Catalytic wet air oxidation of organics-laden wastewater: in situ catalyst regeneration and process scale-up

Abstract

The catalytic wet air oxidation (cWAO) technique is applied to treat industrial wastewater with a chemical oxygen demand (COD) value of ∼120 000 mg L⁻¹ in a trickle bed reactor packed with the Fe–Ag bimetal-synthesized 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 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-based chemical vapor deposition (650 °C, 1 h). The catalyst is characterized using SEM, EDS, XPS, Raman spectroscopy, and XRD. 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 in the treated water maintained below 2000 mg L⁻¹ at a flow rate of 1 cc min⁻¹ 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 washing with deionized water at 25 °C, dilute acid treatment, washing with acetone, washing with deionized water at 80 °C, steam regeneration, H₂ reduction, and KMnO₄ impregnation, enabling four such regeneration cycles. Upon scale-up, a fourfold increase in the flow rate (4 cc min⁻¹) is possible in the same reactor by increasing the catalyst loading to 100 g. A linear relationship between the catalyst dosage and the treatment capacity confirms that 9.77 g of the 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 ∼45 336 INR or 495 USD per kg of the catalyst. These findings highlight the potential of Fe-Ag-CNF/ACB as an efficient, regenerable, cost-effective, and scalable oxidation catalyst for the treatment of industrial organics-laden wastewater through the continuous operation of a packed-bed reactor.