| In the present study, zero-valent iron (Fe0) films coated on indium tin oxide(ITO)-coated glass substratewas were prepared with electrodeposion. Decolorization ofmethylene blue (MB) and orange II (OII) by the Fe0film in the presence of oxalic acid(OA) and dissolved oxygen (DO) and the influence of photoirradiation and hexavalentchromium (Cr(VI)) have been investigated. The newly developed Fe0/OA/DO system ismore effective in decolorization of MB than its EDTA counterpart, and is of a strongstability.Operating parameters such as pH, OA concentration, Fe0film stay time, addition ofmethanol have significant influence on the decolorizationrate for MB in Fe0/OA/DOsystem. A lower pH value is favorable. Nevertheless, a decolorization rate of32%forMB at pH=7.0(after30min) still reached, which means MB can also be decolorizedeven at circumneutral pH. Dissolved iron measurement results show that high pHenvironment is not favorable to dissolution of iron. There are at least two factorsresponsible for this pH-dependence. A higher pH value is not favorable for theformation of H2O2owing to the descrease of dissolution of iron. On the other hand, ahigher pH value is unfavorable for the activity of·OH due to its lower oxidationpotential. With a higher concentration of OA, MB will be decolorized better. However,when the concentration is too high, enhancement effect is not obvious. In theconcentration range studied,2.4mM is a suitable concentration. The longer Fe0filmstayed in the solution, the better MB was decolorized. The reaction is stoppedimmediately when the Fe0film is removed from the reactor.MB’s decolorization in Fe0/OA/DO system is a heterogeneous dynamics process,that is to say, decolorization reaction occurs in iron surface, rather than in bulk solution.Presence of methanol also inhibits Fe0/OA/DO system’s decolorization of MB, andinhibitory effect increases with the increase in methanol concentration. Even in the caseof higher concentration of methanol, however, MB still has a certain decolorization rate.It has been found that the decolorization of OII in Fe0/OA/DO system (22%, after30min) is much lower than that of MB. The low decolorization rate for OII can bedramatically enhanced by introduction of photoirradiation into the system. Operating parameters such as pH and OA concentration have significant influence on thedecolorization rate for OII in photo-assisted Fe0/OA/DO system. The lower pH andhigher OA’s concentration are favorable for OII’s decolorization, and optimum pH andOA’s concentration are3.0,2.4mM, respectively. The introduction of photoirradiationinto Fe0/OA/DO system, MB’s decolorization rate after30min almost doesn’t change(90%and91%before and after the introduction). Unlike Fe0/OA/DO system(heterogeneous dynamics process), homogeneous fenton reaction mainly occurs in thephoto-assisted Fe0/OA/DO system.Photo-assisted Fe0/OA/DO system has both oxidation and reduction potential,because it can both decolorize MB(OII) and reduce Cr(VI). In the given experimentalconditions, Cr(VI) reduction rate reaches83%after30min. Photo-assisted Fe0/OA/DOsystem allows simultaneous OII decolorization and Cr(VI) reduction, and Cr(VI)promotes the decolorization of OII. When OII and Cr(VI) are present in a mixedsolution, OII will be decolorized preferentially. OII’s decolorization rate slightlydecrease with increase in Cr(VI) concentration, while the Cr(VI) reduction efficiency isreduced significantly. Promotion of the decolorization of OII by Cr(VI) can beexplained by OII photosensitivity and electron transfer between its electronicallyexcited state and Cr(VI). The photo-assisted Fe0/OA/DO system also allowssimultaneous MB decolorization and Cr(VI) reduction, but MB’s decolorization isinhibited by the coexistence of Cr(VI). Additionally, Cr(VI) will be reducedpreferentially in the mixed solution. |
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