P-Nitrophenol degradation by activated persulfate

Abstract

The degradation of p-nitrophenol (PNP), which is listed as a priority pollutant by the Environmental Protection Agency (EPA), was investigated in this work by the activated persulfate oxidation. Different forms of persulfate activation (namely thermal, by medium basification and with an iron salt catalyst) were evaluated, apart from founding the best operating conditions for maximizing process performance (assessed in terms of PNP removal and mineralization — evaluated by the overall total organic carbon (TOC) reduction). For the thermal and alkaline activation of persulfate, the temperature was varied in the range of 30 to 90 ºC and the pH between 3.0 to 9.0, respectively, being found that best performances exist for a reaction temperature of 70 ºC while pH above 7.0 is not necessary. In short reaction times (≤10 min) the oxidation of PNP, mineralization and consumption of persulfate were successfully fitted to a second-order kinetic model and the apparent activation energies determined. The effect of persulfate concentration was determined in the range of 0.8 g/L to 9.6 g/L, being noticed that an increase in its concentration above 6.4 g/L led to worst process performances. An additional improvement was observed with the presence of iron in solution, indicating that the metal salt plays an important role in catalyzing the persulfate activation — again, an optimal iron dose was observed (80 mg/L). The combination of the three ways to activated the persulfate allowed obtaining the highest PNP (94%) and TOC (66%) removals, showing that the process is promising in the degradation of PNP and organics mineralization. Radical scavenging runs allowed concluding that both radicals, sulfate and hydroxyl, are forming during the oxidation process, however the role of SO4•− predominates. The identification and quantification of reaction intermediates was also assessed and the contribution of carboxylic acids (oxalic, maleic and pyruvic) in TOC measured was found to be higher than 95%. Finally, the effect of the matrix was evaluated, and for this purpose a domestic wastewater was spiked with 500 mg/L of PNP; it was found that the degradation of the pollutant was almost not affected by the water matrix.