Dear Karin,

sorry to see that your interesting technical question has not yet been answered by experts in the field. As an inorganic chemist I'm certainly not among these experts. Thus all I can do right now is suggest to you some potentially useful articles which might (or might not) help you in your analysis. Please have a look at these papers:

Classical and intermediate monocytes scavenge non-transferrin-bound iron and damaged erythrocytes

Article Classical and intermediate monocytes scavenge non-transferri...

and

“Pumping iron”—how macrophages handle iron at the systemic, microenvironmental, and cellular levels

Article “Pumping iron”—how macrophages handle iron at the systemic, ...

Fortunately both articles have been posted as public full texts on RG. Thus you can freely download them as pdf files. I also strongly suggest that you search the "Publications" sections of RG for other helpful literature references.

Good luck with your research!