Synthesis Of Functionalized Nano-zero-valent Iron And Its Removal Efficacy On Perfluorooctanoic Acid In Water

Per-and poly-fluoroalkyl substances(PFAS)are widely used in industry due to their high chemical stability,hydrophobic and oleophobic properties.At the same time,as a persistent and refractory organic pollutant,PFAS has attracted more and more attention because of its harm to the environment.Perfluorooctanoic acid(PFOA)is a typical PFAS and has high concentration,wide distribution and multiple toxic effects in water environment.Traditional sewage treatment technology could not remove PFOA effectively,thus,it is necessary to establish a simple and efficient removal method for PFOA.In this study,layered double hydroxides(LDHs)supported nano-zero-valent iron(NZVI@LDH)was prepared,which could remove PFOA by adsorption and degradation routes.The optimal synthesis method of hydrotalcite and hydrotalcite-supported zero-valent iron nanocomposites was explored.Besides,the removal efficiency and enhanced removal method of PFOA by NZVI@LDH were investigated.Finally,based on the identification of intermediate products,the reaction mechanism of NZVI@LDH on PFOA was proposed.The detailed conclusions are listed as follows:(1)Preparation and characterization of composites,including the synthesis of carrier materials and the loading of nano-zero-valent iron on the surface of the carrier.Mg-Al hydrotalcite(Mg/Al-LDH)and Mg-Fe hydrotalcite carriers(Mg/Fe-LDH)were prepared by coprecipitation method.Performance characterization results show that the carrier hydrotalcite has high crystallinity and good uniformity.Nano-sized zero-valent iron was synthesized by sodium borohydride reduction method.TEM observation showed that NZVI particles agglomerated into chain-like aggregates.Then using Mg/Al-LDH and Mg/Fe-LDH as carriers,NZVI@LDH was prepared by liquid phase reduction method and impregnation reduction method.The structural characterization and morphology revealed that NZVI@LDH was successfully prepared by two synthesis paths,and the loading effectively improved the dispersibility of NZVI,furthermoer the surface iron content of the liquid-phase reduction method was higher.(2)Carriers LDH,NZVI and NZVI@LDH were used to treat PFOA in water,and the removal performance of PFOA by different materials was explored.First of all,the PFOA reduction effect of carrier Mg/Al-LDH,Mg/Fe-LDH and their calcined products was investigated.The results showed that the PFOA removal rate of Mg/Fe-LDH was as high as 56.74 %,and the removal rate of Mg/Al-LDH was 15%,NZVI only reached9.16%.Calcination could not improve the PFOA removal performance of carrier NZVI.The kinetics of PFOA removal by composite materials was further studied.It was found that the composites obtained by functionalized modification of nano-zero-valent iron could significantly improve the removal efficiency of PFOA.Among the three materials of NZVI,Mg/Al-LDH and NZVI@Mg/Al-LDH,the removal rate of PFOA by composite materials is higher than that of NZVI and Mg/Al-LDH.(3)The PFOA removal effect of NZVI and NZVI@Mg/Al-LDH was enhanced by pH adjustment and persulfate(PS)addition,and the decomposition mechanism of PFOA by NZVI and composite materials was explored.The change of pH had a significant effect on the removal efficiency of PFOA by NZVI@Mg/Al-LDH.The PFOA removal rate decreased first and then increased with pH increaseing from 3 to 9.Under the condition of pH 9,the removal rate was the highest,reaching 45.66%,but the effect of NZVI was little affected by pH.In the range of pH 3?9,the PFOA removal rate was below 10%.On the contrary,the NZVI removal rate increased significantly after the addition of PS,from 7.61% to 71.90% at pH 9.In comparison,the PS addition had little effect on the NZVI@Mg/Al-LDH removal rate,and the removal rate fluctuated within 3%before and after adding.Fluoride ions were detected in NZVI+PS and NZVI@LDH+PS systems by ion chromatography,moreover short-chain perfluorocarboxylic acids,perfluoroalcohols and perfluoroalcohols/aldehydes were identified by UPLC-QTOF techniques.It is suggested that decarboxylation mediated by sulfate radical and gradual reduction of carboxylic acids to unstable alcohols/aldehydes may be the main ways to enhance the degradation of PFOA.