Effect And Mechanism Of Buffer Systems On The Sulfidated Zero Valent Iron For Cr(?)Sequestration

In recent years,heavy metal pollution such as chromium pollution has arousing wide concern because of its serious threat to human and wildlife.Chromium(Cr)usually comes from ore processing,printing and dyeing industry,metal surface treatment,leather tanning,etc.Hexavalent chromium(Cr(?))can cause serious health hazards to humans owing to its carcinogenic and mutagenic,and Cr(?)is 100 times more toxic than Cr(?)in the environment.Zero valent iron(ZVI)shows great potential in heavy metals sequestration due to its stronger reduction ability,relatively inexpensive,available and environment-friendly.Nevertheless,the application of ZVI has showed some drawbacks including a lower chemical activity and poor electron selectivity because of the production of(hydro)xide on ZVI surface which slowed reaction of active sites and contaminants.Because of its large specific surface area and high reactivity,sulfidation of zero valent iron(S-ZVI)technology has become a research hotspot in the field of strengthening zero-valent iron reduction ability.In environmental researches,buffers were usually chosen to maintain the pH of reaction system;it is also beneficial to analyze the reaction mechanism to decouple the influence of the variations of pH.This study explored the effect of different buffer system(HAc-NaAc,Na2HPO4-citric acid and citric acid-citrate)during sequestration of Cr(VI),and considering that research field focus on the metal ion-buffer complexes and Cit's strong complex-forming capacity,citric acid-citrate buffer was selected in this work as buffer system on the sequestration of Cr(?)for exploring reaction mechanism.Firstly,this study explored the effect of different buffer systems during sequestration of Cr(?)by iron material(ZVI and S-ZVI),compared with Na2SO4 system,buffer systems can significantly improve the removal of Cr(?)by iron materials(ZVI and S-ZVI),and different buffer systems have different effects on the removal of pollutant Cr(?)by iron materials(ZVI and S-ZVI).Among them,Cr(?)declines the fastest in H3Cit-Cit buffer system and the slowest in HAc-NaAc buffer system.Among the four reaction systems,the buffering system promoted the reaction of iron materials(ZVI and s-zvi)to remove the pollutant Cr(?)and tendency are HAc-NaAc<Na2HPO4-H3Cit<H3Cit-Cit.Based on the different buffer systems,the promoting effect of S-ZVI is obviously different.We explored the influence of functional groups on Cr(?)removal of S-ZVI from the perspective of molecular structure.It was found that the reaction media based on functional groups could effectively improve the removal efficiency of Cr(?)by zero valent iron sulfide(S-ZVI).FeSx layer on the surface of S-ZVI showed obvious specific chemosorption of alcohol molecules(ROH)containing hydroxyl functional groups.In the reaction of Cr(?)removal by S-ZVI,the hydroxylated FeSx surface can accelerate the migration rate of complex formed by Cr(?)and carboxylic acid molecules(RCOOH)to S-ZVI through hydrogen bonding,so that Cr(?)can fully contact with S-ZVI,thus improving the removal efficiency of Cr(?.In addition,it was found that-OH and-COOH with different molecular weight had a significant effect on the removal effect of Cr(?).The reaction rate was fastest when the ratio between the amount of ethanol(-OH)and the-COOH molecule in acetic acid reached 4:1.When a single molecule containing both-OH and-COOH functional groups is present,the migration process can be further shortened and the removal rate of Cr(?)can be more significantly improved.At the same time,based on the best effect of S-ZVI in removing hexavalent chromium in the H3Cit-Cit buffer system,H3Cit-Cit buffer was selected to further explore the influence of buffer system for providing theoretical basis for metal ion-buffer complex system to participate in Cr(?)removal by S-ZVI.The results on the effects of S-ZVI dosage(0?0.05?0.1?0.2 g/L),buffer concentration(0?0.005?0.01?0.05?0.1 M)and buffer pH(3.0?4.0?5.0?6.0?6.7).Then,SEM,EDS,XPS,FTIR and other characterization methods were used to analyze the samples before and after the reaction and Fe(?)masking experiments were conducted to explore the mechanism of Cr(?)removal of pollutants by S-ZVI.The results showed that the optimum condition are dosage of S-ZVI is 0.2 g/L,the buffer concentration is 0.05 M,the buffer pH is 4.0,and the coupling of depassivation and reduction of metal ion-buffer complexes significantly improves Cr(?)sequestration in H3Cit-Cit buffer system,which is ascribed to inhibited the precipitation that resulted from strong solubility of Fe(?)-citrate,Fe(?)-citrate and Cr(?)-citrate complexes,and the reduction of Fe(?)-citrate for Cr(VI)sequestration.SEM-EDS and XPS indicated that the absence of peaks for Cr(O)3 and/or a mixed phase CrxFe1-x(OH)3 and the peak attenuated as the reaction progress.The Fe(?)masking experiments indicated aqueous Fe(?)of solutions is not totally responsible for the Cr(?)removal in Citric acid-Citrate buffer system,and the effect of Fe(?)-citrate can't be ignored.