The agglomeration of zero-valent iron nanoparticles (Fe-0 NPs) can significantly decrease the effective surface area of nanoparticles and thus reduce their catalytic performance. To avoid such agglomeration, a two-part Fe-0 NP-immobilization approach was developed: (1) The fabrication of a macroporous alginate substrate (MAS) that provided a large surface area capable of sustaining a high load of stable and well-dispersed Fe-0 NPs (26.06 wt.%). (2) A facile chemical reductive growth procedure to generate Fe-0 NPs (ca. 50-100 nm) that are covalently anchored to the surface of the MAS. The macroporous alginate substrate-supported Fe-0 nanoparticles (Fe-0 NPs/MAS) removed >96.5% of nitrates from an aqueous solution within 30 min, whereas unsupported Fe-0 NPs removed only 44.7% of nitrates over a longer period of time. These results demonstrate that MAS acts in a way to prevent the agglomeration of Fe-0 NPs and, in turn, to promote their redox activity compared to unsupported Fe-0 NPs. On the basis of our experimental results, a grow mechanism of Fe-0 NPs on the MAS was proposed, and potential implications for environmental applications of Fe-0 NPs/MAS were discussed. (C) 2016 Elsevier B.V. All rights reserved.