dc.description.abstract | The purpose of this study was aimed to investigate the effects of inorganic ions, such as ferrous ion, calcium ion, and sulfate ion on the reductive dechlorination of trichloroethylene (TCE) with batch tests in aqueous solution by zero-valent iron (ZVI). Also, the observation of the surface of metal iron by scanning electron microscopy with energy dispersive X-ray spectrum (SEM-EDS) and X-ray photoelectron spectrometer (XPS) analysis were carried out to identify the formation of hydroxide precipitations on the surface of metal iron and that of effects of TCE degradation.
Experimental results indicated that the metal iron without acid washing pretreatment could not reduce TCE effectively because the passive film of maghemite (Fe2O3) might coat on the surface of metal iron by the observation of XPS analysis. The precipitations of maghemite on the surface of metal iron could hinder the electrons released from the oxidation of metal iron to the bulk solution and affect the reductive dechlorination of TCE. However, the degradation rate of TCE with ZVI would increase after pretreatment of metal iron with acid washing. At the 37 h of reaction time, the removal percentage of TCE would reach 100 %. The observation of the surface of metal iron by XPS analysis showed that the content of maghemite would decrease and the conformation of oxides on the surface of metal iron with acid washing changed from maghemite to more hydrated α-FeOOH, therefore, the reactive specific area and the ability of oxidation of metal iron might increase. Additionally, the ferrous ion could not directly reduce TCE in this study. According to the Nernst equation, the reductive dechlorination of TCE by the ferrous ion should be at the low pH and high concentration of ferrous ion. Experimental results indicated that in the presence of ferrous ion with unacid washing metal iron in the Fe0-TCE system, the degradation of TCE would not occur during the initial 10 min of the reaction time. However, the degradation rate of TCE increased with the increase of reaction time. Also, the degradation rate increased with the initial concentration of ferrous ion. When the concentration of ferrous ion increased from 0 mg/L to 300 mg/L, the residual concentration of TCE in the solution decreased from 90 mg/L to 20 mg/L after the 77 h of reaction time. The reasons of the increase rate of TCE degradation with unacid washing metal iron was that ferrous ion absorbed to the lattice of maghemite which coating on the surface of metal iron and converse maghimite to the semiconductive materials of magnetite(Fe3O4) that was permitted electrons to pass those precipitations from the metal iron surface and reduce TCE in the solution, therefore, the degradation rates of TCE was enhanced.
On the other hand, when the ferrous ion coexisted with acid washing metal iron, the rate constant of TCE decreased from 0.084 h-1 to 0.056 h-1 as the concentration of ferrous ion increase from 0 mg/L to 300 mg/L. The observation of the surface of metal iron by SEM-EDS and XPS analysis showed that the ferrous ion would form the ferrous hydroxide coating on the surface of ZVI and decreased the degradation rate of TCE during the initial reaction time. With the increase of reaction time, the ferrous hydroxide precipitations would be oxidized to form magnetite and had not hindered the electrons to contact with TCE for reductive dechlorination. However, the transfer rate of electrons from magnetite on the surface of metal iron was still less than that of ZVI. Consequently, it revealed that the removal rate of TCE would decrease when the ferrous ion coexisted with acid washing acid.
Additionally, the concentrations of calcium ion and TCE would not be different in the solution during the 77 h of reaction time when calcium ion coexisted in the Fe0-TCE system. This was because the both concentration of OH- and carbonate was not enough to form the precipitations of calcium hydroxide and calcium carbonate on the surface of ZVI in this study. Also, the calcium ion have no an effect on the reductive dechlorination of TCE.
When the sulfate ion coexist in the Fe0-TCE system, the sulfate ion would react with the ferrous hydroxide that be oxidized by the ZVI, and form the soluble ferrous sulfate (FeSO4) in the solution. Further, it avoided that the ferrous hydroxide was oxidized to form the magnetite continually, thus, the reactive specific area and the ability of electrons releasing of ZVI could decrease. Consequently, the sulfate ion would enhance the degradation rate of the TCE by ZVI in the Fe0-TCE system | en_US |