Synthesis and characterization of cryptomelane (α-MnO2) nanoparticles: Influence of phthalic acid adsorption and oxidation on the mineral structure

Abstract

Cryptomelane (α-MnO2) nanoparticles synthesis was achieved via a Sol-Gel route in mild conditions. XRD analysis showed that the α-MnO2 phase was of high purity and crystallized in the monoclinic space group I2/m, with 12 nm average crystallite size (D). After interaction with an organic pollutant (Phthalic acid, PA), excepting the pronounced decrease in β angle, α-MnO2 cell parameters slightly changed, probably resulting from leaching of K+ cations. Adsorption isotherm of PA on α-MnO2 nanoparticles was fitted in L2 type Langmuir model. Accordingly, an outer-sphere PA-Mn surface complex was suggested. Further, the mineral oxide exerted a low activity in the decarboxylation of PA. Raman analysis illustrated the major bands, of α-MnO2, and their evolution after adsorption indicated the effect of PA on the phase structure, as revealed by the XRD Rietveld refinement. AFM analysis showed stick shaped nanoparticles with D = 15 nm, but the interaction with PA induced aggregation.