Catalytic Wet Air Oxidation of Organics-Laden Wastewater: In Situ Catalyst Regeneration and Process Scale-up

Abstract

Catalytic wet air oxidation (cWAO) technique is applied to treat an industrial wastewater with the chemical oxygen demand (COD) of ~120,000 mg/L in a trickle bed reactor packed with the Fe-Ag bimetal-tipped carbon nanofiber (CNF) catalyst supported on activated carbon beads (Fe-Ag-CNF/ACB). The catalyst is synthesized using a multi-step thermochemical method involving the suspension polymerization of phenol-formaldehyde co-polymers doped in situ with the Fe and Ag salts, followed by carbonization (900 °C, 2 h), steam activation (900 °C, 1 h), hydrogen reduction (650 °C, 2 h), and acetylene-chemical vapor deposition (650 °C, 1 h). The catalyst is characterized using the SEM, EDS, XPS, Raman, and XRD spectroscopy. The cWAO of the wastewater performed under continuous flow conditions in a packed bed tubular reactor shows 99% COD reduction at 244 °C and 27 bar-oxygen pressure. A 25 g-catalyst dose showed consistent performance for 12 h, with the COD measured less than 2000 mg/L in the treated water (1 cc per min) during the continuous operation of the packed bed reactor. The spent catalyst was successfully regenerated in situ through a well-defined seven-step sequential protocol involving deionized water washing at 25 °C, dilute acid treatment, acetone washing, deionized water washing at 80 °C, steam regeneration, H2 reduction, and KMnO4 impregnation, enabling one fresh cycle followed by four regeneration cycles. Upon scale-up, a fourfold increase in the flow rate (4 cc/min) is accommodated in the same reactor by increasing the catalyst loading to 100 g. A linear relationship between catalyst dosage and treatment capacity confirms that 9.77 g of catalyst is required for treating 1 L of wastewater. The overall cost including catalyst synthesis and four in situ regeneration cycles is determined to be ~28,500 INR or 320 USD per kg of the catalyst. The findings highlight the potential of Fe-Ag-CNF/ACB as an efficient, regenerable, cost-effective, and scalable oxidation catalyst for the treatment of an industrial organics-laden wastewater through the continuous operation of a packed bed reactor.