Optimization of the electro-Fenton and solar photoelectro-Fenton treatments of sulfanilic acid solutions using a pre-pilot flow plant by response surface methodology

A central composite rotatable design and response surface methodology were used to optimize the experimental variables of the electro-Fenton (EF) and solar photoelectro-Fenton (SPEF) degradations of 2.5L of sulfanilic acid solutions in 0.05M Na ₂SO ₄. Electrolyses were performed with a pre-pilot flow plant containing a Pt/air diffusion reactor generating H ₂O ₂. In SPEF, it was coupled with a solar photoreactor under an UV irradiation intensity of ca. 31Wm ^-2. Optimum variables of 100mAcm ^-2, 0.5mM Fe ²⁺ and pH 4.0 were determined after 240min of EF and 120min of SPEF. Under these conditions, EF gave 47% of mineralization, whereas SPEF was much more powerful yielding 76% mineralization with 275kWhkg ^-1 total organic carbon (TOC) energy consumption and 52% current efficiency. Sulfanilic acid decayed at similar rate in both treatments following a pseudo-first-order kinetics. The final solution treated by EF contained a stable mixture of tartaric, acetic, oxalic and oxamic acids, which form Fe(III) complexes that are not attacked by hydroxyl radicals formed from H ₂O ₂ and added Fe ²⁺. The quick photolysis of these complexes by UV light of sunlight explains the higher oxidation power of SPEF. NH ₄ ⁺ was the main inorganic nitrogen ion released in both processes.