Reduction of p-nitrophenol in an airlift electrochemical reactor with iron electrodes

Abstract

Kinetic experiments of removal of p-nitrophenol (p-NP) from aqueous solution were performed in an airlift electrochemical reactor at 19.3 ± 0.5 °C and 1 atm. It was demonstrated that a simple and low-cost electrochemical reacting system with iron electrodes is able to reduce efficiently p-NP to a more friendly environment product. The rate limiting step of p-NP reduction and production of p-aminophenol at apparent rate constants between 6.03 ± 0.22 × 10–5 and 9.85 ± 0.45 × 10–5 s−1 was the one-electron transfer reaction at the liquid-cathode interface. The reaction involved the substituent nitro group as electron withdrawer with p-NP adsorbed on the cathode in a quasi-equilibrium condition. The residual amount of soluble ferrous ion, the major product of electrolytic oxidation of zero valent iron at the anode, was lower than the allowable limit of 2.68 × 10–4 mol L−1. The kinetics of oxidation of Fe2+ to ferric ion and the hydrolysis of Fe3+ to iron hydroxides was properly described by a reaction rate that was proportional to the partial pressure of oxygen, first- and nth-order in Fe2+ and hydroxyl anion, respectively. The rate constant of the overall oxidation/hydrolysis reaction of Fe2+ was 6.31 ± 0.63 × 1014 L2 mol−2 atm−1 s−1 for pH from ∼5.2–6.6 (n = 2) and 3.75 ± 0.88 × 10–8 mol L−1 atm−1 s−1 for pH from ∼6.9–8.8 (n = −1).