Remediation of water from perfluorooctanoic acid (PFOA) with ozonation and photocatalysis: Experimental studies and numerical modeling

Authors: Anastasios Melitsiotis, Konstantinos Christodoulis, Michalis Karavasilis, Nadia Bali, Maria Theodoropouloua, Masho Hilawie Belay, Davide Rotondo, Elisa Robotti, Emilio Marengo, Francesco Dondero, Christos D. Tsakiroglou

Abstract: The treatment of relatively high concentration (10–50 mg/L) perfluorooctanoic acid (PFOA) aqueous solutions is tested at ambient pH (5–7.5) in an annular photo-reactor with recirculation, and in a semi-batch bubble flow reactor with ozonation. Zinc oxide and iron-doped zinc oxide (Fe-ZnO-SL) photocatalytic nanoparticles were synthesized and immobilized on Duranit (ZnO-DN), and soda lime (Fe-ZnO-SL) beads by dip-coating and thermal annealing. In photoreactor, the recirculating rate was fixed to 30 mL/min, and a 6 W UV-C lamp emitting at 254 nm was used. Ozone was generated at concentrations ~5–50 g/m3 from oxygen at flow rates 0.1 and 0.2 L/min. The methylene blue active substances with UV-Vis spectrophotometry (MBAS-UV-Vis) was used to measure the PFOA concentration, and evaluated with reference to Ultra High Performance Liquid Chromatography-High Resolution Mass Spectrometry (UHPLC-HRMS). The advection-dispersion-reaction model, and tank-in-series model were used to estimate with inverse modeling the kinetic parameters of PFOA degradation by photocatalysis and ozonation, respectively. The experimental results obtained with the cost-effective and fast MBAS method were comparable to those of the expensive, time-consuming, and high accuracy UHPLC-HRMS method. The photocatalytic degradation of PFOA lasted a couple of days with the apparent kinetic constant depending on the catalyst type, ageing, and air injection, and the PFOA removal efficiency ranging from 22 % to 93 %. The PFOA removal efficiency by ozonation varied from 32 % to 78 %, depending on the gas flow rate, the initial PFOA concentration, and ozone concentration. Numerical modeling reveals that ozonation is dominated by the direct ozone oxidation over early times, and indirect oxidation by generated active species over late times. Short-chain congeners, such as the perfluoroheptanoic acid (PFHpA), perfluorohexanoic acid (PFHxA) and perfluoropentanoic acid (PFPeA), were detected as intermediate products at very low concentrations, and trifluoroacetic acid (TFA) was identified as the main byproduct of ozonation. For photocatalysis, the highest PFOA removal efficiency (RE) and lowest energy expenditure (Em) were achieved with virgin (RE~93 %, Em~0.19 kWh/mg-PFOA) or aged (RE~70 %, EE~0.11 kWh/mg-PFOA) ZnO nanoparticles immobilized on Duranit beads. For ozonation, the highest PFOA removal efficiency was achieved at low gas flow rate and low (~10 mg/L) initial PFOA concentration (RE~70–78 %), while the energy expenditure was minimized (Em~0.075–0.085 kWh/mg- PFOA) for high values (~50 mg/L) of the PFOA concentration.

Keywords: Perfluorooctanoic acid, Trifluoroacetic acid, Ozonation, Photocatalysis, Energy expenditure, Numerical modeling