*2.2.3.3. Activated carbon catalysts for CWAO*

Another promising catalyst could be activated carbon (AC) that shows good properties as adsorbent for both organic materials and oxygen because of its porous structure and high surface area [60, 61]. Activated carbon is stable in highly acidic and basic media and it is also easy to prepare, which is why it is used as a catalyst for different reactions [62], and also as a support for other oxidation catalysts [63, 64].

Activated carbon can also catalyze the polymerization reactions in the presence of oxygen via oxidative coupling. Phenol oxidation over activated carbon in trickle bed reactor has been investigated [65, 66]. The activated carbon was found less active than metal oxide catalysts but more stable and more environmentally accepted, and of course cost-effective [65, 66].

Phenol conversion was compared using copper catalyst and activated carbon [67]. In the long run, copper catalyst was found to lose its activity due to leaching of copper phase. On the other hand, activated carbon also exhibited a continuous drop in phenol conversion, starting from nearly complete and finally reaching about 48%. However, the loss of activated carbon efficiency could be ascribed to its consumption during experiments; thus, the absolute activity of activated carbon remained stable during the long term.

One of the recent studies was about the CWAO of paracetamol on activated carbon [68]. The CWAO of paracetamol was investigated both as a water treatment technique and as a regen‐ erative treatment of the carbon after adsorption in a sequential fixed bed process. They used three ACs as catalysts: a microporous basic AC and meso- and micro-porous acidic ACs. During the first CWAO experiment, they noticed that the adsorption capacity and catalytic performance of fresh basic activated carbons (S23 and C1) were higher than those of the fresh microporous acidic one (L27) despite its higher surface area. It seems that this situation changed after reuse, as finally L27 gave the best results after five CWAO cycles. Respirometric tests were also done with activated sludge and it was mentioned that in the studied conditions the use of CWAO enhanced the aerobic biodegradability of the effluent. The group also checked the different ageing by measuring the physico-chemical properties of activated carbons.
