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[Display omitted] • Exploring the defluorination performance by different crystalline MnO 2. • Lanthanum-modified MnO 2 with four crystal forms for defluorination were prepared. • Lanthanum loading significantly enhanced the fluoride ions adsorption. • The quantity of surface hydroxyl groups strongly influenced the defluoridation. • Ion exchange was the main process for fluoride ions adsorption. MnO 2 has a large potential for defluoridation, however, the influences of its crystal forms on the performance and mechanism of defluoridation are unknown. The research systematically investigated the defluoridation performance and mechanism of a total of eight materials with different crystal forms of MnO 2 (α, β, γ, δ) before and after lanthanum modification, separately. It was indicated that the largest Langmuir adsorption of α-MnO 2 and La-γ-MnO 2 for fluoride achieved 5.04 mg/g and 42.39 mg/g, correspondingly. The surface hydroxyl group and lanthanum loading of the material proved to be the major contributors to defluoridation. For the adsorption performance of eight materials on fluoride, pH was a significant influencing factor, with the highest adsorption performance at pH 5 ∼ 6. The eight materials showed consistent selectivity for fluoride adsorption in the existence of different anions, with the order of influence on the removal efficiency: CO 3 2–> NO 3 –> SO 4 2–> Cl–. The studies of FTIR, XPS, and variation of solution pH further illustrated that the adsorption mechanism of the eight materials on fluoride was the ion-exchange interaction and the complexation reaction. In addition, the effect of crystal forms on the adsorption of fluoride ions was further elaborated by DFT analysis. Finally, this study provided an in-depth discussion of the parameters of the dynamic process as well as the nature of the penetration curves by applying the Thomas, Clark, and BDST models. [ABSTRACT FROM AUTHOR]

At present, the potential mechanism of manganese oxide (MnO 2) activation of PI and the key active sites of PI activation are still unclear and controversial. To this end, three different crystal forms of MnO 2 were prepared in this study and used to activate PI to degrade pollutants. The results showed that different crystal types of MnO 2 showed different catalytic abilities, and the order was γ-MnO 2 > α-MnO 2 > β-MnO 2. Through quenching experiments, EPR tests, Raman experiments and in situ electrochemical experiments, it has been confirmed that electron transfer-mediated non-free radical process is the main mechanism of pollutant degradation, in which the active substance is the highly active metastable intermediate complex (MnO 2 /PI*). Hydroxyl radical (HO•), superoxide radical (O 2 •−), singlet oxygen (1O 2) and iodine radical (IO 3 •) did not participate in pollutant degradation. The quantitative structure-activity relationship analysis confirmed that the catalytic performance of MnO 2 was highly positively correlated with the surface Mn(IV) content, which indicated that the surface Mn(IV) site was the main active site. Overall, this study will be of great help to the design and application of manganese dioxide activation for periodate degradation of pollutants. [Display omitted] • α-MnO 2 , β-MnO 2 and γ-MnO 2 could all effectively activate PI. • The α-MnO 2 /PI, β-MnO 2 /PI and γ-MnO 2 /PI systems were non-radical processes dominated by electron transfer. • The high activity of metastable intermediate complex (MnO 2 /PI*) was the main factor for organic pollutants removal. • The catalytic activities of MnO 2 were positively correlated with the content of Mn (IV) on the catalyst surface. [ABSTRACT FROM AUTHOR]