Fenton-like oxidation and mineralization of phenol using synthetic Fe(II)–Fe(III) green rusts

Abstract

Background, aim, and scope

In literature, the environmental applications of green rust (GR) have mainly been pointed out through the reduction of inorganic contaminants and the reductive dechlorination of chlorinated organics. However, reactions involving GR for the oxidation and mineralization of organic pollutants remain very scantly described. In this study, the ability of three synthetic Fe(II)–Fe(III) green rusts, GR(CO ²⁻₃ ), GR(SO ²⁻₄ ), and GR(Cl⁻), to promote Fenton-like reaction was examined by employing phenol as a model pollutant. Unlike the traditional Fenton’s reagent (dissolved Fe(II) + H₂O₂), where the pH values have to be lowered to less than 4, the proposed reaction can effectively oxidize the organic molecules at neutral pH and could avoid the initial acidification which may be costly and destructive for the in situ remediation of contaminated groundwater and soils. The green rust reactivity towards the oxidative transformation of phenol was thoroughly evaluated by performing a large kinetic study, chemical analyses, and spectroscopic investigations.

Materials and methods

The kinetics of phenol removal was studied at three initial phenol concentrations for three green rusts under similar conditions (pH = 7.1; 1 g L⁻¹ of GR; 30 mM H₂O₂) and reaction rates were calculated based on mass and surface area. The oxidation rate constants are compared with that of magnetite, a well-known mixed iron (II, III) oxide. The mineralization of phenol was investigated at various H₂O₂ doses and GR concentrations. In order to describe the phenol transformation in GR/H₂O₂ system, several investigations were performed including HPLC and ion exclusion chromatography analysis, TOC, dissolved iron, and H₂O₂ concentration measurements. Finally, X-ray powder diffraction and Raman spectroscopy were used to identify the oxidation products of GRs.

Results and discussion

In GR/H₂O₂ system, the kinetics of phenol removal at neutral pH was very fast and independent of the initial phenol concentration. No aromatic intermediates were detected and final by-products are mainly of short chain organic acids (oxalic acid and formic acid). Green rusts exhibit different reactivity toward Fenton-like oxidation of phenol. Both on mass and surface area basis, the reactivity of Fe(II)–Fe(III) species toward the oxidation of phenol was highest for GR(Cl⁻), little less for GR(SO ²⁻₄ ) or GR(CO ²⁻₃ ), and even less for magnetite (Fe₃O₄). Phenol degradation pseudo-first order rate constants (k _surf) values were found to be: 13 × 10⁻⁴, 3.3 × 10⁻⁴, 3.5 × 10⁻⁴, and 0.4 × 10⁻⁴ L m⁻² s⁻¹ for GR(Cl⁻), GR(SO ²⁻₄ ), GR(CO ²⁻₃ ), and Fe₃O₄, respectively. The mineralization yield of phenol as well as the decomposition rate of H₂O₂ was higher for GR(Cl⁻) than for GR(SO ²⁻₄ ) or GR(CO ²⁻₃ ), mainly due to the higher Fe(II) content of GR(Cl⁻). Both X-ray diffraction analysis and Raman spectroscopy showed that the oxidation of GR with H₂O₂ may lead to feroxyhyte (δ-FeOOH), with possible formation of poorly crystallized goethite (α-FeOOH), depending on GR type.

Conclusions

This original work shows that the heterogeneous Fenton-like reaction using GR/H₂O₂ is very effective toward degradation and mineralization of pollutants. In summary, this study has demonstrated that the green rust-promoted oxidation reaction could contribute to the transformation of water contaminants in the presence of H₂O_2.

Recommendations and perspectives

These results could serve as the basis for the understanding of the transformation of organic pollutants in iron-rich soils in the presence of chemical oxidant (H₂O₂) or for the development of wastewater treatment process. However, some experimental parameters should be optimized for a high-scale application. Further work needs to be done for the reactive transport and transformation of organic compounds in a green rust-packed column. The reusability of GR in mineral-catalyzed reaction should be also investigated.