HDEHP and complexes formed in extraction
We need some more details such as what diluent you used. Maybe you were using pure DEHPA as the organic phase which is something which I would suggest that you do not do.

I would like to know if you did triplicate experiments, sorry if I offend you but I have no idea how good you are at the measurement of D values by solvent extraction. I like to think I am good at it, but I always use triplicate experiments. I will shake three different vials for each set of conditions. This allows me to see how reproduceable my work is, it also guards against me doing something stupid with one of the vials.

If you only do single extractions then it is possible that a random error causes the experiment to give misleading results. I hold the view that a beginner needs triplicate shaking experiments more than anyone. When you are new to solvent extraction you are more likely to make errors, also when you have done it for a while you might get careless and then you would need the triplicate experiments again to guard against mistakes.

The triplicate experiments will not prevent you making mistakes, but they will help to alert you to the fact that something has gone wrong.

I also worry about the use of very low acid concentrations, the Eu will tend to form hydroxides.

I think that you need to pay attention to the activity coefficents. I would be much more happy if you had used sodium nitrate to keep the ionic strength of the aqueous phase constant. I would want to do the experiment with a constant nitrate concentration to allow you to separate the effects of changing the nitrate concentration from that of the solvated protons.

I would like to point out that at very high nitrate concentrations the extraction mechanism for lanthanides with DEHPA can change from the normal ion exchange mechanism to a solvation mechanism which is similar to the extraction of plutonium from nitric acid by tributyl phospahte (PUREX process),