Degradation of p-nitrophenol by heat and metal ions co-activated persulfate

• Temperature has significant impact on the degradation of PNP.
• Co-activated persulfate exhibited chaotic and complicated effect on PNP degradation.
• The formation of precipitate restrained the diffusion of persulfate and SO4-.
• Degradation pathway of PNP proposed according to the intermediates formed.

The degradation of p-nitrophenol (PNP) by thermal and metal ions (Fe2+ or Cu2+) co-activated persulfate was studied in this work. The activation of persulfate toward PNP degradation was investigated at four temperatures viz. 25, 50, 60, 70 and 90 °C, and three molecular ratios between Metal ions:PS of 1:100, 1:10, 1:1 (PS was 30 mM). The results showed that the degraded rate of PNP could be enhanced either by increasing the temperature or the molecular ratio between Metal ions:PS. However, the co-activated persulfate exhibited a complicated effect on PNP degradation rather than a solely synergistic enhancement. At higher temperatures of 70 and 90 °C, the additive metal ions could act as scavenger toward SO4- at such higher temperatures, then would slow down the increasing trend. In the case of co-activation system at 70 °C, a decline in PNP degradation was observed beyond 90 min comparing to the sole thermal activation. It was caused by the precipitate formed during the process, which controlled and restricted the diffusion of persulfate and SO4-. The concentration of persulfate remaining in the solution was measured by an iodometric method. The TOC value during the process was detected and the TOC removal efficiency reached to 85.81% at 70 °C. Furthermore, the intermediates formed during the degradation process were detected and identified by high performance liquid chromatography and mass spectrometry, and an interpretation of the degradation pathway was made based on the findings.