Degradation of tyrosol by a novel electro-Fenton process using pyrite as heterogeneous source of iron catalyst

• Novel electro-Fenton process with pyrite as heterogeneous source of Fe2+ catalyst.
• Optimum operating conditions achieved with low pyrite load of 1.0 g L−1 at pH 3.0.
• About 90% mineralization of tyrosol solutions attained using a BDD/carbon-felt cell.
• Absolute rate constant of 3.57 × 109 M−1 s−1 determined for tyrosol hydroxylation.
• Identification of 5 aromatic products, o-benzoquinone and 5 aliphatic carboxylic acids as intermediates.

Tyrosol (TY) is one of the most abundant phenolic components of olive oil mill wastewaters. Here, the degradation of synthetic aqueous solutions of 0.30 mM TY was studied by a novel heterogeneous electro-Fenton (EF) process, so-called EF-pyrite, in which pyrite powder was the source of Fe2+ catalyst instead of a soluble iron salt used in classical EF. Experiments were performed with a cell equipped with a boron-doped diamond anode and a carbon-felt cathode, where TY and its products were destroyed by hydroxyl radicals formed at the anode surface from water oxidation and in the bulk from Fenton's reaction between Fe2+ and H2O2 generated at the cathode. Addition of 1.0 g L−1 pyrite provided an easily adjustable pH to 3.0 and an appropriate 0.20 mM Fe2+ to optimize the EF-pyrite treatment. The effect of current on mineralization rate, mineralization current efficiency and specific energy consumption was examined under comparable EF and EF-pyrite conditions. The performance of EF-pyrite was 8.6% superior at 50 mA due to self-regulation of soluble Fe2+ by pyrite. The TY decay in this process followed a pseudo-first-order kinetics. The absolute rate constant for TY hydroxylation was 3.57 × 109 M−1 s−1, as determined by the competition kinetics method. Aromatic products like 3,4-dihydroxyphenylethanol, 4-hydroxyphenylacetic acid, 4-hydroxybenzoic acid, 3,4-dihydroxybenzoic acid and catechol, as well as o-benzoquinone, were identified by GC–MS and reversed-phase HPLC. Short-chain aliphatic carboxylic acids like maleic, glycolic, acetic, oxalic and formic were quantified by ion-exclusion HPLC. Oxalic acid was the major and most persistent product found. Based on detected intermediates, a plausible mineralization pathway for TY by EF-pyrite was proposed.