Removal of polycyclic aromatic hydrocarbons (PAHs) from groundwater by heterogeneous photocatalysis under natural sunlight

Polycyclic aromatic hydrocarbons (PAHs) are a class of persistent organic pollutants of special concern because they are carcinogenic and mutagenic compounds. In this paper, photodegradation of a mixture of six PAHs in groundwater at pilot plant scale is reported. Semiconductor materials (ZnO and TiO2) as photocatalysts in tandem with Na2S2O8 as oxidant under natural sunlight were used. The PAHs were benzo[a]pyrene, benzo[b]fluoranthene, benzo[ghi]perylene, benzo[k]fluoranthene, fluoranthene, and indene[1,2,3-cd]pyrene. As expected, the influence of both semiconductors on the degradation of PAHs was very significant in all cases. Photocatalytic experiments show that the addition of photocatalyst, especially for ZnO/Na2S2O8 system, strongly improves the elimination of PAHs in comparison with photolytic tests; significantly increasing the reaction rates. The first-order equation (monophasic model) satisfactorily explained the disappearance process although it ignores small residues remaining late in the process. These residues are important from an environmental point of view and the Hoerl function (biphasic model) better predict the results obtained. In our conditions, the time required for 90% degradation was in the range 7–15 min and 18–76 min for ZnO and TiO2 systems, respectively. Thus, the use of the tandem ZnO/Na2S2O8 makes possible the economical decontamination of groundwater containing non-biodegradable pollutants like PAHs.