Polycyclic aromatic hydrocarbons (PAHs) are hydrophobic aromatic compounds consisting of two or more condensed benzene rings, having greater teratogenicity, carcinogenicity, mutagenicity and toxicity. The most representative PAHs is benzo(a)pyrene (B[a]p), which is composed of five benzene rings, and is classified as a Group I carcinogen. The treatment technologies for B[a]p removal include bioremediation, physical remediation and chemical remediation. The advanced oxidation technology based on sulfate radicals becomes a recent research hotspot because of its high redox potential, high reactivity and environmental friendliness. Nevertheless, persulfate needs to be activated through various approaches to produce a series of reactive species, and iron-based materials are widely used due to the abundant reserves, environmental friendliness and excellent activation performance. In this paper, pyrite (FeS₂) was used as a cheap and non-toxic sulfide mineral, allowing for a slow release of Fe²⁺ as well as the cycling of Fe²⁺ and Fe³⁺. Kaolinite was chosen as a suitable carrier to solve the agglomeration of FeS₂ nanoparticles. A composite with FeS₂ and kaolinite was prepared to obtain efficient persulfate activation and B[a]p degradation.

Ferrous sulfate heptahydrate (FeSO₄·7H₂O), sodium thiosulfate (Na₂S₂O₃), cetyltrimethylammonium bromide and kaolinite were ultrasonically mixed and stirred for 30 min, and then the solution was transferred to a reactor for the hydrothermal reaction. At the end of the reaction, the solution was centrifuged and washed for three times, and then put into a vacuum oven for 12 h to obtain the composite catalyst.

The B[a]p solution, desired dosage of catalysts and peroxymonosulfate (PMS) solution were added in a centrifuge tube of 50.00 mL, and then conducted in a rotating mixer at room temperature. Before adding PMS solution, the adsorption-desorption equilibrium process was carried out under 30 min continuous stirring. Moreover, 1.00 mL of the mixture was taken at the setting time points and quenched with 0.50 mL of MeOH. Finally, the mixture was analyzed by high-performance liquid chromatography (HPLC) after filtration.

Also, 4.00 g B[a]p-contaminated soil, desired dosage of catalysts and PMS were added in a centrifuge tube of 50.00 mL, and then conducted in a rotating mixer for 60 min at room temperature. After 60 min reaction, the supernatant was poured after centrifugation, and then the soil sample with an extraction fluid (MeOH and acetonitrile in a volume ratio of 1∶1) was ultrasonicated for 60 min to obtain 20.00 mL of soil extract. Finally, the supernatant was withdrawn and filtered for HPLC to test the residual B[a]p concentration in soil samples after centrifugation.

According to the analysis by scanning electron microscopy and energy dispersive spectroscopy, a composite with kaolinite and layered FeS₂, can be prepared. The specific surface area of FeS₂/kaolinite slightly increases, and the particle size of FeS₂/kaolinite is smaller than that of FeS₂, which is attributed to the elimation of the agglomeration via the introduction of kaolinite and the improvement of the dispersibility of FeS₂, resulting in the superior catalytic performance for PMS activation and B[a]p removal.

The results show that the B[a]p removal rate is 93.00% (liquid-phase) and 69.02% (soil) under the optimum conditions (i.e., 2.0 g/L catalyst, 20 mg/L B[a]p concentration, 2 mmol/L PMS concentration (liquid-phase) and 50 mg/g catalyst, 80 mg/g PMS, 3∶1 water/soil ratio (in mass), respectively). The acidic and neutral conditions are more suitable for the PMS activation and B[a]p removal. Also, the B[a]p removal is affected in the presence of H₂PO₄^2- and HCO₃^-. The existence of Cl^- and NO₃^- promotes the B[a]p degradation efficiency. Based on the quenching experiments, the reactive species including ¹O₂, SO₄^•- and O₂^•- are proved to be involved in the process of PMS activation and B[a]p degradation, and ¹O₂ and O₂^•- are further confirmed to be the dominant reactive species.

Based on the mechanism of PMS activation in FeS₂/kaolinite/PMS, the content of Fe²⁺, S₂^2- and S⁰ is decreased after the catalytic reaction, indicating that Fe²⁺ is the main driving force for the PMS activation to generate ¹O₂, SO₄^•- and O₂^•-, and the cycling of Fe²⁺ and Fe³⁺ is facilitated in the presence of S₂^2- and S⁰ possessing reductivity. In addition, the oxygen-containing functional groups contained in kaolinite can be also beneficial to the PMS activation.

A composite of FeS₂/kaolinite was synthesized by an one-step hydrothermal method for PMS activation and B[a]p degradation. The degradation results demonstrated that 93.00% B[a]p was removed within 15 min under the optimized conditions (i.e., 2.0 g/L FeS₂/kaolinite, 20 mg/L B[a]p concentration, and 2 mmol/L PMS concentration). ¹O₂ and O₂^•- played a dominant role in FeS₂/kaolinite/PMS for PMS activation and B[a]p degradation. The excellent catalytic performance of FeS₂/kaolinite was also confirmed in the degradation experiment of B[a]p removal in soil, with 69.02% B[a]p degradation efficiency under the optimized conditions (i.e., 50 mg/g FeS₂/kaolinite, 80 mg/g PMS, 3∶1 water/soil ratio and pH value 3). The high catalytic performance of FeS₂/kaolinite was mainly due to the cycling of Fe²⁺ and Fe³⁺ as well as the reduction reaction between Fe³⁺ and S₂^2-/S⁰.