Degradation of trinitrophenol by sequential catalytic wet air oxidation and solar TiO2 photocatalysis

Catalytic wet air oxidation (CWAO) and solar TiO2 photocatalysis were investigated as advanced oxidation processes to degrade trinitrophenol (TNP) in model aqueous solutions. An activated carbon (AC) treated with sulphuric acid of different concentrations (5, 10 and 18 M) at two different temperatures (353 and 423 K) was investigated as a metal-free CWAO catalyst, while a commercially available P25 TiO2 powder was used as a photocatalyst. CWAO experiments were conducted at 448 K, 0.7 MPa oxygen pressure (4.7 MPa of total pressure), 1.3 g L−1 AC loading and 270 mg L−1 TNP concentration, while photocatalytic experiments were conducted at ambient temperature, 1 g L−1 photocatalyst loading, 500–1000 W m−2 irradiance provided by a solar simulator and 32–270 mg L−1 TNP concentration. Treatment efficiency was assessed by measuring the concentrations of TNP and nitrates, total organic carbon (TOC) and biochemical oxygen demand (BOD5). Up to 90% TNP degradation was attained during CWAO over 120 min, from an initial concentration of 270 mg L−1. For the same TNP concentration, TiO2 photocatalysis gives only 13% conversion over the same 120 min. However, for TNP concentrations below 144 mg L−1, photocatalysis can be effectively used: 100 and 80% TNP degradation obtained in 120 min of irradiation for initial TNP concentrations of 64 and 144 mg L−1, respectively. In this respect, CWAO and photocatalysis were employed sequentially to treat TNP; complete TNP conversion being achieved after 120 min of CWAO followed by 60 min of photocatalysis at 1000 W m−2 irradiance, and this was accompanied by 82% TOC reduction, as well as an increase of BOD5/TOC ratio from 0 to 2.28.

• Removal of trinitrophenol, also known as picric acid, from aqueous streams.
• Integrated treatment comprising catalytic wet air oxidation and solar-driven heterogeneous photocatalysis.
• Carbon materials treated with sulphuric acid are active metal-free catalysts for wet air oxidation.
• Photocatalysis can be effectively used to polish trinitrophenol aqueous solutions pre-treated by catalytic wet air oxidation, increasing the effluent biodegradability.
• Low molecular weight carboxylic acids are the main by-products of trinitrophenol oxidation.