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)] set at 0.1 mmol L 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 (k) 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 HO 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.