Application Of Iron And Iron Corrision Products In The Treatment Of Environmental Pollutants

As a novel reduction method, zero-valent iron （ZⅥ or Fe⁰） for aqueousenvironmental pollutant treatment or remediation has been developed since1980s. Fe⁰has been intensively studied for the Cr（Ⅵ） removal in waters because of its low priceas electron donor，relatively high redox potential and rapid reaction rate. However，theconventionally employed Fe⁰，are particles that are covered with a surface oxide layer.The oxide layer greatly impedes the Fe⁰reduction activity. In order to remove theadverse effect of this oxide layer, this thesis for the first time introducedphotoirradiation during the reduction of Cr（Ⅵ） by Fe⁰powders in the presence of citricacid. The results indicate that introduction of photoirradiation not only dramaticallyincreases the reduction rate of Cr（Ⅵ） in the presence of citric acid but also effectivelystabilizes the activity of Fe⁰powders for multi-cycle use, which is attributed to thephotoreductive dissolution of the oxide layer on Fe⁰surface. The co-existence ofphotoirradiation and citric acid can inhibit the formation of passivation layer onFe⁰surface during the reduction of Cr（Ⅵ）. The reduction rate of Cr（Ⅵ） in thepresence of citric acid and photoirradation is influenced by factors like pH values andconcentration of citric acid. A reduction rate of87%can be obtained for30min underconditions of1.6mM citric acid, pH3.0,0.8g/L Fe⁰powders,25mL of Cr（Ⅵ）solution （0.20mM）, a300W metal halide lamp used as photoirradation source.Furthermore, activity of Fe⁰powders does not exhibit any significant loss after fiverecycles. Apart from Fe⁰powders, the similar results for self-made corroded Fe⁰filmand corroded low carbon steel were also observed.Based on above experimental results, photoreduction of Cr（Ⅵ） with iron corrosionproducts （ICPs） and tartaric acid under simulated natural light was further investigated.ICPs were collected from the cast iron pipes and characterized with X-ray diffraction（XRD）, scanning electron microscopy （SEM） and energy dispersive X-ray spectroscopy （EDS）. ICPs contain Fe₃O₄and Fe₂O₃, and the weight percentage of Fe is82.51%. The specific surface area of ICPs is51m2/g as measured by N2adsorption/desorption isotherm analyses. The photoreduction efficiency of Cr（Ⅵ） andthe reduction mechanism with ICPs was investigated and compared in differentsystems. The results indicate ICPs are efficient heterogeneous catalysts and have highstability during photocatalytic reduction of Cr（Ⅵ） in the presence of tartaric acid. Thecatalytic reduction reaction takes place primarily in the bulk solution. Thephotoreduction rate is93%for30min in the co-presence of tartaric acid andphotoirradation of simulated natural light, and even under the photoirradation of solarlight, the system also shows catalytic effect. The reduction rate of Cr（Ⅵ） is influencedby factors like ICPs dosage, concentration of tartaric acid, pH value, light intensity andnature of organic acids. The photoreduction efficiency increases with increasing ICPsdosage from0.0to0.4g/L. However, further increasing ICPs dosage leads to adecrease in the efficiency. The photoreduction efficiency also increases with increasinginitial concentration of tartaric acid from0.0to1.6mM. However, further increasingconcentration of tartaric acid is adverse for the efficiency. Effective reduction of Cr（Ⅵ）is favored at low pH value and with stronger light intensity. The photoreductionefficiency is related to the nature of the organic acids, and the order of the effect onphotoreduction of Cr（Ⅵ） is: formic acid＜humic acid＜oxalic acid＜tartaric acid.Organic dye was choosed as a target pollutant, and photodecolorization of orangeII （OII） in heterogeneous system composed of oxalic acid and ICPs was alsoinvestigated in this thesis. The results indicate ICPs are efficient heterogeneousphotocatalysts, and can form an efficient photo-Fenton-like system together withoxalic acid. The catalytic reaction takes place primarily in the bulk solution. Thephotodecolorization rate of OII for30min is83%and44%under simulated naturallight and solar light, respectively. The latter result indicates that this system is aneconomically viable and environment-friendly method for photodecolorization OII.The photodecolorization efficency depends significantly on various operating factorsincluding the ICPs dosage, initial concentration of oxalic acid, the initial pH value, light intensity and nature of organic acids. The photodecolorization efficiencyincreases with increasing ICPs dosage from0.0to0.4g/L. However, further increasingICPs dosage leads to a slight decrease in the efficiency. The photodecolorizationefficiency also increases with increasing initial concentration of oxalic acid from0.0to1.6mM. However, further increasing concentration of oxalic acid is adverse for theefficiency. Effective photodecolorization of OII is favored at low pH value and withstronger light intensity. ICPs can also form a photo-Fenton-like system together withtartaric or citric acid to decolorize OII solution, but the efficiency is lower than that inthe presence of oxalic acid. In addition, the system composed of ICPs and oxalic acidis also able to effectively photodecolorize non-azo dye like methylene blue andrhodamine B solutions under photoirradation of simulated natural light.