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Graphical abstract Highlights • δ -MnO 2 and γ -MnO 2 showed different APAP removal kinetics at different pH. • Strong acidity largely promoted the APAP removal by MnO 2 , especially for γ -MnO 2. • Low pH values promoted nucleophilic addition and dimerization of APAP by MnO 2. • Dimerization and hydrolysis-oxidation were two major APAP transformation pathways by MnO 2. • Low pH values largely decreased the environmental risk of APAP transformation products by MnO 2. Abstract Manganese dioxides (MnO 2) govern the polymerization of phenolic and anilinic pollutants in nature water body. The studies on the chemistry of different phases of MnO 2 was relatively limited. Here we compared two MnO 2 , layer δ -MnO 2 and tunnel γ -MnO 2 , on their reaction kinetics and removal pathways with acetaminophen (APAP) as model compound. We found the kinetics of acetaminophen transformation by MnO 2 were largely influenced by pH values and MnO 2 structures. In neutral systems, APAP removal by δ -MnO 2 was faster than by γ -MnO 2. This was attributed to the higher catalytic potential of δ -MnO 2. In acidic systems, APAP removal by γ -MnO 2 was faster than by δ -MnO 2. This was attributed to the larger surface area and weaker electrostatic repulsion of γ -MnO 2 to APAP in acidic conditions. Furthermore, the pathways of acetaminophen transformation were influenced by pH values. Dimerization and hydrolysis-oxidation were the two major APAP transformation pathways by both MnO 2 in neutral and acidic systems. Low pH values induced the nucleophilic addition between quinoneimine (the product of hydrolysis-oxidation) and anilines (e.g. , APAP and its dimers). Low pH values also promoted the dimerization degrees of APAP. The hydrophilicity of APAP transformation products by MnO 2 were significantly reduced by low pH values. This work reconsidered the significances of acidity in the pollutants transformation by MnO 2 , and provided new perspectives to study the pollutants transformation by different phases of MnO 2. [ABSTRACT FROM AUTHOR]

• Global softness of the NHCs was employed to correlate with the ozonation efficiency. • Higher global softness of NHCs resulted in a greater degradation efficiency. • Fewer N atoms in heterocyclic ring generated more OH from ozone dissociation. • Reactive sites for both O 3 and radical attack were located by FO theory. • Combined DFT and TOF-MS results were used to identify the hydroxylation products. Nitrogen-containing heterocyclic contaminants (NHCs) have aroused intense environmental issues in waterbody because of their high toxicity and strong recalcitrance against natural degradation. In this study, ozonation degradation on four typical NHCs - benzotriazole (BTA), benzimidazole (BMZ), indazole (IDZ), and indole (IDO) was carried out to investigate the effects of N atoms on degradation efficiency. Global softness of the NHCs was calculated by density functional theory (DFT) and correlated with ozonation efficiency. It was discovered that higher global softness of the NHCs resulted in the greater degradation efficiency. Mineralization results together with the quenching tests results suggested that NHCs with ortho-positioned N atoms in heterocyclic ring are easier to be mineralized, while the presence of para-positioned N atoms increased the recalcitrance for destruction. Apart from hydroxyl radicals, superoxide radicals and singlet oxygens were also identified by electron paramagnetic resonance (EPR) spectra. Condensed Fukui index was employed to probe the potential active sites for both direct ozone oxidation and radical-based attack. Single point energies were further calculated among these active sites to seek the initial-stage hydroxylation intermediates. Combined the calculation results with the identified intermediates from time-of-flight mass spectroscopy (TOF-MS), degradation routes of the NHCs were proposed. This study discovered structure-dependent behavior of NHCs on ozonation efficiency and envisaged a more accurate strategy for establishing the degradation pathway. [ABSTRACT FROM AUTHOR]