Merits and limitations of TiO₂-based photocatalytic pretreatment of soils impacted by crude oil for expediting bioremediation

Abstract

Heavy hydrocarbons (HHCs) in soils impacted by crude oil spills are generally recalcitrant to biodegradation due to their low bioavailability and complex chemical structure. In this study, soils were pretreated with varying concentrations of ultraviolet radiation A (UVA) or ultraviolet radiation C (UVC) activated titanium dioxide (TiO₂) (1%–5%) under varying moisture conditions (0%–300% water holding capacity (WHC)) to enhance biodegradation of HCCs and shorten remediation timeframes. We demonstrate that pretreatment of impacted soils with UVC-activated TiO₂ in soil slurries could enhance bioremediation of HHCs.  Total petroleum hydrocarbon (TPH) removal after 24 h exposure to UVC (254 nm and 4.8 mW/cm²) was (19.1±1.6)% in slurries with 300% WHC and 5 wt-% TiO₂. TPH removal was non-selective in the C15-C36 range and increased with moisture content and TiO₂ concentration. In a 10-d bioremediation test, TPH removal in treated soil increased to (26.0±0.9)%, compared to (15.4±0.8)% for controls without photocatalytic pre-treatment. Enhanced biodegradation was also confirmed by respirometry. This suggests that addition of UVC-activated TiO₂ to soil slurries can transform recalcitrant hydrocarbons into more bioavailable and biodegradable byproducts and increase the rate of subsequent biodegradation. However, similar results were not observed for soils pretreated with UVA activated TiO₂. This suggests that activation of TiO₂ by sunlight and direct addition of TiO₂ to unsaturated soils within landfarming setting may not be a feasible approach. Nevertheless, less than 1% of UVA (7.5 mW/cm²) or UVC (1.4 mW/cm²) penetrated beyond 0.3 cm soil depth, indicating that limited light penetration through soil would hinder the ability of TiO₂ to enhance soil bioremediation under land farming conditions.

Graphical abstract

Keywords

TiO₂ pretreatment / bioremediation / total petroleum hydrocarbons / ultraviolet

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Higher Education Press and Springer-Verlag Berlin Heidelberg