Electrochemical treatment of recalcitrant waste: A study of chlorophenols and nitroaromatic compounds

Loading...
Thumbnail Image

Date

Authors

Rodgers, James Donald

Journal Title

Journal ISSN

Volume Title

Publisher

University of Guelph

Abstract

Electrochemical treatment was investigated as a treatment option for the oxidation of chlorophenols and the reduction of nitroaromatics. Chlorophenols are used in the manufacturing of pesticides and herbicides and are added to many formulations to prevent microbial growth. Nitroaromatics are associated with the explosives industry and have caused widespread contamination as a result of their use and manufacturing. Both compounds pose environmental threats as a result of their recalcitrance to chemical or biological oxidation. In the case of chlorinated phenols, we explored the problem of anode fouling which has previously hampered electrolytic treatment of phenolic compounds. Linear sweep voltammograms at a Pt anode were examined for phenols differing in the extent of chlorination. Passivation increased in parallel with the uncompensated resistance of the solution and occurred only at potentials at which water is oxidized, suggesting that the formation of the oligomer film involves attack of hydroxyl radicals on electrochemically oxidized substrate. Chronopotentiometry was used to deduce the oxidation potential of the chlorophenols (~1.3 V vs. saturated calomel electrode) and the number of electrons involved in oxidation; monochlorophenol and trichlorophenol required four electrons while pentachlorophenol required only two. During electrolysis, relative reactivities of congeners were anode-dependent, which we interpreted as different mechanisms of oxidation: direct electron transfer oxidation at PbO2 and hydroxyl radical attack at SnO2 and IrO 2. Voltammetry of nitroaromatics (2,4-dinitrotoluene, 2,6-dinitrotoluene, 2,4,6-trinitrotoluene) at pH 2 and at a mercury electrode was consistent with literature values (e.g. 2,4,6-trinitrotoluene; -0.18 V, -0.35 V, -0.48 V vs. saturated calomel electrode) suggesting electrolysis as a viable alternative to current techniques such as granular activated carbon and advanced oxidation processes. Dinitrotoluene was reduced at several cathodes with the most promising result at Ni-plated Ni wire were at 0.1 mA cm-2 with current efficiencies >80%. Subsequent treatment of the reduction products (aminotoluenes) was shown to be possible with enzymatic catalysis, air sparging and anodic oxidation. The latter alternative was the most successful in terms of efficiency and simplicity.

Description

Keywords

recalcitrant waste, electrochemical treatment, oxidation, chlorophenols, nitroaromatic compounds, environmental threat

Citation