A sustainable method for oxidizing phenanthrene in tropical soil using natural iron as a catalyst in a slurry phase reactor with persulfate assistance†

The presence of impurities is a significant restriction to the use of natural iron minerals as catalysts in the advanced oxidation process (AOP), especially if applied for soil remediation. This study evaluated the catalytic activity of tropical soil, which has relatively low impurities and naturally contains iron, for the remediation of phenanthrene (PHE) contamination. The system showed good performance, and the best result was 81% PHE removal after 24 h under experimental conditions of pH 7, [PHE]₀ = 300 mg/50 g soil, temperature 55 °C, air flow = 260 mL min⁻¹, and [persulfate]₀ = 20 mg kg⁻¹, while the mineralization was 61%. Nevertheless, certain limitations were noted in the soil matrix following the remediation procedure, including the appearance of cracks in the soil aggregate, reduction in the crystal size of the soil particles, and decline in the iron and aluminium contents. The results confirmed that the radicals play a major role in the remediation process. SO₄˙⁻ was more dominant than O₂˙⁻, while HO˙ played a minor role. Additionally, the by-products were detected by gas chromatography-mass spectroscopy (GC-MS), and the degradation pathway of PHE is proposed. Toxicity assessment tests were performed by using a computational method. In spite of the challenges, this research achieved notable progress in soil remediation, taking a significant step forward in implementing the AOP without catalysts to activate oxidants and remove PHE within the soil. Also, this approach supports sustainability by reducing the need for extra materials and providing an environmentally friendly way of soil remediation.