The Mechanism Study On Selenite Removal In Aqueous Solution By Ferrous Hydroxyl Complex(FHC)

With the development of industrialization,the heavy-metal pollution is more and more serious in waters.As one of the metalloid,selenium has rather special physical properties and it exists in two species of selenate and selenite form in the solution,in which the Se（Ⅳ）has much higher toxicity,and they are very soluble without the restriction of solubility.Due to the extremely strong mobility of liquid selenium,there is little effect of the treatment using porous solid materials to dispose wastewater containing selenium,while Se species reduced from Se（Ⅳ）or Se（VI）to oxidation states 0,–I,and –II are low solubility solids.So the reductive precipitation isconsidered to be the most effective way to immobilize Se.The ferrous hydroxyl complex（FHC）,namely structural Fe（II）,has been synthesized by precipitation method and the exploratory researches of treatment to industrial wastewater containing selenite were investigated by applying the excellent reactivity of FHC.The main content of this paper includes: firstly,it was carried out to investigate the influences of different conditions of the FHC dosage,initial p H,the [Fe（II）]/[OH^-] ratio and the dissolved oxygen in system.The results showed that Se（Ⅳ）removal by FHCs has a large available range of pH.The removal reactivity of FHC was different with various mole ratios of [Fe²⁺] to [OH^-].Theremoval efficiencies of selenite with FHC（1:1）and FHC（1:2）were higher than those of FHC（1:3）and FHC（1:4）,and FHC（2:1）was moderate.The results of research showed that the existence of dissolved oxygen reduced the Se（Ⅳ）removal efficiency.Kinetics study and thermodynamics study of Se（Ⅳ）removal by FHCs were conducted and the corresponding removeal model was simulated too.Secondly,with the analyses of scanning electron microscopy（SEM）images of the products after the reaction between FHC and Se（Ⅳ）,it indicated that FHC（1:1）and FHC（1:2）had larger specific surface area and higher reduction activity and there was a phenomenon of an agglomeration formation.XRD（X-ray diffraction）analyses showed that the products of FHC（1:2）with Se（Ⅳ）were maghemite（PDF: 25-1402）and the products of FHC（1:4）with Se（Ⅳ）lepidocrocite（PDF: 13-0087）.When studying the effect of co-existing ions on the structures and the activity of FHC,it was found that several metal ions had a promotion effect on Se（Ⅳ）removal by FHC and the promotion efficiency degree was Ag⁺>Cu²⁺>Mn²⁺.However,anions had a negative effect on Se（Ⅳ）removal by FHC,and the inhibition efficiency degree was H2PO₄^->SiO₃^2->HCO₃^-and the removal efficiency was merely 30% when the nitrite ion concentration was 0.2mol/L.Thirdly,based on the analyses of XPS（X-ray photoelectron spectroscopy）of various mole ratios of [Fe²⁺] to [OH^-],it showed that the mechanism of removal selenite by FHC（2:1）was mainly electrostatic adsorption and the mechanism of removal selenite by FHC（1:4）was considered to be mainly reductive immobilization.And the mechanism of removal selenite by FHC（1:1）included two different mechanisms of electrostatic adsorption and reductive immobilization.Using sequential extraction technology on the products got from the selenite removal by FHC with various mole ratios of [Fe²⁺] to [OH^-],it showed the removal efficiency by FHC（1:1）was higher than FHC（1:4）and FHC（2:1）and element selenium was the main product after reaction and especially in the reduction product from the removal selenite by FHC（1:1）,element selenium accounted for 77%.With the analyses results of the sequential extraction technology on Cu²⁺/FHC,Ag⁺/FHC,NO₂^-/FHC removal selenite products,it indicated that selenide and element Se were the main reduction products and the exchangeable form of Se was the minority product which indicated that the reduction immobilization was mainly the selenite removal route.The research results showed that the removal of selenite by FHC has the characteristics of simple operation,high efficiency and good practicability.It plays an important role of the technique application of selenite removal by FHC for the treatment of industrial wastewater containing selenite in the future development.