Fenton-like oxidation of 2,4,6-trinitrotoluene using different iron minerals

Abstract

Degradation of 2,4,6-trinitrotoluene (TNT) was investigated in presence of different oxidants (Fenton's reagent, sodium persulfate, peroxymonosulfate and potassium permanganate) and different iron minerals (ferrihydrite, hematite, goethite, lepidocrocite, magnetite and pyrite) either in aqueous solution or in soil slurry systems. Fenton's reagent was the only oxidant able to degrade TNT in solution (k_app = 0.0348 min^− 1). When using iron oxide as heterogeneous catalyst at pH 3, specific reaction rate constants per surface area were k_surf = 1.47.10^− 3 L min^− 1 m^− 2 and k_surf = 0.177 L min^− 1 m^− 2 for magnetite and pyrite, respectively while ferric iron minerals were inefficient for TNT degradation. The major asset of iron mineral catalyzed Fenton-like treatment has been the complete oxidation of the pollutant avoiding the accumulation of possible toxic by-products. In soil slurry systems, 38% abatement of the initial TNT concentration (2 g/kg) was reached after 24 h treatment time at neutral pH. Rate limiting steps were the availability of soluble iron at neutral pH together with desorption of the TNT fraction sorbed on the clay mineral surfaces.

Introduction

2,4,6-trinitrotoluene (TNT) has been the most widely used nitroaromatic explosive and TNT concentrations have been reported to range from 10 to 12,000 mg/kg at contaminated sites (Rodgers and Bunce, 2001). Most of these sites contain also contaminated groundwater. TNT is a US EPA priority compound and has been recognized as a mutagenic chemical (Lachance et al., 1999). Consequently, the health advisories for TNT have been issued to less than 0.002 mg/L by the US EPA (US EPA, 2002). In response to the need for clean up of contaminated soils and waters, a variety of physico-chemical remediation techniques has been implemented. Alkaline hydrolysis and zero valent iron treatments have been successfully applied for TNT degradation in water and soil treatment (Bandstra et al., 2005, Emmrich, 1999). However, these two technologies proceed through reductive pathways leading to the formation of an uncharacterized polymeric material, mainly in soil, upon prolonged TNT treatment (Thorn et al., 2004). The toxicities and susceptibilities to microbial and chemical degradation of the polymeric materials remain unknown and this issue is still a matter of concern for the broad application of these technologies. Fenton's reaction has also been used to treat TNT contaminated aqueous solution often resulting in complete degradation and partial mineralization with accumulation of short chain carboxylic acids (e.g. oxalic acid) at the end of the reaction time (Chen et al., 2005, Yardin and Chiron, 2006). Analogous data on Fenton-like reactions using iron minerals as heterogeneous catalysts is scarcely available (Hess and Scharder, 2002), although the efficiency of such processes has been clearly demonstrated for others organic pollutants (Huang et al., 2001, Kwan and Voelker, 2003). Heterogeneous catalysis offers significant advantages. Unlike Fenton's reagent, the reaction of iron oxides with H₂O₂ can effectively catalyse the oxidation of organic contaminants at circumneutral pH. Use of iron oxides instead of dissolved iron may be especially advantageous for in situ remediation of contaminated groundwater where pH cannot be adjusted. The catalyst can be easily recovered by sedimentation or filtration for further uses. Finally, results obtained in heterogeneous Fenton's reaction may be exploited for in situ chemical oxidation (ISCO) treatment where endogenous iron oxides have been already used for the elimination of few recalcitrant chemical families mainly, chlorinated ethenes (Kim and Gurol, 2005), PAHs (Flotron et al., 2005), pesticides (Mecozzi et al., 2006), and hydrocarbons (Kang and Hua, 2005). In this paper, the catalytic activity of six iron minerals for TNT oxidation was evaluated in aqueous solution. These iron minerals were selected because of their widespread abundance in natural soils and sediments, where they might be exploited for in situ degradation of TNT. The main purpose of this work was to investigate the feasibility of Fenton-like chemical oxidation processes for TNT remediation in water and soil slurry. The scope includes the following steps:

• The assessment of different oxidants in aqueous solution.

• The assessment of different iron oxides in aqueous solution.

• The assessment of the efficiency of modified Fenton's reagent in soil slurry systems.