Degradation of atrazine photoinduced by Fe(III)-pyruvate complexes in the aqueous solution

Unravelling structural features of small molecules for photochemical transformation of environmental contaminants

Small molecules, including natural metabolites, organic matter decomposition products, and engineered oxidation byproducts, are widespread in aquatic environment. However, the limited understanding of the photochemical interactions of these small molecules with water pollutants hampers the development of effective environmental protection strategies. This study explores the structural features governing the photochemical transformation of toxic oxyanions by α- and β-dicarbonyl compounds. By integrating experimental observations with quantum chemical calculations, a robust correlation network was constructed. The correlation network reveals that the reactivity of small organic molecules with oxyanions could be quantitively predicted by their intrinsic properties, such as electronic transition energy, bond dissociation energy, molecular softness, molecular orbital gap, atomic charge, and molecular surface local ionization energy. This network maps the relationship between the molecular architecture of chemicals and their photochemical behaviors. This perspective offers fresh insights into the photochemical behaviors of small molecules in diverse environmental and chemical contexts and are helpful for developing advanced water treatment strategies toward a sustainable future.

Phenanthrene degradation using Fe(III)-EDDS photoactivation under simulated solar light: A model for soil washing effluent treatment

In this work, for the first time, the nonionic surfactant polyoxyethylene-(20)-sorbitan monooleate (Tween 80, C₆₄H₁₂₄O₂₆) aided soil washing effluent was treated by enhanced activation of persulfate (PS) using Fe(III)-EDDS (EDDS: ethylenediamine-N, N-disuccinic acid) complexes under simulated solar light irradiation. The performance of this system was followed via the production and reactivity of radical species (SO₄^-, HO, Cl₂^-) and degradation of phenanthrene (PHE) used as a model pollutant in soils. Different physico-chemical parameters such as the concentration of reactive species and pH were investigated through the PHE degradation efficiency. The second-order rate constants of the reactions for generated radicals with PHE and Tween 80 in solution were identified through competitive reaction experiments under steady-state conditions and application of nanosecond laser flash photolysis (LFP) as well. A kinetic approach was applied to assess the selectivity and reactivity of photo-generated radicals in aqueous medium in order to explain the observed degradation trends. This work proposes an innovative technology of management of soil washing solutions using Fe(III)-EDDS complexes and solar light for the activation of persulfate.

NiFe(C2O4)x as a heterogeneous Fenton catalyst for removal of methyl orange

This paper studies a heterogeneous Fenton catalyst NiFe(C₂O₄)_x, which showed better catalytic activity than Ni(C₂O₄)_x and better re-usability than Fe(C₂O₄)_x. The methyl orange removal efficiency was 98% in heterogeneous Fenton system using NiFe(C₂O₄)_x. The prepared NiFe(C₂O₄)_x had a laminated shape and the size was in the range of 2-4 μm, and Ni was doped into catalyst's structure successfully. The NiFe(C₂O₄)_x had a synergistic effect of catalyst of 24.7 for methyl orange removal, and the dope of Ni significantly reduced the leaching of Fe by 77%. The reaction factors and kinetics were investigated. Under the optimal conditions, 0.4 g/L of catalyst dose and 10 mmol/L of hydrogen peroxide concentration, 98% of methyl orange was removed within 20 min. Analysis showed that hydroxyl radicals and superoxide radicals participated in the reaction. With NiFe(C₂O₄)_x catalyst, the suitable pH range for heterogeneous Fenton system was wide from 3 to 10. The catalyst showed good efficiency after five times re-use. NiFe(C₂O₄)_x provided great potential in treatment of refractory wastewater with excellent property.

Degradation of aniline by heterogeneous Fenton's reaction using a Ni-Fe oxalate complex catalyst

A Ni-Fe oxalate complex catalyst was synthesized and characterized by means of Brunauer-Emmet-Teller (BET) method, scanning electron microscope (SEM) and X-ray photo-electron spectroscopy (XPS). The catalyst showed good catalytic activity for aniline degradation by heterogeneous Fenton's reaction, in which the synergetic index was 9.3. The effects of reaction temperature, catalyst dosage, hydrogen peroxide concentration and initial pH were investigated. Under the optimum conditions (T = 293 K, catalyst dosage = 0.2 g/L, H2O2 concentration = 4 mmol/L and initial pH = 5.4), 100% aniline could be removed within 35 min, and approximately 88% deamination efficiency was achieved in 60 min. The aniline degradation process followed the pseudo-first-order kinetic (k = 0.177 min(-1)) with activation energy (Ea) of 49.4 kJ mol(-1). Aniline could be removed in a broad initial pH (3-8) due to the excellent pH-tolerance property of the catalyst. The detected ammonium ion indicated that deamination occurred during aniline degradation. It was proposed that deamination synchronized with aniline removal, and aniline was attacked by free radicals to generate benzoquinonimine and phenol. This system is promising for the removal of aniline from water.

A review of iron species for visible-light photocatalytic water purification

Iron species are one of the least toxic and least expensive substances that are photocatalytic in the visible region of the spectrum. Therefore, this article focuses on iron-based photocatalysts sensitive to visible light. Photo-Fenton reactions are considered with respect to those assisted by and involve the in situ production of H₂O₂. The possible role that photoactive iron species play by interacting with natural organic matter in water purification in the natural environment is considered. The review also considered photosensitization by phthalocyanines and the potential role that layered double hydroxides may have not only as catalyst supports but also as photosensitizers themselves. Finally, photocatalytic disinfection of water is discussed, and the desirability of standardized metrics and experimental conditions to assist in the comparative evaluation of photocatalysts is highlighted.

Oxidation of aquatic pollutants by ferrous-oxalate complexes under dark aerobic conditions

This study evaluates the ability of Fe(II)-oxalate complexes for the generation of OH through oxygen reduction and the oxidative degradation of aquatic pollutants under dark aerobic conditions (i.e., with oxygen but without light). The degradation of 4-chlorophenol (4-CP) was rapid in the mixture of Fe(2+) and oxalate prepared using ultrapure water, but was absent without either Fe(2+) or oxalate. The formation of Fe(II)-oxalate complexes enables two-electron reduction of oxygen to generate H2O2 and subsequent production of OH. The significant inhibition of 4-CP degradation in the presence of H2O2 and OH scavenger confirms such mechanisms. The degradation experiments with varying [Fe(2+)], [oxalate], and initial pH demonstrated that the degradation rate depends on [Fe(II)(Ox)2(2-)], but the degree of degradation is primarily determined by [Fe(II)(Ox)2(2-)]+[Fe(II)(Ox)(0)]. Efficient degradation of diverse aquatic pollutants, especially phenolic pollutants, was observed in the Fe(II)-oxalate complexes system, wherein the oxidation efficacy was primarily correlated with the reaction rate constant between pollutant and OH. The effect of various organic ligands (oxalate, citrate, EDTA, malonate, and acetate) on the degradation kinetics of 4-CP was investigated. The highest efficiency of oxalate for the oxidative degradation is attributed to its high capability to enhance the reducing power and low reactivity with OH.

Distinct effects of oxalate versus malonate on the iron redox chemistry: Implications for the photo-Fenton reaction

The dicarboxylic acids oxalate (Oxal) and malonate (Mal) are frequently detected as the final low-molecular-weight organic acids during oxidative degradation of aromatic compounds. Here a distinct effect of Oxal versus Mal on iron-based photocatalytic technologies was reported by testing the degradation efficiency of the dye rhodamine B. The rates of dye degradation in irradiated Fe(III) solutions depended on Fe(III/II) speciation, photoreactivities of Fe complexes and reactivities of Fe(II) complexes with H2O2. Photolysis of the Fe(III)-oxalato complex was favorable due to the formation of O2-, HO2 and OH for oxidizing the dye; however, an excess of H2O2 could quench the excited state of ferrioxalate, decreasing the degradation efficiency. In contrast, activities of UV/Fe(III) in the presence of Mal were significantly diminished because Fe(III)-Mal complexes, with much lower quantum yield of Fe(II) from photoreduction, dominated Fe(III) speciation. The results provide data for an understanding of the mechanism of iron redox (photo)chemistry mediated by diacids, which will aid in selecting appropriate Fe ligands, screening photo-Fenton conditions and designing UV/Fe(III) treatability.

Bottom-up approach for the reaction of xenobiotics and their metabolites with model substances for natural organic matter by electrochemistry-mass spectrometry (EC-MS)

Risk assessment of xenobiotics requires a comprehensive understanding of their transformation in the environment. As most of the transformation processes usually involve a redox reaction or a hydrolysis as the first steps of the transformation, we applied an approach that uses an electrochemical cell to investigate model "redox" reactions in aqueous solutions for environmental processes. We investigated the degradation of a variety of xenobiotics from polar to nonpolar and analyzed their degradation products by on-line coupling of electrochemistry with mass spectrometry (EC-MS). Furthermore, we evaluated possible binding reactions with regard to the generation of non-extractable residues with some model substances (catechol, phthalic acid, γ-L-Glutamyl-L-cysteinyl-glycine (GSH) and L-histidine) deduced from a natural organic matter (NOM) structure model and identified possible binding-sites. Whereas typically investigations in soil/water-systems have been applied, we used to our knowledge for the first time a bottom-up approach, starting from the chemicals of interest and different model substances for natural organic matter to evaluate chemical binding mechanisms (or processes) in the EC-MS under redox conditions. Under oxidative conditions, bindings of the xenobiotics with catechol, GSH and histidine were found, but no reactions with the model compound phthalic acid were observed. In general, no chemical binding has yet been found under reductive conditions. In some cases (i.e. benzo[a]anthracene) the oxidation product only underwent a binding reaction, whereas the xenobiotic itself did not undergo any reactions. EC-MS is a promising fast and simple screening method to investigate the environmental behavior of xenobiotics and to evaluate the potential risks of newly synthesized substances.

Changes in physiological activity of algae Desmodesmus quadricauda after active bioaccumulation of newly prepared and characterized Fe(III) complexes with pyridine-3-carboxamide (pca) by living algal cells

The study characterized five iron(III) complexes with heterocyclic N-donor ligand pyridine-3-carboxamide (pca) [FeCl(3)(pca)(3)], [Fe(H(2)O)(2)(pca)(3)](ClO(4))(3), [Fe(2)O(ac)(2)(pca)(6)]Cl(2).3H(2)O, [Fe(NO(3))(3)(pca)(3)].3H(2)O, [Fe(Cl(2)ac)(3)(pca)(3)] (ac=acetate, Cl(2)ac=dichloroacetate) and their effects on biomass, chlorophylls (a, b), photosynthetic oxygen production and iron biosorption in algae Desmodesmus quadricauda. The effects of Fe(III) complexes were compared with control and those of FeCl(3).6H(2)O. While pca coordination to iron atom through the nitrogen atom of its heterocyclic ring mostly increased iron inhibitory effect on algal biomass and chlorophylls production, oxygen production was enhanced. The exceptions were observed only for [Fe(2)O(ac)(2)(pca)(6)]Cl(2).3H(2)O complex effect on biomass and oxygen production and [Fe(H(2)O)(2)(pca)(3)](ClO(4))(3) complex effect on chlorophylls production. Complexation increased iron biosorption in algal biomass and iron accumulated amount in algae was 2.8-20 times higher than that from FeCl(3).6H(2)O with maximal accumulation from dimeric complex [Fe(2)O(ac)(2)(pca)(6)]Cl(2).3H(2)O.