Solar photoelectro-Fenton degradation of paracetamol using a flow plant with a Pt/air-diffusion cell coupled with a compound parabolic collector: Process optimization by response surface methodology

The degradation of 10L of 157mgL^-1 paracetamol solutions in 0.05M Na₂SO₄ has been studied by the solar photoelectro-Fenton (SPEF) method. A solar flow plant with a Pt/air-diffusion electrochemical cell and a compound parabolic collector (CPC) photoreactor was used operating under recirculation mode at a liquid flow of 180Lh^-1 with an average UV irradiation intensity of about 32Wm^-2. A central composite rotatable design coupled with response surface methodology was applied to optimize the experimental variables. Optimum SPEF treatment was achieved by applying a current of 5A, 0.40mM Fe²⁺ and pH 3.0 at 120min of electrolysis, being reduced total organic carbon (TOC) by 75%, with an energy cost of 93kWhkg^-1 TOC (7.0kWhm^-3) and a mineralization current efficiency of 71%. Initial N was partially converted into NH₄⁺ ion. Under these optimized conditions, paracetamol decays followed a pseudo first-order kinetics. HPLC analysis of the electrolyzed solution allowed the detection of hydroquinone, p-benzoquinone, 1,2,4-trihydroxybenzene, 2,5-dihydroxy-p-benzoquinone and tetrahydroxy-p-benzoquinone. All aromatics were destroyed by the attack of OH. Maleic, fumaric, succinic, lactic, oxalic, formic and oxamic acids were identified as generated carboxylic acids, which form Fe(III) complexes that are quickly photodecarboxylated under UV irradiation of sunlight. A reaction sequence involving all the detected byproducts was proposed for the SPEF degradation of paracetamol.