Oxidation of 2,4-dichlorophenol and 3,4-dichlorophenol by means of Fe(III)-homogeneous photocatalysis and algal toxicity assessment of the treated solutions

Chlorophenols are used worldwide as broad-spectrum biocides and fungicides. They have half-life times in water from 0.6 to 550 h and in sediments up to 1700 h and, due to their numerous origins, they can be found in wastewaters, groundwaters or soils. Moreover, chlorophenols are not readily biodegradable.Recently, classic Advanced Oxidation Processes (AOP) have been proposed for their abatement in an aqueous solution. This paper investigates the oxidation of 2,4-dichlorophenol and 3,4-dichlorophenol, at starting concentrations of 6.1 · 10−5 mol L−1, in aqueous solutions through Fe(III)/O2 homogeneous photocatalysis under UV light (303 ÷ 366 nm). The Fe(III)/O2 homogeneous photocatalysis is less expensive than using H2O2 due to the capability of Fe(III) to produce OH radicals, if irradiated with an UVA radiation, and of oxygen to re-oxidize ferrous ions to ferric ones when dissolved in solution. The results show that the best working conditions, for both compounds, are found for pH = 3.0 and initial Fe(III) concentration equal to 1.5·10−4 mol L−1 although the investigated oxidizing system can be used even at pH close to 4.0 but with slower abatement kinetics. Toxicity assessment on algae indicates that treated solutions of 2,4-dichlorophenol are less toxic on algae Pseudokirchneriella subcapitata if compared to not treated solutions whereas in the case of 3,4-dichlorophenol only the samples collected during the runs at 20 and 60 min are capable of inhibiting the growth of the adopted organism.The values of the kinetic constant for the photochemical re-oxidation of iron (II) to iron (III) and for HO attack to intermediates are evaluated by a mathematical model for pH range of 2.0–3.0 and initial Fe(III) concentrations range of 1.5 · 10−5–5.2 · 10−4 mol L−1.

Research highlights
► In the present work the oxidation of 2,4-DCP and 3,4-DCP through the system Fe(III)/O2/UV was studied in the pH range 2.0–4.0.
►  It is possible to achieve a complete removal of the substrates by increasing the iron (III) concentration.
► High degrees of mineralization were obtained whereas the percentage of chlorine converted was lower than 45%.
► The toxicity decreasing during the treatment process.
► The adoption of a model enabled to best estimate the values of the kinetic constants.