In this study, three trivalent Mn (oxyhydr)oxides, bixbyite (alpha-Mn2O3), hausmannite (Mn3O4) and manganite (gamma-MnOOH) were synthesized and used as persulfate (PS) activators in phenol oxidation. Of these three Mn (oxyhydr)oxides, only gamma-MnOOH shows a good catalytic activity in PS activation with more than 80% phenol oxidized within 360 min in the full pH range. With increasing concentrations of PS and gamma-MnOOH, phenol removal efficiency increased. TOC results also suggest that phenol oxidation at pH 11 is more complete than those at pH 7 and 3. We further confirm that mechanism involved in phenol oxidation is pH dependent in gamma-MnOOH/PS system. Although SO4 center dot- and (OH)-O-center dot were both found under acidic and alkaline conditions, they did not contribute equally under different pHs. At pH 11, SO4 center dot- and (OH)-O-center dot generated from PS activation efficiently oxidize phenol. At pH 3, the strong oxidation activity of gamma-MnOOH and an oxidative intermediate formed between S2O82- and gamma-MnOOH primarily contribute to phenol oxidation. At pH 7, an oxidative intermediate formed between gamma-MnOOH and S2O82- is the predominant oxidative species in phenol oxidation. (C) 2015 Elsevier B.V. All rights reserved.