox active moieties are not only distributed on the surface of biochar,but also in the pores of biochar.Therefore,the pore structure of biochar can be adjusted through the classification of the particle size of the biochar.This study found that the two-compartment first-order kinetics model can simulate the removal process of PNP by biochar.The distinction between adsorption and degradation found that adsorption mainly occurs in the initial stage of the reaction,and degradation continues throughout the removal process.The correlation analysis between the degradation rate constant and the specific surface area of biochar shows that at a relatively low concentration of PNP(200 mg/L),the degradation of PNP by biochar mainly depends on the active moieties on the outer surface of the biochar.At a relatively high concentration of PNP(800 mg/L),the degradation of PNP by biochar is affected by the active moieties on the microporous surface of biochar.In summary,this study found that the redox ability of biochar plays an important role in the degradation of PNP by promoting the electron transfer between biochar and PNP.In addition,the degradation of PNP by biochar is also related to the threedimensional structure of biochar.This study also found that the degradation of PNP by DOM does not involve free radicals,which is mainly controlled by the redox ability of DOM.In addition,DOM may block the pore structure of biochar to play a certain inhibitory effect during the degradation of PNP by biochar.This study clarified the degradation mechanism of PNP by biochar and its influencing factors expanded the current understanding of the degradation mechanism of PNP mediated by biochar,and provided technical guidance and theoretical support for the treatment of organic pollutants in the water environment.It will guide the preparation of biochar with high PNP degradation efficiency and promote its application in environmental remediation.