Evaluation of degradation and mineralization of glyphosate pollutant in wastewater using catalytic wet air oxidation over Fe-dispersed carbon nanofibrous beads

Abstract

• Complete degradation of PMG is possible by CWAO over porous Fe-CNF/ACB. • Kinetic model based on L-H type mechanism was proposed for degradation of PMG. • Toxicity assay confirmed treated solution to be environmentally safe. The current study introduces catalytic wet air oxidation (CWAO) as an efficient technique for complete mineralization of glyphosate or N-(phosphonomethyl) glycine (PMG) in wastewater using the iron nanoparticle (Fe NP)-dispersed carbon nanofibers (CNFs) decorated over activated carbon beads (ACBs) (~0.8 mm size). A high BET specific surface area of Fe-CNF/ACB (~338 m 2 /g), good Fe-loading (~4 mg/g), high thermal stability (25 – 1000 °C), augmented exposure of the Fe NPs to the surrounding liquid, and graphitic characteristics (I D /I G = 0.946) of the CNFs, all contribute towards efficient CWAO of the aqueous PMG. The experimental data show complete degradation of PMG at 100 mg/L at 220 °C and 25 bar within 6 h, using the catalyst dose of 0.75 g/L. The turnover frequency value was calculated to be 2.056 × 10 −2 s −1 , indicating an efficient role of the active Fe metal sites in degradation of PMG. Additionally, 80% chemical oxygen demand removal was measured in 6 h using Fe-CNF/ACB. Assuming the pseudo-first-order rate kinetics, the rate constant and apparent activation energy were calculated to be 0.01 min −1 at 220 °C, and ~50 kJ/mol, respectively. Initiated by the reactive hydroxyl radicals generated in the aqueous solution, PMG was converted to CO 2 , H 2 O, and some inorganic ions via formation of sarcosine and amino methyl phosphonic acid intermediates. The treated water showed no toxicity. The method of preparation for the Fe-CNF/ACB catalyst and the CWAO technology are scalable and should be explored to treat the other recalcitrant herbicide, pesticide and pharmaceutical organic compounds.