Humic acids adsorption and decomposition on Mn2O3 and α-Al2O3 nanoparticles in aqueous suspensions in the presence of ozone

Abstract

The removal and decomposition of humic acids (HAs) in the presence of ozone and aqueous suspensions of Mn2O3 and α-alumina (Al2O3) nanoparticles was investigated. Mn2O3 presented lower BET specific surface area (15.6 m2 g-1 vs 45.8 m2 g-1) but a higher point of zero charge (PZC) (5.9 vs 4.2) than α-Al2O3. Solution pH played a key role in the adsorption of HAs and catalytic oxidation on the surface of α-Al2O3 and Mn2O3 nanoparticles. The adsorption capacity of α-Al2O3 at the natural pH of HAs in water (pH 5.5) was up to 2.903 gHAs g-1, but no adsorption occurred onto the Mn2O3 nanoparticles, due to the unfavorable surface charge at pH 5.5. In consequence, although Mn2O3 was a more efficient catalyst (khet = 0.7 L-1 min-1 g-1) than α-Al2O3 (khet = 0.2 L-1 min-1 g-1) for the decomposition of O3, Mn2O3 did not exhibited catalytic action during the ozonation of HAs at pH 5.5. Instead, the Mn2O3 catalytic action was significant at pH equal to PZC (catalytic rate constant ratio k1-HAcat/ k1-HA = 1.562). Overall, α-Al2O3 exhibited the highest catalytic removal rate of HAs during ozonation (k1-HAcat/ k1-HA = 2.298) due to favorable surface charge and larger specific surface area. The main mechanism for HAs removal in the presence of α-Al2O3 involves simultaneous adsorption of both HAs and O3, the reaction of ozone from the bulk solution and the catalytic decomposition of HAs on the solid surface by ROS, through complex series-parallel reactions. The α-Al2O3 dosage up to 0.5 g L-1 required to remove HAs by catalytic ozonation was significantly lower than in other studies employing granular activated carbon, iron coated zeolite or γ-alumina catalysts.