Photocatalytic degradation of TNT from water in UV-VIS/Fe-TiO2 System

2,4,6-Trinitrotoluene(TNT) is one of most common toxic pollutant identified in
wastewater generated from ammunitions plants. Due to its potential carcinogenic
characteristics, TNT presence in water bodies represents a risk for human health and
aquatic life. Among modern treatment methods, TiO2 photocatalysis was successfully
applied in order to remove toxic pollutants. Fe-TiO2 assisted photocatalytic degradation
of TNT in aqueous media, under UV-VIS irradiation was studied. The effects of
operating parameters on photocatalytic process performances, kinetic and mechanism
of pollutant degradation were investigated. Solutions with (0.27-2.72) x 10-4 M TNT
content were photo-oxidized using a medium pressure Hg lamp as UV-VIS light source
(λ = 320 – 550 nm), in the following working conditions: pH = 7; photocatalyst dose =
50 – 500 mg/L; irradiation time = 30 - 240min. Prior to irradiation, the photocatalyst
was added to samples, and resulted suspension was bubbled with air (50 L/h). In order
to evaluate the effect of the main active species involved in Fe-TiO2 assisted
photocatalytic degradation of TNT we suppressed the free •.OH radicals mediated
process by addition of 16 x 10- 3M iso-propanol (i-PrOH) scavenger. Lock of •.OHads
radicals’ production on the catalyst surface was assured by addition of 16 x 10- 3M
sodium iodide (NaI). The initial and irradiated samples were analysed for TNT, NO3-,
NO2- and NH4+ concentrations by Gas Chromatography (GC), and Ion
Chromatography respectively. In the tested experimental conditions, at 2.72 x 10-4 M
pollutant concentration, the increase of catalyst load up to 200 mg/L leads to the
enhancement of initial TNT degradation rate up to 0.64 x 10-7 Ms-1. Since, ten times
increase of initial TNT content has a negative effect on pollutant degradation rate
constant, in similar experimental condition, prolonged irradiation time from 60 to 240
min was needed in order to assure pollutant advanced degradation efficiencies (≥
99.9%). The TNT degradation and its inorganic by-products formation obeyed a
pseudo-first-order kinetics. The experimental results of the reactive species quenching
showed that •.OH radicals was the predominant oxidant species participated in
reaction, and the pollutant degradation occurred mainly on the surface of catalyst.