Photocatalytic Mechanisms for Peroxymonosulfate Activation through the Removal of Methylene Blue: A Case Study

Industrial activity is one of the most important sources of water pollution. Yearly, tons of non-biodegradable organic pollutants are discharged, at the least, to wastewater treatment plants. However, biological conventional treatments are unable to degrade them. This research assesses the efficiency of photocatalytic activation of peroxymonosulfate (PMS) by two different iron species (FeSO₄ and Fe³⁺-citrate) and TiO₂. These substances accelerate methylene blue removal by the generation of hydroxyl and sulfate radicals. The required pH and molar ratios PMS:Fe are crucial variables in treatment optimization. The kinetic removal is reduced by the appearance of scavenger reactions in acidic and basic conditions, as well as by the excess of PMS or iron. The best performance is achieved using an Fe³⁺-citrate as an iron catalyst, reaching the total removal of methylene blue after 15 min of reaction, with a molar ratio of 3.25:1 (1.62 mM of PMS and 0.5 mM Fe³⁺-citrate). Fe³⁺-citrate reached higher methylene blue removal than Fe²⁺ as a consequence of the photolysis of Fe³⁺-citrate. This photolysis generates H₂O₂ and a superoxide radical, which together with hydroxyl and sulfate radicals from PMS activation attack methylene blue, degrading it twice as fast as Fe²⁺ (0.092 min^-1 with Fe²⁺ and 0.188 min^-1 with Fe³⁺-citrate). On the other hand, a synergistic effect between PMS and titanium dioxide (TiO₂) was observed (S = 1.79). This synergistic effect is a consequence of PMS activation by reaction with the free electron on the surface of TiO₂. No differences were observed by changing the molar ratio (1.04:1; 0.26:1 and 0.064:1 PMS:TiO₂), reaching total removal of methylene blue after 80 min of reaction.