Iron-containing nanoparticles, such as green rusts, can be formed by either chemical (c-GR) or biological (b-GR) pathways. It is known that c-GRs display very high reactivity toward organic and inorganic contaminants and thus have great potential for the remediation of contaminated environments, whereas b-GRs are very weakly reactive. This reactivity difference is usually attributed to much higher surface/volume ratio of c-GR compared to b-GR Using atomic and chemical force microscopy to probe the reactivity at the nanoscale of both types of nanoparticles, we are able to show that the primary reason for the low reactivity of b-GR is not the low surface/volume ratio but the passivation of the surface due to the presence of biological exopolymers (EPS). This conclusion should hold true for many biological nanoparticles and allows us to explain their often observed low, yet unexplained, reactivity.