Role of Fe(III)-carboxylates in AMZ photodegradation: A response surface study based on a Doehlert experimental design.

Photochemical redox reactions of Fe(III) complexes in surface waters are important sources of radical species, therefore contributing to the sunlight-driven elimination of waterborne recalcitrant contaminants. In this study, the effects of three Fe(III)-carboxylates (i.e., oxalate, citrate, and tartrate) on the UVA photoinduced oxidation of the herbicide amicarbazone (AMZ) were investigated. A Doehlert experimental design was applied to find the Fe(III):ligand ratios and pH that achieved the fastest AMZ degradation rate. The results indicated optimal ratios of 1:10 (Fe(III):oxalate), 1:4 (Fe(III):citrate), and 1:1 (Fe(III):tartrate), with the [Fe(III)]0 set at 0.1 mmol L-1 and the best pH found to be 3.5 for all the complexes. In addition, a statistical model that predicts the observed degradation rate constant (kobs) as a function of pH and Fe(III):carboxylate ratio was obtained for each complex, enabling AMZ-photodegradation predictions based on these two variables. To the best of our knowledge, this is the first time that such models are proposed. Not only the pH-dependent speciation of Fe(III) in solution but also the time profiles of photogenerated OH, Fe(II), and H2O2 gave appropriate support to the experimental results. Additional experiments using a sampled sewage treatment plant effluent suggest that the addition of aqua and/or Fe(III)-oxalate complexes to the matrix may also be effective for AMZ removal from natural waters in case their natural occurrence is not high enough to promote pollutant degradation. Therefore, the inclusion of Fe(III)-complexes in investigations dealing with the environmental fate of emerging pollutants in natural waterbodies is strongly recommended.

2,4-Dichlorophenoxyacetic acid (2,4-D) degradation promoted by nanoparticulate zerovalent iron (nZVI) in aerobic suspensions.

Reactive species generated by Fe(0) oxidation promoted by O2 (catalyzed or not by ligands) are able to degrade contaminant compounds like the herbicide 2,4-dichlorophenoxyacetic acid. The degradation of 2,4-D was influenced by the concentrations of zero valent iron (ZVI) and different ligands, as well as by pH. In the absence of ligands, the highest 2,4-D degradation rate was obtained at pH 3, while the highest percentage degradation (50%) was achieved at pH 5 after 120 min of reaction. Among the ligands studied (DTPA, EDTA, glycine, oxalate, and citrate), only ethylenediaminetetraacetic acid (EDTA) and diethylenetriaminepentaacetic acid (DTPA) significantly enhanced oxidation of 2,4-D. This increase in oxidation was observed at all pH values tested (including neutral to alkaline conditions), indicating the feasibility of the technique for treatment of contaminated water. In the presence of EDTA, the oxidation rate was greater at pH 3 than at pH 5 or 7. Increasing the EDTA concentration increased the rate and percentage of 2,4-D degradation, however increasing the Fe(0) concentration resulted in the opposite behavior. It was found that degradation of EDTA and 2,4-D occurred simultaneously, and that the new methodology avoided any 2,4-D removal by adsorption/coprecipitation.