Reply to the Comment on 'FeS colloids – formation and mobilization pathways in natural waters' by S. Peiffer, D0EN00967A

In this response to the comment by S. Peiffer, Environ. Sci.: Nano, 2021, we wish to first underline the main objective of the paper by V. Noel, et al., Environ. Sci.: Nano, 2020, 7, 2102–2116, which was to better characterize the chemical parameters controlling the generation of Fe–S-colloids under anaerobic conditions. Export of highly reactive FeS-compounds from reducing to more oxidizing environments has down-stream consequences for electron transfer and biogeochemical reactivity. Thus, detailed knowledge of formation, nature, and stability of these colloids is critical for developing conceptual models to predict Fe remobilization under sulfidic conditions in natural environments. However, our understanding of the biogeochemical behavior of Fe–S-colloids is not sufficient to develop such models. This should not be interpreted as indicating that S transformations and speciation in these systems are not important, as we have previously emphasized in several publications. On the contrary, we show that detailed examination of the Fe chemistry provides clear data in terms of the Fe–S-colloid composition (i.e., S-bearing colloids), as we demonstrate in this response.