Schwertmannite (Sch), a good geo-sorbent for immobilizing molybdate oxyanion in natural environment, has been used as an efficient Fenton-like catalyst in the remediation of organic pollutants. However, what's the influence of immobilized molybdate (Mo-imm) on the catalytic activity of Sch and what's the catalytic difference between Mo-imm and dissolved molybdate (Mo-diss) in H2O2 activation are still unknown. In this study, we first find a dual mechanism of Mo-Sch involved in phenol oxidation. At low Mo-imm loadings (0.01 Mo-Sch and 0.05 Mo-Sch), phenol oxidation is predominantly mediated by HO center dot radicals generated from dissolved Fe(III) or surface Fe(III) on Sch, whereas at high loadings (1Mo-Sch and 10Mo-Sch), (OOH)-O-center dot as well as O-1(2) generated from monomeric Mo complexes on Sch under acidic conditions also play important roles in phenol oxidation. Unlike Mo-Sch, Mo-diss forms dimeric Mo2O3 (O-2)(4)(2-) with H2O2 under acidic conditions, which can oxidize phenol directively or via (OOH)-O-center dot radicals, generated from dissolved Mo2O3(O-2)(4)(2-). The dimerization of Mo-diss with H2O2 inhibits O-1(2) generation. During the experiments, Mo-diss can also interfere with the chain reactions between dissolved Fe(III) and H2O2, accelerating H2O2 decomposition via an O-2 generation pathway, which significantly reduces the utilization efficiency of H2O2 in phenol oxidation. (C) 2015 Elsevier B.V. All rights reserved.