Study On The Degradation Of Methyl Paraben By Hydroylamine Enhanced Nano-zero-valent Iron Activated Persulfate

Methylparaben(MP)is a widely used preservative and antimicrobial agent in food,pharmaceutical and personal care products,the environmental pollution and human health problems associated with the massive use and unreasonable emission of such substances have raised more and more concerns.Advanced oxidation techniques based on sulfate radicals have shown excellent performance in the degradation of organic pollutants.Among the many methods of persulfate activation,the activation of persulfate by zero-valent metals such as nano-zero-valent iron has become a hot topic of research.However,the regeneration problem of Fe2+ in this system is the rate-limiting step for persulfate activation and pollutant degradation,therefore,this study aims to improve the performance by introducing the reducing agent hydroxylamine(HA)into the nZVI/PDS system to achieve Fe3+/Fe2+cycling and thus accelerate the degradation rate of contaminants.Firstly,the effect of various factors on MP degradation in the nZVI/PDS system was investigated by single-factor test with MP degradation rate as the main index,and the process parameters were optimized by response surface method,and the best test conditions were obtained as follows:the initial pH value was 2.75,the PDS dosage was 2.18 mM,and the nZVI dosage was 2.87 mM,at which the degradation rate could reach 95.62%.The effects of common inorganic anions and large organic matter humic acid on the degradation of pollutants were also investigated,and it was found that all substances except Cl-inhibited the degradation of MP through free radical quenching.Secondly,the effects of HA dosage and time,nZVI dosage,PDS dosage and initial pH of the solution on the degradation of MP in the HA/nZVI/PDS system were investigated,and the causes of MP degradation were elaborated by measuring the changes of pH,Fe2+and Fe3+concentrations during the reaction.The optimum conditions obtained by single-factor test were:HA dosing amount of 1 mM,dosing time of 60 min of reaction,nZVI dosing amount of 1 mM,PDS dosing amount of 2 mM and initial pH of solution of 3.The MP degradation rate at this time was 94.26%,which was almost the same as the degradation level of nZVI/PDS system under the optimum conditions.It was also found that when the pH was greater than 3,the MP degradation rate was significantly enhanced by adding HA at the beginning of the reaction compared with the system without HA,That means the addition of HA could greatly broaden the effective pH range of the system.Then,EPR tests and free radical quenching tests determined that SO4·and ·OH acted simultaneously in the degradation of pollutants and that SO4·was dominant under acidic conditions.The recycling test showed that the degradation of MP decreases continuously with the increase of zero-valent iron use due to the surface oxide coverage.Finally,the mineralization test was conducted and the results showed that the addition of HA could improve the mineralization rate of the nZVI/PDS system to some extent,but the TOC degradation rate was always lower than the MP degradation rate,indicating that the pollutants were not completely converted into CO2 and H2O.LC-MS was used to detect the intermediates generated during the MP degradation process,and a variety of possible intermediates were inferred based on the detection results The degradation history of the pollutants was also discussed,and the analysis obtained that hydroxylation was the most important pathway of MP degradation,accompanied by decarboxylation reaction and dehydrogenation coupling.