Catalytic performance of thermochemically treated sludge generated in the Fenton process for AOX degradation

• Fenton catalysts were prepared via the pyrolysis of iron-rich Fenton sludge. • Optimal pyrolysis conditions to produce Fenton catalysts were 600 °C and 30 min. • Pyrolyzed Fenton sludges allowed for over 50 % AOX removal from pulp bleaching wastewater. • High catalytic activity was kept over three consecutive cycles without regeneration. • Most AOX was removed by oxidation – less than 10 % by adsorption on the sludge surface. Although the scientific community recognises the issue of iron wastage as ferric sludge following Fenton processes, there has been limited focus on valorising of this sludge. This work proposes a circular solution to valorise Fenton sludge as catalyst of the Fenton process in the treatment of industrial wastewater. To achieve this, sludges were collected after treating pulp bleaching wastewater with Fenton processes catalysed either by iron sulphate (homogeneous Fenton) or residual iron dust (heterogeneous Fenton). These sludges were then pyrolyzed for 30 min at 600 °C. Subsequently, raw wastewater was treated using four different catalysts: two untreated sludges and two pyrolyzed sludges from both Fenton processes. All materials achieved similar AOX removal ranging 50 – 58 % within 30 min. Untreated sludges maintained consistent catalytic activity over three consecutive treatment cycles without regeneration, while the AOX removal with pyrolyzed sludges decreased to approximately 30 % after three cycles. However, pyrolyzed sludges released up to 90 % less iron into the wastewater, reducing secondary pollution and suggesting a longer lifespan. Most AOX removal was due to oxidation, primarily driven by • OH, with less than 10 % attributable to adsorption onto the catalyst surface.