Removal Of As(Ⅲ) And Sb(Ⅲ) From Water With Potassium Ferrate

Heavy metals pare one group of major water pollutants,which are highly toxic,difficult to degrade,and ready to bioaccumulate in organisms.It is a great threat to the safety of water environment and human health.Therefore,the development and application of“green”and effective water treatment technologies is essential to meet the global demand for clean water.Among them,iron-based agents and related technologies are the most attractive,since they are environmental-friendly and the raw materials are widely available in nature.In addition,the iron-containing agents have other advantages,such as easy recovery,large specific surface area,and strong adsorption capacity for heavy metals.Therefore,the application is more practical in large scale water treatment.This research took two heavy metals,arsenic（As）and antimony（Sb）,as the target pollutants.The removal efficiency of As（Ⅲ）and Sb（Ⅲ）in water by potassium ferrate（K₂FeO₄）was studied under various pH values,mass ratios,initial pollutant concentrations,amounts of natural organic matters,and other parameters.Scanning electron microscope（SEM）,Energy Dispersive Spectrometer（EDS）,X-ray powder diffraction（XRD）,Fourier infrared spectrometer（FTIR）,and X-ray photoelectron spectroscopy（XPS）were employed to characterize the precipitated products.Combining with batch experimental data analysis,the removal mechanism was revealed,and the main results are listed as follows:（1）The optimal removal efficiency of As and Sb occurred under the acidic to neutral conditions,while the removal rate was significantly reduced under alkaline condition.The removal efficiency of As and Sb increased with the increase of mass ratio of Fe to As and Sb.At pH=6.5 and Fe/As mass ratio of 4.6,the removal rate of As reached at 99.55%,while At pH=4.5and Fe/Sb mass ratio>6.8,the removal rate of Sb was more than 90%.When the Fe/Sb mass ratio was improved to 22.6,the removal rate of Sb reached at 99.61%.In addition,once the mass ratio was fixed,the removal efficiency of As and Sb increased with the increase of initial concentration and K₂FeO₄ dosage.（2）In the process of K₂FeO₄ treating water samples containing both As（III）and Sb（III）,both oxidation and flocculation characteristics of K₂FeO₄ was investigated,while FeCl₃（only flocculation）were also investigated for As（III）and Sb（III）removal effect as comparison.The results showed that when the reaction progressed to five minutes,the removal rates of As and Sb were 98.39%and 11.22%,respectively.With the increasing time,As was removed completely first.When using FeCl₃（no oxidation）to remove As（III）and Sb（III）simultaneously,during the first five minutes,the removal rates of As（III）and Sb（III）were 15.61%and 69.68%,respectively.As reaction time going,Sb was removed first.This suggests that in complex polluted water samples,the affinity of As（Ⅲ）for Fe（Ⅲ）is not as strong as that of As（V）produced by oxidation.This makes As（V）more easily adsorbed by Fe（Ⅲ）compounds produced by the reduction of K₂FeO₄.On the contrary,for Sb,the neutral binding of Sb（Ⅲ）and Fe（Ⅲ）is stronger than that of Sb（V）produced by oxidation.（3）Humic acid（HA）,nitrogen（N）,phosphorus（P）and other coexisting substances in water inhibit the removal of As and Sb by K₂FeO₄.Within the range of HA concentration of 0²0 mg/L,the efficiency of K₂FeO₄ removing As and Sb was less than 2.27%and 37.36%,respectively.The results show that HA inhibited the removal of As and Sb by K₂FeO₄.The N,P and other pollutants in the actual water body will also impair the removal efficiency of As and Sb,especially for Sb.（4）The SEM scan shows that the precipitated products were spherical nanoparticles during the removal process,and they adsorbed and cohered with each other,presenting the typical characteristics of ferrihydrite.The analysis results of EDS and XPS full scan spectra show that the presence of As and Sb elements were detected in the precipitates.The XRD spectrum shows the presence of FeAsO₄ diffraction peaks in the precipitated product.The FTIR spectrum also confirmed the existence of FeAsO₄,as well as the stretching vibration caused by Fe-O-Sb and the formation of As-O-Sb bonds.At the same time,the stretching vibration of the As（Ⅴ）-O and Sb（Ⅴ）-O bands implies the fact that K₂FeO₄ oxidized As（Ⅲ）and Sb（III）into As（V）and Sb（V）.The analysis of the XPS narrow spectrum of the precipitated product also confirmed the fact that As（Ⅴ）and Sb（Ⅴ）formed by oxidation adsorbed to ferrite.It was also found that the surface iron hydroxyl（Fe-O-H）components increased,suggesting that As（V）oxygen anions were more likely to bond with-OH.The level of bonding ability can be used to clarify the difference in the removal rates of As（Ⅲ）and Sb（III）by K₂FeO₄.